nmp chapter 41 -...

Chapter IV : The System Zn0.25CuxMn0.75-xFe2O4

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4.1 Introduction

Ferrites are mixed metal oxides which contain iron oxide as

major component. These ferrites are classified into three types viz.

spinel, garnet and hexaferrite [1]. In the spinel structure of general

formula AB2O4 there are twice as many octahedral (B) sites as tetrahedral

(A) sites. The spinels are further classsified into two types depending

upon position of M2+ ion. If M2+ occupies only the A sites, the spinel is

normal; if it occupies only the B sites, the spinel is inverse.

Mixed metal oxides possessing spinel structure and its property

have been investigated by number of workers [2-7]. The properties of

these materials dependent on the nature of ions and their distribution

among tetrahedral and octahedral sites.

Ferrospinels have interesting structural, electrical and magnetic

properties and are widely used in many important components such as

magnetic resonance imaging [8], magnetic drug delivery [9], magnetic

recording media, magnetic storage fluids [10], sensors [11], pigments

[12] and photocatalysts [13] etc. The method of preparation plays a very

important role with regard to the chemical, structural and magnetic

property of mixed spinel ferrites .Various methods have been developed

for the synthesis of ferrites [14-18]. Wet chemical methods such as, co-

precipitation [19-22], sol-gel [23-25], citrate-gel [26], hydrothermal [27-

29], solvothermal [30-31], mechanical alloying [32-33] have been

reported for obtaining desired properties of materials.

In present investigation, an attempt is made to prepare copper

substituted Mn-Zn ferrite by co-precipitation method, which neither

requires sophisticated instrument nor high sintering temperature. Their

structural, electrical and magnetic properties were characterized by using

X-ray diffraction (XRD), EDAX, Thermal analysis (TGA-DTA),

Scanning electron microscopy (SEM), IR spectroscopy, Electrical


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conductivity. Magnetic study of synthesized compounds has also been

investigated. Photocatalytic activity of the system was studied with

photodegradation of congored in presence of ultra-violet light. Various

gases were used to check the gas sensing performance of these ferrites at

operating temperature. Different gas sensing parameters were studied in

detail to express the mechanism and activity of these ferrites.

4.2 Materials and Methods

The various compounds of the system Zn0.25CuxMn0.75-xFe2O4 were

prepared by co-precipitation method [34]. To prepare spinel compounds

of high purity, water soluble sulphates such as manganese, copper, zinc

and iron precursor were dissolved separately. Appropriate amount of

these solutions were mixed together in 1000ml beaker with constant

stirring. Then resulting solution was precipitated as hydroxide precursor

using 10% NaOH solution by maintaining pH at 9.5-10.The hydroxides

were then oxidized using 30% H2O2 (100vol.) solution and the obtained

precipitate was digested in water bath for 4 hr. at 90-950C. Then

precipitate was washed thoroughly with distilled water to remove

unreacted sulphates and excess alkali and filtered through the Whatman

filter paper No. 41. The precipitate was dried in oven at 110oC. The dried

powder was then heated at different temperatures to get resulting mixed-

metal oxides. The above powders were heated separately at 900°C for 4

hrs to get the final product.

The spinel powders were then mixed with binder (2% PVA in

acetone) and pressed into pellets of the size 1 cm in diameter and about

0.3 cm thickness under a pressure 10000 psi. The pellets were initially

heated at 400°C for 6 hrs to remove binder. These pellets were then used

for determination of various properties.


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The powder X–ray diffraction patterns were recorded on Philips

PW-1710 X-ray diffractometer by using CuKα radiation. The lattice

parameters were calculated using high angle reflection of XRD by using

the following formula,

---------------- 4.1 Crystallite size was calculated by using the Scherrer formula,

-------------- 4.2

The percentage porosity was calculated by using the formula,

-------------------- 4.3 where, the notations have their usual meaning.

The FT-IR spectra were recorded in the range of 400 to 1000

cm-1 on Perkin Elmer – IR spectrophotometer (model E-2829) in KBr

pellets.

The SEM micrograph of the samples was obtained using

scanning electron microscope (JEOL JSM 6360). The grain size of all the

samples was calculated by Cottrell’s method.

Magnetic measurements of all compositions of the system were

made by using a high field hysteresis loop tracer. The measurements were

done at room temperature. Saturation magnetization (Ms), coercive field

(Hc) and remenant magnetization (Mr) of the samples were studied from

the hysteresis loops of respective curves.

