The sintering of magnesium oxide compacts with seventeen kinds of magnesium phosphate additions (1≤MgO/P₂O₅ ratios of the additives≤3) in the concentrations of 1.0mol% and 3.0mol% was investigated. The phase-change processes and the densification processes were studied from room temperature up to 1400°C. Relative densities and porosities of these compacts, heated at 1400°C for 5h, were measured to evaluate their sinterabilities. The sintering of MgO compact was accelerated by the addition of Mg₂P₂O₇(MgO/P₂O₅=2); such promotion of the densification may be accomplished by the enough active mass transfer through the solid state to form Mg₃(PO₄)₂ and an amorphous phase and then by an increase in liquid phases formed above 1100°C in Mg(PO₃)₂-Mg₂P₂O₇ system, in Mg₂P₂O₇-Mg₃(PO₄)₂ system, and in MgO-Mg₃(PO₄)₂ system. MgO compacts, sintered by the addition of magnesium phosphates, showed higher values of relative densities than that of the pure MgO compact; this may be chiefly due to the reduction of open porosity.

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A concept of two-stage melting was proposed in order to design an efficient furnace, in which the melting process was strictly divided into two-stages; rough melting and refining. The melt down characteristics were studied using a 40l pot set in a gasfired furnace, and the behavior of sodium sulfate in the two-stage process was simulated using a 300ml platinum crucible in electric furnaces. The thickness of foam layer was found to be determined by the amount of SO₃ released from the melt during the refining process. The number of residual sand grains was found to depend not only on the melting time but also on the intensity of agitation of the melt by the released SO₃. The number of bubbles was found to be determined mainly by the amount of SO₃ retained in the melt at the end of the refining process. The following conclusions were derived for rough melting and refining processes, respectively. (1) The condition of rough melting can be determined mainly by the melt down rate, for which relatively low temperature (1250°-1400°C) is sufficient. For effective refining, the rough melting should be conducted below the decomposition temperature of Na₂SO₄ (1450°C), so that the content of SO₃ in the melt might be kept at a proper level at the end of the rough melting. The rough melting time should be between one and two hours for effective refining. (2) The refining temperature should be above the decomposition temperature of Na₂SO₄ to remove bubbles in the melt effectively.

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Cation exchange properties of layered titanate, H₂Ti₃O₇, were examined in LiOH aqueous solutions of various concentrations. Three phases of exchanged products were observed; L-1 phase having interlayered water, L-2 and L-3 phases having approximate compositions of Li_xH_2-x(Ti₃O₇) where x=0.8-1.0 and Li₂Ti₃O₇ respectively. These phases could be converted to the original H₂Ti₃O₇ by the treatment in 0.5M HCl solution, i.e. the ion exchange occurred without causing any change in the host (Ti₃O₇) layer structure. Cation exchange for lithium ions was performed under hydrothermal conditions in the temperature range of 100° to 300°C in an autoclave. The cation exchange for lithium ions was observed up to 190°C but TiO₂ (anatase) was additionally formed above 150°C. The contents of TiO₂ increased with increasing temperature, and finally the cation exchanged products disappeared at 200°C. H₂Ti₃O₇ acted as an ion-exchanger even after 30 cycles of reversible ion-exchange reactions in 0.2M LiOH aqueous solution and 0.5M HCl solution. Only 0.09% and 0.18% of titanium in H₂Ti₃O₇ eluted into 0.1M and 0.5M HCl solutions at room temperature over a period of 3 days respectively.

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The structural damage due to crystallization of a soluble salt (Na₂SO₄⋅10H₂O) from supersaturated solution in pores of a model brick was investigated by taking the cooling direction into consideration. It was found that Na₂SO₄⋅10H₂O crystal was easy to grow along b-axis and was easy to be orientated b-axis perpendicular to the cooling direction in pores of the brick sample as in a supersaturated bulk solution. The artificial crystals of Na₂SO₄⋅10H₂O are columnar in which b-axis is elongated due to the inherent crystal habit. The cracks were developed parallel to the cooling direction because of the growth of such columnar crystals making bridges between opposing internal surfaces of a pore in the brick sample.

