Influence of Excess Magnesia on the Properties of Forsterite Porcelain

Abstract

It was reported already that the forsterite porcelain containing excess silica has the narrow range of sintering temperatur and the low softening temperature under load, and the present paper concerns with the influence of excess magnesia on the properties.A mixture of Mg(OH)2 and SiO2 (α-quartz) corresponding to the molar ratio of 2.2 MgO: 1.0 SiO2 was prepared from the materials of high purity.It was heated with the velocity of 200°C/hr., to various temperatures, kept constant for 15 min., and cooled rapidly.X-ray analysis was used to identify the minerals in the fired specimens. It was confirmed that the formation of forsterite began at about 1000°C and was completed at 1460°C. The amount of quartz and periclase decreased with temperature; the former disappeared at about 1400°C, while the latter remained even at the highest temperature of the heat treatment. The inversion of quartz occured at 1240°C, but it disappeared at 1460°C.The specimen clacined previously at 1460°C and mixed with 5% kaolin could be sintered between 1380°-1500°C.By X-ray method only the existence of forsterite was detected from the sintered body. The dielectric loss of this specimen was very small.Forsterite porcelains made from sea water magnesia and silicious stone were also studied; six batches having the compositions, MgO 57-70, SiO2 42-30% were calcined until the X-ray proved the termination of the forsterite forming reaction. They were ground and mixed with 5% of kaolin. The sintering temperatures were 1340°-1520°C, increasing with the increase of MgO.The coefficient of thermal expansion, the thermal conductivity, and the thermal shock resistance have increased with the increase of MgO- content. The trend of the softening temperature was reflected the relations in the phase diagram. The bending strength became lower with increasing MgO, which presumably came from the effect of the aggregation of forsterite crystals.Although X-ray failed to detect the periclase clystals in the specimens containing 58-62% MgO, it is highly probable that the mineral composition are the same with the forsterite porcelain which does not contain excess of MgO.These two, however, showed some difference in the properties; especially, the softening temperature under load of the former was considerably higher than that of the latter. Microscopic observation showed that the excess MgO was disolved in the glass matrix, and made the refractive index higher. Moreover, such a texture may be considered as being responsible for the higher softening temperature under load.The electrical properties of the specimens made from pure chemical as well as natural raw materials were excellent.It was concluded that the forsterite porcelain having the base composition of 58-62% MgO are the best for using as the vacuum tube.