Masahiko Tajika

Effect of extended annealing cycles on the thermal conductivity of AlN/Y2O3 ceramics

Abstract The effect of extended annealing cycles (up to 50 h at 1800°C) on the thermal conductivity of polycrystalline AlN, doped with 5 wt% Y 2 O 3 , has been studied. The microstructural evolution upon annealing has also been characterized in detail, using quantitative scanning electron microscopy (SEM) observation and energy dispersive X-ray analysis (EDX). As-sintered AlN/Y 2 O 3 composites typically contained a dilute yttrium aluminate secondary phase well distributed and completely wetting the AlN grains. Upon annealing, the AlN matrix grains isotropically grew, while the grain-boundary yttrium aluminate phase tended to segregate to triple grain junctions. This segregation process produced a collapse of the grain-boundary film thickness, thus resulting in a completely different AlN microstructure dispersed with isolated yttrium aluminate grains. Equilibrium of the microstructural morphology was achieved after annealing times in the interval 5–10 h. As a consequence of microstructural changes, the thermal conductivity of the annealed AlN polycrystal exceeded that of the as-sintered material. A discussion is given about the variation of thermal properties in terms of both segregation to the triple-grain junctions of the intergranular Y 2 O 3 -phase and grain-growth of the bulk AlN grains.

Sintering behavior of direct nitrided AlN powder

Abstract Classified direct nitrided (DN) aluminum nitride (AlN) powders were examined by dilatometry to study the effect of particle size on sinterability. The shrinkage behavior of three DN powders was compared to that of a commercial, carbothermally produced, AlN powder. Yttrium oxide was added to AlN powder as a sintering aid. Two distinct features of the resulting shrinkage curves, corresponded to particle rearrangement coincident with yttrium aluminate formation and high temperature sintering. The presence of coarse particles in the original DN powder, greater than 8 μm, distinguished it from the other powders studied and appeared to have the greatest influence on the sintering behavior. For fine powders without coarse particles (>8 μm), the synthesis method did not seem to affect the shrinkage. The grain size distribution of the sintered parts mirrored the particle size distribution of the four powders studied. Thermal conductivity (TC) of the sintered AlN bodies was strongly dependent on the oxygen content of the starting powders.

Microstructures and properties in aluminum nitride–titanium nitride composite ceramics

Abstract Aluminum nitride–titanium nitride (AlN–TiN) composites (TiN content; 0, 5 and 10 vol.%) were prepared using mixtures of fine AlN and TiN powders. Pressureless sintering of the AlN–TiN powder mixtures was done at 1850°C for 1 to 20 h, using yttrium oxide powder as a sintering aid. Sintered samples achieved almost full density. While AlN grains grew isotropically with sintering time, TiN particles in the composite maintained their small and nearly round shape. The TiN grains were present at both grain boundary and within AlN grains. Heat treatment of these composites caused both microstructural change and improvements in thermal diffusivity. Inhibition of AlN grain growth was greatest for the 10 vol.% TiN–AlN composites.

Microstructural development in AlN composite ceramics

AlN-BN composite ceramics were fabricated by sintering a mixture of fine AlN and BN powders containing Y2O3 as the sintering aid at 1850°C in nitrogen atmosphere. BN contents up to 15 vol% were studied. The heat treatment of sintered AlN-BN composite was performed at 1800 °C in nitrogen atmosphere. BN particles in the composite showed large anisotropic grain growth during sintering, and the shape of grown BN particles appeared to be plate-like. Moreover, the heat treatment gave very interesting microstructures of the composite, in which many more interfaces between AlN grains were observed and most of the secondary phase was located at triple points of AlN grains. The resultant microstructural change is thought to be due to the change in the ratio of interfacial energy (between grain and liquid) and grain boundary energy (between grains).

Syntheses of Bulky Y-Zeolite by Hydrothermal Hot-Pressing (HHP) Technique

Synthesis of bulky Y-zeolite was attempted by a hydrothermal hot-pressing (HHP) method. These bulky products synthesized under hydrothermal conditions were identified as Y-zeolite single phase. Especially, bulky Y-zeolite having translucency, high density and large surface area was obtained by HHP treatment at 423K for 2h with 17wt% of 5M-NaOH solution. This solidified zeolite like single-crystal was considered to be made by dissolution and precipitation mechanism. High-density bulky zeolites can be expected as novel molecular sieves and catalysts with high activity.

Preparation of Porous Hydroxyapatite Using Extrusion of PAN

In this study, porous hydroxyapatite (HAp) bodies were prepared by the simple process. We fabricated porous fibers consisting of polyacrylonitrile (PAN) and HAp by an extrusion method of the mixture of PAN and HAp in dimethylformamide (DMF) solution. Fibers obtained by this extrusion method were chopped and molded. Subsequently, these compactions from chopped fibers consisting of HAp and PAN were sintered at various temperatures in air atmosphere. The sintered HAp bodies had high porosity and a number of pores with diameter some hundreds μm. These porous HAp bodies can be used as a scaffold in the body.