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Si3N4 whisker (Si3N4w) reinforced composites were prepared by a near-net shaping process, i.e., gel-casting of the Si3N4w preform followed by polymer infiltration and pyrolysis (PIP) densification using polysilazane as precursor. The densification process by PIP was described mathematically, after which several key parameters affecting densification efficiency were discussed. The small pore size (0.04 ~ 1 μm) of Si3N4w preform can cause filtration effect (low permeability of precursor with a molecular size bigger than pore size), which resulted in the density gradient of the composites. Porosity (P) dependence of flexural strength and elastic modulus of Si3N4w/Si3N4 followed a power law of (1 – P). With increasing density, the response of Si3N4w when confronting cracks transformed from whisker debonding to whisker fracture, which was supposed to be due to the increase of whisker/matrix interface strength. The Si3N4w/Si3N4 developed by us achieved a good balance between high strength and low dielectric constant, making it promising for high-temperature wave-transparent application.  相似文献   
3.
To improve the ablation resistance of C/C composites, a Hf-Ta-Si-C coating was fabricated by chemical vapor co-deposition. The ablation resistances of Hf-Si-C and Hf-Ta-Si-C coatings were characterized comparatively. After introducing Ta, the ablation resistance, smoothness and integrity of the ablated coating improved. Hf-Si-O and Hf-Ta-Si-O glass wires were observed. To investigate the reaction processes of the two types of glass wires, TEM observation was carried out and coupled to molecular dynamics simulations. These illustrated that Hf and Ta atoms tend gather together at high temperature. To investigate the effect of Hf-Ta-O precipitation on the ablation behavior, supplementary experiments were carried out on bulk ceramics with the same target composition, confirming that precipitation reaction could accelerated the formation of a smooth and intact oxide layer, playing a positive role in protecting C/C composites.  相似文献   
4.
The sensitivity analysis functions on globularized fraction of α lamellae were established using a physically-based microstructure model and gradient method. These functions were applied to the sensitivity analysis on globularized fraction of α lamellae in TC17 alloy. The material constants in these functions are determined using the genetic algorithm-based objective optimization technique. The globularized fraction of α lamellae during isothermal compression of TC17 alloy was quantitatively analyzed based on scanning electron microscopy (SEM) observation. The results show that α lamellae mostly change to equiaxed α grains at a deformation temperature of 1083 K, a strain rate of 0.01 s?1 and a strain of 1.2. The globularized fraction decreases with increasing strain rate because lower strain rate provides enough time for the spheroidization. The effect of deformation temperature on the globularized fraction is controlled by the strain rate. And, the predicted derivations of globularized fraction with respect to processing parameters show good agreement with the experimental values.  相似文献   
5.
移动位置服务中一种空间智能发布系统模型   总被引:1,自引:0,他引:1  
与现有的位置服务模式相比,基于Push(推)模式的位置服务有着更加广阔的市场前景,是目前国内外研究的焦点.该模式的位置服务依赖于现存的空间发布系统,而目前空间发布系统由于条件的限制不能有效地完成空间事件的预订和发布.阐述了目前空间发布系统中两种常见的空间事件以及空间事件的预订,随后提出一种空间智能发布系统模型,有效地解决了无线通信的带宽限制问题并提高了空间发布系统的性能.  相似文献   
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《Composites》1984,15(4):259-265
The mechanical properties of woven carbon fibre-reinforced epoxy laminates were compared with those of non-woven laminates with equivalent (0,90), (±45) and (0,90 ± 45) lay-ups, (0,±45) laminates with only the ±45° layers replaced by woven layers were also studied. Static strengths and stiffnesses were measured and toughness was assessed by means of notched tensile tests and residual strength tests after low energy impact. The results of the tests suggest that certain forms of woven material should be seriously considered for high performance applications. The implications of this are discussed in detail.  相似文献   
8.
《Solar Energy Materials》1984,1(1-2):75-83
The dark electrochemical behavior of polycrystalline p-WSe2 electrodes is investigated. The dominant role of tungsten bonds for both the equilibrium behavior of the electrode and the reactions that take place at different electrode potentials (hydrogen tungsten bronze formation and oxidation, hydrogen evolution reaction, etc.) is analyzed.  相似文献   
9.
《Acta Metallurgica》1984,32(3):423-434
A quantitative study of cavitation damage and fracture of a superplastic copper alloy, Coronze 638, has been made. Cavities are found to nucleate at large particles present in the form of stringers. The size and shape of cavities, as well as the level of damage up to fracture are essentially independent of strain rate over regions I and II of the σ−ϵ curve, as are the true strain to fracture and the development of t instabilities. As the strain rate increases into region III, the level of damage to failure decreases, while the true failure strain increases and necks become sharper. Extensive cavity coalescence is observed up to strains of about 1.5, producing a number of large (> 100 μm) cavities which exhibit a high stability, and little tendency to coalescence. This allows the sample to sustain a very high level of cavitation without failure. The mechanism of cavity growth for small isolated cavities (< 10 μm) is thought to be diffusive growth constrained by matrix creep at low strain rates, with a transition to plasticity controlled growth at large strain rates. For larger cavities growth appears to be entirely creep controlled. Final fracture occurs by the material exhaustion in the ligaments between voids once the reduction in the cross section exceeds about 30%. No large instability either in flow or damage seems to be involved in this process.  相似文献   
10.
为提高半监督分类的性能,提出一种基于SOM神经网络的半监督分类算法SSC-SOM。结合SOM的聚类特性,基于先聚类后标记的思想,充分利用有标记样本和未标记样本训练SOM分类器;将聚类的形成和有标记样本分配到各个聚类中同时进行,并根据有标记样本计算各个聚类的聚类中心;在整个未标记样本的范围内,根据聚类中心,使用K近邻算法对未标记样本进行标记,挖掘未标记样本的隐含信息。在UCI数据集中进行分类实验,其结果表明,SSC-SOM的分类率比SSOM提高2.22%,且收敛性较好。  相似文献   
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