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.     

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。     

光伏组件单元的热应力和内力分析

由于层间热失配效应和边界效应,在层合结构边缘存在局部剥离应力和界面剪切应力,引起光伏组件单元的破坏。针对此类问题,首先采用张能辉提出的两变量方法,给出层合结构内部轴向正应力和边界区域界面剥离内力的解析模型。随后进行有限元数值模拟,通过对比证明解析预测具有较高的准确性;同时讨论硅片层弹性模量和厚度对层间剥离内力和轴向正应力的影响。最后考虑温度变化,讨论导热层对光伏组件单元热应力和剥离内力的影响。     

石墨浆做背接触层的碲化镉薄膜太阳电池及组件的研究

使用掺杂石墨浆作为碲化镉薄膜太阳电池的背接触层.研究了石墨浆的成分、涂覆了石墨浆后的热处理工艺对单元电池器件性能的影响.使用优化工艺制备了大面积集成碲化镉薄膜太阳电池.结果表明:使用石墨浆背接触层,可将单元电池效率从3.8%提高到10.2%;将优化了的石墨浆处理工艺应用到27.0cm×36.7cm的大面积集成碲化镉薄膜太阳电池上,得到了7.4%的转化效率.     

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。     

月牙肋钢岔管设计中若干问题的探讨

针对月牙肋钢岔管顶部出现较大局部应力使管壳厚度大为增加、有限元计算肋板应力受网格剖分影响较大的问题,以安徽绩溪水电站钢岔管为例,探讨了钢岔管基本锥腰线转折角对管壳腰部和顶部局部应力的影响,比较分析了肋板不同单元类型和网格疏密对计算结果的影响。结果表明,钢岔管基本锥腰线适宜转折角为6°～14°,月牙肋采用实体单元模拟更精确,采用板壳单元模拟更简便,肋板网格疏密对应力分布规律影响不明显,仅对最大应力值有影响。     

效率超过19%的CdTe薄膜太阳电池

从电池结构、关键制备技术与流程、功能层材料与器件性能方面详细阐述了本研究组在高效率宽光谱CdTe薄膜太阳电池方向的研究工作。提出了新的扩散预制层制备工艺,拓展梯度带隙吸收层制备和组分调控工艺窗口;协同调控梯度吸收层预制结构与Se扩散相关的吸收层制备热过程和活化热过程,优化梯度吸收层组分分布和能带结构;解决前电极窗口层与传统制备技术的兼容性问题,消除限制转换效率的前界面势垒;制备得到两种主流结构的CdTeSe薄膜太阳电池,获得19.1%的器件光电转换效率。     

基于弯扭耦合的自适应风力机叶片设计

探讨了叶片耦合设计的有限元方法.分别运用偏轴碳纤维粱帽与蒙皮两种结构,对1.5MW叶片进行了弯扭耦合等综合参数的设计与评估.分析结果表明,两种方法均能使叶片产生高效的弯扭耦合;但叶片各层纤维的最大拉伸与压缩应变会随偏轴角的增加而增大,最大层间剪切应力也增大,而最大Von Mises应力则会相应下降;同时分析了叶片固有频率随偏轴角的变化,最后将中轴铺层设计运用于叶片非耦合结构中,提高了叶片的抗疲劳性.     

PTB7∶PC_(61)BM聚合物太阳电池的制备与研究

在各种有机聚合物太阳电池活性层材料中,聚苯并二噻吩(PTB7)与富勒烯衍生物(PCBM)的共混材料所制作的电池取得现有单层体相异质结太阳电池领域中最好的光电转换效率。本实验在空气环境中制作结构为ITO/PFN/PTB7∶PC61BM/Mo O3/Al的反型有机聚合物太阳电池,对制作过程中溶液处理、薄膜干燥方式和活性层厚度控制等步骤进行不同条件的对比;并运用紫外可见吸收光谱和J-V曲线测试等方法研究具体制作步骤对太阳电池性能的影响;最后通过测试电池放置不同时间的J-V特性曲线,证明该反型电池在空气中具有良好的稳定性。     

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.     