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For controlling the scattering of fatigue life, four-point bending fatigue tests for hot-pressed silicon nitride ceramics indented with knoop pyramid have been carried out at room temperature. The scattering of fatigue life was controlled when test specimens were indented at 49N load, The gradient of the S-N curve in the reversed bending fatigue test (tension-compression stress) was larger than that of the fluctuating bending fatigue test (tension-tension stress), and the fatigue life in the former was shorter than that in the latter at lower stress levels. Consequently it was concluded that crack propagation and/or degradation of mechanical properties was accelerated by the compression at lower stress levels. A slight difference was observed between the S-N curve for the indented specimens and that for the pre-loaded specimens. This disagreement was attributed mainly to the influence of residual stress generated by indentation. In order to estimate the residual stress, the specimens indented at 49N load were either annealed in nitrogen gas (1300°C, 1atm) or ground to remove surface layer, and fracture toughness for these specimens were calculated from room temperature bending strength and initial crack dimension. The residual stress levels at crack tip for the specimens indented at 9.8N load and at 49N load were about 70MPa and 100MPa, respectively.

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The melt of 3CaO⋅Al₂O₃ (G-C₃A) composition in the presence of a small amount of SiO₂ was synthesized by rapid quenching after fusion. Furthermore, the physical properties and the hydration behavior of G-C₃A were investigated. G-C₃A results in a lowering of lattice constant, a decrease of specific gravity and an increase of free CaO compared with C₃A. The hydration was not completed even on 10 days curing, and 3CaO⋅Al₂O₃⋅6H₂O hydrate grew very slowly. It is thought that Al(OH)₄^- eluted from G-C₃A is remarkably depressed against Ca²⁺. The molar ratio (CaO/Al₂O₃) in case of G-C₃A was given 2.7 after 10 days curing but closely approached to 1.0 after 6 hours curing in case of C₃A. Namely, as glass phase is formed on G-C₃A surface, the strain occurs in inner layer. Therefore, it is difficult to elute Al(OH)₄^- by breaking -Al-O-Al- bonds. Although the elution of Al(OH)₄^- by NaOH is considerable, the growth of the hydrate cannot be expected because of depression of the elution of Ca²⁺. The results of surface analysis by ESCA showed that the binding energy of inner shell electron of Ca, Al, and Si combined with O (oxygen) in G-C₃A is 0.7-0.8eV higher than C₃A. Consequently, as the elution of Ca²⁺ and Al(OH)₄^- is depressed, the hydration reaction becomes inactive.

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Dry gels were synthesized by the sol-gel method from a mixture of Si(OCH₃)₄, water and several kinds of alcohols. Gelation of the solution was carried out at 45°C and the obtained wet gel was allowed to dry by making a pinhole on the closed container at 60°C. The shrinkage and surface area of the dry gel were measured to obtain porosity and average pore diameter. Residual carbon in the dry gel was analyzed. As the molecular weight of the alcohol increased, the pore diameter and porosity of the dry gel increased (Table 1). When a mixture of methanol and tert-butanol was used as solvent, the dry gel's porosity increased with increasing tert-butanol ratio, even if tert-butanol was added to the wet gel after the gelation (Fig. 1). Addition of water, methanol, tert-butanol or a mixed solvent to the wet gel before the drying process starts affected the porosity and residual organic component of the dry gel (Figs. 3 and 4). The change in composition of the wet gel solution during the drying process was calculated (Fig. 7) using a model analysis of the wet gel's drying process (Figs. 5 and 6). The porosity of the dry gel was correlated with the calculated water concentration of the wet gel solution when the solution had reached one-tenth reduction in weight ([H₂O]_1/10) by drying. When methanol or tert-butanol was added to the wet gel, the [H₂O]_1/10 decreased and the gel's porosity increased (Fig. 8). When several kinds of alcohols were used as a solvent for the solution, the gel's porosity was inversely related to the [H₂O]_1/10 (Fig. 9). Residual carbon content was related to the [H₂O]_1/10 (Fig. 10). When wet gel involved only water and methanol, the residual carbon content was inversely related to the [H₂O]_1/10. When wet gel involved tert-butanol, the residual carbon preferred the form of the tert-butoxy group and its content increased (Figs. 10 and 11).

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It has been reported that a unique polytype transformation (6H→4H) in the sintering process of SiC ceramics with BeO addition was observed by means of analytical electron microscopy, in which α-SiC powder was used as a starting material. In this report, the sintering process of β-SiC was investigated. It was expected that the transformation would occur more remarkably in β-SiC powder than in α-SiC because there exists α β to α transformation in the sintering process. SiC powders with and without addition of BeO, AlN and B+C were prepared to study the effects of additives on the transformation. Sintered SiC bodies were hot-pressed at 1850°, 1950°, 2050° and 2200°C, and the structural changes were examined by means of X-ray diffraction and analytical electron microscopy. The results are summarized as follows. Without additives, grain growth was observed. However, the β to α transformation did not occur up to 2050°C. With additives, the transformation occurred at a temperatures dependent on the additives. With addition of BeO or AlN, α rapid β to α (4H) transformation occurred between 1850° and 1950°C. The transformation with B+C addition occurred above 2050°C and 6H polytype dominated. The different transformation temperatures might be responsible for the difference in polytypes after the transformation. The 4H structure formed in the presence of BeO or AlN was stable even at 2200°C.

