Titania-silica glasses prepared by sintering alkoxide derived spherical colloids

Titania-silica glasses prepared by sintering alkoxide derived spherical colloids

Journal of Non-Crystalline Solids 108 (1989) 163-168 North-Holland, A m s t e r d a m 163 TITANIA-SILICA GLASSES PREPARED BY SINTERING ALKOXIDE DERI...

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Journal of Non-Crystalline Solids 108 (1989) 163-168 North-Holland, A m s t e r d a m

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TITANIA-SILICA GLASSES PREPARED BY SINTERING ALKOXIDE DERIVED SPHERICAL COLLOIDS W.T. M I N E H A N , G.L. MESSING and C.G. PANTANO Department of Materials Science and Engineering, The Pennsylvania State University, University Park, PA 16802, USA Received 12 August 1988 Revised manuscript received 3 November 1988

Titania-silica glasses of 3.1 to 8.5 wt% titania were prepared by sintering alkoxide synthesized colloidal powders. The process capitalizes on the advantages of sol-gel chemistry for powder synthesis while circumventing the problem of drying gels. Green bodies formed by colloidal gelation, uniaxial dry pressing and tape casting were sintered from 1200 to 1500 ° C to yield crystallite-free glasses. Complete solution of up to 8.5 wt% titania was inferred from T E M and F T I R analysis. Thermal expansion coefficients ranged from 2.6 × 1 0 - 7 to - 0.38 × 1 0 - 7 / o C between 25 and 700 o C for glasses containing 3.1 to 8.5% TiO2, respectively.

1. Introduction

Vitreous silica doped with less than 10 wt% titania has a coefficient of thermal expansion lower than that of fused silica [1]. Although the physical properties of the titania-silica glasses such as the thermal expansion coefficients, FTIR and Raman spectra are well documented, processing methods required to achieve and maintain an atomic distribution of titanium have been little investigated until recently. Conventional melt technology is limited for these glass compositions by the high melting temperature, high melt viscosity and phase separation of titania-rich phases at elevated temperature [2]. Clear titania-silica glass of 7.4 wt% TiO 2 (ULE®-code 7971) has been commercially produced by Coming Glass Works for over 20 years by a chemical vapor deposition (CVD) process [1,3]. Although this method produces clear glasses containing as much as 16 wt% titania, deposition rates are relatively slow and shaped glass articles must be machined from the bulk glasses. Recently, sol-gel processes have been investigated for synthesis of TiO2-SiO 2 glasses [4-10]. Polymeric gels offer advantages of molecular mixing at room temperature and extremely high pur0022-3093/89/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

ity relative to conventional processes. Furthermore, the alkoxide derived gels densify at temperatures far below melting temperatures, thereby reducing the tendency for phase separation and crystallization. However, sample sizes and drying times for these polymeric gels are limited without the use of recently developed processes. Deng et al. [10] recently reported a colloidal sol-gel approach to TiO2-SiO 2 glass formation whereby colloidal fumed silica was dispersed in an aqueous solution of hydrolyzed titanium alkoxide and allowed to gel in 2-4 days. After a careful drying procedure, the gels were fired at 1450-1500 ° C to produce transparent microstructure free glasses. Their glasses had thermal expansion properties similar to Corning's ULE ® glass, and small disks and rods could be consistently produced. They reported that drying was the limiting parameter in the production of larger glass samples. An alternative method for producing monolithic glass is to synthesize titania-silica powders from organic precursors and use conventional powder fabrication processes such as slip casting, tape casting and uniaxial dry pressing to form green compacts. In this spirit, Sacks et al. [11] demonstated that filter-pressed compacts of col-

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loidal silica powder, produced by the St~Sber, Fink and Bohn process [12], could be sintered to full density. Furthermore, metal alkoxides have been shown to be well suited as precursors for synthesis of monosized particles of m a n y other inorganic compositions including TiO2, ZnO, ZrO2, HfO2, and boron doped SiO 2 [13,14]. In this paper we report a process whereby spherical TiO2-SiO 2 colloids were synthesized from metal alkoxides and then consolidated into compacts using conventional ceramic forming operations such as colloidal gelation, dry pressing and tape casting. The properties of TiO2-SiO 2 glasses obtained by sintering these compacts are reported and compared to U L E ®.

