CHAPTER 3 RESULTS AND DISCUSSION -...

63

CHAPTER 3

RESULTS AND DISCUSSION

This chapter deals with the results obtained from experimental methods

used for synthesis, characterization and applications of metal oxide nanoparticles such

as Al2O3, ZrO2 and TiO2. The obtained results were used to explore the purity, particle

size, morphology, structural and textural properties of metal oxide nanoparticles along

with their physico-chemical properties for industrial applications.

3.1 SYNTHESIS, CHARACTERISATION AND APPLICATION OF Al2O3 NANOPARTICLES

The work described in this chapter is about the synthesis of Al2O3

nanoparticles from raw bauxite and its application in anti-corrosive coating. A novel

processing method has been used to produce the non-aggregated Al2O3 nanoparticles

from the synthetic Bayer liquor obtained from natural bauxite. The synthesis of nano

Al2O3 particles from inorganic salt solutions is an economical method for large scale

production. The objective of this research includes the production of Al2O3

nanoparticles with very fine crystallite size, low degree of the crystallite aggregation,

low production cost, high production rate and a relatively easy scale-up for mass

production. A review of the literature proves that this is an innovative first time effort

for the direct production of alumina nanoparticles from raw bauxite.

64

In this work, the synthesis of Al2O3 nanoparticles follows three different

methods such as precipitation, sol-gel and spray pyrolysis. All the three production

methods were optimised and identified in terms of particle size, surface area,

morphology, purity and yield. The optimised nano Al2O3 particles were applied for

nanostructured surface protective coatings. Major efforts were paid on fabrication of

nanostructured Al2O3 coating on stainless steel (SS304) surface for the protection of

acid corrosion. Design and construction of complex nanostructured Al2O3 coating on

stain steel specimen was performed by a dip coating technique. The thickness of nano

Al2O3 filled silica coating on steel specimen was controlled and varied by using layer

by layer coating method. The effect of nano coating on anti-corrosive properties of

stain steel in acid media was performed using conventional weight loss method.

3.1.1 Precipitation

In this method, the synthesis of Al2O3 nanoparticles follows two main

stages, namely precipitation and ball milling. The soft aggregates obtained by the

precipitation method are disaggregating during a subsequent ball milling step. In terms

of particle size distribution, average particle size and particle morphology, both

precipitation and ball milling methods were compared. The successful synthesis of

uniform sized pure -alumina nanoparticles with the cubic phase and the influence of

mineral acid precipitants, such as HCl, H2SO4 and HNO3 on crystallite size, particle

size, surface area, pore volume, pore size and shape of -alumina are reported. The

formation of hydrous alumina (Al(OH)3 H2O) from aluminium salts such as Al2(SO4)3,

AlCl3 and Al(NO3)3 by aqueous precipitation under acidic conditions was studied

extensively (Jin et al. 2007, Pang and Bao 2002). However, very little is known about

the precipitation of hydrous alumina from the synthetic Bayer liquor using mineral acid

precipitants like H2SO4 (a), HCl (b) and HNO3 (c). In addition, the influence of mineral

acid precipitants on the physico-chemical properties of nano alumina is an area of great

interest. In this process, three hydrous alumina (Al(OH)3 H2O) samples are prepared

65 with three precipitants namely H2SO4, HCl and HNO3 under identical conditions. The

textural properties and particle size of Al2O3 samples precipitated with different

precipitants are presented in Table 3.1.

Table 3.1 Particle size and textural properties of Al2O3 samples precipitated with different precipitants

The pH value of the synthesised Bayer liquor is 13. The precipitate of

hydrous alumina (Al(OH)3 H2O) is obtained after adding H2SO4. Wet cake is obtained

by filtering the solution and is weighed to determine the pH value with the maximum

solid product. Figure 3.1 shows the yield of wet cake (Al(OH)3 H2O) versus pH of

precipitation. It indicates that the solid content of the solution will vary with the

controlled pH value of the synthetic Bayer liquor. The weight of the yield of wet cake

is 29 g at pH 11 and increases with a decrease in pH value to a maximum weight of 60

g at pH 7. When the pH value is lower than 7, the weight decreases with decrease in

Sample Precipitant BET particle size dBET (± 0.5)

nm

Particle size distribution dPSD (± 3) nm

TEM particle size dTEM (± 5)

nm

a H2SO4 9.3 89 100

b HCl 8.2 71 75

c HNO3 5.6 58 50

Sample Precipitant BET

surface area

m2g-1 (± 6)

Total pore volume

cm3g-1

Average pore diameter

Å

a H2SO4 114 0.0576 20.06

b HCl 128 0.0634 20.02

c HNO3 190 0.0938 19.72

66 pH value. The above study reveals that pH 7 is the controlled value for neutralization

step.

The XRD patterns of the prepared samples are shown in Figure 3.2. The

diffraction pattern of the synthesised samples (Figure 3.2a, b and c) is assigned to the

-Al2O3 phase with

cubic symmetry. The obtained XRD patterns of all the samples (a, b and c) agree well

with the standard powder diffraction data (JCPDS File No.: 79-1558). From the XRD

data, it is found that all the samples (a, b and c) have similar crystalline phases with

different crystallite sizes. -Al2O3) is

approximately 9.5, 5 and 3 nm, respectively for samples a, b and c. The average

crystallite size of calcined alumina decreases from 9.5 to 3 nm with the change of

precipitants from H2SO4 to HNO3. Hence, it is evident that during the precipitation, the

precipitant affects the surface properties of hydrous alumina suspension through the

surface adsorption of anions on the surface of metal hydroxide. Detailed discussions

about the surface adsorption of anions on metal hydroxide were studied extensively

(Jin et al. 2007 and Vayssieres 2009). Figure 3.3 illustrates the DTA and TGA curves

of the hydrous alumina precipitated using HNO3 precipitant. Two endothermic peaks

are observed in the DTA curve at 383 K and 572 K. The three step transformation

sequence for the molecular decomposition of Al(OH)3 H2O into -Al2O3 are as follows:

Scheme 3.1 Schematic representation for conversion of hydrous alumina into -Al2O3

2Al(OH)3 . H2O Hydrous Alumina

383 K Al2O3 . 3H2O + 2H2O

572 K

673-723 K -Al2O3 Al2O3 + 3 H2O Amorphous

67

Figure 3.1 Yield of wet cake obtained versus pH of precipitation

Figure 3.2 XRD patterns of Al2O3 samples precipitated with different precipitants: a) H2SO4, b) HCl and c) HNO3

68

Figure 3.3 DTA/TGA curves of the hydrous alumina precipitated using HNO3 precipitant

The observed intense endothermic peak at 383 K is assigned to elimination

of physically absorbed water (Yang et al. 2003). The second endothermic peak

observed at 572 K is related to decomposition of the hydrous alumina to produce

amorphous alumina involving elimination of OH groups. The broad exothermic peak

centered at 683 K is attributed to the transformation of amorphous alumina to -Al2O3

(Park et al. 2005). The XRD pattern of the hydrous alumina calcined at 773 K is also

an evident for the formation of -Al2O3. The weight loss during burning is equal to

18.6% and 46.5% respectively at 383 and 572 K. The experimentally observed second

weight loss of 46.5% is in close agreement with theoretically calculated loss of 46.7%

by the reaction given in scheme 3.1. It can be observed from TGA results that the wet

chemical methods yield hydrous samples with physically adsorbed water molecules.

Figure 3.4 presents the energy dispersive spectrum of synthesised Al2O3 precipitated

with H2SO4, HCl and HNO3. It can be observed from Figure 3.4 that the major

69 elements present in the sample precipitated with H2SO4 are O (44.15%), Al (49.65%),

Na (2.9%) and sulphur (2.3%). Similarly, the sample precipitated with HCl is O

(44.99%), Al (50.61%) and Na (3.2%), while the sample precipitated with HNO3 is O

(46.55%) and Al (52.35%). It is evident that the sample precipitated with H2SO4

contains 2.9% sodium and 2.3% sulphur while the sample precipitated with HCl

contains 2.9% sodium. On the other hand, the sample precipitated with HNO3 does not

contain any sodium or sulphur in its composition.

The precipitation of hydrous alumina with H2SO4 leads to sodium sulphate

as a by product. The SO42 present in the mother liquor is adsorbed on the surface of

hydrous alumina because of the better coordination ability of sulphate ions (Jin et al.

2007). Conversely, hydrous alumina precipitated with HCl leads to sodium chloride as

a by product and the Cl present in the mother liquor is chemisorbed on the hydrous

alumina surface (Vayssieres 2009). The imbalanced sodium ions in both cases (a and

b) are also chemisorbed on the surface of hydrous alumina. During the calcination of

hydrous alumina, the adsorbed sulphur and sodium ions enter into the powder

composition, whereas the adsorbed chloride ions are eliminated during calcination

because of their poor coordination ability (Jin et al. 2007). The adsorbed sulphur and

sodium ions influence the physico-chemical and textural properties of alumina

nanopowder. Chemical analysis shows that the samples a, b and c contain respectively

90.5%, 92.4% and 95.7% of Al2O3. On the other hand, EDS (Figures 3.4a, b and c)

results show that the samples a, b and c contain respectively 93.8, 95.6 and 98.9% of

Al2O3. The results obtained from quantitative elemental analysis (EDS) are in close

agreement with those obtained from the chemical analysis. Figure 3.5 shows the FTIR

spectra of alumina samples precipitated with H2SO4, HCl and HNO3 after calcination at

773 K. The band observed at 1633 cm 1 is common in all three samples and is assigned

to the O–H bending vibration of weakly bounded water molecules (Teoh et al. 2007).

70

Figure 3.4

Energy dispersive spectrum of Al2O3 samples precipitated with different precipitants

a) Precipitated with H2SO4 b) Precipitated with HCl

c) Precipitated with HNO3

71

Figure 3.5

FT-IR spectra of Al2O3 samples precipitated with different precipitants: a) H2SO4, b) HCl and c) HNO3

72

The absorption peaks obtained at 1510, 1480 and 1410 cm 1 are due to the

presence of the carbon–carbon (C–C) deformation. The band at 1100 cm 1 is assigned

to the Al–OH bending vibration of Al–OH–Al groups (Teoh et al. 2007). The

absorption peaks observed between 600 and 800 cm 1 in all the samples are attributed

to the stretching and vibrational modes of AlO4, AlO6 and OH groups (Teoh et al.

2007). The peak observed at 734 cm 1 is ascribed due to the bending vibration of AlO4

groups. The bands observed at 879, 833, 475 and 502 cm 1 are assigned to the

stretching and bending vibrations of Al–O bond (Teoh et al. 2007). The above results

summarise the finding that the Al2O3 powder consists of adsorbed water molecules and

residual carbon.

Figure 3.6 presents the PSD of Al2O3 samples precipitated with H2SO4, HCl

and HNO3 before ball milling. It is clear from Figure 3.6 that the sample precipitated

with H2SO4 has large particle size i.e., 452 nm (d50) with narrow which PSD is in the

range of 376 (d10) to 533 (d90) nm. On the other hand, the samples precipitated with

HCl and HNO3 have small particle size, i.e. 388 (d50) and 272 (d50) nm with the wider

PSD which is in the range of 300 (d10) to 482 (d90) nm and 168 (d10) to 464 (d90) nm

respectively. Specially, compared with HCl, the sample precipitated with HNO3 has a

smaller particle size (272 nm). It is the standpoint that the change of precipitant has a

dramatic effect on the particle size and size distribution of alumina powder. It is

concluded that the adsorption of SO4 and Cl ions on the surface of hydrous alumina

takes place when using precipitants such as H2SO4 and HCl. During calcination, the

chemisorbed anions and sodium ions are induced in the growth of alumina powder

grains, which leads to higher particle size, when HNO3 is used as a precipitant, NO3

ions are not adsorbed on the surface of hydrous alumina. It is removed as NaNO3 by a

subsequent washing process after the precipitation and it does not affect the calcination

process. Hence, it is clear that the samples precipitated with H2SO4 (a) and HCl (b)

have large particle size than the sample precipitated with HNO3 (c).

73 Figure 3.6 Particle size distributions of Al2O3 samples precipitated with different

precipitants: a) H2SO4, b) HCl and c) HNO3, before ball milling Figure 3.7 Particle size distributions of Al2O3 samples precipitated with different

precipitants: a) H2SO4, b) HCl and c) HNO3, after ball milling

74

Apart from the mean particle size, size distribution is the most important

information obtained from the PSD data. PSD measures the broadness and the degree

of asymmetry of size distribution. Physically, it is a measure of homogeneity in the size

of the particles. The effect of ball milling on the PSD and average diameter was studied

with the help of PSD analysis. In addition, the width of the PSD can be directly derived

from the scattering parameters obtained from the PSD results. The particle distribution

of Al2O3 samples after ball milling for 3 h at 500 rpm is shown in Figure 3.7. It can be

seen from the Figure 3.7 that the prepared alumina powder after ball milling yields

mean particle size of 89 nm (d50) for sample (a), 71 nm (d50) for sample (b) and 58 nm

(d50) for sample (c) with distribution range respectively 38 (d10) to 187 (d90) nm, 12

(d10) to 202 (d90) nm and 10 (d10) to 214 (d90) nm (Figures 3.7a, b and c). Hence, it is

evident that precipitation followed by calcination yields aggregated alumina particles,

which are further disaggregated by ball milling. The mean diameter of aggregated

alumina particles are reduced four to five times employing the ball milling process.

From the PSD results, we conclude that the precipitation, drying and

calcination steps lead to aggregated particles that are further disaggregated by a

subsequent ball milling process. The influence of ball milling on particle size and

morphology is also well established with SEM studies. Figure 3.8 shows the SEM

image of Al2O3 samples precipitated with H2SO4 (a) before (b) after ball milling, HCl

(c) before (d) after ball milling and HNO3 (e) before (f) after ball milling. Similarly,

with PSDs (Figures 3.6 and 3.7), the SEM micrographs of the prepared alumina before

ball milling (Figures 3.8a, c and e) show a highly aggregated surface with higher

particle size. Particle aggregation is apparent, suggesting the formation of a continuous

particulate network in the suspension structure, whereas the SEM micrographs of the

ball milled alumina samples (Figures 3.8b, d and f) show disaggregated particles with

quite reduced particle size and highly shaped spherical morphology.

