# Photoconductive TiO\({}_{2}\) Dielectrics Prepared by Plasma Spraying
## 1 Introduction
As the reduction proceeds, a neighboring Ti\({}^{4+}\) may change its charge state to Ti\({}^{3+}\), and the other electron would become a free-moving charge carrier. The ivory-yellow-white color of the fully stoichiometric version turns into a progressively darker blue-gray-black color of a heavily anion-depleted Ti-O sample. The color change is caused by the absorption of red light (photons with energy below the band gap [14; 15]) by the conduction electron [11]. Titanium oxides with temperature-stable and environmentally stable dielectric properties characterized by high relative permittivity and low dielectric loss have recently attracted attention due to their impact on the fabrication of novel microelectronic devices and microwave communication systems [16]. Titanium dioxide coatings are widely used in various applications, such as photo-catalysis, bone implants, electric devices, renewable energy systems' components, and gas sensors [1]. Taking this into account, a gradual transition of TiO\({}_{2}\) coating materials from protective surfaces to active functional components has been welcomed and promoted by the whole thermal spray industry. The plasma spraying of TiO\({}_{2}\) using gas-stabilized plasma systems is well governed [1; 13; 14; 15; 16]. In addition to the high cooling rate, low heat transfer to the spray powder, resulting from low hydrogen content and low plasma power, increases the content of anatase in Ar/H\({}_{2}\) plasma-sprayed coatings [10]. Coatings deposited at a higher temperature are characterized by reduced porosity and increased density [17]. In the case of TiO\({}_{2}\), the anatase-to-rutile phase transition is observed to occur between 500 \({}^{\circ}\)C and 900 \({}^{\circ}\)C, with the exact temperature being a sensitive function of sample preparation conditions [18; 19]. At the Institute of Plasma Physics ASCR (IPP), plenty of various spray parameters have been tested to produce TiO\({}_{2}\) coatings from a conventional crushed [18] powder as well as agglomerated nano-powder [5], and various properties of the coatings have been studied. Plasma spraying belongs to the well-established branch of thermal spraying. It has been proved that water-stabilized plasma (WSP) spraying is a complementary and useful method of thermal spraying. Plasma units based on this principle produced in the Czech Republic have been successfully applied in several countries in Europe, the USA, and Japan. High energy density and high temperatures result in a very high throughput, in some cases being one order of magnitude higher than that of gas-stabilized systems. Therefore, the best use of WSP is for large-area coatings, for the production of self-supporting ceramic parts, and for powder processing. Water-stabilized plasma exhibits substantially different plasma properties in comparison to common gas-stabilized torches and, thus, provides unique capabilities in technological applications. The WSP torch is based on a vortex-stabilized atmospheric DC arc. Water, injected tangentially into the arc chamber, creates a vortex that confines the arc to the center of the chamber. The arc burns between a rod cathode and a rotating copper disc anode, which is placed outside the main body in front of the exit nozzle. The cathode is either a consumable graphite rod or a copper rod with a zirconium tip. Both the anode and the cathode are cooled by water. Continuous evaporation of water walls inside the arc chamber leads to overpressure, which accelerates the plasma (consisting of substances produced due to heating, dissociation, and ionization of steam) towards the exit nozzle, and the plasma jet is injected rapidly into a stagnant environmental atmosphere. The sprayed material is introduced into the plasma jet by one or more external feeders. The feeding gas can be compressed air or an inert gas. Rutile TiO\({}_{2}\) melts at 1912 \({}^{\circ}\)C and subsequently dissociates to the liquid Magneli-phase compositions Ti\({}_{4}\)O\({}_{7}\) and Ti\({}_{3}\)O\({}_{5}\).