arXiv subjects

Miaomiao Jin

Publications and source records attributed to Miaomiao Jin.

At least 19 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

Atomistic mechanism of corrosion-induced grain boundary migration in NiCr alloys in molten FLiNaK

Corrosion of Ni-Cr structural alloys in molten fluoride salts is a persistent material degradation problem, yet the atomistic role of grain boundaries in this process remains poorly understood. Here we use reactive molecular dynamics to investigate corrosion of NiCr alloys in molten FLiNaK across four representative grain boundaries ($\Sigma3(111)$, $\Sigma11(113)$, $\Sigma5(012)$, and $\Sigma5(013)$) and corresponding bulk surfaces. Surface crystallography controls the initial dissolution stage, while grain boundary character governs the spatial localization and longer-time evolution of corrosion. We further identify a corrosion-driven grain boundary migration mechanism in which fluorine localization, preferential chromium dissolution, and vacancy-mediated mobility together drive interfacial motion away from the dealloyed region. The coherent $\Sigma3(111)$ boundary suppresses these processes, indicating low-energy special boundaries as targets for grain boundary engineering of corrosion-resistant Ni-Cr alloys.

cond-mat.mtrl-sci↗

arXiv2026

Demonstrating Quadratic Monte Carlo Speedup via Quantum Amplitude Estimation: Nuclear Engineering Examples

We demonstrate quantum amplitude estimation (QAE) as a route to quadratic speedup for Monte Carlo-type expectation values in nuclear engineering. Using QPE-based QAE, we study two examples: a discrete fission-neutron-yield expectation and a U-238 resonance integral under a $1/E$ slowing-down spectrum. The toy problem is implemented as a gate-level Qiskit circuit, while the resonance-integral example is simulated through an exact eigendecomposition of the Grover operator to avoid state-preparation decomposition bottlenecks. In both cases, the squared error scales as $O(1/T^2)$ with the number of oracle calls $T$, compared with the classical Monte Carlo scaling $O(1/N)$. For the U-238 example, QAE recovers the resonance integral to approximately $0.03%$ relative error with $m=14$ phase-estimation qubits.

quant-ph↗

arXiv2026

Understanding Chemical Short-Range Order in CoNiV via Mode Analysis

We analyze chemical short-range order in equiatomic fcc NiCoV using molecular-dynamics snapshots generated with a machine-learned interatomic potential. Radial distribution functions identify stable coordination shells, while shell-resolved Warren-Cowley parameters and bond probabilities reveal continued chemical ordering after the radial structure has largely converged. The dominant signal is V-V avoidance in the first shell and V-V enrichment in the second shell, consistent with an L1$_2$-like local ordering tendency, while the third-shell response remains weak. Lagged Jensen-Shannon diagnostics show that bond statistics relax more slowly than the RDF. Principal component analysis of per-replica-centered bond probabilities resolves three collective modes: a V-sublattice ordering amplitude, a Ni-Co redistribution mode, and a Co-V exchange-like mode. These results show that scalar RDF convergence can miss slow chemical relaxation, and that shell-resolved bond statistics provide a compact route for tracking SRO development in multicomponent alloys.

physics.comp-ph↗

arXiv2026

Electric-field effects on defect migration energetics in GaN

A predictive understanding of defect transport in GaN under operating electric fields is critical for assessing device reliability in high-power and radiation environments. In this work, a ReaxFF reactive force field for GaN is developed using a density-functional-theory training set that includes structural, thermodynamic, and defect properties. The force field yields various properties such as lattice parameters, cohesive energies, and defect formation and migration energies in close agreement with prior first-principles and experimental results. Under externally applied electric fields, we find that migration barriers can be strongly modulated, with changes that depend on defect type and field orientation. Notably, the electric fields do not simply linearly bias defect motion in GaN, but can anisotropically modify migration barriers through charge-lattice coupling, leading to nonlinear transport behavior. The response arises from field-induced partial charge redistribution and local lattice distortion. These results demonstrate that electric fields can complexly modify the defect migration landscape, providing new insight into defect transport in GaN under high-field conditions.

