drying and sintering of sol-gel derived large sio2 monoliths

Journal of Sol-Gel Science and Technology 6,203-217 (1996) @ 1996 Kluwer Academic Publishers. Manufactured in The Netherlands.

Drying and Sintering of Sol-Gel Derived Large SiO2 Monoliths

FIKRET KIRKBIR, HIDEAKI MURATA, DOUGLAS MEYERS, S. RAY CHAUDHURI AND ARNAB SARKAR

YTC America Inc., 550 Via Alondra, Camarillo, CA 93012, USA

Received January 2, 1996; Accepted January 9, 1996

Abstract. This review article summarizes the development of drying and sintering techniques for the production of sol-gel derived, large silica glass components. Gels may be synthesized using particulate or metal alkoxide precursors, or both in combination. Rapid fracture-free drying has been achieved easily with particulate gels because of their large pore size (100-6000 A). Alkoxide gels, which generally have small pores (<ZOO A), were initially difficult to dry without cracking. However, recent studies have shown that large alkoxide gel monoliths can also be dried in reasonably short times (~10 days). During subsequent heat treatment, alkoxide gels tend to have high shrinkage rates, which may cause trapping of hydroxyl ions or organic groups remaining on the gel surface. Although the removal of these species is easier for particulate gels, their large pore size necessitates heating above 1400°C to achieve full consolidation. Sintering at such temperatures was observed to deteriorate glass quality, through crystallization, warping, and/or sagging. Extensive optimization of the entire process has shown that on a laboratory scale, high-optical-quality glass can be produced from both alkoxide and particulate gels. It remains to be seen whether sol-gel process will be feasible for the manufacture of high-quality glass products on a commercial scale.

Keywords: silica, monolith, sol-gel, pore size, strength

1. Introduction

High purity bulk SiOz glass products, such as lenses, prisms, refractory tubing, muffles, holders and cru- cibles, are currently produced by fusion of naturally occurring quartz crystals. The optical and mechanical quality of fused quartz products is limited, however, by impurities in the raw material such as iron, zirconium, and hydroxyl groups. Also, the high processing temperatures (> 1725°C) can result in contamination by refractory materials, while consuming large quantities of energy.

The highest-quality optical components (e.g., very low IR absorbing windows for high-energy-laser transmission, high-UV-transmitting stepper camera lenses, and photomask substrates used in microlithography) are produced by another commercial process, chemical vapor deposition (CVD). In this process, fine

SiO2 particles are produced from Sic14 in an oxygen- hydrogen flame or plasma, then deposited on a hot surface forming a porous or dense SiO2 monolith, depending on the surface temperature. If the deposited SiO2 is porous, it is later consolidated at temperatures above 1400°C to produce a dense glass body. However, during the deposition of SiOz, 40-&O% of the particles generated in the flame can be lost to the gas stream, resulting in low yield [ 11. Furthermore, only disks and rods can be produced directly. Further shaping causes additional material losses due to cutting and grinding of the glass.

The sol-gel process has three potential advantages over these commercial processes. First, near-net-shape objects can be produced directly by casting and gelation of the sols in molds, at or near room temperature. This minimizes material losses due to processing as well as shaping. Also, dried gels have small pore sizes relative

204 Kirkbir et al.

to CVD monoliths, so full densification can be achieved at temperatures lower than 1300°C. Finally, the optical quality of sol-gel derived SiOz glass can be equal or superior to that of glass produced by CVD. Hence, utilization of the sol-gel process could significantly reduce the production costs of high quality optical grade SiOz components.

scope of this review. Only processes which yield dry gels at sub-critical conditions are reviewed herein.

2. Drying and Pore Size

2.1. Drying Theory and Cracking of Gels

However, there are several technical hurdles which have thus far prevented successful commercialization of this process. The main problems are the fracture and/or warping of wet gels during drying, and the fracture, crystallization, warping, sagging, bloating and/or contamination of dry gels during sintering. The aim of this paper is to review the efforts of several researchers to overcome these difficulties by optimizing the process conditions.

Gels are usually prepared using either of two routes. In the first approach, SiOz particles are dispersed in a solution and then gelled. The pores of the particulate gels can range from about 100 A to 6000 8, depending on the size of particles, the catalysts and dispersing agents used. The relatively large pores facil- itate crack-free drying. However, to fully consolidate these gels requires temperatures higher than 1400°C at which crystallization and deformation of the glass may occur. In the second synthesis technique, silicon alkoxides are used as precursors. The alkoxide gels usually have pores smaller than 200 A. The small pore size can induce fracture during drying due to enormous capillary forces. However, the small pores also pro- vide a great advantage in that the gels can be sintered at temperatures below 1300°C. Crystallization and deformation are therefore minimized and a considerable amount of energy is saved.

In the next section, the traditional drying theory is reviewed and a recent advancement in drying of large alkoxide gels is discussed. In Sections 3 and 4, the properties of alkoxide gels are compared with those of particulate gels. Particulate-alkoxide hybrid gels are discussed in Section 5. The various problems encoun- tered in heat treatment and sintering are summarized in Section 6, and in Section 7 the level of glass quality reached using sol-gel methods is reviewed. In the text, the strength refers to that of the wet gel and the pore size (i.e., average pore diameter) refers to that of the dry gel (xerogel), unless indicated otherwise.

Supercritical conditions of temperature and pressure are often used in sol-gel research to achieve rapid fracture-free drying. However, this process requires expensive high pressure autoclaves and is beyond the

During drying, shrinkage of the gel occurs due to the capillary pressure, Pc, which may be expressed as:

Pc = - 2y,, cos(e)/r,. (1)

Here ylv is the surface tension of the pore liquid at the liquid vapor interface, 8 is the contact angle of the liquid, and rh is the hydraulic pore radius, i.e.,

rh = 2v,,/s,,. (2)

V, and S, are pore volume and surface area, respec- tively. The negative sign in Eq. (1) indicates that the liquid is in tension.

During drying, pressure differences through the bulk causes development of stress. When the stress becomes greater than the strength of the gel, cracking occurs. For a gel plate with thickness L, this stress, a,, can be expressed as [2, p. 4851:

ox(L) = LPt VE/~D (3)

where PL is the viscosity of the pore liquid, V, is the drying rate, and D is the permeability of the gel.

The gel continues to shrink due to capillary pressure until a critical point is reached. The shrinkage then stops and the drying front starts to penetrate into the gel. The stress at the critical point can be written as [2, p. 4861:

a,(L,)x(L2~~13DK~)P~,max (at critical point) (4)

where Ko is the bulk viscosity of the network. Accord- ing to the Kozeny-Karman equation, the permeability is related to the pore radius:

D c( (1 -p) rh2 (5)

where p is the relative density of the network. As shrinkage due to the capillary force proceeds, the pore size is reduced which in turn reduces permeability, according to Eq. (5). This increases the stress in the gel, as shown by Eqs. (3) and (4).

