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At least 847 records · Page 47
All sourcesNASA NTRSDOE OSTI
DOE OSTI2016

Difficulty of carrier generation in orthorhombic PbO

Polycrystalline β-PbO films were grown by pulsed laser deposition in atmospheres ranging from oxygen-poor (the oxygen pressure of 0.01 Pa) to oxygen-rich (13 Pa) conditions, and the oxygen chemical potential was further enhanced by ozone annealing to examine hole doping. It was found that each of the as-grown β-PbO films showed poor electrical conductivity, σ < 1.4 × 10{sup −7} S cm{sup −1}, regardless of the oxygen pressure. The density functional calculations revealed that native defects including Pb and O vacancies have deep transition levels and extremely high formation enthalpies, which indicates difficulty of carrier generation in β-PbO and explains the experimentally observed poor electrical conductivity. The analysis of the electronic structures showed that the interaction between Pb 6s and O 2p orbitals is weak due to the deep energy level of Pb 6s and does not raise the valence band maximum (VBM) level unlike that observed in SnO, which is also supported by ultraviolet photoemission spectroscopy measurements. The deep acceptor transition levels of the native defects are attributed to the deep VBM of β-PbO. On the other hand, annealing β-PbO films in reactive oxygen-containing atmospheres (i.e., O{sub 3}) led to a significantly enhanced electrical conductivity (i.e., σ > 7.1 × 10{sup 2} S cm{sup −1}) but it is the result of the formation of an n-type PbO{sub 2} phase because oxygen chemical potential exceeded the phase boundary limit. The striking difference in carrier generation between PbO and SnO is discussed based on the electronic structures calculated by density functional theory.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Electric field induced lattice strain in pseudocubic Bi(Mg{sub 1/2}Ti{sub 1/2})O{sub 3}-modified BaTiO{sub 3}-BiFeO{sub 3} piezoelectric ceramics

Contributions to the piezoelectric response in pseudocubic 0.3BaTiO{sub 3}-0.1Bi(Mg{sub 1/2}Ti{sub 1/2})O{sub 3}-0.6BiFeO{sub 3} ceramics were investigated by synchrotron X-ray diffraction under electric fields. All of the lattice strain determined from the 110, 111, and 200 pseudocubic diffraction peaks showed similar lattice strain hysteresis that was comparable to the bulk butterfly-like strain curve. It was suggested that the hysteresis of the lattice strain and the lack of anisotropy were related to the complex domain structure and the phase boundary composition.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Detecting topological phases in silicene by anomalous Nernst effect

Silicene undergoes various topological phases under the interplay of intrinsic spin-orbit coupling, perpendicular electric field, and off-resonant light. We propose that the abundant topological phases can be distinguished by measuring the Nernst conductivity even at room temperature, and their phase boundaries can be determined by differentiating the charge and spin Nernst conductivities. By modulating the electric and light fields, pure spin polarized, valley polarized, and even spin-valley polarized Nernst currents can be generated. As Nernst conductivity is zero for linear polarized light, silicene can act as an optically controlled spin and valley field-effect transistor. Similar investigations can be extended from silicene to germanene and stanene, and a comparison is made for the anomalous thermomagnetic figure of merits between them. These results will facilitate potential applications in spin and valley caloritronics.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Effect of (Sr{sub 0.7}Ca{sub 0.3})TiO{sub 3}-substitution on structure, dielectric, ferroelectric, and magnetic properties of BiFeO{sub 3} ceramics

