Plasma Enhanced Electrochemical Preparation for Photo-Luminescent Metal Oxide Films on Al Foil

Electrochemical coating technologies of metal surface have been widely used for decades in various industries, because it provides the thick and anti-corrosive films with a simple and cheap process [1]. Especially, plasma electrolytic oxidation (PEO), which is a high electrical power-induced technology, has much attention in recent years due to their excellent film property. The PEO produces adherent ceramic-like coatings, which are beneficial to enhance the corrosion and wear resistance, so the oxidized film can be applied in many industrial fields [2-4]. During PEO process, a high–voltage discharge zone is generated with spark or arc plasma micro-discharge, and the thick ceramic coating is directly formed on the surface of metals together with elements in the electrolyte. At the discharge zone, the extreme environments with high temperature and pressure are generated, and estimated temperature is around 8000 K at a hot core and 1800 K at a cold circumferential area [5].

 In this study, we applied the plasma discharge phenomenon in PEO process to generate the photo-luminescent oxide film by adding the rare-earth ions in the electrolytes, and the white light emitting oxide film was successfully formed on aluminum foil. So far, the luminescent film was fabricated by screen printing technique, combustion CVD, spin coating, sol-gel soft lithography, and radio-frequency magnetron sputtering, but those kinds of phosphor films showed many problems such as poor uniformity of film thickness and low adhesion to the substrate [6-8]. In case of PEO process, the rare-earth ions are directly incorporated during oxidation process, so well adhesive luminescent films can be obtained.

 In order to prepare photo-luminescent oxide film, the plasma electrolytic oxidation of Al foil was conducted in rare-earth ion containing base electrolyte with constant current density of 3 A/dm² using DC pulse power supply. Various electrolytes and rare earth ions were employed and their effects on the photoluminescence properties of the obtained films were examined. To find out the correlation of photo-luminescence and micro arc generation in early stage, cathode-luminescence properties were investigated in the films formed around break-down voltage with SEM images. When europium was present in the electrolyte, a white light emitting ceramic-like film covering from NUV to visible was formed on aluminum foil (Fig. 1). The film was mainly composed of γ-Al₂O₃, and the emission spectrum consisted of two emission peaks centered at 420 nm and 500 nm with a broad FWHM (~197.6 nm). The origin of these emission peaks will be discussed in this presentation. In addition, the effects of electrolyte composition and rare-earth ions on the phase content and photoluminescence properties of the Al₂O₃films will be presented.

Reference

[1] G. Helen Annal Therese and P. Vishnu Kamath, Chem. Mater.,12, 1195 (2000)

[2] A. Kuhn, Met. Finish, 101, 44 (2003)

[3] X. Nie, E. I. Meletis, J. C. Jiang, A. Leyland, A. L. Yerokhin, and A. Matthews, Surf. Coat. Technol., 149, 245 (2002)

[4] C. Blawert, W. Dietzel, E. Ghali, and G. Song, Adv. Eng. Mater., 8, 511 (2006)

[5] A. L. Yerokhin, X. Nie, A. Leyland, A. Matthews, and S. J. Dowey, Surf. Coat. Technol., 122, 73 (1999)

[6] K.Y. Jung, Y.C. Kang, Mater. Lett, 58, 2161 (2004)

[7] Z.T. Kang, Y. Liu, B.K. Wagner, R. Gilstrap, M. Liu, C.J. Summers, J. Lumin., 121, 595 (2006)

[8] W.-H. Chao, R.-J. Wu, T.-B. Wu, J. Alloys Compd., 506, 98 (2010)