High-Temperature Strength of AIN Hot-Pressed with A12O3 Additions

The flexural strength of hot-pressed AIN with alumina was investigated at temperatures up to 1500°C. The strength depended on the crystal phases which formed from raw materials in the compact. The hot-pressed specimens consisted of AIN and a pseudopolytype of AIN and showed considerable flexural strength up to 1500°C.     

Submicrometer Transparent Alumina by Sinter Forging Seeded γ-Al2O3 Powders

Seeding boehmite with α-Al₂O₂, followed by calcination at 600°C, results in an agglomerated alumina powder (<53 μm) that can be sinter forged to full density at 1250°C. Compressive strains as high as ɛ_x=−0.9, and radial flow (ɛ_x= 1.0) during sinter forging remove large, interagglomerate pores. The fully dense alumina has a grain size of 0.4 pm and is visually transparent. It is proposed that deformation of dense agglomerates is the primary mecha- nism responsible for large pore elimination and compact densification. The sinter forging of sol-gel-derived alumina powders offers a new technology to prepare highly transparent, optical ceramics at lower temperatures than conventional routes.     

Fabrication of Transparent Yttria Ceramics at Low Temperature Using Carbonate-Derived Powder

A new preparation method for a highly sinterable yttria powder has been developed, the resultant powder characterized, and its sinterability studied. As a precursor of the yttria powder, a fine and needle-shaped yttrium carbonate was prepared by a precipitation and aging method. A fine and low-agglomeration yttria powder was obtained by calcining the carbonate precursor at 1100°C. The primary crystallites measured ∼0.1 μm and weakly agglomerated to a size of ∼0.3 μm. The powder had a very high sinterability, so that the powder compact could be sintered to transparency by normal sintering under vacuum without additives at low temperatures, over 1600°C. The sintered transparent ceramics exhibited a homogeneous microstructure with no abnormal grain growth.     

The Emissivity of Transparent Materials

The emission of thermal radiation by transparent materials is reviewed from first principles and is compared with the more familiar emission of radiation by opaque surfaces. The comparison leads to an expression for volume emissive power, which is an important concept for discussions of radiative effects in glass. The present treatment differs from that of McMahon in that it takes account of the diffuse character of radiation. As a result, it also constitutes a simple proof of the often overlooked fact that the radiant flux within a transparent radiator exceeds that emitted into air by a factor approximately equal to the square of the refractive index. Using these concepts, the spectral emissivities of isothermal transparent sheets are expressed in terms of their thickness and the optical properties of their materials. The results are illustrated by a discussion of the total hemispherical emissivities of sheets of window glass at various temperatures. The commonly accepted value of about 0.91 is the same for all glasses having a refractive index of 1.5. However, it applies only for sheets above a certain minimum thickness. For window glass this ranges from 3/16 in. at 200°C. to as much as 8 in. at 1000°C. At 1000°C. a sheet 3/16 in. thick has an emissivity of only 0.59. The application of results to calculations of the radiative cooling of transparent sheets is briefly indicated.     

Synthesis of Nanocrystalline Enstatite Fiber Via Alkoxide Sol–Gel Process

An alkoxide sol–gel route to precursors to transparent enstatite fibers is described. The fibrous gels were drawn from acetic acid-modified viscous solutions. The amount of acetic acid was found to be the crucial factor in the spinnability. Infrared studies indicated that the acetate anion behaved as a ligand and modified the molecular structure of the enstatite gel. The fibrous gel was amorphous after drying by X-ray, and began to convert to enstatite at temperature as low as ∼600°C. The enstatite fibers were nanocrystalline up to 1100°C. At 1300°C, transparent and dense enstatite fibers were produced, with the room temperature dielectric constants of around 5.2–5.8 (at 1 MHz).     

