Zbigniew S. Wronski

Particle size, grain size and γ-MgH2 effects on the desorption properties of nanocrystalline commercial magnesium hydride processed by controlled mechanical milling

The hydrogen desorption properties of commercial nanocrystalline magnesium hydride (Tego Magnan® from Degussa–Goldschmidt) processed by controlled mechanical milling (CMM) are investigated. A profound effect of the powder particle size on the hydrogen desorption characteristics has been observed. The onset (TON) and peak hydrogen desorption temperatures measured by differential scanning calorimetry (DSC) decrease initially slowly with decreasing mean particle size of hydride, and when the particle size reaches a certain critical threshold value, the desorption temperatures start decreasing more rapidly with further decrease of particle size. The total drop of desorption temperature from its initial value for the as-received MgH2 to the value attained for the milled MgH2 having a particle size of ~500–600 nm is within the range 40–60 °C. The metastable γ-MgH2 hydride coexists with the stable nanocrystalline β-MgH2 in the microstructure of the MgH2 powders ball milled for 10 h and longer. Quantitative evidence shows that two factors, namely the refined powder particle size and the γ-MgH2 phase residing within the powder particles, acting additively, are responsible for a substantial reduction of the hydrogen desorption temperature of MgH2 hydride.

Feasibility study of the direct mechano-chemical synthesis of nanostructured magnesium tetrahydroaluminate (alanate) [Mg(AlH4)2] complex hydride

The present work reports a feasibility study of the direct mechano-chemical synthesis by controlled reactive mechanical alloying (CRMA) in a magneto-ball mill of the nanostructured magnesium tetrahydroaluminate (magnesium alanate) Mg(AlH(4))(2) complex hydride. Three stoichiometric Mg-2Al mixtures, (a) elemental Mg and Al powders, (b) elemental Al powder and commercial AZ91 alloy (Mg-Al-Zn alloy) and (c) powder of as-cast Mg-2Al alloy, have been used. No successful synthesis of Mg(AlH(4))(2) has been achieved. The only nanocrystalline hydride formed up to 270 h of CRMA is beta-MgH(2), and it does not react with Al and H(2) to form Mg(AlH(4))(2). It has been found that there is strong competition between formation of Al(Mg) solid solution and the beta-MgH(2) hydride occurring to a various extent up to approximately 10 h of CRMA in all three Mg-2Al mixtures. It is hypothesized that the presence of Al(Mg) solid solution inhibits the reaction of beta-MgH(2), Al and H(2) to form Mg(AlH(4))(2). Furthermore, despite the fact that after prolonged milling the Al(Mg) solution eventually decomposes into secondary Al(s) (derived from solid solution), the latter retains its physico-chemical characteristics of the former solid solution which still inhibits the reaction to form Mg(AlH(4))(2). Experimental evidence from DSC measurements shows increasing ranges of the melting enthalpy with increasing amounts of Al(Mg) solid solution and consequently the secondary Al(s) for all the three Mg-2Al mixtures. This strongly supports the hypothesis about the different nature of Al(Mg) and the secondary Al(s) as compared to the primary elemental Al powder.

Progress in Hydrogen Storage in Complex Hydrides

Complex hydrides are potential candidates for solid state hydrogen storage due to their high gravimetric hydrogen capacities. The present chapter focuses on the most recent progress in the understanding of the hydrogen storage behavior of light metal-based complex hydrides and their (nano)composites. Hydrides such as sodium (NaAlH 4 ) and lithium (LiAlH 4 ) alanate containing various catalyzing additives, complex hydride (nano)composite systems containing LiAlH 4 , lithium amide (LiNH 2 ) and magnesium hydride (MgH 2 ) as well as manganese borohydride (Mn(BH 4 ) 2 ), which was synthesized by the mechanochemical activation synthesis (MCAS) are all discussed. In particular, the phenomena of mechanical dehydrogenation and slow hydrogen discharge during a long-term storage after processing by ball milling are thoroughly discussed. Furthermore, isothermal dehydrogenation behavior of the above mentioned complex hydride systems is thoroughly discussed.

The role of interfaces in the application of rapidly solidified metal ribbons as reinforcements for composites

Abstract The use of rapidly solidified (RS) metallic ribbons as reinforcement for composites is encouraged by their good mechanical properties and geometries which provide higher transverse strengths and surface-to-volume ratios than fiber reinforcements in uniaxial laminates do. As with continuous fiber-reinforced composites, the quality of the interface between components is of primary importance in achieving optimum mechanical properties. The focus of this paper is the investigation of the interfacial zones formed during consolidation of metallic glass ribbons into both polymer and metal matrices. For the polymeric system, RS Fe40Ni40B20 ribbons were incorporated into a thermoplastic polypropylene matrix by sandwich and thin film methods. The results of pull-out tests and microstructural investigation indicate that a much better interface is obtained in the sandwich method of fabrication. The thin film method results in a very imperfect interface containing numerous air bubbles trapped during fabrication. For the metallic system, RS Ni75Al25B1Zr1 ribbons were consolidated into an aluminum matrix by casting.

Nanomaterials for Hydrogen Storage Produced by Ball Milling

Abstract Three methods of hydrogen desorption temperature reduction and desorption kinetics improvement of nanostructured hydrides processed by mechanical (ball) milling are discussed. The first method is based on a simultaneous particle size refinement of MgH2 hydride and the formation of an unstable γ-MgH2 phase. The second method utilizes catalytic effects of nanometric Ni (n-Ni) additives. The third method is based on the compositing of nanohydride mixtures such as NaBH4+MgH2 and MgH2+LiAlH4 where the first hydride in a pair has higher decomposition temperature than the second one. The low decomposition temperature hydride results in the destabilization of the high temperature constituent hydride.

