Simulation and optimization of heat flow via anisotropic material thermal conductivity

This article is focused on heat flow control within a composite material that has designed anisotropic thermal conductivity. The optimal conductive heat transfer path in the composite is specified via customized local scale properties, where the physical parameter distribution is found using an iterative procedure that couples a gradient based optimization routine with a finite element solver. A sample optimization result is presented to illustrate the procedure, and the final solution is translated into a physical embodiment having heterogeneous material properties. Numerical experiments were performed both on this synthesized material and a baseline homogeneous material with the same filler volume fraction. Heat transfer results indicate a substantial reduction in overall temperature with effective concentration of thermal power density in the designed material.     

Radiative heat transfer in transient hot-wire measurements of thermal conductivity

New measurements of the thermal conductivity of liquid toluene between 300 and 550 K have been used to study the importance of radiative heat transfer when using the transient hot-wire technique. The experimental data were used to obtain the radiation correction to the hot-wire temperature rises. Radiationcorrected values of thermal conductivity are reported. This study shows that the transient hot-wire method is much less affected by radiation than steady-state techniques.     

Determination of thermal conductivity from specific heat and thermal diffusivity measurements of plasma-sprayed cermets

The thermal conductivities of three plasma-sprayed cermets have been determined over the temperature range 23–630°C from the measurement of the specific heat, thermal diffusivity, and density. These cermets are mixtures of Al and SiC prepared by plasma spray deposition and are being considered for various applications in magnetic confinement fusion devices. The samples consisted of three compositions: 61 vol% Al/39 vol% SiC, 74vol% Al/26vol% SiC, and 83 vol% Al/17 vol% SiC. The specific heat was determined by differential scanning calorimetry through the Al melt transition up to 720°C, while the thermal diffusivity was determined using the laser flash technique up to 630°C. The linear thermal expansion was measured and used to correct the diffusivity and density values. The thermal diffusivity showed a significant increase after thermal cycling due to a reduction in the intergrain contact resistance, increasing from 0.4 to 0.6 cm²·^–1 at 160°C. However, effective medium theory calculations indicated that the thermal conductivities of both the Al and the SiC were below the ideal defect-free limit even after high-temperature cycling. The specific heat measurements showed suppressed melting points in the plasmasprayed cermets. The 39 vol% SiC began a melt endotherm at 577°C, which peaked in the 640–650°C range depending on the sample thermal history. Chemical and X-ray diffraction analysis indicated the presence of free silicon in the cermet and in the SiC powder, which resulted in a eutectic Al/Si alloy.Paper presented at the Ninth Symposium on Thermophysical Properties, June 24–27, 1985, Boulder, Colorado, U.S.A.     

Ultralow thermal conductivity of isotope-doped silicon nanowires

The thermal conductivity of silicon nanowires (SiNWs) is investigated by molecular dynamics (MD) simulation. It is found that the thermal conductivity of SiNWs can be reduced exponentially by isotopic defects at room temperature. The thermal conductivity reaches the minimum, which is about 27% of that of pure 28Si NW, when doped with 50% isotope atoms. The thermal conductivity of isotopic-superlattice structured SiNWs depends clearly on the period of superlattice. At a critical period of 1.09 nm, the thermal conductivity is only 25% of the value of pure Si NW. An anomalous enhancement of thermal conductivity is observed when the superlattice period is smaller than this critical length. The ultralow thermal conductivity of superlattice structured SiNWs is explained with phonon spectrum theory.     

Enhanced thermal conductivity and retained electrical insulation for polyimide composites with SiC nanowires grown on graphene hybrid fillers

Polyimide (PI) composites containing one-dimensional SiC nanowires grown on two-dimensional graphene sheets (1D–2D SiC_NWs-GSs) hybrid fillers were successfully prepared. The PI/SiC_NWs-GSs composites synchronously exhibited high thermal conductivity and retained electrical insulation. Moreover, the heat conducting properties of PI/SiC_NWs-GSs films present well reproducibility within the temperature range from 25 to 175 °C. The maximum value of thermal conductivity of PI composite is 0.577 W/mK with 7 wt% fillers loading, increased by 138% in comparison with that of the neat PI. The 1D SiC nanowires grown on the GSs surface prevent the GSs contacting with each other in the PI matrix to retain electrical insulation of PI composites. In addition, the storage modulus and Young’s modulus of PI composites are remarkably improved in comparison with that of the neat PI.     

