Study On The Precursor Freezing Ang Drying Process Of Synthesis Of Nanopowder By Vacuum Freeze-Drying

In the recent years, the synthesis of nanopowder has brought broad attention because of its special physical and chemical function. And many researchers began to research on this subject. There are many methods to synthesize nanopowder. But the vacuum freeze-drying technology is more excellent as an recently developed technique. Because the powder synthesized by this method has more excellent characteristics such as regular particle shape, small and even diameter, weak agglomeration and good chemical homogeneity due to its special technical process.There are usually four steps of the synthesis of nanopowder by vacuum freeze-drying. They are precursor solution preparation, solution freezing and its freeze-drying and the freeze-dried product heat-treatment. And in the whole process, steps of solution freezing and its freeze-drying respectively finished the first and the second particle formation. So in order to obtain the powder with even compositon and ideal diameter, the study on its process characteristic is very important.In this paper, the solution was freezed by vacuum evaporation. The expereiments of different solvent:water, organic solvent TBA and TBA/water cosolvent have been implemented. According to the data recorded by the experimental testing system, curves of the mass, temperature and pressure variating with time have been ploted. The experimental phenomenon has been observed and the variation of mass,temperature and pressure has been analyzed. The phenomenon of supercooling and boiling has been analyzed theoretically. The freezing experiment of NaCl solution with different concentrations has also been carried out in order to investigate the influence of concentration.The theoretical study on the freezing process of solution was also carried out. The thermodynamic model which describes the changing relation of mass and temperature and the dynamic model which describes the changing law of pressure and flow of mixture gas in the vacuum and condensation chamber were both established. And then the model was simulated by the Matlab Software. The theoretical curve was obtained which can describe the rule of freezing process by vacuum evaporation quantitatively.In the theoretical study of the freeze-drying process, the upper surface was heated by radiation only and the lower surface was adiabatic and the heat transfer model for sublimation drying was established. Expressions of temperature sublimation interface moving velocity, time needed for sublimation dryiny and temperature of radiation board were obtained by calculating. And through selecting the physical parameters of solution, the thickness of product and temperature of radiation board reasonably, the freeze-drying rate can be impoved and then the energy consumption can be reduced. The powder synthesized will has more excellent characteristics.