Effect of Kaolin Addition on Properties of Forsterite Porcelain

Abstract

The stability of the forsterite porcelian in hydrogen at high temperatures, which is required in the application of the forsterite porcelain to a high frequency insulator, is governed by additives used to promote sintering. Among several additives examined kaolin has the effect to make the porcelain stable in hydrogen at high temperatures, so giving it favorable properties for Mo-Mn metallizing processed under such conditions.In this paper 0-10% of kaolin was added to forsterite porcelains which were prepared from Fukushima siliceous stone and sea-water magnesia in the molar ratio of MgO: SiO2=2:1.By the addition of kaolin the temperature range for sintering was remarkably extended and the firing shrinkage was almost constant within this range. The coefficient of linear thermal expansion decreased with the increase of added kaolin, and no considerable change was found by repeated heat treatments. With the increase of kaolin content from 0 to 10%, the softening temperature under load lowered to 1160°C from 1280°C. The electric resistance at elevated temperatures lowered gradually, but was still sufficiently high for practical use. The flexural strength decreased and the modulus of elasticity lowered gradually in the temperature range from room temperature to 300°C with the increase of kaolin content. The modulus of elasticity, however, became larger with the rise of temperature in this temperature range.Within 5% of kaolin added, the dielectric constant and dielectric loss were unvaried and these values were excellent for the high frequency insulator.The result of X-ray diffraction gave only the pattern of the forsterite. By microscopic observation, forsterite crystal grain changed from allotriomorphic into idiomorphic and glass matrix increased with the kaolin content.It is concluded that the addition of 2.5-5.0% of kaolin was most suitable for preparing the forsterite porcelain for this purpose.