Determination of Precise Unit-Cell Parameters of the α-Tricalcium Phosphate Ca3(PO4)2 Through High-Resolution Synchrotron Powder Diffraction

Unit-cell parameters of the α-tricalcium phosphate [TCP; Ca₃(PO₄)₂] were investigated using high-resolution synchrotron powder diffraction and the Rietveld method. The diffraction experiment was conducted at 29°C at the BL-15XU experimental station of SPring-8, Japan. Precise unit-cell parameters of the α-TCP were obtained; a =12.87271 (9), b =27.28034(8), c =15.21275(12) Å, α=γ=90°, and β=126.2078(4)°. The calculated density of α-TCP (2.8677 g/cm³) is smaller than that of β-TCP, indicating the "looser" structure of α-TCP.     

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.     

Texture and Formation Mechanism of Fibrous Calcium Hydroxyapatite Particles Prepared by Decomposition of Calcium–EDTA Chelates

The texture of fibrous calcium hydroxyapatite (Ca₁₀-(PO₄)₆(OH)₂, CaHAP) particles that were prepared by the decomposition of calcium–ethylenediaminetetraacetic acid (calcium–EDTA) chelates at 100°C under various pH conditions (pH values of 5–10) was investigated by various means. Well-crystallized fibrous CaHAPs were produced at pH .6. The stoichiometry of the CaHAPs with a chemical formula of Ca_{10− x} (HPO₄) _x (PO₄)_{6− x} (OH)_{2− x} (H₂O) _x was improved by increasing the decomposition pH. All the CaHAPs had unit-cell dimensions of a = 0.9436 ± 0.0003 nm and c = 0.6881 ± 0.0006 nm, exhibiting an enlarged a value. The finding of mesoporosity of CaHAPs by nitrogen gas (N₂) adsorption measurement indicated that the CaHAPs were produced by an agglomeration of primary particles. Furthermore, the nonstoichiometric CaHAPs that formed at pH 6 developed ultramicropores, which were accessible to water (H₂O) molecules but not to N₂ molecules, by the elimination of H₂O molecules that were adsorbed in interstices of primary particles in less-orderly crystallized CaHAPs and/or by dehydration of HPO₄²⁻ groups. These findings by gas adsorption techniques could give evidence for the agglomeration mechanism to attain a polycrystalline CaHAP, although they exhibited good crystallinity with large size.     

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.     

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.     

Hydration in the System Ca2SiO4–Ca3(PO4)2 at 90°C

Compositions along the Ca₂SiO₄–Ca₃(PO₄)₂ join were hydrated at 90°C. Mixtures containing 15, 38, 50, 80, and 100 mol% Ca₃(PO₄)₂ were fired at 1500°C, forming nagelschmidtite + a ¹-CaSiO₄, A -phase and silicocarnotite and a -Ca₃(PO₄)₂, respectively. Hydration of these produces hydroxylapatite regardless of composition. Calcium silicate hydrate gel is produced when Ca₂SiO₄≠ 0 and portlandite when Ca₂SiO₄ is >50%. Relative hydration reactivities are a -Ca₃(PO₄)₂ > nagelschmidtite > α ¹-Ca₂SiO₄ > A -phase > silicocarnotite. Hydration in the presence of silica or lime influences the amount of portlandite produced. Hydration in NaOH solution produces 14-A tobermorite rather than calcium silicate hydrate gel.     

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.     

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.     

