Work function performance of a C12A7 electride surface exposed to low pressure low temperature hydrogen plasmas

The calcium aluminate electride [Ca24Al28O64]4+(e−)4 (C12A7:e−) is chemically and thermally stable in ambient atmosphere, offers strong electron donating capabilities and exhibits an intrinsic work function of 2.4 eV. Thus, it is an attractive material for electron emitter and low work function applications. For an implementation in negative hydrogen ion sources, where the electride could serve as a converter surface for the H− production, compatibility with moderate vacuum conditions and exposure to low pressure low temperature hydrogen plasmas must be ensured. Hence, the stability and work function performance of a polycrystalline C12A7:e− sample are investigated under ion source relevant vacuum (∼10−6 mbar) and plasma conditions (electron temperature ∼2 eV, electron density ∼1016m−3). The work function is measured via the photoelectric effect, showing that long-term hydrogen plasma exposure results in a steady-state surface work function of 2.75±0.10 eV. Since the electride is very sensitive to surface contaminations, an increase of the surface work function occurs as soon as the plasma is switched off. Vacuum heat treatment of the degraded C12A7:e− surface after plasma activation leads to a work function minimum of 2.9 eV for temperatures in the range of 670–770 K. Exceeding a temperature of ∼770 K leads to an increase of the C12A7:e− surface work function. The sample is stable during long-term hydrogen plasma exposure (up to 10 h total plasma-on time tested) and no plasma-induced erosion is observed.