Radiofrequency and Microwave Dielectric Properties of Plasma Sprayed and Annealed Thick Layers of Titanium Dioxide
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Radiofrequency and Microwave Dielectric Properties of Plasma Sprayed and Annealed Thick Layers of Titanium Dioxide Pavel Ctibor1,2 • Josef Sedla´cˇek1 • Va´clav Papezˇ1 • Frantisˇek Luka´cˇ2
Submitted: 27 March 2020 / in revised form: 28 May 2020 Ó ASM International 2020
Abstract Titanium dioxide (TiO2) was plasma sprayed using a high feed-rate spray system with hybrid water– argon stabilization torch (WSP-H). TiO2 powder agglomerated from nanometric particles was used as the feedstock. The produced coating had thickness over 4 mm, to be comparable with bulk ceramic bodies. The deposit was removed from the substrate and annealed in air to reoxidize the oxygen deficient as-sprayed titania and to obtain material with a proper dielectric behavior. Besides the microstructure studies, radiofrequency dielectric spectroscopy and microwave dielectric spectroscopy were in the focus of investigation. The approach to annealing of an extremely thick coating is studied: The significance of annealing of a large-area thick coating is discussed, as the role of grain interior and grain boundaries in the charge transport is influenced. Relative permittivity er 94 and the Q*f product 12,800 GHz were reached at frequency 4 GHz, whereas the frequency response under 1 MHz was mapped continuously and the values (i.e., low and stable loss tangent) confirmed that a sprayed and air-annealed TiO2 is able to approach or also overperform dielectric parameters typical for TiO2 sintered bulk.
Electronic supplementary material The online version of this article (https://doi.org/10.1007/s11666-020-01065-y) contains supplementary material, which is available to authorized users. & Pavel Ctibor [email protected] 1
Department of Electrotechnology, Faculty of Electrical Engineering, Czech Technical University in Prague, Technicka´ 2, 166 27 Prague 6, Czech Republic
2
Institute of Plasma Physics ASCR, v.v.i, Za Slovankou 3, Prague, Czech Republic
Keywords TiO2 annealing electrical properties microstructure plasma spraying
Introduction Plasma spraying of robust self-supporting plates (SSP) as well as tubes of various wall thickness-to-diameter ratios by a high feed-rate water-stabilized plasma spray system (WSP) has been performed in the past. First results introduced Al2O3, ZrO2, mullite (Al6Si2O13) and zircon (ZrSiO4) as the multi-purpose materials (Ref 1). Later on, various other materials such as garnets, basalt, forsterite (Ref 2) or diopside (Ref 3) were sprayed and tested. TiO2 was applied routinely for spraying of self-supporting tubes with numerous applications in chemical industry and as refractory components in glass processing. For photocatalytic use, thinner TiO2 coatings on flat substrates were more typical (Ref 4, 5). Titania coatings from agglomerated nanopowder were documented to exhibit rather good and stable mechanical properties despite the actual spray system parameters when a high-enthalpy torch is used (Ref 6), whereas sensitivity on in-flight temperature was described for a low enthalpy torch (Ref 7
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