Bragg coherent imaging of nanoprecipitates: role of superstructure reflections

Coherent precipitation of ordered phases is responsible for providing exceptional high-temperature mechanical properties in a wide range of compositionally complex alloys. Ordered phases are also essential to enhance the magnetic or catalytic properties of alloyed nanoparticles. The present work aims to demonstrate the relevance of Bragg coherent diffraction imaging (BCDI) for studying bulk and thin-film samples or isolated nanoparticles containing coherent nanoprecipitates/ordered phases. The structures of crystals of a few tens of nanometres in size are modelled with realistic interatomic potentials and are relaxed after introduction of coherent ordered nanoprecipitates. Diffraction patterns from fundamental and superstructure reflections are calculated in the kinematic approximation and used as input to retrieve the strain fields using algorithmic inversion. First, the case of single nanoprecipitates is tackled and it is shown that the strain field distribution from the ordered phase is retrieved very accurately. Then, the influence of the order parameter S on the strain field retrieved from the superstructure reflections is investigated. A very accurate strain distribution can be retrieved for partially ordered phases with large and inhomogeneous strains. Subsequently, the relevance of BCDI is evaluated for the study of systems containing many precipitates, and it is demonstrated that the technique is relevant for such systems. Finally, the experimental feasibility of using BCDI to image ordered phases is discussed in the light of the new possibilities offered by fourth-generation synchrotron sources.

The compliant centrifugal pendulum as the vibration absorber with second-order elasto-kinematic approximation

Centrifugal dampers are used to reduce the vibrations and irregularities of motion of rotating parts subjected to torque perturbations. Compliant mechanisms can be used for building compact and simple embodiments of such devices. On the other hand, the elasto-kinematic behaviour of compliant structures requires a more accurate design with respect to standard solutions. The study discusses a methodology for designing compliant centrifugal dampers based on the arrangement of a collection of leaf flexure hinges connecting peripheral masses. The design is based on the equations of motion written under the approximation of an equivalent pseudo-rigid assembly. To preserve accuracy, the pseudo-rigid surrogate model is deduced taking into account second-order kinematic invariants and Euler–Savary equations, thus providing second-order approximation of the relative motion. This strategy combines accuracy and simplicity. The design can benefit from design equations for choosing the dimensions of the flexible links and the weight of the peripheral masses. An example of application is also discussed and a comparison with a flexible multibody model is also presented to demonstrate the accuracy of the proposed methodology.

Glacier runoff variations since 1955 in the Maipo River basin, in the semiarid Andes of central Chile

As glaciers adjust their size in response to climate variations, long-term changes in meltwater production can be expected, affecting the local availability of water resources. We investigate glacier runoff in the period 1955–2016 in the Maipo River basin (4843 km2, 33.0–34.3∘ S, 69.8–70.5∘ W), in the semiarid Andes of Chile. The basin contains more than 800 glaciers, which cover 378 km2 in total (inventoried in 2000). We model the mass balance and runoff contribution of 26 glaciers with the physically oriented and fully distributed TOPKAPI (Topographic Kinematic Approximation and Integration)-ETH glacio-hydrological model and extrapolate the results to the entire basin. TOPKAPI-ETH is run at a daily time step using several glaciological and meteorological datasets, and its results are evaluated against streamflow records, remotely sensed snow cover, and geodetic mass balances for the periods 1955–2000 and 2000–2013. Results show that in 1955–2016 glacier mass balance had a general decreasing trend as a basin average but also had differences between the main sub-catchments. Glacier volume decreased by one-fifth (from 18.6±4.5 to 14.9±2.9 km3). Runoff from the initially glacierized areas was 177±25 mm yr−1 (16±7 % of the total contributions to the basin), but it shows a decreasing sequence of maxima, which can be linked to the interplay between a decrease in precipitation since the 1980s and the reduction of ice melt. Glaciers in the Maipo River basin will continue retreating because they are not in equilibrium with the current climate. In a hypothetical constant climate scenario, glacier volume would reduce to 81±38 % of the year 2000 volume, and glacier runoff would be 78±30 % of the 1955–2016 average. This would considerably decrease the drought mitigation capacity of the basin.

