| Nanoparticle materials have a high specific surface area,thermal conductivity lower than that of still air at normal temperature and pressure,ultra-high porosity,low density,and extremely light weight per unit volume,making them acoustic and optical.Electromagnetics,especially thermal insulation and other related fields have shown excellent performance,and the particle insulation materials are environmentally friendly and have great economic and social value.The particle size of the nanoparticle material is small,and there are a large number of pores inside,which makes the effective thermal conductivity of the material more obvious by the thermal conductivity of the gas phase.It is of great significance to explore the variation law of the thermal conductivity of the gas phase for the popularization and application of the material.In this paper,the transient tropical method(THS)is used to measure the thermal conductivity of granular materials under nitrogen atmosphere.Based on the THS method,the high pressure vessel and vacuum vessel measuring system is provided to provide the required pressure range.The pressure of the vacuum vessel can be adjusted from 10-2 Pa to atmospheric pressure,and the high pressure vessel system can provide pressure in the range of atmospheric pressure to 30 MPa.The microstructure of the granular material was obtained by scanning electron microscopy and transmission electron microscopy.The specific surface area,pore size distribution and corresponding pore volume of the experimental material were measured by nitrogen adsorption method at a temperature of 77.538 K using a fully automatic surface analyzer manufactured by American Micron.Based on the widely used Kaganer and Zeng models,the gas phase thermal conductivity of nanoparticle materials is analyzed in detail by the factors and their changes,and compared with the experimental measurements.The results show that the mean free path and gas phase thermal conductivity of gas molecules inside the particle material are directly related to the particle diameter and pore diameter of the material,but the material microstructure is not well characterized by randomness,and the specific surface area and density of the material are analyzed.The relationship between the two is more practical in engineering.The mean free path and gas phase thermal conductivity of the gas molecules inside the granular material decrease with the increase of the specific surface area and density of the material,but the trend with density changes more obviously.When the specific surface area and density of the material are both large,it has a significant effect on limiting the gas phase thermal conductivity inside the material.The measured effective thermal conductivity of the granular porous material is "S" type with the change of pressure,and the thermal conductivity increases greatly in low pressure section,and when the pressure is close to the normal pressure,there is a plateau region with a gentle change of thermal conductivity.Different from previous studies,it is found that the thermal conductivity will increase significantly with the further increase of pressure,and the growth rate will decrease when the pressure reaches 10 MPa.For the measured particulate porous material,when the pressure is less than 100pa,the effective thermal conductivity tends to a certain value,and the gas phase thermal conductivity of the material can be neglected at this time.It provides a reasonable effective value for estimating the effective thermal conductivity of the material in absolute vacuum,which makes up for the error that the experiment cannot measure the absolute vacuum condition.For the porous material with solid single pore distribution,due to the effect of agglomeration effect,the two materials with the same size of the particle size have the same effect on the gas phase heat transfer in the vacuum to 10 MPa pressure range,and the particle size does not have a significant effect on the gas phase thermal conductivity. |
PDF Full Text Request