Non-destructive characterisation of structural ceramics using impedance spectroscopy

Abstract

The aim of this project was to explore the potential applications of impedance spectroscopy (IS) in the non-destructive characterisation of structural ceramics. A major advantage in the use of the IS technique is its capability in distinguishing the properties of different microstructural origins in materials, e.g., grains and grain boundaries, etc. In this thesis, a review of the theoretical aspects of IS is presented. An analytical approach and numerical analyses are conducted to illustrate how impedance spectra become resolved and why the spectral resolution is dependent on the representation formalism as well as on the difference between the electrical properties of different microstructural features. Three categories of structural ceramics, i. e. A1203/SiC nanocomposites, thermal barrier coatings and clay-based ceramics, were used as the model materials for this IS study. Both sintering and degradation phenomenon have been examined. Conventional analytical techniques, such as SEM, XRD, EDS, TGA and dilatometry, were used to aid in the understanding of IS and to prove the reliability of impedance measurements. The research results indicate: * For A1203/SiC nanocomposites, IS can be used to examine the conducting mechanisms for the materials containing different quantities of SiC particles. The content of SiC can be correlated to the impedance spectral features and dielectric constant of the material. The oxidation scale formed at the surface gives a separate relaxation process. Based on the capacitive effect of this relaxation process, the thickness can be determined non-destructively using IS. * For thermal barrier coatings, three relaxation processes can be found in the impedance spectra, which correspond to the top coating, oxidation layer and microstructural defects in the top coating, respectively. The thickness of the oxidation layer can be quantitatively related to the diameter of the corresponding semicircle in electrical modulus spectrum. In the meantime, the top coating degradation could also be monitored using IS. * For clay-based ceramics, the impedance spectra consist of a high frequency semicircle and a low frequency tail, which correspond to bulk effect and electrode effect, respectively. The variation of the bulk conductivity with sintering time can be quantitatively correlated to the densification of the material during sintering. The electrode effect tail is directly related to the capacitive effect of the electrode/specimen interface, which could be an effective indicator of the electrode temperature. Therefore IS is a useful technique for non-destructive characterisation of structural ceramics.