Edge-Defined Film-Fed Growth of β-Alumina and Mg-Substituted β-Alumina

High Na vapor pressure, peritectic decomposition, and high reactivity of the melt complicate the growth of α-alumina crystals. These difficulties were overcome by using a high-pressure (300 psig) growth chamber, Na₂O-rich melts, and Ir for all surfaces in contact with the melt. These procedures were combined with the edge-defined film-fed growth technique to produce single-crystal β alumina tubes and ribbons.     

Phase Equilibrium Investigations on Na–K–(β+β")-Alumina

The activities of Na₂O and K₂O dissolved in mixed-alkali Na–K–(β+β")-Al₂O₃ (NKBA) have been determined by using yttria stabilized zirconia (YSZ) as a solid electrolyte in the following galvanic cells: The approach enables to verify in situ the establishment and maintenance of the β/β"-equilibrium, and to characterize it as a function of the phase composition of NKBA. The results can be expressed as follows:      

Topotactic Growth of β-Alumina on α-Alumina

Thin foils of polycrystalline α-alumina were reacted with a potassium-rich vapor at ≤900°C. Potassium β-alumina formed along α-alumina grain boundaries and protruded from holes in the foils. Conventional transmission electron microscopy was used to analyze the α-alumina/β-alumina phase boundary for possible orientation relations.     

Kinetics and Microstructural Evolution of Heterogeneous Transformation of θ-Alumina to α-Alumina

Ultrafine (<0.1 μm) high-purity θ-Al₂O₃ powder containing 3–17.5 mol%α-Al₂O₃ seeds was used to investigate the kinetics and microstructural evolution of the θ-Al₂O₃ to α-Al₂O₃ transformation. The transformation and densification of the powder that occurred in sequence from 960° to 1100°C were characterized by quantitative X-ray diffractometry, dilatometry, mercury intrusion porosimetry, and transmission and scanning electron microscopy. The relative bulk density and the fraction of α phase increased with annealing temperature and holding time, but the crystal size of the α phase remained ∼50 nm in all cases at the transformation stage (≤1020°C). The activation energy and the time exponent of the θ to α transformation were 650 ± 50 kJ/mol and 1.5, respectively. The results implied the transformation occurred at the interface via structure rearrangement caused by the diffusion of oxygen ions in the Al₂O₃ lattice. A completely transformed α matrix of uniform porosity was the result of appropriate annealing processes (1020°C for 10 h) that considerably enhanced densification and reduced grain growth in the sintering stage. The Al₂O₃ sample sintered at 1490°C for 1 h had a density of 99.4% of the theoretical density and average grain size of 1.67 μm.     

Preparation Method of Submicrometer-Sized α-Alumina by Surface Modification of γ-Alumina with Alumina Sol

A method is introduced to prepare almost-spherical submicrometer-sized α-alumina via surface modification of γ-alumina with an alumina sol. Milled γ-alumina, in the presence of 3 wt% of α-alumina with a median particle size ( d ₅₀) of 0.32 μm (AKP-30), produced irregularly shaped α-alumina with d ₅₀∼0.3 μm after heat treatment at 1100°C for 1 h. γ-alumina that had been surface-modified by milling in the presence of 3 wt% of the alumina sol resulted in almost-monosized, spherical α-alumina ∼0.3 μm in size after heat treatment at 1100°C for 1 h. Furthermore, almost-spherical α-alumina 0.1—0.2 μm in size was obtained by milling γ-alumina with 3 wt% of AKP-30 alumina in the presence of 3 wt% of the alumina sol, followed by heat treatment at 1100°C for 1 h. The alumina sol that has been introduced in this work seems to act as a dispersant, in addition to helping to form a spherical shape.     

