The rate and the activation energy of the surface grain growth of alumina containing 2 or 5wt% of MgO, Co₃O₄, TiO₂ and MnO₂ were measured in the temperature range from 1400° to 1650°C.
In general, the relation between grain size and heating time can be described by the following equation:
D=At^m (1)
In the present investigation D was expressed by the mean value of the diameter of 20 largest grains. The results obtained were as follows:
1) The equation (1) held good for all of the specimens.
2) The m value increased with temperature rise and its increasing rate was larger according as the grain growth was remarkable.
3) The relation between grain growth and sintering was proportional.
4) The addition of the above metal oxides promoted grain growth, but the extent was different by materials and their quantities. The specimens with 5% of MnO₂ showed remarkable grain growth particularly.
The grain growth was generally related to time by the following equation.
D-D₀=Ktⁿ (2)
Exactly the value of n was dependent on temperature but approximately constant at 0.33.
The addition of metal oxide decreased the activation energy for grain growth. Particularly the value for the specimen with MnO₂ (75kcal/mole) was about half of that for the specimen without additives (135kcal/mole).

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For the purpose of studying the formation and the color development of titanium-aluminium spinels containing Co²⁺ and Ni²⁺ ion, the gradual substitution according to following formula was carried out for each titanium spinel.
Mg²⁺(Zn²⁺)+Ti⁴⁺→←2Al³⁺
Thus, CoO-MgO-TiO₂-Al₂O₃, CoO-ZnO-TiO₂-Al₂O₃, NiO-MgO-TiO₂-Al₂O₃, NiO-ZnO-TiO₂-Al₂O₃, CoO-NiO-MgO-TiO₂-Al₂O₃ and CoO-NiO-ZnO-TiO₂-Al₂O₃ systems were investigated. Each mixture was calcined at 1350°C for 1 hour. The reflectance between 400-760mμ was recorded by spectrophotometer to pursue the displacement of absorption and to represent the results by C. I. E. color specification. X-ray analysis was also carried out to observe the spinel formation and calculate the lattice constant. The results were sum marized as follows.
1. CoO-MgO-TiO₂-Al₂O₃ system.
Samples were prepared according to 0.2CoO⋅(1.8-x)⋅MgO⋅(1-x)TiO₂⋅xAl₂O₃, 0.5CoO(1.5-x)MgO⋅(1-x)TiO₂⋅xAl₂O₃, CoO⋅(1-x)⋅MgO⋅(1-x)⋅TiO₂⋅xAl₂O₃, and single spinel was obtained except x=0.6. When x=0, brilliant hues ranging from greenish blue (0.2CoO⋅1.8MgO⋅TiO₂) through bluish green (0.5⋅CoO⋅1.5⋅MgO⋅TiO₂) to green (CoO⋅MgO⋅TiO₂) developed, and when x=1, clear hues so-called cobalt blue was revealed. In this system coordination number of Co²⁺ ions was invariably 4. Each absorption of tetrahedral Co²⁺ ions shifted towards the violet region with increasing amount of Al³⁺ ions, owing to the contraction of lattice, and color changed from bluish green to fresh blue.
2. CoO-ZnO-TiO₂-Al₂O₃ system.
Samples with the composition 0.2CoO⋅(1.8-x)ZnO⋅(1-x)TiO₂⋅xAl₂O₃, CoO⋅(1.5-x)⋅ZnO⋅(1-x)TiO₂⋅xAl₂O₃, CoO⋅(1-x)ZnO⋅(1-x)TiO₂⋅xAl₂O₃ were prepared. Having strong tetrahedral preference, Zn²⁺ ions occupied preferentially tetrahedral interstices in cobalt-substituted 2ZnO⋅TiO₂, xCoO⋅(2-x)ZnO⋅TiO₂. These spinels revealed fairly strong absorption of octahedral Co²⁺ ions and weak one of tetrahedral Co²⁺ ions. This tendency was remarkable in the region poor in Co²⁺ ions and brown developed. On the other side, cobalt-substituted ZnO⋅Al₂O₃, xCoO⋅(1-x)ZnO⋅Al₂O₃, Co²⁺ and Zn²⁺ ions occupied tetrahedral interstices only, and clear hues as observed cobalt-substituted MgO⋅Al₂O₃, xCoO⋅(1-x)MgO⋅Al₂O₃, developed. Therefore, coordination number of the greater part of Co²⁺ ions changed from 6 to 4 in this systems. Single spinel formed in the range of x≤0.2 and x≥0.9. Brown developed in the region rich in Ti⁴⁺ ions, and in the region rich in Al³⁺ ions, colors ranging from greenish blue to fresh blue were obtained.
3. NiO-MgO-TiO₂-Al₂O₃ system.
In this system samples were prepared according to 0.2NiO⋅(1.8-x)MgO⋅

