The present paper describes the effect of firing atmosphere and additive Cu₂O on the non-linearity of SiC varistors; the non-linearity is expressed by a equation, I=(V/C)^α. The SiC varistor fired in air has a low resistance and shows a high α which remarkably decreases with an increase in the current flowing through the SiC varistor while the SiC varistor fired in H₂ and N₂ has a high resistance and shows a low α which slightly decreases with the current. Partial pressure of oxygen in N₂ has a strong effect on the resistance of SiC varistor; the lowest resistance is obtained with about 5mmHg of oxygen partial pressure. When SiC varistor incorporated with a minor amount of Cu₂O is fired in N₂ or H₂, it produces a low resistance and a high α which is constant with a current flowing through the SiC varistor, An effect of additive Cu₂O is discussed in view of the X-ray microanalyser.

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Morphological changes of the surfaces normal and approximately parallel to [0001] axis of a titanium (III)-containing corundum crystal by heating it in air at 1300°C were investigated using an optical reflection microscope and an electron microscope. The surfaces to be examined were chemically etched before heating. Of the observations, a noticeable one was formation of ridges, probably of corundum, along the intersections between the (0001) surface and twin boundaries or boundaries similar to the twin boundary. Preliminary consideration was made of the causeof formation of the corundum ridge.

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Initial-stage sintering of the active BeO powder was investigated by measuring the isothermal shrinkage of its powder compacts continuously with a thermo-dilatometer consisting of a linear variable differential transformer activating an X-Y recorder, in vacuum for the temperature range 1250° to 1450°C.
The results obtained were summarized as follows:
(1) Initial-stage sintering of the active BeO powder was found to be a rapid densification process represented by the straight line with a slope of unity, when logarithm shrinkage was plotted against logarithm time.
(2) Large amounts of the initial shrinkage for a short time period due to this process markedly shortened the time to complete the densification. The phenomenon showed characteristically one of many properties of the active BeO powder, which is considered to be sinterable.
(3) This process was followed by the neck growth due to vacancy-volume diffusion.

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Powder mixtures of zirconium diboride and lanthanum hexaboride were hot-pressed at the temperatures ranging from 1700° to 2200°C in graphite mould under the pressure of 200kg/cm². During hot-pressing at temperatures below 2000°C, shrinkage of the compact was measured by means of dial gauge which was connected with the ram table of hot-press furnace. The ram table moved in consequence of the densification of the compact at high temperatures.
Final density was measured and density change of the compact during hot-pressing was calculated. Microstructures were observed with microscope and electron probe microanalyzer.
Densification of zirconium boride compacts was accelerated by addition of 3 wt% lanthanum boride and the effect was seemed to be mostly due to quicker densification of lanthanum boride. From the densification data, log (1-ρ) were plotted against t according to Murray's equation and fairly good linearities were observed between them except the early period. The viscosities, η, of the compacts were calculated from the slopes of the above mentioned plots. On the basis of their temperature dependences activation energies were observed to be 76kcal/mol for zirconium boride compact, 68kcal/mol for zirconium boride compact with 10 wt% lanthanum boride and 60kcal/mol for lanthanum boride compact, respectively.
Mutual penetration of Zr or La element to another boride particles was not detected. These compacts did not show good oxidation resistance.

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Mullite specimens were subjected to corrosion at high temperatures by alkaline (Na₂CO₃) vapor and many kinds of reaction products were formed containing corundum, nepheline, carnegieite, carnegieite-sodium aluminate solid solution and sodium aluminate.
In the first stage of the corrosion process, mullite decomposed into carnegieite and corundum. As the supply of alkaline vapor was continued, the latter gradually changed into sodium aluminate, which in turn dissolved into carnegieite to form carnegieite-sodium aluminate solid solution.
In the low content of sodium aluminate, that solid solution could be converted, on cooling, to nepheline structure like as the case with pure carnegieite. With increasing amount of sodium aluminate, it was found to disturb this conversion and to stabilize the solid solution as carnegieite.
The final crystalline product by the corrosion was sodium aluminate. However, after a long time of exposure to the alkaline vapor, this phase gradually changed into glassy phase resulting in the wear of mullite refractory.

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