Jun-ichi Matsushita

Solid-state diffusion bonding of closed-cell aluminum foams

Solid-state diffusion bonding (DB) was demonstrated for joining closed-cell aluminum foams (ALPORAS). A superplastic 5083 aluminum alloy sheet was inserted between the foams to assist the DB process. Microscopic observation revealed that the cell wall of the foams penetrated into the 5083 alloy sheet and their boundary partly disappeared. Energy dispersion X-ray spectrometer (EDS) confirmed the diffusion of magnesium element from the 5083 alloy to the aluminum foam regions. The bonding strength was evaluated by four-points bending tests. The obtained flexure stress was about 50% of the original foam at room temperature and was more than 60% at 423 K. The advantage of the DB process in the high temperature applications was discussed comparing with the adhesive bonding of aluminum foams.

Sintering of indium–tin–oxide with vanadium oxide additive

Abstract Indium–tin–oxide (tin-doped indium oxide) powders and the mixture of indium oxide In 2 O 3 powders and tin oxide SnO 2 powders were sintered pressurelessly in air with vanadium oxide V 2 O 5 additive (1.0 wt.%) whose melting point (≈690°C) is much lower than the sintering temperature. Densification was remarkably observed without sacrificing the electroconductivity; e.g. relative density of 96% and resistivity of 5.5×10 −4 Ω cm was obtained when sintered at 1400°C. The mechanism of the densification was discussed in terms of the liquid phase sintering and the formation of V–In–O compound.

Sintering of tin-doped indium oxide (Indium-Tin-Oxide, ITO) with Bi2O3 additive

Tin-doped indium oxide (Indium-Tin-Oxide, ITO) is known as a poorly sinterable material. Densification of ITO powders with relatively large particle size (1–2 μm) was enhanced remarkably by the additive (Bi2O3) whose melting point is lower than the sintering temperature. The maximum bulk density of 6.75 g/cm3 (relative density; 95%) was obtained when pressurelessly sintered in air at 1500°C for 5 hours using the starting material containing 2.0 mass % i2O3, while the density was approximately equal to the green density when sintered using the starting material without Bi2O3. Increase of electrical resistivity caused by the additive was suppressed successfully when a small amount of Bi2O3 (1.0 mass %) was added and heated at 1500°C. The bismuth was eliminated from the sintered body to achieve the low resistivity (8.1 × 10−4 ohm · cm) which was approximately equal to that of the pure ITO.

Oxidation of Calcium Boride at High Temperature

The oxidation of CaB6 powders at high temperatures was investigated. The sample oxidized at 873 to 973 K for 25 hours exhibited weight gain with increasing oxidation temperature; the oxidation proceeded in accordance with the parabolic law during the initial oxidation stage. On the other hand, the weight gain of the sample oxidized at and above 1073 K for 4 h was approximately 80%; however even if the oxidation time was prolonged, an additional weight change did not occur. Based on the results of the X-ray diffraction analysis, calcium borate (CaB4O7) was present on the surface of the sample oxidized at 1073 K. The sample showed a good oxidation resistance at 1273 to 1373 K, because the surface film of calcium borate (CaB2O4) formed by oxidation acted as an oxidation resistant layer.

Sintering and Mechanical Properties of Chromium Boride - Chromium Carbide Composites

Several boride sintered bodies such as TiB2, ZrB2, and SiB6 were previously reported. In the present study, the sinterability and physical properties of chromium boride (CrB2) containing chromium carbide (Cr3C2) sintered bodies were investigated in order to determine its new advanced material. The samples were sintered at desired temperature for 1 hour in vacuum under a pressure by hot pressing. The relative density of sintered bodies was measured by Archimedes’ method. The relative densities of CrB2 addition of 0, 5, 10, 15 and 20 mass % Cr3C2 composites were 92 to 95 %. The Vickers hardness of the CrB2 with 10 and 15 mass % Cr3C2 composites were about 14 and 15 GPa at room temperature, respectively. The Vickers hardness at high temperature of the CrB2 addition of 10 mass % Cr3C2 composite decreased with increasing measurement temperature. The Vickers hardness at 1273 K of the sample was 6 GPa. The Vickers hardness of CrB2 addition of Cr3C2 composites was higher than monolithic CrB2 sintered body. The powder X-ray diffraction analysis detected CrB and B4C phases in CrB2 containing Cr3C2 composites.

