热循环对硅太阳电池伏安特性的影响

试验研究了太阳电池结构的热循环效应.结果表明:经100次热循环作用后太阳电池伏安特性曲线下移,输出功率降低6‰～7‰;各层胶接材料的热物理性能相差较大,热循环过程中热错配应力累积,使太阳电池胶接结构可能出现裂纹或层间剥离现象.     

船用柴油机缸盖缩尺试样的热失效研究

为研究船用柴油机缸盖在低周热循环载荷下的渐进失效问题,开展了“四孔平板”缩尺试样的热失效模拟试验,建立了基于温度依赖的Chaboche组合硬化材料模型和改进Ostergren塑性能量损伤模型的仿真方法,研究了塑性变形演化、疲劳开裂的热失效行为和机理。研究表明：仿真预测具有较高精度,圆孔失圆变形预测最大偏差不超过10.5%,准确预测了裂纹的萌生位置,预测寿命与实际失效循环数目接近;缩尺试样受到循环变化的温度梯度影响,整体结构表现为热弯曲变形失配,产生的局部非对称拉压应力引发了塑性变形;随着循环次数增加发生应力松弛,塑性变形呈现先快后慢的非线性减少趋势;冷却阶段的拉伸塑性应变能和应力三轴度增加,引发了圆孔鼻梁高应力区开裂损坏。     

采用热天平研究煤粉燃烧特性时的零维燃烧模型

探讨了采用热天平研究煤粉燃烧特性时试样量对试验结果的影响，得到当试样层厚度δｍ与煤粉粒径ｄｐ比δｍ／ｄｐ≤１．３－１．５×１０＾－４时，煤粉燃烧特性试验结果与试亲量无关，即可认为试样层为内氧浓度分布均匀及反应表面积有交利用率相同。提出了适合于采用热天平研究煤粉特性进的零维煤烧模型。     

热循环作用下太阳电池板单元结构寿命预测

试验测试了热循环过程中太阳电池板单元结构各被粘接层的热应变值,结合有限元MSC.Marc模拟的结果提出了以热应变极大值或残余热应变作为热循环条件下胶接结构的损伤参量,建立了预测太阳电池板寿命的数学模型。     

热循环载荷下BGA封装体热应力特性

构建了热循环条件下球栅阵列(ball grid array,BGA)封装体传热和应力耦合的非稳态理论模型,通过器件自身发热功率随时间变化来实现循环热载荷,研究工作过程中流场、温度场、应力场的动态变化,并采用有限元方法进行数值求解,分析了热循环载荷对器件所处物理场的影响。研究结果表明:热循环过程中,器件整体温度与方腔内自然对流强度在高温保温时间开始时刻出现峰值,在低温保温时间结束时刻出现谷值;BGA封装体最高温点均位于作为热源的芯片上,承受应力最大点位于阵列最外拐点与上下侧材料的连接部位;随着循环次数的增加,每个热循环周期中关键焊点上端点处的最大等效应力不断增加。     

热循环下太阳电池板单元结构热应力演变规律研究

利用MSC.Marc大型通用软件研究了热循环条件下太阳电池板单元结构热应力分布及演变规律。结果表明:被粘接层正应力极大值位于结构中点,极小值出现在结构两端;胶层剪切应力极大值位于结构两端,极小值出现在中点;聚酰亚胺薄膜层取得太阳电池板单元结构最大正应力,粘接聚酰亚胺薄膜与碳纤维复合材料的硅橡胶层取得结构最大剪切应力;结构在低温保温阶段的最大正应力及最大剪切应力远大于高温保温阶段的最大应力;太阳电池板单元结构的最大应力值随热循环次数的增加而增加。     

RDF预处理对热解动力学的影响

通过对城市生活垃圾衍生燃料（RDF）进行酸洗、压模成型等预处理，改变其空间结构如比表面积、孔结构等，然后在TB－1型热天平中测试不同温度下的热解TG曲线，分析各种动力学参数，得出RDF性质、空间结构对热解反应速度、活化能、提前因子等动力学参数的影响规律。试验结果表明，压型试样的反应速率明显低于未压型试样，最佳热解反应温度段为550℃－650℃。部分压型试样的活化能和指前因子之间存在差补偿效应。RDF经酸洗后，热解反应速率有明显提高，活化能降低，反应活性增加。     

