Vacuum electron apparatus has a wide application in civil and military field, such as communication, broadcastings and aviation industry [1][2][3]. An increase of current density cathode at a given working temperature is a main trend in the development of thermionic cathode for different in vacuum devices. Among all the electron emitters, Scandate cathodes have aroused great attention among all the thermionic cathodes due to their high current density [4][5][6].In this paper, scandate doped pressed (SDP) cathodes were prepared by a new method of spray drying combined with two-step hydrogen reduction, instead of traditional mechanical mixing of scandium oxide and barium-calcium aluminates with tungsten. The microstructure, element distribution, and emission properties of the cathodes have been characterized by scanning electron microscope, energy dispersive spectrometer, and an emission test system, respectively.The SEM image of SDP cathode and corresponding EDX results are shown in Fig.l. It can be seen that the cathode prepared by spray drying method have a smooth surface and good sub-micrometer quasi-spherical tungsten grain structure with homogeneous distribution of scandia and barium-calcium aluminates, which are dispersed over and among tungsten grains.The SEM image of cathode cross section and related EDX results [ F ig.2 (a) and (b)] also show that the cathode consists of sub-micrometer quasi-spherical tungsten grains with Sc and barium-calcium aluminate which exist at the interface of the tungsten grains. The sub-micrometer microstructure of cathode is favorable for the active substance diffusing from the inside to the cathode surface and improved the distribution homogeneity of scandia in tungsten Fig.l (a) SEM image of SOP cathode surface and (b) corresponding EOX results Fig.2 (b) SEM image of SOP cathode cross section and (b) corresponding EOX results 978-1-4244-6644-3/10/$26.00 ©2010 IEEE 168

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Recently, high power magnetrons as the commercial microwave power source for various civil accelerators have gained more and more attentions because of rapid development of microwave radiation therapy and non-destructive detection [ll. Secondary electron emission (SEE) cathodes play an important role in the operation of such devices. Up to now, the ordinary secondary electron emitters, such as Barium dispenser cathodes, Oxide cathodes, can't be used in such devices because of their non-resistance of violent electron strike [2l. Rare earth oxide Molybdenum ceramic (REO-Mo) cathodes are considered to be the potential emitters for such magnetrons because of its excellent secondary electron emission property.In this paper, the effect of different preparation process on the secondary electron emission performance was investigated. The different doping methods and preparation techniques such as Solid-Solid doping(SS), Liquid-Solid (LS) doping, Liquid-Liquid (LL) doping, Spark Plasma Sintering (SPS), Air Plasma Spray (APS) method, were introduced into this kind of materials. The microstructure of samples was observed with Hitachi S-3500N Scanning Electron Microscopy, equipped with Oxford Inca EDX equipment. The self-designed apparatus was used to evaluate secondary electron emission yields (8) of samples in the ultra-high vacuum chamber. The secondary emission properties were measured at 600°C in order to avoid the influence of the thermionic emission. Before measurement, some samples were annealed in the hydrogen atmosphere at high temperature.Fig.1 illustrate surface of different of samples. It is shown that uniform mixing of rare earth oxides was realized in the LS and LL doping samples. It is also noticed that there are aggregate of rare earth oxides on the surface of SS doping and APS-sputtered samples, as shown in Fig.1 a) and d). It is obvious that the distribution of rare earth oxide will lead to the variation of secondary electron emission property. Thus, it can be inferred that there are better SEE performance of SL and LL doping samples than that of SS doping and APS-sputtered samples. a) SS doping+SPS b )SL doping+SPS c) LL doping+SPS d) APS-sputtered Fig.1 SEM pictures of surface of different samples 978-1-4244-6644-3/1 0/$26.00 ©20 1 0 IEEE 121