Impacts of anisotropy coefficient and porosity on the thermal conductivity and P-wave velocity of calcarenites used as building materials of historical monuments in Morocco

It is essential to study the porosity,thermal conductivity,and P-wave velocity of calcarenites,as well as the anisotropy coefficients of the thermal conductivity and P-wave velocity,for civil engineering,and conservation and restoration of historical monuments.This study focuses on measuring the thermal conductivity using the thermal conductivity scanning(TCS)technique and measuring the P-wave ve-locity using portable equipment.This was applied for some dry and saturated calcarenite samples in the horizontal and vertical directions(parallel and perpendicular to the bedding plane,respectively).The calcarenites were selected from some historical monuments in Morocco.These physical properties were measured in the laboratory to find a reliable relationship between all of these properties.As a result of the statistical analysis of the obtained data,excellent linear relationships were observed between the porosity and both the thermal conductivity and porosity.These relationships are characterized by relatively high coefficients of determination for the horizontal and vertical samples.Based on the thermal conductivity and P-wave velocity values in these two directions,the anisotropy coefficients of these two properties were calculated.The internal structure and the pore fabric of the calcarenite samples were delineated using scanning electron microscopy(SEM),while their chemical and mineral compositions were studied using the energy dispersive X-ray analysis(EDXA)and X-ray diffraction(XRD)techniques.

Thermal Conductivity and Heat Transfer Coefficient of Concrete

A very simple model for predicting thermal conductivity based on its definiensis was presented. The thermal conductivity obtained using the model provided a good coincidence to the investigations performed by other authors. The heat transfer coefficient was determined by inverse analysis using the temperature measurements. From experimental results, it is noted that heat transfer coefficient increases with the increase of wind velocity and relative humidity, a prediction equation on heat transfer coefficient about wind velocity and relative humidity is given.

Thomson and rotation effects during photothermal excitation process in magnetic semiconductor medium using variable thermal conductivity

This study investigates a strong magnetic field acting over an elastic rotator semiconductor medium.The Thomson effect due to the magnetic field during the photothermal transport process is studied,and the thermoelectricity theory is used to explain the behavior of waves in the homogenous and isotropic medium under the effect of variable thermal conductivity.The variable thermal conductivity is considered as a linear function of the temperature.The two-dimensional deformation equations are used to describe the overlaps among plasma,electrical,thermal,and magneto-elastic waves.The charge density of inertia-particles is considered as a function of time for studying the induced electric current.The normal mode analysis is used to obtain the exact solutions of the physical field distributions as part of this phenomenon.To obtain the complete solutions of the physical field quantities,the certain mechanical loads,electromagnetic effects,thermal effects,and plasma recombination process are applied herein.The results of the physical distributions are graphically depicted and discussed in consideration of the internal heat source,rotation,and Peltier coefficient.

Thermal conductivity of systems with a gap in the phonon spectrum

An original theoretical model for describing the low-temperature thermal conductivity in systems with a region of forbidden values （a gap） in the phonon spectrum is proposed. The model is based on new experimental results on the temperature dependence of the phonon diffusion coefficient in nanoceramics and dielectric glasses which showed a similar anomalous behavior of the diffusion coefficient in these systems that may be described under the assumption of a gap in the phonon spectrum. In this paper, the role of the gap in low-temperature behavior of the thermal conductivity, ~＇（T）, is analyzed. The plateau in the temperature dependence of the thermal conductivity is shown to correlate with the position and the width of the gap. The temperature dependence of thermal conductivity of such systems when changing the scat- tering parameters related to various mechanisms is studied. It is found that the umklapp process （U-processes） involving low-frequency short-wavelength phonons below the gap forms the behavior of the temperature dependence of thermal con- ductivity in the plateau region. A comparison of the calculated and experimental results shows considerable possibilities of the model in describing the low-temperature thermal conductivity in glass-like systems.

