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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 289 records · Page 16
All sourcesNASA NTRSDOE OSTI
DOE OSTI2013

Thermal transport in boron nitride nanotorus—towards a nanoscopic thermal shield

Nanotori, or nanorings, are topological variants of nanotubes and are conceived to have different properties from their tubular form. In this study, the toroidal arrangement of boron nitride is introduced. Using classical molecular dynamics simulations, the thermal behaviour (thermal conductivity and thermal stability) of the boron nitride nanotorus and its relationship with the structural characteristics are investigated. Its circumferential thermal rectification strength displays a linear dependence on the bending coefficient of the nanostructure. Surface kinks are relatively inconsequential on its circumferential mode of conduction, as compared to its axial sense. The circumferential conductivity in the diffusive regime is calculated to be approximately 10 W/m K, while the axial conductivity is more than tenfold of this value. All nanotori with different toroidal characters show excellent thermal stability at extremely high temperatures approaching 3400 K. With consideration to its favourable properties, a thermal shield made up of a parallel row of nanotori is proposed as a nanoscale thermal insulation device.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2013

Charge transport properties of p-CdTe/n-CdTe/n{sup +}-Si diode-type nuclear radiation detectors based on metalorganic vapor-phase epitaxy-grown epilayers

Charge transport properties of p-CdTe/n-CdTe/n{sup +}-Si diode-type nuclear radiation detectors, fabricated by growing p-and n-type CdTe epilayers on (211) n{sup +}-Si substrates using metalorganic vapor-phase epitaxy (MOVPE), were studied by analyzing current-voltage characteristics measured at various temperatures. The diode fabricated shows good rectification properties, however, both forward and reverse biased currents deviate from their ideal behavior. The forward current exhibits typical feature of multi-step tunneling at lower biases; however, becomes space charge limited type when the bias is increased. On the other hand, the reverse current exhibits thermally activated tunneling-type current. It was found that trapping centers at the p-CdTe/n-CdTe junction, which were formed due to the growth induced defects, determine the currents of this diode, and hence limit the performance of the nuclear radiation detectors developed.

36 MATERIALS SCIENCE↗
DOE OSTI2013

Phase-change radiative thermal diode

A thermal diode transports heat mainly in one preferential direction rather than in the opposite direction. This behavior is generally due to the non-linear dependence of certain physical properties with respect to the temperature. Here we introduce a radiative thermal diode which rectifies heat transport thanks to the phase transitions of materials. Rectification coefficients greater than 70% and up to 90% are shown, even for small temperature differences. This result could have important applications in the development of future contactless thermal circuits or in the conception of radiative coatings for thermal management.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2013

Stimulated electroluminescence emission from n-ZnO/p-GaAs:Zn heterojunctions fabricated by electro-deposition

In this study, n-ZnO thin films were electrochemically deposited on p-GaAs:Zn substrates. The XRD results of ZnO thin films deposited on p-GaAs:Zn substrates at potentials varied from −0.9 V to −1.2 V show a strong c-axis (002) orientation and homogeneity. The current-voltage characteristics exhibit rectification, proving a low turn-on voltage and an ideality factor of 4.71. The n-ZnO/p-GaAs heterostructures show blue-white electroluminescence (EL) emission, which is composed of broad emission bands. In addition to these broad peaks, stimulated emission also appear on the top of the spectra due to the multiple reflections from the mirror like surfaces of ZnO-ZnO and ZnO-GaAs interfaces. Besides, three broad photoluminescence (PL) emission peaks have also been observed peaking at respectively around 3.36 eV, 3.28 eV and 3.07 eV generally attributed to the near bandedge emission, the residual donor level and deep level emission due to the localized defects, respectively.

36 MATERIALS SCIENCE↗
DOE OSTI2013

Spin-current noise from fluctuation relations

We present fluctuation relations that connect spin-polarized current and noise in mesoscopic conductors. In linear response, these relations are equivalent to the fluctuation-dissipation theorem that relates equilibrium current-current correlations to the linear conductance. More interestingly, in the weakly nonlinear regime of transport, these relations establish a connection between the leading-order rectification spin conductance, the spin noise susceptibility and the third cumulant of spin current fluctuations at equilibrium. Our results are valid even for systems in the presence of magnetic fields and coupled to ferromagnetic electrodes.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2013

Investigation of structural and electrical properties of flat a-Si/c-Si heterostructure fabricated by EBPVD technique

Flat amorphous silicon - crystal silicon (a-Si/c-Si) heterostructure were prepared by ultra-high vacuum electron beam evaporation technique on p-Si (111) and n-Si (100) single crystal substrates. Structural analyses were investigated by XRD, Raman and FEG-SEM analysis. With these analyses we determined that at the least amorphous structure shows modification but amorphous structure just protected. The electrical and photovoltaic properties of flat a-Si/c-Si heterojunction devices were investigated with current-voltage characteristics under dark and illumination conditions. Electrical properties of flat a-Si/c-Si heterorojunction; such as barrier height Φ{sub B}, diode ideality factor η were determined from current-voltage characteristics in dark conditions. These a-Si/c-Si heterostructure have good rectification behavior as a diode and exhibit high photovoltaic sensitivity.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Efficient terahertz wave generation from GaP crystals pumped by chirp-controlled pulses from femtosecond photonic crystal fiber amplifier