Drying and Sintering of Sol-Gel Derived Large SiOz Monoliths 205

Pore Diameter, A

Figure I. Effect of pore diameter on magnitude of capillary pressure in liquid, ] PC], calculated from Eq. (1) assuming f3 = 0.

Then, the most common approach to solve this prob- lem is to reduce the capillary pressure by increasing the pore size of the wet gel. The effect of pore size on capillary pressure is shown in Fig. 1 (pore liquid is mainly composed of water and ethanol). As indicated by Eq. (l), the pore liquid is under enormous tension when the pore size is smaller than 200 A. The drying shrinkage is expected to be high for such gels. When the pore size is larger than 200 A, the shrinkage will be less and cracking will be less likely to occur.

Indeed, most of the research reviewed here seems to support this theory. Initially, alkoxide gels were difficult to dry rapidly without fracture. The stresses were reduced by decreasing the drying rate. Of course, this had the undesirable effect of increasing the drying time. From Eq. (3), it has been estimated that for an alkoxide gel with a typical strength of 0.1 MPa, it would take 4 days to dry a 1 cm thick plate without cracking [2, p. 4851. Using the particulate gel synthesis route, much larger pores were produced and the drying rate was successfully increased. Large monolithic particulate gels several centimeters thick were dried in a matter of a few days.

2.2. Observation of an Anomaly in Drying of Alkoxide Gels

The theory summarized above assumes that the stress develops through the entire gel thickness, L. Accord- ingly, drying should always start from the gel surface and gradually proceed inwards, as shown in Fig. 2(a).

Wang et al. [3] showed that this assumption is not valid for all gels nor for all drying conditions. In their

( 1 a m Figure 2. Photographs of wet alkoxide gels in a drying chamber. In case (a), drying started from the gel surface and gradually proceeded in wards (xerogel pore diameter = 80 A, wet gel diameter = 3.2 cm and length = 19.6 cm). In case (b), dry (white) pockets formed in the bulk (xerogel pore diameter = 40 A, wet gel diameter = 5.2 cm and length = 19.6 cm). From Murata et al. [75].

experiments, drying started not from the gel surface, but in the bulk through the formation of drying pockets, as shown in Fig. 2(b). Gels exhibiting such behavior were dried rapidly without cracking. Monoliths from 5 to 7.5 cm in diameter and 20 to 25 cm in length were obtained by this approach with 7-10 days (Fig. 3). Drying was repeatable and more than 30 crack-free gels were produced in each size. It appears that the initia- tion (or nucleation) of drying from inside the gel prevented the stresses from extending through the entire characteristic gel thickness. Contrary to conventional wisdom, the small pore size gels (~40 A) were easier to dry than larger pore size gels. These experimen- tal results were subsequently explained by a theory of cavitation [4].

This study also showed that large alkoxide gels could be dried as fast as particulate gels, and that drying time could be reduced to a commercially viable length.

206 Kirkbir et al.

F&WZ 3. Photograph ofa monolithic alkoxide derived xerogel. This gel was dried within 7 days. Xerogel pore diameter = 40 A, diameter = 5.9 cm, length = 25 cm. From Murata et al. [20].

3. Strength and Pore Size of Alkoxide Gels

Tetramethyl orthosilicate (TMOS) or tetraethyl orthosilicate (TEOS) are usually used as precursors for preparation of alkoxide gels. The reaction between these alkoxides and water proceeds according to the following stoichiometric relation:

Si(OR)d + 2 Hz0 + SiOz + 4 ROH (6)

where R is an alkyl, such as --CHs, -CzHs etc. The elementary reactions are believed to consist of

-Si-OR + Hz0 + -Si-OH + ROH

(Hydrolysis) (7)

-Si-OR + -Si-OH + -Si-O-Si- + ROH

(Condensation) (8)

-Si-OH + -Si-OH -+ -Si-O-Si- + Hz0

(Condensation) (9)

A catalyst is usually used to accelerate the sol-gel reactions. Since TMOS and TEOS are immiscible with water, a solvent such as methanol or ethanol may be used to form an homogeneous sol.

Research on alkoxide gels for the production of fused silica has focused primarily on the development of wet gels with high strength and xerogels with large pores. High strength is desired to circumvent capillary forces during drying. Large pores allow quicker and more ef- ficient purification by chemical processes at high temperatures (>2OO”C), prior to densification.

Microstructure and strength of the gel are influenced by water, solvent and catalyst concentration, catalyst type, reaction temperature, aging time, aging temperature and the composition of the pore liquid during aging.

3. I. Effects of Water and Alcohol Concentration

The molar ratio of Hz0 : Si(OR)4 in the sol should be at least 2 : 1 (i.e., stoichiometric ratio) to approach


Tile 1. Effects of single acid catalysis. From Kirkbir et al. 1161.

Wet gel Xerogel

Gelation Aging* Rupture Shear Drying Average Total pore Surface Molar ratio Solution time shrinkage modulus modulus shrinkage pore diam. volume area TEOS/EtOH/H~O/catalyst pH (days) (%) WW (MW W) (‘4 (cm3k) Wg)

l/3/4/0.02 HCI 06 3 10 0.46 1.72 38 22 0.49 920

l/3/4/0.02 HNOs 1.1 25 11 0.50 1.62 41 20 0.43 840

l/3/4/0.02 H2SO4 1.1 2 11 0.50 1.89 40 22 0.41 790

l/3/4/0.02 Oxahc acid 2.1 25 9 0.31 1.26 45 20 0.44 850

l/3/4/0.02 HF 3.8 0.13 3 0.11 0.63 25 172 2.40 670

* Gels were aged for 7 days at 7O’C.

complete hydrolysis of the alkoxide. Extremely long gelation times have been reported when this ratio is less than 2 : 1 [5]. By increasing the water concentration, chemical reactions are accelerated and gelation times decrease [5,6]. Strengthening and stiffening of the gel network is also observed, which in turn reduces drying shrinkage [6]. As a result, the xerogel pore size increases [6]. A similar effect can be observed if the gels are aged in aqueous solution [7,8].

Although alcohol is usually used to produce an homogeneous sol, some investigators have prepared sols by directly reacting water with the alkoxides. In these studies, high acid concentrations were used to accelerate reactions at the alkoxide/water interface [9, lo]. The reactants eventually intermix due to the alcohol by-product. It has also been shown that ultrasonic treatment of alkoxide/water mixtures can promote hydrolysis and condensation reactions in the absence of a solvent [ 111. As a result of this procedure, gelation rates increase. The so-called sonogels are denser and stronger, and have larger surface area to volume ratios, than conventional alkoxide gels [12-141. Pore sizes from 20 to 50 8, were produced using this technique.

One might think that the pore size could be enlarged simply by reducing the volume fraction of solids at the sol mixing stage, by diluting with alcohol. However, experiments have shown that while increasing alcohol concentration elongates gelation time and weakens gel structure, it may not significantly affect the xerogel pore size [6, 15, 161. Wet gels synthesized with high alcohol concentrations are less dense, as expected [5]. However, these gels tend to experience greater drying shrinkage [6, 161, which suggests that the xerogel pore size cannot easily be manipulated by alcohol concentration.

