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.     

Sintering of Spherical Glass Powder under a Uniaxial Stress

The sintering of spherical borosilicate glass powder (particle size 5 to 10 μm) under a uniaxial stress was studied at 800°C. The experiments allowed the measurement of the kinetics of densification and creep, the viscosities for creep and bulk deformation, and the sintering stress which was found to increase with density. The data show excellent qualitative agreement with Scherer's theory of viscous sintering. In addition, the quantitative comparison between theory and experiment shows good agreement; the measured viscosity of the bulk glass was ∽1×10⁹ P (∽1×10⁸ Pa·s) compared to ∽3×10⁹ P (∽3 Pa·s) obtained by fitting the data with Scherer's theory.     

Effect of Particle Size Distribution on the Sintering of Alumina

The effect of particle size distribution on the sintering of Al₂O₃ was investigated. Samples could be sintered to high relative density (∼99%), small average grain size (1 μm), and no growth of exaggerated grains using powders with either broad or narrow particle size distribution. However, the broad particle size distribution provided the advantage that powder compacts could be prepared with higher green density and, therefore, samples could be densified with less total shrinkage.     

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.     

The Effect of Conformation Method and Sintering Technique on the Densification and Grain Growth of Nanocrystalline 8 mol% Yttria-Stabilized Zirconia

Uniaxial dry pressing (DP) and slip casting (SC) were used to form green bodies of nanocrystalline 8 mol% yttria-stabilized zirconia powder processed via the glycine-nitrate combustion method. The SC method was shown to be a more efficient, yielding more homogenous green bodies with higher green density (60% theoretical density) which contained smaller pores with narrower distribution. Improved green properties resulted in lowering the sintering temperature of SC bodies by about 200°C compared with DP compacts. Consequently, the grain growth in sintered bodies formed by SC was relatively abated. By taking the benefits of the wet conformation method, the final grain size of nearly full dense (>97% TD) structures was reduced by 39% (i.e. from 2.15 to 1.3 μm). To reveal the effect of sintering technique, DP bodies were sintered via both microwave and two-step sintering methods. While the grain size of two-step sintered samples was <300 nm, sintering via microwave radiation yielded a nearly full dense structure with a mean grain size of 0.9 μm. The results show that conventionally sintered SC bodies posses higher indentation fracture toughness (FT) (∼3 MPa·m^1/2) compared with DP samples (1.6 MPa·m^1/2). Interestingly, it was shown that, without applying any modified sintering technique, the hardness and FT of SC bodies with coarser structures are completely close to those of samples sintered via microwave heating.     

Synthesis of Dense Lead Titanate Ceramics with Submicrometer Grains by Spark Plasma Sintering

Dense PbTiO₃ ceramics consisting of submicrometer-sized grains were prepared using the spark-plasma-sintering (SPS) method. Hydrothermally prepared PbTiO₃ (0.1 μm) was used as a starting powder. The powder was densified to ≳98% of the theoretical X-ray density by the SPS process. The average grain size of the spark-plasma-sintered ceramics (SPS ceramics) was ≲1 μm, even after sintering at 900°–1100°C, because of the short sintering period (1–3 min). The measured permittivity of the SPS ceramics showed almost no frequency dependence over the range 10¹–10⁶ Hz, mainly because pores were absent from the ceramics. The coercive field of the SPS ceramics was somewhat higher than that of conventionally sintered ceramics, which could be attributed to the small-grained microstructures of the SPS ceramics.     

Influence of Cesium Nitrate and Heating Rate on Densification and Microstructure of Cs-Deficient Pollucite Sintered Body

Cs-deficient pollucite, Cs_0.9Al_0.9Si_2.1O₆, a sintered body with a low thermal expansion coefficient of 1.26 × 10⁻⁶ K⁻¹ in the temperature range of 298 to 1273 K, has been fabricated using calcined powders with an amorphous phase. The calcined powder was prepared by heating mixtures of CsNO₃, γ-Al₂O₃, and amorphous SiO₂ powders at 798 K in air for 20 h, repeated 10 times. The repetition of the heat treatment at 798 K was performed to decompose CsNO₃ thoroughly in the mixed raw powders and to obtain calcined powders with a high specific surface area. The relative density of the sintered bodies fabricated from the green compact of the amorphous calcined powder increased with increasing sintering temperature and heating rate. The sintered body with a relative density of ca. 96%, of a well-crystallized pollucite single phase and with the grain size in the range of 0.75–4.75 μm, was fabricated by heating the green compact of the amorphous powders at 1673 K in air for 20 h at a heating rate of 20 K/min.     

