Welds are often located at mechanical-stress concentrations, and welds can be regarded as a form of metallurgical notch due to degraded local material properties, defects, and residual stresses. The combination of high stresses, reduced material properties, and probable defects often leads to failure by fracture, fatigue, stress corrosion cracking, or even creep cavitation.

Although developing materials for demanding applications is essential, joining these materials will be a major difficulty. Welds and other methods of joining are inevitably the weak point due to metallurgical inhomogeneity, defects, residual stresses, and dissimilar mechanical properties. Well-characterized residual stresses are essential for assessing the structural integrity of welds. In a similar manner to the stress redistribution that may occur with high temperature, irradiation can change the residual stress distribution in the weld (Fig. 4.1).

Residual stresses may occur between layers in composites such as the com­monly-used austenitic (stainless steel) cladding on ferritic pressure vessels (Fig. 4.2). These layered composites have two forms of stress, any residual stress due to the bonding process, and a thermal mismatch which is a function of the different coefficients of thermal expansion, and the difference between the bonding temperature and the current (measurement or operational) temperature. In some cases the cladding is too thin for residual stress measurements and only the residual stresses in the base material can be measured [5].

The welds and heat-affected zones (HAZs) are areas of concern for SCC because of the presence of as-fabricated flaws, high residual stresses, elevated plastic strains, chemical heterogeneity, and microstructural differences relative to base metals. Dissimilar metal welds are critical areas in nuclear-power systems due to higher residual stresses than for similar-metal welds, and additional thermal stresses during operation due the different coefficients of thermal expansion.

Fig. 4.2 Residual stresses in base material only in Stellite-clad steel specimens. Reprinted with permission from (H. Kohler, K. Partes, J. R. Kornmeier, F. Vollertsen, Phys. Procedia 39, 354 (2012)) [5]. Copyright (2012) Elsevier

Many of the materials of interest for nuclear-power systems are difficult for neutron diffraction due to large grain size in the weld (stainless steels, U, Zr), low scattering (Zr, Ti), or strong attenuation (W). Hexagonal and orthorhombic crystal structures (Ur, Zr) can complicate residual stress measurement due to type II (inter-granular) stresses from elastic, thermal, and plastic anisotropy which are superimposed on the type I macroscopic stresses.

Dissimilar metal welds (austenitic to ferritic) or bonding system (e. g. copper — tungsten composites for the plasma facing component in fusion systems) requires measurement of different reflections necessitating a different instrument configuration for each material.

Inevitably components will need to be modified or repaired and weld repairs complicate an already complex residual stress field. Repair welds are of special interest as they are often made without post-weld heat treatment, producing welds with higher levels of residual stress (and sometimes hydrogen) than conventional welds. Nearly

half of repair welds made on high-energy components in the power-generation industry subsequently fail. Repair welds are more complex than normal fabrication welds as the repair may have significant stop/start thermal fields, may possess further transformation stresses, and overlay an existing residual stress field. Edwards et al. [6] and Bouchard et al. [7] made neutron-diffraction measurements of residual stresses in typical repair welds for the nuclear industry. Figure 4.3 shows a good comparison between residual stresses measured by deep hole drilling and by neutron diffraction for a short weld-repair in a 20 mm thick 316 stainless vessel.

Neeraj Sharma and Marnix Wagemaker

Abstract The effort of material scientists in the discovery, understanding, and development of Li-ion batteries largely depends on the techniques available to observe the relevant processes on the appropriate time and length scales. This chapter aims at demonstrating the role and use of different neutron-scattering techniques in the progress of Li-ion battery electrode and electrolyte properties and function. The large range in time and length scales offered by neutron-scattering techniques is highlighted. This illustrates the type of information that can be obtained, including key parameters such as crystal structure, Li-ion positions, impact of nano-particle size and defects, ionic mobility, as well as the Li-ion distribution in electrodes and at electrode-electrolyte interfaces. Special attention is directed to the development of in situ neutron-scattering techniques providing insight on the function of battery materials under realistic conditions, a promising direction for future battery research.