2. Powder synthesis Colloidal titania-silica powders were synthesized [15] by mixing a 1 : 1 molar ratio of deionized water and tetraethoxysilane (TEOS) in pure ethanol. The solution also contained 0.003 mol/1 HC1 to catalyze hydrolysis. The system was refluxed for 90 min at 5 0 ° C to ensure the complete reaction of TEOS with water. Titanium isopropoxide was added to the sol in a dry box to avoid hydrolysis of the titanium alkoxide by atmospheric moisture. The TiO2-SiO 2 solution was refluxed at 5 0 ° C for 5 h to allow the titanium alkoxide to react with the partially hydrolysed TEOS. This mixture was then mixed in a 1 : 2 volumetric ratio with a 0.69 M N H a O H solution. Precipitation occurs spontaneously to produce a 2.7 wt% powder dispersion resembling milk. For the experiments reported here the titania content of the powders was varied between 3.1 and 11.5 wt% on an oxide basis. Following precipitation colloid compacts were produced by evaporation of the solvents below their boiling temperature under constant agitation until the dispersion showed a tendency towards gelation. This concentration is estimated to be about 18 vol% powder (33 wt%). The gel powder compacts were dried in air at 25 ° C for less than one week and then dried at 70 ° C for an hour. Powders used for dry pressing and tape casting were produced by ball milling dried colloid compacts in trichlorotrifluoroethane for 24 h to avoid

water contamination and sieving to < 200 ~m. Powders were uniaxially pressed without binder addition into 2.54 cm diameter pellets at 34.4 MPa. Tape-cast samples were prepared by dispersing the gel powder in a solution of trichloroethane, methyl ethyl ketone and an acrylate binder sytstem. Tapes were cast 0.028 cm thick, dried and punched i n t o - 5 . 0 8 cm square sheets. Ten sheets were uniaxially pressed at 50 MPa to obtain 0.26 cm thick laminate blocks. The organic binder and residual organics from synthesis were removed by heating to 600 o C in 10 h. These compacts were heated in air at rates of up to 40 o C / m i n to temperatures between 1200 o and 1530 ° C to complete the sintering cycle. Most sintering was done in air, with the exception of a few of the colloid compacts which were sintered in helium from 600 to 1200°C at 1 0 ° C / m i n and held for 1 h. These samples were reheated in air at 1440°C for 1 min to reoxidize the titania and to remove residual porosity. Green densities were estimated from sample dimensions after calcination at 600 ° C. Densities of sintered glasses were obtained by Archimedes technique. Characterization included transmission electron microscopy (TEM), scanning electron microscopy (SEM), X-ray diffraction, Fourier transform infrared (FTIR) spectroscopy and microprobe analysis. Most T E M samples were prepared by dipping a 300 mesh carbon-coated copper grid into an ethanol diluted powder dispersion. Thermal expansion was measured between 25 and 7 0 0 ° C by differential dilatometry using U L E * glass as the reference [16]. These glasses were then ground and dispersed in acetone or ethanol for subsequent examination by TEM. F T I R spectroscopy was performed on glasses and powders ground to-200 mesh, dispersed in KBr and pressed into pellets. The average glass composition and large scale chemical homogeneity were measured by electron microprobe analysis.

3. Results and discussion 3.1. Powder and green body characteristics F r o m the T E M micrograph in fig. 1 the powders are characterized by a porous interior with an

w.T. Minehan et al. / Titania-silica glasses

ultrafine gel-like infrastructure, coated with a very thin layer of relatively dense material on the surface as suggested b y the dark outline surrounding each particle. The particles have a fairly large particle size distribution ranging from 0.02 to 0.7 /~m and an average size of about 0.3 btm. The skeletal density of the gel network was 2.02 g / c m 3 for a 7.3% TiO 2 gel powder after it was heated at 500 o C. F r o m X-ray and electron diffraction no crystalline phases were detected in the powder. The BET surface area of the 8.0% TiO 2 powder was 275 m 2 / g when heated at 5 0 0 ° C . These physical characteristics are strongly dependent on the reaction conditions during formation, including the relative concentrations of TEOS, water, titanium isopropoxide, m e t h o d of mixing and the p H of water used during final hydrolysis. A detailed account of these effects will be published later [15]. The colloid compacts shrink about 7 to 8 linear percent between gelation and complete drying, resulting in a m a x i m u m dried green density o f 25%. Some large-scale cracking occurs during drying leading to m a x i m u m crack-free gel sizes of

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Fig. 2. Glasses obtained by sintering colloid compacts at 1410°C in air (7.2% titania, center) and in helium (7.1% titania, left; and 8.0% titania, right) followed by annealing in air at 1430 o C.

about 4.25 x 4.25 × 0.3 cm 3. Pressed pellets and laminated tapes had bulk densities of 33 and 36%, respectively, and were characterized with a microstructure consisting of < 7/~m diameter colloidal gel agglomerates. 3.2. Glass densification

The colloidal gels shrink by - 40 linear percent and sinter to > 99.5% of theoretical density when

Fig. 1. Typical titania-silica particles (6.3% TiO2) synthesized from metal alkoxides.

