Thermal Properties Of Low-dimensional Nano Materials

With the characteristic size of materials decreasing down to nanoscale,the heat transport law in the macroscopic thermodynamics will become no longer applicable.Due to the quantum confinement effect caused by the dimensional collapse,The low-dimensional materials exhibit various transport mechanisms and excellent properties in mechanics,thermology and electricity,different from the bulk material.It has great engineering and academic value in the field of thermal transport and thermal management.In order to explore the novel thermal transport behavior of low-dimensional materials,we developed a measurement system which accurately measures the heat transport properties of low-dimensional materials.We studied the physical principle of high anisotropic thermal conductivity of two-dimensional composite film system,we also studied the thermal properties of one-dimensional metallic glass fiber system and their similarities and differences with bulk materials.Based on montmorillonite/reduced graphene oxide system,we prepared composite films with component thickness close to the phonon average free path at room temperature by improved vacuum filtration method.The 3ωmethod was taken to measure the cross-plane thermal conductivity of the composite films at room temperature.From the relation of the cross-plane thermal conductivity of the composite films with their deposition cycle and temperature,we found that the phonons exhibit incoherent heat transport behavior at the cross-plane upper interface.The cross-plane thermal conductivity of the composite films is much lower than any component of the films,which indicates that the way of assembling of the films can effectively reduce the cross-plane thermal conductivity.We also analyzed the possibility of forming phononic crystals on the cross-plane montmorillonite/reduced graphene oxide system,which offers a hint for the preparation of highly anisotropic thermal management materials.We used the improved steady-state method to measure in-plane thermal conductivity of montmorillonite/reduced graphene oxide composite film at room temperature.Compared with the traditional thermal laser method,we pointed out the shortcomings of existing research methods for ultra-thin composite film system,and we put forward a new method for the thermal characterization of ultra-thin composite film system.For the first time the accurate thermal conductivity anisotropy parameters of ultra-thin composite films were obtained and analyzed.The measurement results show that the way of assembling of the films can effectively improve the anisotropy.We found that the cross-plane and in-plane thermal conductivity as well as the anisotropy of thermal conductivity of composite films are independent of the deposition cycles,and this characteristic has application potential for developing new low-dimensional materials for thermal management under the condition of limited environmental size.In addition,based on one-dimensional amorphous system,we prepared one-dimensional palladium copper nickel phosphorus alloy(Pd₄₀Cu₃₀Ni₁₀P₂₀)metal glass fiber material by free load traction method.3ωmethod was taken to study the thermal property of the one-dimensional metallic glass fiber,including thermal conductivity,thermal diffusivity and heat capacity.For the first time the thermal parameters of micron sized metallic glass fiber was measured,which provides guidance for the subsequent thermal research for nano metallic glass fiber materials.On the basis of accurate measurement,we also studied the relation of the thermal properties of metallic glass fibers with their diameter.It is found that the contribution of phonons to thermal conductivity will increase with the decrease of the fibers diameter,and that a large inherent heat capacity exists at low temperature in metallic glass fiber materials.A reason for this large inherent heat capacity is that the temperature drop of low diameter metallic glass fiber during tensile cooling is different from that of metallic glass rod,resulting in the superposition of Boson peaks excited by multi phonons.