Thermal Conductivity Measurement of Anisotropic Biological Tissue In Vitro

The accurate determination of the thermal conductivity of biological tissues has implications on the success of cryosurgical/hyperthermia treatments. In light of the evident anisotropy in some biological tissues, a new modified stepwise transient method was proposed to simultaneously measure the transverse and longitudinal thermal conductivities of anisotropic biological tissues. The physical and mathematical models were established, and the analytical solution was derived. Sensitivity analysis and experimental simulation were performed to determine the feasibility and measurement accuracy of simultaneously measuring the transverse and longitudinal thermal conductivities. The experimental system was set up, and its measurement accuracy was verified by measuring the thermal conductivity of a reference standard material. The thermal conductivities of the pork tenderloin and bovine muscles were measured using the traditional 1D and proposed methods, respectively, at different temperatures. Results indicate that the thermal conductivities of the bovine muscle are lower than those of the pork tenderloin muscle, whereas the bovine muscle was determined to exhibit stronger anisotropy than the pork tenderloin muscle. Moreover, the longitudinal thermal conductivity is larger than the transverse thermal conductivity for the two tissues and all thermal conductivities increase with the increase in temperature. Compared with the traditional 1D method, results obtained by the proposed method are slightly higher although the relative deviation is below 5 %.     

Anisotropic Effective Thermal Conductivity Measurement of Various Kinds of Metal Fiber Materials

Recently, metal fiber materials were made by laminating metal fibers with a diameter of about 30 μm to 300 μm. Since the almost metal fibers were oriented in the horizontal direction (the major axis of the fiber), these metal fiber materials are estimated to be anisotropic with an effective thermal conductivity. However, there is little quantitative data on the anisotropic effective thermal conductivity of the various kinds of metal fiber materials. The purpose of this study is to investigate the anisotropic effective thermal conductivity of various metal fiber materials experimentally and theoretically. In order to measure the horizontal and vertical effective thermal conductivities of these metal fiber materials, new measurement devices were developed. As a result, it is found that the anisotropic effective thermal conductivity of the various metal fiber materials was confirmed, and the horizontal and vertical effective thermal conductivities of these metal fiber materials depend on the bulk density or porosity, Young’s modulus, the fiber length, and fiber diameter. And a dimensionless correlation equation for predicting the vertical and horizontal effective thermal conductivities of the various kinds of metal fiber materials was derived in terms of various dimensionless parameters.     

Estimation of the Mean Thermal Conductivity of Anisotropic Materials

An estimation method of the plane directional thermal conductivity of fibrous insulations using the cyclic heat method and the transient hot-wire method is proposed. By assuming that the thermal conductivity _h of anisotropic materials measured by the transient hot-wire method is equivalent to that of the isotropic materials which have the same bulk density and specific heat c as the anisotropic materials, the thermal conductivity _h is shown to be equal to , which is a geometrical mean of the thermal conductivities in the direction of the plane _x and the thickness _y of the anisotropic materials. For an alumina silica blanket (=125 kg·m^–3), the thermal conductivities _h , _x , and _y were measured in the temperature range between –140 and 300°C using the transient hot-wire method for _h and the cyclic heat method for _x and _y . In the same way, the thermal conductivities _h , _x , and _y of a rock wool (=121 kg·m^–3) insulation were also measured in the temperature range, 100 to 600°C. From a comparison of the measured results with the estimated values of _x , it is confirmed that the proposed method can estimate the measured values reasonably well.     

Thermal Conductivity of AlN and SiC Thin Films

The thermal conductivity of AlN and SiC thin films sputtered on silicon substrates is measured employing the 3ω method. The thickness of the AlN sample is varied in the range from 200 to 2000 nm to analyze the size effect. The SiC thin films are prepared at two different temperatures, 20 and 500°C, and the effect of deposition temperature on thermal conductivity is examined. The results reveal that the thermal conductivity of the thin films is significantly smaller than that of the same material in bulk form. The thermal conductivity of the AlN thin film is strongly dependent on the film thickness. For the case of SiC thin films, however, increased deposition temperature results in negligible change in the thermal conductivity as the temperature is below the critical temperature for crystallization. To explain the thermal conduction in the thin films, the thermal conductivity and microstructure are compared using x-ray diffraction patterns.     

