Makio Kinoshita

Characterization of crystalline zirconium phosphates and their isomerization activities

The catalytic activities for ring-opening isomerization of cyclopropane and isomerization of butenes have been examined on crystalline zirconium phosphates. e-Zr(HPO4)2 catalyst was highly crystallized during the dehydration of zirconium phosphate gel with concentrated phosphoric acid solution under reduced pressure. This catalyst, which was evacuated at higher temperatures (above 770 K), exhibited higher catalytic activities (based on unit surface area) than α-Zr(HPO4)2 · H2O catalyst or other conventional solid acid catalysts. The coisomerization of d0− and d8−-1-butene suggests that isomerization would proceed on protonic acid sites even after heat treatment at 1100 K. After evacuation at 773 K, most of phosphate groups were removed, with consequent loss of water, due to the condensation of phosphate groups between each zirconium atom layer. However, a trace amount of residual phosphate groups still remained on the surface. After heat treatment at higher temperatures, the stretching and bending vibration of POP appeared in infrared studies; their intensities increased with increasing temperatures of evacuation. Even though the protonic concentrations decreased, the reaction rates for isomerization were drastically enhanced, because of the presence of POP bonds which could withdraw the electrons from the residual phosphate groups on the surface. Thus some enhancement of acid strength of protons of phosphate groups may occur.

Sinterability of various high-purity magnesium oxide powders

The sinterability of high-purity MgO powders with different production histories was investigated to make clear the relationship between the powder characterization, the densification processes, and the changes in microstructure both with increasing temperature at a rate of 10° C min−1 and at a fixed temperature of 1450° C for 5 h. The densification behaviour and the changes in microstructure of these compressed bodies were affected chiefly by their original surface activity and degree of agglomeration, depending on the production histories: (i) the ultra-fine and well-dispersed powder prepared by the vapour-phase oxidation process showed that densification proceeded with an appreciable grain growth with few closed pores remaining; (ii) powder derived from the sea-water magnesia process showed that the densification behaviour was affected by the species of magnesium salt, i.e. basic magnesium carbonate or magnesium hydroxide, used as a precursor; however, whichever magnesium salt was used, its sintered compact showed similar closed porosities and grain-size distributions; (iii) powder derived from the spark-discharge process contained skeletons of the original Mg(OH)2 particles; however, the densification proceeded gradually with slow grain growth, reflecting the fact that the powder has a moderate surface area (36 m2 g−1). The sintered compact from (iii) had a small closed porosity and the smallest grain-size distribution among the compacts used in this investigation.

Agglomeration of magnesium oxide particles formed by the decomposition of magnesium hydroxide

Agglomeration of magnesium oxide (MgO) particles was studied by decomposing magnesium hydroxide (Mg(OH)2). The properties of agglomerates varied according to the decomposition temperature region: (i) below 650° C, (ii) 650° C to 850° C, (iii) 850° C to 1050° C, and (iv) 1050° C to 1200°C. In region (i), the original Mg(OH)2 frameworks or pseudomorphs remained in the powder and showed agglomeration. The strength of agglomerates containing the pesudomorphs was about 50 MPa; the primary particles in pseudomorphs are bonded chemically by the interaction of MgO and residual water. In region (ii) the pseudomorphs began to show some fragmentation: the bonding strength of these pseudomorphs reduced rapidly. In region (iii), both crystallite and primary particles were grown by the sintering; this growth may be due to an increase in contact area based on the collapse of pseudomorphs. The primary particles whose necks were grown by the sintering could be easily pulled apart by grinding. In region (iv) pore growth due to the rearrangement of primary particles caused the suppression of both densification rate and crystal growth of MgO.

Densification and microstructure development during the sintering of submicrometre magnesium oxide particles prepared by a vapour-phase oxidation process

The densification behaviour and microstructure development of MgO compacts fired from room temperature up to 1700°C at a heating rate of 10°C min−1 were examined. Starting materials were seven kinds of MgO powder with primary particle sizes ranging from 11–261 nm; these powders were produced by a vapour-phase oxidation process. The original powders contained agglomerates, due to the spontaneous coagulation of primary particles, which ranged in size from 100–500 nm. The MgO compacts densified during firing by three types of sintering: sintering within agglomerates; sintering between agglomerates and grains; and rearrangement of agglomerates and grains. The MgO compact with the lowest primary particle size (11 nm) densified by the first and second types of sintering, but the effects of these two types of sintering decreased when the primary particle size became 44 nm; here the rearrangement of agglomerates and grains primarily contributed to densification of the compact. All three types of densification became less complete with further increases in primary particle size up to 261 nm. The relative densities of the MgO compacts with smaller primary particle sizes (11–44 nm) became 96–98% when the compacts were fired up to 1700°C.

