A novel method to determine effective thermal conductivity of porous materials

1 Introduction Due to some unique structure and characteristics, the porous material behaves as afunctional material and is finding more and more applications in a variety of fields suchas aviation, chemistry and energy[1]. In such cases, the effective thermal conductivity ofthe porous materials is a vital parameter that dominates application performances. Gen-erally speaking, the effective thermal conductivity can be measured experimentally, butin some cases (for example the stuffing is l…     

多孔材料有效导热系数的试验研究

本文根据多孔材料的传热特点,修正了气孔内辐射的传热计算,发展了辐射当量导热系数的新概念,建立了多孔材料有效导热系数的数学模型,推导出计算多孔材料有效导热系数的计算式。试验结果和有关文献的试验结果均证明:该模型更加符合多孔材料的实际情况,计算精度更高。     

非均匀吸放湿过程对建材导热系数的影响

在针对含湿建材导热系数的研究中,预测模型一般假设材料含湿均匀,试验测量多采用均匀增湿方案,但实际使用过程中,受室内外湿度及气象条件影响,造成吸放湿过程中湿组分非均匀分布,相同含湿量下吸放湿过程中的材料导热系数存在差异。通过对孔隙特征不同的加气混凝土(AAC)、发泡水泥(FC)、红砖(RB)材料进行增减湿试验,控制试件在试验环境中的放置时间获得质量含湿量不均匀的试件,选择瞬态导热系数测量方案,通过分析含湿多孔建材增/减湿全过程导热系数变化趋势,研究非均匀湿分布材料导热系数变化特性。结果表明：AAC、FC、RB加湿过程导热系数相较干燥状态下分别增长了279%、266%、106%。增减湿过程中,含湿量相同的AAC导热系数差值范围约为-20.2%~63.4%,FC差值范围约为14.5%~53.3%,RB差值范围约为-18.0%~37.4%。     

形状因子对应态原理研究流体及混合物的导热系数

采用作者提出的形状因子对应态原理，由纯流体的热力学性质求得保形参数。关联了纯流体及混合物的导热系数。本文方法不但适用于汽、液两相，而且能从二元体系的导热系数推算三元体系的导热系数，结果满意。     

离子喷涂W-Cu电子封装材料的组织与性能

采用等离子喷涂工艺制备了W Cu电子封装材料，并对该电子封装材料的微观组织和热物理性能进行了研究. 结果表明，在内部送粉条件下，复合材料中钨的收得率要高于外部送粉条件下复合材料中钨的收得率；在内部送粉条件下，功率对铜氧化的影响并不明显；而在外部送粉条件下，随着功率的提高，铜的氧化明显增多；由于氧化物和孔隙等的存在，利用大气等离子喷涂工艺制备的复合材料的导热率远远低于根据混合法则计算的理论值. 这为后续实验研究提供了重要的参考依据.     

高分子基多孔炭材料的研究进展

近年来多孔炭材料已经逐渐成为国内外的研究热点并且得到了广泛的应用.本文从高分子基多孔炭材料的制备、应用前景等方面进行了综述,并重点介绍了由不同种类的高分子前驱体制备多孔炭材料的研究进展.     

等离子喷涂W-Cu电子封装材料的组织与性能

采用等离子喷涂工艺制备了W-Cu电子封装材料，并对该电子封装材料的微观组织和热物理性能进行了研究．结果表明，在内部送粉条件下，复合材料中钨的收得率要高于外部送粉条件下复合材料中钨的收得率；在内部送粉条件下，功率对铜氧化的影响并不明显；而在外部送粉条件下，随着功率的提高，铜的氧化明显增多；由于氧化物和孔隙等的存在，利用大气等离子喷涂工艺制备的复合材料的导热率远远低于根据混合法则计算的理论值．这为后续实验研究提供了重要的参考依据．     

Investigation of microgravity effect on solidification of medium-low-melting-point alloy by drop tube experiment

The solidification microstructure of Al-Ni, Al-Cu, Ag-Cu, Al-Pb and Cu-Co alloys quenched in silicone oil before and after free fall in evacuated 50 m drop tube were investigated contrastively. The effect of microgravity on the solidification process of medium-low-melting-point eutectic, monotectic and peritectic alloys were analyzed and discussed. The results indicated that the effects of microgravity on the eutectic cell shape, the pattern in eutectic cell and the inter-eutectic spacing were different for different types of eutectic systems; the size distribution of Pb particles in Al-5wt% Pb monotectic alloy was significantly changed by microgravity; and the shape of retained primary α-Co phase in Cu-10wt%Co peritectic alloy was also changed by microgravity. These results are beneficial for people to further identify and analyze the solidification behavior of multiphase alloys under microgravity.     

