Effect of Dopants on the Distribution of Aluminum and Oxygen Fluxes in Polycrystalline Alumina Under Oxygen Potential Gradients at High Temperatures

The oxygen permeability of Lu‐, Y‐, and Hf‐doped polycrystalline alumina wafers under steep oxygen potential gradients was evaluated at temperatures above 1773 K. Oxygen permeation occurred by the grain‐boundary (GB) diffusion of oxygen from the higher oxygen partial pressure (P_O2) surface to the lower P_O2 surface, and was coincident with GB diffusion of aluminum in the reverse direction. Lu‐ and Y‐doping both suppressed oxygen permeation to the same extent, owing to the decrease in oxygen mobility, but neither had any significant effect on aluminum mobility. Hf‐doping had the opposite effect. The fluxes of oxygen and aluminum at the inflow side in all wafers were significantly smaller than those at the outflow side, regardless of whether or not these dopants were added. Consequently, the intersection of the fluxes shifted to the lower P_O2 side upon Lu‐ and Y‐doping, and to the higher P_O2 side upon Hf‐doping. Furthermore, the effect of dopants on the mass transfer in scales formed by oxidation of FeCrAl‐based alloys at 1300–1500 K was analyzed through predictions of the flux distributions of oxygen and aluminum in the scales.     

高温下电解槽衬用耐火材料的渗透

铝电解槽阴极所用耐火挡板砖的蚀损主要是由氟化物熔体和钠蒸汽的化学侵蚀引起的。当砖与腐蚀介质起化学反应时，它们经历着一些物理变化和化学变化，最终将削弱电解槽的工作能力，甚至会缩短它的寿命。近来对硅酸铝耐火材料性能的研究显示出在材料的渗透性和它们的抗化学侵蚀性之间存在一定的联系。由于真实的环境不能再现，本文所述例子中的性能，均在室温条件下测出。在当前研究中．为了研究从室温上升到700qC渗透性是如何变化的，运用了最新开发的设备进行了各种不同的实验，并试图再现挡板砖的实际环境。对室温和高温下的结果进行了对比和分析，得出选择铝电解槽衬的最好方法。     

Tensile Stress Relaxation of Alumina at High Temperature

The development of a tensile testing methodology for ceramics which enables a stress vs strain-rate response to be measured at high temperature is described. The test involves a carefully controled stress relaxation test at constant total strain using an experimental procedure and phenomenological analysis previously developed for metallic materials. It is demonstrated here with preliminary tests on alumina at 1050° and 1150°C. This offers, with further development, the possibility of establishing design stresses associated with low strain-rate behavior for structural applications. The results demonstrate that data covering four decades of strain rate may be generated in tests lasting a few hours. The inelastic strain consists of substantial anelastic recoverable strain in addition to a permanent creep strain.     

Oxidation of ZrB2 Nanoparticles at High Temperature under Low Oxygen Pressure

The oxidation of ZrB₂ nanoparticles was observed at high temperature of 1500°C under low oxygen partial pressure of 5 × 10^?2 Pa by an environmental transmission electron microscope. The results demonstrate that the oxidation starts on the surface of ZrB₂ nanoparticles with decomposition of ZrB₂ into ZrO₂ and B₂O₃. The nucleation and growth of ZrO₂ on the surface of ZrB₂ proceed with B₂O₃ being evaporated.     

高铝质耐火材料常温耐磨性能研究

选用棕刚玉为磨料,在压缩空气加速磨粒的冲蚀磨损试验机上分别对高铝质浇注料和高铝耐火砖的原始成型面和砖体内切割面进行了冲蚀磨损实验.探究了高铝质耐火材料的原始成型面和砖体内切面在不同冲蚀角度、磨料速度及冲蚀时间下的体积冲蚀磨损率,采用扫描电子显微镜(SEM)观察经不同冲蚀时间冲蚀后试样的微观形貌.实验结果表明:在不同的冲蚀角度、磨料速度及冲蚀时间的试验条件下,高铝质耐火材料内切面的耐冲蚀性均优于其原始成型面的耐冲蚀性,且在冲蚀过程中,原始成型面存在加速和稳态两个冲蚀过程.     

A Layered Alumina Composite Tested at High Temperature in Air

An oxidation-resistant interphase for layered alumina composites was prepared by aerosol spray deposition of submicrometer alumina powder. A model composite specimen was made by placing the interphase between thin layers of monolithic alumina. The composite sandwich was hot-pressed to control the interphase fracture resistance for successful crack deflection. Specimens were tested under four-point bending in air at two crosshead speeds at ambient temperature, 1000°C, and 1200°C. The fracture behavior was temperature dependent, with a higher work of fracture at higher temperatures. Interphase delamination and composite toughening behavior were very pronounced at all temperatures. At the highest temperature, the transition to multiple widely distributed cracks and increased crack deflection may be related to inelastic deformation in the alumina.     

