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.     

Synthesis and Photoluminescence Properties of Eu3+-Doped Calcium Phosphates

Eu³⁺-doped (1–12 mol%) calcium hydroxyapatite (HA) and β-tricalcium phosphate (β-TCP) powders were synthesized by a precipitation method and their photoluminescence (PL) properties were investigated. Eu has a limited solubility in HA and Eu₂O₃ was detected by X-ray diffraction above 3% doping, whereas a nearly single β-phase was obtained up to 6% doping in TCP but EuPO₄ appeared in the 12% Eu-doped specimen. Electron spin resonance measurement confirmed that europium ions exist as Eu³⁺ in both samples. Eu³⁺-doped HA exhibited a strong emission at 575 nm with several minor peaks at 610–640 nm and Eu³⁺-doped TCP had an intense emission at 613 nm with secondary peaks at 590–600 nm, which were consistent with the earlier reports determined at low temperatures. In Eu-doped TCP, the PL intensity increased with increasing calcination temperature without phase transformation. The more the Eu added, the higher the PL intensity obtained in both cases.     

Synthesis of Er3+ Doped Y2O3 Nanophosphors

The synthesis and characterization of yttrium hydroxyl carbonate (Y(OH)CO₃²⁻) and yttrium nitrate hydroxide hydrate (Y(OH)NO₃H₂O) precursor materials as well as Y₂O₃ nanoparticles are reported. The resultant precursor particle size is about 10–12 nm with a narrow size distribution by the enzymatic decomposition method, whereas the particle size was smaller than those acquired by the homogeneous and alkali precipitation methods. The formation of Y(OH)CO₃²⁻ and Y(OH)NO₃H₂O species was also evident from the fourier-transform infrared spectrometry (FT-IR) analysis. Precipitated Y(OH)CO₃²⁻ precursors have an amorphous nature whereas Y(OH)NO₃H₂O precursors have a crystalline nature, which was manifested from the XRD analysis. Moreover, precipitated Y(OH)NO₃H₂O precursors were found in the agglomerated form and Y(OH)CO₃²⁻ was established in the monodispersed form, as determined from the FE-SEM, TEM and DLS measurements. It was demonstrated that calcination of precursor materials at 900°C eventually removed the inorganic anions from the precursors and consequently produced crystalline Y₂O₃ nanoparticles, which was evident from the XRD and FT-IR analysis. The EDS analysis confirms Er³⁺ doping in the Y₂O₃ nanoparticles. The morphology and the size of the Y₂O₃ nanoparticles are almost unchanged before and after the calcination.     

Influence of Synthesis Conditions on the Characteristics of Biphasic Calcium Phosphate Powders

Mixtures of hydroxyapatite (HAp) and tricalcium phosphate (TCP) powders with different Ca/P ratios were prepared by a wet chemical precipitation synthesis of calcium-deficient hydroxy apatite (CDHA) and subsequent calcination. The influence of precipitation method and calcination temperature on the chemical and physical properties of the resulting powders were evaluated. Different ratios between HAp and TCP were obtained by variation of the velocity of orthophosphoric acid addition into calcium hydroxide suspension. Slow addition (1 mL/min, S-powder) of H₃PO₄ into Ca(OH)₂ suspension lead to a higher amount of CO₃ groups substituting PO₄ in the apatite lattice compared to powders synthesized by fast addition (200 mL/min, F-powder). Consequently, the Ca/P molar ratio of S-powder (1.55) was higher than that of F-powder (1.52), leading to a higher amount of HAp in the resulting calcinated powders. Both powders show significant differences in their calcinations behavior. Whereas for F-powders a temperature of 750°C was adequate for a complete transformation of CDHA to TCP and HAp, 950°C were required for S-powders.     

