Effect of Sc<sub>2</sub>O<sub>3</sub> doping on thermal emission properties of rare-earth refractory yttrium salt cathode

To improve the thermionic emission performance of the rare-earth refractory yttrium salt cathode used in the magnetron, the influence of Sc<sub>2</sub>O<sub>3</sub> doping on its thermionic emission properties is investigated. Cathodes are fabricated by incorporating different weight percentages of Sc<sub>2</sub>O<sub>3</sub> into the rare-earth refractory yttrium salt matrix, and their thermionic emission properties are systematically evaluated. The experimental findings reveal that the doping of Sc<sub>2</sub>O<sub>3</sub> significantly enhances the thermionic emission capability of the cathode. Notably, Sc<sub>2</sub>O<sub>3</sub> with a doping concentration of 3% has the most significant improvement in emission performance. The 3% Sc<sub>2</sub>O<sub>3</sub>-doped cathode can achieve a thermionic emission current density of 3.85 A/cm<sup>2</sup> under an anode voltage of 300 V at 1600 ℃. In contrast, under the same conditions, the undoped cathode provides a current density of only 1.66 A/cm<sup>2</sup>, indicating a 132% increase in thermionic emission efficiency when doped with 3% Sc<sub>2</sub>O<sub>3</sub>. By using the Richardson line method coupled with data-fitting algorithms, the absolute zero work functions for undoped and Sc<sub>2</sub>O<sub>3</sub>-doped cathodes (3%, 7%, and 11%) are determined to be 1.42, 0.93, 0.98, and 1.11 eV, respectively. The lifespan assessment indicates that at 1400℃ the cathode doped with 3% Sc<sub>2</sub>O<sub>3</sub> remains stable for over 4200 h under an initial load of 0.5 A/cm<sup>2</sup> without significant degradation. Finally, those cathodes are analyzed by the XRD, SEM, EDS, AES respectively. The analyses show that during thermionic emission testing, the Sc<sub>2</sub>O<sub>3</sub> and Y<sub>2</sub>Hf<sub>2</sub>O<sub>7</sub> undergo substitutional solid solution reactions, forming the Sc<sub><i>x</i></sub>Y<sub>(2–<i>x</i>)</sub>Hf<sub>2</sub>O<sub>[7+(3/2)<i>x</i>]</sub> solid solution. This process causes lattice distortion in the Y<sub>2</sub>Hf<sub>2</sub>O<sub>7</sub>, which makes it in a high-energy state, thus reducing the work function on the cathode surface. At the same time, Sc from Sc<sub>2</sub>O<sub>3</sub> displaces Y in the Y<sub>2</sub>Hf<sub>2</sub>O<sub>7</sub> unit cells, with the displaced Y existing in the form of metal, which enhances the electrical conductivity of the cathode surface. Additionally, the Sc<sub><i>x</i></sub>Y<sub>(2–<i>x</i>)</sub>Hf<sub>2</sub>O<sub>[7+(3/2)<i>x</i>]</sub> solid solution generates a substantial number of Vo<sup>2+</sup> oxygen vacancies and free electrons, thereby further augmenting surface conductivity. All in all, these mechanisms contribute to significantly improving the thermionic emission capability of the cathode.