Numerical Determination of Damping in Metal Matrix Composites

Damping in metal matrix composites is mainly caused by inelastic matrix deformation induced by the great difference in the mechanical properties of the single constituents of the materials. In this study, the finite-element method in combination with a highly accurate material model is employed to examine the effects of both the fiber volume fraction and the external loading amplitude on the energy dissipation process in an Al/SiC composite under a cyclic mechanical load.     

ATLEED研究6H-SiC(0001)(3~(0.5)×3~(0.5))R30°重构表面

低能电子衍射 (LEED)对 6H SiC(0 0 0 1) (3× 3)R30°表面的研究结果表明 ,该表面有 1/3单层的Si原子吸附在T4 空位上与第一个SiC复合层中的三个Si原子键接 ,它们之间的垂直距离为 0 171nm .通过对该表面 10个非等价垂直入射衍射束的自动张量低能电子衍射 (ATLEED)计算 ,得到“最佳结构”由于表面SiC复合层堆积顺序不同而产生的三种表面终止状态 (surfacetermination)的混合比例为S1∶S2∶S3 =15∶15∶70 ,理论计算与实验I V曲线比较得到可靠性因子RVHT=0 .16 5 ,RP=0 .142 ,表明表面生长符合能量最小化的台阶生长机制     

低压Ar气氛下用PS/OCS/Si (111)叠层热解制备单晶4H-SiC薄膜及层错缺陷抑制机理

在一定压力的Ar气氛中对Si (111) 衬底上的PS/OCS(硅的有机化合物)凝胶叠层进行热处理,制备出单晶4H-SiC薄膜.用XPS、XRD、TEM、TED和SEM研究了热处理温度和压力对薄膜结晶质量和晶型的影响.XPS分析显示薄膜中C/Si比为1.09.SEM分析表明薄膜的表面平整,SiC/Si(111)界面清晰、无层错缺陷形成.进一步讨论了层错缺陷形成及抑制的机理.     

Structures and Properties of Cast Irons Reinforced by Trace Addition of Modified SiC Nanopowders

"To improve the performance of traditional cast iron, trance amount of surface modified nanometer SiC powders were added into the melted iron. The microstructures, the mechanical properties, as well as the wear resistance were investigated. The trace addition of SiC nano-powders were active due to the presence of structural defects arising from the treatment, they were eoient in affecting not only the generation and growth of crystals but also change the morphology of graphite. On the other hand, the addition of SiC nanopowders as heterogeneous seedings in the crystallization of liquid metals lead to the changing of supercooling temperature, so the ratio of ferrite and pearlite was changed. The mechanical characteristics and wear resistance were enhanced as a result of the improved graphite shape and changed matrix composition caused by the trace addition of SiC nanopowders (in amounts of about 0.01% mass). The strengthening mechanism and the free gap between powders were also discussed. It is suggested that the tensile strength, toughness, as well as the wear resistance can be improved simultaneously, which indicates the novel strengthening technology by trace addition of nanopowders is promising to extend to large-scale industrial production."     

SiC based Si/SiC heterojunction and its rectifying characteristics

The Si on SiC heterojunction is still poorly understood, although it has a number of potential applications in electronic and optoelectronic devices, for example, light-activated SiC power switches where Si may play the role of an light absorbing layer. This paper reports on Si films heteroepitaxially grown on the Si face of (0001) n-type 6H-SiC substrates and the use of B₂H_6 as a dopant for p-Si grown at temperatures in a range of 700--950~\du. X-ray diffraction (XRD) analysis and transmission electron microscopy (TEM) tests have demonstrated that the samples prepared at the temperatures ranged from 850~℃ to 900~℃ are characterized as monocrystalline silicon. The rocking XRD curves show a well symmetry with FWHM of 0.4339° Omega. Twin crystals and stacking faults observed in the epitaxial layers might be responsible for widening of the rocking curves. Dependence of the crystal structure and surface topography on growth temperature is discussed based on the experimental results. The energy band structure and rectifying characteristics of the Si/SiC heterojunctions are also preliminarily tested.     

