arXiv subjects

Ananya Renuka Balakrishna

Publications and source records attributed to Ananya Renuka Balakrishna.

18 recordsLinked to original sources

arXiv2026

Micromechanics of Nanoporous Alloying Electrodes

Nanostructured electrodes with voids or interconnected pores accommodate large volume changes, shorten ion diffusion pathways, and enhance the structural reversibility of alloying electrodes. While these nanoporous features improve the performance of architected electrodes over bulk electrodes, they also act as geometric irregularities that localize and concentrate internal stresses. In this work, we investigate the hierarchical interplay between phase boundaries and nanoporous features at the microstructural scale and their collective role in mitigating chemo-mechanical failure at the engineering scale. Using Sb$\to$Li$_2$Sb$\to$Li$_3$Sb as a model system, we develop a continuum framework coupling lithium diffusion and reaction kinetics with the finite deformation of alloying electrodes. We analytically show that the interfacial stresses (and not volume changes alone) induce fracture during the Sb $\to$ Li$_2$Sb transformation. Nanoporous geometries accommodate volume changes and in turn lower these interfacial stresses that are detrimental to the structural stability of architected electrodes. We analytically study the elastic-plastic deformation accompanying the Sb $\to$ Li$_2$Sb transformation, and develop a micromechanical model using a hyperelastic neo-Hookean material law to predict the deformations accompanying the Li$_2$Sb$\to$Li$_3$Sb transformation. Our results reveal how diffusion and reaction kinetics shape phase boundary morphology, predict stress concentrations at defects, identify crack geometries likely to propagate, and show how carefully architected electrodes relieve stresses. These findings highlight critical design principles to optimize electrode lifespan and demonstrate a potential application of our continuum model as an electrode design tool.

cond-mat.mtrl-sci↗

arXiv2025

CuPyMag: GPU-Accelerated Finite-Element Micromagnetics with Magnetostriction

We introduce CuPyMag, an open-source, Python-based framework for large-scale micromagnetic simulations with magnetostriction. CuPyMag solves micromagnetics with finite elements in a GPU-resident workflow in which key operations, such as right-hand-side assembly, spatial derivatives, and volume averages, are tensorized using CuPy's BLAS-accelerated backend. Benchmark tests show that the GPU solvers in CuPyMag achieve a speedup of up to two orders of magnitude compared to the CPU codes. Its runtime grows linearly/sublinearly with problem size, demonstrating high efficiency. Additionally, CuPyMag uses the Gauss-Seidel projection method for time integration, which not only allows stable time steps (up to 11 ps) but also solves each governing equation with only 1-3 conjugate-gradient iterations without preconditioning. CuPyMag accounts for magnetoelastic coupling and far-field effects arising from the boundary of the magnetic body, both of which play an important role in magnetization reversal in the presence of local defects. CuPyMag solves these computationally intensive multiphysics simulations with a high-resolution mesh (up to 3M nodes) in under three hours on an NVIDIA H200 GPU. This acceleration enables micromagnetic simulations with non-trivial defect geometries and resolves nanoscale magnetic structures. It expands the scope of micromagnetic simulations towards realistic, large-scale problems that can guide experiments. More broadly, CuPyMag is developed using widely adopted Python libraries, which provide cross-platform compatibility, ease of installation, and accessibility for adaptations to diverse applications.

physics.comp-ph↗

arXiv2025

Lattice Compatibility and Energy Barriers in Intercalation Compounds

We present a continuum model for symmetry-breaking phase transformations in intercalation compounds, based on Ericksen's multi-well energy formulation. The model predicts the nucleation and growth of crystallographic microstructures in Li$_{2}$Mn$_{2}$O$_{4}$ -- a representative intercalation compound -- with twin boundary orientations and volume fractions that closely match experimental observations. Our chemo-mechanically coupled model not only generates geometrically accurate microstructures through energy minimization, but also reveals a subtle interplay between twinned domains and electro-chemo-mechanical behavior. A key finding is that intercalation compounds satisfying specific compatibility conditions (e.g., $λ_{2} = 1$ or $|\det \mathbf{U} - 1| = 0)$ show lower elastic energy barriers, require smaller driving forces, and display narrower voltage hysteresis loops. Furthermore, we show that twinned domains act as conduits for fast Li-diffusion. These results establish quantitative design guidelines for intercalation compounds, which focuses on tailoring lattice deformations (rather than suppressing them) and reducing energy barriers to mitigate structural degradation and enhance the electrochemical performance of battery electrodes.