Role of Particle Substructure in the Sintering of Monosized Titania

Monosized titania particles (∼0.35-μ diameter) prepared by controlled hydrolysis of titanium tetraethoxide in ethanol were found to be porous agglomerates of ∼6-nm primary particles. The sintering behavior of compacts constituted of monodispersed agglomerates was evaluated, and changes in macroscopic dimensions were correlated with changes in particle microstructure and chemistry. The total volume shrinkage during sintering was ≥87%. Five contributions to the total shrinkage and the temperature ranges for the associated processes were identified: removal of chemisorbed water (from ambient to 250°C), crystallization to anatase (between 250° to 425°C), intra-agglomerate densification (425° to 800°C), conversion of anatase to rutile (600° to 800°C), and inter-agglomerate densification (>800°C). Approximately one-half the compact shrinkage was the result of agglomerate substructure changes. Studies of the agglomerate structural evolution indicated the intra-agglomerate densification and crystallite growth rates are the secondary factors, after compact packing, that influenced microstructure development.     

Infrared-Transparent Mullite Ceramic

Mullite ceramic, transparent in the infrared, was prepared by hot-pressing and hot-isostatically pressing starting materials derived from alkyloxides. A composition with 72.3 wt% Al₂O₃ yielded transparent, submicrometer grain size bodies at 1630°C, whereas higher temperatures produced glass-containing microstructures. A composition with 76 wt% A1₂O₃ formed precipitates of α-Al₂O₃ at the consolidation temperature, which could be removed by subsequent annealing between 1800° and 1850°C. Spectral transmittance and absorption coefficients of the bodies are reported. The formation of the second phases was linked to phase equilibria and grain growth that promoted compositional equilibration of the mullite phase. The results suggest adjustments to phase boundaries in the high-temperature segment of the SiO₂-Al₂O₃ phase diagram.     

Microstructure Development and Dielectric Properties of Potassium Strontium Niobate Ceramics

Sintering of a KSr₂Nb₅O₁₅ powder compact at 1350°C resulted in a duplex structure. Prefiring of the compact between 1200° and 1300°C inhibited the abnormal grain growth responsible for the duplex structure. The Curie temperature and dielectric constant were dependent on the microstructure.     

Solvothermal Reaction of Rare-Earth Metals in 2-Methoxyethanol and 2-Aminoethanol

In a previous paper, we reported that the reaction of cerium metal in 2-methoxyethanol yielded a transparent colloidal solution of ultrafine (2 nm) ceria particles [M. Inoue, M. Kimura, T. Inui, Chem. Commun ., 957 (1999)]. In this paper, the solvothermal reaction of other rare-earth metals in 2-methoxyethanol has been examined. Reactions of Sm and Yb metals in 2-methoxyethanol at 300°C yielded transparent colloidal solutions of Sm₂O₃ and Yb₂O₃ particles. However, the reaction of rare-earth metals of Y, Eu, Gd, and Tb in 2-methoxyethanol at 300°C resulted in recovery of the starting materials. Transparent colloidal solutions of the oxides of these elements were obtained by reaction in the presence of a small amount of acetic acid. The solvothermal reaction of rare-earth metals in 2-aminoethanol was also examined.     

Yttrium Segregation and YAG Precipitation at Surfaces of Yttrium- Doped α-A12O3

Auger electron spectroscopy and low-energy ion scattering were used to assess surface composition of yttrium-doped polycrystalline and single-crystal α-aluminas which were heated under vacuum to temperatures above 1500°C. Extensive surface and intergranular precipitation of yttrium aluminum garnet was observed in a compact of nominal Y content of 210 wt ppm, while apparent Y segregation to surface cation sites predominated in a second compact with a Y content of 160 wt ppm. In the latter case, the segregation enthalpy of yttrium to the alumina surface in the temperature range 1700° to 1900°C was estimated to be of the order –44 kJ/mol with a maximum surface cation site occupancy of ∼13%.     

Transparent Hydroxyapatite Ceramics through Gelcasting and Low-Temperature Sintering

Transparent hydroxyapatite (HAP) was prepared by sintering gel-cast powder compacts at 1000°C for 2 h; the resultant HAP material was studied using X-ray diffractometry, transmission electron microscopy, scanning electron microscopy, and microhardness measurement. Nanoscale HAP crystallites were prepared using a precipitation method that involved calcium nitrate and ammonium dihydrogen orthophosphate solutions; the preparation was conducted at a temperature of 0°C. The precipitate was gel-cast and sintered at 1000°C in the form of a transparent ceramic that had a uniform grain size of 250 μm. The maximum Vickers microhardness obtained for a sample sintered at 1000°C was 6.57 GPa. The sintering behavior of gel-cast samples prepared from high-temperature-precipitated HAP was compared with that of material prepared at 0°C.     

