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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 217 records · Page 12
All sourcesNASA NTRSDOE OSTI
DOE OSTI2025

Meso-scale modeling of UO2 nuclear fuel to high burnup

To improve the economics of light water reactors for commercial nuclear energy generation, utility operators are seeking to obtain regulatory approval to run UO2 fuel to higher levels of burnup. One potential impediment to obtaining this approval is the phenomenon of fuel fragmentation, relocation, and dispersal (FFRD). FFRD can result when fuel experiences a rapid temperature transient, such as that occurring during a Loss Of Coolant Accident (LOCA). FFRD has historically been most associated with the rim region in UO2 fuel pellets, where the phenomenon of fragmentation is also referred to as pulverization due to the small size of the fragments. More recent evidence suggests that the so-called “dark zone” (due to its appearance in micrographs) that can be observed in the mid-radial regions of high burnup fuel is also susceptible to FFRD. Although empirical fuel performance models have been developed that can adequately predict pulverization in the rim region under typical LWR conditions, a scientific understanding of what underlies fuel restructuring and subsequent FFRD is lacking even in the rim region, and no models are currently available for the behavior the dark zone. To address these challenges, the U.S. Department of Energy’s Nuclear Energy Advanced Modeling and Simulation (NEAMS) program has employed a multi-scale modeling approach to improve scientific understanding and develop new fuel performance models. In this talk, I will focus on meso-scale efforts, which form a crucial link between atomic-scale and engineering-scale models. Phase-field modeling combined with cluster dynamics is used to predict the restructuring process in the rim region. Phase-field fracture modeling, informed by atomistic simulations, is used to predict the onset of pulverization in the rim region. Combining these techniques together allows the extent of rim pulverization to be predicted. The formation and evolution of the dark zone has also been simulated with the phase-field method, using an improved approach to vacancy source term parameterization. The work shows the important impact of microstructure on fuel performance.

fracture↗
DOE OSTI2024

Anomalous localization in a kicked quasicrystal

Abstract Quantum transport can distinguish between dynamical phases of matter. For instance, ballistic propagation characterizes the absence of disorder, whereas in many-body localized phases, particles do not propagate for exponentially long times. Additional possibilities include states of matter exhibiting anomalous transport in which particles propagate with a non-trivial exponent. Here we report the experimental observation of anomalous transport across a broad range of the phase diagram of a kicked quasicrystal. The Hamiltonian of our system has been predicted to exhibit a rich phase diagram, including not only fully localized and fully delocalized phases but also an extended region comprising a nested pattern of localized, delocalized and multifractal states, which gives rise to anomalous transport. Our cold-atom realization is enabled by new Floquet engineering techniques, which expand the accessible phase diagram by five orders of magnitude. Mapping transport properties throughout the phase diagram, we observe disorder-driven re-entrant delocalization and sub-ballistic transport, and we present a theoretical explanation of these phenomena based on eigenstate multifractality.

Physics↗
DOE OSTI2025

Redox-Induced Microstructure and Phase Dynamics in Nickel: Insights from In Situ Synchrotron X-ray Diffraction

Using in situ synchrotron X-ray diffraction, we interrogate the microstructural and phase evolution of polycrystalline nickel (Ni) during redox cycling in O 2 , H 2 , and H 2 O environments. Oxidation in O 2 promotes strong (111) texturing in both the NiO overlayer and the underlying Ni substrate. However, this crystallographic alignment is lost following reduction in H 2 and subsequent reoxidation, demonstrating irreversible microstructural changes. H 2 exposure leads to proton dissolution into the Ni lattice, triggering a localized phase transition from face-centered cubic (FCC) to hexagonal close-packed (HCP) Ni in hydrogen-saturated regions. In H 2 O-containing atmospheres, dissociative H 2 O adsorption produces protons that permeate the NiO layer, forming γ-NiOOH within the NiO lattice and HCP Ni beneath the NiO overlayer as protons accumulate. Kinetic analysis via the Johnson-Mehl-Avrami–Kolmogorov model uncovers distinct growth mechanisms: preoxidized Ni surfaces follow one-dimensional (1D) kinetics for NiO, γ-NiOOH, and HCP growth, whereas pristine Ni exhibits three-dimensional (3D) kinetics due to island-like nucleation and growth of NiO. Furthermore, these results highlight the critical interplay between H 2 O dissociation, hydrogen permeation, and redox-driven phase transformations, with practical implications in engineering nickel-based catalysts and hydrogen storage systems through controlled microstructural and phase evolution.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Training for Decommissioning and Waste Management - Competence in the Field of Nuclear Engineering - 20137

