Lossy material

Abstract

Dielectrics that exhibit electromagnetic loss at microwave frequencies are used extensively in coupled-cavity traveling-wave tubes (CCTWTs), and less frequently in klystrons, gyrotrons, and gyro-klystrons as rf terminations, to suppress unwanted oscillations, and to reduce rf cavity Q's. Over the years, a number of rf lossy materials have been developed for these applications, including flame-sprayed Kanthal (an iron-chromium-aluminum alloy), Al/sub 2/O/sub 3//SiC, AlN/SiC, MgO/SiC, and BeO/SiC. In these lossy dielectrics, absorbed microwave energy is converted into heat, which must be removed from the circuit; consequently, for high average power applications, the thermal conductivity of the lossy material is critical. At 130 W/m-K, BeO/SiC has a room temperature thermal conductivity that is more than a factor of two higher than the next best commercially-available lossy dielectric; with a loss tangent of >0.25 for frequencies above one GHz. BeO/SiC has become the vacuum electronics industry's material of choice for applications with high average power densities. Unfortunately, under some circumstances, beryllium oxide presents a health hazard. Indeed, the principal disadvantage of BeO/SiC is the increased processing and handling requirements associated with hazardous material. The main goals of this paper include qualifying a new source of BeO/SiC, evaluating new alternative materials, and developing a common standard for the evaluation of the dielectric properties of lossy ceramics.