Low-Temperature Synthesis and Properties of LaMnO3±d and La0.67R0.33MnO3±d (R = Ca, Sr, Ba) from Citrate Precursors

LaMn_{1− y} ³⁺Mn _y ⁴⁺O_3±d and La_0.67R_0.33Mn_{1− y} ³⁺Mn _y ⁴⁺O_3±d (R = Ca, Sr, Ba) phases were synthesized at 350°C by using very reactive, amorphous precursors obtained from the stoichiometric citrate solutions. The chemical process was optimized with respect to the solution concentration, pH, and additives. The precursor reactions were investigated as a function of the cation stoichiometry and the additive by simultaneous thermal and thermogravimetric analysis and X-ray diffraction. The reaction pathway was found to be independent of the cation stoichiometry, but related to the acid or base additive. The annealing temperature was systematically increased in the 350–1200°C interval and the La_0.67Sr_0.33MnO_3±d properties (i.e., crystal sizes, Mn average valence, Curie temperature, magnetization, magnetic susceptibility) were measured and found to vary consistently as a function of it.     

Phase Diagram of the Pseudobinary System Ga2O3-Bi2-x3+Bix5+O3+x2-

The pseudobinary system Ga₂O₃-Bi_{2- x} ³⁺Bi _x ⁵⁺O_{3+ x} ^2- was studied in view of its importance for growth of SrGa₁₂O₁₉ crystals from a bismuth oxide flux. A subsolidus transition of γ*-Bi₂O₃ to a β'-phase and a strictly stoichiometric 1:2 phase with 33.3 mol% bismuth oxide were found. The single-crystal data for the compound indicated space group P2₁2₁2, with lattice constants a=0.79180±0.0003 nm, b =0.8288±0.0003 nm, and c =0.5889±0.0003 nm; the measured density was 7.1±0.3 g/cm³ and the cell content (Z) 2.     

Thermodynamic Stability of K+-β-Alumina

K₂O activity in K⁺-(α+β)-alumina was determined from emf data of the galvanic cell Pt,O₂,(α+β)-alumina/K⁺-β-alumina/K₂SO₄,SO₂+SO₃+O₂,Pt. K₂O activity in the K⁺-(α+β)-alumina was expressed by the equation log a_k2o (±0.038)=(−18295±120)(K/T)+(0.998±0.110), where 961 K+-β-alumina are discussed: the standard Gibbs energy of formation of K₂O·11Al₂O₃ from K₂O and α-Al₂O₃ and from K, O, and Al; the thermodynamic stabilities of K⁺-β-alumina in the atmospheres of SO_x (x=2, 3) and CO₂; the equilibrium vapor pressure of potassium over K⁺-β-alumina under a constant O₂ pressure; and the stability of K⁺-β-alumina in the molten Na-K alloy.     

Porous Photocatalytic TiO2 Thin Films by Aerosol Deposition

Porous photocatalytic TiO₂ thin films were fabricated by the leaching technique, followed by aerosol deposition. Mixed powders of TiO₂ and β-tricalcium phosphate (TCP) were aerosol deposited at room temperature for the initial fabrication of composite films. After the β-TCP phases were leached out from the composite films in a diluted HCl aqueous solution for 24 h, porous TiO₂ films remained on the substrate. To fabricate these porous films, the β-TCP content was varied from 10 to 45 wt% and submicrometer-sized pores were formed after leaching. The porous TiO₂ films showed strong initial photocatalytic activities due to the adsorption effect of the pores and the enlarged surface area.     

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.     

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.     

High-Temperature Creep Behavior of Mixed Conducting La0.5Sr0.5Fe1−xCoxO3-δ (0.5≤x≤1) Materials

Steady-state compressive creep rate of La_0.5Sr_0.5Fe_0.5Co_0.5O_3−δ (LSFC) and La_0.5Sr_0.5CoO_3−δ (LSC) is reported in the temperature region 900°–1050°C and stress range 5–28 MPa. The stress exponents for the two materials were 1.71±0.18 and 1.24±0.15, respectively. The activation energy for creep was considerably higher for LSC (619±56 kJ/mol) than for LSFC (392±28 kJ/mol). The grain size exponent for LSC was 1.28±0.14. Considerably higher creep rates were observed for both materials in N₂ compared with air. Relaxation by creep of chemical-induced stresses in oxygen-permeable membranes is addressed, especially at low partial pressure of oxygen.     

