Study On Characteristics Of Electron Emission Cathode For Thermionic Energy Conversion

Energy issues have always been the focus of global attention.The current energy conversion is mainly based on the principle of thermal cycling,and the conversion efficiency is very limited.Thermionic energy conversion technology is a way to directly convert thermal energy into electrical energy.It has huge development potential in the recovery of waste heat,the efficient use of solar nuclear energy and the field of aerospace.Thermionic energy conversion relies on the positive contact potential difference between the emitter and the emitter to receive the heat and emits current,which can realize high-current DC output at low voltage.The energy conversion device has a simple structure,safe and reliable operation,and the energy conversion efficiency is relatively high ideally.But as far as the actual thermal electron emission energy conversion device is concerned,its conversion efficiency is greatly limited,and the most important limiting factor is the hot cathode material.At a certain temperature,if you want to ensure higher efficiency,you need to improve the emission ability of the cathode.This can be achieved by developing an emitter material with a low work function.Work function is an important parameter to evaluate the emission performance of hot cathode,and the research on hot cathode materials with low work function performance has also become a hot topic of current research.Based on this research idea,explore some low work function materials that are suitable for as a hot cathode.This paper sorts out and compares the emission mechanisms,the using occasions and other issues of various types of hot cathodes.Among them,the oxide cathode is focused on with its lower operating temperature and higher emission efficiency.The emission mechanism of the oxide cathode is relatively complicated,and there is no mature emission theoretical model.At present,the semiconductor model is the most appropriate model theory.Composite oxide mayenite(C12A7)is a new type of ceramic material,and its electronic derivatives have received widespread attention due to its controllable conductivity,low work function value,and stable chemical properties at room temperature.Based on the C12A7crystal structure,this paper analyzes its electrical characteristics and emission performance in detail,and believes that it has the potential of hot cathode materials.By evaluating the performance of C12A7 electride(C12A7:e-)obtained by different preparation methods,this study used high-temperature direct synthesis to prepare polycrystalline C12A7 electride.Under a nitrogen atmosphere of 1600°C,stoichiometric ratio(12:7)of calcium carbonate and alumina mixed powder was annealed at high temperature in a high-purity graphite crucible.Due to the different placement of the crucible in the furnace,two types of samples with different properties were obtained.XRD(X-ray diffraction technology)technology was used to detect sintered two types of crystal samples and believed that they all contained C12A7phase.The obvious unevenness at the surface of the C12A7 electride prepared by the above-mentioned way can be clearly observed from the enlarged view of the crystal.In order to further investigate the influence of this non-uniformity on the electrical properties and emission performance of C12A7 electride crystals,further physical property testing of samples is needed.XRD and electrical characteristics test results are in good agreement.The internal composition of the two types of samples is the same,both of which contain a small amount of C-doped C12A7 electride.Because different degrees of electronization show different conductivity,this study uses the conductivity results to guess the approximate range of electron concentration.Using a four-probe resistance meter to measure its electrical characteristics,the results showed poor conductivity.It behaves as a semiconductor,and its electron concentration is estimated to be about 1×10¹⁸cm^-3.The main component of the silver surface layer of the first type of sample is C,which shows that the C coverage of the surface layer affects the internal electronic substitution during the preparation process,so the reduction reaction does not proceed completely.The surface layer of the second type of sample is a C12A7 electride with a more complicated structure.The surface of the second type of sample is a C12A7 electronic compound with a relatively complex structure.In view of this complex situation,this paper uses the Kelvin probe technique to measure the distribution of its surface work function.Through the analysis of the work function test results,the sample collected a very small work function value,only about 1e V,and this value is very advantageous for the hot cathode.And this non-uniformity is very valuable,because such unevenness also exists on the surface of atomic thin film cathode and oxide cathode with good emission performance.