Bi{sub 1−x}(Sr{sub 0.7}Ca{sub 0.3}){sub x}Fe{sub 1−x}Ti{sub x}O{sub 3} ceramics were prepared by a standard solid state reaction process, and the influence of Sr/Ca ratio on structure and properties for Bi{sub 1−x}(Sr,Ca){sub x}Fe{sub 1−x}Ti{sub x}O{sub 3} system was discussed by comparing with Sr{sub 0.5}Ca{sub 0.5}TiO{sub 3}-modified BiFeO{sub 3} ceramics. Rietveld analysis of X-ray diffraction data revealed that the crystal structure changed from rhombohedral R3c (x ≤ 0.4) to orthorhombic Pnma (x = 0.6) with Sr{sub 0.7}Ca{sub 0.3}TiO{sub 3} substitution, and biphasic structure (R3c + Pnma) was determined at x = 0.5, while that for Bi{sub 1−x}(Sr{sub 0.5}Ca{sub 0.5}){sub x}Fe{sub 1−x}Ti{sub x}O{sub 3} system was at x = 0.4. This indicated that the morphotropic phase boundary in Bi{sub 1−x}(Sr,Ca){sub x}Fe{sub 1−x}Ti{sub x}O{sub 3} system shifted toward (Sr,Ca)TiO{sub 3} side with increasing Sr/Ca ratio. The Raman spectrometric analysis and selected area electron diffraction analysis also confirmed this transition. The dielectric relaxation could be well fitted by Arrhenius law, and the different activation energies were attributed to the different origins of the dielectric relaxations with increasing temperature. The current density-field (J-E) curves indicated that the leakage current was reduced to about five orders of magnitude with Sr{sub 0.7}Ca{sub 0.3}TiO{sub 3} substitution. The P-E hysteresis loops obtained by three different methods indicated the enhanced ferroelectricity at x = 0.4, and it could be attributed to the decrement of leakage current. Meanwhile, the magnetization was enhanced with Sr{sub 0.7}Ca{sub 0.3}TiO{sub 3} substitution, and the maximum remanent magnetization was determined at x = 0.2. The enhanced magnetization originated from the partial substitution of Fe{sup 3+} by Ti{sup 4+}.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

A review on the performance and modelling of proton exchange membrane fuel cells

Proton Exchange Membrane Fuel Cells (PEMFC), are energy efficient and environmentally friendly alternative to conventional energy conversion for various applications in stationary power plants, portable power device and transportation. PEM fuel cells provide low operating temperature and high-energy efficiency with near zero emission. A PEM fuel cell is a multiple distinct parts device and a series of mass, energy, transport through gas channels, electric current transport through membrane electrode assembly and electrochemical reactions at the triple-phase boundaries. These processes play a decisive role in determining the performance of the Fuel cell, so that studies on the phenomena of gas flows and the performance modelling are made deeply. This paper gives a comprehensive overview of the state of the art on the Study of the phenomena of gas flow and performance modelling of PEMFC.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2016

Growth of L1{sub 0}-ordered crystal in FePt and FePd thin films on MgO(001) substrate

Formation of L1{sub 0}-oredered structure from disordered A1 phase has been investigated for FePt and FePd films on MgO(001) substrates employing a two-step method consisting of low temperature deposition at 200 °C followed by high-temperature annealing at 600 °C. L1{sub 0}-(001) variant crystal with the c-axis perpendicular to the substrate grows preferentially in FePd films whereas L1{sub 0}-(100), (010) variants tend to be mixed with the L1{sub 0}-(001) variant in FePt films. The structure analysis by X-ray diffraction indicates that a difference in A1 lattice strain is the influential factor that determines the resulting L1{sub 0}-variant structure in ordered thin films. Misfit dislocations and anti-phase boundaries are observed in high-resolution transmission electron micrographs of 10 nm-thick Fe(Pt, Pd) film consisting of L1{sub 0}-(001) variants which are formed through atomic diffusion at 600 °C in a laterally strained FePt/PeFd epitaxial thin film. Based on the experimental results, a nucleation and growth model for explaining L1{sub 0}-variant formation is proposed, which suggests a possibility in tailoring the L1{sub 0} variant structure in ordered magnetic thin films by controlling the alloy composition, the layer structure, and the substrate material.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Direct view at colossal permittivity in donor-acceptor (Nb, In) co-doped rutile TiO{sub 2}

