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111.
A method for growing graphene on a sapphire substrate by depositing an SiC buffer layer and then annealing at high temperature in solid source molecular beam epitaxy (SSMBE) equipment was presented. The structural and electronic properties of the samples were characterized by reflection high energy diffraction (RHEED), X-ray diffraction Φ scans, Raman spectroscopy, and near edge X-ray absorption fine structure (NEXAFS) spectroscopy. The results of the RHEED and Φ scan, as well as the Raman spectra, showed that an epitaxial hexagonal α-SiC layer was grown on the sapphire substrate. The results of the Raman and NEXAFS spectra revealed that the graphene films with the AB Bernal stacking structure were formed on the sapphire substrate after annealing. The layer number of the graphene was between four and five, and the thickness of the unreacted SiC layer was about 1--1.5 nm. 相似文献
112.
All metallic, hollow sandwich cylinders having ultralight two-dimensional (2D) prismatic cores are optimally designed for maximum thermo-mechanical performance at minimum mass. The heated cylinder is subjected to uniform internal pressure and actively cooled by forced air convection. The use of two different core topologies is exploited: square- and triangular-celled cores. The minimum mass design model is so defined that three failure modes are prevented: facesheet yielding, core member yielding, and core member buckling. The intersection-of-asymptotes method, in conjunction with the fin analogy model, is employed to build the optimization model for maximum heat transfer rate. A non-dimensional parameter is introduced to couple the two objectives—structural and thermal—in a single cost function. It is found that the geometry corresponding to maximum heat transfer rate is not unique, and square-celled core sandwich cylinders outperform those having triangular cells. The eight-layered sandwich cylinders with square cells have the best overall performance in comparison with other core topologies. Whilst a sandwich cylinder with shorter length is preferred for enhanced thermo-mechanical performance, the influence of the outer radius of the cylinder is rather weak. 相似文献
113.
114.
《Particuology》2023
The energy density of Li-ion batteries is closely related to the capacity and average voltage of cathode materials. Unfortunately, current cathode materials either have low capacity or voltage, which limits the development of high-energy-density Li-ion batteries. This has given challenge to many attempts to develop new cathode materials with high capacity and voltage. In this study, we find that Li easily inserts into the (111) plane of SiC in the presence of Si, and a well-organized Li-rich SiC twin crystal is formed. Ultraviolet–visible diffuse reflectance spectra and electrochemical test results suggest that this Li-rich SiC twin crystal possesses the band gap energy of 3.5 eV and charging capacity of 1979 mAh/g at the current density of 200 mA/g, making it a promising candidate for the cathode material in high-capacity Li-ion batteries. X-ray photoelectron spectroscopy and high-resolution transmission electron microscopy results reveal that Si-induced Li insertion contributes to the changes in the surface species and structure of pristine SiC. These findings suggest that the Li-rich SiC twin crystal raises new possibilities for the development of high-capacity cathode materials and merits further investigation to expand its application scope. 相似文献
115.
T. J. Heindel F. P. Incropera S. Ramadhyani 《International Journal of Heat and Fluid Flow》1995,16(6):511-518
Coupled conduction and natural convection transport within a discretely heated cavity have been investigated numerically. One vertical wall of the cavity is composed of discrete, isoflux heat sources mounted in a substrate of finite thermal conductivity. The opposite vertical wall and the horizontal walls are assumed to be isothermal and adiabatic, respectively. The governing steady-state partial differential equations for the fluid and solid region are solved simultaneously using a control volume formulation, coupled with an additive correction multigrid procedure that increases the convergence rate of the solution. The fluid Prandtl number and heater/fluid thermal conductivity ratio are fixed at 25 and 2350, respectively, corresponding to a dielectric fluid (FC-77) and heaters manufactured from silicon. With increasing modified Rayleigh number (104 < RaLz* < 109), the cavity flow becomes more boundary layer-like along the vertical walls, and multiple fluid cells develop in the central region. Thermal spreading in the substrate increases with decreasing modified Rayleigh number and with increasing values of the substrate/fluid thermal conductivity ratio (10−1 <- Rs ≤ 103). For large Rs, the discrete heat sources lose their thermal identity, and the streamlines and isotherms resemble those associated with a differentially heated cavity. Thermal spreading in the substrate also has a significant effect on circulation in the cavity and on maximum surface temperatures. 相似文献
116.
