Hydrogen behavior in Mg + -implanted graphite
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A graphite wafer has been implanted with Mg+ to produce a uniform Mg concentration. Subsequent H+ implantation covered the Mg+-implanted and -unimplanted regions. Ion-beam analysis shows a higher H retention in graphite embedded with Mg than in regions without Mg. A small amount of H diffuses out of the H+-implanted graphite during thermal annealing at temperatures up to 300 °C. However, significant H release from the region implanted with Mg+ and H+ ions occurs at 150 °C; further release is also observed at 300 °C. The results suggest that there are efficient H trapping centers and fast pathways for H diffusion in the Mg+-implanted graphite, which may prove highly desirable for reversible H storage.
Many metals and metallic alloys possess outstanding properties1,2 for absorbing hydrogen as hydrides, where the volumetric density of hydrogen (>100 gH2/l) can exceed that of liquid hydrogen (∼70 gH2/l). For an onvehicle storage system, however, the main obstacle in using those materials is the low gravimetric storage capacity (when transition metals are major components) or the high decomposition temperature of the hydrides (usually above 300 °C). The sluggish transport/diffusion process of hydrogen in the hydrides is also considered an issue. On the other hand, porous materials, such as clathrates and zeolites, can promote physisorption or chemisorption of hydrogen on surfaces and may have desirable release/absorption kinetics.3 However, their relatively low gravimetric density is unlikely to meet the challenge for on-vehicle storage applications.4 Magnesium hydride (MgH2) has attracted considerable attention for H storage because of its favorable volumetric and gravimetric densities. It has been reported5 that H is weakly bonded to Mg in MgH2, thus making the metal hydride one of the promising candidates for future reversible H storage material. Recent Mg powder studies6,7 have indicated that the hydrogen absorption and desorption kinetics is significantly improved because of the formation of extensive boundary layers that provide fast diffusion paths for hydrogen. In addition, the probabilities of surface absorption and creation of extra sites for H absorption
a)
Address all correspondence to this author. e-mail: [email protected] b) This author was an editor of this journal during the review and decision stage. For the JMR policy on review and publication of manuscripts authored by editors, please refer to http://www.mrs. org/publications/jmr/policy.html. DOI: 10.1557/JMR.2006.0121
also dramatically increase with a decrease in particle size.6,7 Currently, the Mg particle size that has been achieved using ball-milling method is around 10 nm.7 Nanocomposite materials that contain efficient H absorption centers in a highly H-diffusive matrix could be one of the important materials for reversible H storage. High-dose ion implantation followed by thermal annealing can result in nucleation and growth of clusters of the implanted species with atomic to nanometer dimensions, depending on the experimental conditions
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