cond-mat.mtrl-sci↗

arXiv2025

A Micromechanical Model for Light-interactive Molecular Crystals

Molecular crystals respond to a light stimulus by bending, twisting, rolling, jumping, or other kinematic behaviors. These behaviors are known to be affected by, among others, the intensity of the incident light, the aspect ratios of crystal geometries, and the volume changes accompanying phase transformation. While these factors, individually, explain the increase in internal energy of the system and its subsequent minimization through macroscopic deformation, they do not fully explain the diversity of deformations observed in molecular crystals. Here, we propose a micromechanical model based on the Cauchy-Born rule and photoreaction theory to predict the macroscopic response in molecular crystals. By accounting for lattice geometry changes and microstructural patterns that emerge during phase transformation, we predict a range of deformations in a representative molecular crystal (salicylideneamine). Doing so, we find that the interplay between photoexcited states and the energy minimization pathways, across a multi-well energy landscape, is crucial to the bending and twisting deformations. We use our model to analyze the role of particle geometries and the intensity of incident light on macroscopic deformation, and identify geometric regimes for shearing and twisting deformations in salicylideneamine crystals. Our micromechanical model is general and can be adapted to predict photomechanical deformation in other molecular crystals undergoing a solid-to-solid phase change and has potential as a computational design tool to engineer reversible and controllable actuation in molecular crystals.

cond-mat.mtrl-sci↗

arXiv2024

Magnetoelastic Interactions Reduce Hysteresis in Soft Magnets

The width of the magnetic hysteresis loop is often correlated with the material's magnetocrystalline anisotropy constant $κ_1$. Traditionally, a common approach to reduce the hysteresis width has been to develop alloys with $κ_1$ as close to zero as possible. However, contrary to this widely accepted view, we present evidence that magnetoelastic interactions governed by magnetostriction constants, elastic stiffness, and applied stresses play an important role in reducing magnetic hysteresis width, despite large $κ_1$ values. We use a nonlinear micromagnetics framework to systematically investigate the interplay between material constants $λ_{100}$, $c_{11}$, $c_{12}$, $κ_1$, applied or residual stresses $σ_{\mathrm{R}}$, and needle domains to collectively lower the energy barrier for magnetization reversal. A distinguishing feature of our work is that we correlate the energy barrier governing the growth of needle domains with the width of the hysteresis loop. This energy barrier approach enables us to capture the nuanced interplay between anisotropy constant, magnetostriction, and applied stresses, and their combined influence on magnetic hysteresis. We propose a mathematical relationship on the coercivity map: $κ_1 = α(c_{11}-c_{12})(λ_{100}+βσ_{11})^2$ for which magnetic hysteresis can be minimized for a uniaxial residual stress $σ_\mathrm{R} = σ_{11}\hat{\mathbf{e}}_1\otimes\hat{\mathbf{e}}_1$ (and for some constants $α$, $β$). These results serve as quantitative guidelines to design magnetic alloys with small hysteresis, and potentially guide the discovery of a new generation of soft magnets located beyond the $κ_1 \to 0$ region.

cond-mat.mtrl-sci↗

arXiv2023

Coupling Diffusion and Finite Deformation in Phase Transformation Materials

We present a multiscale theoretical framework to investigate the interplay between diffusion and finite lattice deformation in phase transformation materials. In this framework, we use the Cauchy-Born Rule and the Principle of Virtual Power to derive a thermodynamically consistent theory coupling the diffusion of a guest species (Cahn-Hilliard type) with the finite deformation of host lattices (nonlinear gradient elasticity). We adapt this theory to intercalation materials--specifically Li$_{1-2}$Mn$_2$O$_4$--to investigate the delicate interplay between Li-diffusion and the cubic-to-tetragonal deformation of lattices. Our computations reveal fundamental insights into the microstructural evolution pathways under dynamic discharge conditions, and provide quantitative insights into the nucleation and growth of twinned microstructures during intercalation. Additionally, our results identify regions of stress concentrations (e.g., at phase boundaries, particle surfaces) that arise from lattice misfit and accumulate in the electrode with repeated cycling. These findings suggest a potential mechanism for structural decay in Li$_2$Mn$_2$O$_4$. More generally, we establish a theoretical framework that can be used to investigate microstructural evolution pathways, across multiple length scales, in first-order phase transformation materials.

