Tomozo Nishikawa

Thermal conductivity of reaction bonded Si3N4

To investigate the effect of α- and β-Si3N4 content on the thermal conductivity of silicon nitride ceramics, six samples containing various quantities of α-Si3N4 were prepared by the reaction bonding process and the thermal conductivity of the reaction bonded Si3N4 (RBSN) was measured by the transient hot wire method at various temperatures from room temperature to 1000°C. The RBSN samples containing 1, 25, 33, 45, 55 and 66% α-Si3N4 were produced by nitriding the compacts of pure silicon powder in a nitrogen atmosphere at 1480°C for prescribed periods. Bulk density, true density and total porosity of samples were 2.51±0.02g/cm3, 3.08±0.03g/cm3 and 18.6±1.0%, respectively, regardless of the α-content. No significant change in microstructure of RBSN samples was observed by scanning electron microscopy With an increase in the content of α-Si3N4, the thermal conductivity (λ) of BRSN samples decreased from λ=17.4Wm-1K-1 for the sample with α-Si3N4 of 1% (viz. consisted mainly of β-Si3N4) to λ=9.3Wm-1K-1 for that of 66%. It was considered that the α-Si3N4 solid solutions by which the thermo-elastic wave carrying the heat energy in the non-electro-conductive solid materials such as Si3N4 ceramics is scattered, might be formed with the impurities included in silicon powder as a starting material. The thermal conductivity of all RBSN samples decreased simply with increasing temperature due to the increase of the mutual scattering of the thermo-elastic wave, and the decreasing rate of thermal conductivity for the RBSN with lower α-contents was found to be larger than that for those with higher α-contents.

The Flow Behaviour of Concentrated Silicon Nitride Slurries under Vibration.

To obtain concentrated silicon nitride slurries appropriate for a vibro-casting (solid casting) to fabricate large scale products such as valve cocks or refractory blocks used in glass industry, the influence of pH on deflocculation/flocculation for silicon nitride fine powder/water suspensions was investigated. The viscosity of the concentrated slurries with various pH values was measured by means of pulling up ball method under vibration.The silicon nitride fine powder-water suspensions were found to be deflocculated in the range of 2-4 and 10-12 of pH, while they were flocculated primarily between 5 and 9 of pH, and secondarily below 2 or over 13 of pH.The apparent viscosity of the concentrated and deflocculated silicon nitride slurries under vibration was lower than that of the flocculated slurries. The apparent viscosity of both deflocculated and flocculated silicon nitride slurries decreased with increasing frequency and amplitude of vibration. However, feed of vibration with high frequency and large amplitude to silicon nitride slurries raised their apparent viscosity instead.

Hot-Extrusion of Bi-Superconducting Ceramic Powder

Bi2Sr2CaCu2Ox ceramic powder prepared by a soled-state reaction was encapsulated in Ag pipe with a dimension of 6φ(O.D.) -4φ(I.D.) x 200lmm. The outer diameter of the pipe was decreased to 1φmm by using a rotary swaging machine. The thin wires thus obtained were made up into a bundle, and the bundle was again processed by swaging to make a rod with multiple cores. A bullet was prepared by charging the rod into Cu can, and it was hot-extruded at relatively low degree of sectional area decrease. This procedure produced a highly efficient superconducting ceramic wire with multiple cores.

Prediction of Thermal Conductivity of Ceramic Fibrous Insulator

The thermal conductivity of alumino-silicate fibrous insulators with various bulk densities was measured by the transient hot wire method under various reduced pressures in order to propose a simple equation for predicting their thermal conductivities. The results obtained were summarized as follows:(1) The thermal conductivity of ceramic fibrous insulators with various bulk densities decreased gradually with decreasing pressure ranging from 760 to 10mmHg and converged to an extremely low constant value under 10-2mmHg after a sharp decrease between 10 and 10-1mmHg, due to the pressure dependence of thermal conductivity of air. Their thermal conductivity increased with increasing bulk density all over the pressure range covered in the thermal conductivity measurement.(2) The thermal conductivity of ceramic fibrous insulators in helium, oxygen and freon-12 gas atmospheres also increased with an increase in bulk density, and their thermal conductivity in a gas atmosphere having high thermal conductivity such as helium was higher than those in gases with lower thermal conductivity such as freon-12.(3) In alumino-silicate fibrous insulators, the following experimental relation among λ (thermal conductivity of insulator), λs (that of fiber: 1.07W/m·K), λf (that of gas being filled between fibers) and φ (volume fraction of fiber) was obtained.λ=aλs+bλf+(1-a-b)λsλf/(1-y)λs+yλfa=0.169φ1.57b=(1-φ)1.16y=0.81-0.043lnφThis equation enabled us to calculate the thermal conductivity of the fibrous insulators in various gas atmospheres (-0.2W/m·K) at room temperature within the error of about ±10%.

