考虑杂散电感影响的风电变流器IGBT功率模块动态结温计算及热分布

针对绝缘栅双极型晶体管(IGBT)模块内部并联芯片间动态不均流导致损耗分布存在差异,传统结温计算方法无法准确反映模块内部热分布的问题,提出考虑模块内部封装杂散电感影响的IGBT功率模块动态结温计算方法。首先,建立考虑杂散电感的IGBT等效电路模型,仿真证明功率模块内部电流分布不均的机理。其次,定量推导杂散电感与开通损耗的关系,提出基于多芯片电热耦合影响的IGBT模块内部动态结温计算方法,并通过实验验证。最后,结合双馈风电机组控制策略,建立计及杂散电感影响的风电变流器功率模块电热仿真模型,并与传统模型进行比较。结果表明,所提模型可准确反映模块内部各芯片结温均值及波动幅值的差异性。

半导体功率器件动态结温的实时在线预测算法

半导体功率器件的结温是决定其能否安全工作,以及功率循环寿命长短的关键参数。目前获取结温的方法主要分为直接测量、热敏电参数换算、功耗热阻计算三大类,但都存在一定弊端。在此背景下,提出基于离散拟合热阻的半导体功率器件动态结温的实时在线预测算法,在不增加系统硬件成本的前提下,消耗极少的处理器运算资源,实时、准确的获得动态结温。首先,详细阐述提出的动态结温实时在线预测算法的步骤和原理;然后,通过Matlab仿真搭建算法模型,在各种工况下进行仿真,并与Melcosim仿真的结果进行对比;最后,定制内埋高响应热电偶的IPM(Intelligent Power Module)模块,搭建实验平台进行电机加载、堵转等实验。仿真和实验的结果证明该算法正确有效。

用于IGBT模块结温预测的热-电耦合模型研究

随着IGBT模块功率等级及密度的提高,因功率损耗而导致芯片温升加剧进而导致变流系统崩溃的问题愈发突出。对功率器件及散热系统的深入研究有助于功率器件封装设计、器件选型、系统布局以及逆变器的可靠运行。本文通过有限元分析方法对IGBT模块和散热系统的瞬态热阻抗进行了提取,所得结果与厂商数据手册吻合,而且通过所用方法验证了热阻抗曲线的普适性,最后利用热特性RC等效网络建立热-电耦合模型,可对芯片动态结温进行预测。

Microstructure evolution of Zn-8Cu-0.3Ti alloy during hot deformation

The hot deformation behavior of homogenized zinc alloy was investigated through uniaxial compression test on a Gleeble-1500 thermal-mechanical simulator within a temperature range of 230-380°C and a strain rate range of 0.01-10 s -1 ,the corresponding flow curves and their characters were determined and analyzed,and microstructures were studied by optical,SEM and TEM microscopy.The results indicated that the microstructure evolution of zinc alloy during hot deformation involves the spheroidization of the phase of TiZn15,coarsening of the precipitated phase and dynamic recrystallization（DRX）of the phase of matrix,leading to the formation of the polyphase（η＋ε＋TiZn15）structure.The spheroidization of the phase of TiZn15 during hot deformation was beneficial to the particle nucleation stimulated and then promoted to DRX of matrix.The dynamic recrystallization grain size of the matrix phase decreased firstly and then increased with elevating the temperature,and the degree of DRX became more complete when the strain rate and strain became larger.Hot deformation accelerated the diffusion of Cu atom,which resulted in the coarsening of the precipitated phase.Thus,the microstructure was refined owing to the pinning effect of the precipitated phase.

Dynamic recrystallization of single-crystal nickel-based superalloy

The dynamic recrystallization behavior of single-crystal（SC） superalloy SR.R99 at low strain rate was investigated by high-temperature creep testing.The results show that dynamic recrystallization may take place after the uncoated samples have been creep-tested in air at high temperature and low stress for a long time.Both the threshold temperature and strain for the dynamic recrystallization of SC superalloy SRR99 at low strain rate are lower than those for the static recrystallization.Dynamically recrystallized grains with the depth less than 15 μm are only located in the surface γ＇-free layers,and the recrystallized grains are well-developed grains without columnar y＇precipitates within them.The dynamic recrystallization behavior of SC superalloy SRR99 at low strain rate is mainly related to high-temperature oxidation.Suitable protective coating can effectively prevent the dynamic recrystallization of SC superalloy components in service.In addition,the dynamic recrystallization behavior of SC superalloy SRR99 at high strain rate was also studied by high-temperature compression testing.At high strain rate,a higher temperature and larger strain are needed for the occurrence of dynamic recrystallization than at low strain rate,and the recrystallized grains have cellular structures with an amount of columnar γ＇ precipitates within them.

