Self-templated synthesis and thermal conductivity investigation for ultrathin perovskite oxide nanowires

The large thermal conductivity of bulk complex metal oxides such as SrTiO(3), NaCo(2)O(4), and Ca(3)Co(4)O(9) has set a barrier for the improvement of thermoelectric figure of merit and the applications of these materials in high temperature (≥1000 K) thermoelectric energy harvesting and solid-state cooling. Here, we present a self-templated synthesis approach to grow ultrathin SrTiO(3) nanowires with an average diameter of 6 nm in large quantity. The thermal conductivity of the bulk pellet made by compressing nanowire powder using spark plasma sintering shows a 64% reduction in thermal conductivity at 1000 K, which agrees well with theoretical modeling.     

Impact of irradiation induced dislocation loops on thermal conductivity in ceramics

Experimental work aimed at understanding the role of dislocation loops in limiting phonon mediated thermal transport in ceramics is presented. Faulted dislocation loops, having diameters of a few nanometers, were introduced by irradiating a polycrystalline cerium dioxide sample with 1.6 MeV protons at 700°C. XRD analysis indicated that irradiated samples retained their crystalline structure and exhibit very little lattice expansion suggesting a low concentration of point defects. Further microstructure characterization using transmission electron microscopy revealed that interstitial type faulted dislocation loops were primarily created as expected for these irradiation conditions. Thermal conductivity of the damaged layer was measured using a modulated thermoreflectance approach. Analysis of the experimental data using the classical Klemens-Callaway approach reveals that the conductivity reduction is primarily due to dislocation loops, while point defects and voids play only a minor role. These results provide experimental confirmation that faulted loops offer a unique arrangement for displaced atoms that leads to an unusually large reduction of thermal conductivity.     

Hydrogenation induced deformation mode and thermal conductivity variations in graphene sheets

In this study, we adopt molecular dynamics simulation and first principles theory calculation to investigate the deformation modes of graphene sheets induced by patterned hydrogenation stripes, as well as thermal conductivity variation with respect to the doping density and deformation modes. We demonstrate that the deformation modes can be controlled by the hydrogenation patterning parameters. Both the doping density and morphology contribute to the thermal conductivity variation of the graphene sheet. With the control of hydrogenation patterning parameters, desired deformation modes and thermal conductivity of graphene can be achieved.     

Methods for predicting the thermal conductivity of composite systems: A review

The thermal conductivity of a solid or gas filled polymer is used in processing or end use application calculations. Numerous theoretical and empirical correlations are found in the literature. A careful review of these models indictes that no one correlation or technique accurately predicts the thermal conductivity of all types of composites. The investigation indicated that for solid filled composites the Lewis and Nielsen equation fitted the experimental data best for the range of fillers tested. However, for a gas filled polymer, none of the theoretical models proved adequate. The semi-empirical approach of Harding showed considerable merit.     

含油气盆地中热传导与岩石热导率研究

含油气盆地中热传导是最主要的热量传递方式,在大地热流一定的情况下制约热传导的主要因素是热导率的大小。前人对岩石热导率与温度、压力、岩石孔隙度的关系以及岩石热导率的各向异性进行了研究。本文对前人的研究成果进行了综合分析,认为下一步应该加强沉积岩平行层面方向和垂直层面方向热导率差异的研究及热导率与岩石输导性能关系的研究。     

Gold nanoparticle-modulated conductivity in gold peapodded silica nanowires

We report the enhanced electrical conductivity properties of single gold-peapodded amorphous silica nanowires synthesized using microwave plasma enhanced chemical vapor deposition. Dark conductivity of the gold-peapodded silica nanowires can be adjusted by controlling the number of incorporated metal nanoparticles. The temperature-dependent conductivity measurement reveals that the band tail hopping mechanism dominates the electron transport in the gold-peapodded silica nanowires. The high conductivity in the nano-peapodded nanowires with more embedded gold-nanoparticles can be explained by the higher density of hopping states and shorter hopping distance. These Au-embedded amorphous silica nanowires have provided a new approach to enhance not only the electron conduction, but also the chemical-sensor response/sensitivity.     