cond-mat.mtrl-sci↗

arXiv2026

Anomalous bulk dealloying below the parting limit

Dealloying has been extensively studied both as a corrosion degradation mechanism in structural materials, including those used in nuclear, aerospace, or marine environments, and as a versatile method to fabricate porous materials for catalysts and other functional applications. Classical dealloying theory in aqueous environments predicts a critical reactive-element concentration (parting limit) for continuous selective dissolution at temperatures where bulk diffusion does not dominate; this threshold is commonly reported around 50~60 at.%. Yet recent studies show that molten salt environments can generate extensive bulk dealloying below this threshold. Despite the importance of this anomalous dealloying behavior in many energy systems and electrochemical applications, its fundamental origin remains elusive. Here, we address this critical gap, revealing a grain boundary (GB)-assisted bulk dealloying mechanism. Using three-dimensional (3D) reconstruction of the dealloyed regions correlated with crystallographic and elemental analyses, we directly map the 3D GB-void architecture and reveal that diffusion-induced recrystallization (DIR) generates a high-density GB network, which then promotes molten-salt infiltration and can drive bulk dealloying far-below the conventionally reported parting limit, producing a distinctive morphology reminiscent of discontinuous precipitation (DP). Understanding this dynamic GB-void interplay is crucial for the prediction and control of dealloying in complex electrochemical environments.

cond-mat.mtrl-sci↗

arXiv2026

Atomistic Mechanisms of Stress-Dependent Molten Salt Corrosion in NiCr Alloys

Ni-based structural alloys in molten salt environments often experience simultaneous mechanical loading and corrosive attack, yet the mechanisms governing stress-corrosion interactions remain unclear. Prior studies largely emphasize tensile stress, while the role of compressive stress has received limited attention. Here, reactive molecular dynamics simulations are used to investigate the coupled effects of applied strain and corrosion in Ni$_{0.75}$Cr$_{0.25}$ exposed to molten FLiNaK at 800$^\circ$C. A $\Sigma5(210)$ grain boundary model is subjected to tensile (+4%) to compressive (-4%) uniaxial strains, and corrosion behavior is evaluated through fluorine adsorption, charge redistribution, and grain boundary evolution. Tensile strain accelerates intergranular corrosion by reducing local atomic packing through elastic dilation and increasing excess free volume at the grain boundary, which enhances atomic mobility and salt infiltration. In contrast, compressive strain suppresses corrosion by promoting the formation of a ridge-like surface layer along the grain boundary, limiting salt access to the underlying alloy. These results provide atomistic insight into how stress states influence grain boundary corrosion in molten salts.

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

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

Percolating Corrosion Pathways of Chemically Ordered NiCr Alloys in Molten Salts

Recent experiments have shown that chemical ordering in NiCr alloys can significantly accelerate corrosion in molten salt environments. However, the underlying mechanisms remain poorly understood. Using reactive molecular dynamics and first-principles calculations, we show that long-range ordered Ni$_2$Cr in Ni-33at.%Cr alloys corrodes far more rapidly in FLiNaK salt at 800°C than short-range ordered or random solid solutions. This accelerated attack originates from percolating Cr pathways that enhance near-surface diffusion and a lowered energetic barrier for Cr dissolution, as confirmed by first-principles calculations. Contrary to earlier explanations that attributed this behavior to residual stresses, our stress-free simulations demonstrate that ordering alone accelerates the degradation. These results establish percolation as a critical link between chemical ordering and corrosion kinetics, offering a mechanistic basis for experimental observations.

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

Anomalous Ionic Conductivity along the Coherent $Σ$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 $Σ3(111)$ GB in thorium dioxide (ThO$_2$), despite its compact and coherent structure, exhibits significantly higher oxygen ionic conductivity compared to the more open GB ($Σ19$ as an example). Using atomistic simulations based on a machine learning interatomic potential, we revealed that the high conductivity in the $Σ3$ GB arises from a collective diffusion mechanism involving highly correlated atomic motion reminiscent of a superionic state. In contrast, the $Σ19$ 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↗

arXiv2025

Surface Orientation-dependent Corrosion Behavior of NiCr Alloys in Molten FLiNaK Salt

The corrosion behavior of NiCr alloys in molten FLiNaK salt is governed by complex Cr-F chemical interactions, necessitating a fundamental understanding for enhancing alloy performance in harsh environments. However, significant gaps remain in our understanding of the dynamic atomic-scale processes driving the progression of molten salt corrosion. This study employs reactive force field-based molecular dynamics simulations to unravel the influence of crystallographic orientation, temperature, and external electric fields on corrosion kinetics. The (100), (110), and (111) orientations of Ni$\mathrm{_{0.75}}$Cr$\mathrm{_{0.25}}$ alloys are evaluated at temperatures from 600 to 800°C, with and without electric fields. Results reveal that Cr dissolution and near-surface diffusion drive pitting-like surface morphology evolution. The (110) surface shows the highest corrosion susceptibility, while the (100) and (111) surfaces exhibit greater resistance, with (111) being the most stable. The corrosion activation energy, derived from the Arrhenius relation, ranges from 0.27 eV to 0.41 eV, aligning well with limited experimental data yet significantly lower than bulk diffusion barriers. This finding indicates that corrosion progression is primarily a kinetically controlled near-surface process, rather than being limited by bulk diffusion as suggested in previous understanding. Additionally, electric fields perpendicular to the interface are found to asymmetrically modulate corrosion dynamics, where a positive field (+0.10 V/Å) promotes Cr dissolution. In comparison, a negative field (-0.10 V/Å) largely suppresses corrosion, which can be effectively used to mitigate corrosion. These findings, along with atomistic details into the corrosion mechanisms, offer strategic perspectives for designing corrosion-resistant materials in advanced high-temperature molten salt applications.

