Soluble Silicates

18 S i l i c o n A l k o x i d e s i n G l a s s Technology

Downloaded by UNIV OF ARIZONA on October 27, 2015 | http://pubs.acs.org Publication Date: June 1, 1982 | doi: 10.1021/bk-1982-0194.ch018

L. C. KLEIN and G. J. GARVEY Rutgers University-The State University of New Jersey, Ceramics Department, Piscataway, NJ 08854

The sol-gel process for forming glasses from silicon alkoxides is described. The processing steps are forming the solution, gelling, drying and firing. The chemical reactions hydrolyzation and polymerization occur in solution depending on combinations of the variables pH, electrolyte, percent water, solvent and temperature. The advantages of the process are high purity, homogeneity and low temperature. Commercial applications of sol-gel glasses include coatings, microballoons, fibers, substrates and porous monolithic shapes. Though ethyl silicates and other silicon alkoxides have been commercially available for some time (1), their use in glass technology has only recently been well publicized (2). Perhaps the reason for so few investigations in the past into their use in glass technology is that the traditional ideas about glass formation have always involved high temperature. That is to form a glass, a material is heated above its liquidus temperature to disrupt its crystalline structure and, because of its viscous nature, the random liquid structure is trapped by a rapid quench. Once at room temperature, the glass is an unstable solid which is isotropic and in some cases transparent. Accepting this the formation of an isotropic, transparent amorphous material at low temperature is in conflict with this definition. Nevertheless, such a material can be made at room temperature through a sequence of chemical reactions including hydrolyzation and polymerization with silicon alkoxides. In the broader sense of glass formation, this paper will cover the raw materials for what is called the sol-gel process, the processing steps and variables, the applications of the technology and its advantages over traditional methods.

0097-6156/82/0194-0293$06.00/0 © 1982 American Chemical Society In Soluble Silicates; Falcone, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

SOLUBLE

294

SILICATES


Raw M a t e r i a l s Most s i l i c a t e glasses are made w i t h sand g r a i n s that range from a few to hundreds of microns i n s i z e . The process of melt i n g and homogenizing these glasses r e q u i r e s high temperatures and long times f o r s o l i d s t a t e r e a c t i o n s t o occur. Suppose the source of the s i l i c a i n the s i l i c a t e g l a s s was a f l u i d or a l i q u i d . This would e l i m i n a t e the long times needed f o r r e a c t i o n s . C o l l o i d a l s i l i c a s (3) and s o l u b l e s i l i c a t e s (4) can be used as f l u i d sources o f s i l i c a . S i l i c o n alkoxides can be used as w e l l , and i n p a r t i c u l a r the c l e a r l i q u i d t e t r a e t h y l o r t h o s i l i c a t e (TEOS from Dynamit-Nobel) was s e l e c t e d f o r t h i s study. Of the a v a i l a b l e s i l i c o n a l k o x i d e s , t e t r a e t h y l o r t h o s i l i c a t e (TEOS) appears to be the most popular. T h i s i s because i t r e a c t s more slowly with water than tetramethyl o r t h o s i l i c a t e , comes t o e q u i l i b r i u m as a complex s i l a n o l and i n a p a r t i a l l y hydrolyzed s t a t e i s s t a b l e over longer periods of time (5) . The c l e a r TEOS l i q u i d i s the product of the r e a c t i o n of S i C l 4 w i t h ethanol. The r e a c t i o n produces HCl along w i t h the e s t e r Si(0C2H5)4 (1). T h i s c o l o r l e s s l i q u i d of a density o f about 0.9 g/cm3 i s easy to han d l e and through m u l t i p l e d i s t i l l a t i o n extremely pure. T e t r a e t h y l s i l i c a t e i s i n s o l u b l e i n water ( 6 ) . In order to i n i t i a t e the h y d r o l y s i s r e a c t i o n , the TEOS and water must be introduced i n t o a mutual s o l v e n t . In t h i s study the mutual solvent i s ethanol. A t y p i c a l mixture i s 43 volume % TEOS, 43 volume % ethanol and 14 volume % water. For multicomponent g l a s s e s , the d e s i r e d a d d i t i o n s may be i n the form of a l k o x i d e s (7) or s o l u b l e s a l t s such as acetates and n i t r a t e s ( 8 ) . In t h i s way glasses c o n t a i n i n g B, A l , T i , Na, K, t r a n s i t i o n metals, r a r e earths and others are r e l a t i v e l y s t r a i g h t forward i n p r a c t i c e to prepare. A longer chain a l c o h o l such as propanol may be used to slow the r a t e s of the chemical r e a c t i o n s and allow adequate time f o r complete mixing. In some cases where the high p u r i t y of TEOS i s not needed, a p a r t i a l l y condensed m a t e r i a l may be used. Such l i q u i d s have up to 40% by weight S 1 O 2 and d e n s i t i e s of about 1.05 g/cm . When i t i s p o s s i b l e t o s t a r t with t h i s p a r t i a l l y condensed TEOS, the advantage i s reduced weight l o s s i n the conversion t o an i n organic g l a s s . These raw materials, are p r a c t i c a l i n small s c a l e operations as w e l l as l a r g e s c a l e . For s p e c i a l t y a p p l i c a t i o n s such as coat ings f o r s o l a r c e l l s , the m a t e r i a l s o f f e r high p u r i t y and ease of a p p l i c a t i o n ( 9 ) . For c o a t i n g window g l a s s e i t h e r t o improve chemical d u r a b i l i t y or to reduce r e f l e c t i o n l o s s e s , the s t a b i l i t y of the s o l u t i o n makes the c o a t i n g of many square meters of g l a s s a continuous process (10). In e i t h e r case, i t i s p r a c t i c a l t o r e c y c l e ethanol generated by the chemical r e a c t i o n s i n the s o l u t i o n back i n t o the production of the raw m a t e r i a l , thus producing more TEOS. 3

