Microstructure of 96% Alumina Ceramics: I, Characterization of the As-Sintered Materials

Characterization of the microstructure and microchemistry of a group of commercial 96% Al₂O₃ ceramics, using analytical and conventional electron microscopy techniques, was conducted. A continuous glassy grain-boundary phase was found in all samples, in addition to a number of intragranular and intergranular crystalline second phases; the phases present depended on the original boundary-glass composition. The faceted nature of many of the Al₂O₃-glass interfaces was studied and is thought to be an equilibrium structure.     

The effect of an Al2O3 dispersion on the adhesion of Al2O3 scales on aluminized Co-10% Cr alloys

The beneficial effect of dispersions of reactive-metal oxide particles on the adhesion of Cr₂O₃ and Al₂O₃ scales formed on heat-resisting alloys is wellknown. It has been shown that an Al₂O₃ dispersion in an alloy can improve the adhesion of a Cr₂O₃ scale, and it is of particular interest in assessing the various theoretical proposals for the effect to determine whether such a dispersion can affect the adhesion of an Al₂O₃ scale. In this investigation, a Co–10% Cr–1 % Al alloy was first internally oxidized to form an Al₂O₃ dispersion. This alloy was then aluminized so that on subsequent oxidation an Al₂O₃ scale developed. It was shown that the dispersion did indeed improve the scale adhesion. The implications of this result are discussed.     

Microstructure-Mechanical Property Relationships in Hot Isostatically Pressed Alumina and Zirconia-Toughened Alumina

The rates of densification and the mechanical properties of pure Al₂O₃ and ZrO₂-toughened Al₂O₃ (ZTA) have been investigated as a function of the temperatures and time schedules used for hot isostatic pressing (HIP) as a postsintering heat treatment for samples which had already been pressureless sintered in air at 1460°C for 45 min. ZTA hot isostatically presed at 1400°C had a finer grain size and a narrower grain size distribution than ZTA hot isostatically pressed at 1600°C. At both HIP conditions, the density which could be obtained was almost the maximum theoretical density. The amount of grinding-induced and fracture-induced monoclinic ZrO₂ formed as a result of the tetragonal → monoclinic martensitic transformation in ZTA was higher in the samples hot isostatically pressed at 1400°C. ZTA hot isostatically pressed at 1600°C and 100 MPa had fewer flaws and higher strengths than ZTA hot isostatically pressed at 1400°C for the same time, with a gradual improvement in mechanical properties with increasing HIP time at each of these two temperatures. The best mechanical properties were obtained from ZTA hot isostatically pressed at 100 MPa and 1600°C for 1 h: these specimens had a four-point bend strength of 940 ± 15 MPa at room temperature and 540 ± 15 MPa at 1000°C and an indentation fracture toughness at room temperature of 9.4 ± 0.2 MPa·m^1/2.     

Structural Studies of Anomalous Behavior in the Silica-Alumina Gel System

This paper compares the structure ofAl₂O₃-SiO₂ xerogels prepared with Al₂O₃ contents ranging from 21 to 75 wt% (13 to 64 mol%). The 47 wt% Al₂O₃ xerogel (Al/Si ≅ 1) exhibits anomalously low surface area (≅1 m²/g) and skeletal density compared with other Al₂O₃-Si₂ compositions. Based on the results of nitrogen adsorption/condensation, helium displacement, high-resolution transmission electron microscopy (HRTEM), small-angle X-ray scattering (SAXS), and small-angle neutron scattering (SANS), we attribute the low surface area and skeletal density (density of the solid phase which is inaccessable to helium) to the formation of closed micropores, whereas the higher surface area materials exhibit a slightly coarser texture comprising open pores with radii of ≅1 nm. X-ray diffraction (XRD) and ²⁹SI and ²⁷Al magic-angle spinning nuclear magnetic resonance (MASNMR) indicate no anomalous behavior in the 47% sample on molecular length scales. HRTEM indicates the presence of a small fraction of crystallites which is supported by the SAXS results, but it is unknown if this crystallinity is related to low surface area. Low-field ¹H NMR spin-lattice relaxation measurements show that the physical structure of all of the 'wet' gels is similar implying that pore closure occurs during drying. Consistent with this idea, gel surface area and density increased significantly when the pore fluid (water) was replaced with a lower surface tension fluid (formamide, dioxane, ethanol. water/surfactant).     

