Increase in Phase Transition Temperature of Activated Alumina with Nano-Zirconia Synthesized by Pulsed Wire Discharge

Activated alumina powders have been synthesized by using a novel dry process of pulsed wire discharge. The temperature of phase transition from activated alumina to α-alumina and its variation caused by adding zirconia have been investigated. A mixture of activated alumina and zirconia was formed by mixing zirconium plasma with aluminum plasma and cooling together in an oxygen atmosphere. It was found that the transition temperature increased when the zirconia content ratio was increased. On the other hand, results of X-ray diffraction and X-ray photoelectron spectroscopy analysis indicated no substitution of zirconium in an alumina lattice. Thus, most of the zirconium atoms were located in zirconia particles on the surface and/or the grain boundary of alumina grains. Thus, it appears that the increase in the phase transition was caused by retardation of atomic diffusion at zirconia particles.     

Phase Transformation and Densification of an Attrition-Milled Amorphous Yttria-Partially-Stabilized Zirconia–20 mol% Alumina Powder

Phase transformations during consolidation treatments of an attrition-milled amorphous yttria-partially-stabilized zirconia (Y-PSZ: ZrO₂–3 mol% Y₂O₃)–20 mol% Al₂O₃ powder and the resulting microstructures have been investigated. A metastable cubic phase ( c -ZrO₂ solid solution) together with an α-Al₂O₃ phase is formed in the amorphous matrix by consolidation at temperatures below 1204 K. The metastable cubic phase transforms to a stable tetragonal phase ( t -ZrO₂ solid solution) with an increase in the consolidation temperature. Fully dense bulk samples consisting of extremely fine tetragonal grains together with a small amount of α-Al₂O₃ particles could be obtained by consolidation at temperatures above 1432 K. Important features concerned with the densification behavior are as follows: (1) Marked increase in the relative density occurs after cubic crystallization and subsequent cubic-to-tetragonal transformation. (2) All of the consolidated bulk samples show extremely fine grain structure with grain sizes of several tens of nanometers, irrespective of the consolidation temperature. (3) The regularity of the lattice fringe contrast in each tetragonal grain seems to be kept in the vicinity of grain boundaries. These results suggest that densification of the attrition-milled amorphous powder proceeds via superplastic flow and/or diffusional creep, rather than viscous flow of the initial amorphous phase before crystallization.     

Particle Interactions in Zirconia-Toughened Alumina

In order to determine the effect of the coarse tail of the ZrO₂ size distribution of the monoclinic ZrO₂ content of zirconia-toughened alumina, quantitative hot-stage X-ray measurements were made on (1) A1₂O₃-10% Zr0₂ (submicrometer with a coarse tail >1 μm), (2) A1₂O₃-10% ZrO₂ with the coarse tail of the ZrO₂ carefully removed by centrifugation, and (3) Al₂O₃-10% ZrO₂ in which 10% of Zr0₂(1) and 90% of Zr0₂(2) were used. The monoclinic content of (3) was compared with the weighted average of(1) and (2). A disproportionate amount of monoclinic ZrO₂ was found in (3) and it was concluded that transformation of the coarse particles promotes transformation in finer particles. Microstructural examination supports this conclusion. Results were interpreted as being due both to autocatalytic transformation and to constraint relief from microcracking.     

低温烧结氧化铝陶瓷的动力学研究

用CaO-MgO-SiO2玻璃(CMS-G)和TiO2为烧结助剂,在1450℃下实现了氧化铝陶瓷的致密烧结.探讨了CMS-G和TiO2质量百分含量对氧化铝陶瓷烧结性能、显微结构以及其烧结动力学的影响.结果表明:CMS-G和TiO2可有效促进氧化铝陶瓷的致密化,当CMS-G含量为3%、TiO2含量为1%时,氧化铝陶瓷的相对密度达到98.25%;液相烧结激活能为113.4kJ/mol,表明扩散控制了烧结过程.     

氧化铝上乙醇脱水制乙烯的动力学

为开发多段绝热床生物质乙醇脱水制乙烯反应工艺,在Φ10 mm等温积分反应器中以自研发的氧化铝为催化剂研究了生物乙醇脱水制乙烯反应动力学。实验在排除内外扩散的影响的条件下,考察了反应温度、反应空速和水醇比对反应的影响,建立了生物质乙醇脱水制乙烯的半理论半经验的幂函数本征动力学模型。模型方程具有统计意义上的可靠性,并与实验结果吻合良好。     

