Influence of Ge and GeO2 on the microstructure and varistor properties of TiO2–Ta2O5–CaCO3 ceramics

This study investigated the effect of elemental crystal Ge or/and GeO₂ doping on the microstructure and varistor properties of TiO₂–Ta₂O₅–CaCO₃ varistor ceramics, which were prepared via the traditional ball milling–molding–sintering process. X-ray diffraction, scanning electron microscopy, scanning transmission electron microscopy-energy dispersive X-ray spectroscopy, scanning electron microscopy-energy dispersive X-ray spectroscopy, and X-ray photoelectron spectroscopy demonstrated that co-doping with Ge and GeO₂ changed the microstructure of TiO₂–Ta₂O₅–CaCO₃ ceramics, thereby increasing the nonlinear coefficient and decreasing the breakdown voltage. The optimum doping concentrations of Ta₂O₅, CaCO₃, Ge, and GeO₂ exhibited the highest nonlinear coefficient (α=14.6), a lower breakdown voltage (E_B=18.7 V mm⁻¹), the least leakage current (J_L=10.5 μA cm⁻²), and the highest grain boundary barrier (Φ_B=1.05 eV). In addition, Ge and GeO₂ function as sintering aids, which reduce the sintering temperature because of their low melting points.     

铬刚玉陶瓷的制备及性能研究

本文研究在刚玉陶瓷中引入Ｃｒ２Ｏ３添加剂对其结构与性能的影响,并分析其机理。     

The controllable microstructure of porous Al2O3 ceramics prepared via a novel freeze casting route

In traditional aqueous slurry freezing casting processing, the growth method of ice crystals is hard to control, resulting in the uncontrollable pore's morphologies of the porous ceramics. In the experimental, the pure Al₂O₃ sol was used to substitute water as a medium for preparing ceramic slurry. With Al₂O₃ sol addition, it becomes easy to control the microstructure and pore's morphologies of the porous Al₂O₃ ceramics via adjusting of the solid loading, composition of the ceramic slurries, as well as the cooling methods. The SEM micrographs showed that the sol-contained ceramic slurry combined with freeze casting processing can easily prepare the porous Al₂O₃ ceramics with different pore sizes and different morphologies. The porous Al₂O₃ ceramics prepared from 70 wt.% to 90 wt.% solid loading sol-contained Al₂O₃ slurries and sintered at 1500 °C for 2 h have open porosities from 81.7% to 64.6%.     

Sol–gel deposition of multiply doped thermographic phosphor coatings Al2O3:(Cr,M) (M = Dy,Tm) for wide range surface temperature measurement application

A promising method of measuring surface temperatures in harsh environments is the use of thermographic phosphor coatings. There, the surface temperature is evaluated from the phosphorescence decay lifetime following a pulsed laser or flash lamp light excitation. Depending on the used dopant, single doped M³⁺:α-Al₂O₃ (M = Cr, Dy, Tm) emit at 694 nm (Cr³⁺), 488 nm (Dy³⁺), 584 nm (Dy³⁺), and 459 nm (Tm³⁺), respectively. However, the accessible temperature range with a single dopant is limited: for the Cr³⁺-transition from 293 K up to 900 K, and for the Dy³⁺ and Tm³⁺-transitions both from 1073 K up to 1473 K. In the present study a new approach is followed to extend these limitations by co-doping two dopants using the sol–gel method and dip coating of α-Al₂O₃ thin films. For that application (Dy³⁺ + Cr³⁺) co-doped thin α-Al₂O₃ films and (Tm³⁺ + Cr³⁺) co-doped α-Al₂O₃ films with thicknesses of 4–6 μm were prepared, and the temperature-dependent luminescence properties (emission spectra and lifetimes) were analysed after pulsed laser excitation in the UV (355 nm). The phosphorescence lifetime as a function of temperature were measured between 293 K and 1473 K. A considerably extended range for surface temperature evaluation was established following this new approach by combining different dopants on the molecular level.     

Properties of alumina 10 vol% zirconia composites—The role of zirconia starting powders

Zirconia toughened alumina (ZTA) materials are applied for cutting tools, wear parts and in biomedical applications. Due to the constraint of the rigid alumina matrix, ZTA materials with up to 10 vol% zirconia addition (AZ10) do not require addition of stabilizer oxides. AZ10 materials based on submicron sized alumina and four different submicron to nanoscale zirconia powders were manufactured by hot pressing at temperatures between 1475?1600 °C. Results show that the powder choice has a strong influence on mechanical properties, evolution of microstructure and phase composition. Best results with strength up to 850 MPa, fracture toughness values of 8.5 MPa√m and invulnerability to overfiring were obtained with zirconia powders showing the coarsest yet most homogeneous primary particle size and a low degree of agglomeration. Ultrafine but hard agglomerated powders lead to materials with extremely inhomogeneous microstructure and inferior properties.     

