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.     

Low-Temperature Synthesis of β-Ca2SiO4 from Hillebrandite

Experiments on hydrothermal synthesis were conducted using quartz or silicic acid and lime as starting materials at Ca/Si = 2.0. It is possible to synthesize pure hillebrandite (Ca₂(SiO₃)(OH)₂) having the theoretical composition by heating at 200°C for 10 h or at 250°C for 5 h. The synthesized product is fibrous, open at each end, and has a length of 20 to 30 μm. Calcium silicate hydrate gels are produced at the initial stage of the reaction. These react further with the unreacted lime to give hillebrandite. However, when silicic acid is used as silica, hillebrandite with tricalcium silicate hydrate is observed at 250°C because of the high reaction rate of silica. On heating, hillebrandite starts to decompose at about 500°C and produces low-crystalline β-Ca₂SiO₄, which is stable at room temperature and has a remarkably large specific surface area of about 7 m²/g. The decomposition reaction rate in a single crystal is rapid, and the reaction is considered to proceed topotactically.     

Thermal Expansion of Homogeneous and Gradient Solid Solutions in the System KZr2(PO4)3─KTi2(PO4)3

Low-thermal-expansion ceramics having arbitrary thermal expansion coefficients were synthesized from homogeneous solid solutions in the system KZr₂(PO₄)₃─KTi₂(PO₄)₃ (KZP–KTP). Dense and strong ceramics were fabricated by sintering at 1100° to 1200°C with 2 wt% MgO. The thermal expansion coefficient increased from 0 to +3 × 10⁻⁶/°C with increasing x in KZr_{2 − x}Ti_x (PO₄)₃ (KZTP). In addition, a functionally gradient material with respect to thermal expansion was prepared by forming a series of KZTP solid solutions in a single ceramic body. By heating a pile of KZP and KTP ceramics in contact with each other, KZP and KTP bonded together to form a KZTP gradient solid solution near the interface.     

Preparation and Compressive Strength Behavior of Porous Ceramics with β-Ca(PO3)2 Fiber Skeletons

High-strength calcium metaphosphate fibers, which are expected to show good biocompatibility, are extracted from crystallized products of ultraphosphate glasses by aqueous leaching. Porous ceramics with skeletons of β-Ca(PO₃)₂ fibers for biomedical use are prepared by the sintering of the fibers. In the present work, porous ceramics having a large porosity of −70% were obtained. The compressive test of these porous ceramics showed that large strains of 0.2–0.3 are requisite for their fracture; the calcium phosphate porous ceramics in this work show much higher flexibility than conventional ceramics. The present work discusses the influence of the preparation conditions of the porous ceramics on their compressive strength behavior.     

High-Strength Ca(PO3)2 Glass-Ceramics Prepared by Unidirectional Crystallization

New CaO-P₂P₅ glass ceramics were prepared by crystallizing glass rods or plates unidirectionally in a temperature-gradient furnace. The unidirectionally crystallized glass showed extremely high bending strength (∼500 to 600 MN/m²) even when subjected to a severe abrasion treatment with Sic paper. Scanning electron microscopy of fracture surjaces revealed a typical fiber-reinforced-composite structure. Crystalline phases and crystallographic directions were checked by X-ray diflraction. The fracture mechanism is discussed in terms of microstructure.     

Preparation of High-Strength Calcium Phosphate Ceramics with Low Modulus of Elasticity Containing β-Ca(PO3)2 Fibers

Novel calcium phosphate ceramics were fabricated by hot-pressing fibrous products extracted from crystallized products of calcium ultraphosphate glasses by aqueous leaching. The ceramics were dense materials with a relative density of >95%; these ceramics were composite materials that consisted of β-Ca(PO₃)₂ fibrous crystals with CaO–P₂O₅ glass, which was formed during hot pressing, as the matrix phase. These ceramics showed a high bending strength of 150–220 MPa and a low Young's modulus of 30–60 GPa. The high toughness contributed to the high strength, with fiber pull-out and crack deflection observed as the primary toughening mechanism.     

