52.77.-j Plasma applications
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With the growing importance of advanced lighting technologies, customers expect additional functionality and higher comfort from fluorescent lamps. However, the ability to regulate light intensity (dimmed operation), in particular, exerts enormous stress on fluorescent lamps’ electrodes, leading to increased electrode erosion and significantly reduced lifetimes. During the operation of a fluorescent lamp, free barium (the main compound of the electrode emitter) is produced at the electrode responsible for lowering the work function in order to enable energy-efficient and durable electrodes with lifetimes of up to 20,000 hours. Despite their relatively long lifetimes, electrodes remain the lifetime-limiting factor of a fluorescent lamp. Therefore, for practical applications (e.g., maintaining quality control, adjusting operational parameters, and evaluating new electrode designs), electrode erosion is of special interest. The actual erosion-measurement methods determine a time-averaged erosion level over several hundred operation hours. Thus, a quasi-instantaneous measuring method (short measurement) is still necessary to determine erosion during operation. Such a method would allow us to compare erosion under different discharge conditions (currents, frequencies, or heating currents) from the same electrode in the same lamp. This work focuses on the determination of absolute electrode erosion during the stationary operation of commonly used fluorescent lamps. Commercial T8 lamps (fluorescent lamps with a diameter of 8/8 inch) are investigated at the operating mode of commonly used electronic ballasts with frequencies of several kHz. Operations under standard and dimmed conditions with an additional heating current to reduce electrode erosion are investigated. Electrode erosion is characterized by the erosion of barium, the main compound of the electrode. Therefore, laser-induced fluorescence (LIF), which is the most sensitive method for this application, is applied to determine the absolute densities of the eroded barium in the electrode region. These densities are affected by the plasma in the electrode region and do not directly represent the absolute barium erosion. To overcome this limitation, a new method based on a special measurement technique in combination with a barium-diffusion-model is developed to determine the absolute barium erosion based on the measured densities. It has been found that the barium densities in the electrode region are lower than the equilibrium pressures produced by the reduction of the barium oxide. This could be caused either by a reduced reaction rate, the reduced diffusion of the reactant (primarily barium oxide) or by reduced barium transport through the porous emitter. However, these results suggest that barium erosion depends on temperature and emitter structure, which vary over an electrode’s lifetime. For currents significantly higher than the nominal lamp current, a drastic increase in emitter evaporation is found. Such, an increase in the lamp current from 300 mA to 500 mA leads to an increase in emitter evaporation by a factor of five. Using the lamp for a long period of time under these conditions therefore reduces the lifetime by a factor of five. Notably, at this dramatically increased erosion level, the hot spot temperature only increases from 1120 K to 1170 K. Investigation of various frequencies from 50 Hz to 5 kHz revealed no significant dependence of emitter evaporation on frequency.
In the framework of the current work has been the plasma initiated and surface catalysed species conversion studied in low pressure and atmospheric plasmas. The aim of the work is to improve the understanding of the internal processes in order to increase the energy efficiency as well as the selectivity of the reaction products of future plasma devices. Beside many technical applications of plasmas, air purification shows great potential. Over the last decades, plasma based pollution control has proofed its ability to remove harmful contaminants or annoying odours from an air stream. However, the energy efficiency and the selectivity of the products are a remaining challenge.
Motivated by these issues, a multi stage packed-bed reactor has been used to remove admixed ethylene and toluene from an air stream. It has been found that the maximum toluene destruction has been 60%, whereas ethylene has been nearly completely removed. The specific energy β has been between 120 and 1600 JL-1. Fourier Transform Infrared spectroscopy, FTIR spectroscopy, has been used to identify and quantify the species H2O, CO2, CO, O3, HNO3, HCN, CH2O, CH2O2, N2O and NO2. However, none of these experiments led to the detection of NO.
