Effect of porosity and microstructure on the microwave dielectric properties of rutile

The dielectic properties of rutile were strongly affected by porosity and microstructure. The relative permittivity, loss tangent, and temperature coefficient of resonant frequency of titanium oxide in microwave frequency were explored over a broad density range which was controlled through sintering temperature. Permittivity values range between 15 and 115 as the sintering temperature increased from 500 to 1550 °C. The measured permittivity were compared with several types of mixing rules which assumed titania and pores as the constituents. A coherent potential formula was modified to produce the best fit over the largest porosity range. General constitutive formulas were also developed to describe tanδ and TCF and these models work well for intermediate porosity ranges (0.08 < Pore fraction < 0.4); however there were significant deviations at the high and low porosity ranges (Pore fraction < 0.08 and > 0.4). The deviations were attributed to local reduction of the rutile grains and grain boundaries.