A noncontact method for measuring thermal conductivity and thermal diffusivity of anisotropic materials

A noncontact method for measuring the thermal conductivity and thermal diffusivity of anisotropic materials is proposed. This method is based on the fact that the surface temperature variation with time depends on the thermal properties of the material when its surface is heated locally. The three-dimensional transient heat conduction equation in the material is solved numerically. The dimensionless average surface temperature variations are obtained along each principal axis: that is, thex andy axes. The relation between the dimensionless temperature and the Fourier number is expressed by a polynomial equation and used as a master plot, which is a basic relation to be compared with measured temperature variation. In the experiments, the material surface is heated with a laser beam and the surface temperature profiles are measured by an infrared thermometer. The measured temperature variations with time are compared with the master plots to yield the thermal conductivity λ _x and thermal diffusivityx _v in thex direction and the thermal conductivity ratioE _xy (=λ _y λ _x ) simultaneously. To confirm the applicability and the accuracy of the present method, measurements were performed on multilayered kent-paper, vinyl chloride, and polyethylene resin film, whose thermal properties are known. From numerical simulations, it is found that the present method can measure the thermophysical properties λ _x , α _x andE _xy within errors of ±6, ±22, and ±5%, respectively, when the measuring errors of the peak heat flux, the heating radius, and the surface temperature rise are assumed to be within ±2, ±3%, and ±0.2 K, respectively. This method could be applied to the measurement of thermophysical properties of biological materials.     

Molecular dynamics calculations of InSb nanowires thermal conductivity

Non-equilibrium molecular dynamics calculations were performed in order to obtain the thermal conductivity coefficients of InSb nanowires in comparison with the bulk system value. For bulk, the value obtained was 16 ± 2 Wm⁻¹ K⁻¹ which is in very good agreement with experimental value of 15 Wm⁻¹ K⁻¹, and the thermal conductivity of the nanowires were 0.54 ± 0.01 Wm⁻¹ K⁻¹ and 0.50 ± 0.01 Wm⁻¹ K⁻¹ for a 5 and 4 unit cells square cross-sectional nanowires, respectively, which is almost two orders of magnitude smaller. This result is in agreement with other simulations performed for other materials.     

Electrical conductivity of random silver-potassium chloride composites

The complex alternating current (a.c.) impedance of random silver-potassium chloride (Ag-KCl) composite specimens near the percolation threshold has been measured in the frequency range 5 Hz to 13 MHz. The impedance of these composites as a function of metal volume fraction and direct current (d.c.) potential field are presented. This a.c. response, which is correlated with structural and compositional characterization of these composites, is used to examine the applicability of several different theoretical models as well as to determine the important factors in the electrical conductivity of such materials.     

Effective thermal conductivity and electrical conductivity of anisotropic solids of low porosity

Equations are derived to determine the effective thermal and electrical conductivities of anisotropic media of low porosity. The influence of porosity on the thermal and electrical conductivities of anisotropic ternary alloys is established.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 30, No. 4, pp. 686–692, April, 1976.     

Fabrication and effective thermal conductivity of multi-walled carbon nanotubes reinforced Cu matrix composites for heat sink applications

A novel particles-compositing method was used for the first time to disperse different contents of multi-walled carbon nanotubes (CNTs) in micron sized copper powders, which were subsequently consolidated into CNT/Cu composites by spark plasma sintering (SPS). Microstructural observations showed that the homogeneous distribution of CNTs and dense composites could be obtained for 0–10 vol.% CNT contents. The CNT clusters were appeared in the powder mixture with 15 vol.% CNTs, which resulted in an insufficient densification of the composites. The effective thermal conductivity of the composites was analyzed both theoretically and experimentally. The addition of CNTs showed no enhancement in overall thermal conductivity of the composites due to the interface thermal resistance associated with the low phase contrast of CNT to copper and the random tube orientation. Besides, the composite containing 15 vol.% CNTs led to a rather low thermal conductivity due possiblely to the combined effect of unfavorable factors induced by the presence of CNT clusters, i.e. large porosity, lower effective conductivity of CNT clusters themselves and reduction of SPS cleaning effect. The CNT/Cu composites may be a promising thermal management material for heat sink applications.     

Specific heat of superconductor-normal metal composites

The specific heat of Th-Nb composites has been measured to determine the extent to which a dense array of Nb rods will enhance the superconducting energy gap of the surrounding Th matrix at temperatures well belowT _c of the Th. A directional solidification technique has been used to prepare an acicular composite having a triangular array of 500-nm-diameter Nb rods in a Th matrix with a center-to-center spacing of 1500 nm. At low temperatures, where both metals are superconducting, the specific heat indicates that the Th matrix has an energy gap about 10% larger than bulk Th, so there is some enhancement by the Nb rods. BetweenT _c of Nb andT _c of Th there is clear evidence of an induced energy gap in the Th, but as yet there is no theory for the specific heat in this regime. A field as small as 20 mT will quench all evidence of proximity-induced superconductivity in the Th, indicating a strong field dependence of the Cooper pair decay lengthK _N ^–1 .This work has been authored by a contractor of the U.S. Government under contract No. W-7405-eng-82. Accordingly, the U.S. Government retains a nonexclusive, royalty-free license to publish or reproduce the published form of this contribution, or allow others to do so for U.S. Government purposes.Operated for the U.S. Department of Energy by Iowa State University under contract No. W-7405-Eng-82. This research was supported by the Director for Energy Research, Office of Basic Energy Sciences, WPAS-KC-02-02-02.     

