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1.
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.  相似文献   

2.
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.  相似文献   

3.
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.  相似文献   

4.
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.  相似文献   

5.
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.  相似文献   

6.
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:   相似文献   

7.
β-Alumina, the electrolyte of choice of sodium/sulfur and sodium/metal chloride batteries and alkali-metal thermal-electric converters, was sintered from precursor phases to high β-phase purity in less than 15 s from the onset of densification by rapid pass-through rf induction coupled plasma sintering. The maximum instantaneous shrinkage rate was 1.8%/s. The resistivity was measured to be 13.8 ± 1.4 Ω·cm. The rapid conversion found is a significant improvement over conventional processing of β-alumina, which requires extended postsintering annealing times to obtain high β-phase purity.  相似文献   

8.
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.  相似文献   

9.
10.
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.  相似文献   

11.
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.  相似文献   

12.
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.  相似文献   

13.
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.  相似文献   

14.
Single-crystal thin foils of a-alumina were annealed i:i air and examined by transmission electron microscopy. Weak reflections, which are kinematically forbidden for the a-alumina unit cell, were obscrxed in selected-area diffraction patterns obtained from foils prepared parallel to (0001), (1012), and (1120) planes. These reflections are interpreted as giving evidence for surface periodicities which differ from those expected for the simple termination of the bulk crystal.  相似文献   

15.
16.
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.  相似文献   

17.
The polarization behavior of lithia-stabilized sodium β-alumina in the form of polycrystalline pressed and sintered specimens was studied at 100° to 400°C by observing the isothermal values of the complex admittance from ≊5 Hz to 500 kHz. Analyzing these data in the complex admittance plane in terms of an equivalent circuit for the system made it possible, when ion-blocking electrodes were used, to distinguish the separate contributions of the grain boundaries and the bulk crystals to the total electrolyte resistance. These results gave activation energies of 0.32 to 0.41 eV for the grain-boundary resistance and 0.11 to 0.14 eV for the bulk resistance. The prefactors in the Arrhenius expressions for these resistances appeared to be more sensitive to factors such as specimen composition, microstructure, and specimen preparation conditions than are the activation energies.  相似文献   

18.
A simple method is described to prepare submicrometer α-alumina by burning carbon supported on the surface of γ-alumina in oxygen flow at a temperature of 800°C. The burning of carbon generates a large amount of heat and leads to a rapid increase in the local temperature inside the pores of alumina. When the temperature is high enough for the phase transformation, α-alumina is obtained in a very short time. It was found that, for carbon contents between 6 and 10 wt%, all the γ-alumina could transform into α-alumina after burning of carbon in oxygen for a short time, and the transformed particle sizes of α-alumina were mostly no more than 1 μm.  相似文献   

19.
The dehydration, transformation, and densification of boehmite (γ-AlOOH) are enhanced by addition of γ-Al2O3 seed particles. α-Al2O3 microstructures with uniform 1- to 2-μm grain size and sintered densities 98% of theoretical are achieved at 1300°C Thermal analysis shows that γ-Al2O3 seed particles transform to α-Al2O3 before the matrix, thus controllably nucleating the transformation of θ-AI2O3 to α-Al2O3.  相似文献   

20.
Partially stabilized zirconia (PSZ) /β-alumina composites exhibit extensive non-elastic deformation during fracture. Repeated loading/stable fracture/unloading experiments were performed on chevron-notched four-point-bend specimens of the composite and pure PSZ. The energy consumption during the propagation of long cracks in the composite (∼500 J/m2) is 1 order of magnitude larger than for PSZ (∼50 J/m2). Breaking strengths of 127 MPa were obtained with a Weibull modulus of 43.  相似文献   

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