Due to the phase-out of nuclear power and the reorganization of the participating organizations in Germany, the demand for qualified personnel has increased. But the number of nuclear study programs has rapidly decreased in Germany, although qualified employees are needed for the upcoming tasks in nuclear decommissioning. The solution for those challenging tasks can only be achieved through a strategic approach, involving all stakeholders. AiNT supports this process by offering a tailor-made Education and Training (E and T) program in cooperation with a wide range of partners and initiating the project BEAST (Building competence - Expert knowledge - Applied techniques - Safe decommissioning - Train fellows) as part of the OECD NEA framework Nuclear Education, Skills and Technology (NEST) program. This paper describes the current situation in Germany, provides insight into the education and training program of AiNT, and presents the project BEAST. We are currently looking for international partner institutions that would like to cooperate with us within the framework of the BEAST project and expand the range of topics. (authors)

12 MANAGEMENT OF RADIOACTIVE AND NON-RADIOACTIVE W↗
DOE OSTI2021

Robust and ultrafast state preparation by ramping artificial gauge potentials

Abstract The implementation of static artificial magnetic fields in ultracold atomic systems has become a powerful tool, e.g. for simulating quantum-Hall physics with charge-neutral atoms. Taking an interacting bosonic flux ladder as a minimal model, we investigate protocols for adiabatic state preparation via magnetic flux ramps. Considering the fact that it is actually the artificial vector potential (in the form of Peierls phases) that can be experimentally engineered in optical lattices, rather than the magnetic field, we find that the time required for adiabatic state preparation dramatically depends on which pattern of Peierls phases is used. This can be understood intuitively by noting that different patterns of time-dependent Peierls phases that all give rise to the same magnetic field ramp, generally lead to different artificial electric fields during the ramp. As an intriguing result, we find that an optimal choice allows for preparing the ground state almost instantaneously in the non-interacting system, which can be related to the concept of counterdiabatic driving. Remarkably, we find extremely short preparation times also in the strongly-interacting regime. Our findings open new possibilities for robust state preparation in atomic quantum simulators.

Wang, Botao (ORCID:0000000282202452)↗
DOE OSTI2025

Topological Defect Mediated Helical Phase Reorientation by Uniaxial Stress

Strain engineering enables precise, energy-efficient control of nanoscale magnetism. However, unlike well-studied strain-dislocation interactions in mechanical deformation, the spatial evolution of strain-induced spin rearrangement remains poorly understood. Using Lorentz transmission electron microscopy, we manipulate and observe helical domain reorientation under quantitatively applied uniaxial tensile stress. Our findings reveal striking similarity to plastic deformation in metals, where the critical stress for propagation vector ( Q ) reorientation depends on its angle with the stress direction. Magnetic defects mediate reorientation via “break-and-reconnect” or “dislocation gliding–annihilation” processes. Simulations confirm that strain-induced anisotropic Dzyaloshinskii-Moriya interaction may play a key role. These insights advance strain-driven magnetism and offer a promising route for energy-efficient magnetic nanophase control in next-generation information technology. Published by the American Physical Society 2025

36 MATERIALS SCIENCE↗
DOE OSTI2026

Tensile creep properties of NbTaTi and NbTaTiV refractory complex concentrated alloys

Refractory complex concentrated alloys (RCCAs) have emerged as promising candidates for high-temperature structural applications due to their high melting points and potential for exceptional strength, whereas their creep properties remain relatively underexplored. Here, we investigate the tensile creep behavior of NbTaTi and NbTaTiV in high vacuum to elucidate the influence of alloying on creep mechanisms. NbTaTiV exhibits higher creep resistance than NbTaTi, which is attributed to V-induced edge-dislocation-glide-controlled deformation. Microstructural analysis reveals the formation of Ti- and interstitial impurity-rich secondary phases during creep, which enhances the creep resistance without significant embrittlement. Stress exponents (∼3) and activation energies (∼those for vacancy diffusion) align with thermally activated dislocation glide-controlled creep. Despite NbTaTiV surpassing Ni-based superalloys in yield strength at elevated temperatures, its single-phase BCC structure limits creep resistance under engineering-relevant conditions, highlighting the necessity of deliberate secondary phase design to achieve competitive high-temperature performance.

