Internal structure of the electrodeposited nanocrystalline Al–Mg alloy dendrites Sankara Sarma V. Tatiparti ⁎, Fereshteh Ebrahimi Materials Science and Engineering Department, University of Florida, Gainesville, FL, 32611, USA abstract article info Article history: Received 20 April 2011 Accepted 1 May 2011 Available online 4 May 2011 Keywords: Al–Mg Electrodeposition Porosity Internal structure Dendrite Porous nanocrystalline supersaturated face centered cubic (fcc)-Al(Mg) dendrites with globular morphology were produced via electrodeposition. The cross-section of the globules revealed compact–disperse–compact structure along the growth direction. Initially compact globules formed due to high potential (or current density) which decreased eventually resulting in disperse-entity growth. Overlapping spherical diffusion zone formation over the disperse-entities was attributed as a reason for the compact growth at later stages. The internal structure of the globules was explained by the global potential (E)–time curve and the estimated local current densities ahead of deposit front. A growth mechanism for globular morphology was proposed using the results presented. © 2011 Elsevier B.V. All rights reserved. 1. Introduction Electrodeposition is a versatile technique to produce metal/alloy- based dendrites with different properties. Among these, dendrites consisting “cauliflower-like” or “globular” morphology received wide attention [1,2] for their suitability in applications like H 2 O 2 decom- position, O 2 reduction/evolution catalysis (Ni–Co) [3] and H 2 -storage (Al–Mg) [4]. Most of these applications require large specific surface area which can be obtained through porous dendrites. However, electrodeposition of porous dendrites attracted less attention. Fundamentally, electrodeposition is a bottom-up process where dendrites form via several nucleation-growth events. To introduce porosity in dendrites these events need to be hindered. The electrode- position of Co resulted in agglomerates the cross-sections of which suggested a compact–disperse–compact structure imparting porosity to these agglomerates [5]. The compact deposit formation at later stages was explained by possible overlapping spherical diffusion zones forming locally above the disperse-entities. However, a detailed mechanism of the compact–disperse–compact structure formation is yet to be investigated and is necessary for controlling the dendrite porosity. Although the current density/potential at global level decides the general shape of deposit, the internal structure of morphology is governed by the local current density/potential [6] which is extremely difficult to quantify. We were able to produce nanocrystalline, supersaturated Al–Mg dendrites with globular morphology for H 2 -storage application [7]. Externally this morphology looks nearly spherical suggesting isotropic growth. However, interestingly, the internal structure revealed porosity due to compact– disperse–compact structure formation. In this study we investigated the internal structure of the globular morphology using E–time curve collected at global level and the estimated local current densities over the growth front. A growth mechanism was proposed for the globular morphology using the results presented. 2. Experimental Al–Mg dendrites were electrodeposited using an organometallic- based electrolyte in a rotating-cylinder-cell-setup (200 rpm) for 15 min at 60 °C and current density: i applied = 150 mA cm -2 according to a previously reported process [7,8]. Mg was introduced into electrolyte by pre-electrodeposition process [9]. The deposits were cleaned [8] and characterized using JEOL 6400 scanning electron microscope (SEM) for morphology. The cross-sections along the length of the dendrites were prepared by slicing the dendrites longitudinally with 100–300 pA Ga-ion-current using Dual-Beam Strata DB235 Focused ion beam milling (FIB). 3. Results and discussion Fig. 1(a) shows the SEM image of a typical Al–Mg deposit in as- deposited condition. Dendritic nature of the deposit is evident from the figure suggesting that i applied N i L (limiting current density = 38 mA cm -2 [7]) and indicates the diffusion controlled nature of the deposition process. The global diffusion layer thickness was estimated for the rotating-cylinder-cell-setup using Eq. (1) [10]: δ = 12:64d 0:3 ν 0:344 D 0:356 M V -0:7 ð1Þ where δ: global diffusion layer thickness, d: electrode diameter (6 mm), v: kinematic viscosity of electrolyte (organometallic Materials Letters 65 (2011) 2413–2415 ⁎ Corresponding author at: General Motors Technical Center India Pvt. Ltd., 3rd Floor, Creator Building, ITPL, Bangalore, 560 066, India. Tel.: +91 80 4118 4000; fax: +91 80 4115 8562. E-mail addresses: sankara@ufl.edu, sankarasarma.tatiparti@gm.com (S.S.V. Tatiparti). 0167-577X/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2011.05.001 Contents lists available at ScienceDirect Materials Letters journal homepage: www.elsevier.com/locate/matlet