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In this paper, we report the microstructure and first hydrogenation properties of TiFe cast with 4 wt.% of Zr. Measurements were made in as-cast state and after processing by ball milling and cryomilling. It was found that ball milling and cryomilling significantly reduced the particle/crystallite sizes with most of the reduction occurring during the first 15 min of milling. While the cryomilled sample did not absorb any hydrogen, ball milling improved the initial kinetics of processed powders compared with the as-cast sample. However it also reduced the hydrogen storage capacity. The observed increased kinetics was likely caused by the reduction of crystallite sizes with longer ball milling times. However, the longer ball milling times might also be the reason of the capacity loss due to formation of new grain boundaries. Faster kinetics was not due to a change in the rate-limiting step, as all kinetics curves were fitted with the 3D growth, diffusion controlled with decreasing interface velocity model. Keywords: TiFe alloy, First hydrogenation kinetics, Rate limiting step, Ball milling, Cryomilling

International audience; The main objective of the present work is to study the effect of rapid solidification on the electrochemical performance of Zr-based Laves type alloy with a nominal composition Ti12Zr21.5V10Cr7.5Mn8.1Co8Ni32.2Al0.4Sn0.3. The samples were prepared from the as-cast arc melted buttons by melt spinning at different copper wheel rotation speeds of 5, 16.5, 33, and 100 Hz, which are equivalent to linear speeds of 6.3, 21, 41, and 62.8 m s−1 respectively using a cooling wheel with a diameter of 20 cm. The phase composition and morphology of the ribbons were analyzed by X-Ray diffraction (XRD) and scanning electron microscopy (SEM). The microstructural changes of the ribbons induced by the variations in the wheel rotation speed were found to be closely related to the electrochemical performances. High discharge capacities exceeding 400 mAh∙g−1 were achieved for the melt spun samples during the measurements at low current densities. Furthermore, melt spun casting performed at the highest wheel rotation speed of 100 Hz resulted in the best rate performance of the alloy. As this alloy has the smallest crystallite size, this resulted in the shortest H atoms diffusion distances, and thus increased the efficient H diffusion rate and improved the electrochemical performance.

A commercially feasible, nontoxic material that would convert heat into electricity in an efficient and cheap way has not been yet identified. Half-Heusler compounds are one the most interesting candidates but to become competitive their thermoelectric properties still need to be improved. Atomic substitution and excess Ni in the crystal structure of ternary MNiSn half-Heuslers are among the most successful approaches used to boost their performance. Here, we report on the effect of both methods applied simultaneously to the series of polycrystalline Ti1−xHfxNi1.0/1.1Sn, x = 0.00, 0.10, 0.15, 0.20, samples. High-resolution synchrotron powder X-ray diffraction studies combined with transmission electron microscopy demonstrate the crystallization of single or multiple Hf-substituted half-Heusler phase(s). The analysis of their long-range atomic structures shows that most of them contain interstitial Ni atoms disorderly distributed at the nominally vacant 4d sites. However, the short-range atomic correlations suggest that, for some compositions, excess Ni creates an orderly arranged additional atomic plane at the vacant fcc sublattice, which can be seen as a defective half- and/or full-Heusler phase. The results also show that the Ni-rich samples crystallize with the micrometer-sized full-Heusler phases, while all Hf-incorporating compositions present dispersion of HfO2 nanoprecipitates in the grains of the half-Heusler matrix. This research is complemented by thermoelectric transport measurements of the studied compositions in the range of 300–750 K. The results suggest that neither Seebeck coefficient nor electrical resistivity shows an obvious correlation with the observed microstructures. These finding are discussed with respect to previously proposed transport mechanisms.

Metal hydrides are known as a potential efficient, low-risk option for high-density hydrogen storage since the late 1970s. In this paper, the present status and the future perspectives of the use of metal hydrides for hydrogen storage are discussed. Since the early 1990s, interstitial metal hydrides are known as base materials for Ni – metal hydride rechargeable batteries. For hydrogen storage, metal hydride systems have been developed in the 2010s [1] for use in emergency or backup power units, i. e. for stationary applications. With the development and completion of the first submarines of the U212 A series by HDW (now Thyssen Krupp Marine Systems) in 2003 and its export class U214 in 2004, the use of metal hydrides for hydrogen storage in mobile applications has been established, with new application fields coming into focus. In the last decades, a huge number of new intermetallic and partially covalent hydrogen absorbing compounds has been identified and partly more, partly less extensively characterized. In addition, based on the thermodynamic properties of metal hydrides, this class of materials gives the opportunity to develop a new hydrogen compression technology. They allow the direct conversion from thermal energy into the compression of hydrogen gas without the need of any moving parts. Such compressors have been developed and are nowadays commercially available for pressures up to 200 bar. Metal hydride based compressors for higher pressures are under development. Moreover, storage systems consisting of the combination of metal hydrides and high-pressure vessels have been proposed as a realistic solution for on-board hydrogen storage on fuel cell vehicles. In the frame of the “Hydrogen Storage Systems for Mobile and Stationary Applications” Group in the International Energy Agency (IEA) Hydrogen Task 32 “Hydrogen-based energy storage” different compounds have been and will be scaled-up in the near future and tested in the range of 500 g to several hundred kg for use in hydrogen storage applications, The research for the lab-scale compressor is part of the activities of SCCER HaE, which is financially supported by Innosuisse - Swiss Innovation Agency . The authors thank the Alexander von Humboldt Foundation in the frame of the post-doctoral fellowship of Dr. J. Puszkiel (No. 1187279 STP ) as well as the European Union for their funding of projects STORHY (contract Nr. SES6-CT-2004-502667 , FP6-2002-Energy-1, 6.1.3.2.2), NESSHY (contract Nr. 518271 , FP6-2004-Energy-3, 6.1.3.2.2) and the EU Horizon 2020 /RISE project HYDRIDE4MOBILITY. Financial support from the S02 and KP8 S05), the European Union's Seventh Framework Programme ( FP7/2007e2013 ) for the Fuel Cells and Hydrogen Joint Technology Initiative under grant agreement no. 256653 (SSH2S), from the European Fuel Cells and Hydrogen Joint Undertaking in the framework of BOR4STORE (Grant agreement no. 303428 ), from the Australian Research Council for grants LP120101848 , LP150100730 , and LE0989180 , The Innovation Fund Denmark (project HyFill-Fast), DST within Hydrogen South Africa/HySA programme (projects KP3 National Research Foundation/NRF of South Africa , incentive funding grant number 109092 and the Research Council of Norway (project 285147 ) is thankfully acknowledged

