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At least 973 records · Page 54
All sourcesNASA NTRSDOE OSTI
DOE OSTI2025

Asymmetry of the Ferroelectric Phase Transition in BaTiO 3

Symmetry changes during phase transformations fundamentally determine the behavior of thermodynamic systems, governing phenomena as diverse as water evaporation, fermion condensation, and epidemic spreading. Phase transitions are conventionally divided into two classes, and this classification is typically assumed to remain invariant upon reversing the transition. Here, an asymmetric phase transformation is uncovered in the ferroelectric–paraelectric transition of single-crystal BaTiO 3 , a model system for first-order transitions. Under slow temperature variation (≤0.1 °C min −1 ), thermodynamic, dielectric, and domain-structure measurements reveal that the ferroelectric-to-paraelectric transition exhibits latent heat, phase coexistence, and a discontinuous order parameter, while these signatures are absent upon cooling. Complementary phase-field simulations demonstrate similar behavior, attributing it to distinct elastic strain energy accumulation and release during heating and cooling. These findings reveal a first-order character upon heating but a second-order-like behavior upon cooling, challenging the conventional paradigm of symmetric phase-transition classification and suggesting new possibilities for ferroelectric-based energy storage.

Hershkovitz, Asaf [Technion-Israel Institute of Te↗
DOE OSTI2017

First-Principles Simulation of the (Li–Ni–Vacancy)O Phase Diagram and Its Relevance for the Surface Phases in Ni-Rich Li-Ion Cathode Materials

Despite several reports on the surface phase transformations from a layered to a disordered spinel and a rock-salt structure at the surface of the Ni-rich cathodes, the precise structures and compositions of these surface phases are unknown. The phenomenon, in itself, is complex and involves the participation of several contributing factors. Of these factors, transition metal (TM) ion migration toward the interior of the particle and hence formation of TM-densified surface layers, triggered by oxygen loss, is thermodynamically probable. Here in this study, we simulate the thermodynamic phase equilibria as a function of TM ion content in the cathode material in the context of lithium nickel oxides, using a combined approach of first-principles density functional calculations, the cluster expansion method, and grand canonical Monte Carlo simulations. We developed a unified lattice Hamiltonian that accommodates not only rock-salt like structures but also topologically different spinel-like structures. Also, our model provides a foundation to investigate metastable cation compositions and kinetics of the phase transformations. Our investigations predict the existence of several Ni-rich phases that were, to date, unknown in the scientific literature. Our simulated phase diagrams at finite temperature show a very low solubility range of the prototype spinel phase. Finally, we find a partially disordered spinel-like phase with far greater solubility that is expected to show very different Li diffusivity compared to that of the prototype spinel structure.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Shear-Assisted Formation of Cation-Disordered Rocksalt NaMO 2 (M = Fe or Mn)

Herein we report the synthesis of cation-disordered rocksalt NaMO 2 (M = 3d transition metal), i.e., NaFeO 2–δ (δ ~ 0.18), for the first time via a martensitic-like phase transformation from β-NaFeO 2 . This disordered rocksalt NaFeO 2–δ does not exhibit long-range cation ordering, as evidenced by synchrotron X-ray diffraction, neutron diffraction, and transmission electron microscopy analyses. Pair distribution function analysis revealed that although the cation sublattice mostly preserved the rocksalt symmetry, O atoms were significantly displaced to accommodate the different Na + –O, Fe 2+ –O, and Fe 3+ –O bond lengths (~2.4, 2.1, and 2 Å, respectively). In addition, we confirmed the presence of O vacancies and partial Fe reduction from 3+ to 2+ in NaFeO 2–δ based on 57 Fe Mössbauer and X-ray absorption spectroscopy analyses. Controlled experiments revealed that three factors are essential for the successful synthesis of the rocksalt structure: (1) an appropriate precursor structure, (2) the application of a large stress by high-energy milling, and (3) the creation of O vacancies via carbon reduction. Density functional theory calculations demonstrated that these factors substantially affect the energetics of the minimum-energy reaction path of the phase transformation, making the highly metastable disordered rocksalt phase accessible. Our work suggests the effectiveness of mechanical milling to access metastable phases that are structurally related to the precursors via a shearing transformation. Utilizing this mechanism, we were able to predict and synthesize another cation-disordered rocksalt structure, NaMnO 2 , and explain the syntheses of several other related materials.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2012

