Study On Thermal Process Of Ultrafine Particles Fabricated By Evaporation-condensation Method

Ultrafine particles have been used in different industries due to their advantage over regular particles.In all the preparation methods for ultrafine particles fabrication,particles produced by gas phase method have the advantages of clear interface,controllable particle size and morphology.During the gas phase technology,transient energy transfer controls nucleation and growth processes for ultrafine particles in the shock cooling process.In order to master influence from fabrication conditions on characteristic of particles,one has to reveal transient thermal mechanisms and micro mechanisms for nucleation and growth during the shock cooling process.From the perspective of thermodynamics and kinetics for energy transformation and transfer during nucleation process,this paper explores the governing effects of transient energy transfer on nucleation and growth,and then obtains influence rules of fabrication conditions for gas phase method on characteristic of ultrafine particles.For ultrafine particles produced by inert gas condensation method,this paper studied influences of fabrication conditions on mean particle size,particle size distribution and particle morphology.Firstly,we analyzed thermal process for ultrafine particles formation,and established the energy model for homogeneous nucleation based on energy analysis during the process.By solving the model,we predicted productive rate and mean particle size for three kinds of metals under different fabrication conditions.Secondly,we established general dynamics equations for particle size distribution,and used improved multiple Monte Carlo method to solve the equations.Based on the dynamics equations,we obtained the particle size distributions for ultrafine Mg particles fabricated under different conditions by inert gas condensation method.We prove our method to predict particle size distribution to be right by the relative low errors between calculated and experimental results.Thirdly,this paper established Gibbs free energy and energy variation model for morphology prediction.Take ultrafine Al particles for example;we predicted morphology of A1 particles under different cooling rates and pressure in the condensation room.For ultrafine particles fabricated by chemical vapor deposition method,this paper studied influences of fabrication conditions on mean particle size,and proposed a numerical simulation method for thermal conductivity calculation of particles accumulation.Firstly,take ultrafine silicon nitride fabricated from chemical reaction between ammonia gas and silicohydride gas for example,we studied the whole process from chemical reaction to nucleation and deposition,and established energy analysis model.Based on the model,this paper obtains mean particle size of ultrafine silicon nitride fabricated under different conditions,and revealed nucleation mechanisms for ultrafine silicon nitride.Secondly,we proposed a simulation method for particle accumulation.Then,based on the results of particle accumulation simulation,after gridding computing and binarization processing,we used explicit difference scheme to solve temperature field distribution,and obtained thermal conductivity accordingly.This paper studied thermal process of gas phase method for ultrafine particles fabrication,and provided a way to obtain process conditions for particle size and morphology control of ultrafine particles fabricated by gas phase method,and proposed a method for thermal conductivity calculation for particles accumulation.We hope our work can shed some light on precise control over ultrafine particles fabricated by gas phase method.