Thin Film Dispenser Cathodes for Thermionic Micro-Devices
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Thin Film Dispenser Cathodes for Thermionic Micro-Devices Kevin R. Zavadil1 and Donald B. King2 1 Materials and Process Science Center, Sandia National Laboratories 2 Nuclear and Risk Technologies Center, Sandia National Laboratories Albuquerque, NM 87185-0888 ABSTRACT Electron emissive thin films that possess the thermionic properties of macroscale dispenser cathodes have been fabricated using standard RF sputter deposition techniques. These films are based on a compositionally modulated structure using W and a Ba-containing ternary oxide. These films exhibit uniform work function values of 1.9 to 2.2 eV with high emission coefficients of 2 to 26 A·cm-2K-2. Electron microscopy shows that W layer coalescence and continued particle growth occurs with prolonged annealing, eventually disrupting the original surface layer. Chemical modification of the oxide during deposition may provide a route to limit the extent of W coalescence and preserve surface metallization layers on these films. These films are designed to be integrated into thermionic micro-devices. INTRODUCTION A variety of thermionic-based technologies could benefit from the development of a microthermionic device. Applications include radiation resistant vacuum transistors, tuned micro-wave emitters for directed energy applications, klystrons for particle acceleration, and energy conversion. A critical component of a micro-device is the cathode or electron emitter that will support a high electron flux at the operational temperature of the device. Electron emission is governed by the Richardson-Dushmann equation: j = AT2e-φ/kT
(1)
where j is the current density, A is a theoretical maximum emission coefficient of 120 A·cm-2K-2, φ is the work function, T is the absolute temperature, and k is Boltzmann’s constant [1]. The efficiency of a device becomes a function of the material dependent work function and emission coefficient. The primary advantage of a micro-thermionic device is in the minimization of the inter-electrode gap, which can either eliminate or greatly reduce the space charge produced by the release of thermal electrons. Space charge creates an additional energy barrier that lowers the achievable current density. Modeling has shown that power densities of 1 to 10 W·cm-2 are possible for an identical emitter and collector separated by gap size of 10 to 1 µm possessing a 1 eV work function and 10 A·cm-2K-2 emission coefficient with the emitter and collector operating at 1200 and 700 K, respectively [2]. Candidates for thermionic electrodes include conventional emissive metal and oxides, as well as low electron affinity materials. The candidate material must be generated in thin film form due to the use of a small inter-electrode gaps and the need to integrate it into small-scale devices. Extensive research and development by the electron source community has produced macroscopic dispenser cathodes that exhibit a high field work function of 1.2 eV and an emission coefficient of 7 A·cm-2K-2 [3,4]. These emitters work on the principle of Ba reduction
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