Field-assisted sustainable O− ion emission from fluorine-substituted 12CaO·7Al2O3 with improved thermal stability

We examined the electric field-assisted thermionic emission of atomic oxygen radical anion (O−) in a vacuum from fluorine-substituted derivatives of 12CaO·7Al2O3 (C12A7) with a composition of (12 − x)CaO·7Al2O3·xCaF2 (0 ≤ x ≤ 0.8). Unsubstituted C12A7 easily decomposed into 5CaO∙3Al2O3 (C5A3) and 3CaO∙Al2O3 (C3A) above 830 °C during the emission experiment in a vacuum. The decomposition temperature range became narrower as the amount of F− ion substitution increased, e.g. the sample with x = 0.4 kept a single phase after the emission experiment at 900 °C. The emitted anionic species from the x = 0.4 sample were dominated by O− ions (∼ 92%) together with a small amount of O2− ions (∼ 4%) and F− ions (∼ 4%). The absence of an O2 gas supply to the opposite side of the emission surface led to a nearly steady co-emission of O− ions and electrons with a ratio of < 1/1. The O2 gas supply markedly enhanced the O− ion emission, and suppressed the electron emission. A sustainable and high-purity O− ion emission with a current density of 11 nA cm− 2 was achieved at 830 °C with the supply of 40 Pa O2 gas. The similarity in these emission features to the unsubstituted C12A7, together with the improved thermal stability demonstrates that the F− ion-substituted C12A7 is a promising material for higher intensity O− ion emission at higher temperatures.