Microwave absorption characteristics of composites with tungsten coated filler particles

This study compares the dielectric relaxation, measured at microwave frequencies, of composites made with four distinct types of filler particles. The particles studied are 3Mtrade glass bubbles, with diameter approximately 60 mum, sputter coated with tungsten. Prior to sputter coating, one batch of glass bubbles was dried at 150degC and another at 350degC. The drying process removes bound water from the de-aggregating agent (Volanreg) used to coat the glass bubbles. Of these two batches, some particles were further coated with aluminum oxide (AlOx) to prevent conduction between the particles when in contact. To measure the permittivity, the particles were dispersed in a matrix of paraffin wax at various volume fractions. The electromagnetic properties were measured in the frequency range 2 to 18 GHz using a 7 mm coaxial reflection/transmission line method. Dielectric loss was observed in this frequency range for all four types of filler particle. The composites containing the particles coated with AlOx were observed to have a higher real and imaginary relative permittivity than those containing non-AlOx-coated particles. For AlOx-coated particles, the drying temperature of 150degC yielded stronger relaxation than that of 350degC. This behavior is attributed to traces of bound water remaining in the particles dried at 150degC and its effect on the morphology of the tungsten layer in these particles.