[in Japanese]

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[in Japanese]

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Aqueous suspensions of α-Ca₃(PO₄)₂ powder were treated at 80°C and at various initial and fixed values of pH by using the “pH spontaneous drift” and “pH-stat” methods. The products were characterized by X-ray diffraction, chemical analysis, infrared absorption spectroscopy, thermogravimetry and electron microscopy.
Products obtained above pH 4.6 consisted of a single phase of apatite having various Ca/P ratios. As the pH was controlled higher, the Ca/P molar ratio tended to be higher values ranging from 1.50 to 1.67, while the rate of apatite formation was decreased by the controlling. The shape of the apatite crystals formed by the “pH-spontaneous drift” method was thin leaves elongated along the {1010}. In the “pH-stat” experiments, the apatite crystals were either thin leaves or cylindrical rods depending upon the fixed pH. In the range of pH 4.6 to 4.3 mixtures of apatite and octacalcium phosphate crystallized as thin blades. In the range of pH 4.2 to 3.0 platelet CaHPO₄ crystals were a major constituent of products. The apatites prepared by using this reaction system had a high crystallinity compared with that of apatites obtained by usual precipitation reactions, The dehydration process of apatites was similar to that of octacalcium phosphate, and the process was divided into, at least, three regions: below 140°C, 140°-600°C, and 750°-850°C. The amount of dehydration in the each region increased as the Ca/P ratio decreased. HPO₄^2- groups in the apatite structure converted to P₂O₇^4- groups near 250°C. This temperature was considerably lower than that (-400°C) for CaHPO₄. Consequently, it was suggested that these apatites were close to octacalcium phosphate with respect to the nature of water and HPO₄^2- contained in both the structures.

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Strength and fracture behavior of hot-pressed silicon nitride were studied in vacuum at high temperatures in three-point bending. Two kinds of silicon nitride were compared; SN-3 containing Mg, Al and Fe and HP (N) containing Al, Ti, Fe and Y. Furthermore surface flaws of controlled size and shape were produced in the specimens with a Vickers hardness indenter and the specimens were annealed at high temperatures in vacuum, in order to know the effect of the annealing on the room temperature strength in air of the specimens.
The results obtained were as follows:
(1) The annealing precracked specimens led the increase of the room temperature strength due to the relaxation of residual stress at crack at lower temperature and due to the evaporation-condensation of the material at higher temperature.
(2) The fracture surface of both SN-3 and HP(N) showed that crack transgranularly propagated at room temperature whether the specimens were precracked or not and whether they were annealed or not.
(3) Both specimens showed the decrease of strength at high temperature which coincided with that for the the beginning of slow crack growth. The slow crack growth occurred intergranularly and caused the decrease of the strength at high temperatures.
(4) The slow crack growth occurred above about 1000°C for SN-3 and about 1200°C for HP(N) not only in uncracked but also in precracked specimen. The difference of the nature of glassy phase or additives in both specimens may account for the difference of the beginning temperatrue of slow crack growth. Slow crack growth occurred in the bending at 1400°C in air as well.
(5) The critical stress intensity factor, K_IC, was obtained using a fracture mechanics formula for semielliptical surface flaws in bending: 450kg/cm^3/2 for SN-3 and 575kg/cm^3/2 for HP(N) at room temperature. At high temperatrue K_IC apparently increased when using the size of crack propagated by slow crack growth.

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Sinterability of submicron powders of β-SiC prepared by reactions of carbon black with silicon, silicon monoxide, or silicon dioxide was studied by measuring shrinkages of those compacts contained 1wt% of boron and 1wt% of carbon as a function of temperature. And then, polytypes in sintered pellets were identified and the microstructures were observed.
The obtained results were summarized as follows:
While compact of Si-derived β-SiC powder did not attain high bulk density, that of SiO₂-derived powder attained -95%TD, and both sintered pellets consited of grains of about 100μm of β-SiC with α-SiC. The maximum bulk density reached in the present study was -96%TD for the sintered pellet of SiO-derived powder. The sintered pellet consisted of grains of about 10μm of β-SiC and a few percent of α-SiC.
By mixed acid (fluoric and nitric acids) treatment, green densities of compacts of treated powders were reduced to -40%TD regardless of raw materials of silicon. Their compacts began to shrink at 1300° to 1400°C which was lower by 200° to 300°C than the initiation temperature of shrinkage in non-treated powders.