Investigating Parameter Transferability across Models and Events for a Semiarid Mediterranean Catchment

Physically based distributed hydrologic models (DHMs) simulate watershed processes by applying physical equations with a variety of simplifying assumptions and discretization approaches. These equations depend on parameters that, in most cases, can be measured and, theoretically, transferred across different types of DHMs. The aim of this study is to test the potential of parameter transferability in a real catchment for two contrasting periods among three DHMs of varying complexity. The case study chosen is a small Mediterranean catchment where the TIN-based Real-time Integrated Basin Simulator (tRIBS) model was previously calibrated and tested. The same datasets and parameters are used here to apply two other DHMs—the TOPographic Kinematic Approximation and Integration model (TOPKAPI) and CATchment HYdrology (CATHY) models. Model performance was measured against observed discharge at the basin outlet for a one-year period (1930) corresponding to average wetness conditions for the region, and for a much drier two-year period (1931–1932). The three DHMs performed comparably for the 1930 period but showed more significant differences (the CATHY model in particular for the dry period. In order to improve the performance of CATHY for this latter period, an hypothesis of soil crusting was introduced, assigning a lower saturated hydraulic conductivity to the top soil layer. It is concluded that, while the physical basis for the three models allowed transfer of parameters in a broad sense, transferability can break down when simulation conditions are greatly altered.

Modeling Hydrological Response to Climate Change in a Data-Scarce Glacierized High Mountain Astore Basin Using a Fully Distributed TOPKAPI Model

Water scarcity is influencing environmental and socio-economic development on a global scale. Pakistan is ranked third among the countries facing water scarcity. This situation is currently generating intra-provincial water disputes and could lead to transboundary water conflicts. This study assessed the future water resources of Astore basin under representative concentration pathways (RCP) 4.5 and 8.5 scenarios using fully distributed TOPographic Kinematic APproximation and Integration (TOPKAPI) model. TOPKAPI model was calibrated and validated over five years from 1999–2003 with a Nash coefficient ranging from 0.93–0.97. Towards the end of the 21st century, the air temperature of Astore will increase by 3°C and 9.6 °C under the RCP4.5 and 8.5 scenarios, respectively. The rise in air temperature can decrease the snow cover with Mann Kendall trend of –0.12%/yr and –0.39%/yr (p ≥ 0.05) while annual discharge projected to be increased 11% (p ≤ 0.05) and 37% (p ≥ 0.05) under RCP4.5 and RCP8.5, respectively. Moreover, the Astore basin showed a different pattern of seasonal shifts, as surface runoff in summer monsoon season declined further due to a reduction in precipitation. In the spring season, the earlier onset of snow and glacier melting increased the runoff due to high temperature, regardless of the decreasing trend of precipitation. This increased surface runoff from snow/glacier melt of Upper Indus Basin (UIB) can potentially be utilized to develop water policy and planning new water harvesting and storage structures, to reduce the risk of flooding.

Limitations of the kinematic approximation in neutron reflectivity measurements for the analysis of bilayers

The limitations of a phenomenological fitting approach compared to simulations of the optical model including reflection and refraction at all interfaces are demonstrated using the example of hydrogen loading in ultra-thin vanadium layers. Fe/V superlattices are loaded with deuterium and the lattice expansion and deuterium concentration are extracted from neutron reflectivity data. A noticeable difference is found between the extraction of concentrations and bilayer thicknesses directly from the superlattice peaks and fits of the density profile using the Parratt formalism. The results underline the importance of carefully considering the limitations of phenomenological approaches, in order to obtain robust results. The limitations of the kinematic approximation for the analysis are discussed in detail.

Effect of the solid/liquid interface structure on X-ray diffraction in nano-biocomposites

It is shown that periodic modulation of electron density in a liquid layer surrounding a nanocrystal may influence considerably the width of a conventional diffraction profile taken along the normal direction to the liquid/solid interface. The kinematic approximation is used to develop an analytical expression for the diffraction profile, assuming that the degree of order in the modulated liquid-like layers diminishes exponentially with the distance from the interface, which is characterized by the correlation length, L. Owing to the above-mentioned modulation, the sizes of nanocrystals extracted from the width of diffraction profiles will appear larger than they really are. Molecular ordering is destroyed by mild annealing or pressure application, resulting in substantial broadening of X-ray diffraction lines. This effect may be most significant in nano-biocomposites, such as bone and tooth dentin, comprising substantial amounts of water (tens of percent). It is calculated that for L ≃ 1 nm and a net crystallite thickness of T < 50 nm, the relative change in profile width can reach a few percent, which is easily measurable. The obtained simulation results are compared with existing experimental data.