Fabrication of Hydroxyapatite–Zirconia Composites for Orthopedic Applications

Dense hydroxyapatite–zirconia (HAp–ZrO₂) composites are expected to have desired mechanical and biological properties for orthopedic applications. However, due to some processing problems, to date, this material can only be prepared by special techniques. In this paper, we report for the first time a facile route to prepare HAp–ZrO₂ composites. Initially, HAp and ZrO₂ powders were dispersed in aqueous media with polyacrylic acid and glutamic acid as the dispersants. The slurries exhibited a well-stabilized state at a high solid content (>50 vol%) and therefore green samples with high density (>60%) can be obtained after slip casting. These HAp–ZrO₂ green samples can be easily densified by pressureless sintering at 1450°C with 2 h holding. After sintering, only hydroxyoxyapatite Ca₁₀(PO₄)₆O _x (OH)_{2(1− x )} (HOA), ZrO₂, and trace amounts of α-tricalcium phosphate phases were detected. No obvious reactions between HAp and ZrO₂ phase were observed. The HAp–ZrO₂ samples showed excellent mechanical and biological properties. For 40 vol% HAp–60 vol% ZrO₂ samples sintered at 1450°C, the flexural strength and toughness were 220 MPa and 4.37 MPa·m^1/2, respectively. In addition, we observed the attachment, spreading, and proliferation of mesenchymal stem cells on the HAp–ZrO₂ samples' surface. The results showed that the proposed colloidal processing and pressureless sintering process is feasible for preparing HAp–ZrO₂ composites with high mechanical properties and promising bioactivity for orthopedic applications.     

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.     

Porous Glass-Ceramics with a Skeleton of the Fast-Lithium-Conducting Crystal Li1+xTi2−xAlx(PO4)3

Porous glass-ceramics with a skeleton of the fast-lithium-conducting crystal Li_{1+ x} Ti_{2− x} Al _x (PO₄)₃ (where x = 0.3–0.5) were prepared by crystallization of glasses in the Li₂O─CaO─TiO₂─Al₂O₃–P₂O₅ system and subsequent acid leaching of the resulting dense glass-ceramics composed of the interlocking of Li_{1+ x} Ti_{2− x} Al _x (PO₄)₃ and β-Ca₃(PO₄)₂ phases. The median pore diameter and surface area of the resulting porous Li_{1+ x} Ti_{2− x} Al _x (PO₄)₃ glass-ceramics were approximately 0.2 μm and 50 m²/g, respectively. The electrical conductivity of the porous glass-ceramics after heating in LiNO₃ aqueous solution was 8 × 10⁻⁵ S/cm at 300 K or 2 × 10⁻² S/cm at 600 K.     

Cation-Exchange Characteristics of Sintered Hydroxyapatite in the Strongly Acidic Region

The ion-exchange behavior of hydroxyapatite (Ca₁₀(PO₄)₆-OH)₂) (HAp), fired at high temperatures, has been investigated in the strongly acidic region (pH 2 and 3). According to the present study, HAp fired above 1000°C can exchange ions from Ca²⁺ to Pb²⁺, even at pH 2, without dissolution. The molar ratios of Pb²⁺/Ca²⁺ after ion exchange are approximately unity in all cases. Ion exchange occurs in a thin layer near the surface of HAp particles fired at 1300°C. After ion exchange, Pb-Cl-apatite crystals are created in the strongly acidic region (pH 2).     

Structure and Microwave Dielectric Characteristics of Ca1+xNd1−xAl1−xTixO4 Ceramics

CaNdAlO₄ microwave dielectric ceramics were modified by Ca/Ti co-substitution, and their dielectric characteristics were evaluated along with their structure and microstructures. Ca_{1+ x} Nd_{1− x} Al_{1− x} Ti _x O₄ ( x =0, 0.025, 0.05, 0.10, 0.15, 0.20) ceramics with the relative density of over 95% theoretical density were obtained by sintering at 1400°–1450°C in air for 3 h, where the K₂NiF₄-type solid solution single phase was determined from the compositions of x <0.20, while a small amount of CaTiO₃ secondary phase was detected for x =0.20. With Ca/Ti co-substitution in CaNdAlO₄ ceramics, the dielectric constant (ɛ_r) increased with increasing x , and the temperature coefficient of resonant frequency (τ_f) was adjusted from negative to positive, while the Q × f ₀ value increased significantly at first and reached an extreme value at x =0.025 and the maximum at x =0.15. The best combination of microwave dielectric characteristics were achieved at x =0.15 (ɛ_r=19.5, Q × f ₀=93 400 GHz, τ_f=−2 ppm/°C). The improvement of the Q × f ₀ value primarily originated from the reduced interlayer polarization with Ca/Ti co-substitution, while the decreased tolerance factor, the subsequent increased interlayer stress, and the appearance of CaTiO₃ secondary phase brought negative effects upon the Q × f ₀ value.     