cond-mat.mtrl-sci↗

arXiv2022

Designing shape-memory-like microstructures in intercalation materials

During the reversible insertion of ions, lattices in intercalation materials undergo structural transformations. These lattice transformations generate misfit strains and volume changes that, in turn, contribute to the structural decay of intercalation materials and limit their reversible cycling. In this paper, we draw on insights from shape-memory alloys, another class of phase transformation materials, that also undergo large lattice transformations but do so with negligible macroscopic volume changes and internal stresses. We develop a theoretical framework to predict structural transformations in intercalation compounds and establish crystallographic design rules necessary for forming shape-memory-like microstructures in intercalation materials. We use our framework to systematically screen open-source structural databases comprising n > 5000 pairs of intercalation compounds. We identify candidate compounds, such as Li$_x$Mn$_2$O$_4$ (Spinel), Li$_x$Ti$_2$(PO$_4$)$_3$ (NASICON), that approximately satisfy the crystallographic design rules and can be precisely doped to form shape-memory-like microstructures. Throughout, we compare our analytical results with experimental measurements of intercalation compounds. We find a direct correlation between structural transformations, microstructures, and increased capacity retention in these materials. These results, more generally, show that crystallographic designing of intercalation materials could be a novel route to discovering compounds that do not decay with continuous usage.

cond-mat.mtrl-sci↗

arXiv2022

Crystallographic design of intercalation materials

Intercalation materials are promising candidates for reversible energy storage and are, for example, used as lithium-battery electrodes, hydrogen-storage compounds, and electrochromic materials. An important issue preventing the more widespread use of these materials is that they undergo structural transformations (of up to ~10% lattice strains) during intercalation, which expand the material, nucleate microcracks, and, ultimately, lead to material failure. Besides the structural transformation of lattices, the crystallographic texture of the intercalation material plays a key role in governing ion-transport properties, generating phase separation microstructures, and elastically interacting with crystal defects. In this review, I provide an overview of how the structural transformation of lattices, phase transformation microstructures, and crystallographic defects affect the chemo-mechanical properties of intercalation materials. In each section, I identify the key challenges and opportunities to crystallographically design intercalation compounds to improve their properties and lifespans. I predominantly cite examples from the literature of intercalation cathodes used in rechargeable batteries, however, the identified challenges and opportunities are transferable to a broader range of intercalation compounds.

cond-mat.mtrl-sci↗

arXiv2021

Design of soft magnetic materials

We present a strategy for the design of ferromagnetic materials with exceptionally low magnetic hysteresis, quantified by coercivity. In this strategy, we use a micromagnetic algorithm that we have developed in previous research and which has been validated by its success in solving the "Permalloy Problem" -- the well-known difficulty of predicting the composition 78.5% Ni of lowest coercivity in the Fe-Ni system -- and by the insight, it provides into the "Coercivity Paradox" of W. F. Brown. Unexpectedly, the design strategy predicts that cubic materials with large saturation magnetization $m_s$ and large magnetocrystalline anisotropy constant $κ_1$ will have low coercivity on the order of that of Permalloy, as long as the magnetostriction constants $λ_{100}, λ_{111}$ are tuned to special values. The explicit prediction for a cubic material with low coercivity is the dimensionless number $(c_{11}-c_{12}) λ_{100}^2/κ_1 = 81$ for $\langle 100 \rangle$ easy axes. The results would seem to have a broad potential application, especially to magnetic materials of interest in energy research.

cond-mat.mtrl-sci↗

arXiv2021

Film strains enhance the reversible cycling of intercalation electrodes

A key cause of chemo-mechanical degradation in battery electrodes is that they undergo abrupt phase transformation during the charging/discharging cycle. This phase transformation is accompanied by lattice misfit strains that nucleate microcracks, induce fracture and, in extreme cases, amorphize the intercalation electrode. In this work, we propose a strategy to prevent the chemo-mechanical degradation of intercalation electrodes: we show that by engineering suitable film strains we can regulate the phase transformations in thin-film intercalation electrodes and circumvent the large volume changes. We test this strategy using a combination of theory and experiment: we first analytically derive the effect of film strain on the electrochemical response of a thin-film intercalation electrode and next apply our analytical model to a representative example (LixV2O5 with multiple phase transformations). We then test our theoretical predictions experimentally. Specifically, we electrochemically cycle thin-film V2O5 electrodes with different film strains and measure their structure, voltage, and stress responses. Our findings show that tensile film strains lower the voltage for phase transformations in thin-film V2O5 electrodes and facilitate their reversible cycling across a wider voltage window without chemo-mechanical degradation. These results suggest that film strain engineering is an alternative approach to preventing chemo-mechanical degradation in intercalation electrodes. Beyond thin-film electrodes, our findings from this study are applicable to the study of stress-induced phase transformations in particle-based electrodes and the thin surface layers forming on cathode particles.

