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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 1,081 records · Page 60
All sourcesNASA NTRSDOE OSTI
DOE OSTI2021

Room-temperature large magnetoelectricity in a transition metal doped ferroelectric perovskite

There is increasing interest in novel magnetoelectric (ME) materials that exhibit robust ME coupling at room-temperature (RT) for advanced memory, energy, spintronics, and other multifunctional device applications, by making use of the ability to control polarization with a magnetic field and/or magnetization via an electric field. Obtaining ME materials with strong ME coupling, understanding the origin, and manipulating its processing along with composition to realize large ME coefficients at RT constitute an important step in multiferroic research. To address this, we have investigated the multiferroic and ME properties of Ni-doped Pb(Zr 0.20 Ti 0.80 )O 3 (PZT). We find that the ferroelectric (T C ~ 700 K) and weak ferromagnetic (~ 602 K) phase transitions of Ni-doped PZT are well above RT, leading to a strong ME coupling coefficient (α E,31 ) of 11.7 mVcm -1 Oe -1 (H ac = 1 Oe and f = 1 kHz). While X-ray diffraction suggests a single-phase material, high resolution transmission electron microscopy reveals regions with and without Ni present; thus magnetoelectric coupling between two phases is possible. First-principle calculations suggest the (Ni Pb ) × defect is likely to be responsible for the experimental observed magnetism and ME coupling in Ni-doped PZT. Furthermore, we demonstrate that Ni-doped PZT exhibits low loss tangent, low leakage current, large saturation polarization and weak ferromagnetism. Ultimately, our work demonstrates that Ni-doped PZT is a cost-effective RT multiferroic with strong ME coupling.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

Universal relation between doping content and normal-state resistance in gate voltage tuned ultrathin Bi 2 Sr 2 CaCu 2 O 8 + x flakes

We report gate voltage tunable ultra-thin high-T c cuprates supply a unique platform to investigate the electronic phase diagram and superconductor-insulator transition. One of the challenges in this field is the precise determination of the doping content in the underdoped non-superconducting region. Here we report the discovery of a universal relation between the doping content p and the normal-state resistance at a fixed temperature R(T f ), p = α + β ln[1/R(T f )], in the ultra-thin Bi 2 Sr 2 CaCu 2 O 8+x flakes. The in-depth analysis shows that the evolution of carrier scattering probability with doping content and the change of effective mass caused by superconductor-insulator transition are two key factors leading to this logarithmic relation. Based on our finding, the more precise electronic phase diagram can be established. In addition, the superconductor-insulator transition is verified to be a quantum phase transition using a finite size scaling analysis. The scaling exponent z v is found to have a close correlation with the disorder levels. The present result provides an important foundation to investigate the fascinating electronic states in the ultra-thin cuprates.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

Spin-triplet superconductivity from intervalley Goldstone modes in magic-angle graphene

Here, we consider magic-angle graphene in the doping regime around charge neutrality, and we study the connection between a recently proposed intervalley coherent insulator at zero doping and the neighboring superconducting domes. The magic-angle graphene continuum model has an emergent U(1) valley-charge conservation symmetry and an emergent SU(2) symmetry corresponding to opposite spin rotations in the two valleys. The intervalley coherent insulator spontaneously breaks both these emergent symmetries, and as a result has four Goldstone modes that couple to doped charge carriers. We derive the effective interaction mediated by the Goldstone modes, and we study its role in electron pair formation. The SU(2) Goldstone modes generate a ferromagnetic interaction, which is attractive in spin-triplet pairing channels and repulsive in spin-singlet channels. From a weak-coupling BCS calculation, we find the leading superconducting instability in the p-wave channel.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Shubnikov-de Haas quantum oscillations reveal a reconstructed Fermi surface near optimal doping in a thin film of the cuprate superconductor Pr 1.86 Ce 0.14 CuO 4 ± δ