The Wulff Shape of Alumina: I, Modeling the Kinetics of Morphological Evolution

The rate at which fully facetted nonequilibrium shaped particles and pores approach their equilibrium (Wulff) shape via surface diffusion was modeled, and calculations relevant to alumina were performed to guide experimental studies. The modeling focuses on 2-D features, and considers initial particle/pore shape, size, surface energy anisotropy, and temperature (surface diffusivity) as variables. The chemical potential differences driving the shape change are expressed in terms of facet-to-facet differences in weighted mean curvature. Two approaches to modeling the surface flux are taken. One linearizes the difference in the mean chemical potential of adjacent facets, and assumes the flux is proportional to this difference. The other approach treats the surface chemical potential as a continuous function of position, and relates the displacement rate of the surface to the divergence of the surface flux. When consistent values for the relevant materials parameters are used, the predictions of these two modeling approaches agree to within a factor of 1.5. As expected, the most important parameters affecting the evolution times are the cross-sectional area (volume in 3-D) and the temperature through its effect on the surface diffusivity. Pores of micrometer size are predicted to reach near-equilibrium shapes in reasonable times at temperatures as low as 1600°C. The detailed geometry of the initial nonequilibrium shape and the Wulff shape appear to have relatively minor effects on the times required to reach a near-equilibrium shape.     

Strengthening of Zirconia–Alumina During Cyclic Fatigue Testing

Recent fatigue results of advanced ceramics are reviewed. It is pointed out that current statements of fatigue properties often relate to specimens with long artificial cracks; in this study, a Y-ZrO₂+Al₂O₃ ceramic is tested under cyclic stresses using specimens with natural flaws. Cyclic fatigue is restricted to a narrow range of stresses near the ultimate strength. Survivor specimens reveal increased residual strength, possibly resulting from a higher ZrO₂ phase transformation encouraged by long-term cyclic stresses.     

相变动力学和热分析动力学在干燥动力学研究中应用的可能性

对相变动力学理论、热分析动力学理论及薄层干燥模型进行简单的介绍和比较。探讨了相变动力学理论及热分析动力学理论在干燥动力学研究中应用的可能性。     

酸浸提取煤矸石中氧化铝工艺优化及其动力学

以贵州盘县煤矸石为原料,采用单因素实验和正交实验方法,以浓硫酸为酸浸介质酸浸提取煤矸石中氧化铝,考察了酸浸反应主要影响因素,研究了浸出动力学. 结果表明,最佳酸浸提取条件为：酸矸质量比1.4,反应时间4 h,反应温度170℃. 该条件下氧化铝的浸取率达98.47%. 该酸浸反应过程符合收缩未反应核模型的粒径不变缩核模型,反应受界面化学反应控制,反应动力学方程为1-(1-x)1/3=kt,反应活化能为61.32 kJ/mol.     

Effect of Monovalent Cation Additives on the γ-Al2O3-to-α-Al2O3 Phase Transition

The effect of monovalent cation addition on the γ-Al₂O₃-to-α-Al₂O₃ phase transition was investigated by differential thermal analysis, powder X-ray diffractometry, and specific-surface-area measurements. The cations Li⁺, Na⁺, Ag⁺, K⁺, Rb⁺, and Cs⁺ were added by an impregnation method, using the appropriate nitrate solution. β-Al₂O₃ was the crystalline aluminate phase that formed by reaction between these additives and Al₂O₃ in the vicinity of the γ-to-α-Al₂O₃ transition temperature, with the exception of Li⁺. The transition temperature increased as the ionic radii of the additive increased. The change in specific surface area of these samples after heat treatment showed a trend similar to that of the phase-transition temperature. Thus, Cs⁺ was concluded to be the most effective of the present monovalent additives for enhancing the thermal stability of γ-Al₂O₃. Because the order of the phase-transition temperature coincided with that of the formation temperature of β-Al₂O₃ in these samples, suppression of ionic diffusion in γ-Al₂O₃ by the amorphous phase containing the added cations must have played an important role in retarding the transition to α-Al₂O₃. Larger cations suppressed the diffusion reaction more effectively.     

非均相化学反应的类分形动力学

大多数非均相反应的动力学不遵从经典的动力学规律。随着分形几何理论在表面科学中的应用和发展 ,非均相反应的类分形动力学研究取得了可喜的成果。综述了分形表面上反应的动力学特征及非均相反应的类分形动力学的研究进展。     

Phase Equilibria and Phase Transformation in the Aluminum Nitride-Aluminum Oxycarbide Pseudobinary System

Samples in the pseudobinary AIN–AI₂OC were fabricated by hot-pressing mixtures of ANI, AI₄C₃, and AI₂O₃ powders in graphite dies in an atmosphere of nitrogen. The resulting dense samples were subjected to thermal treatments over a range of temperatures from 1550° to 1950°C. The hot-pressed as well as annealed samples were examined using optical microscopy, scanning transmission electron microscopy, and X-ray diffraction. An electron microprobe was used to determine composition. X-ray diffraction showed that AI₂OC dissolved in AIN up to 44 mol% at 1800°C. Thermal treatment at lower temperatures led to the decomposition of the solid solution into two isostructural phases. In samples containing ∼14 to 60 mol% AIN, the morphology of the precipitates was lenticular. Diffraction contrast analysis showed that the precipitates were rich in ANI. Lattice images showed that the (001) planes of the 2H structure were continuous between the matrix and the precipitates. These precipitates appear to be similar to Guinier-Preston zones observed in metallic alloys. When annealed for long periods of time, interface dislocations formed, signifying partial loss of coherency. In some compositions (∼61 to 64 mol% AIN), a lamellar microstructure developed similar to that observed in cellular phase separation. Also, in some of the compositions, an additional phase was observed whose composition and structure were not determined.     

