arXiv subjects

Beihan Chen

Publications and source records attributed to Beihan Chen.

7 recordsLinked to original sources

arXiv2026

Thermal Transport in Defective Uranium Nitride: Effects of Point Defects, Anharmonicity, and Electronic Contributions

The impact of point defects on thermal transport in uranium nitride (UN) is investigated using a MLIP combined with Green-Kubo (GK) and normal mode analysis (NMA) methods over 300-1500 K. In pristine UN, temperature-dependent calculations of lattice thermal conductivity reveal that four-phonon scattering is essential yet sufficient to accurately capture high temperature anharmonic phonon transport, as evidenced by close agreement between GK and ShengBTE calculations including three- and four-phonon processes. In defective systems, all types of point defects significantly reduce thermal conductivity at low temperature. Mode-resolved analysis further shows that interstitial defects introduce new phonon states due to a stronger local strain effect. Notably, the uranium interstitial leads to strong defect-phonon scattering over broad phonon spectrum, while the other point defects produce more selective scattering, with even reduced phonon scattering for some acoustic modes. The optical contribution to thermal conductivity remains nearly constant in the presence of IU, but decreases with increasing temperature for pristine and the other defect types. The total thermal conductivity, incorporating electron-phonon coupling and an estimated electronic contribution, yields excellent agreement with experiment in the pristine system, with electronic contributions dominating thermal transport above 600 K. Moreover, with defect-electron contribution introduced through a semiclassical electron-defect scattering model, it is found that (i) the total conductivity degradation follows IU, VU, IN, and VN in descending order, and (ii) electron-phonon coupling becomes negligible in defective systems. These results provide a unified understanding of defect-dependent thermal transport in UN.

cond-mat.mtrl-sci↗

arXiv2026

Chemical Short-Range Order Regulates Hydrogen Energetics and Hydrogen-Dislocation Interactions in CoNiV

Chemical short-range order (CSRO) has emerged as a critical structural feature in concentrated alloys, yet its coupling with hydrogen remains an active discussion. Here, we develop a machine-learning interatomic potential for the Co-Ni-V-H system and investigate how CSRO regulates hydrogen energetics and dislocation behavior in CoNiV, an alloy with reported strong resistance to hydrogen embrittlement. We identify strong V-centered ordering that suppresses V-V clustering and significantly reshapes the hydrogen solution landscape. Compared to a chemically random alloy, the ordered state exhibits higher average hydrogen solution energies and a reduced population of strongly binding sites, indicating lower bulk hydrogen uptake. At partial dislocations, hydrogen preferentially segregates to tensile core regions, acting as a shallow, reversible trap with a much weaker effect compared to chemical trapping states. These results demonstrate that local chemical order strongly regulates hydrogen-dislocation coupling and provide an atomistic understanding for tuning hydrogen-assisted deformation in concentrated CoNiV alloys.

cond-mat.mtrl-sci↗

arXiv2025

Evaluation of Structural Properties and Defect Energetics in Al$_x$Ga$_{1-x}$N Alloys

Al$_x$Ga$_{1-x}$N alloys are essential for high-performance optoelectronic and power devices, yet the role of composition on defect energetics remains underexplored, largely due to the limitations of first-principles methods in modeling disordered alloys. To address this, we employ a machine learning interatomic potential (MLIP) to investigate the structural and defect-related physical properties in Al$_x$Ga$_{1-x}$N. The MLIP is first validated by reproducing the equation of state, lattice constants, and elastic constants of the binary endpoints, GaN and AlN, as well as known defect formation and migration energies from density functional theory and empirical potentials. We then apply the MLIP to evaluate elastic constants of AlGaN alloys, which reveals a non-linear relation with alloying effect. Our results reveal that nitrogen Frenkel pair formation energies and the migration barriers for nitrogen point defects are highly sensitive to the local chemical environment and migration path. In contrast, Ga and Al vacancy migration energies remain relatively insensitive to alloy composition, whereas their interstitial migration energies exhibit stronger compositional dependence. These results provide quantitative insight into how alloying influences defect energetics in AlGaN, informing defect engineering strategies for improved material performance.

cond-mat.mtrl-sci↗

arXiv2025

Machine learning interatomic potential for predicting the thermal properties of uranium nitride

