Influence of the Kapitza resistance on the thermal conductivity of filled epoxies

The thermal conductivity of epoxy resins filled with copper powder was measured as a function of grain size and filler concentration between 1.5 and 20 K. In addition, the thermal boundary layer resistance (Kapitza resistance) between epoxy resin and copper was measured. As a consequence of this resistance the thermal conductivity is strongly dependent on grain size in the lower temperature range. Below a characteristic temperature dependent on grain size, thermal conductivity is reduced by adding filler. A simple formula is presented for calculation of the thermal conductivity of filled resins.     

The thermal expansion of chrysotile asbestos and its composites

The principal linear thermal expansion coefficients of chrysotile asbestos have been measured over the approximate temperature range 80 to 270 K. Implications of the results on: (i) the influence of thermal strain upon the specific heat capacity, and (ii) the temperature variation of the reduced volume dependence of the lattice vibrational frequencies have been assessed as far as available ancillary data permit. The principal linear thermal expansion coefficents of specimens prepared from composite bars consisting of phenol formaldehyde resin reinforced with chrysotile fibre in random and preferential dispositions, have also been measured over this temperature range. An appraisal of the results in the neighbourhood of ambient temperature has been undertaken in terms of a simple structural model. It transpires that a satisfactory account of the thermal expansion results for the composites is provided in terms of the thermal and elastic properties of the constituents for directions parallel to the pressing directions. The limited ancillary data available leave the principal causes of the less satisfactory agreement in directions at right angles to the pressing directions an open question, though a suggestion is advanced concerning the direction which further investigations might take in order to resolve this uncertainty.     

Diameter-dependent thermal transport in individual ZnO nanowires and its correlation with surface coating and defects

A systematic study of the thermal transport properties of individual single-crystal zinc oxide (ZnO) nanowires (NWs) with diameters in the range of ～50-210 nm is presented. The thermal conductivity of the NWs is found to be dramatically reduced by at least an order of magnitude compared to bulk values, due to enhanced phonon-boundary scattering with a reduction in sample size. While the conventional phonon transport model can qualitatively explain the temperature dependence, it fails to account for the diameter dependence. An empirical relationship for assessing diameter-dependent thermal properties is observed, which shows an approximately linear dependence of the thermal conductivity on the cross-sectional area of the NWs in the measured diameter range. Furthermore, it is found that an amorphous-carbon layer coating on the NWs does not perturb the thermal properties of the NW cores, whereas 30 keV Ga(+) ion irradiation at low dose (～4 × 10(14) cm(-2)) leads to a remarkable reduction of the thermal conductivity of the ZnO NWs.     

Influence of Radiation Losses on Thermal Conductivity Determination at Low Temperatures

The thermal conductivity of electrolytic iron has been measured in the temperature range from 100 to 390 K. Electrolytic iron is a standard material for the measurement of thermal conductivity. The thermal conductivity was measured on a commercial device Thermal Transport Option (TTO) of a Physical Properties Measurement System (PPMS) produced by the Quantum Design company. The temperature gradient on the sample was determined using small highly accurate Cernox chip thermometers. The thermal conductivity of the standard material showed higher values than those cited by NIST for the temperature range from 100 to 390 K (NIST’s “Report of Investigation” for SRM 8420). The maximum deviation reached 30% at 390 K. Detailed analyses of the measured data and of the commercial software of the measuring device revealed that the large differences resulted from radiative losses of the interior parts of the device. The determination of the radiative losses takes into account the sample geometry, contacts, and cooling part of the device, and these differences in the thermal conductivity values were substantially reduced after accounting for these losses.Paper presented at the Seventeenth European Conference on Thermophysical Properties, September 5–8, 2005, Bratislava, Slovak Republic.     

Thermal conductance of an individual single-wall carbon nanotube above room temperature

The thermal properties of a suspended metallic single-wall carbon nanotube (SWNT) are extracted from its high-bias (I-V) electrical characteristics over the 300-800 K temperature range, achieved by Joule self-heating. The thermal conductance is approximately 2.4 nW/K, and the thermal conductivity is nearly 3500 Wm(-1)K(-1) at room temperature for a SWNT of length 2.6 mum and diameter 1.7 nm. A subtle decrease in thermal conductivity steeper than 1/T is observed at the upper end of the temperature range, which is attributed to second-order three-phonon scattering between two acoustic modes and one optical mode. We discuss sources of uncertainty and propose a simple analytical model for the SWNT thermal conductivity including length and temperature dependence.     

