The Relation Between the Microstructure and the Electrical and Mechanical Properties of Forsterite Porcelain

Forsterite Porcelain as High Frequency Insulator, V

Abstract

Three kinds of forsterite batches of molar ratios, MgO:SiO₂=2.00:0.82 (specimen M), 2.00:1.00 (specimen F), and 2.00:1.05 (specimen S) were calcined at 1300°C, added 5% of Kaolin and fired at various temperatures.
Firing shrinkage, apparent specific gravity, water adsorption, dielectric loss, insulation resistance and bending strength of the fired specimens were measured to discuss the relation between the properties and the microstructure.
Above physical properties gave the maximum or the minimum values at certain firing temperatures. The specimen M fired at 1550°C, F at 1390°C, and S at 1370°C gave the maximum values of firing shrinkage, apparent specific gravity, T_e values of volume resistivity, and bending strength, and the minimum values of water adsorption and dielectric loss.
At these maximum or minimum points each specimen showed a similar microstructure being composed of the forsterite crystals of about 10μ. The texture was dense but not yet the perfect mosaic structure which came out by firing at higher temperature through the rapid aggregation of forsterite crystals.
In such a specimen cracks were observed in crystal and matrix which leads to the lowering of many properties.
Comparing the three specimens the aggregation of forsterite crystals by overfiring and concequently the fall of properties was the most remarkable in the specimen S.
The electrical properties of the specimen M were the best, but the proper firing temperature were considerably high.
It was suggested that the best composition for forsterite porcelain should lie between M and F.