High interfacial thermal resistance induced low thermal conductivity in porous SiC-SiO2 composites with hierarchical porosity

The incorporation of a thermally insulating secondary phase can significantly increase the interfacial thermal resistance attributed to its low intrinsic thermal conductivity and the creation of multiple phonon scattering interfaces between adjacent SiC particles. The newly developed porous SiC-33 wt% SiO₂ composites with SiO₂ as a thermally insulating secondary phase exhibited a very low thermal conductivity (0.047 Wm⁻¹ K⁻¹, 72.4 % porous), which is an order of magnitude lower than the previously reported lowest thermal conductivity (0.14 Wm⁻¹ K⁻¹, 76.3 % porous) for powder processed porous SiC ceramics and is even lower than the thermal conductivity (0.060 Wm⁻¹ K⁻¹, 87.9% porous) of SiO₂ aerogel. The porous SiC-(16–73 wt%) SiO₂ composites processed from nano β-SiC and a 40 wt% carbon template exhibited a hierarchical (meso-/macro-porous) pore structure that transformed to a trimodal (micro-/meso-/macro-porous) porous structure when polysiloxane was added and sintering was performed at 600–1000 °C in air.     

Defect engineering in development of low thermal conductivity materials: A review

Low thermal conductivity is the key property dominating the heat insulation ability of thermal barrier coatings (TBC). Reducing the intrinsic thermal conductivity is the major topic for developing advanced TBCs. Defect engineering has attracted much attention in seeking better TBC materials since lattice defects play a crucial role in phonon scattering and thermal conductivity reduction. Oxygen vacancies and substitutions are proven to be the most effective, while the accompanying lattice distortion is also of great importance. In this paper, recent advances of reducing the thermal conductivity of potential thermal barrier coating materials by defect engineering are comprehensively reviewed. Effects of the mass and size mismatch between the defects and the host lattice are quantitatively estimated and unconventional thermal conductivity reduction caused by the lattice distortions is also discussed. Finally, challenges and potential opportunities are briefly assessed to further minimize the thermal conductivity of TBC materials in the future.     

A strategy for fabricating textured silicon nitride with enhanced thermal conductivity

C-axis textured Si₃N₄ with a high thermal conductivity of 176 W m⁻¹ K⁻¹ along the grain alignment direction was fabricated by slip casting raw α-Si₃N₄ powder seeded with near-equiaxed β-Si₃N₄ particles and Y₂O₃–MgSiN₂ as sintering additives in a rotating strong magnetic field of 12 T, followed by gas pressure sintering at 1900 °C for 12 h at a nitrogen pressure of 1 MPa. The green material reached a relative density of 57%, with slip casting and the sintered material exhibited a relative density of 99% and a Lotgering orientation factor of 0.98. The morphology of the β-Si₃N₄ seeds had little effect on the texture development and thermal anisotropy of textured Si₃N₄. The technique developed provides highly conductive Si₃N₄ with conductivity to the thickness direction, which is a major advantage in practical use. The technique is also simple, inexpensive and effective for producing textured Si₃N₄ with high thermal conductivity of over 170 W m⁻¹ K⁻¹.     

一种计算泡沫金属等效热导率的新模型

提出一种针对泡沫金属等效热导率预测的新模型,该模型基于泡沫金属Kelvin十四面体元胞结构建模,以凹面三棱柱近似金属韧带,并考虑韧带交汇处的结点特点,通过热阻分析得到计算等效热导率的表达式。研究表明：该模型对充填不同介质的不同材质泡沫金属等效热导率均有较高的预测精度（平均偏差均小于10%）,与文献其他半经验模型相比能更好兼顾预测精确性和通用性。     

A method for measuring the thermal conductivity of isotropically compressed powders

This paper proposes a method for measuring the thermal conductivity of compressible powders, in particular of powders which do not permit a calculation of sphere models. The apparatus, which is briefly duscussed, is designed to compress powders hydrostatically up to 600 bars while measuring the thermal conductivity with a hot wire lance. With this range of compression it is possible to determine the thermal conductivity for almost every plastic powder, non-compressed powders as well as quasi-solid materials. Experimental results are given for polytetrafluorethylene (PTFE) and polyethylene (PE) powders. The effect of crystallization on thermodynamic properties of polyethylene under high pressure is discussed.     

Calorimetric thermal conductivity measurement for high-temperature materials

A K1952 instrument has been upgraded to determine the thermal conductivities of materials by the GOST 12170 stationary method. The application of that method has been extended to the determination of thermal conductivity for industrial forms of refractory materials and for research in material science, design, and thermal insulation calculation. The K1952 system has been used in testing standard specimens. It has been confirmed that the parameter measurements conform to the technical documentation for standard specimens. A certificate has been obtained to confirm the type of means of measurement RU. E.32.001.A No. 30197, which was issued by the Federal agency on technical regulation and metrology. Translated from Novye Ogneupory, No. 2, pp. 47 – 49, February, 2009.     