75

Figure 3.8 SEM images of Al2O3 samples obtained through precipitation route

a) Precipitated with H2SO4 before ball milling

b) Precipitated with H2SO4 after ball milling

c) Precipitated with HCl before ball milling

d) Precipitated with HCl after ball milling

e) Precipitated with HNO3 before ball milling

f) Precipitated with HNO3 after ball milling

76

Aggregated and non-aggregated alumina could be well distinguished from

the SEM results. The SEM findings not only justify the aggregation of the particles but

also show the particle morphology. The TEM images of Al2O3 samples precipitated

with H2SO4, HCl and HNO3 are shown in Figure 3.9. From the Figure 3.9, the

morphologies and particle size of alumina nanoparticles can be confirmed. It can be

seen from the TEM images that the sample precipitated with HNO3 has predominantly

spherical particles with an average particle size of 50 nm while the sample precipitated

with HCl has spherical particles (75 nm) with a slightly agglomerated surface. On the

other hand, the sample precipitated with H2SO4 is said to have nearly spherical

morphology with an average particle size of 100 nm with an agglomerated surface. The

above results are in good agreement with SEM observations, which are presented in

Figures 3.8(b, d and f).

Table 3.1 summarizes the particle size and textural properties of Al2O3

samples precipitated with H2SO4, HCl and HNO3. In Table 3.1, the mean particle size

measured in the TEM images (dTEM) has close resemblance to the results obtained from

the PSD measurements (dPSD). On the other hand, the average particle diameter

measured from the BET surface area (dBET) does not agree with the results obtained

from the TEM (dTEM) and PSD (dPSD) measurements. It is understood that the

synthesised alumina samples have the primary particles (dBET), which is in the range

from 9.3 to 5.6 nm. To explain the differences in particle size, we compared the results

obtained from PSD, TEM and BET measurements (Table 3.1). It is evident that 9 to 10

primary particles are grown in a combined manner into a single secondary particle,

which is in the range of 89 - 58 nm (dPSD) and 100 - 50 nm (dTEM). It is also evident

that the ball milling process efficiently reduces the aggregation of the secondary

particles and not the primary particles. It can be seen from Table 3.1 that the prepared

alumina powder after ball milling yields an SSA of 114, 128 and 190 m2 g 1

respectively for the samples a, b and c.

77 Figure 3.9 TEM images of Al2O3 samples precipitated with different precipitants

a) Precipitated with H2SO4 b) Precipitated with HCl

c) Precipitated with HNO3

78 Figure 3.10 N2 adsorption-desorption isotherms of Al2O3 samples precipitated with different precipitants: a) H2SO4, b) HCl and c) HNO3

79

The sample precipitated with H2SO4 yields a smaller pore volume

(0.0576 cm3 g 1) and higher pore size (20.06 Å) when compared with the sample

precipitated with HNO3. To explore the overall textural properties, we compared the

area, volume, pore size and the average particle diameter of the alumina powders. An

increase in the total pore volume (0.0576-0.0938 cm3 g 1) and a decrease in the average

diameter of the pore (20.06-19.72 Å) and particles (9.3-5.6 nm) were noted while

changing the precipitants from H2SO4 to HNO3. The above result suggests that the

change of precipitants from H2SO4 to HNO3 leads to higher surface area and lower

particle size. Figure 3.10 show the N2 adsorption-desorption isotherms of Al2O3

samples precipitated with H2SO4, HCl and HNO3. The shape of the isotherms (Figure

3.10a, b and c) shows the presence of micropores, mesopores and macropores. It can be

seen from Figure 3.10 that the hystereses for samples (a) and (b), i.e., the samples

precipitated with H2SO4 and HCl, are associated with slit shaped pores (Boer, 1958)

whereas hysteresis (c), i.e., the samples precipitated with HNO3 is attributed to ink

bottle pores (Boer, 1958). Hence, it is evident that the change of precipitants from

H2SO4 to HNO3 induces a change in particle size, surface area, pore size, volume and

shape.

3.1.2 Sol-Gel

Sol-gel is one of the ecofriendly routes proposed in this work for mass

production of Al2O3 nanoparticles from natural bauxite. It is a commonly used

simplified technique to produce mesoporous Al2O3 nanoparticles without any use of

expensive instrumental facilities. In the present study, reasonable efforts have been

made to produce Al2O3 nanoparticles with higher surface area, low particle size and

uniform spherical morphology. The XRD pattern of the sol-gel derived nano alumina

particles after calcination at 773 K is shown in Figure 3.11(a). It can be seen that nano

alumina particles obtained by sol-gel route has -alumina phase with cubic symmetry.

All the observed XRD peaks of alumina particles are well matched with reported

80 -alumina phase (JCPDS File No.: 79-1558) and confirms -Al2O3

phase. The XRD peaks observed in Figure 3.11(a) is broad in nature and confirms the

presence of nanosized crystallites of -Al2O3. Using the Scherrer formula, the mean

crystallite size of -Al2O3 particles was determined to be 12 nm. FTIR characterisation

of the Al2O3 particles obtained through sol-gel route is presented in Figure 3.11(b). The

broad absorption band centered at 720 cm-1 arises from stretching and bending modes

of AlO4 and AlO6 groups (Teoh et al. 2007). The obtained slight slope changes at 1100

cm-1 are related to the stretching modes of Al-OH-Al groups. The vibration bands

appear at 1400 cm-1 and 1520 cm-1 are correlated to the presence of carbon-carbon (C-

C) and carbon-hydrogen (CH3) deformation. The vibration band obtained at 1630 cm-1

is due to the bending vibration of weakly bounded water molecules (Teoh et al. 2007).

It can be revealed from FTIR studies that the sol-gel derived Al2O3 particles consist of

residual carbon and physically adsorbed hydroxyls.

Figures 3.11(c) and (d) show the particle size distribution (PSD) of Al2O3

nanoparticles before and after ball milling. It can be revealed from PSD measurements

that the Al2O3 nanoparticles obtained before milling yields a mean agglomerate size

(d50) of 192 nm with particle size distribution (d10 - d90) of 130 - 282 nm (Figure 3.11c).

The soft agglomerates of Al2O3 nanoparticles can easily be broken through ball milling

as shown in Figure 3.11(d). It is interesting to note that the ball milling technique

affords a mean particle size (d50) of 42 nm with size distribution (d10 - d90) is of 28 - 53

nm. An important finding resulting from the PSD observations that the sol-gel route

yields agglomerated Al2O3 nanoparticles which were further disagglomerated by ball

milling process. Hence, it is evident that the ball-milling process contributes four to

five times reduction of the mean diameter of agglomerated Al2O3 nanoparticles.

81

Figure 3.11 Characterisation of sol-gel derived Al2O3 nanoparticles

a) XRD pattern b) FTIR spectra

c) PSD obtained before ball milling d) PSD obtained after ball milling

82

Figure 3.12 SEM and TEM images of sol-gel derived Al2O3 particles

a) SEM image obtained before ball milling

b) SEM image obtained after ball milling

c) TEM image obtained before ball milling

d) TEM image obtained after ball milling

83

Physical and chemical analysis of sol-gel derived Al2O3 nanoparticles is

shown in Table 3.2. Before ball milling, the Al2O3 sample has the BET surface area of

38 m2g-1, which is again increased to 225 m2g-1 after ball milling. Another prediction

from BET surface area measurement is that the particle size and surface area are inter

related. Thus, it is evident that ball milling process easily breaks the soft agglomerates

(192 nm) and brings the reduced particle size (40 nm) with increased surface area

(225 m2g-1). The Al2O3 nanoparticles obtained at the end of ball milling process was

analysed using XRF which is a semiquantitative technique and the results are presented

in Table 3.2. It can be seen from Table 3.2 that the chemical components present in the

sol-gel derived Al2O3 sample are 95.3 % (Al2O3), 4.5% (SiO2) and 0.14% (Na2O). It is

explored from XRF chemical analysis that the sol-gel derived Al2O3 particles have a

chemical purity of 95.3%.

Table 3.2 Physical and chemical analysis of sol-gel derived Al2O3 nanoparticles

The scanning electron microscope (SEM) and transmission electron

microscope (TEM) pictures of Al2O3 samples before and after ball milling are

presented in Figure 3.12. It can be seen from Figure 3.12(a) and (c), before ball

milling, the sol-gel derived Al2O3 samples have agglomerated particles with shapeless

irregular morphology. Whereas the SEM and TEM observations of ball milled samples

(Figure 3.12 b and d) provide an evidence for non-agglomerated, well defined Al2O3

Physical analysis XRF chemical analysis

Parameters Before milling After milling Component Weight, %

BET surface area 38 m2g-1 225 m2g-1 Al2O3 95.30±0.1

Mean particle size - 40 nm SiO2 04.50±0.1

Mean aggregate size 192 nm - Na2O 00.14±0.1

84 nanoparticles with spherical shape and diameter ranging from 30 to 50 nm. It is quite

worthy to note that the PSD results are most comparable with SEM and TEM. The

effect of ball milling on particle size, morphology and surface area are well established

with PSD, SEM, TEM observations and BET surface area measurements. The above

experimental evidence suggests that, ball milling is highly required to produce the

spherical shaped particles with quite reduced particle size and high surface area.

3.1.3 Spray Pyrolysis

The third part of this chapter is devoted to the production of high surface

area Al2O3 particles with free flowing structure. Precipitation and sol-gel process have

the advantage of high productivity but control of size is quite difficult. The spray

pyrolysis has been identified as the desirable method for large scale production of

monodispersed Al2O3 nanoparticles with high surface area and quite uniform diameter.

The present spray pyrolyser is an automated system which provides significant benefits

such as scale up the productivity in a manufacturing environment.

The XRD pattern of Al2O3 nanoparticles produced through spray pyrolysis

is shown in Figure 3.13 (a). The diffraction patterns of Al2O3 nanoparticles are

assigned to the cubic phase, indicating that the Al2O3 nanoparticles have -

Al2O3 phase with cubic symmetry. The obtained XRD pattern of Al2O3 nanoparticles is

in well agreement with the standard powder diffraction data (JCPDS File No.: 79-

1558). From the XRD data, it is found that Al2O3 particles has cubic crystalline phase

with an average crystallite size of 5 nm. Figure 3.13 (b) shows the FTIR spectra of

Al2O3 nanoparticles after calcination at 773 K. The band observed at 1633 cm 1 is

assigned to the O–H bending vibration of a weakly bounded water molecule (Teoh et

al. 2007). The absorption peaks obtained at 1510 and 1410 cm 1 are due to the

presence of the carbon–carbon (C–C) deformation. The band observed at 1100 cm 1 is

assigned to the Al–OH bending vibration of Al–OH–Al groups (Teoh et al. 2007).

85

Figure 3.13 Characterisation of Al2O3 nanoparticles produced through spray pyrolysis


d) TEM picture c) PSD

86

The absorption peaks observed between 600 and 800 cm 1 are attributed due

to the stretching and librational modes of AlO4, AlO6 and OH groups. The band

observed at 502 cm 1 is assigned to the stretching and bending vibrations of the Al–O

bond (Teoh et al. 2007). The above results summarise the finding that Al2O3

nanoparticles consists of adsorbed water molecules and residual carbon. Figure 3.13(c)

presents the PSD of Al2O3 nanoparticles produced through spray pyrolysis. It can be

revealed from Figure 3.13(c) that the synthesised powder consists of particle size in the

range (d10-d90) of 12-51nm and the maximum distribution (d50) of particle is at 25 nm.

Chemical analysis and effect of feed rate on production rate, particle size and surface

area of the Al2O3 nanoparticles produced through spray pyrolysis are presented in

Table 3.3.

Table 3.3 Chemical analysis and effect of feed rate on production rate, particle size and surface area of the Al2O3 nanoparticles produced through spray pyrolysis

In order to optimise the spray pyrolysis for better yield, only the feed rate

has been varied while keeping the parameters such as spray air pressure (30-40 PSI),

temperature of reaction chamber (773 K), speed of hot air blower (1800 rpm) and

liquid density (1.02 g cm-1) of precursor as constant. It is inferred from Table 3.3 that

Density of precursor solution

g cm-3 (±0.005)

Feed rate

L h-1

Al2O3

production rate

g h-1

Mean particle size (d50)

nm (±3)

Surface area

m2 g-1 (±6)

XRF chemical analysis

Component Weight

% (±0.01)

1.02 0.10 1.6 20 339 Al2O3 99.20

1.02 0.15 2.7 23 337 SiO2 00.54

1.02 0.20 3.5 25 336 Na2O 00.25

1.02 0.25 4.6 49 231 CaO 0.01

1.02 0.30 5.9 71 176 - -

87 the liquid feed rate of the precursor is increased from 0.1 to 0.3 L h-1 which

corresponds to an Al2O3 production rate from 1.6 to 5.9 g h-1, the mean particle

diameter (d50) increases from 20 to 71 nm while the specific surface area decreases

from 339 to 176 m2 g-1. Whereas, the liquid feed rate of 0.2 L h-1 (3.5 g h-1) yields the

mean particle size of 25 nm and the surface area of 336 m2 g-1 which is the optimised

feed rate in terms of the production rate along with comparable and negligible changes

of particle size and surface area. The optimised and net specific surface area of the

nano Al2O3 particles calculated using BET method is 336 m2 g-1. The nano Al2O3

particles maintained a larger surface area above 200 m2 g-1 even after calcination at

1023 K. It can be revealed from BET analysis that the spray pyrolysis yields high

surface particles with free flowing structure. Further, it is evident from the above

studies that the particles are freely functionalized and easily fabricated to different

nanostructures depending on the requirements.

It can be seen from XRF results (Table 3.3) that the Al2O3 nanoparticles

produced through spray pyrolysis consists of 99.2% (Al2O3), 0.54% (SiO2),

0.25 (Na2O) and 0.01% (CaO). From the XRF chemical analysis, it was concluded that

the spray pyrolysis yields Al2O3 nanoparticles with 99.2% chemical purity along with

silica, sodium and calcium oxide impurities. The TEM image of Al2O3 nanoparticles

produced through spray pyrolysis is shown in Figure 3.13 (d). From the TEM image,

the morphology and size of Al2O3 nanoparticles can be confirmed. It can be seen from

the TEM images that the Al2O3 particles exhibit monodispersed spherical particles with

quite uniform diameter of 20 nm. The observed results confirm that the spray pyrolysis

yields monodispersed uniform spherical Al2O3 nanoparticles with quite uniform

particle diameter of 25 nm with high surface area of 336 m2 g-1.