Anisotropic Thermal Expansion of β-Ca2SiO4 Monoclinic Crystal

Crystals of β-Ca₂SiO₄ (space group P 12₁/ n 1) were examined by high-temperature powder X-ray diffractometry to determine the change in unit-cell dimensions with temperature up to 645°C. The temperature dependence of the principal expansion coefficients (α_i) found from the matrix algebra analysis was as follows: α₁= 20.492 × 10⁻⁶+ 16.490 × 10⁻⁹ ( T - 25)°C⁻¹, α₂= 7.494 × 10⁻⁶+ 5.168 × 10⁻⁹( T - 25)°C⁻¹, α₃=−0.842 × 10⁻⁶− 1.497 × 10⁻⁹( T - 25)°C⁻¹. The expansion coefficient α₁, nearly along [302] was approximately 3 times α₂ along the b -axis. Very small contraction (α₃) occurred nearly along [     01]. The volume changes upon martensitic transformations of β↔α_L' were very small, and the strain accommodation would be almost complete. This is consistent with the thermoelasticity.     

Grain Size-Microcracking Relation for NaZr2(PO4)3 Family Ceramics

The grain size-microcracking relation was examined for low thermal expansion NaZr₂(PO₄)₃ family ceramics. By measurements of the strength, Young's modulus, thermal expansion, and grain size of polycrystalline ceramics sintered at appropriate conditions, the critical grain size for microcracking was determined. The critical grain size was proportional to the inverse square of the maximum thermal expansion difference.     

Thermal Expansion Behavior of NaZr2(PO4)3Type Compounds

The thermal expansion of the skeletal framework was essentially zero for NaZr₂(PO₄)₃-type compounds; the interstitialion, e.g., Na⁺, was primarily responsible for the total thermal expansion. The composition dependence of the thermal expansion is discussed in terms of the amounts, crystallographic sites, and ionic radii of the interstitial ions. The mechanism which results in low thermal expansion was clarified, particularly for KZr₂(PO₄)₃, in which a larger ion is substituted for Na⁺, and NbZr(PO₄)₃, which does not contain Na⁺. Polycrystalline ceramics formed from these crystals might be useful as thermal-shock-resistant materials.     

Spark Plasma Sintering of LiTi2(PO4)3-Based Solid Electrolytes

Solid electrolytes, LiTi₂(PO₄)₃ (LTP), Li_1.3Al_0.3Ti_1.7(PO₄)₃ (LATP), and Li_1.3Al_0.3Ti_1.7(PO₄)_2.9(VO₄)_0.1 (LATPV), were prepared by conventional sintering (CS) and spark plasma sintering (SPS) methods, and the Li⁺ ion conductivity of the sintered pellets was examined using an impedance analyzer. SPS remarkably improved the densification compared to CS and resulted in dense ceramics (95–97% of theoretical density) irrespective of the substituted ions. The highest conductivity of 2.6 × 10⁻⁴ S/cm was found for the LATPV specimen sintered by spark plasma at 1100°C. LATP and LATPV exhibited an order of magnitude higher ionic conductivity than LTP in the specimens of similar densities. The results demonstrated that the enhanced conductivity in substituted LTP is not due to the enhanced densification alone. The other possible explanations are discussed in terms of bottleneck size, lithium content, and grain boundary characteristics.     

Effect of Al(OH)3 Addition on β-LiAlO2 Crystal Growth

Crystal growth of rod-shaped β-LiAlO₂ was previously reported by us, and the rod-shaped β-LiAlO₂ crystals were 1.5 μ in diameter and 10 to 15 μm long. In the present study needle-shaped β-LiAlO₂ crystals which were thinner and had larger aspect ratios (length/diameter) than the rodshaped β-LiAlO₂ crystals were grown by using LiOH–Al₂O₃–Al(OH)₃–NaOH as the raw material. These crystals were 0.7 to 1 μm in diameter, 9 to 13 μm long, and had aspect ratios of about 10 to 13.     