Colloidal Processing and Ionic Conductivity of Fine-Grained Cupric-Oxide-Doped Tetragonal Zirconia

CuO-doped tetragonal ZrO₂ (3-mol%-Y₂O₃-doped tetragonal zirconia, 3Y-TZ) green bodies were consolidated from zirconia slurries with Cu²⁺ by a pressure filtration method. The slurries were prepared by dispersing 3Y-TZ powder in a solution of [NH₄OH + NH₃NO₃] = 0.1 M at pH 11 and adding an appropriate amount of Cu(NO₃)·3H₂O solution. Green bodies with narrow pore-size distribution were obtained after cold isostatically pressing the pressure-filtrated bodies. Small amounts of CuO-doped samples were densified fully at 1200°C. The size of a grain of 0.16-mol%-CuO-doped 3Y-TZ sintered at 1200°C was 84 nm. Bulk and grain-boundary conductivities are measured by a complex impedance method. The bulk conductivity of the CuO-doped 3Y-TZ was almost equal to the undoped one, but the grain-boundary conductivity decreased with CuO addition.     

Populations and Emission Properties of the 5I6 and 5I7 Levels in Ho3+ Doped into PbO–Bi2O3–Ga2O3 Glasses

Emission properties of PbO–Bi₂O₃–Ga₂O₃ glasses doped with Ho³⁺ were investigated for fiber-optic amplification at the 1.18 μm wavelength region. When the glasses were doped with Ho³⁺ ions only, there was a weak emission at 1.18 μm with a lifetime of ∼200 μs. However, when Yb³⁺ ions were codoped, the lifetime of the 1.18 μm emission increased to 630 μs together with a significant increase in intensity. A similar enhancement in the intensity and lifetimes was realized for the 2.05 μm emission. These effects are due to energy transfer from the Yb³⁺:²F_5/2 to the Ho³⁺:⁵I₆ level. Devitrification of the ternary PbO–Bi₂O₃–Ga₂O₃ glasses was efficiently suppressed by adding 10 mol% GeO₂. Optimum Ho³⁺ concentration was ∼0.4 mol%, whereas Yb³⁺ ions can be added up to the solubility limit.     

Wick Debinding of Molybdenum-Silicon-Boron Extrudates

Wick debinding was investigated as possible means of binder removal for the Mo-Si-B extrudates. With increased temperature the amount of binder wicked out of the extrudate increased because of a decrease in viscosity, 2513 mPa·s at 75°C down to 7 mPa·s at 250°C. The permeability of the binder in the 1 μm wicking powder was higher than that in the 0.05 μm powder, 3.1 × 10⁻¹³ m² compared to 6.024 × 10⁻¹⁵ m². The amount of binder removed at a given temperature was considerably lower, which could be attributed to the larger capillary pressure difference between the 0.05 μm wick and the extrudate (1.55 × 10⁵ Pa compared to 1.46 × 10⁴ Pa for the 1 μm powder). Wicking removed approximately 80% of the binder in <10 h at 250°C with no defect formation.     

Rearrangement of Porous CaO Aggregates During Calcite Decomposition in Vacuum

High-resolution SEM photographs, N₂ adsorption isotherms, Hg porosimetry, and micrometer measurements were used to characterize CaO particle shapes and pore-size distributions that result when calcite crystals are decomposed in vacuum at 686°C. The surface area of the CaO produced from large calcite crystals is constant at 116.4 m²/g independent of the extent of reaction. The volume occupied by a CaO aggregate is 98±2% that of the original calcite crystal. The ∼54% total porosity is comprised of 42% pores of ∼5 nm cross section and 12% pores of ∼10 μ m cross section. The duplex pore structure is formed by a diffusionless repacking of CaO particles that initially form with a more uniform distribution of particles and pores.     

