Wettability And Interfacial Chemistry Between Aluminum Or Copper And Mono-/polycrystalline Zirconia Under The Application Of Direct Current

Zirconia is well known for its outstanding properties such as high-temperature ionic conductivity,low thermal conductivity,good toughness and thermal stability as well as excellent wear and corrosion resistance.Therefore it is widely used in the fields of solid oxide fuel cells,oxygen sensors,thermal-resistant devices,etc..In many applications,metal-ZrO₂ bonding is usually encountered and the wettability and adhesion at the metal-ZrO₂ interface play a crucial role,especially when a liquid phase is involved.Therefore,investigations on the wettability of ZrO₂ by liquid metals and careful control of the interfacial structures are of vital importance in determining the performance of the components or devices.Therefore,in this paper,based on the high temperature ionic conductivity of zirconia,we utilized the current coupled seat drop devices to investigate the wettability of Al/single crystal ZrO₂-9 mol%Y₂O₃?FSZ?,Cu/polycrystalline Zr O₂-8mol%Y₂O₃?FSZ?,Cu/polycrystalline ZrO₂-3 mol%Y₂O₃?PSZ?and Cu/single crystal ZrO₂-8 mol%Y₂O₃?FSZ?systems.The main results obtained in this thesis are listed as follows:?1?The wettabiliy of single crystal FSZ by molten Al was greatly improved by applying a micro-direct current,and the mechanisms were the formation of nonstoichiometric zirconia and the release of metallic Zr at the FSZ/Cu interface.?2?The effects of current intensity,temperature,grain boundary and crystal orientation on the wettability of FSZ by molten Al were revealed.The final contact angle of molten Al on single crystal FSZ decreased with the increase of current intensity,and the wettalility transformed from non-wetting to partial wetting when the current intensity was greater than or equal to 5 mA.The higher the temperature,the worse the wettability,which was mainly related to the thickening of the oxide film on the Al surface caused by the increase in the oxygen content released by the FSZ at a high temperature.The grain boundary inhibited the spreading of Al on FSZ,and the contact angle of Al on single crystal FSZ is significantly smaller than that on polycrystalline FSZ at the same current intensity.The coarse ZrAl₃phase formed at the polycrystalline FSZ/Al interface significantly increased the roughness of the substrate,which inhibited the spreading of molten Al.Besides,Al had the best wettability on single crystal?110?-oriented FSZ and the poorest wettability on single crystal?111?-oriented FSZ.?3?The application of DC,regardless of its polarity,significantly promotes the wettability of FSZ by molten Cu.However,the wetting mechanisms vary greatly with the current polarity.When FSZ was connected to anode and Cu to cathode,the wetting was promoted by the formation of nonstoichiometric zirconia and the release of metallic Zr at the FSZ/Cu interface;whereas,when the FSZ was connected to the cathode and Cu to the anode,the wetting was primarily driven by the dissolution of considerable amount of oxygen in Cu and the formation of O-Cu clusters at the FSZ/Cu interface as well as at the Cu surface,thus decreasing the solid-liquid and liquid-vapor interfacial energies.?4?The interfacial chemistry can be also manipulated by the application of DC.When the FSZ was connected to the anode and Cu to the cathode,the FSZ/Cu interface was rich in Zr and deficient in oxygen,thereby developing substantial metallic bonds.In contrast,when the FSZ was connected to the cathode and Cu to the anode,the FSZ/Cu interface was rich in oxygen,forming a larger number of O-Cu clusters.?5?The effects of ZrO₂ substrate,grain boundaries and crystal orientation on the wetting of FSZ by molten Cu were revealed.The contact angle of Cu on PSZ was lower than that on polycrystalline FSZ at the same current intensity,which was more obvious when FSZ was connected to cathode and Cu to anode.The effect of grain boundary on the wettability of Cu on FSZ is closely related to the current polarity.When FSZ was connected to anode and Cu to cathode,the wettability of Cu on the single crystal FSZ was better than that on polycrystalline FSZ at the same current intensity,however,when FSZ was connected to cathode and Cu to anode,it was the opposite.The influence of crystal orientation on wettability was related to the polarity of the DC power.When FSZ was connected to the cathode and Cu to the anode,the wettability of Cu on the single crystal FSZ had no relevance to the crystal orientation;when FSZ was connected to the anode and Cu to the cathode,the contact angles of Cu on single-crystal?100?-oriented FSZ were much smaller than that on single crystal?110?and?111?-oriented FSZ.