3.2. Effects of Catalyst Type and Catalyst Concentration

As shown in Table 1, inorganic catalysts such as HCl, HzS04, and HNOs usually produce gels with very narrow pores (<2aO A) [16, 171, and relatively high strength. The use of organic acid decreases the gel strength, without affecting the pore size. Although pores larger than 80 A can be produced using HF, these gels tend to be very weak. Still larger pore sizes can be achieved through the synthesis of NHdOH-catalyzed TMOS-based SOIS. These gels tend to have broad pore size distributions, between 15 and 100 8, [18], and they are very weak gels, as one might expect. In one instance, gels made from TMOS, HzO, methanol, and NHbOHin themolarratio 1 : 3.7 : 4.8 : 1.5 x 10d3 had a rupture modulus of 0.08 MPa and a shear modulus of 0.48 MPa in the as-synthesized, unaged condition [ 191.

Gelation times are generally longer when the pH of the sol is low. The addition of HF or NHaOH as a second catalyst to a sol initially catalyzed by HCl, HzS04 or HNOs can increase the rate of condensation reactions and reduce the gelation time. Also, as shown in Table 2, these gels have higher strength and larger pores (40-150 A) than gels prepared with a single acid catalyst [6, 201. The properties of these gels can be customized by manipulating various pa- rameters including the timing of the catalyst additions and their concentrations. Hench et al. [21] have controlled xerogel pore sizes between 64 and 100 A using a HF/I-INOs dual catalysis technique. Sakka et al. [84] observed by SEM that dry gels prepared from sol composition of HCl/TMOS = 0.01 did not have any particulate microstructure and transparent, whereas HCl/TMOS > 0.15 yielded opaque gels formed by fused particles.

208 Kirkbir et al.

Table 2. Effects of double acid catalysis. From Murata et al. [75].

Molar ratio TEOS/EtOH&O/catalysts

l/3/4/0.03 HF/O.O2 HCl

l/3/4/0.03 HlVO.02 HN03

l/3/4/0.03 HF10.02 H2S04

l/3/4/0.03 HF/0.02 Oxalic A

l/3/4/0.0002 HCVO.00 1 NH3

Wet gel

Gelation Aging* Rupture Shear Solution time shrinkage modulus modulus

PH (days) (%) WW (MPa)

1.0 0.33 14.0 0.76 5.47

1.4 N.A. 14.5 0.69 5.36

1.4 N.A. 14.5 0.63 5.07

2.4 N.A. 12.5 0.55 4.66

8 <0.67 6.3 0.38 1.98

Xerogel

Drying Average Total pore Surface shrinkage pore diam. volume

(%) (-4 (cm3/g) (r?&

29 40 0.87 1020

29 40 0.87 1040

30 40 0.73 1020

32 40 1 .oo 1020

34 72 1.67 959

* Gels were aged for 7 days at 70°C.

It is also possible to synthesize gels without the addition of a catalyst. Yamane et al. [ 181 and Katagiri and Maekawa [15] have prepared such gels from TMOS and reported a pore size of 15 to 20 A.

3.3, Effects of Reaction Temperature

The temperature at which the hydrolysis and condensation reactions take place can also have a profound influence on gel properties. For example, Yamane [22] has reported that increasing the reaction temperature causes both gelation time and drying shrinkage to decrease. As a result, xerogels synthesized at higher temperatures have lower density.

3.4. Effects of Aging

It is possible to increase the pore size by aging gels in their original pore liquid. However, pore enlargement proceeds very gradually this way, and for short aging times there is little noticeable effect. In one instance, aging for 60 days resulted in an increase of the xerogel pore size from 24 to 42 A [23]. Aging in the original pore liquid is usually carried out for the purpose of strengthening the gel network rather than altering the xerogel pore size.

There are several mechanisms by which the wet gel may be strengthened through aging: (1) Increasing degree of completion of condensation and silanol cross- linking reactions between the gel surfaces (which is also responsible for aging shrinkage) [24]; (2) Disso- lution and reprecipitation of SiO:! (i.e., Ostwald ripen- ing) [25]; and (3) Deposition on the gel surfaces of unreacted oligomers remaining in the pore liquid [26]. By increasing the aging temperature, the rates of these processes can be accelerated [27]. The practical upper limit for aging temperature is the boiling point of the

pore liquid. The aging temperature can be increased further by placing the gels in a pressurized vessel. However, West et al. [27] suggested that the wet gel may actually be weakened by this high temperature process.

It is common practice to carry out the aging for extended lengths of time. Gels can continue to gain strength even after a few months of aging [23, 281. However, there are limits to this approach. Longer time spent in aging makes the entire process less feasible economically. Also, the gels may begin to weaken if aged too long. Dissolution or coarsening of the gel network may occur, as observed in HF-catalyzed gels [ 161. A weakening effect was also observed in HCl/NI&OH catalyzed gels [6].

Aging effects may be enhanced by immersing wet gels in various solutions [29, 301. Scherer [24] reported that higher water concentrations in the aging liquid caused faster shrinkage and greater stiffening of the gels. Haereid et al. [8] aged TMOS-based gels in water at various temperatures and found that the gel strength first increased with aging time (by a factor of 1.5) and then decreased due to the continued coarsening of the microstructure. Pope et al. [31] reported considerable pore coarsening in wet gels which were hydrothermally treated in an autoclave. The pore size of these gels increased from 130 8, to 1200 A.

Liu and Hench [23] were able to vary the xerogel pore size between 24 and 400 8, by treating their gels in N&OH solutions. Aggressive aging conditions dam- aged the surfaces of the gels. They suggested that the aging procedure which results in a pore size of 90 A yields glasses suitable for optical applications. The microstructure could be modified by this process with aging times of less than 300 minutes.

Another promising approach recently developed is the aging of gels in solutions containing TEOS [32].


During this process SiO2 is formed from the additional alkoxide and precipitates on the gel surface. This strengthens the gel and restricts shrinkage during drying. Using this process, the shear modulus was increased from 0.48 MPa to 7.4 MPa and the modulus of rupture from 0.08 MPa to 0.48 MPa [19]. A 52% increase in gel weight was also observed, and the peak pore size of the xerogel was around 200 A [33]. This dramatic modification of the gel structure was achieved by aging the gels in a solution of 70 ~01% TEOS in methanol for 144 hours. Gels were dried crack-free at atmospheric pressure and had physical properties similar to aerogels.

3.5. Effects of Chemical Additives and Pore Liquid on Drying

Some studies have shown that the addition of surfac- tants to the sol can improve drying yield. These additives reduce the capillary stresses by influencing the surface tension and the contact angle as in Eq. (1) [34, 351. However, these gels have a greater tendency to crystallize during subsequent densification [35].

Other investigators have added various chemicals at the sol preparation stage intending to control the evap- oration rates of water and alcohol, which form the bulk of the pore liquid after gelation. These Drying Control Chemical Additives, or DCCAs, include formamide [36], NJ/-dimethyl formamide (DMF) [37], ethylene glycol [lo], acetonitrile, and 1,4 dioxane [38], etc.