Effects of Ca/Ti Cosubstitution upon Microwave Dielectric Characteristics of CaSmAlO4 Ceramics

(Ca_{1+ x} Sm_{1− x} )(Al_{1− x} Ti _x )O₄ (0≤ x ≤0.4) ceramics were synthesized by solid-state reaction method and their microstructures and microwave dielectric properties were investigated. X-ray diffraction analysis and energy-dispersive X-ray analysis indicated that the matrix phase was a solid solution with a composition represented by the chemical formula (Ca_{1+ x} Sm_{1− x} ) (Al_{1− x} Ti _x )O₄ and minor amount of (Ca,Sm)(Al,Ti)O₃ secondary phase was detected. Ca/Ti cosubstitution could significantly improve the microwave dielectric characteristics of CaSmAlO₄ ceramics, and the excellent microwave dielectric characteristics were obtained in the modified ceramics as ɛ_r=19–23, Q × f =49 100–118 700 GHz, and τ_f=−15–15 ppm/°C.     

Orthophosphates and Diphosphates Containing Zinc and Copper and Zinc and Cobalt

Solid solutions of diphosphates of zinc and copper and of zinc and cobalt were synthesized from mixtures of pure diphosphates at temperatures up to 1000°C. Their X-ray diffractometry patterns varied continuously from one end member to the other. Solid solutions of orthophosphates of composition Zn_3−xCox(PO₄)2, with x = 0.4–1.6, were formed at temperatures up to 950°C; all exhibited the structure of γ-Zn₃(PO₄)2. Solid solutions of orthophosphates of composition Zn_3−xCu_x(PO₄)2 exhibited more-complex behavior. At 1000°C and copper contents of 20–80 mol%, a phase that is related to Cu₃(PO₄)2, termed here the "ε-phase," predominated. At 850°–950°C and in the region from 20 mol% to ∼33 mol% of copper, the solid solutions (the "η-phase") adopted the structure of graftonite. At 800°–900°C and 10–15 mol% of copper, the solid solutions exhibited a new structure (the "δ-phase"), which we found to be related to the mineral sarcopside. At temperatures 950°C, the solutions that contained 5–15 mol% of copper (the "β-phase") had the structure of β-Zn₃(PO₄)2, whereas at 800°–850°C, solutions with 5 mol% of copper (the "-phase") exhibited the structure of γ-Zn₃(PO₄)2. Attempts to synthesize Cu⁺ZnPO₄ and Cu⁺Cu²⁺Zn₃(PO₄)3 were unsuccessful.     

Polymorphism of Calcium Orthosilicate

Therecentobservation of orthorhombic α'-Ca₂Si0₄ (bredigite) at all temperatures between about 850° and 1450°C. leads to a rational interpretation of the polymorphism of this substance, which is very satisfactory from the crystal-chemical point of view. The so-called β phase, of as yet unknown, complex structure, exists only meta-stably, and not above but below about 675°C, where γ is the stable phase. The monotropic β phase forms on cooling from α'near 675°C. as the result of the inhibition of the α'→γ inversion, at 850°C. The inhibition is caused by the need for a considerable atomic rearrangement and a 12% volume increase which accompanies the change of the coordinations CaO₂ and CaOlo, in α', to CaO₆, in γ. Among the solid phase equilibria with, Mg₂SiO₄ and Ca₃(PO₄)₂, unlimited solid solubility between γ-Ca₂SiO₄ and Mg₂SiO₄ is predicted, whereas the solubility of Mg₂SiO₄ is a limited one in α'and still more so in a-Ca₂SiO₄, as a result of the substitution, for calcium, of the smaller magnesium.     

