HIGH PARTICLE CHARGE STATES AT ELEVATED TEMPERATURE: DUE TO STRONGLY REDUCED AEROSOL PARTICLE WORK FUNCTION? A. Schiel, A. P. Weber and G. Kasper Institut für Mechanische Verfahrenstechnik und Mechanik, Universität Karlsruhe (TH), Am Forum 8, 76128 Karlsruhe, Germany, annette.schiel@mvm.uni-karlsruhe.de Keywords: particle charging, high temperature, effective work function INTRODUCTION In situ measurements of the charge state of aerosol particles at elevated temperatures have resulted in fairly high average charges per particle. Furthermore, the charge depends highly on particle material and temperature. However, a model calculation of the particle charge taking into account the charge recombination process, results only in a fraction of the measured particle charges. Latest experiments indicate that at higher temperatures the activation energy for particle charging is very low, which could explain the experimentally found high particle charge states. EXPERIMENTAL METHOD The experiments were carried out at high temperatures (T > 1300 °C). Aerosol particles of different materials (Pt, Ni, TiO 2 , SiO 2 , ZnO and Al-O-Si) passed a tube furnace in order to heat the particles (1600 °C). At the furnace exit at a temperature of 1000 °C an electrostatic precipitator device was mounted to induce particle separation. Subsequently, the particle size distribution was measured, using a Scanning Mobility Particle Sizer (SMPS), which covers a particle size range from 10 to 600 nm. The experimental setup and the corresponding temperature axial temperature profile are shown in Fig. 1. tube furnace aerosol generator particle size distribution temperature length, time 1600 °C 1000 °C electrodes Fig.1: Block diagram of the experimental setup and the corresponding axial temperature profile. The principle approach to determine the charge per particle is to measure the particle size distribution of the aerosol after passing the high temperature furnace. The so-called grade efficiency T(x) can be calculated in the common way from the aerosol number concentrations measured with and without an electrical field. The separation efficiency can be related to a dimensionless transport number comparing the residence time to a characteristic drift time. This transport number in turn is a function of the particle charge [1]. RESULTS AND DISCUSSION The experimentally determined average particle charges for all materials are shown in Fig. 2. The particle charge increases with the particle diameter to the power of close to 2. Also, a higher thermal charging state is observed for metal particles (Pt, Ni), which may be due to their special electronic band structure. Abstracts of the European Aerosol Conference 2004 S1187