Thermally decomposed alumina from aluminum sulfate has a superior sinterability. Transparent polycrystalline alumina body may be obtained from this alumina atuarelatively low temperature and short sintering time in vacuum.
The main subject of this study was to investigate the appearance process of the transparency of the above bodies. Two series of specimens were prepared, i.e. with and without addition of 0.05wt% MgO. Specimens were sintered in vacuum (5×10^-4mmHg) at the temperature range from 1450°C to 1750°C.
The relationship among porosity, transmission, and grain size for the sintered bodies at varied temperatures were examined and discussed.
The results obtained were as follows:
1) The relationship between the porosity (P) and the transmission (T) can be described by the following equation:
T=Ae^-mP
The values of A and m obtained from this experiment were 56% and 0.35 respectively.
2) From MgO added specimens transparent bodies were obtained by firing at above 1600°C. This might be ascribed to the superior sinterability of the decomposed alumina material.
3) The porosity for the series of specimen fired at above 1550°C without additives showed no decrease and the value was maintained at about 7%. The specimens of this showed poor transparency. The closed pore which might affect the transparency increased in the temperature range from 1500°C to 1550°C.
The above temperature range was in accordance with that of exaggerated grain growth. The existence of some relationship between the occurrence of closed pore and exaggerated grain growth may be suggested.

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The relationship between the polytypes of SiC and the supersaturation of the solution has been studied at 1800°C with the synthetic method using molten silicon. The results obtained are summarized as follows;
1) β-SiC crystallizes from highly supersaturated solution. The crystals obtained at the condition of low supersaturation, however, consist of mainly α-SiC such as 4 H, 15 R and 6 H.
2) Relative amount of 4 H increased markedly with decreasing the supersaturation.
3) From the results stated above, it is concluded that 4 H is the most stable structure at 1800°C among the basic polytypes of SiC, 3 C, 4 H, 15 R and 6 H.

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Self-diffusion of sodium ions in some silicate glasses was measured by means of RI tracer technique. At the binary system of Na₂O-SiO₂ glasses, the diffusion coefficient of sodium ions increases exponentially with the increase of this ion concentration. The introduction of Li₂O, K₂O, CaO and La₂O₃ into the glass brings about the lowering of diffusion coefficient of sodium ions. The effect of Ca, Al and La ions on the diffusion of sodium ions is explained in terms of the polarizing power of these cations to oxygen electron shell, being similar to the behavior of di-valent ions such as Mg, Sr, Ba, Zn, Cd and Pb. But Li and K ions are considerably different from di-, and tri-valent cations, and the behavior of both ions are probably related to “mixed alkali effect”. The reasons for this effect are not understood at present.
The calculation of so-called correlation factors (f) was carried out by the modified Nernst-Einstein equation, and gave the values about 0.2-0.3 for each glass. By analogy with the relation between correlation factor and diffusion mechanism in ionic crystal, it may be supposed from the observed f-value that transport of sodium ions are by some type of vacancy or interstitialcy mechanism. However, such estimation is too simple for glasses having a more complicated structure than ionic crystal.

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The liquid-liquid phase separation in the glass having the chemical composition of Na⋅NaCa⋅Mg₅⋅Si₈O₂₂F₂ was studied by examining the electron micrographs and infrared spectra of the glass samples heated at various temperatures ranging from 525°C to 650°C. The results obtained were as follows:
1) A small number of the phase-separated droplets of under 250 Å in diameter precipitated already in mother glass as poured from 1400°C.
2) In the early stage of heat-treating of the glass, the total volume and number of droplets in unit volume of the glass and the droplet size increased with heating time at each temperature, as a result of further nucleation and growth of the droplets. The maximum nucleation rate was found at about 600°C and the growth rate increased with temperature under 650°C.
3) In the middle stage, the total number of droplets decresed with increasing heating time and temperature as a result of coalecence, while the total volume of droplets remained constant.
4) In the final stage, the total volume and number of droplets did not change with heating time at each temperature.
5) The apparent activation energy of growth of droplet in the early stage was obtained as 63kcal/mol., which was larger than that for transformation of the glass at the same temperature range. It was considered that the diffusion processes for these two reactions are quite different.
6) Information on the transformation, phase separation and crystallization of the glass was obtained from the infrared spectra of the glass subjected to heat treatment at various temperatures.

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Despite of the marked difference in physical properties which are significant for practical purpose between α- and β-form of CaSO₄⋅1/2H₂O, the fundamental correlation between these two has not been clarified yet.
From the fact that the solubility of β⋅CaSO₄⋅1/2H₂O is larger than that of α⋅CaSO₄⋅1/2H₂O and from the investigation which showed that CaSO₄⋅1/2H₂O is meta-stable, namely, it does not hydrate in water at the temperature range of 84°-97°C, the possibility of the transformation from β⋅CaSO₄⋅1/2H₂O to α⋅CaSO₄⋅1/2H₂O is anticipated by means of holding β⋅CaSO₄⋅1/2H₂O in water at this temperature range. From this point of view, the authors treated β⋅CaSO₄⋅1/2H₂O hydrothermally at the temperature range of 84°-98°C in order to cause transformation reaction from β-form to α-form and to prepare reaction products at different steps of transformation. The rate of this transformation was determined by measuring the characteristic exothermic peak of α⋅CaSO₄⋅1/2H₂O which appeared in the vicinity of 220°C in D. T. A. Moreover, in connection with this experiment, the result obtained on the mesa-stability of CaSO₄⋅1/2H₂O in water is also reported.
The results are summerized as follows:
1) The transformation to α⋅CaSO₄⋅1/2H₂O did not occure when single β⋅CaSO₄⋅1/2H₂O was treated hydrothermally. However, when α⋅CaSO₄⋅1/2H₂O was added as seed to β⋅CaSO₄⋅1/2H₂O, the transformation proceeded after an induction period, but at the same time, the hydration of hemihydrate to dihydrate started after an induction period thereof, and afterwards the transformation reaction was extinguished. In this experiment in which 10 wt-% of α⋅CaSO₄⋅1/2H₂O was added as seed, the transformation attained a maximum rate of 20%.
2) It was indicated that CaSO₄⋅1/2H₂O is not so-called meta-stable in water at the temperature of 84°-97°C but hydrates after an induction period, depending on the treatment temperature and time. In this experiments, the relation between the temperature of water Y (°C) and treatment time required to hydrate completely X (hr) was represented as Y=81.33+5.34logX(84≤Y<98).

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The tensile and bending strength of glass fiber containing TiO₂ and CaF₂ as nucleating agents was investigated in connection with effects of devitrification, UV radiation and X-ray radiation. The glass fiber of the composition (SiO₂ 50; Al₂O₃ 20; Li₂O 5; TiO₂ 7; CaF₂ 13; MgO 5% by weight) was prepared by pot method in air at 1350°C. The bending tests on various fibers showed an average strength of 5000kg/cm² which is almost identical to that before heattreatment. In case of non-heat-treatment, no difference was observed in the tensile and bending strength between the specimen not-irradiated and those irradiated with UV or X-ray. The properly heat-treated glass fiber was found to retain the tensile and bending strength almost identical to those of non-treated.

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