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This investigation represented the effects of addition of MgO on firing process of bone china bodies prepared by use of the bone phosphate. On the standard body consisting of bone phosphate, limestone, clays, feldspar and quartz, and on the two bodies, the composition of which were modified from the standard one by replacing partially sericite clay by talc or replacing limestone by magnesite. Thermal expansion and shrinkage behaviours of the green bodies were investigated. On the fired bodies, the microstructure were investigated by X-ray diffraction, polarization microscope and a scanning electron microscope. Further, bulk specific gravity, modulus of elasticity, transverse strength, and electrical properties were measured. On the basis of the results obtained the effects of the ion replacement on the vitrification process were discussed.
The addition of MgO had the effect to promote the vitrification of bone china body. The Mg ion breaks P-O-P bond in Ca₂P₂O₇ and induces the formation of Ca₃(PO₄)₂. This effect of Mg is efficient to produce phosphate glass in the body. Magnesia act also as a glass-former together with silica, that is to say, they compose a Mg-SiO_x complex, which dissolving Ca-PO₄ complex, becomes a component of the matrix glass in the bone china body. By replacement of K by Mg in the bone china body, their mechanical strength was increased, which might be caused by increase of ion bonding force in the matrix glass due to an increase of ion density in the glass.

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Graphite-boron carbide bodies were made from calcined pitch coke powder with boron carbide addition in the range from 0 to 50wt% using hot-pressing method at the temperature from 1800° to 2200°C under the pressure of 200kg/cm², and the effects of amount of boron carbide added and hot-pressing temperature on some properties of the hot-pressed bodies were investigated.
Some properties of the hot-pressed bodies varied with temperature and amount of boron carbide added. Above 2000°C, particularly at 2200°C, variation of boron carbide content was found to have a remarkable influence for densification of the hot-pressed bodies and graphitization behaviour of coke.
In the case of the hot-pressed bodies made at 2200°C, relative density increased with increase of boron carbide content and showed about 96% over the range above about 30wt% boron carbide, and bending strength also increased up to about 1500kg/cm² at 30wt% and about 1800kg/cm² at 50wt% with increase of boron carbide content. While graphitization of coke proceeded very rapidly to the minimum value of d₍₀₀₂₎ spacing of 3.358Å at about 5wt% and then the value gradually increased with increase of boron carbide content. Electric resistivity also showed the minimum value of 1×10^-3ohm·cm at about 5-10wt% and then had a tendency of slight increase with increase of boron carbide content.
Content of boron carbide in the hot-pressed bodies decreased by 3-5wt% as compared to that of the one added before hot-pressing. Boron atoms of the boron carbide disappeared were considered to diffuse into coke particles and also into graphite mold and punches. The densification of the hot-pressed bodies and graphitization of coke were considered to be accelerated by the diffusion of a small amount of boron atoms into coke particles above 2000°C under the pressure.

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As a preliminary study on the decomposition kinetics of aluminium titanate, factors affecting the rate of decomposition were examined using different reaction-sintered samples and samples obtained by milling and/or annealing.
Sigmoidal decomposition isotherms were obtained for powdered and sintered samples. The decomposition rate increased as milling time increased, but annealing at 1310°C made the decomposition more sluggish especially in finer particles. The decomposition rate of the sintered samples lowered with decreasing forming pressure and increasing sintering time, and especially by annealing. These facts imply that the decomposition process is nucleation and growth-controlled and that the nucleation at the grain boundaries of sintered samples is favored by densification and checked by grain growth.
Avrami plots for the decomposition of sintered samples gave (n=3) except for the early stage in fast decomposing samples (n=2) and for the annealed ones, (n=4) where n is Avrami index. It was inferred that compression stress in the grain boundaries caused by anistropic thermal expansion increased the nucleation rate and brought about “site-saturation” for nucleation, so that (n=2) and (n=3) correspond to edge nucleation and three dimensional growth with site-saturation respectively, and (n=4) to three dimensional growth with a constant low nucleation rate.

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