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Amorphous carbon and amorphous materials in general are of particular importance for high resolution electron microscopy, either for bulk materials, generally covered with an amorphous layer when prepared by ion milling techniques, or for nanoscale objects deposited on amorphous substrates. In order to quantify the information of the high resolution images at the atomic scale, a structural modeling of the sample is necessary prior to the calculation of the electron wave function propagation. It is thus essential to be able to reproduce the carbon structure as close as possible to the real one. The approach we propose here is to simulate a realistic carbon from an energetic model based on the tight-binding approximation in order to reproduce the important structural properties of amorphous carbon. At first, we compare this carbon with the carbon obtained by randomly generating the carbon atom positions. In both cases, we discuss the limit thickness of the phase object approximation. In a second step, we show the influence of both carbons models on (i) the contrast of Cu, Ag, and Au single atoms deposited on carbon and (ii) the determination of the long-range order parameter in CoPt bimetallic nanoalloys. [ABSTRACT FROM AUTHOR]

The aim of this paper is to investigate the consequences of atomic size mismatch on the thermodynamics and the topology of binary phase diagrams of face centred cubic alloys. Simple pairwise interatomic potentials with few controlling parameters are used to identify general tendencies. Thermodynamic states are computed by Monte Carlo simulations on a non-rigid lattice. A special attention has been paid to the comparison between calculations in the canonical ensemble, where composition–temperature phase diagrams are determined through van der Waals loops, and in the grand canonical ensemble, where phase diagrams are computed using an interface migration technique. It is shown that these two procedures lead essentially to the same incoherent phase diagram. In the case of phase separating systems, we argue that the introduction of a size mismatch leads to a shrinkage of the solid solution domain and that the asymmetry of the miscibility gap is essentially controlled by the anharmonicity of the heteroatomic potential. Finally, in the case of ordering systems, we show that the asymmetry of the phase diagram may be due to the anharmonicity of the pair potentials or to the differences between their curvatures, the former effect being dominant if the atomic size mismatch is large. [ABSTRACT FROM AUTHOR]

An elasto-viscoplastic model is developed to study the microstructural evolution during creep loading in a model AM1 superalloy. Elastic anisotropy and inhomogeneity, as well as the description of long-range order in the γ ' phase, are included in the model. Plastic activity is introduced using a continuum crystal plasticity framework at the mesoscale. Special attention is paid to the corresponding parameter identification from experiments. Two-dimensional simulations of creep in the [100] direction are performed, and the results are compared to the predictions of an elastic phase field model, in order to characterize the influence of plastic activity on the microstructural evolution. In particular, our simulations show that plastic activity in the γ channels significantly increases the rafting kinetics and allows misalignments of rafts with respect to cubic directions. The simulation results are critically discussed and improvements to the model are proposed. [ABSTRACT FROM AUTHOR]

Chemically ordered bimetallic nanoparticles are promising candidates for magnetic-storage applications. However, the use of sub-10 nm nanomagnets requires further study of possible size effects on their physical properties. Here, the effects of size and morphology on the order–disorder phase transition temperature of CoPt nanoparticles (TCNP) have been investigated experimentally, using transmission electron microscopy, and theoretically, with canonical Monte Carlo simulations. For 2.4–3-nm particles, TCNP is found to be 325–175 ∘C lower than the bulk material transition temperature, consistent with our Monte Carlo simulations. Furthermore, we establish that TCNP is also sensitive to the shape of the nanoparticles, because only one dimension of the particle (that is, in-plane size or thickness) smaller than 3 nm is sufficient to induce a considerable depression of TCNP. This work emphasizes the necessity of taking into account the three-dimensional morphology of nano-objects to understand and control their structural properties. [ABSTRACT FROM AUTHOR]

Investigates the relation between metal electronic structure and morphology of metal compounds inside carbon nanotubes. Characteristics of nanocomposite materials; Filling mechanism and materials; Use of the arc-discharge method; Existence of an incomplete electronic shell in the stable ionic state of the metal.

Extended abstract of a paper presented at Microscopy and Microanalysis 2012 in Phoenix, Arizona, USA, July 29 – August 2, 2012. [ABSTRACT FROM PUBLISHER]

Extended abstract of a paper presented at Microscopy and Microanalysis 2010 in Portland, Oregon, USA, August 1 – August 5, 2010. [ABSTRACT FROM PUBLISHER]

Starting from an atomic interaction model of an alloy with a clustering tendency, a mesoscopic thermodynamic simulation method is developed. The alloy is represented by a mesoscopic rigid lattice of cells characterized by the number of solute atoms they contain and a discrete cellular Monte Carlo (CMC) algorithm is designed to perform simulations at this scale. The central quantity is a mesoscopic free energy, whose components (local free energy and stiffness parameter) are extracted from the equilibrium properties of single-cell and two-cell systems. Particular attention has been paid to properly incorporate, at mesoscale, the correlations between neighboring cells with identical concentrations. Finally, the ability of the CMC method to compute thermodynamic properties such as phase diagrams, interface profiles, and equilibrium fluctuation spectra is illustrated on a model alloy as well as on an Ising model with interactions up to the second-nearest neighbors, previously developed for body-centered-cubic Fe-Cu alloys. The very good agreement between the CMC calculation results for different cell sizes and direct computations at the atomic scale indicate that the proposed scale change is thermodynamically consistent [ABSTRACT FROM AUTHOR]

We demonstrate the use and benefits of quantum annealing approaches for the determination of equilibrated microstructures in shape memory alloys and other materials with long-range elastic interaction between coherent grains and their different martensite variants and phases. After a one dimensional illustration of the general approach, which requires to formulate the energy of the system in terms of an Ising Hamiltonian, we use distant dependent elastic interactions between grains to predict the variant selection for different transformation eigenstrains. The results and performance of the computations are compared to classical algorithms, demonstrating that the new approach can lead to a significant acceleration of the simulations. Beyond a discretization using simple cuboidal elements, also a direct representation of arbitrary microstructures is possible, allowing fast simulations with currently up to several thousand grains., (© 2023. The Author(s).)

