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{bullet operator}3Al₂O₃ (C5A3) and 3CaO{bullet operator}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.