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Free carbon in a silicon carbide raw material powder was determined by a combustion method. A sample of 0.5g was weighed out into a 3ml-quartz crucible, and the crucible was placed in a graphite-sealed quartz cell. The cell was raised into the induction furnace combustion tube and heated in oxygen flow at 850°C for 10min by controlling the voltage applied to the furnace. The amount of CO₂ produced by the combustion of free carbon in silicon carbide was measured with a thermal conduction detector. Heating at 850°C induced oxidation of silicon carbide. Therefore, the amount of free carbon in silicon carbide was calculated by the correction method in ANSI (B74.15). We also used the correction method based on the change of oxygen concentration. For the sample dried at 105°C for 1h, the values obtained by two correction methods did not agree each other. When the samples were heated at 500°C in an argon atmosphere for 1h, the values obtained by the two correction methods agreed well. Free carbon in five commercial silicon carbide powders were determined to be 0.26-0.92wt%, and the precision (relative standard deviation) was 1-11%.

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Effects of four additives on the microstructure and bending strength of aluminum titanate ceramics were investigated. Five wt% of MgO, Fe₂O₃, Y₂O₃ and ZrO₂ was added to a synthesized aluminum titanate powder which had a particle size of about 1μm. Firing temperature was 1300°, 1350° and 1400°C, and firing period was 2h. The specimens were cooled in the furnace. MgO and Fe₂O₃ dissolved into aluminum titanate as MgTi₂O₅ and Fe₂TiO₅, respectively. All specimens containing additives had crystalline phases besides aluminum titanate. In the specimens fired at 1300°C and containing ZrO₂, Y₂O₃, and no additive, corundum and rutile, decomposition products of aluminum titanate, were observed. Addition of MgO and Fe₂O₃ increased the sinterability of aluminum titanate, but addition of ZrO₂ decreased it. The grain size of aluminum titanate in fired specimens increased with increasing firing temperature, but decreased with the addition of MgO, Y₂O₃ and ZrO₂. Grain boundary microcracking during cooling due to the marked anisotropy of the aluminum titanate crystals increased with increasing firing temperature, and specimens containing MgO showed very little microcracking. Only MgO-containing specimens, especially those fired at high temperatures, exhibited transgranular fracture. These results suggest that the addition of MgO increases fracture surface energy of the grain boundary. Thus, the bending strength of MgO-added specimens was highest, 1850kgf/cm² at 1350°C. Other specimens containing additives showed also higher bending strengths, about 500-800kgf/cm² higher than blank specimens when fired at 1350° and 1400°C. The thermal expansion of the fired specimens containing MgO was about 0.7% at 1000°C, which was caused by very little microcracking during cooling.

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A plate of hot-pressed silicon nitride (HPSN), which is difficult to machine, was cut through by laser machining at the speed equal to that of usual slicing with a diamond wheel. Then the influences of laser machining on the strength and the composition of the material were investigated. The affected layer by laser machining was composed of three sites; site 1, melted granular materials sticked together, site 2, cracks were observed, and site 3, the residual stress existed without structural change. In the presence of this affected layer, the strength of the work decreased to one third of the strength of the material. From the changes of the strength caused by surface removal removing and the hardness in the cross section, the thickness of the affected layer that reduced the strength was estimated about 70μm except site 1. But the change of the composition was observed only in boundary layer between site 1 and site 2 which was rich in yttria and alumina. Since an X-ray diffraction pattern showed that site 1 was composed of silicon, it was confirmed that silicon nitride was decomposed into silicon and nitrogen by laser machining.

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In the system of 2R^I₂O⋅R^II₂O⋅7 SiO₂ glasses (R^I: Na, Cs and R^II: Li, Na, K, Rb, Cs) the electrical coductivity in the temperature range of 150°-1200°C and the self-diffusion coefficient of R^I ions at 600°C have been measured. The change in the mobility of the host alkali ion, R^I, has been examined by introduction of various guest alkali ions, R^II, The electrical conductivity and the self-diffusion coefficient of R^I decreased with an increase in the difference in the ionic radii of R^I and R^II in a series with same R^I. The dependence of alkali mobility on ionic sizes could be explained by the thermodynamics of molten alkali salt mixtures with a common anion. As the size parameter calculated from the theory on molten salt mixtures increased, the self-diffusion coefficients of R^I decreased in the molten state of glass. It means that Coulombic repulsion force between the alkali ions as next-nearest neighbors tends to be more weak, the greater the difference in the both ionic radii. This lowering of the repulsion causes the mixed alkali effect. The results in the solid state of glass have been discussed from points of view for the binding force of alkali ions and the frozen temperature of glass mentioned in the previous reports.

