Search NASA⌕ Search

SEARCH · Search NASA

Results for “sharp transition”

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.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 433 records · Page 24
All sourcesNASA NTRSDOE OSTI
DOE OSTI2015

The influence of temperature induced phase transition on the energy storage density of anti-ferroelectric ceramics

Anti-ferroelectric (AFE) composite ceramics of (Pb{sub 0.858}Ba{sub 0.1}La{sub 0.02}Y{sub 0.008})(Zr{sub 0.65}Sn{sub 0.3}Ti{sub 0.05})O{sub 3}-(Pb{sub 0.97}La{sub 0.02})(Zr{sub 0.9}Sn{sub 0.05} Ti{sub 0.05})O{sub 3} (PBLYZST-PLZST) were fabricated by the conventional solid-state sintering process (CS), the glass-aided sintering (GAS), and the spark plasma sintering (SPS), respectively. The influence of the temperature induced phase transition on the phase structure, hysteresis loops, and energy storage properties of the composite ceramics were investigated in detail. The measured results of X-ray diffraction demonstrate that the composite ceramics exhibit the perovskite phases and small amounts of non-functional pyrochlore phases. Compared with the CS process, the GAS and SPS processes are proven more helpful to suppress the diffusion behaviors between the PBLYZST and PLZST phases according to the field emission scanning electron microscopy, thereby being able to improve the contribution of PBLYZST phase to the temperature stability of the orthogonal AFE phase. When the ambient temperature rises from 25 °C to 125 °C, CS and GAS samples have undergone a phase transition from orthorhombic AFE phase to tetragonal AFE phase, which results in a sharp decline in the energy storage density. However, the phase transition temperature of SPS samples is higher than 125 °C, and the energy storage density only slightly decreases due to the disorder of material microstructure caused by the high temperature. As a result, the SPS composite ceramics obtain a recoverable high energy storage density of 6.46 J/cm{sup 3} and the excellent temperature stability of the energy storage density of 1.16 × 10{sup −2} J/°C·cm{sup 3}, which is 1.29 × 10{sup −2} J/°C·cm{sup 3} lower than that of CS samples and about 0.43 times as that of GAS samples.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Atomically sharp jagged edges of chemical vapor deposition-grown WS 2 for electrocatalysis

Chemical vapor deposition (CVD)-grown 2D transition metal dichalcogenides can adopt faceted edges. To investigate how sharp these sites can be, we utilize aberration-corrected annular dark-field scanning transmission electron microscopy (ADF-STEM) to resolve the atomic structure of two-dimensional (2D) WS 2 domains that show jagged edges. Nanoscale triangular edge structures with S zigzag terminations are observed. Both the peak and valley regions exhibit near-atomic sharpness. The peaks are as sharp as two atoms in width. Highly ordered valley sites display a minimum width of three atoms, and prospective single-atom valleys appear as two-atom-wide sites in the ADF-STEM contrast. Regarding the kinetics, density-functional theory (DFT) calculations indicate that the 2-W-atom peak would not evolve into a single-W-atom peak even though the latter configuration is also stable with a high enough binding energy under the growth conditions. These results help deepen our understanding of the possible structuring at the nanoscale and the atomic-scale limits of peaks and valleys formed via intersection of two zigzag edges. The enriched edge sites lead to higher catalytic activities for the hydrogen evolution reaction (HER).

36 MATERIALS SCIENCE↗
DOE OSTI2020

Nematic transitions in iron pnictide superconductors imaged with a quantum gas

The Scanning Quantum Cryogenic Atom Microscope (SQCRAMscope) uses an atomic Bose–Einstein condensate to measure magnetic fields emanating from solid-state samples. The quantum sensor does so with unprecedented d.c. sensitivity at micrometre resolution, from room to cryogenic temperatures. An additional advantage of the SQCRAMscope is the preservation of optical access to the sample so that magnetometry imaging of, for example, electron transport may be performed in concert with other imaging techniques. Here, we apply this multimodal imaging capability to the study of nematicity in iron pnictide high-temperature superconductors, where the relationship between electronic and structural symmetry breaking resulting in a nematic phase is under debate. We combine the SQCRAMscope with an in situ microscope that measures optical birefringence near the surface. This enables simultaneous and spatially resolved detection of both bulk and near-surface manifestations of nematicity via transport and structural deformation channels, respectively. By performing local measurements of emergent resistivity anisotropy in iron pnictides, we observe sharp, nearly concurrent transport and structural transitions. More broadly, these measurements demonstrate the SQCRAMscope’s ability to reveal important insights into the physics of complex quantum materials.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2013

