Nanostructures from Hydrogen and Helium Implantation of Aluminum
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Nanostructures from Hydrogen and Helium Implantation of Aluminum Markus D. Ong1, Nancy Yang1, Ryan J. Depuit2, Bruce R. McWatters3, and Rion A. Causey1 1
Energy Nanomaterials Department, Sandia National Laboratories, California, P.O. Box 969, Livermore, CA 94551, U.S.A. 2 Department of Chemical Engineering, University of California, Santa Barbara, Santa Barbara, CA 93106-5080, U.S.A. 3 Radiation-Solid Interactions Department, Sandia National Laboratories, New Mexico, P.O. Box 5800, Albuquerque, NM 87185, U.S.A. ABSTRACT This study investigates a pathway to nanoporous structures created by hydrogen and helium implantation in aluminum. Previous experiments for fusion applications have indicated that hydrogen and helium ion implantations are capable of producing bicontinuous nanoporous structures in a variety of metals. This study focuses specifically on implantations of hydrogen and helium ions at 25 keV in aluminum. The hydrogen and helium systems result in remarkably different nanostructures of aluminum at the surface. Scanning electron microscopy, focused ion beam, and transmission electron microscopy show that both implantations result in porosity that persists approximately 200 nm deep. However, hydrogen implantations tend to produce larger and more irregular voids that preferentially reside at defects. Implantations of helium at a fluence of 1018 cm-2 produce much smaller porosity on the order of 10 nm that is regular and creates a bicontinuous structure in the porous region. The primary difference driving the formation of the contrasting structures is likely the relatively high mobility of hydrogen and the ability of hydrogen to form alanes that are capable of desorbing and etching Al (111) faces. INTRODUCTION With many renewable energy sources of intermittent nature, energy storage will be important for the implementation of clean energy. This study investigates a pathway to nanoporous structures created by hydrogen implantation in metals. Nanoporous materials have high specific surface areas, making them ideal as supercapacitor electrodes [1]. In experiments for fusion applications, formation of nanoporous films several micrometers thick was observed in tungsten surfaces exposed to helium plasma [2], aluminum exposed to helium [3], and beryllium surfaces exposed to hydrogen [4]. Nanoscale features were produced near the irradiated surfaces, and the resulting porosity created bicontinuous structures. Other routes to formation of nanoporous metals such as dealloying [5,6] or chemical reduction [7-11] work well with noble metals, but cannot be used with electropositive metals such as aluminum or tungsten. This study focuses specifically on aluminum and the resulting morphology after implantation of hydrogen and helium ions. The formation and presence of volatile alane (AlH3) contributes significantly to the resulting nanostructure when aluminum is implanted with hydrogen. The findings of complementary experimental and computational modeling of this system are presented here.
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