A Study of Scandia-Doped-Impregnated Cathode Fabricated by Spray Drying Method

Abstract

Scandia-doped-impregnated (SDI) cathodes were fabricated with scandia-doped tungsten powders using spray drying (SD) combined with two-step reduction in hydrogen atmosphere. The temperature-programmed reduction results show that the addition of Sc 2 O 3 leads to a decrease of the reduction temperature of scandia-doped tungsten oxide powders. Using the obtained scandia-doped tungsten powders with homogenous distribution of nanosized scandia, a submicrometer porous matrix cathode composed of tungsten grains with active substances distributing uniformly around these grains has been obtained. For cathodes with the above matrices and impregnated with 411 or 612 Ba, Ca aluminates (named SDI/SD-411 or SDI/SD-612 cathodes, respectively) and space charge limited current densities of ∼72.5 A/cm 2 have been reliably reached at 900°C b . After proper activation, the cathode surface is covered by a Ba-Sc-O active substance layer with preferable atomic ratios, leading to its good emission property. The average evaporation rate of 8.45 × 10 −9 gcm −2 s −1 at 1150°C b for SDI/SD-612 cathode is the lowest one among Ba-W, W/Re/Os-M, SG-411 (the scandia-doped cathode fabricated by sol-gel method with impregnating 411 aluminates), and SDI/SD-612 cathodes.Index Terms-Evaporation behavior, scandate cathode, thermionic emission.

Study on Low-Temperature Emission Performance of Scandate Cathode with Micro-Blade-Type Arrays

“…One of the views is that this effect is caused by the uneven work function at the surface of the scandium-containing cathodes. When the cathode is in the transition zone, some areas of the cathode surface enter the temperature limited zone, but other parts remain in the space charge limited zone [11,29]. From the perspective of a semiconductor model, another viewpoint is that the increase of emission is due to a decrease of the work function in the semiconductor layer caused by external field penetration.…”

“…Impregnated scandate cathodes are considered as the most promising type of cathode, due to them having the highest electron emission capacity of all kinds of thermionic cathodes. Many scholars have devoted a lot of energy to the research of scandate cathodes; for example, preparation of the scandia-doped tungsten matrix [9][10][11][12][13], fabrication of the active salt [14,15], and top-layer scandate cathodes [16][17][18]. These studies have further improved the properties of scandate cathodes.…”

Phase selectivity and environmental stability of 612‐aluminate synthesized by sintering Co‐precipitated precursor

“…For instance, the M-type cathode impregnated with 612-aluminate can provide a current density of 8 A/cm 2 at 1080 • C B (bright temperature) in dc mode and an average lifetime of 19,799 h. 11 Additionally, the emission current density at 1100 • C B can be enhanced by a factor of 2.0 ̶ 4.3 after doping the 612-aluminate with scandia. 12,13 Barium calcium aluminates are susceptible to moisture but exposure to air is inevitable during the preparation of the active materials and the manufacturing of cathodes. 14 Therefore, the environmental stability of impregnants is important to ensure the repeatability and reliability of the electron emission properties of manufactured cathodes.…”

“…For instance, the M‐type cathode impregnated with 612‐aluminate can provide a current density of 8 A/cm 2 at 1080°C B (bright temperature) in dc mode and an average lifetime of 19,799 h 11 . Additionally, the emission current density at 1100°C B can be enhanced by a factor of 2.0 ̶ 4.3 after doping the 612‐aluminate with scandia 12,13 …”