Phase Transformation Contributions to Heat Capacity and Impact on Thermal Diffusivity,Thermal Conductivity,and Thermoelectric Performance

The accurate characterization of thermal conductivity κ, particularly at high temperature, is of paramount importance to many materials, thermoelectrics in particular. The ease and access of thermal diffusivity D measurements allows for the calculation of κ when the volumetric heat capacity, ρc_p, of the material is known. However, in the relation κ = ρc_pD, there is some confusion as to what value of c_p should be used in materials undergoing phase transformations. Herein, it is demonstrated that the Dulong–Petit estimate of c_p at high temperature is not appropriate for materials having phase transformations with kinetic timescales relevant to thermal transport. In these materials, there is an additional capacity to store heat in the material through the enthalpy of transformation ΔH. This can be described using a generalized model for the total heat capacity for a material $$\rho {\mathit{c}}_p={\mathit{C}}_{p?}+\mathrm{\Delta }\mathit{H}{(??\mathrm{?}/\mathrm{?}?\mathit{T})}_p$$ where φ is an order parameter that describes how much latent heat responds “instantly” to temperature changes. Here, C_pφ is the intrinsic heat capacity (e.g., approximately the Dulong–Petit heat capacity at high temperature). It is shown experimentally in Zn₄Sb₃ that the decrease in D through the phase transition at 250 K is fully accounted for by the increase in c_p, while κ changes smoothly through the phase transition. Consequently, reports of κ dropping near phase transitions in widely studied materials such as PbTe and SnSe have likely overlooked the effects of excess heat capacity and overestimated the thermoelectric efficiency, zT.     

具有低热导率的Skutterudite类新型热电材料

结合声子散射机制介绍了近来报道的几种降低skutterudite类热电材料热导率的途径，为研究和发展skutterudite类热电材料提供一些思路和方法。     

相变材料导热性能强化的研究进展

相变储能复合材料对于缓解能源紧张状况,保护环境和提供舒适健康的生活环境具有积极的意义,在太阳能热发电、工业热利用及余热回收方面有显著优势。但是相变材料本身存在导热性能不高,容易腐蚀容器等问题,所以本文主要综述了相变材料导热性能强化的研究进展。首先介绍相变储能材料的现状以及存在的问题,然后讨论了以金属、陶瓷和碳质纳米材料作为导热强化材料在相变储能复合材料中的研究现状和成果,最后展望了经导热强化后的相变储能复合材料的前景。     

Cu2S相变过程中热扩散系数的精确测量和解析

材料发生相变时, 其结构和物理性能可能会发生剧烈的变化。采用激光闪射法测量热扩散系数时, 激光照射样品可能会伴随有光吸收/发射现象以及温度的显著升高, 导致其测量值偏离真实值。本工作以Cu₂S为研究对象, 发现激光照射样品后, 光吸收/发射的影响很小可以忽略, 但样品温度的升高则会明显影响热扩散系数的测量。通过构建具有不同石墨层厚度的石墨/Cu₂S双层结构, 利用石墨层减弱激光照射时Cu₂S样品的温度增加幅度, 成功使热扩散系数出现显著降低的起始温度接近采用DSC测量材料发生相变的起始温度。本研究进一步建立了石墨/Cu₂S双层结构样品的热流输运模型, 从石墨/Cu₂S双层结构样品的实验测试热扩散系数中解析出了Cu₂S在相变区间的本征热扩散系数。本工作对于理解和精确表征具有相变特征的离子导体热电材料、光敏、热敏材料的热扩散系数具有重要的意义。     

纳米氧化钇稳定氧化锆/钛酸锶复合热电材料的热导率研究

通过添加纳米尺度的低热导率添加剂YSZ,形成钛酸锶复合热电材料,研究发现,虽然电子热导率有明显提高,但声子热导率降低得更加明显,进而导致总热导率显著下降。声子热导率的降低主要是因为纳米YSZ在钛酸锶体材料中弥散分布,声子在纳米YSZ与钛酸锶界面处被大量散射,导致声子平均自由程明显降低,进而导致声子热导率明显降低。     

热电材料中的晶格热导率

随着可再生能源及能源转换技术的快速发展, 热电材料在发电及制冷领域的应用前景受到越来越广泛的关注。发展具有高热电优值材料的重要性日益突出, 如何获得低晶格热导率是热电材料的研究重点之一。本文阐述了热容、声速及弛豫时间对晶格热导率的影响, 介绍了本征低热导率热电材料所具有的典型特征, 如强非谐性、弱化学键、本征共振散射及复杂晶胞结构等, 并分析了通过多尺度声子散射降低已有热电材料晶格热导率的方法, 其中包括点缺陷散射、位错散射、晶界散射、共振散射、电声散射等多种散射机制。此外, 总结了几种预测材料最小晶格热导率的理论模型, 对快速筛选具有低晶格热导率的热电材料具有一定的理论指导意义。最后, 展望了如何获得低热导率热电材料的有效途径。     

