Masamitsu Imai

Improvement of the mechanical properties of hot-pressed silicon-carbide-fiber-reinforced silicon carbide composites by polycarbosilane impregnation

Abstract Green sheets of SiC with Al 2 O 3 –Y 2 O 3 –CaO sintering additives prepared by the doctor-blade method and polycarbosilane (PCS)-impregnated Hi-Nicalon cloth with a BN coating were used for the fabrication of SiC-fiber-reinforced SiC (SiC/SiC f ) composites by hot-pressing. Two kinds of SiC/SiC f composites with different fiber volume fractions were fabricated and their room-temperature mechanical properties were investigated. These composites showed non-brittle fracture behavior. The maximum strength of a composite with 52 vol.% of fibers was about twice as high as that of a composite with 40 vol.% of fibers, and the composite hot-pressed at 1700°C showed the highest maximum strength. In this fabrication process, PCS-impregnation into Hi-Nicalon cloth was an effective way of forming the matrix between fibers.

Processing and microstructure of silicon carbide fiber-reinforced silicon carbide composite by hot-pressing

Abstract Continuous 2D woven fiber-reinforced SiC composites were fabricated by hot-pressing in Ar at 1750°C under a pressure of 40 MPa using Al–B–C or Al 2 O 3 –Y 2 O 3 –CaO system as sintering additives. In this study, fracture behavior and microstructure of the composites fabricated by this process were investigated. These composites achieved nearly full density in both cases. In the case of the composite with Al–B–C additives, the load-displacement behavior of the composite with non-coated Hi-Nicalon cloths showed completely brittle fracture, whereas that of the composite with BN-coated Hi-Nicalon cloths showed ductile fracture with a lot of fiber pull-out. On the contrary, in the case of the composite with Al 2 O 3 –Y 2 O 3 –CaO additives, the load-displacement behavior of the composite with non-coated Hi-Nicalon cloths showed slight ductile fracture with small tails, whereas that of the composite with BN-coated Hi-Nicalon cloths showed completely brittle fracture.

Helium Release and Physical Property Change of Neutron-Irradiated α-SiC Containing B4C of Different 10B Concentrations

Sintered α-SiC ceramics containing B4C with various 10B concentrations were neutron-irradiated, and the helium release rate during increasing annealing temperature was observed. The lattice parameter and the macroscopic length changes of the same specimens were also observed. The helium release rate from powdered specimens increased markedly above 650°C, and showed a sharp peak at ~1,160°C independent of the 10B concentration, but the rate was higher for the specimen containing higher concentration of 10B. The macroscopic volume of the SiC specimen with higher 10B concentration was expanded larger than that of the lower one by the irradiation. The unit cell volume change was mostly independent of 10B concentration, whereas the each axis changed different manner. A-axis started to shrink around the irradiation temperature, and contracted mostly linearly with increasing annealing temperature. On the other hand, contraction of the c-axis was not parallel to that of a-axis, and was retarded between 500–1,100°C. It is supposed that helium will induce some lattice defects, which expands c-axis length of SiC. These defects retained up to 1,100°C with support of helium migration. During the annealing, the expansion of macroscopic length occurred at higher temperature than 1,300°C, where the higher 10B concentration specimen gave the larger expansion, indicating the formation of grain boundary helium bubbles.

Fabrication of Two Dimensional Silicon Carbide Fiber-Reinforced Silicon Carbide Composite by Electrophoretic Deposition and Hot-Pressing

In this study, Tyranno SA fiber cloth was coated with carbon black and SiC powder containing sintering aids by means of electrophoretic deposition method, and SiC/SiC composites with three different fiber volume fractions were fabricated using the Tyranno SA cloth by hot-pressing at 1700oC. The sufficient formation of the SiC matrix between each fiber could be observed. The composite fractured in non-brittle manner, and bending strength decreased with increasing fiber volume fraction. The crack propagation and fracture behavior depended on the fiber volume fraction. These differences in bending strength and fracture behavior would be caused by the difference in the interfacial bonding between fiber cloth and the matrix.