The objective of the present work was to study an interrelation between the conditions of the rapid solidification casting and electrochemical properties of a C15 Laves-type Zr-based metal hydride battery anode alloy. A 7-component overstoichiometric with B/A>2 alloy (Ni + Mn + V + Fe/Zr + Ti + La = 2.078) was Zr- and Ni-rich. Melt spinning was performed using as cast mother ingot at different rotation speeds of 5, 16.5, and 33 Hz with a 20-cm diameter Cu cooling wheel. The phase composition and morphology of the ribbons were analyzed by X-ray diffraction (XRD) and scanning electron microscopy (SEM) along with X-ray energy dispersive spectroscopy (EDS). Partial Mn vaporization took place during the melt spinning and became more pronounced with the increase in the quenching rate at higher wheel rotation speeds. The SEM study showed a successful refinement and a more homogeneous distribution of La in the melt-spun alloys. However, the alloys demonstrated a more difficult activation performance as compared to the as-cast sample which was associated with introduction of Zr into the composition of the secondary La-and Ni-rich phase when quenching the melt. Presence of Zr in the secondary phase results in inferior rate of its hydrogenation. The formed during the rapid solidification (La,Zr) (Ni,Mn) secondary phase also contained a significant amount of oxygen additionally worsening the activation performance. The grain size gradually decreased from 3.5 μm in the as-cast alloy to approximately 250 nm for the melt spun sample obtained at the highest cooling rate. The two-phase alloys contained a mixture of two Laves type intermetallics, C15 and C14 ones. The abundance of the secondary C14 phase decreased with increase of the cooling rate as compared to the as-cast sample. The electrochemical performance was significantly affected by the compositional and microstructural changes associated with the variation in the cooling rate. The highest electrochemical discharge capacity of 414 mAh/g was obtained for the 16.5 Hz melt spun rate because of an optimized composition of the Laves phase. Furthermore, the diffusion coefficient of hydrogen in this alloy was the highest in a full state of charge, which indicates that faster hydrogen diffusion and shorter hydrogen diffusion paths were achieved via an application of the rapid solidification process. An optimization of the rapid solidification conditions is required to improve an activation behavior of the studied Laves types metal hydride anode alloys. © 2019 The Authors. Published by Elsevier B.V. Open Access. CC BY-NC-ND 4.0

The impact of ultra-short milling at liquid nitrogen temperatures on structural and magnetic properties of cobalt ferrite (CoFe 2 O 4 ) powders has been explored for the first time. Cryomilling for only up to 9 min increases the coercivity of the isotropic powder from 139 to 306 kA/m (1.74–3.85 kOe) and results in its modifications comparable with milling for hours at room temperature. A thermal treatment of processed CoFe 2 O 4 enables further optimization of powder magnetic properties and leads to a high value of energy product (13.5 kJ/m 3 ) for the sample treated at 600 °C. Systematic studies, comprising analysis of structural and microstructural properties, based on synchrotron powder X-ray diffraction, scanning and transmission electron microscopy demonstrate the high efficiency of cryomilling in reduction of crystallite sizes and formation of lattice strain in the processed cobalt ferrite samples.

Thermoelectric application of half-Heusler compounds suffers from their fairly high thermal conductivities. Insight into how effective various scattering mechanisms are in reducing the thermal conductivity of fabricated XNiSn compounds (X = Hf, Zr, Ti, and mixtures thereof) is therefore crucial. Here, we show that such insight can be obtained through a concerted theory-experiment comparison of how the lattice thermal conductivity κLat(T) depends on temperature and crystallite size. Comparing theory and experiment for a range of Hf0.5Zr0.5NiSn and ZrNiSn samples reported in the literature and in the present paper revealed that grain boundary scattering plays the most important role in bringing down κLat, in particular so for unmixed compounds. Our concerted analysis approach was corroborated by a good qualitative agreement between the measured and calculated κLat of polycrystalline samples, where the experimental average crystallite size was used as an input parameter for the calculations. The calculations were based on the Boltzmann transport equation and ab initio density functional theory. Our analysis explains the significant variation of reported κLat of nominally identical XNiSn samples, and is expected to provide valuable insights into the dominant scattering mechanisms even for other materials.