Akira Kishioka

Characterization of hydroxyapatite powders prepared by ultrasonic spray-pyrolysis technique

The hydroxyapatite (HAp) powder was prepared by the ultrasonic spray-pyrolysis technique; the characterization of the resulting powders was performed. Five kinds of the starting solutions with the Ca/P ratio of 1.67 were prepared by mixing Ca(NO3)2, (NH4)2HPO4 and HNO3; the concentrations of Ca2+ and PO43− were in the ranges of 0.10 to 0.90 mol · dm−3 and 0.06 to 0.54 mol · dm−3, respectively. These solutions were sprayed into the heating zone to prepare the HAp powders. The heating zone was composed of two electric furnaces; the lower furnace was used for the evaporation of the solvent from the droplets (300–500°C) and the upper furnace for the pyrolysis of the precipitated metal salts (750–900°C). The easily sinterable HAp powder was prepared by spray-pyrolysing the solution with Ca2+ (0.50 mol · dm−3) and PO43− (0.30 mol · dm−3) at the temperatures of 800°C (the upper furnaces) and 400°C (the lower furnaces). The resulting powder was composed of the spherical particles with diameters of ∼1 μm or below. Even without the calcination and grinding operations, the relative densities of the compacts fired at 1150 and 1200°C for 5 h attained maxima ∼95%. The microstructure of the sintered compacts appeared to be uniform; the average grain size was ∼3 μm. The activation energies for the grain growth of the sintered HAp compacts were 120 to 147 kJ · mol−1 · K−1.

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.

Effect of starting particle size on hot-pressing of magnesium oxide powder prepared by vapour-phase oxidation process

Effect of starting particle size on hot-pressing of magnesium oxide (MgO) powder 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 (compacts) were hot-pressed at a temperature between 900 and 1300°C. The densifications of these compacts during the hot-pressing proceeded via (i) the sintering of primary particles within secondary particles and the rearrangement of secondary particles/grains (900°C), (ii) the gradual grain growth controlled by the pore migration (900∼1100°C) and (iii) the rapid grain growth due to the active mass transfer (1300°C); the grain sizes of MgO compacts hot-pressed at and below 1100°C were <1 μm, while those at 1300°C attained 20∼30 μm. The transluscent compact with the relative density of 99.7% could be obtained when the compressed powder with the average primary particle size of 44 nm was hot-pressed at a temperature as low as 1100°C for 1 h.

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.

Sinterability of spinel (MgAl2O4)-Zirconia composite powder prepared by double nozzle ultrasonic spray pyrolysis

Abstract Two kinds of spinel (MgAl 2 O 4 )-zirconia (ZrO 2 ) composite powders were prepared by double nozzle ultrasonic spray pyrolysis; the aqueous solutions in the Mg(NO 3 ) 2 -Al(NO 3 ) 3 and ZrOCl 2 -YCl 3 systems were spray-pyrolysed in a hot zone of an electric furnace heated at 900 °C, using two ultrasonic vibrators. The compositions of the composite powders were as follows: (1) Sample No.1: MgAl 2 O 4 93.36 mol% and yttria-stabilized tetragonal ZrO 2 polycrystals (Y-TZP) 6.64 mol% and (2) Sample No. 2: MgAl 2 O 4 75.51 mol% and Y-TZP 24.49 mol%. While MgAl 2 O 4 and Y-TZP were present in both powders, MgO was additionally detected from Sample No. 2; such MgO disappeared when it was heated up to 1100 °C or higher. The composite powders contained spherical particles with diameters of below μm and, in part, acicular particles with long axis lengths of 1 to 2μm. The wet-milled powder of Sample No.1 showed an excellent sinterability; when the composite powder compact was fired at 1700 °C for h, the relative density attained 97.6%.

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.

Preparation of magnesium silicon nitride powder by the carbothermal reduction technique

Abstract Magnesium silicon nitride (MgSiN 2 ) powder was prepared by carbothermally reducing a magnesium metasilicate whose chemical composition corresponded to MgO SiO 2 or MgSiO 3 . About 0.2 g of the powder mixture of magnesium metasilicate and carbon (C) with the molar ratio of C to MgO SiO 2 equal to 6.0 was heated at 1250°C for 7 h in nitrogen atmosphere. The crystalline phase of the carbothermally reduced powder was only MgSiN2. The residual carbon could be removed by heating the powder at 600°C for 1 h in air. The yield of MgSiN 2 powder was ∼ 70%. The resulting powder contained 3.22 mol% oxygen. The primary particle sizes were ranging from 0.1 to 0.5 μm.

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.