Crystallization of SiO2Al2O3MgO gel glasses

Journal of Non-Crystalline Solids 48 (1982) 205 -217 North-Holland Publishing Company

205

CRYSTALLIZATION OF SiO2-Al203-MgO GEL GLASSES Wolfram H()LAND Otto-Schott-Institut der Sektion Chemie, Friedrich-Schiller-Universit~-Jena, DDR

Emile R. PLUMAT and Paul H. DUVIGNEAUD Universitb libre de Bruxelles, Facultb Sciences appliqubes, Chimie industrielle et Chimie des Solides, Belgium

Basic glasses are prepared by chemical polymerization in a sol-gel process. Nucleation and crystallization of these glasses are analyzed in dependence of the composition of the basic glasses and "additions" of TiO 2 and LiO 2. A comparison of gel glasses with conventionally molten glasses is made. Gel glass-ceramics are prepared as bulk materials and thin coatings.

1. Introduction The first fundamental paper on the formation of glass-ceramics from molten glasses was published by Stookey [1]. He investigated the crystallization of lithium-aluminosilicate glasses and found that the final glass-crystal-product had better properties than the glass. The nucleating substance was TiO 2. This discovery stimulated the determination of new types of glass-ceramics. Today many products like machinable glass-ceramics or products with special electrical properties or high mechanical strength have been developed [2,3]. Glass-ceramics can also be developed from molten SiO2-A1203-MgO glasses [4-7]. These include machinable glass-ceramics with a new type of fluorophlogopite, which are produced by adding alkali- and fluorine ions to the basic ternary glass [4]. Another result has been a combination of magnetic properties with machinability. This glass-ceramic material contains iron oxides. Phlogopite or biotite are needed for the machinability, whereas the spinel is responsible for the magnetic properties [5]. A third application of glass-ceramics from molten glasses are products with high mechanical strength. Additions of TiO 2 to the ternary SiO2-A1203-MgO glass are used, after a thermal treatment of this glass a highly strengthed glass-ceramic with quartz-like crystals and titanates [6,16] or enstatite and titanates is obtained [7]. The latter publications on glass-ceramics with high mechanical strength and the results of Dislish [8], Yoldas [9], Brinker and Mukherjee [10], Carturan and Gottardi [1 l] on gel-glasses have been the basis for the investigations of gel-glasses in the SiO2 -AI203 - M g O system. The main 0022-3093/82/0000-0000/$02.75 © 1982 North-Holland

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W. Ha~and et aL / Crystallization of SiO2-AI203-MgO gel glasses

goal of this paper is the analysis of the crystallization of gel-glasses during the formation of glass-ceramics.

2. Experimental Different gel-glasses have been prepared by chemical polymerization. Four basic glasses of the ternary SiO 2-A1203-MgO system were of interest. Three gel-glasses having the same compositions as those molten glasses obtained by Plumat [6,7,16] were prepared. It was of interest to compare directly the nucleation and crystallization of gel-glasses with those of molten glasses. The three glasses hav e compositions corresponding to the tridymite field [R = (mol% A1203)/(mol% MgO)=0.5], the cordierite field (R =0.71) and the mullite field (R = 1.3). Fig. 1 shows the phase diagram according to Osborn [12]. A fourth composition was chosen with a high SiO 2 content. The composition corresponds to the 2-liquids field of molten glasses (fig. 1); that means, that

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IF. HiJland et al. / Crystallization of SiO2-Al203-MgO gel glasses

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glass formation is not possible by melting. The liquidus temperature is higher than 1700°C. It was thought to be of interest to investigate if glass formation can be obtained by the sol-gel technology and how the nucleation and crystallization of this glass proceeds. g ~ - [oemation

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A sol-gel processing method was apphed, which corresponds to that developed by Dislich [8] and Yoldas [9]. This procedure is very useful for low temperature glass formation in the SiO2-A1203-MgO system. Fig. 2 shows the reaction scheme of gel formation. The network-forming dements were introduced as alkoxides and the network-modifying ions as acetates. The strict control of the added water, temperature treatment between room temperature and 90°C and the reaction time made possible a kinetically controlled reaction [13]. Chemical polymerization took place; it was possible to form a half polymerized liquid from all compositions - independent of additions like Tior Li- components. Thin coatings on ceramics and glass substrates ans a bulk gel sample could be produced from this half polymerized liquid. The results of the crystallization process of the gel glasses described in this paper concern bulk gel glass samples of a few millimeter thickness. The method used to convert gels to gel glasses was a thermal treatment of the gel up to 500-700°C with a heating rate of 1-5 K min- I. Thermal analysis was used to characterize the processes, which took place during the thermal treatment of the gel. X-ray diffraction measurements and electron microscopic inCestigations were performed at room temperature after thermal treatment of the samples.

