Fine-Grained Silicon Carbide Ceramics with Oxynitride Glass

By using an oxynitride glass composition from the Y-Mg-Si-Al-O-N system as a sintering additive, the effect of atmosphere on densification was investigated during the liquid-phase sintering of SiC, and the resulting microstructure and mechanical properties of the sintered and subsequently annealed materials were investigated. SiC ceramics that were densified with 10 wt% oxynitride glass showed higher sinterability in a nitrogen atmosphere. Oxynitride glass enlarged the stability region of β-SiC and suppressed β→ alpha phase transformation, which resulted in an equiaxed microstructure. Grain growth of fine-grained SiC in some extent (up to ∼300 nm) was beneficial in improving both room-temperature strength and toughness. The best results were obtained when the ceramics were hot-pressed at 1800°C for 1 h in a nitrogen atmosphere and subsequently annealed at 1900°C for 3 h in an argon atmosphere. The room-temperature flexural strength and fracture toughness of the material were 847 MPa and 3.5 MPa·m^1/2, respectively.     

Microstructural Analysis of Liquid-Phase-Sintered β-Silicon Carbide

The microstructures of fine-grained β-SiC materials with α-SiC seeds annealed either with or without uniaxial pressure at 1900°C for 4 h in an argon atmosphere were investigated using analytical electron microscopy and high-resolution electron microscopy (HREM). An applied annealing pressure can greatly retard phase transformation and grain growth. The material annealed with pressure consisted of fine grains with β-SiC as a major phase. In contrast, the microstructure in the material annealed without pressure consisted of elongated grains with half α-SiC. Energy-dispersive X-ray analysis showed no differences in the amount of segregation of aluminum and oxygen atoms at grain boundaries, but did show a significant difference in the segregation of yttrium atoms at grain boundaries along SiC grains for the two materials. The increased segregation of yttrium ions at grain boundaries caused by the applied pressure might be the reason for the retarded phase transformation and grain growth. HREM showed a thin secondary phase of 1 nm at the grain boundary interface for both materials. The development of a composite grain consisting of a mixture of β/α polytypes during annealing was a feature common to both materials. The possible mechanisms for grain growth and phase transformation are discussed.     

Phase Transformation and Texture in Hot-Forged or Annealed Liquid-Phase-Sintered Silicon Carbide Ceramics

Starting from three powder mixtures of 80 vol% SiC (100α, 50α/50β, 100β) and 20 vol% YAG, liquid-phase-sintered silicon carbide ceramics were prepared by hot pressing at 1800°C for 1 h under 25 MPa, and then by hot forging or annealing at 1900°C for 4 h under an applied stress of 25 MPa in argon. The phase transformation and texture development in the as-hot-pressed, hot-forged, and annealed SiC ceramics were investigated via X-ray diffraction (XRD) and the pole figure measurements. The 6H → 4H polytypic transformation was observed in samples consisting of both α- and β-SiC phases when subjected to compressive deformation but absent in the case of annealing, suggesting the deformation-enhanced solubility of aluminum in SiC. Deformation was also found to enhance the 3C → 4H transformation in the sample containing entirely β-phase, which is due to the accelerated solution-precipitation process assisted by grain boundary sliding. The current study showed that the β- →α-phase transformation had little effect on texture development in SiC. Hot forging generally produced the strongest texture, with the calculated maximum of 2.2 times random in samples started with pure α-SiC phase. The mechanism for texture development was explained based on the microstructural observations.     

Rate of emulsion polymerization of styrene

In emulsion polymerization, the Smith and Ewart theory gives about two or three times the number of polymer particles obtained by experiment. In this paper, a reaction model is proposed which, from the standpoint of reactor design, can give an adequate explanation of the whole course of an emulsion polymerization of monomer highly insoluble in water. Among other things, the generating process of polymer particles is examined in detail. It is demonstrated experimentally that a new parameter proposed here, which represents the degree of difficulty of monomer initiation in micelles, is indispensable in explaining that process. Also confirmed is that monomer initiation takes place more easily in polymer particles than in micelles. According to the new model, the progress of polymerization, i.e., monomer conversion, the number of the polymer particles, and properties of polymer thus produced can be estimated with satisfactory accuracy. Furthermore, approximate equations are derived for easier estimation.     

