Thermionische Studien zu Bestimmung elektronischer und ionischer Austrittsarbeiten durch Zählen von Ladungsträgern

The subject of this thesis is the determination of the ionic and electronic work functions of different materials, with a focus on the work functions of different cathode materials. The work functions were determined using thermionic emission. For this purpose, an MCP detector was used to quantify the emission current, which allows the emission current to be determined by counting charge carriers. The ionic work functions of different lithium phosphate glasses were determined. For lithium ultra phosphate glass the electronic work function could be determined as well. For this material, the ionic work function was 1.99 eV and the electronic work function was 1.64 eV. Ionic and electronic work functions of lithium iron phosphate and lithium manganese oxide were determined for samples of different lithium contents. For lithium manganese oxide, the electronic work functions could not be determined using thermionic emission and were therefore determined by cooperation partners using photoelectron spectroscopy (XPS). For some of the measured, mainly electronic, emission currents, a decrease in emission current was observed. This decrease cannot be explained by the standard models for thermionic emission. Influences on the work function could be minimized by taking into account the decrease of the emission current with time. First the ionic work function of lithium iron phosphate showed no and later only a small change with the lithium concentration. The ionic work functions of lithium iron phosphate obtained were compared with results from molecular dynamics calculations carried out by a cooperation partner. These showed an accumulation of lithium ions at the surface during emission, which could explain the low dependence of the work functions on the ion concentration in the bulk. The calculated ionic work functions behaved similarly to those determined experimentally. For the electronic work functions of lithium iron phosphate, an increase in the electronic work function with decreasing lithium content in the sample and later an area with stable electronic work functions was observed. For lithium manganese oxide, in contrast to lithium iron phosphate, there was no change in the ionic and electronic work function with the lithium content in the respective samples. The work functions obtained for the cathode materials were used in a Born cycle already established in an earlier work. This Born cycle allows the estimation of the cell voltage of a battery system with the respective cathode material. However, this resulted in a difference of the estimated cell voltage to known literature values of the cell voltages for both lithium manganese oxide and lithium iron phosphate. It is believed that this difference may be caused interface effects. The Born cycles obtained for a lower lithium content of the respective samples showed no change in the difference for lithium iron phosphate, while for lithium manganese oxide a change in the difference between the estimated cell voltage and literature values could be determined. In addition, the ionic work function of lithium trisilicate glass was determined to be 2.95 eV. This material will be used in isotope-pure form as a lithium emitter in CAIT (charge attachment induced transport) experiments in the Weitzel group. Since the ionic emission also showed a slight decrease in the emission current over time, the temperature of the sample was increased whenever the emission fell below a specified threshold value. An emission of at least 50 nA could be achieved over a period of 10 days, which corresponds to approximately 7% of the lithium content of the sample used.