Synthesis of Calcium Hydroxyapatite-Tricalcium Phosphate (HA-TCP) Composite Bioceramic Powders and Their Sintering Behavior

Composite (biphasic) mixtures of two of the most important inorganic phases of synthetic bone applications-namely, calcium hydroxyapatite (Ca₁₀(PO₄)₆(OH)₂ (HA)) and tricalcium phosphate (Ca₃(PO₄)₂ (TCP))-were prepared as submicrometer-sized, chemically homogeneous, and high-purity ceramic powders by using a novel, one-step chemical precipitation technique. Starting materials of calcium nitrate tetrahydrate and diammonium hydrogen phosphate salts that were dissolved in appropriate amounts in distilled water were used during powder precipitation runs. The composite bioceramic powders were prepared with compositions of 20%-90% HA (the balance being the TCP phase) with increments of 10%. The pellets prepared from the composite powders were sintered to almost full density in a dry air atmosphere at a temperature of ~1200°C. Phase-evolution characteristics of the composite powders were studied via X-ray diffractometry as a function of temperature in the range of 1000°-1300°C. The sintering behavior of the composite bioceramics were observed by using scanning electron microscopy. Chemical analysis of the composite samples was performed by using the inductively coupled plasma-atomic emission spectroscopy technique.     

Mechanochemical Synthesis of Hydroxyapatite from Calcium Oxide and Brushite

Fine hydroxyapatite (HA) powders were prepared by mechanically activating a mixture of calcium oxide and brushite powders in a high-energy shaker mill. A defective HA phase was formed when the starting powder mixture was activated for ≤20 h. When the mixture was calcined at 800°C, it was converted to β-tricalcium phosphate. In contrast, a nanocrystalline HA phase was formed when the mechanical activation was extended to 30 h. The material was transformed to a HA compound (Ca₁₀(PO₄)₆(OH)₂) of high crystallinity when it was calcined at 800°C.     

Gel-to-Ceramic Conversion during Hydroxyapatite Synthesis

Conversion of hydroxyapatite (HA, Ca₁₀(PO₄)₆(OH)₂) synthesis from gel to ceramic using solid-state nuclear magnetic resonance (NMR) spectroscopy and X-ray diffraction was studied. The sol was prepared by dissolving calcium nitrate tetrahydrate and triethyl phosphate in 2-methoxyethanol. In solid-state ³¹P magic-angle spinning nuclear magnetic resonance (MAS NMR) spectra, gel calcined at 250°, 350°, and 600°C showed a conversion from gel to glassy phosphate, then HA ceramic. Spin-lattice relaxation experiments of samples calcined at 600°C indicated that the existence of a CaO impurity might affect OH···O=PO₃ linkages in a HA unit cell and result in longer ³¹P spin-lattice time.     

Near-Net-Shaped Calcium Hydroxyapatite by the Oxidation of Machinable, Calcium-Bearing Precursors (the Volume Identical Metal Oxidation, or VIMOX, Process)

A novel VIMOX (volume identical metal oxidation) route to near-net-shaped calcium hydroxyapatite, Ca₁₀(PO₄)₆(OH)₂, is demonstrated: the oxidation of machinable Ca—Ca₂P₂O₇ precursors. Mechanically alloyed mixtures of Ca and β-Ca₂P₂O₇ were compacted into disk- and bar-shaped preforms. The latter preforms could be machined into cylinders using a metalworking lathe (200 rpm, hardened steel tooling). After oxidation at 600°C in O₂, and then postoxidation annealing in H₂O/O₂ mixtures at 850°C and 1150°C, phase-pure hydroxyapatite was obtained. Because of offsetting volume changes from calcium oxidation and hydroxyapatite formation, porous hydroxyapatite bodies were produced that retained the shapes and dimensions (within 1%) of the machined precursors.     

Influence of Pyridine–Borane on the Sintering Behavior and Properties of α-Silicon Carbide

In the present study, α-SiC powder is coated with pyridineborane (BH₃·C₅H₅N), a liquid molecular compound, which forms a boron carbonitride (BC_3.5N) layer by heat treatment at 1000°C under argon. The precipitation method leads to an improved chemical homogeneity in the compacted powder resulting in enhanced densification and significant reduction in grain growth during subsequent sintering at temperatures exceeding 2070°C. Thus, small average grain sizes of d ₅₀= 1.3 μm and a narrow grain size distribution ( d ₁₀= 0.6 μm, d ₉₀= 2.2 μm) are detected in the liquid-phase-processed sample sintered at 2200°C for 0.5 h in argon. Final densities of at least 98% of theoretical could be obtained by pressureless sintering at 2100°C. These results as well as the microstructural distribution of the sintering aids in the densified samples are discussed.     