The studies have shown that DCCAs not only influence the drying behavior, but also affect gel synthesis, aging and drying processes in a complicated manner. Hydrolysis reaction rates may be retarded or accelerated depending on the type of chemical added [38]. Also, chemicals such as formamide can be chemically adsorbed on the gel surface [39]. Pore coarsening (from 24 A to 60 A) has been observed as well after addition of a DCCA [40]. Among the various chemicals used as DCCAs, the most successful results were achieved with DMF. In one study, after adding DMF to the sol and then elevating the drying temperature to 150°C xerogel pore sizes were enlarged from 10-20 A to 50-250 A [41].

DCCAs have shown a clear advantage for the successful drying of large monoliths within reasonable times. However, the chemicals tend to be difficult to remove from the dried gel because of their higher boiling points, and probably because of their irreversible chemical adsorption on the gel surface. The latter may cause bloating of the glass [2, p. 5001, and insufficient

Figure 4. Effect of pore fluid surface tension on pore radius of alkoxide derived xerogels. From Deshpande et al. [44]. Reprinted by permission of Elsevier Science.

oxygenation may leave carbon impurities in the sintered glass [22].

More recent studies have focused on the manipula- tion of interfacial energy through the use of different pore liquids. It was previously thought that the contact angle can be reduced by using heavier alcohol solvents (propanol, butanol, etc.) [42]. However, recently it was shown that the contact angle of all these alcohols is zero [43]. It was further proposed that the pores enlarged not as a result of decreasing contact angle, but because of decreasing gel strength resulting from es- terification reactions occurring between the pore liquid and the gel surface. Aporatic compounds (i.e., compounds that do not react with the gel surface) have also been used as pore liquids and it was found that the pore size increases by using liquids with lower surface tension [44] (see Fig. 4).

A very interesting effect has been achieved through the chemical modification of the gel surface [45]. In this instance, the gel shrank up to 40% during drying. Since the modified surfaces did not react with each other during drying, the shrinkage was reversible. Thus, the gel returned to its original size after drying, i.e., the size of the wet gel. This approach allowed the researchers to produce aerogels at atmospheric conditions [46].

4. Particulate Gels, Pore Size and Sintering Temperature

Particulate (or colloidal) gels are prepared using SiO2 particles smaller than 1 wrn in diameter. These are produced either from Sic14 in an oxygen-hydrogen flame, or by precipitation in aqueous solution of a silica

210 Kirkbir et al.

Figure 5. Photographs of complex shapes prepared by direct casting of particulate gels in molds. Machined 30 cm porous silica preform and sintered 15 cm mirror core segment (left). Before and after sintering of hemispherical silica monolith (right). From Shoup [85]. Reprinted by permission of ASM International.

precursor such as TEOS. Gelation is achieved by dispersing the particles in a solution and then altering the pH of the liquid medium.

The primary advantage of particulate gels for the production of synthetic silica is their large pore size. Because they usually have higher density and lower pore volume, particulate gels may be stronger than alkoxide gels [47,48]. This type of microstructure also tends to suffer less drying shrinkage, typically 7-9%. Fine SiOz particles are also cheaper than silicon alkoxides (except for particles prepared by precipitation of TEOS).

Scherer [49] first showed that the pore size of the gel could be increased by increasing the particle size. Then, Scherer and Luong [50] synthesized gels by dispersing Aerosil OX-50 particles (Degussa Co.) in chloroform by an adsorbed layer of n-decanol, then inducing gelation with ammonia vapors or an amine. Dry gels 2.5 cm diam. x 19 cm length were obtained in 3 days. The xerogel pore size was around 600-1000 A. Full densification was achieved at 1450°C.

Clasen [5 l] dispersed Aerosil OX-50 particles in water and induced gelation with ammonium flouride. For gels 1.6-3.0 cm in diameter, the drying time was reduced to a few hours (the length of these gels was not reported). The xerogel pore size was around 700 A. Sintering was achieved in a zone furnace at temperatures between 1400 and 1500°C.

Shoup [52] mixed a colloidal SiO2 sol (Ludox HS- 40, Du Pont Co.) with potassium silicate (Kasil 1, PQ Corp.). This mixture was gelled by the addition

of formamide. The pore sizes of these gels were controlled between 100 8, and 3600 8, by varying the potassium silicate concentration in the sol. The total alkali level was decreased to 0.02 wt% by leaching the wet gels in weakly acidic solutions. Shoup recommended synthesizing wet gels with pores larger than 600 A, preferably between 1000 and 2000 A, to obtain crack- free dry gels. Sintering was achieved at 1400-1500°C [53]. Because of their large surface area, the gels could be held at these temperatures for no more than 10-15 min to avoid surface nucleation of cristobalite.

Shoup was also able to cast, dry and sinter gels in various sizes and shapes using this process (Fig. 5). “Egg- crate”-shaped gels 25.5 cm square x 0.6 cm thick were dried in a microwave oven at medium power within 2- 3 h [53]. Glass plates 13 cm square x 5 cm thick, disks 27 cm diam. x 3 cm thick, etc. were also obtained (Fig. 6) [54]. (The drying time for these large size gels was not indicated).

Another significant feature of this process was that the gels could be machined while still wet. It was also noted that the drying yield decreased with increasing gel size, from 50% for 7.6 cm triangle mirror core seg- ments to 1% for 30 cm square structures [54]. For larger and thicker castings, the formation of significant density gradients was observed which caused fracture during sintering [53].

Rabinovich et al. [55] developed the so-called double-dispersion process. At the first dispersion step, 120 A diameter Cab-0-Sil Grade M5 (Cabot Co.) particles were used. This yielded a dried gel with


Fr~uw 6. Photograph of a particulate dewed glass SiOs disk (27 x 3 cm thick). Courtesy of Robert Shoup, Coming Co.

narrow pores, i.e., 100-200 A. The drying shrinkage was high and a very slow drying rate was necessary to obtain crack-free gels. Instead, the gels were rapidly dried at 150°C and allowed to crack. Then, the dried gel fragments were heat treated at 900°C. Finally, in the second dispersion step, the fragments were re- dispersed in water and gelled again by addition of ammonia and hydrochloric acid.

This second gel had a bimodal pore size distribu- tion, with smaller pores in the 100-200 A range and larger pores around 1 to 10 pm. The drying shrinkage was low (~4%) and the drying rate was very rapid. Crack-free dry gels 2.4-3.6 cm diam. x 20 cm length (rods and tubes) were obtained with 10 hours at 90- 110°C. However, it was necessary to increase the sintering temperature above 1500°C to eliminate residual porosity and produce clear glass [56]. Addition of about 3.5 wt% BzOs decreased the sintering temperature and prevented cristobalite formation, but at the expense of glass purity.