Investigation on hot deformation behavior of AZ31 magnesium alloy

The hot compressive deformation of extruded AZ31magnesium alloy with the mass fractions of Al and Zn equal to 3 % and 1 % respectively is studied by a Gleeble-1500D thermal mechanical simulator over the temperature range from 200 ℃ to 400 ℃ and the strain rate from 10-3 s-1 to 100 s-1. The true stress-strain curves of the strain of 65 % are tested. The deformation activation energy is obtained and the flow stress model is established by analyzing the effects of strain rate and temperature on the flow stress. Zener-Hollomon parameter is introduced to describe the softening behaviors of AZ31 magnesium alloy resulted from dynamic recrystallization during the hot compressive deformation, whose natural logarithm is linear with the critical strain of dynamic recrystallization.

Influence of temperature and strain rate on flow stress behavior of twin-roll cast, rolled and heat-treated AZ31 magnesium alloys

The effects of temperature and strain rate on the flow stress behavior of twin-roll cast, rolled and heat-treated AZ31 magnesium alloys were investigated under uniaxial tension. At high temperatures, dynamic recovery, continuous dynamic recrystallization, grain boundary sliding and the activation of additional slip systems lead to an improvement of the ductility of the alloys. The elongation to failure is nearly independent of the strain rate between 473 and 523 K at 10-2 s-1 and 10-1 s-1, which is related to the strain rate dependence of the critical resolved shear stress（CRSS） for nonbasal slip. Despite the high temperature, twins are even observed at 573 K and 10-3 s-1 because they have a low CRSS.

Modeling on dynamic recrystallization of aluminium alloy 7050 during hot compression based on cellular automaton

The dynamic recrystallization(DRX) process of hot compressed aluminium alloy 7050 was predicted using cellular automaton(CA) combined with topology deformation. The hot deformatation characteristics of aluminium alloy 7050 were investigated by hot uniaxial compression tests in order to obtain the material parameters used in the CA model. The influences of process parameters(strain, strain rate and temperature) on the fraction of DRX and the average recrystallization grain(R-grain) size were investigated and discussed. It is found that larger stain, higher temperature and lower strain rate(less than 0.1 s^(–1)) are beneficial to the increasing fraction of DRX. And the deformation temperature affects the mean R-grain size much more greatly than other parameters. It is also noted that there is a critical strain for the occurrence of DRX which is related to strain rate and temperature. In addition, it is shown that the CA model with topology deformation is able to simulate the microstructural evolution and the flow behavior of aluminium alloy 7050 material under various deformation conditions.

Flow behaviour constitutive model of CuCrZr alloy and 35CrMo steel based on dynamic recrystallization softening effect under elevated temperature

In order to study the effect of dynamic recrystallization on the metal flow behavior during thermal deformation,the elevated temperature compression experiments of CuCrZr alloy and 35CrMo steel are carried out using Gleeble-3810 thermal simulator.It is proved that the samples underwent obvious dynamic recrystallization behavior during thermal deformation by microstructure observation of deformed specimens.The size of recrystallized grains increases as the temperature improved and the strain rate decreased.Meanwhile,the net softening rate caused by dynamic recrystallization is determined based on the stress-dislocation relationship.It can be found that the value of net softening rate increases quadratically as the Z parameter decreases,and the dynamic recrystallization net softening rate of CuCrZr alloy and 35CrMo steel are calculated to be 21.9%and 29.8%,respectively.Based on the dynamic recrystallization softening effect proposed,the novel elevated temperature flow constitutive models of two different alloys are proposed,and the related parameters are well defined and solved in detail.The predicted values of the obtained models are agreed well with the experimental values.

Characterization of hot deformation behavior of Al-Zn-Mg-Mn-Zr alloy during compression at elevated temperature

The hot deformation behavior of Al-6.2Zn-0.70Mg-0.30Mn-0.17 Zr alloy and its microstructural evolution were investigated by isothermal compression test in the deformation temperature range between 623 and 773 K and the strain rate range between 0.01 and 20 s^(-1).The results show that the flow stress decreased with decreasing strain rate and increasing deformation temperature.At low deformation temperature(≤673 K) and high strain rate(≥1 s^(-1)),the main flow softening was caused by dynamic recovery;conversely,at higher deformation temperature and lower strain rate,the main flow softening was caused by dynamic recrystallization.Moreover,the slipping mechanism transformed from dislocation glide to grain boundary sliding with increasing the deformation temperature and decreasing the strain rate.According to TEM observation,numerous Al_3Zr particles precipitated in matrix,which could effectively inhibit the dynamic recrystallization of the alloy.Based on the processing map,the optimum processing conditions for experimental alloy were in deformation temperature range from 730 K to 773 K and strain rate range from 0.033 s^(-1) to 0.18 s^(-1) with the maximum efficiency of 39%.