Formation and Properties of Hydroxyapatite–Calcium Poly(vinyl phosphonate) Composites

Ceramic–polymer composites composed of hydroxyapatite (Ca₁₀(PO₄)₆(OH)₂, HAp) and calcium poly(vinyl phosphonate) salt were prepared by warm-pressing powder mixtures of tetracalcium phosphate (Ca₄(PO₄)₂O, TetCP) and poly(vinyl phosphonic acid) (PVPA) at a weight ratio of 3.5:1. The effects of temperature (to 300°C), pressure (to 690 kpsi), or compaction time (to 1 h) on the extent of conversion were studied using X-ray diffraction, Fourier transform infrared spectroscopy, differential scanning calorimetry, and scanning electron microscopy coupled with energy dispersive spectroscopy. The conversion of TetCP to HAp and formation of the calcium poly(vinyl phosphonate) salt was enhanced at higher temperature, pressure, and/or longer compaction time. Mechanical property determinations showed both the tensile strengths and elastic moduli continuously increase with increasing temperature, pressure, and compaction time. However, the glass transition temperature values of the composites were only minimally higher than that of the unreacted polymer.     

Superplasticity of Hot Isostatically Pressed Hydroxyapatite

Dense and translucent hydroxyapatite polycrystals (Ca₁₀(PO₄)₆(OH)₂ with a grain size of 0.64 μMm) were obtained by hot isostatic pressing at 203 MPa and 1000°C for 2 h in argon. The material exhibited superplastic elongation (>150%) in a tension test at temperatures from 1000° to 1100°C and at strain rates from 7.2×10⁻⁵ to 3.6 × 10⁻⁴ s⁻¹. Extensive strain hardening was observed. The stress exponent of the yield stress was larger than 3.     

Formation of Calcium Phosphate Whiskers in Hydrogen Peroxide (H2O2) Solutions at 90°C

A novel, one-pot technique of synthesizing calcium phosphate whiskers was developed. Commercially available β-tricalcium phosphate (β-Ca₃(PO₄)₂) powders were aged in unstirred 30% H₂O₂ solutions at 90°C for 48 h in ordinary glass media bottles. Resultant samples consisted of whiskers (200 nm wide and 5 μm-long) of a biphasic mixture of octacalcium phosphate (OCP: Ca₈H₂(PO₄)₆·5H₂O) and carbonated apatitic (apatite-like) calcium phosphate (Ap-CaP). As-formed whiskers possessed a Ca/P molar ratio of 1.46 and a BET surface area of 8 m²/g. Upon soaking these whiskers in a Tris-HCl-buffered SBF solution of 27 mM HCO₃⁻ for 6 days, Ca/P molar ratio and surface area values were increased to 1.60 and 52 m²/g, respectively. The technique, owing to its simplicity, may prove useful in providing large amounts of biocompatible short whiskers for numerous technology sectors.     

Preparation and Sintering Behavior of Fine-Grained A12O3-SiO2 Composite

A fine, uniform A1₂O₃-SiO₂ powder was prepared by heterocoagulation of narrow Al₂O₃ and SiO₂ powders. This composite powder was dispersed, compacted, and fired in air at 900° to 1580°C for 1 to 13 h. Full density was achieved at 1550°C with the formation of a mullite phase. Relative densities of 83% and 98% (0.3 μm grain size) were measured for samples sintered at 1200°C for 13 h and at 1400°C for 1 h, respectively.     

Sinterability of β-Calcium Orthophosphate Powder Prepared by Spray-Pyrolysis

Mixed solutions of Ca(NO₃)₂ and (NH₄)₂HPO₄ with Ca/P = 1.50 were spray-pyrolyzed at 600°C to produce β-calcium orthophosphate (β-Ca₃(PO₄)₂) powder; the spray-pyrolyzed powder was ground and then calcined at 600°C for 1 h. The best crystalline β-Ca₃(PO₄)₂ powder was obtained from the solution with 1.80 mol.L^–1 Ca(NO₃)₂, 1.20 mol.L^–1 (NH₄)₂HPO₄. The resulting powder was composed of primary particles with sizes of <0.5 μm. Dense β-Ca₃(PO₄)₂ ceramics with a relative density of 96.1% could be fabricated by firing this compressed powder at 1070°C for 5 h.     