Experimental study of rotation effect on film cooling over the flat wall with a single hole 总被引:3,自引:0,他引:3
Zhi Tao Xiaojun Yang Shuiting Ding Guoqiang Xu Hongwei Wu Hongwu Deng Xiang Luo 《Experimental Thermal and Fluid Science》2008,32(5):1081-1089
A new rotating test rig was set up to investigate the rotation effect on the film cooling over the flat wall. A simple flat blade with an inclined 30° film hole, which is parallel to the hot mainstream, was installed. And different rotation orientations were selected to simulate the blade pressure or suction side of a turbine blade. A steady liquid crystal technique was applied to obtain detailed distribution of the temperature over the blade surface. And the average adiabatic film cooling effectiveness of the area adjacent to the film hole was selected to evaluate the cooling effect. Five different rotational speeds, i.e., 0, 300, 500, 800, 1000 r/min, were considered. Experimental results indicate that the film trajectory could bend under the rotating condition. With the increase of the rotational speed, on the pressure side, the film trajectory inclines centripetally firstly and then centrifugally; whereas, on the suction side the film trajectory bends centrifugally. On the other hand, as the rotational speed increases, the cooling effect is improved firstly and then worsened when Ω > 500–600 r/min on the pressure side. On the suction side, however, the cooling effect is not sensitive to the rotational speed. 相似文献
117.
表面涂层对非晶薄带爆炸焊接温度影响探讨 总被引:2,自引:0,他引:2
针对在非晶薄带爆炸复合制备块体非晶复合材料过程中涂层对温度的影响,提出对非晶薄带实施涂层后再进行爆炸复合。利用放缩法将涂层后的薄带简化为均匀材料,并对30 m厚的Fe78B13Si9非晶涂层前、后爆炸焊接层内温度场进行系列分析计算,表明:(1) 高速撞击产生的热量主要集中在涂铜层上,涂层处理能够显著减小界面撞击引起的热影响区域;(2) 相对于非涂层的非晶爆炸复合,文中方案平均温升降低约280 K;(3) 铜涂层后,碰撞界面冷却速率高达107 K/s。研究表明,具有表面涂层的非晶薄带在爆炸复合制备层合块体非晶复合材料过程中,能够更好地保持非晶材料的非晶结构,并有效扩大爆炸焊接窗口。 相似文献
118.
NiAl-Cr(Mo)-Cr_xS_y自润滑复合材料的摩擦磨损特性 总被引:2,自引:1,他引:2
采用滑动磨损试验方法测试了NiAl-Cr(Mo)-CrxSy自润滑复合材料与SiC陶瓷配副在110~960℃的摩擦磨损特性.结果表明:200~400℃,纳米CrxSy晶粒在复合材料摩擦表面形成较完整的润滑膜,产生自润滑性能;700~900℃,复合材料摩擦表面生成了1~3μm厚、完整的玻璃陶瓷润滑膜,产生了自润滑耐磨性能.2种润滑膜材料均可向SiC表面转移,消除了复合材料/SiC的摩擦状态.随着温度的升高,2种润滑膜材料的强度降低,SiC微凸体压入润滑膜,导致润滑膜的剥落加剧,复合材料的摩擦系数与磨损率升高. 相似文献
119.
An important goal of spray cooling research is the ability to predict local heat transfer from the spray hydrodynamics. It is postulated that the local normal pressure exerted by the spray onto the heated surface can be used to obtain the local heat transfer coefficient. This hypothesis was tested using data obtained from hollow cone, full cone, and linear sprays at four nozzle pressures and three stand-off distances. A correlation between the pressure and heat transfer coefficient was determined from the data, then used to “predict” the heat transfer coefficient to verify the accuracy of the correlation. The area averaged heat transfer coefficient could be predicted within 25%, indicating that pressure can be used to predict the local heat transfer coefficient in the single-phase regime. 相似文献
120.