Technological Significance of Sol-Gel Process and Process-Induced Variations in Sol-Gel Materials...

Journal of Sol-Gel Science & Technology 1:65-77 (1993) © Kluwer Academic Publishers. Manufactured in The Netherlands

Technological Significance of Sol-Gel Process and Process-Induced Variations in Sol-Gel Materials and Coatings

BULENT E. YOLDAS PPG Industries, Inc. Chemical Group Technical Center, Monroeville, PA 15146

A b s t r a c t . A significant aspect of sol-gel technology is the capability it provides to affect the substructure of materials by controlling the nature and the kinetics of chemical reactions. This capability allows us to produce novel materials, design unique molecular and pore morphologies, circumvent high-temperature reactions, and modify material properties. The modifications include strongly thermodynamic-dependent high-temperature properties such as sintering, crystallization, and viscosity in glass and ceramic materials. A particularly exciting area for investigation is the optical- electronic field, where a significant dependence of electro-optical properties and photosensitivity on process-induced molecular-structural variations occurs. Understanding the basis for the creation of structural variations in sol-gel processes should have significant impact on the technologies and systems that use these materials. In this article, some fundamental aspects of alkoxide-based, sol-gel processes and thermochemical bases for process-induced structural variates are discussed.

Key words: Materials modification, polycondensation, optical films, process effects, photosensitivity

I n t r o d u c t i o n

In the last decade, advancing technologies re- quired material functions and capabilities that the classical material-forming methods could not deliver. As a result, a number of novel material- forming processes emerged. One of the emerg- ing processes in the ceramic field is the non- thermal formation of ceramic substructures by chemical polymerization, i.e., polymeric sol-gel method [1,9].

The chronological evolution of the sol-gel process can be placed in three progressive stages:

1. Use of colloids and metal-organic compounds as starting materials

2. Formation of monolithic materials from gels

3. Modification of material properties by molecular structural variations.

The last of these may prove to be the most significant development in this field since it provides the following technological capabilities to material scientists:

• Creation of stable non-equilibrium materials by avoiding high-temperature reactions

• Molecular design (in morphology and in organic-inorganic polymerization)

• Low-temperature stabilization of structures • Nano-scale composites • Creation and design of molecular and micro-

porosity, e.g., aerogels, membranes • Unusual coordination, valency, and stoichio-

metric states, e.g., laser, photosensitivity • Room-temperature liquid state (for optical

and electronic coatings)

Molecular composites produced by copolymerization of inorganic and organic precur- sors combine very different properties of these materials in a single material [10-15]. Low- temperature stabilization of structures allows preservation of high surface areas and other properties for catalysts and catalyst support [16- 19]. Designing nano-scale porosity has significant implications in molecular filters, membranes, and aerogels [20-25]. For example, for a given porosity, reducing the pore size from 100 nm to 10 nm decreases the thermal con- ducitivity of the gaseous phase in aerogels ap-

66 YoMas

C o

E 13)

O ¢.)

,-t

E ,,.=

03

FROZEN-IN STATES i= 1 •

" ° . I • - - "*'J O > C !

o ~'~ "~'~ 1 ~ I MOLTEN STATE • -~ ~ " r e . . _ ~=

o - ,-,,, / . - I ) I--.-..~ I t 1

a,; I f ' - z i ; " I >-

G2el I structural relaxations and|.-.-.~':-'~ Z l network connectivity J I ~ Go

. . . . . 1]" . ,i .,,t,"- . . ~ . . > ~ [equillbriuml

r~

T e m p e r a t u r e

Fig. I. Chemical (I) and thermal (II) paths to materials formation. In the chemical path, initial variations in the condensates and gels, bond cleavage variations during the thermal degradation, and diffusion-dependent restoration of stoichiometry creates structural variations.

proximately tenfold. These type of materials are also an interesting host material for laser dyes and for non-linear optic applications [27-30]. It is now well-established that design of materials in nano-scale can lead to mechanical, magnetic, and optical properties radically different than those in micro-scale [31-33].

Properties and performances of glass and ceramics are essentially a cooperative phenomenon established by the microstructure. In classical thermal processes, thermodynamics (i.e. time, temperature), is the main determining factor of the microstructure for a given composition. Syn- thesis of these materials by chemical polymerization at low temperatures, on the other hand, permits significant non-compositional modifications of the ultrastructure.