Elemental analysis was carried out by using the energy dispersive

X-ray spectroscopy equipped with Scanning electron microscopy on

instrument Phi560 XPS/AES/SIMS UHV system.


69

Two probe technique was employed to measure the D.C.

resistivity of the samples in the temperature range of room temperature to

500°C and specific resistivity was determined using the relation,

ρ = (πd2 / 4t) R ----------------- 4.4

The activation energy (eV) was calculated from the plots of log ρ

Vs 103/T. Silver paste was applied to both the surfaces of the pellets for

good ohmic contacts.

In presence of various oxidizing and reducing gases, gas

sensing performance of Cu-substituted Mn-Zn ferrite was tested . The

electrical resistance of a sensor in dry air is measured by use of

conventional circuitry in which the sensor is connected to an external

resistor at circuit voltage of 10 V. The gas response (S) is defined as the

ratio of ∆R, i.e. the change in resistance of the sensor in air (Ra) and in

presence of gas (Rg), normalized to the value of sensor resistance in air.

(%) S = Ra – Rg / Ra -------------- 4.5

Photocatalytic performance was studied by using organic dye like

congored. Congored is considered as a model of a series of common azo-

dyes, used in the industry .The photocatalytic activity was studied for

congored dye in presence of Ultra-violet light with different times.

4.3 Results and Discussion

4.3.1 X-ray Diffraction Analysis

X-ray powder diffraction patterns of all the compositions of

system Zn0.25CuxMno.75-xFe2O

4 sintered at sintering temperature 900oc are

shown in Fig. 4.1. From the X-ray analysis it is observed that all the

compositions of the system have cubic unit cells. Indexed X-ray

diffractometer patterns are listed in Tables 4.1 -4.4. For all spinel ferrites

(311) reflection is the more intense one. The dhkl

and 2θ values were

compared with the values reported in the JCPDS 10-467.


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The interplanar distance d for each diffraction plane was

calculated using formula,

The structural parameters for different compositions of the

system Zn0.25CuxMn0.75-xFe2O4are listed in Tables 4.1 to 4.4. This reveals

that the lattice constant goes on increasing from 8.3170 to 8.4142 Å. This

value also increases with increasing Cu2+ concentration in the

system.This can be attributed to the ionic size difference between Mn2+

(0.67Å) and Cu2+ (0.73Å) . The crystallite size, X-ray density ,lattice

parameter have been listed in the Table 4.6. The crystallite size lies in the

range 29.31 to 29.59 nm and it shows linear variation with increasing

Cu2+ content. X-ray density (dx) shows irregularity from x= 0.0 to x =

0.75 and lies between the range 5.3638 and 5.2903 g/cc.

Fig.4.1 XRD pattern for Zn0.25CuxMn0.75-xFe2O4system sintered at 900

0C

(0.0 ≤ x ≤ 0.75).


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Table 4.1 Indexed X-ray diffraction pattern of Zn0.25 Mn0.75Fe2O4

Structure: Cubic Lattice constant: a = 8.3170 Å

Table 4.2 Indexed X-ray diffraction pattern of

Zn0.25 Cu0.25 Mn0.5 Fe2O4

.Sr.No. 2 θ d (Å)

(observed) d(Å)

(calculated) hkl a(Å)

1 30.13 2.7440 2.7420 2 2 0 8.2065 2 35.72 2.5250 2.523 3 1 1 8.3296 3 49.58 1.8264 1.8262 4 2 1 8.4183 4 54.20 1.7085 1.7082 4 2 2 8.2833 5 57.71 1.6102 1.6100 5 1 1 8.2934 6 62.06 1.5033 1.5030 5 2 1 8.3193 7 62.75 1.4798 1.4796 4 4 0 8.3687


.Sr.No. 2 θ d(Å)

(observed) d(Å)


1 30.14 2.9625 2.9700 2 2 0 8.3793

2 35.48 2.5279 2.5280 3 1 1 8.3841

3 37.13 2.4192 2.4249 2 2 2 8.3808

4 43.13 2.0956 2.1001 4 0 0 8.3824

5 53.48 1.7119 1.7147 4 2 2 8.3865

6 56.99 1.6145 1.6160 5 1 1 8.3892

7 62.57 1.4832 1.4849 4 4 0 8.3906


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Table 4.3 Indexed X-ray diffraction pattern of

