醇类有机物热传导的分子动力学模拟及微观机理研究

传热是化工生产的基本问题之一,热导率是化工产品生产工艺设计中一类重要的热力学数据。通过非平衡分子动力学方法模拟了8种液态醇类有机物在不同温度下的导热过程。热导率计算值与实验值的平均相对偏差为3.77%。通过对热流的分解发现,分子动能、分子间库仑相互作用和分子内的二面角对醇类有机物的热传导影响较大。同时随着分子链增长,通过分子内相互作用进行的热传导逐渐占主导作用,表明醇类有机物的热能传输机理与分子结构有显著关系。此外,随着温度的升高,通过分子的动能、分子间库仑作用和分子内键角、键伸缩作用项传输的热流增大,表明温度对液态醇类有机物的热传导也有一定影响。本工作从微观分子间和分子内作用分析了液态醇类有机物结构和温度对热导率的影响,为液态有机物的热传导研究提供了微观依据。     

基于热传导反问题的各向异性材料热物性预测方法

采用不同方法对基于热传导反问题的固体热导率预测进行了研究。分别采用采用不同方法对基于热传导反问题的固体热导率预测进行了研究。分别采用Bayesian统计方法、Levenberg-Marquardt和遗传算法对二维各向异性材料的热物性进行了预测;并进行了分析比较。研究结果表明;Bayesian方法中热传导反问题的解是其后验概率密度的数学期望;而后验概率密度函数（PPDF）通过测定的温度进行计算获得;用Markov chain Monte Carlo算法计算后验状态空间以得到未知热导率的统计估计;采用Metropolis-Hasting算法进行数据采样构造Markov chain;并截取收敛后的样本进行分析。遗传算法是一种相对较新的用于最优化估计的方法;也可以用于求解反问题。     

炭黑填充橡胶的热传导理论模型及应用研究

在研究炭黑填充橡胶传热机理的基础上建立热传导模型,推导出热导率计算方程。用该式分别估算了炭黑N330和N134填充橡胶的热导率,与试验结果对比发现,在较高炭黑用量下,热导率的理论预测值与试验值具有较好的一致性。对理论公式进行修正后获得炭黑填充橡胶热导率预测的半经验公式。     

金属基覆铜板的研究与应用

介绍了金属基覆铜板的结构组成与特点,从树脂本征热导率与无机填料的选择等两个层面阐述了提高树脂中间层热导率的途径,分析了金属基覆铜板领域亟待解决的问题,指出了金属基覆铜板的研究重点与发展方向。     

多孔莫来石的热导率

采用激光闪光法测定了温度达1 000℃、孔隙度为35%~60%(体积)的多孔莫来石材料的热扩散系数。这些材料的制备是基于淀粉在水悬浮液中的膨胀性直接合成的,主要为直径30μm的球形气孔。从热导率角度出发,它们表现为在连续莫来石基质中分散的空隙的双相材料。不同孔隙度的热导率与温度的关系符合一个简单的方程式,考虑到了莫来石基质的热传导、气孔内气体以及热辐射。基质的热导率来自于对致密莫来石的测定。     

炭黑填充胶热导率模型的推导及应用

研究了炭黑填充胶复合材料的传热机理;基于最小热阻力法则和比等效导热系数相等法则,建立了热传导模型,并推导出热导率计算方程;用该方程分别估算了炭黑N234和N539填充胶的热导率,并与实验结果进行比较。结果表明,当炭黑体积分数低于20%时,热导率的预测值与实验值有较好的一致性;随着炭黑填充量的增加,炭黑填充胶的热导率逐渐增加;填充量相同时,N 234填充胶的热导率大于N539填充胶,体积分数越低,两者的热导率相差越小,反之则越大。     

基于分形理论的纳米颗粒多孔介质真空导热特性

采用分形理论,描述了纳米颗粒多孔介质材料的微尺度空间结构,建立了分形等效单元体模型,分析了影响其真空下有效热导率关键因素为固体基质热导率、填充率、分形维数、分形直径、残余气体压力及热导率等,并导出了气相、固相热传导计算公式和热辐射等效热导率计算公式及多孔介质材料总有效热导率计算公式。研究表明,纳米颗粒有效热导率随着分形直径、残余气体压力的增大而增大,并给出了纳米颗粒多孔介质材料作为真空材料的最佳直径。同时,模型计算值与实验测量值比较,具有较好的一致性。该理论分析方法对新型真空绝热材料的研制和绝热性能的提高具有实用价值。     

纤维增强复合材料热导率分析

纤维增强复合材料的热导率有方向性,如碳、石墨纤维增强复合材料,沿纤维方向比垂直纤维方向的热导率大得多。以树脂为基体的复合材料,沿垂直铺层方向的热导率往往很小,是一种良好的隔热保温材料,因而在火箭,导弹、航天飞机方面得到广泛应用。纤维增强复合材料的热导率不仅与纤维和基体的热导率有关,还与纤维的排列及方向有关。本文从理论上得出纤维增强复合材料各方向热导率的估算公式,并与试验结果进行了比较,理论计算值与实测值较符合。     

