The destabilizing effect of (X) group IV elements (C, Si, Sn and Ge) on NaH + Al was investigated. A significant decrease in the desorption temperature as well as the reaction enthalpy of NaH was achieved with additions of C. Si, Ge and Sn due to the formation of NaAlGe and NaAlSi ternary and NaSn binary compounds. Compared to a reaction enthalpy of 114 kJ mol(-1) H-2 for NaH above 400 degrees C, lower reaction enthalpies of 94 kJ mol(-1) H2, 72 kJ mol(-1) H-2, 20 kJ Mol(-1) H-2 and 2 kJ mol(-1) H-2 were obtained for the NaH + Al + C, NaH + Al + Si, NaH + Al + Ge and NaH + Al + Sn mixtures with onsets at 270 degrees C, 220 degrees C, 180 degrees C, and 130 degrees C respectively. Reversible hydrogenation was partly achieved in the NaH Al Si system with the formation of NaAIH(4) + Si. (C) 2011 Elsevier Ltd. All rights reserved.

The influence of additives on the reaction kinetics and microstructure refinement in LiBH4–MgH2 composites is investigated in detail. Indications of the rate-limiting processes during the reactions are obtained by comparison of the measured reaction kinetics with simulations with one specific rate-limiting process. The kinetics of the sorption reactions are derived from volumetric measurements as well as from in situ X-ray diffraction measurements. During desorption, the hydrogen is released at a constant rate, which is possibly correlated with the one-dimensional growth of MgB2 platelets. In contrast, the kinetic curves of the absorption reactions exhibit the typical shape of contracting-volume controlled kinetics. The microscopical interpretation of kinetic measurements are supported by transmission electron microscopy images confirming the formation of additive-nanostructures in the grain boundaries upon cycling. The present investigations underline the importance of the additives as nucleation substrates and the influence of microstructure on the reaction kinetics.

In the framework of the EC project STORHY (Hydrogen Storage for Automotive Applications), the prototype of a solid storage tank for hydrogen based on sodium alanate was developed. A storage tank containing 8 kg sodium alanate was designed and manufactured with the objective of fast refueling. To obtain the optimum design of the storage tank a simulation tool was developed and validated by experiments with a laboratory-scale tubular reactor. Application of the simulation tool to different storage concepts and geometries yielded the final design. The chosen concept is modular, enabling simple scale-up. This is the basis for the future development of fuel cell vehicle storage tanks containing 5 kg of hydrogen.

In this work, the effect of powder bed size on the absorption and desorption kinetics of NaAlH4 catalyzed with TiCl4 was studied experimentally. For this purpose, volumetric titration measurements were performed using cells of different diameters. The temperature was measured during the process at different positions inside the hydride bed, providing detailed information about the influence of heat conduction. Experimental results show that, under the applied conditions up to a critical size, larger diameters can lead to faster kinetics for the first and second absorption reactions. At larger cell diameters, however, temperatures up to 200 °C were measured during the first absorption step in the hydride bed. This leads to a significant delay in the start of the second absorption step, reducing the overall rate of the process. Reasons for the observed behaviour are discussed and measures for optimization are proposed.

Screening experiments were performed in order to investigate the formation of Al-based quaternary hydrides on the basis of multi-component mixtures treated by reactive ball milling under hydrogen pressure. The data indicated that the milling parameters and in particular the milling speed and milling time are of great importance to the formation of any new phase obtained by reactive ball milling. Indeed, a higher milling speed was shown to favour the formation of the new phases. In the case of (MgH2 + Al + LiH) and (MgH2 + LiAlH4) mixtures, the formation of a new phase was observed, which exhibits relatively fast decomposition kinetics.

This work demonstrates that hydrogen can be reversibly stored in a composite of NaF and Al. NaF and Al reacts to a mixture of Na3AlF6 and NaAlH4 via hydridofluoride phases of the form Na3AlH6−xFx. The analysis of thermodynamics based on literature standard enthalpies of formation yields the technically favourable enthalpy of reaction of roughly 35 kJ/mol H2 for a theoretical gravimetric hydrogen storage capacity of 3.3 wt%. Reaction mechanisms are discussed with respect to substitution of hydrogen by fluorine in complex hydrides.

