Controlling the {111}/{110} Surface Ratio of Cuboidal Ceria Nanoparticles

The ability to control size and morphology is crucial in optimizing nanoceria catalytic activity as this is governed by the atomistic arrangement of species and structural features at the surfaces. Here, we show that cuboidal cerium oxide nanoparticles can be obtained via microwave-assisted hydrothermal synthesis in highly alkaline media. HRTEM revealed that the cube edges were truncated by CeO2{110} surfaces and the cube corners by CeO2{111} surfaces. When adjusting synthesis conditions by increasing NaOH concentration, the average particle size increased. Although this was accompanied by an increase of the cube faces, CeO2{100}, the cube edges, CeO2{110}, and cube corners, CeO2{111} remained of constant size. Molecular Dynamics (MD) was used to rationalise this behaviour and revealed that energetically, the corners and edges cannot be atomically sharp, rather they are truncated by {111} and {110} surfaces respectively to stabilise the nanocube; both experiment and simulation agreed a minimum size of ~1.6 nm associated with this truncation. Moreover, HRTEM and MD revealed {111}/{110} faceting of the {110} edges, which balances the surface energy associated with the exposed surfaces, which follows {111}>{110}>{100}, although only the {110} surface facets because of the ease of extracting oxygen from its surface, which follows {111}>{100}>{110}. Finally, MD revealed that the {100} surfaces are ‘liquid-like’ with a surface oxygen mobility 5 orders of magnitude higher than that on the {111} surfaces; this arises from the flexibility of the surface species network that can access many different surface arrangements due to very small energy differences. This finding has implications for understanding the surface chemistry of nanoceria and provides avenues to rationalize the design of catalytically active materials at the nanoscale

A 2xxx aluminum alloy crept at medium temperature: role of thermal activation on dislocation mechanisms

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A study of the hardening precipitation in a 2650 aluminium alloy for aeronautics

International audienceThis paper focus on the hardening precipitation in a 2650 Al(CuMg) aluminium alloy and on its interactions with the aluminium matrix. The study has been carried out using High Resolution Transmission Electron Microscopy (HRTEM) images analysis and simulated images. Two types of precipitation (an homogeneous one and an heterogeneous one) and two types of precipitates (fine needles and S-laths) can be distinguished. A tomographic atom probe analysis of the fine needles reveals a strong copper and magnesium atom enrichment. The lath shaped precipitates can be sorted into two types, each one presenting specific interface morphology and orientation relationship with respect to the matrix

A study of the hardening precipitation in a 2650 aluminium alloy for aeronautics

International audienceThis paper focus on the hardening precipitation in a 2650 Al(CuMg) aluminium alloy and on its interactions with the aluminium matrix. The study has been carried out using High Resolution Transmission Electron Microscopy (HRTEM) images analysis and simulated images. Two types of precipitation (an homogeneous one and an heterogeneous one) and two types of precipitates (fine needles and S-laths) can be distinguished. A tomographic atom probe analysis of the fine needles reveals a strong copper and magnesium atom enrichment. The lath shaped precipitates can be sorted into two types, each one presenting specific interface morphology and orientation relationship with respect to the matrix

High-resolution transmission electron microscopy and tomographic atom probe studies of the hardening precipitation in an Al-Cu-Mg alloy

International audienceThe hardening precipitation of an Al-Cu-Mg aluminium alloy designed for aeronautics was investigated using high-resolution transmission electron microscopy (HREM) and tomographic atom probe techniques. The observed precipitates clearly belong either to the Guinier-Preston-Bagaryatskii (GPB) zones type or to the so-called S-Al2CuMg precipitation. We analysed a large number of precipitates in order to obtain statistical information on the precipitation. We focused on the structural and/or chemical composition of the different precipitates. It was found, in particular, that the very numerous GPB zones do not present a single chemical composition. Evidence is also given for the presence of two different kinds of S-precipitate/matrix orientation relationships, strongly linked to the morphology of the precipitate. The structure of the S precipitates was confirmed by direct comparison with simulated HREM images. Particular attention was paid to the nature of the S-precipitate/matrix interfaces

High-resolution transmission electron microscopy and tomographic atom probe studies of the hardening precipitation in an Al-Cu-Mg alloy

International audienceThe hardening precipitation of an Al-Cu-Mg aluminium alloy designed for aeronautics was investigated using high-resolution transmission electron microscopy (HREM) and tomographic atom probe techniques. The observed precipitates clearly belong either to the Guinier-Preston-Bagaryatskii (GPB) zones type or to the so-called S-Al2CuMg precipitation. We analysed a large number of precipitates in order to obtain statistical information on the precipitation. We focused on the structural and/or chemical composition of the different precipitates. It was found, in particular, that the very numerous GPB zones do not present a single chemical composition. Evidence is also given for the presence of two different kinds of S-precipitate/matrix orientation relationships, strongly linked to the morphology of the precipitate. The structure of the S precipitates was confirmed by direct comparison with simulated HREM images. Particular attention was paid to the nature of the S-precipitate/matrix interfaces

Cross-slip and glide in {001} planes of Al-Mg-Si alloy 6056

International audienceIn-situ straining experiments and post-mortem observations in a transmission electron microscope have been performed between 20 and 170C on Al-Mg-Si alloy 6056. Evidence is found for activation of cross-slip and occurrence of glide in the {001} planes. These mechanisms occur more readily as temperature increases. Their existence is discussed in the light of microstructural characteristics of the alloy and an attempt is made to correlate them with macroscopic properties

Realizing field-dependent conduction in ZnO nanowires without annealing

We report on the low-temperature fabrication of field-effect transistors by bridging pre-patterned electrodes using ZnO nanowires grown in situ, which operate without requiring post-growth processing or annealing. The devices show good performance using as-grown nanowires, with on–off ratios of 105 and threshold voltages of 2 V. Electron microscopy shows the field-dependent nanowires hierarchically nucleate from larger ZnO nanorods, and both are oriented along a common c-axis. A high nanowire surface-to-volume ratio allows depleting electron traps on the nanowire surface to compensate intrinsic electron donors present throughout the nanowire bulk. This eliminates the need to reduce the electron concentration through high-temperature annealing, making the nanowires naturally field-dependent in their as-grown state

Growth and relaxation mechanisms in La0.66Sr0.33MnO3 manganites deposited on SrTiO3(0 0 1) and MgO(0 0 1)

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