Fig. 3. Tape cast (square) and pressed samples (disks) yield translucent 7.3% titania silica glass when sintered at 1500 o C in air (right).

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Fig. 4. T E M micrographs and selected area diffraction of (a) 7.4% and (b) 11.1% titania glasses (bar = 50 nm).

heated in air at 4 0 ° C / m i n to 1200°C and held for 30 min. At this stage the glass is slightly cloudy due to the presence o f - 200 /zm pores. Rapid heating of the gels to 1410 o C at 40 o C / m i n with dwell times less than 3 min reduces some of this porosity as shown in fig. 2. Longer times at temperatures greater than 1400°C result in pore expansion due to the softening of the glass, and decreased density and optical transmittance of the glass. The remaining pores are difficult to remove presumably because they are filled with nitrogen or oxygen, which does not readily diffuse through the glass matrix.

To reduce this problem, some samples were sintered in helium to closed porosity. The helium filled pores could then be eliminated due to the relatively higher diffusivity of helium in the glass matrix. At this stage the glass has a slightly bluish tint that is attributed to the presence of Ti 3+ that arises from reduction during He sintering [1]. A 2 min oxidizing reheat at 1430 o C removes the bluish tint as a result of the oxidation of any reduced titania and also allows helium to diffuse out of the large pores. Examples of the improved clarity are shown in fig. 2. The densities of the clearest glasses (colloid compacts sintered in He, thinned

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and polished) were 2.205 and 2.207 g / c m 3 for 7.1% TiO 2 and 8.0% TiO 2 glasses, respectively. These values are comparable to the measured density of Corning's U L E ® glass of 2.198 g / c m 3. Although the colloidal gels densified to clear glass in air, the complete densification of powders pressed into pellets and cast as tapes was more difficult due to powder agglomeration during drying. Nevertheless, > 98% density was achieved under conditions similar to those used for the colloid compacts and samples became more transparent with longer times at > 1400 o C as shown in fig. 3. Sintering temperatures of 1400 ° C and above also caused slumping due to the lowering viscosity of the glass. Better glasses should result when techniques are developed for dispersing the particles during powder drying and shape forming. 3. 3. Glass characterization

TEM analysis of sintered glasses revealed that glasses with < 8.5 wt% titania were microstructure- and crystallite-free (fig. 4(a)). Occasional crystallites were observed in the 8.5 wt% glass, but their phase composition could not be identified by electron diffraction because of their scarcity and size. When the titanium content in the glass was increased to 11.5%, the amount of crystallite formation increased significantly (fig. 4(b)). Separate areas of rutile and anatase crystallites were identified by electron diffraction, but their individual formation has not been linked to specific thermal conditions at this time. Similar observations of metastable anatase crystallite formation from titania-silica gels has been cited elsewhere [8,9]. The thermal expansion of glasses produced from colloid compacts sintered in air are shown in fig. 5, along with the U L E ® glass. The thermal expansion curves show a slightly higher expansion coefficient than Corning's ULE * glass below 300 o C, but a value closer to zero between 300-700°C. The effect of increased titania doping on the thermal expansion is clearly exhibited by this set of curves. It must be noted that the slight phase separation of the 8.5 wt% TiO 2 glass may have modified the expansion properties of that sample to some degree. The glass with 11.5 wt% TiO 2 undergoes a larger amount of crystallization and

Thermal Expansion of Titania-Silica Glasses 20O

•~

I00

R

-10 0

200

400

600

800

Temperature (°C) Fig. 5. Thermal expansion curves of titania-silica glasses prepared by sintering colloid compacts.