温度变化对S25C低碳钢塑性的影响

在室温到４００℃范围内的恒温和变温条件下，对Ｓ２５Ｃ低碳钢的塑料进行了研究。通过对薄壁管式试样施加轴向载荷和扭转载荷，进行了混合载荷试验，材料的寝 始屈服状态遵循端斯卡定律，但屈服后的应力－－应变关系既不遵循端斯卡定律，也不遵循迈兹定律，扭转上的包辛格曲线有别于轴向载荷条件下的包辛格曲线。在不同温度下获得由拉伸预应变所产生的应力－应变关系式。将实验结果与等温应力－－应变关系进行了对比。     

秸秆类生物质热解特性及其动力学研究

该文对秸秆类生物质的热解行为进行了热重分析（TG）和差分热重分析（DTG）研究。加热速率分别为10K／min、20K／min和30K/min，加热终温为1173K；采用高纯氮气做保护气；样品粒径为250μm－1000μm。通过对TG、DTG曲线的分析，深入研究了加热速度、温度、加热时间等对热解过程的影响，建立了北方典型的秸秆类生物质的反应动力学方程，得出了该类物质热解反应动力学参数、表观活化能和频率因子，并提出了相应的热解机理。     

太阳电池板结构应力-应变状态分析

多层胶接是太阳电池板的结构特点,由于不同材料的弹性模量、热膨胀系数和泊松比不同,在温度场 作用下会产生热应力应变,在多次热交变过程中热应力．应变累积最后导致结构层间剥离,因此研究温度场作用 下太阳电池板结构应力应变状态具有非常重要的实际意义。本文推导了模拟太阳电池板结构应力-应变状态的一 维模型,该模型同样适用于分析多层胶接结构应力应变状态。     

FREQUENCY DEPENDENCE OF THE MAGNITUDE OF THERMAL STRESSES IN A FLAT PLATE SUBJECTED TO RAPID THERMAL CYCLING BY CONVECTIVE HEATING AND COOLING

Abstract

A study was conducted of the effect of the thermal stresses in a plate subjected to thermal cycling by changes in ambient temperature involving convective heat transfer, at frequencies sufficiently high that thermal equilibrium (;i.e., temperature uniformity) is not achieved within each cycle. The results obtained indicate that at any specific value of the Biot number the magnitude of maximum thermal stress within any cycle decreases with increasing frequency. Furthermore, at any given frequency the magnitude of the peak stress exhibits its maximum value during the first cycle and decreases to a constant value from cycle to cycle with increasing number of cycles     

Low temperature thermal safety performance of soft packaged lithium iron phosphate battery

本文以剩余容量接近80%的软包磷酸铁锂电池为研究对象,研究其在-10 ℃低温充放电循环后的安全性能.对低温和常温循环后的电池进行热失控实验分析,同时解剖电池并测试电池材料的锂元素含量和热稳定性能.测试结果表明,电池低温循环过程中容量急剧衰减,低温循环后电池热失控温度明显降低,低温循环过程中电池负极析出了锂单质,电池材料的热稳定性也发生了变化.另外,还对低温循环后的电池进行了满电状态下的常温搁置实验,实验过程中电池全部产生胀气现象,通过进一步测试分析发现,气体以CO和H₂为主.与新电池对比发现,剩余容量接近80%的软包磷酸铁锂电池低温下充放电循环更容易产生锂枝晶,造成其电化学性能发生严重的不可逆衰退,热失控温度明显提前,因此剩余容量接近80%的磷酸铁锂电池应避免在低温下运行.     

Modeling of thermal stresses and lifetime prediction of planar solid oxide fuel cell under thermal cycling conditions

A typical operating temperature of a solid oxide fuel cell (SOFC) is above 600 °C, which leads to severe thermal stresses caused by the difference in material mechanical properties during thermal cycling. Interfacial shear stress and peeling stress are the two types of thermal stresses that can cause the mechanical failure of the SOFC. Two commonly used SOFC configurations (electrolyte-supported and anode-supported) were considered for this study. The paper developed a mathematical model to estimate the thermal stresses and to predict the lifetime of the cell (Ni/8YSZ-YSZ-LSM). Due to the mismatch of the material mechanical properties of the cell layers, a crack nucleation induced by thermal stresses can be predicted by the crack damage growth rate and the initial damage distribution in the interfacial layer for each thermal cycle. It was found that the interfacial shear stress and peeling stress were more concentrated near the electrode free edge areas. The number of cycles needed for failure decreased with the increase in the porosity of electrode. The number of cycle for failure decreased with increase in electrolyte thickness for both anode- and electrolyte-supported SOFC. The model provides insight into the distribution of interfacial shear stress and peeling stress and can also predict damage evolution in a localized damage area in different SOFC configurations.     

