Fabrication of HAp-Coated Micro-Channelled t-ZrO2 Bodies by the Multi-Pass Extrusion Process

HAp-coated micro-channelled t -ZrO₂ bodies were fabricated using a multi-pass extrusion process in which carbon powders were used to facilitate the formation of pores and ethylene vinyl acetate was used as a binder. The unidirectional pores and HAp coating layers can be obtained easily, using the extrusion process. The micro-channelled bodies were 180 μm in diameter, and the HAp layer was uniformly coated on the pore walls. However, after being sintered above 1200°C, the HAp decomposed and was transformed into β-TCP, in which a large number of pores were observed. The maximum values of bending strength of HAp-coated and uncoated porous t -ZrO₂ bodies sintered at 1500°C were about 116 and 173 MPa, respectively, and their relative densities were about 57% and 63%, respectively.     

Processing–Property Relations in Grain-Oriented Lead Metaniobate Ceramics Fabricated by Layered Manufacturing

Highly grain-oriented lead metaniobate (PbNb₂O₆; PN) ceramics were prepared by a layered manufacturing (LM) process. This process has enabled us to obtain a net-shaped piezoelectric ceramic component with grain orientation of f ∼89%, and improvement in electromechanical properties. The LM feedstock (filament) includes equiaxed (matrix; 90 vol%) and anisometric PN templates (10 vol%), both dispersed uniformly in a thermoplastic binder. The needle-like PN templates were synthesized by molten salt synthesis technique, while the equiaxed PN powder was prepared by conventional ceramic processing methods. The processing conditions were studied and optimized to obtain orthorhombic phase fine powder and anisometric templates.
Samples were obtained through layer-by-layer deposition of the filament through a small diameter (500 μm) nozzle. After binder removal, the PN samples were sintered in a temperature range of 1150°–1300°C for 1 h. SEM observation revealed strong grain orientation perpendicular to the deposition direction. Relative permittivity at the Curie point ( T _c: 560°C) was 18 100 and 14 600 for the LM and random polycrystalline samples, respectively. Improved properties in piezoelectric figure of merit by 71%, d ₃₃ by 23%, and g ₃₃ by 31% were observed in the grain-oriented samples. Remnant polarization also showed about 80% improvement, increasing from 4.5 to 8.1 μC/cm² for the grain-oriented LM samples.     

Formation and Sintering Characteristics of Aluminum Borate Whiskers

A boric acid-stabilized aluminum acetate powder was decomposed to aluminum borate (Al₁₈B₄O₃₃) by calcining at 1000°C. The powder was then ball-milled, compacted, and fired to temperatures of 1500°, 1600°, and 1700°C for a period of 1 h. The resulting aluminum borate ceramic had a whiskerlike grain morphology, with the whisker length approaching 20 μm and a diameter of 2–3 μm. The sintered compact showed no shrinkage upon firing, had a porosity of ∼50%, a narrow pore size distribution, and an average pore size of 1–3 μm.     

Dense Nanostructured Hydroxyapatite Coating on Titanium by Aerosol Deposition

In order to improve biocompatibility of Ti metal substrates, 1-μm-thick nanostructured hydroxyapatite (HAp) coatings were deposited on the substrates through aerosol deposition, which sprays HAp powder with an average particle size of 3.2 μm at room temperature in vacuum. The original HAp particles were fractured into nanoscale fragments to form highly dense coating during the deposition process. Density of the HAp coating was 98.5% theoretical density (TD). Transmission electron microscopy observation revealed that the as-deposited coating consisted of HAp crystallites with average grain size of 16.2 nm and amorphous phase. Tensile adhesion strength between the coating and the substrate was 30.5±1.2 MPa. Annealing up to 500°C in air increased crystallinity and grain size in the coating without any delamination or crack according to X-ray diffraction analysis and electron microscopy. MTS assay and alkaline phosphatase activity measurements with MC3T3-E1 preosteoblast cell revealed that the biocompatibility was greatly improved by postdeposition heat treatment at 400°C in air due to well-crystallized HAp with average grain size of 29.3 nm. However, further heat treatment at 500°C deteriorated biocompatibility due to rapid growth of HAp grains to average size of 99 nm. Cross section of the coating on a commercially available Ti dental implant revealed full coverage of the surface with HAp.     

