F. S. Howell

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.

Preparation of spherical apatite particles by the homogeneous precipitation method in the presence of magnesium ions and their ion-exchange properties

Abstract Spherical apatite particles were prepared by the homogeneous precipitation method in the presence of magnesium ions. The starting solutions were prepared by mixing 0.167 mol·L −1 of Ca(NO 3 ) 2 , 0.100 mol·L −1 of (NH 4 ) 2 HPO 4 , 1.00 mol·L −1 of (NH 2 ) 2 CO, 0.10 mol·L −1 of HNO 3 , and a small amount of Mg(NO 3 ) 2 . The carbonate-containing apatite powders were obtained by heating these solutions at 80∼95°C for 48∼192 h. Although fibrous particles with long-axis lengths of 30 to 60 μm were obtained from the magnesium-free solution, spherical agglomerates with diameters of ∼10 μm, which contained minute plate-like particles, were present in the apatite powders derived from the solutions with 5 mass% of magnesium ions. The ion-exchange test for the harmful ions (Pb 2+ , Cd 2+ , and Ni 2+ ) showed that the ion-exchange abilities of the apatite powders containing magnesium ions were much better than the ability of the magnesium-free apatite powder. The ion-exchanged amounts of the apatite powder containing magnesium ions were arranged in the following order: Pb 2+ >> Cd 2+ > Ni 2+ .

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.

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.

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 magnesium-oxide powder containing spherical agglomerates

The magnesium-oxide (MgO) powders were prepared by calcining basic magnesium carbonate (4MgCO3·Mg(OH)2·4H2O; BMC) powder at a temperature between 600°C and 1200°C for 1 to 5 h. The resulting MgO powders contained spherical agglomerates with diameters of 10–50 μm; the external shapes of these BMC agglomerates remained unchanged even after the calcination. With increasing compaction pressure, the densification of MgO powder compacts proceeded by (i) the rearrangement of agglomerates (≲50 MPa), (ii) the collapse of agglomerates (50–100 MPa), and (iii) the closer packing of primary particles (≳100 MPa). The MgO compact was fired at 1400 °C for 5 h. The relative density of the sintered MgO compact whose starting powder was prepared by calcining the BMC at 1000°C for 3 h attained 98.0%. The bending strength of this sintered MgO compact attained 214 MPa.