Improvement of barium tungsten cathode and investigation of thermionic emission performance

Abstract

<sec>The Ba-W cathode consists of the porous W matrix and the aluminate. During cathode operation, the Ba atoms are generated in the pores through the thermal reaction between the W and aluminate, and then diffuse along the pore channels to the W surface, lowering the work function. Therefore, the Ba yield and the Ba diffusion are significantly influenced by the micro pore structure of the matrix and the phase composition of the aluminate.</sec><sec>Firstly, the matrix is fabricated with the narrow particle size distribution powder by the spark plasma sintering (SPS) technique, which shows the narrow pore size distribution (FWHM = 0.43 μm). Then the spherical powder with good fluidity and high tap density is prepared using the RF induction thermal plasma. The matrix prepared with spherical powder exhibits narrower pore size distribution (FWHM = 0.4 μm), smooth pore channels and good inter-pore connectivity. The two matrixes prepared with narrow particle powder and spherical powder are named N-matrix and S-matrix, respectively.</sec><sec>The aluminates are prepared using the solid phase method and the liquid phase method, separately. The particles of solid phase aluminate precursor present all shapes and all sizes, while the particles of the liquid phase aluminate precursor are uniform in size and identical in shape. The phase of solid phase aluminate and the phase of liquid phase aluminate are analyzed by XRD, the results show that the former consists of the effective Ba<sub>3</sub>CaAl<sub>2</sub>O<sub>7</sub> phase and other impurity phases, while the latter is composed of two effective phases of Ba<sub>3</sub>CaAl<sub>2</sub>O<sub>7</sub> and Ba<sub>5</sub>CaAl<sub>4</sub>O<sub>12</sub>.</sec><sec>The N+S and S+S cathodes are obtained by using the solid phase aluminate to impregnate the N-matrix and the S-matrix, and the <i>U</i>-<i>j</i> characteristics of the two cathodes are investigated. The double logarithmic curves of <i>U</i> and <i>j</i> show that the slope of 1.37 in the space charges limited (SCL) region for the S + S cathode is higher than that of 1.25 for the N+S cathode, so the S+S cathode exhibits better emission uniformity. The current density at the deviation point (<i>j</i><sub>DEV</sub>) of the N+S cathode and that of the S+S cathode are 6.6 A·cm<sup>–2</sup> and 6.96 A·cm<sup>–2</sup>, respectively. So the improvement on the matrix obviously raises the emission uniformity of cathode, but the current density is increased less.</sec><sec>Based on the excellent matrix of the S+S cathode, the S+L cathode is obtained by improving the aluminate of the S+S cathode with liquid phase aluminate. The <i>U</i>-<i>j</i> characteristics show the slope of the S+L cathode reaches to 1.44, and the <i>j</i><sub>DEV</sub> is 21.2 A·cm<sup>–2</sup>. So the improvement on the aluminate not only increases the uniformity, but also raises the current density.</sec><sec>The present study shows that the <i>U-j</i> curve calculated from the classical thermionic emission (TE) theory accords well with that of the S + L cathode at 1000 ℃, which indicates that the Ba-W cathode follows the classical TE theory rather than other emission theories, and the Ba-O dipole layer just changes the work function of the cathode.</sec>