Subsolidus Phase Relationships in the Ga2O3–Al2O3–TiO2 System

Subsolidus phase relationships in the Ga₂O₃–Al₂O₃–TiO₂ system at 1400°C were studied using X-ray diffraction. Phases present in the pseudoternary system include TiO₂ (rutile), Ga_{2−2 x} Al_{2 x} O₃ ( x ≤0.78 β-gallia structure), Al_{2−2 y} Ga_{2 y} O₃ ( y ≤0.12 corundum structure), Ga_{2−2 x} Al_{2 x} TiO₅ (0≤ x ≤1 pseudobrookite structure), and several β-gallia rutile intergrowths that can be expressed as Ga_{4−4 x} Al_{4 x} Ti _{n −4}O_{2 n −2} ( x ≤0.3, 15≤ n ≤33). This study showed no evidence to confirm that aluminum substitution of gallium stabilizes the n =7 β-gallia–rutile intergrowth as has been mentioned in previous work.     

Accelerated Curing of Compacted Calcium Silicate Mortars on Exposure to CO2

The parameters affecting strength development in compacted cylinders of 3CaO- SiO₂ and β-2CaO- SiO₂ mortars exposed to CO₂ were investigated. Strength increased with time up to 81 min, the duration of the longest detailed study. The β-2CaO- SiO₂ develops strength more slowly initially, but both silicates achieved compressive strengths of 7,000 to 10,000 psi. The rate of increase in strength depends on both the amount of water used in molding the compact and the amount of water present in the CO₂. Increasing CO₂ pressures from 1 to 2 atm increased the rate of reaction, but a further increase to 4 atm had little additional effect. Carbonation occurs mainly in the outer portions of the cylindrical compacts. The initial reaction on exposure to CO₂ appears to be accelerated hydration of the silicates to a CaO-SiO₂-H₂O-like gel and calcite. The gel has a stoichiometry similar to that found in conventional hydration. Further reaction results in progressive carbonation of the gel, which decreases its lime content. The reaction products appear to be intimately dispersed in the microstructure.     

Combustion Synthesis of Si3N4–SiC Composite Powders

Fine Si₃N₄-SiC composite powders were synthesized in various SiC compositions to 46 vol% by nitriding combustion of silicon and carbon. The powders were composed of α-Si₃N₄, β-Si₃N₄, and β-SiC. The reaction analysis suggested that the SiC formation is assisted by the high reaction heat of Si nitridation. The sintered bodies consisted of uniformly dispersed grains of β-Si₃N₄, β-SiC, and a few Si₂N₂O.     

Alkoxide–Hydroxide Route for the Preparation of γ-LiAlO2-Based Ceramics

An alkoxide-hydroxide route has been developed to prepare Li_{4 + x} Al_{4 − 3 x} Si_{2 x} O₈ (0 ≤ x ≤ 0.25) powders by taking into account fundamental aspects of the sol-gel process. This technique allows one to prepare powders which exhibit the β-LiAlO₂ type of structure after drying at 150°C. The β→γ-LiAlO₂ topotactic transformation spreads over a large temperature range (746–839°C for x = 0.125) with no significant dilatometric and enthalpic change. Stoichiometric γ-LiAlO₂-based ceramics with a large variety of uniform microstructures are fabricated by a direct sintering of β-LiAlO₂ powders in the temperature range of 900–1100°C.     

Sintering, Crystallization, and Properties of B2O3/P2O5-Doped Li2O·Al2O3·4SiO2 Glass-Ceramics

Sintering, crystallization, microstructure, and thermal expansion of Li₂O·Al₂O₃·4SiO₂ glass-ceramics doped with B₂O₃, P₂O₅, or (B₂O₃+ P₂O₅) have been investigated. On heating the glass powder compacts, the glassy phase first crystallized into high-quartz s.s., which transformed into β-spodumene after the crystallization process was essentially complete. The effects of dopants on the crystallization of glass to high-quartz s.s. and the subsequent transformation of high-quartz s.s. to β-spodumene were discussed. The major densification occurred only in the early stage of sintering time due to the rapid crystallization. All dopants were found to promote the densification of the glass powders. The effect of doping on the densification can fairly well be explained by the crystallization tendency. All samples heated to 950°C exhibited a negative coefficient of thermal expansion ranging from about −4.7 × 10^-6 to −0.1 × 10^-6 K^-1. Codoping of B₂O₃ and P₂O₅ resulted in the highest densification and an extremely low coefficient of thermal expansion.     