Homoepitaxial growth of 6H–SiC thin films by metal-organic chemical vapor deposition using bis-trimethylsilylmethane precursor

Homoepitaxial silicon carbide (SiC) films were grown on 3.5° off-oriented (0 0 0 1) 6H–SiC by metal-organic chemical vapor deposition (MOCVD) using bis-trimethylsilylmethane (BTMSM, C₇H₂₀Si₂). A pronounced effect of the growth conditions such as source flow rate and growth temperature on the polytype formation and structural imperfection of the epilayer was observed. The growth behavior was explained by a step controlled epitaxy model. It was demonstrated by high-resolution X-ray diffractometry and transmission electron microscopy that high-quality 6H–SiC thin films were successfully grown at the optimized growth condition of substrate temperature 1440°C with the carrier gas flow rate of 10 sccm.     

Thermal redistribution of indium in amorphous silicon layers

A new impurity redistribution mechanism is reported for low temperature annealing (525°C) of (100) Si samples implanted with high indium doses. The redistribution is a strong function of implant dose and is believed to be stress related.     

Investigation of the electrical properties of XLPE/SiC nanocomposites

The interface between nanoparticles and the polymer matrix, which dominates the electrical properties of nanocomposites, can effectively improve the DC breakdown and suppress space charge accumulation in nanocomposites. To research the interface characteristics, XLPE/SiC nanocomposites with concentrations of 1 wt%, 3 wt% and 5 wt% were prepared. The DC breakdown, dielectric properties and space charge behavior were examined using pulsed electro-acoustic (PEA) equipment and a dielectric analyzer. The test results show that the nanocomposites with concentrations of 1 wt% and 3 wt% have higher DC breakdown field strength than neat XLPE. In contrast, there is a lower DC breakdown strength at a concentration of 5 wt%, possibly due to the agglomeration of nanoparticles. Nanoparticle doping increases the real and imaginary permittivities over those of neat XLPE. Furthermore, with increasing concentration, a larger increase in the permittivity amplitude was observed. Based on the space charge behavior, all nanocomposites could suppress space charge accumulation, but the nanocomposite with a concentration of 1 wt% exhibited the best effect. Meanwhile, heterocharge accumulation near electrodes was observed in neat XLPE and the nanocomposite with a concentration of 5 wt%. In contrast, homocharge accumulation near electrodes was observed in the nanocomposite with a concentration of 3 wt%. This phenomenon may be due to different amounts of shallow traps in nanocomposites with different concentrations, which might lead to differing electron or hole mobility.     

Photocatalytic reduction of carbon dioxide to methanol by Cu2O/SiC nanocrystallite under visible light irradiation

The Cu2O/SiC photocatalyst was obtained from SiC nanoparticles (NPs) modified by Cu2O. Their photocatalytic activities for reducing CO2 to CH3OH under visible light irradiation have been investigated. The results indicated that besides a small quantity of 6H-SiC, SiC NPs mainly consisted of 3C-SiC. The band gaps of SiC and Cu2O were estimated to be about 1.95 and 2.23 eV from UV-Vis spectra, respectively. The Cu2O modification can enhance the photocatalytic performance of SiC NPs, and the largest yields of methanol on SiC, Cu2O and Cu2O/SiC photocatalysts under visible light irradiation were 153, 104 and 191μmol/g, respectively.     

Temperature-dependence of acoustic fatigue life for thermal protection structures

Acoustic fatigue life evaluation is essential for thermal protection structures due to the severe thermo-acoustic load in service. A study on temperaturedependence of acoustic fatigue life for a C/SiC panel is presented in this paper. Effects of temperature on both the structural responses and the S-N curves are investigated. The Dirlik method is adopted to predict the fatigue life of a C/SiC panel at three different temperatures respectively. Significant differences are observed from the results of numerical simulations between the fatigue lives of the panel in the three cases. The temperature-dependence of acoustic fatigue life of a C/SiC panel is verified, and fatigue test of the material needs to be more attentively performed.