Thermal Conductivity Studies On Mesoporous Silica Particles With A Bimodal-pore Distribution

Mesoporous silica particles are a kind of typical materials with a Bimodal-pore Distribution.Because of its excellent physical and chemical properties and controllable microstructure,it has great practical application value in energy,chemical industry,architecture,environmental protection,medicine and other fields.As the basic theoretical basis of related research,the heat and mass transfer law of particle is of great significance to the development and promotion of materials.Therefore,this paper analyzes the influencing factors of mesoporous silica thermophysical properties from two aspects of experimental measurement and theoretical simulation,and further explores the actual heat conduction mechanism of its dual pore structure.In this paper,the pore unit model of MCM-41 particles is built by MS software,and it is modified by crystal optimization and annealing cooling.The model was verified by small angle diffraction pattern drawn by XRD technology,which confirmed the existence of mesoporous pores.At the same time,based on the molecular dynamics theory,the thermal conductivity components of solid skeleton along different directions are calculated by Green-Kubo formula.The simulation data show that the pore size of mesoporous silica is close to the average free path of phonon motion due to its small pore size and wall size.Therefore,the microscale effect of particles is prominent,and when the pore size and porosity increase,the phenomenon is more significant.For the purpose of experimental study,MCM-41 and SB A-15 were selected as sample materials,and the transmission electron microscope,field emission scanning electron microscope and nitrogen adsorption/desorption method were used to characterize the microstructure of the particles.The effective thermal conductivity of the material was measured under the environment of 0?30MPa and 25?550? by the controllable temperature and pressure regulating system.The experimental results show that MCM-41 and SBA-15 have well-organized hexagonal mesoporous pores with high specific surface area and stable properties,and their accumulated porosity is very high in conventional state.In addition,the thermal conductivity of mesoporous particles shows a great correlation with the ambient temperature and pressure.When the measurement environment is in the low pressure,the thermal conductivity of the material is very low,and it increases with the increase of atmosphere pressure,especially in the range of 100Pa?0.1MPa.At the same time,the higher the ambient temperature is,the greater the effective thermal conductivity is.In order to further analyze the influence of dual pore structure on the heat and mass transfer process of mesoporous silica,this paper used the parallel form to build the theoretical thermal conductivity calculation model,and verified the rationality and accuracy of the model through the above experimental data.According to the related inference,the difference of microstructure has a great influence on the thermal conductivity of the materials with a bimodal-pore distribution.For example,the increase of porosity usually leads to the decrease of thermal conductivity,which is due to the fact that the increase of gas phase thermal conductivity is less than the decrease of solid phase thermal conductivity.However,there is an extreme point in this process,and the increase of porosity will lead to the enhancement of radiation.At the same time,the particle size and BET specific surface area are also important factors.The larger the particle size is,the smaller the specific surface area of the mesopore is,and the higher the effective thermal conductivity of the material is.