In Soluble Silicates; Falcone, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

18.

K L E I N AND GARVEY


Processing Steps and

Silicon

Alkoxides

in Glass

Technology

295

Nomenclature

The apparatus needed f o r l a b o r a t o r y s c a l e processing i s r e l a t i v e l y simple. As shown i n Figure 1, i t c o n s i s t s of a t h r e e necked f l a s k , a mechanical s t i r r e r , a r e f l u x condenser and a temperature probe c o n t r o l l i n g a constant temperature bath. The i n g r e d i e n t s are TEOS, ethanol and water. An e l e c t r o l y t e such as HCl or N H 4 O H may be used. The neck of the f l a s k f i l l e d by the temperature probe may a l s o be used f o r e l e c t r o l y t e a d d i t i o n s or sampling. The processing steps are, i n s h o r t , forming the s o l , g e l l i n g , d r y i n g , and f i r i n g . During the f i r s t step, a l l components must be mixed to form a c l e a r s o l . Cloudiness or p r e c i p i t a t i o n i n d i cates a segregation of components which needs to be c l e a r e d up by an e l e c t r o l y t e a d d i t i o n or d i f f e r e n t s o l v e n t . Once a l l of the components are mixed, the water and alkoxides r e a c t to begin the g e l l i n g , the second step. While being continuously s t i r r e d the f l u i d s o l w i l l become i n c r e a s i n g l y more v i s c o u s . At a d e f i n i t e p o i n t the v i s c o u s s o l becomes an e l a s t i c g e l , and at t h i s p o i n t bubbles cease r i s i n g . One way to p i c t u r e the g e l i s as an e l a s t i c sponge now f i l l i n g the volume once f i l l e d by the s o l . During the t h i r d step, the porous g e l w i l l exude l i q u i d and s h r i n k . While d r y i n g organics and water trapped i n pores w i l l escape. E v e n t u a l l y , the d r i e d g e l comes to an e q u i l i b r i u m w i t h ambient c o n d i t i o n s , and t h i s amorphous r i g i d s o l i d i s from then on f a i r l y i n s e n s i t i v e to moisture. I f the goal of t h i s process i s to make a m a t e r i a l with the same p h y s i c a l p r o p e r t i e s as g l a s s , the f i n a l step i s to heat the i n o r g a n i c sponge, d r i v e o f f absorbed water, react hydroxyls to form b r i d g i n g oxygens l i n k i n g the S 1 O 2 network, c o l l a p s e pores and s i n t e r to dense g l a s s . A l l of t h i s can be accomplished at 1/3 to 1/2 lower temperatures i n °K than used i n the c o n v e n t i o n a l method, w i t h (11) and without pressure ( 8 ) , i n vacuum and i n a i r (12). At the s o l - g e l t r a n s i t i o n , the molecular s t r u c t u r e of the g e l determines the p r o b a b i l i t y that a g e l w i l l dry i n one p i e c e or w i l l break i n t o fragments. I t i s convenient to t h i n k of t h i s t r a n s i t i o n as the formation of the l a s t bond needed to c r e a t e an i n f i n i t e molecule. However, t h i s t r a n s i t i o n has not been defined i n terms of thermodynamics, so i t may be m i s l e a d i n g to c a l l i t the s o l - g e l t r a n s i t i o n at a l l . Yet, i n p r a c t i c e the t r a n s i t i o n i s determined by q u a l i t a t i v e i n s p e c t i o n when an abrupt i n c r e a s e i n v i s c o s i t y occurs. The goal of t h i s work w i t h TEOS i s to f i n d the optimum combination of v a r i a b l e s which gives a s t r u c t u r e at the s o l - g e l t r a n s i t i o n which can be processed to form v a r i o u s l y f i b e r s , beads, f r i t s , m i c r o b a l l o o n s , shapes, s e a l s , coatings or i n o r g a n i c sponges.


