Multiple cation substitution in the B site of the perovskite oxide, ABO₃, has been investigated mainly for the 3d transition metal cations under usual preparation conditions, and the cationic valencies have been determined using the electrochemical concept. The maximum number of the kinds of transition metal cations in equivalent amounts were nine for the perovskite oxide prepared in the present study, whose composition is La (TiVCrMnFeCoNiCuZn)₁O₃. It has been concluded from the map of the cationic valencies calculated in the present study that all cations, even Zn and Cu, in the B site exist as M³⁺ in LaMO₃ and that Nb exists as Nb⁵⁺ in La_1-xSr_xMO₃. This has been proved from the measurement of the lattice constants of the perovskites. The maps for the cationic valence have been found to be very useful for the determination of the distribution of the cationic valencies in the B site of the various perovskite oxides.

View full abstract

A NbN powder has been prepared from NbCl₅ and a stoichiometric amount of a reducing metal at the temperature ranging 200°C to 1100°C in a nitrogen stream (150-200ml/min). The influences of Al, Zn and Mg metals on nitridation of NbCl₅ were examined. In the NbCl₅-Al-N₂ system, NbN was formed above 500°C. After heating the powder mixture at 1100°C for 1h, the N/Nb ratio in the product was 0.88. The ratio was slightly higher than the theoretical value. On the other hand, in the NbCl₅-Zn-N₂ system, formation of NbZn₃ alloy was observed from 400°C to 500°C, and NbN was formed above 700°C. After heating the powder mixture at 900°C for 1h, the N/Nb ratio in the product was 0.63, but the N/Nb ratio decreased above 900°C. In the NbCl₅-Mg-N₂ system, NbN was formed above 500°C, and MgO formed from oxygen, which was present in trace amounts, was removed by washing the product with 1N HCl solution. The particle size of NbN powder ranged from 0.1 to 0.3μm. After heating the powder mixture at 1100°C for 1h, the N/Nb ratio in the product was 0.91. When heated at 1100°C for 4h, the N/Nb ratio in the product was 0.94, which was the highest value among the three reducing agents. The above results indicated that Mg was the most effective reducing agent.

View full abstract

Oh and Finnie developed the statistical theory of fracture location which can be used to estimate not only the fracture stress but also fracture location. However, this study is restricted only to one type of fracture origin. In fact, brittle materials like ceramics involve many types of fracture origins. Recently, we established a new theory by combining the statistical theory of fracture location with the competing risk theory. In the present study, we developed a new method of estimating Weibull's shape parameters on the basis of this theory, which has made it possible to estimate Weibull's shape parameters only from fracture location data. Using 3-point bending test data for silicon nitride, we estimated the shape parameters by the present method. The estimated results were compared with those obtained by the multi-maximum likelihood method. It was found that they coincide with each other satisfactorily. Further, the implications of Weibull's shape parameters for the fracture location in the 3-point bending test are discussed.

View full abstract

Dense two-phase composite bodies of cordierite and mullite with various volume fractions were fabricated by firing at 1450°C. The bending strength of the composites increased with increasing mullite fraction. It was revealed that their linear thermal expasion coefficients are proportional to the volume fraction of the constituent mineral. The expansion coefficients of the composites were investigated referring to Kerner's and Turner's equations, and the reason why Ker-ner's equation was better applicable to the composites was discussed. Stresses due to the difference in expansion coefficients between cordierite and mullite were verified by the X-ray diffraction method. Microcracks surrounding mullite particles formed in the cordierite phase of the composite body. And the shape of those cracks looked as if they had formed in a glassy matrix.

