Substitution Model of Monovalent (Li, Na, and K), Divalent (Mg), and Trivalent (Al) Metal Ions for β-Tricalcium Phosphate

The maximum substitution of monovalent, divalent, and trivalent metal ions for β-tricalcium phosphate (β-TCP) was investigated, and a substitution model of these metal ions for β-TCP was proposed. Monovalent metal ions (M^I) could replace Ca(4) site and vacancy ( V _Ca(4)) in β-TCP as 2M^I=Ca²⁺+ V _Ca(4) and the maximum substitution was about 9.1 mol%. In the case of divalent metal ions (M^II), Ca(4) and Ca(5) sites were replaced by divalent metal ions as M^II=Ca²⁺, and the maximum substitution was about 13.6 mol%. Trivalent metal ions (M^III) could be substituted for Ca(4) site and vacancy as 3M^III=2Ca²⁺+ V _Ca(4), and the maximum substitution was about 9.1 mol%.     

Formation of Strontium-Stabilized β-Tricalcium Phosphate from Calcium-Deficient Apatite

A series of strontium-stabilized β-tricalcium phosphate (β-TCP) was prepared via the aqueous precipitation method, and characterized by X-ray diffraction and infrared fourier spectrometer techniques. The general principle of preparing calcium-deficient apatite and thereby heat-treating the resultant apatite above 700°C to form single-phase β-TCP was attempted to create strontium-substituted β-TCP. The results have proved that single phase β-TCP could be formed with the substituted strontium with only an influence on the increase in lattice constants with an increase in the concentration of strontium. Further, it is also stated from the present findings that strontium has a specific role in the crystallinity of the resultant β-TCP.     

Lath-like Structure of β-Tricalcium Phosphate in Orthopedic Implants

The lath-like β-tricalcium phosphate (β-TCP) in the sintered porous β-TCP implants was revealed by transmission electron microscope (TEM). Samples of sintered porous β-TCP implants were extracted from rabbit tibia after implanted for 1–6 months. Although the majority of sintered β-TCP particles are in a granular shape, the lath-like structures of implants are observed occasionally. The length of laths is on the order of 1 μm, while the thickness of laths is on the order of 10 nm. The X-ray energy dispersive spectroscopy and the electron diffraction indicate that the lath-like material is β-TCP with its rhombohedral (     ) plane parallel to the longitudinal direction of laths. High resolution TEM imaging also confirms the finding of electron diffraction. This abnormal morphology of β-TCP have raised our attention, even though its formation mechanism and effects on osseointegration is not yet certain.     

Synthesis and Structure Refinement of Zinc-Doped β-Tricalcium Phosphate Powders

Zinc substituted β-tricalcium phosphate [β-Ca₃(PO₄)₂] was formed by substituting a zinc precursor in calcium-deficient apatite through aqueous precipitation technique. Heat treatment at 1000°C led to the formation of well crystalline β-Ca₃(PO₄). Refinement technique was used to determine the influence of incorporated zinc in the β-Ca₃(PO₄) structure. The structural data for all the four different zinc substituted β-Ca₃(PO₄) ranging from 0–9 mol% of zinc investigated in the present study confirmed the rhombohedral structure of β-Ca₃(PO₄) in the hexagonal setting (space group R 3 c ). The incorporation of lower sized Zn²⁺ (0.745 Å for sixfold coordination with O) at the higher sized Ca²⁺ (1.00 Å for sixfold coordination with O) site in the β-Ca₃(PO₄) structure led to the contraction of unit cell parameters. The added zinc prefers to occupy the Ca(5) site of β-Ca₃(PO₄) structure.     

Mechanical Properties of Polycrystalline β-Sic

The mechanical properties of chemically vapor-deposited β-Sic were measured in bending between room temperature and 1400°C. Material with grain diameters from less than 1 to 15 μm was tested. No grain-size dependence of the bend strength of dense (<99% of theoretical) Sic was observed at any test temperature. The fracture strength of dense Sic remained approximately constant between room temperature and about 900°C and then increased sharply up to the maximum test temperature of 1215° to 1400°C. This increase in fracture stress coincided with the onset of plastic yielding detectable in the stress-strain curves. The fracture mode of this material was transgranular cleavage at all test temperatures. The fracture stress of Sic of lower density, which was characterized by the presence of grain boundary flaws, decreased slightly at high temperature. The fracture mode of the low-density (3.17 g/cm³) β-Sic underwent a transition from predominantly transgranular at room temperature to predominantly intergranular at high temperature.     