Fabrication of Thin-Film SrCe0.9Eu0.1O3−δ Hydrogen Separation Membranes on Ni–SrCeO3 Porous Tubular Supports

SrCe_0.9Eu_0.1O_3−δ thin-film (∼30 μm) tubular hydrogen separation membranes were developed in order to obtain high hydrogen fluxes. Fifteen centimeters long, one end closed, NiO–SrCeO₃ tubular supports were fabricated by tape casting, followed by rolling the green tape on a circular rod. SrCe_0.9Eu_0.1O_3−δ powders were prepared by the citrate process and coated on partially sintered NiO–SrCeO₃ tubular supports. Leakage-free hydrogen membrane cells were obtained by adjusting the presintering and final sintering temperatures to reduce the difference of linear shrinkage rates between SrCe_0.9Eu_0.1O_3−δ thin films and NiO–SrCeO₃ supports. A hydrogen flux of 2.2 cm³/min was obtained for the SrCe_0.9Eu_0.1O_3−δ on Ni–SrCeO₃ tubular hydrogen separation membranes at 900°C using 25% H₂ balanced with Ar and 3% H₂O as the feed gas and He as the sweep gas. Thus, a 40% single pass yield of pure H₂ was achieved with this membrane.     

Physical Properties of an Illitic Clay Due to Specific Base-Exchange Cations

An illitic clay from Grundy County, Illinois, was fractionated at 5μ. The base-exchange cations of the < 5μ fraction were exchanged for Ca⁺⁺, Mg⁺⁺, Na⁺, and NH₄⁺ cations to study the changes in physical properties due to specific base-exchange cations. Studies on water of hydration, bulk density of dry-pressed clay, viscosity, and fired properties showed that the base-exchange cation had a definite effect on the physical properties of the clays studied.     

Sintering Behavior and Electrical Properties of Nanosized Doped-ZnO Powders Produced by Metallorganic Polymeric Processing

Homogeneous and nanosized (28 nm crystallite size) doped-ZnO ceramic powders were obtained by a metallorganic polymeric method. Calcining and granulating resulted in green compacts with uniform powder packing and a narrow pore-size distribution (pore size 19 nm). Dense ceramic bodies (>99% of theoretical) were fabricated by normal liquid-phase sintering at 850° and 940°C for 1–5 h. Apparently, the low pore-coordination number allowed a uniform filling of the small pores by the liquid formed in the early stages of sintering, and, consequently, high shrinkage and rapid densification occurred in a short temperature interval (825°–850°C). At these sintering temperatures, limited grain growth occurred, and the grain size was maintained at <1 μm. Ceramics so-fabricated showed a nonlinear coefficient, α, of ≥70, and a breakdown voltage, V _b (1 mA/cm²), of ≥1500 V/mm. The high electrical performance of the doped-ZnO dense ceramics was attributed to liquid-phase recession on cooling, which enhanced the ZnO-ZnO direct contacts and the potential barrier effect.     

Ceramics With Ultra-Low Density Fabricated by Gelcasting: An Unconventional View

Gelcasting is conventionally used to acquire high-density ceramic parts; however, in this work, alumina (Al₂O₃) ceramics with ultra-low density (8%–40% theoretical density) were successfully fabricated by this method. In this research, polymerization of acrylamide was realized in tert-butyl alcohol/Al₂O₃ slurries with solid loading ranging from 5 to 15 vol%. Green bodies with ultra-low density could be dried with very small shrinkage, and relatively high green strengths (1–3 MPa) were achieved. By choosing different initial solid loadings and sintering temperatures, ceramic microstructures could be effectively controlled, with the porosity ranging from 60% to 92% and pore sizes from 0.1 to 2.2 μm. Sintered Al₂O₃ showed high open porosity (90%), high specific area (14 m²/g) and high compression strength (>10 MPa), which was attributed to the connection of Al₂O₃ particles. This technique is considered potentially useful in many applications, and introduces a new application field of gelcasting.     

Surface Pretreatments of Silicon Nitride for CVD Diamond Deposition

The efficiency of different surface pretreatments (four standard chemical etchings and four diamond powder abrasive procedures) on silicon nitride (Si₃N₄) substrates for chemical vapor deposition (CVD) of diamond has been systematically investigated. Blank Si₃N₄ samples were polished with colloidal silica (∼0.25 μm). Diamond nucleation and growth runs were conducted in a microwave plasma chemical vapor deposition apparatus for 10 min and 6 h, respectively. Superior results concerning nucleation density ( N _d∼ 10¹⁰ cm⁻² after 10 min), film uniformity, and grain size (below 2 μm after 6 h) were obtained for the mechanically microflawed samples, revealing that chemical etchings (hot and cold strong acids, molten base or CF₄ plasma) are not crucial for good CVD diamond quality on Si₃N₄.     

