Preparation of Na2OSiO2 glasses in the metastable immiscibility region

Journal of Non-Crystalline Solids 82 (1986) 177-182 North-Holland, Amsterdam

177

PREPARATION OF N a z O - S i O 2 G L A S S E S IN T H E M E T A S T A B L E IMMISCIBILITY REGION A. Y A S U M O R I , S. I N O U E and M. Y A M A N E

Department of Inorganic Materials, Facultyof Engineering, Tokyo Institute of Technology,Japan

Preparation of Na20-SiO 2 glasses in the metastable immiscibility region has been examined by means of the sol-gel method. Gels of the binary system Na20-SiO 2 of composition 10 Na 20-90 SiO2 mol.% were obtained by the hydrolysis of tetramethylorthosilicate with an aqueous solution containing NaNO3, NaOCH3, CH3COONa. The experiment was carried out mainly on the dense and transparent gel of composition 10 Na20-90 SiO2 tool.% obtained by using CH3COONa. CH3COO- groups contained in the gel decomposed completely at 460°C, about 50°C below the Tgof 10 Na20-90 SiO 2 glass by the conventional method. The surface area of the gel decreased with heat-treatment at this temperature. The gel, however, became opaque while being held at the glass transition temperature of conventional glass. This opaqueness was considered to be caused by phase separation in the gel.

I. Introduction Glasses of the N a 2 0 - S i O 2 binary system containing less than 20 mol.% of N a 2 0 is k n o w n to show phase separation. Application of the sol-gel process in preparing single phase glasses of this system by avoiding the phase separation [1] is a subject of interest. There is, however, only one paper that reports the synthesis of glass-like material in the metastable immiscibility region [2] a m o n g the reports on this system [3-5]. So, it is still uncertain whether the preparation of a single phase glass by the sol-gel process is really possible or not. In this study, the preparation of a glass (composition 10 N a 2 0 - 9 0 SiO 2 mol.%) by the sol-gel process has been tried in order to find the answer to the above question. The changes in properties of alkoxy-derived gels in the heating process were investigated by small angle X-ray scattering (SAXS), infrared spectroscopy (IR) and measurement of specific surface area (Sg). The results were c o m p a r e d with the p h e n o m e n a observed in the development of phase separation in glasses prepared by the conventional melting method.

2. Experiment 2.1. Preparation of gels After a preliminary study on gel preparation using tetramethylorthosilicate, ( T M O S or Si(OCH3)4) and various salts such as sodium acetate, sodium 0022-3093/86/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

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nitrate, and sodium methoxide, sodium acetate (NaOAc or CH3COONa ) was selected as the source of Na20. NaOAc dissolved in an aqueous solution of acetic acid with pH = 2 was mixed under stirring with TMOS and methanol (MeOH). The molar ratio among TMOS, MeOH and water was 1 : 5 : 4 . The mixture was placed in a cylindrical glass container and covered with aluminum foil to control the evaporation of the solvent, followed by aging at room temperature. The gel thus obtained was dried completely at 250°C before being subjected to heat treatment for densification. 2.2. H e a t treatment

The gels were heated at a rate of 20°C h - 1 up to 360°C, the first exothermic peak of the DTA trace, and held there for 12 h. The samples were again heated at l l ° C h -~ up to 460°C, the second peak of the DTA curve, and were held at that temperature for 24 h before the final treatment at 515°C for various periods of time. In order to promote the decomposition of residual organic compounds, oxygen gas was fed into the furnace up to 515°C, and after that, helium gas was fed in to remove the H 2 0 or O H - radical from the gel.

3. Results The gel was transparent up to 460°C but turned translucent while being held at 515°C. Change in Ss of the gel decreased from an original value of 160 m2/g to 50 m2/g by heating up to 515°C and the subsequent treatment at that temperature as shown in fig. 1. IR spectra of the gel heated to various temperatures are shown in fig. 2. It is evident that the absorptions at 1580 cm-1 and 1420 cm-1, both of which are assigned to the carboxylic acid salts, - C O O - group, decreased markedly by the treatment up to 360°C for 8 h, and almost disappered at 460°C for 0 h. The spectrum of the sample heated at 515°C for 48 h was similar to that of 10 N a 2 0 - 9 0 SiO 2 glass by the conventional method, except for the very weak absorPtion due to the CO 2- group. The SAXS intensity of the gels after various heat-treatments are plotted against the scattering vector, S ( = 4~r sin(e/2)/)~), in fig. 3. The changes in SAXS intensity for conventional glass are also shown in fig. 4. In both gel and glass, the SAXS intensity and slope in the Guinier region (log S < - 1.7) which is represented by the radius of gyration, Rg, in the figures, increased with heat treatment, showing the increase in number and the growth in size of the scattering particles. The increase in the fractal dimension, the slope D in the Porod region (log S > - 1.7), is also observed for both of the samples, indicating that the density of scattering particles also increases with heat treatment [6]. This is attributed to the development of phase separation in both samples.