镍泡沫定型相变材料制备与性能研究

聚光光伏热电耦合系统(CPV-TE) 通过聚光增大光照强度, 提升热电模块两侧的温差进而提高热电效率, 但聚光同时也带来光伏电池温度过高等问题。提高冷端换热能力是降低光伏温度, 提升热电冷热端温差的有效手段。研究了不同光强下, 冷端冷却温度、冷却水流速以及不同冷却介质对CPV-TE 的影响。研究发现降低冷却水温度 和提高介质流速, 不仅降低光伏电池温度、提高光电转化效率, 而且增大热电模块两侧的温差提高热电效率。采用MWCNT- 水纳米流体作为换热介质, 可以进一步提升冷却效果进而提高系统整体输出。     

Study on undercooling of metal droplet in rapid solidification

In 1950, Turnbull et al.[1] dispersed 18 kinds of metals including Cu, Ni, etc. into droplets with diameters of 10-100 μm and obtained a large undercooling of 0.18 TL. Volkmann et al.[2] have successfully obtained the undercooling of 0.3 TL in the Co-Pd alloy by means of the molten glass purified technique. As yet, the maximum undercool-ing in liquid metals is acquired by Perepezko et al.[3], who makes the Ga with a diameter of 20 μm reach an undercooling of about 0.58 TL by use of the …     

多孔低介电常数材料研究进展

摘要: 具有低介电常数的多孔材料适合于集成电路方面的应用.从组成与结构、制备方法和介电性能等方面,分别介绍了以无机材料、有机材料、无机/有机复合相为基体的多孔低介电常数材料,其介电常数分别可以降低至1.99、1.50、1.99.以有机材料为基体的多孔低介电常数材料的使用温度达到450 ℃;以无机材料为基体的多孔低介电常数材料的抗弯强度达到136 MPa.在获得低介电常数的同时,改善材料由于引入孔隙导致的材料力学性能下降、介电损耗升高等问题,可以进一步拓展材料的应用空间.     

热电材料研究的现状与发展趋势

热电材料作为以废热和太阳能为热源的发电技术的关键,受到世界各国的高度关注。近年来,随着纳米材料制备技术的不断进步和对纳米尺度下电子、声子输运机制研究的不断深入,热电材料的研究取得了很大突破。本文简要介绍了热电发电技术的能量转换原理,综述了热电材料在原子尺度、纳米尺度和微米尺度下的研究进展,指出从增加简并能带数、调控有序结构、形成共振能级等方面提高功率因子是热电材料研究未来发展的趋势。     

Directional solidification and physical properties measurements of the zinc-aluminum eutectic alloy

Zn-5wt％ Al eutectic alloy was directionally solidified with different growth rates (5.32-250.0 tm/s) at a constant temperature gradient of 8.50 K/mm using a Bridgman-type growth apparatus.The values of eutectic spacing were measured from transverse sections of the samples.The dependences of the eutectic spacing and undercooling on growth rate are determined as λ=9.21 V-0.53 and △T=0.0245 V0.53,respectively.The results obtained in this work were compared with the Jackson-Hunt eutectic theory and the similar experimental results in the literature.Microhardness of directionally solidified samples was also measured by using a microhardness test device.The dependency of the microhardness on growth rate is found as Hv=115.64V0.13.Afterwards,the electrical resistivity (r) of the casting alloy changes from 40× 10-9 to 108× 10-9 Ω·m with the temperature rising in the range of 300-630 K.The enthalpy of fusion (△H) and specific heat (Cp) for the Zn-Al eutectic alloy are calculated to be 113.37 J/g and 0.309 J/(g.K),respectively by means of differential scanning calorimetry (DSC) from heating trace during the transformation from liquid to solid.     

Cu-Zr合金定向凝固组织的研究

采用定向凝固方法制备了Cu 12 .2 5Zr自生复合材料,并通过扫描电镜(SEM)、透射电镜(TEM)和X射线衍射等方法对定向凝固组织进行了研究.结果表明,在温度梯度一定的前提下,生长速度对定向凝固组织的形貌有着重要的影响,随着生长速度的增加,凝固组织由层片状依次转化为棒状、树枝状和集群状组织;在所制备的Cu 12 .2 5Zr自生复合材料中,第2相主要是Cu5Zr,并且Cu5Zr与基体α(Cu)之间存在着一定的取向关系,另外还有少量的Cu8Zr3 相和Cu10 Zr7相;在Cu 12 .2 5Zr合金枝晶端部存在明显的Zr元素富集.     