88 3.1.4 Optimisation of the Al2O3 Nanoparticles Production Method for Multilayered Nanostructured Protective Coating

On comparison with precipitation and sol-gel route, the spray pyrolysis has

been found to be a useful route to produce the mass quantity of nanoparticles through

continuous process. It was found that the material produced through this process owns

quite uniform spherical morphology with narrow particle size distribution and almost

free flowing structure with reduced particle size. All the techniques result in

nanocrystalline alumina particles, however, the spray pyrolysis technique is beneficial

particularly to produce uniform spherical Al2O3 nanoparticles with high surface area

and free flowing structure which is note worthy for multilayered nanostructured

ceramic coating. Hence, the Al2O3 nanoparticles produced through spray pyrolysis

were optimised and applied as nanofiller for multilayered nanostructured protective

coating for anti-corrosive application of SS304 in acid (1M HCl) media.

3.1.5 Application of Al2O3 Nanoparticles

i) Nano Al2O3 Filled Silica Sol Characterisation

Nano Al2O3 particles were filled in silica sol to form the transparent coating

solution of nano Al2O3 filled monophase silica sol. The PSD of nano Al2O3 filled silica

sol is shown in Figure 3.14. It is observed from Figure 3.14 that the nano Al2O3 filled

silica sol was found to have particle size distribution in the range (d10-d90) of 13-78 nm

and mean particle size (d50) of 32 nm. It can be explored from Figure 3.13(c) and 3.14,

the mean diameter of Al2O3 particles obtained through spray pyrolysis is 25 nm while

the nano Al2O3 filled silica sol has mean particle diameter of 32 nm. It is used to frame

the discussion that the dispersion of free formed Al2O3 particles in silica sol yields

hybrid particles i.e., Al2O3 particles were surrounded by silicate network which

increase the mean particle size of free formed Al2O3 particles from 25 to 32 nm. Hence,

it is clear from the PSD data in Figure 3.14 that the free formed Al2O3 particles were

89 chemically coordinated with the silicate network and formed monophase nano Al2O3

filled silica sol.

Figure 3.14 Particle size distribution of nano Al2O3 filled silica sol

ii) Coating Microstructure and Phase Analysis

Employing sol-gel technique, nano Al2O3 filled silica multilayer coatings

were deposited on a SS304 surface whose chemical composition is shown in Table 3.4.

Table 3.4 shows that the elemental composition of uncoated and nano Al2O3 filled

silica coated stainless steel specimens. It was confirmed from spark optical emission

studies (Table 3.4) that the uncoated stainless steel specimen is SS304. It was also

observed from Table 3.4 that the nano Al2O3 filled silica coated SS304 shows an

increased amount of Al and Si composition and the remaining elements are almost

unaltered. It is clear from the data in Table 3.4 that the increase in composition of Si

and Al explore the presence of nano Al2O3 filled silica coating on SS304 surface. The

90 presence of complex nanostructed Al2O3 filled silica coating on SS304 was well

established with XRD and AFM findings.

Figure 3.15 X-ray diffraction patterns of SS304 specimens: a) Uncoated SS304 and b) Nano Al2O3 filled silica (six layers) coated SS304

The XRD patterns of two representative samples of uncoated and coated

SS304 specimens were compared in Figure 3.15 which shows the presence of

nanocrystalline Al2O3 filled silica coating on SS304 surface. From Figure 3.15(a), it is

clear that the uncoated specimen presents a single phase -Iron with sharp peaks

(Ghoranneviss et al. 2007) which is typical of annealed SS304 materials (Jain and

Christmana 1996). The observed (111), (200) and (220) reflections are well matched

with reported XRD data of SS304. The XRD pattern of nano Al2O3 filled silica coated

SS304 is show in Figure 3.15(b). According to standard powder diffraction data

(JCPDS File No.: 89-0890, 79-1457 and 79-1558), the nano Al2O3 filled silica coating

consists of mullite -Al2O3 phases. It should be noted that the marked XRD

patterns seen in Figure 3.15(b) are assigned to the orthorhombic phase with mullite

91 (aluminosilicate) symmetry while unmarked XRD patterns -

alumina phase with cubic symmetry.

Table 3.4 Chemical composition of uncoated and nano Al2O3 filled silica (six layers) coated SS304

Uncoated SS304 specimen Coated SS304 specimen

Elements Composition (%) Elements Composition (%)

C 0.05 C 0.07 Si 0.47 Si 1.88 Mn 1.14 Mn 1.15 P 0.037 P 0.035 S 0.011 S 0.01 Ni 8.53 Ni 8.55 Cr 18.92 Cr 18.90 Mo 0.22 Mo 0.21 V 0.12 V 0.15 Cu 0.15 Cu 0.16 W 0.013 W 0.033 Ti 0.021 Ti 0.020 Sn 0.02 Sn 0.022 Co 0.13 Co 0.12 Al 0.004 Al 0.96 Pb 0.02 Pb 0.04 B 0.001 B 0.001 Sb 0.092 Sb 0.093 Nb 0.034 Nb 0.031 Zr 0.022 Zr 0.024 Bi 0.051 Bi 0.050 Ca 0.001 Ca 0.003 Mg 0.005 Mg 0.004 Zn 0.013 Zn 0.011 Ce 0.002 Ce 0.001 La 0.008 La 0.006 Fe 69.78 Fe 67.466

92

Hence, the nano Al2O3 filled silica coating consists of major mullite

(orthorhombic) and minor -alumina (cubic) phase. During the evolution of the coating

microstructure, it can be noted that the Al:Si ratio is insufficient to complete nucleation

of mullite phase. However, the presence of major mullite phase indicates that the

concentration Al2O3 particles within the silica coating are almost sufficient to maintain

the stoichiometry ratio of Al:Si to initiate the nucleation of mullite crystals. The

average crystalline size of mullite phase obtained through Scherrer formula is 36 nm

which is in close resemblance with mean particle diameter (32 nm) of nano Al2O3 filled

silica sol. It is important to note that the nano Al2O3 filled silica coating on SS304

intimately keeps the mean diameter of nano Al2O3 filled silica sol during the film

construction process such as curing and firing. Thus, it is evident from XRD analysis

that the nano Al2O3 filled silica coating forms the complex nanocrystalline coating on

SS304 surface.

Surface microstructure, topography and three dimensional surface profile of

uncoated and nanostructured six layered Al2O3 filled silica coated SS304 specimens

were obtained using tapping mode of AFM. Figure 3.16 shows the AFM images of the

SS304 surface before and after coating. The 2D and 3D view of the AFM images of

polished SS304 surface is shown in Figure 3.16 (a) and (b). The SEM micrograph

(3.17a) and AFM images (Figure 3.16a & b) reveals that the surface is uniform and the

parallel features seen on the surface may be associated with polishing scratches with

lip heights about 1µm. Figure 3.16 (c) and (d) shows the 2D and 3D view of the AFM

images of SS304 specimen after six layered nano Al2O3 filled silica coating. The nano

Al2O3 filled silica coated SS304 surface exhibits a nanometer scale roughness (Figure

3.16c & d) with nearly uniform distribution of islands and valleys of different sizes and

shapes. Pinholes were also identified in the particle layout, likely as a result of solvent

molecule evaporation during the curing and firing step (Fanizza et al. 2007).

93

Figure 3.16 AFM topographic images of a SS304 surface before and after coating

a) 2D view of the polished SS304 surfaces

c) 2D view of the nano Al2O3 filled silica (six layers) coated SS304 surface

b) 3D view of the polished SS304 surfaces

d) 3D view of the nano Al2O3 filled silica (six layers) coated SS304 surface

94

Figure 3.17 SEM micrographs of SS304 surfaces

d) Nano Al2O3 filled silica (six layers) coated SS304 after immersion in 1M HCl for 24 h

c) Uncoated SS304 after immersion in 1M HCl for 24 h

a) Polished SS304 b) Nano Al2O3 filled silica (six layers) coated SS304

95

The existence of complex nanostructured Al2O3 filled silica coating on

SS304 surface was confirmed with clear vision through AFM analysis. Furthermore,

the presence of nano Al2O3 filled silica coating on SS304 surface was also revealed by

SEM micrographs as shown in Figure 3.17(b).

iii) Influence of Nano Al2O3 Filled Silica Multilayer Coating on Acid Corrosion of SS304

Table 3.5 shows the effect of silica and nano Al2O3 filled silica protective

coating on acid corrosion of SS304 in 1M HCl for 24 h. It was observed from Table

3.5 that an increase in coating thickness increases the inhibition efficiency and

decreases the corrosion rate. This suggests that an increase in thickness of silica and

nano Al2O3 filled silica coating decreases the diffusion of H+ ions over the SS304

surface which inturn reduces the corrosion rate and improve the protection efficiency

against acid corrosion. It can be seen from Table 3.5 that nano Al2O3 filled silica

coating shows better acid corrosion resistance (92%) than silica coating (70%). This

may be due to the pin holes present in the silica coating which were arrested by

dispersion of nano Al2O3 particles in silica matrix. In addition, filling of nano Al2O3

particles in silica matrix induces the better densification of silica protective coating

over the SS304 surface. Nano Al2O3 filled silica coated SS304 isolates the underlying

metal from the corroding environment. The anti-corrosion studies reveals that the nano

Al2O3 filled silica coating is acting as an effective barrier to block the H+ ion diffusion

over the SS304 surface. Therefore, the coated barrier effectively reduces the H2

evolution and inturn control the release of metal ions when it is in contact with acid

solution. Further, it is noted that an increase in coating thickness increases the

protection rate against H+ ion diffusion over SS304 surface. The multilayer coating

shows an effective corrosion barrier (92%) against HCl corrosion when compared to

single layer coating (60%).

96 Table 3.5 Effect of silica and nano Al2O3 filled silica protective coating on acid corrosion of SS304 in 1M HCl for 24 h

Acid corrosion is an interfacial phenomenon, occurring at solid surfaces in

contact with acid media. Acid corrosion in SS304 is a macroscopic consequence.

However, corrosion typically begins at the atomic level. These factors make use to

study the acid corrosion through a highly sensitive surface technique that is atomic

force microscopy. Figure 3.18(a) and (b) shows the AFM images (2D and 3D) of the

SS304 surface after immersion in 1M HCl for 24 h. It can be revealed from AFM

images (Figure 3.18a and b) and SEM micrographs (Figure 3.17c) that the surface was

covered with high density of pits with depth of cavities about 55 nm. It can be

observed from Figures 3.18 (a) and (b) that the acid (1M HCl) solution effectively

damages the SS304 specimen through the pitting corrosion of 55 nm depth due to the

effective diffusion of H+ ions over SS304 surface. The AFM images (2D and 3D) of

nano Al2O3 filled silica coated SS304 after immersion in 1M HCl for 24 h are shown in

Figures 3.18 (c) and (d). It can be noted from Figure 3.18 (c) and (d) that the surface

displayed a nanometer scale roughness, likely to be the surface (Figure 3.16 c and d) of

nano Al2O3 filled silica coated SS304 before immersion in1M HCl for 24 h.

SS304 specimen coatings

Number of layers

Thickness

(± 0.1)

Inhibition efficiency

% (± 5)

Corrosion rate

mg dm-2day-1(mdd)

Without coating 0 - - 256

Silica coating 1 0.2 40 154 3 0.5 63 95 6 0.8 70 77

Nano Al2O3 filled silica coating

1 0.3 60 102 3 0.7 88 31 6 0.9 92 20

97

Figure 3.18 AFM topographic images of SS304 surfaces before and after immersion in 1M HCl for 24 h

a) 3D view of the uncoated SS304 after immersion in 1M HCl for 24 h

c) 2D view of the nano Al2O3 filled silica (six layers) coated SS304 after immersion in 1M HCl for 24 h

d) 3D view of the nano Al2O3 filled silica (six layers) coated SS304 after immersion in 1M HCl for 24 h

b) 2D view of the uncoated SS304 after immersion in 1M HCl for 24 h

98

It is quite worthy to note that the Figures 3.18(c) and (d) show the close

resemblance with Figures 3.16(c) and (d). This suggests that the nano Al2O3 filled

silica coating has the ability to resist and protect H+ ion diffusion without leaching and

degradation over the SS304 surface. The SEM micrographs seen in Figure 3.17 are

also in good agreement with the AFM observations. It was reported from AFM images

(Figure 3.18) and SEM micrographs (Figure 3.17c and d) that the nano Al2O3 filled

silica coating have the massive potential to protect the acid corrosion of SS304 in 1M

HCl. Ultimately, AFM and SEM observations are in close agreement with corrosion

studies that the nano Al2O3 filled silica coating shows better corrosion barrier against

acid (1M HCl) corrosion of SS304. Influence of nano filled silica coating on the

cumulative weight loss of the SS304 in acid (1M HCl) media was clearly demonstrated

in comparison with uncoated and silica coated SS304 with respect to immersion time

in days.

Figure 3.19 The variation of cumulated weight loss of uncoated, silica and nano Al2O3 filled silica (six layers) coated SS304 specimens versus time in acid (1M HCl) media

99

The variation of cumulated weight loss of uncoated, silica and nano Al2O3

filled silica coated SS304 specimen against time in the acid (1M HCl) media is shown

in Figure 3.19. Nano Al2O3 filled silica coating shows relatively low cumulated weight

loss when compared to uncoated and silica coated SS304. It can be noted from Figure

3.19 that the cumulated weight loss of nano Al2O3 filled silica coated SS304 is

30 mg dm-2 after 100 days of immersion period. Whereas the cumulated weight loss of

silica coated and uncoated SS304 is respectively 1324 mg dm-2 and 3018 mg dm-2 after

100 days of immersion period. Hence, it is significant to note that the nano Al2O3 filled

silica coating has the ability to protect the weight loss of SS304 against acid corrosion

for long time period.

100

3.2 SYNTHESIS, CHARACTERISATION AND APPLICATION OF ZrO2 NANOPARTICLES

The main objective of this investigation is to investigate a new method for

direct synthesis of ZrO2 nanoparticles from zircon sand and its application in high

temperature oxidation resistance coating. More specifically, the low cost method was

formulated for mass production of nanocrystalline ZrO2 particles using inexpensive

precursor developed from zircon, a simple reaction scheme and easy scalable methods.

The development of inexpensive methods for producing mass quantities of highly

crystalline and monodispersed nano ZrO2 particles, remains an area of extensive

interest. On the basis of precursor, two kinds of chemical precursor namely zirconium

(II) salts and zirconium alkoxides can be used for the production of ZrO2 nanoparticles.

Mainly zirconium alkoxides are used as precursor for production of nano ZrO2

particles however, it is more expensive for bulk production. Production of ZrO2

nanopowder from zircon sand is an innovative effort for bulk production of ZrO2

nanoparticles.