Thermal- and stress-induced lattice distortions in a single Kevlar49 fibre studied by microfocus X-ray diffraction

Understanding the macroscopic physical and mechanical properties of poly(p-phenylene terephthalamide) (PpTA) fibres as a function of temperature requires an understanding of how temperature influences its microscopic structure. This study investigates lattice distortions in single PpTA fibres using the high brilliance of a synchrotron radiation microbeam. Lattice distortions are studied over a temperature range of 110–350 K and the influence of tensile deformation is also considered. The results reveal linear thermal expansion behaviour for all unit cell axes, in general agreement with literature. Expansion/contraction is greatest along the [100] direction whilst being reduced along [010] by inter-chain hydrogen bonding. During macroscopic deformation, longitudinal crystal strain dominates with respect to axial lattice distortions induced by temperature changes. There is only a small change in the [100] coefficient of thermal expansion, with the [010] and [001] directions being largely unaffected.     

Thermal expansion behavior of aluminum alloys reinforced with alumina planar random short fibers

The thermal expansion behavior of two aluminum alloys (Al-4%Cu and Al-12%Si) reinforced with alumina planar random short fibers has been studied, both experimentally and theoretically. The metal matrix composites (MMCs) were manufactured by pressure infiltration of molten metal into short fiber preforms with a planar random distribution of fibers. Dilatometric testing was used to investigate the influence of fiber volume fraction and architecture, and the effects of thermal cycling between 25 °C to 560 °C. Thermal expansion measurements showed that, by increasing the fiber content in the composites, both the thermal strains and the effective coefficient of thermal expansion (CTE) were reduced in the whole temperature range. Furthermore, the thermal strains of MMCs increased almost linearly up to a critical temperature, T _cr, where the metallic matrix began to yield macroscopically due to internal thermal stresses. For temperatures higher than T _cr the thermal strains of MMCs showed a marked hysteresis during heating/cooling cycles due to the elasto-plastic response of the metallic matrix. In this temperature range, the thermal expansion curves deviated appreciably from linearity and the planar (in the plane of fibers) and transverse (normal to the plane of fibers) responses were very different: while the planar CTE was strongly reduced, the transverse CTE increased sharply with temperature, being even larger than the CTE of the unreinforced alloy. Thermal cycling produced a net dimensional change of composites during the first 2-3 cycles but, on the subsequent cycles, the permanent deformation disappeared almost completely and the successive thermal expansion curves were identical. Experimental results were compared to the theoretical predictions of an analytical model based on the Eshelby's equivalent inclusion method, and an excellent agreement was obtained.     

Electrical and thermal conduction in atomic layer deposition nanobridges down to 7 nm thickness

While the literature is rich with data for the electrical behavior of nanotransistors based on semiconductor nanowires and carbon nanotubes, few data are available for ultrascaled metal interconnects that will be demanded by these devices. Atomic layer deposition (ALD), which uses a sequence of self-limiting surface reactions to achieve high-quality nanolayers, provides an unique opportunity to study the limits of electrical and thermal conduction in metal interconnects. This work measures and interprets the electrical and thermal conductivities of free-standing platinum films of thickness 7.3, 9.8, and 12.1 nm in the temperature range from 50 to 320 K. Conductivity data for the 7.3 nm bridge are reduced by 77.8% (electrical) and 66.3% (thermal) compared to bulk values due to electron scattering at material and grain boundaries. The measurement results indicate that the contribution of phonon conduction is significant in the total thermal conductivity of the ALD films.     

Predicting the thermal conductivity of aluminium alloys in the cryogenic to room temperature range

Aluminium alloys are being used increasingly in cryogenic systems. However, cryogenic thermal conductivity measurements have been made on only a few of the many types in general use. This paper describes a method of predicting the thermal conductivity of any aluminium alloy between the superconducting transition temperature (approximately 1 K) and room temperature, based on a measurement of the thermal conductivity or electrical resistivity at a single temperature. Where predictions are based on low temperature measurements (approximately 4 K and below), the accuracy is generally better than 10%. Useful predictions can also be made from room temperature measurements for most alloys, but with reduced accuracy. This method permits aluminium alloys to be used in situations where the thermal conductivity is important without having to make (or find) direct measurements over the entire temperature range of interest. There is therefore greater scope to choose alloys based on mechanical properties and availability, rather than on whether cryogenic thermal conductivity measurements have been made. Recommended thermal conductivity values are presented for aluminium 6082 (based on a new measurement), and for 1000 series, and types 2014, 2024, 2219, 3003, 5052, 5083, 5086, 5154, 6061, 6063, 6082, 7039 and 7075 (based on low temperature measurements in the literature).     