Demixing of Materials under Chemical Potential Gradients

A steady-state demixing of initially homogeneous solid solution which occurs in binary oxides under oxygen partial pressure gradient is treated by the path probability method of irreversible statistical mechanics from an atomistic point of view. The larger the difference between the diffusion coefficients of constituent species is, the larger is the degree of demixing. Essential features of demixing by diffusion, however, do not depend on whether the driving force is the chemical potential gradient, the electric field, or others.     

Oxygen Interaction with Hexagonal OsB2 at High Temperature

The stability of ReB₂‐type hexagonal OsB₂ powder at high temperature with oxygen presence has been studied by thermogravimetric analysis, differential scanning calorimetry, SEM, EDS, and high‐temperature scanning transmission electron microscopy and XRD. Results of the study revealed that OsB₂ ceramics interact readily with oxygen present in reducing atmosphere, especially at high temperature and produces boric acid, which decomposes on the surface of the powder resulting in the formation of boron vacancies in the hexagonal OsB₂ lattice as well as changes in the stoichiometry of the compound. It was also found that under low oxygen partial pressure, sintering of OsB₂ powders occurred at a relatively low temperature (900°C). Hexagonal OsB₂ ceramic is prone to oxidation and it is very sensitive to oxygen partial pressures, especially at high temperatures.     

Penetration of Polycrystalline Alumina by Glass at High Temperatures

An analysis of interfacial energies in a solid-liquid system showed that complete penetration of a liquid along grain boundaries in a microstructure forming during liquid-phase sintering is not necessarily associated with complete wetting or spreading at the solid-liquid interface. Investigations were made on the penetration of CaO-Al₂O₃-SiO₂ glasses into dense polycrystalline aluminas. Initial penetration was along three-grain junctions or grain boundaries by a solution/reaction between the liquid and defect-containing alumina along interfaces. Degree of penetration was variable, and it was dependent on internal strain and MgO addition. Extended heat treatments reduced liquid penetration.     

Effect of Surface Concentration of Lu on Oxygen Permeation of Polycrystalline Alumina at High Temperatures

The effect of Lu surface concentration on oxygen permeation in polycrystalline α‐alumina wafers was determined at 1773 K under limited oxygen potential gradients (Δ), where the two surfaces of the wafer were deliberately subjected to different oxygen partial pressures [ (I) ?  (II)]. When oxygen permeation occurred mainly by oxygen GB diffusion under a Δ generated by a combination of low values, the Lu‐coating on the (I) [ (II)] surface decreased [increased] the oxygen permeability constants. When Δ was the result of a combination of high values, where oxygen permeation proceeded mainly by aluminum GB diffusion, the oxygen permeability constants were decreased only by the Lu‐coating on the (I) surface. The analysis of mass transfer parameters, such as the chemical potentials, GB diffusion coefficients, and fluxes of aluminum and oxygen in the wafers, suggested that ambient oxygen molecules were effectively attracted toward Lu‐coated surfaces exposed to low‐ environments, leading to a change in oxygen permeability.     

低温熔盐法制备片状氧化铝

研究了以氢氧化铝为原料,氯化钠一氯化钾为熔盐,以及熔盐加入量对试样物相的影响。研究结果表明,熔盐加入量为67％时,在1000℃保温4h,得到了结晶完整的,边长为5～10μm,厚度为0．6～1．4μm的片状α～Al2O3;熔盐加入量为50％时,不能得到片状氧化铝。     

化学结合法低温制备氧化铝泡沫陶瓷的研究

以磷酸二氢铝为粘结剂、电熔α氧化铝粉末为原料、聚氨酯海绵为模板低温制备出了氧化铝泡沫陶瓷.研究了浆料固相含量对氧化铝泡沫陶瓷微观形貌、线收缩率、体积密度和抗折强度的影响.结果表明:随浆料固相含量的增加,烧成氧化铝泡沫陶瓷的通孔率逐渐降低、体积密度与抗折强度逐渐提高,线收缩率均低于1％.在浆料固相含量为60％时,可制备出体积密度为0.83 g/cm3、抗折强度为(2.21±0.43) MPa、通孔率优异的泡沫陶瓷.     

Considerable Enhancement of Field Emission of SnO2 Nanowires by Post-Annealing Process in Oxygen at High Temperature

The field emission properties of SnO₂ nanowires fabricated by chemical vapor deposition with metallic catalyst-assistance were investigated. For the as-fabricated SnO₂ nanowires, the turn-on and threshold field were 4.03 and 5.4 V/μm, respectively. Considerable enhancement of field emission of SnO₂ nanowires was obtained by a post-annealing process in oxygen at high temperature. When the SnO₂ nanowires were post-annealed at 1,000 °C in oxygen, the turn-on and threshold field were decreased to 3.77 and 4.4 V/μm, respectively, and the current density was increased to 6.58 from 0.3 mA/cm² at the same applied electric field of 5.0 V/μm.     