Reliable and high quality nanoparticles are readily available using

precipitation and sol-gel technologies. The nanoparticles obtained by the above

methods are with high chemical purity but are relatively expensive. Early work showed

that zircon ore could be fused at 873 K using NaOH and mixed with water to form a

hydrolysed zirconate, which was further treated with HCl to form a zirconium

oxychloride prior to precipitation (Gilbert et al. 1954). This is an inexpensive process

however, the particles derived from zirconium oxychloride leads to agglomerated

particles with low surface area and irregular morphology. The innovation of the present

work is to use a modified process based on the previous work to form an inexpensive

precursor i.e. zirconyl nitrate. This can be used for low cost production of ZrO2

nanoparticles with high surface area and quite uniform spherical morphology.

101

The use of ball milling as a means for breaking down the loose aggregates

and agglomerates is also explored. It was inferred that this route was less expensive

than hydrothermal (Reynen 1981) or other industrial processes which are currently

used (Dudnik et al. 1993). The nanoparticles produced by the above methods are

suitable for applications requiring high surface area with uniform spherical

morphology, specifically for nanofillers and nanoceramic coatings. The overall

objective of this chapter is to optimise an inexpensive process to produce non-

agglomerated nanometer sized ZrO2 particles with monodispersed spherical

morphology with high surface area and free flowing structure. Ultimately, an attempt

has been made to design the nano ZrO2 filled hybrid silica sol for the construction of

complex nanostructured coating on SS304 surface to protect the high temperature

oxidation corrosion at 1273 K.

3.2.1 Precipitation

The focus of this work is to investigate the precipitation process for the

production of ZrO2 nanparticles from zirconyl nitrate and to understand the key issues

on stabilisation of crystalline phase, particle aggregation and morphological features.

The hydrolysis behaviour of zirconyl nitrate in an aqueous solution has been

investigated in detail by changing the pH value of solution and calcination temperature.

In addition, the effect of pH value on crystalline phase formation, particle size and

surface area were also discussed. The processing factors for the production of non-

stabilised monoclinic ZrO2 particles from zirconyl nitrate solution were clarified and

used for the production of sodium stabilized cubic ZrO2 particles. Figure 3.20 (a) and

(b) displays the DTA and TGA curves of the hydrous zirconia synthesised at pH 7 after

being dried at 393 K. TGA results indicate that the samples obtained by the wet

methods are hydrous samples. The controlled thermal decomposition of hydrous

zirconia (ZrO(OH)2 xH2O) and its crystallisation temperature are established with the

help of TG/DTA analysis.

102

When it is heated, a part of the structural H2O molecules is decomposed and

results in the formation of ZrO2 xH2O structure. The self controlled three steps thermal

decomposition behaviour of hydrous zirconia is shown in scheme 3.2.

Scheme 3.2 Schematic representation of decomposition behaviour of hydrous zirconia

In the DTA curve, the observed broad endothermic peak centered at 493 K

is attributed due to the loss of the physically adsorbed water and the weight loss on

heating is equal to 10.6% at 421 K and 12.8% at 456 K. The endothermic effect

occurring at 704 K is assigned to a loss of chemically co-ordinated (chemisorbed)

water molecules present in hydrous zirconia and the weight loss on heating which is

equal to 23.7% at 704 K. The total 24.3% weight loss on heating to 753 K results from

the molecular decomposition of ZrO(OH)2.H2O into ZrO2 and 2H2O. It is noted that

hydrous zirconia is progressively decomposed into amorphous zirconia due to

dehydroxylation with increase in temperature (704 K). The observed sharp exothermic

peak at 753 K is associated with the enthalpy of the transformation of the amorphous

zirconia into crystalline zirconia. The kinetic process of dehydration and crystallisation

of hydrous zirconia take place respectively at 704 and 753 K. The above thermal

decomposition behaviour of hydrous zirconia is in good agreement with the data

obtained for undoped zirconia nanoparticles synthesised by other methods (Guo and

Chen 2005, Mondal and Ram 2004).

ZrO2.xH2O + H2O Amorphous

ZrO(OH)2.xH2O Amorphous

421- 456 K

ZrO2 Crystallised Polymorphs

704 – 753 K

456-704 K

ZrO2 + xH2O Amorphous

103

Figure 3.20 a) DTA and b) TGA curves of the hydrous zirconia precipitated at pH 7 after dried at 393 K

Figure 3.21 FTIR spectra of ZrO2 samples precipitated at a) pH 7, b) pH 10 and c) pH 13 after calcination at 773 K

104

Figure 3.21 displays the FTIR spectra of zirconia samples precipitated at pH

values of 7, 10 and 13 after calcination at 773 K. The IR active modes between 490

cm 1 and 1030 cm 1, corresponding to the asymmetric and symmetric stretching

frequencies of Zr–O–Zr vibrational bands (Zhang et al. 2009a). The absorption peaks

obtained at 1360 cm 1 and 1630 cm 1 show respectively, the O-H stretching and

bending vibrations of ZrO2 xH2O and H2O molecules. The OH group is distinguished

easily in the H2O molecule by its bending vibrations which appear in a single band at

nearby 1630 cm 1. Further, the O-H bending vibration of the hydroxyl group of

ZrO2 xH2O molecule appears at nearby 1360 cm 1. The above results summarise the

finding that the ZrO2 powder consists of chemisorbed hydroxyl groups and adsorbed

water molecules.

Figure 3.22 XRD patterns of ZrO2 samples precipitated at pH 7 after calcination at a) 773 K, b) 873 K, c) 973 K and d) 1073 K

105

Table 3.6 Crystalline phase, phase composition and average grain size of ZrO2 samples precipitated at pH 7, 10 and 13

Three samples of zirconia were synthesised at three different pH levels,

namely pH 7, 10 and 13 to investigate the effects of pH on crystalline phase. The

crystalline phase, phase fraction and average crystallite size of all zirconia samples are

presented in Table 3.6. Figure 3.22a-d shows the XRD patterns of the zirconia sample

precipitated at pH 7 after calcinations at (a) 773 K, (b) 873 K, (c) 973 K and

(d) 1073 K for 6 h. Figure 3.23 displays XRD data of ZrO2 synthesised at pH 10 after

being calcined at 773 K for 6 h. Similarly, the XRD patterns of the zirconia sample

Precipitation pH (± 0.1)

Calcination temperature

K

Crystalline phase

Phase fraction

% (± 1)

Average particle size

nm

7 773 Monoclinic Orthorhombic

90 10

22

873 Monoclinic Orthorhombic

92 08

27


93 07

32


95 05

38

10 773 Monoclinic Cubic

86 14

18

13 773

Cubic Tetragonal

96 04

11

873 Cubic Tetragonal

96 04

16


95 05

19


92 08

22

106

precipitated at pH 13 after calcination at (a) 773, (b) 873, (c) 973 and (d) 1073 K for

6 h are given in Figure 3.24a-d.

The diffraction peaks of all the samples were identified and assigned with

standard powder diffraction data using the JCPDS Files 81-1314 (monoclinic), 87-2105

(orthorhombic), 49-1746 (orthorhombic), 50-1089 (tetragonal) and 65-0461(cubic).

From the XRD pattern (Figure 3.22a), it is found that the ZrO2 particles synthesised at

pH 7 yield a predominantly monoclinic (90%) phase along with some fraction of

orthorhombic (10%) zirconia. The XRD pattern (Figure 3.23) of the sample

synthesised at pH 10 results in a monoclinic structure (86%) coexisting with some

fraction of cubic zirconia (14%), whereas the XRD pattern (Figure 3.24a) of the sample

synthesised at pH 13 exhibits a cubic phase (96%) along with some fraction of

tetragonal (4%) zirconia. It can be seen from the XRD results that the phase

transformation of zirconia is influenced by the pH value of the precipitation. A greater

quantity (90%) of monoclinic zirconia (m-ZrO2) is noticed at pH 7, which is reduced

(86%) for the sample synthesised at pH 10.

In contrast, the minor orthorhombic phase (10 %) obtained at pH 7

disappeared at pH 10. The minor orthorhombic zirconia phase obtained at pH 7 was

transformed into cubic zirconia (14 %) at pH 10, while both the monoclinic and

orthorhombic structures disappeared in the sample synthesised at pH 13. Instead of

both monoclinic and orthorhombic structures, cubic (96%) and tetragonal (4%)

zirconia is detected at pH 13. The above structural transitions are explained in terms of

the non-stabilised monoclinic and sodium stabilised cubic zirconia (Canton et al.

1999). In an effort to understand the effect of pH and sodium ions on the crystal

structure of zirconia, the samples synthesised at pH 7 and 13 were examined by

qualitative elemental analysis.

107

Figure 3.23 XRD patterns of ZrO2 sample precipitated at pH 10 after calcination at 773 K

Figure 3.24 XRD patterns of ZrO2 samples precipitated at pH 13 after calcination at a) 773 K, b) 873 K, c) 973 K and d) 1073 K

108

Table 3.7 shows that the chemical composition of ZrO2 samples precipitated

at pH 7 and pH 13 after calcinations at 773 K. It can be observed from Table 3.7 that

the major components present in the sample precipitated at pH 7 are 97.8 wt.% of

ZrO2, 0.2 wt.% of Na2O, 0.4 wt.% of SiO2, 0.012 wt.% of CaO and 1.5 wt.% of Hf

whereas the samples precipitated at pH 13 are 95.3 wt.% of ZrO2, 2.6 wt.% of Na2O,

0.5 wt.% of SiO2, 0.016 wt.% of CaO and 1.5 wt.% of Hf. It is obvious that the sample

synthesised at pH 13 has 2.6 wt.% of sodium in its composition whereas the sample

synthesised at pH 7 has 0.2 wt.% sodium in its composition. The sodium content in the

composition is very high nearly 13 times higher in pH 13 than in pH 7.

Table 3.7 Chemical compositions of ZrO2 samples precipitated at pH 7 and 13

It is used to frame the discussion that the presence of sodium in the crystal

structure is influenced by the phase formation of zirconia. The above result agrees with

the known fact that a fraction of Na+ cations present in the mother liquor after the

precipitation at pH 13 is electrostatically chemisorbed on the surface of

ZrO(OH)2 xH2O. It is noteworthy that the strong negative surface potential (Vayssieres

2009) of ZrO(OH)2 xH2O facilitates the chemisorption of Na+ cation on the precipitate

surface at the basic pH value of 13. The chemisorbed sodium ions enter the structure of

zirconia during the crystallisation to form sodium stabilised cubic zirconia. Formation

of sodium stabilised cubic zirconia under basic pH is studied experimentally in detail

by other methods developed from chemical precursors (Canton et al. 1999). At neutral

Precipitation pH (± 0.1)

XRF chemical composition (wt.%)

ZrO2 Na2O SiO2 CaO Hf

7 97.8 ± 0.1 0.2 ± 0.1 0.4 ± 0.1 0.012 ± 0.01 1.5 ± 0.1

13 95.3 ± 0.1 2. 6 ± 0.1 0.5 ± 0.1 0.016± 0.01 1.5 ± 0.1

109

pH 7, the surface potential of ZrO(OH)2 xH2O is equal to zero i.e. electro kinetically

uncharged (Vayssieres 2009). In this case, Na+ cations are not chemisorbed on the

uncharged surface of ZrO(OH)2 xH2O and lead to monoclinic zirconia. The monoclinic

to cubic phase transformation is clearly established by varying the pH values from 7 to

10 and from 10 to 13.

It is evident from XRD results that the neutral pH 7 has no cubic phase

whereas the pH values of 10 and 13 yield respectively 14 and 96 % of cubic phases.

The above observation confirms that when the pH of the medium is higher than the pH

of the isoelectric point i.e. pH 7, a negative charge is developed on the surface of

hydrous zirconia along with the chemisorptions of sodium ions. An increase in pH

value during precipitation results in an increase in the interactions between the sodium

ions and hydrous zirconia which leads to an increase in sodium ion chemisorption and

formation of cubic zirconia. The XRD patterns of zirconia samples synthesised at pH 7

(Figure 3.22a-d) and pH 13 (Figure 3.24a-d) indicate the influence of calcination

temperature on the crystalline phase and size of zirconia in the temperature range of

773-1073 K. It can be seen from Table 3.6 that the calcination temperature of the

sample that is synthesised at pH 7 increases from 773 to 1073 K, the percentage of

monoclinic phase increases from 90 to 95% while decreasing the percentage of the

orthorhombic phase from 10 to 5% (Figure 3.22a–d). A complete (> 98%)

transformation of the orthorhombic to the monoclinic phase was observed after

calcination at 1273 K.

As shown in Figure 3.22a, after calcination at 773 K for 6h, the average

crystallite size of monoclinic zirconia is about 22 nm, which increases to 27 nm at 873

K, 32 nm at 973 K and 38 nm at 1073 K (Table 3.6). Table 3.6 indicates that crystallite

size and orthorhombic to monoclinic phase transformation are inter related. While

increasing the calcination temperature, the monoclinic phase fraction starts to increase

significantly with an increase in the crystallite size. The major monoclinic zirconia

110

appears only in the samples synthesised at pH 7, but even at pH 13, cubic zirconia is

predominant in the sample. The enhanced durability of the cubic phase in this case can

be explained by partial incorporation of sodium into the zirconia structure (Canton et

al. 1999). When the calcination temperature of the sample synthesised at pH 13 is

increased from 773 to 1073 K (Figure 3.24a–d), the percentage of the cubic phase

decreases from 96 to 92% and the percentage of the tetragonal phase increases from 4

to 8%. A complete (>96%) transformation of the cubic to the monoclinic phase was

observed after calcination at 1473 K. Table 3.6 shows that the average crystallite size

of cubic zirconia calcined at 773, 873, 973 and 1073 K are respectively 11, 16, 19 and

22 nm. The above results indicate that the grain size of cubic zirconia is increased

considerably with an increase in the calcination temperature which notably induces the

cubic to tetragonal phase transformation.

In general, the results that are observed show that the degree of crystallinity

and the crystallite size of both monoclinic and cubic zirconia increases with increase in

calcination temperature. Further, the temperature seems to be the dominant factor for

the grain size and phase contents. The results that are obtained in accordance with

earlier reports (Canton et al. 1999 and Tyagi et al. 2006) and reveals that an increase in

the calcination temperature is accompanied by an increase in crystallite size, which

leads to phase transformation in zirconia. The present work shows a close agreement

with earlier studies (Benedetti et al. 1990 and Benedetti et al. 1989) that cubic zirconia

has been stabilized by incorporation of sodium in zirconia gel during the precipitation

of hydrous zirconia under alkaline conditions. Conversely, Chang et al. (2000) showed

that the monoclinic phase with increased crystallite size was formed at 1273 K while

increasing the sodium content. However, our results indicate that an increase in the

sodium content is responsible to the stabilisation of cubic zirconia with decreased

crystallite size at 773 K. This is due to effect of high energy ball milling which leads to

incorporation of sodium in zirconia crystal structure and form cubic zirconia with

111

reduced crystallite size. Further, the present work shows that the sodium stabilised

cubic zirconia has been destabilised at higher calcination temperature (>1273 K) to

form monoclinic zirconia with increased crystallite size. It is in line with early work

(Shi et al. 1997) that the sodium destabilises the metastable i.e., tetragonal zirconia and

lead to the formation of monoclinic phase at higher sintering temperature.