Preparation and Characterization of Macroporous α-Alumina

α-Al₂O₃ powders with three-dimensionally ordered or randomly positioned macropores were synthesized by templating with poly(methyl methacrylate) colloidal crystals. Aluminum nitrate was precipitated with ammonium hydroxide within the interstices of the template; calcination removed the polymer and converted the inorganic precursors into a macroporous skeleton of α-Al₂O₃. Subsequent calcination at higher temperatures and hot stage transmission electron microscopy experiments were performed to study sintering effects on the product morphology. These materials combine the thermal and chemical stability of corundum with a very open structure of uniform macropores that can permit facile transport of guest molecules in potential catalysis, filtration, and sensing applications.     

Synthesis and Properties of Strontium β-Alumina

It has been shown that strontium β-alumina is a stable phase at compositions close to SrAl₁₀O₁₇ and can be further stabilized over a composition range in the presence of MgO in the SrO–MgO–Al₂O₃ system. In the absence of MgO, at temperatures greater than 1673 K, the magnetoplumbite phase is readily formed. Conductivity measurement carried out at temperatures from 1073 to 1773 K has demonstrated that both the strontium β-alumina and the magnetoplumbite phases have reasonably similar magnitudes of conductivity and activation energy.     

Resistivity-Microstructure Relations in Lithia-Stabilized Polycrystalline β"-Alumina

The sodium ion resistivity of lithia-stabilized polycrystalline β"-alumina was measured as a function of temperature for fine-grained and coarse-grained specimens with a chemical composition of 8.80 Na₂0-0.75 Li₂O-90.45 A1₂O₃ (wt%). A model is presented which explains the dependence of sodium ion resistivity on grain size. Using the model the activation energy was determined for the transport of sodium ions across a grain boundary in this form of sodium β"-alumina.     

Nonuniformity of Potassium Ions in β-Alumina Ceramics

A nonuniform distribution of K was found in β -alumina that had been in contact with sodium polysulfide or sodium nitrate melts containing small amounts of K. Large β -alumina grains in contact with the melt had approximately the equilibrium concentration of K expected from studies on single crystals of β -alumina, but surrounding grains contained little or no K. Thus the mobility of K ions in grain boundaries in β -alumina appears to be very low. Stresses from K concentration at the ceramic surface could enhance other incipient sources of failure such as seal stresses, cracks, and other defects.     

Preparation of α-Alumina Platelets by Laser Scanning

A laser scanning with gas jet process was developed to prepare alumina platelets from an alumina powder. When the carbon-dioxide laser scanned the alumina powdery coatings prepared using an electrospraying technique, the alumina particles were heated to a melting state. The coaxial gas ejection force pushed the melting particles to obtain tabular shape grains that recrystallized into alumina platelets in the subsequent rapid-cool solidification. The phase and morphologies of powder bed were characterized by XRD and SEM. Results show that only α-alumina platelets were formed in the scanning process and the average edge length and thickness is 10 μm and 1–2 μm, respectively. Laser processing parameters such as laser energy density, scanning speed, and gas pressure were expected to play a vital role in the melting-crystallization-solidification process for obtaining platelike grains from powder beds. The preliminary experiment showed that the laser-scanning technique could be an effective means of tailoring the morphologies of particles to meet application requirements.     

Surface Characteristics of Sodium (β-Alumina Electrolytes

Surfaces of sodium β/β"-alumina ceramics were characterized via techniques such as scanning electron microscopy, secondary ion mass spectroscopy, and electron spectroscopy for chemical analysis to determine the nature of the resistive surface film that leads to the phenomenon of asymmetric polarization in a sodium-sulfur cell. The results indicated that the resistive surface film is soda rich, has submicrometer thickness, and is removable by a surface treatment. A mechanism consistent with these observations is proposed for the formation of the resistive surface film on sodium β/β"-alumina during the sintering process.     