MOLECULAR SIMULATION OF SOME THERMOPHYSICAL PROPERTIES AND PHENOMENA

Molecular dynamics study has been performed on ultra-thin liquid film sheared between two solid surfaces, which has a direct relation to lubrication. Energy and momentum transfer in the liquid film and at the solid-liquid interface accompanied by viscous heating are analyzed. The system consists of liquid film where molecules are modeled by the Lennard-Jones (12-6) potential and two parallel solid walls having a spacing of several nanometers. The solid walls have a constant temperature and move at a velocity in the opposite directions to each other, which causes a shear in the liquid film. A layered structure has been formed in the liquid by the effect of interaction with solid molecules, in which highly nonequilibrium distribution of thermal energy among the degrees of freedom for molecular motion is observed.     

Numerical investigation on chill-down and thermal stress characteristics of a LH2 tank during ground filling

To investigate the thermal and structural characteristics of a flight-scale LH₂ tank during ground fillings, a CFD model and a structural analysis model are established to simulated the chill-down process and the induced thermal stress behavior of the tank, respectively. Results show that, at the early stage of filling, a severe temperature gradient appears at the liquid level, leading to a remarkable local concentration of thermal stress, while the maximal thermal deformation is at the outlet region. After the local wall is chilled down sufficiently, the temperature jump at the interface vanishes as well as the local thermal stress, while the maximal thermal deformation is located at the middle height of tank. The thermal stress is most serious at the beginning stage of filling and the maximum appears at the tank bottom. Moreover, the non-uniformity of the temperature distribution and the average thermal stress level within the tank wall both increase with the filling rate. At a filling rate of 7.5 kg·s⁻¹, the maximal thermal stress and thermal deformation of the target tank are more than 70 MPa and 30 mm.     

Dendritic constructal heat exchanger with small-scale crossflows and larger-scales counterflows

This paper describes the constructal route to the conceptual design of a two-stream heat exchanger with maximal heat transfer rate per unit volume. The flow structure has multiple scales. The smallest (elemental) scale consists of parallel-plates channels the length of which matches the thermal entrance length of the small stream that flows through the channel. This feature has two advantages: it eliminates the longitudinal temperature increase (flow thermal resistance) that would occur in fully developed laminar flow, and it doubles the heat transfer coefficient associated with fully developed laminar flow. The elemental channels of hot fluid are placed in crossflow with elemental channels of cold fluid. The elemental channel pairs are assembled into sequentially larger flow structures (first construct, second construct, etc.), which have the purpose of installing (spreading) the elemental heat transfer as uniformly as possible throughout the heat exchanger volume. At length scales greater than the elemental, the streams of hot and cold fluid are arranged in counterflow. Each stream bathes the heat exchanger volume as two trees joined canopy to canopy. One tree spreads the stream throughout the volume (like a river delta), while the other tree collects the same stream (like a river basin). It is shown that the spacings of the elemental and first-construct channels can be optimized such that the overall pumping power required by the construct is minimal. The paper concludes with a discussion of the advantages of the proposed tree-like (vascularized) heat exchanger structure over the use of parallel small-scale channels with fully developed laminar flow.     

Numerical simulation of stress distribution for CF/EP composites in high temperatures

The high-performance carbon fiber materials can be obtained by decomposing carbon fiber reinforced resin matrix composites using thermally activated oxide semiconductors. This paper established the representative volume element (RVE) of carbon fiber/epoxy resin composites, and investigated the structure destruction of composites in the recycling process based on analysis of the stress and strain distribution by the thermomechanical coupling module of Digimat. The results indicated that the maximum thermal stress of the epoxy resin appeared in the poor resin region, while the minimum appeared in the resin-rich region; the stress of the carbon fibers in poor resin region was greater than that in the resin-rich region; the maximum stress of composites appeared in the interface layers when the temperature ranged from 350 to 500?°C, and the maximum thermal stress was 196.9–281.3?MPa, as well as the maximum shear stress was 98.2–140.3?MPa; the maximum peeling stress perpendicular to the fiber directions was 53.7–157.3?MPa; the strain of the interface layers and carbon fibers were the smaller than that of the resin matrix, whose maximum strain ranged from 0.0622 to 0.0889. The structure destruction of the composites was caused by both the peeling stress and the interfacial shear stress in recycling.     

