Preparation of ZrC/SiC porous self-supporting monoliths via sol-gel process using polyethylene glycol as phase separation inducer

We present a straightforward method via sol-gel process using polyethylene glycol (PEG) as phase separation inducer to prepare zirconium carbide/silicon carbide (ZrC/SiC) porous monoliths. Organic/inorganic hybrid gels are prepared using zirconium oxychloride, furfuryl alcohol, and tetraethyl orthosilicate as major starting materials. In the presence of PEG, crack-free hybrid monoliths are obtained by drying the wet gels under ambient pressure, whereas in the absence of PEG, the wet gels break into pieces as expected. PEG plays a key role in maintaining the macroscopic shape of the monoliths. After ceramization at 1300–1500?°C, ZrC/SiC porous monoliths are obtained. SEM and mercury intrusion porosimetry data show that PEG also has strong influence on the microstructures of the monoliths. The compressive strengths of the ceramic monoliths are in the range of 0.3 to 0.7?MPa. And their compressive behavior starts to differ due to the changes in their microstructures, especially the pore structure.     

Constructal Enhancement of Heat Conduction with Phase Change

For enhancing the heat energy release from a phase change material (PCM) the conductive fin is designed according to a proposed constructal rule, in which the high conductivity material (HCM) should be located at the place where the heat flux density is the largest. The local temperature gradient integration over time is taken as a criterion to determine where to distribute the limited HCM during a given time period. This rule is applicable to heat conduction of steady and unsteady conditions, as well as to the problems with and without phase change. Numerical simulations show that the constructal design of a conductive fin has much better performance than arbitrary ones. The constructal rule is an effective technique that designs the fin with high performance for enhancing heat conduction.Paper presented at the 7th Asian Thermophysical Properties Conference, August 23–28, 2004, Hefei and Huangshan, Anhui, P. R. China.     

红外成像技术对电气设备故障的诊断分析研究

分析了红外成像技术诊断电气设备故障之机理，故障的演变与发展、性能劣化过程；分别对‘电气设备外部及内部故障的各种类型、热像特征进行了论述。     

An efficient diagnosis system for detection of Parkinson’s disease using fuzzy k-nearest neighbor approach

In this paper, we present an effective and efficient diagnosis system using fuzzy k-nearest neighbor (FKNN) for Parkinson’s disease (PD) diagnosis. The proposed FKNN-based system is compared with the support vector machines (SVM) based approaches. In order to further improve the diagnosis accuracy for detection of PD, the principle component analysis was employed to construct the most discriminative new feature sets on which the optimal FKNN model was constructed. The effectiveness of the proposed system has been rigorously estimated on a PD data set in terms of classification accuracy, sensitivity, specificity and the area under the receiver operating characteristic (ROC) curve (AUC). Experimental results have demonstrated that the FKNN-based system greatly outperforms SVM-based approaches and other methods in the literature. The best classification accuracy (96.07%) obtained by the FKNN-based system using a 10-fold cross validation method can ensure a reliable diagnostic model for detection of PD. Promisingly, the proposed system might serve as a new candidate of powerful tools for diagnosing PD with excellent performance.     

Erosion resistance of needled carbon/carbon composites exposed to solid rocket motor plumes

The erosion resistance of the needled carbon/carbon composite leading edge was investigated by exposure to the solid rocket motor (SRM) plume. The erosion morphology and microstructure were examined and associated erosion mechanisms discussed. The composite leading edge was severely damaged with large recession and erosion rate after exposure. The results suggest that the erosion behavior was determined by the weak needling structure of the leading edge windward with small thickness in the severe environment of the plume impingement. Erosion mechanisms were combined effects of thermochemical ablation by oxidizing species and thermomechanical erosion by both aerodynamic force and alumina particles. The former weakens the strength of the composite while the latter contributes primarily to high erosion rate in the oxy-lean SRM plume environments.     

Optimal analysis of a space solar dynamic power system

The major purpose of the present study is the theoretical modeling, numerical simulation and optimal analysis of a space solar dynamic power system. Using the method of system analysis, a mathematical and physical model is developed to describe the process of energy transfer and conversion in a space solar dynamic power system. As a new assessing criterion for total launch mass, it is proposed to combine the system mass and aerodynamic drag area into a unified criterion. The effects of the configurations and operating parameters on the system performance are analyzed and the optimal scheme of a space solar dynamic power system is obtained by numerical simulation.     

Ablation of pierced C/C composite nozzles in an oxygen/ethanol combustion gas generator

The ablation of pierced C/C composite nozzles was investigated via hot-fire testing by an oxygen/ethanol combustion gas generator simulating combustion environments of the liquid rocket engine. Ablation microstructures and mechanisms of the composite nozzle were discussed. The results showed that both linear ablation rate and mass ablation rate increased with increasing mass ratio of oxygen/ethanol. The ablation of the pierced C/C composite was attributed to the heterogeneous reactions between carbon and oxidizing species and could be interpreted by the thermochemical ablation model. The ablation rate of the composite was determined by both chemical kinetics and diffusion. The ablation of the composite resulted in cone-shape carbon fibers and shell-shape matrix with pits and grooves left on the surfaces.     

First-principle high-throughput calculations of carrier effective masses of two-dimensional transition metal dichalcogenides

Two-dimensional group-VIB transition metal dichalcogenides (with the formula of MX2) emerge as a family of intensely investigated semiconductors that are promising for both electronic (because of their reasonable carrier mobility) and optoelectronic (because of their direct band gap at monolayer thickness) applications.Effective mass is a crucial physical quantity determining carriers transport,and thus the performance of these applications.Here we present based on first-principles high-throughput calculations a computational study of carrier effective masses of the two-dimensional MX2 materials.Both electron and hole effective masses of different MX2 (M =Mo,W and X =S,Se,Te),including in-layer/out-of-layer components,thickness dependence,and magnitude variation in heterostructures,are systemically calculated.The numerical results,chemical trends,and the insights gained provide useful guidance for understanding the key factors controlling carrier effective masses in the MX2 system and further engineering the mass values to improve device performance.     

基于CPN的OAuth协议建模与分析

在云计算环境下,网络安全协议的执行环境变得更为复杂,应用Web安全问题开放授权协议,可以提高信息共享的安全性. 本文采用CPN（Colored Petri Net）对OAuth协议进行建模,使用仿真工具CPNTools分析OAuth协议授权码模式的相关性质,并通过仿真实验表明授权码模式可以基于令牌进行验证与授权,防止针对授权码的CSRF注入攻击.     

3D C/SiC复合材料喷管在小型固体火箭发动机中的烧蚀规律研究

采用小型固体火箭发动机研究了3D C/SiC复合材料喷管的烧蚀性能,分析了3D C/SiC的烧蚀机理及燃气参数对烧蚀性能的影响.结果表明,喷管喉部线烧蚀率为0.128±40.088mm/s,质量烧蚀率为0.166kg/(m2.s);受喷管内燃气组分、温度、压强和流速等环境参数的影响,3D C/SiC的烧蚀涉及不同机理的非均匀烧蚀.喉部及其上下游过渡区域烧蚀最严重,收敛段其次,扩散段烧蚀最弱.烧蚀过程是热物理化学侵蚀和机械剥蚀综合作用的结果:涉及SiC的分解流失,SiC和碳纤维的氧化烧蚀,还涉及低速Al2O3大粒子的机械化学侵蚀,高速Al2O3小粒子的机械侵蚀等.