Crack tip plasticity in single crystal UO2: Atomistic simulations

The fracture behavior of single crystal uranium dioxide is studied using molecular dynamics simulations at room temperature. Initially, an elliptical notch is created on either {111} or {110} planes, and tensile loading is applied normal to the crack planes. For cracks on both planes, shielding of crack tips by plastic deformation is observed, and crack extension occurs for crack on {111} planes only. Two plastic processes, dislocation emission and phase transformation are identified at crack tips. The dislocations have a Burgers vector of ?110?/2, and glide on {100} planes. Two metastable phases, the so-called Rutile and Scrutinyite phases, are identified during the phase transformation, and their relative stability is confirmed by separate density- functional-theory calculations. Examination of stress concentration near crack tips reveals that dislocation emission is not an effective shielding mechanism. The formation of new phases may effectively shield the crack provided all phase interfaces formed near the crack tips are coherent, as in the case of cracks residing on {110} planes.

36 MATERIALS SCIENCE↗
DOE OSTI2019

A review of computational modeling techniques in study and design of shape memory ceramics

Shape memory ceramics are a unique family of shape memory materials with a wide variety of applications, such as ultra-high energy dissipation and high-temperature actuation. Along with significant progress in the experimental study of zirconia-based shape memory ceramics in recent years, computational simulations have exhibited powerful capabilities in revealing nano/microstructure-dependent deformation and failure mechanisms in these materials. In this work, we review the recent progress in understanding the phase transformation behavior in shape memory ceramics, focusing on computational modeling of zirconia-based ceramics. Electronic structure calculations have provided new data on the phase stability and surface properties of shape memory ceramics. At the nanometer length scale, molecular dynamics simulations have captured fundamental information about martensitic phase transformation and the effects of grain boundaries and defects on mechanical response of bi- and polycrystalline zirconia-based ceramics. At the micrometer length scale, advanced phase-field models have shown the ability to predict morphological evolution of microstructures and the corresponding mechanical responses in good agreement with experimental observations. Despite the recent multiscale computational advancements, further developments are required to establish processing-structure-property-performance relations that will lead to reliable and practical strategies for designing zirconia-based (or other) ceramics with improved and sustained shape memory responses. This article critically reviews current computational modeling techniques and provides an outlook for the study and design of the next generation of shape memory ceramics.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Phase Field Modeling of Chemical Reaction Related Damage Evolution in Environmental Barrier Coatings

The advent of next-generation engines necessitates materials capable of withstanding temperatures beyond the reach of current superalloys. SiC-based ceramic matrix composites, augmented with environmental barrier coatings (EBCs), present a promising materials solution. Given the active search for effective and durable EBCs, there is a pressing need for modeling tools to understand and predict damage evolution in these materials to help accelerate their development. This study introduces a phase-field model (PFM) designed to simulate the thermally grown oxides (TGO) and phase transformation in the degradation and failure of EBCs. The model accounts for the severe volume expansion due to oxidation, alongside phase transformations and porosity evolution during thermal cycling, offering a comprehensive view of the damage processes. Simulation results are validated against experimental findings reported in the literature, establishing the model's potential as a significant tool for understanding and improving the resilience of EBCs in cyclic oxidative environments.

fast-diffusion path↗
DOE OSTI2016

Exploring phase stability, electronic and mechanical properties of Ce–Pb intermetallic compounds using first-principles calculations