正交异性钢桥面构造细节的日照温度次应力分析

为研究正交异性钢桥面板构造细节的日照温度次应力行为,多次在高温和强日照天气下现场实测了某自锚式悬索桥钢箱梁外周和实腹式横隔板上温度场,基于观测到的最大顶底板温差拟合了横隔板竖向温度梯度表达式;在ANSYS中建立了钢箱梁节段有限元模型并施加外周温度,计算其24 h温度场,并与横隔板实测竖向温度的对比校验了温度场模拟的合理性;开展了钢箱梁节段和子模型的精细化热应力分析,得到了纵肋-横隔板和弧形切口共4个构造细节的温度应力时程曲线。研究表明：在强太阳辐射和高温环境下,钢箱梁横向温差不明显,但存在明显的竖向温度梯度,横隔板竖向温度梯度可拟合为四折线形式,最大温差明显小于欧规值。正交异性钢桥面板产生热变形并在构造细节处产生明显热应力集中,特别是弧形切口热应力大。仅日照竖向温度梯度作用,或仅桥面货车通行加载,弧形切口均为无限疲劳寿命;但二者共同作用产生的应力幅,大于构造细节的常幅疲劳极限,可能是该构造细节出现疲劳开裂的原因。     

SiO2@TiO2核壳结构及其在高温下的隔热性能表征（英文）

以SiO2@TiO2核壳结构粒子为新型功能填料,采用旋涂方法制得了高温隔热涂层.TEM照片显示通过溶胶-凝胶法,成功在SiO2核上包覆上厚度为50 nm的TiO2壳层.通过自开发的测试设备表征了填料加入前后涂层在1300~1500℃的隔热性能.结果表明:SiO2@TiO2核壳结构粒子加入后,隔热涂层能将从热源辐射出的热流减少50%,在热源温度达1500℃时涂层试样的表背温度差为260℃.加入SiO2@TiO2核壳粒子的涂层在高温下隔热效果明显,是一种很有前景的高温隔热涂层填料.     

一种高精度薄膜铂电阻温度计测量迟滞性的研究

为了研究热迟滞性对工业铂电阻温度计测量不确定度的影响,选取了8支高精度铂电阻温度计进行实验。在-50～150℃内,选择3个温度区间,采用两种标准方法(IEC 60751,ASTM E644)测量水三相点(0.01℃)和所选温度范围内的中间点的迟滞性变化。实验结果表明:4支薄膜铂电阻温度计在两种标准方法测量下,随着温度区间跨度增大,热迟滞性影响增大,IEC 60751标准方法测量的热迟滞性最大值为14.2mK,ASTM E644标准方法测量的热迟滞性最大值为20.5mK;选取4支铂丝铂电阻温度计在温度范围为-50～150℃测量时,IEC 60751和ASTM E644标准方法测量的热迟滞性数据最大值分别为1.1mK和0.9mK;铂丝铂电阻温度计热迟滞性明显小于薄膜铂电阻温度计。     

粉末冶金法制备SiC_p/Al_2W_3O_(12)/Al复合材料及其热膨胀性能

通过添加适量的Al_2W_3O_(12)负热膨胀粉体来优化碳化硅颗粒增强铝基(SiC_p/Al)复合材料的热膨胀系数。实验采用固相法制备负热膨胀性能的Al_2W_3O_(12)粉体,并按10%,20%,30%的体积比添加至SiC_p/Al复合粉体中,利用粉末冶金工艺制备SiC_p/Al_2W_3O_(12)/Al复合材料。实验结果表明:制备的复合材料组织分布均匀,致密度良好。室温到200℃内,在Al基体质量分数不变的前提下,Al_2W_3O_(12)的加入有效降低了复合材料的热膨胀系数。     

A test of the modified Enskog theory for the transport properties of liquids

The modified Enskog theory (MET) has been applied to various fluids in the liquid range (between the triple point and the critical point), and the viscosity, thermal conductivity, and self-diffusion coefficients have been calculated. The temperature dependence of the covolume has been introduced explicitly, bypassing the use of virial coefficients. The agreement is generally acceptable and sometimes good. There is an evident regularity in the results when the reduced temperature is introduced as an independent variable.     