It is evident from the chemical analysis that the sample synthesised at pH 7

contains 97.6% of ZrO2, whereas the sample synthesised at pH 13 contains 92.5% of

ZrO2. It can be seen from XRF (Table 3.7) results that the samples synthesised at pH 7

contains 97.8% of ZrO2, while the sample synthesised at pH 13 contains 95.3% of

ZrO2. The results obtained from quantitative elemental analysis (XRF) are in close

agreement with the results obtained from the chemical analysis. From the above

studies, it was concluded that the non-stabilised monoclinic zirconia synthesised at pH

7 shows 97.6% chemical purity whereas the sodium stabilised cubic zirconia

synthesised at pH 13 shows 92.5% chemical purity. Commercial high purity zirconia

powders have Na content which are less than 0.05 wt.%. Not only do high sodium

contents impede densification (Shi et al. 1997) but they are detrimental to strength and

ionic conductivity, which are important properties of zirconia based materials. While

zirconia containing sodium would still be useful for thermal barrier coatings, it is clear

that the sodium contents in the present powders are too high to be acceptable. Keeping

the pH value as low and lowering the sodium content is therefore critical issues to

address if this method is to be pursued.

Figure 3.25 presents the PSD of ZrO2 samples after calcination at 773 K, a)

pH 7 and b) pH 13 samples before milling, c) pH 7 and d) pH 13 after milling for 3 h at

500 rpm. It can be seen from Figure 3.25 (a) and (c) that the monoclinic zirconia before

ball milling yields a particle size in the range of 269-444 nm (Figure 3.25a), whereas

the ball milled monoclinic zirconia yields a particle size in the range of 5-193 nm

(Figure 3.25c). Before ball milling, the mean diameter of monoclinic zirconia is about

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327 nm, which decreases to 64 nm after ball milling (Figure 3.25a and c). Hence, it is

evident that precipitation followed by calcination yields aggregated zirconia particles

which are further disaggregated by ball milling. Four to five times reduction of the

mean diameter of zirconia particles result with the aid of the ball milling process.

It can be seen from Figure 3.25(b), before ball milling the calcined cubic

zirconia yields a particle size in the range of 122-300 nm with an average diameter of

193 nm, while the ball milled cubic zirconia (Figure 3.25d) yields a particle size in the

range of 26-54 nm with an average diameter of 39 nm. From the above observations, it

is clear that the calcined cubic zirconia yields highly aggregated particles (193 nm),

which are disaggregated (39 nm) by the ball milling process. From the above results, it

can be understood that the ball milling process helps the diminution of the particle size

of the aggregated zirconia by four to five times. The effect of ball milling on the PSD

and average diameter were studied with help of PSD analysis. In addition, the width of

the PSD can be directly derived from the scattering parameters obtained from the PSD

results. Figure 6 shows that calcined zirconia has a wide PSD width when compared to

the ball milled samples. From the PSD results, we conclude that the precipitation,

drying and calcination steps lead to aggregated particles which are further

disaggregated by a subsequent ball milling process. While planetary mixing at a high

ball to charge ratio was effective in increasing the surface area, two problems are

obvious: 1) as the process is scaled up from a laboratory process to a commercially

viable process it is not possible to keep the ball to charge ratio as high as in this

process, and 2) high surface area means that particle packing is difficult. Commercial

powders are typically having surface areas of the order of 5-20 m2 g-1. The important

point from the present study is that it shows that this process results in soft

agglomerates which are easy to break down.

113

Figure 3.25

Particle size distribution of ZrO2 samples obtained through precipitation method after calcination at 773 K

a) Precipitated at pH 7 before ball milling

b) Precipitated at pH 13 before ball milling

c) Precipitated at pH 7 after ball milling for 3 h at 500 rpm

d) Precipitated at pH 13 after ball milling for 3 h at 500 rpm

114

Figure 3.26 presents the effect of calcination temperature on the particle size

of ZrO2 samples synthesised at (a) pH 7, (b) pH 10 and (c) pH 13. Figure 3.26 shows a

steady progression towards larger particle size as the temperature increases. It is noted

(Figure 3.26) that the variation in the calcination temperature from 773 to 1073 K leads

to variations in average particle size, which is in the range of 64-135 nm, 56-110 nm

and 39-86 nm, respectively for the samples synthesised at pH values of 7, 10 and 13.

The above result indicates that the powders that are obtained by high temperature

calcination are aggregated because the aggregates will be hard, which will be difficult

to break efficiently by a subsequent ball milling process. To obtain a well dispersed

nano sized particle product, the processing temperature must be kept as low as possible

and high temperature calcination should be avoided. It is interesting to note that the

particle size distribution (PSD) results are in good agreement with SEM studies. The

influence of ball milling on particles size and morphology is also well established with

SEM observations.

Figure 3.26 Effect of calcination temperature on the particle size of ZrO2 samples, a) pH 7, b) pH 10 and c) pH 13

115

Figure 3.27 SEM images of ZrO2 samples produced through precipitation route after calcination at 773 K

c) Precipitated at pH 7 after ball milling for 3 h at 500 rpm

d) Precipitated at pH 13 after ball milling for 3 h at 500 rpm

a) Precipitated at pH 7 before ball milling

b) Precipitated at pH 13 before ball milling

116

Figure 3.28 TEM images of ZrO2 samples calcined at 773 K after ball milling for 3 h at 500 rpm

Figure 3.27 displays the SEM images of ZrO2 samples after calcination at

773 K, a) pH 7 and b) pH 13 samples before milling, c) pH 7 and d) pH 13 after

milling for 3 h at 500 rpm. Similarity with PSD (Figure 3.25a), the SEM micrograph of

monoclinic zirconia before ball milling (Figure 3.27a) is shows a highly aggregated

surface. Particle aggregation is apparent, suggesting the formation of a continuous

particulate network in the suspension structure, whereas the SEM micrograph of the

ball milled monoclinic zirconia (Figure 3.27c) shows the disaggregated particles with

almost spherical morphology. It can be seen from Figure 3.27(b) that synthesised cubic

zirconia has an aggregated surface with higher particle size whereas the ball milled

(Figure 3.27d) cubic zirconia gets disaggregated particles with quite reduced particle

size with a spherical morphology. Aggregated and non-aggregated zirconia could be

well distinguished from the SEM results. The SEM findings not only justify the

aggregation of the particles but also show the particle morphology. Figure 3.28

displays the TEM images of ZrO2 samples precipitated at pH 7 and pH 13 calcined at

a) Precipitated at pH 7 b) Precipitated at pH 13

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773 K after ball milling for 3 h at 500 rpm. It can be seen from the TEM images that

cubic zirconia (pH 13) has predominantly spherical particles with a slightly

agglomerated surface whereas monoclinic zirconia (pH 7) shows almost spherical

morphology with some rectangular shaped particles. The above results are in good

agreement with PSD and SEM observations, which are presented respectively in

Figures 3.25 and 3.27.

Table 3.8 BET surface area, mean aggregate and particle size of ZrO2 samples before and after ball milling for 3 h at 500 rpm

Precipitation

pH (± 0.1)

Calcination temperature

K

Before milling After milling

Mean aggregate size

nm

BET surface area

m2g-1

Mean particle size

nm

BET surface area

m2g-1

7 773 327 18 64 126

10 773 220 26 56 190

13 773 193 35 39 227

Table 3.8 summarises BET surface area, mean aggregate and particle size of

ZrO2 samples before and after ball milling for 3 h at 500 rpm. Before ball milling, the

mean aggregate size of zirconia is 327, 220 and 193 nm respectively for the samples

precipitated at pH 7, 10 and 13. After ball milling, the mean particle size of zirconia is

64, 56 and 39 nm respectively for the samples precipitated at pH 7, 10 and 13 (Table

3.8). Whereas the average crystallite size decreases from 22 to 18 and 18 to 11 nm

respectively with an increase in the pH value of precipitation from 7 to 10 and 10 to 13

(Table 3.6). It can be noticed from Table 3.8 that the surface area of aggregated

particles are 18, 26 and 35 m2g 1 respectively for the samples precipitated at pH 7, 10

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and 13 (Table 3.8). After milling, the surface area of the disaggregated particles are

126, 190 and 227 m2 g 1 respectively with increasing the pH value of precipitation from

7 to 10 and 10 to 13 (Table 3.8). Disaggregated particles have six to seven times higher

specific surface area than the aggregated particles. Aggregated and non-aggregated

particles are well distinguished from the observed surface area of the zirconia samples.

When increasing the pH (7-13) values of precipitation, a decrease in the mean

aggregate and particle size with an increase in the surface area of the zirconia samples

are noticed. The above results conclude that the monoclinic to cubic phase

transformation leads to higher surface area and lower particle size.

The particle size of zirconia depends on the nucleation of hydrous zirconia

sols during the precipitation process. Further, the nucleation of hydrous zirconia

depends on the experimental conditions, including pH, temperature, concentration of

reactants, nature of anions, and stirring and ageing time (Milonjic, 1996). It is used to

frame the discussion that the pH value of precipitation affects the size distribution of

the zirconia particles. In a neutral value of pH, the surface potential of hydrous zirconia

is almost zero i.e., near iso-electric (Vayssieres, 2009), which induce surface

aggregation of hydrous zirconia sols because of hydrogen bonding caused by Van der

Waals attraction between the hydrolysed hydrous zirconia molecules. At the basic pH

value of 10 and 13, the surface of hydrous zirconia has a negative zeta ( ) potential

which in turn, reduces the surface aggregation of hydrous zirconia sols due to the

strong electrostatic repulsion between the negatively charged hydrous zirconia

molecules. Thus, the aggregation behaviour of hydrous zirconia is strongly depends on

the pH value according to the Derjaguin and Landau, Verwey and Overbeek (DLVO)

theory (Hunter, 2001). In addition, above a pH value of 7, i.e., basic pH value of 10

and 13, the present process has free sodium ions in the mother liquor, which protects

the coalescence of hydrous zirconia molecules because of the chemisorption of sodium

ions on the negative surface of hydrous zirconia. The justification of the above

119

argument shows that the basic pH value of 13 has a positive effect on particle size

reduction.

3.2.2 Sol-Gel

In this work, the sol-gel process is used for the production of highly smooth

and spherical ZrO2 particles with uniform particle size. Considering the different

behaviour of ZrO2 particles depending on the size, the objective of the present work is

aimed to study the influence of the sol-gel process on particle size and surface area of

ZrO2 particles. The effect of ball milling on particle size and morphology were

examined using PSD and SEM. Figure 3.29 (a) presents the XRD pattern of ZrO2

particles obtained through sol-gel process after calcination at 773 K. It can be observed

from Figure 3.29 (a) that all the diffractions were recognised and assigned with

standard powder diffraction data (JCPDS File No. 81-1314). The observed results are

revealed with the existence of monoclinic zirconia with an average crystalline size of

15 nm. Figure 3.29 (b) shows FTIR spectra of zirconia particles obtained by sol-gel

method. In Figure 3.29 (b), the broad absorption band observed between 830 and 405

cm-1 is attributed to Zr-O vibrations (Zhang et al. 2009b). The strong absorption band

observed at 1030 cm-1 is due to existence of Zr-O linkage. This indicates that the

presence of Zr-O-Zr networks. The IR active modes observed at 1100 and 1140 cm-1

are assigned to asymmetric stretching vibration of C-O group (Luo et al. 2008). The

bands observed at wave numbers of 1200, 1260, 1380, 1490 and 1720 cm-1 can be

ascribed due to the carbon related impurities, which exist in the ZrO2 sample calcined

at low temperature 773 K (Zhou et al. 2007).

120

Figure 3.29 Characterisation of sol-gel derived ZrO2 nanoparticles


c) PSD d) EDS

121

Figure 3.30 SEM and TEM images of sol-gel derived ZrO2 particles

a) SEM image obtained before ball milling

b) SEM image obtained after ball milling

c) TEM image obtained before ball milling

d) TEM image obtained after ball milling

122

Figure 3.29 (c) displays the particle size distribution of sol-gel derived ZrO2

sample before and after ball milling. PSD results indicate that the sol-gel derived ZrO2

sample before milling yields an agglomerated particles in the range (d10-d90) of 170 -

432 nm and a mean particle size (d50) of 294 nm whereas the ball milled ZrO2 sample

yields particles in the range (d10-d90) of 28-50 nm and a mean particle size (d50) of 35

nm. It is important to note that the ball milling process facilitates the seven to eight

times reduction of particle size with narrow distribution.

Table 3.9 Physical and chemical analysis of sol-gel derived ZrO2 nanoparticles

EDS analysis (Figure 3.29d) reveals that the synthesized zirconia

nanopowder is having purity with more than 98 wt.% of ZrO2 content and 1.5 wt.% of

residual carbon. The presence of residual carbon is also indicated by FTIR analysis.

PSD measurements are in close agreement with SEM and TEM observations. The SEM

and TEM images of ZrO2 particles are displayed in Figure 3.30(a)-(d). The SEM and

TEM micrographs shown in Figure 3.30(a) and (c) reveals that the ZrO2 sample

obtained before milling yields an irregular shaped aggregated particles (300 nm) with

highly agglomerated surface. Whereas the ball milled ZrO2 sample (Figure 3.30b and

d) affords non-agglomerated monodispersed particles (40 nm) with well shaped



BET surface area 23 m2g-1 240 m2g-1 ZrO2 98.10±0.10

Mean particle size - 35 nm SiO2 00.40±0.10

Mean aggregate size 294 nm - CaO 00.02±0.01

- - - Hf 01.40±0.10

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spherical morphology. PSD, SEM and TEM observations reveals that the sol-gel

process yields an agglomerated particles which are further non-agglomerated by ball

milling process. It is quite worthy to note that the ball milling process offers reduced

particle size with high surface area and spherical morphology. The textural properties

of nano ZrO2 particles produced through sol-gel route is presented in Table 3.9. It can

be observed from BET surface area measurements that the soft agglomerates (294 nm)

of zirconia obtained before milling has the specific surface area of 23 m2 g-1 while the

non-agglomerated particles (35 nm) of zirconia after milling has the specific surface

area of 240 m2 g-1. It is interesting to note that, ball milling process not only influences

the particle size reduction and but also well enhances the specific surface area of sol-

gel derived ZrO2 particles. The XRF chemical composition of as produced ZrO2

particles is tabulated in Table 3.9. It can be seen from Table 3.9 that the ZrO2 particles

obtained through sol-gel process consists of 98.1 wt.% of ZrO2 while the remaining

consists of 0.4 wt.% of SiO2, 1.4 wt.% of Hf and trace amount of CaO. Hence, it can be

revealed from EDS (Figure 3.29d) and XRF analysis (Table 3.9) that the sol-gel

derived zirconia contains 98.1 wt.% of ZrO2.

