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.     

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.     

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.     

Rapid Hardening of Cement by the Addition of a Mechanically Activated Al(OH)3–Ca(OH)2 Mixture

Rapid hardening of cement was achieved in the present study by adding a mechanically activated Al(OH)₃–Ca(OH)₂ mixture to the starting cement paste. Among the dominant parameters for hardening were the mechanical treatment time for the Al(OH)₃ powder and the Al(OH)₃/Ca(OH)₂ ratio. The hardening mechanisms are discussed here in terms of the ionic concentration of the solution and the hydration products created when the Al(OH)₃–Ca(OH)₂ mixture was added to water. Mechanical activation of the Al(OH)₃ powder accelerated dissolution into an aqueous alkaline solution and induced the formation of calcium aluminate hydration products. Those hydration products increased the compressive strength of the cement paste at a very early stage of hardening.     

Characterization and Mechanical Performance of the Mg-Stabilized β-Ca3(PO4)2 Prepared from Mg-Substituted Ca-Deficient Apatite

The preparation of Mg-stabilized β-tricalcium phosphate (β-TCP) was carried out by an aqueous precipitation method and the characterization of the powders was performed by powder X-ray diffraction, FT-IR spectra, Raman spectroscopy, and elemental analysis. The transformation of calcium-deficient apatite into β-TCP has occurred in the range of 700°–800°C. The calculated values for lattice parameters confirm the stabilization role played by Mg. The thermal stability of the Mg-stabilized β-TCP powders was evident until 1400°C, thus broadening the sintering temperature range without transformation into the undesirable α-TCP. Accordingly, the mechanical properties of the Mg-stabilized β-TCP were improved in comparison with those of pure β-TCP.     

Saturated Solutions of Anhydrous Phases in the System Lime-Silica-Water: Example of β-C2S

Saturated solutions rerely form when the anhydrous constituents of aluminous and portland cement are stirred in water or in lime solutions of increasing concentrations. Apart from monocalcium aluminate, concentration of ions in solution cannot exceed maximum supersaturation with respect to the hydrate most likely to precipitate. The present work shows such a behavior for β-C₂S suspended in water and in lime solutions at low concentration. In more concentrated lime solutions, a short lifetime saturation state with respect to β-C₂S seems to be reached.     

Structural Transformations in the Decomposition of Mg(OH)2 and MgCO3

A transmission electron microscopy study has been carried out to understand the structural transformation mechanisms of decomposition of Mg(OH)₂ and MgCO₃ under vacuum. Both Mg(OH)₂ and MgCO₃ decompose topotactically to yield porous pseudomorphic MgO of normal structure with definite orientation relationships. The decomposition of Mg(OH)₂ yields MgO with a single orientation relationship, but that of MgCO₃ leads to one major and two minor orientation relationships with two, six, and three variants, respectively. The major orientation relationships in the formation of 2- to 3-nm cubic MgO particles are discussed in terms of a correlation between oxygen octahedra in the reactant solids. The small cubic MgO particles may aggregate spontaneously to reduce the excess surface energy and result in minute cracks.     

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.     

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.     

Shape and Size of Crystalline MgO Particles Formed by the Decomposition of Mg(OH)2

Decomposition of Mg(OH)₂ at 300° to 400°C yields MgO crystals with often unequal edge lengths which, from counting of crystal planes in high-resolution transmission electron micrographs, range from 0.8 to 2.4 nm, in agreement with conclusions of Moodie and Warble. Optical diffractograms and electron diffraction patterns yield concordant results. An origin for discordant X-ray diffraction estimates of particle size for MgO produced from Mg(OH)₂ in the same temperature range is suggested.     

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.     

Nanosized Zinc-Oxide Particles Derived from Mechanical Activation of Zn5(NO3)2(OH)8·2H2O in Sodium Chloride

Nanosized ZnO particles are successfully synthesized via mechanical activation of a zinc nitrate hydroxide hydrate (Zn₅(NO₃)₂(OH)₈·2H₂O) precursor in NaCl matrix for 15 h. The ZnO particles obtained are in the nanosize range of ∼20 nm, with a well-established hexagonal morphology. They compare favorably with those derived from conventional calcination of the precursor. The decomposition of Zn₅(NO₃)₂(OH)₈·2H₂O precursor and formation of nanocrystalline ZnO cannot be completed by mechanical activation in the absence of NaCl, which acts as both an effective dispersing matrix and drying agent although it remains chemically inert during mechanical activation. The powder derived from calcination at 400°C does not possess powder characteristics comparable to that of the powder derived from the mechanical activation in NaCl, because of the extensive particle coarsening and aggregation at the calcination temperature.     

Fiberizable Si(OC2H5)4-H20-C2H5OH Sols with Stabilized Viscosity

Sols in which the viscosity changes by only 4%/h in the range 1.5 to 150 Pa.s were produced, and fibers with diameters between 20 and 500 pm were hand-drawn from them. Sols remain fiberizable for <140 h at room temperature. Periodic additions of alcohol increase the time available for fiberizing to ≥ 300 h.     

Thermodynamic Solubility Constant of Ca(OH)2 in Simulated Radioactive Sulfate Liquid Waste

The thermodynamic solubility product constant ( K _sp) of Ca(OH)₂ in simulated radioactive sulfate liquid waste was experimentally determined at temperatures of 25° and 40°C. Synthetic solutions were prepared with pH ranging from 12.6 to >14 and ionic strength ranging from 0.25 M to 3.7 M . All the solutions were saturated in CaO and contained Na₂SO₄ to simulate the salts accompanying radioactive liquid waste. The Bromley model was applied to estimate the global ionic activity coefficients, and the K _sp value was compared with those found in the literature. The stability ranges of the different compounds formed as a function of pH were determined by X-ray diffractometry analysis, and their quantification was conducted by thermal analysis.     

Electrical Properties of β-LixV2O5 Thin Layers Prepared by the Sol—Gel Process

The electrical properties of a highly oriented thin layer of lithium vanadium beta bronze, β-Li_xV₂O₅, were investigated through dc and ac conductivity measurements. The ionic transference number was determined by a simplified polarization method employing ion-blocking electrodes. The electronic conductivity was 10 ^–3to 10^–2 S cm^–1 in air or N₂ at 300–400°C, which was lower than that of a randomly oriented sintered specimen. The ionic conductivity was l0^–2–10^–1s cm^–1 at 350°C, resulting in a high ion transference number of 0.8–0.9; it was higher than that reported for a bulk specimen. The lower electronic conductivity and higher ionic conductivity were assumed to be due to (1) the oriented microstructure, i.e., alignment of the b -axis along the substrate direction which enhanced Li⁺ conduction through tunnels along the b -axis, and (2) an open crystallo-graphic structure, which promoted ionic motion in an open channel between adjacent ( a-c ) places and suppressed the electron hopping process in the b -axis direction.