Zn0.25 Cu0.5 Mn0.5 Fe2O4

.Sr.No. 2 θ d (Å)

(observed) d (Å)


1 30.14 2.9625 2.9708 2 2 0 8.3793

2 35.48 2.5279 2.5335 3 1 1 8.3841

3 37.10 2.4211 2.4257 2 2 2 8.3547

4 43.13 2.0956 2.1002 4 0 0 8.3824

5 53.45 1.7127 1.7152 4 2 2 8.3909

6 56.99 1.6145 1.6171 5 1 1 8.3892

7 62.57 1.4832 1.4854 4 4 0 8.3906


Table 4.4 Indexed X-ray diffraction pattern of Zn0.25 Cu07.5 Fe2O4

.Sr.No. 2 θ d(Å)

(observed) d (Å)


1 30.02 2.9740 2.9755 2 2 0 8.4137

2 35.36 2.5362 2.5375 3 1 1 8.4116

3 37.20 2.4290 2.4293 2 2 2 8.4107

4 42.98 2.1025 2.1040 4 0 0 8.4100

5 49.61 1.8359 1.8365 4 2 1 8.4135

6 53.27 1.7101 1.7179 4 2 2 8.4171

7 56.75 1.6207 1.6196 5 11 8.4217

8 62.36 1.4877 1.4877 4 4 0 8.4160

Structure: Cubic Lattice constant: a = 8.4142Å


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Table 4.5 Lattice constants, crystallite size, X-ray density for

Zn0.25CuxMn0.75-x Fe2O4 System(0.0 ≤ x ≤ 0.75).

4.3.2 Thermo Gravimetric Analysis

The simultaneous differential scanning calorimetry and

thermogravimetry analysis (DSC–TGA) were carried out in presence of

air between the range 10 to 1000oC for the typical samples of system

Zn0.25CuxMn0.75-xFe2O

4 . Fig.4.3 shows DSC–TGA curves of synthesized

sample prepared by co-precipitation method .TGA curve indicates

continuous weight loss up to 600oC .DSC curve shows exothermic peak

about 165oC can ascribed to evaporation of absorbed water and

decomposition of Zn(OH)2, Cu(OH)2, Mn(OH)2, Fe(OH)2 precipitate.

The second exothermic peak observed from 200 to 400 oC .This

represents solid state reaction between ZnO, CuO, MnO and Fe2O3[35-

37].Evaluation of various kinetic parameters from TGA curves

summerised in Table 4.6 to 4.9.To evaluate the kinetic parameters for

oxide material,Coat-Redfern method is used.The decomposition steps

studied by this method are shown in Fig.4.2 while activation energy

calculated by this method are listed in table 4.9.

Sr.

No. Sample composition

Lattice

constant 'a'

(Ao)

Crystallite

Size( nm)

X-ray

density(gm/cc)

1 Zn0.25Mn0.75Fe2O4 8.3170 29.3121 5.3638

2 Zn0.25Cu0.25 Mn0.5Fe2O4 8.3847 29.5824 5.2219

3 Zn0.25Cu0.5 Mn0.25 Fe2O4 8.3912 29.5925 5.2660

4 Zn0.25 Cu0.75 Fe2O4 8.4142 29.5948 5.2903


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Table 4.6 Evaluation of Kinetic parameters of Zn0.25Mn0.75Fe2O4.

W0 = 7.0800 mg

Temp oc

residual wt.(mg)

α = Wt - Wf W0 - Wf ln α /T2 gα ln gα /T2 1000/T

50 7.0241 0.9354 -11.6220 1.4918 -11.1553 3.0950

100 6.8751 0.7634 -12.1131 1.0272 -11.8663 2.6800

150 6.7074 0.5698 -12.6572 0.6884 -12.4681 2.3640

200 6.5490 0.3870 -13.2675 0.4342 -13.1524 2.1147

250 6.3814 0.1935 -14.1616 0.2040 -14.1087 1.9120

300 6.2789 0.0752 -15.2893 0.0768 -15.2586 1.7450

350 6.2416 0.0322 -16.3048 0.0326 -16.2925 1.6051

400 6.2323 0.0214 -16.8678 0.0216 -16.8585 1.4850

450 6.223 0.0107 -17.7043 0.0108 -17.6950 1.3831

Table 4.7 Evaluation of Kinetic parameters of Zn0.25 Cu0.5 Mn0.25 Fe2O4.