Complex concentrated alloys↗
DOE OSTI2025

A stress-sensitive precipitate nucleation model beyond classical nucleation theory

The dynamic evolution of precipitates and second phases dictates the strength and stability of most engineering alloys. By design, or as a consequence of thermo-mechanical aging, engineering metals and alloys often form precipitates of second phases when subjecting to diverse thermal and mechanical loads. Precipitation is governed by several factors, including the alloy’s composition, processing/operating temperature, and stresses — either as a result of external loads or from residual stresses. However, state-of-the-art models for precipitate nucleation (i.e., classical nucleation theory) typically lacks consistent method to capture the effects of externally applied and/or internal stresses on nucleation; thereby severely limiting the applicability of these models to complex materials systems and to representative loading scenarios. Here, in this work, we extend upon classical nucleation theory to account for the effect of stresses on precipitation kinetics and thermodynamics. This is achieved via the use of an Eshelbian micromechanics framework keeping track of (i) the stress build up resulting from second phase formation as a function of mechanical load and, (ii) the effects of dislocations on precipitate formation. This new model is applied to σ precipitate in Fe–Cr binary alloys and M 23 C 6 precipitate in 316H stainless steel (SS). Simulations demonstrate the important role of both the remotely applied loads and dislocation pile ups on precipitate nucleation.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Performance, Combustion and Emissions Evaluation of Liquid Phase Port-Injected LPG on a Single Cylinder Heavy-Duty Spark Ignited Engine

Liquefied petroleum gas (LPG), like many other alternative fuels, has witnessed increased adoption in the last decade, and its use is projected to rise as stricter emissions regulations continue to be applied. However, much of its use is limited to dual fuel applications, gaseous phase injection, light-duty passenger vehicle applications, or scenarios that require conversion from gasoline engines. Therefore, to address these limitations and discover the most efficient means of harnessing its full potential, more research is required in the development of optimized fuel injection equipment for liquid port and direct injection, along with the implementation of advanced combustion strategies that will improve its thermal efficiency to the levels of conventional fuels. This paper focuses on the development of a liquid phase port-injection system for LPG, the design of a reference piston, and the baseline evaluation of the performance, combustion, and emissions characteristics of a single cylinder research engine to establish a benchmark comparable to existing LPG engines. A sweep of start of injection (SOI) timing is performed by injecting liquid LPG at several closed and open intake valve timings, which demonstrates no significant variation in engine performance, but accounts for a 10% reduction in bsCO with the optimal SOI timing. Spark timing sweep demonstrates the 50% burn crank angle location related to maximum brake torque (MBT) point with a brake thermal efficiency (BTE) of ~34% for the tested load case. The effect of equivalence ratio is also presented with optimal SOI timing at MBT condition. The engine starts exhibiting knocking combustion at 140kPa intake manifold air pressure (IMAP) with a peak torque of 253Nm and a 5% reduction in brake specific fuel consumption compared to the naturally aspirated scenario.

Fosudo, Toluwalase↗
DOE OSTI2022

Carrier Mobility Modulation in Cu 2 Se Composites Using Coherent Cu 4 TiSe 4 Inclusions Leads to Enhanced Thermoelectric Performance

Carrier transport engineering in bulk semiconductors using inclusion phases often results in the deterioration of carrier mobility (μ) owing to enhanced carrier scattering at phase boundaries. Here, we show by leveraging the temperature-induced structural transition between the α-Cu 2 Se and β-Cu 2 Se polymorphs that the incorporation of Cu 4 TiSe 4 inclusions within the Cu 2 Se matrix results in a gradual large drop in the carrier mobility at temperatures below 400 K (α-Cu 2 Se), whereas the carrier mobility remains unchanged at higher temperatures, where the β-Cu 2 Se polymorph dominates. The sharp discrepancy in the electronic transport within the α-Cu 2 Se and β-Cu 2 Se matrices is associated with the formation of incoherent α-Cu 2 Se/Cu 4 TiSe 4 interfaces, owing to the difference in their atomic structures and lattice parameters, which results in enhanced carrier scattering. In contrast, the similarity of the Se sublattices between β-Cu 2 Se and Cu 4 TiSe 4 gives rise to coherent phase boundaries and good band alignment, which promote carrier transport across the interfaces. Interestingly, the different cation arrangements in Cu 4 TiSe 4 and β-Cu 2 Se contribute to enhanced phonon scattering at the interfaces, which leads to a reduction in the lattice thermal conductivity. The large reduction in the total thermal conductivity while preserving the high power factor of β-Cu 2 Se in the (1–x)Cu 2 Se/(x)Cu 4 TiSe 4 composites results in an improved ZT of 1.2 at 850 K, with an average ZT of 0.84 (500–850 K) for the composite with x = 0.01. Furthermore, this work highlights the importance of structural similarity between the matrix and inclusions when designing thermoelectric materials with improved energy conversion efficiency.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Exploring the Spatial Control of Topotactic Phase Transitions Using Vertically Oriented Epitaxial Interfaces