Structure and ferroelectric studies of (Ba{sub 0.85}Ca{sub 0.15})(Ti{sub 0.9}Zr{sub 0.1})O{sub 3} piezoelectric ceramics

Graphical abstract: - Highlights: • (Ba{sub 0.85}Ca{sub 0.15})(Ti{sub 0.9}Zr{sub 0.1})O{sub 3} (BCTZO) ceramic was synthesized by the ceramic method. • In situ XRD and Raman spectra showed the phase transition of BCTZO around 360 K. • The ceramics showed a tunability of 82% at 40 kV cm{sup −1} electric field. • BCTZO exhibited good quality factor of 111 at microwave frequencies. • Piezoforce microscopy studies indicated the switchability of ferroelectric domains. - Abstract: We have synthesized and studied the structural and ferroelectric properties of lead-free 0.5(Ba{sub 0.7}Ca{sub 0.3})TiO{sub 3}–0.5Ba(Zr{sub 0.2}Ti{sub 0.8})O{sub 3} ceramics in the temperature region of its ferroelectric transition. The synthesized material showed high dielectric constant, low loss and good pyroelectric figure of merit. From the temperature dependent X-ray diffraction measurements, we determined the tricritical point to be in the temperature range of 303–400 K. The dielectric measurements indicate a diffuse ferroelectric phase transition (DPT) around 360 K in agreement with the X-ray measurements. We studied the evolution of Raman spectra with temperature to understand the nature of phase transition in BaTiO{sub 3} (BTO) and the BCTZO. The results indicates that the transition of ferroelectric–paraelectric state is not sharp as in the case of BTO and the polar state persists through the paraelectric state. In general, our study indicates that there are ferroelectric domains of nanometer size beyond the commonly defined transition temperature. The observation of local piezoelectric hysteresis loop indicated the existence of intrinsic ferroelectric property of the ceramic at the nanoscale. The ceramics exhibited electric field tunable dielectric properties with a tunability of 82% at an applied DC field of 40 kV cm{sup −1}, low dielectric loss of 0.001 and room temperature pyroelectric coefficient of 6 × 10{sup −8} C cm{sup −2} K{sup −1} and the detectivity of 1.9 × 10{sup −8} C cm{sup −1} J{sup −1}; larger than those reported for other BaTiO{sub 3}-based materials. Overall, our results indicate that BCTZO ceramics with coexistence of rhombohedral–tetragonal phases is a promising candidate for lead-free ferroelectric applications.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Tunable UV-Emitters through Graphene Plasmonics

Control over the spontaneous emission of light through tailored optical environments remains a fundamental paradigm in nanophotonics. The use of highly confined plasmons in materials such as graphene provides a promising platform to enhance transition rates in the IR-THz by many orders of magnitude. However, such enhancements involve near-field plasmon modes or other kinds of near-field coupling like quenching, and it is challenging to use these highly confined modes to harness light in the far-field due to the difficulty of plasmonic outcoupling. Here in this paper, we propose that through the use of radiative cascade chains in multilevel emitters, IR plasmons can be used to enhance far field spectra in the visible and UV range, even at energies greater than 10 eV. Combining Purcell-enhancement engineering, graphene plasmonics, and radiative cascade can result in a new type of UV emitter whose properties can be tuned by electrically doping graphene. Varying the distance between the emitter and the graphene surface can change the strength of the far-field emission lines by 2 orders of magnitude. We also find that the dependence of the far-field emission on the Fermi energy is potentially extremely sharp at the onset of interband transitions, allowing the Fermi energy to effectively serve as a “switch” for turning on and off certain plasmonic and far-field emissions.

