Effect of Surface Stresses on Microwave Surface Resistance and Its Impact for Cavity Perturbation Measurements

For microwave characterization of materials over a wide temperature range, it is often desirable to perform cavity perturbation measurements at elevated temperatures. Careful cavity design, involving the minimization of electrical contacts, ensures high measurement stability as the temperature varies. However, examining the Q-factor of a cylindrical cavity resonator during heating has shown that heat treatment can permanently modify the surface resistance of an aluminum surface. X-Ray diffraction measurements of Al-6082 confirm that the source of change can be due to changes in surface stress and the appearance of solution precipitates. Here we discuss the necessity to pretreat microwave cavities to stabilize their Q-factors in order to reliably measure material properties via cavity perturbation across a wide temperature range.