cond-mat.mtrl-sci↗

arXiv2025

Parameterizing empirical interatomic potentials for predicting thermophysical properties via an irreducible derivative approach: the case of ThO$_2$ and UO$_2$

The accuracy of classical physical property predictions using molecular dynamics simulations is determined by the quality of the interatomic potentials. Here we introduce a training approach for empirical interatomic potentials (EIPs) which is well suited for capturing phonons and phonon-related properties. Our approach is based on direct comparisons of the second- and third-order irreducible derivatives between an EIP and the Born-Oppenheimer potential within density functional theory (DFT) calculations. Irreducible derivatives fully exploit space group symmetry and allow for training without redundant information. We demonstrate the fidelity of our approach in the context of ThO$_2$ and UO$_2$, where we optimize parameters of an embedded-atom method potential in addition to core-shell interactions. Our EIPs provide thermophysical properties in good agreement with DFT and outperform widely utilized EIPs for phonon dispersion and thermal conductivity predictions. Reasonable estimates of thermal expansion and formation energies of Frenkel pairs are also obtained.

cond-mat.mtrl-sci↗

arXiv2024

First-Principles Investigation of Grain Boundary Effects on Fluorine-Induced Initial Corrosion of NiCr Alloys

Chromium depletion at grain boundaries (GBs) due to selective attack is a critical issue in the molten salt corrosion of NiCr alloys. Despite the importance of GBs in this process from numerous experimental studies, most theoretical work has predominantly focused on fluorine interactions with idealized crystalline surfaces, neglecting the complexity of GB local environments. This study aims to bridge that gap by employing density functional theory (DFT) to investigate the atomic interactions and Cr dissolution mechanisms at GB in NiCr alloys under molten fluoride salt environments. Specifically, a $Σ$5(210)/(001) symmetrical tilt GB is constructed to explore the adsorption energies of fluorine on Ni(100) and Cr-doped Ni(100) surfaces. We find that fluorine exhibits a strong preference for binding at GB sites, with Cr doping amplifying this effect, leading to higher adsorption energies compared to bulk Ni surfaces. Fluorine bonding with Cr significantly alters the interaction between Cr-F complexes and Ni substrate, and the consequent dissolution barriers for Cr atoms; the formation of CrF$_3$ largely reduces the energy barrier for Cr dissolution. This work highlights the essential role of GBs in enhancing fluorine adsorption and accelerating Cr depletion, providing new insights into the mechanisms of early-stage corrosion in NiCr alloys.

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↗

arXiv2024

Examining composition-dependent radiation response in AlGaN

Al$_x$Ga$_{1-x}$N materials have become increasingly important for electronics in radiation environments due to their robust properties. In this work, we aim to investigate the atomistic mechanisms of radiation-induced damage in AlGaN compounds, providing insights that bridge the gap between high-length-scale experimental data and detailed atomic-level processes. Through extensive molecular dynamics simulations, we reveal the compositional dependence of radiation-induced defect production in Al$_x$Ga$_{1-x}$N systems with $x$ ranging from 0 to 1. The damage accumulation characteristics observed in our simulations align notably well with available experimental data at temperatures up to room temperature. Our findings indicate that alloy composition significantly influences defect production and microstructural evolution, including the formation of dislocation loops and defect clusters. Specifically, with increasing Al content, defect production from individual recoil events is reduced; however, extended interstitial defects are more likely to form considering cumulative effects, leading to enhanced damage at high doses. Among the compositions studied, we find that 25\% Al content leads to the least overall radiation damage, suggesting an optimal alloying strategy for mitigating radiation effects. These findings underscore the interplay between defect formation, dynamic annealing, and cascade effects, offering insights for optimizing AlGaN materials for radiation resistance in practical applications.

cond-mat.mtrl-sci↗

arXiv2024

Understanding the Sampling Algorithm for Watt Spectrum

We provide details in understanding the Watt spectrum sampling method. The algorithm is given in "R12" from "3rd Monte Carlo Sampler" without detailed derivation. We rederive the algorithm by optimizating the sampling efficiency of the rejection method.

physics.comp-ph↗