W0 = 6.8220 mg

Temp

oc residual wt.(mg)


50 6.6447 0.7329 -11.1250 0.9664 -10.8485 3.0950

100 6.6083 0.6781 -11.6074 0.8654 -11.3635 2.6800

150 6.5583 0.6028 -12.0614 0.7396 -11.8569 2.3640

200 6.4355 0.4179 -13.1907 0.4742 -13.0643 2.1147

250 6.3536 0.2945 -13.7416 0.3202 -13.6579 1.9120

300 6.2763 0.1781 -14.4271 0.1870 -14.3784 1.7450

350 6.2490 0.1370 -14.8568 0.1422 -14.8196 1.6051

400 6.2308 0.1096 -15.2344 0.1128 -15.2056 1.4850

450 6.2172 0.0891 -15.5848 0.0912 -15.5615 1.3831

500 6.2081 0.0754 -15.8855 0.0770 -15.8645 1.2936

550 6.1899 0.0480 -16.4624 0.0486 -16.4500 1.2150

600 6.1671 0.0137 -17.8342 0.0138 -17.8269 1.1454


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Table 4.8 Evaluation of Kinetic parameters of Zn0.25 Cu0.75Fe2O4.

W0 = 8.8370

Temp oc residual wt.(mg)


50 8.7347 0.7658 -11.8220 1.0322 -11.5236 3.0950

100 8.6557 0.5854 -12.3786 0.7124 -11.9522 2.6800

150 8.6045 0.4684 -12.8531 0.5418 -12.4026 2.3640

200 8.5719 0.3940 -13.2495 0.4432 -13.1319 2.1147

250 8.5487 0.3400 -13.5950 0.6820 -13.2518 1.9120

300 8.4929 0.2135 -14.2458 0.2264 -14.1872 1.7450

350 8.4510 0.1178 -15.0078 0.1216 -14.9761 1.6051

400 8.4278 0.0648 -15.7599 0.0680 -15.7415 1.4850

450 8.4138 0.03289 -16.5814 0.0324 -16.5964 1.3831

500 8.4020 0.0059 -18.4282 0.0060 -18.4165 1.2936

550 8.4000 0.0013 -20.0188 0.0014 -19.9971 1.2150

Table 4.9 Kinetic Parameters for precursors of

Zn0.25CuxMn0.75-xFe2O4system by TGA/DSC

Composition

(x) Method Step

Activation energy(∆E)

KJ/mol

0.0 Coat-Redfern I 38.29

II 127.57


II 94.70


II 245.27


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Fig.4.3. TGA-DSC Spectrum for Zn0.25 Cu0.5 Mn0.25 Fe2O4.

1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0 3.2

-18

-17

-16

-15

-14

-13

-12

-11

Zn0.25

Mn0.75

Fe2O

4

ln g

αα αα/T

2

1000/T

Fig.4.2 Coats and Redfern method.Graph of ln[g(α)/T2] Vs 1000/T for

Zn0.25Mn0.75Fe2O4


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4.3.3 Scanning Electron Microscopy

The particle surface morphology Zn0.25CuxMn0.75-xFe2O4 system

(0.0 ≤ x ≤ 0.75). was studied using Scanning Electron Microscopy(SEM)

technique (JEOL JSM 6360). The average grain size was measured by

Cottrell’s method [38]. The data given in Table 4.10 indicates grain size

with compositional variation.

The studies from the micrographs reveal that all samples show

fine grains with varying porosity. The average grain size of ferrite

increases with increasing copper content (0.1-0.3µm). The grain size

depends upon the interaction of grain boundary and porosity along with

the sintering temperature [39-40].

(a) X=0.0


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(b) x=0.25

(c) x =0.5 Fig. 4.4 Scanning electron micrographs Zn0.25CuxMn0.75- xFe2O4System ( 0 ≤ x ≤ 0.75).


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Table 4.10 Grain Size from SEM of the Zn0.25CuxMn0.75- xFe2O4 system (0≤ x ≤0.75).

