ALBERT

Notes: Composite (biphasic) mixtures of two of the most important inorganic phases of synthetic bone applications-namely, calcium hydroxyapatite (Ca10(PO4)6(OH)2 (HA)) and tricalcium phosphate (Ca3(PO4)2 (TCP))-were prepared as submicrometer-sized, chemically homogeneous, and high-purity ceramic powders by using a novel, one-step chemical precipitation technique. Starting materials of calcium nitrate tetrahydrate and diammonium hydrogen phosphate salts that were dissolved in appropriate amounts in distilled water were used during powder precipitation runs. The composite bioceramic powders were prepared with compositions of 20%-90% HA (the balance being the TCP phase) with increments of 10%. The pellets prepared from the composite powders were sintered to almost full density in a dry air atmosphere at a temperature of ~1200°C. Phase-evolution characteristics of the composite powders were studied via X-ray diffractometry as a function of temperature in the range of 1000°-1300°C. The sintering behavior of the composite bioceramics were observed by using scanning electron microscopy. Chemical analysis of the composite samples was performed by using the inductively coupled plasma-atomic emission spectroscopy technique.

Notes: Monodisperse, spherical Si3N4 powder composed of fine particulates was synthesized by pyrolyzing spherical organo-silica powder under nitrogen. The organo-silica powder was prepared by hydrolyzing a mixture of phenyltrimethoxysilane (PTMS) and tetraethoxysilane (TEOS) in a methanol solution of water and ammonia. The organo-silica powder consisted of 81.3 at.% silicon units derived from PTMS and 18.7 at.% silicon units derived from TEOS. During the pyrolysis under nitrogen, the organo-silica powder decomposed to a mixture of free carbon and silica, with an increase of the surface area, at 500°-600°C, followed by the formation of alpha-Si3N4, with ß-Si3N4 as a minor phase, at 1450° and 1500°C and ß-SiC at 1550°C. The pyrolyzed powders, which retained the spherical shape and monodispersity of the organo-silica powders, with a reduction in mean particle diameter, were composed of fine particulates that were ~40 nm in size.

Notes: A new approach to tensile creep testing and analysis based on stress relaxation is described for sintered silicon nitride. Creep rate data covering up to 5 orders of magnitude are generated in tests lasting less than 1 day. Tests from various initial stresses at temperatures up to 1300°C are analyzed and compared with creep rates measured during conventional constant load testing. It is shown that at least 40% of the creep strain accumulated under all test conditions is recoverable, and that the deformation may properly be described as viscoelastic. A regime which approximated as Newtonian viscous behavior (creep rate directly proportional to stress) was observed during decreasing stress at temperatures between 1200° and 1300°C. This resulted in anomalous behavior at low strains in pseudo stress-strain curves generated from the stress relaxation data. However, the otherwise systematic rate dependence provides a possible basis for design in terms of a secant modulus analysis. The anelastic, recoverable component of creep may lead to complex deformation history-dependent phenomena.

Notes: Continuous fiber ceramic composites (CFCCs) based on oxides are of interest for high-temperature applications owing to their inherent oxidative stability. An enabling element is a matrix with an optimum combination of toughness and strength, which may be achieved by incorporating a controlled amount of fine, well-distributed porosity. Implementation of this concept by vacuum infiltration of aqueous mullite-alumina slurries into two-dimensional woven preforms of alumina fibers has been investigated. Evaluation of these materials shows stress-strain characteristics similar to other CFCCs, especially carbon-matrix composites. Moreover, promising notch and creep properties have been found. Microstructural and processing issues relevant to the attainment of these behaviors are discussed.

Notes: BaTiO3 is widely used as the dielectric in ceramic chip capacitors and multilayer capacitors, because of its high dielectric constant and ferroelectric properties. Multilayer capacitors provide fairly high capacitance per unit volume (volumetric efficiency); however, processing difficulties in the preparation of ultrathin layers limit further enhancement. Tantalum solid electrolytic capacitors, on the other hand, provide very high volumetric efficiencies, because of the large surface area of the sintered, porous tantalum anode on which the dielectric Ta2O5 is electrochemically deposited. Recent developments in electrochemical methods to deposit BaTiO3 on titanium substrates provide an opportunity to fabricate barium titanate electrolytic capacitors using sintered, porous titanium anodes. The high dielectric constant of BaTiO3 and the high surface area of the sintered, porous anode provide a good combination to achieve larger volumetric efficiencies. Current work involves the fabrication and characterization of barium titanate electrolytic capacitors. Effects of electrochemical processing parameters on the formation of BaTiO3 on the surface of sintered titanium anodes are described. Influence of the purity of titanium powder, the porosity of the sintered anode, and the post-deposition heat treatment on the dielectric properties of the fabricated capacitors is discussed. Complete penetration of the electrolyte solution and a thin uniform coating of TaTiO3 over the entire titanium surface was achieved using high-porosity (35%-40% of theoretical density) sintered titanium anodes. Samples treated for 8 h in 0.5M Ba(OH)28H2O electrolyte solutions at 100°C with an applied cell voltage of 12 V show the formation of a dense, uniform BaTiO3 coating on the surface of the titanium anode. High-purity, chloride-free titanium powder provides smaller dissipation factors at low frequencies. Heat treatment at 400°C significantly increases the capacitance at all frequencies, whereas the heat treatment lowers the dissipation factors at low frequencies. Calculated volumetric efficiencies are comparable to those typically obtained for tantalum solid electrolytic capacitors but are not as high as expected for barium titanate electrolytic capacitors. Penetration of the colloidal-carbon (external) electrode was limited to a depth of ~300 µm, which might have caused the lower volumetric efficiencies.