Nanocrystalline Seeding Effect on the Crystallization of Two Leucite Precursors

The effect of nanocrystalline leucite seeding with leucite precursors prepared by sol–gel and hydrothermal methods on the leucite crystallization process and the microstructure of its sintered porcelain was studied. The introduced seeds lowered the crystallization temperature of leucite by 100° and 50°C for the precursor prepared by hydrothermal and sol–gel methods, respectively. The crystallization process was changed after the seeds were introduced. As the transition phase during leucite crystallization, kalsilite did not appear after the seeds were added. When the seeded hydrothermally derived precursor was treated at 650° and 700°C, part of the cubic leucite was stabilized to room temperature. This stabilization was due to the crystallization of nanocrystalline leucite on the seeds at a low temperature. The leucite synthesized by the hydrothermal method with seeding at 800°C had an average particle size of 0.4 μm that grew to about 0.6 μm in the sintered porcelain.     

Low-Temperature Fine-Grained Alumina–Aluminum Titanate Composites Produced from a Titania-Coated Alumina Precursor

A method for the preparation of alumina–aluminum titanate (AT) composites, which can be sintered to high density with a fine-grained microstructure at <1450°C, is reported. The composite precursor is alumina particles coated by sol–gel-derived titania, which reacts during sintering to form AT in situ at temperature as low as 1300°C. The composite can be sintered at 1350°C to 98% density with 1.5–2.0 μm grain size. Other composites containing 5–50 wt% AT are also investigated.     

The densification of zeolite/apatite composites using a pulse electric current sintering method: A long-term assurance material for the disposal of radioactive waste

Pulse electric current sintering (PECS) method was applied to the fabrication of zeolite, hydroxyapatite (HAp) and fluorapatite (FAp) sintered bodies that should be long-term assurance materials for the disposal of radioactive waste. The weight ratio of zeolite and apatite was ca. 3/7. Zeolite powder evenly covered with HAp thin layers prepared by a hydrothermal method and spherical FAp powder by spray dryer were employed for the PECS; the sample was sintered at 900 °C for 10 min at a rate of 50 °C/min under a uniaxial pressure of 50 MPa and then cooled to 600 °C at 5 °C/min in vacuum. The powder X-ray diffractions indicated that the structure of zeolite changed to the amorphous. The zeolite powder was well dispersed in FAp matrix as the results of element mapping analyses by energy dispersive X-ray spectrometer. The observations by a scanning transmission electron microscope indicated that amorphous zeolites were covered with needle-like HAp crystals of which layer completely coupled with sintered FAp grains. The HAp thin layers thus play an important role for improving the affinity between FAp matrix and the zeolite. The microhardness and three-point bending strength of the sintered bodies were also elucidated by a dynamic-ultra microhardness tester and a universal tester, respectively.     

Processing and Properties of Cellular Silica Synthesized by Foaming Sol-Gels

A novel process for fabricating lightweight, cellular ceramics from sols is presented. The process utilizes the rapid viscosity change during gelation to stabilize the structure of a foamed silica sol. Manipulation of gel viscosity and foaming agent concentration resulted in a minimum cell size of 90 μm at 31% density and minimum density of 17% with average cell size of 400 μm. The flexural strength compared favorably with space shuttle tiles and sintered hollow glass spheres. From −50° to 150°C, the dielectric constant ranged from 1.51 to 1.55 for a 20% dense foam and was slightly dispersive, whereas the dielectric loss was comparable with fused silica.     

Lower-Temperature Hot-Pressed Dy-α-Sialon Ceramics with an LiF Additive

Hot-pressed Dy-α-sialon ceramics, using LiF as a sintering additive, were fabricated at lower temperatures (≤1650°C). Some of the densified samples possessed higher hardness and fracture toughness up to 19.00–20.00 GPa and 4.00–6.00 MPa·m^1/2, respectively. The amount of LiF had a strong effect on the densification behavior in sialon preparation. As one of the experimental results, the sample with 0.1 wt% of LiF additive sintered at 1600°C produced an optical translucence of about 50% in the range of 1.5–5.0 μm wavelengths. The maximum infrared transmission reached ∼60% at a wavelength of 2.4 μm. It is inferred that these more easily sintered materials would be practical for optical applications in certain fields.     

