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.     

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.     

Effect of Substitutional Monovalent and Divalent Metal Ions on Mechanical Properties of β-Tricalcium Phosphate

β-tricalcium phosphate (β-TCP) powder doped with monovalent or divalent metal ions was hot pressed at 1100°C, and the effect of substitutional monovalent and divalent metal ions on mechanical properties of β-TCP was investigated. The sinterability of β-TCP would be enhanced by the substitution of monovalent and divalent metal ions for β-TCP. Sintered β-TCP doped with 7.6 mol% of Mg²⁺ ion showed a bending strength of 160 MPa. It was found that the substituition of Mg²⁺ ion up to 9.6 mol% and a small amount of monovalent metal ions for β-TCP is effective to improve the mechanical strength of β-TCP.     

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.     

Elucidation of the Formation Mechanism of β-SiAlON from a Zeolite

β-SiAlON was synthesized from Y-type zeolite and its formation mechanism was determined using magic-angle spinning nuclear magnetic resonance (NMR) spectroscopy and powder X-ray diffraction (XRD). Y-type zeolite and carbon powders were mixed for 36 h, fired at 1050° to 1450°C for 0–2 h in flowing gas (0.5 L/min N₂), then heat treated at 700°C for 2 h in air to remove residual carbon. The resulting product was, according to XRD analysis, mono-phase β-SiAlON; but minor amounts of unreacted SiO₂ and SiC (possibly amorphous) were detected in the NMR spectra. It was found that β-SiAlONs with higher and lower Z values are formed by different reaction pathways.     

Sintering and Microstructure Formation of β-Silicon Carbide

The effect of additions of B, Al, and B + Al on the pressureless sintering of β-Sic was examined. The influence of the sintering atmosphere and heating schedule on densification behavior, polytype transformation, and microstructure development was also studied. High densities were obtained at 1940°C by the simultaneous addition of B and Al. The decrease in the sintering temperature is attributed to the presence of a liquid phase which results in the formation of platelets (up to 200 # in size) of an α-polytype, predominantly 4H and 6H. Polytype transformation and exaggerated grain growth could be prevented by annealing the compact at 1650° to 18500°C for 0.5 to 1 h. This procedure results in a better redistribution of the sintering aids, giving a fine-grained microstructure, constituted primarily of the cubic 3C polytype.     

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.     

Standard Gibbs Energy of Formation of β-Silicon Nitride

Experimental thermochemical data (temperature, pressure) corresponding to the equilibrium conditions between finegrained β-SiC and β-Si₃N₄ for carbon activity a (C) = 1 are presented. Based on these data, the temperature dependence of ΔG°_f(β-Si₃N₄) has been expressed for standard states Si( s ), C( s ), and p(N₂) = 0.1 MPa by the equation ΔA°_f(β-Si₃N₄) = (-995.9 + 0.4547 T/K) kJ mol for T/K ε〈1650; 1968〉.     

Formation of Carbide-Derived Carbon on β-Silicon Carbide Whiskers

Carbon was synthesized on β-SiC whiskers by extraction of Si atoms from SiC. In this study, three different elevated temperature extraction methods were used to remove Si atoms from SiC: treatments in either Cl₂ or HCl and vacuum decomposition. In all chlorination experiments and vacuum treatment at 1700°C, carbon preserved the original shape of SiC whiskers. At higher temperatures (2000°C), vacuum decomposition led to a distortion in the shape of the whiskers. High-resolution transmission electron microscopy and Raman spectroscopy showed that the structure of carbide-derived carbon depends on the Si extraction method and the process parameters. Chlorination of SiC resulted in the formation of mostly amorphous nanoporous carbon. High-temperature treatment of SiC in HCl environment produced fullerene-like structures, while high-temperature vacuum decomposition resulted in the formation of graphite. Transmission electron microscopy studies of the carbon coating thickness produced in Cl₂ at various chlorination times revealed linear reaction kinetics at 700°C. Raman studies showed that the carbon structure became more ordered with increasing chlorination temperature. The results obtained demonstrate that by using the silicon extraction technique, one can precisely control the thickness and morphology of the carbon coating.     

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%.     

Mullite–β-Spodumene Composites from Aluminosilicates

Sintering studies were conducted using kaolin, metakaolin, zeolite 4A, and various synthetic mixtures of Al₂O₃ and SiO₂ in the presence of Li₂CO₃ and LiCl as fluxing agents. Various compositions of the above were prepared, and conventional sintering studies were conducted at temperatures of 900°–1450°C with soaking periods of 1–3 h. Kaolin, metakaolin, and amorphized kaolin in the presence of Li₂CO₃ showed nucleation centers of β-spodumene as pink specks, whereas synthetic mixtures of Al₂O₃ and SiO₂ failed to behave in the same manner. To determine whether the pink specks formed were color centers or F centers, the samples were subjected to UV, IR, and X-ray irradiation; however, the samples showed no tenebrescence properties. External addition of iron as an impurity in a nonlayered system also resulted in pink speck formation. This observation indicated that impurities present in the natural kaolin were the cause of this phenomenon. Moreover, the LiCl-based samples did not result in pink specks, even though the kaolinitic samples contained iron as an impurity. Therefore, although β-spodumene was formed in aluminosilicates in the presence of Li₂CO₃ and LiCl, the pink variety of β-spodumene (kunzite) formation occurred only in the presence of lithium-rich aluminosilicates and in the presence of iron as an impurity. The phase identification and microstructure were explained based on XRD, DTA, and SEM studies.     