Temperature-Dependent Thermal Conductivity and Absorption Coefficient Identification of Quartz Window up to 1100 K

The temperature-dependent absorption coefficient and thermal conductivity of a quartz window are obtained through experimental tests at a wide range of temperatures.A Fourier transform infrared spectrometer with a heated cavity is used for performing the transmittance measurements.The spectral absorption coefficient of the quartz window is inverted by the transmittance information at different temperatures using a genetic algorithm.Then,a quartz window-graphite plate-quartz window multilayer structure is designed,and the transient response of the structure subjected to high-temperature heating is recorded by a self-designed setup.Cooperating with the above absorption coefficient,a non-gray radiative-conductive heat transfer model is built for the multilayer structure,and the intrinsic thermal conductivity of the quartz window is identified.Finally,the effects of the temperature-dependent absorption coefficient and spectral selective features of the medium on the heat transfer characteristics are discussed.The results show that the absorption coefficient gradually increases with temperature.The intrinsic thermal conductivity of the quartz window varies from 1.35 to 2.52 W/(m·K)as the temperature rises,while the effective thermal conductivity is higher than the intrinsic thermal conductivity due to thermal radiation,specifically 26.4%higher at 1100 K.In addition,it is found that the influence of the temperature-dependent absorption coefficient on temperature of unheated side shows a trend of first increasing and then decreasing.When the absorption coefficient varies greatly with wavelength,a non-gray radiative-conductive heat transfer model should be built for the semitransparent materials.

Drift Mobility, Diffusion Coefficient of Randomly Moving Charge Carriers in Metals and Other Materials with Degenerated Electron Gas

In this short review some aspects of applications of free electron theory on the ground of the Fermi statistics will be analyzed. There it is an intention to attempt somebody’s attention to problems in widespread literature of interpretation of conductivity of metals, superconductor in the normal state and semiconductors with degenerated electron gas. In literature there are many cases when to these materials the classical statistics is applied. It is well known that the electron heat capacity and thermal noise (and as a consequence the electrical conductivity) are determined by randomly moving electrons, which energy is close to the Fermi energy level, and the other part of electrons, which energy is well below the Fermi level can not be scattered and change its energy. Therefore there was tried as simple as possible on the ground of Fermi distribution, and on random motion of charge carriers, and on the well known experimental results to take general expressions for various kinetic parameters which are applicable for materials both without and with degenerated electron gas. It is shown, that drift mobility of randomly moving charge carriers, depending on the degree degeneracy, can considerably exceed the Hall mobility. Also it is shown that the Einstein relation between the diffusion coefficient and the drift mobility of charge carriers is valid even in the case of degeneracy. There also will be presented the main kinetic parameter values for different metals.

Improving tribological and thermal properties of Al alloy using CNTs and Nb nanopowder via SPS for power transmission conductor

This study aimed at improving the tribological and thermal properties of Al alloy using CNTs and Nb nanopowder as reinforcements and spark plasma sintering(SPS)as the fabrication method.The SPS was conducted at 630℃,30 MPa,10 min,and 200℃/min.The tribology test was run with ball-on-disc tribometer using steel ball as the counter body.And the thermal test was processed with thermogravimetric analyzer(TGA)and laser flash apparatus(LFA).Results showed that the addition of 8 wt.%CNTs and 8 wt.%Nb reinforcements respectively decreased the coefficient of friction(COF)of the composite by 79%.The wear volume of the composite was decreased by 23%,and so was the wear rate.However,the thermal conductivity of the composite was equally improved by 44%.The tribology improvement was stimulated by a C film generated by CNTs and a protective Nb2O5 formed by Nb nanopowder.The thermal conductivity was improved by the grain refining property of Nb and the high thermal conductivity of CNTs.Therefore,these results indicated that Al-CNTs-Nb composite is a robust material for high transmission conductor capable of reducing sag and ensuring the durability of the composite.

Research on influence factors of rock-soil thermal properties by laboratory test

In order to discover the relation between rock-soil thermal properties and strata during the process of engineering investigation,the authors studied the measuring principle of Thermal Conductivity Scanner( TCS)and measured the thermal properties of 45 drilling samples from Qinghai with TCS in the laboratory. The results show that the specific heat capacity( SHC) decreases while the thermal conductivity( TC) increases with the increase of the depth. With the lithological change,the specific heat capacity and thermal conductivity have the opposite trend. The depth and lithology have a greater influence on the thermal conductivity than the specific heat capacity.

