填充型聚合物基复合材料的导电和导热性能

研究了高密度聚乙烯为基体、炭黑和炭纤维为填料复合体系的导电和导热性能。发现当导电填料的含量达到渗流阈值时,复合材料的电导率急剧升高;而在渗流阈值附近,其热导率未出现突变。这表明电导渗流现象不完全是由导电粒子通过物理接触生成导电链所致。其导电机制是相当数量的导电粒子相互发生隧道效应。     

Thermal and Electrical Conductivity Measurements on Aluminum Foams

Metal foams are one of the most interesting types of materials although there is limited information concerning their thermal and electrical conductivity. Closed cell different density Alporas foams are investigated, which has one of the most homogeneous cell size distribution recently. Comparative method has been chosen to determine the thermal conductivity of the samples in the function of the temperature at 30, 100, 200, 300, 400, 500 °C. For measuring the electrical conductivity of aluminium foams a special low frequency eddy current measuring apparatus was used. The ratio of thermal and electrical conductivity was calculated and shown an increasing function by the density of the foams.     

Measurements of Thermal Conductivity and Electrical Conductivity of a Single Carbon Fiber

In this paper, the thermal conductivity of a single carbon fiber under different manufacturing conditions is measured using the steady-state short-hot-wire method. This method is based on the heat transfer phenomena of a pin fin attached to a short hot wire. The short hot wire is supplied with a constant direct current to generate a uniform heat flux, and both its ends are connected to lead wires and maintained at the initial temperature. The test fiber is attached as a pin fin to the center position of the hot wire at one end and the other end is connected to a heat sink. One-dimensional steady-state heat conduction along the hot wire and test fiber is assumed, and the basic equations are analytically solved. From the solutions, the relations among the average temperature rise of the hot wire, the heat generation rate, the temperature at the attached end of the fiber, and the heat flux from the hot wire to the fiber are accurately obtained. Based on the relations, the thermal conductivity of the single carbon fiber can be easily estimated when the average temperature rise and the heat generation rate of the hot wire are measured for the same system. Further, the electrical conductivity of the single carbon fiber is measured under the same conditions as for the thermal conductivity using a four-point contact method. The relation between the thermal conductivity and electrical conductivity is further discussed, based on the crystal microstructure.     

霜层综合导热系数的研究

提出了霜层综合导热系数概念。认为霜层导热系数除了应考虑霜层结构复杂性还应计及蒸汽凝华潜热释放的影响。从霜层能量平衡分析出发，导出霜层综合导热系数是霜层有效导热系数和蒸汽有效导热系数之和。据此计算的霜层综合导热系数与实验数据符合较好。     

Electrical Resistivity and Thermal Conductivity of Pure Aluminum and Aluminum Alloys up to and above the Melting Temperature

Experimental improvements of a four-probe technique to measure the electrical resistivity of molten metals led to an improvement of the measurement uncertainty to less than 6%. Measurements of pure Al and AlSi-, AlSiMg-, and AlSiCu-alloys with a systematic variation of individual components are described. The problem of the calculation of the thermal conductivity has been investigated, and the resulting values of the binary system have been discussed in terms of behavior during melting and solidification. There is a remarkable difference of the thermal conductivity before melting and after resolidification. Paper presented at the Seventeenth European Conference on Thermophysical Properties, September 5–8, 2005, Bratislava, Slovak Republic.     

复合烧结助剂对Si3N4陶瓷力学性能和热导率的影响

Si₃N₄陶瓷因兼具优异的力学和热学性能, 成为第三代半导体陶瓷基板的首选材料之一。本研究以7种不同离子半径的稀土氧化物(RE₂O₃, RE=Sc、Lu、Yb、Y、Gd、Nd、La)与非氧化物(MgSiN₂)作复合烧结助剂, 通过热压烧结和退火热处理制备了高强、高热导Si₃N₄陶瓷, 并系统研究了复合烧结助剂中RE₂O₃种类对Si₃N₄陶瓷物相组成、微结构、力学性能和热导率的影响规律。热压后Si₃N₄陶瓷力学性能优越, 其中添加Nd₂O₃-MgSiN₂的样品弯曲强度达到(1115±49) MPa。退火处理后Si₃N₄陶瓷的热导率得到大幅提升, 呈现出随稀土离子半径减小而逐渐增大的规律, 其中添加Sc₂O₃-MgSiN₂的样品退火后的热导率从54.7 W·m^-1·K^-1提升至80.7 W·m^-1·K^-1, 提升了47.6%。该结果表明, 相较于国际上通用的Y₂O₃-MgSiN₂和Yb₂O₃-MgSiN₂烧结助剂组合, Sc₂O₃-MgSiN₂有望成为制备高强度、高热导Si₃N₄陶瓷的新型复合助剂。     

