Study On Thermal Shrinkage And Insulation Properties Of Silica Nano-powder Forming Materials

Developing new thermal insulation materials is considered to be one of the most effective means to optimize the development and use of energy resources.In recent years,nanoparticle based porous thermal insulation materials have been widely used in the field of thermal insulation because of their excellent thermal insulation properties and very low thermal conductivity,among which silica nanoparticles are the most widely used.However,in the environment of medium and high temperature,the volume shrinkage of the thermal insulation materials prepared by silica nanoparticles is easy to occur,which leads to the loss of thermal insulation performance.Therefore,the further optimization of high temperature performance becomes urgent.In view of the above problems,this thesis aims to use SiO₂ nanoparticles as raw materials,focusing on the impact of size effect on the volume shrinkage,microstructure and adiabatic properties of powder molding materials in the middle and high temperature environment,and explore the volume thermal shrinkage mechanism of nanoparticles molding insulation materials and the change rule of adiabatic properties.Specific research contents are as follows:（1）SiO₂ nanoparticle packed bed（SPP）was prepared by cold pressing method.XRD,SEM,TEM and BET were used to characterize the structural changes of the materials at high temperature.The results show that the melting phenomenon of SiO₂nanoparticles has occurred in the environment below the melting point temperature of bulk SiO₂.The fusion between particles leads to the collapse of the internal structure of the packed bed,and finally leads to the contraction of the macro volume of the packed bed,and the collapse trend gradually weakens with the increase of the particle size.XRD results show that the volume shrinkage of the packed bed is only related to the melting of SiO₂ nanoparticles,and there is no crystal phase change and no impurity doping.Finally,combined with the particle melting model and BET test results,it was determined that the structural failure inside the particle packed bed was mainly concentrated in the mesoporous pores between particles.（2）Based on the instantaneous tropical method,a high temperature thermal conductivity measurement platform was built.According to the thermal conductivity changes of SPP at high temperature,the relationship between the macroscopic volume shrinkage of the particle packed bed and the layered melting phenomenon of the microscopic particles was explored.The effects of sample preparation pressure,particle morphology and particle size on the adiabatic properties of packed bed at medium and high temperature were studied.The results show that the spherical SiO₂ particles have less fusion degree and less volume and thermal insulation loss than irregular SiO₂particles at the same temperature.Compared with smaller particles,the pore volume of the stacked bed with larger particles changes less,so the adiabatic property changes less.At the same time,it is found that pressure has little effect on the thermal shrinkage of the sample,but a large pressure will reduce the initial porosity of SPP and weaken the adiabatic performance.（3）The insulation performance of SiO₂ particles was further optimized by doping wood chips.Based on cold pressing and vacuum carbonization,a new type of particle packed bed insulation material with high temperature and humidity resistance was prepared.The effect of the amount of sawdust on the microstructure and thermal insulation properties of the carbonized sawdust mixed SiO₂ particle packed bed（WSPP）was investigated.The results show that WSPP exhibits excellent structural stability in both normal and high temperature environments.Due to the pore structure and pore-making ability of sawdust,when the amount of sawdust is 20%,the lowest thermal conductivity of WSPP can reach 0.03 W/（m·K）.Thanks to the mass reduction of wood chips after carbonization,WSPP can reduce density by up to 35 percent while enhancing insulation.At the same time,WSPP shows excellent performance stability and service life in humid and high temperature environment,and has great practical application potential.