We present a combined computational and experimental investigation of the thermal properties of uranium nitride (UN), focusing on the development of a machine learning interatomic potential (MLIP) using the moment tensor potential (MTP) framework. The MLIP was trained on density functional theory (DFT) data and validated against various quantities including energies, forces, elastic constants, phonon dispersion, and defect formation energies, achieving excellent agreement with DFT calculations, prior experimental results and our thermal conductivity measurement. The potential was then employed in molecular dynamics (MD) simulations to predict key thermal properties such as melting point, thermal expansion, specific heat, and thermal conductivity. To further assess model accuracy, we fabricated a UN sample and performed new thermal conductivity measurements representative of single-crystal properties, which showed strong agreement with the MLIP predictions. This work confirms the reliability and predictive capability of the developed potential for determining the thermal properties of UN.

cond-mat.mtrl-sci↗

arXiv2025

Anomalous Ionic Conductivity along the Coherent $\Sigma$3 Grain Boundary in ThO2

Understanding oxygen diffusion along grain boundaries (GBs) is critical for controlling ionic conductivity in oxide ceramics. GBs are typically thought to enhance ionic transport due to structural disorder and increased free volume. In this study, we report an unexpected anomaly: the $\Sigma 3(111)$ GB in thorium dioxide (ThO$_2$), despite its compact and coherent structure, exhibits significantly higher oxygen ionic conductivity compared to the more open GB ($\Sigma 19$ as an example). Using atomistic simulations based on a machine learning interatomic potential, we revealed that the high conductivity in the $\Sigma 3$ GB arises from a collective diffusion mechanism involving highly correlated atomic motion reminiscent of a superionic state. In contrast, the $\Sigma 19$ GB follows conventional pipe diffusion, consistent with its more open structure. This comparison highlights that enhanced GB conductivity is not simply correlated with free volume, but can occur from specific structural motifs that enable collective transport. These findings provide new guidance for designing GB-engineered oxides with targeted ionic transport properties for energy applications.

cond-mat.mtrl-sci↗

arXiv2024

Phonon modal analysis of thermal transport in ThO2 with point defects using equilibrium molecular dynamics

Defects can significantly degrade the thermal conductivity of ThO2, an advanced nuclear fuel material as well as a surrogate for other fluorite-structured materials. We investigate how point defects in ThO2 impact phonon mode-resolved thermal transport. By incorporating phonon modes from lattice dynamics, we decompose the trajectory and heat flux to phonon normal mode space and extract key phonon properties, including phonon relaxation times and their contributions to thermal conductivity. We implement two methods. The first method is based on the Green Kubo formalism to resolve the contribution of each phonon mode to thermal conductivity. The second resolves the lifetime of individual phonon modes and the thermal conductivity is calculated using the Boltzmann transport equation within relaxation time approximation. Notably, a lower contribution of acoustic modes is revealed compared to perturbative approaches considering only three phonon scattering processes. The effects of four types of point defects are evaluated. The strongest impact on a reduction in thermal conductivity is from Th interstitials, followed by Th vacancies. O interstitials/vacancies have a similar impact, albeit smaller than defects on the thorium sublattice. These observations are consistent with previous studies.

cond-mat.mtrl-sci↗

arXiv2023

Extended defects-enhanced oxygen diffusion in ThO2

Oxygen self-diffusion is key to understanding stoichiometry and defect structures in oxide nuclear fuels. Experimentally, low activation-barrier oxygen migration was found in ThO$_2$, a candidate nuclear fuel, possibly due to short-circuit diffusion mechanisms. Here, we perform extensive molecular dynamics simulations to show that various types of extended defects can enhance oxygen self-diffusion with a much-reduced activation barrier in ThO$_2$. In this work, we consider extended defects including 1D (dislocation), 2D (grain boundary), and 3D (void) defects. Due to the distinct characteristics of each type of extended defect, the modulation of oxygen diffusion varies. These results provide a quantitative description of oxygen transport, which is significantly enhanced within a close distance (nanometer scale) from the extended defects. Among all these defects, grain boundary, particularly the $\Sigma 3$ twin boundary with a low formation energy, exhibits the strongest effect on increasing oxygen transport.

cond-mat.mtrl-sci↗