Topical observations of colossal permittivity (CP) with low dielectric loss in donor-acceptor cations co-doped rutile TiO{sub 2} have opened up several possibilities in microelectronics and energy-storage devices. Yet, the precise origin of the CP behavior, knowledge of which is essential to empower the device integration suitably, is highly disputed in the literature. From spectromicroscopic approach besides dielectric measurements, we explore that microscopic electronic inhomogeneities along with the nano-scale phase boundaries and the low temperature polaronic relaxation are mostly responsible for such a dielectric behavior, rather than electron-pinned defect-dipoles/grain-boundary effects as usually proposed. Donor-acceptor co-doping results in a controlled carrier-hopping inevitably influencing the dielectric loss while invariably upholding the CP value.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Phonon scattering mechanisms dictating the thermal conductivity of lead zirconate titanate (PbZr 1- x Ti x O 3 ) thin films across the compositional phase diagram

This paper represents a thorough investigation of the thermal conductivity (κ) in both thin film and bulk PbZr 1–x Ti x O 3 (PZT) across the compositional phase diagram. Given the technological importance of PZT as a superb piezoelectric and ferroelectric material in devices and systems impacting a wide array of industries, this research serves to fill the gap in knowledge regarding the thermal properties. The thermal conductivities of both thin film and bulk PZT are found to vary by a considerable margin as a function of composition x. Additionally, we observe a discontinuity in κ in the vicinity of the morphotropic phase boundary (MPB, x = 0.48) where there is a 20%–25% decrease in κ in our thin film data, similar to that found in literature data for bulk PZT. The comparison between bulk and thin film materials highlights the sensitivity of κ to size effects such as film thickness and grain size even in disordered alloy/solid-solution materials. A model for the thermal conductivity of PZT as a function of composition (κ(x)) is presented, which enables the application of the virtual crystal approximation for alloy-type material systems with very different crystals structures, resulting in differing temperature trends for κ. We show that in the case of crystalline solid-solutions where the thermal conductivity of one of the parent materials exhibits glass-like temperature trends the compositional dependence of thermal conductivity is relatively constant for most values of x. Finally, this is in stark contrast with the typical trends of thermal conductivity with x in alloys, where the thermal conductivity increases dramatically as the composition of the alloy or solid-solution approaches that of a pure parent materials (i.e., as x = 0 or 1).

36 MATERIALS SCIENCE↗
DOE OSTI2017

Role of flexoelectric coupling in polarization rotations at the a-c domain walls in ferroelectric perovskites

Ferroelectric and ferroelastic domain walls play important roles in ferroelectric properties. However, their couplings with flexoelectricity have been less understood. Here, we applied phase-field simulation to investigate the flexoelectric coupling with ferroelectric a/c twin structures in lead ziconate titanate thin films. Local stress gradients were found to exist near twin walls that created both lateral and vertical electric fields through the flexoelectric effect, resulting in polarization inclinations from either horizontal or normal orientation, polarization rotation angles deviated from 90°, and consequently highly asymmetric a/c twin walls. Furthermore, by tuning the flexoelectric strengths in a reasonable range from first-principles calculations, we found that the transverse flexoelectric coefficient has a larger influence on the polarization rotation than longitudinal and shear coefficients. And as polar rotations that commonly occur at compositional morphotropic phase boundaries contribute to the piezoelectric enhancement, this work calls for further exploration of alternative strain-engineered polar rotations via flexoelectricity in ferroelectric thin films.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Mechanisms underpinning the ultrahigh piezoelectricity in Sm-doped 0.705Pb(Mg 1/3 Nb 2/3 )O 3 -0.295PbTiO 3 : Temperature-induced metastable local structure and field-induced polarization rotation