激光导热仪在橡胶导热性能研究中的应用

简要介绍了激光导热仪的测试原理以及测试工作的注意事项等,详细介绍了如何使用激光导热仪测试炭黑／橡胶复合材料的导热性能,并对测试结果作了恰当的分析。     

Locally ordered nano-domains as novel microstructure defects suppressing the phonon transport in SnTe thermoelectrics

The phonon transport is intensively related to microstructure defects. Herein, a unique high-pressure sintering technology (GPa-level) was applied to modify the microstructure of pristine SnTe. There are many nano-domains exist in the high pressured NaCl-type SnTe binary compounds. The fast Fourier transform images show the domain phases have two sets of reflections with comparable intensity and exhibit doublet periodicities of special lattice planes, which demonstrates the domain phases are the CuPt and CuAu-I-type variants evolved from the NaCl-type matrix phase. An in-depth investigation indicates the constructions of the domain phase are related to locally off-stoichiometric of SnTe and the introduced strong inner-stress during high pressure sintering process. Additionally, a calculation of the phonon transport discloses the nano-domains restrict the phonon relaxation time by orders of magnitude over a wide phonon frequency range. As a result, a wide frequency phonon scattering network can be established to lower the lattice thermal conductivity.     

A novel facile synthesis and characterization of molybdenum nanowires

We describe a straightforward technique to synthesize pure Mo nanowires (NWs) from Mo₆S_yI_z (8,2 <y + z ≤ 10) NWs as precursor templates. The structural transformations occur when Mo₆S_yI_z NWs are annealed in Ar/H₂ mixture leading to the formation of pure Mo NWs with similar structures as initial morphologies. Detailed microscopic characterizations show that large diameters (>15 nm) Mo NWs are highly porous, while small diameters (<7 nm) are made of solid nanocrystalline grains. We find NW of diameter 4 nm can carry up to 30 μA current without suffering structural degradation. Moreover, NWs can be elastically deformed over several cycles without signs of plastic deformation.     

Thermal conductivity of a single carbon nanocoil measured by field-emission induced thermal radiation

The spectrum of thermal radiation induced by field emission from a single carbon nanocoil (CNC) is obtained, from which its thermal conductivity is investigated. It is found that the resistivity of the CNC shows an exponential decrease with increasing temperature. The spectrum calculated by the temperature distribution along the CNC coincides with that calculated by the single maximum temperature in the wavelength range below 1250 nm, but shows considerable differences in intensity and peak position for higher wavelengths. Based on this understanding, the thermal conductivity of the CNC is evaluated to be 38 W/m K by a one-dimensional thermal conduction model.     

Preparation and microstructure evolution of novel ultra-low thermal conductivity calcium silicate-based ceramic foams

In this study, municipal solid waste incineration fly ash (MSWI FA) was used as a new raw material for the ceramics industry and a novel ultra-low thermal conductivity calcium silicate-based foams (CSFs) was prepared by the direct foaming method. The effects of the addition of foam and borax on the sintering behavior and microstructural evolution of the CSFs were investigated. With the optimal amount of foam, the CSFs had an apparent porosity of 63.43%–67.49%, bulk density of 0.75–0.84 g/cm³, compressive strength of 1.83–3.21 MPa, and room-temperature thermal conductivity of 0.213–0.235 W/(m·K). Notably, the whisker morphology, pore structure, and sintering behavior of the samples can be controlled by changing the amount of borax. The prepared ceramic foams can be applied in the fields of thermal insulation, filtration, and catalyst carriers.     

A corresponding states model for the thermal conductivity of gases and liquids

A new model for the prediction of the thermal conductivity of pure fluids and mixtures is presented. The model is based on the corresponding states theory, and to use the model only well-known, pure component parameters plus the transport properties of a reference substance (here methane) are required. No P—V—T information is needed. The model is applied to the lower hydrocarbons (up to pentane), nitrogen, carbon dioxide, and mixtures of these at reduced temperatures between 0.6 and 3.0 and reduced densities between 0.0 and 2.7. For over 600 data points, the deviation between experimental and predicted values is typically between 2 and 5%.     

有机物混合液导热系数的预测

文章根据液体物质的导热系数与密度的关系,导出了估算有机物混合液导热系数的计算模型。利用该模型计算了53个体系369个数据点的二元有机物混合液导热系数,计算值与实验值的总平均相对偏差为1.434%,计算值与实验数据吻合较好,计算准确性优于文献方法;文章方法简单方便,只需要混合液各组分的临界温度、临界体积和导热系数数据,就可以直接预测各种温度和组成的有机物混合液的导热系数。