Thermal Conductivity of Suspensions Containing Nanosized SiC Particles

Nanosized SiC suspensions were prepared, and their thermal conductivities were measured using a transient hot-wire method. The experimental results showed that the thermal conductivities of the studied suspensions were increased as expected, and the enhancement was proportional to the volume fraction of the solid phase, but the increasing ratio of the thermal conductivity was not significantly related to the base fluid. The effects of the morphologies (size and shape) of the added solid phase on the enhancement of the thermal conductivity of the nanoparticle suspension are reported for the first time.     

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.     

Molecular Dynamics Simulations for Anisotropic Thermal Conductivity of Borophene

The present work carries out molecular dynamics simulations to compute the thermal conductivity of the borophene nanoribbon and the borophene nanotube using the Muller-Plathe approach. We investigate the thermal conductivity of the armchair and zigzag borophenes, and show the strong anisotropic thermal conductivity property of borophene. We compare results of the borophene nanoribbon and the borophene nanotube, and find the thermal conductivity of the borophene is orientation dependent.The thermal conductivity of the borophene does not vary as changing the width of the borophene nanoribbon and the perimeter of the borophene nanotube. In addition, the thermal conductivity of the borophene is not sensitive to the applied strains and the background temperatures.     

固化吸附剂导热系数测量参数选择

在开发高导热吸附剂过程中经常需要测量材料的导热系数,稳态平板法测量导热系数因方法简单直接而应用广泛.通过数值模拟和实验发现侧面散热对导热系数测量影响很大,采用大空间自然对流换热实验关联式对试验数据处理公式进行修正,可得到较满意的实验结果,并进一步研究了热源温度、样品厚度、试样导热系数对侧面散热的影响.为减少侧面散热以及热电偶波动对测量结果的影响,并考虑到试样加工难易程度,就所用实验装置而言,试样厚度在12mm较为适宜,热源温度选用55℃较为适宜.     

Measurement of the Thermal Conductivity of Nanodeposited Material

The small size of nanomaterials deposited by either focused ions or electron beams has prevented the determination of reliable thermal property data by existing methods. A new method is described that uses a suspended platinum hot film to measure the thermal conductivity of a nanoscale deposition. The cross section of the Pt film needs to be as small as 50 nm × 500 nm to have sufficient sensitivity to detect the effect of the beam-induced nanodeposition. A direct current heating method is used before and after the deposition, and the change in the average temperature increase of the Pt hot film gives the thermal conductivity of the additional deposited material. In order to estimate the error introduced by the one-dimensional analytical model employed, a two-dimensional numerical simulation was conducted. It confirmed the reliability of this method for situations where the deposit extends onto the terminals by (1 μm or more. Measurements of amorphous carbon (a-C) films fabricated by electron beam induced deposition (EBID) produced thermal conductivities of 0.61 W · m⁻¹ · K⁻¹ to 0.73 W · m⁻¹ · K⁻¹ at 100 K to 340 K, values in good agreement with those of a-C thin films reported in the past.     

Thermal Conductivity Measurement of an Electron-Beam Physical-Vapor-Deposition Coating

An industrial ceramic thermal-barrier coating designated PWA 266, processed by electron-beam physical-vapor deposition, was measured using a steady-state thermal conductivity technique. The thermal conductivity of the mass fraction 7 % yttria-stabilized zirconia coating was measured from 100 °C to 900 °C. Measurements on three thicknesses of coatings, 170 μm, 350 μm, and 510 μm resulted in thermal conductivity in the range from 1.5 W/(m·K) to 1.7 W/(m·K) with a combined relative standard uncertainty of 20 %. The thermal conductivity is not significantly dependent on temperature.     

In Situ Measurement of Thermal Diffusivity in Anisotropic Media

A knowledge of thermal conductivity/diffusivity is essential in several situations in engineering. This material property serves also as a measure of the quality of the manufactured materials. The thermal conductivity and diffusivity are measured by specialized labs using commercially available equipment. Even though both the number of such sites and the available measurement techniques are quite large, non-destructive, fast, and reliable techniques are still demanded. The developed technique, due to its rapidity and nondestructive character, can be embedded in a manufacturing process. As opposed to most methods, it does not require preparation of samples of a special shape (e.g., a small cylinder, a thin foil, cuboid). Moreover, one measurement cycle of the proposed technique yields two principal components of the diffusivities of orthotropic materials.     