cond-mat.mtrl-sci↗

arXiv2021

A solution to the permalloy problem

We propose a solution to the longstanding permalloy problem$-$why the particular composition of permalloy, Fe$_{21.5}$Ni$_{78.5}$, achieves a dramatic drop in hysteresis, while its material constants show no obvious signal of this behavior. We use our recently developed coercivity tool to show that a delicate balance between local instabilities and magnetic material constants are necessary to explain the dramatic drop of hysteresis at 78.5% Ni. Our findings are in agreement with the permalloy experiments and, more broadly, provide theoretical guidance for the discovery of novel low hysteresis magnetic alloys.

cond-mat.mtrl-sci↗

arXiv2020

A tool to predict coercivity in magnetic materials

Magnetic coercivity is often viewed to be lower in alloys with negligible (or zero) values of the anisotropy constant. However, this explains little about the dramatic drop in coercivity in FeNi alloys at a non-zero anisotropy value. Here, we develop a theoretical and computational tool to investigate the fundamental interplay between material constants that govern coercivity in bulk magnetic alloys. The two distinguishing features of our coercivity tool are that: (a) we introduce a large localized disturbance, such as a spike-like magnetic domain, that provides a nucleation barrier for magnetization reversal; and (b) we account for magneto-elastic energy -- however small -- in addition to the anisotropy and magnetostatic energy terms. We apply this coercivity tool to show that the interactions between local instabilities and material constants, such as anisotropy and magnetostriction constants, are key factors that govern magnetic coercivity in bulk alloys. Using our model, we show that coercivity is minimum at the permalloy composition (Fe-21.5Ni-78.5) at which the alloy's anisotropy constant is not zero. We systematically vary the values of the anisotropy and magnetostriction constants, around the permalloy composition, and identify new combinations of material constants at which coercivity is small. More broadly, our coercivity tool provides a theoretical framework to potentially discover novel magnetic materials with low coercivity.

cond-mat.mtrl-sci↗

arXiv2018

Li-diffusion accelerates grain growth in intercalation electrodes: a phase-field study

Grain boundary migration is driven by the boundary's curvature and external loads such as temperature and stress. In intercalation electrodes an additional driving force results from Li-diffusion. That is, Li-intercalation induces volume expansion of the host-electrode, which is stored as elastic energy in the system. This stored energy is hypothesized as an additional driving force for grain boundaries and edge dislocations. Here, we apply the 2D Cahn-Hilliard$-$phase-field-crystal (CH-PFC) model to investigate the coupled interactions between highly mobile Li-ions and host-electrode lattice structure, during an electrochemical cycle. We use a polycrystalline FePO$_{4}$/ LiFePO$_{4}$ electrode particle as a model system. We compute grain growth in the FePO$_{4}$ electrode in two parallel studies: In the first study, we electrochemically cycle the electrode and calculate Li-diffusion assisted grain growth. In the second study, we do not cycle the electrode and calculate the curvature-driven grain growth. External loads, such as temperature and stress, did not differ across studies. We find the mean grain-size increases by $\sim11\%$ in the electrochemically cycled electrode particle. By contrast, in the absence of electrochemical cycling, we find the mean grain-size increases by $\sim2\%$ in the electrode particle. These CH-PFC computations suggest that Li-intercalation accelerates grain-boundary migration in the host-electrode particle. The CH-PFC simulations provide atomistic insights on diffusion-induced grain-boundary migration, edge dislocation movement and triple-junction drag-effect in the host-electrode microstructure.

cond-mat.mtrl-sci↗

arXiv2018

Nanoscale domain patterns and a concept for an energy harvester

The current work employs a phase-field model to test the stability of nanoscale periodic domain patterns, and to explore the application of one pattern in an energy harvester device. At first, the stability of several periodic domain patterns with in-plane polarizations is tested under stress-free and electric field-free conditions. It is found that simple domain patterns with stripe-like features are stable, while patterns with more complex domain configurations are typically unstable at the nanoscale. Upon identifying a stable domain pattern with suitable properties, a conceptual design of a thin film energy harvester device is explored. The harvester is modelled as a thin ferroelectric film bound to a substrate. In the initial state a periodic stripe domain pattern with zero net charge on the top electrode is modelled. On bending the substrate, a mechanical strain is induced in the film, causing polarized domains to undergo ferroelectric switching and thus generate electrical energy. The results demonstrate the working cycle of a conceptual energy harvester which, on operating at kHz frequencies, such as from vibrations in the environment, could produce an area power density of about 40W/m2.

