Preparation Process and Performance Analysis of Oxygen-containing Coating on Transition Metal

As candidate materials for the collector in thermionic energy converters (TECs), reducing the surface work function of transition metals can lower the barrier index between TEC electrodes, thereby improving their thermoelectric conversion efficiency and promoting the application of TEC in the fields of aerospace and deep space exploration. In order to obtain the coatings of transition metals with low work function, investigation into the preparation process of the oxygen-containing coatings on the surface of transition metals has been conducted, and the testing technique for surface work functions was also studied. The magnetron sputtering physical vapor deposition (MS-PVD) technology was used to acquire the oxygen-containing coatings on three transition metals, including Mo, Nb and Ni. The methods on how to control the oxygen content value inside the deposited coating were analyzed by calculating the standard Gibbs free energy variation of the reactions between transition metals and oxygen. And several sets of refractory metal oxygen-containing coatings with different oxygen contents were prepared. Then, the surface morphology of the prepared coatings was observed using field emission scanning electron microscopy, and the changes in element content along depth of the coatings were detected using a nano scanning Auger system. Meanwhile, the bonding strength between the deposited coating and the substrate was tested and evaluated using an automatic scratch tester for coating adhesion. Finally, surface work functions of transition metal substrates and their coatings were tested using ultraviolet photoelectron spectroscopy (UPS). The results show that the smooth and dense oxygen-containing coatings of transition metals with high bonding strength can be obtained by using MS-PVD technology, and the oxygen content with controllable sizes is evenly distributed along the coating thickness. After adsorption of oxygen on the surface of transition metals, the work function value slightly increases, while after adsorption of cesium, the work function value decreases. At the same time, after adsorption of oxygen and cesium, the work function further decreases. Among them, the work function of Mo absorbing cesium after oxygenation decreases to about 1.3 eV, Nb decreasing to about 1.4 eV, and Ni decreasing to 1.6 eV. Based on the research results, it can be concluded that as preparing the oxygen-containing coatings on metal surfaces by adopting MS-PVD technology, the oxygen content inside the coating can be controlled by adjusting the O2/Ar flow ratio, which is directly proportional to the standard Gibbs free energy change of reactions between the deposited metal and oxygen. That is, the easier the metal reacts with oxygen, the lower the O2/Ar value required to obtain the same range of oxygen content. Moreover, after depositing oxygen-containing coatings on the surface of transition metals, the work function after cesium absorption can be further reduced compared to the substrate.