锂电池卷绕结构有效热导率研究

卷绕结构因具有更优异的电学性能而被广泛应用到锂电池中,为了更准确地研究锂电池热特性,应同时考虑锂电池卷绕结构径向及周向热传导。通过数值模拟的方法研究了卷绕层数、卷绕张力以及正极材料热导率对卷绕结构有效热导率的影响。结果表明:随着卷绕层数的增加,卷绕结构的有效热导率逐渐减小,有效热导率的变化幅度也减小;在相同的卷绕层数下,随着卷绕张力的增加,有效热导率逐渐增大;对于低卷绕层数结构,卷绕张力对有效热导率的影响显著,而对于高卷绕层数结构,有效热导率的变化相对平缓。提高正极材料的热导率可以在一定程度上改善卷绕结构的等效热导率,但是对于卷绕层数较大的结构,正极材料的热导率超过40 W/(m·K)后,对有效热导率的影响很小,因此,应通过改善多种材料的导热性能来改善锂电池的有效热导率。     

高强度聚乙烯纤维的热导率及其应用

<正>高分子材料的应用特性中,导热性是有关冷热感、隔热性等的重要特性。如图1所示,大多数高分子的热导率比金属的热导率低,因此主要被用作绝热材料。由共价键构成的分子链轴向有很高的热传导性,例如聚乙烯(PE),也即高结晶性、高取向性的聚合物在取向方向有高的导热性。高强度PE纤维(东洋纺的Dyneema)及聚对苯撑苯并双噁唑纤维(PBO,东洋纺的ZylonR)在纤维方向有类金属的高导热性,如图2所示。     

二维氮化铝材料传热性能的模拟研究

二维氮化铝材料是一种新型Ⅲ-Ⅴ族二维材料,具有与石墨烯相似的分子结构和材料性能,受到了广泛的关注,然而其导热性能尚未被充分探讨。应用分子动力学模拟的方法研究了单层二维氮化铝在不同温度的热稳定性和导热性能,并分析了其声子频谱。结果表明,单层二维氮化铝材料可以在极高温度（3500 K）下保持结构稳定性,同时在常温情况热导率可达264.2 W·m^-1·K^-1;在500 K以上温度时,声子色散现象使得该材料热导率明显降低。为二维氮化铝材料导热过程的调控和高温导热材料的应用提供了理论指导。     

The dependence of thermal conductivity of binder residues on heat treatment temperature

The dependence of the thermal conductivity of carbon residues of one pitch and one polyfurfuryl alcohol binder on the heat treatment temperature has been investigated. Specially prepared graphites made with these two binders were used for the experiments. The thermal conductivity data were analyzed in terms of a two-component system and the binder residue thermal conductivity calculated. Both binder residues show an increase in thermal conductivity with increasing heat treatment temperature.     

The use of drying experiments in the study of the effective thermal conductivity in a solid containing a multicomponent liquid mixture

The effective thermal conductivity of a porous solid containing multicomponent liquid mixtures has been studied. To achieve this, the liquid composition, liquid content and temperature distributions have been measured in a cylindrical sample dried by convection from the open upper side and heated by contact with a hot source at the bottom side. A quasi-steady state reached at high source temperatures permits to calculate the total heat flux from temperatures measured on the surface and the gas stream. The simulations performed and compared with experimental data made it possible to estimate the adjusting geometric parameter of Krischer's model for the effective thermal conductivity. The effective thermal conductivity has been widely studied for two-phase systems, mostly with regard to thermal insulation elements. The calculation of this transport parameter includes the contribution to heat transfer of the evaporation–diffusion–condensation mechanism undergone by the multicomponent mixture. The influence of liquid composition and temperature on the thermal conductivity due to the evaporation–diffusion–condensation mechanism and the effective thermal conductivity is described. The results reveal that in this case the resistance to heat transfer seems to correspond to a parallel arrangement between the phases.     

Measurement of thermal conductivity of Green River oil shales by a thermal comparator technique

A thermal comparator method has been developed for the measurement of the thermal conductivity of oil shales from the Green River formation. Oil shales from two locations in the above formation were studied. Measurements were carried out in the temperature range 25–350 °C on oil shales assaying between 28.3 and 333.0 ml/kg (6.8 and 79.9 U.S. gal/short ton). Predictive equations showing the variation of thermal conductivity with temperature and shale grade are presented. A simple model is proposed to explain the anisotropic effects observed in the thermal-conductivity values for heat flowing in directions parallel and perpendicular to the shale bedding planes. The relative merits of the thermal comparator technique are discussed in the light of techniques previously employed for the measurement of thermal-conductivity factors of Green River oil shales.     

