Dielectric properties of materials are related to their microstructure, defects and compositional variations. Traditional impedance measurement of dielectric properties is an average performed on the length scale of the microwave wavelength, which is not sensitive to local structure and compositional variations. The nondestructive mapping technique of near-field scanning microwave microscopy (SMM) has been shown to be an effective technique for investigating the local dielectric properties variation.
The development of near-field SMM and its application in characterization of dielectric ceramics are presented in this work. The local surface dielectric properties of a variety of bulk specimens were characterized with SMM, while their microstructures were characterized with backscattered electron scanning electron microscopy (SEM) and polarized optical microscopy. The compositions and phases were identified by energy dispersive spectroscopy (EDS) analysis and X-ray powder diffraction (XRD). The local dielectric properties variations causing the contrast in SMM images were correlated to the local microstructures and chemical variations, such as defects, nonstoichiometric compositions, solid-solution, phase separations, and so on.
SMM characterization has been used to detect defects in single crystals, such as twinning structure in a LaAlO3 single crystal; to present topographic and grain boundary effects in bulk polycrystalline yttria stabilized zirconia (YSZ); to differentiate chemical variations, such as oxygen-deficient “cored” titania crystal, and Zn/Co varied BZCN312 matrices; to characterize inhomogeneities of dielectric properties in a co-fired CMT30/CMT40 ceramic; to discover a new phase with unknown dielectric properties, such as BZCN816 phase in BZCN312 matrices; to investigate stabilized components, such as La2/3TiO3 phase stabilized by LaAlO3 phase; to study solid solution, such as LT3-LAO solid solution and LAO-STO solid solution; to study phase separation and development, LaAlO3-TiO2 and CaTiO3-MgTiO3 diffusion couples; to observe critical feature size effect, such as in CT-MT eutectic system; and to rank the dielectric constant of dielectric materials in these demonstrated systems.
All of theses studies indicate that near-field scanning microwave microscopy is a powerful nondestructive technique to characterize local dielectric properties variations, and relate them to local microstructure and chemical variations. It permits one to easily study the dielectric properties of bulk ceramic materials.