Relating to the results of sinterings in the system urania-thoria in the author's preceding report, their processes were investigated by means of the thermogravimetric balance method and the dilatometry, and by the quenching experiments and the X-ray examinations. From their data the chemical and dimensional changes of their systems observed at the sintering at temperatures up to 1500°C were discussed. Specimens used in this study are the same as that described in the preceding paper.
1) The sintering processes of the bar specimens (4×4×30mm) pressed at 3tons/cm² were traced in air by means of the dilatometer (used silica or alumina as tube and rods materials) till 1400°C (Fig. 1). The slight shrinkages of 0.1-0.2% were observed till 300°C on the specimens with the compositions of thoria contents 20-30%. There were rapid shrinkages at about 800°C in all specimens and then followed by essential and large shrinkages in the range from 1100 to 1400°C, as shown in Fig. 1, whose temperature were higher with increasing thoria contents.
2) A vaporisation of uranium component was observed in air at above 1100-1200°C on the specimen with larger uranium contents, but it was not occured in a nitrogen or argon atmosphere. This fact may be based on the presence of U₃O₈ in these specimens. The weight decreases at 1400°C in air are 0.18, 0.08 and 0.06%/hr in bodies of U₃O₈, 30% thoria, and 50% thoria specimens respectively (Fig. 2). Accordingly it is evident that the sintering in air is not so proper at the specimens containing larger content of uranium that the sintering in a neutral atmosphere is recommended.
3) The dimensional changes of the bar specimens in the different atmospheres were determined at 800°C or a constant temperature up to 1400°C by means of the dilatometer (Figs. 4 and 5). It is noted that a small and slow contraction was observed in the change from air to nitrogen and a large and abrupt expansion or contraction in the change from nitrogen to hydrogen. In such a abrupt change at the reduction process, there was the contraction at lower temperature and the expansion at higher temperature. All of the specimens were disintegrated or cracked at the reduction process of 800°C, but these crackings were not observed at above 1200°C.
4) From measurements of weight changes of sintered pellets containing 30% ThO₂ in the change of atmosphere air-nitrogen-hydrogen at 1400°C, the weight change at the airnitrogen process was observed to be comparatively slow, whereas the abrupt increase and the abrupt decrease at oxidation and reduction process respectively (Figs. 6 and 7).
5) The thermogravimetric curves of reduced or oxidized powder samples in the atmosphere of air or nitrogen showed the weight decrease at above 1000-1100°C (Fig. 8). The weight changes here were indicated by O/(U+Th) atomic ratio based upon the weights obtained when the samples were reduced at 800°C in hydrogen.
6) The O/(U+Th) atomic ratio measurements and the X-ray examinations of the pressed specimens (12φ×1-2mm) were carried out after heated at each temperature between 800° and 1500C for 2 hrs in air or nitrogen and quenched into water or mercury. Results of the O/(U+Th) measurements are given in Fig. 9 which shows the decrease of these ratios at 1000°-1400°C.
7) It was found from the X-ray examinations of the quenched samples that U₃O₈ phase only occured at above 2.6 of O/(U+Th) ratio, U₃O₈ phase plus solid solution phase of cubic fluorite type between 2.25 and 2.60, and cubic phase only between 2.0 and 2.25. The cell constants of cubic phases increased vigorously with lowering temperatures in the specimens contained U₃O₈ phase, since these cubic phases have a larger content of thoria than that in the starting specimens

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The ceramic materials are ordinary inhomogeneous, being composed of different materials. The authors' purpose is to make a fundamental study of the properties of such complex materials in relation to their microscopic structure by the use of the mixture of different powdered materials as a simplified model.
In the present study, the dissolving process into water has been investigated for the mixtures of powders of SiO₂ and NaCl.
The dissolving process into water for the mixture of powders was considered to be governed by a diffusion phenomenon expressed by the equation,
∂C/∂t=D∂²C/∂x²,
where C is the NaCl concentration in the water diffused into the interior of the mixture, D is the diffusion constant for NaCl, t and x are parameters of time and distance. From this diffusion equation the following equation was derived.
log(M_∞-M_t/M_∞)=A-Bt
where M_t is the amount of NaCl dissolved into water during the time t, M_∞ is the total amount of NaCl in the mixture, and A, B are constants.
The above theoretical equation was examined by the results of experiments made on the mixtures of NaCl and SiO₂. The results obtained are as follows;
i) The value of log[(M_∞-M_t)/M_∞], which corresponds to the residual amount of NaCl undissolved at the time t in the mixture, was confirmed experimentally to decrease lineary with time.
ii) The diffusion constant, D, is greatly affected by the particle size of insoluble SiO₂. It decreases with the decrease in particle size of SiO₂.
iii) Increasing the content of NaCl in the mixture decreases the diffusion constant because the NaCl powder itself becomes an obstacle for its dissolving into water.
iv) The mixtures containing small amounts of fine powders have the coarse texture. For such mixtures the diffusion constant is greatly affected by their porosity; it increases with increasing the porosity.

