Sintering-viscosity relation for mixed-alkali glass powder compacts
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The sintering of glass powder compacts was studied by dilatometer in the (25 - x) Na 2 O-xK 2 O-75SiO2 glass system. The dilatometrically determined sintering temperature (r sin ) at constant heating rate decreases and the shrinkage at isothermal sintering increases when Na 2 O is replaced by K 2 O up to the mole fraction of K 2 0 / ( N a 2 0 + K2O) = 0.5. This is due to the decrease in viscosity and means that sintering can be possibly accelerated by introduction of mixed alkali oxides in this system. According to calculation using the VFT-equation, the viscosity value at r sin is almost independent of glass composition. From these results it may be supposed that the dilatometrically determined sintering begin temperature (Tsia) can be a characteristic point for the sintering of glass.
I. INTRODUCTION In the last several years the preparation of glass by sintering has received attention because of its advantages compared with the conventional melting technology. The sintering takes place at considerably lower temperature than melting1 and gives a possibility for the manufacture of glass phase contained composites2-3 or porous sintered glasses and glass ceramics for the biotechnology.4 According to sintering theories,5'6 the sintering of glass powders and amorphous powders proceeds by viscous flow; this has been already experimentally supported by many works.7"10 The viscosity is thus the decisive factor for sintering of glass. However, there is no denned viscosity fixed point for sintering of glass. According to recent studies10"12 on the relationship between liquidus line and sintering temperature of glass in binary eutectic systems, the socalled minimal sintering temperature of glass powder compacts decreases with a decrease of their liquidus temperature and lies always between transformation and the softening point of glass. On the basis of these results, the concept of sintering aid material in glasses was introduced. A sintering aid material must be selected as a constituent of glass which improves the sintering; in other words, it decreases the viscosity without bad effects on the desired properties of glass. From the viewpoint of the sintering aid, mixed alkali glasses exhibit interesting characteristics. At an intermediate mixed-alkali composition, the glasses show not only a viscosity minimum, particularly in the silicate systems13"15 containing com-
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Present address: Samsung-Corning Co., R&D Center, 445-970 Suwon, Korea.
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J. Mater. Res., Vol. 10, No. 7, Jul 1995
mercially frequently used alkali oxides such as Na 2 O and K 2 O, but also improved properties, for example, chemical durability and electrical resistance due to the mixed alkali effect.16 In the present work the influence of mixed alkali oxide on the sintering of glass powder compacts was studied in the Na 2 O-K 2 O-SiO 2 system with 25 mol % as total alkali content, and the correlation between the observed results and the viscosity was discussed. II. EXPERIMENTAL PROCEDURE Mixed alkali glasses of mol % composition (25 - x) Na 2 O-xK
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