锆刚玉莫来石－碳化硅复合材料的显微结构

用热压的方法制备了Ａｌ２Ｏ３／莫来石（３／１）＋１５％ＡｒＯ２十１０％ＳｉＣ四元复合材料。采用ＡＥＭ和ＡＥＭ－ＥＤＳ等研究了材料的显微结构以及品界两侧金属元素的分布，确定了本材料晶界直接结合的三种形式。     

Creep mechanisms and microstructure evolution of Nextel™ 610 fiber in air and steam

Creep rates of Nextel™ 610 alumina fibers were measured at 1100 °C and 100–500 MPa in air and steam. Steam increased creep rates and reduced fiber lifetimes. Fiber microstructures were characterized by TEM. The small amounts of grain growth, fiber-axis grain elongation, and pore growth that occur during creep were quantified. To separate the effects of stress and temperature on microstructural evolution, grain growth and elongation were also quantified for fibers heat-treated for 1–100 h in air at 1100–1500 °C. Grain growth laws were determined. The contributions of pore growth and grain elongation to creep strain were quantified. Grain elongation accounts for a large fraction of the strain during creep in air, but little in steam. Pore growth was more pronounced in steam, but does not create significant creep strain. Creep and failure mechanisms consistent with the observed microstructural changes are discussed.     

The formation of core-sheath structure and its effects on thermal decomposition and crystallization of alumina fibers

Thermal decomposition is a critical process for fabricating alumina fibers using sol-gel methods. In this work, effects of preheating time on microstructure of prepared alumina fibers were studied. Results showed that core-sheath structure formed in the fibers when holding time exceeded 2 h at 400 °C. The fibers having core-sheath structure after thermal decomposition showed more residues, smaller crystal grain size and lower density. Thermal decomposition and densification of fibers were investigated. Results showed that core-sheath structure formed because fiber surface and core underwent different decomposition reactions due to the reduction in the number of gas channels on the fiber surface. This work provides a comprehensive study of thermal decomposition of precursor fibers and provides guidelines for developing appropriate calcination schedules.     

Irradiation damage in xenon-irradiated α-Al2O3 before and after annealing

The irradiation damage build-up of α-Al₂O₃ under Xe²⁰⁺ ion irradiation has been investigated by a combination of Raman spectroscopy and transmission electron microscopy. α-Al₂O₃ crystalline was irradiated with 5 MeV Xe²⁰⁺ ions to fluences of 1 × 10¹⁴ cm⁻², 5 × 10¹⁴ cm⁻², 1 × 10¹⁵ cm⁻² and 5 × 10¹⁵ cm⁻² at room temperature. No amorphous phase was formed under the experimental condition. The Raman intensities of feature peaks of Al₂O₃ decrease after Xe ion irradiation. The interstitial-type dislocation loops with Burgers vectors of b = 1/3 < 10-11> on the {10-10} and (0001) habit planes were found. The formation of basal and prism dislocation loops is related to the lattice damage and position. After annealing, the Raman intensities of feature peaks of Al₂O₃ increases with annealing temperature. With annealing at 1500℃ for 30 min, lattice defects were completely annealed out in the near surface region. Meanwhile, long dislocations and facet cavities on long dislocations were found in the Xe deposition region. Some lattice defects beyond the projected region were found due to the diffusion toward deep region during thermal annealing.     

工艺参数对氧化铝陶瓷烧结性能和显微结构的影响

讨论了工艺参数对氧化铝陶瓷致密化行为和显微结构的影响。通过控制工艺参数,如粉料的粒径和粒度分布、生坯密度、温度机制等可以改善烧结性能、改变显微结构。     

Preparation and characterisation of alumina-based composite support layers

Porous ceramic membranes are of special interest owing to their outstanding thermal and chemical stability. However, porous ceramic membranes with permeability usually suffer from low mechanical strength. Therefore, there have been a number of studies of the optimisation of membrane mechanical strength and permeability. In this paper, to avoid a trade-off between mechanical strength and permeability, we attempt to enhance these parameters by incorporating diatomite as both a pore former and a bonding phase. Because the flexural strength and air permeability of alumina support layers cannot be enhanced simultaneously by just changing the sintering temperature, we investigate whether they can be controlled by changing the amount of added diatomite. We study the effectiveness of diatomite as both a pore former and a bonding phase through a comparison of alumina–diatomite and the alumina–pyrophyllite composite support layers.     

