AlN-based lossy ceramics for high average power microwave devices: performance–property correlation

AlN matrix-based ceramics were manufactured by hot pressing, with the objective of producing electromagnetically lossy materials in the 0.5–20 GHz region. These materials are designed to replace BeO–SiC lossy ceramics in high average power vacuum electronic devices to control electromagnetic instabilities, increase bandwidth, reduce reflections, and suppress unwanted modes. During the development of the candidate materials, complex permittivity measurements of samples were obtained using complex reflection, transmission, and cavity measurement techniques. Corresponding lossy buttons were bonded into klystron cavities and the resulting resonant frequency and cavity quality-factors were measured and compared with the measured material permittivity. Although trends could be established, the measured complex permittivity was not sufficient to completely predict the material performance in the cavities, due to large changes in cavity field distributions, material anisotropy, or uncharacterized metal-ceramic interface physics. High-power functional test results of AlN- relative to BeO-based materials and elevated temperature complex permittivity data of selected materials are presented.