フォルステライト磁器の微構造と熱膨脹との関係

高周波用絶縁材料としてのフォルステライト磁器 (第4報)

抄録

The thermal expansion of forsterite porcelain, which is one of the most important properties for use in electron tube, and the effect of different firing temperature, and of repeated heating were investigated. Based on the experimental results the relation between the microstructure and the thermal properties were discussed.
The coefficients of linear expansion along each axis of synthesized forsterite were measured by X-ray with a result; a-axis 10.47, b-axis 12.50, c-axis 11.05×10^-6, in the temperatures 20°-800°C. These values correspond to that of the forsterite porcelain containing much forsterite.
Referring to the data of Navias (J. Am. Ceram. Soc., 37 [8] 329 (1954)) it was pointed out that the elevation of the firing temperature of 25°C would accompany with the increase of the coefficient by 1.0×10^-6, and this is the figure being intorelably large for the vacuum sealing with metal.
To study this phenomenon many batches of forsterite porcelain were prepared, and fired at different temperatures. The specimen made from talc, sea-water magnesia and BaCO₃ showed the marked decrease of the coefficient (about 1×10^-6) with the elevation of firing temperature from 1400° to 1500°C. Under microscope was observed the increase of glassy phase with increasing firing temperature, which probably leads to the reduction of the expansion coefficient.
The coefficient of the specimens containing MgO and SiO₂ in various ratios have also beem measured. Forsterite porcelain having excess of SiO₂ (SiO₂ 47%) showed the increase of the coefficient of about 1.2×10^-6 by the elevation of firing temperature from 1400° to 1500°C, which, however, decreased with increasing MgO. The specimens containing excess MgO showed only a very small variation.
By microscopic observation of the specimens containing excess SiO₂, the forsterite crystal showed a marked aggregation by the elevation of firing temperature, causing the decrease of the coefficient, while in the forsterite containg excess of MgO the aggregation was scarecely recognized, and consequently gave the constant coefficient.
Moreover, for many kinds of forsterite porcelain, the variations of the coefficients of linear thermal expansion by the repeated heating were measured. They could be classified into two groups, i.e., one coefficients increased, and the other coefficient did not nearly or entirely increased with repeated heating. The former has more glassy substance than the latter, and it was concluded that such a glassy substance decreased the coefficient by the repeated heating.
The authors have confirmed that the composition of forsterite porcelain for use in electron tube should be in excess of MgO and the glassy substance should be reduced as far as possible.