Tohdite, 5Al₂O₃⋅H₂O, is a metastable phase which is formed in the corundum stable P-T region under hydrothermal conditions when certain materials are added as mineralizers, The additives found to be effective for tohdite formation are fluorides, borates, titanium sulfate ammonium metavanadate and sodium bismuthate. The present investigation concerns the dependence of the mineralizer effect on the concentration and the mol ratio to the starting material, boehmite. The effect of fluoride ion has been studied in detail and a probable mechanism has been proposed. For the metastable tohdite formation structures of Poly aluminate ions in the solution and the interference of fluoride ion with them are critically important. The effect of fluoride ion actually depends on the ratio of the concentration of fluoride ions to the concentration of aluminum in the solution.

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With the aid of the sublimation method and the method with silicon melts, the relationship between the basic polytypes of SiC and their growth temperatures has been studied experimentally in the range from 1600°C to 2600°C under the condition of low supersaturation. From the results of these experiments, it is concluded that 3 C, 4 H, and 6 H are stable up to 1600°C, at about 1800°C, and between 2200°C and 2600°C, respectively. The 15 R type has been found as stable as 6 H at about 2200°C, and if a stable region exists for this type, it is considered to be between 2000°C and 2200°C.

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Aluminium nitride (AlN) sublimates at 2450°C under ordinary pressure without forming liquid phase. It has been believed that AlN products with sufficiently high density and mechanical strength for commercial uses have not been able to obtain by sintering moulded AlN powder.
In this paper, it is described that AlN rods with high strength and low porosity were produced by sintering fine AlN powder.
The process of sintering of AlN powder was observed. Sample rods 8mm in diameter and 15mm in length were formed of raw powder of particle size 4.4, 1.5 and 0.78μ in average, respectively. The pressure of forming was 5t/cm², and the samples were sintered at the temperature range from 1600° to 2000°C in N₂ atmosphere at 760mmHg. The time of sintering ranged from 10 to 240min. Density, flexural strength and shrinkage of sintered rods were measured in relation with sintering time, temperature and the particle size of raw powder. The structure of sintered rods were examined microscopically.
Shrinkage, density and flexural strength of the samples increased with increase of sintering temperature. At constant soaking temperature, they increased rather rapidly in the early stage and slowly in the later stage of sintering. In the process of densification, crystal grains in sintered rods grew concurrently, and the rate of grain growth was faster with finer particle size of raw powder and higher sintering temperature. At the final stage of sintering, it was observed that the mechanical strength fell with increase of grain size.

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2H-type SiC whiskers were obtained in cool parts of a Lely furnace in an argon ambient. From the observation, 2H-type is considered to be more stable structure than cubic, β-type below ca. 1300°C.

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