Effects Of Near-field Radiation And Surface&Grain Boundary Scattering On Heat Tansfer In Cylindrical Mesoporous Materials

With the development of the space industry, highly integrated electronic device and micro-electro-mechanical systems (MEMS), the material with special property has drawn wide attention to meet the requirement of thermal design or temperature control. In this paper, it is focused on the thermal transport mechanism of the ordered mesoporous composites through theoretical analysis and experimental measurments, as well as the contributions of near-field radiation and surface&grain boundary scattering to the thermal conductivity of the mesoporous composites, in order to provide theoretical support for thermal design of the potential material.Firstly, for the ordered mesoporous substrates, such as mesoporous silica (MCM-14and SBA-15) and mesoporous alumina(AAO), the reconstruction and characterization of the substrates’ structure were carried out, and the thermal conductivities of the mesoporous substrates were investigated by experimental measurements and theoretical analysis. Meanwhile, near-field radiation model at a curved surface was established, and the near-field radiation across a spherical/cylindrical pore was calculated with the effect of pore diameter and temperature. Furthermore, a theorecrical model was established to combine the thermal conductivity of substrate, the near-field radiation and the thermal conductivity of confined air in the pore, which can obtain the effective thermal conductivity of mesoporous substrate. The study shows that:The thermal conductivity of the mesoporous material are anisotropic and decreases with pore diameter increasing or wall thickness decreasing; the heat flux and equivalent thermal conductivity of near-field radiation across a pore decreases with pore diameter increasing or temperature decreasing, and the heat flux of near-field radiation is2-7orders higher than that of far-field radiation. The smaller the pore diameter, the near-field radiation effect is more significant. The effective thermal conductivities of mesoporous substrates are consistent with the measurements and decrease with the pore diameter increasing or temperature decreasing. The effective thermal conductivities of mesoporous substrates with near-field radiation considered are almost0.2～10%higher than those without consideration.Secondly, the contributions of phonon and electron transport to the thermal conductivities of metallic nanowires were studied theoretically. The effects of surface and grain boundary scatterings were involved. While the molecular dynamic simulation and Green-Kubo formulation were used to get the lattice thermal conductivity, a model derived from Boltzmann transport equation and the Wiedemann-Franz relation were applied to calculate electronic thermal conductivities. In addition, diffuse mismatch model (DMM) was used to calculate thermal resistance of grain boundary to modify the lattice thermal conductivity, meanwhile, Mayadas-Shatzkes (MS) model was used to consider the impact of grain boundary scattering on the electronic thermal conductivity. By coupling the lattice and electronic thermal conductivities, the effective thermal conductivity of nanowire was obtained. On this base, the influences of size and temperature were analyzed. At the same time, the quantum modification of in-plane thermal conductivity of metallic nanowire can be obtained by using the linear response theory and the matrix of electron density.It turns out that, the contribution of electron transport to the thermal conductivities of nanowire is dominated, but the contribution of phonon transport cannot be ignored in the nanoscale. The thermal conductivities of nanowires decrease due to the grain boundary scattering. And it decreases with increasing temperature or decreasing size. The contribution of phonon transport becomes more important in the case of smaller size. When the lengths of nanowires increase up to200nm, the thermal conductivities of nanowires increase to a stable value. If the length is greater than the cross-section size of the nanowire, the in-plane(XY) thermal conductivity is lower than axial(Z direction) thermal conductivity.Meanwhile, near-field radiation between two nanowires can be calculated by two parallel plates’model, and the effect of the gap and temperature were analyzed. The study shows that:the radiation heat flux decrease with the gap increasing or temperature decreasing. With increasing separation, the equivalent thermal conductivitiesof radiation increase firstly untill reaches a peak, where d is around30nm, then decrease. At nanoscale, the contribution of polarization of near-field evanescent wave is greater than the contribution of the rest at least one order of magnitude. The main contribution to the radiation is the s-polarization wave of near-field. However, with decreasing separation distance, p-polarization waveof near-field has a sharp increase. As the gap is greater than1000nm, the near-field effect gradually disappears, and the far-field plays a dominant role.Thirdly, non-equilibrium molecular dynamics simulations was used to calculate the interface thermal resistance between mesoporous substrate and nanowire, and compared with the acoustic mismatch model and diffuse mismatch model. It turns out that, the interface thermal resistance incease with temperature decreasing or material difference increasing.Finally, the combined thermal conductivities of mesoporous composites (Cu-nanowire/MCM-41, Ag-nanowire/SBA-15and Ag-nanowire/AAO) were got by establishing thermal network model to combine the thermal conductivities of substrate, nanowire with surface&grain boundary scattering considered, near-field radiation and the interface thermal resisitance between mesoporous substrate and nanowire. Meanwhile, double current method and transient heat source method were used to measure the materials’ thermal conductivities. The measured results can verify the theoretical results, with the effect of near-field and surafe&grain boundary scattering were analyzed.It turns out that, the combined thermal conductivities of the mesoporous composites are anisotropic and decreases in all directions with the gap between nanowires increasing or length of nanowire decreasing, i.e., the filling ratio decreasing; with pore diameter increasing, the effective thermal conductivities of the mesoporous composites along X, Z directions decrease, as well as have a peak along Y direction. Along the X, Y directions, the reduced thermal conductivity by the suface&grain boundary scattering are greater than the increased thermal conductivity generated by near-field radiation; the effective thermal conductivities of the mesoporous composites with near-field radiation and suface&grain boundary scattering considered are lower than not considered; the opposite phenomina are apeared along the Z direction. When the filling ratio of material is high enough, it showed a significant increase of the effective thermal conductivity; interface thermal resistance can reduce the thermal conductivity of mesoporous composites less than20%.