Frequency Dependence of Microwave Permeability in Composite Materials Filled with Gadolinium Particles: Effects of Temperature and Filler Fraction

The microwave permeability of composites containing microsized gadolinium powder in paraffin wax was studied as a function of frequency, temperature and gadolinium fraction for a series of composites with a powder volume fraction of 5% to 50%. The constitutive parameters of the composites were measured by the Nicolson-Ross-Weir technique in a standard 7 × 3 mm coaxial line in the frequency range of 0.1 to 10 GHz and the temperature range of 5 to 26 °C. In the indicated ranges, the permittivity was independent of frequency and temperature, and the real part of the permittivity only depended on the gadolinium powder volume fraction. It was found that the peak of the magnetic losses shifts towards lower frequencies with an increase in temperature. The Curie temperature of the composites found from the temperature dependence of the static permeability was close to 15 °C, being practically independent of the gadolinium volume fraction. To retrieve the intrinsic permeability of the filler particles, the Odelevsky mixing rule was used. The measured dependence of the static permittivity on the gadolinium volume fraction was in good agreement with the fitting by the Odelevsky mixing rule. Using the values of the percolation threshold and the depolarization factor obtained as a result of the fitting of the static permittivity, the frequency dependence of the intrinsic permeability of the filler particles was retrieved. The temperature effect on the intrinsic permeability was also analyzed. The obtained results may be useful for designing microwave devices, including temperature-tunable microwave screens and sensors.