phase separation and had a thermal expansion similar to the 3.1% glass, but was excluded from fig. 5. The average glass composition was measured using electron microprobe analysis with a probe beam diameter of 5 /~m and compositions were measured in 5 # m steps across the glass sample as described by Deng et al. [10]. Compositions of 7.99 and 7.93% titania with standard deviations of 0.18 and 0.11, respectively were obtained from 25 measurements on two glasses estimated to contain 8.0% titania from molar ratios of alkoxides. Furthermore, under the stated experimental conditions, the microprobe analysis did not detect any difference in the compositional homogeneity between glasses produced in this study and glasses produced by CVD or sol-gel methods [10]. Finally, glasses and powders were examined using F T I R to compare the molecular structure of the gel powders and glass with previously reported data. Figure 6 compares infrared transmittance patterns of 7.1% titania powders and the colloidal compact-derived glass to a pattern obtained from Corning's ULE ® glass. The gel powder shows an absorption band at 3660 cm-1 due to the stretching of S i - O H bonds. The absence of this band in the densified glasses indicates that O H groups adsorbed on the surface of colloidal particles are significantly reduced before densification to the final glass product. Absorption bands characteristic of pure silica at 1100-1200, 800 and 460 cm-1 are also present in these titania-silica powders and glasses. An additional absorption band occurring in the 920-960 cm-1 range is also present.

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W.T. Minehan et al. / Titania-silica glasses I

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median of 0.3 /~m. FTIR demonstrated that titanium was tetrahedrally coordinated in both the gel and glass structures. Microstructure- and crystallite-free titaniasilica glass containing up to 8.5 wt% titania was produced by sintering the alkoxide derived powder. The powders were also processed through more traditional shape-forming operations such as dry pressing and tape casting, thereby providing evidence of the excellent potential of this powder to produce shaped titania-silica glass articles. Characterization of the sintered glasses showed that the chemical homogeneity, amorphous structure, densities, and thermal expansion coefficients are all comparable to Corning's ULE ® glass. The authors gratefully acknowledge the Air Force Weapons Lab (F2960-85-c-0056) for financial support.

4000 5500 5000 2500 2000 1500 1000 500 WAVENUMBERS (cm-I) Fig. 6. Fourier-transform infrared (FTIR) absorption spectra of 7.1% titania-silica powder and sintered colloid compact derived glass compared to the CVD processed glass (Corning's ULE®).

This band has previously been reported to be due to the rigid cage mode of titania tetrahedra in the structure [3,8]. The presence of this peak in the gel powder demonstrates that at least some of the titanium is tetrahedrally coordinated before densification. However, this peak is slightly more intense for densified powders, showing that titanium remains in tetrahedral coordination during heat treatment, resulting in the characteristic low thermal expansion properties of this composition.

4. Conclusions

Amorphous titania-silica spherical powders were synthesized by hydrolysis and condensation of metal alkoxides with ammoniated water. Particles were spherical and had a porous interior and a thin dense layer coating the particle. The powder had a surface area of 275 mZ/g, and the particle size distribution ranged from 0.02 to 0.7/tm with a

References [1] P.C. Schultz and H.T. Smyth, in: Amorphous Materials, eds. R.W. Douglas and B. Ellis (Wiley, London, 1970) p. 453. [2] D.G. Ostrizhko and G.A. Pavlova, Neorg. Mater. 6 (1970) 74. [3] P.C. Schultz, J. Amer. Ceram. Soc. 59 (1976) 214. [4] K. Kamiya and S. Sakka, J. Mater. Sci. 15 (1980) 2937. [5] C.J.R. Gonzalez-Oliver, P.F. James and H. Rawson, J. Non-Cryst. Solids 48 (1982) 129. [6] B.E. Yoldas, J. Non-Cryst. Solids 38&39 (1980) 81. [7] M. Yamane, S. Inoue and K. Nakazawa, J. Non-Cryst. Solids 48 (1982) 153. [8] R. Jabra, J. Phalippou and J. Zarzycki, J. Non-Cryst. Solids 42 (1980) 489. [9] C.P. Scherer and C.G. Pantano, J. Non-Cryst. Solids 82 (1986) 246. [10] Z. Deng, E. Breval and C.G. Pantano, J. Non-Cryst. Solids 100 (1988) 364. [11] M.D. Sacks and T. Tseng, J. Amer. Ceram. Soc. 67 (1984) 526. [12] W. StSber, A. Fink and E. Bohn, J. Coll. Interf. Sci. 26 (1968) 62. [13] K.S. Mazdiyasni, Ceram. Intern. 8 (1982) 42. [14] B. Fegley and E.A. Barringer, in: Better Ceramics Through Chemistry, eds. C.J. Brinker, D.E. Clark and D.R. Ulrich (North-Holland, New York, 1984) p. 187. [15] W.T. Minehan and G.L. Messing, to be published. [16] W.A. Plummer (Coming, NY) and K. Raffalski (Theta, Port Washington, NY), in Theta-Operating InstructionsExpansion Reference Manual (1975) personal correspondence.