In the following sections, basic process- induced modifications and their consequences in sol-gel derived glass and ceramic materials are discussed.

Process-Induced Variations in Sol.Gel

Fig. 1 illustrates thermal and chemical paths to ceramic and glass formation. The fundamental difference between the two methods is the orig- ination point of the structure. In thermal methods, the materials structure essentially is established at fusion temperature or in a molten state and proceeds towards room temperature, a condition that is diametrically reversed in the sol- gel method. This crucial difference determines the relative roles thermal and chemical forces play in the establishment of the initial structure. Even among the sol-gel derived glasses, significant structural differences will arise due to variations in the following stages:

1. Initial gel structures: condensation-induced differences in the molecular size, topology, and terminal bonds

2. Thermal degradation paths: heat-treatment- induced variations in bond cleavages; oxygen deficiency

3. Chemically pinned configurations: diffusion-

Sol-Gel Process and Process-Induced Variations in Sol-Gel Materials and Coatings 67

dependent nonstoichiometry, chemically restricted network connectivity and structural relaxations

Chemical basis of these variations are discussed in the following sections.

C h e m i c a l P o l y m e r i z a t i o n o f O x i d e N e t w o r k s

PRECURSOR:

M{OR) m M: Metal (Al,Zr, T i o r Si, P etc) m: Valence of M

R: A lky l (CH=,CaH¢C=H 7 etc)

C o n d e n s a t i o n o f Oxide N e t w o r k s and C o n d e n - s a t i o n - I n d u c e d Var ia t ions

Alkoxides of Group III and Group V metals, which constitute a large portion of glass and ceramic forming systems, are readily synthesized and commercially available at a reasonable cost. These compounds do not occur naturally. Al- though alkoxy derivatives of silicon have been investigated as early as 1846, only in this cen- tury has the alkoxide chemisty become widely known. Metal alkoxides have the general for- mula M(OR)n where M is a metal having a valence n, and R is an alkyl group Ck + H2k+l. They can either be considered derivatives of alcohol, R(OH), or derivatives of hydroxides M(OH), . Due to the electro-negativity of oxygen, the M-O-C bonds are often highly polar- ized. The nature of the M-O bond, i.e. cova- lency, varies not only with the electronegativity of the central metal, M, but also by the induc- tive effect of the alkyl group. Detailed chemistry of these compound has been covered by D.C. Bradley and his coworkers [34]. We will only deal with their reactions leading to the formation of inorganic polymers, i.e., oxide glasses and ceramics.

In most chemistry books, hydrolysis of metal alkoxides is often represented by one of the following equations:

M(OR)n

M(OR)n

M(OR) n

+ n /2H20 --, MO./2 + mR(OH) (1)

+ nH20 ~ M(OH)~ + mR(OH)

+ (n - 1 )S20 ~ M(OS)n_xOR)~(2 )

+ (n - 1)R(OH) (3)

All these reaction representations are somewhat incorrect, not because they are mere oversim- plifications but because they do not represent the chemical makeup of the end product and vail the most significant aspects of the sol-gel

CONDESATION;

Hydrolysis; R ROH R o k o I /1--'-.,. I

R O - - M - O ~ q + H ~ H - , R O - M - O H I . . . . I O O R R

ROH Polymer izat ion ' . . ~

P~= = M - O~_ -I-_H__O~'/- M_ = ----,-~ M - O - a - _ t . . . . .

Pa: = M - C~H + HO)- M - - - - - - P - M - O - M - -

~Hp PRODUCT :

R H R O O O I l I

H O - - M - O - M - O - - M - - I I I

O O H I

R O - M - I M,, Ol.u - ~.~ +~,)a] (O H )., (O R)~, O H

Fig. 2. Schematic representation of oxide network formation by hydrolytic polycondensation of alkoxides. Note that oxide main chains must be always framed by terminal OH and OR groups.

process. For example, the hydrolysis product of metal alkoxides can never be a pure oxide as equation (1) implies (because of the terminal bonds). The reactions (2) and (3) ignore the presence of bridging oxygen and imply that only monomers are formed.