添加CaF2-YF3的AlN陶瓷的热导率

用CaF2和YF3做添加剂，在1750℃制备了热导率高于170W/m.K的的AlN陶瓷，并用XRD和SEM研究了AlN陶瓷在烧结过程中的相组成，微结构以及晶格参数的变化，并讨论了其对热导率的影响，研究发现，当使用CaF2-YF3做添加剂时，微结构差异对AlN陶瓷热导率的影响很小，AlN陶瓷的热导率主要由AlN晶格氧缺陷浓度决定，由于CaF2-YF3能有效降低AlN颗粒表面的氧含量，从而有利于获得高的热导率。     

热障涂层热导率的研究进展

简要回顾了热障涂层体系的发展,讨论了氧化锆陶瓷材料的传热规律,包括涂层微观结构、陶瓷成分等因素的影响.同时指出了改进陶瓷涂层热导率的方法和开发适用于更高温度下的陶瓷涂层材料的指导原则,并详细介绍了改善热障涂层热导率的研究现状.     

Thermal and electrical conductivities of an improved 9 Cr-1 Mo steel from 360 to 1000 K

The electrical and thermal conductivities and Seebeck coefficients of three 9 Cr-1 Mo samples were measured over the temperature range 360–1000 K. All of the samples were in the normalized and tempered condition and two of the samples were from different heats of a new, modified alloy. The thermal conductivity of the third sample, which was from a commercial heat, was found to agree well with the ASME code values for this steel. The two heats of modified 9 Cr-1 Mo were found to have significantly higher thermal conductivities and this difference appears to be due to the lower Si content of the modified alloy. The results were compared with the predictions of standard transport theory and data on bcc Fe. These comparisons show that phonon energy transport is important and quite dependent upon the Si content.     

Imaging of Thermal Conductivity with Sub-Micrometer Resolution Using Scanning Thermal Microscopy

With the development of new emerging technologies, many objects in scientific research and engineering are of sub-micrometer and nanometer size, such as microelectronics, micro-electro-mechanical systems (MEMS), biomedicines, etc. Therefore, thermal conductivity measurements with sub-micrometer resolution are indispensable. This paper reports on the imaging of various micrometer and sub-micrometer size surface variations using a scanning thermal microscope (SThM). The thermal images show the contrasts indicating the differences of the local thermal conductivity in the sample. Thermal resistance circuits for the thermal tip temperature are developed to explain the heat transfer mechanism between the thermal tip and the sample and to explain the coupling between the local thermal conductivity and the topography in the test results.     

聚氨酯/二氧化硅包覆多壁碳纳米管复合材料的导热与电绝缘性能

通过溶胶-凝胶法制备了厚度为30nm-50nm的二氧化硅(SiO2)包覆多壁碳纳米管(SiO2-MWNTs),并与聚氨酯(PU)复合制备了PU/SiO2-MWNT复合材料。研究了SiO2-MWNTs对PU导热电绝缘性能的影响。结果表明,SiO2包覆层增强了MWNTs与PU之间的界面相互作用,促进了MWNTs在PU中的分散。由于SiO2包覆层的电绝缘作用,PU/SiO2-MWNT复合材料保持了PU的电绝缘性能。同时SiO2包覆层作为过渡层,降低了PU与MWNTs间的模量失配,减少了声子的界面散射,提高了PU/SiO2-MWNT复合材料的导热性能。当SiO2-MWNTs的质量分数为0.5%和1.0%时,PU/SiO2-MWNT复合材料的热导率分别提高了53.7%和63.8%。     

Thermal Conductivity during Phase Transitions

Thermal conductivity is a very basic property that determines how fast a material conducts heat, which plays an important and sometimes a dominant role in many fields. However, because materials with phase transitions have been widely used recently, understanding and measuring temperature‐dependent thermal conductivity during phase transitions are important and sometimes even questionable. Here, the thermal transport equation is corrected by including heat absorption due to phase transitions to reveal how a phase transition affects the measured thermal conductivity. In addition to the enhanced heat capacity that is well known, it is found that thermal diffusivity can be abnormally lowered from the true value, which is also dependent on the speed of phase transitions. The extraction of the true thermal conductivity requires removing the contributions from both altered heat capacity and thermal diffusivity during phase transitions, which is well demonstrated in four selected kinds of phase transition materials (Cu₂Se, Cu₂S, Ag₂S, and Ag₂Se) in experiment. This study also explains the lowered abnormal thermal diffusivity during phase transitions in other materials and thus provides a novel strategy to engineer thermal conductivity for various applications.     