Sintering of Mixtures of Seeded Boehmite and Ultrafine α-Alumina

α-Alumina was fabricated by dry pressing mixtures of seeded boehmite and fine α-alumina (i.e., 0.2 and 0.3 μm diameter) to reduce the large shrinkage of boehmite-derived α-alumina. The maximum green density was obtained with mixtures containing ∼70%α-alumina for both alumina powders. The ∼15% linear shrinkage and microstructures of these samples were comparable to 100% alumina powder samples. Samples with 0.2 μm alumina sintered to densities >95% at 1300°C whereas 1400°C was needed for samples with 0.3 μm alumina. These results indicate that boehmite can be used as a substitute for relatively expensive ultrafine α-alumina powders.     

Thermal Softening Behavior and Application to Transparent Thick Films of Poly(benzylsilsesquioxane) Particles Prepared by the Sol–Gel Process

Thick films of poly(benzylsilsesquioxane) (BnSiO_3/2) particles, prepared by the sol–gel process, were deposited onto indium-tin-oxide-coated glass substrates by electrophoresis for the application to the micropatterning of the films. BnSiO_3/2 particles were thermally softened and sintered at ∼50°C, which was above the glass transition temperature of the particles. The films prepared by the electrophoretic sol–gel deposition consisted of aggregates of particles with diameters of 0.2–1 μm and were opaque. The film shrank from 2.5 μm to 1.4 μm in thickness and became transparent upon sintering of the particles during heat treatment at temperatures >100°C.     

Electrophoretic Deposition of Ferroelectric Barium Titanate Thick Films and Their Dielectric Properties

Barium titanate thick films 10–70 μm thick were obtained using a unique electrophoretic deposition technique, which utilizes a mixed ethanol–acetylacetone suspension as the dispersive medium. The capacitance of the BaTiO₃ films was measured as 25–70 nF, at temperatures of up to 200°C, giving a dielectric constant of 4000–12000 and dielectric loss (tan δ) of 0.13–0.51. In the sintered films thus deposited, (111) orientation was observed, and the degree of orientation was found to increase with decrease of the thickness of the films. In so-oriented films, the Curie point was confirmed to be near 120°C.     

Sintering and Properties of Monazite-Type CePO4

Monazite-type CePO₄ powder (average grain size 0.3 μm) was dry-pressed to disks or bars. The green compacts began to sinter above 950°C. Relative density ≧ 99% and apparent porosity <1% were achieved when the specimens were sintered at 1500°C for 1 h in air. The linear thermal expansion coefficient and thermal conductivity of the CePO₄ ceramics were 9 × 10⁻⁶/°C to 11 × 10⁻⁶/°C (200° to 1300°C) and 1.81 W/(m · K) (500°C), respectively. Bending strength of the ceramics (average grain size 4 μm) was 174 ± 28 MPa (room temperature). The CePO₄ ceramics were cracked or decomposed by acidic or alkaline aqueous solutions at high temperatures.     

Influence of Pyridine–Borane on the Sintering Behavior and Properties of α-Silicon Carbide

In the present study, α-SiC powder is coated with pyridineborane (BH₃·C₅H₅N), a liquid molecular compound, which forms a boron carbonitride (BC_3.5N) layer by heat treatment at 1000°C under argon. The precipitation method leads to an improved chemical homogeneity in the compacted powder resulting in enhanced densification and significant reduction in grain growth during subsequent sintering at temperatures exceeding 2070°C. Thus, small average grain sizes of d ₅₀= 1.3 μm and a narrow grain size distribution ( d ₁₀= 0.6 μm, d ₉₀= 2.2 μm) are detected in the liquid-phase-processed sample sintered at 2200°C for 0.5 h in argon. Final densities of at least 98% of theoretical could be obtained by pressureless sintering at 2100°C. These results as well as the microstructural distribution of the sintering aids in the densified samples are discussed.     

Low-Temperature Sintering of Aluminum Oxide

A fine-sized (∼0.1 μm), agglomerate-free Al₂O₃ dispersion was used to prepare homogeneous green bodies with ∼69% relative density and ∼10-nm median pore radius. Samples could be sintered at 1150°C to a relative density >99.5% and an average grain size of 0.25 μm.     