In reality, hydrolytic polycondensation reactions of metal alkoxides are strongly affected by molecular separation of interacting species, their ratios, the reaction medium, the catalyst (pH), the reaction length, and temperature. Each of these parameters affects the network structure in a fundamental way, producing polymers having linear, cyclic, or highly branched structure with different terminal bonds and size [35]. The chemical composition of condensates continu-

68 Yoldas

ously changes with molecular morphology and size. Fig. 2 shows a schematic representation of hydrolytic polycondensation reactions leading to oxide network formation in these systems. As is obvious from Fig. 2, the condensate can never be a pure oxide.

A hydrolytic polycondensation equation which would take into account this variability of the chemical composition, and the polymeric nature of the condensates can be written:

riM(OR)4 + m H 2 0 ---* MnO2n-(z+y)/2

(OH)=(OR)y + ( 4 n - y ) R ( O H ) (4)

where n is the number of metal ions polymerized in an oxide network, and x and y are the num- bers of terminal OH and OR groups framing that network.

The principal variations in single-component condensates fall into three basic categories:

1. Molecular size (extent of oxide networks) 2. Network topology (linear, cyclic, etc.) 3. Nature of terminal groups (OR, OH)

Molecular size and topology play an important role when these gels are processed or when fibers and films are drawn from these solutions. However, it is the nature of the terminal groups that has the most influence in the processing of these materials. When the boundaries are made up entirely of alkyl groups, the molecular encounters cannot lead to further network formation [29]. On the other hand, when a number of terminal bonds are hydroxyl groups, network expansion occurs during the aging or concentration of solutions by the reactions:

P 1 - OH + RO - P2 ~ Pa + ROH (5)

P 1 - OH + HO - P2 ~ P3 + H20 (6)

where P1 ,P2 and P3 are oligomers or polymeric species where P3 > P1,P2. This expansion of molecular weight is shown in Fig. 3.

A homogeneous doping or copolymerization of multi-component systems for alkoxide often presents problems due to differences in the hydrolysis rates of metal alkoxides. It is found that alkoxide mixtures form a more homogeneous oxide network when the chemical encounter rates with water are reduced significantly during the

(OR} FRAMED SPECIES (OH) FRAMED SPECIES

4 HiGH MW.

Retention Time {min.}

Fig. 3. Presence of hydroxyl terminal groups leads to expansion of oxide networks (molecular weight, MV, logarithmi- cally related to retention time).

hydrolytic polycondensation [37]. The chemical encounter rates can be reduced by first diluting the alkoxides in an inert solvent, then introduc- ing water in very small quantities. For example, humidity hydrolysis of silicon and alumina alkoxide mixtures in high molecular alcohols (e.g, tertiary butanol) at low concentrations (1%- 3%, equivalent oxide) results in a homogeneous copolymerization by retarding the hydrolysis.

xAI(OR)3 , y Si(OR)4 , z SLV + w H20

= AI-O-Si = +ROH, z SLV (7)

where w < z, y < z and SLV is an inert solvent. This principle is applicable to other systems and can be further manipulated by the type of solvent and reaction temperature. In alumina-slica systems, a less kinetically sensitive condensation method was devised, which eliminates the rate sensitivity in the polymerization process [38].

A molecular-level network homogeneity and network bonding attained by polymeric sol-gel processes have the effect of changing compound formation reactions, e.g., mullite from silica and alumina:

T > 1250c °

3A1203 + 2SIO2 , A16Si2013 (8)

to a polymorphic transformation:

oA16SizO13(amorphous gel) 98°c°

A]6Si2O13 (9)


Thermal energy of the enthalpy change in reaction (9) being non-displacive and instantaneous is expanded in densification. Sol-gel derived amorphous polymeric mullite gels show up to 35% linear shrinkage and appearance of fusion during this transformation [37].

Thermal Degradition-lnduced Variations

Terminal groups must be thermally removed to obtain oxides:

MnO[2.-(x+u)/2] (OH)z(OR)u --*

nMOn/2 + H20, Carbon Compounds (10)

Molecular size, n, along with the nature of terminal bonds, z and y, in M,O[2,_(=+u)/2](OH)~(OR)y determine the oxide content of the condensate. Analysis of the above equation shows an increase in the equivalent oxide content with increasing n. The initial increase in n occurs rapidly, then it levels off. For a detailed discussion of the relationship between n and the oxide content of the polysilox- anes, see Yoldas [5]. In sillica and titania systems, condensates varying in oxide content from -,~70% to 90% by weight can readily be obtained [39].

