Jiri Kotlan

Dielectric properties of CaTiO3 coatings prepared by plasma spraying

Abstract This paper presents a study of dielectric properties, namely the relative permittivity and loss factor dependence on the frequency of a weak electric field. Perovskite CaTiO3 was studied in the form of coatings and self-supporting plates made by plasma spraying. A conventional gas stabilised plasma gun (GSP) as well as a water stabilised plasma gun (WSP) were employed. It was observed that plasma sprayed titanates exhibit a strong relaxation of permittivity and loss factor decrease with a frequency rise. These properties are influenced by spray technique and spraying parameters, but the relaxation character in general is preserved in all cases. The volume resistivity of the samples was studied as well. Several aspects of the structural features of plasma deposits, especially the phase composition, porosity character, and their influence on dielectric properties are discussed.

Photocatalytic and electrochemical properties of single- and multi-layer sub-stoichiometric titanium oxide coatings prepared by atmospheric plasma spraying

We studied the photocatalytic and electrochemical impedance properties of two different sub-stoichiometric titania powders deposited using plasma spraying. Two different commercial powders with markedly diverse mean size as well as size distribution were chosen. Thermal oxidation of these as-received powders was carried out to restore the O/Ti ratio to be 2.0. By this way, another two kinds of feedstock powders were obtained. Total of four kinds of feedstock powders were used in the experiments. All powders were sprayed using identical plasma spraying parameters. For some spraying runs, carbon steel served as the substrate, and for others, the previously sprayed titania coating from a different kind of feedstock. Combinations of single- and double-layer coatings were studied. Porosity, microstructure, phase composition, chemical composition, band gap based on reflectance measurement, and photocatalytic activity were examined. Electrochemical impedance spectroscopy demonstrated substantial differences between samples sprayed from the fine and coarse powders. Coatings from oxidized powders were slightly more photocatalytic. High spraying distance used for several of them was good for obtaining low band gap, whereas surface roughness and phase composition were not substantially different compared to standard spraying distance coatings.

Controlling Microstructure of Yttria-Stabilized Zirconia Prepared from Suspensions and Solutions by Plasma Spraying with High Feed Rates

Introduction of suspension and solution plasma spraying led to a breakthrough in the deposition of yttria-stabilized zirconia (YSZ) coatings and enabled preparation of new types of layers. However, their deposition with high feed rates needed, for example, for the deposition of thermal barrier coatings (TBCs) on large-scale components, is still challenging. In this study, possibility of high-throughput plasma spraying of YSZ coatings is demonstrated for the latest generation of high-enthalpy hybrid water-stabilized plasma (WSP-H) torch technology. The results show that microstructure of the coatings prepared by WSP-H may be tailored for specific applications by the choice of deposition conditions, in particular formulation of the liquid feedstock. Porous and columnar coatings with low thermal conductivity (0.5-0.6 W/mK) were prepared from commercial ethanol-based suspension. Dense vertically cracked coatings with higher thermal conductivity but also higher internal cohesion were deposited from suspension containing ethanol/water mixture and coarser YSZ particles. Spraying of solution formulated from diluted zirconium acetate and yttrium nitrate hexahydrate led also to the successful deposition of YSZ coating combining regions of porous and denser microstructure and providing both low thermal conductivity and improved cohesion of the coating. Enthalpy content, liquid-plasma interaction and coating buildup mechanisms are also discussed.