In this work, we study magnetotransport properties of the electron-doped superconductor Pr 2-x Ce x CuO 4±δ with x = 0.14 in magnetic fields up to 92 T, and observe Shubnikov-de Haas magnetic quantum oscillations. The oscillations display a single frequency F = 255 ± 10 T, indicating a small Fermi pocket that is ~1 % of the two-dimensional Brillouin zone and consistent with a Fermi surface reconstructed from the large holelike cylinder predicted for these layered materials. Despite the low nominal doping, all electronic properties including the effective mass and Hall effect are consistent with overdoped compounds. In conclusion, our study demonstrates that the exceptional chemical control afforded by high quality thin films will enable Fermi surface studies deep into the overdoped cuprate phase diagram.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2020

Adsorption-controlled growth of MnTe(Bi2Te3)n by molecular beam epitaxy exhibiting stoichiometry-controlled magnetism

We report the growth of the intrinsic magnetic topological system MnTe ( Bi 2 Te 3 ) n by molecular beam epitaxy. By mapping the temperature and the Bi:Mn flux ratio, it is shown that there is a narrow growth window for the n = 1 phase Mn Bi 2 Te 4 with 2.0 < Bi : Mn < 2.6 at 225°C. In this work, the films are stoichiometric and excess Bi and Te is not incorporated. At higher flux ratios (Bi:Mn≥4.5) it is found that the n = 2 Mn Bi 4 Te 7 phase is stabilized. Transport measurements indicate that the Mn Bi 2 Te 4 and Mn Bi 4 Te 7 undergo magnetic transitions around 25 and 10 K, respectively, consistent with antiferromagnetic phases found in the bulk. Further, for Mn-rich conditions (Bi:Mn<2), ferromagnetism emerges that exhibits a clear hysteretic state in the Hall effect, which likely indicates Mn-doped Mn Bi 2 Te 4 . Understanding how to grow ternary chalcogenide phases is the key to synthesizing new materials and to interface magnetism and topology, which together are routes to realize and control exotic quantum phenomena.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Transition metal elements as donor dopants in CdO

CdO has been shown to achieve a high electron concentration N (> 10 21 cm -3 ) and at the same time a high mobility μ (> 100cm 2 /V s) when doped with conventional shallow dopants (In or Ga), and consequently making it a transparent conducting oxide with very low resistivity ρ <10 -4 $\Omega$ cm. In this work, the properties of CdO thin films doped with a series of transition metal elements (CdO:TM) with partially filled 3d and 4d shells, including Sc, Ti, V, Cr, Fe, Y, Mo, and W, were investigated. We find that doping with these TM elements can effectively increase the N in CdO to a maximum N (N max ) of ~(7-12)×10 20 cm -3 with a dopant concentration x max of 4-7 %. However, unlike CdO:In, the μ of CdO:TM films drops rapidly from > 100 to < 10cm 2 /Vs as the dopant concentration x increases, so that they can only achieve a minimum ρ of ~ (1-2) × 10 -4 $\Omega$ cm, ~ a factor of 2-3 higher than that in CdO:In. As a result, free-carrier absorption and plasma reflection effects limit their optical transparency to <1200 nm. For most 3d TM dopants, a qualitatively higher d-donor level E d,donor gives rise to higher E F,max or a higher N max . Although at low x, the optical band gap E opt of CdO:TM follows the calculated values due to free-carrier effects, as x increases, E opt values are significantly higher than the calculated values. This is believed to be an effect of the anticrossing interaction of the localized d-levels and the extended CdO conduction-band (CB) states, giving rise to a lower occupied E - and an upper unoccupied E + subband. In conclusion, the restructured CBs have much flatter dispersion, which also results in a much higher effective mass $m^{\star}_{e}$, hence it can also explain the much lower μ of CdO:TM films with high N.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Phase Separation in (Ga,Mn)As Layers Obtained by Ion Implantation and Subsequent Laser Annealing

In this paper, we present the results of studies of the spectral, temperature, and field dependences of the transversal Kerr effect in Ga{sub 1 –} {sub x}Mn{sub x}As (x = 0.0066–0.033) layers produced by ion implantation and subsequent pulsed laser annealing. The complicated nonmonotonous nature of the temperature dependences of the transversal Kerr effect and its dependence on the measurement range indicate a magnetic inhomogeneity of the layers. The reasons for the inhomogeneity can be the Gaussian distribution of Mn over the thickness of the layers and the electron phase separation in them. The appearance of new features in the spectra of the transversal Kerr effect is explained by the presence in the doped semiconductor matrix of nanoregions with a higher carrier concentration and a higher Curie temperature and a shift of the Fermi level into the valence band leading to an increase in the energy of optical transitions.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Novel Hydrocarbon Ionomers for Durable Proton Exchange Membranes