Zeolitic imidazolate frameworks (ZIFs) constitute a sub-family of MOFs. They somewhat bridge the gap between zeolites and MOFs. The structural topologies of ZIFs are similar to zeolite or zeolite-like topologies [25]. ZIFs have tetrahedral frameworks where transition metals (Zn, Co, etc.) are linked by imidazolate ligands, the angle formed by imidazolates when bridging transition metals being close to the Si-O-Si bond angle in zeolites (^145°). These new materials exhibit exceptional chemical and thermal stability.

Although ZIFs can have large cavities, the windows apertures whereby mass — transport proceeds have rather small diameters. In ZIF-8, the linker between Zn atoms is 2-methylimidazolate, and the diameter of the window is approximately

1.4 A [25]. The diffusivities computed from MD, even for H2, have been found to be quite sensitive to the mobility of the linker [26]. This is illustrated in Fig. 2.9, where a comparison between experimental and calculated self-diffusivities shows that an agreement is only reached for a flexible framework. As detailed in Sect. 2.3.2, quantum corrections were included via the Feynman-Hibbs approach [26]. The influence of flexibility on computed diffusivities in ZIF-3, which has a larger window diameter (4.6 A), was found to be less pronounced, the agreement with QENS values being not as good as in ZIF-8 [26].

Active samples pose additional difficulties; safe handling and sample preparation, transport, shielding, and disposal may all add to the experimental complexity. The standard methods of reducing operator exposure include limiting the amount or size of the sample, increasing the distance from the sample to the operator, limiting exposure time, and the use of shielding [63].

For SANS and texture measurements, the sample size can be favourably small (*10 mm3 in some cases). The sample size in radiography is generally defined by the project scope and the scale of the component to be imaged. When measuring residual stresses, reducing the sample size changes the constraint conditions which can significantly alter the measured residual stress; there are simplified methods of assessing the allowable sample size [64]. An additional issue is that reducing sample size by cutting active samples increases operator exposure and leads to active waste. All these factors must be considered for the safe conduct of the experiment and as well as additional conditions subject to the safety policy of the laboratory. However, in order to understand the real effects of neutron irradiation damage to reactor materials the benefits can sometimes outweigh the complexities involved. By nature, neutron facilities have detailed radiological safety procedures already in place, making the study of radioactive samples perhaps more viable than at other facilities.

Structure is determined using neutron diffraction (ND). The structure factor, S(Q), describes scattered neutrons of the beam in terms of the wave-vector transfer, Q, where Q = 4nsin0/X, and 0 is the angle of the scattered neutrons with X being the incident-neutron wavelength. For a single crystal, the scattering will consist of Bragg peaks. In an ideal powder sample, small crystallites are randomly oriented and scattering from a particular set of lattice planes corresponds to the scattering obtained by turning a single crystal. In powder samples, Debye-Scherrer cones are obtained in place of Bragg peaks, where intensity from the cones can be determined simultaneously using large-area detector arrays.

Currently, postcombustion capture methods, which separate CO2 at low partial pressures from N2 in flue streams, are the most economically viable CO2 capture methods in the short — to mid-term as they can be easily retrofitted to existing power plants. Due to the dilute amount of CO2 present in these flue-gas streams, pro­spective materials for this purpose are required to have a high selectivity for CO2 over N2, which may be achieved by surface or pore functionalisation with strongly — polarising chemical groups. This may be achieved via the incorporation of open — metal cation sites, often exposed upon desolvation or “activation” of the frame­work, which provide strong, highly-charged binding sites for CO2 [55, 56]. Another strategy is the introduction of strongly-polarizing organic functional-groups into the pore. Functional groups investigated previously for CO2 separation from other gases include amines [22, 57], carboxylic acids, nitro, hydroxy, and sulfone groups [23]. Although more strongly polarizing groups enhance CO2 adsorption mani­fested through higher isosteric heats of adsorption, this factor must be balanced against the ease of regenerability of the resulting material in an industrial setting.