Preparation of Zirconia Coatings by Hydrolysis of Zirconium Alkoxide with Hydrogen Peroxide

Coatings of 8.8-mol%-yttria-doped zirconia were fabricated using a transparent and spinnable sol prepared by hydrolysis of zirconium alkoxide with hydrogen peroxide and nitric acid. The sol gave a crack-free coating film consisting of fine grains. The crystalline phase was cubic after heating at 1000° and 1200°C and cubic and tetragonal at 1350°C, with the coating being highly oriented in the (111) plane, especially at 1000°C. Activation energy of the coating films was higher than that of the bulk. Transmittance through a film thickness of about 0.3 μm on each side was 75%.     

Microstructure and Optical Properties of Hot Isostatically Pressed Nd:YAG Ceramics

Neodymium-doped transparent yttrium-aluminum garnet (Y₃Al₅O₁₂, YAG) (Nd:YAG) ceramics for solid-state laser material were fabricated by a solid-state reaction method using high-purity powders (Al₂O₃, Y₂O₃, and Nd₂O₃) as starting materials and capsule-free hot isostatic pressing (HIP). The mixed powder compacts were presintered at 1600°C for 3 h under vacuum, hot isostatically pressed at 1500°–1700°C for 3 h under 9.8 or 196 MPa of argon gas pressure, and then sintered again at 1750°C for 20 h under vacuum. Although the presintered specimen approached full density after HIP, its optical transmittance was quite low (∼5% at 1000 nm) because of lack of grain growth. Grain growth was observed in the specimens that were hot isostatically pressed and vacuum sintered at 1750°C for 20 h, but numerous pores occurred around the surface of these specimens. Consequently, the optical transmittance of Nd:YAG ceramics that were treated by HIP was inferior to that of the same ceramics that were sintered under vacuum only because of light scattering that was caused by the pores (at the grain boundaries) that were produced during the HIP treatment.     

Fabrication of Transparent Yttria Ceramics by the Low-Temperature Synthesis of Yttrium Hydroxide

Thin flakes of yttrium hydroxide agglomerated in a manner resembling houses of cards with aging at 10°C. The agglomerate then dissociated into fine yttria particles with calcination at >800°C. The particle size of the calcined powder increased appreciably as the calcination temperature increased. The shrinkage curve indicated similar densification behavior among undoped yttria powders calcined at 800°–1000°C, despite considerable particle growth as the calcination temperature increased. Increasing the calcination temperature to >1000°C shifted the shrinkage curve appreciably to the high-temperature region. Sulfate-ion-doped yttria particles had round edges, irrespective of calcination temperature, in contrast to the sharp edges of the undoped yttria particles. A calcination temperature of <1000°C resulted in skeleton yttria particles, which exhibited poor sinterability. At a calcination temperature >1000°C, the skeleton particles dissociated into monodispersed particles that densified easily. When the calcination temperature was >1000°C and the average particle sizes were similar, the undoped and sulfate-ion-doped yttria showed similar densification rates. The transparency of the sintered yttria ceramics was dependent on both the calcination temperature and sulfate-ion doping: that is, sulfate-ion doping and calcining at 1100°C were both necessary conditions for the fabrication of a transparent body.     

Presintering Heat Treatment, Densification, and Mechanical Properties of Silicon Carbide

Silicon carbides were subjected to extended heat treatments at 1400° or 1600°C prior to a 1-h isochronal sintering at 2000°C in argon, and the resulting microstructures, final densities, and room-temperature four-point bend strengths were compared to those of samples prepared without the pretreatments. The samples heat-treated at 1600°C exhibited higher relative densities and flexure strengths. Microstructural observation revealed that the heat treatment at 1600°C modified the initial powder compact microstructure, decreasing the development of large, dense domains in the early and intermediate sintering stages. This evolution was considered to promote a more uniform subsequent densification. The final size and distribution of the grains as well as of the pores were found to be affected favorably by the pretreatment.     

