Alternative ceramic potting materials for dispenser cathodes

Dispenser cathodes operate under ultra high vacuum and high temperature conditions. Because of these harsh conditions the tungsten rhenium heater coil is surrounded by a ceramic potting material to maintain a uniform temperature profile. Many cathodes are scrapped due to cracking in the potting which could lead to thermal hot spots. This study explores alternative ceramic potting materials that pass the stringent requirements of high end cathodes including slight expansion upon sintering. Ternary aluminum nitride based ceramics were proposed with calcium oxide, yttria, and alumina as additions. The standard potting material used by Semicon Associates was also profiled. It was found that a ceramic with composition 70.5 wt% Al2O3, 23.5 wt% AlN, and 6 wt% CaO expanded 1.71 volume percent compared to 0.81 percent expansion of the standard potting. This new Al2O3/AlN/CaO material was measured to have a thermal conductivity of 3 W/mK at 1200°C and the standard potting to have 12 W/mK at 1200°C. All other compositions considered contracted significantly upon sintering, disqualifying each as a viable alternative potting material. This paper is the result of a student project performed by University of Kentucky MSE480 Senior Design class in coordination with the sponsor Semicon Associates of Lexington, KY.