冰生消过程体系导热系数动态变化特性

本文利用基于瞬变平面热源技术的Hot Disk热常数分析仪研究了冰生消过程中导热系数的变化规律。根据体系状态的变化,冰的生消过程可以分为5个阶段:未结冰过程(纯水)、结冰过程(冰水混合)、纯冰过程、融冰过程(冰水混合)、完全融解(纯水)。实验温度从10.3℃降至-11.0℃,然后再升至10.3℃,5个阶段中测得的导热系数分别为0.592~0.669 W/(m·K)、0.603~2.284 W/(m·K)、2.019~3.106 W/(m·K)、0.611~1.945 W/(m·K)和0.596~0.598 W/(m·K)。结果表明:水结冰融冰过程中,体系导热系数发生动态变化,并且体系导热系数值可能不是纯冰或冰水混合物的导热系数;当温度接近冰点时,水或冰的结构可能改变,导致体系导热系数突变。由于冰和水的密度不同,在结冰融冰过程中体系局部微环境的冰水两相可能存在微弱的自然对流,进而影响体系的导热系数。在实际生产活动中冰的生消过程一般接连发生,考虑体系导热系数动态变化的因素,可以避免热平衡失调影响生物环境或涉冰构筑物结构性能。     

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.     

New Transient Hot-Bridge Sensor to Measure Thermal Conductivity, Thermal Diffusivity, and Volumetric Specific Heat

A high sensitivity thermoelectric sensor to measure all relevant thermal transport properties has been developed. This so-called transient hot bridge (THB) decidedly improves the state of the art for transient measurements of the thermal conductivity, thermal diffusivity, and volumetric specific heat. The new sensor is realized as a printed circuit foil of nickel between two polyimide sheets. Its layout consists of four identical strips arranged in parallel and connected for an equal-ratio Wheatstone bridge. At uniform temperature, the bridge is inherently balanced, i.e., no nulling is required prior to a run. An electric current makes the unequally spaced strips establish an inhomogeneous temperature profile that turns the bridge into an unbalanced condition. From then on, the THB produces an offset-free output signal of high sensitivity as a measure of the properties mentioned of the surrounding specimen. The signal is virtually free of thermal emf’s because no external bridge resistors are needed. Each single strip is meander-shaped to give it a higher resistivity and, additionally, segmented into a long and short part to compensate for the end effect. The THB closely meets the specific requirements of industry and research institutes for an easy to handle and accurate low cost sensor. As the key component of an instrument, it allows rapid thermal-conductivity measurements on solid and fluid specimens from 0.02 to 100 W· m⁻¹·K⁻¹ at temperatures up to 250°C. Measurements on some reference materials and thermal insulations are presented. These verify the preliminary estimated uncertainty of 2% in thermal conductivity.     

Thermal Conductivity of High-Temperature Multicomponent Materials with Phase Change

This work focuses on the investigation of the effective thermal conductivity (λ_eff) of heterogeneous materials consisting of a phase change material (PCM) and expanded graphite (EG). These composites may be employed in latent heat storage systems, where a PCM stores energy by being heated to a temperature higher than its melting point (T _m), and releases it during solidification. For the determination of λ_eff, the steady-state comparative method was used and modified to measure composite samples at temperatures above T _m. Results were compared with the thermal conductivity of the pure PCMs, and a significant increase could be observed. The dependence of λ_eff on temperature, as well as the influence of the material microstructure on the enhancement of λ_eff, were examined. Paper presented at the Seventeenth European Conference on Thermophysical Properties, September 5–8, 2005, Bratislava, Slovak Republic.     

Thermal diffusivity and conductivity in low-conducting materials: A new technique

A comparative method is presented, suitable to measure both thermal diffusivity and conductivity of low-conducting solids. The repeatibility of the measurements of thermal conductivity is 3%, whereas for diffusivity is 6%. Data for some low-conducting materials are given, consistent with those reported in the literature.     

A Nonstationary Method and Experimental Equipment for Measuring the Thermal Conductivity of Heat Insulators

A method of measuring the thermal conductivity of solid heat-insulating materials based on the integral form of Fourier's equation, obtained by an integro-interpolation method is described. The theoretical basis for the calculation formula of the method, the results of investigations of the formula using a thermal model, the circuit of a device which realizes the theoretical formula, and the accuracy and time characteristics of the proposed method are presented. __________ Translated from Izmeritel'naya Tekhnika, No. 8, pp. 38–43, August, 2005.     