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.     

Flexible Ceramics in the System KZr2(PO4)3–KAlSi2O6 Prepared by Mimicking the Microstructure of Itacolumite

A ceramic composite mimicking the pervasively cracked microstructure of flexible sandstone (itacolumite) was successfully synthesized by sintering two ceramic materials with different thermal expansion coefficients. A combination of granular KZr₂(PO₄)₃ (high thermal expansion) and powdered KAlSi₂O₆ (low thermal expansion) resulted in a material with a jigsaw-like three-dimensional cracking microstructure similar to that of itacolumite. The synthesized composite was found to exhibit ductile deformation.     

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.     

Preparation of Porous Glass-Ceramics with a Skeleton of NASICON-Type Crystal CuTi2(PO4)3

A novel porous glass-ceramic with a skeleton of CuTi₂(PO₄)₃ was prepared by controlled crystallization of a glass and subsequent chemical leaching of the resulting dense glass-ceramic. A volume-crystallized dense glass-ceramic composed of CuTi₂(PO₄)₃ and Cu₃(PO₄)₂ whose surface was covered by a CuO thin layer was prepared by reheating a glass with a nominal composition of 50CuO·20TiO₂30P₂O₅ (in mol%) glass in air. When the resultant glass-ceramic was leached with dilute H₂SO₄, Cu₃(PO₄)₂ and CuO phases were dissolved out selectively, leaving a crystalline CuTi₂(PO₄)₃ skeleton. The specific surface area and the average pore radius of the porous glass-ceramic obtained were approximately 45 m₂g_-1 and 9 nm, respectively. The porous glass-ceramic showed catalytic activity in the conversion reaction of propene into acrolein.     

Formation Processes of β-C2S by the Decomposition of Hydrothermally Prepared C-S-H with Ca(OH)2

A mixture of CaO and silicic acid prepared with a Ca/Si ratio of 2.0 was hydrothermally synthesized at 80° to 200°C, and the thermal decomposition behavior of the products (C-S-H with Ca(OH)₂) was analyzed using XRD, ²⁹Si MAS NMR, and the trimethylsililation method (TMS). It was found that the main silicate anion structure of C-S-H was a mixture of a dimer and a single-chain polymer (larger than Si₅O₁₆) and that polymerization advanced with an increase of the synthesizing temperature. On heating, the products decomposed to form β-C₂S. It was found that the decomposition was gradual and that the-higher the temperature of hydrothermal synthesis, the lower was the temperature of the decomposition into β-C₂S. Although the decomposition proceeded to form a monomer (β-C₂S) from the polymer and dimer, this dimer was resistant to heat and did not decompose unless heated to above 400°C.     

Kinetics of the β"-to-β Transformation in the System Na2O-A12O3

Mixtures of α-A1₂O₃ and NaAlO₂ corresponding to the eutectic composition 65 mot % A1₂O ₃ were heated in sealed platinum crucibles above the eutectic temperature (measured to be 1572°C in this study). Once melting was complete, the liquid was quenched and the resultant solid examined by X-ray diffraction. Sodium aluminate, beta alumina, and beta" alumina were detected in all cases. However, the relative amounts of beta and beta" present were a function of the cooling rate, with the amount of beta" increasing with a decreasing cooling rate. Subsequent annealing (in platinum envelopes) of the solidified eutectic at 1400° and 1530°C showed that the amount of beta" present decreased with increasing annealing time. Based on this observation, it is apparent that, in the temperature range studied, beta"-is metastable. Analysis of the time-transformation data shows that the reaction kinetics are consistent with the diffusion-controlled growth of the beta phase at the expense of the beta"-phase.