Creep Behavior and Grain-Boundary Sliding in Polycrystalline A12O3

The creep properties of polycrystalline A1₂O₃ (grain size 14 to 65 μm) were examined under compressive stresses of between 4,000 and 18,000 psi (27.6 and 124 MPa) in the range 1600° to 1700°C. Two distinct types of behavior were observed. The creep rate of medium-grained specimens (14 to 30 μm) could be described by ασ^1.2 / d² where σ is the applied stress and d is the grain size. These results are consistent with the Nabarro-Herring creep mechanism. For the coarse-grained (65 μm) specimens, the creep rate was related to the stress by ασ^2.6. This behavior was not related to cracking; instead, a dislocation mechanism was thought to be rate-controlling. Considerable evidence for grain-boundary sliding was seen, and measurements showed that grain-boundary sliding contributed between 46 and 77% of the total strain in the 3 medium-grained specimens examined and between 38 and 50% in the 3 coarsegrained specimens examined.     

Synthesis of Micron-Scale Platelet BaTiO3

Micron-scale platelet barium titanate was synthesized using a twostep molten salt and topochemical technique. Plate-like BaBi₄Ti₄O₁₅ was first synthesized as a precursor by molten salt synthesis. Then, Bi³⁺ in the precursor was replaced by Ba²⁺ and formed perovskite-structure BaTiO₃ through a topochemical reaction. The BaTiO₃ single crystals have an average size of 5–10 μm and a thickness of 0.5 μm. The purpose of this article is to control the particle shape with desired structure. High aspect ratio BaTiO₃ platelets are suitable templates to obtain textured ceramics (especially Pb(Mg_1/3Nb_2/3)O₃–32.5PbTiO₃) by the templated grain growth process.     

Influence of B site Substituents on Lanthanum Calcium Chromite Nanocrystalline Materials for a Solid-Oxide Fuel Cell

Highly reactive and nanocrystalline powders of LaCrO₃based compositions, having the general formula La_0.9Ca_0.1Cr_{1− x} M _x O_3−δ (0≤ x ≤0.1, and M=Al, Co, or Mg), suitable for solid-oxide fuel cell (SOFC) applications, have been synthesized using an auto-combustion technique with ammonium dichromate as the chromium source. Owing to very fine crystallite size (ranging from 10 to 50 nm) and the high reactivity of the powders (surface area as high as 25 m²/ g ), the sintering temperature reduces drastically and a highly dense, uniform, and fine-grained microstructure is obtained. A dramatic improvement in densification (nearly theoretical density) is observed for aluminum substitution, when sintered at as low a temperature as 1300°C. The microstructure shows a uniform distribution of grains having an average grain size of ∼0.5 μm. Depending on the substituent, the electrical conductivities of the sintered samples in air, at 1000°C, were found to be in the range of 10–45 S/cm, and are more than that of the values required for SOFC application. The thermal expansion coefficients, as obtained, are also comparable with the other SOFC cell components.     

Pr3+/Er3+ Codoped Ge-As-Ga-S Glasses as Dual-Wavelength Fiber-optic Amplifiers for 1.31 and 1.55 μm Windows

Fluorescence emissions at both 1.31 and 1.55 μm communication windows were observed from Pr³⁺/Er³⁺ codoped Ge-As-Ga-S glasses with a single wavelength pumping at 986 nm. The lifetime of the Er³⁺:⁴ I _11/2 level decreased as the Pr³⁺ concentration increased, and that of the Pr³⁺:¹ G ₄ level increased as the Er³⁺ concentration increased. Energy transfer from the Er³⁺:⁴ I _11/2 level to the Pr³⁺:¹ G ₄ level was responsible for emission of the 1.31 μm fluorescence from the Pr³⁺:¹ G ₄ level. Ge-As-Ga-S glasses that have been doped with Pr³⁺ and Er³⁺ cations are promising amplifier materials for both 1.31 and 1.55 μm communication windows.