Composition Grain Size (µm)

Zn0.25 Mn0.75Fe2O4 0.1 Zn0.25 Cu0.5 Mn0.25 Fe2O4 0.2

Zn0.25 Cu07.5 Fe2O4 0.3 4.3.4. Fourier transfer Infra-red spectroscopy

The system Zn0.25CuxMn0.75-x Fe2O4 was characterized by the IR

spectra in the region 400 to 1000 cm-1 as shown in the Fig. 4.5. The

spectrum provides a clear indication of the spinel ferrite phase. Two

metal- oxygen bands observed around 600 cm−1

and 400 cm−1

are usually

assigned to vibration of ions in the crystal lattice [41]. All the samples

show two prominent absorption bands. The high frequency band υ1 is in

the range 534-644 cm-1and the lower frequency band υ2 is in the range

396-463 cm-1. The high frequency band is due to the vibration of the

tetrahedral M---O bond and the low frequency band is due to the

vibration of the octahedral M---O bond.


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(a) X=0.0

(b) x=0.25


81

(c) x =0.5

(d) x =07.5

Fig.4.5 Infrared spectra for Zn0.25CuxMn0.75-xFe2O4 System(0 ≤ x ≤ 0.75)


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Table 4.11 Infrared spectroscopic bands for Zn0.25CuxMn0.75-xFe2O4 System (0 ≤ x ≤ 0.75).

Composition v

1(cm

−1) v

2 (cm

−1)

Zn0.25Mn0.75Fe2O4 560 433

Zn0.25Cu0.25 Mn0.5Fe2O4 534 399

Zn0.25Cu0.5 Mn0.25 Fe2O4 593 396

Zn0.25 Cu0.75 Fe2O4 644 463

4.3.5 Energy dispersive X–ray spectroscopy

The typical EDAX (energy dispersed analysis by X-rays) spectra

for the elemental analysis of Zn0.25CuxMn0.75-xFe2O

4 system with x = 0.0-

0.75 are shown in the Fig. 4.6. The quantitative analysis of EDAX

spectrum revealed the atomic percentage of system which are close to the

expected values for Zn0.25CuxMn0.75-xFe2O4 (x = 0.0, 0.5,0.75).The data

of the EDAX analysis for this system is given in Table 4.12.


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(a) X=0.0

(b) x =0.5


84

( c ) x = 0.75

Fig. 4.6 Energy Dispersive Spectra of Zn0.25CuxMn0.75-xFe2O4System (0≤ x ≤0.75).

Table 4.12 Atomic percentage value for the Zn0.25CuxMn0.75-x Fe2O4

system by EDAX analysis.

Zn Atomic %

Mn Atomic %

Cu Atomic %

Fe Atomic %

Composition

x Theo. Obs. Theo. Obs . Theo. Obs. Theo. Obs.

0.00 9.56 10.36 24.34 26.18 --- ---- 65.99 63.46

0.50 9.41 8.82 7.91 6.76 18.30 17.33 64.35 64.64

0.75 9.30 9.10 ----- ----- 27.12 25.61 63.57 65.29


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4.3.6 Electrical Resistivity

The temperature variation of dc resistivity for all the compositions

of Zn0.25CuxMn0.75-xFe2O

4 system with x = 0.0- 0.75 is shown in Fig.4.7.

The d.c. resistivity of all the samples was measured in the temperature

range 100 to 500 oC which varied between 102–10

8 Ω-cm. The resistivity

of ferrite materials decreases with increase in temperature, indicating

the semi-conducting nature of all samples[42]. The extent of decrease in

resistivity up to the temperature of about 150oC is very slow. The

measurement of resistivity indicates linear relationship with the

temperature. The electrical conductivity has been related to the hopping

of electrons between Fe2+ and Fe3+ [43-44]. The conduction mechanism in

ferrite is quite different from semiconducrors. In ferrites the temperature

dependence of mobility affects the conductivity and carrier concentration

is almost affected by temperature variation.

Electrical properties of ferrites are affected by distribution of

cations in the sites. Also by non-magnetic and magnetic substitutions, by

amount of Fe2+present,sintering conditions, grain size and grain gowth

effect. At higher temperature the oxidation state of the Fe and Mn

ions fluctuates. The electron hopping energy between Mn3+ ↔ Mn2+ is

larger than that between Fe3+↔ Fe2+. During the oxidation of Fe and Mn,

more electrons are released and large electron hopping is expected.

Hence the resistivity decreases with increase in temperature [45-48.]. In

the present case, it is also observed that as the content of Cu

increases, resistivity decreases. This decrease in resistivity is

expected because at higher temperature probability of higher

oxidation state of metal ions and hence increasing the possibility of

electron hopping.