42 ENGINEERING↗
NASA NTRS1982

Altitude transitions in energy climbs

The aircraft energy-climb trajectory for configurations with a sharp transonic drag rise is well known to possess two branches in the altitude/Mach-number plane. Transition in altitude between the two branches occurs instantaneously, a 'corner' in the minimum-time solution obtained with the energy-state model. If the initial and final values of altitude do not lie on the energy-climb trajectory, then additional jumps (crude approximations to dives and zooms) are required at the initial and terminal points. With a singular-perturbation approach, a 'boundary-layer' correction is obtained for each altitude jump, the transonic jump being a so-called 'internal' boundary layer, different in character from the initial and terminal layers. The determination of this internal boundary layer is examined and some computational results for an example presented.

Weston, A. R.↗
DOE OSTI2015

Multiphase Nanostructure of a Quinary Metal Oxide Electrocatalyst Reveals a New Direction for OER Electrocatalyst Design

Ce-rich mixed metal oxides comprise a recently discovered class of ­electrocatalysts for the oxygen evolution reaction (OER). In particular, at current densities below 10 mA cm -2 , Ni 0.3 Fe 0.07 Co 0.2 Ce 0.43 O x exhibits ­superior activity compared to the corresponding transition metal oxides, despite the relative inactivity of ceria. To elucidate the enhanced activity and underlying catalytic mechanism, detailed structural characterization of this quinary oxide electrocatalyst is reported in this paper. Transmission electron microscopy imaging of cross-section films as-prepared and after electrochemical testing reveals a stable two-phase nanostructure composed of 3–5 nm diameter crystallites of fluorite CeO 2 intimately mixed with 3–5 nm crystallites of transition metal oxides alloyed in the rock salt NiO structure. Dosing experiments demonstrate that an electron flux greater than ≈1000 e Å -2 s -1 causes the inherently crystalline material to become amorphous. A very low dose rate of 130 e Å -2 s -1 is employed for atomic resolution imaging using inline holography techniques to reveal a nanostructure in which the transition metal oxide nanocrystals form atomically sharp boundaries with the ceria nanocrystals, and these results are corroborated with extensive synchrotron X-ray absorption spectroscopy measurements. Finally, ceria is a well-studied cocatalyst for other heterogeneous and electrochemical reactions, and our discovery introduces biphasic cocatalysis as a design concept for improved OER electrocatalysts.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2026

Broad-range tuning of ferroelectric switching of La x Bi 1−x FeO 3 epitaxial films via digital doping using off-axis co-sputtering

To investigate the scope of ferroelectric behavior in La-substituted BiFeO 3 films, La x Bi 1−x FeO 3 epitaxial films were synthesized using off-axis co-sputtering on SrTiO 3 (001) and DyScO 3 (110) substrates with a SrRuO 3 bottom electrode layer. A digital-doping deposition method was used to enable precise control and continuous tuning of La concentration in high-quality LaxBi 1−x FeO 3 films across a wide range of x = 0.05–0.60, which was systematically investigated using piezoresponse force microscopy. Robust and reversible out-of-plane ferroelectric switching has been observed up to x = 0.35, while films with x ≥ 0.37 exhibit no measurable ferroelectric behavior, indicating a sharp ferroelectric-to-paraelectric phase transition between x = 0.35 and 0.37. This represents the highest reported La concentration in LaxBi 1−x FeO 3 films that retains ferroelectric ordering, highlighting opportunities to engineer ferroelectric and multiferroic properties in complex oxide heterostructures.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2020