Later, this process was improved by ball milling the gels dried in the first step. This eliminated the large inter-aggregate pores (3-4 @m) and thereby decreased the sintering temperature [57, 581. It was necessary to carry out the ball milling in borosilicate jars with

fused SiO, milling media to prevent contamination. After ball milling the fragments for 64 h, the pore size of the final dried gels was about 160 8, and the sintering temperature was 1300°C (the drying shrinkage, however, increased back to 14%). Glass tubes and rods 0.7-2.3 cm in diameter and 25 cm in length were produced [59]. The drying times were not clearly stated either in the journal articles [58,59] or in the patent [57].

In a recent and promising study, fumed SiOz particles were dispersed in water with tetramethyl ammonium hydroxide. Gelation was achieved using methyl formate in the presence of additives such as glycerin and/or a polymer in small quantities (~0.5%) [60]. It was argued that the SiO;! particles were partially cov- ered by these additives, causing the formation of a flex- ible network which prevented cracking. Gel tubes 6.35 cm OD x 2.86 cm ID x 100 cm in length were dried within 2 weeks with a pore size of 500 A. The additives were successfully removed by heating the dry gels to 300°C in air. Full densification was achieved at 1300- 1500°C. The main aim of this research was to produce a cheaper, lower quality cladding material for optical fiber manufacturing.

In conclusion, rapid crack-free drying of large monoliths can be achieved with particulate gels because of

212 Kirkbir et al.

their large pore sizes. However, these large pores ne- cessitate sintering above 14Oo”C, at which the hetero- geneous nucleation of cristobalite may deteriorate the properties of the glass. Chemical additives can be used to manipulate the pore size and sintering temperature, but these materials reduce the purity of the sintered glass and thereby limit its potential applications.

5. Particulate-Alkoxide Gels

The alkoxide gels can easily be sintered at temperatures below 1300°C and it is relatively easy to dry particulate gels. It is possible to benefit from the advantages of each process by developing a hybridized process. Thus, some investigators have synthesized gels by dispersing SiO2 particles in alkoxide solutions.

Toki et al. [61, 621 prepared 15 cm x 15 cm x 0.5 cm glass plates by sintering particulate-alkoxide gels dried at 60°C for 10 days. The sintering temperature was 1200-1300°C. It was found that the porosity increased with increasing SiOz particle content. When the particle content was increased to 65% of the total weight of SiOn, large pores (about 300 A) were formed. Although the pore size could be increased to 800 8, by using 90% SiOa particles, these gels cracked during drying. They also found that the glass purity could be improved by using SiOz particles precipitated from a TEOS solution, instead of Aerosil OX-50. Mori et al. [63] also demonstrated the possibility of sintering a large tube, 2.6 cm OD x 1.3 cm ID x 100 cm length, utilizing the same process.

Okazaki et al. [64] followed a similar approach, but using fluorinated TEOS. Gel strength increased with both increasing fluorine and Si0.z powder content. Crack-free xerogel rods 8.5 cm D. x 20 cm L. were dried within 7 days at 60-12O”C, and sintered successfully at 1350°C.

6. Sintering of Xerogels

6.1. Hydrocarbon Removal

Organic species, including alkoxides such as Si-GCHs or Si-GCzHs, remain on the dried gel surface because of processing conditions or incomplete chemical reactions. These organic residues are removed by burn- ing in air or some other oxygen-containing atmosphere to increase glass purity and prevent bloating of the glass.

(4

,a: Smm I’ I

E ;

I@

-----__- I

:: 3’

. ._____ ___ ’

(6) T (‘C1

Figure 7. DTA and TGA curves of au alkoxide derived xerogel. The monolith schematically shown in the figure was slightly more porous in the surface region (A) than in the hulk region (B). From Kawaguchi et al. [65]. Reprinted by permission of Elsevier Science.

T I’01

Figure 8. Thermal expansion of an alkoxide gel at different states; the initial wet gel, and the xerogel heat treated at indicated temperatures for 2 h. From Kawaguchi et al. [65]. Reprinted by permission of Elsevier Science.

TGA-DTA data shown in Fig. 7 indicate that most of the alkoxides can be removed between 200 and 300°C [65]. During this removal, the weight of the gel suddenly starts to decrease. The reactions are greatly exothermic and release heat quite suddenly inside the gel. Since these highly porous materials are among the best existing thermal insulators, this may cause sharp temperature gradients within the gel body. Also, after a monotonous expansion the removal of organic species is accompanied by a rapid shrinkage of the gel (Fig. 8) [65]. The sudden creation of gaseous species in the pores, their expansion in a porous media with low permeability, abrupt temperature increases, and sudden shrinkages all occurring together can easily lead to catastrophic failure of the fragile gel microstructure. This effect will be more pronounced as gel size is increased.

Cracking of dry gels during the removal of organ- its has been observed in several instances [66, 671.


0.6

0.4

02

00 I I ’ I ’ I ’ I ’ I ’ I

0 200 400 600 800 1000 1200

Process Temperature(“C)

Figure 9. The sintering of alkoxide derived xerogels which have various pore sizes. The numerals next to the curves are average pore radiuses of xerogels. From Hench et al. [82]. Reprinted by permission of Kluwer Academic Publishers.

Fracture may easily occur during the heat treatment of alkoxide gels having small pores, but can be overcome by reducing the heating rate. For particulate gels having larger pores, the removal of hydrocarbons is not difficult.

6.2. Removal of Hydroxyl Groups

If not removed from the dried gel, chemically adsorbed water can cause bloating in the glass during or after sintering. This water can be removed by thermal treatment above 150°C so long as the pores remain open. In Fig. 9, curves of pore volume vs. temperature re- veal the differences in sintering behavior of gels having various pore sizes. Gels with smaller pores densify at lower temperatures. Increasing the pore size shifts the pore closure point to higher temperatures and aids the thermal removal of water.

Silanol (Si-OH) is the most stable form of adsorbed water. Although the hydroxyl concentration can be reduced by thermal treatment, to attain optical quality glass may require concentrations lower than 1 ppm (and lower than a few ppb for optical fiber applications). These low levels can only be achieved through the chemical treatment of dried gels, using gases containing halogen ions, such as Clz, CClb, Fa, NI&F, HF, etc., [68-701. Chlorine, for example, replaces the hydroxyl groups above 700°C [68,71]:

2 Si,,-OH + 2 Cl2 -+ 2 Si,-Cl + 2 HCl + 02 (10)

It has also been pointed out that the levels of other contaminants such as transition metals, alkalis, etc., could also be reduced by the chlorination process [72,73].

Figure 9 shows that the pores may start to collapse at or above 700°C which can prevent the complete removal of hydroxyl groups. For this reason, halo- genated compounds which are effective at low temperatures have been used to chlorinate small pore size alkoxide gels. For example, chemical removal of water by CC14 can be achieved between 350 and 600°C [70,74].

