Dielectric properties of plasma sprayed silicates subjected to additional annealing

Several silicate materials were plasma sprayed and characterized by the authors in recent years from the point of view of their 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 and steatite 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 form of plates with a smooth surface. Part of samples was later annealed in air. These samples – after coverage by metal electrodes functioning as monoblock capacitors – were 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. It is shown that the relative permittivity of plasma as-sprayed silicates is less stable compared to bulk in the whole studied frequency range. However, thermal annealing modifies the structure much closer to the sintered bulk. This is reflected also in dielectric properties. Insulating ability of plasma-sprayed silicates with and without annealing is discussed in consequence with chemical changes and phase changes induced by annealing.