Some properties of aluminium nitride powder synthesized by low-pressure chemical vapour deposition

Aluminium nitride (AIN) powders were synthesized by a low-pressure chemical vapour deposition, i.e. reactions of vaporized aluminium with various compositions of NH3-N2 gases at 1050°C under a pressure of 0.1–1.3 kPa. The properties of the resulting powders were divided into three categories, according to the NH3 content in the NH3-N2 gases: (i) 0 ⩽ NH3 < 40%, (ii) 40⩽NH3⩽60%, and (iii) 60<NH3⩽100%. In Region (i), the unreacted aluminium adhered to the AIN crystallites to form spherical primary particles; in Region (ii), the spherical agglomerates with diameters of 0.2–0.5 μm, composed of primary particles, were present as minimum units of secondary particles; in Region (iii), the crystal growth of AIN was enhanced with increasing NH3 contents. The primary particles formed by the reaction of aluminium vapour with NH3-N2 gases containing NH3⩾40% were single crystals.

Some properties of aluminium-nitride powder prepared by metal-organic chemical vapour deposition

Abstract Ultrafine aluminium-nitride (AlN) powders with primary particle sizes of 10–20 nm were prepared by a metal-organic chemical vapour deposition (MOCVD): trimethylaluminium (Al(CH 3 ) 3 ), triethylaluminium (Al(C 2 H 5 ) 3 ) and triisobutylaluminium (Al(i-C 4 H 9 ) 3 )-vapours were reacted with ammonia (NH 3 ) at 1050 °C. The sintered compacts with relative densities of ~95% could be fabricated by firing these compressed powders at temperatures as low as 1600 °C. The relative densities of the Al(CH 3 ) 3 -derived and Al(C 2 H 5 ) 3 -derived compacts fired at 1800 °C for 10 h attained ~98%. The oxygen contents of these compacts were 1.8 and 4.7%, respectively.

Formation of porous calcium phosphate films on partially stabilized zirconia substrates by the spray-pyrolysis technique

Porous calcium phosphate films could be formed on partially stabilized zirconia (3YZ) substrates by a spray-pyrolysis technique. The use of calcium metaphosphate as a binder was effective to enhance the binding strengths of these films to the substrates. The crystalline phase in the resulting films was mainly β-calcium orthophosphate. This phase was thermally stabilized by solid solution with Y3+. The thickness of the film (30–150 μm) was dependent upon the spraying time; the pore size was about 15 μm. The films were still present on the substrate after Scotch tape (810) was adhered to the film side and then taken off from the substrate. The films prepared in this study were found to bind strongly to the substrate.

Some properties of mullite powders prepared by chemical vapour deposition

The chemical vapour deposition (CVD) technique based upon reaction among aluminium chloride (AlCl3), silicon chloride (SiCl4) and oxygen was applied to produce submicrometresized mullite (3Al2O3 ·2SiO2) powder. The conditions for preparing the best crystalline mullite were as follows: (i) the reaction temperature, 1200 °C; (ii) the flow rate of carrier gas (Ar) of AlCl3, 0.3 dm3 min−1, and that of SiCl4, 0.3 dm3 min−1; (iii) the sublimation temperature of AlCl3, 180 °C, and the evaporation temperature of SiCl4, 25 °C; and (iv) the flow rate of oxygen, 0.9 dm3 min−1. The as-prepared powder contained mullite, a small amount of γ-Al2O3 (Al-Si spinel) and amorphous material; this powder was composed of spherical primary particles of ∼ 0.05 μm diameter. Although only mullite was present at the calcination temperature of 1300 °C, a small amount of α-Al2O3 was formed at 1400–1700 °C. Agglomeration due to primary particle growth started at temperatures exceeding 1400 °C.

Sintering of magnesium oxide powder prepared by vapour-phase oxidation process — Relationship between particle size and mechanical properties of consolidated specimens

Relationship between powder properties and bending strengths of the sintered magnesium oxide (MgO) specimens was examined using seven kinds of MgO powders prepared by a vapour-phase oxidation process; the average primary particle sizes were 11, 25, 32, 44, 57, 107 and 261 nm. These compressed powders (specimens) were fired at 1600 or 1700 °C for 1 to 15 h. Although the densification behaviours of the specimens varied with the primary particle size of the starting powders, the relative densities of the specimens fired at 1700 °C for 5 h were all in the range of 97–98%. The relationships between bending strengths and grain sizes of these sintered specimens could be classified into two categories, according to the primary particle size of the starting powder: (i) at and below 32 nm and (ii) 44–261 nm. In range (i), the bending strengths of the sintered specimens were as low as ∼ 120 MPa; the grain size was reduced from 50.7 to 35.8 μm as the primary particle size decreased from 32 to 11 nm. In range (ii), as the primary particle size increased from 44 to 261 nm, the bending strength of the sintered specimen was enhanced from 162 to 183 MPa, while the grain size was reduced from 28.3 to 13.7 μm.

Sinterability of Yttria-Stabilized Zirconia Powder Prepared by Ultrasonic Spray Freeze-Drying Technique

Abstract Tetragonal zirconia poly crystals (Y-TZP)containing 3·0 mole% yttria were prepared by (i) the ultrasonic spraying of solutions containing desired amounts of components into a chamber chilled by liquid nitrogen, (ii)the subliming of the ice from the frozen materials and (iii) the calcining of the remaining materials. The resulting Y-TZP powder was composed of ultrafine primary particles with sizes below 0.05 um. The relative density of Y-TZP compacts fired at 1300°C for 5 hrs attained a maximum of 97·3%%