The effect of terminal bonds goes beyond the solution state. A consequence of certain chemical reactions during thermal treatment is densification. The driving force for these chemical reactions can be orders of magnitude larger than the driving force for, i.e., the surface area reduction, for thermal sintering. For example:

Thermal

AFz

nlSi02 + n2Si02 ---* mSi02 (9)

Thermochemical

AF1 (10)

- S i - OH + H O - S i -

= Si-O-Si = +ROH

5

-~ ~o 3"- CO

z ._1

Z0 • OH FRAMED CONDENSATES

25 ~ . ~ I I I I I I RM 500 600 700 800 900 I000

T (C)

$i

m

m

FROM (OR) FRAMED FROM [OH] FRAMED

OONOENSATES CONDENSATES

Fig. 4. Both silica films (A) and bulk materials (B) produced from (OH) terminated molecules show significantly higher densification during the thermal treatment. (Bulk silicas shown in (A) were produced from condensates by hot pressing at 1200°C for 15 minutes).

AF1 >> AF2 (11)

Thus, the presence of hydroxyl bonds in the intimate molecular boundaries in these materials leads to chemical activated sintering [39] as seen in Fig. 4.

Another interesting effect of terminal bonds occurs in creation of nonstoichiometry and photosensitivity [40]. The way in which the organic R groups are attached to silicon-oxygen main chains is schematically represented below for R=CHa in silica system.

= Si - O - CH3 (12)

The organics can only be removed from the network either by cleavage of carbon-oxygen bond or by cleavage of oxygen-silicon bond. In

70 Yoldas

5, 84E B5

C

+-

C Si(

~,13SiO n

0. OZE ~0

40s.s s2s.ss 65 . o

Wevel ength (nm) Fig. 5. Normalized emission spectra in chemically polymerized materials in A1203-SiO 2 binary under 365 nm irradiation.

either case, a defect is created:

= Si - O - CH3 , = Si - 0 (13)

___ Si - O - CH3 ~ - Si + (14)

In the first case, an electron hole will be localized in a 2p orbital of single-bonded oxygen. In the second case, an electron will be localized in a dangling Sp 3 orbital of three-coordinated silicon in SiOa units. These give rise to photosensitivity through electron-photon coupling (Fig. 5). Fig. 6 shows the effect of the condensation conditions on various properties of the resultant material in silica system.

In these materials, external surface consti- tutes an insignificant portion of the total surface area, typically lcm 2 of 106cm2/g. Thus, thermal degradation takes place essentially in the self-generated internal environment. Chem- ical nature of that environment is determined by the gaseous products of decomposition and modified by diffusive exchange with external at- moshpere. This diffusion dependence makes the

heating rate an important structural parameter. Thermal degraditon of organic groups of a silica gel essentially takes place between 250°C and 300°C when in close equilibrium with air, e.g., l°C/min, heating rate. In the absence of oxygen, e.g., in nitrogen, the bond cleavages shift to 400-500°C range (Fig. 7). A slower heating rate in air shifts the intermolecular chemical environment towards a more oxidizing condition, thus a corresponding shift in the bond cleavages Occurs .

For example, due to the high energy of t~H2 formation (,,~383, kJ/mole), the initial cleavage of the carbon-carbon bonds =Si-O-CH2-CHa chains, modifies the intermolecular chemical environment so as to favor the cleavage of the silicon-oxygen bonds next. This also removes a portion of the network oxygen along with carbon. Removed organics and organic radical may deposit carbon by various reactions, for example:

4t~Ha ~ C + 3CH4, etc. (15)

which also contributes to photosensitivity and


other property modifications. 15 f Sin0zn_~.(0R)k(0H) p 90

Chemical ly Restrictive States and Nonstoi - HIGH: Connectivity~

chiometry C0nnectabihty, , Sinlerobilily

Removal of structural organics by thermal degradation is a fundamentally different process than removal of organics by hydrolysis. Hydroly- sis reactions remove the organics by replacing them with hydroxyl and bridging oxygens under equilibrium conditions. Thermal degradation necessitates cleavage of the structural chains at a "configurationally frozen" state. Thus, the structural differences in precursor sols not only persist in their oxides at high temperatures, but they also modify glass structures when they are used as a component. Fig. 8 shows two different silica sols inducing two different structures in the glasses produced from these sols.