S O L U B L E SILICATES


296

Figure

1.

Schematic

of apparatus for preparing

gels from silicon

alkoxides.


18.

K L E I N A N D GARVEY

Silicon

Alkoxides

in Glass

Technology

297


Chemical Reactions and Reaction Rates The chemical r e a c t i o n s that occur when water and TEOS are d i s s o l v e d i n ethanol are h y d r o l y z a t i o n and condensation polymer i z a t i o n . The s o l u t i o n i s a c t i v a t e d once the water r e a c t s w i t h alkoxy groups on the s i l i c o n t o form hydroxyl groups and a l c o h o l . T h i s h y d r o l y z a t i o n produces complex s i l a n o l s and ethanol w i t h TEOS, but t h i s never goes to completion. That i s the water i s not used up to form s i l i c i c a c i d . Instead condensation polymer i z a t i o n takes p a r t i a l l y hydrolyzed u n i t s and makes l a r g e r u n i t s with b r i d g i n g oxygens. T h i s condensation p o l y m e r i z a t i o n regen erates water. While h y d r o l y z a t i o n uses water as a r e a c t a n t , p o l y m e r i z a t i o n regenerates water as a product. The k i n e t i c s of t h i s process are very complex. In f a c t , the mechanism f o r r e a c t i o n s c a t a l y z e d by a c i d i s d i f f e r e n t from that c a t a l y z e d by base (5, 12). To moni tor the extent of these r e a c t i o n s , an experiment was devised t o simultaneously measure ethanol and water content i n the r e a c t i o n f l a s k (7). An i n c r e a s e i n ethanol content would i n d i c a t e prog r e s s i n h y d r o l y z a t i o n . A minimum i n water with a subsequent r i s e would i n d i c a t e progress i n p o l y m e r i z a t i o n . The experiment i n v o l v e s p e r i o d i c sampling of the s o l u t i o n i n the r e a c t i o n f l a s k . With a s y r i n g e , a sample i s e x t r a c t e d f o r ethanol a n a l y s i s i n a c a l i b r a t e d gas chromatograph (Bendix 2600 w i t h 6 foot Porapak S column). At the same time, the s o l u t i o n i s t i t r a t e d with K a r l F i s h e r reagent to give s e m i q u a n t i t a t i v e water a n a l y s i s . The data c o l l e c t e d are p l o t t e d i n F i g u r e 2. The open symbols are the volume percent e t h a n o l . N o t i c e that the ethanol l e v e l reaches a p l a t e a u . The time at which the ethanol l e v e l reaches t h i s p l a t e a u corresponds t o the minimum i n the water l e v e l . The f i l l e d symbols are the volume per cent water. The data were c o l l e c t e d a t 20°C, 60°C and 80°C, the r e f l u x i n g temperature f o r the s o l u t i o n . An i n t e r e s t i n g f e a t u r e i s that the p l a t e a u i n the ethanol l e v e l i s the same f o r a l l three temperatures. When the p l a t e a u i s reached, the p o l y m e r i z a t i o n process appears t o dominate the h y d r o l y z a t i o n process. Another i n t e r e s t i n g f e a t u r e i s that the water l e v e l does not go t o zero. There i s a minimum i n d i c a t i n g p o l y m e r i z a t i o n has begun before complete h y d r o l y s i s of a l l alkoxy groups. Beyond the minimum, the water l e v e l i n c r e a s e s l o g a r i t h m i c a l l y with time. The r a t e o f i n c r e a s e o f the water l e v e l i n c r e a s e s w i t h i n c r e a s e d temperature i n d i c a t i n g that p o l y m e r i z a t i o n speeds up with temperature. The data f o r water l e v e l extends t o the g e l p o i n t , so i n c r e a s e s i n temperature shorten the time t o g e l . The behavior f o r volume percent ethanol and volume percent water i n F i g u r e 2 i s t y p i c a l f o r s o l u t i o n s of TEOS i n ethanol with enough water f o r complete h y d r o l y s i s . The time to reach the ethanol p l a t e a u and the slope of the volume percent water