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 of a Simulated Fuel with Dissolved Fission Products

The thermal diffusivity of a simulated fuel with fission products forming a solid solution was measured using the laser-flash method in the temperature range from room temperature to 1673 K. The density and the grain size of the simulated fuel with the solid solutions used in the measurement were 10.49 g · cm⁻³ (96.9% of theoretical density) at room temperature and 9.5 μm, respectively. The diameter and thickness of the specimens were 10 and 1 mm, respectively. The thermal diffusivity decreased from 2.108 m² · s⁻¹ at room temperature to 0.626 m² · s⁻¹ at 1673 K. The thermal conductivity was calculated by combining the thermal diffusivity with the specific heat and density. The thermal conductivity of the simulated fuel with the dissolved fission products decreased from 4.973 W · m⁻¹ · K⁻¹ at 300 K to 2.02 W · m⁻¹ · K⁻¹ at 1673 K. The thermal conductivity of the simulated fuel was lower than that of UO₂ by 34.36% at 300 K and by 15.05% at 1673 K. The difference in the thermal conductivity between the simulated fuel and UO₂ was large at room temperature, and decreased with an increase in temperature. Paper presented at the Seventeenth European Conference on Thermophysical Properties, September 5–8, 2005, Bratislava, Slovak Republic.     

Thermal diffusivity of solids with a low expansion coefficient: A dilatometric technique

A dilatometric method is presented, suitable to obtain both thermal diffusivity and conductivity of low-conducting solids with a low expansion coefficient. The repeatibility of the measurements of thermal conductivity is 3%, whereas that for diffusivity is 5 %. Data for fused silica at room temperature are given, consistent with those reported in the literature. Since the method is based on detecting the thermal expansion of a copper disk in thermal contact with the specimen, its range of applicability is linked to the sensitivity by which the dilation of copper can be measured: no difficulty arises between liquid nitrogen and 1000°C.     

空调蓄冷用相变材料的研究进展

本文综述了空调用相变蓄冷材料的国内外研究进展,简要介绍了相变蓄冷材料的性能要求和基本分类,总结了已有相变蓄冷材料存在的不足,并指出了优化性能的方法,特别是改善无机类材料的过冷、相分离,有机类材料的导热性能差等问题。对比了无机和有机两类材料在相变蓄冷中的优劣,并对今后空调蓄冷用相变材料的发展方向提出了建议。本文侧重强调满足空调蓄冷温区的材料,并针对典型材料展开深入调研和评述,为更好的改善空调蓄冷用相变材料的性能和推动其实际应用提供依据。     

Measurements of Thermal Conductivity and Thermal Diffusivity of CVD Diamond

The thermal conductivity of natural, gem-quality diamond, which can be as high as 2500 Wm^–1 K^–1 at 25°C, is the highest of any known material. Synthetic diamond grown by chemical vapor deposition (CVD) of films up to 1 mm thick exhibits generally lower values of but under optimal growth conditions it can rival gem-quality diamond with values up to 2200 Wm^–1 K^–1. However, it is polycrystalline and exhibits a columnar microstructure. Measurements on free-standing CVD diamond, with a thickness in the range 25–400 m, reveal a strong gradient in thermal conductivity as a function of position z from the substrate surface as well as a pronounced anisotropy with respect to z. The temperature dependence of in the range 4 to 400 K has been analyzed to determine the types and numbers of phonon scattering centers as a function of z. The defect structure, and therefore the thermal conductivity, are both correlated with the microstructure. Because of the high conductivity of diamond, these samples are thermally thin. For example, laser flash data for a 25-m-thick diamond sample is expected to be virtually the same as laser flash data for a 1-m-thick fused silica sample. Several of the techniques described here for diamond are therefore applicable to much thinner samples of more ordinary material.     

AlN/玻璃复合材料的相分布对导热性能的影响

以热压烧结方法在850～1000℃的低温下制备了Al/玻璃复合材料,通过组成变化研究了复合材料热导率的变化规律,利用SEM、TEM、XRD等方法观察了AlN/玻璃复合材料的显微结构和相组成与分布,讨论了复合材料的显微结构和相组成对热导率的影响,结果表明:AlN/玻璃复合材料的热导率随玻璃相的减少而增加,低温烧结的复合材料的热导率可以达到10W/m·K以上,材料的相分布对热导率的影响表现为:当玻璃相减少,AlN晶粒相互接触时,有利于热导率的提高;当材料内部以AlN晶相为主时,材料出现晶体的导热特征,即热导率随温度的升高而下降。     

Measurements of the Thermal Conductivity and Thermal Diffusivity of Polymer Melts with the Short-Hot-Wire Method

In this paper, the thermal conductivity and thermal diffusivity of four kinds of polymer melts were measured by using the transient short-hot-wire method. This method was developed from the hot-wire technique and is based on two-dimensional numerical solutions of unsteady heat conduction from a wire with the same length-to-diameter ratio and boundary conditions as those in the actual experiments. The present method is particularly suitable for measurements of molten polymers where natural convection effects can be ignored due to their high viscosities. The results have shown that the present method can be used to measure the thermal conductivity and thermal diffusivity of molten polymers within uncertainties of 3 and 6%, respectively. Further, the thermal conductivity and thermal diffusivity of solidified samples were also measured and discussed.