Engineering oxygen vacancy formation and distribution is a powerful route for controlling the oxygen sublattice evolution that affects diverse functional behavior. The controlling of the oxygen vacancy formation process is particularly important for inducing topotactic phase transitions that occur by transformation of the oxygen sublattice. Here we demonstrate an epitaxial nanocomposite approach for exploring the spatial control of topotactic phase transition from a pristine perovskite phase to an oxygen vacancy-ordered brownmillerite (BM) phase in a model oxide La 0.7 Sr 0.3 MnO 3 (LSMO). Incorporating a minority phase NiO in LSMO films creates ultrahigh density of vertically aligned epitaxial interfaces that strongly influence the oxygen vacancy formation and distribution in LSMO. Combined structural characterizations reveal strong interactions between NiO and LSMO across the epitaxial interfaces leading to a topotactic phase transition in LSMO accompanied by significant morphology evolution in NiO. Using the NiO nominal ratio as a single control parameter, we obtain intermediate topotactic nanostructures with distinct distribution of the transformed LSMO-BM phase, which enables systematic tuning of magnetic and electrical transport properties. The use of self-assembled heterostructure interfaces by the epitaxial nanocomposite platform enables more versatile design of topotactic phase structures and correlated functionalities that are sensitive to oxygen vacancies.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Engineering lithium-ion battery cathodes for high-voltage applications using electromagnetic excitation

Microwave radiation (MWR), a type of electromagnetic excitation source, reduces the synthesis temperature and processing time for chemical reactions compared to traditional synthesis methods. Recently, we demonstrated that MWR can engineer ceramics with different crystal phases compared to traditional methods [ Journal of Materials Chemistry A 5 , 35 (2017)]. Here, we further apply the MWR-assisted technique to improve the electrochemical performance of LiCoO 2 cathodes by engineering TiO 2 and ZrO 2 ceramic coatings. Electrochemical tests suggest that the TiO 2 coating improves the rate capability of the LiCoO 2 electrode. Both TiO 2 and ZrO 2 coatings improve the high-voltage (4.5 V) cycling stability of LiCoO 2 . The capacity remaining is improved from 52.8 to 84.4% and 81.9% by the TiO 2 coating and the ZrO 2 coating, respectively, after 40 cycles. We compare these results with existing studies that apply traditional methods to engineer TiO 2 /ZrO 2 on LiCoO 2 , and find that the MWR-assisted method shows better performance improvement. X-ray photoelectron spectroscopy measurements suggest that the improved cycling stability arises from the formation of metal fluorides that protect the electrode from side reactions with electrolytes. This mechanism is further supported by the reduced Co dissolution from TiO 2 /ZrO 2 -coated LiCoO 2 electrode after cycling. This study provides a new toolbox facilitating the integration of many delicate, low melting point materials like polymers into battery electrodes.

25 ENERGY STORAGE↗
DOE OSTI2022

Quadratic strong coupling in AlN Kerr cavity solitons

Photonic platforms with χ (2) nonlinearity offer new degrees of freedom for Kerr frequency comb development. Here, we demonstrate Kerr soliton generation at 1550 nm with phase-matched quadratic coupling to the 775 nm harmonic band in a single AlN microring and thus the formation of dual-band mode-locked combs. In the strong quadratic coupling regime where the χ (2) phase-matching window overlaps the pump mode, the pump-to-harmonic-comb conversion efficiency is optimized. However, the strong quadratic coupling also drastically modifies the Kerr comb generation dynamics and decreases the probability of soliton generation. By engineering the χ (2) phase-matching wavelength, we are able to achieve a balance between high conversion efficiency and high soliton formation rate under the available pump power and microring quality factors. Our numerical simulations confirm the experimental observations. These findings provide guidance on tailoring single-cavity dual-band coherent comb sources.

