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.     

Ni-P-PTFE化学复合镀层导热性能的研究

采用Ni-P-PTFE化学复合镀对铜管进行表面处理能有效减少污垢在换热表面上形成。然而,在实际应用上,复合镀层对铜管导热性能的影响是必须考虑的问题。实验利用热阻法对铜基Ni-P-PTFE复合镀层的导热系数进行测量,并利用Wilson plot方法处理数据最终得到Ni-P-PTFE复合镀层的导热系数。分析了镀层各组分镍(Ni),聚四氟乙烯(PTFE),碳(C),磷(P)的质量分数对其导热性能的影响规律。结果表明,Ni-P-PTFE复合镀层的导热系数随PTFE和P的质量分数增大而降低,随C的质量分数增加而增大。当PTFE,C以及P的质量分数w(PTFE)=1.76%,w(C)=3.82%和w(P)=10.81%时,最大值为23.12 W/(m.K)。尽管复合镀层的导热系数不高,但由于其厚度很小,镀层产生的热阻仅为9.91×10-5~1.6×10-4(m2.K)/W,所以复合镀层铜管仍保持很高的导热系数值314.88~357.55W/(m.K)。     

热辐射对碳纳米管纱名义热导率的影响

碳纳米管纱是完全由碳纳米管构成的宏观材料,因其类似“气凝胶”的结构,具有较大的比表面积,相对于普通宏观材料,会有更多的热量通过表面热辐射耗散出去。本文详细地推导了电加热时,准一维热传播材料表面温度分布和热导率计算公式,并引入热辐射项对上述传热过程进行修正,获得表面真实温度分布,进而结合碳纳米管纱的物理性质,分析了测试样品长度对碳纳米管纱名义热导率(指测试获得的热导率)的影响。对于长度为5 mm的碳纳米管纱测试样品,名义热导率约为真实热导率的4倍。     

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.     

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.     

Measurement of the Intrinsic Thermal Conductivity of a Multiwalled Carbon Nanotube and Its Contact Thermal Resistance with the Substrate

The intrinsic thermal conductivity of an individual carbon nanotube and its contact thermal resistance with the heat source/sink can be extracted simultaneously through multiple measurements with different lengths of the tube between the heat source and the heat sink. Experimental results on a 66‐nm‐diameter multiwalled carbon nanotube show that above 100 K, contact thermal resistance can contribute up to 50% of the total measured thermal resistance; therefore, the intrinsic thermal conductivity of the nanotube can be significantly higher than the effective thermal conductivity derived from a single measurement without eliminating the contact thermal resistance. At 300 K, the contact thermal resistance between the tube and the substrate for a unit area is 2.2 × 10^?8 m² K W^?1, which is on the lower end among several published data. Results also indicate that for nanotubes of relatively high thermal conductance, electron‐beam‐induced gold deposition at the tube–substrate contacts may not reduce the contact thermal resistance to a negligible level. These results provide insights into the long‐lasting issue of the contact thermal resistance in nanotube/nanowire thermal conductity measurements and have important implications for further understanding thermal transport through carbon nanotubes and using carbon nanotube arrays as thermal interface materials.     

以物体表面温度推算变导热系数的测试原理及技术

提出了一种根据第三类边界条件下物体表面温度的测定值推算随温度线性变化的物体导热系数的测试原理和方法。利用设计的测试系统对几种材料的导热系数进行了测量和计算。结果表明，测试计算方法简单可行。为测定物体变导热系数提供了一个有效的方法。     

Vapor-Phase Thermal Conductivity Measurements of Refrigerants

The paper reports further developments of the transient hot-wire technique. The particular development of interest is the extension of the technique to study polar, or electrically-conducting gases with a relatively low thermal conductivity but a high thermal diffusivity, circumstances which occur at low density and therefore low pressure, for gases of high molecular weight. The theory of the transient hot-wire instrument is examined again in order to guide a revised design of the thermal conductivity cell with this particular application in mind. Test measurements have then been conducted on helium, argon, and propane at low and moderate pressures to confirm that the instrument operates in accordance with the theory of it. The satisfactory completion of these tests demonstrates that the new equipment overcomes many of the defects observed in earlier variants of the instrument for application to the study of refrigerant gases.     

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.     

