Dielectric properties and vacancy‐like defects in plasma‐sprayed barium titanate

Positron annihilation spectroscopy was employed for investigation of vacancy‐like defects in plasma‐sprayed barium titanate. Defect studies were combined with measurement of dielectric properties of barium titanate coatings. Samples prepared by gas‐stabilized plasma spray (GSP) torch and by plasma torch with the hybrid water‐argon stabilization (WSP‐H) were studied. Processing parameters were selected so that GSP coatings were sprayed in reductive conditions, whereas WSP‐H coatings were prepared in oxidizing environment. As‐sprayed GSP coating is dark, whereas WSP‐H one is light. The dielectric properties of WSP‐H coating are superior to those for GSP one. Defect studies revealed that both GSP and WSP‐H coatings contain titanium vacancies. However, GSP coating contains in addition a considerable concentration of oxygen vacancies. Some fraction of oxygen vacancies in GSP coating is coupled with titanium vacancies forming partial Schottky defect. The structure of WSP‐H coating is less disordered and contains only a low concentration of oxygen vacancies. This is consistent with reductive and oxidizing conditions in GSP and WSP‐H spraying, respectively. Annealing at elevated temperatures in air leads to removal of oxygen vacancies which are filled by oxygen diffusing into the samples.