Current Research Projects Hydrogen in Metals nanostructured storage materials Growth of nano-structures using STM Physics of GaN and ZnO Hydrogen in Metals nanostructured storage materials 1. Introduction We are carrying out experimental studies of the absorption and desorption of hydrogen in light metal alloys thin films with the goal of identifying systems that can be useful for the storage and transportation of hydrogen fuel in cars and boats. We have strived to find a system where (1) the weight fraction of hydrogen is as high as possible, (2) the temperature needed to release the hydrogen is close to room temperature, and (3) the time needed to release the hydrogen gas is short enough. We have, in particular, focused on magnesium-based alloys. Thin film samples have been be prepared by sputter-deposition and the hydrogen uptake characterized as well as the atomic structure within and on the surface of the films. Nano-scale (1-20 nm) composites have been made and are currently being characterized. The background for this research and development opportunity is combination of competences from Uppsala University and University of Iceland. The goal is to provide thin film model structures that open up opportunity for new materials for hydrogen storage. Thin films of hydride material are ideal model system to start with for studying the effect of mechanical constrains. The clamping of the grown film to the substrate can withstand an enormous amount of stress or around 5-20 Giga Pascal depending on the thickness of the film. In multilayered structure, additional layer can be inserted in between hydride layers increases this clamping effect further. The clamping leads to the effect that only the out of plane expansion is possible for the hydride material. The binding energy is greatly affected by this constrains since the natural relaxed structure cannot be reached and new phase behavior is seen. Thin films of Mg:C composite can for example open up a completely new field of research since the mechanical constrains become 3-dimensional and not only 2-D like. By co-sputtering the Mg and C, two materials that do not mix it is possible to condense the carbon into web like structure of graphite or chains of carbon that lock down Mg grains in vice like fashion. The lowering of the binding energy of the MgHx grains is brought by 3-4 physical energy terms. 1. Surface energy of small MgHx grains becomes considerable when the grain size is 1-5 nm. 2. The mechanical constrains hinders the formation of most stable phase since lattice expansion ΔV/V of 15% for MgH2 phase is hindered. 3. In small grains surface segregation of Mg or H to surface of the grains can alter the binding energy greatly. 4. The binding energy of H shows a spectrum that lowers the hydrogen storage capacity but the practical energy range -0.20-0.25 eV/H atom will be reached. 5. By adding the third element into this Mg:C composite all the above terms can be altered and tuned. 2. Experimental Sample preparation was done in a newly constructed sputtering system at the experimental facility of Science Institute University of Iceland. The system has three sputtering sources with control of sputtering parameters as gas flow and sputtering power. The sample can be sputtered at sputtering temperature of RT-1000°C. The sample holder can be biased with voltage in order to facilitate energetic ionic bombardment during growth. It is possible to construct in-situ resistance sample holder to follow the development of the film during growth, post annealing treatment and hydrogenation  Figure 1. The resistance of the sample is used to monitor hydrogen uptake of the new Mg based material Different new material types can be made by growing multilayers or graded multilayeres. Infinite number of possible arrangements is available depending on deposition sequence of materials, thickness, growth, temperature, post annealing treatment and energetic ion flux during deposition. Post annealing treatment can for example easily lead to phase separation of material A into small grains in the Mg matrix.  Figure 2. One out of many possible structure of the new material 3. Results 3.1 Hydrogen uptake in Mg films Until now resistance measurements have not been used to study the hydrogen uptake of Mg based materials since magnesium hydride becomes insulating. Resistance probes therefore only the unloaded Mg part of the sample. This behavior is however different in thin flms. Resistance measurements were done on 100 nm thin magnesium films covered with 10 nm palladium in temperatures ranging from 60 to 100°C. The samples were grown by DC sputtering system with a base pressure of ∼ 10-8 mbar. The measurements were done in situ in the sputtering chamber where the samples where grown, without breaking the vacuum to minimize contamination. Results show similar thermodynamic behavior to that found in bulk samples and samples made by ball milling. Resistance measurements of thin films could therefore be a useful tool in screening for changes in the binding energy of hydrogen in alloyed thin Mg films.  Figure 3. P-R-T Isotherms for the palladium covered magnesium film Owing to the significantly lower loading times of thin films at this temperature range, a wide range of new materials consisting of nano-scale structures of Mg and other elements can be produced through sputtering and studied with the equipment. 3.3 Hydrogen uptake in Mg:C films Magnesium:Carbon films 25 nm thick, with approximately 40% carbon content and a 5 nm Pd capping layer, were co-sputtered in a DC magnetron sputtering chamber. The films were found to be X-ray amorphous. The hydrogen uptake was studied by in-situ resistance measurements. The uptake showed one fast process with a time span of a few seconds followed by much slower resistance change, indicating structural relaxation. The isotherms show similar behavior as found in amorphous materials with a broad distribution of binding energies. A significant part of the film showed a reduction in the binding energy of hydrogen in MgH2 that would yield a release temperature of ∼ 200°C at 1 bar.  Figure 4. Fast kinetics of uptake and amorphous behavior of M:C hydrogen storage material 4. Conclusions of work The work has shown significant development in using thin films as means to find and characterize new hydrogen storage materials. First experimental steps have been taken to study the thermodynamic properties of nano-scale Mg carbon based hydrogen storage material. The results are encouraging showing reduction in binding energy. 5. References Thermodynamics of hydrogen uptake in Mg films studied by resistance measurements A. S. Ingason and S. Olafsson Journal of Alloys and Compounds Volumes 404-406, 8 December 2005, Pages 469-472 Influence of MgO nanocrystals on the thermodynamics, hydrogen uptake and kinetics in Mg films A. S. Ingason and S. Olafsson Thin Solid Films, Volume 515, Issue 2, 25 October 2006, Pages 708-711 Resistivity changes in Cr/V(0 0 1) superlattices during hydrogen absorption A.K. Eriksson, A. Liebig, S. Ólafsson and B. Hjörvarsson accepted Journal of Alloys and Compounds 2007 Hydrogen uptake in Mg:C thin films A. S. Ingason A. K. Eriksson S. Olafsson accepted Journal of Alloys and Compounds 2007 This work is supported by University of Iceland research fund, Icelandic Research fund and Nordic Center of Excellence on Hydrogen Storage Materials funded by the Nordic Energy research fund Growth of nano-structures using STM Physics of GaN and ZnO