Densification and Mechanical Behavior of HfC and HfB2 Fabricated by Spark Plasma Sintering

Hafnium diboride (HfB₂)- and hafnium carbide (HfC)-based materials containing MoSi₂ as sintering aid in the volumetric range 1%–9% were densified by spark plasma sintering at temperatures between 1750° and 1950°C. Fully dense samples were obtained with an initial MoSi₂ content of 3 and 9 vol% at 1750°–1800°C. When the doping level was reduced, it was necessary to raise the sintering temperature in order to obtain samples with densities higher than 97%. Undoped powders had to be sintered at 2100°–2200°C. For doped materials, fine microstructures were obtained when the thermal treatment was lower than 1850°C. Silicon carbide formation was observed in both carbide- and boride-based materials. Nanoindentation hardness values were in the range of 25–28 GPa and were independent of the starting composition. The nanoindentation Young's modulus and the fracture toughness of the HfB₂-based materials were higher than those of the HfC-based materials. The flexural strength of the HfB₂-based material with 9 vol% of MoSi₂ was higher at 1500°C than at room temperature.     

Preparation of Mullite Fiber

Transparent mullite fibers have been prepared using aluminum carboxylates (ACs) and tetraethyl orthosilicate (TEOS) as starting materials. The ACs are derived from the catalyzed dissolution of elemental aluminum in a mixture of formic acid and acetic acid. The solubility of aluminum in the acids is influenced by the concentrations of the acids, water, and additives and the preparation temperature. A 1:4:3:24 molar ratio of aluminum, formic acid, acetic acid, and water dissolves the aluminum completely to give a colorless, clear solution suitable for fiber synthesis. The mixture of the ACs and TEOS, in the presence of ethyl alcohol as a mutual solvent at 50°–60°C, is concentrated to give a spinnable dope, from which mullite precursor fibers can be drawn. Heat treatment of the precursor at 1250°C yields crystallized and transparent mullite fibers.     

Rapid Densification of Nano-Grained Alumina by High Temperature and Pressure with a Very High Heating Rate

A novel method for producing full-dense nano-grained alumina was reported. The heat generated by the combustion reaction (or self-propagating high-temperature synthesis (SHS)) was applied to act as a heating source. An alumina compact was loaded inside the combustion reactants. After reaction, the temperature of the alumina compact was increased at a heating rate of 1600°C/min. A large mechanical pressure was applied, when the temperature reached the maximum. Two kinds of α-Al₂O₃ powders (200 and 600 nm) were used as the raw materials. The rapid densification process was performed within 2 min. Microstructure analysis indicated that the grains had no significant growth.     

Bubble Rise in Glassmelts

Bubble-rise experiments in borate glassmelts were conducted in a transparent furnace at T =800° to 1000°C using video techniques to record and analyze the bubble rise. The results of these experiments over a range of Reynolds numbers from 10 ⁻⁷ to 10° clearly indicate that the bubble motion is governed by the Rybezynski-Hadamard equation and hence that the glassmelts are not subject to interfacial contamination.     

High-Temperature Proton-Conducting Lanthanum Ortho-Niobate-Based Materials. Part II: Sintering Properties and Solubility of Alkaline Earth Oxides

The sintering properties and microstructure of La_{1− x} A _x NbO₄ powders ( x =0, 0.005, and 0.02 and A=Ca, Sr, and Ba), prepared by spray pyrolysis have been investigated. Dense materials (>97%) were obtained by conventional sintering at 1200°C and by hot pressing (25 MPa) at 1050°C, respectively. Homogeneous materials were obtained and the average grain size obtained by the two densification methods was ∼2.0 and ∼0.4 μm, respectively, for the 2% doped materials. Pure lanthanum ortho-niobate (LaNbO₄) showed a higher degree of grain growth. In the acceptor-doped materials, secondary phases were observed to inhibit grain growth at 1200°C. At 1400°C or higher, molten secondary phases in the Ba-doped materials resulted in severe grain growth, causing microcracking during cooling due to crystallographic anisotropy. A low solubility of AO (A=Ca, Sr, and Ba) in LaNbO₄ is inferred from the presence of secondary phases, and 1 mol% solubility of SrO in LaNbO₄ was found by electron microprobe analysis. The electrical conductivity in wet hydrogen of the materials demonstrated that the main charge carrier was protons up to 1000°C and reached a maximum value of ∼8·10⁻⁴ S/cm at 900°C.