The embedment of packing material into a plasma volume leads to increased surface effects. In order to study them, the inner side of a tube reactor, made of Pyrex, served as the surface under study and has been exposed to a rf plasma for 1h. The surface effects of the plasma treatment have been investigated indirectly by studying the oxidation of NO into NO2. After the plasma exposure, the reactor has been evacuated and filled with a gas mixture of 1% NO in N2 / Ar. Both species have been measured using quantum cascade laser absorption spectroscopy, QCLAS. It has been found that, using oxygen containing plasmas, the NO concentration decreased whereas the NO2 concentration increased. Therefore, oxygen containing plasmas are able to deposit oxygen on the surface. The filling with NO leads to the oxidation via the Eley-Rideal mechanism. A simplified model calculation supports these assumptions.
For a more comfortable application of the QCLAS, a compact multi channel spectrometer has been developed, TRIPLE Q. It combines the high time resolution with the possibility to measure the concentration of at least three infrared active species simultaneously. Due to the high time resolution, a huge number of spectra have to be analysed. In order to calculate absolute number densities, an algorithm has been developed which automatically treats typical phenomena like pulse jitter, rapid passage effect or variations of the intensity of the laser pulses.
The gas temperature is an important parameter in plasma physics. Using the TRIPLE Q system, the gas temperature has been determined for pulsed dc plasmas. For this case, NO has been used as a probe gas. From the spectra, the temperature has been calculated using the line ratio method. The relative intensity of the absorption structures of NO at 1900.5cm-1 and 1900.08cm-1 depend on the temperature. Therefore, the ratio has been used to calculate the gas temperature with a time resolution in the μs range.
Vibrationally excited nitrogen can be an energy reservoir that plays an important role in plasma chemistry. In N2 / N2O plasmas, vibrationally excited N2 can undergo relaxation via a resonant vibration vibration coupling between vibrationally excited N2 and N2O. Due to such an efficient energy transfer, the method allows one to study the relaxation of vibrationally excited N2. Using this method, molecules, which are not infrared active, can be monitored. This approach has extended the field of scientific and commercial applications of the QCLAS.
A fluorescent lamp driven with an 'instant start electronic control gear' starts in a glow mode. In the glow mode, which lasts typically for tens of milliseconds, the cathode fall exceeds hundreds of volts. This causes high energy ion bombardment of the electrode which heats the electrode, and induces a transition from glow to arc mode. In the arc mode the electrode emits thermionically and the cathode fall drops to the 12 – 15 V range. Unfortunately, the high energy ion bombardment during the glow mode leads also to intense sputtering of electrode material, including tungsten as well as emitter. Thus, instant started fluorescent lamps often suffer from early failures due to coil fracture. Therefore, the investigation of tungsten erosion during instant start is necessary and was the main goal of this work.
The density of neutral atomic tungsten is determined by laser-induced fluorescence (LIF) and optical emission spectroscopy measurements (OES). Investigations are performed on a low-pressure argon dc discharge and on commercial fluorescent lamps. To include the entire temperature profile along the electrode the diffuse and spot operation modes of the dc lamp are studied experimentally and theoretically. The measured dependencies of the cathode temperature along the coil on the discharge and heating parameters are compared with the calculated results. For the first time the tungsten erosion during instant start of commercial fluorescent lamps was experimentally investigated in this work. The erosion process could be related to sputtering. A reconstruction of the temporal evolution of the absolute tungsten population density of the ground state during the glow mode was presented. The sputtered tungsten density increases immediately with the ignition, reaches a maximum where the discharge contracts at the end of the glow mode, and decreases some milliseconds before the glow-to-arc transition takes place. The maximum tungsten density was observed within a region of a few hundred micrometers only located at the discharge attachment point. The main result achieved in this work is that during the whole glow mode tungsten is sputtered. Therefore, the lifetime of instant started fluorescent lamps can be enhanced by reducing the duration of the glow mode. Additionally, the need for the application of different types of diagnostics for the observation of lamp ignition was shown due to different results of LIF, AAS and OES: The observation of excited tungsten atoms by OES shows the maximum emission signal at the glow-to-arc transition whereas by LIF and AAS measurements of tungsten atoms in the ground state the maximum density is found during the whole glow mode. This can be explained by the fact that the intensity of the spontaneous emitted light is related not only to the density but also to the degree of excitation.