Studies on the formability of powder metallurgical aluminum–copper composite

Formability is concerned with the extent of deformation that the materials undergo before failure; thus its investigation is critical for successful processing of materials during bulk deformation. The present investigation has been undertaken to generate the forming limit diagrams for powder metallurgical aluminium–copper composites for different initial relative densities and copper contents. Sintered aluminium–copper composite compacts of 2%, 4% and 6% copper content with different initial relative densities have been prepared by applying recommended powder compaction pressures. The material properties such as apparent strain hardening exponent and strength coefficient were determined using stage wise compression test to generate the formability limit diagram. Densification curves were plotted to investigate the effect of initial relative density and copper content on the pore closure phenomena during deformation. Theoretical and experimental investigations using standard ring compression test were carried out to determine friction factor between tool and work piece interfaces for different initial relative density and copper content. The critical transition densities vide the forming limit diagram were found to be 84%, 85.3%, 86% and 87.5% for pure sintered aluminium, Al–2%Cu, Al–4%Cu and Al–6%Cu composites respectively. The friction factor between tool and work piece interfaces has showed increasing pattern for all the cases with decrease in the initial relative density and increase in the copper content of the composite.     

车载设备对地铁车厢地板振动影响及评估方法

通过对某地铁车辆辅助变流器分别进行装车通电测试和车间台架通电测试,确定了导致车厢地板振动过大的原因,证明了当前车载设备振动评估指标的不合理性;在此基础上,提出了一种可在车间或实验室评估变流器等车载设备对地铁车厢地板振动影响的方法。试验测试结果表明,该评估方法误差在10%左右。该方法能够指导变压器、风机等车载设备的选型及避免大批量装车振动故障的出现,具有较高的工程应用价值。     

Modeling of material deformation behavior in micro-forming under consideration of individual grain heterogeneity

This study aims to develop a model to characterize the inhomogeneous material deformation behavior in micro-forming. First, the influence of individual grain heterogeneity on the deformation behavior of CuZn20 foils was investigated via tensile and micro-hardness tests. The results showed that different from thick sheets, the hardening behavior of grains in the deformation area of thin foils is not uniform. The flow stress of thin foils actually only reflects the average hardening behavior of several easy-deformation-grains, which is the reason that thinner foils own smaller flow stress. Then, a composite modeling method under consideration of individual grain heterogeneity was developed, where the effects of grain orientation and shape are quantitatively represented by the method of flow stress classification and Voronoi tessellation, respectively. This model provides an accurate and effective method to analyze the influence of individual grain heterogeneity on the deformation behavior of the micro-sized material.     

Modeling the threshold voltage instability in SiC MOSFETs by multiphonon-assisted tunneling

Threshold voltage instability is a main reliability issue of silicon carbide MOS transistors submitted to gate bias stress. A new time and temperature-dependent TCAD model based on multiphonon-assisted tunneling is proposed. The dynamics during both stress and recovery phase are studied and are compared with experimental data.     

Microstructure and mechanical properties of Al−Mg−Si alloy U-shaped profile

To get a full understanding of hot extrusion, solid solution treatment and aging process on the Al−0.56Mg− 0.63Si alloy, the microstructure and mechanical properties of a U-shaped profile were studied through optical microscopy, scanning electrical microscopy, transmission electrical microscopy, hardness, and tensile tests. The coarse equiaxed grains existed near the profile edge as a result of the dynamic recrystallization nucleation and exceeding growth during hot extrusion. The fibrous deformed and sub-structured grains located between the two coarse grain layers, due to the occurrence of work-hardening and dynamic recovery. Perpendicular needle β′′ precipitates were distributed inside the grain, and obvious precipitates-free zone appeared after aging treatment. The tensile strength, yield strength and elongation of the aged Al−Mg−Si alloy U-shaped profile were no less than 279.4 MPa, 258.6 MPa, and 21.6%, respectively. The fracture morphology showed dimple rupture characteristics. The precipitates and grain boundaries played key role in the strengthening contribution.     

