(Cu-50TiH_2)+SiC_p复合粉体焊料连接石墨/铜接头的微观结构

碳材料与铜的连接在汽车及能源等领域具有潜在的应用。采用Cu、TiH2和SiC粉末组成的复合粉体焊料在950℃/10min工艺条件下真空钎焊石墨与铜,并研究复合粉体焊料中SiC含量对石墨/铜接头剪切强度的影响。结果表明,复合焊料中添加SiC有利于接头室温剪切强度的提高。当SiC体积分数为10%时,接头室温剪切强度最高,为19.2MPa。微观分析表明,连接过程中,复合粉体焊料中TiH2分解产生的Ti与SiC发生原位反应,生成TiC、Ti5Si3及Ti3SiC2等反应产物;另外,Ti与石墨母材发生界面反应形成厚度为2~3μm的TiC反应层,Ti和Cu则形成Ti3Cu4等金属间化合物。由于Ti3SiC2在高温下具有塑性,可在一定程度上缓解石墨/铜接头的残余热应力。同时,晶须状和颗粒状的反应产物弥散分布在连接层中,对接头起强化作用,也有利于石墨/铜接头性能的提高。     

弥散燃料颗粒裂纹起源的有限元模拟分析

通过建立含多气泡的燃料颗粒模型,采用有限元方法分析了燃料颗粒在裂变气体气泡内压作用下的应力分布,统计了燃料颗粒内部气泡位置对气泡内壁处的最大拉应力的影响,并结合实验结果探寻了弥散燃料颗粒在辐照后退火时的裂纹起源。结果表明:当弥散燃料颗粒内部含有多个裂变气体气泡时,受气泡内压作用,气泡内壁径向应力为压应力,环向应力为拉应力;气泡位置距燃料颗粒心部越远,气泡内壁处的最大环向拉应力越大;表层气泡的最大环向拉应力远大于心部气泡的;燃料颗粒裂纹起源于表层气泡内壁。     

球磨时间和氧化铝含量对氧化铝弥散增强铜基复合材料组织与性能的影响

采用高能球磨法,通过改变Al2O3含量(质量分数)和调节球磨时间等参数,烧结制备出不同含量Al2O3颗粒弥散增强的铜基复合材料;通过显微组织观察、相对密度、电导率和硬度的测试,研究了Al2O3含量和球磨时间对烧结体组织和性能的影响.结果表明:球磨转速为270 r·min<'-1>时,随球磨时间延长,铜晶粒细化和晶格畸变...     

膨润土润滑脂的热稳定和抗氧化性能研究

研究助分散剂、温度和抗氧化剂对膨润土润滑脂稠度的影响,认为加热温度为40℃是合适的合成温度,对膨润土润滑脂的性能和结构表征结果表明在采用丙酮为极性助分散剂和胺类添加剂作为抗氧化剂的条件下,可制备出热稳定性和抗氧化性能良好的膨润土润滑脂。     

Equilibrium hydrogen pressures in the V–H system from first principles

Equilibrium hydrogen pressures over hydrogenated vanadium, i.e., crystalline VH_x with $0\le x\le 2$, are estimated based on first-principles calculations to assess the suitability of V-based alloys for use as hydrogen storage materials. For intermediate H contents (x = 0.5 and 1) corresponding to partial site occupancies, a multistep process was used to determine the ground-state structures. First, a large number of configurations with H on tetrahedral (T) or octahedral (O) sites in bcc–type lattices were ranked in terms of energetical stability sans vibrational contributions. Next, phonon calculations were carried out for the lowest-energy structures identified in the first step and zero-point energies added to the electronic energies. Hydrogen pressures for V-VH_0.5 and VH-VH₂ equilibria were then calculated using these ground-state free energies and thermodynamic equations. Calculation of vibrational energies was simplified by identifying the linear relationship between H content and zero-point energy, which depends only on the type of sites occupied by H atoms. Whereas H atoms preferentially occupy T sites when vibrational terms are ignored, their inclusion leads to H atoms preferentially occupying O sites in both VH_0.5 and VH at 0 K. Inclusion of vibrational energy also leads to a better match with reported experimental hydrogen pressures highlighting its importance when assessing the stabilities of different H distributions in metal lattices. The method outlined consists of a more exhaustive configurational search than has been considered previously, and should be useful for systematic investigation of a wide variety of V-based alloys.     