石墨泡沫有效导热系数研究综述

石墨泡沫作为一种新型的轻质多孔材料,具有孔隙率高、比表面积大、密度低、导热系数高等优点,可应用于诸多领域。本文综述了三种研究石墨泡沫有效导热系数的方法,分别为实验研究、理论分析和数值模拟,概括了三种方法中不同泡沫模型的假设条件及适用范围,并通过分析计算与实验数据间的相对误差,得到每种模型的优、缺点。经比较发现,K.C. Leong模型在模拟泡沫内部孔隙结构上效果最好,为改进石墨泡沫有效导热系数的模型设计提供了很好的参考价值。     

The Thermal Conductivity, Thermal Diffusivity, and Heat Capacity of Gaseous Argon

This paper presents new absolute measurements for the thermal conductivity and thermal diffusivity of gaseous argon obtained with a transient hot-wire instrument. Six isotherms were measured in the supercritical dense gas at temperatures between 296 and 423 K and pressures up to 61 MPa. A new analysis for the influence of temperature-dependent properties and residual bridge unbalance is used to obtain the thermal conductivity with an uncertainty of less than 1% and the thermal diffusivity with an uncertainty of less than 4%. Isobaric heat capacity results were derived from measured values of thermal conductivity and thermal diffusivity using a density calculated from an equation of state. The heat capacities presented here have a nominal uncertainty of 4% and demonstrate that this property can be obtained successfully with the transient hot wire technique over a wide range of fluid states. The technique will be useful when applied to fluids which lack specific heat data.     

影响纳米流体导热系数实验测试的因素

导热系数是衡量纳米流体强化换热的最重要的参数,但在不同学者的研究中,对于同一种纳米流体所测得的导热系数却有很大差别。本文针对影响纳米流体导热系数实验测量的因素进行研究,在相同的实验条件下,分别运用Hotdisk导热仪和闪光法导热仪对水基二氧化钛纳米流体的导热系数进行了测量。实验结果表明,用Hotdisk法的测量结果比用闪光法测得的高21%～34%。通过计算分析发现:自然对流是引起纳米流体导热系数测量结果多变的重要原因之一。     

热处理对热障涂层孔隙率及热导率的影响

热障涂层高温使用过程中不可避免发生烧结,引起涂层失效。为探索烧结过程对涂层可靠性的影响,采用等离子喷涂制备氧化钇稳定氧化锆(YSZ)涂层,并对其进行1 000,1 100,1 200,1 300℃高温烧结试验,研究其高温烧结过程中的微观结构及热导率演变规律。结果表明:高温热处理引起热障涂层组织结构和热导率均发生变化,稳定状态的孔隙率为12%~14%,在热障涂层服役温度范围内热导率增加到1.25~1.45 W/(m·K)。     

细颗粒掺杂石墨高温热导率的研究

通过球磨分散方法制备了具有优良热机械性能的细颗粒掺杂石墨.该掺杂石墨材料具有致密的结构,细小的碳化物颗粒均匀分布在石墨基体中.研究表明;球磨分散方法制备的掺杂石墨比常规方法制备的石墨材料具有更优良的高温热性能以及抗热震性能.同时,本文对材料的性能与其微观结构进行关联.     

The Prediction of the Emissivity and Thermal Conductivity of Powder Beds

The particle size distribution and packing (loose bulk and tapped density) of a mixture of ground biomass from Douglas fir wood particles was characterized by different practical methods: sieving, digital imaging and scanning electron microscopy. The ground mixture was analyzed using a set of 14 wire mesh sieves. The calculated mean diameter of mixture was 251 µm. The mixture was divided into four size fractions of mean size ranging from 74 to 781 µm. Particle length measured by imaging technique were 3–4 times larger than the mean diameter determined by sieve analysis. Similarly, particle width was 1.0–2.5 times larger than mean particle diameter. The sphericity of particles in each of the four fractions increased with decreasing size of the sieve indicating that smaller particles also have a smaller aspect ratio. Empirical power law equations were developed to correlate the packing and flow ability of ground particles (HR) to the mean diameter, with R² values of 0.88 and 0.91, respectively. The HR values indicated good flow ability for the large particles and poor flow ability for the smallest particles and the entire mixture. HR and porosity ratio reached an asymptote for particles larger than 400 µm.     