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基于目标模型的遗传算法在走廊辨识中的应用

为了保证移动机器人能够沿着走廊自动行走并具有一定的智能水平，工作环境的辨识就显得至关重要．同时考虑到控制的实时性，提出了一种基于目标模型的模型匹配的走廊辨识方法．利用遗传算法对未处理的灰度图像中的走廊影像进行匹配，从而使移动机器人自动地适应复杂环境，实现了模型对灯光条件变化以及包括干扰物等环境噪声的鲁棒性，并经实际走廊图像试验验证了辨识效果．     

Microstructure and press formability of a cross-rolled magnesium alloy sheet

Cross-rolling, in which the roll axis is tilted by 7.5° towards the TD-direction, was carried out on a commercial magnesium alloy. The (0002) texture intensity of the cross-rolled specimen was lower than that of the unidirectionally rolled specimen, and the (0002) texture of the cross-rolled specimen was inclined about 10° towards the TD-direction. Also, the grain size of the cross-rolled specimen was smaller than that of the unidirectionally rolled specimen. As a result of the Erichsen tests at 433-493 K, the press formability of the cross-rolled specimen was higher than that of the unidirectionally rolled specimen. The high formability of the cross-rolled specimen is attributed to both the modification of (0002) texture and the enhancement of grain refinement by the cross-rolling.     

Pulsed Laser Processing of Carbon and Silicon Clusters and Thin Films

Abstract

Several new processes have been developed for the preparation of fullerenes and thin films by using a pulsed excimer laser. The irradiation of a pulsed KrF excimer laser beam onto a C₆₀ powder target produced single phase C₆₀ thin films when the laser energy fluence was in the range between 40 and 50mJ/cm². By atomic force microscopy, the laser-deposited C₆₀ thin film was verified to have a surface far smoother that the surfaces of films produced by the conventional evaporation method. The stainless steel rods coated with this film exhibited an excellent tribological property. Cluster formation from SiC and other carbides MC_n(M=Ti,W,B) was investigated by laser desorption time-of-flight mass spectrometry. No clear indication was observed for the production of such clusters as (SiC)₆₀ and (M_xC_60-x) from the sintered targets directly as well as from the films laser deposited from the targets. However, C₆₀ and C₇₀ were found to exists in the laser-deposited films, indicating a new applicability of pulsed laser processing for segregative cluster synthesis from solid solution. Preliminiary results on thin film deposition via pulsed ablation of (Ba,Na)_xSi₄₆ clathrate were also presented.     

Photoluminescence of Rare-Earth-Doped Ca-α-SiAlON Phosphors: Composition and Concentration Dependence

Rare-earth-doped Ca-α-SiAlON phosphors, with the compositions of (Ca_{1−3/2 x} RE _x ) _{m /2}Si_{12− m − n} Al _m+n O _n N_{16− n} (RE=Ce, Sm, and Dy, 0.5≤ m =2 n ≤3.0), were prepared by sintering at 1700°C for 2 h under 10 atm N₂. The concentration of rare earths varied from 3 to 30 at.% with respect to Ca. The photoluminescence (PL) properties were investigated as functions of the composition of the host matrix (i.e., m ) and the concentration of rare earths (i.e., x ). The results show that the emission properties can be optimized by tailoring m and x . The Ce³⁺ luminescence originating from the 4 f –5 d interconfigurational transitions is greatly affected by the environment surrounding the Ce³⁺ ions, which differs from the Sm³⁺ or Dy³⁺ luminescence arising from the 4 f –4 f intraconfigurational transitions. X-ray diffraction and scanning electron microscopy were used to explain the composition and concentration dependence of PL properties.