Joining of Calcium Phosphate Invert Glass-Ceramics on a β-Type Titanium Alloy

A bioactive calcium phosphate invert glass-ceramic containing β-Ca₃(PO₄)₂ crystals could be joined strongly with a Ti–29Nb–13Ta–4.6Zr alloy consisting of a β-titanium phase by heating the metal on which the mother glass powders with a composition 60CaO·30P₂O₅·7Na₂O·3TiO₂ (mol%) were placed, at 800°C for 1 h in air; the tensile joining strength was estimated to be ∼26 MPa on average. A compositionally gradient layer was developed on the metallic substrate during the heating. When the metal with glass powders on it was heated at 850°C in air, the phosphate glassy phase flowed viscously, permeating the oxide layer formed around the surface of the metal, which was thicker than that formed by heating at 800°C; a compositionally gradient layer was not developed, and a strong joining was not realized. The joining between the glass-ceramic and the metal is suggested to be controlled by viscous flow of the glassy phase in the glass-ceramic and by reaction of the glassy phase with the oxide phase formed around the surface layer of the metal.     

Edge-Defined Film-Fed Growth of β-Alumina and Mg-Substituted β-Alumina

High Na vapor pressure, peritectic decomposition, and high reactivity of the melt complicate the growth of α-alumina crystals. These difficulties were overcome by using a high-pressure (300 psig) growth chamber, Na₂O-rich melts, and Ir for all surfaces in contact with the melt. These procedures were combined with the edge-defined film-fed growth technique to produce single-crystal β alumina tubes and ribbons.     

Apatite Formation on Calcium Phosphate Invert Glasses in Simulated Body Fluid

Calcium phosphate invert glasses, which contain P₂O₇²⁻ and PO₄³⁻ ions, have been prepared via the addition of a small amount of TiO₂. The formation of bonelike calcium phosphate apatite on the surface of the phosphate invert glasses was examined in simulated body fluid (SBF) at a temperature of 37°C. Soaking for 20 d resulted in the deposition of leaflike apatite particles on 6CaO·3P₂O₅·TiO₂ invert glass (based on molar ratio). The glass had much-greater chemical durability against SBF, in comparison with a metaphosphate glass; P ions were not dissolved excessively from the 6CaO·3P₂O₅·TiO₂ glass, so the apatite formation was not suppressed.     

New Approach to the β→α Polymorphic Transformation in Magnesium-Substituted Tricalcium Phosphate and its Practical Implications

The transformation β→α in Mg-substituted Ca₃(PO₄)₂ was studied. The results obtained showed that, contrary to common belief, there is, in the system Mg₃(PO₄)₂–Ca₃(PO₄)₂, a binary phase field where β+α-Ca₃(PO₄)₂ solid solutions coexist. This binary field lies between the single-phase fields of β- and α-Ca₃(PO₄)₂ solid solution in the Ca₃(PO₄)₂-rich zone of the mentioned system. In the light of the results and the Palatnik–Landau's Contact Rule of Phase Regions, a corrected phase equilibrium diagram has been proposed. The practical implications of these findings with regard to the synthesis of pure α- and β- Mg-substituted Ca₃(PO₄)₂ powders and to the sintering of related bioceramics with improved mechanical properties are pointed out.     

Thermal Expansion of Synthetic β-Spodumene and β-Spodumene—Silica Solid Solutions

Synthetic β-Spodumene and several β-Spodumene-silica solid solutions were prepared, and their thermal expansion behavior was studied using high-temperature X-ray diffraction techniques. Data indicate that all β-Spodumenes are characterized by a pronounced anisotropy of thermal expansion much the same as that of the silica polymorph keatite. Anisotropy increases slightly and the volume expansion decreases as the compositions become richer in SiO₂. Specimens with more than 80.8 wt% SiO₂ at 130O°C and 1 atm contain P-cristobalite along with β-Spodumene. The lattice parameters of β-Spodumene compositions throughout the solid solution range were determined. The structural aspects of the anisotropy of thermal expansion are discussed.     

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.     

A Novel Biphasic Bone Scaffold: β-Calcium Phosphate and Amorphous Calcium Polyphosphate

Calcium polyphosphate (CPP) was added to hydroxyapatite (HA) to develop a novel biphasic calcium phosphate (BCP). The effects of varying CPP dosage on the sintering property, the mechanical strength, and the phase compositions of HA were investigated. Results showed that CPP reacted with HA and produced β-calcium phosphate (β-TCP) and H₂O and that an excessive dosage of CPP (>10 wt%) obtained a novel BCP of β-TCP/amorphous-CPP, while a lesser dosage of CPP (<10 wt%) obtained a traditional BCP (HA/β-TCP). The porous β-TCP/amorphous-CPP scaffolds (porosity of 66.7%, pore diameter of 150–450 μm, and compressive strength of 6.70±1.5 MPa) were fabricated and their in vitro degradation results showed a significant improvement of degradation with the addition of CPP.     

Thermomechanical Properties of β-Sialon Synthesized from Kaolin

β-Sialon powder was synthesized by the simultaneous reduction and nitridation of Hadong kaolin at 1350°C in an N₂–H₂ atmosphere, using graphite as a reducing agent. The average particle size of β-sialon powder was about 4.5 μm. The synthesized β-sialon powder was pressureless sintered from 1450° to 1850°C under a N₂ atmosphere. The relative density, modulus of rupture, fracture toughness, and microhardness of β-sialon ceramics sintered at 1800°C for 1 h were 92%, 248 MPa, 2.8 MN/m^3/2, and 13.3 GN/m², respectively. The critical temperature difference (ΔT_c) in water-quench thermal-shock behavior was about 375°C for the synthesized β-sialon ceramics.