Notes: Sintering of In2O3 has been carried out in air to full density. Because of the difference in densification between the agglomerates and the matrix, large interagglomerate pores were observed to form at the initial stage of sintering. Such pore formation could be prevented by applying a small external pressure which resulted in the beneficial rearrangement of agglomerates.

Notes: We propose a flow method to produce barium hexaferrite (BaO6Fe2O3) particles with hydrothermal crystallization in supercritical water. Aqueous iron(III) and barium nitrate solution at room temperature was pressurized to 30 MPa and then mixed with potassium hydroxide solution (OH:NO3 = 4) at the same conditions to generate metal hydroxides. This mixture was then rapidly heated to 400°C by mixing with supercritical water and then fed into a tubular reactor. Residence time was ~1 min. The reaction was terminated by cooling at the exit of the reactor. The Ba:Fe mole ratio was varied over a range of 0.1-2. When the Ba:Fe ratio was ~1/12, which is the stoichiometric ratio for BaO6Fe2O3, the main products were alpha-Fe2O3. However, for the case of Ba:Fe 〉 0.5, fine particles of single-phase BaO6Fe2O3 were produced. Batch experiments (380°C, 30 MPa) at Ba:Fe = 0.5 in supercritical water at a reaction time of 10 min produced a mixture of alpha-Fe2O3 and BaO6Fe2O3. This product transformed to the equilibrium phase, BaO2Fe2O3, in 4 h as the reaction time increased, which suggests that the BaO6Fe2O3 that formed in supercritical water with our proposed flow method under nonstoichiometric conditions was an intermediate but stable product. Furthermore, the nonstoichiometric and nonequilibrium (dynamic) conditions are important for producing single-phase BaO6Fe2O3 particles. The single-phase particles are highly stable and can be produced continuously in a reaction time of 〈1 min.

Notes: The piezoelectric properties of Pb[Zr0.45Ti0.5-xLux(Mn1/3-Sb2/3)0.05]O3 ceramics, with 0 lessthan equal to x lessthan equal to 0.03, have been investigated. The partial substitution of Ti4+ with Lu3+ permitted improvement of the electromechanical coupling factor (kp), the dielectric constant (epsilonT33), and the piezoelectric constant (d33), while the dielectric loss (tan delta) increased and the mechanical quality factor (Qm) decreased with an increase of x. A pertinent piezoelectric material for actuator applications was Pb[Zr0.45Ti0.48Lu0.02(Mn1/3Sb2/3)0.05]O3, and the piezoelectric properties were kp = (58.5 ± 0.5)%, epsilonT33 = 32 ± 25, d33 = (373 ± 6) 10-12 C/N, Qm = 714 ± 22, and tan delta = (0.98 ± 0.03)%.

Notes: In the soda-lime-silica glass family, the effect of each constituent of the composition on the brittleness was first investigated. Vickers indentation was employed to estimate the brittleness (ratio of harness (H) to fracture toughness (Kc)) by measuring the C/a ratios (where C and a are the characteristic crack and indentation diagonal lengths, respectively). It was observed that a higher silica content and a lower lime content helped to lower the brittleness. Substitution of potash and magnesia for soda and calcia, respectively, was effective in lowering the brittleness. From these results, a higher molar volume was found to be a key factor for reducing the brittleness. A new low-brittleness glass was then developed with a brittleness as low as 5.1 µm-1/2 as compared with the brittleness of 7.1 µm-1/2 for commercial soda-lime-silica glass. The crack initiation load (P*), measured by the Vickers indentation method, for this new low-brittleness glass was almost 10 times as high as P* of commercial soda-lime-silica glass. The new glass shows lower hardness and higher fracture toughness than the commercial soda-lime-silica glass.

Notes: Intermetallic/ceramic composites represent an interesting class of materials for high-temperature structural and functional applications. These materials can be prepared via high-energy milling of pure metals with Al2O3 as well as of aluminum with metal oxides. During subsequent compaction via pressureless sintering, the components react to form dense composites that consist of interpenetrating networks of the ceramic and intermetallic phases. Microstructural investigations, mechanical properties, and resistivity and wear resistance measurements of selected composites are presented. Improved fracture toughness and bending strength, with respect to monolithic Al2O3, have been achieved.