Proton exchange membrane fuel cells (PEMFCs) are considered efficient, environmentally friendly power sources. However, there are no commercially available hydrocarbon-based proton exchange membrane (PEM) capable of achieving the DOE’s 2020 technical targets, some of which include 0.02 Ω.cm 2 area specific proton resistance at maximum operating temperature and water partial pressures from 40-80kPa, durability of 20,000 cycles (Mechanical), >500Hours (Chemical), and 20,000 cycles without 20% loss in OCV (Chemical and Mechanical), maximum operating temperature of 120°C, and $20/m 2 (at 500,000 systems/year). While significant progress has been made over the previous two decades in the development of polymer electrolytes for fuel cells, their widespread commercialization has been largely hindered by the high-cost and low-durability. NanoSonic has developed novel, cost-effective, quaternary ammonium- functionalized hydrocarbon ionomers doped with phosphoric acid as intermediate temperature (120°C) PEMs for fuel cells as energy efficient transportation alternative to internal combustion engines. These next-generation polymer membranes possess excellent film forming and mechanical properties, exceptional hydrated dimensional stability, have good proton conductivity at low relative humidity (RH) and exhibit superb stability over current state of the art perfluorosulfonic acid (PFSA) ionomers used under the harsh operating conditions of PEM fuel cells. NanoSonic’s engineered and manufactured a library of membrane types, identified a subset of best performers, and are evaluating these materials in partnership with Los Alamos National Laboratory. Membrane electrode assembly (MEA) fabrication was successfully demonstrated in during Phase I. Initial results utilizing these novel polymer membranes and non-optimize MEAs generated during this project demonstrate good proton conductivity at 80-200°C at zero humidification (RH=0%) and have notably low high frequency resistance (HFR). NanoSonic’s membranes show superior acid retention in comparison to the reference doped PBI membranes, an attribute credited to the formation of stable ionic acid pairs, in addition to polar nteractions with the polymer backbone.

30 DIRECT ENERGY CONVERSION↗
DOE OSTI2020

Evaluation of Doped-LaCrO3 ceramics for high temperature sensor applications

The main objective of this work was to investigate the applicability of the doped-LaCrO3 system for electrical interconnects and sensor applications at temperatures >800°C. Various A- and B-site doping strategies were investigated, and the effect on their electrical conductivity, Seebeck coefficient, and thermal coefficient of expansion were characterized up to 1500ºC. Solid-state, co-precipitation and sol gel methods were used to synthesize the compositions, and the chemical/thermal stability, microstructural evolution, sintering and grain growth kinetics were investigated for these compositions by XRD, SEM and Rietveld methods. The materials showing the greatest promise were included within thick film thermistor and thermocouple sensor designs that were tested to 1500ºC in various gas environments (various pO2 concentrations).

20 FOSSIL-FUELED POWER PLANTS↗
NASA NTRS1991

X-ray photoelectron spectroscopy study of chemically-etched Nd-Ce-Cu-O surfaces

Acetic acid, Br2, and HCl solutions are investigated for removing insulating species from Nd(1.85)Ce(0.15)CuO(4-delta) (NCCO) thin film surfaces. X-ray photoelectron spectroscopy (XPS) shows that the HCl etch is most effective, yielding O 1s spectra comparable to those obtained from samples cleaned in vacuum and a clear Fermi edge in the valence band region. Reduction and oxidation reversibly induces and eliminates, respectively, Fermi level states for undoped samples, but has no clearly observable effect on the XPS spectra for doped samples. Reactivity to air is much less for NCCO compared to hole superconductors, which is attributed to the lack of reactive alkaline earth elements in NCCO.

Vasquez, R. P.↗
NASA NTRS1994

Advanced p-MOSFET Ionizing-Radiation Dosimeter

Circuit measures total dose of ionizing radiation in terms of shift in threshold gate voltage of doped-channel metal oxide/semiconductor field-effect transistor (p-MOSFET). Drain current set at temperature-independent point to increase accuracy in determination of radiation dose.