The Prediction of the Emissivity and Thermal Conductivity of Powder Beds

The present study attempts to understand drying characteristics of rubber wood sawdust in a tray dryer as it is the simplest and oldest of the dryers known commercially. An increase in temperature, flow rate of the heating medium, and initial moisture content was found to increase the drying rate. However, an increase in the particle diameter and bed height was found to reduce the drying rate. The increase in drying rate with temperature and moisture content was attributed to increase in the diffusion coefficient, while the increase due to the flow rate is attributed to reduction in the external mass transfer resistance during early stages of drying while the drying rate was high. An increase in bed height as well as particle size increases the diffusion path length for moisture, which contributes to the reduction in drying rate. The experimental data were modeled using Fick's diffusion equation, and the effective diffusivity coefficient was evaluated by minimizing the error between the experimental data and the prediction using the model equation. The effective diffusion coefficient was found to increase with increase in temperature, initial moisture content, and the flow rate of the heating medium, while it was found to decrease with increase in particle size. The diffusion coefficient was not found to vary with the bed height/solid loading. The effective diffusion coefficient was found to vary within 9.1 × 10^?9to 22 × 10^?9 m²/min. The standard deviation of error between the experimental data and prediction using the model, using the estimated effective diffusivity coefficient, was found to be less than 0.07 for the entire set of data, indicating the appropriateness of the model in predicting drying kinetics.     

Effect of Moisture on the Thermal Conductivity of a Cementitious Composite

Measurements of the thermal conductivity of a cement-based composite material are performed as a function of moisture content from a dry state to a fully water-saturated state using an impulse technique. Then, the obtained data are analyzed using Brugemann and Wiener homogenization formulas. The validity of applied homogenization techniques is assessed comparing the measured and calculated results. On the basis of the experimental data and the homogenization analyses, the effects of total pore volume, pore distribution, and moisture content on the thermal conductivity are discussed.Paper presented at the Seventeenth European Conference on Thermophysical Properties, September 5–8, 2005, Bratislava, Slovak Republic.     

Realizing the Accurate Measurements of Thermal Conductivity over a Wide Range by Scanning Thermal Microscopy Combined with Quantitative Prediction of Thermal Contact Resistance

Quantitative thermal performance measurements and thermal management at the micro-/nano scale are becoming increasingly important as the size of electronic components shrinks. Scanning thermal microscopy (SThM) is an emerging method with high spatial resolution that accurately reflects changes in local thermal signals based on a thermally sensitive probe. However, because of the unclear thermal resistance at the probe-sample interface, quantitative characterization of thermal conductivity for different kinds of materials still remains limited. In this paper, the heat transfer process considering the thermal contact resistance between the probe and sample surface is analyzed using finite element simulation and thermal resistance network model. On this basis, a mathematical empirical function is developed applicable to a variety of material systems, which depicts the relationship between the thermal conductivity of the sample and the probe temperature. The proposed model is verified by measuring ten materials with a wide thermal conductivity range, and then further validated by two materials with unknown thermal conductivity. In conclusion, this work provides the prospect of achieving quantitative characterization of thermal conductivity over a wide range and further enables the mapping of local thermal conductivity to microstructures or phases of materials.     

The Prediction of the Emissivity and Thermal Conductivity of Powder Beds

A simple model for estimating the emissivity of the surface of a powder bed by knowing only the bed porosity and its solid emissivity is presented. Estimates by the model are compared with experimental measurements for powder beds of alumina, silicon carbide, and iron. Agreement within the uncertainty of the measurements of±10% is obtained. A view factor that adopts the predicted emissivity of the powder beds into its term is suggested for the calculation of the conductivity by radiation. For the prediction of the thermal conductivity of the powder beds, the authors compared the existing models in literature. They rederived the Zehner-Schlunder equation (1970) and made some modifications to it. Comparison of predictions by this equation with 424 measured values shows the predictions to be accurate to within a ±30% relative error.     

环境温度和导热系数条件下保温箱壁厚分析

目的为了优化保温箱壁厚设计,以保温箱的保温性能为表征,探究不同环境温度和材料导热系数条件下的最优保温箱壁厚。方法建立有限元模型,对比实验和有限元结果,验证有限元模型的准确性;改变模型中材料的导热系数和外界环境温度,探究不同壁厚对箱体保温性能的影响。结果在环境温度为20～50℃的条件下,导热系数为0.01～0.075 W/(m·K)的保温箱壁厚从10 mm增加到35 mm时,保温性能变化较大;壁厚从35 mm增加到45 mm时,保温性能变化缓慢;同一温度条件下,增加相同壁厚,导热系数越小的保温箱保温性能增加越显著;同一导热系数的保温箱,增加相同壁厚,环境温度越低,保温性能增加越显著。结论在不同环境温度和材料导热系数下,保温箱对应的最佳壁厚为35～40mm,为保温箱的优化设计奠定理论基础。