A chirp-tunable femtosecond 10 W, 42 MHz photonic-crystal-fiber oscillator-amplifier system that is capable of delivering sub-60 fs light pulses at 1040 nm is used to demonstrate high-efficiency terahertz radiation generation via optical rectification in GaP crystals only a few millimeters in length. The optimization of the chirp of the fiber-laser pulses is shown to radically enhance the terahertz output, indicating one possible way to more efficiently use these extended nonlinear crystals in compact fiber-pumped terahertz radiation sources.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Origin of attendant phenomena of bipolar resistive switching and negative differential resistance in SrTiO{sub 3}:Nb/ZnO heterojunctions

Epitaxial ZnO thin films were grown on SrTiO{sub 3}:Nb (NSTO) substrates by pulsed laser deposition. The NSTO/ZnO heterojunctions exhibit a typical rectification characteristic under a small voltage, while two attendant behaviors of bipolar resistive switching and negative differential resistance appear under a large voltage. The NSTO/ZnO heterojunctions show extremely weak resistance switching hysteresis without applying a forward bias. However, when the forward bias increases to some extent, the hysteresis becomes more and more prominent and negative differential resistance gradually appears. Furthermore, the high resistance state is obtained when sweeping from negative to positive voltage bias, and vice versa. We propose a model for these behaviors at NSTO/ZnO interface, in which the space charge region in ZnO is wide in high resistance state when the interface state is unoccupied, while the space charge region becomes narrower in low resistance state due to Fermi pinning when the interface state is completely occupied, and the low resistance state is remained until electrons are detrapped from the interface state.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Spin-orbit torque opposing the Oersted torque in ultrathin Co/Pt bilayers

Current-induced torques in ultrathin Co/Pt bilayers were investigated using an electrically driven ferromagnetic resonance technique. The angle dependence of the resonances, detected by a rectification effect as a voltage, was analysed to determine the symmetries and relative magnitudes of the spin-orbit torques. Both anti-damping (Slonczewski) and field-like torques were observed. As the ferromagnet thickness was reduced from 3 to 1 nm, the sign of the sum of the field-like torque and Oersted torque reversed. This observation is consistent with the emergence of a Rashba spin orbit torque in ultra-thin bilayers.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Characterization of local heat fluxes around ICRF antennas on JET

When using Ion Cyclotron Range of Frequency (ICRF) heating, enhanced power deposition on Plasma-Facing Components (PFCs) close to the antennas can occur. Experiments have recently been carried out on JET with the new ITER-Like-Wall (ILW) to characterize the heat fluxes on the protection of the JET ICRF antennas, using Infra-Red (IR) thermography measurement. The measured heat flux patterns along the poloidal limiters surrounding powered antennas were compared to predictions from a simple RF sheath rectification model. The RF electric field, parallel to the static magnetic field in front of the antenna, was evaluated using the TOPICA code, integrating a 3D flattened model of the JET A2 antennas. The poloidal density variation in front of the limiters was obtained from the mapping of the Li-beam or edge reflectometry measurements using the flux surface geometry provided by EFIT equilibrium reconstruction. In many cases, this simple model can well explain the position of the maximum heat flux on the different protection limiters and the heat-flux magnitude, confirming that the parallel RF electric field and the electron plasma density in front of the antenna are the main driving parameters for ICRF-induced local heat fluxes.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2014

A contactless microwave-based diagnostic tool for high repetition rate laser systems

We report on a novel electro-optic device for the diagnostics of high repetition rate laser systems. It is composed of a microwave receiver and of a second order nonlinear crystal, whose irradiation with a train of short laser pulses produces a time-dependent polarization in the crystal itself as a consequence of optical rectification. This process gives rise to the emission of microwave radiation that is detected by a receiver and is analyzed to infer the repetition rate and intensity of the pulses. We believe that this new method may overcome some of the limitations of photodetection techniques.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2014

Transport of active ellipsoidal particles in ratchet potentials

Rectified transport of active ellipsoidal particles is numerically investigated in a two-dimensional asymmetric potential. The out-of-equilibrium condition for the active particle is an intrinsic property, which can break thermodynamical equilibrium and induce the directed transport. It is found that the perfect sphere particle can facilitate the rectification, while the needlelike particle destroys the directed transport. There exist optimized values of the parameters (the self-propelled velocity, the torque acting on the body) at which the average velocity takes its maximal value. For the ellipsoidal particle with not large asymmetric parameter, the average velocity decreases with increasing the rotational diffusion rate, while for the needlelike particle (very large asymmetric parameter), the average velocity is a peaked function of the rotational diffusion rate. By introducing a finite load, particles with different shapes (or different self-propelled velocities) will move to the opposite directions, which is able to separate particles of different shapes (or different self-propelled velocities)