298 SOLUBLE


SILICATES


18.

KLEIN AND

GARVEY

Silicon

Alkoxides

in Glass

Technology

299

vs time curve can be changed by changing the f o l l o w i n g v a r i a b l e s : pH, e l e c t r o l y t e , percent water, nature of solvent and temperature. F i r s t , the pH can be changed by adding more of an e l e c t r o l y t e , f o r example IN HCl. In Figure 3, the e f f e c t of a d d i t i o n s of IN HCl on the v i s c o s i t y i s shown on a p l o t of v i s c o s i t y vs time. These measurements were made with a B r o o k f i e l d Viscometer at 50 RPM. The i n t e r e s t i n g f e a t u r e i s that the shape of the curve remains the same whereas the p o s i t i o n of the s o - c a l l e d knee s h i f t s to longer times with l a r g e r a c i d a d d i t i o n s . The knee s t a r t s at about 30 c e n t i p o i s e . For p r a c t i c a l purposes the s o l g e l t r a n s i t i o n i s at 2000 cp. The e f f e c t of these small a c i d a d d i t i o n s would appear to be a r e t a r d a t i o n of the bond formation needed to set to a g e l , though the eventual s t r u c t u r e i s p r e t t y much the same. Second, i t has already been mentioned that base c a t a l y z e d r e a c t i o n s are d i f f e r e n t from a c i d c a t a l y z e d r e a c t i o n s . Some p r e l i m i n a r y observations i n t h i s study were that an a c i d such as HCl drove the h y d r o l y s i s r e a c t i o n w h i l e impeding g e l l i n g . Then a base such as NH4OH l i m i t e d h y d r o l y s i s which made g e l l i n g im p o s s i b l e . However a s a l t such as NH4CI postponed h y d r o l y s i s , but t h i s was q u i c k l y followed by g e l l i n g . In a crude way, i t can be suggested that to speed up the o v e r a l l s o l - g e l process, i n i t i a l treatment should be with a c i d followed by a f i n a l t r e a t ment w i t h base. T h i r d , the e f f e c t of water a d d i t i o n s to a s o l u t i o n w i t h 20 weight % Na20 i s l i s t e d i n Table I. With small a d d i t i o n s , the g e l l i n g time was s e v e r a l weeks and the product was p a r t i c u l a t e . With a d d i t i o n s greater than 1 mole water per mole of ethoxy groups, or 4 moles of water per mole of TEOS, the g e l l i n g time was l e s s than 40 seconds and the product was a f r i a b l e shape. Intermediate a d d i t i o n s gave a shape which stayed i n one p i e c e , but the i n t e r f e r e n c e of pores with l i g h t transmission made the piece opaque. In general an increase i n water increases the r a t e of g e l l i n g . Table I - E f f e c t of Water A d d i t i o n on G e l l a t i o n Time For 20 Weight % Na20-80 Weight % S1O2 S o l u t i o n Water A d d i t i o n moles H20/moles Ethoxy Group 0.18 0.36 0.53 0.71 0.89 1.07 1.25 >1.25

Time to Gel 3 weeks 3 weeks 36 sec 48 sec 43 sec 40 sec

Soluble Silicates

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