The activation energy of electrical conduction was found to

decrease with increasing Cu ion content which is shown in Table 4.13.


86

Fig.4.7 .Variation of electrical resistively with temperature for Zn0.25CuxMn0.75-x Fe2O4System( 0 ≤ x ≤ 0.75).

Table 4.13 Activation energy (∆E) , Band gap values for the system

Zn0.25CuxMn0.75-x Fe2O4

Sr.No. Composition

Activation energy

(∆E) eV Band gap eV

1 Zn0.25Mn0.75Fe2O4 0.5484 1.096

2 Zn0.25Cu0.25 Mn0.5Fe2O4 0.6904 1.380

3 Zn0.25Cu0.5 Mn0.25 Fe2O4 0.5052 1.010

4 Zn0.25 Cu0.75 Fe2O4 0.4925 0.985


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4.3.7 Magnetic Hysteresis

The hysteresis studies of system was carried out using a magenta

B-H loop tracer. The hysteresis loops of Zn0.25CuxMn0.75-x Fe2O4 system

with x = 0.0- 0.75 are shown in Fig.4.8. The variation of magnetic

moment (µB), saturation magnetization (Ms), remanent magnetization

(Mr) and coersive field (Hc) is given in Table 4.14 The variation of the

saturation magnetization (Ms) depends on the cation distribution in a

spinal lattice as well as particle size of the sample. The resultant

magnetization is thus the difference between magnetization of octahedral

lattice (B) and that of the total tetrahedral lattice (A).From the hysteresis

loops, increasing trend in saturation magnetization (Ms) (Fig.4.8) and

increasing values of coercivity (Hc) are observed with substitution of

Cu2+ ions. The increasing trend in saturation magnetization with

decreasing concentration of manganese is due to the A--O--B interactions

in ferrites [49]. The ions such as Zn2+ and Mn2+ occupy A sites while

Cu2+, Fe3+ ions occupy B sites respectively. There is possibility of A-B

interaction between Mn2+ – Fe3+ in which spins of A site and B site

cancel each other. A resultant saturation magnetization due to Cu2+ions of

the B site. Hence, in the present system, as content of Cu increases,

the saturation magnetization increases.

The experimental magnetic moment is calculated by the following

relation [50].

µB = Mw x Ms /5585 --------------------

4.7

Where, Mw = Molecular weight of composition (in grams)

Ms =Saturation magnetization (in Oe) and

5585 = Magnetic factor.


88

( a) x = 0.0

(b) x = 0.25


89

( c) x=0.5

(d) x= 0.75

Fig. 4.8 Hysteresis loops for the Zn0.25CuxMn0.75-x Fe2O4 system ( 0.0 ≤ x ≤ 0.75).


90

Table 4. 14 Magnetic data for the Zn0.25CuxMn0.75-x Fe2O4 system (0 ≤ x ≤ 0.75).

Sample Ms (emu/gm)

Hc (Oe) Mr (emu/gm)

Magnetic Moment

(nB )

Zn0.25Mn0.75Fe2O4 10.27 56.82 4.47 0.4289

Zn0.25Cu0.25 Mn0.5Fe2O4 30.38 68.18 2.10 1.2804

Zn0.25Cu0.5 Mn0.25 Fe2O4 41.88 84.42 3.40 1.7813

Zn0.25 Cu0.75 Fe2O4 47.81 76.30 5.11 2.0519

4.3.8 Gas sensing property

Presently the atmospheric pollution has become a global issue.

Gases from auto and industrial exhausts are polluting the environment.

The sensors are required basically for measurement of physical quantities

and for monitoring working environments. Liquified petroleum gas

(LPG) is used in almost every kitchen all over the world. It is therefore,

referred as a town gas or cooking gas. Along with inevitable domestic

use, it is utilized in large extent for industrial purposes and in laboratories

as fuel. Cooking gas consists chiefly of n- butane , which is a colourless

and odourless gas. It is usually mixed with compounds of sulphur (viz.

methyl mercaptan and ethyl mercaptan) having foul smell, so that its

leakage can be noticed easily. This gas is potentially hazardous because

explosion accidents might be caused when it leaks out by mistake. It has

been reported that, at the concentration up to noticeable leakage, it is very

much more than the lower explosive limit (LEL) of the gas in air.