预报纤维增强复合材料有效热导率的随机微结构胞元模型

发展了能预报复合材料有效性质的随机微结构胞元模型以预测单向纤维增强复合材料横向热导率。研究了能反映宏观有效性质的模型最小化问题, 探讨了微结构影响宏观有效热传导性能的机制。结果表明: 通过对模型指定周期边界条件并且以多个合适的小规模模型计算的平均值取代大模型计算, 可大大改进收敛性并提高计算效率, 从10×10个到30×30个子胞的模型, 所得有效热导率计算结果的最大相对变化量仅为0.6%。不同纤维排列引起热流穿过热阻大的基体的路径长度改变, 造成有效热导率不同; 纤维热导率远大于基体热导率时, 纤维随机分布造成纤维偏聚, 部分纤维接触形成"热流通道", 使得有效热导率增大, 揭示了某一体积分数下有效热导率急剧增加是由"热流通道"贯通引起。与实验结果的比较说明了微结构随机性研究的必要性和本文工作的实用价值。     

陶瓷热障涂层的热导率和热扩散率测量

提出了应用3ω法进行等离子喷涂热障涂层材料的热导率和热扩散率测量的方法。测试了室温下2种典型的热障涂层材料Y2SiO5和La2Zr2O7的热导率和热扩散率,测试结果与文献中的结果吻合良好。实验中对不同孔隙率的样品的热导率在室温附近的温度区间内进行测试,结果表明,孔隙率的变化对热导率有明显的影响。另外,孔隙率对热扩散率有双向的影响,即存在某一孔隙率值使得涂层样品的热扩散率最大。     

Estimation of the thermal conductivity of composites

In this article we introduce the concept of homogenization for the approximation of the effective thermal conductivity of composites. A simple algebraic approximation method is proposed and shown to yield an upper bound for the effective conductivity. Numerical results are given for uni-directional carbon-carbon composites which demonstrate the validity of the approach.     

Unguarded cryostat for thermal conductivity measurements of multilayer insulations

A cryostat for measurements of the effective thermal conductivity of multilayer insulations in conditions similar to those in normal cryogenic liquid containers has been constructed. Results for two multilayer insulations are presented.     

The thermal conductivity of Kevlar fibre-reinforced composites

The thermal conductivities of a series of blocks consisting of Shell DX210/BF₃400 resin reinforced with Kevlar 49 fibre are reported in the approximate temperature range 180–270 K. The results are used to calculate the thermal conductivities of the fibres in directions parallel and perpendicular to their length. Varying the angle between the principal fibre directions of bidirectional laminates produced in-plane results that varied in a manner which was quantitatively consistent with expectation. The out-of-plane results proved to be independent of fibre orientation, as expected. In-plane and out-of-plane results for a Kevlar 49 fabric reinforced laminate proved to be essentially similar to results for a laminate reinforced with unwoven fibres of the same type, arranged in a 90° cross-plied disposition at the same fibre volume density.     

Low-temperature thermal conductivity of two fibre-epoxy composites

Thermal conductivity data are presented for two fibre-epoxy composites. The fibre in one composite is glass and an aramid-class polymer in the other. Measurements were conducted on a glass-epoxy specimen in the direction perpendicular to the fibres at temperatures from 14 to 100 K. The aramid-epoxy specimen was measured parallel to the fibres at 5, 76, 196, and 276 K. Data are presented both in graphical and tabular form.     

The thermal conductivity of carbon fibre-reinforced composites

Measurements of the thermal conductivity between approximately 80 and 270 K of a series of unidirectional and bidirectional specimens of epoxy resin DX210/BF₃400 reinforced with Morganite high modulus (HMS) and high strength (HTS) carbon fibres are reported for in-plane and out-of-plane directions. The main features of the results conform with expectations based upon known structural properties of the fibres and predictions based upon current theoretical models. Employing the results for the composites in association with results for the pure resin, the account concludes with an assessment of some of the heat transmission characteristics of the fibres.     

Thermoelastic properties and conductivity of composites reinforced by spherically anisotropic particles

The present paper is concerned with the overall thermoelastic properties and conductivity of composites reinforced by spherically anisotropic particles. Based on the concept of the replacement particle an equivalent bulk modulus, thermal expansion coefficient and thermal conductivity are derived for the spherically anisotropic particle. Such equivalent properties can be employed in the micromechanical models to predict the overall behavior of the composite. In addition to these, the shear loading is considered. The effective shear modulus is evaluated on the basis of Mori and Tanaka's approximations and the dilute phase concentration factors are derived from exact solutions of an auxiliary boundary value problem.