ANALYSIS OF TRANSIENT THERMAL BENDING MOMENTS AND STRESSES OF THE WORKPIECE DURING SURFACE GRINDING

Geometrical inaccuracy is often induced by heat generated during grinding. Furthermore, the transient thermal process is the main cause for the residual stresses on theground surface. The objective of this article is to investigate the three-dimensional transient temperature distribution of the workpiece using the finite difference method,and based on the acquired temperature and beam theory, the thermal moment and thermoelastic stress as calculated using Simpson's multiple numerical integral method. The energypartition is the key factor in accurately predicting the temperature distribution, on which the solution of the thermal moment and stress rely. As the heat conductivity of the workpiece decreases, the stress and moment increase near the wheel-workpiece contact zone and the peaks move closer to the contact position. A smaller thickness results in higher thermal stress and lower thermal moment. Enhancing cooling in grinding effectively reduces temperature and the induced stress.     

生物质颗粒成型机的环模特性研究

借助Pro/Engineer实现环模的三维建模,通过ANSYS对环模温度场进行数值模拟,揭示其温度场分布规律.对环模进行结构静力学、结构动力学、结构热耦合数值模拟,揭示环模的应力场分布规律.重点探讨了参数模孔长径比对环模应力的影响.模拟结果显示模孔长径比为5:1时应力最小,与相关资料报道吻合.     

THERMAL STRESSES IN A FLAT PLATE SUBJECTED TO CONVECTIVE HEATING INDUCED BY PIECEWISE LINEAR VARIATION OF AMBIENT TEMPERATURE

The problem of determining the stress state in a plate subjected to a thermal transient is often encountered in engineering practice. Available solutions are limited to special cases and are not easy to use. The aim of this work is to provide a simple tool for stress and strain calculations due to piecewise linear variation of ambient temperature. A variational approach is applied to obtain approximate temperature and stress distributions within the plate in a simple analytic form. Stress diagrams derived from the exact temperature distribution are used to assess the accuracy of the method. The method is finally used to determine the magnitude of thermal stresses induced by thermal cycling. The results are shown to be in agreement with those of previous studies     

MOLECULAR DYNAMICS SIMULATION OF THERMAL CONDUCTIVITY OF NANOSCALE THIN SILICON FILMS

Molecular dynamics simulations are performed to explore the thermal conductivity in the cross-plane direction of single-crystal thin silicon films. The silicon crystal has diamond structure, and the Stillinger-Weber potential is adopted. The inhomogeneous nonequilibrium molecular dynamics (NEMD) scheme is applied to model heat conduction in thin films. At average temperature T = 500 K, which is lower than the Debye temperature Θ_D = 645 K, the results show that in a film thickness range of about 2–32 nm, the calculated thermal conductivity decreases almost linearly as the film thickness is reduced, exhibiting a remarkable reduction as compared with the bulk experimental data. The phonon mean free path is estimated and the size effect on thermal conductivity is attributed to the reduction of phonon mean free path according to the kinetic theory.     

EFFECT OF THE HISTORY OF TEMPERATURE ON THE THERMOELASTOPLASTIC SOLUTION

The stress paths in the transient thermoelastoplastic analysis are strongly dependent upon the history of temperature, whereas stresses in the thermoelastic analysis are determined uniquely by the distribution of temperature.

This paper treats the effect of the history of temperature on the thermoelastoplastic solution of a thick-walled tube. The transient state of stresses in the tube when subjected to inner-surface heating in which the temperature has an exponential time history is discussed in detail, on the assumption that thermal and material properties depend on temperature.     

Thermal behaviors of Ni-MH batteries using a novel impedance spectroscopy

In this paper, a novel impedance spectroscopy was used to describe the thermal behaviors of Ni-MH batteries. The impedance functions were derived similarly to electric impedance functions. The square of current was treated as a current equivalent and heat-flow as a voltage equivalent. The impedance spectra of batteries during charge showed that the combination of hydrogen and oxygen increased rapidly when charge rate was higher than 0.5 C. Thermal runaway might happen when battery was charged at temperature above 348 K even at a low charge rate. The cycling test showed that the charge efficiency of battery was the highest after cycling at high-rate for 10–100 cycles and decreased after more cycles. Different batteries showed different thermal behaviors which may be caused by the different structures of batteries.