High-Temperature Thermoelectric Properties of Silicon Boride Ceramics as a Smart Material

Several silicon boride phases such as SiB4, SiB6, SiB6-x, SiB6+x, and Si11B31, were previously reported. Among them, SiB6has proved to be a potentially useful material because of its excellent electrical conductivity, high degree of hardness, moderate melting point, and low specific gravity. The sintering conditions and thermoelectric properties of silicon boride (SiB6) ceramics produced by hot pressing were investigated in order to determine the suitability of this material for high-temperature thermoelectric applications as a smart material. The relative density increased with increasing sintering temperature. With a sintering temperature of 1923 K, a sintered body having a relative density of more than 99% was obtained. X-ray diffraction analysis showed no crystalline phase other than SiB6 in the sintered body. The specimens were prepared for measurement of the electrical conductivity and Seebeck coefficient by the D.C. four-terminal method. The thermal conductivity of SiB6 was obtained by calculation from the thermal diffusivity and specific heat capacity of the specimen. The electrical conductivity of SiB6 increased with increasing temperature. The electrical conductivity of the polycrystalline SiB6 (99% dense) was 0.5 to 1.1 × 103 S/m at 298 to 1273 K. The thermal conductivity decreased with increasing temperature in the range of room temperature to 1273 K. The thermal conductivity was 9.1 to 2.5 W/mK in the range of room temperature to 1273 K. The Seebeck coefficient of SiB6 increased with increasing temperature. The Seebeck coefficient of SiB6 was 140 × 10−6 V/K at 1273 K. The figure of merit Z of SiB6 increased with increasing temperature. The Z of SiB6 reached 8.1 × 10−6/K at 1273 K. The ZT value is useful to evaluate the ability of thermoelectric materials. The ZT value reached 0.01 at 1273 K. Based on the results, SiB6 showed very good thermoelectric material characteristics at high temperature.

Oxidation of Tantalum Nitride

Tantalum (Ta) can be use a suture for operation and implant material in order not to react with body fluid and stimulate a human body. In this study, the stable oxide of a tantalum, tantalum oxide layer produced by oxidation of the tantalum nitride, TaN powders by high temperature oxidation were investigated in order to determine the possibility of its a distributed aid for biomaterial composite such as an artificial root etc. The sample, TaN powder oxidized at high temperature exhibited a steady mass gain with increasing oxidation temperature. Based on the results of the XRD, tantalum oxide, Ta2O5 was detected on the samples. It is considered, the TaN showed a good oxidation film produced by high temperature oxidation.

Oxidation Behavior of Tantalum Boride Ceramics

The oxidation behavior of tantalum diboride (TaB2) powder at high temperature was investigated in order to determine the possibility of the use of advanced high temperature structural materials. Unfortunately, monolithic TaB2 were known to be chemical stability up to high temperatures. To date, there have been few reports regarding the properties of TaB2 ceramics. The samples were oxidized at room temperature to 1273 K for 5 minutes to 25 hours in air. The weight changes were measured to estimate the oxidation resistance. The oxidation of samples oxidized for short oxidation time of 5 minutes started at 873 K, and the weight gain increased with increasing oxidation temperature. On the other hand, at the oxidation time of above 1 hour, a maximum weight gain value at 973 to 1073 K was observed. However, even if the oxidation temperature was increased an additional weight change slightly occurred. The weight gain of the sample oxidized at 1273 K for 5 minutes to 25 hours was about 40 to 20 % of the theoretical oxidation mass change. According to the powder X-ray diffraction date, the oxidized TaB2 sample was changed to Ta2O5 at 873 K. Finally, the TaB2 showed a good oxidation resistance at high temperature, because the surface film of tantalum oxide (Ta2O5) formed by oxidation acted as an oxidation resistant layer.

Oxidation of Pentatitanium Trisilicide (Ti5Si3) Powder at High Temperature

The oxidation of pentatitanium trisilicide (Ti5Si3) powder at high temperature was investigated in order to determine the suitability of this ceramic material for advanced application in an oxidation atmosphere at high temperature. Titanium silicide has been attracted for years as an engineering ceramics due to its high hardness, high melting point, and good chemical stability. The samples were oxidized from 300 to 1000 °C for 1 to 5 h in air. The mass changes were measured to estimate the oxidation resistance of the sample. The mass gain of the sample oxidized at 1000 °C for 5 h was about 26 % of the theoretical oxidation mass change. The commercial powder, Ti5Si3 showed an excellent oxidation resistance at 1000 °C, because the surface film of both titanium dioxide and silicon dioxide formed by oxidation acted as an oxidation resistant layer.

Preparation of Si-B-C system powder using silicon, boron, and boron carbide

Boron carbide composites in Si-B-C System have been widely studied and applied in excellent engineering materials. These materials are usually used at high temperature. Unfortunately, amorphous Si-B-C ceramics have been few reports regarding the properties of Si-B-B4C ceramics. In this study, the preparation of crystallized Si-B-C system compounds using Si, B, and B4C powders was investigated to determine their potentially for applications as high hardness composites. The samples were prepared at 1673 K for 2 hours in Ar atmosphere. The sintered bodies were cut into 5 х 5 х 10 mm blocks and polished with diamond disk for Vickers hardness. The samples were subjected to X-ray diffraction (XRD) analysis for phase evolution using a powder X-ray diffractometer. The fracture surfaces of the specimens were observed using a scanning electron microscope (SEM) included an energy dispersive X-ray fluorescence spectrometer (EDX) system to estimate the microstructures.