A Potential Approach to Enhance the Seebeck Coefficient of UHMWPE by Using the Graphene Oxide

Thermoelectric materials have been a competent source for the production of energy in the present decade.The most important and potential parameter required for the material to have better thermoelectric characteristics is the Seebeck coefficient.In this work,ultra high molecular weight polyethylene(UHMWPE)and graphene oxide(GO)nanocomposites were prepared by mechanical mixing by containing 10000ppm,50000ppm,70000ppm,100000ppm,150000ppm,and 200000ppm loadings of graphene oxide.Due to the intrinsic insulating nature of UHMWPE,the value of Seebeck for pristine UHMWPE and its nanocomposites with 10000ppm&50000ppm of GO concentration was too low to be detected.However,the Seebeck coefficient for composites with 70000ppm,100000ppm,150000ppm,and 200000ppm loadings of GO was found to be 180,206,230,and 235μV/K,respectively.These higher values of Seebeck coefficients were attributed to the superior thermal insulating nature of UHMWPE and the conductive network induced by the GO within the UHMWPE insulating matrix.Although,the values of the figure of merit and power factor were negligibly small due to the lower concentration of charge carriers in UHMWPE/GO nanocomposites but still reported,results are extremely hopeful for considering the composite as the potential candidate for thermoelectric applications.

Thermal and Electrical Properties of Liquid Metal Gallium During Phase Transition

Liquid metal gallium has been widely used in numerous fields, from nuclear engineering, catalysts, and energy storage to electronics owing to its remarkable thermal and electrical properties along with low viscosity and nontoxicity. Compared with high-temperature liquid metals, room-temperature liquid metals, such as gallium(Ga), are emerging as promising alternatives for fabricating advanced energy storage devices, such as phase change materials, by harvesting the advantageous properties of their liquid state maintained without external energy input. However, the thermal and electrical properties of liquid metals at the phase transition are rather poorly studied, limiting their practical applications. In this study, we reported on the physical properties of the solid–liquid phase transition of Ga using a custom-designed, solid–liquid electrical and thermal measurement system. We observed that the electrical conductivity of Ga progressively decreases with an increase in temperature. However, the Seebeck coefficient of Ga increases from 0.2 to 2.1 μV/K, and thermal conductivity from 7.6 to 33 W/(K·m). These electrical and thermal properties of Ga at solid–liquid phase transition would be useful for practical applications.

航天器用界面导热填料应用状态接触传热系数分析

为解决航天器用RKTL-DRZ-2型导热脂及RKTL-DRNJ-1型导热凝胶2种新开发导热填料无实际应用状态下接触传热系数数据的问题,采用试验方法对其接触传热系数进行研究,得到在航天器上设备典型尺寸及不同固定螺钉分布情况下的接触传热系数。试验结果表明:在相同安装状态下,与目前常用的RKTL-DRZ-1型导热脂相比,RKTL-DRZ-2型导热脂对界面接触传热性能改善效果较好,RKTL-DRNJ-1型导热凝胶较差。此外,获取了保证导热填料填充效果的实施经验,并给出了导热填料的选用建议。试验分析结论可应用于选用导热填料时的航天器热设计及热控实施。

高熵稀土氧化物热障涂层材料研究进展

热障涂层(Thermal barrier coating,TBC)材料在航空发动机和燃气轮机的热防护中具有保护高温合金基底免受氧化及腐蚀,并降低高温合金的工作温度的重要作用。新型热障涂层材料中存在许多高熵稀土氧化物,能够实现比单一主成分稀土氧化物更优异的热学、力学、高温相稳定性以及抗烧结、耐腐蚀等性能。但是目前对高熵稀土氧化物的研究仍然停留在初步阶段,其中稀土元素对材料性能的作用尚未完全明确,且没有形成统一标准。简要概述了热障涂层的基本结构,并重点总结了高熵锆酸盐、铈酸盐、铪酸盐、钽酸盐和铌酸盐等5种高熵稀土酸盐的晶体结构、热物理性能与力学性能。对比分析了其与相应的单一组分稀土酸盐的差异,并探讨了影响其性能优劣的多种因素。相比于单一组分稀土氧化物,高熵稀土氧化物的热导率、热膨胀系数和相稳定性均有明显改善。最后,展望了未来高熵稀土热障涂层的发展方向。