The phase stability, electronic and mechanical properties of Ce–Pb intermetallics have been investigated by using first-principles calculations. Five stable and four metastable phases of Ce–Pb intermetallics were verified. Among them, CePb{sub 2} has been confirmed as HfGa{sub 2}-type structure. For Ce{sub 5}Pb{sub 3}, the high pressure phase transformation from D8{sub m} to D8{sub 8} with trivalent Ce has been predicted to occur at P=1.2 GPa and a high temperature phase transformation has been predicted from D8{sub m} to D8{sub 8} with tetravalent Ce at 531.5 K. The calculated lattice constants of the five stable phases are in good agreement with experimental values. The electronic density of states, charge density and electron localization function of Ce{sub 3}Pb have been calculated, which indicated that the Ce and Pb show ionic behavior. The polycrystalline bulk modulus, shear modulus, Young's modulus, and Poisson's ratio are also estimated from the calculated single crystalline elastic constants. All of the calculated elastic constants satisfy mechanical stability criteria. The microhardness and mechanical anisotropy are predicted. The anisotropic nature of the Ce–Pb intermetallic compounds are demonstrated by the three-dimensional orientation dependent surfaces of Young's moduli and linear compressibility are also demonstrated. The longitudinal, transverse and average sound velocities and the Debye temperatures are also obtained in this work. The Ce{sub 3}Pb has the largest Debye temperature of 192.6 K, which means the Ce{sub 3}Pb has a highest melting point and high thermal conductivity than other compounds. - Graphical abstract: The convex hull plots of the enthalpies of formation for Ce–Pb binary systems calculated at 0 K. - Highlights: • The five stable and four metastable phases in the Ce–Pb binary system were predicted. • The crystal structure of CePb{sub 2} has been confirmed as HfGa{sub 2}-type.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2023

In situ formed partially disordered phases as earth-abundant Mn-rich cathode materials

Abstract Earth-abundant cathode materials are urgently needed to enable scaling of the Li-ion industry to multiply terawatt hours of annual production, necessitating reconsideration of how good cathode materials can be obtained. Irreversible transition metal migration and phase transformations in Li-ion cathodes are typically believed to be detrimental because they may trigger voltage hysteresis, poor kinetics and capacity degradation. Here we challenge this conventional consensus by reporting an unusual phase transformation from disordered Li- and Mn-rich rock salts to a new phase (named δ ), which displays partial spinel-like ordering with short coherence length and exhibits high energy density and rate capability. Unlike other Mn-based cathodes, the δ phase exhibits almost no voltage fade upon cycling. We identify the driving force and kinetics of this in situ cathode formation and establish design guidelines for Li- and Mn-rich compositions that combine high energy density, high rate capability and good cyclability, thereby enabling Mn-based energy storage.

25 ENERGY STORAGE↗
DOE OSTI2021

On the existence of orthorhombic martensite in a near-α titanium base alloy used for additive manufacturing

Additive manufacturing is a state-of-the-art production technology to produce tailor-made and highly complex parts. Among various other alloys, Ti base alloys are frequently used in this manufacturing technique due to their well-balanced properties and their wide range of applications. Allotropic phases and the occurrence of athermal phase transformations are the main reasons why these alloys hold a great development potential and are the basis of extensive use. High cooling rates during manufacturing lead to martensitic phases and the formation of nanometer-sized microstructures resulting in extraordinary strength. Simultaneously, such high cooling rates cause a high amount of lattice defects and the occurrence of residual stresses, which finally may result in delamination effects and cracks. Usually, a common approach to reduce residual stresses during additive manufacturing is to decrease thermal gradients by increasing the heat input or preheating the building platform. Instead of applying the typical approaches to lower thermal gradients, this study deals with the origin of the ‘softer’ orthorhombic martensite by accelerating the solidification process. The implementation of the orthorhombic phase in bulk components was inspired by a new phase transformation herein reported for the first time in the powder material, which also validates the possible occurrence of two martensitic phases in the same alloy. Various sophisticated characterization techniques like high energy and high-temperature X-ray diffraction, high-resolution transmission electron microscopy as well as atom probe tomography were applied to characterize this softer orthorhombic martensitic phase in detail aiming to highlight the opportunities accompanied by this new approach for additive manufacturing of titanium alloys.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Analytical modeling of localized surface plasmon resonance in heterostructure copper sulfide nanocrystals