Thermal and Electrical Conductivity of Copper-Graphene Heterosystem: An Effect of Strain and Thickness

Copper-graphene (Cu/Gr) composite carries high thermal (κ) and electrical (σ) conductivities compared with pristine copper film/surface. For further improvement, strain is applied (compressive and tensile) and thickness is changed (of both copper and graphene). It is observed that electronic thermal conductivity (κ_e) and σ enhance from 320.72 to 869.765 W mK⁻¹ and 5.28 × 10⁷ to 23.01 × 10⁷ S m⁻¹, respectively, by applying 0.20% compressive strain. With the increase in copper thickness (three to seven layers) in Cu(111)/single-layer-graphene (SLG) heterosystem, κ_e increases from 320.72 to 571.81 W mK⁻¹ while electrical resistivity (ρ ∝ (1/σ)) decreases from 0.189 × 10⁻⁷ to 0.117 × 10⁻⁷ Ωm. Furthermore, with the increase in graphene thickness (one to four layers) in seven-layer Cu(111)/multilayer-graphene (MLG) heterosystem, κ_e enhances upto 126% while ρ decreases upto 70% compared with the three-layer Cu(111)/SLG. A large available state near Fermi level (of Cu/Gr heterosystem) offers the conduction of more electrons from valence to conduction bands. The increasing thickness broadens this state and enhances conduction electrons. The electron localization function decreases with increasing thickness, suggesting electrons are delocalized at copper-graphene junction, resulting in an increase of free electrons that enhance κ_e and σ. Herein, it is useful in advancing the thermal management of electronic chips and in applying hybrid copper-graphene interconnects.     

一维高导热C/C复合材料的制备研究

以三种沥青作为基体前驱体, 实验室自制的AR中间相沥青基纤维为增强体, 通过500℃热压成型, 随后经炭化和石墨化处理制备出一维炭/炭(C/C)复合材料。研究了前驱体沥青种类和热处理温度对复合材料导热性能的影响, 并采用扫描电子显微镜和偏光显微镜对其石墨化样品的形貌和微观结构进行表征。结果表明; C/C复合材料在沿纤维轴向的室温热扩散系数和导热率均随热处理温度的升高而逐渐增大; 由AR沥青作为基体前驱体所制备的C/C复合材料具有更加明显的沿纤维轴向取向的石墨层状结构以及最好的导热性能, 其3000℃石墨化样品沿纤维轴向的室温热扩散系数和导热率分别达到594.5 mm²/s和734.4 W/(m·K)。     

Thermal conductivity and electrical resistivity of cadmium arsenide (Cd3As2) in the temperature range 4.2–40 K

Results on electrical resistivity and thermal conductivity measured in the temperature range 4.2–40 K are presented for single-crystal and polycrystalline samples of Cd₃As₂. Hall effect has been studied at temperatures of 4.2, 77, and 300 K. The calculated value of the conduction electron concentration was in the range 1.87–1.95 10²⁴m^–3. Electrical resistivity of all investigated samples was independent of temperature up to about 10K and increased slowsly at higher temperatures. The thermal conductivity shows a maximum in the region in which the lattice component of thermal conductivity dominates. The strong anisotropy of the lattice component determines the anisotropy of the total thermal conductivity. The electronic component of thermal conductivity does not exhibit any anisotropy and shows a maximum at a temperature of about 300 K.Paper submitted to the Ninth Symposium on Thermophysical Properties, June 24–27, 1985, Boulder, Colorado, U.S.A.     