Sm_(2)Y_(1-x)Al_(x)TaO_(7)陶瓷材料的热物理性能

以高纯度Al_(2)O_(3)、Y_(2)O_(3)、Sm_(2)O_(3)和Ta_(2)O_(5)为原材料,采用高温烧结方法制备了Sm_(2)Y_(1-x)Al_(x)TaO_(7)系列陶瓷材料,对其晶体结构、显微组织、元素组成、热导率和热膨胀性能进行了系统研究。结果表明,除Sm_(2)Y_(0.5)Al_(0.5)TaO_(7)具有焦绿石结构之外,其余三种氧化物均具有单一的萤石晶体结构。其块体材料相对致密度均在90%以上,晶界清晰,各元素摩尔比与其化学式基本一致。由于Al^(3+)和Y^(3+)之间较大的离子半径和原子量差别,增大了对声子的散射,其热传导性能随着Al_(2)O_(3)含量的增加而降低。虽然Al_(2)O_(3)含量增加降低了其热膨胀系数,其大小与YSZ基本相当,所制备陶瓷在室温至1400℃范围内具有良好的相稳定性能。

(Ho_(0.25)Lu_(0.25)Yb_(0.25)Eu_(0.25))_(2)SiO_(5) high-entropy ceramic with low thermal conductivity,tunable thermal expansion coefficient,and excellent resistance to CMAS corrosion

Low thermal conductivity,compatible thermal expansion coefficient,and good calcium–magnesium–aluminosilicate(CMAS)corrosion resistance are critical requirements of environmental barrier coatings for silicon-based ceramics.Rare earth silicates have been recognized as one of the most promising environmental barrier coating candidates for good water vapor corrosion resistance.However,the relatively high thermal conductivity and high thermal expansion coefficient limit the practical application.Inspired by the high entropy effect,a novel rare earth monosilicate solid solution(Ho_(0.25)Lu_(0.25)Yb_(0.25)Eu_(0.25))_(2)SiO_(5)was designed to improve the overall performance.The as-synthesized(Ho_(0.25)Lu_(0.25)Yb_(0.25)Eu_(0.25))_(2)SiO_(5)shows very low thermal conductivity(1.07 W·m-1·K-1 at 600℃).Point defects including mass mismatch and oxygen vacancies mainly contribute to the good thermal insulation properties.The thermal expansion coefficient of(Ho_(0.25)Lu_(0.25)Yb_(0.25)Eu_(0.25))_(2)SiO_(5)can be decreased to(4.0–5.9)×10^(-6)K^(-1)due to severe lattice distortion and chemical bonding variation,which matches well with that of SiC((4.5–5.5)×10^(-6)K^(-1)).In addition,(Ho_(0.25)Lu_(0.25)Yb_(0.25)Eu_(0.25))_(2)SiO_(5)presents good resistance to CMAS corrosion.The improved performance of(Ho_(0.25)Lu_(0.25)Yb_(0.25)Eu_(0.25))_(2)SiO_(5)highlights it as a promising environmental barrier coating candidate.

相变储热技术在空气源热泵供暖中的应用研究现状

空气源热泵技术因具有节能、环保、高效等优点,在我国建筑供暖领域得到了广泛应用。文章分析了空气源热泵系统在供暖过程中存在的问题,介绍了利用相变储热技术优化空气源热泵运行性能的基本方式及原理。通过分析相变材料自身热物性以及相变储热单元在系统中的应用,从材料及系统两方面简述了相变储热技术在空气源热泵供暖中的研究现状及进展。最后,得出结论并针对相变储热技术在空气源热泵供暖中的应用难点,对未来发展进行了展望。

导热胶导热性能的研究进展与探讨

随着消费电子产品的迅速普及和技术的不断进步,导热胶市场需求不断增加,发展潜力巨大。但在小型化、集成化和便携性电子产品快速发展的同时,也对导热胶的导热性能提出了更高的要求。为此,本文介绍了填充型导热胶的导热机理,论述了传统导热模型的应用范围及有限元模拟预测导热系数的方法。综述了近年来导热胶胶体的研究成果,并着重论述了导热胶填料的种类、形状、粒径、改性及导热填料的混合使用对导热胶导热系数的影响规律,总结了提升导热胶导热系数的方法。

New class of high-entropy pseudobrookite titanate with excellent thermal stability,low thermal expansion coefficient,and low thermal conductivity