The Effects of Oxidized and Oxide-Free Boron on the Mg-B-H Nanohydrides Transformation in the Nearly Nanosized Powders

In this work oxidized and oxide-free amorphous boron (a-B) powder and elemental Mg were used in an attempt to directly synthesize the Mg(BH4)2 complex hydride by controlled reactive mechanical alloying (CRMA) under hydrogen in a magneto-mill up to 200h. The particle size was refined to the 100-200nm range. Nanocrystalline MgH2 (~6nm crystallite size) was formed within the particles when an oxidized a-B is used. In contrast, a mixture of MgB2 and an amorphous hydride MgHx was formed when an oxide-free a-B was used. Differential scanning calorimetry (DSC) test up to 500°C produced a single endothermic heat event at 357.7°C due to hydrogen desorption. In addition, desorption conducted in a Sieverts-type apparatus revealed ~1.4wt.% of hydrogen release. The X-ray diffraction pattern after DSC test of the 200h milled sample made with oxide-free boron showed the presence of MgB2.

Nanonickel Catalyst for Kinetic Destabilization of LiAlH4 (Lithium Alanate) for Facile Discharge of Hydrogen

Abstract. Microstructure and catalytic properties of new nanometric-scale Ni powders produced by a carbonyl nickel chemical vapour deposition process, have been investigated in respect to kinetic destabilization of LiAlH4 for hydrogen release during continuous heating conducted in a directpower-compensation differential scanning calorimeter. The endothermic only heat flow was observed for the nanonickiel-catalyzed LiAlH4, which was prepared by short-time milling of 5 wt% nano-Ni with Li alanate. The endothermic heat that occurs around 160 ̊C corresponds well to release 4.4 wt% H2 into a 1 bar reservoir, as determined in the Hiden Model IGA gravimetric gas analyzer. None of the three thermal events can be attributed to melting, and even 15 min milling resulted in receding of the melting peak at 170 ̊ C in the as-received LiAlH4. Therefore, nanonickel-catalyzed LiAlH4 desorbs hydrogen from solid crystal, without melting and with no exothermic desorption step reported in many previous studies. These results are discussed in terms of a catalytic promotion of thermodynamic destabilization that can occur in complex hydrides.

Nanostructured Hydrides for Solid State Hydrogen Storage for Vehicular Applications

The energy supply to mankind in the last two centuries was solely based on fossil fuels such as coal in the 19th century and crude oil and natural gas in the 20th century. Unfortunately, this fossil fuel-based economy has led to a number of new challenges facing all of mankind in the 21st century, such as global warming and the following climate changes due to the release of growing amounts of greenhouse gas CO2, poor urban air quality, and reduction in the world crude oil supply, which could reach the so-called Hubbert’s Peak around 2011–2020. It is also noted that no major oil field has been discovered since 1970 [1]. Since the mid-1970s the concept of an ecologically clean “Hydrogen Economy” has been gaining momentum as essentially the only viable remedy for the growing world energy problems. The Hydrogen Economy offers a potential solution to satisfying the global energy requirements while reducing (and eventually eliminating) carbon dioxide and other greenhouse gas emissions and improving energy security. Hydrogen is a very attractive alternative fuel or more precisely energy vector. It is ubiquitous, clean, efficient, and can be produced directly from sunlight and water by biological organisms and using semiconductor-based systems similar to photo-voltaics. Hydrogen can also be produced indirectly via thermal processing of biomass or fossil fuels where the development of advanced technological processes combined with CO2 sequestration [2] have the potential to produce essentially unlimited quantities of hydrogen in a sustainable manner.

X-Ray Diffraction Study of Stress in a Magnesium-Hydrogen System Produced by High-Energy Milling of Powders

There is great interest in metal-hydrogen systems. When small amounts of hydrogen are absorbed by elemental metals and alloys, the engineering materials made of the systems exhibit strong changes in their physical and mechanical properties. These changes, and hydrogen embrittlement, can be considered detrimental to the structural performance of the materials. However, the changes, often studied by metallurgists and metal physicists, can present new possibilities and applications. An example is the interest in safe, solid-state hydrogen storage in metallic lattices. The effect of elasto-plastic deformation on hydrogen sorption in light metals and hydrides has been studied in an MTL project. It has been observed that high-rate impacting of pure magnesium has an effect on hydrogen storage capacity. Furthermore, analysis of the state of residual stress performed by X-ray diffraction method for the first time in such high-rate, milled materials, indicates a very complex stress distribution dependent on the time of milling. The milling in increments of 10 minutes from 0, original un-milled material, to 30 minutes, indicates extensive changes in the stress states.

Mechanosynthesis of Nanocrystalline MgB2 Ceramic Powders in Hydrogen Alloying Mills via Amorphous Hydride Intermediate

In the present work we report on the synthesis of nanocrystalline MgB2 by mechanochemical reaction (mechanosynthesis) conducted in a high-energy mechanical alloying mill filled with hydrogen. The solid-state reaction of mechanochemical alloying between Mg and B with H (hydrogen alloying) leads to formation of an intermediate amorphous (Mg,B)Hx hydride. This amorphous intermediate is subsequently annealed (devitrified) to nucleate and grow nanocrystalline boride. The first stage of synthesis was carried out at room temperature from elemental Mg and B powders in a high-energy ball mill under sequential supply of hydrogen. The subsequent annealing of the amorphous product led to nearly single-phase MgB2, with only small fraction of MgO impurity. The easy room-temperature synthesis renders the method promising for production of MgB2, which recently gained attention as a new 39K ceramic superconductor. The amorphous intermediate itself can be studied further for its capacity to store ca. 2 wt% H in a metastable hydride phase. The effort was undertaken to predict formation of amorphous hydride phase through analysis of atomic volume mismatch between atoms of Mg, B, and H.