Influence of humidity and temperature on the dielectric properties of thermally sprayed ceramic MgAl2O4 coatings

Thermally sprayed ceramic coatings can be used as electrically insulating materials for example in high temperature applications (e.g. fuel cells) or in other demanding conditions where ceramic-based solutions are needed instead of e.g. polymers. The dielectric properties of thermally sprayed ceramic coatings are strongly affected by external conditions. The aim of this paper is to characterize the dielectric properties of thermally sprayed ceramic MgAl₂O₄ coatings; especially the effects of ambient conditions on certain dielectric properties of thermally sprayed coatings are studied. DC resistivity at various electric field strengths as well as permittivity and losses at different frequencies is reported in the paper for MgAl₂O₄ samples made by three different thermal spray techniques. These measurements were performed at three temperatures as well as at two different relative humidities. The DC breakdown strength was studied at one condition. Due to the slightly open porous microstructure of the studied coatings, increasing humidity particularly increases the dc conductivity and relative permittivity.