Broadband Dielectric Measurement Techniques and Systematic Dielectric Studies of Ceramic Composites for Vacuum Electronics Applications

Summary form only given. Ceramic materials, including ceramic composites, are used extensively in vacuum electronic (VE) devices for windows, electrode and structure supports, and for controlling electromagnetic dispersion and losses. While many of the more common low-loss materials are well characterized, detailed dielectric studies of specialized lossy ceramics of particular importance to VE devices are much less common. Furthermore, the measurement techniques and systems commonly used with low-loss dielectrics need to be modified to provide accurate data, particularly in W-band. This paper will first present broadband (0.1-18 GHz) room temperature measurements of the microwave properties of a variety of vacuum compatible lossy ceramics, including those based on SiC or carbon imbedded in BeO and AlN ceramic matrices. The microwave properties of a family of lossy ceramics composed of pyrolytic carbon in an aluminum silicate matrix ceramic that exhibits tailored frequency response and exceptional dielectric relaxation strength will be highlighted. The impact of changes in the synthesis conditions on dielectric properties, and the correlation between microstructure and frequency response, will be explored. The observed dielectric behavior will be compared to the results of first-principles, microstructure-level simulations of composite dielectric properties. A second portion of the paper will deal with measurement techniques for accurately obtaining the W-band (75-110 GHz) dielectric properties of lossy ceramics. This will include improved WR-10 waveguide-based, room temperature measurements of ceramic composites and preliminary results of variable-temperature measurements based on a resonant cavity system