As a type of titanate,the pseudobrookite(MTi_(2)O_(5)/M_(2)TiO_(5))exhibits a low thermal expansion coefficient and thermal conductivity,as well as excellent dielectric and solar spectrum absorption properties.However,the pseudobrookite is unstable and prone to decomposing below 1200℃,which limits the practical application of the pseudobrookite.In this paper,the high-entropy pseudobrookite ceramic is synthesized for the first time.The pure high-entropy(Mg,Co,Ni,Zn)Ti_(2)O_(5) with the pseudobrookite structure and the biphasic high-entropy ceramic composed of the highentropy pseudobrookite(Cr,Mn,Fe,Al,Ga)_(2)TiO_(5) and the high-entropy spinel(Cr,Mn,Fe,Al,Ga,Ti)_(3)O_(4) are successfully prepared by the in-situ solid-phase reaction method.The comparison between the theoretical crystal structure of the pseudobrookite and the aberration-corrected scanning transmission electron microscopy(AC-STEM)images of high-entropy(Mg,Co,Ni,Zn)Ti_(2)O_(5) shows that the metal ions(M and Ti ions)are disorderly distributed at the A site and the B site in high-entropy(Mg,Co,Ni,Zn)Ti_(2)O_(5),leading to an unprecedentedly high configurational entropy of high-entropy(Mg,Co,Ni,Zn)Ti_(2)O_(5).The bulk high-entropy(Mg,Co,Ni,Zn)Ti_(2)O_(5) ceramics exhibit a low thermal expansion coefficient of 6.35×10^(−6) K^(−1) in the temperature range of 25-1400℃ and thermal conductivity of 1.840 W·m^(−1)·K^(−1) at room temperature,as well as the excellent thermal stability at 200,600,and 1400℃.Owing to these outstanding properties,high-entropy(Mg,Co,Ni,Zn)Ti_(2)O_(5) is expected to be the promising candidate for high-temperature thermal insulation.This work has further extended the family of different crystal structures of high-entropy ceramics reported to date.

基于ANSYS/APDL的织物导热性能影响因素

为研究织物导热性能的影响因素,提出了一种基于ANSYS的织物热传递有限元仿真二次开发系统。利用ANSYS/APDL软件对织物进行参数化建模,模拟分析在不同织物组织、织物紧度、经纬纱线材料条件下,织物中的热量传递过程及温度分布特征,并计算各织物的导热系数和蓄热系数。使用C-THERM TCi导热仪对织物样本的导热系数和蓄热系数进行测试,与模拟计算结果对比,发现模拟同实验测试结果的绝对误差值在4%以内,验证了该模型的有效性,并进一步探究了织物导热性能的影响因素。结果表明:在相同条件下,平纹织物的蓄热系数和导热系数最高,斜纹次之,缎纹最低;随着织物紧度的增加,织物的导热系数和蓄热系数也升高;随着纱线导热系数的上升,织物的导热系数和蓄热系数升高。研究结果对织物导热性能的参数化模拟仿真及良好导热性能的织物开发具有重要意义。

金刚石粒径对金刚石/Cu-B合金复合材料热物理性能的影响

采用Cu-B合金为基体,选用粒径分别为110、230、550μm的金刚石颗粒作为增强体,利用气压熔渗工艺在1100℃、10 MPa气体压力下制备金刚石/Cu-B合金复合材料,研究金刚石颗粒粒径对复合材料组织结构、界面相分布及热物理性能的影响。结果表明,随着金刚石粒径的增大,复合材料热导率上升,热膨胀系数减小,复合材料界面处硼碳化合物含量增加,界面结合情况得到改善。由金刚石颗粒粒径为550μm时,复合材料热导率最高,可达680.3 W/(m·K),热膨胀系数最小,为4.905×10^(−6)K^(−1),符合高效热管理器件对金刚石/金属基复合材料的热物理性能要求,在电子产品散热器件方面具有良好的应用前景。

高温熔盐储罐基础中陶粒的材料性能试验研究

高温熔盐储罐中储热介质温度约600℃,要求每天温降不超过1℃,为满足保温要求,储罐基础常以陶粒作为保温材料。通过对不同级配下5种典型陶粒的化学成分、材料特性和高温热工参数等进行试验研究,得到了陶粒的物理力学性能、相变温度、热膨胀系数和不同温度(20~600℃)下的导热系数等关键参数,揭示了陶粒的导热特性和变形机理。