The electrical conductivity characteristics of Fe/Cu nano-scale multilayer materials

A mathematical model for describing the relationship between electrical conductivity and the thickness of bilayer, ratio of sublayer thickness of a nano-scale multilayer material (MLM) is presented. Fe/Cu MLM was synthesized by electron beam physical vapor deposition (EB-PVD) technique, and the dependence of electrical conductivity of Fe/Cu MLM on the bilayer thickness and ratio of sublayer thickness were investigated. It is shown that the electrical conductivity of Fe/Cu MLM with fixed ratio of sublayer thickness decreases sharply when the thickness of bilayer becomes thinner than 30 nm. When the bilayer thickness is kept constant, the electrical conductivity linearly decreases with the increasing ratio of sublayer thickness. The values of parameters in the model were obtained by fitting the measured results of electrical conductivity of Fe/Cu MLM with fixed ratio of sublayer thickness. It is found that the calculated values agree well with measured ones.     

透水铺装材料湿物理性质测定

为获取典型透水铺装材料的湿物理性质,以用于进行透水铺装热湿传递计算,进而科学评价其蒸发降温效果。以中国居住区广泛使用的3种透水铺装材料：烧结陶瓷透水砖（TC）、再生骨料混凝土透水砖（ZS）和普通透水砖（PT）为研究对象,参考相关标准,采用真空饱和实验测试了透水铺装材料的表观密度ρ、开放孔隙率φ及真空饱和含湿量w_vac;采用单面浸泡实验测试了毛细饱和含湿量w_cap及吸水系数A;采用渗透实验测试了其渗透系数k_T。通过对测试结果进行误差分析,建立了透水铺装材料与液态水相关的湿物理性质数据库。通过对比3种材料测试数据发现：保水性能方面,ZS最优、PT次之、TC最差;吸水性能方面,TC最优、PT次之、ZS最低,表明材料保水与吸水性能并非正相关;渗透性能方面,TC性能最优、PT次之、ZS最弱,在满足渗透性能的前提下,PT最有利于吸水后的蒸发降温。     

固体氧化物燃料电池连接体研究进展

首先介绍了作为固体氧化物燃料电池(SOFC)连接体材料所必备的条件和特性,然后对目前已被开发的SOFC连接体材料在高温环境下存在的问题作了概括性的评述,并指明了各种连接体材料的今后发展趋势.最后着重指出含稀土元素及其它微量元素的铁素体不锈钢是今后平板型SOFC连接体材料研究的一个重要方向.     

孔隙率对Al2O3高孔隙率多孔介质EHC的影响

新型多孔介质燃烧技术主要采用泡沫陶瓷 ( Ceramic Foams)、波纹状陶瓷 ( Fabric LamellaStructure)等高孔隙率多孔介质。基于气、固相局部热平衡假设的有效导热系数 EHC( EffectiveHeat Conductivity)描述了气流与固体骨架中导热、弥散和热辐射多种传热方式的综合效果 ,是一个重要的传热特性 ,对其研究非常不足。通过实验与数值模拟确定不同孔隙率 Al2 O3 陶瓷的 EHC:基于测定的温度分布 ,给定 EHC的初值 ,采用有限体积法进行流场数值模拟 ;比较测量与计算的温度均方根差 ,通过对 EHC搜索寻优 ,间接确定泡沫陶瓷的有效导热系数。结果表明 Al2 O3 陶瓷的EHC随温度增加而增加 ,对速度不敏感 ;孔隙率高的陶瓷其 EHC较大。给出试样 EHC与温度和空管速度的拟合式 ;将颗粒床 EHC的 Z&B模型外推到 Al2 O3 高孔隙率陶瓷 ,并给出 EHC预示结果     

Frequency-dependent dynamic effective properties of porous materials

The frequency-dependent dynamic effective properties (phase velocity, attenuation and elastic modulus) of porous materials are studied numerically. The coherent plane longitudinal and shear wave equations, which are obtained by averaging on the multiple scattering fields, are used to evaluate the frequency-dependent dynamic effective properties of a porous material. It is found that the prediction of the dynamic effective properties includes the size effects of voids which are not included in most prediction of the traditional static effective properties. The prediction of the dynamic effective elastic modulus at a relatively low frequency range is compared with that of the traditional static effective elastic modulus, and the dynamic effective elastic modulus is found to be very close to the Hashin-Shtrikman upper bound.     

水热法制备低热导率 PbS 热电材料

通过纳米结构或声子工程降低热导率是改善PbS热电材料性能的重要途径。利用水热法制备Pb_(1-x)Bi_xS纳米热电材料,并对其热导率进行测试。结果表明,制备的纳米PbS的热导率比文献中PbS的热导率低10%左右。随着掺入Bi元素的增加,增加了晶体内部缺陷, Pb_(0.9)Bi_(0.1)S样品的热导率比PbS单体的热导率降低了33%。