Buehler, Martin G.↗
NASA NTRS1996

Charge Collection in Simple EPI Diodes

Charge collection from ion tracks in epi diodes is investigated by computer simulations. As previously noted by others, collected charge can exceed charge liberated in the epi layer. Several cases are compared to illustrate the effect of changing ion LET, epi doping density, and doping types. It is found that the n(sup +)-p-p(sup +) diode displays a funneling regime and a diffusion regime (as previously noted by others),, but the p(sup +)-n-n(sup +) diode does not. Simple models are proposed for quantitative estimates of collected charge. A qualitative two-state picture of funneling is discussed.

epi diodes↗
NASA NTRS2003

Dyakonov-Perel Effect on Spin Dephasing in n-Type GaAs

A paper presents a study of the contribution of the Dyakonov-Perel (DP) effect to spin dephasing in electron-donor-doped bulk GaAs in the presence of an applied steady, moderate magnetic field perpendicular to the growth axis of the GaAs crystal. (The DP effect is an electron-wave-vector-dependent spin-state splitting of the conduction band, caused by a spin/orbit interaction in a crystal without an inversion center.) The applicable Bloch equations of kinetics were constructed to include terms accounting for longitudinal optical and acoustic phonon scattering as well as impurity scattering. The contributions of the aforementioned scattering mechanisms to spin-dephasing time in the presence of DP effect were examined by solving the equations numerically. Spin-dephasing time was obtained from the temporal evolution of the incoherently summed spin coherence. Effects of temperature, impurity level, magnetic field, and electron density on spin-dephasing time were investigated. Spin-dephasing time was found to increase with increasing magnetic field. Contrary to predictions of previous simplified treatments of the DP effect, spin-dephasing time was found to increase with temperature in the presence of impurity scattering. These results were found to agree qualitatively with results of recent experiments.

Ning, C. Z.↗
DOE OSTI2017

Palladium and platinum-based nanoparticle functional sensor layers for selective H2 sensing

The disclosure relates to a plasmon resonance-based method for H.sub.2 sensing in a gas stream utilizing a hydrogen sensing material. The hydrogen sensing material is comprises Pd-based or Pt-based nanoparticles having an average nanoparticle diameter of less than about 100 nanometers dispersed in an inert matrix having a bandgap greater than or equal to 5 eV, and an oxygen ion conductivity less than approximately 10.sup.-7 S/cm at a temperature of 700.degree. C. Exemplary inert matrix materials include SiO.sub.2, Al.sub.2O.sub.3, and Si.sub.3N.sub.4 as well as modifications to modify the effective refractive indices through combinations and/or doping of such materials. The hydrogen sensing material utilized in the method of this disclosure may be prepared using means known in the art for the production of nanoparticles dispersed within a supporting matrix including sol-gel based wet chemistry techniques, impregnation techniques, implantation techniques, sputtering techniques, and others.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2019

Palladium and platinum-based nanoparticle functional sensor layers and integration with engineered filter layers for selective H2 sensing

The disclosure relates to a method for H2 sensing in a gas stream utilizing a hydrogen sensing material. The hydrogen sensing material is comprised of Pd-based or Pt-based nanoparticles having an average nanoparticle diameter of less than about 100 nanometers dispersed in an inert matrix having a bandgap greater than or equal to 5 eV, and an oxygen ion conductivity less than approximately 10−7 S/cm at a temperature of 700° C. Exemplary inert matrix materials include SiO2, Al2O3, and Si3N4 as well as modifications to modify the effective refractive indices through combinations and/or doping of such materials. Additional exemplary matrix materials consist of zeolitic and zeolite-derivative structures which are microporous and/or nanoporous such as the alumino-silicates and the dealuminated zeolite NaA structures. Additional sensing layers may be comprised of (1) a single “nanocomposite” layer comprised of Pd- or Pt-based particles dispersed within an inert matrix, (2) multi-layered sensing layers comprised of a nanocomposite layer with a filter overlayer, (3) core-shell layers comprised of matrix materials surrounding a core of Pd-based or Pt-based nanoparticles, and any combinations of the above.

Ohodnicki, Jr., Paul R.↗