3.2.3 Spray Pyrolysis

The present work focuses on the production of high surface area ZrO2

nanoparticles suitable for ceramic coating applications. It is important to note that the

production of nano ZrO2 particles in large quantity is essentially required for their

extensive industrial applications. Spray pyrolysis has been identified as suitable

method for mass production of nano ZrO2 particles for industrial applications. Our

objective is to achieve the production of nanocrystalline ZrO2 with high specific

surface area using automated spray pyrolysis method. The morphology, size

distribution, surface area and flow ability of the spray pyrolysed powder are discussed

in detail. XRD powder patterns of ZrO2 nanoparticles produced through spray

pyrolysis are shown in Figure 3.31(a). The peak positions of the XRD patterns were

124

recognised and compared with JCPDS (File No.:81-1314 and 80-2155). The crystallite

size of the produced nano ZrO2 has been obtained employing the Scherrer’s equation

(Zhang et al. 2000). From the XRD data, it was found that the synthesised nano ZrO2

particles have 96% monoclinic and 4% tetragonal crystal structure which is well agreed

with the standard powder diffraction data (JCPDS File No.:81-1314 and 80-2155). An

average crystallite size of monoclinic crystal phase is obtained as 18 nm from the

observed eleven different reflections.

On the other hand, the grain size of tetragonal phase is found to be 26 nm

All the reflection peaks are well recognised

and are wide widths with uniform crystalline size distribution in the range from 11 to

31nm. From the above XRD analysis, it is evident that spray pyrolysis yields

crystalline ZrO2 particles with low grain size. The grain size of both monoclinic and

tetragonal phases of prepared ZrO2 particles has close resemblance with the nano ZrO2

particles obtained from chemical precursors employing mechanochemical method

(Avvakumov and Karakchiev 2004). Table 3.10 presents the XRF chemical

composition of ZrO2 nanoparticles produced through spray pyrolysis. The obtained

results confirm that the ZrO2 nanoparticles consist of 98.3 wt.% of ZrO2, 0.3 wt.% of

SiO2 and 1.3 wt.% of Hf with trace amount of CaO. The above studies confirm that the

synthesised sample contains more than 98% chemical purity.

Table 3.10 Chemical analysis of ZrO2 nanoparticles produced through spray pyrolysis

XRF chemical composition (wt.%)

ZrO2 SiO2 CaO Hf

98.30±0.10 00.30±0.10 00.04±0.01 01.30±0.10

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Figure 3.31 Characterisation of ZrO2 nanoparticles produced through spray pyrolysis



126

The FTIR spectrum of ZrO2 nanoparticles prepared by the spray pyrolysis is

shown in Figure 3.31(b). The IR active modes observed at 600 cm-1 and 1070 cm-1

show the asymmetric stretching frequencies of Zr-O-Zr vibrational bands and the bands

observed at 445 cm-1 and 500 cm-1 confirms the existence of symmetric stretching

frequencies of Zr-O-Zr bonds. The absorption peaks at observed 1380 cm-1 and 1631

cm-1 are show respectively the O-H stretching and bending vibrations of ZrO2.xH2O

and H2O molecules.

The OH group is distinguished easily in H2O molecule by its bending

vibrations which appear in a single band at 1631 cm-1 confirms the presence of water in

ZrO2 sample. Further, the O-H bending vibration of hydroxyl group of ZrO2.xH2O

molecule appears at 1360 cm-1 (Lopez et al. 2001, Sahu and Rao 2000). The FTIR

results summaries that the ZrO2 powder consists of chemisorbed hydroxyl groups and

water molecules. Figure 3.31(c) shows that the particle size distribution of ZrO2

nanoparticles synthesised using spray pyrolyser. It can be seen from Figure 3.31(c) that

the synthesised powder consist of particle size in the range (d10-d90) of 15-64 nm and

the maximum distribution (d50) of particles is at 30 nm. Figure 3.31(d) shows the TEM

images of ZrO2 nanoparticles. TEM analysis of ZrO2 particles confirms that the powder

consist of monodispersed particles with quite uniform spherical morphology with a

mean diameter of 25 nm. It can be seen from Figure 3.31(c) and (d) that the particle

size distribution measurements (30 nm) are in close agreement with the TEM

observations (25 nm).

127

Table 3.11 Effect of feed rate on production rate, particle size and surface area of the ZrO2 nanoparticles produced through spray pyrolysis

The effect of feed rate on production rate, particle size and surface area of

ZrO2 nanoparticles are presented in Table 3.11. In order to achieve the maximum

efficiency in production rate for better yield, the spray pyrolysis was optimized. The

parameters such as spray air pressure (30-40 PSI), temperature of reaction chamber

(773 K), speed of hot air blower (1800 rpm) and liquid density (1.031 g cm-1) of

precursor are kept constant by varying the feed rate of the precursor. As the liquid feed

rate of the precursor is increased from 0.1 to 0.3 L h-1 results in an increase in the ZrO2

production rate from 2.4 to 7.3 g h-1 and the mean particle diameter (d50) from 28 to

112 nm while the specific surface area decreases from 283 to 98 m2 g-1 (Table 3.11).

Whereas, the liquid feed rate of 0.15 L h-1 (3.6 g h-1) yields the mean particle size of 30

nm and the surface area of 280 m2 g-1. Due to the production rate and negligible

changes of particle size and surface area, the liquid feed rate of 0.15 L h-1 has been

optimised for better yield. The optimised and net specific surface area of the nano ZrO2

particles calculated using BET method is 280 m2 g-1. The nano ZrO2 particles

maintained a larger surface area above 120 m2 g-1even after calcination at 1023 K. It

can be revealed from BET analysis that the spray pyrolysis yields high surface area

Density of precursor solution

g cm-3 (±0.005)

Feed rate

L h-1

ZrO2 production rate

g h-1

Mean particle size

nm (±3)

Surface area

m2 g-1 (±6)

1.031 0.10 2.4 28 283

1.031 0.15 3.6 30 280

1.031 0.20 4.8 46 195

1.031 0.25 6.0 93 141

1.031 0.30 7.3 112 98

128

particles with free flowing structure. Furthermore, it is evident from the above studies

that the particles are easily functionalised and flexibly fabricated to different

nanostructures depending on the requirements.

3.2.4 Optimisation of the ZrO2 Nanoparticles Production Method for Multilayered Nanostructured Protective Coating

Based on specific surface area and particle size distribution, the spray

pyrolysis has been optimised for multilayered nanostructured coating for protection of

high temperature oxidation corrosion of SS304 at 1273 K. In addition to that, the spray

pyrolysis has some advantages such as ease scaling for mass production, uniform

morphology with free flowing structure and controlled particle size over other classical

process, particularly precipitation and sol-gel route. Furthermore, the spray pyrolyser is

one of the direct processes for conversion of zirconyl nitrate solution into ZrO2

nanopowders and has advantages of short formation and decomposition time, mass

production needs simple protocol, little restriction in the nature of precursor, and no

requirement for hydrolysing and precipitating agents with recyclable byproducts.

3.1.5 Application of ZrO2 Nanoparticles

i) Nano ZrO2 Filled Silica Sol Characterization

Engrafting the ZrO2 nanoparticles within silica sol to make the hybrid

coating solution of monophase nano ZrO2 filled silica sol. Figure 3.32 displays the

PSD of nano ZrO2 filled silica sol. It can be seen from Figure 3.32 that the nano ZrO2

filled silica sol was found to have the particle size distribution in the range (d10-d90) of

20-150 nm and a mean diameter (d50) of 50 nm. It can be revealed from Figures 3.31(c)

and 3.32 that the mean diameter of ZrO2 particles obtained by spray pyrolysis is 30 nm

while the nano ZrO2 filled silica sol has a mean particle diameter of 50 nm. It is used to

frame the discussion that the dispersion of free formed ZrO2 particles in silica sol

129

yields hybrid particles i.e., ZrO2 particles was encircled by silicate network which

increase the mean diameter of free formed ZrO2 particles from 30 to 50 nm. Hence, it

is clear from the PSD observations (Figure 3.31c and 3.32) that the free formed ZrO2

particles are chemically coordinate with silicate network and form monophase nano

ZrO2 filled silica sol.

Figure 3.32 Particle size distribution of nano ZrO2 filled silica sol

ii) Coating Microstructure and Phase Analysis

The oxidation resistance of the SS304 at high temperature can be made by

producing an additional barrier action protective coating i.e., a nano ZrO2 filled silica

ceramic coating deposited by the sol-gel method on its surface. The sol–gel method is

often used to produce multilayer nanostructured ceramic coatings. The protective

properties of the coating are determined by its crystal structure and bulk density of the

film. The SS304 was covered with nano ZrO2 filled silica coatings using the sol-gel dip

coating method in order to increase its resistance to high temperature oxidation and

thermal shock. Elemental and composition analysis of SS304 and nano ZrO2 filled

silica coated SS304 is shown in Table 3.12. It was revealed by composition analysis

130

(Table 3.12) that the uncoated stainless steel specimen is SS304. Further, it was

detected through optical spark emission studies (Table 3.12) that the nano ZrO2 filled

silica coated SS304 shows an increase in content of Zr and Si while the remaining

elements are almost unaltered. It is clear from the above observations (Table 3.12) that

the increase in composition of Si and Zr explores the presence of nano ZrO2 filled silica

coating on SS304 surface. The presence of nanostructured multilayered ZrO2 filled

silica coating on SS304 was well demonstrated with XRD and AFM observations.

XRD examinations were carried out to determine the crystal structure of

coated and bare SS304 specimens. The XRD patterns of two representative samples of

bare and coated SS304 specimens are compared as shown in Figure 3.33. It shows the

presence of nanocrystalline ZrO2 filled silica coating on SS304 surface. According to

the XRD analysis (Figure 3.33a), it can be obtained that the bare specimen shows a

-iron with sharp peaks (Ghoranneviss et al. 2007) which are typical of

annealed SS304 materials. The observed (111), (200) and (220) reflections are well

matched with reported XRD data of SS304 (Jain and Christmana 1996). The stainless

steel specimen chosen in the present study was SS304 whose chemical composition

and crystal structure was clearly established respectively with optical emission

spectroscopy and XRD analysis (Jain et al. 1996). Figure 3.33(b) presents the XRD

pattern of nano ZrO2 filled silica (six layers) coated SS304. The standard powder

diffraction data reported in JCPDS File No.: 85-0659 and 81-1314 evidences the nano

ZrO2 filled silica coating consists of tetragonal (T) ZrSiO4 and monoclinic (M) ZrO2

phases. It should be noted that the T marked XRD patterns seen in Figure 3.33(b) are

assigned to the zirconium silicate (ZrSiO4) phase with tetragonal symmetry, while the

M marked XRD patterns are assigned to the pure zirconia phase with monoclinic

symmetry. Hence, the nano ZrO2 filled silica coating consists of almost 75% of ZrSiO4

(tetragonal) and nearly 25% of pure zirconia (monoclinic) phases. During the evolution

131

of the coating microstructure, it can be noted that the Zr:Si ratio is insufficient to

complete (100%) nucleation of ZrSiO4 phase.

Figure 3.33 X-ray diffraction patterns of the SS304 specimens: a) Bare SS 304 and b) Nano ZrO2 filled silica (six layers) coated SS304

132

Table 3.12 Chemical composition of uncoated and nano ZrO2 filled silica (six layers) coated SS304

Uncoated SS304 specimen Coated SS304 specimen

Elements Composition (%) Elements Composition (%)

C 0.05 C 0.07 Si 0.47 Si 2.01 Mn 1.14 Mn 1.12 P 0.037 P 0.034 S 0.011 S 0.013 Ni 8.53 Ni 8.51 Cr 18.92 Cr 18.89 Mo 0.22 Mo 0.26 V 0.12 V 0.10 Cu 0.15 Cu 0.12 W 0.013 W 0.011 Ti 0.021 Ti 0.020 Sn 0.02 Sn 0.015 Co 0.13 Co 0.127 Al 0.004 Al 0.005 Pb 0.02 Pb 0.018 B 0.001 B 0.003 Sb 0.092 Sb 0.088 Nb 0.034 Nb 0.031 Zr 0.022 Zr 0.665 Bi 0.051 Bi 0.048 Ca 0.001 Ca 0.002 Mg 0.005 Mg 0.004 Zn 0.013 Zn 0.015 Ce 0.002 Ce 0.001 La 0.008 La 0.006 Fe 69.78 Fe 67.814

133

However, the presence of 75% of ZrSiO4 phase indicates that the

concentration ZrO2 particles within the silica coating are sufficient to maintain more

than 50% nucleation of ZrSiO4 crystals. The average crystallite size of ZrSiO4 phase

obtained through Scherrer formula is 47 nm which is in close agreement with mean

particle diameter (50 nm) of nano ZrO2 filled silica sol. It is quite worthy to note that

the nano ZrO2 filled silica coating almost maintain the mean diameter of nano ZrO2

filled silica sol during the film formation process such as curing and firing. Thus, it is

evident from XRD analysis that the nano ZrO2 filled silica coating produce the

nanostructured crystalline film on SS304 surface. Surface microstructure, topography

and three dimensional surface profile of uncoated and nanostructured six layered ZrO2

filled silica coated SS304 specimens were obtained using tapping mode of AFM.

Figure 3.34 presents the AFM images of the SS304 surface before and after

coating. The 2D and 3D view of the AFM images of polished SS304 surface is shown

in Figure 3.34 (a) and (b). The SEM micrograph (3.35a) and AFM images (Figure

3.34a & b) reveals that the surface is uniform and the parallel features seen on the

surface may be associated with polishing scratches with lip heights about 1.5 µm.

