Synthesis and thermionic properties of tungsten-barium titanate composites

The potential of novel tungsten-barium titanate composites as thermionic emitters is explored. Composites ranging from 20% to 80% tungsten by mass were prepared by sintering in an Ar-H2 atmosphere. XRD and SEM studies indicate four major micro-constituents; W, BaTiO3, Ba4(Ti,Fe)12O27 and BaWO4. Richardson work functions (φR) and Richardson constants (AR) were determined using a Schottky diode arrangement at temperatures ranging from 1223 to 1473 K. Work functions ranged from 2.67 eV to 3.32 eV with a shallow minimum at 40% by mass W and were relatively constant (∼2.7-2.8 eV) in the range 30-70% by mass W. The decrease in work function was accompanied by a strong decrease in AR from 39.3 A cm−2 K−2 to 0.02 A cm−2 K−2 over the range 20-70% by mass W. The reduction in both φR and AR was associated with the major conversion of the surface to BaWO4 and Ba4Ti12O27 during the activation treatment before emission testing.