Effects of Sulfate, Pyrophosphate, and Citrate Ions on the Physicochemical Properties of Cements Made of β-Tricalcium Phosphate-Phosphoric Acid-Water Mixtures

Several additives were selected to increase the setting time of calcium phosphate cements made of β-tricalcium phosphate (β-TCP; β-Ca₃(PO₄)₂)-phosphoric acid-water mixtures. The effects of the additives, i.e., sulfate, pyrophosphate, and citrate ions, are presented in this study. The results show that they all increased the setting time of the cement. Their effectiveness at increasing the setting time is in the order pyrophosphate > citrate > sulfate. The effect of sulfate ions on the setting time is increasingly large below a concentration of 0.1 M . Above that concentration, calcium sulfate dihydrate (CSD; CaSO₄-2H₂O) crystals nucleate and act as nuclei for dicalcium phosphate dihydrate (DCPD; CaHPO₄-2H₂O) crystals, which are the normal product of the setting reaction. This decreases the setting time and decreases the DCPD crystal size, resulting in an increase of the tensile strength of the cement.     

Ionic Substitutions in Biphasic Hydroxyapatite and β-Tricalcium Phosphate Mixtures: Structural Analysis by Rietveld Refinement

The structural information on the influence of ionic additions in biphasic (hydroxyapatite (HAP) and β-tricalciumphosphate (β-TCP)) mixtures ranging from single ionic substitutions to combined ionic substitutions of most of the essential ions embedded in biological apatite was analyzed through the Rietveld refinement technique. The results have proved that the determined quantitative phase composition of HAP and β-TCP in biphasic mixtures was dependent on the initial calcium (Ca) deficiency of the precursor powders precipitated from the different molar concentrations used in the synthesis. The substitution of cations (Na⁺, Mg²⁺, and K⁺) improved the stabilization of the β-TCP structure whereas anions (F⁻ and Cl⁻) were found incorporated at the OH⁻ site of the HAP phase. Rietveld analysis of X-ray powder diffraction data from the present study proved to be a powerful technique to describe the position and occupancy of certain ions like Mg²⁺ and Cl⁻ in the biphasic mixtures. However, it has also shown limitations in tracking back other ions like Na⁺, K⁺, and F⁻, which require the use of other complementary characterization methods.     

Modeling of the Hydrolysis of α-Tricalcium Phosphate

Some of the formulations of apatitic calcium phosphate bone cements are based on the hydrolysis of α-tricalcium phosphate (α-Ca₃(PO₄)₂, α-TCP). In this work the hydrolysis kinetics of α-TCP are studied, taking into account the particle-size distribution of the initial powder, to identify the mechanisms that control the reaction in its successive stages. The temporal evolution of the depth of reaction is calculated from the degree of reaction data, measured by X-ray diffractometry. A kinetic model is proposed, which suggests the existence of two rate-limiting mechanisms: initially, the surface area of the reactants and, subsequently, the diffusion through the hydrated layer formed around the reactants. For the specific particle size and preparation used, the controlling mechanism changeover takes place after 16 h of reaction.     

The Mechanical Properties of β-SiAlON Ceramics Joined Using Composite β-SiAlON-Glass Adhesives

The mechanical properties of β-SiAlON ceramics joined using β-SiAlON-glass-forming adhesives consisting of mixed Si³N⁴, Y²O³, Al²O³, and SiO² powders are described. Use of adhesives with a β-SiAlON:glass ratio of 60:40 gave an optimum joint strength of 650 MPa in four-point bending mode, i.e., 85% of that of unbonded material, when joining was carried out at 1600°C for 10 min, under an applied uniaxial pressure of 2 MPa. Bonding pressures in excess of 2 MPa caused excessive compressive creep distortion during the joining operation. The strengths of postjoined HIPed material and HIPed, unbonded material, differed by only 4%, i.e., 975 and 1010 MPa, respectively, which indicates that HIPing reduces the size of critical defects in the joint. Fracture toughness of the joint also improved upon HIPing.     

Optical and Mechanical Properties of α/β Composite Sialons

Two-phase α/β composites have been produced with a combination of high hardness, fracture toughness, and strength. Compared with a single-phase α-sialon, the composite showed around a twofold increase in both fracture toughness and bending strength, with only minimal reduction in hardness. Despite being a two-phase material, the optical properties of the composite were very good, showing transparency in sections of around 0.5 mm thickness. The optical properties were in fact better for the composite than for the single-phase α-sialon. Work to date on transparent sialons has focused on single-phase α-materials, which have inherently low fracture toughness unless elongated microstructures are developed. However, this microstructural development appears to adversely affect optical transparency. In this work it has been shown that good combination of mechanical properties can be achieved while maintaining optical transparency in two-phase composite sialons. The development of such materials should widen their range of application.     

