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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 1,315 records · Page 73
All sourcesNASA NTRSDOE OSTI
DOE OSTI2014

Effect of deposition times on structure of Ga-doped ZnO thin films as humidity sensor

Gallium doped zinc oxide (GZO) has good electrical property. It is widely used as transparent electrode in photovoltaic devices, and sensing element in gas and pressure sensors. GZO thin film was prepared using magnetron sputtering. Film deposition times were set at 10, 15, 20, 25 and 30 minutes to get samples of different thickness. X-ray diffraction (XRD) was used to determine the structure of GZO thin films. Structure for GZO thin film is hexagonal wurtzite structure. Morphology and thickness of GZO thin films was observed from FESEM micrographs. Grain size and thickness of thin films improved with increasing deposition times. However, increasing the thickness of thin films occur below 25 minutes only. Electrical properties of GZO thin films were studied using a four-point probe technique. The changes in the structure of the thin films lead to the changed of their electrical properties resulting in the reduction of the film resistance. These thin films properties significantly implying the potential application of the sample as a humidity sensor.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Thermal annealing effects on ultra-violet luminescence properties of Gd doped AlN

We studied energy transfer from AlN to doped Gd{sup 3+} ions as a function of the post-thermal annealing temperature. Gd-doped AlN thin films were deposited on fused-silica substrates using a reactive radio-frequency magnetron sputtering technique. The film is a c-axis oriented polycrystal. The intra-orbital electron transition in Gd{sup 3+} showed an atomically sharp luminescence at 3.9 eV (318 nm). The photoluminescence (PL) excitation spectrum exhibited a resonant peak, indicating efficient energy transfer from the host AlN crystal to Gd{sup 3+} ions. The PL intensity increases approximately ten times by thermal annealing. The PL decay lifetime becomes long with annealing, and mid-gap luminescence relating to the crystal defects in AlN was also found to be reduced by annealing. These results suggest that energy dissipation of excited carriers in AlN was suppressed by annealing, and the efficiency of energy transfer into Gd{sup 3+} was improved.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Origin of p-type conductivity of Sb-doped ZnO nanorods and the local structure around Sb ions

To probe the origin of p-type conductivity in Sb-doped ZnO, a careful and detailed synchrotron radiation study was performed. The extended X-ray absorption fine structure and X-ray photoelectron spectroscopy investigations provided the evidence for the formation of the complex defects comprising substitution Sb ions at Zn sites (Sb{sub Zn}) and Zn vacancies within the Sb-doped ZnO lattice. Such complex defects result in the increases of Sb-O coordination number and the Sb valence and thereby lead to the p-type conductivity of Sb-doped ZnO. The back-gate field-effect-transistors based on single nanorod of Sb-doped ZnO were constructed, and the stable p-type conduction behavior was confirmed.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Radiation-Induced Effects in Ce{sup 3+}- and Eu{sup 2+}-Doped Al{sub 5}O{sub 6}N

Polycrystalline Al{sub 5}O{sub 6}N samples have been prepared by sol–gel synthesis followed by carbothermal reduction and nitridation. X-ray luminescence (XRL) spectra and thermoluminescence (TL) curves of Ce{sup 3+}- and Eu{sup 2+}-doped aluminum oxynitride have been measured in the temperature range 5–380 K and analyzed. Working TL peaks have been detected in the range 150–250 K for the Al{sub 5}O{sub 6}N:Ce{sup 3+} sample and in the range 140–200 K for the Al{sub 5}O{sub 6}N:Eu{sup 2+} samples. There are also lower temperature peaks: in the ranges 10–60 and 20–100 K. The XRL spectra of the samples have been measured at temperatures of 8 (for the first time) and 300 K.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Simulation of the effective concentration profiles in InGaAs/GaAs heterostructures containing δ-doped layers

InGaAs/GaAs heterostructures containing quantum wells and δ-doped layers are studied theoretically and experimentally. On the basis of the procedure of self-consistently solving the Schrödinger equation and Poisson equation, the differential capacitance and the apparent electron concentration profiles are numerically calculated for structures with different mutual arrangements of the quantum well and the δ layer. The results of the calculations are compared with the result of analyzing the experimental capacitance-voltage characteristics of the structures. The systematic features of the behavior of the apparent concentration profiles and capacitance-voltage characteristics in relation to the geometric properties of the structure, the temperature, and the doping level are established.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Effect of helium on irradiation creep behavior of B-doped F82H irradiated in HFIR

The diameter of pressurized tubes of F82H and B-doped F82H irradiated up to similar to 6 dpa have been measured by a non-contacting laser profilometer. The irradiation creep strains of F82H irradiated at 573 and 673K were almost linearly dependent on the effective stress level for stresses below 260 MPa and 170 MPa, respectively. The creep strain of (BN)-B-10-F82H was similar to that of F82H IEA at each effective stress level except 294 MPa at 573K irradiation. For 673K irradiation, the creep strain of some (BN)-B-10-F82H tubes was larger than that of F82H tubes. However, the generation of similar to 300 appm He did not cause a large difference in the irradiation creep behavior at 6 dpa.