Figure 3.34 (c) and (d) displays the 2D and 3D view of the AFM images of SS304

specimen after six layered nano ZrO2 filled silica coating. The nano ZrO2 filled silica

coated SS304 surface displayed a nanometer scale roughness (Figure 3.34c & d) with

nearly uniform distribution of islands and valleys of different sizes and shapes.

Pinholes were also identified in the particle layout, likely as a result of solvent

molecule evaporation during the curing and firing step (Fanizza et al. 2007). The

existence of multilayered nanostructured ZrO2 filled silica coating on SS304 surface

was confirmed with clear vision through AFM analysis. Furthermore, the presence of

nano ZrO2 filled silica coating on SS304 surface was also revealed by SEM

micrographs as shown in Figure 3.35(b).

134

Figure 3.34 AFM topographic images of a SS304 surface before and after coating

a) 2D view of the polished SS304 surfaces

b) 3D view of the polished SS304 surfaces

c) 2D view of the nano ZrO2 filled silica (six layers) coated SS304 surface

d) 3D view of the nano ZrO2 filled silica (six layers) coated SS304 surface

135

Figure 3.35 SEM micrographs of SS304 surfaces

b) Nano ZrO2 filled silica (six layers) coated SS304 after 100h isothermal oxidation at 1273 K

c) Uncoated SS304 after 100h isothermal oxidation at 1273 K

a) Polished SS304 d) Nano ZrO2 filled silica (six layers) coated SS304

136

iii) Influence of Nano ZrO2 Filled Silica Multilayer Coating on High Temperature Oxidation Corrosion of SS304

The SEM micrograph of uncoated SS304 after 100 h isothermal oxidation at

1273 K is shown in Figure 3.35(c). It can be seen from SEM micrograph that the

lustrous of oxide scale along with some white tinges were formed on the surface of

SS304 after 100 h (100 cycles) isothermal oxidation at 1273 K. During initial cycles,

light lustrous of oxide scale formed, however at the end of cycles, some whitish tinges

were also observed on the surface of SS304 (Figure 3.35c). The oxide scale shows two

regions namely with dark and light whitish grey scales, wherein the random

distribution of whitish grey scale with white tinges is observed. Figures 3.36(a) and (b)

show the AFM images (2D and 3D) of the SS304 surface after 100 h isothermal

oxidation at 1273 K. It can be visualised from AFM images (Figure 3.36a and b) that

the surface was almost completely covered with high density of dark and light whitish

grey scales with depth of 700 nm. A complex oxide scale with large spallation formed

on uncoated SS304 surface which indicates that SS304 has relatively poor cyclic

oxidation resistance at 1273 K.

It can be confirmed from SEM (Figure 3.35c) and AFM (Figures 3.36a and

b) observation that the high temperature (1273 K) isothermal oxidation effectively

damages the SS304 specimen through the high temperature corrosion of 700 nm depth

due to the effective diffusion of static air oxygen over SS304 surface. The AFM images

(2D and 3D) of nano ZrO2 filled silica coated SS304 surface after 100 h isothermal

oxidation at 1273 K are shown in Figures 3.36 (c) and (d). It can be noted from Figure

3.36 (c) and (d) that the surface exhibited a nanometer scale roughness (24 nm) with

random distribution of nano rods. It can be used to frame the discussion that the long

time isothermal treatment of coated SS304 at 1273 K for 100 h only induce an inter

diffusion of particles and thus, stimulate particle growth along with spallation of rod

like particles on SiO2-ZrO2 composite layer.

137

Figure 3.36 AFM topographic images of SS304 surfaces

a) 2D view of the uncoated SS304 after 100 h isothermal oxidation at 1273 K

b) 3D view of the uncoated SS304 after 100 h isothermal oxidation at 1273 K

c) 2D view of the nano ZrO2 filled silica (six layers) coated SS304 after 100 h isothermal oxidation at 1273 K

d) 3D view of the nano ZrO2 filled silica (six layers) coated SS304 after 100 h isothermal oxidation at 1273 K

138

However, no spallation of SiO2-ZrO2 composite layer could be observed on

SS304 surface (Figure 3.36c and d). The SEM micrograph seen in Figure 3.35(d) is

also in good agreement with the AFM (Figure 3.36c and d) observations. It is

important to note that the scale spallation was observed at 1273 K on the uncoated

(3.35c) SS304 while no spallation of the SiO2-ZrO2 composite coating (3.35d) occurred

on the coated SS304 during the tests. This suggests that the nano ZrO2 filled silica

coating has the ability to withstand and protect high temperature oxidation without

leaching and degradation of SiO2-ZrO2 composite layer over the SS304 surface. It was

reported from AFM images (Figure 3.34 and 3.36) and SEM micrographs (Figure 3.35)

that the nano ZrO2 filled silica coating have the massive potential to protect the high

temperature oxidation corrosion of SS304 at 1273 K. Ultimately, AFM and SEM

observations indicate that nano ZrO2 filled silica composite coating could act as a

barrier to spatially separate oxygen and the SS304 substrate.

Figure 3.37 Isothermal oxidation kinetics of coated and bare SS304 at 1273K for 100h: a) Bare SS304, b) Silica three layer coating, c) Silica six layer coating, d) Nano ZrO2 filled silica three layer coating and e) Nano ZrO2 filled silica six layer coating

139

The isothermal oxidation kinetics of various samples at 1273 K in static air

is shown in Figure 3.37. Influence of nano ZrO2 filled silica coating on the mass

change of SS304 against cyclic isothermal oxidation was clearly demonstrated in

comparison with uncoated and silica coated SS304 with respect to oxidation time in

hours. The mass gains of nano ZrO2 filled silica coated SS304 (Figure 3.37d and e) is

much higher than the silica coated and uncoated SS304. The oxidation kinetics of

composite coating did not follow parabolic law and the mass loss of silica coated and

uncoated SS304 is increased gradually when an increase in the exposure or oxidation

time. While nano ZrO2 filled silica coated SS304 shows almost linear progression in

mass gain while increasing the oxidation time. In order to make comparison, six and

three layers of silica and nano ZrO2 filled silica was coated on SS304 and tested at

1273 K for 100 h. It can be observed from Figure 3.37 that an increase in coating

thickness increases the mass gain and decreases the oxidation rate. It can be seen from

Figure 3.37 that the six layered nano ZrO2 filled silica coating shows better high

temperature oxidation resistance than three layered nano ZrO2 filled silica and silica

coating.

This suggests that an increase in thickness of silica and nano ZrO2 filled

silica coating reduces the thermal conductivity along with oxygen diffusion over the

SS304 surface which inturn reduces the oxidation rate and improves the protection

efficiency against high temperature oxidation corrosion. Thus, the above observation

clearly indicates that the nano ZrO2 filled silica composite coating changes the

oxidation mechanism with a reduction in oxidation rate.

140 3.3 SYNTHESIS, CHARACTERISATION AND APPLICATION

OF TiO2 NANOPARTICLES

The present chapter accounts for the synthesis and characterisation of TiO2

nanoparticles from raw ilmenite and its application in silica refractory. An inexpensive

process was developed for mass production of TiO2 nanoparticles from ilmenite

mineral. The focus of this work is to investigate in detail the process and to understand

the key issues for TiO2 nanoparticles production from natural mineral namely ilmenite.

The hydrolysis behaviour of titanium sulphate in an aqueous solution was investigated

by changing the production process. The aim of this present investigation is to achieve

large scale production of high surface area TiO2 nanoparticles from inexpensive

precursors through solution based chemical process. The understanding of particles

formation chemistry is expected to play an important role in designing new techniques

and developing new processes that can be used economically for the mass production

of high surface area TiO2 nanoparticles. In this chapter, the results based on

precipitation and sol-gel processes are discussed in detail for the mass production of

high surface area TiO2 nanoparticles.

3.3.1 Precipitation

The thermal decomposition and phase evolution behaviour of dried hydrous

titania were studied through TG-DTA to ascertain the formation of phase pure TiO2.

The TGA and DTA curve for the sample of hydrous titania are shown in Figure 3.38.

The TG curve shows weight loss in two stages with endothermic and exothermic peaks

respectively at 383 and 572 K. The total weight loss of around 55% is observed

between 308 and 623 K in two stages. It can be proposed from experimentally

observed total weight loss that the molecular structure of hydrous titania is

Ti(OH)4.4H2O or TiO2.6H2O. The thermal decomposition behaviour of hydrous titania

is given in scheme 2.3.

141

Scheme 3.3 Schematic representation for conversion of hydrous titania into titania

Figure 3.38 TGA - DTA curves for a hydrous titania powder obtained by the precipitation route

Ti(OH)4.4H2O Hydrous Titania

383 K

572 K

TiO2 .2H2O + 4H2O

TiO2 + 2H2O Titania

142

The weight loss is attributed to the loss of absorbed water in the titanium

peroxide gel and the conversion of peroxide to oxide, respectively. It can be observed

from TGA results that the wet chemical method yields hydrous samples with

physically adsorbed water molecules. Further, the obtained little shoulder at 773 K in

TGA curve may be due to the presence of chemically coordinated hydroxyl groups.

Figure 3.39(a) shows the XRD patterns for the TiO2 powders synthesized using

precipitation process. A typical XRD patterns (Figure 3.39a) indicates that the TiO2

nanoparticles obtained after calcination at 573 K are crystalline and phase pure which

exhibits a tetragonal structure with rutile symmetry (JCPDS File No.: 21-1276).

According to the Scherrer’s equation, the crystallite size of the prepared TiO2

nanoparticles is about 38 nm. The XRD patterns (Figure 3.39a) of the samples obtained

after calcination at 673 and 773 K also shows the tetragonal structure with rutile

symmetry. However, the average crystalline size increases from 38 to 44 nm and 44 to

49 nm respectively for the sample calcined at 673 K and 773 K. It can be seen from

the overall XRD observations (Figure 3.39a) that the calcination process induces the

crystal growth without any impact on crystalline phase and structure of the sample.

Figure 3.39(b) display the FTIR spectra of the TiO2 nanopowder produced

through precipitation route after calcination at 673 K. The broad band centered at 620

cm-1 is the characteristic peak of the Ti-O stretching mode (Zhang et al. 2005 and Jiang

et al. 2003). The vibration band observed at 1045 cm-1 corresponding to the stretching

and bending modes of the Ti-OH surface group. The wave band observed at 1405 cm-1

is corresponds to the stretching vibrations of C-C bond. Further, the observed vibration

band at 1620 cm-1 correspond to the stretching and bending modes of the O-H group of

absorbed water molecules (Zhang et al. 2005, Sun et al. 2003 and Yu et al. 2002a). It

can be revealed from FTIR spectra (Figure 3.39b) that the synthesized nano TiO2

powder contains chemisorbed water molecules with residual carbon.

143

Figure 3.39 Characterisation of TiO2 nanoparticles produced through precipitation route



144

Table 3.13 Particle size, textural and chemical analysis of TiO2 nanoparticles produced through precipitation route

It can be observed from Table 3.13 that the precipitation route affords

91.2 wt.% of TiO2 with 7.9 wt.% of Fe2O3 impurity. The XRF chemical analysis

reveals that the precipitation route yields 91.2 wt.% TiO2 chemical purity. Figure

3.39(c) shows the particle size distribution of TiO2 nanoparticles obtained by

precipitation method. It can be seen from Figure 3.39(c) that the TiO2 sample calcined

at 573 K yields a mean particle diameter (d50) of 385nm while the ball milled TiO2

sample (Figure 3.39c) yields a mean particle diameter (d50) of 60 nm. Before ball

milling, the precipitation route yields a particle size in the range of 335-450 nm

whereas the ball milled rutile titania yields a particle size in the range of 35-90 nm. It

can be revealed from particle size analysis (Figure 3.39c) that the ball milling process

influences a reduction in the mean particle size of TiO2 particles in the order of five to

six times. Table 3.13 shows the textural properties of the TiO2 nanoparticles. Table

3.13 clearly reveals that the ball milling process not only reduces the mean particle size

from 385 to 60 nm which also increases the specific surface area from 28 to 70 m2 g-1.

Figure 3.39(d) presents the TEM picture of ball milled TiO2 nanoparticles. It can be

confirmed from TEM observation (Figure 3.39d) that the particle size of TiO2

nanoparticles is around 60 nm. It is interesting to note that the mean particle size

obtained by PSD analysis is in close agreement with TEM observation.



BET surface area 28 m2g-1 70 m2g-1 TiO2 91.2 ± 0.1

Mean particle size - 60 nm Fe2O3 07.9 ± 0.1

Mean aggregate size 385 nm - - -

145 3.3.2 Sol-Gel

A nanostructured TiO2 rutile phase powder with a large specific surface area

was produced by the sol-gel process at very low temperature. The sol-gel process was

developed from titanium sulphate precursor which is extracted from ilmenite

employing the acid digestion and leaching process. The prepared nano TiO2 was

characterised for its quality. The cost effective and high quality process has been

proposed for mass production of high surface area TiO2 nanoparticles. Figure 3.40a-c

presents XRD patterns of as synthesised and calcined samples which show the phase

and crystallite dimensions of sol-gel derived nano TiO2 particles. All the

experimentally observed reflections are assigned on the basis of reported rutile titania

phase (JCPDS File No.:21-1276) with tetragonal symmetry. The entire XRD patterns

(Figure 3.40a-c) clearly indicate that the obtained product has fine rutile structure with

tetragonal symmetry. The crystallite size is obtained by XRD peak analysis based on

Scherer’s equation (Mazloumi et al. 2006 and Cullity, 1978). The average crystallite

size distributions of nano rutile TiO2 calcined at 573, 673 and 773 K are respectively

34, 38 and 41 nm. It confirms that the crystallite size increases with increase in

sintering temperature of the synthesised sample. The dried samples which are calcined

at 573, 673 and 773 K show no diffraction peaks of anatase (Figure 3.40a-c) which

suggests that amorphous TiO2 does not exist in the prepared sample (Shafi et al. 2001).

It can be seen from Figure 3.40 that the existence of pure rutile structures with an

average crystal sizes in the range between 34 and 41 nm.

146

Figure 3.40 XRD patterns of nano TiO2 powder calcined at different temperatures a) 573 K, b) 673 K and c) 773 K

Figure 3.41 FTIR spectra of a) titanium hydroxide gel dried at 353 K and b) nano titania sintered at 573 K

a)

b)

147

The infrared spectra of titanium hydroxide gel dried at 353 K and nano

rutile titania calcined at 573 K are presented in Figure 3.41. The observed broad

absorbance peaks between 500 and 900 cm-1 in Figure 3.41(a) & (b) is originated from

the titanium dioxide. The peaks observed at 1644 cm-1 in Figure 3.41(a) & (b)

corresponds to the hydroxyl groups and the surface adsorbed water. The peaks

observed at 983, 1145, and 1400 cm-1 in Figure 3.41(a) corresponds to Ti-OH, C-N and

–CH3 bonds (Shafi et al. 2001, Zhang et al. 2005, Yu et al. 2002b and Jiang et al.

