Effective thermal and electrical conductivity of graphite nanoplatelet composites

The relationship between the thermal/electrical conductivity enhancement in graphite nanoplatelets （GNPs） composites and the properties of filling graphite nanoplatelets is studied. The effective thermal and electrical conductivity enhancements of GNP-oil nanofluids and GNP-polyimide composites are measured. By taking into account the particle shape, the volume fraction, the thermal conductivity of filling particles and the base fluids, the thermal and electrical conductivity enhancements of GNP nanofluids are theoretically predicted by the generalized effective medium theory. Both the nonlinear dependence of effective thermal conductivity on the GNP volume fraction in nanofhiids and the very low percolation threshold for GNP-polyimide composites are well predicted. The theoretical predications are found to be in reasonably good agreement with the experimental data. The generalized effective medium theory can be used for predicting the thermal and electrical properties of GNP composites and it is still available for most of the thermal/electrical modifications in two-phase composites.

Effects of thermal radiation on MHD viscous fluid flow and heat transfer over nonlinear shrinking porous sheet

This paper investigates the effects of thermal radiation on the magnetohy- drodynamic （MHD） flow and heat transfer over a nonlinear shrinking porous sheet. The surface velocity of the shrinking sheet and the transverse magnetic field are assumed to vary as a power function of the distance from the origin. The temperature dependent viscosity and the thermal conductivity are also assumed to vary as an inverse function and a linear function of the temperature, respectively. A generalized similarity transfor- mation is used to reduce the governing partial differential equations to their nonlinear coupled ordinary differential equations, and is solved numerically by using a finite difference scheme. The numerical results concern with the velocity and temperature profiles as well as the local skin-friction coefficient and the rate of the heat transfer at the porous sheet for different values of several physical parameters of interest.

Theoretical calculations of thermophysical properties of single-wall carbon nanotube bundles

Carbon nanotube bundles are promising thermal interfacial materials due to their excellent thermal and mechanical characteristics. In this study, the phonon dispersion relations and density of states of the single-wall carbon nanotube bundles are calculated by using the force constant model. The calculation results show that the inter-tube interaction leads to a significant frequency raise of the low frequency modes. To verify the applied calculation method, the specific heat of a single single-wall carbon nanotube is calculated first based on the obtained phonon dispersion relations and the results coincide well with the experimental data. Moreover, the specific heat of the bundles is calculated and exhibits a slight reduction at low temperatures in comparison with that of the single tube. The thermal conductivity of the bundles at low temperatures is calculated by using the ballistic transport model. The calculation results indicate that the inter-tube interaction, i.e. van der Waals interaction, hinders heat transfer and cannot be neglected at extremely low temperatures. For （5, 5） bundles, the relative difference of the thermal conductivity caused by ignoring inter-tube effect reaches the maximum value of 26% around 17 K, which indicates the significant inter-tube interaction effect on the thermal conductivity at low temperatures.

Optimization of thermoelectric properties of n-type Bi_2(Te,Se)_3 with optimizing ball milling time

The thermoelectric properties at elevated temperature were investigated for n-type Bi2（Te,Se）3 which is obtained from ball milling processed powder with various milling times. Electrical properties such as electrical resistivity and Seebeck coefficient are clearly dependent on milling time, in which the carrier concentration is attributed to the change of the electrical properties. The concentrations of the defects are also varied with the ball milling time, which is the origin of the carrier concentration variation. Even though finer grain sizes are obtained after the long ball milling time, the temperature dependence of the thermal conductivity is not solely understood with the grain size, whereas the electrical contribution to the thermal conductivity should be also considered. The highest figure of merit value of ZT = 0.83 is achieved at 373 K for the optimized samples, in which ball milling time is 10 h. The obtained ZT value is 48% improvement over that of the 0.5-h sample at 373 K.

Novel methods by using non-vacuum insulated tubing to extend the lifetime of the tubing

The analysis of the failure mechanics, namely hydrogen permeation of vacuum insulated tubing （VIT）, indicated that the failure of VIT could be decreased but could not be avoided. To solve this problem, some measures by using non-vacuum materials were proposed and analyzed in this paper. The results show that to fill the tubing with foam-glass beads or high pressure argon may lead to a good performance.

Thermal conduction across the one-dimensional interface between a MoS2 monolayer and metal electrode

The thermal conductance across the one-dimensional （1D） interface between a MoS2 monolayer and Au electrode （edge-contact） has been investigated using molecular dynamics simulations. Although the thermal conductivity of monolayer MoS2 is 2-3 orders of magnitude lower than that of graphene, the covalent bonds formed at the interface enable interfacial thermal conductance （ITC） that is comparable to that of a graphene-metal interface. Each covalent bond at the interface serves as an independent channel for thermal conduction, allowing ITC to be tuned linearly by changing the interfacial bond density （controlling S vacancies）. In addition, different Au surfaces form different bonding configurations, causing large ITC variations. Interestingly, the S vacancies in the central region of MoS2 only slightly affect the ITC, which can be explained by a mismatch of the phonon vibration spectra. Further, at room temperature, ITC is primarily dominated by phonon transport, and electron-phonon coupling plays a negligible role. These results not only shed light on the phonon transport mechanisms across 1D metal-MoS2 interfaces, but also provide guidelines for the design and optimization of such interfaces for thermal management in MoS2-based electronic devices.

Manipulation of interfacial thermal conductance via Rhodamine 6G

Interfacial thermal conductance plays a sig- nificant role in the heat transfer efficiency of nanoscale systems. The thermal conductance across Al/SiO_2 inter- faces, which is subjected to the change in concentration of sandwiched Rhodamine 6G solution, is measured with time domain transient thermoreflectance technique. The thermal conductance of the interface between Al and SiO_2 logarithmically decreases with an increase in the concen- tration of the Rhodamine 6G solution. This study reveals that heat transport efficiency across an interface can be conveniently manipulated according to the demand of thermal engineering by introducing organic moleculars between both sides of the interface.