Correlation among Grain Boundary Internal Friction, Wear and Purity of Polycrystalline Ferrites

Internal friction of polycrystalline Ni-Zn ferrite and Mn-Zn ferrite were measured. Raw materials of industrial grade and specially-prepared grade (with very high purity) were used for the preparation of these ferrite. Internal friction value of grain boundary in Ni-Zn ferrite did not change with the purity of raw material. Mn-Zn ferrite with high purity showed smaller internal friction than the ferrite from industrial grade raw material. When these ferrite were used as head core of video tape recorder, the ferrite which had smaller grain boundary internal friction showed smaller wear. It seems to be that the ferrite which has smaller internal friction of grain boundary has better resistance to wear, and then the ferrite which has high purity has better resistance to wear, especially in Mn-Zn ferrite.

Sodium-Ion Conduction of Polycrystalline β-Alumina

In order to examine the conduction mechanism of Na-ion in polycrystalline β-Al2O3, the temperature dependence of the conductivity was investigated. The plot 1/T vs. σT for the polycrystal which was sintered well at a higher temperature, showed a bending point in the vicinity of a temperature of 250°C. The apparent activation energy of the conduction from the plot was 4-5kcal/mol in the higher temperature range, and 6-10kcal/mol in the lower temperature range. Consideration about the conduction paths of Na-ion in the polycrystal gives the conclusion that Na-ion diffusion within the easy conduction plane of β-Al2O3 single crystal controls the conduction in the higher temperature range, and Na-ion diffusion along grain boundary is the controlling process in the lower temperature range. On the other hand, the plot of 1/T vs. σT for the polycrystal which was sintered at a lower temperature, did not show the bending point. The apparent activation energy from the plot was almost the same as that in the lower temperature range for the well-sintered polycrystal. This means that the temperature range in which Na-ion diffusion along grain boundary is the controlling process extends to higher temperature side with decreasing the sintering temperature of the polycrystal.

Abrasion and Tempepature Dependence of Internal Friction of Some Polycrystalline Ferrites

Temperature dependences of Youngs modulus and internal friction of polycrystalline nickel-zinc and manganese ferrites were measured with sonic resonance technique. The effects of porosity of ferrite and small quantity of additives to ferrite on the temperature dependences were also examened. Exfoliation of grain particles from a polycrystalline ferrite was discussed with respect to the internal friction of the grain boundary. The results were as follows.1) Youngs moduli of nickel-zinc and manganese ferrites were measured in the temperature range from room temperature to about 1100°C. The temperature dependence of Youngs moduli showed a discontinuous change in the vicinity of Curie temperature in both nickel-zinc and manganese ferrites.2) In the temperature dependence curve of internal friction of nickel-zinc ferrite, large internal friction which must have resulted from anelasticity of grain boundary was observed. This large internal friction increased with increasing temperature without showing any maximum point.3) The temperature in which internal friction resulting from anelasticity of grain boundary appears, shifted to lower with larger porosity in nickel-zinc ferrite.4) The temperature dependence curve of internal friction of manganese ferrite showed a maximum in the vicinity of 700°C. This maximum is reasonably attributed to anelasticity of grain boundary, but not to the precipitated α-Fe2O3 on grain boundaries.5) Small quantity of some additives to manganese ferrite effected on the magnitude of internal friction and the temperature in which internal friction showed its maximum. The addition of small quantity of SiO2 to manganese ferrite decreased the internal friction and shifted the maximum temperature of internal friction to higher temperature side. And the manganese ferrite with small quantity of SiO2 showed little exfoliation of grain particles from the mother polycrystalline ferrite when the ferrite was rubbed with γ-Fe2O3 magnetic tape.