Thermal Hysteresis for the α'L↫β Transformations in Strontium Oxide-Doped Dicalcium Silicates

A series of Sr-bearing Ca₂SiO₄ solid solutions (C₂S( ss )), (Sr _x Ca_1−x)₂SiO₄ with 0 ≤ x ≤ 0.12, was prepared. They were examined by high-temperature powder X-ray diffractome-try to determine the start and finish temperatures of the α'_L-to-β and β-to-α'_L martensitic transformations. The thermal hysteresis was positive with x < 0.045, nearly equal to zero at x = 0.045, and negative with x > 0.045. The zero and negative hysteresis were consistent with the thermoelasticity of the transformations. With increasing x from 0.02 to 0.08, the hysteresis decreased steadily from positive to negative, suggesting a continuous increment in the stored elastic energy.     

Phase Transformation during Plasma Spraying of Hydroxyapatite–10-wt%-Zirconia Composite Coating

This study aims to explore phase transformation in plasma-sprayed hydroxyapatite (HA) + 10 wt% ZrO₂–8-mol%-Y₂O₃composite coating, using separately prepared HA and ZrO₂–8-mol%-Y₂O₃coatings as a control. Changes in the phase and chemistry of the coatings are characterized by X-ray diffractometry, with lattice-constant measurement (Cohen's method), and by transmission electron microscopy. Experimental results show evidence of diffusion, in the liquid state, of calcium ions from the HA matrix into the ZrO₂. This behavior causes the formation of the following structural features in the composite coating: (i) a CaO-doped ZrO₂solid solution (ZrO₂–7.7 mol% Y₂O₃–4.4 mol% CaO); (ii) a mixture of ZrO₂and CaZrO₃having a crystal-orientation relationship; (iii) an amorphous phase containing elements of calcium, phosphorus, zirconium, and yttrium; and (iv) a remaining CaO-poor HA matrix (Ca_{10− x} (HPO₄) _x (PO₄)_{6− x} (OH)_{2− x} ; x = 0.06). Rationales for the greatly decreased impurity phases of CaO and Ca₄P₂O₉found in the composite coating are discussed.     

Hydrolysis of Pure and Sodium Substituted Calcium Aluminates and Cement Clinker Components Investigated by in Situ Synchrotron X-ray Powder Diffraction

The hydrolysis of pure and sodium-substituted calcium aluminates and cement clinker phases was investigated in situ in the temperature range 25°–170°C, using the angle dispersive powder synchrotron powder X-ray diffraction technique. The final hydrolysis product in all cases was Ca₃Al₂(OH)₁₂. The intermediate phase Ca₄Al₂O₇·19H₂O was formed from the pure calcium aluminates, and the intermediate phases Ca₄Al₂O₇· x H₂O, x = 11, 13, or 19, were formed from the cement clinker phases.     

Glass-Free KxBa1−xGa2−xGe2+xO8 Ceramics for Low-Temperature Cofired Ceramics Technology: Synthesis, Phase Transitions, Sintering, and Microwave Dielectric Properties

K _x Ba_{1− x} Ga_{2− x} Ge_{2+ x} O₈ (0.6≤ x ≤1) polycrystalline ceramics are potential materials for glass-free low-temperature cofired ceramics (LTCC) substrates. We have made a comprehensive study of the kinetics of the monoclinic-to-monoclinic P 2₁/ a ⇔ C 2/ m phase transition. The low-temperature-stable P 2₁/ a phase with a high Q × f value was synthesized using a subsolidus method and was well sintered at the LTCC temperature with a H₃BO₃ additive. A good combination of low sintering temperature (910°–920°C), high Q × f values (96 700–104 500 GHz), low permittivities (5.6–6.0), and a small temperature coefficient of resonant frequency (∼−20 ppm/°C) was obtained for ceramics with x =0.67 and 0.9 and with 0.1 wt% of H₃BO₃.