CERAMIC TO METAL SEALS FOR HIGH-TEMPERATURE THERMIONIC CONVERTERS

A rather broad investigation was carried on in the materials and methods which are considered possibly useful in forming ceramic to metal seals and composite structures for use in the environment of cesium-plasma thermionic converters. Initially, studies of five specific meands of joining refractory metals to alumina were undertaken, as well as studies of metallizing methods, of forming graded tungsten-alumina composites, and of the possible metals and ceramics which could be utilized best in structures operated at 2000 deg C. The five joining methods studied are: (1) Formation of a continuous electroplated layer of refractory metal across the adjacent surfaces of metal and ceramic. (2) Welding of a metal member to the ceramic by ultrasonic welding techniques; i.e., without heat. (3) Use of high temperature brazing alloys. (4) Formation of a bond between metal and ceramic by the diffusion of compatible intermediate materials into both structural members. (5) Welding of single oxide ceramics directly to refractory metals using electron-beam welding equipment. The study of metallizing methods was started because the first three of the joining methods listed require the use of a metallized surface of the alumina. It was determined that three of the five methods for joining refractory metals to alumina which were under study were not practically fessible. The methods considered impractical are: formation of a continuous electroplated joint, welding by ultrasonic energy, and welding with electron beam techniques. The studies on these methods were ended. Significant achievements have been made in the other studies. A metallizing process has been developed by which a strongly adherent sintered tungsten film can be applied to commercially available, high-purity, silica-free alumina. No fluxing materials or active metals which are susceptible to attack by cesium are used in this process. Bonding of alumina to molybdenum has been successful by several methods developed from the diffusion seal study. Some of these have been tested at 1500 deg C successfully, i.e., with neither structural failure nor loss of hermeticity resulting from the test. Methods of fabricating graded-composition, tungsten-alumina emitter and envelope stractures have been developed. Techniques for making brazed joints between refractory metals and high-purity alumina metallized with tungsten, using high temperature brazing alloys, are considered to be well enough developed for use of these structures near 1500 deg C. Diffusion joining methods for molybdenum or niobium and alumina are also considered to be processes which are immediately applicable to joining molybdenum or niobium and commercially available high-purity alumina for use at this temperature. Tungsten, rheniuan, hafnia, thoria, and yttria have been identified as the most promising materials for 2000 deg C service. The techniques developed for fabrication of tungsten-alumina composite bodies are considered to be directly applicable to formation of composite structures utilizing combinations of the materials for 2000 deg C service. Diffusion bonding of tungsten or rhenium to hafnia, thoria, or yttria demonstrates a high potential capability as a means of joining these materials for use at 2000 deg C. (auth