Gong, Zheng (ORCID:0000000301704870)↗
DOE OSTI2024

Imaging nanomagnetism and magnetic phase transitions in atomically thin CrSBr

Since their first observation in 2017, atomically thin van der Waals (vdW) magnets have attracted significant fundamental, and application-driven attention. However, their low ordering temperatures, T c , sensitivity to atmospheric conditions and difficulties in preparing clean large-area samples still present major limitations to further progress, especially amongst van der Waals magnetic semiconductors. The remarkably stable, high-T c vdW magnet CrSBr has the potential to overcome these key shortcomings, but its nanoscale properties and rich magnetic phase diagram remain poorly understood. Here we use single spin magnetometry to quantitatively characterise saturation magnetization, magnetic anisotropy constants, and magnetic phase transitions in few-layer CrSBr by direct magnetic imaging. We show pristine magnetic phases, devoid of defects on micron length-scales, and demonstrate remarkable air-stability down the monolayer limit. We furthermore address the spin-flip transition in bilayer CrSBr by imaging the phase-coexistence of regions of antiferromagnetically (AFM) ordered and fully aligned spins. Our work will enable the engineering of exotic electronic and magnetic phases in CrSBr and the realization of novel nanomagnetic devices based on this highly promising vdW magnet.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Modular, Crushed-Rock Thermal Energy Storage Pilot Design (Final Report)

The goal of this project was to design a next-step pilot to advance near-term energy storage integrated with a fossil plant to provide a facility capable of being viable and effective in a market with growing penetration of variable renewable energy (VRE). Thermal energy storage (TES) represents an ideal technology for this purpose. The completed effort included a feasibility study to prepare for the Phase II pre-front end engineering design (pre-FEED) for implementing a crushed-rock TES system integrated with a natural gas combined cycle (NGCC) plant. The crushed rock storage technology, which is being developed by Brenmiller Energy, is a modular TES system termed bGen™, which can accommodate both thermal and electrical inputs and output steam, hot water, or hot air. For this application, the estimated efficiency is 80% thermal to thermal. For the feasibility study, the Brenmiller technology was designed to operate on a slipstream from NYPA’s Eugene W. Zeltmann Power Project (Zeltmann) NGCC plant in Astoria, New York. The projected size of the system will be up to 4 MWe with at least 4 hours of storage duration, or 16 MWh-e total. The study also included a techno-economic evaluation of a 200 MWh commercial-scale demonstration. Prior to this project, EPRI had reviewed Brenmiller’s technology, which is being built to demonstrate bGen™ at 1.7 MWe on a solar plant (Rotem) and has been designed for an NGCC facility in Italy, assessing it at technology readiness level (TRL) 5. Brenmiller is also conducting a separate 1-MWth pilot with NYPA that pairs a bGen™ module with a microturbine for a combined-heat-and-power (CHP) application to improve efficiency and provide flexibility. The next-step pilot being designed as part of this project would represent a 5-fold increase in scale, versus Rotem, and would show the technology’s ability to provide effective and economical energy storage, bringing the technology to TRL 6. This pilot would represent the next-to-last demonstration scale before the technology could be commercial ready at GWh-e scales in the 2030 timeframe. The main objective of the work completed by the Electric Power Research Institute, Inc. (EPRI), Brenmiller Energy (Brenmiller), New York Power Authority (NYPA), and United E&C (formerly AECOM) was to perform a Phase I feasibility study on the integration of a crushed-rock thermal energy storage (TES) with a fossil plant. Under this project, the EPRI-led team successfully completed a feasibility study to prepare for the potential future Phase II pre-front end engineering design (pre-FEED) to implement a crushed-rock TES system integrated with a natural gas combined cycle (NGCC) plant. Specific deliverables under this project included the Technology Maturation Plan, Conceptual Study, Techno-Economic Assessment, Technology Gap Assessment, Project Plan for Phase II (submitted as Phase II Renewal Application), Commercialization Plan, and the Final Report (this document). This Final Report includes a compilation of the various summary reports that were prepared during the 12-month schedule of the Phase I project execution under award DE-FE0032017.