Effect of CO32− Incorporation on the Mechanical Properties of Wet Chemically Synthesized β-Tricalcium Phosphate (TCP) Ceramics

Calcium-deficient hydroxyapatite (CDHA) powders were synthesized by a wet chemical precipitation method using Ca(OH)₂ and H₃PO₄ solutions. Single-phase β-tricalcium phosphate (β-TCP) powder with a molar (Ca+Mg)/P ratio of 1.5 was obtained after calcination of CDHA synthesized under vacuum. During synthesis in air, CO₂ can be adsorbed and HBO₄²⁻ is partly substituted by CO₃²⁻, resulting in a lower phosphorous content and consequently an increase of the molar (Ca+Mg)/P ratio to 1.53. A two-phase β-TCP powder containing 20 wt% hydroxyapatite (HA) was obtained after calcination. Samples prepared from β-TCP powders synthesized under vacuum achieved a compressive strength of 301±23 MPa at 99.6% fractional density, while TCP/HA samples prepared in air achieved a maximum compressive strength of 132±29 MPa at 91.7% fractional density. This decrease in strength can be correlated to the porosity retaining due to CO₂ release during sintering and residual tensile stresses in the TCP matrix caused by the thermal expansion mismatch of β-TCP and HA.     

Phase Evolution during the Formation of α-Tricalcium Phosphate

The formation of α-tricalcium phosphate from monobasic ammonium phosphate and calcium carbonate was investigated using a number of complementary techniques. Differential thermal analysis showed five distinct thermal events attributed to melting of the ammonium phosphate, decomposition of acidic calcium orthophosphate into an amorphous calcium metaphosphate, crystallization of β-calcium metaphosphate, crystallization of β-calcium pyrophosphate, and calcination of calcium carbonate. X-ray diffraction analyses as a function of temperature supported the evolution of these events and phases and also showed the formation of other intermediates, including an amorphous phase, apatite, lime, and β-tricalcium phosphate. Thermal gravimetric analysis (TGA) showed a large weight loss occurring between 150° and 250°C due to reaction between the acidic phosphate liquid and the calcium carbonate. This resulted in an amorphous intermediate with a Ca/P ratio between 0.5 and 1.0 from which both β-calcium metaphosphate and β-calcium pyrophosphate crystallized. Mass spectroscopy indicated that the ammonia and carbon dioxide were evolved in four different steps, while water was evolved in at least five steps. These decomposition steps correlated with those observed by TGA. Scanning electron microscopy indicated the formation of an intermediate phase that coated the calcium carbonate by 250°C. The proposed mechanistic reaction path to alpha-tricalcium phosphate involves the formation and consumption of the following sequence of intermediates: an acidic amorphous condensed phosphate; β-calcium metaphosphate and β-calcium pyrophosphate; lime, apatite, and β-tricalcium phosphate.     

Amorphous α-Tricalcium Phosphate: Preparation and Aqueous Setting Reaction

The mechanical and setting properties of calcium phosphate cements are considerably determined by the pretreatment of the constituents. In this report we show for the first time that prolonged high-energy ball milling of α-tricalcium phosphate (α-TCP) led to mechanically induced phase transformation from the crystalline to the amorphous state. The amorphous material demonstrated a high reactivity such that the time for substantially complete conversion of α-TCP to calcium-deficient hydroxyapatite in 2.5% Na₂HPO₄ solution decreased from about 20 h (1 h of grinding in ethanol, 85% relative crystallinity) to 4–6 h for a material with a crystallinity of 8% (24 h of grinding). This reactivity could be attributed to an increased thermodynamic and kinetic solubility of the ground materials. Mechanically activated α-TCP cements were produced with compressive strengths of up to 80 MPa and setting times of 5–16 min. The effect of reactant preparation and cement mixing parameters on the physical and chemical properties of mechanically activated α-TCP cement was investigated. By comparing cements of similar porosity and degree of conversion it was demonstrated that apatite specific surface area has a strong influence on cement mechanical performance, which highlights the importance of this previously overlooked parameter in improving strength.     

Nonuniformity of Potassium Ions in β-Alumina Ceramics

A nonuniform distribution of K was found in β -alumina that had been in contact with sodium polysulfide or sodium nitrate melts containing small amounts of K. Large β -alumina grains in contact with the melt had approximately the equilibrium concentration of K expected from studies on single crystals of β -alumina, but surrounding grains contained little or no K. Thus the mobility of K ions in grain boundaries in β -alumina appears to be very low. Stresses from K concentration at the ceramic surface could enhance other incipient sources of failure such as seal stresses, cracks, and other defects.     

