Thermal conductances between Cu and graphene covered carbon nanotubes (gCNTs) are calculated by molecular dynamics simulations. The results show that the thermal conductance is about ten times larger than that of Cu-CNT interface. The enhanced thermal conductance is due to the larger contact area introduced by the graphene layer and the stronger thermal transfer ability of the Cu-gCNT interface. From the linear increasing thermal conductance with the increasing total contact area, an effective contact area of such an interface can be defined.
Abstract A series of ZnGa 2 O 4 :Tb 3+ ,Ce 3+ persistent luminescence nanophosphors were prepared by high temperature solid‐state reaction. The X‐ray diffraction (XRD) patterns of the representative samples are consistent with that of the ZnGa 2 O 4 standard card. Scanning electron microscope (SEM) images show that the particle size of phosphor is enlarged when the Tb 3+ ion is doped, however, it becomes smaller again after the Ce 3+ ion is codoped. For ZnGa 2 O 4 :Tb 3+ nanophosphors, there exist intense 492 and 551 nm blue/green emissions under 280 nm excitation, and the optimum Tb 3+ doping concentration is 1.7 mol% and the long afterglow time is about 140 min. The PL and long afterglow characteristics of ZnGa 2 O 4 :1.7Tb 3+ nanophosphor are significantly improved once the Ce 3+ ion is codoped. The PL intensity of ZnGa 2 O 4 :1.7Tb 3+ nanophosphor is enhanced by 1.5 times and the long afterglow time is extended to 210 min when Ce 3+ doping concentrations are 0.3 and 0.5 mol%, respectively. The internal mechanism of the long afterglow effect of ZnGa 2 O 4 :1.7Tb 3+ ,0.5Ce 3+ nanophosphor is discussed in detail with the help of measurement results of thermoluminescence spectra.
Microscale gas gaps commonly exist in gas thermal conductance related microdevices, such as micro-hot-plate gas sensors and micro-Pirani vacuum gauges. In these devices, thermal conduction of the gas gaps is an important issue for their performance. Although simulations for size effect of the thermal conduction in microscale gas gaps have been carried out, experimental results are still rare. In this paper, four microhot plates that contain four gas gaps from 220 nm to 21 μm have been fabricated by a standard CMOS process and some additional post-CMOS processes. The thermal convection coefficient can be obtained as large as 1242 Wm 2 K -1 from the convection dominate 21-μm gap. The effective thermal conductivity of 220-nm gap is as low as 1.2 × 10 -3 Wm -1 K -1 . Both of them indicate that size effect of gaseous heat transport is significant in such microscale devices.
Molecular dynamics simulations were performed to evaluate temperature-dependent thermal conductivity of bent carbon nanotubes. Thermal conductivities of bent nanotubes are predicted to be smaller than those of straight nanotubes. This is due to the suppression of high frequency phonons from the density of states calculations. It was found that for the defect-free bent nanotubes, the ratio of thermal conductivity of bent nanotubes to that of the straight ones are temperature and diameter independent, while significantly relies on the bent characteristic size. The more is the nanotube bent, the smaller is thermal conductivity obtained. For the larger nanotubes, the buckled defects were observed after bending and the ratio decrease rapidly. The ratios of thermal conductivity of the buckled nanotubes to that of the straight ones increase with the increasing temperatures until a maximum is obtained.
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