The surfactant used during a colloidal synthesis is known to control the size and shape of metallic nanoparticles. However, its influence on the nanoparticle (NP) structure is still not well understood. In this study, we show that the surfactant can significantly modify the lattice parameter of a crystalline particle. First, our electron diffraction measurements reveals that NiPt nanoparticles around 4 nm in diameter covered by a mixture of oleylamine and oleic acid (50:50) display a lattice parameter expansion around 2% when compared to the same particles without surfactant. Using high-resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS), and energy-dispersive X-ray spectroscopy (EDX) techniques, we show that this expansion can not be explained by crystal defects, twinning, oxidation, or atoms insertion. Then, using covered NPs in the 4-22 nm size range, we show that the lattice parameter evolves linearly with the inverse of the NP size, as it is expected when a surface stress is present. Finally, the study is extended to pure nickel and pure platinum NPs, with different sizes, coated by different surfactants (oleylamine, trioctylphosphine, polyvinylpyrrolidone). The surfactants induce lattice parameter variations, whose magnitude could be related to the charge transfer between the surfactant and the particle surface.

Phase field modeling offers an extremely general framework to predict microstructural evolutions in complex systems. However, its computational implementation requires a discretization scheme with a grid spacing small enough to preserve the continuous character of the theory. We present here a new formulation, which is intrinsically discrete, in which the interfaces are resolved with essentially one grid point with no pinning on the grid and an accurate rotational invariance, improving drastically the numerical capabilities of the method. We show that interfacial kinetic properties are reproduced with a high accuracy. Finally, we apply the model to a situation where conserved and nonconserved fields are coupled.

A method for measuring and correcting the row displacement errors in lattice images acquired using scanning based methods is presented. This type of distortion is apparent in lattice-resolved images acquired using scanning-based techniques such as scanning transmission electron microscopy (STEM) and translates to vertical streaks convolving every feature in Fourier space. This paper presents a method to measure and correct the distortion based on the phase analysis of the streaks in Fourier space. The validity and the precision of the method is demonstrated using a model image and two experimental STEM images of Si <110> thin film and a 5 nm CoPt disordered nanocrystal. The algorithm is implemented in a freely available Digital Micrograph™ script., (Copyright © 2012 Elsevier B.V. All rights reserved.)

Ostwald ripening has been broadly studied because it plays a determinant role in the evolution of cluster size during both chemical and physical synthesis of nanoparticles. This thermoactivated process causes large particles to grow, drawing material from the smaller particles, which shrink. However, this phenomenon becomes more complex when considering the coarsening of metallic alloy clusters. The present experimental and theoretical investigations show that the relative composition of CoPt nanoparticles can be strongly modified during high temperature annealing and displays a size-dependent behavior. This compositional change originates from the higher evaporation rate of Co atoms from the nanoparticles. More importantly, this effect is expected in all alloy clusters containing species with different mobilities.

We propose an original route to prepare magnetic alloy nanoparticles with uniform size and shape by using nanosecond annealing under pulsed laser irradiation. As demonstrated here on CoPt nanoparticles, flash laser annealing gives an unprecedented opportunity to control the size and the shape of bimetallic nanoparticles without changing their composition. The mechanisms involved in the complete reshaping of the nanoparticle thin films are discussed and it is also shown that order-disorder phase transformations occur under laser irradiation. This technique is then very interesting for magnetic alloy nanoparticles studies and applications because it opens up a new way to fabricate size-controlled spherical nanoparticles with narrow size dispersion.

We present the first derivation of the phase field equations using a coarse-graining procedure on a microscopic master equation. The procedure leads to a mesoscopic nonlinear Fokker-Planck equation equivalent to a Cahn-Hilliard equation supplemented with a noise, but with specific prescriptions for the mobilities and the noise term. All the ingredients (chemical potentials, mobilities, stiffness coefficient) depend on the coarse-graining size. Finally, we show the ability of the phase field equations to describe a precipitation kinetics involving a nucleation and growth mechanism.

Core/shell CuAg and alloyed CoPt have been synthesized using two vapor phase deposition techniques. For CuAg prepared by Thermal Evaporation (TE), the size and the morphology of the Cu cores are the key parameters to promote the formation of the core/shell arrangement. For CoPt synthesized by Pulsed Laser Deposition (PLD), the growth kinetics of nanoparticles, depending on the deposition rate, the substrate nature and the temperature, controls the nanoparticle morphology. The competition between the growth and the ordering kinetics governs the nanoparticle structure. By reducing the growth kinetics, as-grown L1(0) ordered nanoparticles are obtained according to the bulk phase diagram.