View full abstract

Single crystals of NbC were grown using Ni or Co metal, or Ni-Co alloy as a flux at soaking temperatures of 1600°-1900°C. When Ni flux was used, the crystal size increased with increasing temperature to 1800°C. Neither effect of keeping time (4-16h) nor cooling rate (0.7-7°C/min) on the crystal size at 1600°and 1700°C was found, while the longer keeping time and slower cooling rate at 1800°C resulted in the decrease of the crystals. The crystal of a maximum size of 1.8mm was grown at 20wt% NbC concentration when the melt was kept at 1800°C for 4h, and cooled at 7°C/min. The golden black cubic crystals obtained have a lattice constant of a₀=4.4695±0.0005Å, with nearly stoichiometric compositions The grown faces of the crystals were {100} family, and the dislocation density of 10⁸/cm² on these faces was measured for the crystals grown from 1800°C. The crystals grown after keeping at 1800°C for 4h contained Ni impurity of about 2900ppm, which then decreased to 600ppm in 16h. In the case of Co flux, the crystal size decreased with increasing temperature from 1600°to 1800°C, amaximum size being of 1.0mm. Neither effect of keeping time nor cooling rate on the crystal size was found at any temperature. The lattice constant and grown faces of the crystals were the same as the case of Ni flux. The crystals were found to contain a free carbon of 1wt% and Co impurity of 1250-7900ppm depending on the growth temperature of 1600°-1800°C. Ni-Co alloy flux with the wt% composition of Co/Ni=1/2 and 4/1 gave the crystals of 1.5 and 1.2mm size, respectively, at the NbC concentration of 15wt% and the growth temperature of 1700°C.

View full abstract

The temperatures of glass transition (T_g), crystallization (Tc) and melting (T_m) during heating were measured for compositions of 50-70CaO⋅15-40Ga₂O₃⋅0-20GeO₂. The crystallization temperature (T_k) during cooling and the fraction of precipitated crystals were measured from DTA data. Time-temperature-transformation (T-T-T) curves were determined experimentally. The position of the nose was 30s and 1050°C for the composition of 65CaO⋅25Ga₂O₃⋅10GeO₂. Cooling rate (Q)-temperature-transformation (X) (Q-T-T) curve is obtainable by plotting -Qⁿ⋅ln (1-X) vs. temperature, where n depends on crystallization mechanism and other factors and is 4 and 1 or 2 above and below 1150°C, respectively.

View full abstract

The internal friction of alkali aluminosilicate glasses containing divalent ions was measured from -100° to 550°C at a frequency of about 1Hz and two peaks were obseved. The peak observed at low temperature is due to the motion of alkali ions, i.e. the low temperature peak. Since the compositional dependence of the peak observed at high temperature was similar to that of the mixed-cation peak in mixed alkali glasses, it was suggested that its peak was the mixed-cation peak which was attributed to coexistence of alkali and divalent ions. The relaxation mechanism of the mixed-cation peak is probably associated with the interaction between alkali and divalent ions, and is essentially the same as that of the mixed-cation peak in mixed alkali glasses. There was the close correlation that the heights of the low-temperature peak and the mixed-cation peak were inversely proportional each other. Consequently, the relaxation mechanisms of the mixed-cation peak must have the same kind of relaxation element of the low-temperature peak, i.e. alkali ions.

View full abstract

The low grade pottery stone containing brown layer, of which color is due to the presence of high concentrated ferric ions, was reduced in H₂ atmosphere in order to obtain ferromagnetic materials. The results of Mössbauer spectrum, characteristics of magnetization vs. magnetic field, and X-ray diffraction patterns of brown layer before and after reduction treatment indicated that ferric ions are present in amorphous ferric hydroxide before the reduction treatment, and that ferromagnetic Fe₃O₄ or α-Fe is produced by the reduction treatment. The reduction treatment increased the magnetization of brown layer by a factor of 50. Thus, the brown layer was easily separated from the colorless part of pottery stone by applying the magnetic field to samples. When brown layer attracted to magnetic was immersed in dilute hydrochloric acid, ferromagnetic compounds were dissolved faster than that of raw pottery stone. This treatment lowered the Fe₂O₃ content in brown layer to 0.36%.

View full abstract

The PTCR (positive temperature coefficient of resistivity) characteristics in hot-pressed semiconducting barium lead titanate ceramics with the composition of Ba_0.35Pb_0.65TiO₃ have been investigated and compared with those in the porous ceramics with the same composition, which have been confirmed to exhibit the PTCR effect of more than four orders of magnitude. Influences of heat-treatments and the addition of Mn on the PTCR characteristics in the hot-pressed materials were also examined. The results obtained are summarized as follows:
(1) Too much densification of PTCR materials causes a smaller PTCR effect.
(2) Heat-treatment above 1200°C yields a complicated influence on the electrical conduction of the material through both grain growth and change of the composition due to the vaporization of PbO.
(3) The addition of Mn in the hot-pressed materials resulted in no significant improvement of the PTCR characteristic, but a slight improvement was observed at a doping concentration of less than 0.03mol%.

View full abstract