Neutron Scattering and Other Nuclear Techniques for Hydrogen in Materials

The interaction of hydrogen with materials is a wide field of research. The simplicity of the hydrogen atom allows us to gain fundamental insights into crystal structures, phase transitions, diffusion, absorption, adsorption, and magnetism in hydrogen-containing metals. These insights could then be used to develop a variety of practical systems for the advancement of our society. One key example is the development of a sustainable and environmentally friendly energy production and distribution systems. Hydrogen is a well-known energy carrier and could be a solution for some of our energy and environmental issues. Hydrogen can be generated from various primary energy sources, especially renewables such as thermal solar, photovoltaic, wind, and hydroelectricity. Hydrogen is the only fuel that can be converted to and from electricity easily, thus making it a potential partner with electricity in the global energy distribution system. As most renewable energy sources are intermittent, using hydrogen to store energy is an elegant and ecological solution. Therefore, hydrogen is expected to be used widely, along with electricity, as a major energy carrier by mid-twenty-first century.

Neutron scattering is an important method for characterization of materials. The chapter starts with a presentation of the properties of neutrons and how they are produced. The interaction with matter is the main part of this chapter. The notion of scattering length and cross section are explained as well as the differences between coherent and incoherent scattering. The chapter ends with a short overview of elastic, inelastic and quasi-elastic neutron scattering. The goal of this chapter is to provide to a beginner the essential background for the comprehension of the other neutron related chapters of this book.

Neutron powder diffraction is extensively used for the development and fundamental understanding of metal hydrides. In this chapter we will first present the basic principles of neutron diffraction. This will be followed by a discussion about the two main neutron sources used for diffraction experiments: reactors and spallation sources. Analysis of diffraction patterns by Rietveld refinement will be discussed next: we will expose the principle of the method and the significance of various parameters to be refined. The special case of locating the hydrogen using neutron powder diffraction will be discussed in a separate section. The final section is a presentation of a few selected examples of the application of neutron powder diffraction to metal hydrides.

Total neutron scattering is in many ways an extension of neutron powder diffraction (Chapter 3). However, rather then just considering the shape Bragg peaks, the total scattering from the sample is measured and analyzed. This includes the weaker diffuse scattering which contain information about short-range atomic strutural features. This chapter gives an introduction to total neutron scattering theory and instrumentation and gives an overview of total neutron scattering investigations of hydrogen storage materials.

Neutron Reflectometry (NR) is capable to determine the chemical depth profile of thin film samples from the sub-nanometer regime up to about 200 nm thickness. The high sensitivity of neutrons to hydrogen and deuterium enables NR to detect absolute hydrogen concentrations in the at.% range even in nm-thick layers. Therefore, NR is an ideal tool to study in-situ the hydrogen/deuterium absorption and desorption properties of thin films on a nanometer scale—without the need of a calibration sample. After an introduction to the NR technique and required instrumentation, this chapter provides a comprehensive overview of NR applications in various scientific areas, e.g. hydrogen absorption and desorption of Mg-based alloys and thin Nb films, electrochemical measurements on thin Zr and Ti films, and tuning of the oscillating magnetic exchange coupling in Fe/V and Fe/Nb multilayers with hydrogen.

Recent progress on materials for hydrogen storage have pointed out that kinetics and thermodynamics can be modified by nano-confinement of hydrides in porous scaffolds. The investigation of the structural features of these particular systems is challenging with conventional methods, for instance due to the lack of peaks in the diffraction pattern. Small angle neutron scattering is suitable for studying porous systems containing hydrogen and it has been invaluable for the characterisation of the new hydride systems. This chapter presents the basic principles of the technique and gives examples of its application to the field of nano-confined materials for hydrogen storage.

Neutron imaging methods are appropriate to investigate hydrogen distributions in several metallic systems. The large total neutron cross section of hydrogen compared to those of elements or isotopes, respectively, in usual structural materials like steels or zirconium alloys allows the detection even of small amounts of hydrogen in such materials. The dependence of the total macroscopic neutron cross section of the sample or component on the hydrogen concentration can be determined experimentally by means of calibration specimens with known hydrogen concentrations. Such a calibration allows for a full quantitative determination of the local hydrogen concentration with a spatial resolution better than 20 μm. Because neutron radiography is fast and non-destructive, in situ investigations of time-dependent processes like hydrogen absorption and release or hydrogen bulk diffusion can be performed. This chapter gives an introduction into the main neutron imaging methods, radiography and tomography, and gives as examples results of neutron imaging investigations of hydrogen in different steels and in zirconium alloys, respectively.

