Dielectric properties of plasma-sprayed silicates

Several silicate materials were plasma sprayed and characterized by the authors in recent years from the point of view of their sprayability, chemical and phase compositions, microstructure and mechanical as well as thermal properties. Present work is concerned with selected dielectric properties of these deposits. Synthetic mullite, steatite and spodumene as well as natural olivine–forsterite were plasma sprayed using the water-stabilized plasma system (WSP®). The deposits were striped-out, ground and polished to produce samples in a shape of planparallel plates with a smooth surface. These samples—in principle monoblock capacitors—were then tested in the alternative low voltage electric field to measure capacity and loss factor in the frequency range from 200 Hz to 1 MHz. Relative permittivity was calculated from the measured capacity. Volume resistivity was measured in the direct electric field. In addition dielectric strength of steatite was measured at 50 Hz ac. It is shown that the relative permittivity of plasma-sprayed silicates is less stable compared to bulk in the whole studied frequency range. Insulating ability of plasma-sprayed silicates is discussed in comparison with the bulk ceramics with the same composition. Paths of the electrical breakdown of plasma-sprayed steatite are observed by microscopy to help to resolve the failure mechanism.