Fred Fietzke

Pulse-magnetron-sputtered long-term superhydrophilic coatings have been synthesized to functionalize the surfaces of solid-state cooling devices, e.g., electrocaloric heat pumps, where not only a complete wetting of the surface by a fluid is intended, but also fast wetting and dewetting processes are required. The coatings consist of a (Ti,Si)O₂ outer layer that provides lasting hydrophilicity thanks to the mesoporous structure, followed by an intermediate WO₃ film that enables the reactivation of the wettability through visible light irradiation, and a W underlayer which can work as a top electrode of the electrocaloric components thanks to its suitable electrical and thermal conductivity properties. Process parameter optimization for each layer of the stack as well as the influence of the microstructure and composition on the wetting properties are presented. Finally, water contact angle measurements, surface energy evaluations, and a contact line dynamics assessment of evaporating drops on the coatings demonstrate that their enhanced wetting performance is attributed not only to their intrinsic hydrophilic nature but also to their porous microstructure, which promotes wicking and spreading at the nanometric scale.

This study focuses on the use of superhydrophilic titanium dioxide (TiO₂) coatings applied to the surfaces of water-bearing systems to prevent surface colonization and biofilm formation. Biofilms in water-bearing systems are a problem in many industrial areas and are associated with risks to hygiene and health, material damage, and high costs for cleaning and maintenance. We investigated the suitability of TiO₂ coatings activated by UVA irradiation to achieve a superhydrophilic surface. The well-adherent coatings were deposited on flat and curved substrates (stainless steel, Al₂O₃) by pulsed magnetron sputtering. Surface characteristics, wettability, and the influence on microbial surface colonization were evaluated by WCA measurements, SEM, and XRD. For microbiological evaluation, Escherichia coli and Staphylococcus warneri were used. An adapted and specialized regime for sample conditioning and testing was developed that allows comparability with upcoming studies in this field. The superhydrophilicity was stable for up to 4 days, and an additional UVA reactivation step revealed comparable results. The microbiological studies proved a successful prevention of bacterial colonization on the activated coatings, which is attributed to their superhydrophilicity. The results demonstrate the potential of UV-activated TiO₂ as a long-term coating of water-bearing systems, like pipes, on which it assists in avoiding biofilm formation.

Hollow cathode arc discharges have been studied for some decades in basic research and different fields of practical application. Especially in the field of physical vapor deposition (PVD) they are often used for metal evaporation and the generation of plasma. Problematical aspects were found in the high throughput of working gas necessary for stable operation and in the inhomogeneous, locally concentrated ionization efficiency. A promising way out of this dilemma was found in a special arrangement of hollow cathode and annular anode in an axial magnetic field. This configuration allows a drastic reduction of working gas flow without loss of discharge stability. Moreover, with reduction of gas flow a strong increase in ion current density was observed together with a highly expanded volume of plasma plume.