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In the previous paper, the present authors reported a new phase of alumina hydrate, tohdite 5Al₂O₃⋅H₂O, which was formed under hydrothermal conditions. In the present experiments, various aluminas and alumina hydrates have been used as the starting material in order to investigate the effect of the starting material on tohdite formation. The effect of certain mineralizers has also been discussed. The results can be summarized as follows. (1) Tohdite is formed by hydrothermal treatment of the following species; bayerite, boehmite (obtained from bayerite), η-alumina, γ-alumina (from bayerite), ρ-alumina, χ-alumina and κ-alumina, without any mineralizer. Gibbsite, boehmite (from gibbsite), γ-alumina (from gibbsite) and θ-alumina, however, did not give rise to tohdite but are transformed directly into corundum. (2) Tohdite is an intermediate phase of the transformation to corundum and is formed in the corundum stable region of the phase diagram of the system Al₂O₃-H₂O. (3) Addition of some fluorides or Ti(SO₄)₂ is favorable to the formation of tohdite. Fluorine ion acts as a strong inhibitor against the nucleation of corundum. CuSO₄ has a slight effect upon tohdite formation. (4) Some of the impurities were proved to be in solid solution in Tohdite.

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Glazes which are applicable to the lithia-ceramic bodies having extremely low thermal expansion coefficients [α<20×10^-7/°C (room temperature-500°C)] were developed. The chemical composition of the glaze which showed the best result when applied to the lithiaceramic having the linear thermal expansion of about 5×10^-7/°C, is 50.4 SiO₂, 29.2Al₂O₃, 5.9Li₂O, 1.7ZrO₂, 2.6P₂O₅, 2.6TiO₂, 1.0Na₂O, 1.0K₂O, 2.8B₂O₃, 2.8PbO wt.%.
The process of its application is as follows: A frit of the composition described above is added with 5% clay powders and 0.05% PVA and pulverized to a finess over 200 mesh by the wet process. The body to be applied with the glaze slip is prepared by firing the pressed mixture of petalite and bentonite powders (100:5 in wt. ratio) at 1260°C for one hour.
The body applied with the slip is dried and heated first at 1290°C in a furnace until the glaze melts and spreads over the surface of the body (for 5-10min.). It is then taken out from the furnace, put into another furnace previously set at 750°C, kept there for about 60min. and then taken out from the furnace. By this second heat-treatment the glaze converts from glassy state to crystalline aggregates consisting almost of β-eucryptite crystallites, thus lowering its thermal expansion to fit that of the petalite body. The chemical durability of this glaze is satisfactory for use as dinner wares: No loss in gloss is observed for its surface after immersed in the boiling 2% citric acid solution. It also withstands quenching from 500°C into cold water without showing any cracking.

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The authors have proposed a revised phase diagram for the system Al₂O₃-H₂O. In the system, some transformation mechanisms are very difficult nucleation processes for the more stable phase. The mechanism is as discussed in the previous paper. The authors propose that it must be checked whether seed crystals of the more stable phase is present beforehand or not when an equilibrium curve is measured and illustrated in the diagrams. Addition of seed crystals of the more stable phase often causes very remarkable effects upon the initiation of transformation into the more stable phase. And it must be checked whether the transformation of A into B is a reversible one along the equilibrium curve between A and B illustrated in the diagrams. The transformations of boehmite into corundum and diaspore into corundum have been confirmed to be reversible ones along the curves in the present revised diagram. Other transformations are also discussed. The authors clarify three types of transformation in the phase diagram which are those between two stable forms, between a stable form and a metastable one, and between two metastable forms. Spontaneous transformations are illustrated in the diagram. The present diagram is useful in order to discuss hydrothermal reactions of alumina polymorphs appearing in it.

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