Measurement of thermal properties of liquids with an AC heated-wire technique

An apparatus for the simultaneous absolute measurement of the thermal activity, thermal diffusivity, thermal conductivity, and heat capacity of nonconducting liquids with the AC heated-wire (strip) technique is described. The main advantage of this technique is that the temperature oscillations field can be confined around the sensor in a liquid layer thin enough to suppress the hydrodynamic currents. This leads to the elimination of the convective heat transport. Carrying measurements at different frequencies, the inertia of the sensor can be considered, and the radiative heat transport can be estimated for liquids with known optical properties. The apparatus was constructed and tested using six different liquids in a limited temperature range. The thermal properties of these liquids at 20°C are reported. The thermal conductivity data of toluene and n-heptane (recommended as proposed thermal conductivity standards) are given in the temperature range 10–40°C. Good agreement was found with data reported by other investigators at 20°C, but there is still a considerable discrepancy in the temperature coefficient of thermal conductivity.     

Hot Isostatic Pressing of Plasma Sprayed Thermal Barrier Coating Systems

Thermal barrier coatings (TBC) are important to aerospace and high performance gas turbine engines because they help to keep the temperature experienced by the base metal low; thus, prolonging the life span of the material. Plasma spraying is a technique commonly used to deposit the ceramic-based TBC. An intermediate layer is applied to enhance the bond between the substrate and the ceramic top coat. However, the oxidation of the bond coat due to the infiltration of gas through the porous ceramic layer is a major problem encountered in TBC. This in turn leads to spalling and eventual destruction of the whole coating system. Hot isostatic pressing (HIP) was performed on a number of plasma sprayed thermal barrier coating systems to investigate the effects the process has on micro structure and other physical properties. Due to the fact that the majority of TBC is exposed to thermal cycling and thermal fatigue, it is hoped that the changes brought about by HIP in the porosity and microstructure will improve the life span and performance of TBC. HIP was performed in the temperature range 750-1300° C and pressures of 50-200 MPa. The bond coats that were studied include Ni-5% Al, Ni-20 percnt; Al, NiCrAl and NiCrAlY, while the ceramic coat was Zr0₂-5 wt percnt; CaO. Characterization of the coatings was carried out using scanning electron microscopy (SEM) and image analyser. The results showed the porosity of the coatings to be dramatically reduced to near zero levels. In addition, the other physical properties like hardness and Young's modulus increased over a wide temperature range.     

降压装置对电动车动力电池制冷剂直接冷却系统的影响

动力电池使用制冷剂直接冷却具有成本低,冷却效率高、重量轻,安全性高等优点,但存在蒸发温度过低和电池冷却温度不均的问题。本文通过实验研究了直冷板后二次节流对直冷板温度调节的有效性。结果表明:在直冷板后接人降压装置(固定节流孔装置或可调节开度压力调节阀)可以改变直冷板的出口压力,减小直冷板出口过热度,从而提升直冷板的蒸发温度,并改善制冷剂直冷方式的均温性;但固定开度的降压装置无法主动调节直冷板出口压力,直冷板出口压力随着热负荷增加而增加,因此在热负荷变化时较难将电池温度控制在合适的范围;而可变开度的降压装置可根据电池的运行热负荷将直冷板的蒸发压力调整到合适的目标值,既能避免直冷板的蒸发温度过低,又能改善直冷板的均温性,以取得较佳的电池冷却效果。     

3D针刺C/SiC-TaC复合材料的热膨胀性能

采用液相加压浸渗法将TaC渗入到三维针刺毡中, 并结合反应熔体渗透法(RMI)制得C/SiC-TaC复合材料。采用热膨胀仪测量了热处理前后复合材料从室温到1400 ℃温度范围内的热膨胀系数(CTE), 发现C/SiC-TaC的CTE数值较C/SiC的高。从材料内部热应力的变化、 制备方法及添加物和基体的性能方面定性地分析了CTE的变化机制。研究表明, C/SiC-TaC复合材料的膨胀性能在各个温度段的变化机制不同, 低温段(1100 ℃以下)CTE的不断上升主要由90°无纬布、 TaC和SiC基体贡献, 该阶段的起伏波动主要由复合材料的结构应力和孔隙分布不均及残余Si产生; 高温段(1100 ℃以上)的热膨胀性能主要由0°无纬布和界面热应力决定。热处理降低了复合材料在1100 ℃以下的CTE, 也改变了高温段的变化规律。      

A new optically sensed thermal element for precise AC-DC conversion

An optically sensed isothermal differential thermal converter has been reduced to practice using a commercially available optical infrared pyrometer. Eliminating the conventional bimetallic thermocouples avoids draining thermal energy from the heater, reduces the Thomson effect error, and minimizes shunt paths along the heater which bypass higher frequency signal components. The optically sensed thermal element provides the potential for greater accuracy over a wide frequency range, faster response time, and improved dynamic range over conventional wire or planar thermal elements using Seebeck, thermoresistive or semiconductor temperature sensing