Mohamed Zbiri, Lucas A. Haverkate, Gordon J. Kearley, Mark R. Johnson and Fokko M. Mulder

Abstract Organic-based photoconverters are subject to a considerable interest due to their promising functionalities and their potential use as alternatives to the more expensive inorganic analogues. We introduce the basic operational mechanisms, limitations and some ideas towards improving the efficiency of organic solar cells by focusing on probing the morphological/structural, dynamical, and electronic aspects of a model organic material consisting of charge-transfer discotic liquid — crystal system hexakis(n-hexyloxy)triphenylene/2,4,7 trinitro-9-fluorenone (HAT6/ TNF). For the electronic ground-state investigations, neutron-scattering techniques play a key role in gaining deeper insight into structure and dynamics. These measurements are complemented by Raman and nuclear magnetic resonance probes, as well as resonant Raman and UV-vis spectroscopies that are used to explore the low-lying excited states, at the vibronic level. Synergistically, numerical simulations, either classical via empirical force fields, or first-principles via density functional theory, are used for the analysis, interpretation and predictions.

6.1 Introduction

Research on organic devices is focused mainly on three concept organic photovoltaic (OPV) designs in the solid-state phase by using either: (i) vacuum-deposited mole­cules by evaporation of two or more n-and p-dopants [1], (ii) solution-processed polymeric macromolecules [2], and, (iii) the Gratzel concept [3] to build dye-sensi-

M. Zbiri (H) • M. R. Johnson

Institut Max von Laue-Paul Langevin, 71 avenue des Martyrs,

CS 20156, 38042 Grenoble, France e-mail: zbiri@ill. fr

L. A. Haverkate • F. M. Mulder

Faculty of Applied Sciences, Reactor Institute Delft, Delft University of Technology,

Mekelweg 15, 2629 JB Delft, The Netherlands

G. J. Kearley

Australian Nuclear Science and Technology Organisation,

Lucas Heights, NSW, Australia

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DOI 10.1007/978-3-319-06656-1_6

Fig. 6.1 Schematic representation of a p-n heterojunction (a) and a bulk heterojunction (BHJ) of interpenetrating networks of p-and n-dopants (b)

tized devices based on mesoporous nanocrystalline thin-films of TiO2 covered by a dye layer. In terms of architectures, solar cells based on cases (i) and (ii) are similar and can be set either into a heterojunction or abulk heterojunction (BHJ) device (Fig. 6.1).

The former represent the “standard” way of building a p-n junction and the donor-acceptor (DA) interface at which the exciton dissociation into charge carriers should take place. The latter architecture is based on a more elaborated technique of enhancing the p-n junction of setup (i) and making it as an interpenetrating network of the n-and p-dopant materials. It should be noted that in the case of polymer-based OPV both heterojunction and BHJ setups are made by a solution process because of the impossibility of evaporation due to the large macromolecular weight. The Gratzel-type solar cell is based on a different device architecture comprising mes — oporous dye-covered TiO2 films connected through a pore-penetrating liquid electrolyte or an organic n-dopant material, to a transparent counter electrode. Each of these OPV concepts presents different advantages and limitations in terms of efficiency and production costs. The high-vacuum deposition of small molecules ensures good processability and reproducibility of the device in contrast to cases (ii) and (iii). However, it is an expensive and complex technology which limits its use. In this respect, polymer-based OPVs are better because they can be fabricated under ambient conditions. Moreover, polymers in this case can be made soluble and spray-deposited on plastic substrate, which considerably increases their flexibility with obvious benefits for commercialization [4]. The dye-sensitized (Gratzel) solar cell performs the best [5] for power-conversion efficiency (PCE), but because a heat treatment is needed for TiO2 a flexible realization of this architecture is limited.

Herve Jobic

Abstract Catalysis helps to save energy and to produce less waste. Hydrogen will possibly be the energy carrier for the future, but it will not replace oil before several decades so the efficiency of the catalytic processes in petroleum refinery and petrochemistry still has to be improved. Numerous physical techniques are being used to follow catalytic processes. The samples can be subjected to several probes: electrons, photons, ions, neutrons; and various fields can be applied: magnetic, electric, acoustic, etc. Apart from the basic catalyst characterization, the various methods aim to observe surface species (intermediate species are much more tricky), the reaction products, and the influence of diffusion. Coupling of two, three, or more techniques is now common and very powerful. The biggest challenge has always been to perform measurements during the reaction, the term in situ being sometimes replaced by the more recent one operando, when the catalyst is under working conditions of pressure, temperature, flow, and avoiding diffusion limitations.

