Direct synthesis of [Ca₂₄Al₂₈O₆₄]⁴⁺(4e^–) electride and its thermionic emission performance

Abstract

[Ca<sub>24</sub>Al<sub>28</sub>O<sub>64</sub>]<sup>4+</sup>(4e<sup>–</sup>) eletride, as the first room-temperature stable inorganic electride, has attracted intensive attention because of its fascinating chemical, electrical, optical, and magnetic properties. However, it usually needs synthesizing through a complicated multistep process involving high temperature (e.g., 1350 °C), severe reduction (e.g., 700–1300 ℃ for up to 240 h in Ca or Ti metal vapor atmosphere) and post-purification. Owing to the H<sub>2</sub>O sensitivity of mayenite, the post-purification is quite troublesome once impurities are introduced. High-density, loosely bound encaged electrons with a low work function make it promise to possess practical applications. Therefore the facile method of massively producing the high-quality C12A7:e<sup>–</sup> with high Ne is extremely desired. In this work, C12A7:e<sup>–</sup> bulks are for the first time synthesized by simple spark plasma sintering process directly from a mixture of C12A7, CA and Ca powders under milder conditions (e.g., sintered at 1070 ℃ for 10 min in a vacuum). The obtained electride, which exhibits a relative density of 99%, an electron concentration of ~2.3×10<sup>21</sup> cm<sup>–3</sup> and an obvious absorption peak at 2.5 eV, is obtained via SPS process at 1100 ℃ for 10 min. Electronic structure is also investigated by electron paramagnetic resonance. The occurrence of Dysonian characteristic, a typical feature of good electronic conductors, strongly suggests that the electrons are trapped in mayenite cavities. Furthermore, the obtained C12A7:e<sup>–</sup> exhibits good sinterabilty on a crystal scale of 5–40 μm. Thermionic emission test results show that the thermionic emission begins to occur at 700 K and a large current density of 1.75 A/cm<sup>2</sup> is obtained in the electron thermal emission from a flat surface of the polycrystalline C12A7:e<sup>–</sup> with an effective work function of 2.09 eV for a temperature of 1373 K with an applied electric field of ~35000 V/cm in a vacuum. Owing to no external reductant is needed, this developed route exhibits notable superiority over the conventional reduction method for phase-pure C12A7:e<sup>–</sup>. Therefore, these results not only suggest a novel precursor for fabricating mayenite electride but also make it possible to produce efficiently the electride in large volume.