Flat Bands and Chiral Optical Response of Moiré Insulators

Here, we present a low-energy model describing the reconstruction of the electronic spectrum in twisted bilayers of honeycomb crystals with broken sublattice symmetry. The resulting moiré patterns are classified into two families with different symmetry. In both cases, flat bands appear at relatively large angles without any magic-angle condition. Transitions between them give rise to sharp resonances in the optical absorption spectrum at frequencies well below the gap of the monolayer. Owing to their chiral symmetry, twisted bilayers display circular dichroism, that is, different absorption of left and right circularly polarized light. This optical activity is a nonlocal property determined by the stacking. In hexagonal boron nitride, sensitivity to the stacking leads to strikingly different circular dichroism in the two types of moirés. Our calculations exemplify how subtle properties of the electronic wave functions, encoded in current correlations between the layers, control physical observables of moiré materials.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Super-Heat Resistant Polymer Nanocomposites Based on Heterocyclic Networks: Structure and Properties

The heterocyclic polymer network nanocomposites obtained from bisphtalonitrile and various (from 0.03 to 5 wt %) modified silicate montmorillonite (MMT) nanolayers are studied. The nanostructure together with thermal, relaxation, and elastic properties of the composites are characterized with transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy (EDXS), dynamic mechanical analysis (DMA), and thermogravimetric analysis (TGA). DMA and TGA measurements were performed in air and nitrogen mediums in the temperature range from 20 to 600–900°C. There was a various degree of exfoliation of MMT in the matrix to form single nanolayers, as well as thin and “thick” packs of MMT nanolayers, depending on its amount. We showed that there are strong constraining dynamics effects of the matrix by nanoparticles and sharp dynamic heterogeneity in the glass transition. We found that there are possibilities of complete suppression of the latter and of preservation of elastic characteristics of the composites unchanged at the temperatures from 20 to 600°C. The nanocomposites exhibit uniquely high thermal properties. A glass transition temperature of 570°C and satisfactory thermal stability are achieved with preservation of integrity of the material up to ~500°C in air and up to ∼900°C in nitrogen.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
NASA NTRS1975

Analysis of atmospheric flow over a surface protrusion using the turbulence kinetic energy equation

Atmospheric flow fields resulting from a semi-elliptical surface obstruction in an otherwise horizontally homogeneous statistically stationary flow are modelled with the boundary-layer/Boussinesq-approximation of the governing equation of fluid mechanics. The turbulence kinetic energy equation is used to determine the dissipative effects of turbulent shear on the mean flow. Mean-flow results are compared with those given in a previous paper where the same problem was attacked using a Prandtl mixing-length hypothesis. Iso-lines of turbulence kinetic energy and turbulence intensity are plotted in the plane of the flow. They highlight regions of high turbulence intensity in the stagnation zone and sharp gradients in intensity along the transition from adverse to favourable pressure gradient.

Frost, W.↗
NASA NTRS1975

Analysis of atmospheric flow over a surface protrusion using the turbulence kinetic energy equation with reference to aeronautical operating systems

Flow over surface obstructions can produce significantly large wind shears such that adverse flying conditions can occur for aeronautical systems (helicopters, STOL vehicles, etc.). Atmospheric flow fields resulting from a semi-elliptical surface obstruction in an otherwise horizontally homogeneous statistically stationary flow are modelled with the boundary-layer/Boussinesq-approximation of the governing equation of fluid mechanics. The turbulence kinetic energy equation is used to determine the dissipative effects of turbulent shear on the mean flow. Iso-lines of turbulence kinetic energy and turbulence intensity are plotted in the plane of the flow and highlight regions of high turbulence intensity in the stagnation zone and sharp gradients in intensity along the transition from adverse to favourable pressure gradient. Discussion of the effects of the disturbed wind field in CTOL and STOL aircraft flight path and obstruction clearance standards is given. The results indicate that closer inspection of these presently recommended standards as influenced by wind over irregular terrains is required.

Frost, W.↗
NASA NTRS1988

Essentially non-oscillatory shock capturing methods applied to turbulence amplification in shock wave calculations

ENO (essentially non-oscillatory) schemes can provide uniformly high order accuracy right up to discontinuities while keeping sharp, essentially non-oscillatory shock transitions. Recently, an efficient implementation of ENO schemes was obtained based on fluxes and TVD Runge-Kutta time discretizations. The resulting code is very simple to program for multi-dimensions. ENO schemes are especially suitable for computing problems with both discontinuities and fine structures in smooth regions, such as shock interaction with turbulence, for which results for 1-D and 2-D Euler equations are presented. Much better resolution is observed by using third order ENO schemes than by using second order TVD schemes for such problems.

