Electrical impedance characterization of ceramic-matrix composites with various inclusions

A thorough understanding of the AC electrical behavior vs. microstructure relationships in short fiber-reinforced composites should enable the use of AC-impedance spectroscopy (AC-IS) as a diagnostic tool to characterize composite microstructure. This work explores the use of an electrical AC-IS technique to characterize microstructure of composites with various inclusions, including short fibers, disc-shaped inclusions, fiber-bridged cracks, and carbon nanotubes. AC-IS is sensitive to the presence of conductive fibers and other shapes of inclusions (e.g., spheres or disc-shaped inclusions, both conductive and insulating) in specific type of composites. The requirements for such composites include a highly conductive inclusion in a moderately conductive matrix, and a high-impedance interface or coating between them. Due to the ease of fabrication, cement-based composites were used as a model system to establish the AC-IS characterization method. However, the developed AC-IS technique is quite general and can be extended to other ceramic-matrix composites satisfying the above requirements. Below percolation, composite theory describes the relationship between the composite conductivity and volume fraction of the inclusions in terms of the intrinsic conductivity, which is a function of shape, orientation and electrical properties of the inclusions. Such relationships (for example, for spherical inclusions, fibers, and oblate spheroidal inclusions) can be related to the resistance values derived from the AC-IS measurements to establish a foundation for AC-IS electrical characterization. Based on the intrinsic conductivity approach, a universal equivalent circuit model was developed to quantitatively describe the impedance response of composites containing particulates or discontinuous fibers. Deviations from the behavior predicted by the model is an indication of inhomogeneity and/or preferred orientation of the inclusions. The various microstructural issues that were characterized by the AC-IS technique in the present work include fiber dispersion, orientation of disc-shaped inclusions, damage accumulation, and the degree of fiber percolation.