Thermal Conductivity of Ionic Liquids: Measurement and Prediction

This study reports thermal-conductivity data for a series of [EMIM] (1-ethyl-3-methylimidazolium)-based ionic liquids (ILs) having the anions [NTf₂] (bis(trifluoromethylsulfonyl)imide), [OAc] (acetate), [N(CN)₂] (dicyanimide), [C(CN)₃] (tricyanomethide), [MeOHPO₂] (methylphosphonate), [EtSO₄] (ethylsulfate), or [OcSO₄] (octylsulfate), and in addition for ILs with the [NTf₂]-anion having the cations [HMIM] (1-hexyl-3-methylimidazolium), [OMA] (methyltrioctylammonium), or [BBIM] (1,3-dibutylimidazolium). Measurements were performed in the temperature range between (273.15 and 333.15) K by a stationary guarded parallel-plate instrument with a total measurement uncertainty of 3 % (k = 2). For all ILs, the temperature dependence of the thermal conductivity can well be represented by a linear equation. While for the [NTf₂]-based ILs, a slight increase of the thermal conductivity with increasing molar mass of the cation is found at a given temperature, the [EMIM]-based ILs show a pronounced, approximately linear decrease with increasing molar mass of the different probed anions. Based on the experimental data obtained in this study, a simple relationship between the thermal conductivity, molar mass, and density is proposed for the prediction of the thermal-conductivity data of ILs. For this, also densities were measured for [EMIM][OAc], [EMIM][C(CN)₃], and [HMIM][NTf₂]. The mean absolute percentage deviation of all thermal-conductivity data for ILs found in the literature from the proposed prediction is about 7 %. This result represents a convenient simplification in the acquisition of thermal conductivity information for the enormous amount of structurally different IL cation/anion combinations available.     

Thermal Conductivity Measurement of Nanoparticle Based Solid Propellant Mixtures

A Miniaturised Pyrotechnic Initiator (MPI) is a device used to ignite a charge. It is reduced in scale in order to be suitable for application in miniaturised satellites. Miniaturising the system presents challenges requiring careful consideration of propellant selection. Thermal conductivity tests have been conducted on different mixtures of composite solid propellant materials in order to determine the thermal conductivity of the propellant in the temperature range 0‐100 °C and to study the effect of the presence of porosity and bubbles on the thermal conductivity of the propellant. The applicability of semiempirical Ziebland Relation to predict the thermal conductivity of the composite propellant once the values of the single components are known has been assessed. Furthermore, mixtures of Al and Potassium Perchlorate powders with different compression factors have been examined in order to understand the influence of the processing conditions on thermal conductivity     

热线法测量半透明固体材料的导热系数

建立了适用于热线半透明固体材料导热系数测试系统的简化解析模型。针对一种半透明固体材料—Ｋ９冕玻璃，在２９７～１２３０Ｋ温度范围内进行了测试研究。     

碳化硅纳米线增强多孔碳化硅陶瓷基复合材料的制备

构建多孔碳化硅纳米线(SiCNWs)网络并控制化学气相渗透(CVI)过程,可设计并获得轻质、高强度和低导热率SiC复合材料。首先将SiCNWs和聚乙烯醇(PVA)混合,制备具有最佳体积分数(15.6%)和均匀孔隙结构的SiCNWs网络;通过控制CVI参数获得具有小而均匀孔隙结构的SiCNWs增强多孔SiC(SiCNWs/SiC)陶瓷基复合材料。SiC基体形貌受沉积参数(如温度和反应气体浓度)的影响,从球状颗粒向六棱锥颗粒形状转变。SiCNWs/SiC陶瓷基复合材料的孔隙率为38.9%时,强度达到(194.3±21.3) MPa,导热系数为(1.9 ± 0.1) W/(m∙K),显示出增韧效果,并具有低导热系数。     

导热系数测量系统的数值模拟

对中国计量科学研究院的稳态保护热平板导热系数测量系统的温度场分布进行数值计算,并在此基础上对实验材料内测温点的选择进行了分析.结果表明,测温点复盖了实验材料内温度的最低和最高点,且呈线性分布,满足实验材料导热系数测量所需温度梯度测量的要求.此外,对热流密度测量的探讨发现,热流密度测量范围的确定是实现精密测量导热系数的关键.