3. Results and discussion Figs. 3 and 4 show the results of the thermal analysis of a gel and gel glass in stagnant air. Fig. 3 gives the results of the thermogravimetric analysis and

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W. Hi, land et al. / Crystallization of SiO 2 - A I 2 0 3 - M g O gel glasses

T'G-anatysis

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Fig. 3. Thermal analysis of gel and gel glass (400°C/24 h), composition from the cordierite field, TGA and DTGA.

the first derivative curves of T G from a simultaneous D T A - T G analysis. The values for the gel show that there is a weight loss of 32% up to 360°C in one step and a second one up to 700°C. It can be assumed, that the first weight loss is due to the physical desorption of water and that the second is caused by the pyrolysis of residual organic groups. The gel glass was thermally treated at 4 0 0 ° C / 2 4 h and after sorption of water in air also showed a weight loss of surface water. The second weight loss should be a result of the pyrolysis of

DTA

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Fig. 4. As for fig. 3 but DTA.

W.. H6land et al. / Crystallization of SiO2-A1203-MgO gel glasses

209

organic residues, which was not completed during the thermal treatment at 400°C. DTA shows the endothermic reaction of water desorption as the first step and the exothermic combustion of organic residues as a second reaction between 400 and 600°C (fig. 4). Protuberances like little hills are visible on the surface of gel glasses (fig. 5) and make it understandable why the surface is very rough; but no crystals were identified in the glass. Today it is well known that each molten glass tends to phase separate and the nucleation process in glasses can be steered to a special crystallization process (Vogel [14]). The question is: what are the driving forces of the nucleation in the gel glasses? The process of nucleation was investigated in more detail in SiO2-AI203MgO glasses without additions of TiO 2 or Li20. The studied composition was selected in the 2-liquids field. The gel was converted to a bulk glass after thermal treatment of the SiO2-rich gel at 400-700°C. The X-ray diffraction pattern does not include any diffraction peaks from crystals. Moreover, the investigations with the electron microscope at a rough surface (no etching !) do not show any crystals (fig. 6); but phase separation in very little droplets is visible. The electron replica micrographs (figs. 7 and 8) were performed from a fractured surface after etching with H F / H N O 3 for 10 s. The gel was thermally treated at 600°C for 24 h. The glass matrix has the typical microstructure of a highly phase separated glass with very little droplets. But there are also inhomogeneities which could be caused by different compositions of the microheterogenic structure of the glass and may result from the heating of the

Fig. 5. Scanning electron mierograph of a gel glass, composition from the cordierite field. Protuberances like small hills are visible after thermal treatment at 400°C for 24 h.

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W. H61and et al. / Crystallization of Si02-AI203-MgO gel glasses

Fig. 6. Electron replica micrograph from a fracture surface of a gel glass, composition from the 2 liquids field. Phase separation takes place after thermal treatment at 700°C for 24 h.

Fig. 7. Electron replica micrograph from a fracture surface of gel glasses, composition from the 2 liquids field, after etching with H F / H N O 3. Thermal treatment: 600°C for 24 h. Microheterogenic droplets are growing and inhomogeneities are visible.

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211

Fig. 8. As for fig. 7 but thermal treatment: 700°C for 24 h.

gel to a bulk glass. This process should be expected to proceed between 400 to 700°C. Fig. 8 shows an electron replica micrograph from a fractured surface, which was etched with H F / H N O 3 for 10 s; its thermal treatment was carried out at 100 K more than the previous one. The microheterogenic droplets grow and also have ranges of different solubility. The crystallization process was initiated during a thermal treatment of the same gel glass at 1000°C for 5 h. The result is shown in fig. 9. The scanning electron micrograph of this gel glass-ceramic has been obtained directly from the surface (no fracture) after etching with HF. High quartz mixed crystals can be identified by X-ray diffraction. The little particles in fig. 9 are interpretated as high quartz mixed crystals, which are distributed all over the glass volume. Some residues of glass are also visible, but it can be seen that the crystals are situated below the glass. Thus, the glass does not isolate the crystals and the glass is in this case really an "unreacted" residue on the surface of the sample. The nucleation and crystallization behaviour of this glass can be summarized: three facts should be pointed out: (1) it is possible to extend the glass forming region by using a sol-gel processing method of glass formation (this corresponds to the results of Yoldas [9] in binary systems). A new definition of the glassy state would be interesting: how to describe the glass-like state thermodynamically. (2) Phase separation takes place as the most important nucleating step during the formation of glass ceramics. The formation of little droplets

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IV.. H61and et al. / Crystallization of SiO 2 - A 1 2 0 s - M g O gel glasses

Fig. 9. Scanning electron micrograph of a gel glass-ceramic, composition from the 2 liquids field, thermal treatment: 1000°C for 5 h. High-quartz mixed crystals are precipitated.