Due to experimental uncertainties, there is considerable variation in residual stress measurements; a common strategy is to validate measurements by using two or more methods. Some neutron residual stress measurements have been validated (Figs. 4.4 and 4.5) by modelling [8], deep hole drilling [7], or contour methods [8, 9].

Due to the time and complexity of residual stress measurements, computer modelling of residual stresses has been pursued by many groups. The development and validation of models relies heavily on residual stress measurements. As there are significant uncertainties in both modeling and in residual stress measurements, the two activities inform each other’s results. Agreement between modelling and measurements, or between different measurement techniques, improves confidence in both sets of results.

The Versailles Project on Advanced Materials and Standards (VAMAS) Technical Working Area (TWA) 20 ring-and-plug strain round-robin specimen has been used to validate neutron diffraction measurements with good correspondence in results [11].

The European network on neutron techniques standardization for structural integrity (NeT) round robin had a number of samples for both residual stress measurement (by neutron diffraction, with some deep hole drilling and also using the contour method) and modelling [12-15]. The results show good correspondence (Fig. 4.6), although there were some systematic shifts in modelling and contour-method results, when compared to other methods. This led to changes in material descriptions used in modelling, and an appreciation of the effects of localized yielding on contour-method results during cutting.

There are difficulties comparing results as the neutron method averages stresses within the gauge volume. Finite-element analysis (FEA) results are discrete so the results should be volume-averaged to produce values over similar gauge volumes to neutron diffraction results. As real welds often have significant distortion, the spatial position of neutron results should be considered, if they were taken in a straight line they will often be at different distances from the surface while FEA results made on a ‘straight’ weld will be from different areas.

Electrochemical energy storage is attractive, having very high storage efficiencies typically exceeding 90 %, as well as relatively-high energy densities. Li-ion battery technology provides the highest energy densities of commercialized battery-tech­nologies and has found widespread use in portable electronic applications. Appli­cation of Li-ion batteries in electrical vehicles and as static storage media is emerging, however, improved performance and reduced cost, combined with safety

N. Sharma (H)

School of Chemistry, The University of New South Wales, Sydney, NSW 2052, Australia e-mail: neeraj. sharma@unsw. edu. au

M. Wagemaker

Faculty of Applied Sciences, Radiation, Science and Technology Department, Delft University of Technology, Mekelweg 15, 2629 JB Delft, The Netherlands e-mail: m. wagemaker@tudelft. nl

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DOI 10.1007/978-3-319-06656-1_7

Electronic conductor

•• Lithium storage material Binder

enhancements, are required. This has initiated a worldwide research effort for Li-ion electrode and electrolyte materials that combine desirable properties such as high energy and power density, low cost, high abundance of component elements, and electrochemical stability [1—4]. In the current generation of Li-ion batteries, insertion materials that reversibly host Li in the crystal structure form the most important class of electrodes. Although the future of Li-ion batteries looks bright, it should be noted that the availability of a number of relevant transition metals and possibly Li itself is a topic of interest [2].

In a Li-ion battery two insertion-capable electrodes[11] with a difference in Li chemical potential (change in free energy upon Li addition) are in contact through an electrolyte (an ionic conductor and electronic insulator) and a separating membrane, see Fig. 7.1. The Li will flow from the insertion material in which Li has a high chemical-potential towards the electrode in which Li has a low chemical-potential. Only Li-ions can flow through the electrolyte and the charge compensation requires electrons to follow via the external circuit which can be used to power an application. By applying a higher electrical potential than the spontaneous equilibrium open circuit polarization the process can be reversed. High energy-density requires a large
specific-capacity of ions in both electrodes and a large difference in chemical potential. High power (and fast insertion/extraction) requires both electrons and Li-ions to be highly mobile throughout the electrode materials and electrolyte.