(12c) In-Situ Process Monitoring for Plasma Synthesis of Alumina Nanoparticles

Among the gas phase nanoparticle synthesis routes, the inductively coupled thermal plasma (ICP) process presents the advantage of a high production rate, a controlled atmosphere leading to high purity products and a high flexibility concerning the materials to be synthesized. Typically solid precursors are fed into the plasma where they are evaporated and, by a controlled condensation with cold gas, engineered nanoparticles are formed. The final powder properties like the particle size and size distribution, the phase and chemical composition, as well as the morphology are determined by process factors involving plasma properties and powder trajectories, and interactions between powders and plasma. Therefore it is an important task for process control and optimization to monitor the on-going process steps, in order to understand plasma-powder interactions and to find a correlation between process parameters and synthesised powder properties. In this investigation, the influence of the swirling central gas flow of an inductively coupled Ar-H2 thermal plasma used for nanopowder synthesis is experimentally investigated using various plasma diagnostic and powder monitoring methods. Enthalpy probe technique is applied to characterize the plasma properties such as specific enthalpy, temperature and velocity for different central gas flow rates under powder-free condition. For determination of the temperature, a mass spectrometer is connected to the enthalpy probe device. Furthermore, enthalpy probe measurements are also carried out under quenching conditions. Quenching is a significant process step for controlling the nanoparticle properties such as particle size and crystallographic phases. The evaporation of micro-scaled alumina precursors injected axially into the plasma is monitored on-line simultaneously by optical emission spectroscopy (OES) and laser extinction (LE) measurements. The complete evaporation of the solid precursors is a precondition for the control of the nanoparticle synthesis. Finally, the synthesized powders are collected in a sampling unit and ex-situ characterized by various techniques to validate the in-situ diagnostic measurements. An increase in specific enthalpy and cooling rate in the plasma core is measured with decreasing central gas flow rate. A steeper gradient and an off-axis maximum of plasma enthalpy over axial and radial axis are observed for low flow rates of central gas (LCG) compared to high flow rates of central gas (HCG). A higher quenching efficiency is measured at LCG conditions. Chemical analysis of powder-plasma interaction by OES shows much higher intensities of emission spectral lines of aluminium vapour resulting from vaporisation of injected alumina powders for LCG, which can be caused by high plasma enthalpy. The laser extinction measurements confirm the favourable low central gas flow rate conditions for the precursor evaporation. Moreover, the OES measurements allow determining the optimum regime between precursor deficiency and energy deficiency. The in-situ observations are validated by ex-situ particle size distribution analyses of the collected powders indicating that more precursor powders are plasma-treated and more nanoparticles are synthesized as central gas flow decreases. XRD analysis presents a phase transition from alpha- to delta-alumina for LCG featuring higher quenching efficiency, while theta phased alumina is synthesized for HCG. The investigated in-situ monitoring techniques have proved their potential for the synthesis of nanoparticles by thermal plasmas. The precursor feed rate could be maximized with OES and laser extinction measurements. These techniques could be suitable for the development of an evaporation on-line sensor in plasma environment aiming to guarantee a process quality control for nanoparticle synthesis.