Analysis of Radio Frequency Induction Plasma Process for Mass Production of Ni Nanoparticles

Abstract

As the demand for nickel nanopowders is recently increasing as electrode material of microelectronic devices such as multilayer ceramic capacitors (MLCCs), it is essential for the industrial applications to develop the economical method for the mass production of nickel nanoparticles. Especially, spherical nickel nanoparticles with narrow size distribution are favored for high-tech MLCCs with high performance. However, conventional nanoparticle synthesis routes such as ball milling, sol-gel and spray pyrolysis methods have a limitation in manufacturing nickel nanoparticles which meets the industrial needs. Meanwhile, radio frequency (RF) induction plasmas have attracted much attention as a powerful tool to prepare spherical nanopowders. In spite of the advantages of induction plasma process in the synthesis of spherical nanopowders, the difficulty in scale-up of RF induction plasma systems and the resultant low production rate are frequently mentioned as the weakness in industrial applications. In order to overcome this weakness, a high powered RF induction plasma process for mass production of nickel nanoparticles is studied through the numerical modelling and experiment. The primary purpose of this dissertation is to develop a practical RF induction plasma process for the mass production (~ 16.7 g/min) of well-dispersed spherical nickel nanoparticles with high purity and good crystallinity using solid nickel precursor. In order to vaporize the nickel precursor at the high feed rate of ≥ 16.7 g/min, a high-powered RF thermal plasma should be generated and maintained. For the stable operation at high power level of ≥ 50 kW, Ar-N2 RF-ICP (Radio Frequency Inductively Coupled Plasma) was proposed because Ar only ICP is difficult to scale up the plasma power higher than 30 kW due to the rapid increase of radiation loss. The existence of N2 in Ar-N2 induction plasmas is expected to alleviate the radiation loss of the plasma and to improve the thermal conductivity of plasma, which enhances the heat transfer from plasma to the injected precursor and the mass production of Ni nanopowders. In order to find the power-up process of Ar-N2 induction plasma higher than 50 kW, thermal flows and the electrical characteristics of Ar-N2 ICPs were investigated based on the numerical results of 2-dimensional MHD (Magneto-Hydro Dynamic) equations for Ar-N2 RF-ICP combined with the basic circuit theory. Based on these numerical studies, the optimum N2 content are presented for Ar-N2 ICPs with the power level of 50 kW. A simplified PSI-Cell model were developed to consider the plasma-particle interaction with dense loading of raw nickel powders conditions for the mass production of nickel nanoparticles. From the computational study, effects of process parameters on the plasma characteristics and the behaviors of particles injected into the induction torch are investigated. Firstly, the effects of carrier gas flow on the flow fields of induction plasma and the behavior of a single particle injected into an induction torch are investigated. The introduction of cold carrier gas for powder feeding makes a low temperature zone downstream from the position of powder injection probe tip in the central region of RF induction torch. The low temperature zone expands and elongates with the increase of the carrier gas flow rate. As the carrier gas flow rate increases, the initial axial velocity of particle loaded in carrier gas flow is increased. As a result, the residence time of particle in high temperature region decreases, which is disadvantageous to particle heat treatment. Therefore, the carrier gas flow rate should be minimized as much as possible for the complete evaporation of the powder injected. Secondly, the effects of N2 mole fraction of the plasma gas on the plasma characteristics and the behavior of particles injected into induction torch are studied. The addition of N2 can reduce down the percentage of radiation power loss. The highest temperature region of ~ 10,000 K is significantly diminished for Ar-N2 induction plasma with the N2 mole fraction of 20 %, which is attributed to the increase of specific heat, enthalpy and thermal conductivity for N2 addition. The Ar-N2 mixture gas is advantageous to vaporize nickel particles due to the enhanced plasma-particle interaction. Thirdly, the effects of nickel feed rate on the temperature field of induction plasma and the behavior of particles injected into an induction torch are investigated. As the nickel feed rate increases, the plasma temperature drop near the particle trajectories increases due to the plasma-particle interaction. When the plasma is cooled down below the boiling point of nickel, plasma-particle heat transfer is reduced and the complete vaporization of the injected particles is difficult. The particles moving along with the outer trajectories far away from the symmetric axis of the torch can be completely vaporized because they pass through the high temperature region of induction plasma. However, the particles moving along with the inner trajectories close to the centerline of the torch can be partly vaporized because they pass through the low temperature region of induction plasma. Fourthly, the increase of RF power input affects two opposing effects. On the one hand, the increase of RF power input raises the plasma temperature, which has the favorable effects on the vaporization of the particles. On the other hand, the rise of RF power input increases the plasma axial velocity, which is disadvantageous to the vaporization of the particles. Consequently, the optimal RF power input to evaporate nickel powders is expected to exist. In order to demonstrate the feasibility of the RF induction plasma process for the mass production of nickel nanoparticles, the RF induction plasma system for the preparation of nickel nanoparticles was constructed and the experimental work was carried out based on the computational study. Well-dispersed spherical nickel nanoparticles are successfully produced at an RF power input of 50 kW, 20 mol % of N2 and a nickel feed rate of 20 g/min with the production yield of 65 wt. %. The mass production of Ni nanoparticles with spherical shape, high dispersion, high crystallinity and high purity is achieved via RF induction plasma process.