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Hot pressed (HP) and reaction bonded (RB) silicon carbide (SiC) ceramics were ground with a # 400 diamond wheel, and the induced damage was evaluated by the specimen strength measured by four-point bending test. The surface layers of the ground specimens were removed by polishing with a # 3000 diamond disc. The relationships between the strength and the thickness of removed layers were examined. In HP-SiC and RB-SiC, when the thickness was smaller than about 10μm, the strength was lower than that of the ground specimen. The maximum strength reductions were more than 30% for HP-SiC and more than 10% for RB-SiC of those of the original ground specimens, respectively. In addition, when the removed surface layers were more than about 10μm, the strength of each SiC ceramics recovered, and each showed a constant value regardless of the thickness. The strength seemed to have a similar value to the inherent strength of each SiC ceramics, and it was considered to be independent of the surface damage which was induced during grinding. In order to explain these reduction in strength, a simple model for the surface damage caused by grinding was proposed by considering internal cracks near the surface. The relationships between the thickness of removed layer and the theoretical specimen strength based on the model showed the same tendency as the experimental results.

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The correlation between temperature transforming into bloating of a formed sample and nominal temperature equivalent to its refractoriness in the JIS method was positive. This result suggested that refractoriness of Amakusa pottery stone is determined by thermal expansion curves. The various thermal expansion curves were measured under different conditions of milling time of samples, pellet thickress, forming pressure, heating rate and loading. Anomalous expansion due to bloating was observed, and the temperature (T₅) giving a linear expansion coefficient of 5×10^-4cm/cm/°C was independent of the pellet thickness, forming pressure and loading. The rectification method of T₅ under the different condition of particle size distribution and heating rate was discussed. The thermal expansion curves of 58 samples were measured under the fixed conditions: milling time: 0.5min, pellet thickness: 3mm, forming pressure: 2500kg/cm², heating rate: 10°C/min and loading: 10g. Hence values of T₅ from the above mentioned thermal expansion curves were calculated. The nominal temperature (T_sk) equivalent to its refractoriness of the same samples was measured by the JIS method. A correlative equation between the rectified temperature (T₅′(°C)) from T₅ using the contents of particles larger than 10μm, and T_sk (°C) was calculated by the method of least squares as follows,
T_sk=-1.931×10^-3T₅′²+6.288T₅′-3464.
The correlation coefficient between T_sk calculated by this equation and T_sk measured by JIS method was 0.980. The result showed that refractoriness for Amakusa pottery stone can be determined by thermal expansion.

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A simple constant stress, high precision compression creep machine is described for use at very low stress levels. The primary advantage of this machine is that it can be operated upto a temperature of 2023K under a minimum feasible load of about 1MPa. Some typical creep results are present for MgO single crystals with measured steady state strain rates down to about 10^-8S^-1. It is shown that the results obtained from this machine coincide with those from an ordinary creep machine at high stress levels. This machine has an additional advantage of studying stress-change behavior during creep.

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Electrical properties of (001) and (110) oriented SnO₂ films with SnO₂ crystallites oriented in the same direction as the rutile TiO₂ substrates, were investigated. The oriented films exhibited relatively low electric conductivities of 10^-2-10¹Scm^-1 (carrier concentrations of 10¹⁶-10¹⁹cm^-3 and mobilities of 0.3-16cm²V^-1s^-1) in comparison with polycrystalline films used as transparent conducting coatings. No anisotropy of the conductivity was noticeable in spite of the alignment of the SnO₂ particles along a particular crystallographic axis.

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Pr₆O₁₁-doped Si₃N₄, with or without simultaneous addition of other oxides were fired at 1900°-2050°C under 1MPa N₂ pressure. In Si₃N₄ containing 15mol% Pr₆O₁₁, it was assumed that an oxynitride having a structure similar to that of CeSiO₂N is formed at grain boundaries, resulting in highly refractive ceramic without any strength degradation from room temperature to 1300°C. When sintered at above 2000°C, nearly full-dense Si₃N₄ was obtained with 7.5mol% Pr₆O₁₁ and 7.5mol% Y₂O₃. The sample, having yttrium oxynitrides as grain boundary phases, showed a strength of over 800MPa at room temperature and of 600-700MPa at 1300°C. Si₃N₄ with 7.5mol% Pr₆O₁₁ and 7.5mol% Al₂O₃ a indicated significant strength degradation at 1300°C.

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