Dielectric and electrochemical properties through-thickness mapping on extremely thick plasma sprayed TiO2

An extremely thick TiO2 deposit was plasma sprayed using a water stabilized (WSP) spray system. Inexpensive natural rutile TiO2 was used as feedstock powder. Reaching the required enormous thickness, 15 mm, was relatively fast thanks to a high feed-rate WSP gun, much more powerful than conventional spray systems. The massive deposits were cut into slices aligned parallel with the substrate and studied. Their microstructure is lamellar with defects typical for plasma sprayed ceramics. Differences between the near substrate slices, medium space slices and near surface slices of the deposit were found namely in physical properties. Besides the microstructure, dielectric spectroscopy and electrochemical impedance spectroscopy were in the focus of investigation. The role of the grain interior and grain boundaries in the charge transport and the role of dipoles of various sizes are different near the substrate, in the middle and near the surface. The deposit was annealed to remove the oxygen sub-stoichiometry of the semiconductive as-sprayed titania and turn the behavior of the material to dielectric. Various differences are present also in the near substrate, medium space and near surface slices of the annealed deposit since they are inherited from the as-sprayed state, which was not completely overlaid by annealing.