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A dissolving phenomenon of the mixed system (SiO₂-Na₂CO₃ system) has been studied in this experiment. Thirty two kinds of mixed system were prepared by mixing the fixed amounts, e.g. Ag (A=2, 4, 6, 8), of Na₂CO₃ (Anhydrous) of constant meshes (40-80, 120-150 mesh) with (10-A) g of SiO₂ of constant meshes (20-30, 35-60, 60-80, 80-120 mesh). Each mixture in the state of complete mixing was put into water in a special vessel. The amounts of Na₂CO₃ which dissolved into 1l H₂O at a constant temperature, 40°C, for fixed times, 1, 2, 3 and 4 minutes, were determined by titrating with 0.01N HCl.
As described in the previous paper, the relation between the amounts of dissolved Na₂CO₃ and the time was expressed by the following experimental formula,
log(1-M_t/M_∞)=Bt,
M_t: Amounts of dissolved Na₂CO₃ at time t,
M_∞: Amounts of Na₂CO₃ which is able to dissolve.
An agreement of the experimental results with this formula was examined by the analysis of variance. It was found that the dissolving phenomenon can be considered as a diffusion process with an apparent diffusion constant.
As -B in the formula is regarded as a diffusion rate of Na₂CO₃, it can be used as a measure of the dissolving phenomenon. The relations between -B and the weights of Na₂CO₃ in the mixed system are shown in Fig. 1 and Fig. 2, which show that the dissolving rate of Na₂CO₃ decreases with increasing the weights of Na₂CO₃.
Eight experimental formulae showing the relations among -B, the valume percentage of each constituent, and the porosity of the mixture were obtained by the regression analysis, and from these equations it was found that the volume percentage of Na₂CO₃ gives a negative factor but the porosity a positive factor to the dissolving rate, and the volume percentage of SiO₂ gives a negative factor, with the exception of the case of SiO₂ 20-30 mesh.
The value of B calculated by the use of Eq. (4. 3. 4) and the observed value of B were found to be in good agreement (Table 10 and 11). From this results, it was ascertained that -B satisfies the aclitivity relationship in reference to the volume percentage of Na₂CO₃, the volume percentage of SiO₂ and the porosity as follows;
-B=L[Na₂CO₃]volume%+N[SiO₂]volume%+L[Porosity]
L=0.0648-0.00250x-0.000392y+0.0000138xy
N=0.0380-0.00104x-0.000247y+0.0000064xy
M=-0.0226+0.00085x+0.000169y-0.0000051xy
x: mean SiO₂ mesh, y: mean Na₂CO₃ mesh.

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The microstructure of magnesium oxide calcined at temperature of 1400°C to 1750°C and the effect of various oxide additions to magnesia on the microstructure of products were investigated by electron and petrographic microscopy. The results obtained are as follows:
Growth of hexagonal periclase crystal is observed for magnesium oxides of relatively high purity obtained at calcining temperatures of 1650° to 1750°C. Fig. 1 (2), (3) show the well formed crystals in the specimens having high load-bearing capacity at temperature of 1680°C.
Additions such as CaO, SiO₂ and Fe₂O₃ to magnesia promote the sintering by increasing the amount of vitreous phase. However, degree of crystal growth and microstructure of the products depend on the kind of oxide additions and the relative amounts of the additives. For example, additions of CaO and SiO₂ in amounts of 1.0 or 2.0molal ratio CaO/SiO₂ improve the load-bearing capacity at high temperature. The matrix in the products tends to include compounds indicating high melting point which have high refractoriness under load. Additions of Fe₂O₃ and CaO produce samples containing fine MgO⋅Fe₂O₃ together with liquid phase and also promote the growth of periclase crystal. Additives such as CaO, SiO₂ and Fe₂O₃ improve the resistance to hydration of MgO. The hydration rate may be reduced by coating the crystallities with the protective layer of matrix.

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Forsterite porcelains for use as electrically resistant materials for high frequency and cordierite porcelains having characteristic of low thermal expansion were prepared from the electrically fused dunite which had been obtained by the author's previous work (J. Ceram. Assc., Japan, 68 [10] 237 (1960)).
Forsterite porcelain The electrically fused dunite was milled with or without additions of flux agents, such as MnO₂, Ni₂O₃, Co₃O₄, Ca₃(PO₄)₂, Mg(NH₄)PO₄⋅6H₂O, Pb₃O₄, PbO⋅B₂O₃⋅H₂O, etc., formed by dry pressing into a shape of small disk, and fired at SK 10-29 for 2 hrs. The firing temperature for obtaining the vitrified bodies was high, but its range was fairly wide, i.e., SK 16-20, even in the case without additions of the flux agents. The color of the vitrified bodies was white or pale greenish white. The ease of vitrification of the bodies increasing with the decreasing particle size of dunite, the increasing forming pressure, and also the increasing firing temperature, of which the last factor gave the most marked effect. The power factor (tan δ) become small with the advancement of sintering, and, accordingly, with the growth of the size of crystallites in the bodies. The addition of Mg(NH₄)PO₄⋅6H₂O, Ca(PO₄)₂, or PbO⋅B₂O₃⋅H₂O as the flux agent was found to be effective for lowering both the devitrification temperature and the value of power factor. The sintered bodies had the following properties; firing shrinkage 10-14%, thermal expansion coefficient 10-11×10^-6, tan δ 2-9×10^-4, ε approximately 7.
Cordierite porcelain The pressed mixtures of the dunite and the kaolin powders with or without additions of flux agents, such as ZnO, Pb₃O₄, etc., were fired at SK 13-SK 18 for 2 hrs. The thermal expansion coefficient of the vitrified bodies decreased with the increasing amount of kaolin added, reaching the minimum value when the proportion of dunite and kaolin in the mixtures became 20:80. Although the range of the firing temperature for the above composition was fairly wide, i.e., SK 13-18, the use of the flux agent, such as ZnO or Pb₃O₄, was found to be necessary to achieve the thorough vitrification. The vitrified bodies had almost the same electrical properties as those of the conventional one.

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