Synthesis of Nanocrystalline α-Alumina Powder Using Triethanolamine

Nanocrystalline α-Al₂O₃ powders have been prepared by pyrolysis of a complex compound of aluminum with triethanolamine (TEA). The soluble metal-ion–TEA complex forms the precursor material on complete dehydration of the complex of aluminum-TEA. The single-phase α-Al₂O₃ powder has resulted after heat treatment at 1025°C. The precursors and the heat-treated final powders have been characterized by X-ray diffractometry, thermogravimetric and differential thermal analysis, and transmission electron microscopy (TEM). The average particle sizes as measured from X-ray line broadening and TEM are ∼25 nm. The powder has crystallite sizes of the same order indicates the poor agglomeration of crystallites.     

Preparation and Properties of Hydrothermally Stable γ-Alumina Membranes

Supported mesoporous γ-Al₂O₃ membranes deteriorate and blister in steam-containing environments at high temperatures. This deterioration led us to the development of a new type of supported γ-Al₂O₃ membrane with significantly improved stability under hostile conditions. Two measures were taken to achieve this result. First, the γ-Al₂O₃ itself was stabilized by an addition of 6 mol% La₂O₃ to suppress pore growth of the mesoporous structure. Second, the adherence of the γ-Al₂O₃ membrane to the α-Al₂O₃ support was significantly improved by application of phosphate bonding between the membrane layer and the support, using an Al(H₂PO₄)₃ precursor solution. Membranes applied without phosphate bonding were separated from the α-Al₂O₃ support during high-temperature steam treatment, resulting in complete loss of separative properties. The newly developed membranes could be operated for 100 h at 600°C in H₂O/CH₄= 3/1 (by volume) at 2.5 MPa total pressure with no delamination or cracking in the membrane–support interface and with no significant pore growth in the γ-Al₂O₃ membrane.     

Tribological Characteristics of α-Alumina at Elevated Temperatures

The tribological characteristics of a high-purity α-alumina sliding on a similar material under unlubricated conditions are divided into four distinct regimes. At low temperatures, T < 200°C, tribochemical reactions between the alumina surface and water vapor in the environment control the tribological performance. The coefficient of friction in this temperature range is approximately 0.40 and the wear coefficient is less than 10⁻⁶, independent of contact load. At intermediate temperatures, 200°C < T < 800°C, the wear behavior depends on the contact load. At low loads, wear occurs by plastic flow and plowing; the coefficient of friction is approximately 0.60 and the wear coefficient is less than 10⁻⁶. At loads larger than a threshold value, severe wear occurs by intergranular fracture. The coefficient of friction increases to 0.85 and the wear coefficient increases to a value greater than 10⁻⁴. At temperatures above 800°C, formation of a silicon-rich layer on the wear track by diffusion and viscous flow of the grain-boundary phase reduces the coefficient of friction to 0.40, and the wear coefficient is reduced to a value less than 10⁻⁶. The results of the wear tests and observations of the fundamental mechanisms controlling the tribological behavior of this material are consolidated in a simple wear transition diagram.     

Chemical Polishing Behavior of Sodium β"-Alumina Electrolytes

Sodium ion conducting solid electrolytes based on sodium β"-alumina phase, useful in sodium-sulfur batteries, were exposed to phosphoric acid to determine the chemical polishing conditions. Under suitable conditions of acid concentration, exposure time, and exposure temperature, samples of sodium β"-alumina were polished by phosphoric acid. The kinetics of polishing was studied, and a polishing mechanism is proposed. Insignificant effect of chemical polishing on the fracture strength of sodium β"-alumina electrolyte was obtained.     

Asymmetric Polarization Behavior of Sodium β"-Alumina Electrolyte

The roles of electrolyte composition and surface treatments in the phenomenon of asymmetric polarization in sodium β"-alumina ceramics are investigated. The results indicate that Asymmetric polarization is only displayed by electrolyte compositions containing 80 vol%β"-alumina phase and that a chemical treatment utilizing phosphoric acid, or a water treatment, is effective in circumventing the asymmetric polarization via removal of the asymmetric-polarization-causing resistive surface film. The surface film is soda rich, 20 to 40 nm thick, and there is a considerable concentration gradient from the surface into the bulk of an as-sintered sodium β"-alumina electrolyte.