Localized surface plasmon resonance (LSPR) in semiconductor nanocrystals is a relatively new field of investigation that promises greater tunability of plasmonic properties compared to metal nanoparticles. A novel process by which the LSPR in semiconductor nanocrystals can be altered is through heterostructure formation arising from solution-based cation exchange. Herein, we describe the development of an analytical model of LSPR in heterostructure copper sulfide-zinc sulfide nanocrystals synthesized via a cation exchange reaction between copper sulfide (Cu 1.81 S) nanocrystals and Zn ions. The cation exchange reaction produces dual-interface, heterostructure nanocrystals in which the geometry of the copper sulfide phase can be tuned from a sphere to a thin disk separating symmetrically-grown sulfide (ZnS) grains. Drude model electronic conduction and Mie-Gans theory are applied to describe how the LSPR wavelength changes during cation exchange, taking into account the morphology evolution and changes to the local permittivity. The results of the modeling indicate that the presence of the ZnS grains has a significant effect on the out-of-plane LSPR mode. By comparing the results of the model to previous studies on solid-solid phase transformations of copper sulfide in these nanocrystals during cation exchange, we show that the carrier concentration is independent of the copper vacancy concentration dictated by its atomic phase. The evolution of the effective carrier concentration calculated from the model suggests that the out-of-plane resonance mode is dominant. The classical model was compared to a simplified quantum mechanical model which suggested that quantum mechanical effects become significant when the characteristic size is less than ~8 nm. Overall, we find that the analytical models are not accurate for these heterostructured semiconductor nanocrystals, indicating the need for new model development for this emerging field.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Energy dependence on formation of TiO{sub 2} nanofilms by Ti ion implantation and annealing

Graphical abstract: - Highlights: • Fabrication of TiO{sub 2} films by ion implantation and annealing strongly depends on ion energy. • Best photocatalytic activity is achieved in the TiO{sub 2} nanofilm annealed at 1000 °C. • Phase transformation of TiO{sub 2} appears under annealing temperature of 900 °C. - Abstract: TiO{sub 2} nanofilms were fabricated by a solid-phase-growth progress. The silica glass slides were implanted with Ti ions to the fluence of 1.84 × 10{sup 17} ions/cm{sup 2} at accelerate voltages of 20, 50, and 80 kV, respectively. The samples were annealed in oxygen atmosphere at 700, 800, 900, and 1000 °C for 4 h, respectively. The influence of the ion energy and the annealing temperature on the formation and phase transformation of the TiO{sub 2} films was studied. It was found that anatase TiO{sub 2} nanofilms instead of embedded rutile TiO{sub 2} nanoparticles on the substrate surfaces when the energy of implanted Ti atoms was 20 kV.

36 MATERIALS SCIENCE↗
DOE OSTI2022

High Performance Computing to Quantify the Evolution of Microscopic Concentration Gradients During Flash Processing