An Alternative Chemical Route for the Synthesis and Thermal Stability of Chemically Enriched Hydroxyapatite

Hydroxyapatite (HAp: Ca₁₀(PO₄)₆(OH)₂) was synthesized by aqueous precipitation using CaCl₂ and Na₃PO₄ with NaOH added to ensure completion of the reaction at room temperature. The HAp powder prepared using stoichiometric amounts of NaOH was stable even at 1200°C, but the HAp prepared with sub-stoichiometric amounts of NaOH resulted in its transformation into β-tricalcium phosphate at 600°C. The reaction pH, X-ray diffraction, thermal analysis, scanning electron microscopy, Fourier transform infrared analyses and inductively coupled plasma-optical emission spectroscopy were used to characterize the phase purity, thermal stability, morphology, and chemical composition of the synthesized HAp powder.     

Nanometer-Sized ZrO2 Particles Prepared by a Sol–Emulsion–Gel Method

ZrO₂ powder was prepared by a sol–emulsion–gel method at temperatures below 140°C from ZrO(NO₃)₂· n H₂O. The asprepared powder was amorphous, but crystallized into the tetragonal structure by 600°C. The metastable tetragonal powder (600°C) was comprised of ultrafine 4- to 6-nm size particles. On heat treatment, the tetragonal form completely transformed into the monoclinic state at 1100°C. Preliminary studies indicate good sinterability with densities greater than 94% at 1100°C and with a grain size of 0.25 μ.     

Preparation of Porous Ca10(PO4)6(OH)2 and β-Ca3(PO4)2 Bioceramics

Submicrometer-sized, pure calcium hydroxyapatite (HA, (Ca₁₀(PO₄)₆(OH)₂)) and β-tricalcium phosphate (β-TCP, Ca₃(PO₄)₂) bioceramic powders, that have been synthesized via chemical precipitation techniques, were used in the preparation of aqueous slurries that contained methyl cellulose to manufacture porous (70%–95% porosity) HA or β-TCP ceramics. The pore sizes in HA bioceramics of this study were 200–400 μm, whereas those of β-TCP bioceramics were 100–300 μm. The pore morphology and total porosity of the HA and β-TCP samples were investigated via scanning electron microscopy, water absorption, and computerized tomography.     

Comparison of the Formation of Calcium and Barium Phosphates by the Conversion of Borate Glass in Dilute Phosphate Solution at near Room Temperature

The formation of phosphates of calcium or barium at near room temperature by the direct conversion of borate glass in dilute phosphate solution was investigated. Borate glass particles (150–300 μm), with the composition 20Na₂O·20AO·60B₂O₃ (mol%), where A is the alkali-earth metal Ca or Ba, were prepared by conventional processing, and immersed in 0.25 M K₂HPO₄ solution at 37°C and with a starting pH value of 9.0 or 12.0. The effects of the borate glass composition and the solution pH on the rate of formation, the chemical composition, and the structure of the phosphate products formed in the conversion reaction were investigated using weight loss and pH measurements, X-ray diffraction, X-ray fluorescence, scanning electron microscopy, and Brunauer–Emmett–Teller surface area. At both pH values, the calcium borate glass particles were completely converted to hydroxyapatite, Ca₁₀(PO₄)₆(OH)₂, with a high surface area (160–170 m²/g). At pH=9.0, the barium borate glass particles converted completely to a barium phosphate product that was isostructural with BaHPO₄, whereas at pH=12.0, the barium phosphate product was isostructural with Ba₃(PO₄)₂, with both products having much lower surface areas (4–8 m²/g). The formation of the phosphate products was pseudomorphic, retaining the external shape of the original glass particles.     

Kinetic Model for α–Tricalcium Phosphate Hydrolysis

A mechanistic model for the kinetics of hydrolysis of α-tricalcium phosphate (α–Ca₃(PO₄)₂ or α-TCP) to hydroxyapatite (Ca_{10− x} (HPO₄) _x (PO₄)_{6− x} (OH)_{2− x} or HAp) has been developed. The model is based on experimental hydrolysis rate data obtained using isothermal calorimetry. Analysis of the kinetic data according to the general kinetics models in terms of the fractional degree of reaction and time suggests the hydrolysis to be controlled by different rate-limiting mechanisms as reaction proceeds. Initially, the hydrolysis kinetics depend on the surface area of the anhydrous α-TCP. Subsequently, they change to a dependence on the rate of HAp product formation controlled by a nucleation and growth mechanism. The model predicts that HAp nuclei form at essentially one time and growth occurs in two dimensions, leading to a platelike morphology. The change in the reaction mechanism occurs at a fractional degree of hydrolysis, which does not change significantly with temperature in the range of 37°–56°C.     