As shown in Eq. (lo), as the hydroxyl groups are removed from the gel, chlorine is incorporated into the structure. This may also cause the formation of bubbles at temperatures above 1600°C [68]. In fact, recent research has indicated that residual chlorine may cause bubbling at temperatures as low as 1350°C [75]. In one study, it was recommended that chlorine concentrations be reduced to ~0.53% to avoid foaming [68]. To prevent this foaming, chlorine should be removed from the gel by oxygenation as follows [68]:

2 Si,Cl + 02 -+ 2 Si,-0 + Cl2 (11)

This dechlorination process becomes effective only above 900°C. The reduction of hydroxyl content by chlorination does raise the pore closure temperature. Still, pore closure may occur before the oxygenation is complete, trapping chlorine in the glass matrix.

The Si-F bond is much stronger than Si+l. If fluorine is used rather than chlorine during hydroxyl group removal, the bubbling can be prevented [69]. However, for the same reason it is difficult to remove fluorine from the glass. This may be undesirable for some applications because the presence of fluorine lowers the refractive index of the glass.

It is in the chlorination and dechlorination processes that the particulate gels offer their greatest advantage over alkoxide gels. The considerably larger pores of the particulate gels remain open at much higher temperatures, allowing more complete removal of chemically adsorbed water.

6.3. Crystallization of Particulate Gels

The large pore size, on the other hand, is the primary reason for problems in the densification of particulate gels. Sintering has been attempted at 1300°C

214 Kirkbir et al.

F&W-~ IO. Photograph of the glasses sintered from the xerogels shown in Fig. 3. These glasses have surface cracks. From Murata et al. [20].

[55, 561, but the samples did not become fuly transparent, possibly because of un-dissolved SiOz particles or incomplete densification. To obtain full density and optical clarity, the particulate gels usually require sintering above 1400°C. However, extended heating at such temperatures should be avoided, since the onset of devitrification of sol-gel SiO2 has been detected at 1200-1250°C [76,77].

The optimum sintering temperature for particulate gels was determined to be between 1400 and 1500°C [73]. Below 14OO”C, sintering may be incomplete, and between 1500 and 1700°C the crystallization reaction was found to be very fast. Above 17OO”C, crystallization was not observed, but the glass started to sag and deform as the liquidus temperature of SiOz is 1725°C [54, 731. Moreover, at such high processing temperatures the sol-gel method loses its main advantages (i.e., low temperature sintering, near net shape production, etc.) over CVD and fusion techniques.

6.4. Pore Size Gradients in Drying and Sintering

Pore size gradients may develop in the gel body during either the drying or sintering processes. Kawaguchi et al. [65] has reported the formation of such a gradient during drying. One dried alkoxide gel, schematically

shown in Fig. 7, had smaller pores near the surface (region A) than in the core (region B).

The formation of pore size gradients during sintering has also been reported [20]. When the large alkoxide gels shown in Fig. 2 were heated to 9OO”C, it was found that the pores near the surface of the gel were smaller than those of the interior [75]. Prior to heating, the dry gel had a uniform pore size of about 40 A. The formation during sintering of a glassy crust surrounding the porous inner structure also indicated a structural gradient. The most likely cause of pore collapse near the surface, and its progres- sion towards the center, is the formation of a thermal gradient in the gel during sintering. This is not sur- prising considering the thermal-insulating capabilities of these materials. The temperature non-uniformity may be responsible for the formation of cracks on the glass surface. Gels with characteristic lengths less than 0.3 cm were sintered crack-free, whereas gels with larger cross-sections developed surface cracks, as shown in Fig. 10.

It is important to note that the formation of mi- crostructural gradients is not an innate tendency of silica gel, and may depend on sol composition or the processing conditions of wet and dry gels. Further research is required for elucidation of such phenomena.

Drying and Sintering of Sol-Gel Derived Large SiO2 Monoliths 215

7. Quality of Sol-Gel Derived SiOz Glass

Since the production of sol-gel derived SiO2 glass requires the use of expensive precursor materials, the most likely application of this technique is in the area of expensive, high-quality optical components. These glasses are currently produced by CVD (see, for example, [78-801). Therefore, the quality of sol-gel-derived glass should be at least equivalent to that of CVD glass in order to be a viable alternative.

It can easily be shown that properties such as density, refractive index, and thermal expansion coefficient are similar in sol-gel SiO2 and CVD SiO2. However, for optical applications, the glass should also have high chemical purity, low inclusion content (i.e., gas bubbles and solid particles), and high optical homogene- ity. High transmission over a wide wavelength range, particularly in the infrared and ultra-violet regions, is necessary as well.

Figure II. Ultraviolet optical transmission of an alkoxide derived glass (Type V Silica) compared with two commercml CVD glasses (Type III Silicas). From Hench [83]. Reprinted by permission of John Wiley and Sons Inc.

The CVD process can yield glass with less than l- 2 ppm metallic impurities. This level of purity has been achieved with alkoxide gels [70, 811 and with particulate-alkoxide gels in which TEOS-derived pow- ders were used [62]. Purity is generally lower with particulate gels because of the processing technique [54], or the presence of impurities in the SiO2 powder [54, 621.

Some careful measurements of other glass properties have been conducted [70, 731. They indicate that glasses prepared from alkoxide gels can have low inclusion levels (class 0) and high optical homogene- ity (3 x 1 Op6), comparable to CVD glasses [70]. Their UV cut-off (50% transmission) is about 155-168 nm which is better than some commercial CVD glasses (Fig. 11). Also, commercial high quality optical glasses have poor transmission in either the UV or IR region, as shown in Fig. 12. Sol-gel glass has been produced with complete broad-band transmission (0.17-3.4 pm). The transmission capability over the entire W-Vis-IR spec- trum is one of the primary advantages of sol-gel over CVD. This level of quality was attained through the distillation of precursors, micro-filtration of sols, and handling of precursors and sols in inert atmospheres [70,75]. The properties of glass prepared by the particulate process approach those of alkoxide-derived glass and can be used in the production of reflective optics [54].

z2 ,$ : ,’ I : : :

1 2

I >,’

z I: 50-1 E : 2 l

F. : __ YTCA Silica ’ -- UV Trans. Silica

25- -----. Low OH Silica

200 300 400 2500 3000

Wnvelength (nm)

Figure 12. Opt& transmission of aglass from an alkoxide derived glass (YTCA Silica) compared with two commercml CVD glasses (UV Trans. Silica and low OH Silica). From Murata et al. [75]

1 m in diameter. The production of such large glass pieces by the sol-gel method has not yet been shown.

8. Conclusions

Sol-gel glass products having the above properties A considerable amount of research in the field of are commercially available up to 4 cm in diameter [Sl]. sol-gel science has been devoted to the drying and sin- The CVD technique can produce glasses greater than tering of large silica monoliths. Both particulate and

216 Kirkbir et al.

alkoxide-derived gels have been studied in depth, and both types have their respective advantages and disad- vantages for the production of high-quality glass.

Significant progress has been made in the last few years in the drying of alkoxide gels. The development of controlled-atmosphere drying systems has allowed drying rates to be accelerated, so that large alkoxide gel monoliths can now be dried sufficiently fast for commercial application. Using this type of equipment, it was also discovered that drying is easier for small pore size gels (<40 A).

