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Influence of void ratio on thermal stress of PCM (phase change material) canister for heat pipe receiver under microgravity is numerically simulated. LiF-CaF2 (80.5-19.5 mol%, melting point: 763 degrees C) PCM is used as a latent thermal energy storage medium for its high latent heats of fusion. Accordingly, mathematical model and numerical method are offered. A judgment for convergence is accomplished. Numerical results show that the thermal stress in PCM canister increases as void ratio increases. Void cavity prevents the heat transfer between canister wall and PCM zone during both sunlight and eclipse periods. The temperature gradient is very significant when void ratio reaches the maximum. So the lifetime of heat pipe receiver may decrease as void ratio increases. The corresponding maximum thermal stress of different void ratios during eclipse periods is lower than that during sunlight periods. If the designed material of PCM canister can satisfy the strength during sunlight periods, it must be safe and reliable during eclipse periods and the necessary overall performance can be maintained. The research results can be used to guide designing the PCM canister for heat pipe receiver. (C) 2015 Elsevier Ltd. All rights reserved.

In this paper, three-dimensional periodic numerical model for fully developed flow in a cross wavy primary surface recuperator for a microturbine system is built. The performance of flow and heat transfer is analyzed. The fields of flow and temperature in a gas and air channel are obtained. Different working conditions are numerically simulated. Numerical results are compared with experimental data concerned. Analysis results show that the flow in the gas and air channel is anti-symmetry along the centre of channel. The flow of fluid is fluctuant. The flow velocity of gas is much higher than that of air. The thermal ratio of cross wavy primary surface recuperator can reach 95.2%. The thermal ratio decreases with the improvement of gas inlet temperature. When gas inlet temperature increases by 100 K, the thermal ratio decreases by about 1%. The thermal ratio increases with the reduction of flow rate in the channel. When flow rate reduces by 40%, the thermal ratio increases by about 4%. The research results can be used to guide checking the performance of a recuperator.

The heat conduction and thermal conductivity for methane hydrate are simulated from equilibrium molecular dynamics. The thermal conductivity and temperature dependence trend agree well with the experimental results. The nonmonotonic temperature dependence is attributed to the phonon inelastic scattering at higher temperature and to the confinement of the optic phonon modes and low frequency phonons at low temperature. The thermal conductivity scales proportionally with the van der Waals interaction strength. The conversion of a crystal-like nature into an amorphous one occurs at higher strength. Both the temperature dependence and interaction strength dependence are explained by phonon inelastic scattering.

Zinc telluride, due to its direct band gap and broadband light absorption, has the good application prospects in terahertz devices, solar cells, waveguide devices, and green light emitting diodes. In the photovoltaic field, it is possible to further improve the photoelectron conversion efficiency of multi-junction tandem solar cells by combining zinc telluride with III-V semiconductors. Ultrafast photo-excited carrier dynamics is fundamental to understand photoelectron conversion process of nanofilm solar cells. In this study, the ultrafast energy carrier dynamics of N-doped polycrystalline zinc telluride is investigated by using the femtosecond laser two-color pump-probe method at room temperature. The polycrystalline zinc telluride nanofilm is grown on a 500 μm GaAs (001) substrate via molecular beam epitaxy and doped by using a nitrogen ratio frequency plasma cell. The laser pulses with a central wavelength of 800 nm are divided into pump beam and probe beam by a beam splitter, after which the pump beam passes through a bismuth triborate crystal and its frequency is doubled to 400 nm. The 400 nm pump beam and 800 nm probe beam are focused on the sample collinearly through the same objective lens. Photo-excited carriers will be generated since the excitation photon energy of 400 nm pump beam (3.1 eV) is higher than the band gap of zinc telluride (～ 2.39 eV). The experimental data are analyzed by using the theoretical fitting model which includes energy relaxation processes of electrons and lattice, and the theoretical curves are consistent well with the experimental data. The fitted results show that the three dominated relaxation processes which affect the initial reflectivity recovery are in sub-picosecond time regime. The positive amplitude electron relaxation process is attributed to inter-band carrier cooling and carrier diffusion through electron-photon interactions, and the deduced decay time of this positive amplitude electron relaxation process is about 0.75 ps. The negative amplitude electron relaxation process is characterized as a photo-generated carrier trapping process induced by defects, and the decay time of this process is about 0.61 ps. The lattice heating process is realized through electron-phonon coupling process, and the calculated time constant of the lattice heating is about 0.86 ps.

To improve the thermal conductivity of polymeric composites, the numerous interfacial thermal resistance (ITR) inside is usually considered as a bottle neck, but the direct measurement of the ITR is hardly reported. In this paper, a sandwich structure which consists of transducer/high density polyethylene (HDPE)/sapphire is prepared to study the interface characteristics. Then, the ITRs between HDPE and sapphire of two samples with different HDPE thickness values are measured by time-domain thermoreflectance (TDTR) method and the results are ~ 2 × 10−7 m2KW−1. Furthermore, a model is used to evaluate the importance of ITR for the thermal conductivity of composites. The model's analysis indicates that reducing the ITR is an effective way of improving the thermal conductivity of composites. These results will provide valuable guidance for the design and manufacture of polymer-based thermally conductive materials.

Heat management at nanoscale is a critical issue across many areas of science and engineering, where the size effect of thermal properties plays an important role. We measure the transient thermoreflectance signals of thin metal films with thicknesses from 50 to 200nm by using the femtosecond laser pump and probe method, and the experimental data are combined with the parabolic two-step model to enable us to measure thermal conductivity of the thin metal films. The measurement results of Ni and Al films show that, in the thickness range from tens to hundreds of nanometer, the thermal conductivity increases with the increasing thicknesses of the films, which agrees well with the previous conclusions.

The reduction in size and the increase in speed of microelectronic device make the probability of nonequilibrium electron-phonon phenomena become greater, leading to the increase of thermal resistance in the device. The measurement of electron-phonon coupling factor in material resistance increasingly becomes important for accurate thermal treatment. The femtosecond laser pump and probe method is used for studing the nonequilibrium heat transfer in nano metal films with different thicknesses. Exploring parabolic two-step model (PTS) to fit the experimental data. During the fitting process, we considered the proportional relationship between the changes of electron temperature and phonon temperature, which affects the reflectivity. By studying the different thicknesses of Ni and Al films electron-phonon coupling factors, we find that the electron-phonon coupling factor does not change with film thickness. In addition, the experimental result verifies that the reflectivity of probe laser is affected by electron temperature and phonon temperature at the same time. Through the data analysis, we also get the influence coefficients of electron temperature and phonon temperature on reflectivity.

We report measurements on the length-dependent thermal conductivity of individual single-walled carbon nanotubes (SWNTs) on a substrate using a four-pad 3ω method with the consideration of heat loss between the sample and substrate. An increase in thermal conductivity with length (0.5—7μm) was observed at room temperature. The length-dependence of thermal conductivity was theoretically predicted by the modified WV model with consideration of second order-3-phonon process. The predicted phonon mean free path is about 175nm. Both the measurement and the prediction show the scale effect of the length on the thermal conductivity of SWNTs.