Silica and Magnesia Dopant Distributions in Alumina by High-Resolution Scanning Secondary Ion Mass Spectrometry

Trace SiO₂ and MgO additive distributions in sintered alumina have been studied using high-resolution scanning secondary ion mass spectrometry (SIMS). When doped with each additive individually, evidence is seen for both strong silicon segregation to grain boundaries ( C _gb/ C _grain similar/congruent 300) in SiO₂-doped alumina and strong magnesium segregation to grain boundaries ( C _gb/ C _grain similar/congruent 400) in MgO-doped alumina. When codoped with both SiO₂ and MgO, segregation of both ions to grain boundaries is reduced by a factor of 5 or more over single doping. The additive concentrations increase proportionally in the grains, and both dopants become more uniformly distributed throughout the bulk. It is concluded that codoping with these additives increases their mutual bulk solid solubility and decreases their interfacial segregation over single doping. The beneficial effect of MgO additions in controlling microstructure development in alumina and improving corrosion resistance to aqueous HF stems from its ability to redistribute silicon ions from grain boundaries into the bulk.     

Dielectric Properties of Alumina at High Temperatures

The dielectric constant and dissipation factor of both single-crystal and polycrystalline alumina were measured in air over the frequency range lo² to 3 × 10⁵ cycles per second, in the temperature interval 900° to 1300° C. The three-electrode guard-ring method was used. The observed dielectric losses were due predominately to free-electron conduction mechanisms. The volume conductivity of single-crystal and polycrystalline alumina was calculated from the high-frequency data. The temperature dependences of the volume conductivity for two single crystals of different thicknesses were similar. The large thermal activation energy indicated intrinsic conductivity; the energy band gap of alumina was calculated to be approximately 7.3 electron volts. The bulk conductivities of the polycrystalline alumina were considerably higher than the single-crystal values, presumably because of impurities in the polycrystalline material. The results were compared with values previously reported in the literature.     

降低高铝瓷烧成温度的研究

从我国能源战略的高度和高铝瓷产业生存、发展的角度,阐述论证了低温烧成的必要性和重要性.基于高铝瓷烧结的内在规律和固有特点,指出低温烧结对高铝瓷性能有改善作用,提出了几种降低高铝瓷烧成温度的可行技术方案.     

Tensile Ductility in Zirconia-Dispersed Alumina at High Temperatures

High-temperature plastic flow in Al₂O₃-10 wt% ZrO₂ (2.5 mol% Y₂O₃) has been examined at temperatures between 1400° and 1500°C. Al₂O₃-10 wt% ZrO₂ (2.5 mol% Y₂O₃) exhibits much higher flow stress and smaller tensile elongation below about 1450°C than 0.1 wt% MgO-doped single-phase Al₂O₃. The suppression of grain growth with ZrO₂ dispersion into Al₂O₃ is not effective for improving the tensile ductility. The limited ductility in Al₂O₃-10 wt% ZrO₂ (2.5 mol% Y₂O₃) is associated with the increment of flow stress caused by ZrO₂. The ZrO₂ dispersion or segregation in Al₂O₃/Al₂O₃ boundaries suppresses the grain boundary sliding and hence results in the increased flow stress at high temperatures.     

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.     

Solubility of Magnesia in Polycrystalline Alumina at High Temperatures

High-purity Al₂O₃ compacts were doped with 0–350 ppm (by weight) of MgO using a liquid immersion technique and equilibrated at temperatures between 1700° and 2000°C under hydrogen. The solubility limits of MgO in Al₂O₃ at temperatures of 1720° and 1880°C were very low, ∼75 and 175 ppm, respectively. Variation of MgO solubility with temperature could be represented by the equation, ln Mg/Al = 3.80–2.63 × 10⁴/ T . The small MgO solubilities were understood by the high enthalpy (326 kJ/mol) of solution. The results of this study suggested that previous investigations on sintering and grain-growth mechanisms in MgO-doped Al₂O₃ were probably not done in single-phase Al₂O₃ solid solutions. However, the conclusions on sintering and grain-growth mechanisms in prior research work in MgO-doped A₂O₃ may be correct. The effects of SiO₂ impurity and grain size on MgO solubility are discussed. Previous grain-growth experiments in MgO-doped Al₂O₃ are described that demonstrate the clearest evidence for grain-boundary mobility controlled by a solid-solution mechanism.     

Conductivity of Iron-Doped Polycrystalline Alumina at High Temperatures

Conductivity was studied from 10⁻⁵ to 10⁵ Pa oxygen and 1450° to 1600°C in samples of polycrystalline alumina doped with 0.09, 0.5, and 5 wt% iron. The observed conductivity appeared to be electronic. The electronic defects in the 0.09 and 0.58 samples were controlled by iron at high oxygen pressures and by silicon at low oxygen pressures. The behavior of the 5% samples was controlled by the formation and precipitation of iron-aluminate spinel.