Because of their narrow pores, the alkoxide gels can be sintered at relatively low temperatures (< 1300°C). For the same reason, problems may arise during pre- ceding chemical and physical heat treatments. The gels may crack during the removal of organic residues at 200-5OO”C, due to heat generation and/or vapor expansion. Hydroxyl groups can effectively be removed only above 500°C. At such temperatures, apprecia- ble shrinkage and pore closure may occur, trapping impurities in the glass.

By comparison, removal of impurities from the large pore size particulate gels is not difficult. However, the large pores also cause their consolidation temperature to be higher than 1400°C. Crystallization, warping and/or sagging of the sintered glass are often encoun- tered at such temperatures. Contaminants associated with the SiO2 particles or introduced during gel processing can also deteriorate the glass quality.

The ability to produce high-quality glass on a laboratory scale has been demonstrated with both alkoxide and particulate gels. Cost competitive scale-up for spe- cific products, with adequate control of glass quality remains to be demonstrated.

References

1 J.B. MacChesney and D.J. DiGiovanni, J. Am. Ceramic Sot. 73, 3537 (1990).

26. P.J. Davis, C.J. Brinker, and D.M. Smith, J. Non-Crystalline Solids 142, 189 (1992).

27. J.K. West, R. Nikles, and G. Lattore, in Better Ceramics Through Chemistry III, edited by C.J. Brinker, D.E. Clark, and D.R. Uhich (Mater, Res. Sot. Symp. Proc., Pittsburgh, PA, 1988). vol. 121, p, 219.

2. C J. Brinker and G.W Scherer, Sol-Gel Science (Academic Press, New York, 1990).

28. G.W. Scherer, J. Non-Cry& Solids 109, 183 (1989). 29. T. Mizuno, H. Nagata, and S. Manabe, J. Non-Cryst. Solids 100,

236 (1988). 3. S. Wang, F. Kirkbir, S. Ray Chaudhuri, and A. Sarkar, in Sol- 30. P.J. Davis, C.J. Brinker, D.M. Smith, and R.A. Assink, J. Non-

Gel Optics, edited by J.D. Mackenzie (SPlE-The International Crystalline Solids 142, 197 (1992). Society for Optical Engineering, Washington, 1992), vol. 1758, 3 1, E.J.A. Pope, Y. Sane, S. Wang, and A. Sarkar, US. Patent 5,023, p. 113. 208 (1991).

4. G.W. Scherer and D.M. Smith, J. Non-Crystalline Solids 189, 197 (1995).

32. M.A. Einarsrud and S. Haereid, International Patent, WO92/ 20623 (1992).

5. A.V. Rao, G.M. Pajonk, and N.N. Parvathy, J. Sol-Gel SIX. Techn. 3,205 (1994).

6. D. Meyers, F. Kirkbir, H. Murata, S. Ray Chaudhuri, and A. Sarkar, to be published (1995).

33. M.A. Einarsrud and S. Hrereid, J. Sol-Gel Sci. Tech. 2, 903 (1994).

34. P.G. Simpkins, D.W. Johnson, and D.A. Fleming, J. Am. Ceram. Sot. 72, 1816 (1989).

7. G.W. Scherer, in Better Ceramics Through Chemistry III, edited by C.J. Brinker, D.E. Klark, and D.R. Ulrich (Mater. Res. Sot. Symp. Proc., Pittsburgh, Pennsylvania, 1988). vol. 121, p. 179.

8. S. Haereid, J. Anderson, M.A. Einarsrud, D.W. Hua, and D.M. Smith, J. Non-Crystalline Solids 185,221 (1995).

9. D. Avnir and V.R. Kaufman, J. Non-Crystalline Solids 92, 180 (1987).

10. S. Luo and K. Tian, J. Non-Crystalline Solids 100,254 (1988). 11. M. Tarasevich, Ceram. Bull 63,500 (1984) (abstract only). 12. N. De La Rosa-Fox, L. Esquivias, and J. Zarzycki, J. Mater. Sci.

L&t. 10, 1237 (1991). 13. J. Zarzycki, in Ultrastructure Processing ofAdvanced Ceramics,

edited by D.R. Uhlmann and D.R. Uhich (Wiley, New York, 1992), p. 135.

14. L. Esquivias and J. Zarzycki, in Ultrastructure Processing of Advunced Ceramics, edited by J.D. Mackenzie and D.R. Ulrich (Wiley, New York, 1988). p. 255.

15. T. Katagiri and T. Maekawa, J. Non-Crystalline Solids 134, 183 (1991).

16. F. Kirkbir, H. Murata, D. Meyers, S. Ray Chaudhuri, and A. Sarkar, J. Non-Crystalline Solids 178,284 (1994).

17. L.C. Klein, T.A. Gallo, and G.J. Garvey, J. Non-Crystalline Solids 63,23 (1984).

18. M. Yamane, S. Aso, and T. Sakaino, J. Mater. Sci. 13,865 (1978). 19. S. Hrereid, M.A. Einarsrud, and G.W. Scherer, J. Sol-Gel Sci.

Tech. 3, 199 (1994). 20. H. Murata, E Kirkbir, D.E. Meyers, S. Ray Chaudhuri, and

A. Sarkar, in Sol-Gel Optics III, edited by J.D. Macken- zie (SPIl-The International Society for Optical Engineering, Washington, 1994). vol. 2288, p. 709.

21. L.L. Hench, F.G. Araujo, J.K. West, andG.P.LaTorre, J. Sol-Gel Sci. and Tech. 2,647 (1994).

22. M. Yamane, in Sol-Gel Technology for Thin Films, Fibers, Pre- forms, Electronics and Speciality Shapes, edited by L.C. Klein (Noyes Publications, Park Ridge, New Jersey, 1988), p. 200.

23. S. Liu and L.L. Hench. in Sol-Gel Optics II, edited by J.D. Mackenzie (SPIE-The International Society for Optical Engi- neering, Washington, 1992) vol. 1758, p. 14.

24. G.W. Scherer, J. Non-Cryst. Solids 100.77 (1988). 25. R.K. Iler, The Chemistry of Silica (John Wiley and Sons, New

York, 1979).


35. J. Zarzycki, M. Prassas, and J. Phalippou, J. Mater. Sci. 17,337l (1982).

36. S. Wallace and L.L. Hench, in Bet&r Cerutnic.~ Through Chem- isfry I, edited by C.J. Brinker, D.E. Clark, and D.R. Uhich (El- sevier Science Publishing, Amsterdam, 1984) vol. 32, p. 47.

37 T. Adachi and S. Sakka, J. Mater. Sci. 22,4407 (1987). 38. I. Artaki, T.W. Zerda, and J. Jonas, Mater. Lett. 3, 493

(1985). 39 G. Orcel, J Phalippou, and L. Hench, J. Non-Crystalline Solids

104, 170 (1988). 40 G Orcel, L.L Hench, I. Artaki, J. Jonas, and T.W. Zerda, J.

Non-Crystalline Solids 105, 223 (1988) 41 T Adachi and S. Sakka, J. Non-Crystalline Solids 99, 118

(1988) 42 B M. Mitsyuk, Z.Z. Vysotskii, and M.V. Polyakov, Kokl. Akad.