Explosion accidents destroyed many industries, laboratories, kitchens and

houses, buildings, societies . Therefore, all industries should have an

alarm system detecting and warning for dangerous exhaust gas


91

concentration levels. Depending on the gas and its concentration in the

atmosphere, the electrical conductivity is different.

Semiconducting oxides are widely used as inexpensive and

robust sensors for toxic, hazardous and combustible gases and vapors in

safety and automotive applications. Few semiconducting oxide materials

used in these applications are ZnO, SnO2, Fe2O3, Cr2O3, etc [51-61]. The

gas sensing is a surface phenomenon of gas–solid interaction, where the

conductivity of semi conducting oxides can be altered by adsorption of

gases from ambient. Different spinel ferrites such as NiFe2O4, CdFe2O4,

ZnFe2O4 and CuFe2O4 have been studied for various gas-sensing

applications.

The relative response of a sensor to a target gas can be defined

as the ratio of the change in conductance of a sample before and after

exposure of target gas to the conductance in air. The gas response can be

defined as:

%S =(Ra/Rg)100 ---------4.8

where Ga = conductance in air and Gg = conductance in a target gas.

Specificity or selectivity of the sensor can be defined as the ability of

a sensor to respond to a certain gas in the presence of different gases.

Response time (RST) is defined as the time required for a sensor to attain

the 90% increase in the maximum conductance after exposure of the

sensor surface to a test gas, while recovery time (RCT) is the time taken

by the sensor to decrease the conductance up to 90% of the maximum

conductance [62] in air.

The gas response (%S) against various gases for the

Zn0.25CuxMn0.75-xFe2O

4System (0≤ x ≤0.75) is shown in Fig. 4.9 a to d.

The graph 4.9 a shows that this material is highly selective towards H2S

as compared with the other test gases like Ammonia, H2,

Ethanol,CO2,Cl2 and LPG. While figure 4.9 b, c, d shows that this


92

material is highly selective towards LPG as compared with the other test

gases like ammonia, H2, ethanol, CO2, Cl2 and H2S. The response (%S)

towards LPG at various operating temperatures which indicates 300 oC as

the optimum temperature for the gas response. The copper substituted

Mn-Zn ferrite system showing better gas sensitivity to LPG and to other

gases like NH3, H2, C2H5OH, CO2, Cl2 and H2S having smaller extent.

( a) x = 0.0

(b) x = 0.25


93

( c) x = 0.5

(d) x = 0.75

Fig.4.9 Sensitivity of Zn0.25CuxMn0.75-x Fe2O4System (0.0 ≤ x ≤ 0.75)

4

towards different test gases at different temperature In all type of gas sensing mechanism, oxygen gets adsorbed on the

surface of the sensor at desired operating temperature and electron is

transferred from conduction band to adsorbed oxygen atom resulting in

the formation of ionic species such as O2- or 2O-. The reaction kinetics

may be explained by following reactions,

O2 (gas) O2 (ads) ------------- 4.9

O2 (gas) + e – O2 -(ads) ------------- 4.10

O2 (ads) + e – 2O - (ads) ------------- 4.11


94

The electron transfer from the conduction band to the chemisorbed

oxygen results in decrease in electron conduction of the sensor. As a

consequence, an increase in resistance of the sensor is observed.

The present system gave maximum sensitivity towards LPG.

During the testing the LPG dissociates very slowly into the reactive

reducing components like CH4, C3H8, C4H10, etc on the surface of the

sensor [63-72]. In these substrates, reducing hydrogen species are bound

to the carbon atom. When the nanosized lithium ferrite is exposed to the

reducing gases like LPG, it reacts with chemisorbed oxygen thereby

releasing an electron back to conduction bands which decreases the

resistance of the sensor. The overall reaction mechanism of LPG can be

explained as follow,

CnH2n+2 + O- (ads) H2O + CnH2n-O + e - ----------- 4.12 where CnH2n+2 represents CH4, C3H8, and C4H10

Sensitivity of the material towards a gas is due to physical

adsorption of the gas on the surface of the sensor. During this study it is

seen that the response of sensor is increased with increase in temperature

and after optimum temperature it decreases randomly. Because

adsorption increases with temperature but saturates at a particular

temperature attaining maximum response towards the gas. But at still

higher temperature, desorption of the gas predominates over the

adsorption and hence adsorption decreases showing decreased sensitivity

of the sensor.

