Surface Modification Of Nanoparticles Via Atomic Layer Deposition And Its Applications

Micro-and nano-sized particles have many unique physical and chemical properties compared with bulk materials,deriving from their small dimension effect and quanta dimen-sion effect.Imparting these particles with specific physical and chemical properties by sur-face decoration,modification or coating is the precondition for their widely use in the fields of environment,energy,electronics,medical,military affairs,etc.Atomic layer deposition?ALD?is a unique chemical vapor deposition method to fabricate thin film with many ad-vantages such as good uniformity,high conformity,high density,relatively low reaction tem-perature,and precise thickness control.Nevertheless,it requires large amount of precursors to complete the coating reaction on particles due to their large specific surface area.The particles tend to form agglomerates and a very long reaction time is needed to obtain saturated growth on all particle surfaces,which results in low precursor utilization and low production rate,limiting its practical use.In this dissertation,rotating fluidization is adopted to enhance the precursor transport in the particle bed and facilitate the mass transfer between particles and precursors.The influ-ence mechanism of process parameters on deposition efficiency is explored,and highly effi-cient process is achieved with high coating comformity and uniformity.This research has been used to many specific cases for particle surface modifications.The main research work and contributions of the dissertation are introduced as follows:Multi-phase mechanistic model of precursor diffusion in the reactor scale are built based on computational fluid dynamics.Theoretical model for precursor diffusion inside agglom-erates under molecular flow are also built.The agglomeration and fluidization behaviors of micro-and nano-sized particles are studied.Rotating fluidization technique for particle ALD is proposed to overcome the defects of conventional fluidization of particles and enhance the gas-solid mass transfer.The enhanced collision between particles and the additional centrifugal force are used to obtain the dynamic dismantling of soft agglomerates,exposing every particle surface to precursors.The rotating fluidized bed?RFB?reactor for particle ALD is built,which is verified with good conformity,uniformity and coating ratio.The experimental monitoring and simulation methods are investigated.The mechanism of the influences of fluidizing velocity,rotation speed,precursor mass fraction and bed thick-ness on the process efficiency are studied.By optimizing the above parameters,the process time is reduced to more than one order of magnitude shorter than that of conventional static method,while the precursor utilization is increased to about 98%.Applications of the above approach are explored on the surface modifications of Fe3O4 and AIH₃ nanoparticles.Proper processes are investigated to coat these particles with dense and uniform Al2O3 films.The Fe3O4 nanoparticles are protected from oxidation,and the de-crease of saturated magnetization is less than 10%,which is the highest level in present re-ports.Alumina films act as physical barrier layers to isolate water and oxygen in the air from AIH₃ cores,and also decrease the friction sensitivity from 96%to 68%.This value satisfies the requirement that the friction sensitivity needs to be lower than 70%to ensure the safety during transportation or the preparation of propellant using AIH₃ and other agents.The high overall hydrogen capacity and fast dehydrogenation speed are well retained which indicate this surface passivation via ALD a feasible method to stabilize AlH₃.