arXiv subjects

Alexis Front

Publications and source records attributed to Alexis Front.

4 recordsLinked to original sources

arXiv2025

Density-dependent sodium-storage mechanisms in hard carbon materials

Understanding the sodium-storage mechanism in hard carbon (HC) anodes is crucial for advancing sodium-ion battery (SIB) technology. However, the intrinsic complexity of HC microstructures and their interactions with sodium remain not fully elucidated. We present a multiscale methodology that integrates grand-canonical Monte Carlo (GCMC) simulations with a machine-learning interatomic potential based on the Gaussian approximation potential (GAP) framework to investigate sodium insertion mechanisms in hard carbons with different levels of porosity, achieved by simulating structural models with densities ranging from 0.7 to 1.9 g cm$^{-3}$. Structural and thermodynamic analyses reveal the interplay between pore size and accessibility and the relative contributions of adsorption, intercalation, and pore filling to the overall storage capacity. Low-density carbons favor pore-filling, achieving extremely high capacities at near-zero voltages, whereas high-density carbons primarily store sodium through adsorption and intercalation, leading to lower but more stable capacities. Intermediate-density carbons ($1.3-1.6$ g cm$^{-3}$) provide the most balanced performance, combining moderate capacity (480 and 310 mAh g$^{-1}$), safe operating voltages, and minimal volume expansion ($<10$\%). These findings establish a direct correlation between carbon density and electrochemical behavior, providing atomic-scale insight into how hard carbon morphology governs sodium-storage. The proposed framework offers a rational design principle for optimizing HC-based SIB anodes toward high energy density and long-term cycling stability.

cond-mat.mtrl-sci↗

arXiv2024

Size effect on the structural and magnetic phase transformations of iron nanoparticles

Iron nanoparticles are among the most promising low-dimensional materials in terms of applications. This particularity is attributable to the magnetic properties of these nanoparticles, which exhibit different allotropes as a function of temperature. In this work, we sought to characterise at the atomic scale how their structural and magnetic transformations can be affected by the size. To achieve this objective, we developed a tight-binding model incorporating a magnetic contribution via a Stoner term implemented in a Monte Carlo code to relax the structure and the magnetic state. Using our approach, we show that magnetism is strongly reinforced by the surface, which leads to increase the Curie temperature as the size of the particle decreases contrary to the solid-solid transition temperature. Our work thus provides a deep understanding at the atomic scale of the key factors that determines the structural and magnetic properties of Fe nanoparticles, shedding more light on their unique character which is crucial for further applications.

cond-mat.mtrl-sci↗

arXiv2022

Melting properties of Ag$_x$Pt$_{1-x}$ nanoparticles

At the nanoscale, materials exhibit unique properties that differ greatly from those of the bulk state. In the case of Ag$_x$Pt$_{1-x}$ nanoalloys, we aimed to study the solid-liquid transition of nanoparticles of different sizes and compositions. This system is particularly interesting since Pt has a high melting point (2041 K compare to 1035 K for Ag) which could keep the nanoparticle solid during different catalytic reactions at relatively high temperatures, such as we need in the growth of nanotubes. We performed atomic scale simulations using semi-empirical potential implemented in a Monte Carlo code at constant temperature and chemical composition in canonical ensemble. We observed that the melting temperature decreases with the size (pure systems and alloys) and the composition. We show that the melting systematically passes through an intermediate stage with a crystalline core (pure platinum or mixed PtAg depending on the composition) and a pure silver liquid skin, which strongly questions the idea of having a faceted solid particle in catalytic reactions for carbon nanotubes synthesis.

cond-mat.mtrl-sci↗

arXiv2021

Simulation of thermodynamic properties of magnetic transition metals from an efficient tight-binding model

Atomic scale simulations at finite temperature are an ideal approach to study the thermodynamic properties of magnetic transition metals. However, the development of interatomic potentials explicitly taking into account magnetic variables is a delicate task. In this context, we present a tight-binding model for magnetic transition metals in the Stoner approximation. This potential is integrated into a Monte Carlo structural relaxations code where trials of atomic displacements as well as fluctuations of local magnetic moments are performed to determine the thermodynamic equilibrium state of the considered systems. As an example, the Curie temperature of cobalt is investigated while showing the important role of atomic relaxations. Furthermore, our model is generalized to other transition metals highlighting a local magnetic moment distribution that varies with the gradual filling of the d states. Consequently, the successful validation of the potential for different magnetic configurations indicates its great transferability makes it a good choice for atomistic simulations sampling a large configuration space.

cond-mat.mtrl-sci↗