4.3.9 Photocatalysis

Removal of potentially toxic and dangerous compounds from the

environment has generated a great interest in the last decade. The textile

dyes and dye intermediates with high aromaticity and low

biodegradability have emerged as major environmental pollutants and


95

nearly 10-15% of the dye is lost in the dyeing process and is released in

the wastewater which is an important source of environmental

contamination. Photocatalysis is applied in many areas including the

elimination of contaminants from water and air, odor control, bacterial

inactivation.[73-76]. The photocatalytic degradation of organic molecules

is of great importance in water treatment. In most cases, dyes are studied

as model compounds for large organic molecules. However, ferrites are

also effective in the degradation of many other potential organic

contaminants.

Congored is a well known organic dye and is considered as a

model of a series of common azo-dyes, used in the industry. The

performance of Cu-substituted Mn-Zn ferrite nanoparticles for the

photocatalytic degradation of Congo-red for different hours is presented

in Fig. 4.10.

The catalytic activity of the Zn0.25CuxMn0.75-xFe2O4 system

(0.0 ≤ x ≤ 0.75) was studied by photodegradation rate of congored. The

experiment of photocatalytic reaction was conducted in a 100-ml Pyrex

glass vessel with magnetic stirring and a UV lamp (8 W) with the main

wavelength of 253.7 nm, which was positioned over the glass beaker. The

as-prepared Cu-substituted Mn-Zn ferrite was used as photocatalyst

during the activity. The catalytic activity of the system studied with

respect to time parameter.

In the present system it is observed that the degradation of dye

starts within first hour in presence of catalyst. Further exposer to U.V

light, increases photodegradation rate of congo-red dye with time period.

The composition Zn0.25Cu0.5Mn0.25Fe2O4 shows better catalytic activity

for the decoposition of congored. After 5 hours exposure of light,

79.22%degradation of congored observed in presence of Zn0.25Cu0.5

Mn0.25Fe2O4 sample. Fig. 4.11 shows photodegradation of dye in presence


96

of Zn0.25Cu0.5 Mn0.25Fe2O4 sample.Kinetic study of photodegradation of

congored dye reported in Table 4.15.

Table 4.15 Mean rate constant(K min -1), % degradation of

congored dye.

Composition Mean rate constant of 1hr,2hr,3hr,4hr,5hr

K min -1

% degradation of Congored dye after 5

hr. irradiation

Zn0.25Mn0.75Fe2O4 6.3295 × 10-3 66.70%

Zn0.25Cu0.25 Mn0.5Fe2O4 6.1218 × 10-3 67.44%

Zn0.25Cu0.5 Mn0.25 Fe2O4 7.2697 × 10-3 79.22%

Zn0.25 Cu0.75 Fe2O4 5.2972× 10-3 68.08%

Without catalyst 1.3505× 10-3 39.04%

Fig.4.10 Photocatalytic study for Zn0.25CuxMn0.75-x Fe2O4 system


97

Fig. 4.11 Photodegradation of congored for Zn0.25Cu0. 5 Mn0.25Fe2O4


98

4.4 Conclusions 1. Copper substituted Mn-Zn ferrites were synthesized by co-

precipitation method. The crystal structure studied by X-ray

diffraction method. The system shows cubic phase.

2. From thermal analysis (TGA-DTA) various thermokinetic

parameters were calculated.

3. Two strong absorption bands for tetrahedral and octahedral

stretching were observed at 600 cm-1 and around 400 cm-1

respectively.

4. Scanning electron micrographs indicated increase in grain size with

increase in Cu content in system.

5. Elemental analysis of system studied by Energy dispersive X–ray

spectroscopy. The quantitative analysis of EDAX spectrum

revealed the atomic percentage of system which are close to the

expected values for Zn0.25CuxMn0.75-xFe2O4 (x = 0.0, 0.5, 0.75).

6. The saturation magnetization and Coercivity showed increasing

trend with increase in Cu content in system.

7. Cu-substituted Mn-Zn ferrite shows semiconducting behaviour.

The DC conductivity increases with increase in temperature for all

the samples.

8. Various gases were tested for gas sensing activity of all the

compositions of Cu-substituted Mn-Zn. All the samples shows

remarkable response towards LPG with good selectivity.

9. The photocatalytic activity of Zn0.25CuxMn0.75-xFe2O4 was studied

in presence of U.V. light. The composition Zn0.25Cu0. 5 Mn0.25Fe2O4

shows better catalytic performance for degradation of congored

dye.


99

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