Evaluation of Strut Strength in Open-Cell Ceramics

The strength of cellular materials is dependent on the strength of the solid making up the cellular structure, but this parameter is often difficult to quantify. In order to better evaluate these materials, a technique has been developed to measure the strength ofthe cell struts in open-cell ceramics. The strut strength was measured in two commercially available, open-cell ceramics and evaluated in terms of a two parameter Weibull distribution. The strut strength distribution was found to be very wide with the Weibull modulusin the range 1 to 3. In addition, it was found that the strut strength was invariant withdensity (at constant cell size) but could be dependent on cell size. This behavior was inqualitative agreement with the failure statistics of brittle materials, once the variations in the microstructural geometry are understood. Based on these data and the observations of flaws within the struts, it is clear that careful processing of these materials could significantly improve the strut strength distribution, in terms of reducing its width and increasing its magnitude. Such microscopic improvements would lead to similar benefits in the strength and toughness of the bulk cellular ceramics.     

Phosphorus removal by granular activated alumina

Phosphorus removal from wastewater may be carried out by fixed-bed adsorption using activated alumina. In order to prevent unacceptable head-losses coarse-grained alumina must be used. Such systems have been referred to by several authors in literature. The mass transport characteristics of the system has so far, however, not been given a thorough investigation. This study uses the homogeneous surface diffusion model (HSDM) to describe the process as influenced by the system parameters.A sensitivity analysis is presented to optimize the process design for given conditions. The pH, the alumina particle size and the column length are found to be very important parameters determining the column performance. The process is very well suited for designing a beds-in-series system.     

Fabrication of evacuated glazing at low temperature

An indium-based seal augmented with an adhesive, developed to maintain a vacuum between two sheets of glass, avoids the high temperatures required to produce a seal in evacuated glazings to date. An experimentally-validated three-dimensional transient model has been used to predict heat transfer for an indium/adhesive sealed 1 m² area evacuated window with a highly insulating frame. An overall heat loss coefficient of 0.9 W m⁻² K⁻¹, with a midplane value of 0.36 W m⁻² K⁻¹, can be achieved with 0.72 visible transmittance for two 6 mm panes separated by 0.2 mm diameter pillars 40 mm apart. The conduction through a 3 mm edge-seal was 1.14 W m⁻² K⁻¹. Detailed three-dimensional isothermal contour plots through the system are presented.     

Al2O3 deposited by the oxidation of trimethylaluminum as gate insulators in hydrogen sensors

It is known that stable hydrogen sensors can be made with palladium-aluminum oxidesilicon dioxide-silicon (PdMAOS) structures. An improved way of making thin aluminum oxide films on silicon dioxide is presented. It is based on the oxidation of trimethylaluminum in an argon/oxygen atmosphere at 300–400° C. The optical and electrical properties of Al₂O₃ films made with this metal-organic chemical vapor deposition (MOCVD) method are presented. Finally the properties of completed PdMAOS hydrogen sensors are briefly presented.     

二氧化锆热障涂层的性能与应用

近年来,等离子喷涂二氧化锆热障涂层在航空及工业用燃气轮机上的应用已有很大进展,在一定限度内已经用于燃气轮机的涡轮部分。由于这种涂层可以降低气冷高温部件的温度50～200℃,因此可以显著地改善高温部件的耐久性,或者容许提高燃气温度或减少冷却气体的需用量而保持高温部件目前所承受的温度不变,从而提高发动机的效率。许多研究工作正在继续进行,以期进一步认识、改进及标定涂层的性能。本文将评述这方面的研究工作,简述二氧化锆热障涂层的性能与应用。     

提高溶出机组运转率的生产实践

介绍了拜尔法氧化铝生产中提高溶出机组运转率的生产实践 ,阐述了工艺、设备方面的改进、创新 ,以及科学组织生产 ,强化管理责任 ,提高机组运转率的成功经验。