Osher, Stanley↗
NASA NTRS2003

Quartz Microbalance Study of 400-angstrom Thick Films near the lambda Point

In a recent measurement we observed the thinning of an adsorbed helium film induced by the confinement of critical fluctuations a few millikelvin below the lambda point. A capacitor set-up was used to measure this Casimir effect. In this poster we will present our measurement of an adsorbed helium film of 400 angstroms near the lambda point with a quartz microbalance. For films this thick, we must take into account the non-linear dynamics of the shear waves in the fluid. In spite of the added complications, we were able to confirm the thinning of the film due to the Casimir effect and the onset of the superfluid transition. In addition, we observe a sharp anomaly at the bulk lambda point, most likely related to critical dissipation of the first sound. This work is carried out in collaboration with Rafael Garcia, Stephen Jordon and John Lazzaretti. This work is funded by NASA's Office of Biological and Physical Research under grant.

Chan, Moses H. W.↗
NASA NTRSDate not supplied

Isochemical Characteristics of the Clay-Sulfate Transition in Gale Crater, Mars: APXS Results From Mont Mercou to the Marker Band Valley

During sols 3052-3572 (March 2021 to August 2022), Curiosity traversed a region in Gale crater where orbital observations predicted a vertical stratigraphic transition from a phyllosilicate unit to the overlying layered sulfates of Mt. Sharp [1, 2]. This region was informally named the clay-sulfate transition (CST). The Alpha Particle X-ray Spectrometer (APXS) was systematically deployed to sedimentary bedrock and selected diagenetic features to analyze possible chemical changes with increasing elevation (a proxy for increasing stratigraphic position). The occurrence of Ca- and Mg-sulfate was a particular focus. Here, we present results from the APXS investigation of the CST, as well as results from the first targets of the sulfate unit located in Marker Band Valley (MBV).

J.A. Berger↗
NASA NTRSDate not supplied

Mars Science Laboratory Mastcam Multispectral Investigation of Drill Targets From the Beginning of the Clay Sulfate Transition to Marker Band Valley

In March, 2021 the Mars Science Laboratory Curiosity rover officially started exploring the Clay Sulfate Transition (CST). This area of Mt. Sharp has been interpreted to have hydrated Mg-sulfate spectral signatures in CRISM orbital data, and it no longer has the strong phyllosilicate spectral signature as the previous Glen Torridon region. Recently, the Curiosity rover has also encountered the marker band that was also identified in orbital data [1,3]. This shift from phyllosilicates to hydrated Mg-sulfates suggests a major change in the environmental conditions during which these rocks were deposited. This abstract focuses on changes seen in the Mastcam multispectral data between the CST drill targets.

S R Jacob↗
DOE OSTI2022

Charge-neutral fermions and magnetic field-driven instability in insulating YbIr 3 Si 7

Kondo lattice materials, where localized magnetic moments couple to itinerant electrons, provide a very rich backdrop for strong electron correlations. They are known to realize many exotic phenomena, with a dramatic example being recent observations of quantum oscillations and metallic thermal conduction in insulators, implying the emergence of enigmatic charge-neutral fermions. Here, we show that thermal conductivity and specific heat measurements in insulating YbIr 3 Si 7 reveal emergent neutral excitations, whose properties are sensitively changed by a field-driven transition between two antiferromagnetic phases. In the low-field phase, a significant violation of the Wiedemann-Franz law demonstrates that YbIr 3 Si 7 is a charge insulator but a thermal metal. In the high-field phase, thermal conductivity exhibits a sharp drop below 300 mK, indicating a transition from a thermal metal into an insulator/semimetal driven by the magnetic transition. These results suggest that spin degrees of freedom directly couple to the neutral fermions, whose emergent Fermi surface undergoes a field-driven instability at low temperatures.

36 MATERIALS SCIENCE↗