2003) respectively which are observed from peptised titanium hydroxide with organic

surfactant. The FTIR results clearly show that the mechanism of sol-gel process and

conversion of titanium hydroxide into nano TiO2. It can be observed from chemical

analysis that the chemical purity of the prepared nano TiO2 particles is about 90.8%

which reveals the presence of some impurities such as carbon and nitrogen in sol-gel

derived titania sample. Figure 3.42 shows the BET plot of nano rutile titania powder.

The specific surface area of nano TiO2 is 112 m2 g-1.

Figure 3.42 BET plot of nano rutile TiO2 powder

148

Figure 3.43 Particle size distribution of TiO2 nanoparticles

The observed surface area of the synthesised nano rutile TiO2 is in close

agreement with the surface area of nano rutile TiO2 powders obtained from liquid

hydrolysis of TiCl4 (Li et al. 2002). The obtained high surface area of nano rutile TiO2

particles is essentially required for industrial applications (Philpot et al. 1983). Figure

3.43 shows the particle size distribution of TiO2 nanoparticles. The particle size

distribution obtained in the present results is in the range of 22-120 nm. However, the

maximum distribution (d50) of particles is 50 nm. The TEM micrograph of the powder

calcined at 573 K (Figure 3.44) clearly shows that the particle morphology and size of

the titania particles. Particle size and TEM analysis reveal that the obtained primary

particle size is 50 nm which is larger than the observed grain size of titania from XRD

pattern. The observed larger particle size of the powder is due to the poly crystalline

nature (Philpot et al. 1983) of the powder. Thus, it is evident from the observed results

that the obtained particles have uniform size distribution and spherical morphology

with non-agglomerating surface.

149

Figure 3.44 Transmission electron micrographs of rutile TiO2 nanopowder

a) 50 nm scale bar b) 200 nm scale bar

c) Selected area electron diffraction analysis

150 3.3.3 Optimisation of the TiO2 Nanoparticles Production Method for

Refractory Applications

The chemical reactivity is mainly based on the surface area of the TiO2

particles. TiO2 is used as mineraliser for densification of silica brick. The chemical

affinity and reactivity of TiO2 in silica refractory composite induces the compactness

and densification of the silica brick. Sol-gel derived TiO2 particles have more specific

surface area with reduced mean particle size than the TiO2 particles obtained through

precipitation route. Based on the specific surface area, mean particle size and higher

chemical reactivity, sol-gel derived TiO2 nanoparticles have been optimised for

refractory applications.

3.3.4 Application of TiO2 Nanoparticles in Silica Refractory

Silica bricks have been used as refractory in constructing and restoring

industrial furnaces, including coke oven, glass melting furnace and hot blast furnaces

(Balkevich et al. 1984, Arahori and Suzuki 1987). The volume stability and creep

properties of silica bricks (Arahori and Suzuki 1987) at high temperature are the unique

properties of the heavy duty refractory products. The conventional silica bricks

generally exhibit major difficulties such as degradation (Brown and Wosinski 2001),

lower refractoriness (Chrzan et al. 1924) and thermal expansion (Coler 1930,

Houldsworth and Cobb 1923, Austin and Pierce 1933) due to their poor alkali and

thermal resistance. In order to eliminate the above troubles while using for industrial

applications, nano titania is used as mineraliser (Chaudhuri et al. 1999) to reduce the

calcium oxide binder which prevents the degradation, loss of refractoriness and thermal

expansion of fused silica refractory bricks in furnace environments. The incorporation

of nano metal oxides such as TiO2 (Huang et al. 2007), SiO2 (Xianfeng et al. 2004),

ZrO2 (Ghosh et al. 2007) and MgO (Braulio et al. 2008) into the refractories enhances

151 the physico-chemical and thermo-mechanical properties of refractories and their

composites (Yang et al. 2008).

In manufacturing of silica bricks, quartzite has been used as a starting raw

material and calcium oxide is used as a bonding material as well as a mineraliser to

convert cristobalite into tridymite during the firing of green silica brick. Different

mineralisers such as iron oxide, manganese dioxide, magnesium oxide and titanium

dioxide have been used to speed up the tridymite phase transformation from

cristobalite (Budnikov 1964, Arahori and Suzuki 1987). A further addition of

mineraliser like TiO2 leads to the acceleration of the tridymite phase transformation

due to the decrease in the viscosity of lime silica glass. In the present work, an attempt

has been made to incorporate sol-gel derived nano TiO2 powder into the matrix of

silica brick and to monitor the effect of nano titania on the physical and thermal

properties of silica refractory. The obtained results are revealed to explore the use of

nano titania as an effective mineraliser for the formation of silica refractory. Table 3.14

shows that the physical and thermal properties of silica brick with and without addition

of nano and micron sized TiO2. The role and impact of nano TiO2 addition are explored

by incorporation of nano and micron sized TiO2 powder in silica bricks. The addition

of nano TiO2 can remarkably alter the refractory properties such as bulk density,

apparent porosity, CCS, RUL, creep in compression and reversible thermal expansion.

152

Table 3.14 Physical and thermal properties of silica bricks

A) Bulk Density and Apparent Porosity

Table 3.14 shows that the addition of micron and nano TiO2 in silica brick

leads to an increase in bulk density and decrease in apparent porosity of the brick.

Especially, silica bricks with addition of nano TiO2 have greater density and less

porosity than the silica bricks with micron sized TiO2. It can be seen that, addition of

nano TiO2 induces better densification of silica refractory than the micron sized TiO2.

Parameter Silica brick

without TiO2

Silica brick with micron

TiO2

(0.5 %)

Silica brick with nano TiO2

(0.5%)

Silica brick with nano

TiO2

(1.0 %)

Standard silica brick DIN 1089

Part 1

Apparent Porosity (%) 21.8 ± 0.10 19.5 ± 0.10 16.0 ± 0.10 16.6 ± 0.10 22

Bulk Density (kg m-3) 1.80 ± 0.01 1.85 ± 0.01 1.92 ± 0.01 1.90 ± 0.01 -

Cold Crushing Strength (kg cm-2 )

410 ± 25 480 ± 25 640 ± 25 540 ± 25 350

Refractoriness Under Load

1675 ± 01 1680 ± 01 1680 ± 01 1680 ± 01 1650

Creep in Compression

at 1723 K / 50 h 0.22 ± 0.01 0.19 ± 0.01 0.15 ± 0.01 0.20 ± 0.01 0.35

Reversible Thermal Expansion at 1273 K

1.35 ± 0.01 1.28 ± 0.01 1.15 ± 0.01 1.20 ± 0.01 -

Residual Quartz (%) 0.8 0.7 0.3 0.4 1.5 max

153 This is due to the efficient catalytic effect of nano TiO2 which enhances the

densification process by forming a liquid phase (Chaudhuri et al. 1999).

B) Cold Crushing Strength

Mechanical properties in terms of cold crushing strength (CCS) of the nano

and micron TiO2 incorporated silica bricks are shown in Table 3.14. It can be seen that

CCS of silica brick is increased with addition of nano and micron sized TiO2. The

addition of nano TiO2 efficiently enhances the cold strength of bricks when compared

to micron sized TiO2. This is due to the good compactness of brick induced by nano

TiO2.

C) Refractoriness under Load

Table 3.14 summarizes the refractory properties of both nano and micron

TiO2 incorporated silica bricks. Refractoriness under load (RUL) results is expressed in

terms of silica bricks with and without addition of nano and micron sized TiO2. The

above results confirm that the improved load bearing capacity of silica bricks with

TiO2 additives. Both nano and micron TiO2 additives induce same degree of

refractoriness under load which is better than the silica brick without TiO2.

D) Creep in Compression

The results shown in Table 3.14 demonstrate that the creep in compression

(CIC) of silica brick with and without addition of nano and micron sized TiO2. The

observed result reveals that the silica brick with nano TiO2 can have good creep

resistance than the silica brick with and without micron sized TiO2.

154 E) Reversible Thermal Expansion

As seen from Table 3.14, the silica bricks prepared with addition of nano

TiO2 show low thermal expansion than the silica bricks prepared with and without

addition of micron TiO2. During the firing of green silica brick, the quartz grains start

converting into cristobalite at 1146 K (Houldsworth and Cobb 1923). In silica brick,

the reversible thermal expansion (RTE) is dependent on the amount of mineral phases

such as cristobalite, tridymite and residual quartz. Higher RTE indicates that the higher

amount of cristobalite and lower amount of tridymite. Similarly, the lower RTE

indicates lower amount of cristobalite and higher amount of tridymite (Coler, 1930).

Nano TiO2 has effectively reduced the percentage of thermal expansion of silica brick

due to the formation of more amount of tridymite.

F) Influence of Nano TiO2 on Phase Transformations in Silica Brick

The characteristic XRD patterns of silica brick with and without nano TiO2

are shown respectively in Figures 3.45 and 3.46.

Figure 3.45 XRD patterns of silica brick with nano TiO2 powder

155

Figure 3.46 XRD patterns of silica brick without nano TiO2 powder

XRD patterns of silica brick with nano TiO2 powder (Figure 3.45) show

higher tridymite peak intensity when compared to conventional silica brick. This may

be either due to the higher proportion of tridymite phase or the lower crystal size

distribution of tridymite phase. But, the XRD patterns of silica brick without nano TiO2

powder (Figure 3.46) show a less tridymite peak intensity. This may be either due to

the lower proportion of tridymite phase or the higher crystal size distribution of

tridymite phase. The wide variation of crystal size (40 to 120 microns) of tridymite

phase is witnessed in silica brick without the addition of nano TiO2. The crystal size is

reduced from 80 to 40 microns with the addition of nano TiO2. The tridymite counts

are increased to an appreciable amount as a result of nano TiO2 addition (Figure 3.45)

when compared to the silica brick without addition of TiO2 (Figure 3.46). Silica brick

with nano TiO2 shows a higher tridymite phase distribution and a low thermal

156 expansion. From the thermal expansion data (RTE at 1273 K), it is found that the

thermal expansion is decreased with increase in titania powder. The effect is more

pronounced when nano TiO2 powder is added (0.5 wt.%) optimally. The reduction in

thermal expansion is also an indication of more tridymite formation (Austin and Pierce

1933). In order to reduce the coefficient of thermal expansion in a fired silica brick,

one can use mineraliser like TiO2 which in turn helps in forming the more amount of

tridymite than cristobalite phase.

G) Microstructure

The microstructural evolution of silica bricks fired at 1723 K was evaluated

using the optical micrographs. Figures 3.47 and 3.48 show the optical microscopic

images of silica brick respectively with and without nano TiO2. It is evident that the

optical microscopic characterisation of silica brick with nano TiO2 powder (Figure

3.46) shows the homogeneous size distribution of tridymite phase with cristobalite

degeneration cracks which is almost blind and indistinct. On the other hand, it can be

seen from the optical microscopic characterisation of silica brick without nano TiO2

powder (Figure 3.47) that the heterogeneous size distribution of tridymite phase with

cristobalite degeneration cracks. The ability to control microstructure is of the central

importance in achieving desired properties. The microstructure analysis of silica bricks

reveals that the addition of nano TiO2 induces the greater densification, lower grain

size and well defined morphology.

157

Figure 3.47 Optical micrographs of silica brick with nano TiO2 powder at Figure 3.48 Optical micrographs of silica brick without nano TiO2 powder at

158 H) Optimisation of Nano TiO2 Addition

The optimisation of nano additive (TiO2) on properties of silica brick are

explored interms of quantity and size distribution of tridymite phase in base brick. In

order to establish the optimised content of nano additive (TiO2) in base brick, four

different fractions i.e., 0.25, 0.5, 0.75 and 1 wt.% of nano TiO2 are added to the brick

composition. The addition of 100 % i.e. 1 wt.% micro TiO2 leads to an increase in the

residual quartz content in base brick. Similarly, the addition of 100% i.e., 1 wt.% nano

TiO2 also increases the residual quartz content in base brick. The addition of 0.25 - 0.5

wt.% of nano TiO2 into base brick reduces the residual quartz content gradually. On the

other hand, it increases the tridymite phase content in base brick which results in better

improvement in refractory properties. A highest reduction in thermal expansion is

achieved in silica brick with addition of 0.5 wt.% of nano TiO2 to the base brick

composition. It can be seen from Table 3.14 that 1 wt.% of nano TiO2 addition in base

brick leads to higher apparent porosity, low bulk density and CCS when compared to

the addition of 0.5 wt.% of nano TiO2. In addition to that silica brick with 1 wt.% of

nano TiO2 show a less creep resistance and a higher thermal expansion. The reason

may be due to optimum conversion of quartz into tridymite phase is attained with the

addition of 0.5 wt.% nano TiO2. The excess of nano additive (> 0.5 wt.%) leads to an

increase in the residual quartz contents due to the higher active catalytic effects of

nano TiO2 for reconversion of tridymite into quartz phase in base brick.

The experimental results conclude that the addition of 0.5 wt.% of nano

TiO2 is sufficient to convert maximum content of residual quartz into tridymite phase

and control the microstructure of silica brick. It has been proven that TiO2 is a good

mineralizer (Chester, 1973) for the conversion of silica into its different polymorphs

during the firing of silica brick. The calcium hydroxide present in the brick gets

decomposed into lime (CaO) and it reacts more intensively with silica in the

temperature range from 1088 to 1118 K to form metastable phases such as

159 3CaO.2SiO2 (Budnikov, 1964). Lime acts as a media for the conversion of silica phases

from quartz to tridymite with a simultaneous formation of meta-cristobalite phase. The

formation of meta-cristobalite phases leads to more thermal expansion (Chester, 1973)

than the tridymite phase of silica (Austin and Cobb 1933). At this temperature, a highly

active mineralizer namely TiO2 reinforced the conversion tendency to overcome the

metastable (meta-cristobalite) stage (Chester, 1973). The role of TiO2 in silica brick is

to accelerate the nucleation of tridymite by keeping the formation in labile state. A

complete homogenisation is not achieved effectively due to the incorporation of micron

sized rutile titania powder. However, the addition of nano TiO2 leads to a better

dispersion and homogenisation of tridymite phase due to its higher surface area,

smaller particle size and reactivity. In the absence of mineraliser, the conversion is

called dry alteration and it allows the formation of meta-cristobalite (Ruddlesden,

1954).

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