During the Flash process, the cross section of a plain-carbon or a low-alloy steel is austenitized through rapid heating and transformed on rapid cooling to a predominantly martensite + bainite structure with small amounts of retained austenite. Unlike conventional heat treating, homogeneity is intentionally avoided during Flash processing of steels. The Flash process assembly consists of a pair of rolls that transfer the steel sheets through the heating and cooling stage of the thermal cycle. The initial microstructure of the steel consists of ferrite (body-centered cubic iron) + carbide ((Fe,X)mCn) mixture. The heating rate through the peak temperature is a function of temperature and reaches a peak of about 300-400°C/s and the cooling rate has a maximum value of 3,000-4,000°C/s. The on-heating phase transformations include carbide dissolution, austenite (face-centered cubic iron) nucleation and growth, and diffusion of carbon and other substitutional elements in the steel. The on-cooling phase transformations include formation of martensite (body-centered tetragonal phase containing supersaturated solute) and bainite (ferrite plates with or without fine carbides). In this project, the focus is on Fe-C-Cr steels that are currently Flash processed for armor applications. The modeling effort proposed here will help optimize the Flash thermal cycle for these low alloy steels to achieve the target performance, which is an ongoing effort at SFP Works. A significant feature of Flash processed Fe-C-Cr steels is the presence of scatter in the through-thickness in the sheet. The variability in hardness results from a variability in the bainite + martensite microstructure that is sensitive to the local chemical concentration of C and Cr. Such a chemical inhomogeneity is intentionally obtained in the Flash process. Although such a microstructural gradient is presumably responsible for the exceptional properties of the Flash processed steel, it is very important to quantify the gradients as a function of Flash variabilities in processing parameters and the input microstructure. Understanding the mechanistic pathway that leads to microstructural gradients could be ground-breaking and instrumental for achieving better process control and optimized microstructural state to meet application-specific strength-ductility requirements. Since the final microstructure depends on setting up precise solute concentration gradients through a rapid heating process, and transforming these regions into various phases, it is important to understand how small changes in steel chemistry, input microstructure (carbide size and distribution), and process variables (Flash thermal cycle) will impact the solute concentration gradients.

97 MATHEMATICS AND COMPUTING↗
DOE OSTI2017

Influence of Temperature on the Texture Distribution in Two-Phase Region Hot Deformation of Grain-Oriented Silicon Steel

Hot deformation simulation test of 3% Si grain-oriented silicon (CGO) steel was carried out at different temperatures, and evolution of microstructures and recrystallized textures in two-phase region was analyzed by electron backscatter diffraction and optical microscopy. The “δ → γ” and “γ → α” phase transformation points of the two-phase region measured by differential scanning calorimetry method are 1272 and 819 °C, respectively, which are within the ranges of 1300-1350 and 800-850 °C, measured by water quenching. The hot deformation temperature has a large effect on the microstructure and texture in the steel. A higher hot deformation temperature leads to higher degree of recrystallization and larger average grain size, more γ-fiber textures and cube {100} <001> texture, but the rotated cube {100} <011> texture is stable. The recrystallized microstructure was also affected by the phase compositions. While the cube {100} <001> texture results in typical austenite recrystallization texture, {111} textures lead to typical ferrite rolling textures.

36 MATERIALS SCIENCE↗
DOE OSTI2012

Evaluation of microstructural development in electron beam melted Ti-6Al-4V

In the current work an investigation of the microstructures of EBM built Ti-6Al-4V test bars has been performed using OM, SEM, TEM and XRD. It has been found that the prior {beta} phase, that formed during the initial solidification, possesses a column shaped morphology with growing direction parallel to built direction. Typical ({alpha} + {beta}) structures namely Widmanstaetten {alpha} platelets with rod-like {beta} phase formed on the interfaces of the fine {alpha} grains, have been observed in the columnar prior {beta} grains. Grain boundary {alpha} phase was found to be formed around the boundaries of the columnar prior {beta} grains. Different phases present in the parts, especially the BCC {beta} phases have been characterized. The TEM/EDX results indicate very high V composition in the {beta} phase. Results of TEM/SAED and XRD also revealed that a superlattice structure could be present in the {beta} phase. Phase transformation sequence is discussed according to the processing history and the microstructures observed. - Highlights: Black-Right-Pointing-Pointer {alpha} + {beta} and individual {beta} phase is observed and characterized by SEM, TEM and XRD. Black-Right-Pointing-Pointer {beta}-phase is identified as rod-like structure embedded in {alpha} matrix and platelets. Black-Right-Pointing-Pointer Phase transformation sequence is discussed as per thermal history and microstructures.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Microstructural changes in an extruded Mg−Zn−Y alloy reinforced by quasicrystalline I-phase by small additions of calcium, manganese and cerium-rich mischmetal

