Non-destructive evaluation of ceramic porosity using terahertz time-domain spectroscopy

Abstract Dielectric constants of Al2O3 (alumina) ceramic samples have been measured with non-destructive terahertz time-domain spectroscopy. Using an effective medium approximation, we demonstrated that the relative permittivity can be used to evaluate the porous alumina samples, with porosity fractions ranging from 0% to 20% per volume. This result makes it possible to control sample porosity, a key material parameter related to mechanical strength under compression load, hardness and wear resistance, or thermal stability. We also employed terahertz time-domain spectroscopy to differentiate alumina samples fabricated with different grain sizes, or reveal a small fraction of impurities, such as 5%vol of ZrO2, embedded in non-porous alumina. These results demonstrated the potential of terahertz radiation for non-destructive characterization of low loss high dielectric constant ceramics.