侧面散热对导热系数测量的影响研究

稳态平板法测量导热系数因方法简单直接在测量材料的导热系数中广泛应用。但通过数值模拟和实验发现侧面散热对导热系数测量影响明显,采用大空间自然对流换热实验关联式对实验数据处理公式进行修正,在实验中得到较满意结果。利用数值模拟和实验对影响侧面散热的主要因素如环境温度、热源温度、样品厚度、试样导热系数进行研究,发现环境温度降低、热源温度增加、试样厚度的增加以及试样导热系数的减小,都会导致传热温差增加,侧面散热的绝对量和占总传热量的比例也随之增加。     

Shear-induced anisotropy in thermal conductivity of a polyethylene melt

A rotating concentric-cylinder thermal conductivity cell for polymer liquids is described. Thermal conductivity can be measured at temperatures approaching 200°C and at strain rates up to 400 s^?1, The transient heat flux probe (with inner cylinder as heat source and temperature probe) method is used to permit the separation of the viscous heating effect from the probe heating effect. A polyethylene melt was studied and showed that at 50 s^?1, a 2 percent increase in thermal conductivity occurs, followed by a gradual decrease until a value 10 percent less than the no-shear thermal conductivity was found at 400 s^?1. This effect is due to molecular orientation.     

On thermal conductivity of micro‐ and nanocellular polymer foams

Many theoretical and empirical models exist to predict the effective thermal conductivity of polymer foams. However, most of the models only consider the effect of porosity, while the pore size effect is ignored. The objective of this study is to understand the effect of pore size on the thermal conductivity of polymer foams, especially when it reduces to the micro and nanometer scales. A wide range of pore sizes from 1 nm to 1 mm were studied in conjunction with the porosity effect using finite element analysis and molecular dynamics simulation methods. Experimental data was used to validate the modeling result. It is shown that the pore size has significant effect on thermal conductivity, even for microcellular and conventional foams. The contribution of heat conduction through air is negligible when pore size is reduced to the micrometer scale. The extremely low thermal conductivity of nanofoams is attributed to extensive diffusive scattering of heat carriers in the solid phase of polymer matrix, instead of air. This study provides quantitative understanding of the pore size effect on thermal conductivity of polymer foams. It is also shown that polyetherimide (PEI) nanofoams could have a thermal conductivity as low as 0.015 W/m‐K. POLYM. ENG. SCI., 2013. © 2013 Society of Plastics Engineers     

Estimating effective thermal conductivity in carbon paper diffusion media

Heat management is crucial to polymer electrolyte membrane (PEM) fuel cell commercialization. Numerical modeling is often used to simulate heat transfer in the various components of the cell and specifically the gas diffusion layer (GDL). Due to the porous nature of the gas diffusion layer and its complexity of anisotropy, the effect of the structure on the thermal conductivity is usually taken into account by introducing an effective thermal conductivity. In this study, the effective thermal conductivity of carbon paper diffusion media was estimated numerically. Carbon paper is often used as the GDL in PEM fuel cells due to its ability to efficiently transport electrons, heat and gaseous species. Using the GeoDict code, a realistic three-dimensional pore morphology of carbon paper was used as the modeling domain and the governing mathematical equations were solved using the commercial software package Fluent (6.3.26) and the ThermoDict solver. The geometrical effects on the effective thermal conductivity were investigated for different geometries. It was found that the effective thermal conductivity is highly sensitive to the geometry of the porous material under investigation. The effective thermal conductivity is much larger in the in-plane direction when compared with the value in the through-plane direction. Further, the change of the effective thermal conductivity due to porosity and compression was studied. Finally, correlations for the through-plane and in-plane effective thermal conductivity were developed.     

相变热界面材料导热增强及定形改善的研究进展

将相变时伴随潜热的相变材料（phase change material, PCM）特别是潜热值较大的固-液PCM引入热界面材料（TIM）领域,有望获得兼具储热和导热双功能的新型热界面材料——相变热界面材料（phase change thermal interface material, PCTIM）。然而,鉴于固-液相变材料的热导率普遍较低且存在液相流动泄漏问题,使得增强热传导并同时提升固-液相变材料的定形性成为研制高性能相变热界面材料（PCTIM）的关键。本文系统评述了国内外研究者在提升相变热界面材料热导率以及改善其定形性方面的策略及其研究进展。文中指出,目前强化PCTIM导热的手段主要有添加高导热填料、促使填料有序结构化以及使用低熔点金属等。在改善定形性方面,已运用的策略主要包括使用柔性载体负载固-液PCM以在保证一定柔性的基础上克服其液相泄漏问题,使用固-固PCM来取代固-液PCM来彻底避免液相泄漏问题的出现,以及将固-液PCM封装在微米级或纳米级胶囊内,旨在牺牲借助液相PCM增加柔性的功能,而且通过提高PCTIM的潜热值来提升其抗热流冲击性能。文章指出,当前已研制的PCTIM热...     

Measurements of the thermal conductivity and thermal diffusivity of polymers

In this paper, the thermal conductivity and thermal diffusivity of nine polymers were measured by using the transient short‐hot‐wire method. The corresponding specific heat was measured with a commercial Differential Scanning Calorimeter (DSC). The effects of temperature on the thermal conductivity, thermal diffusivity, and the product of density and specific heat are further discussed. The results show that the transient short‐hot‐wire method can be used to measure the thermal conductivity, thermal diffusivity, and the product of density and specific heat of polymers within uncertainties of 3%, 6%, and 9%, respectively.