The effects of calcium, manganese and cerium-rich mischmetal additions on the microstructure and texture of the extruded Mg−6Zn−1Y (wt.%) alloy have been investigated. The microstructure of the alloys consisted of a magnesium matrix embedding second phase particles aligned along the extrusion direction. The nature and volume fraction of the second phases depended on the alloying element. Thus, Ce-rich mischmetal promoted the formation of T-phase while calcium additions resulted in the formation of a ternary Mg−Zn−Ca compound. Only, manganese additions did not affect the existence of the I-phase present in the ternary alloy. The texture was measured and it was found that calcium addition has a significant effect weakening the extrusion texture. - Highlights: •In-situ HEXRD and DSC techniques are employed to identify phase transformations. •The existence of I-phase is not altered by manganese addition to Mg93Zn6Y1 (wt.%). •Calcium addition promotes Mg{sub 6}Zn{sub 3}Ca{sub 2} formation instead of I-phase. •Mischmetal addition induces the formation of coarse T-phase particles. •Texture depends on the nature of second phases and therefore of the extrusion pressure.

36 MATERIALS SCIENCE↗
DOE OSTI2016

The influence of A-site strontium ion in controlling the microstructure and electrical properties of P{sub 1−x}S{sub x}ZNZT ceramics

A Pb{sub 1−x}Sr{sub x}(Zn{sub 1/3}Nb{sub 2/3}){sub 0.2}(Zr{sub 0.5}Ti{sub 0.5}){sub 0.8}O{sub 3} (P{sub 1−x}S{sub x}ZNZT, 0.00 ≤ x ≤ 0.10) system was prepared through a conventional solid oxide process. Increasing addition of Sr{sup 2+} was found to induce a continuous decrease in grain size from 2.46 μm to 1.14 μm, accompanied by a phase transformation from coexisting rhombohedral and tetragonal phases to tetragonal phase only. The underlying mechanism of the evolution of dielectric and ferroelectric behavior in the P{sub 1−x}S{sub x}ZNZT ferroelectric ceramics was ascribed to the synergy between the grain size effect and the dilution of Pb-O covalency. Additionally, the grain size effect on domain wall displacement was found to be the main origin of the superior permittivity and piezoelectricity of the ceramics at intermediate grain size. The optimal electrical properties achieved for the P{sub 0.95}S{sub 0.05}ZNZT specimen at a grain size of about 1.79 μm, d{sub 33} = 465 pC/N, d{sub 33} × g{sub 33} = 11 047 × 10{sup −15} m{sup 2}/N make this material promising for multilayer energy harvesting device applications.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
NASA NTRS2001

Determination of the Accommodation Coefficient Using Vapor/gas Bubble Dynamics in an Acoustic Field

Nonequilibrium liquid/vapor phase transformations can occur in superheated or subcooled liquids in fast processes such as in evaporation in a vacuum. The rate at which such a phase transformation occurs depends on the "condensation" or "accommodation" coefficient, Beta, which is a property of the interface. Existing measurement techniques for Beta are complex and expensive. The development of a relatively inexpensive and reliable technique for measurement of Beta for a wide range of substances and temperatures is of great practical importance. The dynamics of a bubble in an acoustic field strongly depends on the value of Beta. It is known that near the saturation temperature, small vapor bubbles grow under the action of an acoustic field due to "rectified heat transfer." This finding can be used as the basis for an effective measurement technique of Beta. We developed a theory of vapor bubble behavior in an isotropic acoustic wave and in a plane standing acoustic wave. A numerical code was developed which enables simulation of a variety of experimental situations and accurately takes into account slowly evolving temperature. A parametric study showed that the measurement of Beta can be made over a broad range of frequencies and bubble sizes. We found several interesting regimes and conditions which can be efficiently used for measurements of Beta. Measurements of Beta can be performed in both reduced and normal gravity environments.

Gumerov, Nail A.↗