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NaAlH(4) is the archetypical complex hydride for hydrogen storage. The extraordinary effect of dopants on the sorption kinetics triggered the investigation of this empirical finding. In this paper, a short review of the state of the art is given. To gain further understanding of the mechanisms involved we label the interacting species during the sorption process. This was experimentally realized by hydrogen-deuterium exchange measurements during the decomposition of NaAlH(4) followed by thermogravimetry, Raman spectroscopy and mass spectrometry. By these experiments we are able to obtain specific information on the diffusing species and formation of intermediates. The activation energy of tracer diffusion in NaAlH(4) is found to be 0.28 eV. The results are evidence for a vacancy-mediated desorption process of NaAlH(4).

This work demonstrates that fast sorption kinetics in complex hydrides can be achieved by a simple synthesis method using cost-efficient initial components, if microstructure and powder morphology are optimized. NaH/Al precursors with TiCl4 catalyst were synthesised under varying conditions in argon atmosphere and cycled. The influence of powder morphology and microstructure resulting from different process conditions were studied in detail. It is shown that a homogeneous mixing of the phases and a high surface area of the material is essential for fast kinetics and high reversible capacity. The optimized process can be easily scaled up to a cost-efficient production process for large amounts of storage material and can also be applied for other complex hydrides.

In this work Mg- and K-containing alanates have been investigated as possible hydrogen storage materials. Ball milling was carried out under argon or at moderate/high hydrogen pressure in order to obtain an improved driving force for the formation of potential new alanate phases. Powder X-ray diffraction and volumetric measurements were used in order to identify reaction mechanisms and phases forming in these systems. New unidentified peaks were detected for the mixtures 2MgH2+3Al+KH and 2CaH2+Al+2KH. However, they do not seem to belong to reversible hydride phases.

A detailed analysis of the reaction mechanism of the reactive hydride composite (RHC) MgH2+2LiBH4↔MgB2+2LiH+4H2 was performed using high-pressure differential scanning calorimetry (HP-DSC) measurements and in situ synchrotron powder X-ray diffraction (XRD) measurements along with kinetic investigations using a Sievert-type apparatus. For the desorption the following two-step reaction has been observed: MgH2+2LiBH4↔Mg+2LiBH4+H2↔MgB2+2LiH+4H2. However, this reaction is kinetically restricted and proceeds only at elevated temperatures. In contrast to the desorption reaction, LiBH4 and MgH2 are found to form simultaneously under fairly moderate conditions of 50bar hydrogen pressure in the temperature range of 250–300°C. As found in pure light metal hydrides, significant improvement of sorption kinetics is possible if suitable additives are used.

Light metal hydrides show a high potential for reversible hydrogen storage applications. In view of the potential future storage of large amounts of hydrogen, an economic tonnage scale production will be required. This viewpoint set introduces production methods and discusses the potential for simplifying processing routes and reducing costs in view of an industrial mass production. For this purpose, sodium alanate, for which cost-competitive large-scale production is already considered feasible, is used as an example for future promising hydrides, like complex hydrides or reactive hydride composites.

This paper highlights the influence of a refined microstructure on the wear behaviour of cermets by comparing TiC‐Ni based, VPS‐sprayed microcrystalline and nanocrystalline cermet coatings. The coatings are subjected to two‐body and three‐body abrasive wear at different loads. Fracture behaviour as well as mass loss and surface quality after the wear tests are evaluated. Although at low wear loads, the refined microstructure leads to a higher mass loss, at high loads the wear resistance of the nanocrystalline coating is superior. This behaviour is strongly related to a significantly higher toughness of the nanocrystalline coating. Independent of the wear load, the nanocrystalline microstructure leads to less surface roughness after wear.

A breakthrough in hydrogen storage technology was achieved by preparing nanocrystalline hydrides using high‐energy ball milling and the use of suitable catalysts/additives. These new materials show fast or in case of Mg‐based hydrides very fast absorption and desorption kinetics within minutes, thus qualifying lightweight Mg‐ or Al‐based hydrides for storage applications. This article summarizes our current understanding of the kinetics of Mg‐based light metal hydrides, describes an approach for a cost‐effective processing technology and highlights some promising new developments in lightweight metal hydride research.

