Evaluation of aluminum nitride based RF lossy dielectrics for use in high power microwave devices

Summary form only given. For the past three years, new aluminum nitride (AlN) based ceramics have been under development to replace a hot-pressed ceramic composition made from beryllia (BeO) and silicon carbide (SiC). The BeO-SiC composite is currently the material-of-choice to provide RF loss and high thermal conductivity within the vacuum envelopes of microwave tubes. However, increasingly stringent BeO health and safety regulations and liability issues have caused qualified domestic manufacturers to cease production. Experimental evaluation of the new AlN materials in specific microwave tube configurations is being carried out. Loss buttons of the new materials are fabricated and diffusion bonded into S-band klystron cavities for cold testing. The loaded cavity Q's and frequencies are compared to those obtained with buttons made from the standard BeO-SiC composition. Materials that exhibit RF behavior similar to BeO-SiC are then selected for high-power testing at X-band to determine their suitability for use in the severe environmental conditions existing inside a high power microwave tube.