When the molecular size is small and the terminal bonds are alkyl groups, i.e., species formed with limited water under diluted conditions [41], a very high density of broken bonds is created during thermal degradation. Condi- tions normally occurring in a molten state are transposed into the regime of a fully frozen-in configurational state where healing of the frag- mented bonds, and normalization of the coordination state, are thermodynamically prohibited, e.g., at 300-500°C. In these cases, part of the network oxygen is also removed by network- bonded carbon-creating nonstoichiometry:

8 0 % S i 0 2 ~ . , ~ /

\ \ j : " I = ' kkP

- 70%SiOz ~ ~Z k*p

HIGH: Oxygen Def. Si Undere00rdinati0n Phot01urninescence

1 I I

Si,O2n-k/2(OR) k , SinO2n_k,/2 + RkOk+k,/2

(in some form)

where k'/2 is the net oxygen deficiency created. This reduces network connectivity. Its restoration, and related structural relaxations must, therefore, be chemically restricted until suffi- cient oxygen diffusion takes place. This oxygen deficiency may persist, even in a molten, state for weeks [41]. Consequently, glass properties that are affected by network connectivity and distribution of network bonds, e.g., viscosity, as shown by Fig. 9.

O E to a., O E

(/3

U}

-1-

10 20 30 40

Si(0CzHs)4 Concentration (% wt.)

Fig. 6. The effects of the water/alkoxide ration and the molecular spacing during the hydrolytic polycondensation of Si(OC2H5)4 in ethanol.

Process Effects in Ceramics

Performance of many electronic systems is limited by materials performance and can be significantly enhanced by improvements in the properties of certain key materials. These materials include ferroelectrics, fiber optics, optical coatings (antireflective and reflective coatings, band pass filters, etc.) waveguides, graded index materials and coatings, infrared transparaent materials, IR windows, radomes, and tough ceramics. Ferroelectrics constitute the backbone of electronic technology. Recent applications include pattern recognition, optical processing, radar and sonar signal processing, real-time processing for missile guidance, and imaging and communication through fluctuating media, all of which have tremendous importance in communication and space technologies [42-47].

The distribution of defects and dopants as well as the grain size development during sin-

72 Yoldas

110

100-

9 0 -

~: 8 0 -

70

60

"'OH" Fxchanged

~lr

i I i I , I 7 I i I - 0 100 Z00 :500 400 SO0 600

Temperature (°C)

Fig. 7. Thermogravimetric analysis (TGA) shows that bond cleavage temperatures of organic groups shifts significantly to higher temperatures under nitrogen atmosphere (A vs.C). A fast heating rate also imparts the same effect as atmospheric change (A vs B). (In Curve, D, organic (OR) groups where chemically converted to "OH" groups by solid state hydrolysis in acidic water.)

tering are also of great importance. Classical processing of some of these materials involves repetitive ball millings, sinterings, and calcin- ings to achieve uniformity and reproducibility. These technologies would benefit significantly from sol-gel processing that provides molecular level homogeneity and nano-scale grain size control. Nanocomposites with radical and im- pressive properties have been produced. In this material, not only the size but the morphologi- cal design and interrelationship between various phases in that scale allows creation of unique properties that are too extensive to present here and are discussed in other publication [31-33].

A homogeneous doping in molecular level at room temperature allows stabilization of certain structures that would not survive the high- temperature diffusion requirements of classical thermal process. An example of such a case

is the stablization of alkoxide-derived zirconia [48] and transition alumina structure [49]. For example, high surface area transition aluminas produced from aluminium alkoxides undergo a structural transformation at around 1200°C with the elimination of high surface area. The high surface phase can be stabilized only by doping below 1200°C by the sol-gel process. Doping by as little as 3% SiO2 raises the transformation temperature by ,~ 150°C to 1375°C, pro- viding sintering-resistant porous materials for high-temperature utilization [49].

