Ping-Yen Hsieh^1,2, Ying-Hung Chen^1,2, Ralf Bandorf^2,3, Kai Ortner², Ju-Liang He^{1,2 1}Feng Chia University, Taichung City, Taiwan ²Fraunhofer Innovation Platform for Surface and Production Engineering for Optical and Electrical Systems at the Feng Chia University (FIP-SPE@FCU), Taichung City, Taiwan ³Fraunhofer Institute for Surface Engineering and Thin Films IST, Braunschweig, Germany
Black metal, characterized by a highly porous metallic material, can significantly absorb light, appearing black in color. Due to its electrical conductivity and high specific surface area, black metal has attracted increasing interest for applications in optical imaging, electrochemical sensing, catalysis, and energy harvesting electrode. Gas flow sputtering (GFS) is based on hollow cathode discharge and involves an inert high-flux gas flow to blow high-density plasma and the accompanied cathode material from the source to the substrate for film deposition. By tuning proper deposition parameters, the GFS prepared coating exhibits high film growth rate with variable film structures ranging from porous to dense. For the former case, it would be a good candidate source for growing black metal films. In this study, black aluminum metal and titanium films were respectively deposited on aluminum metal foils to explore the possibility of serving as the porous electrode of electrolytic capacitor foils, alternatively as a new dry process to the existing electrochemical wet process. Experimental results show that the volume capacity of the GFS prepared aluminum porous electrode can reach 201.0 μF/cm²-μm, which is 12 times higher than that of commercial electrochemically etched foils of 16.7 μF/cm²-μm. After the electrochemical formation process, the unit capacity under a formation voltage of 10 V can reach 110.0 μF/cm². When in the case of the GFS deposited black titanium metal film as porous electrode, the volume capacity can further increase to 1795.0 μF/cm²-μm, which is 107 times higher than that of commercial electrochemically etched foils. The unit capacity can be as high as 640 μF/cm² with a formation voltage of 5 V. Hydration test results also confirm the stability of such GFS black metal prepared electrodes for capacitor foil purpose. These findings indicate that GFS to prepare black metal films for electrolytic capacitor foils is highly promising, with large-scale production underway for commercialization. Furthermore, other potential applications of GFS prepared black metal coatings are being explored.