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Fig. 5. TEM of the gels: (a) the dry gel before pyrolysis: (b) the gel heated at 515°C for 48 h: (c) the gel boiled in distilled water for 4 h. The rate of increase in the conventional glass, however, was much smaller than that of gels. Typical examples of the transmission electron micrographs of the gels are shown in figs. 5a-c. The dry gel before pyrolysis (fig. 5a) consists of oval particles of about 100-200 A in size. The micrograph of the gel heated at 515°C for 48 h, which was translucent and had small surface area, did not show any distinct particles or micropores, as is known from fig. 5b. When the same sample had been boiled in distilled water for 4 h, however, its micrograph (fig. 5c) again showed particles of about 100-200 ,~. The existence of inhomogeneity causing nonuniform solubility in water is due to phase separation as was suggested from SAXS results.

4. Discussion It is known from the above results that the decomposition of N a O A c in the gel concerned in the present study was completed by heat treatment up to 460°C. But the gel remained porous until most of the micropores collapsed at 515°C. The heat treatment to promote the micropore collapse, on the other hand, caused the gel to phase separate. And moreover, the rate of increase in SAXS intensity in the gel was much larger than that observed in the phase separation,of a conventional glass of similar composition. This suggests that the elimination of phase separation which has been expected to a sol-gel process is not so easy, though it might not be impossible. N a O A c was selected as the raw material among those tested in the preliminary study for the reason that it gave a homogeneous clear gel of low porosity and was expected to be suitable to densify. It is possible, however. that Na + ions introduced by N a O A c were not taken in the network structure

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of S i - O - S i bonds of primary particles of the gel due to the incompatibility of acetate radicals, but remained on the particle surface when the gel was formed. Na ÷ ions which we hypothesize to have remained on the particle surface even after the decomposition of NaOAc, might be excluded again from the newly developed network of S i - O - S i through the dehydration polycondensation of silanols. Thus, Na ÷ ions would be gradually concentrated in the consolidated particles during the gel to glass conversion. In other words, the microheterogeneity might already have existed in the gel-derived glass just formed by the collapse of micropores. Since Na ÷ ions in the interface can diffuse much more easily than those in the potential well of the S i - O - S i network of conventional glasses, the rate of growth of microheterogeneity in gel-derived glass was larger than that in the melt-formed glass. This explanation of our failure to eliminate the phase separation is not inconsistent with the fact that a transparent pore free non-crystalline solid can be prepared in the stable immiscibility region of the SrO-SiO 2 system [7]. Both the segregation of the components via diffusion of mobile ions and the collapse of the micropore occur in parallel throughout the densification process. If the former proceeds much faster than the latter, the glass will have two phase structure. If the collapse of micropore proceeds much faster than the segregation, on the contrary, the resulting glass will have the appearance of single phase structure, in spite of the fact that there is a microheterogeneity of the order of several to several tens of angstroms. On the basis of the above considerations, it may be concluded that the preparation of a gel with uniform distribution of modifier cations throughout the S i - O - S i network within the primary particles of the gel is essential in order to obtain a single phase glass in a metastable region by the sol-gel method.

Reference [1] J.D. Mackenzie,J. Non-Cryst. Solids 48 (1982) 1. [2] L.L. Hench, J. Non-Cryst. Solids 53 (1982) 183. [3] J. Zarzyckiet al., J. Non-Cryst. Solids 48 (1982) 79. [4] L.L. Hench et al., J. Non-Cryst. Solids 65 (1984) 375. [5] S.H. Wang and L.L. Hench, Better Ceramics Through Chemistry, eds., C.J. Brinker, D.E. Clark, D.R. Ulrich (North-Holland,Amsterdam, 1984) p. 71. [6] D.W. Shaeferand K.D. Keefer, ibid., p. 1. [7] M. Yamane and T. Kojima,J. Non-Cryst.Solids 44 (1981) 181.