Hydriding kinetics of ball-milled nanocrystalline MgH 2 powders
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passov Department of Chemistry, University of Sofia “St.Kl.Ohridski,” 1164 Sofia, Bulgaria (Received 17 April 2007; accepted 17 July 2007)
The kinetics of hydride formation and decomposition described by semiempirical models generally do not involve particle and grain-size dependence. However, ball-milled nanocrystalline powders usually exhibit log-normal grain-size and particle-size distribution. Considering size dependence, a total reacted function for a multiparticle system has been developed. We show that the shape of the measured reaction fraction curves do not determine unambiguously the rate-controlling mechanism of hydrogen sorption, since the kinetics are strongly affected by the microstructure. With the application the convolutional multiple whole profile fitting procedure for nanocrystalline MgH2, the parameters, e.g., the median and variance of the log-normal grain-size distribution have been determined. Taking these values into account, the reaction constants corresponding to different sorption states are considerably modified compared with values obtained from classical single-particle models.
I. INTRODUCTION
The novel approaches to reach hydrogen-based energy systems resulted in a great interest to metal–intermetallic hydride storage solutions. A large variety of materials are used to store hydrogen reversibly under proper conditions.1–3 The practicable use of reversible hydride systems raised a claim to reduce the hydrogen absorption temperature and to improve the kinetics. Magnesium is considered to be one of the most attractive hydrogen-storage materials, mainly because of its high storage capacity (7.6 wt% in MgH2), light weight, and low cost.4 However, because of its high thermodynamic stability (⌬H ⳱ −75 kJ/mol),5 high hydrogen desorption temperature (higher than 400 °C), and relatively poor hydrogen absorption/desorption kinetics at temperatures below 350 °C, the use of Mg in technological applications is impeded. Recently, ball milling (BM) has been developed for processing nanocrystalline Mg (∼10 nm) under a hydrogen gas environment to reach special advantages such as improving the thermodynamic and kinetic properties of hydriding.6–8 By reducing the grain size of Mg to nanocrystalline dimensions, the H-sorption kinetics are
accelerated substantially,9–13 and the hydrogen desorption temperature is decreased by about 100 °C.13–15 It was demonstrated that not solely the nanometer grain size, but the powder particle size (down to
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