锥形量热仪确定聚合物材料导热系数的方法

通过对聚甲基丙烯酸甲酯聚合物材料在锥形量热仪实验条件下加热过程的分析和研究,建立了材料加热过程的数学模型。利用该模型来计算材料的平均导热系数,需要测量的物理量仅仅是物体表面或内部的温度,通过一次测试就可以得到不同温度范围内材料的平均导热系数,方法简单,可靠。利用文中的计算方法,得到PMMA在不同温度范围内的平均导热系数,结果表明,随温度范围的增加,材料平均导热系数值有增大的趋势。计算出的导热系数值在低温度范围与文献值比较接近。研究表明在高的温度范围内不能将传统测量的导热系数值用作火灾条件下材料的参数值。     

Low Temperature Thermal Conductivity of PVC

At very low temperatures, the tunnelling theory for amorphous solids predicts a thermal conductivity κ α T^m, with m = 2. However, most of the data in the literature in the temperature range 0.1–1 K report an m < 2. We want to show that this discrepancy often disappears for T→ 0 K. Here we report the case of Polyvinyl Chloride (PVC) whose thermal conductivity is known in the 0.2–100 K temperature range. A new technique is described which makes the measurement of the exponent m of the thermal conductivity independent of the spurious thermal power. Such technique is particularly useful for measurements of κ when working with a low power refrigerator. We carried out measurements down to 50 mK, obtaining a thermal conductivity W/cm K for our PVC sample below 120 mK.     

The Thermophysical Properties of Solid Solutions of CaLa2S4-La2S3 System

The thermal conductivity coefficient in the temperature range from 275 to 450 K and the coefficient of thermal expansion in the range from 300 to 900 K are experimentally determined for solid solutions of the CaLa₂S₄-La₂S₃ system. The mechanisms of heat transfer in CaLa₂S₄- La₂S₃ samples in the investigated temperature range are discussed, as well as the factors which define the complex concentration dependence of thermal conductivity coefficient. The correlation is treated between the value and temperature dependence of the coefficient of thermal expansion and the variation of the interatomic bond force in the case of variation of the concentration of cation vacancies in the investigated crystals.     

3D Anisotropic Thermal Conductivity of Exfoliated Rhenium Disulfide

ReS₂ represents a different class of 2D materials, which is characterized by low symmetry having 1D metallic chains within the planes and extremely weak interlayer bonding. Here, the thermal conductivity of single‐crystalline ReS₂ in a distorted 1T phase is determined at room temperature for the in‐plane directions parallel and perpendicular to the Re‐chains, and the through‐plane direction using time‐domain thermoreflectance. ReS₂ is prepared in the form of flakes having thicknesses of 60–450 nm by micromechanical exfoliation, and their crystalline orientations are identified by polarized Raman spectroscopy. The in‐plane thermal conductivity is higher along the Re‐chains, (70 ± 18) W m^?1 K^?1, as compared to transverse to the chains, (50 ± 13) W m^?1 K^?1. As expected from the weak interlayer bonding, the through‐plane thermal conductivity is the lowest observed to date for 2D materials, (0.55 ± 0.07) W m^?1 K^?1, resulting in a remarkably high anisotropy of (130 ± 40) and (90 ± 30) for the two in‐plane directions. The thermal conductivity and interface thermal conductance of ReS₂ are discussed relative to the other 2D materials.     

Experimental Study on the Effective Thermal Conductivity and Thermal Diffusivity of Nanofluids

This paper reports measurements of the effective thermal conductivity and thermal diffusivity of various nanofluids using the transient short-hot-wire technique. To remove the influences of the static charge and electrical conductance of the nanoparticles on measurement accuracy, the short-hot-wire probes are carefully coated with a pure Al₂O₃ thin film. Using distilled water and toluene as standard liquids of known thermal conductivity and thermal diffusivity, the length and radius of the hot wire and the thickness of the Al₂O₃ film are calibrated before and after application of the coating. The electrical leakage of the short-hot-wire probes is frequently checked, and only those probes that are coated well are used for measurements. In the present study, the effective thermal conductivities and thermal diffusivities of Al₂O₃/water, ZrO₂/water, TiO₂/water, and CuO/water nanofluids are measured and the effects of the volume fractions and thermal conductivities of nanoparticles and temperature are clarified. The average diameters of Al₂O₃, ZrO₂, TiO₂, and CuO particles are 20, 20, 40, and 33 nm, respectively. The uncertainty of the present measurements is estimated to be within 1% for the thermal conductivity and 5% for the thermal diffusivity. The measured results demonstrate that the effective thermal conductivities of the nanofluids show no anomalous enhancement and can be predicted accurately by the model equation of Hamilton and Crosser, when the spherical nanoparticles are dispersed into fluids.