Nauk SSSR, 155 [6], 416 (1964) (Eng. Trans.). 43. D.J. Stein, A. Maskara, S. Hrereid, J. Anderson, and D.M.

Smith, in Betfer Ceramics Through Chemistry VI, edited by A.K. Cheetham, C.J. Brinker, M.A. Mecartney, and C. Sanchez (Mat. Res. Sot., Pittsburgh, Pennsylvania, 1994). vol. 346, p. 643.

44. R. Deshpande, D. Hua, D.M. Smith, and C.J. Brinker, J. Non- Crystalline Solids 144, 32 (1992).

45. R. Deshpande, D.M. Smith, and C J. Brmker, U.S. Patent Ap- plication (1992).

46 D.M. Smith, D. Stem, J M. Anderson, and W. Ackerman, J. Non-Crystalline Solids 186, 104 (1995).

47. S.A. Pardenek, J.W. Fleming, and L.C. Klein, m Vltrustructure Processing of Advanced Ceramics, edited by J.D. Mackenzie and D.R. Uhich (Wiley, New York, 1988) p. 379.

48 J. Zarzycki, J. Non-Crystalline Solids 100 359 (1988). 49 G.W. Scherer, in Better Cerumics Through Chemistry I, edited

by C.J. Brinker, D.E. Klark, and D.R. Uhich (Elsevier Science Publishing, Amsterdam, 1984). vol. 32, p. 205.

50. G.W. Scherer and J.C. Luong, J. Non-Crystalline Solids 63, 163 (1984)

51. R Clasen, J. Non-Crystalline Solids 89, 335(1987). 52. R.D. Shoup, in Colloid and Inte$ace Science, edited by M.

Kerker (Academic Press, New York, 1976). vol. 3, p. 63. 53 R.D. Shoup, in Ultrastructure Processing of Advanced Cerum-

ic.r, edited by J.D. Mackenzie and D.R. Ulrich (Wiley, New York, 1988) p. 347.

54. R D Shoup, J. Ceram, Sot. Bull. 70, 1505 (1991) 55. E.M. Rabinovich, D.W. Johnson, J.B. MacChesney, and E.M.

Vogel, J. Am. Ceramic Sot. 66,683 (1983). 56. D.W Johnson, E.M. Rabinovich, J.B. MacChesney, and E.M.

Vogel, J. Am. Ceramic Sot. 66,688 (1983). 57 D.W. Johnson, J B. MacChesney, and E.M. Rabinovich, U.S.

Patent, 4,605,428 (1986). 58. D.W. Johnson, E.M. Rabinovich, D.A. Fleming, and J.B. Mac-

Chesney, J. Mater. Sci. 24, 2214 (1989). 59. E.M. Rabinovich, J.B. MacChesney, D.W. Johnson, J.R. Simp-

son, B.W Meagher, EV. Dimarcello, D.L. Wood, and E.A. Sigety, J Non-Crystalline Solids 63, 155 (1984).

60. E.A. Chandross, D.A. Fleming, D.W. Johnson, J.B. MacChes- ney, and F.W. Walz, U.S. Patent, 5,240,488 (1993).

61. M. Toki, S. Miyashita, T. Takeuchi, S. Kanbe, and A. Kochi, J. Non-tryst. Solids 100,479 (1988).

62. M. Toki, T. Takeuchi, S. Miyasita, and S. Kanabe, J. Mater. Sci. 27.2857 (1992).

63. T. Mori, M. Toki, M. Ikejiri, M. Takei, M. Aoki, S. Uchiyama, and S. Kanbe, J. Non-Cryst. Solids 100,523 (1988).

64. H Okazaki, T. Kitagawa, S. Shibata, and T. Kimura, J. Non- Crystalline Solids 116, 87 (1990).

65. T. Kawaguchi, J. Iura, N. Taneda, H. Hishikura, and Y. Kokubu, J. Non-Cryst. Solids 82,50 (1986).

66. E Kirkbir and S. Ray Chaudhuri, in Sol-Gel Opttcs 11, edited by J.D. Mackenzie @PIE--The International Society for Optical Engineering, Washington, 1992), vol. 1758, p. 160.

67. J. Phalippou, in Chemical Processing of Ceramics, edited by B.I. Lee and E.J.A. Pope (Marcel Dekker, New York, 1994). p. 265.

68. I. Matsuyama, K. Susa, S. Satoh, and T. Suganuma, J. Ceram. Sot. Bull. 63, 1409 (1984).

69. E.M. Rabinovich, D.L. Wood, D.W. Jhonson, D.A. Fleming, SM. Vincent, and J.B. MacChesney, J. Non-Crystalline Solids 82,42 (1986).

70 L.L. Hench and S.H. Wang, Phase Transitions 24,785 (1990). 71. T.E. Elmer, J. Am. Ceramic Sot. 64, 150 (1981). 72. J.B. MacChesney, D.W. Johnson, D.A. Fleming, F.W. Walz, and

T.Y. Kometani, Mat. Res. Bull. 22, 1209 (1987). 73. R.D. Shoup, m Vltrustructure Processing of Advanced Ceram-

ics, edited by D.R. Uhlmann and D.R. Ulrich (Wiley, New York, 1992), p. 291.

74. G. De, D. Kundu, B. Karmakar, and D. Ganguli, J. Mater. Lett. 16,231 (1993).

75. H. Murata, D.E. Meyers, F. Kirkbir, S. Ray Chaudhuri, and A. Sarkar, to be published, in J. Sol-Gel Sci. Tech. (1995).

76. J. Zarzycki, J. Non-Crystalline Solids 48, 105 (1982). 77. J. Zarzycki, in Advances in Ceramics: Nucleation and Crystal-

lization in Glosses, edited by J.H. Simmons, D.R. Uhlmann, and G.H. Beall (Am. Ceram. Sot., Columbus, Ohio, 1982) vol. 4, p. 204.

78. Dynasil (Berlin, New Jersey), sales brochure (1995). 79. Corning Co. (Coming, New York), sales brochure (1995). 80. Heraeus Co. (Duluth, Georgia), sales brochure (1995). 81, Geltech Inc. (Alachua, Florida), sales brochure (1995). 82. L.L. Hench, F.G. Araujo, J.K. West, andG.P. LaTotre, J. Sol-Gel

Sci. Techn. 2,647 (1994). 83. L.L. Hench, in Chemical Processing of Advunced Materials,

edited by L.L. Hench and J.K. West (John Wiley, New York, 1992). p. 875.

84 S. Sakka, H. Kozuka, and S.H. Kim, in Ultrastructure Process- ing of Advanced Ceramics, edited by J.D. Mackenzie and D.R. Ulrich (John Wiley, New York, 1988), p. 159.

85. R.D. Shoup, in Engineered Muterials Handbook, Cerumics and Glasses (ASM International, Ohio, 1991). p. 445.

drying and sintering of sol-gel derived large sio2 monoliths

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