Forsterite porcelains for use as electrically resistant materials for high frequency and cordierite porcelains having characteristic of low thermal expansion were prepared from the electrically fused dunite which had been obtained by the author's previous work (J. Ceram. Assc., Japan, 68 [10] 237 (1960)).
Forsterite porcelain The electrically fused dunite was milled with or without additions of flux agents, such as MnO2, Ni2O3, Co3O4, Ca3(PO4)2, Mg(NH4)PO4⋅6H2O, Pb3O4, PbO⋅B2O3⋅H2O, etc., formed by dry pressing into a shape of small disk, and fired at SK 10-29 for 2 hrs. The firing temperature for obtaining the vitrified bodies was high, but its range was fairly wide, i.e., SK 16-20, even in the case without additions of the flux agents. The color of the vitrified bodies was white or pale greenish white. The ease of vitrification of the bodies increasing with the decreasing particle size of dunite, the increasing forming pressure, and also the increasing firing temperature, of which the last factor gave the most marked effect. The power factor (tan δ) become small with the advancement of sintering, and, accordingly, with the growth of the size of crystallites in the bodies. The addition of Mg(NH4)PO4⋅6H2O, Ca(PO4)2, or PbO⋅B2O3⋅H2O as the flux agent was found to be effective for lowering both the devitrification temperature and the value of power factor. The sintered bodies had the following properties; firing shrinkage 10-14%, thermal expansion coefficient 10-11×10-6, tan δ 2-9×10-4, ε approximately 7.
Cordierite porcelain The pressed mixtures of the dunite and the kaolin powders with or without additions of flux agents, such as ZnO, Pb3O4, etc., were fired at SK 13-SK 18 for 2 hrs. The thermal expansion coefficient of the vitrified bodies decreased with the increasing amount of kaolin added, reaching the minimum value when the proportion of dunite and kaolin in the mixtures became 20:80. Although the range of the firing temperature for the above composition was fairly wide, i.e., SK 13-18, the use of the flux agent, such as ZnO or Pb3O4, was found to be necessary to achieve the thorough vitrification. The vitrified bodies had almost the same electrical properties as those of the conventional one.