Thermal Shock Behavior of Iron-Particle-Toughened Alumina

The thermal shock behavior of an alumina monolith and two alumina–iron ceramic-matrix composites has been investigated by superimposing the measured K _R-curves of the materials onto the theoretically generated curves of the thermally induced stress intensity factor. Predictions of the critical-temperature differentials and retained strengths after quenching are in good agreement with the experimental data. The inclusion of metallic particles into an alumina matrix improves the thermal shock resistance, although the increase in toughness is not solely responsible for this improvement. There is a decrease in thermal stress-intensity factor that is generated for the composites; this decrease is due to a reduction in the Young's modulus and/or Biot modulus. However, the increased toughness for large crack lengths may offer increased damage resistance for severe thermal shock treatments.     

Plasticity and Microcracking in Shock-Loaded Alumina

Using a 110-mm-diameter compressed-gas gun, we performed two types of experiments on alumina samples. The first involved plate impacts with wave profile measurements made with a VISAR velocity interferometer. The second type was plate-impact recovery experiments. SEM, TEM, and X-ray diffraction analysis showed that grain plasticity begins below the Hugoniot elastic limit (HEL), leading to a compaction of the material. This compaction is not accompanied by microcracking in pure alumina up to 2 × HEL. In the case of alumina with an intergranular glassy phase, the deformation of the sample is accompanied by a microcracking of the glass. However, the microcracks are not interconnected at a stress level equal to 2 × HEL for an alumina with about 10% glassy phase.     

Effect of Alumina Addition on the Tetragonal-to-Monoclinic Phase Transformation in Zirconia–3 mol% Yttria

The tetragonal-to-monoclinic martensitic phase transformation in ZrO–3 mol% Y₂O₃ (PSZ) containing 0 to 12 wt% Al₂O₃ was investigated by dilatometry, XRD, and SEM-EDS methods. The propagation of the transformation into the specimen interiors was suppressed by the addition of Al₂O₃. The grain size was independent of the addition of Al₂O₃. Both Y₂O₃ and Al₂O₃ segregated at grain boundaries. From this segregation behavior, it was suggested that a certain compound or phase of Y₂O₃–Al₂O₃ could be formed at grain boundaries, which would presumably prevent the propagation of the transformation into interiors of PSZ-containing Al₂O₃.     

Kinetics of Solid Polymorphic Phase Transitions of CL‐20

Solid‐solid phase transitions of the CL‐20 polymorphs ε→γ, α→γ, and β→γ were studied isothermally at ambient pressure by transmission FT‐IR spectroscopy and analyzed by multivariate regression. Activation energies as function of the extent of conversion were obtained using the model‐free isoconversional method. The apparent activation energies for all three solid phase transformations were about 210 kJ/mol at 5% conversion, and compared well with the calculated lattice energies. The apparent activation energies increased to a maximum of about 400 kJ/mol at 40%, 80% and 60% conversion for ε→γ, α→γ, and β→γ, respectively, and then decreased. The mechanisms of the phase transitions of CL‐20 appear to be quite complex. Comparisons are made with the phase transitions of HMX.     

氧化铝源对CA6多孔材料相组成和显微结构的影响

用Al(OH)3、Al2 O3微粉(uf-Al2 O3)、工业Al2 O3粉作为氧化铝源,CaCO3为钙源,通过高温固相反应合成了六铝酸钙(CA6)材料,研究了氧化铝原料对材料物相组成、显微结构和性能的影响.结果表明,以Al(OH)3、Al2O3微粉、工业Al2 O3为氧化铝源的试样在1300℃时开始有CA6相生成,各...     

Kinetic Analysis of Solution-Precipitation During Liquid-Phase Sintering of Alumina

Densification controlled by solution-precipitation during liquid-phase sintering was analyzed for the aluminamagnesium aluminosilicate glass system. As a model system for liquid-phase sintering, narrowly sized alumina powders and up to 20 vol% magnesium aluminosilicate glass samples were isothermally sintered at 1550° to 1650°C. Densification rate increases with increasing liquid content and sintering temperature but decreases with increasing density. For samples with >15% grain growth, the densification rate during the solution-precipitation stage of sintering was proportional to (particle size)⁻² and thus interface reaction-controlled. Activation energies ranged from 270 to 500 kJ/mol over the relative density range of 66% to 96%, respectively. The low activation energy is attributed to densification by particle rearrangement, whereas the higher activation energy is due to densification controlled by interface-reaction-controlled solution-precipitation. Intermediate activation energies are attributed to simultaneous densification by the two mechanisms.