基于频率自适应 PLL 的传动系统牵引电机转速实时估计

为满足传动系统在牵引电机无速度传感器控制、速度传感器异常诊断与预测等领域对牵引电机转速精确估计的要求,提出了一种基于频率自适应锁相环的转速实时估计方法。首先针对传统锁相环(PLL)算法无法实现频率大范围跟踪问题,提出了一种前馈参考频率自适应调节的二阶广义积分锁相环(RFSOGI-PLL)算法,实现定子电流基波频率的大范围实时跟踪;再利用牵引电机转速与定子电流基波频率以及齿槽谐波关系模型,结合电机结构参数,实现了传动系统牵引电机转速大范围实时估计;最后在某型大功率交流传动系统上对所提算法的有效性进行了验证。仿真与试验结果表明,与传统PLL算法相比,所提的RFSOGI-PLL算法可有效提升算法对跟踪频率变化的适应性,满足牵引电机转速估计在快速性和估计误差等指标方面的要求。     

Reliability design of direct liquid cooled power semiconductor module for hybrid and electric vehicles

With the global interests and efforts in popularizing low carbon vehicles, automotive power module has been becoming one of the fastest growing sectors in power semiconductor industry. As working in a harsh environment, the performance and reliability requirements of automotive module are stringent than industrial products. In this work, an integrated direct liquid cooled power module with enhanced reliability for hybrid and electric vehicles (HEV/EV) is developed. The design and assembly of the module were optimized in terms of performance, weight, cost and reliability. The module is integrated Al direct liquid cooling structure, leading to about 40% reduction of weight and cost and almost 50% reduction of junction to heat sink thermal resistance. Therefore, the junction temperature stays below the upper limit at the worst operation case which enhances the thermal reliability and lifetime. By incorporating advanced die lead bonding, the parasitics can be reduced by 50%, which is beneficial to efficiency and reliability. Furthermore, the die and terminal attach technologies are investigated to improve reliability. The lifetime prediction under a typical driving cycle shows that the proposed module is capable of working in the whole vehicle service period.     

Effects of combined NO and forming gas annealing on interfacial properties and oxide reliability of 4H-SiC MOS structures

The effects of NO and forming gas post oxidation annealing treatments on the interfacial properties and reliability of thermal oxides grown on n-type 4H-SiC (0001) Si face have been investigated in this study. The results show that forming gas annealing (FGA) treatment has limited effect on interface trap density (D_it) while it results in an improvement of the insulating properties of thermal oxide with uniform high FN barrier height (2.56 eV), high field-to-breakdown (10.71 MV/cm) and charge-to-breakdown (0.078 C/cm²). On the other hand, NO annealing causes a drastic reduction in D_it in the entire energy level, but in the case of reliability, it is not so effective as FGA, with lower barrier height (2.52 eV), field-to-breakdown (10.08 MV/cm), charge-to-breakdown (0.025 C/cm²) and worse uniformity of oxide. The combined NO&FGA treatment was also studied. It leads to a significant reduction in interface trap density further, especially in deep energy level (E_C-E_T ≥ 0.4 eV). As for reliability, it brings about uniform barrier height (2.69 eV), field-to-breakdown (10.15 MV/cm) and charge-to-breakdown (0.024 C/cm²). Taking interfacial properties and reliability into account, combined NO&FGA treatment is a promising POA technique for fabrication of high-quality SiC MOS devices.     

Drain avalanche breakdown and gate instabilities in 4H-SiC MESFET's

The breakdown characteristic of 4H-SiC MESFET is simulated with a two-carriers model. The simulation results are considerately accurate using the impact ionization, thermal equation and anisotropy models. The breakdown mechanism is explained in terms of impact ionization and conductivity modulation. The thermal effect and gate-bias dependence of the breakdown are analyzed in detail.