Fracture behaviour,microstructure, and performance of various layered-structured Al2O3-TiC-WC-Co composites

In this study, layered-structured Al₂O₃-based composites containing WC-Co, TiC, and MgO additives were prepared using hot-pressing sintering. The best comprehensive mechanical characteristics were acquired for the sample with a layer number (N_LN) of 7 and thickness ratio (η_TR) of 6. Its composite exhibited a fracture toughness of 8.5 and 8.4 MPa m^1/2 in the X and Z directions, respectively. Analysis of the micro characteristics of the fracture surfaces of the Al₂O₃-TiC-WC-Co layered composites revealed a significant enhancement in the bending strength, which could be attributed to the mixed fracture modes in the composite, including intergranular and trans-granular modes. As the displacement increased, first, the bending stress of all the composites increased gradually, after which all the samples showed abrupt elevation in stress. The enhancement in the damage resistance of Al₂O₃-TiC-WC-Co layered composites could be attributed to the microscopic and macroscopic crack deflection, bridging, and partial surface bonding that occurred in the layers. Finally, a new theoretical perspective was employed to discuss the mechanism of the effect of the layered structure on the toughness of the composites.     

Effect of carbon contamination on the sintering of alumina ceramics

The present work aimed with the carbon contamination in alumina ceramics and its influence on sinterability of alumina in low vacuum and atmospheres of argon and nitrogen. The commercially available alumina was coated with carbon and sintered at different atmospheres to investigate the effect of carbon presence on alumina sintering behaviour. The sintering conditions were: heating/cooling rates 5 °C/min and 1.7 °C/min until the maximum temperature of 1400 °C and a dwell time of 2 h. The microstructure of the samples was investigated from fracture and surface, prior to polishing, chemical or thermal etching. The non-densified (porous) surface layer was found in the samples sintered in nitrogen and vacuum, however, sintering in argon atmosphere showed a negligible effect on the surface. The core of investigated specimens exposes a transgranular/intergranular fracture mode and is dense in all cases. In the case of vacuum sintering, the strong carbon diffusivity was also noticeable by the dark grey color of the samples. Interestingly, the formation of aluminium nitride took place during sintering of carbon coated alumina samples in a nitrogen atmosphere at 1400 °C. The thickness of the reactive porous layer was approximately 15 μm beneath the surface. Such a porous layer is inappropriate to the desired features of final ceramic products. Presented results lead to better understanding of the sintering behaviour of ceramic and to suitable selecting of the set-up by densification conditions.     

TiN/Al_2O_3 Composite Material Synthesized by Aluminothermic Reduction Process in Coke Bed

TiN/Al_2O_3 composite material was prepared by aluminothermic reduction of TiO2 in coke bed. The phase composition, lattice parameter and microstructure of products treated at 1 300 ℃, 1 400 ℃, 1 500 ℃ for 3 h respectively were analyzed by XRD, SEM and TEM. The results showed that TiN/Al_2O_3 composite material can be prepared in coke bed via aluminothermic reduction method. However, the treatment temperatures affect the evolution of aluminothermic reaction and morphology of the materials obviously. The con...     

真空电子器件用Al2O3陶瓷

本文综合了Al2O3陶瓷在真空电子器件中的应用背景、沿革和前景,根据应用的不同,提出了金属化用陶瓷和非金属化用陶瓷两种陶瓷的分类方法,两者显微结构迥然不同,前者是粗晶结构（D平=10～20μm）,而后者则是微晶结构（D平=2～3μm）。     

Microstructure,mechanical properties and machining performance of spark plasma sintered Al2O3–ZrO2–TiCN nanocomposites

The effect of addition of nanocrystalline ZrO₂ and TiCN to ultrafine Al₂O₃ on mechanical properties and microstructure of the composites developed by spark plasma sintering (SPS) was investigated. The distribution of the nanoparticles was dependent on their overall concentration. Maximum hardness (21 GPa) and indentation toughness (5.5 MPa m^1/2) was obtained with 23 vol% nanoparticles, which was considered as the optimum composition. The Zener pinning criteria were also satisfied at this composition with grain size of the restraining nanoparticles ～63–65 nm. Hardness of the composites follows the rule of mixtures; crack deflection and crack arrest by nanoparticles at grain boundaries along with mixed fracture mode led to high toughness in the nanocomposites. Cutting tool inserts were developed by SPS with the optimized composition and their machining performance was compared with commercial alumina based inserts. Increased toughness in the nanocomposite inserts reflects in the machining performance as the tool life improves drastically compared to that of the commercial inserts at high cutting speeds ≥500 m min^?1. This was attributed to differences in their failure modes; the commercial inserts fail catastrophically by fracture due to their low toughness whereas the nanocomposite inserts reach the tool failure criteria by crater wear at all machining conditions.