三维混合层中湍流拟序结构对颗粒扩散的影响

对时间发展模式的三维气固两相混合层湍流拟序结构及其对不同尺寸颗粒扩散的影响进行直接数值模拟，对于气相场，应用拟谱方法（Pseudospectral method）直接求解Navier-Stokes方程组；对于颗粒场，对于Lagrangian方法跟踪半场颗粒，针对不同Stokes数的颗粒，分别模拟颗粒场在展向和流向的分布变化，进而分析流场三维大涡结构对不同颗粒扩散的影响，并引入一系列参数，着重定量描述流场展向大涡结构和流向大涡结构对不同尺寸颗粒分布的不同影响。     

Second law analysis of crossflow heat exchanger in the presence of axial dispersion in one fluid

In the present paper second law analysis of crossflow heat exchangers has been carried out in the presence of non-uniformity of flow. This non-uniformity is modeled with the help of axial dispersion model and takes into account the back mixing and flow maldistribution. An analytical model for exergy destruction has been evaluated for the cross-flow configuration. A wide range of study of the operating parameters and non-uniform flow on exergetic behavior of crossflow heat exchangers has been carried out. The results clearly bring out not only the reason behind the maximum entropy paradox in heat exchangers but also the proper perspective of exergy destruction and the consequent optimization of crossflow heat exchangers from the second law viewpoint.     

共挤复合结构高含量色母粒的研制

介绍了复合结构高含量色母粒的结构设计、筛选和研制过程。试验结果表明,采用核/壳复合结构可以解决高含量色母粒难以牵引造粒的问题,其分散效果及与PP/PE树脂的兼容性良好。     

Plane harmonic waves in the theory of thermoviscoelastic materials with voids

In this article we analyze the behavior of plane harmonic waves in the entire space filled by a linear thermoviscoelastic material with voids. We take into account the effect of the thermal and viscous dissipation energies upon the corresponding waves and, consequently, we study the damped in time wave solutions. There are five basic waves in an isotropic and homogeneous thermoviscoelastic porous space. Two of them are shear waves, while the remaining three are dilatational waves. The shear waves are uncoupled, damped in time with decay rate depending only on the viscosity coefficients. The three dilatational waves are coupled and consist of a predominantly dilatational damped wave of Kelvin–Voigt viscoelasticity, other is predominantly a wave carrying a change in the void volume fraction and the third takes the form of a standing thermal wave whose amplitude decays exponentially with time. The explicit form of the dispersion equation is obtained in terms of the wave speed and the thermoviscoelastic homogeneous profile. Furthermore, we use numerical methods and computations to solve the secular equation for some special classes of thermoviscoelastic materials considered in literature.     

Effect of Turbulence on Flame Propagation in Comstarch Dust-Air Mixtures

Following the quantitative determination of dust cloud parameters, this study investigated the flame propagation through cornstarch dust clouds in a vertical duct of 780 mm height and 160×160 mm square cross section, and gave particular attention to the effect of turbulence on flame characteristics. The turbulence induced by dust dispersion process was measured using a particle image velocimetry (PIV) system. Upward propagating dust flames were visualized with direct light and shadow photography. The results show that a critical value of the turbulence intensity can be specified below which laminar flame propagation would be established. This transition condition is about 10 cm/s. The measured propagation speed of laminar flames appears to be in the range of 0.45-0.56 m/s, consistent with the measurements reported in the literature. For the present experimental conditions, the flame speed is little sensitive to the variations in dust concentration. Some information on the flame structure was revealed from the shadow records, showing the typical heterogeneous feature of dust combustion process.