Hydrogen scatters neutrons strongly and incoherently, which makes neutrons very sensitive to the presence of hydrogen in materials (Squires, Introduction to the Theory of Thermal Neutron Scattering, 3rd edn. Cambridge University Press, New York, 2012). This strong incoherent scattering removes neutrons from a transmitted beam, so that hydrogen-rich regions appear as dark spots in neutron radiographs. The scattered neutrons are spread into all directions and appear as a contribution to the background in neutron diffraction patterns, often viewed as a nuisance. In this chapter, we first provide a brief introduction to coherent and incoherent neutron scattering. We then present two examples in which incoherent neutron scattering reveals knowledge of practical value. In the first example, we describe an experiment to determine the smallest change in hydrogen concentration that can be detected in nuclear fuels (~10 wt. ppm) by incoherent neutron scattering. Hydrogenous materials such as water and organic lubricants are often used in the processing of fuels for nuclear reactors, and residual lubricants trapped in the fabricated fuels can lead to complications in fuel performance. Neutron scattering is very sensitive to the presence of hydrogen in bulk specimens, and hence of water or hydrogen-rich organic lubricants. Furthermore, since neutron scattering is non-destructive and simple to apply, it is an ideal technique for quantifying hydrogen-bearing contaminants in fuels for the purpose of quality assurance. A brief description of the uncertainties associated with a typical neutron counting experiment is provided. Though the experiment deals with nuclear fuels, the method is straightforward and easily applicable to other materials. In the second example, we describe an in situ experiment in which both coherent and incoherent neutron scattering are used to study the bulk diffusion of hydrogen into a zirconium alloy in order to accurately determine the solubility limit of hydrogen. In CANDU^® nuclear power reactors, pressurized heavy water coolant flows over fuel bundles in pressure tubes made of Zr–2.5Nb alloy. Over time, deuterium accumulates in the zirconium alloy, resulting in the precipitation of hydrides which can lead to various types of failure, such as Delayed Hydride Cracking (Puls, The Effect of Hydrogen and Hydrides on the Integrity of Zirconium Alloy Components: Delayed Hydride Cracking. Springer, New York, 2012). Accurate quantitative knowledge of the deuterium (hydrogen) concentration at which hydrides start to form is thus critical to inform regulations limiting the acceptable residence time of pressure tubes in power reactors. Again, though the experiment deals with a specific alloy, the technique is straightforward to apply and is applicable to a wide range of materials.

This chapter describes the basic principles of inelastic and quasi-elastic neutron scattering as applied to hydrogen storage systems—in particular to provide an understanding of the vibrations and diffusion of hydrogen and deuterium in the host lattice. The techniques are then illustrated using typical hydrogen storage materials. Incoherent inelastic scattering can be applied to isolated hydrogens—where the protons can be modelled as in an isolated potential well formed by the surrounding atoms. At higher concentrations, the effect of H–H interactions and the role of hydrogen vibrational density of states are described. Ab initio theory becomes important in this case. The advantages of modelling the dynamics of a deuteride by simulation of the polycrystalline coherent inelastic neutron scattering in comparison with ab initio modelling are then described. The final area of application of inelastic scattering is to the case of adsorbed H₂ molecules where particular spin transitions are observed. Here the results provide important information on the geometry of the potential energy surface around the adsorbing site. Quasi-elastic neutron scattering is then described. In particular the Chudley–Elliott model is derived for a Bravais lattice and it is indicated how this approach can be extended to more general cases where there are multiple sublattices which may have differing energies of adsorption. Here the important case of intermetallic Laves phases is described.

Among the family of Ion Beam Analysis techniques for material characterization, Elastic Recoil Detection Analysis (ERDA) exploits the spectroscopy of recoil nuclei moving under the impact of the ions of the beam. This technique is well suited to light elements profiling, especially for hydrogen measurements which can be performed with usual helium-4 beams available in most facilities. This chapter presents an overview of ERDA main features and focuses on a selection of recent papers dealing with hydrogen measurements in materials for miscellaneous applications, divided into four sections: metals, ceramics, minerals, and thin films.

Nuclear reaction analysis (NRA) is a method utilizing an ion beam from an accelerator to determine the hydrogen content in materials. The hydrogen concentration can be quantified absolute and the method can provide depth profiles in the range up to 2–3 μm with a depth resolution in the range of a few nm. Concentrations of 500 at. ppm are easily measurable and with special efforts sensitivities down to 1 ppm have been achieved. The chapter describes the basic principles of the measurements and data analyses and discusses special cases and complications arising with particular samples or when very low concentrations are to be measured.

Nuclear magnetic resonance (NMR) is one of the most versatile methods for studies of the behavior of hydrogen in materials. For metal – hydrogen systems and complex hydrides, this technique provides microscopic information of diverse nature, including the electronic properties, local structural features, and hydrogen dynamics. The present survey discusses the basic principles that make NMR useful for investigations of hydrogen in materials. Such a discussion is complemented by recent examples of application of this technique to studies of metal – hydrogen systems and complex hydrides. The emphasis is put on applications related to hydrogen mobility. The complementarity between NMR and quasielastic neutron scattering is also addressed.

A positron is an anti-particle of an electron and is annihilated with the electron in a sample. When lattice defects with open-volume exist in the samples, positrons are trapped there. Therefore, positron annihilation spectroscopy (PAS) is used to investigate lattice defects in samples. In this chapter, we will introduce the principles of the PAS technique and show some examples of how PAS was applied to studies on hydrogen storage materials. The topics are the formation mechanism of lattice defects by the initial hydrogenation in LaNi₅-based alloys and the effect of hydrogen charging on the formation of lattice defects by tensile strain (hydrogen embrittlement). PAS showed that both vacancies and dislocations are introduced by the initial hydrogenation in LaNi_4.5M_0.5 and by plastic deformation in pure Fe. The presence of hydrogen enhanced the increase in the concentration of vacancies rather than the dislocation density in pure Fe.