Since the oxide network is formed below the crystallization temperature, some unusual amorphous materials can also be formed. For example, amorphous /~-AI203(llAI203-Na20) formed by chemical polymerization of aluminum and sodium alkoxides crystallized to the/3-A1203 at 1200°C [50]. It was observed that the amor-


0 ¢-

N I11

¢r

S i l i c a A

i

Silica from Sol 1. / ~ i

, i I \, /~\ /

/ \\ / / \ / I ~ \ / I I \ f \ / / I ~\ / 1 ,J I I Silica from Sol 2. I

/ /

I

3 SiQ~Na20 Glass

B

Glass from Sol 1. / - - - - - \ \ / ~

/ ",J',l L_

I / '\, ) Glass from Sol 2./ '\ i t - \ . \ j / "x

T - i - r i - . • i - 128~ 1000 B O O 7 ' 0 0 1 0 ~

Wavenumber (era "1) Wavenumber (era "1)

8 O 0

Fig. 8. Infrared absorption spectra showing the molecular structure differences created in the precursor siloxane solutions, as exhibited in the silicas derived from those sols at 1000°C heating (A): also manifest in oxide glasses formed by chemical polymerization of these sols (B). (This difference also shows up in oxygen ls XPS spectra).

phous phase was also a sodium ion conductor. The significance of this is that the amorphous phase can be coated, may be a better ion conductor, and its optimum conductivity may occur at a different Na/A1 ratio than the value observed for the crystalline phase. This has not been investigated.

(Mg2A12SisO14) and mullite (A16Si2013) by chemical polymerization would be of consider- able advantage. Preparation of these materials normally requires high-temperature processing because of the need for thermal diffusion of their refractory constituents. It has recently been shown that mullite can be formed and substantially densified at only 980°C if the Si and A1 are polymerized at room temperature by a sol-gel process. This prevents high-temperature grain growth, resulting in a more desirable microstructure.

In the optical field of graded index optical coatings [51], bulk materials have been de- veloped [52]. Further developments in this area will alleviate spherical aberration in optical lenses and expensive surface polishing and shap-

ing of optical components. Particular attention should be given to the dependence of electro- optical properties on molecular structural variations in certain materials. Basic information obtained from these studies should have an impact on technologies such as electro-optics, optical fibers, interference coatings, ferroelectrics, radomes, and composite materials.

Process-Induced Variations in Coatings

Use of organometallic-derived polymer solutions in coatings have a number of advantages. The important ones are that it requires no vacuum or expensive equipment, allows coating of substrates by a simple dipping procedure regardless of shape and size, and has the capability for varying the index of refraction continuously for various optical interference coatings. Among the disadvantages are the thickness limitation and porosity. The latter can also be advanta- geous in producing low index materials.

Sol-gel coatings have already found uses on

74 Yoldas

Fig. 9. Molecular variations introduced in silica condensates showing this effect at 1800°C in their melting. Top photog- raphy: Si(OC2H4) 4 is hydrolyzed under acidic conditions with 4 moles water in ethanol at 1% (A), 4% (B) and 16% equivalent SiO2 concentrations. Bottom photo shows the effect of hydrolysis water: the condensate were formed by 2 modes (left) and 15 moles (right water hydrolysis of Si(OC2H5)4 •

optical fibers, on photovoltaic cells [53], as IR reflective transparent coatings [54], electronic films [55], and most recently as wide-spectrum- antireflective coatings [56] on transparencies and in optics [51, 56]. The graded index coating provides wide spectrum antireflectivity down to ,-~250 nm on fused sillica, increasing its transmission to better than 99.5% with laser damage thresholds measured at better than 9 J/cm 2 at 350 nm. Antireflective coatings facilitates deliv- ery of ,,~20% more energy in some high-power multi-lens lasers representing a significant eco- nomic value in energy savings.

In the coating process, the solutions are applied on substrates by any liquid application methods, including dipping and spinning.

These two processes involve drastically different speeds and shear rates under which the polymer molecules are aligned and deposited on the substrate. Spin coating dries the film in a few seconds in a dynamic condition. Drain coatings, typically at several cm/min, allow for polymer alignment into a denser configuration [57]. Surface roughness of spin coating films is found to be significantly higher than dip or drain coated films [57]. Thus, morphology created by the application of a given polymeric solution is significantly different for the drain process than for the spin process.

Densification of these films, as shown in Fig. 10, indicates the fundamental differences between the films deposited by spin coating and dip coating. It also elucidates the nature of the crazing in these films. When l#m-thick spin coated film is densified, its thickness is reduced by ~50% indicating it was initially ~50%, porous. The thickness reduction of drain coated films upon densification is approximately half as much as this, indicating that the initial porosity was half as much.

Fig. 10 exhibits two other significant events. The first one is that when densified, the crazing thickness-i.e., 1100 nm for spin coating, 600 nm for dip coat ing-becomes about the same for both types of films: ,,~600 nm. This indicates that the crazing phenomenon in this film is directly related to the density, i.e., t~ 1/d. The second indication is that as the films get thinner, they are deposited denser. This means that a degree of density and refractive index gradation normally occurs in sol-gel deposited films.

