Materials Science Forum Vols. 561-565

Abstract: Energetic nitrogen ion was injected into a TiO2 photocatalyst in order to investigate the optimal local concentration of doped nitrogen for visible-light response. N+-implanted TiO2 samples promoted the photocatalytic activity under visible-light irradiation. N K-edge XANES of the highest activity sample indicated that N replaces the O sites near the surface, whereas in the samples of higher N+ fluence, N−O and/or N−N species formed. Depth-resolved N K-edge ELNES revealed the two types of N, depending on the concentration, and we found the local N concentration effective for visible-light response was less than ∼1 at%. Further, the spatial distributions of the different chemical states of N by energy-filtering TEM (FETEM) supported these findings.

Abstract: A new nitriding method has been devised which requires only a simple vacuum furnace and enables direct nitridation of solid aluminium without any prior surface treatment. It can be used to produce thick aluminium nitride surface layers on aluminium, under nitrogen at atmospheric pressure. A critical element of the process is the use of a magnesium vapour source that reduces/disrupts the natural, protective oxide film on the aluminium surface and facilitates nitriding. The nitride surface layers form through two distinct modes, one growing outward from the aluminium plate surface and the other growing into the aluminium. Studies of the nitride layers utilizing optical microscopy, TEM, SEM, XRD and XPS have been conducted. Details of the composition, structure and growth as well as possible mechanisms for the nitride formation are presented. Understanding of the reaction may have important implications for the production of wear resistant coatings on bulk Al as well as for the production of Al/AlN composites.

Abstract: The densification and phase transformation of high α-phase silicon nitride powder synthesized by SHS with a combination of Y2O3 and Al2O3 sintering additives were investigated. The densification process occurred rapidly from 1400 to 1500 °C and was nearly finished at 1500 °C. However, the α-β transformation of silicon nitride progressed rapidly from 1500 to 1600 °C, and completed at 1600 °C. The phase transformation of silicon nitride lagged behind the densification of Si3N4 ceramics. It showed that the solution and precipitation process did not play the main role in the densification of silicon nitride. The sufficient amount of liquid phase was crucial to complete the densification.

Abstract: Thermoelectric properties of zinc oxide ceramics are largely influenced by the mobility variation because of the formation of double Schottky barrier at grain boundary. It was demonstrated that magnetic texturing enabled to fabricate highly c-axis oriented ceramics with orientation degree of 100 MRD. This high orientation of grains resulted in periodic grain boundary structures along ab -plane, which was confirmed by edge-on HRTEM images. Hall measurement revealed that the mobility was enhanced at 80% by the magnetic texturing. Furthermore, it was estimated that the effect of magnetic texturing was equivalent to the reduction in the density of trap by 5.9×1012 cm-2.

Abstract: In this work, β-sialon ceramics were prepared from high-aluminium fly ash via carbothermal reduction-nitridation (CRN) and the physicochemical properties of the materials such as bulk density, apparent porosity, water absorption and flexural strength were also discussed. The results showed that the percentage of β-sialon phase in the product decreases as the temperature increases from 1400°C and the weight of the sintered specimen experienced an increase during 1350°C~1450°C due to the nitridation reactions, and followed by a gradual decrease till 1550°C for the decomposition of β-sialon. It is indicated that the optimum sintering temperature to obtain the highest yield of β-sialon ~93% lies in 1400°C~1450°C. The SEM images revealed that the prepared β-sialon sintered at 1400°C were mainly in shape of elongated prisms, typically ~5μm in length and 0.5~1μm in width. As the temperature increased to 1500°C and above, β-sialon decomposed and the new phases of SiC and AlN were formed at 1550°C as confirmed by XRD.

Abstract: To investigate doping-free zirconia, pulsed laser ablation technique was applied to synthesize pure zirconia nanoparticles. Various experimental parameters were investigated by means of microscopic, spectroscopic and diffractmetric techniques to reveal morphology and production yield. It is successful to form cubic and monoclinic phases at nanometer-scale fine particles. The cubic phase has relatively small lattice parameters than the stabilized zirconia.

Abstract: Ca-Ru-O compounds were prepared by spark plasma sintering (SPS) using CaCO3 and RuO2 powders at various molar ratio of Ru to Ca (RRu/Ca). CaRuO3 in a single phase was obtained at RRu/Ca = 1.0. A non-stoichiometric solid solution range of CaRuO3 was identified at RRu/Ca = 0.7 to 1.0. The electrical conductivity (σ) showed metallic conduction behavior and increased with increasing RRu/Ca. The electrical conductivity at RRu/Ca = 0.8 to 1.0 increased with decreasing oxygen partial pressure (PO2) and the PO2 dependence of σ became significant with increasing temperature.

Abstract: βSiAlON-cubic boron nitride (cBN) composites were prepared from β-SiAlON and cBN powders using spark plasma sintering (SPS) at firing temperature of 1600-1900oC under pressure of 100MPa, and densification, phase transformation and hardness of the composites were investigated. The phase transformation of cBN to hexagonal BN (hBN) was inhibited in βSiAlON-cBN composite. βSiAlON-cBN composites containing 10-30 vol% cBN fired at 1650oC were densified to 95-98% of theoretical density with no transformation of cBN to hBN. Vickers hardness of the βSiAlON-cBN composite containing 20 vol% cBN fired at 1650oC was 17.5 GPa in maximum value, and the hardness significantly decreased as cBN phase was transformed to hBN in the composites.

Abstract: Carbon fibers of ~9 μ m in diameter were used as templates to fabricate alumina and silicon carbide fibers. The carbon fibers were placed in a vacuum furnace with aluminum and heated at 1100°C for 8 h to form aluminum carbide. Then, the aluminum carbide fibers were oxidized in air at 1500°C. The resulted fibers were hollow and the alumina layer was porous in the interior. To fabricate silicon carbide fiber, carbon fibers were reacted with Si at 1300°C -1500°C in Ar. The thickness of silicon carbide layers increased with reaction temperature and reaction time. Solid fibers could be obtained after reaction at 1400°C for 4 h. In contrast to porous alumina layer, the silicon carbide layer/fibers were dense. The porous alumina hollow fibers were fragile while the solid silicon carbide fibers were flexible. BET surface area measurements revealed that the porous alumina had surface area as high as ~100 m2/g.

Abstract: SiC and AlN form a solid solution in the wide compositional range, expectantly leading to control of the semiconductive property. In the present work, the SiC-AlN composites were fabricated by sintering process, and evaluated with emphasis on the distribution of SiC and AlN and electrical property. SiC and AlN powders were mixed at a molar ratio between 90:10 and 10:90, and sintered at 1900-2100 °C for 30 min under 50 MPa in Ar atmosphere by spark plasma sintering technique. The sintered bodies reached high densities over 95 % of theoretical, and the grain size increased with an increase in the sintering temperature and the AlN content. The SiC-AlN composites had 3C and 2H phases in SiC-rich composition, while 2H phase only in AlN-rich composition, and the mutual dissolution between SiC and AlN was enhanced at high temperatures. The electrical conductivity decreased with dissolution of AlN into SiC because of the increase in band gap.