Stefan Briefi
Spectroscopic Investigation of Indium Halides as Substitutes of Mercury in Low Pressure Discharges for Lighting Applications
Supervisor: Prof. Dr.-Ing. Ursel Fantz [Experimental plasma physics]
Date of oral examination: 05/22/2012
158 pages, english
Low pressure discharges with indium halides as radiator are discussed as substitutes for hazardous mercury in conventional fluorescent lamps. In this work, the applicability of InBr and InCl in a low pressure discharge light source is investigated. The aim is to identify and understand the physical processes which determine the discharge characteristics and the efficiency of the generated near-UV emission of the indium halide molecule and of the indium atom which is created due to dissociation processes in the plasma. As discharge vessels sealed cylindrical quartz glass tubes which contain a defined amount of indium halide and a rare gas are used. Preliminary investigations showed that for a controlled variation of the indium halide density a well-defined cold spot setup is mandatory. This was realized in the utilized experimental setup. The use of metal halides raises the issue, that power coupling by internal electrodes is not possible as the electrodes would quickly be eroded by the halides. The comparison of inductive and capacitive RF-coupling with external electrodes revealed that inductively coupled discharges provide higher light output and much better long term stability. Therefore, all investigations are carried out using inductive RF-coupling. The diagnostic methods optical emission and white light absorption spectroscopy are applied. As the effects of absorption-signal saturation and reabsorption of emitted radiation within the plasma volume could lead to an underestimation of the determined population densities by orders of magnitude, these effects are considered in the data evaluation. In order to determine the electron temperature and the electron density from spectroscopic measurements, an extended corona model as population model of the indium atom has been set up. A simulation of the molecular emission spectra has been implemented to investigate the rovibrational population processes of the indium halide molecules. The impact of the cold spot temperature, the RF-power and the background gas type on the discharge characteristics is investigated for rare gas plasmas containing InBr or InCl. Varying the cold spot temperature yields a maximum efficiency around 210 to 230 degrees centigrade. At low cold spot temperatures the efficiency drops due to a too low indium halide density whereas reabsorption of emitted indium halide and indium photons limits the efficiency at high cold spot temperatures. Reducing the RF-power increases the discharge efficiency as the effect of reabsorption of the generated radiation is reduced. The utilized background gas does not influence the maximum efficiency as the discharge characteristics is solely determined by indium in the relevant cold spot temperature range. As reabsorption effects of the indium radiation are smaller with InCl, the highest efficiency of 24 per cent was obtained at a cold spot temperature of 230 degrees centigrade (40 W power, discharge vessel filled with 1 mbar argon and 1.5 mg InCl). The potential of a low pressure indium halide discharge light source has been demonstrated by simulating the application of phosphors which yields an efficacy of about 60 lm/W. The feasibility of applying low pressure indium halide discharges for lighting purposes has been discussed by outlining a lamp prototype.