Kiyoshi Itatani

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 various calcium-phosphate powders by ultrasonic spray freeze-drying technique

Abstract Various calcium–phosphate powders with Ca/P ratios ranging from 1.00 to 1.67 were prepared by an ultrasonic spray freeze-drying (USFD) technique: (i) 100 cm 3 of aqueous solution containing 0.500 mol·dm −3 of calcium acetate (Ca(CH 3 COO) 2 ), 0.300–0.500 mol·dm −3 of trimethyl phosphate (PO(OCH 3 ) 3 ) and 1.77 mol·dm −3 of acetic acid (CH 3 COOH) were refluxed at 80°C for 100 h; (ii) the droplets with diameters as small as 2–3 μm, which were formed by the ultrasonic spray of aqueous solution, were introduced into a chamber chilled by liquid nitrogen to freeze the droplets; (iii) the water ice in the frozen droplets was sublimed under reduced pressures; and (iv) the freeze-dried materials were heated at 900°C for 1 h to form calcium phosphates. The β-calcium orthophosphate (β-Ca 3 (PO 4 ) 2 ) and carbonate-containing hydroxyapatite powders with stoichiometric Ca/P ratios were prepared from the solutions with Ca/P ratio of 1.44 and 1.67, respectively.

Influence of the pore generator on the evolution of the mechanical properties and the porosity and interconnectivity of a calcium phosphate cement.

Porosity and interconnectivity are important properties of calcium phosphate cements (CPCs) and bone-replacement materials. Porosity of CPCs can be achieved by adding polymeric biodegradable pore-generating particles (porogens), which can add porosity to the CPC and can also be used as a drug-delivery system. Porosity affects the mechanical properties of CPCs, and hence is of relevance for clinical application of these cements. The current study focused on the effect of combinations of polymeric mesoporous porogens on the properties of a CPC, such as specific surface area, porosity and interconnectivity and the development of mechanical properties. CPC powder was mixed with different amounts of PLGA porogens of various molecular weights and porogen sizes. The major factors affecting the properties of the CPC were related to the amount of porogen loaded and the porogen size; the molecular weight did not show a significant effect per se. A minimal porogen size of 40 μm in 30 wt.% seems to produce a CPC with mechanical properties, porosity and interconnectivity suitable for clinical applications. The properties studied here, and induced by the porogen and CPC, can be used as a guide to evoke a specific host-response to maintain CPC integrity and to generate an explicit bone ingrowth.

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.

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+ .

Low-Temperature Deposition of Polycrystalline Titanium Oxide Thin Film on Si Substrate Using Supercritical Carbon Dioxide Fluid

A novel technique for the deposition of oxide thin films using supercritical carbon dioxide (CO2) fluid was proposed for the large-scale circuit integration on a silicon (Si) substrate at a low temperature. Thin films of titanium oxide (TiO2) as a model material were fabricated from organometallic Ti(Oi-Pr)2(dpm)2 source on (100)Si substrates in supercritical CO2 fluid. The film deposition was accomplished using a flow-type supercritical fluid deposition apparatus designed on the basis of metal-organic chemical vapor deposition (MOCVD) systems. Flat films were fabricated on (100)Si substrates at a faster source delivery (fluid flow rate: 2.0–10.0 cm3/min), while granular deposits owing to homogeneous nucleation in the bulk supercritical CO2 fluid appeared on the substrates at a slower source delivery (fluid flow rate: 1.0 cm3/min). The amount of TiO2 deposited decreased gradually with increasing the temperature of CO2 fluid, which would be related to the change in the density of CO2 fluid. The crystalline TiO2 film was deposited on the (100)Si substrate at a substrate temperature of 80–120°C and fluid temperature/pressure of 40°C/8.0 MPa, at which the decomposition of organometallic Ti(Oi-Pr)2(dpm)2 source was accomplished. These results indicated that the supercritical CO2 deposition technique is suitable for the large-scale circuit integration on Si substrates.

Densification and microstructural developments during the sintering of aluminium silicon carbide

Densification and microstructural developments during the sintering of aluminum silicon carbide (Al4SiC4) were examined. Two types of Al4SiC4 powders were prepared by the solid-state reactions between: (i) Al, Si, and C at 1600°C for 10 h (designated as Al4SiC4(SSR)), and (ii) chemically-vapour deposited ultrafine Al4C3 and SiC powders at 1500°C for 4 h (Al4SiC4(CVD/SSR)). The specific surface areas of the Al4SiC4(SSR) and Al4SiC4(CVD/SSR) powders were 2.7 and 15.5 m2 · g−1, respectively. Relative densities of the pressureless-sintered Al4SiC4(SSR) and Al4SiC4(CVD/SSR) compacts were as low as 60–70% for firing temperatures between 1700°C and 2000°C. The relative densities of Al4SiC4(SSR) and Al4SiC4(CVD/SSR) compacts could be enhanced using the hot-pressing technique; the relative density of the Al4SiC4(SSR) compact hot-pressed at 1900°C for 3 h was 97.0% whereas that of the Al4SiC4(CVD/SSR) compact hot-pressed at 1900°C for 1 h attained 99.0%. The former microstructure was composed of plate-like grains of width 10–30 μm and thickness ∼10 μm whilst the latter microstructure was comprised of equiaxed grains with a typical diameter of ∼10 μm. Densification of the Al4SiC4(CVD/SSR) compacts appeared to be promoted compared to the Al4SiC4(SSR) compact and this was attributed to the higher surface area, reduced agglomeration of the starting primary particles, and more homogeneous chemical composition.

A probabilistic approach to the toughening mechanism in short-fiber-reinforced ceramic-matrix composites

The toughening mechanism in brittle ceramic-matrix composites reinforced with short fibers is analytically studied by considering probabilistic aspects of fiber strength. Energy for broken and/or unbroken fibers to be pulled out is given on the assumption that the resistant force at the fiber interface is uniform along the sliding area. The effect of fiber inclination is incorporated by the introduction of a resistant force proportional to the normal component of the bridging force on the fiber and a stress magnification factor. This considers significant local deformation of the fiber near the crack surface. Fiber breakage is assumed to occur at the randomly distributed flaws whose expected number in a unit length follows a power law of the stress. This assumption is consistent with the fact that the strength of a fibre of a certain length is expressed by a Weibull distribution function. The energy dissipated as a result of fiber pull-out for a unit area of major crack propagation is estimated for continuous unidirectional composites, aligned short-fiber composites and two- and three-dimensional random short-fiber composites.

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.