contributes to the enrichment of Mg 2+ and A13+ ions in the nucleation process. High-quartz mixed crystals (containing Mg 2+/AI 3+ ions) grow within the droplets. These crystals are precipitated at 1000°C and are stable at room temperature, because of their Mg 2+ content. The high quartz mixed crystals decompose to low-quartz mixed crystals, cristobalite and enstatite at 1100°C (fig. 10). (3) Fig. 10 also presents a third phenomenon. If Ti components are added, anosovite mixed crystal grows as a primary phase. The low quartz mixed crystals precipitate in the glass in a second step. By comparing the nucleation and crystallization of this glass with those of the ternary glass, it can be shown that the titanates do not influence the crystallization of quartz crystals as kinetic catalysts. Growth of titanates and crystallization of quartz crystals are two independent parallel reactions. Nucleation and crystallization of gel glasses with the compositions corresponding to the tridymite, cordierite or mullite fields are of complex nature. Some parallel solid state reactions take place. The crystallization of glasses with compositions from the tridymite and cordierite fields are similar. Fig. 11 shows the crystallization flux of glasses with R = 0.71 and additions of "TiO2" and "Li20". Spodumene and eucryptite crystallize as the main crystal phases nearly independent of the temperature of thermal treatment. SiO2-rich petalite-like crystals could be determined as the primary phases in the ternary glasses of compositions in the tridymite or cordierite field or with additions of "TiO2". Similar crystals have been de-

W. H~land et a L / Crystallization of SiO2-Al2Os-MgO gel glasses

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W. H61and et al. / Crystallization of Si02-AI203-MgO gel glasses

Fig. 12. Petalite-like and high quartz mixed crystals in a gel-glass-ceramic (cordierite field), scanning electron micrograph after etching.

scribed in molten glasses (Schreyer [15]): LiA1Si4Olo ~- MgAlzSi8020 petalitelike phase. Figs. 12 and 13 show petalite-like crystals in scanning electron micrographs

Fig. 13. As for fig. 12 but higher magnification.

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of a glass-ceramic with the composition from the cordierite field. The interpretation of the micrograph is possible in relationship with the results of X-ray diffraction. The small petalite-like crystals are stable and the first precipitated quartz crystals are attacked by HF. The primary crystal phase is always high quartz, if the concentration of A1203 becomes higher in the ternary glass; i.e. a composition from the mullite field with R = 1.3 (fig. 14). Petalite like crystals do not appear and the titanates change when the temperature of Al-titanates increases. Cordierite is a crystal phase known to grow in surface regions. In the study it was found that the cordierite only precipitates in the absence of TiO 2. The coriderite precipitation is one of the differences which was observed between the crystallization of molten glasses of the same composition. The reaction scheme shown in fig. 15 is the crystallization process of molten glasses with the composition of the mullite field. The modification from high to low quartz takes place at lower temperatures than in gel glasses and also the decomposition to titanates is already finished at 1100°C. These differences between the crystallization of the molten and gel glasses are caused by different oxidation and reduction potentials (Ti III-Ti IV ions) and the different possibilities of surface crystallization. It is of interest to discuss the properties as a function of the crystalline phases in the glass ceramics obtained from molten glasses. The highest mechanical strength could be obtained by precipitation of low quartz and titanates as the main crystal phases in the glass ceramic [16]. Therefore, gel glass ceramics with similar crystalline phases may have an application as thin coatings on substrates with special mechanical properties [13].

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W. H61and et al. / Crystallization of SiO2-AI203-MgO gel glasses

217

All the crystallization reactions of the main crystal phases are demonstrated in fig. 16. Most important are the possibilities of some combinations of crystal phases in one gel glass-ceramic.

References [!] S.D. Stookey, Iad. Eng. Chem. 51 (1959) 805. [2] G.H. Beall, M.R. Montiert and G.P. Smith, Glas-Email-Keramo-Technik 11 (1971) 409. [3] K. Chyung, G.H. Beall and D.G. Grossman, Tenth Int. Congress on Glass, Kyoto, 1974; Ceram. Soc. Japan (1974) 14/33. [4] W. H61and, K. Naumann, H.-G. Seifert and W. Vogel, Zeitschr. Chem. 21 (1981) 108. [5] W. Hi)land, N.A. Dung, E. Heidenreich, E. Tkalcec and W. Vogel, Glastechn. Ber. 55 (1982) in press. [6] E. Heidenreich, F.D. Doenitz, H. Erxleben, G. Metz, R. Ehrt and W. Vogel, Wiss. Zeitschr. der FSU Jena, math. nat. Reihe 28 (1979) 425. [7] E. Plumat, 73rd Annual Meeting of the Am. Ceram. Soc., Chicago (1971). [8] H. Dislich, Glastechn. Ber. 44 (1971) 1. [9] B.E. Yoldas, J. Non-Crystalline Solids 38-39 (1980) 81. [10] C.J. Brinker and S.P. Mukherjee, J. Mat. Sci. 16 (1981) 1980. [11] G. Carturan, V. Gottardi and M. Graziani, J. Non-Crystalline Solids 29 (1978) 41. [12] E.M. Levin, C.R. Robbins and H.F. McMurdie, Am. Ceram. Soc. 4 (1964) 246. [13] E. Plumat, W. H/51and and P.H. Duvigneaud, to be published. [14] W. Vogel, Glaschemie VEB Verlag for Grundstoffindustrie, Leipzig (1979). [15] W. Schreyer and J.F. Schairer, Am. Min. 47 (1962) 90. [16] W. H61and, P. Wange, G. Carl, W. Vogel, E. Heidenreich and H. Erxleben, to be published.