Superplastic deformation of hydroxyapatite ceramics with B2O3 or Na2O addition fabricated by pulse current pressure sintering

High-density and fine-grained transparent hydroxyapatite (Ca₁₀(PO₄)₆(OH)₂: HAp) ceramics with B₂O₃ and Na₂O addition were fabricated using pressureless sintering and pulse-current pressure sintering between 1000 and 1100 °C; the superplastic deformation of these HAp specimens was evaluated. The relative density of pure HAp compacts pulse-current pressure sintered at 1000 °C for 10 min under a pressure of 50 MPa attained 99.9% and exhibited translucency. The tensile elongation of the pure HAp specimen, which was measured at 1000 °C under a strain rate of 1.48 × 10⁻⁴ s⁻¹, was as high as 364%. The relative density of HAp compacts with 3.0 mol.% B₂O₃ addition pulse-current pressure sintered under the same conditions as those of pure HAp compacts was 98.9%, whereas the grain size was as low as 0.24 μm. The elongation of HAp specimens, measured at a test temperature of 1000 °C under a strain rate of 1.48 × 10⁻⁴ s⁻¹, was as high as 578%.     

Novel Design of Microchanneled Tubular Solid Oxide Fuel Cells and Synthesis Using a Multipass Extrusion Process

A novel, microchanneled tubular solid oxide fuel cell was fabricated using a multipass extrusion process, with an outside diameter of 2.7 mm that contained 61 cells. Cell materials used in this work were 8 mol% yttria-stabilized zirconia (8YSZ), La_0.8Sr_0.2MnO₃ (LSM), and NiO–8YSZ (50:50 vol%) as electrolyte, cathode, and anode, respectively. Three stages of heat-treatment processes were applied, at 700°C in N₂ condition, at 1000°C in air, and then sintered at 1300°C for 2 h, respectively. The X-ray diffraction analysis confirmed that no reaction phases appeared after sintering. The microstructures of anode and cathode were fairly porous while the electrolyte had a dense microstructure (relative density >96%). The thickness of electrolyte, anode, and cathode were 20, 30, and 40 μm, respectively, and the diameter of the continuous channels was 150 μm.     

Interconnected Hydroxyapatite Nanofibers in the Biomimetic Coating of a Bioinert Ceramic Substrate

A bioinert ceramic substrate, α-Al₂O₃, has been coated with hydroxyapatite (HAp) by the biomimetic route using simulated body fluid (SBF) solution at room temperature. The substrate was incubated at 37°C in SBF for 6 days with a periodic replacement with freshly prepared SBF at 48-h intervals. After 6 days, continuous nanofiber-like structures of HAp (5–35 μm in length, 0.05 μm diameter) were obtained, connecting the intra- and interglobular clusters, within the coated mineral layer on the substrate surface. This is a unique and new observation, and this phenomenon has been demonstrated by a simple fractal growth model.     

Creep, Strength, Expansion, and Elastic Moduli of Sintered BeO As a Function of Grain Size, Porosity, and Grain Orientation

Physical properties are presented for extruded and sintered BeO from 25° to 1400° C as a function of porosity (0 to 15%), grain size (5 to 100μ), and grain orientation (random to 80% preferred). The elastic constants and linear thermal expansion are sensitive to the degree of preferred grain orientation. Measurements on polycrystalline specimens permitted calculation of the anisotropy in single crystals of BeO for these properties. Modulus of rupture data are treated in terms of the Knudsen equation, and compressive creep data have been used to estimate the diffusion coefficient controlling strain rate at 1200°C.     

Low-Temperature Sintering of Lead Zinc Niobate–Lead Zirconate Titanate Ceramics Using Nano-Sized Powder Prepared by the Stirred Media Milling Process

Lead zinc niobate–lead zirconate titanate (PZN–PZT) nano-sized powders with a diameter of ∼35 nm were fabricated by a high-energy stirred media mill using 50 μm diameter zirconia beads as the milling media at a rotation speed of 4000 rpm for 1 h. The sintering temperature of PZN–PZT was greatly reduced, and a fully densified bulk body was obtained at only 750°C when stirred media milled nanopowder was used. The control of evaporation of lead oxide was very important to obtain high electrical properties due to the increased surface area of nano-sized powders. The ferroelectric hysteresis, piezoelectric d ₃₃ coefficient, and dielectric properties of sintered ceramics using nanopowder were measured and compared with the values obtained from a sintered specimen using conventional milled powders. Remanent polarization, d ₃₃ coefficient, and relative dielectric constant of 750°C sintered stirred media milled powders containing 2% of excess PbO and 1% of 4PbO–B₂O₃ liquid phase were 10.3 μC/cm², 277 pC/N, and 1310, respectively.