A comparative study of high intensity DC and AC vortex stabilized arcs operating in argon (at pressures of one to five atmospheres) has been conducted. The energy balance for both the AC and DC arcs has been determined calorimetrically. From these measurements the radiative efficiency (radiation losses/input power) has been calculated. It was found over the current range examined (150-450 amperes) that the radiative efficiency of the AC vortex stabilized arc was comparable to the DC arc. Since DC vortex stabilized arcs have been used as a high intensity radiation source, these results indicate that the AC vortex stabilized arc shows promise for use as a high intensity radiation source. From the energy balance results the heat transfer to the wall was surprisingly found to scale linearly with the radiation losses. The wall loading is not due to absorption of radiation and is much larger than that expected from laminar radial heat transfer. To investigate this further a simple channel model was developed for the luminous DC arc core. From this model the radius and temperature of the luminous arc core was determined as a function of current. The predicted radii were in good agreement with time integrated photographs of the luminous core of the arc. At high current (I>350 amperes) the DC arc radius was essentially constant. The wall heat transfer continued to increase when the arc radius was essentially constant, so that highly efficient heat transfer processes must be taking place outside the central luminous arc core. It is believed that turbulent mixing might be present in this region and be responsible for the large wall heat transfer. The heat transfer processes to the arc electrodes have been measured calorimetrically and the electrode surface temperature has been measured spectroscopically. For both AC and DC electrodes the heat transfer scaled linearly with the arc current. The electrode voltage drop is strongly dependent on gas flow direction with the voltage drop always larger for flow towards the electrode than for flow away from the electrode. These results are not due to convective heat transfer effects. The geometry of the electrode arc attachment region changes when the flow direction is reversed. It is believed that both the anode and cathode fall potentials are altered when the flow direction is reversed, and this is responsible for the difference in electrode voltage drop when the flow direction is reversed. From the electrode surface temperature measurements the heat transfer to the arc electrodes was shown to be essentially one-dimensional in nature. A model of the AC electrode heat transfer was developed using the DC heat transfer results which predicts results for the electrode voltage drop that are in good agreement with the experimental results. The AC electrode heat transfer was found to be <50% of the anode heat transfer in a DC arc at the same current. In the DC arc the anode heat transfer is much larger than the cathode heat transfer. For a practical DC vortex stabilized arc radiation source anode failure is a serious problem, so that the results for the AC electrode heat transfer is of considerable practical importance. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

Thesis (Ph. D. in Chemistry)--University of California, Berkeley, October 1958. / Part II of the thesis "high intensity light sources" see UCRL-8509. "UCRL-8508" Includes bibliographical references : p. 98-100.

Thesis (Ph.D. (Physics)) -- University of Limpopo, 2018 / Beyond conventional intercalation chemistry, reaction of lithium with sulphur and oxygen (so-called “Li-air” batteries) have the potential to provide 2 to 5 times the energy density of current Li-ion battery systems. However, both Li/S and Li/O2 systems suffer from cycling performance issues that impede their commercial applications: Li/O2 cycling is limited by electrolyte decomposition and large cell polarization; Li/S suffers from the low conductivity of S and the solubility of intermediary polysulfide species during cycling. It has been reported that Se and mixed SexSy represent an attractive new class of cathode materials with promising electrochemical performance in reactions with both Li and Na ions. Notably, unlike existing Li/S batteries that only operate at high temperature, these new Se and Li/SexSy electrodes are capable of room temperature cycling. Initially, stabilities of insoluble discharge products of oxygen and sulphur in the Li-S and Li-O2 batteries were investigated using density functional theory within the generalized gradient approximation, and these were deduced from their structural, electronic and mechanical properties. The structural properties are well reproduced and agree to within 3% with the available experimental data. Li2S, Li2O and Li2O2 and Li2S2 structures all have negative heats of formations indicating that they are stable, however, that of Li2S2 structure was relatively high compared to others. Calculated phonon dispersion and elastic properties revealed that Li2O, Li2S and Li2O2 structures are mechanically stable and great agreement with experimental work. The Li2S2 structure displayed soft modes associated mainly with sulphur atoms vibrations in the a-b plane, hence it is not mechanically stable in agreement with the negative C13. Stable Li2S2 polymorphs were extracted from soft modes of calculated phonon dispersions along the gamma direction in the Brillioun zone. Temperature is known to have a significant impact on the performance, safety, and cycle lifetime of lithium-ion batteries (LiB). In order to explore properties of discharge products associated with Li/S and Li/Se batteries at different temperatures, molecular dynamics and cluster expansion methods were employed. The former was achieved by firstly deriving empirical interatomic potentials of Li2S and Li2Se which were fitted to experimental and DFT calculated data. The potentials were validated against available experimental and calculated structure, elastic properties and phonon spectra. In addition, complex high temperature transformations and melting of Li2S and Li2Se were reproduced, as deduced from molecular dynamics simulations. Both Li2S and Li2Se were found to withstand high temperatures, up to 1250K each which is a desirable in future advanced battery technologies. Furthermore, cluster expansion and Monte-Carlo simulations were employed to determine phase changes and high temperature properties of mixed Li2S-Se. The former generated 42 new stable multi-component Li2S-Se structures and ranked metastable structures by enthalpy of formation. Monte Carlo simulations produced thermodynamic properties of Li2S-Se system for the entire range of Se concentrations obtained from cluster expansion and it demonstrated that Li2S-Se is a phase separating system at 0K but changes to mixed system at approximately 350K which was confirmed by constructed by phase diagram of Li2S-Se system. It was finally demonstrated that molecular dynamics and Monte Carlo simulations techniques yield consistent results on phase separation and high temperature behavior of Li2S-Se at 50% of sulphur and selenium.