Experimental Study On Thermal Conductivity Of Carbon Nanotube Sponges With Temperature Ranging From 10K To 297K

Energy shortage has became one of the most serious energy problems in the contemporary world.Only when human beings establish the correct concept of energy conservation and take effective measures,energy efficiency that benefits our descendants can be improved.The research of thermal insulation material makes the development of energy-saving,environment friendly and low-carbon industry better.As a modern new thermal insulation material,carbon nanotube sponge can be widely used in construction,aerospace,ocean,clothing materials and other fields due to its good thermal stability,mechanical properties,electrical properties,hydrophobic and oleophilic properties.Therefore,the study on the thermal conductivity of carbon nanotube sponge is of great significance to guide the practical work.Based on such background,this topic is proposed carbon nanotube sponges(CNT sponges)thermal conductivity experiment under the wide temperature range.We experimentally investigate the thermal diffusivity and conductivity change rule of carbon sponge samples varying with temperature ranging from 297K to 10K using the transient electro-thermal technique(TET)and the steady-state electro-thermal technique(SET),which will make a little contribution for the theoretical guidance on the application of carbon nanotube sponge and its composites,especially in outer space exploration and unknown deep-sea exploration under low temperature environment.Firstly,this paper reports on thermal diffusivity of CNT sponges at room temperature.By Schuetz’s thermal conductivity model,the intrinsic thermal conductivity of CNT sponges is determined at 45-51 W/(m·K),and the real thermal conductivity reached 0.15 W/(m·K).The thermal transport mechanism of carbon sponge was explained from the perspective of micro-scale heat transfer,and the SEM characterization diagram proved that the structure defect of carbon sponge would bring about the influence of phonon scattering on the sample surface and the increase of contact resistance,thus resulting in the low thermal conductivity of carbon nanotube sponge.The Janis CCS-450 cryogenic system was employed to measure the thermal diffusion coefficient and thermal conductivity of the CNT sponges in a wide temperature range(10-297K).The thermal diffusivity of the CNT sponges with a growth rate of 24.5%increases with the decrease of temperature,and remains basically unchanged below 65K.The main reasons for this phenomenon are Umklapp phonon–phonon scattering(U-scattering)decreases with the decrease of temperature until it almost completely disappears,while the boundary scattering and defect scattering are basically independent of temperature at low temperature.Under the combined action of phonon scattering and specific heat,the thermal conductivity of the CNT sponges which is reduced by about 40%decreases linearly with the decrease of temperature.After analysis,the main reasons for the linear decrease of thermal conductivity of CNT sponges are as follows:as temperature goes down,lattice elastic vibrations in CNT sponges weaken and phonon population decreases;thermal strains intensify the contact of CNT nanofibers,which increase the contact resistance,and then it deteriorates the degree of structural disorder;the experimental sample may have a certain proportion of amorphous structure.By fitting the phonon scattering effect curve caused by defects in the heat conduction of CNT sponges,the Debye temperature is found to be 538.7K for the studied CNT sponges.The corresponding mean free paths from defect and boundary scattering are determined as 6.2nm,namely the crystallite size.The intrinsic residual thermal reffusivity of CNT sponges accounting for 80%of the whole reffusivity at room temperature is 52702.6 s/m~2,which is 470-1226 times higher defect level than the high-crystallized graphite materials and one order of magnitude less than the carbon nanocoils material.This also explains the reasons for the high defect level in the material:large scale defects such as high porosity and grain boundary;a certain proportion of amorphous structure in the material;the chemical component defects formed with the residual chemical elements in the CNT sponges preparation process.