Ultrashort pulse induced elastodynamics in polycrystalline materials. Part II: Thermal–mechanical response

Near-field thermal–mechanical responses in polycrystalline gold films considering grain size effects and temperature-dependent thermophysical properties are examined using the generalized thermo-elastodynamic model formulated in Part I. Ultrashort laser-induced transverse and radial thermal stress waves are attenuative, broad in bandwidth, and dispersive with extremely high frequencies. Grain size effects on thermal response are found to be so prominent that the stress fields induced in the polycrystalline film considered for the study are consistently more intense with decreasing averaged grain diameter. The normal and shear stress fields induced by a below melting threshold fluence are low in amplitude but extremely high in frequency response and power density. With the former not exceeding 20?MPa and the latter of the order of 10¹⁸-to-10¹⁹ [W/m³] in magnitude, non-melting ultrafast heating that generates no plastic deformation is profound with the potential for initiating fatigue micro-cracking in near-field and for inflicting physical damage.     

MULTIPLE SURFACE CRACKING IN GLASS-FIBER-REINFORCED PLASTICS DUE TO A THERMAL SHOCK AT A LOW TEMPERATURE

We consider the transient thermal singular stress problem of multiple surface cracking in glass-fiber-reinforced plastics due to a thermal shock at a low temperature. The layered composite is made of a layer bonded between two layers of different physical properties, and it is suddenly cooled on the surfaces. The surface layers contain parallel arrays of the embedded or edge cracks perpendicular to the boundaries. The thermal and elastic properties of the material are dependent on the temperature. For the case of the crack that ends at the interface between orthotropic elastic materials, the order of stress singularity around the tip of the crack is obtained. Finite element calculations are carried out, and the transient thermal stress intensity factors are shown graphically.     

Thermal Stresses in Thin Auxetic Plates

This article investigates the effect of auxeticity on the thermal stresses of isotropic plates. The thermal stress is non-dimensionalized against the coefficient of thermal expansion, the change in temperature and at least one of the moduli so as to express the dimensionless thermal stresses solely in terms of Poisson's ratio of the plate material. Results show that increasing auxeticity leads to mild and significant drop in the thermal stresses under the conditions of constant Young's modulus and constant shear modulus, respectively. However, increasing auxeticity causes increase in the thermal stress under the condition of constant bulk modulus. It is also shown that increasing auxeticity under the condition of constant product of all the three moduli reduces the thermal stress if Poisson's ratio falls within a wide range of ?1 and 0.303. These results suggest that, under most circumstances, the replacement of conventional plate materials with auxetic solids is useful for reducing thermal stresses therein. The use of auxetic materials, therefore, provides an additional choice for the reduction of thermal stresses in plates other than selecting materials of lower modulus and low coefficient of thermal expansion.     

THERMAL STRESSES IN A PARTIALLY ABSORBING FLAT PLATE ASYMMETRICALLY HEATED BY CYCLIC THERMAL RADIATION AND COOLED BY CONVECTION

Abstract

An analysis is presented for the temperatures and thermal stresses in a partially absorbing flat plate subjected to normally incident cyclic thermal radiation on the front face, and cooled by convection on the rear face.

The resulting temperature and stress responses are cyclic in nature, with the temperatures and stresses in the center and the back face lagging behind those in the front face. The amplitude of the temperature fluctuation is found to be a maximum in the front face. During the initial thermal cycles the maximum stress in the plate is compressive. In contrast, when the plate approaches thermal equilibrium after many cycles, the maximum stress is tensile. The values of the maximum tensile and compressive stresses were found to depend on the frequency of the incident cyclic radiation.