Conversion from β-Ce2O3·11Al2O3 to α-Al2O3 in Tetragonal ZrO2 Matrix

Composites of β-Ce₂O₃·11Al₂O₃ and tetragonal ZrO₂ were fabricated by a reductive atmosphere sintering of mixed powders of CeO₂, ZrO₂ (2 mol% Y₂O₃), and Al₂O₃. The composites had microstructures composed of elongated grains of β-Ce₂O₃·11Al₂O₃ in a Y-TZP matrix. The β-Ce₂O₃·11Al₂O₃ decomposed to α-Al₂O₃ and CeO₂ by annealing at 1500°C for 1 h in oxygen. The elongated single grain of β-Ce₂O₃·11Al₂O₃ divided into several grains of α-Al₂O₃ and ZrO₂ doped with Y₂O₃ and CeO₂. High-temperature bending strength of the oxygen-annealed α-Al₂O₃ composite was comparable to the β-Ce₂O₃·11Al₂O₃ composite before annealing.     

Methane Oxidation on Perovskite-Type Ca(Mn1−xTix)O3−δ

X-ray photoelectron spectroscopy was performed to elucidate the catalytic activity of CH₄ oxidation on perovskite-type Ca(Mn_{1− x} Ti _x )O_3−δ synthesized at 1173 K in a flow of oxygen from a gel with citric acid and ethylene glycol. The Mn ion content decreases and the ratio of the Mn³⁺ ion in the Mn ion increases with increases in x . Ca(Mn_{1− x} Ti _x )O_3−δ has a high catalytic activity of CH₄ oxidation at x =0.4. These results indicate that the catalytic activity strongly depends on the Mn³⁺ ion content of the surface.     

Phase Relations In The Pseudo-Ternary System La2O3–SrO–Fe2O3

The phase relations in the pseudo-ternary system La₂O₃–SrO–Fe₂O₃ have been investigated in air. Isothermal sections at 1100° and 1300°C are presented based on X-ray diffraction and thermal analysis of annealed samples. Extended solid solubility was observed for the compounds Sr _{n +1− v} La _v Fe _n O_{3 n +1−δ} ( n =1, 2, 3, and ∞) and Sr_{1− x} La _x Fe₁₂O₁₉, while only limited solubility of La in Sr_{4− z} La _z Fe₆O_13±δ was observed. At high Fe₂O₃ content, a liquid with low La₂O₃ content was stable at 1300°C.     

Preparation of Ba6−3xNd8+2xTi18O54 via Ethylenediaminetetraacetic Acid Precursor

Ba_{6−3 x} Nd_{8+2 x} Ti₁₈O₅₄ ceramic powders were synthesized by the modified Pechini method using ethylenediaminetetraacetic acid (EDTA) as a chelating agent. A purplish red, molecular-level, homogeneously mixed gel was prepared, and transferred into a porous resin intermediate through charring. Single-phase and well-crystallized Ba_{6−3 x} Nd_{8+2 x} Ti₁₈O₅₄ powders were obtained from pulverized resin at a temperature of 900°C for 3 h, without formation of any intermediate phases. Meanwhile, the molar ratio of EDTA to total metal cation concentration had a significant influence on the crystallization behavior of Ba_{6−3 x} Nd_{8+2 x} Ti₁₈O₅₄. The Ba_{6−3 x} Nd_{8+2 x} Ti₁₈O₅₄ ( x = 2/3) ceramics prepared via EDTA precursor have excellent microwave dielectric characteristics: ɛ= 87, Qf = 8710 GHz.