Thermal Conductivity Improvement by Heat-Treatment in Si3N4 Ceramics Using SiO2-MgO-Y2O3 Additive System

Silicon nitride (Si3N4) ceramics have been interested for electrical substrate applications, because the ceramics can be made highly mechanical strength, fracture toughness, electrical resistivity and high thermal conductivity. Generally, relatively large amount of additives are required to obtain dense Si3N4 ceramics. During sintering, additives react with SiO2 including surface oxide of Si3N4 raw powder to form a liquid phase. Most of liquid phase changed into glassy phase during cooling down. In this study, Si3N4 ceramics were fabricated by gas pressure sintering. Yttrium oxide (Y2O3), silica (SiO2), and magnesia (MgO) were used for liquid-phase-enhanced sintering process. Dense materials were sintered by this process, but their thermal conductivities were not so high (30-40 W/m·K). Therefore, post-sintering heat-treatment process was performed to reduce the excess amount of glassy phase. An additive system (3 mass% SiO2 with 3 mass% MgO and 1-5 mass% Y2O3) was selected as the sintering aid. These ceramics could be sintered to almost full density at relatively low temperature as 1650oC for 2 h under 0.1 MPa-N2 without packing powder. The resulting materials have high bending strength, about 1 GPa, when 5mass% of Y2O3 was added. Based on the creation of low temperature pressureless sintering without packing powder, a novel two-step sintering (once firing) was proposed. The two-step sintering conducted by sintered at 1650oC under 0.1 MPa-N2 for 2 h for densification in the first step. Followed by heated up to and kept at 1950oC for 8 h under 1.0 MPa-N2 in the second step. The Si3N4 ceramics could be fabricated with relatively high thermal conductivity of 90 W/m·K. Mass loss, microstructure, mechanical properties, oxygen content and chemical composition were discussed.

Fabrication and Mechanical Properties of In Situ Monazite-Coated Alumina Fiber-Reinforced Alumina/YAG Composites

Monazite(LaPO4)-coated alumina-fiber/alumina-YAG (Y3Al5O12) matrix composites were fabricated by in-situ coating of monazite followed by hot-pressing, and the effects of coating and sintering condition on mechanical properties of the composite were examined. Alumina powder and YAG powder (weight ratio, 95:5) were used as raw materials for green sheets, which was fabricated by tape casting technique. Monazite was synthesized by the in-situ reaction of La(NO3) solution with H3PO4 on the surface of fibers. After slurry infiltration into the coated fiber bundles, the fiber cloths were laminated with the green sheets alternately, then they were heat-treated, finally sintered by hot-pressing at various temperatures. The mechanical properties of the composites were changed by the fabrication conditions. Non-brittleness of the composites reduced with the increase of sintering temperature. The composites sintered at 1200oC showed the highest Weibull modulus and pseudo-ductility.

Synthesis and Growth Mechanism of SiC/SiO2 Core-Shell Nanowires by Thermal Evaporation Method

In this study, SiC/SiO2 core-shell nanowires (SiCNWs) were fabricated by thermal evaporation method without any catalyst, using pre-oxidized silicon powder and methane (CH4) gas as precursors. The reaction temperature was 1340°C in an inert atmosphere. The SiCNWs produced by this process had a single crystal β-SiC core ranging from 20 to 80 nm in diameter and low-crystalline SiO2 shell about 10-20 nm thick, and up to 1 mm long. The exhaust gas from the production system was analyzed by gas chromatography and the growth activity of SiCNWs was captured by digital camera during a soaking period. From the results, CO gas was detected only when SiCNWs were growing and was not found when methane gas was stopped. It was clarified that CO gas was produced as a by-product during the formation of SiCNWs. The formation mechanism of SiCNWs synthesized by thermal evaporation method was suggested to be oxide-assisted growth mechanism.