physics.app-ph↗

arXiv2018

Nanoscale periodic domain patterns in tetragonal ferroelectrics: A phase-field study

Ferroelectrics form domain patterns that minimize their energy subject to imposed boundary conditions. In a linear, constrained theory, that neglects domain wall energy, periodic domain patterns in the form of multi-rank laminates can be identified as minimum-energy states. However, when these laminates (formed in a macroscopic crystal) comprise domains that are a few nanometers in size, the domain-wall energy becomes significant, and the behaviour of laminate patterns at this scale is not known. Here, a phase-field model, which accounts for gradient energy and strain energy contributions, is employed to explore the stability and evolution of the nanoscale multi-rank laminates. The stress, electric field, and domain wall energies in the laminates are computed. The effect of scaling is also discussed. In the absence of external loading, stripe domain patterns are found to be lower energy states than the more complex, multi-rank laminates, which mostly collapse into simpler patterns. However, complex laminates can be stabilized by imposing external loads such as electric field, average strain and polarization. The study provides insight into the domain patterns that may form on a macroscopic single crystal but comprising of nanoscale periodic patterns, and on the effect of external loads on these patterns.

cond-mat.mtrl-sci↗

arXiv2017

Scale effects and the formation of polarization vortices in tetragonal ferroelectrics

Vortices consisting of $90^\circ$ quadrant domains are rarely observed in ferroelectrics. Although experiments show polarization flux closures with stripe domains, it is as yet unclear why pure single vortices are not commonly observed. Here we model and explore the energy of polarization patterns with vortex and stripe domains, formed on the square cross-section of a barium titanate nanowire. Using phase-field simulations, we calculate the associated energy of polarization patterns as a function of nanowire width. Further, we demonstrate the effects of surface energy and electrical boundary conditions on equilibrium polarization patterns. The minimum energy equilibrium polarization pattern for each combination of surface energy and nanowire width is mapped for both open-circuit and short-circuit boundary conditions. The results indicate a narrow range of conditions where single vortices are energetically favorable: nanowire widths less than about 30nm, open-circuit boundary condition, and surface energy of less than 4N/m. Short-circuit boundary conditions tend to favor the formation of a monodomain, while surface energy greater than 4N/m can lead to the formation of complex domain patterns or loss of ferroelectricity. The length scale at which a polarization vortex is energetically favorable is smaller than the typical size of nanoparticle in recent experimental studies. The present work provides insight into the effects of scaling, surface energy and electrical boundary conditions on the formation of polarization patterns.

cond-mat.mtrl-sci↗

arXiv2017

Combining phase field crystal methods with a Cahn-Hilliard model for binary alloys

During phase transitions certain properties of a material change, such as composition field and lattice-symmetry distortions. These changes are typically coupled, and affect the microstructures that form in materials. Here, we propose a 2D theoretical framework that couples a Cahn-Hilliard (CH) model describing the composition field of a material system, with a phase field crystal (PFC) model describing its underlying microscopic configurations. We couple the two continuum models via coordinate transformation coefficients. We introduce the transformation coefficients in the PFC method, to describe affine lattice deformations. These transformation coefficients are modeled as functions of the composition field. Using this coupled approach, we explore the effects of coarse-grained lattice symmetry and distortions on a phase transition process. In this paper, we demonstrate the working of the CH-PFC model through three representative examples: First, we describe base cases with hexagonal and square lattice symmetries for two composition fields. Next, we illustrate how the CH-PFC method interpolates lattice symmetry across a diffuse composition phase boundary. Finally, we compute a Cahn-Hilliard type of diffusion and model the accompanying changes to lattice symmetry during a phase transition process.

cond-mat.mtrl-sci↗

arXiv2017

Nano-actuator concepts based on ferroelectric switching

The concept of a nano-actuator that uses ferroelectric switching to generate enhanced displacements is explored using a phase-field model. The actuator has a ground state in the absence of applied electric field that consists of polarized domains oriented to form a flux closure. When electric field is applied, the polarization reorients through ferroelectric switching and produces strain. The device is mechanically biased by a substrate and returns to the ground state when electric field is removed, giving a repeatable actuation cycle. The mechanical strains which accompany ferroelectric switching are several times greater than the strains attained due to the piezoelectric effect alone. We also demonstrate a second design of actuator in which the displacements are further increased by the bending of a ferroelectric beam. Phase-field modelling is used to track the evolution of domain patterns in the devices during the actuation cycle, and to study the design parameters so as to enhance the achievable actuation strains.

physics.app-ph↗