The solid solution of (100 – x )%Pb(Mg 1/3 Nb 2/3 )O 3 - x %PbTiO 3 (PMN- x PT) exhibits ultrahigh piezoelectric and dielectric properties near the morphotropic phase boundary compositions and, as such, has been extensively studied in recent years. Recently, 2.5 mol. % Sm-doped PMN-29PT polycrystalline ceramics were reported to possess the highest piezoelectric coefficients (~1500 pC/N) among all reported piezoceramics, but the atomic-scale mechanisms for such high piezoelectric properties are not yet clear. Here, in situ X-ray diffraction and X-ray total scattering measurements during the application of an electric field, together with in situ total scattering measurement at different temperatures, were conducted for 2.5 mol. % Sm-doped PMN-29.5PT (2.5Sm-PMN-29.5PT). Both the largest field-induced strain and the piezoelectric response were found in the crystallites oriented with their &#12296 100 &#12297 PC directions parallel to the applied field. The local Pb displacement was analyzed using the reverse Monte Carlo method based on the pair distribution functions at different temperatures, where a temperature-induced directional change of Pb displacement was observed. Based on the experimental observations, a field-induced polarization rotation is suggested to be the dominant mechanism for the ultrahigh piezoelectricity of the 2.5Sm-PMN-29.5PT ceramic, while the ease of polarization rotation is possibly attributed to the temperature-induced metastable local monoclinic symmetries with their polar axes close to &#12296 111 &#12297 PC .

36 MATERIALS SCIENCE↗
DOE OSTI2020

Inertial dynamics of an interface with interfacial mass flux: Stability and flow fields’ structure, inertial stabilization mechanism, degeneracy of Landau’s solution, effect of energy fluctuations, and chemistry-induced instabilities

This work focuses on the long-standing problem of inertial dynamics of an interface with interfacial mass flux and reports new mechanisms for the interface stabilization and destabilization. The interface is a phase boundary separating fluids of different densities and having interfacial mass flux. To analyze the interface dynamics from a far field, we develop and apply the general matrix method to rigorously solve the boundary value problem involving the governing equations in the fluid bulk and the boundary conditions at the interface and at the outside boundaries of the domain. We find the fundamental solutions for the linearized system of equations and analyze the interplay of interface stability with flow fields’ structure by directly linking rigorous mathematical attributes to physical observables. We find that the interface is stable when the dynamics conserves the fluxes of mass, momentum, and energy; the stabilization is due to an inertial mechanism causing small oscillations of the interface velocity. In the classic Landau’s dynamics, the postulate of perfect constancy of the interface velocity leads to the development of Landau–Darrieus instability. This destabilization is also linked to the imbalance of the perturbed energy at the interface. The classic Landau’s solution is found to have degeneracy; lifting of the degeneracy may lead to singularity and self-similar dynamics. Our results compare well with traditional theories of combustion and propose new experiments to study the dynamics of the interface and the flow fields in combustible systems. We further conduct reactive molecular dynamics simulations to elucidate the complexity of chemical processes, to study the destabilizing effect of energy fluctuations on the interface stability, and to illustrate the chemistry-induced instabilities. In summary, we identify the extreme sensitivity of the interface dynamics to the interfacial boundary conditions, including the formal properties of fundamental solutions and the qualitative and quantitative properties of the flow fields. This provides new opportunities for studies, diagnostics, and control of multiphase flows in a broad range of processes in nature and technology.

42 ENGINEERING↗
DOE OSTI2021

Predictability as a probe of manifest and latent physics: The case of atomic scale structural, chemical, and polarization behaviors in multiferroic Sm-doped BiFeO 3

The predictability of a certain effect or phenomenon is often equated with the knowledge of relevant physical laws, typically understood as a functional or numerically derived relationship between the observations and known states of the system. Correspondingly, observations inconsistent with prior knowledge can be used to derive new knowledge on the nature of the system or indicate the presence of yet unknown mechanisms. In this work, we explore the applicability of Gaussian processes (GP) to establish predictability and uncertainty of local behaviors from multimodal observations, providing an alternative to this classical paradigm. Using atomic resolution scanning transmission electron microscopy (STEM) of multiferroic Sm-doped BiFeO 3 across a broad composition range, we directly visualize the atomic structure and structural, physical, and chemical order parameter fields for the material. GP regression is used to establish the predictability of the local polarization field from different groups of parameters, including the adjacent polarization values and several combinations of physical and chemical descriptors, including lattice parameters, column intensities, etc. We observe that certain elements of microstructure, including charged and uncharged domain walls and interfaces with the substrate, are best predicted with specific combinations of descriptors, and this predictability and associated uncertainties are consistent across the composition series. The associated generative physical mechanisms are discussed. It is also found that certain parameter combinations tend to predict the orthorhombic phase in the cases where rhombohedral phase is observed, suggesting a potential role of clamping and confinement phenomena in phase equilibrium in Sm-BiFeO 3 system close to morphotropic phase boundary. We argue that predictability and uncertainty in observational data offer a new pathway to probe the physics of condensed matter systems from multimodal local observations.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2021