03 NATURAL GAS↗
DOE OSTI2026

Deterministic Control of Sn 3+ Valence and Electronic Phase Evolution in AgSnSe 2

Understanding how unusual oxidation states influence material properties is important for both fundamental science and energy applications. AgSnSe 2 is particularly intriguing because it stabilizes the rare and long-debated Sn 3+ oxidation state, whose true existence and role have remained enigmatic for many years. Here, in this work, we employ X-ray photoelectron spectroscopy, Mössbauer spectroscopy, and X-ray absorption spectroscopy to directly probe the oxidation state of Sn and its evolution under chemical substitution. All experimental evidence consistently confirms the presence of Sn in the +3 oxidation state in AgSnSe 2 . Complementary density functional theory calculations further corroborate this assignment. By substituting Sn with Sb, we systematically control the electronic state and its impact on the material’s physical properties. At low Sb concentrations, AgSnSe 2 retains superconductivity with a transition temperature of ∼5 K, while increasing Sb content deterministically drives a metallic-to-semiconducting transition through progressive suppression of superconductivity. Spectroscopic analyses show that Sb substitution provides deterministic control of the Sn oxidation state, evolving from a uniform +3 configuration in AgSnSe 2 to a mixed +2/+4 valence-skipping regime at higher Sb levels, thereby establishing a direct chemical handle over the material’s electronic phase. This tunability demonstrates that the Sn oxidation state in AgSnSe2 can be precisely engineered through Sb substitution, enabling controlled electronic phase transitions and establishing AgSnSe 2 as a promising platform for quantum and energy-related applications

crystal structure↗
DOE OSTI2020

Structural and Optical Response of Polymer-Stabilized Blue Phase Liquid Crystal Films to Volatile Organic Compounds

Engineering useful mechanical properties into stimuli-responsive soft materials without compromising their responsiveness is, in many cases, an unresolved challenge. For example, polymer networks formed within blue-phase liquid crystals (BPs) have been shown to form mechanically robust films, but the impact of polymer networks on the response of these soft materials to chemical stimuli has not been explored. In this work, we report on the response of polymer-stabilized BPs (PSBPs) to volatile organic compounds (VOCs, using toluene as a model compound) and compare the response to BPs without polymer stabilization and to polymerized nematic and cholesteric phases. We find that PSBPs generate an optical response to toluene vapor (change in reflection intensity under crossed polars) that is sixfold greater in sensitivity than the polymerized nematic or cholesteric phases and with a limit of detection (140 ± 10 ppm at 25 °C) that is relevant to the measurement of permissible exposure limits for humans. Additionally, when compared to BPs that have not been polymerized, PSBPs respond to a broader range of toluene vapor concentrations (5000 vs <1000 ppm) over a wider temperature interval (25–45 vs 45–53 °C). We place these experimental observations into the context of a simple thermodynamic model to explore how the PSBP response reflects the effect of toluene on competing contributions of double-twisted LC cylinders, disclinations, and polymer network to the free energy that controls the PSBP lattice spacing. Overall, we conclude that the mechanical and thermal stability of PSBPs, when combined with their optical responsiveness to toluene, make this class of self-supporting LCs a promising one as the basis of passive and compact (e.g., wearable) sensors for VOCs.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2024

A brief review on strain engineering of ferroelectric K x Na 1- x NbO 3 epitaxial thin films: Insights from phase-field simulations

Strains play a pivotal role in determining the phase equilibrium, domain configuration, and functional properties of the low-dimensional ferroelectrics. There is growing interest in the strain engineering of ferroelectric K x Na 1- x NbO 3 (KNN) epitaxial thin films, which exhibit excellent physical properties and promise as eco-friendly alternatives to lead-based ferroelectrics for microdevice applications. Further, advances have been made in understanding the phase equilibria and transitions, domains and domain walls, and their relations to the physical properties of KNN epitaxial thin films using a combination of experiments and theoretical modeling, particularly phase-field simulations. Here, we review recent progress in these aspects and showcase the phase-field method for establishing strain phase diagrams, elucidating the domain and domain wall structures at equilibrium, and predicting the structure–property relationships in ferroelectric KNN thin films. We also discuss challenges and opportunities to further advance our understanding of KNN thin films and potentially unlock new functionalities by leveraging phase-field simulations.

36 MATERIALS SCIENCE↗