| Energy crisis and environmental issues are two major problems in the world today.Waste heat utilization and improvement of the traditional power plants efficiency are the effective ways to solve these two problems.The new micro-gap thermionic energy converters（TEC）can directly utilize low-quality waste heat to realize thermal to electric conversion,and also can be coupled with the conventional thermal power plants to improve the overall efficiency,which have broad application prospects.In the practical applications,the W emitters of TECs can be oxidized and delaminated in the combustion atmospheres,which constrains the practical applications.Moreover,conventional high temperature oxidation resistant silicon carbide（Si C）coatings would react with W,yielding a variety of silicides and carbides.Therefore,the incorporation of a diffusion barrier and the study of the high temperature stability of the composite emitters are critical to improve the conversion efficiency of the TECs.Based on this,this thesis successfully developed hightemperature stable TEC emitters and discussed the high-temperature interface stability of the emitter structure with and without the diffusion barriers.Furthermore,the Ohmic contact mechanisms of emitters were systematically studied by combining experimental studies and first-principles calculations.The crack formation mechanism of Ti N film during high temperature aging was studied.Because the Ti N film was thick and the atomic energy of the Si C substrate bombarded by the magnetron sputtering method was large,the film strain energy played a dominant role during the depositing process and the Ti N film was mainly oriented in the（111）crystal plane.As the annealing temperature increased,the crystallinity of the Ti N film was improved,and the（111）crystal plane diffraction intensity of Ti N was increased,and also the grain size was increased.Combined with the surface morphology evolution,surface oxygen content,Ti N deficiency and thermal stress calculation of different thickness of Ti N films,oxygen was adsorbed on the surface of Ti N film during high temperature annealing,and then reacted with Ti N to form Ti O2,which produced the internal stress in the film and reduced the fracture toughness of the film.As a result,the energy release rate of surface crack for a certain thickness of Ti N film exceeded the fracture toughness of the film,resulting in microcracks under thermal cycling.Therefore,for the Ti N/Si C system,the optimal Ti N film thickness is 100 nm.The high temperature stability of W/TiN/SiC and W/TaC/SiC systems were investigated.W and Si C would react to form W5Si3,WC and W2 C phases during long-term high-temperature annealing without the diffusion barrier,which led to an inhomogeneous surface structure with island structure,“crater” structure or several large craters and small pores,thereby increasing surface roughness greatly.Incorporation of a Ti N diffusion barrier suppressed the formation of W5Si3,W2 C and WC during aging.Changes in the surface structure and roughness were minimal.The interfaces between W/Ti N and Ti N/Si C were very clear,indicating no elemental inter-diffusion between the interfaces of W/Ti N and Ti N/Si C.For the W/TaC/SiC system,only the diffraction peaks of W and Ta C were observed after 24 h annealing without interfacial reaction product formation.The surface structure remained nearly unchanged and small changes in surface roughness with time were observed,which could be attributed to grain growth within the W layer.The W grains grew to tens of nanometers after 5h annealing and the average grain size exceeded 100 nm after 24 h.The thermodynamic analysis of the W/TiN/SiC and W/TaC/SiC system in the range of 300K-1800 K showed that the Gibbs free energy difference of the interface reaction was greater than zero,that was,the reaction could not occur in the forward direction.Therefore,the Ti N and Ta C barrier layers formed a stable interface with W and Si C,thereby inhibiting the high temperature reaction between W and Si C effectively.The Ohmic contact performance of W/TiN/SiC was studied.Circular Transmission Line Model（C-TLM）could be quite inaccurate when parasitic resistances were present.A new,simple formula based on C-TLM to extract specific contact resistance of Ohmic contacts to Si C with an acceptable error（<2.5%）was developed.Especially,the calculation error was less than 0.5% when the proportion of parasitic resistance was more than 80%.Furthermore,the corresponding experimental results demonstrated that the specific contact resistance extracting from the new simple formula was much closer to the theoretical value.In addition,the Ohmic contact mechanisms of Ti N to Si C were systematically investigated by combining experimental studies with first-principles calculations.The experimental results revealed that the well-arranged Ti N（111）-oriented lattice planes along the direction paralleled to the（0001）Si C-oriented substrate and the interface was atomically abrupt and coherent without any amorphous layers,secondary phase layers,transition regions,voids or cracks,which confirmed a direct contact of Ti N to Si C on the atomic scale.Furthermore,the atomic structure,interfacial charge distribution,bonding nature and the interfacial electronic states of the Si C（0001）/Ti N（111）interface were computed by first-principles calculations.The calculated adhesion energies showed that the Si2 of Si C on-top binding the Ti site of Ti N was determined to be the most stable interfacial configuration.The charge density differences demonstrated that ～0.86 e-would transfer from Ti N to Si C per unit in the slab model.Quantitatively,first-principle calculations revealed that the Schottky barrier height（SBH）was as low as 0.03 e V and that the band gap nearly vanished at the interface,indicating an excellent Ohmic contact of Ti N to 4H-Si C.The interfacial charge polarization and strong coupling of interfacial electronic states could lower Schottky barrier height and then enhance quantum electron transport significantly which were the origins of Ti N Ohmic contact to Si C.The ZrO2 coated W/TiN/SiC multilayer structure and CVD Si C coating were designed to test the high temperature oxidation resistance of Si C and the stability of the emitter structures during flame heating.Si C remained stable and no inter diffusion occurred at the W/TiN/SiC interface after 10 h flame heating.Based on the Deal Grove model,the thickness of Si O2 was only 88 nm for Si C after heating at 1473 K for 10 h,which satisfied the requirements of high temperature protective coatings for TEC emitters.The conversion efficiency of the micro-gap TEC was tested by integrating the emitter,the collectors and the stand-offs.The efficiency reached 12% and the system was simpler,more cost-effective compared with traditional TECs. |
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