Exhaust gas analysis and formation mechanism of SiC nanowires synthesized by thermal evaporation method

Abstract Silicon carbide nanowires (SiCNWs) are a set of promising reinforcement materials due to their superior properties. However, formation mechanism of the SiCNWs synthesized by the thermal evaporation method without metal catalyst is still unclear. To understand the formation mechanism, SiCNWs were synthesized by the thermal evaporation method at 1350 °C using a pre-oxidized Si powder and CH4 gas as precursors. SiCNWs obtained by this method were β-SiC/SiO2 core–shell nanowires with average diameter about 55 nm and with a length up to 1 mm. The exhaust gases during the SiCNWs synthesis process were examined by gas chromatography and the photographs of growth activity of SiCNWs inside the furnace were captured. CO gas was detected during the active formation of SiCNWs. It was clarified that CO gas was one of the byproducts from SiCNWs synthesis process, and the formation reaction of SiCNWs should be 3SiO(g) + 3C(s) → 2SiC(s) + SiO2(s) + CO(g). The formation of SiCNWs was discussed based on the oxide-assisted-growth mechanism.

Sintering of Silicon Carbide Ceramics with Co-Addition of Gadrinium Oxide and Silica and their Mechanical Properties

Effects of simultaneous addition of SiO2 and gadrinium oxide on densification of SiC ceramics were examined, and relation between microstructure and their mechanical properties were discussed. Total 11wt% of Gd2O3 and SiO2 were mixed with fine -SiC powder. The weight of Gd2O3 in (Gd2O3 + SiO2) were set as 0, 20, 40, 60, 80 and 100%. The mixture was hot-pressed at 1950oC under 40 MPa applied pressure for 1 h. In the case of 40Gd2O3 and 80Gd2O3 compositions, the effect of sintering temperature from 1900 to 2000oC was also examined. The bulk density increased with increasing Gd2O3 content at the sintering temperature of 1950oC. Bending strength of the sintered bodies also improved with increasing Gd2O3 content generally, but at 40Gd2O3 composition, the maximum over ~800 MPa was observed. Young’s modulous, Vickers hardness and fracture toughness also increased with increasing Gd2O3 content. The distribution of grain boundary phase was not homogeneous. Evaporation of additives, mainly SiO2, caused non-homogeneous distribution of grain boundary phase between outside and inside of sintered bodies. High temperature bending strength of 80Gd2O3 specimen was superior than that of 40Gd2O3 specimen.

The Effect of Heat-Treatment on Thermal Conductivity of Silicon Nitride Ceramics

Alpha- or beta-Si3N4 powder with larger grain size was uses as starting material, and the effect of heat-treatment on thermal conductivity of Si3N4 ceramics using MgO, Y2O3 and SiO2 as sintering additives was investigated in terms of their microstructure and the amount of grain boundary phase. Most of the components derived from sintering additives existed as glassy phase in sintered Si3N4. After heat-treatment at 1950oC for 8 h, the amount of glassy phase significantly decreased, and then small amount of glassy phase existed in Si3N4 ceramics was crystallized as Y2O3 and Y2Si3N4O3. In the case of Si3N4 ceramics using SN-7 powder, thermal conductivity of heat-treated Si3N4 was around twice of the value of sintered Si3N4, and the thermal conductivity was increased from 41.4 to 87.2 W/m•K due to not only the reduction of grain boundary phase but also the grain growth. In the case of Si3N4 using SN-F1 powder, thermal conductivity of Si3N4 ceramics was also significantly increased from 36.0 to 73.2 W/m•K after heat-treatment. In this case, the reduction of grain boundary phase mainly affected the thermal conductivity of Si3N4 ceramics because the grain size of heat-treated Si3N4 was nearly the same as that of sintered Si3N4. The reduction of grain boundary phase from Si3N4 was effective for the improvement of their thermal conductivity in addition to grain growth of Si3N4.