# Photoconductive TiO\({}_{2}\) Dielectrics Prepared by Plasma Spraying
## 1 Introduction  
Plasma-sprayed coatings are produced by introducing powder particles of feedstock material into a plasma jet, which melts them and propels them towards the substrate. The formation of a coating depends on the interaction between the droplet and the substrate or the previously deposited layers, i.e., spreading of the droplet, the formation of a lamella (called a \"splat\"), and its solidification. A difference in the degree of splat flattening results in differences in the shape and size distribution of porosity. These factors could also affect bonding between lamellae, which affects the cohesion of the coating markedly. In the case of the WSP system, feeding distance and spray distance can affect most of the above-mentioned features. Titanium dioxide (titania) is frequently sprayed using plasma torches of various designs [1; 2; 3; 4; 5]. The resulting coatings are very often oxygen-deficient. In materials research based on inorganic chemistry, a large amount of effort has gone into studying the Ti-O system, and the promising properties of several oxygen-deficient titanium oxide compounds [6; 7; 8; 9; 10; 11; 12] are well known. However, everyone working on the plasma spraying of TiO\({}_{2}\) and materials from the Ti-O system knows that many aspects concerning the spray process remain unsolved. Examples of questions arising in connection with the phenomenon of TiO\({}_{2}\) sensitivity to oxygen loss are as follows:  
* How can the plasma spray system be set up to ensure that a Ti\({}_{x}\)O\({}_{y}\) coating (with x and y being exactly known) is obtained from TiO\({}_{2}\) powder? * Once one has obtained a Ti\({}_{x}\)O\({}_{y}\) coating, how is the responsibility for a change in the addressed property (e.g., electrical resistance) distributed between the phenomena at the (i) lattice level, i.e., plasma-induced oxygen-deficient stoichiometry, and (ii) the macroscopic level, i.e., the specific lamellar character of the sprayed coating with its flat pores and crack network? * Is there an important difference in the coating's oxygen content when the starting powder is in the rutile or anatase phases of TiO\({}_{2}\)? All of these questions are also closely associated with the spraying of coatings targeted toward electrical, optical, and photocatalytic studies and applications. The family of titanium suboxides TiO\({}_{\rm x}\) (1.66 \(<\) x \(<\) 2.00) can be subdivided into several groups [10]. Important intervals of the x value seem to be 1.93 (or 1.89) to 2.00 and 1.75 to 1.89. Within these intervals, individual compounds exist. At above 1.93 (or 1.89), so-called \"higher suboxides\" have ordered structures with (132) crystallographic shear planes. Near to x = 2.0, the (132) crystallographic shear planes are dispersed with varying spacing in the rutile matrix. Due to small differences in the oxygen content of the individual phases, these are difficult to identify. They cannot be segregated in polycrystalline TiO\({}_{2}\) compacts [10], but they can be formed in plasma-sprayed coatings. The composition range of 1.75 \(\leq\) x \(\leq\) 1.89 is a group in which a homologous series of discrete oxides called \"Magneli phases\" was originally described [12]. These phases are characterized by crystallographic shear (121) planes. The oxides of the series are built up of slabs of a rutile-type structure--the slabs are of infinite extension in two dimensions and of characteristic finite widths of (n TiO\({}_{6}\)) octahedra in the third direction [10]. Below x = 1.75, other individual phases exist, but based on research into plasma composition [13], it is nearly impossible to have these phases in a coating in a pure form. The formation of the Magneli phases from the melt relates to destruction of the shear plane structure. It might be preserved in unmolten particles after the spray process only. Furthermore, the O/Ti ratio in the spray powder can be increased by oxidation (e.g., if spraying is conducted in the atmosphere) and decreased by reduction (e.g., if hydrogen is a plasma gas component).