Hot deformation behavior and microstructure evolution of TC4 titanium alloy

The hot deformation behavior of Ti-6Al-4V （TC4） titanium alloy was investigated in the temperature range from 650℃ to 950℃ with the strain rate ranging from 7.7×10^-4 s^-1 to 7.7×10^-2 s^-1. The hot tension test results indicate that the flow stress decreases with increasing the deformation temperature and increases with increasing the strain rate. XRD analysis result reveals that only deformation temperature affects the phase constitution. The microstructure evolution under different deformation conditions was characterized by TEM observation. For the deformation of TC4 alloy, the work-hardening is dominant at low temperature, while the dynamic recovery and dynamic re-crystallization assisted softening is dominant at high temperature.

Low-temperature Properties of Biodiesel:Rheological Behavior and Crystallization Morphology

A soybean oil derived biodiesel was prepared and blended with a conventional No. 0 petrodiesel. The pour points （PP） and the cold filter plugging points （CFPP） of biodiesel blends were evaluated on a low-temperature flow tester. Dynamic viscosities of the blends at different temperatures and different shear rates were measured on a rotary rheometer. The crystal morphologies of biodiesel blends at low temperatures were analyzed using a polarizing microscope. The results indicated that blended fuels demonstrated slight decrease in PPs and CFPPs as compared with those of neat soybean oil derived biodiesel and pure petrodiesel. Below the temperatures of PPs or CFPPs, the dynamic viscosity of biodiesel blends dramatically increased with a decreasing temperature, but decreased with an increasing shear rate, so that biodiesel blends exhibited non-Newtonian behavior. At temperatures higher than PPs or CFPPs, a linear relationship appeared between the dynamic viscosity and shear rate and biodiesel blends became Newtonian fluids. At low temperatures, wax crystals of biodiesel blends grew and agglomerated rapidly. Loss of fluidity for biodiesel blends at low temperatures could therefore be attributed on one hand to the sharp increase of viscosity and on the other hand to the rapid growth and agglomeration of wax crystals.

Corner forming of AZ61A magnesium alloy tube within warm hydroforming

The comers with small radii on cross sections are crucial for forming hydroformed components with polygonal sections. In this paper, warm hydroforming experiments of AZ61A magnesium alloy tubes were cartied out to study the forming regularity of round comers by using a demonstration part with square sections. Effects of temperature on radius forming, thinning ratio distribution and microstructure were revealed and a component with relative outer corner radius of 3.0 was obtained by warm hydroforming at 240℃. The minimum thickness of the formed square section was located in the transition position between the corner and the straight wall. The thinning ratio of the round corner increased with the increase of forming temperature. Fotmability of the magnesium tube was improved by raising temperature under the effect of dynamic recrystallization at 240℃.

Static and Dynamic Properties of Soil Food Web Structure in a Greenhouse Environment

Soil food web structure is fundamental to ecosystem process and function; most studies on soil food web structure have focused on agro-ecosystems under different management practices and natural terrestrial ecosystems,but seldom on greenhouses. This study explored the static and temporal variability of soil food structure in two greenhouses of Shandong Province,North China over a two-year period. The static properties were measured directly by surveying functional group composition and a series of parameters portraying the species properties,link properties,chain properties and omnivory properties of the web,as well as indirectly through calculation of nematode indices,enrichment index(EI),structure index(SI),and channel index(CI). The dynamic variability of greenhouse soil food structure was described by the dynamics of functional groups,Bray-Curtis(BC) similarity and cluster analysis. The results showed that the greenhouse soil food web contained 14 functional groups,with microbes having the highest mean biomass,followed by protozoa. Of the three functional groups of protozoa,flagellates were the dominant group on most sampling dates,amoebae only became the dominant group during the summer,while ciliates were the least prevalent group. All nematodes were assigned into one of the four functional groups,bacterivorous,fungivorous,herbivorous and omnivorous,and the fungivorous nematodes had the lowest mean biomass. Mites were assigned into three functional groups and the omnivorous noncryptostigmatic mites were the dominant group. All the functional groups showed significant seasonal changes. The soil food web connectance was 0.15,the maximum food chain length was 5,and the average food chain length was 3.6. The profiles of the EI and SI showed that the food web was resourcedepleted with minimal structure. The results of CI indicated that the bacterial decomposition pathway was the dominant pathway in the food web of the greenhouse soils studied and the results of BC similarity showed that the soil food web had higher variability and instability over time. The cluster analysis showed that the functional groups located at high trophic levels with low biomass were in a cluster,whereas those at low trophic levels with high biomass were closer. Compared with the food web structure of agroecosystem and natural terrestrial ecosystem soils,the structure of greenhouse soil food web was simple and unstable,which was likely driven by high agricultural intensification,particularly over application of fertilizers.