The present study highlights the advantages of milling NaH/Al under moderate hydrogen pressure as a favourable production step for NaAlH4-based hydrogen storage materials. Firstly, it is demonstrated that NaAlH4 can be obtained by applying a moderate hydrogen pressure (6–12bar) during milling of NaH and Al with and without the presence of an inexpensive catalyst (TiCl4). The yield of NaAlH4 depends critically on process parameters, such as hydrogen pressure and milling time. A fully converted product is capable of reversible hydrogen storage without any activation procedure. Under optimized conditions, a capacity of 4.2wt.% was achieved and kinetics in the first desorption are comparable to NaAlH4 doped with TiCl3. Secondly, the synthesis has been optimized towards shorter milling times. By applying a few absorption/desorption cycles to material that was partially converted during milling, almost full reversible storage capacity can be reached. In addition, kinetics is extremely enhanced. For example, such material exhibits an optimum capacity already after two sorption cycles at 100bar and 125°C and allows to absorb 80% of the reversible hydrogen content within a few minutes.

This study demonstrates the potential of high-energy milling to use nanostructured cermet powders for thermal spraying utilizing a TiC–Ni-based composite as model material. The microstructure of coatings processed by VPS and HVOF spraying of nanostructured composite powders is characterized and compared to the initial microstructure of the feedstock. Thus, the effect of different microstructures, which can be produced by high-energy milling, on the microstructural evolution during spraying is evaluated. Results show that partial dissolution and reprecipitation of hard phase material as well coarsening of the binder phase crystallite size occur during the spraying process. However, a homogeneously dispersed hard phase distribution similar to that of the nanostructured precursor powder with hard phase sizes in the range of 100 nm is formed. Additionally, hard phase particles bigger than of 300 nm are retained during spraying. First results concerning hardness and wear resistance of respective coatings are shown and discussed.

In the present study, the wear behaviour of nanocrystalline coatings of the composition (Ti,Mo)(C,N)–45 vol. % NiCo, prepared by vacuum plasma spraying (VPS) and high-velocity oxy-fuel (HVOF) spraying of high-energy-milled powder, is characterized and compared to microcrystalline coatings of the same composition. Two-body abrasive wear tests, as well as scratch tests, are applied to produce wear traces on the surfaces of the nano- and microcrystalline coatings. While nanocrystalline HVOF coatings are weaker than their microcrystalline counterparts, nanocrystalline VPS coatings show superior wear resistance. The worn surface morphologies are investigated with optical microscopy, scanning electron microscopy and atomic force microscopy. The wear mechanisms and failure of nano- and microcrystalline coatings are distinctly different and are discussed in detail.

Nanocrystalline coatings have a high potential for various engineering applications, e.g. against wear of rolls in the paper fabrication and as corrosion protection. Using vacuum plasma spray (VPS) and high velocity oxy-fuel (HVOF) spray techniques, coating materials are exposed to high flame temperatures only for less than a millisecond. Therefore, high-energy milled powders can be used as feedstock material without losing its nanocrystalline microstructure during the thermal spray process. In this way, homogeneous, dense nanocrystalline coatings can be produced, which show enhanced hardness, thus obtaining promising superior wear resistance. In the present study, (Ti,Mo)(C,N)-45vol.%NiCo nanocrystalline coatings were prepared by VPS- and HVOF-spraying of high-energy milled powders. Abrasive wear tests JIS H 8615 with varying number of strokes and scratch tests with a Vickers indenter were applied to produce wear traces on the polished surfaces of the nanocrystalline coatings. Abrasive wear behaviour was analyzed by investigating the surface morphologies with optical microscopy (OM), scanning electron microscopy (SEM) and atomic force microscopy (AFM). Experimental results indicate that the VPS-coating has higher abrasive wear resistance than the HVOF-coating according to the JIS H8615 test. The abrasive wear mechanism of VPS-sprayed nanocrystalline coatings can be delineated as cutting plus ploughing. For HVOF-sprayed nanocrystalline coatings the abrasive wear mechanism can be described as cutting plus material delamination.

TiC–Ni based nanocrystalline cermet powders for thermal spraying were produced by high-energy milling. Milling experiments were performed in an attrition mill and a vibration mill in kilogram scale, and powder morphologies and microstructures were characterized using scanning electron microscopy, X-ray diffraction, and laser scattering for particle size analysis. Milling time and powder input were optimized with respect to the desired microstructure and particle sizes, and the results using both types of mill were compared. Powders with homogeneously dispersed hard phase particles below 300 nm could be produced in both mills. Additional processes for the refinement of powder morphology and particle size distribution are discussed.