Temperature dependence of field-response mechanisms in 0.4Ba(Zr 0.2 Ti 0.8 )O 3 &#x2013;0.6(Ba 0.7 Ca 0.3 )TiO 3

Understanding the temperature dependence of how ferroelectric materials respond to electric fields is critical for determining how best to use these materials in applications. In this study, temperature dependent in situ x-ray diffraction is used to study how field-response mechanism Ba ( Zr 0.2 Ti 0.8 ) O 3 – x ( Ba 0.7 Ca 0.3 ) TiO 3 evolves with temperature. Increasing the measurement temperature was found to slightly reduce the extent of domain reversal and the magnitude of the 111 lattice strain. Instead, these results indicate that temperature predominantly impacted the angular dependence of the 002/200 lattice strain. We attribute the observed differences in 002/200 lattice strain to the flattening of the free-energy near the tetragonal-orthorhombic phase boundary, where the flattening of the energy landscape promotes a polarization rotation response during the application of electric fields at room temperature.

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

Inference of strength and phase transition kinetics in dynamically-compressed tin

Dynamic compression experiments in condensed matter are of interest in part because they provide opportunities to examine material response under extreme conditions; however, the inference of material behavior from dynamic experiments is challenging in the presence of phase transitions exhibiting kinetic processes. Demonstrating an approach to quantitative interpretation of such dynamic experiments, here we present a Bayesian model calibration of strength and phase transformation parameters to data drawn from pulsed power and gas gun shot experiments. The posterior predictions of the Bayesian model capture the experimental measurements and account for the various uncertainties in the experimental configurations. This holistic approach to model calibration utilizing multiple types of experimental data identifies important cross correlations among kinetics, strength, and the phase boundary. Improved insight into potential sources of current model form error is provided by comparing the differences between calibrations against different subsets of the experimental data.

42 ENGINEERING↗
DOE OSTI2015

Reply to Zayed: Interplay of magnetism and structure in the Shastry–Sutherland model

The connection between electronic and structural degrees of freedom - whether successive, coincident, or causal - suffuses the study of phase transitions. The Shastry–Sutherland model of a planar network of coupled spin dimers (1) and its physical realization in SrCu 2 (BO 3 ) 2 (SCBO) provide a fundamental quantum mechanical test of this connection at the onset of antiferromagnetic order. We summarize in Fig. 1 the current understanding of SCBO’s phase diagram for T < 200 K and an intermediate pressure range of 3.5–6 GPa (2⇓–4). At pressures below ~4–5 GPa, SCBO has a tetragonal structure that hosts several low-temperature magnetic phases. Above this pressure, monoclinic distortions reduce the symmetry of the lattice. In ref. 2, we performed full structural refinements of X-ray and neutron scattering measurements to identify a change in space group at 5.5 GPa as a function of temperature (red circles in Fig. 1). This structural change coincides with the onset of antiferromagnetic ordering as a function of temperature, and we argue that this is not a coincidence but instead represents a cooperative effect between distortions of the lattice, the dimers tilting out of the plane, and the emergence of long-range magnetic order. In his comment on our work, Zayed (5) proposes an alternative scenario, in which the antiferromagnetic ordering onsets at lower pressure, within the tetragonal phase, and is then stabilized by the structural distortion associated with the monoclinic phase. Finally, he further speculates that this earlier onset may be associated with the dome in the phase boundary reported in ref. 4 (dark red region, Fig. 1).