The refractive index measurements of TiO2 indicates that the density of TiO2 films increases with decreasing thickness. It is also reveal- ing that the density differences between the air bake and vacuum bake TiO2 films diminishes shes with decreasing thickness and completely disappears around ,~ 1 0 0 - 150~. A reasonable interpretation of this is that at around ,-~ 100_~ thickness, the TiO2 films become completely dense; a case under which the effect of the bake atmosphere on densification becomes inoperative [58].

Another important aspect of sol-gel coatings is that the spectral transmission of ceramic materi-

C u r v e 749513-A

Spin Coated /

Spin Rate (rpm)

12001000 2000 3000 4000 5000 6000 7000

1100 -

1000 -

9 0 0 -

E 8 0 0 - e-"

7 0 0 - ¢-

';- 6 0 0 -

E .500 - - i ,

400 -

3 0 0 -

2 0 0 -

1 0 0 -

0

I Crazes I I I I


C razes

oB °

A o o °

Dip C

I I I I 2 3 4 5

Pull Rate (cm/min)

Fig. 10. Thickness of SiO2 films as a function of application rate by spin and dip coating before (solid lines) and after (dash lines) the densification. Note that after densification crazing thickness becomes about the same. Also, compare the densification shrinkages, at point 'W' where the as-deposited thickness is the same. Spin coated films shrink twice as much, indicating a correspondingly higher porosity.

als, especially in the IR region, can be modified significantly (Fig. 11). This allows for the development of optical coatings for infrared interference windows and guidance systems. Table 1 summarizes process induced variations in sol-gel coatings.

S u m m a r y

Formation of materials by sol-gel methods creates some novel technological capabilities. Among them: creation of stable non- equilibrium materials, low-temperature doping and stabilization, design of microstructure

and porosity, organic-inorganic polymerization, nano-composites, host materials for laser dies, and room-temperature liquid state for coating applications.

In sol-gel systems processing parameters intro- duce significant molecular structural variations. These variations occur during the initial condensation stage, during the thermal removal of network organics, and during heat treatment, where the restoration of stoichiometry and network connectivity is chemically restricted and strongly diffusion-dependent. In coating systems, effects of processing parameters extend to deposition methods of the films. Thus, processing parameters themselves become powerful tools in modifying materials in sol-gel processes.

76 Yoldas

o o

o o

o o

o 02

bJ

Z • ~C o i=-

IlC ,.~ l-

0 t~

o o

z.o ~.o ~.o b.o

!.'1

't;i 2o, . . . . S o l - G e l

I

tb.o 1~.o ttj.o t~s.o tb.o M l CPIONS

zb.o

Fig. 11. Infrared transmission differences of GeO2 films produced by vacuum deposition and sol-gel method.

Table I. Process-induced variations in sol-gel derived coatings.

1. Precursor Effects

• Alkoxides of lower alkyls produce denser films with higher oxide content.

• Same applies to alcohols. Pore size and surface area strongly dependent on dispersion medium.

2. Condensation Effects

• Hydrolytic condensations under diluted conditions result in smaller molecules. Finer texture, but higher porosity.

• At a given concentration, hydrolysis with higher water results in denser films. Such films are easier to sinter (independent of porosity).

3. Method of Deposition Effects • Surface roughness of optical films deposited by spin coat-

ing is higher than dip coating. Spin coating produces significantly higher porosity than dip coating.

4. Thickness Effects • There is significant density gradient in the first 500.~ of

the film. • In both dip and spin coating the first 100-200/*k layer

appears to be substantially dense.

5. Heat-Treatment Effects

• There are thermal conditions which that to selective cleavage in bonds, resulting in accelerated sintering, carbon deposition, and photosensitivity.

• There are significant atmospheric effects in some oxide systems, e.g., in titania.

R e f e r e n c e s

1. R. Roy, J. Am. Ceram. Soc. 52, 344 (1969). 2. B.E. Yoldas, J. Mater. Sci. 12, 1203 (1977). 3. S. Sakka and K. Kamiya, J. Non-Cryst. Solids 42, 403

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Technological Significance of Sol-Gel Process and Process-Induced Variations in Sol-Gel Materials...

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