The role of grain boundaries in the mechanism of plasma immersion hydrogenation of nanocrystalline magnesium films

doi:10.1016/j.apsusc.2005.06.034

Applied Surface Science

Volume 252, Issue 12, 15 April 2006, Pages 4202-4208

https://doi.org/10.1016/j.apsusc.2005.06.034 Get rights and content

Abstract

In this paper, attention in focused on the nanostructured magnesium films for hydrogen storage. It is shown that 2 μm thick Mg film is transformed into MgH₂ film under high-flux and fluence hydrogen plasma immersion ion implantation at 450 K for 15 min. All hydrogen desorbs at temperature about 530 K, which corresponds to the decomposition of MgH₂ → Mg + H₂↑. The macroscopic and microscopic observations show that magnesium film undergoes a high deformation and restructuring during hydrogenation–dehydrogenation reaction. The suggested hydrogenation model is based upon the incorporation of excess of hydrogen atoms in grain boundaries of nanocrystalline Mg film driven by the increase in surface chemical potential associated with the implantation flux. The results provide new aspects of hydriding of thin nanocrystalline film materials under highly non-equalibrium conditions on the surface.

Introduction

Search for materials to store hydrogen in hydride forming systems is a subject of research for several decades. Nanostructured materials are distinguished from conventional polycrystalline materials by the larger fraction of grain boundaries and hence the larger percentage of surface-like atoms. Surface properties start to determine bulk properties [1]. Clearly, the interactions between bulk properties of the material and its free surface and internal surfaces properties become decisive. Modern technologies allow tailoring fundamental properties of materials by decreasing the length scale to the nanoscale.

In this paper, attention is focused on the nanostructured magnesium films for hydrogen storage. Magnesium is known as a possible hydrogen storage material due to its high hydrogen storage capacity (MgH₂ corresponds to 7.6 wt.% H). However, pure magnesium metal has poor hydrogenation characteristics: the kinetics of hydrogenation–dehydrogenation is very slow and the reaction occurs only at very high temperature [2].

Various attempts have been undertaken to improve the kinetics of magnesium by modifying its surface reducing the particle size of a powder, controlling the surface oxidation or adding various elements [3], [4], [5]. It has been observed that energetic ball milling of nanocrystalline magnesium hydride significantly improves hydrogen sorption kinetics [6]. However, no general mechanism for the hydriding–dehydriding reaction has been proposed despite the existence of numerous experimental data.

In the present work, hydrogen behavior properties in nanocrystalline magnesium films are investigated in order to get additional knowledge about the hydriding/dehydriding kinetics. Magnesium films have been prepared by unbalanced magnetron sputter deposition technique enabling to modify the microstructure of deposited film. Plasma immersion hydrogen ion implantation technique is used for the hydrogenation of the magnesium films.

The description of the hydrogen driving mechanisms into or out the bulk of the film is based on the assumption that the external irradiation from plasma brings the surface of the film to a higher chemical potential. The difference of chemical potentials between the surface and the grain boundaries is assumed to be the driving force for the motion of atoms into the grain boundaries of nanocrystalline films and therefore the creation of compressive stress. Thus, dislocation plasticity within the nanograins and diffusional flow of hydrogen between the grain boundaries and the surface are possible mechanisms acting on the hydrogenation kinetics.

Section snippets

Experimental

The experimental technique combines a conventional, balanced-magnetron sputtering with an independently generated arc discharge plasma that provides for independent control of the sputter-deposition and plasma immersion ion implantation. This capability allows for ion current densities of 1–10 mA cm⁻² or higher for ion bombardment at the substrate, independent of the sputtering rate. Ion-to-atom ratios of 0.1–10 ions/atom are achieved to tailor the microstructure of growing film. Inside the vacuum

Results

Fig. 1 shows the XRD profiles of a Mg film at different stages of the hydrogenation–dehydrogenation cycle. Before hydrogenation, Mg(0 0 2), Mg(0 0 1), and Mg(0 0 4) diffraction reflections are present, indicating that magnesium growth is highly oriented along the c-axis. This particular structure of the deposited film can be modified changing the bias voltage. As hydrogenation occurs, the Mg peaks vanish and the diffraction peaks corresponding to magnesium hydride (MgH) at 2Θ = 48.1° and magnesium

Discussion

Experimental results show that plasma activated surfaces become transparent for hydrogen. The efficiency of hydrogenation (probability of hydrogen accommodation) sharply increases and the role of surface barrier diminishes. After plasma hydrogenation surface morphology changes in the way that the film behaves as an open and porous structure, which makes film transparent for hydrogen supplied through the surface. In order to understand these experimental observations, it is assumed that the

Conclusions

Plasma immersion hydrogen ion implantation was shown to be suitable technique to hydrogenate thin nanocrystalline magnesium films. The fractional volume of MgH₂ in the Mg film has been evaluated in dependence on plasma tratment parameters measuring the amplitude of MgH₂ peaks in XRD patterns. The complete hydrogenation of 2 μm-thick Mg film was done at 450 K for 15 min under high-flux hydrogen ion irradiation.

The hydrogen effusion curves measured by thermal desorption technique include a sharp

Acknowledgments

The work was performed under support from the Hydrogen Program of the US Department of Energy and the Sandia National Laboratories. The Lithuanian State Science and Studies Foundation is gratefully acknowledged. The authors express their appreciation to Dr. G. Sandrock (USA) and Mr. A. Vasys (USA) for encouragement and kind cooperation to this work.

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The role of grain boundaries in the mechanism of plasma immersion hydrogenation of nanocrystalline magnesium films

Abstract

Introduction

Section snippets

Experimental

Results

Discussion

Conclusions

Acknowledgments

J. Alloys Compd.

J. Alloys Compd.

J. Alloys Compd.

J. Alloys Compd.

Appl. Surf. Sci.