Novel Preparation Method of Hydroxyapatite Fibers

A novel method for preparing calcium hydroxyapatite (Ca₁₀(PO₄)₆(OH)₂: HAp) fibers has been developed. HAp fibers can be prepared successfully by heating a compact consisting of calcium metaphosphate (ß-Ca(PO₃)₂) fibers with Ca(OH)₂ particles in air at 1000°C and subsequently treating the resultant compact with dilute aqueous HCl solution. The ß-Ca(PO₃)₂ fibers and the Ca(OH)₂ in the compact were converted into fibrous HAp and CaO phases by the heating, and the CaO phase was removed by acid-leaching. HAp fibers obtained in the present work were 40-150 µm in length and 2-10 µm in diameter. The fibers had almost the same dimensions as those of the ß-Ca(PO₃)₂ fibers.     

A Crystallographic Investigation of Calcium Diferrite

Calcium diferrite has been prepared at 1220°C in air. The lattice constants at 25° C of calcium diferrite have been determined by X-ray powder data and have been based on the hexagonal system. They are a = 5.992 ± 0.004 A and co = 31.121 ± 0.008 A. The coefficients of thermal expansion were evaluated by X-ray methods between-130° and 25°C to be α₁= 9.2 × 10⁻⁶ per°C and ₃= 23 × lop⁶ per °C, where a₃ is along the principal axis and α is perpendicular to it. The diferrite is ferromagnetic with a Curie point of 135°C. Comparison of the results obtained from the present work with those of some recent studies shows that the X-ray data given by previous workers on compounds claimed to be Ca₃FeleO₁₅, Ca₄Fe₁₇O₂₉, and Ca₄Fe₄O₂₅ are for calcium diferrite. The present work, however, also indicates that when mixtures of calcium diferrite plus ferrous oxide (up to 6 wt%) are heat-treated at 1220°C, a new phase appears together with the diferrite.     

Preparation of Nanostructured Hexagonal Boron Nitride Powder

In this communication, we describe an inexpensive and feasible method for the preparation of hexagonal boron nitride (h–BN) nanorods in the absence of metal catalyst. Tertiary calcium phosphate (Ca₃(PO₄)₂) and ammonium biborate hydrate (NH₄HB₄O₇·3H₂O) were selected as starting materials where calcium phosphate was used as a diluting agent to prevent the formation of bulk B₂O₃ during the thermolysis of ammonium biborate hydrate. The mixture was nitrided at 900°C in the flowing ammonia and was transformed into h–BN nanorods after subsequent crystallization. After crystallization at 1650°C for 2 h, the unique microstructure of h–BN nanorods was observed.     

High-Temperature Stability of Alumina in Argon and Argon/Water-Vapor Environments

The high-temperature stability of alumina (Al₂O₃) in argon and argon/water-vapor (Ar/H₂O) environments has been investigated. Samples were exposed at temperatures of 1300°C–1700°C for 10 h. The microstructure, flexural strength, and volume all showed significant changes in the Ar/H₂O environment at 1700°C. Samples also became whiter, because of the oxidation of graphite impurities that had diffused from the hot-processing dies. In the Ar/H₂O environment at 1700°C, grain-boundary etching occurred and was much more severe than in the pure-argon environment, which was very likely caused by the enhanced formation of gaseous Al(OH)₃ and Al(OH)₂ along grain boundaries. In addition, in the Ar/H₂O environment, substantial grain growth occurred in the surface vicinity. This grain growth, together with grain-boundary etching, led to a decrease in flexural strength.     

Double Precursor Solution Coating Approach for Low-Temperature Sintering of [Pb(Mg1/3Nb2/3)O3]0.63[PbTiO3]0.37 Solids

Lead-based piezoelectric ceramics typically require sintering temperatures higher than 1000°C at which significant lead loss can occur. Here, we report a double precursor solution coating (PSC) method for fabricating low-temperature sinterable polycrystalline [Pb(Mg_1/3Nb_2/3)O₃]_0.63-[PbTiO₃]_0.37 (PMN–PT) ceramics. In this method, submicrometer crystalline PMN powder was first obtained by dispersing Mg(OH)₂-coated Nb₂O₅ particles in a lead acetate/ethylene glycol solution (first PSC), followed by calcination at 800°C. The crystalline PMN powder was subsequently suspended in a PT precursor solution containing lead acetate and titanium isopropoxide in ethylene glycol to form the PMN–PT precursor powder (second PSC) that could be sintered at a temperature as low as 900°C. The resultant d ₃₃ for samples sintered at 900°, 1000°, and 1100°C for 2 h were 600, 620, and 700 pm/V, respectively, comparable with the known value. We attributed the low sintering temperature to the reactive sintering nature of the present PMN–PT precursor powder. The reaction between the nanosize PT and the submicrometer-size PMN occurred roughly in the same temperature range as the densification, 850°–900°C, thereby significantly accelerating the sintering process. The present PSC technique is very general and should be readily applicable to other multicomponent systems.