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2026

Roadmap on advanced and / real-time characterisation of solid state materials and devices for energy applications

A strong societal and political drive is motivating the development and optimization of novel energy conversion and storage systems for decarbonization. The successful implementation of solid state devices such as fuel cells and secondary batteries depends, however, on achieving ambitious targets in terms of performance, reliability and cost competitiveness. Research and technology are addressing these needs through a holistic approach including exploration of new materials and nanoarchitectures, as well as system engineering. These significant efforts require the support of appropriate characterization tools capable of assessing nanometer-scale phenomena such as concentration profiles of ionic and electronic charges, local chemical compositions and their evolution over time across interfaces. This roadmap provides an overview of selected advanced characterization techniques for energy materials and devices. Specific focus is put on in situ/operando methods for probing electrochemical phenomena in real-time under realistic working conditions. Experts in the field provide an extensive review of the current state of the art in 2025 and the current and future challenges for the characterization of local chemistry and kinetics in the bulk of the material, in nanoarchitectures (e.g. thin films) and at the interfaces (e.g. grain boundaries, phase contacts, solid/liquid and solid/gas interfaces) . The aim is to provide a detailed guide to the techniques, describing opportunities and bottlenecks for their practical deployment and examples of successful applications.

25 ENERGY STORAGE↗
DOE OSTI2023

How the existence of unstable neutral media restricts the aspect ratio of cold neutral media?

ABSTRACT The ubiquity of very thin and lengthy cold neutral medium (CNM) has been reported by multiple authors in the H i community. Yet, the reason of how the CNM can be so long and lengthy is still in debate. In this paper, we recognize a new type of instability due to the attractive nature of the pressure force in the unstable phase. We provide a new estimation of the average CNM filament aspect ratio with the consideration of force balances at the phase boundary, which is roughly 5–20 in common CNM environment. We show that most of the cold filaments are less filamentary than what usually predicted via MHD turbulence theory or inferred from observations: The average length of CNM filament is roughly 1/2 of that in isothermal MHD turbulence with similar turbulence conditions. This suggests that the ‘cold filaments’ that are identified in observations might not be in pressure equilibrium or generated via other mechanisms.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2026

Anisotropic magnetism and Kondo-lattice behavior in the frustrated antiferromagnet Ce 3 ⁢MgBi 5

Here, we report the synthesis and physical characterization of single-crystalline Ce 3 ⁢MgBi 5 , a previously unexplored member of the Ce 3 ⁢𝑀⁢𝑃⁢𝑛 5 family. This compound crystallizes in the hexagonal 𝑃⁢6 3 /𝑚⁢𝑐⁢𝑚 structure, featuring an anisotropic Ce sublattice composed of zigzag chains along the 𝑐 axis and a distorted kagome-like network in the basal plane. Magnetization measurements reveal antiferromagnetic order below 𝑇 𝑁 ≈ 4.2K with strong magnetic anisotropy and multiple field-induced metamagnetic transitions for fields applied perpendicular to [001], leading to a dome-shaped 𝐻–𝑇 phase diagram. Electrical transport exhibits characteristic signatures of a Ce-based Kondo lattice, including broad resistivity maxima and pronounced field-dependent anomalies in the magnetoresistance and Hall response that track the magnetic phase boundaries. Specific-heat measurements confirm the magnetic transition and show that the full R ⁢ln⁡ 2 entropy expected for a Ce 3+ Kramers doublet is recovered by 20 K, indicating an extended temperature range of magnetic fluctuations consistent with Kondo correlations. Our results establish Ce 3 ⁢MgBi 5 as a platform within the Ce 3 ⁢𝑀⁢𝑃⁢𝑛 5 family for exploring the interplay of geometric frustration, magnetic anisotropy, and Kondo-lattice physics under applied magnetic fields.

Kondo effect↗