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2014

Electrochemical deposition of iron sulfide thin films and heterojunction diodes with zinc oxide

Iron sulfide thin films were fabricated by the electrochemical deposition method from an aqueous solution containing FeSO{sub 4} and Na{sub 2}S{sub 2}O{sub 3}. The composition ratio obtained was Fe:S:O = 36:56:8. In the photoelectrochemical measurement, a weak negative photo-current was observed for the iron sulfide films, which indicates that its conduction type is p-type. No peaks were observed in X-ray diffraction pattern, and thus the deposited films were considered to be amorphous. For a heterojunction with ZnO, rectification properties were confirmed in the current-voltage characteristics. Moreover, the current was clearly enhanced under AM1.5 illumination.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Schottky contacts to In{sub 2}O{sub 3}

n-type binary compound semiconductors such as InN, InAs, or In{sub 2}O{sub 3} are especial because the branch-point energy or charge neutrality level lies within the conduction band. Their tendency to form a surface electron accumulation layer prevents the formation of rectifying Schottky contacts. Utilizing a reactive sputtering process in an oxygen-containing atmosphere, we demonstrate Schottky barrier diodes on indium oxide thin films with rectifying properties being sufficient for space charge layer spectroscopy. Conventional non-reactive sputtering resulted in ohmic contacts. We compare the rectification of Pt, Pd, and Au Schottky contacts on In{sub 2}O{sub 3} and discuss temperature-dependent current-voltage characteristics of Pt/In{sub 2}O{sub 3} in detail. The results substantiate the picture of oxygen vacancies being the source of electrons accumulating at the surface, however, the position of the charge neutrality level and/or the prediction of Schottky barrier heights from it are questioned.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Efficient terahertz-wave generation and its ultrafast optical modulation in charge ordered organic ferroelectrics

Efficient terahertz (THz) wave generation in strongly correlated organic compounds α-(ET){sub 2}I{sub 3} and α′-(ET){sub 2}IBr{sub 2} (ET:bis(ethylenedithio)-tetrathiafulvalene) was demonstrated. The spontaneous polarization induced by charge ordering or electronic ferroelectricity was revealed to trigger the THz-wave generation via optical rectification; the estimated 2nd-order nonlinear optical susceptibility for α-(ET){sub 2}I{sub 3} is over 70 times larger than that for prototypical THz-source ZnTe. Ultrafast (<1 ps) and sensitive (∼40%) photoresponse of the THz wave was observed for α-(ET){sub 2}I{sub 3}, which is attributable to photoinduced quenching of the polarization accompanied by insulator(ferroelectric)-to-metal transition. Modulation of the THz wave was observed for α′-(ET){sub 2}IBr{sub 2} upon the poling procedure, indicating the alignment of polar domains.

36 MATERIALS SCIENCE↗
DOE OSTI2014

1.5 MeV proton irradiation effects on electrical and structural properties of TiO{sub 2}/n-Si interface

In this paper, we report the effect of 1.5 MeV proton beam irradiation dose on the structural and electrical properties of TiO{sub 2} thin films deposited on n–Si substrates. The formation and transformation of different TiO{sub 2} phases in the irradiated thin films were characterized by X-ray diffraction and X-ray photoelectron spectroscopy (XPS). X-ray diffraction measurements revealed that the as grown film was rich in Ti{sub 5}O{sub 9} phase and then converted to mixed phases of TiO{sub 2} (rutile and anatase) after exposure with radiation doses up to 5 × 10{sup 14} cm{sup −2}. The XPS results revealed the formation of oxygen vacancy (negative) traps in the exposed TiO{sub 2} films, which showed strong dependence on the dose. The C-V measurements showed that proton radiations also damaged the Si substrate and created deep level defects in the substrate, which caused a shift of 0.26 ± 0.01 V in the flat band voltage (V{sub FB}). I–V measurements showed that the ideality factor increased and the rectification ratio dropped with the increase in the radiation dose. The present study showed the stability of TiO{sub 2}/Si interface and TiO{sub 2} film as an oxide layer against proton radiations.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Negative differential resistance and resistance switching behaviors in BaTiO{sub 3} thin films

The polycrystalline BaTiO{sub 3} (BTO) thin films were grown on F-doped SnO{sub 2} substrates by pulsed laser deposition. The devices show a rectification at a small voltage, while bipolar resistive switching (RS) and negative differential resistance (NDR) appear at a large voltage. Furthermore, RS remains and NDR disappears when no positive bias is applied, while both RS and NDR behaviors improve when increasing the positive bias. The electrons trapped/detrapped by interface states at Au/BTO interface are proposed to understand the above behaviors.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