Effect of β-to-α Phase Transformation on the Microstructural Development and Mechanical Properties of Fine-Grained Silicon Carbide Ceramics

Ultrafine β-SiC powders mixed with 7 wt% Al₂O_3, 2 wt% Y₂O₃, and 1.785 wt% CaCO₃ were hot-pressed and subsequently annealed in either the absence or the presence of applied pressure. Because the β-SiC to α-SiC phase transformation is dependent on annealing conditions, the novel processing technique of annealing under pressure can control this phase transformation, and, hence, the microstructures and mechanical properties of fine-grained liquid-phase-sintered SiC ceramics. In comparison to annealing without pressure, the application of pressure during annealing greatly suppressed the phase transformation from β-SiC to α-SiC. Materials annealed with pressure exhibited a fine microstructure with equiaxed grains when the phase transformation from β-SiC to α-SiC was <30 vol%, whereas materials annealed without pressure developed microstructures with elongated grains when phase transformation was >30 vol%. These results suggested that the precise control of phase transformation in SiC ceramics and their mechanical properties could be achieved through annealing with or without pressure.     

Mechanical and Thermal Properties of β'-Sialon Prepared by a Slip Casting Method

Various mechanical and thermal properties of β'-sialon ceramics, Si_6–zAl_zO_zN_8–z, prepared from aqueous slurries by a slip casting method were investigated as a function of Z values from 0 to 4. In the present study, Si_6–zAl_zO_zN_8–z occurred in the β'-sialon crystalline phase only near Z = 1. The maximum values for bending strength and fracture toughness, 660 MPa and 5.7 MPa·m^1/2, respectively, were obtained at Z = 0.5. The thermal expansion coefficient exhibited its lowest value, 3.4 × 10⁻⁶· T⁻¹ , at Z = 0.5. In view of the present results, β'-sialon, Si_6–zAl_zO_zN_8–z (Z = 0.5–1), prepared by a slip casting method using aqueous slurries could be adopted, with no problem, for refractory parts.     

Synthesis and Properties of Strontium β-Alumina

It has been shown that strontium β-alumina is a stable phase at compositions close to SrAl₁₀O₁₇ and can be further stabilized over a composition range in the presence of MgO in the SrO–MgO–Al₂O₃ system. In the absence of MgO, at temperatures greater than 1673 K, the magnetoplumbite phase is readily formed. Conductivity measurement carried out at temperatures from 1073 to 1773 K has demonstrated that both the strontium β-alumina and the magnetoplumbite phases have reasonably similar magnitudes of conductivity and activation energy.     

Hydrolysis of α-Tricalcium Phosphate in Simulated Body Fluid and Dehydration Behavior During the Drying Process

The hydrolysis of α-tricalcium phosphate (α-TCP) in a simulated body fluid (SBF) at 37°C was investigated. The hydration rate was found to be slower in SBF than that in deionized water. The concentration of ions in SBF was monitored by ICP. The hydrolysis product, which was characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, Fourier transform infra red, and X-ray photoelectron spectroscopy, was determined to be carbonate-containing, calcium-deficient hydroxyapatite (CO₃−CDHAp) with Mg²⁺, Na⁺, and Cl⁻ impurities similar to the biological apatite. An amorphous layer on the α-TCP surface was found to be the precursor of the apatite phase, which may either form crystalline apatite or may decompose back to α-TCP at a lower temperature.     

Hydration of Sodium β- and β'-Aluminas

The enthalpy and entropy for the hydration of sodium β - and β '-aluminas have been determined by measuring the infrared absorbance of single-crystal samples following equilibration at elevated temperatures in water vapor pressures ranging from 3 to 70 kPa. The equilibrium water concentrations can be varied continuously and reversibly up to saturation concentrations of about 0.9H₂O-Na_1.2AI₁₁O_17.1 for the β-phase and 0.4H₂O.Na_1.67Mg_.67Al_10.33O₁₇ for the β'-phase. The hydration reactions are exothermic; Δ H =−56 ± 2 for sodium β '-aIumina and ΔH =−67 ± 2 kJ/mol for sodium β-alumina. The entropies of hydration are about −143 ± 6 J/(mol-K) for both sodium β - and β '-aIuminas.     

Optical and Magnetic Properties of Some Transition Metal Ions in Barium Phosphate Glass

In barium phosphate glass containing first-row transition metal ions, the ions appear to possess normal oxidation states: Cr³⁺, Mn³⁺, Fe^2+,3+, Co²⁺, Ni²⁺, and Cu²⁺. Room temperature and 77°K spectra for Cr³⁺-, Mn³⁺-, Fe^2+,3+-, Ni²⁺-, and Cu²⁺-containing glasses can be interpreted, on the, basis of octahedrally coordinated metal ions, whereas the spectrum of the Co²⁺ glass correlates with a tetrahedrally coordinated metal ion. Magnetic moments for the glasses are similar to those for the spin-only values of the various metal ions.