Ando, M.↗
NASA NTRS1991

Thermal effects on cavity stability of chromium- and neodymium-doped gadolinium scandium gallium garnet laser under solar-simulator pumping

Results are presented on testing a Cr- and Nd-codoped Gd-Sc-Ga-garnet (Cr:Nd:GSGG) crystal and a Nd:YAG crystal (both of 3.2 mm diam and 76-mm long) for pulsed and CW laser operations using a flashlamp and solar simulator as pumping sources. Results from experiments with the flashlamp show that, at pulse lengths of 0.11, 0.28, and 0.90 ms, the slope efficiency of the Cd:Nd:GSGG crystal was higher than that of the Nd:YAG crystal and increased with pulse width. With the solar simulator, however, the CW laser operation of the Cr:Nd:GSGG crystal was limited to intensities not greater than 1500 solar constants, while the Nd:YAG laser successfully performed for all pump beam intensities available. It was found that the exposure for several minutes of the Cr:Nd:GSGG crystal to pump beam intensity of 3000 solar constants led to its damage by thermal cracking, indicating that a better solar-pumped CW laser performance may be difficult to realize with rod geometry.

Kim, Kyong H.↗
DOE OSTI2015

Study of morphology effects on magnetic interactions and band gap variations for 3d late transition metal bi-doped ZnO nanostructures by hybrid DFT calculations

Using density functional theory (DFT) based electronic structure calculations, the effects of morphology of semiconducting nanostructures on the magnetic interaction between two magnetic dopant atoms as well as a possibility of tuning band gaps have been studied in the case of the bi-doped (ZnO){sub 24} nanostructures with the impurity dopant atoms of the 3d late transition metals—Mn, Fe, Co, Ni, and Cu. To explore the morphology effect, three different structures of the host (ZnO){sub 24} nano-system, having different degrees of spatial confinement, have been considered: a two dimensional nanosheet, a one dimensional nanotube, and a finite cage-shaped nanocluster. The present study employs hybrid density functional theory to accurately describe the electronic structure of all the systems. It is shown here that the magnetic coupling between the two dopant atoms remains mostly anti-ferromagnetic in the course of changing the morphology from the sheet geometry to the cage-shaped geometry of the host systems, except for the case of energetically most stable bi-Mn doping, which shows a transition from ferromagnetic to anti-ferromagnetic coupling with decreasing aspect ratio of the host system. The effect of the shape change, however, has a significant effect on the overall band gap variations of both the pristine as well as all the bi-doped systems, irrespective of the nature of the dopant atoms and provides a means for easy tunability of their optoelectronic properties.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2013

Unification of the negative electrocaloric effect in Bi{sub 1/2}Na{sub 1/2}TiO{sub 3}-BaTiO{sub 3} solid solutions by Ba{sub 1/2}Sr{sub 1/2}TiO{sub 3} doping

The microscopic mechanisms of the negative electrocaloric effect (ECE) of the single-phase (1−x)(0.94Bi{sub 1/2}Na{sub 1/2}TiO{sub 3}-0.06BaTiO{sub 3})-xBa{sub 1/2}Sr{sub 1/2}TiO{sub 3} (BNT-BT-BST) perovskite solid solutions fabricated via the sol-gel technique are explored in this study. Dielectric and mechanical relaxation analyses are employed to investigate the ferroelectric and structural transitions of the samples. The electrocaloric properties of the samples were measured by thermodynamics Maxwell relations. The difference between the depolarization temperature (T{sub d}) and the maximum dielectric constant temperature (T{sub m}) was found to decrease with increasing BST content. Doping with BST stabilized the ferroelectric phase along with unifying the EC temperature changes (ΔT) to only negative values. The origin of the uniform negative ECE of BNT-BT-BST is discussed.

36 MATERIALS SCIENCE↗
DOE OSTI2013

Inhomogeneous Si-doping of gold-seeded InAs nanowires grown by molecular beam epitaxy

We have investigated in situ Si doping of InAs nanowires grown by molecular beam epitaxy from gold seeds. The effectiveness of n-type doping is confirmed by electrical measurements showing an increase of the electron density with the Si flux. We also observe an increase of the electron density along the nanowires from the tip to the base, attributed to the dopant incorporation on the nanowire facets whereas no detectable incorporation occurs through the seed. Furthermore, the Si incorporation strongly influences the lateral growth of the nanowires without giving rise to significant tapering, revealing the complex interplay between axial and lateral growth.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Electron–phonon interaction and ultrafast photoemission from doped monolayer MoS 2

We have examined the effect of electron–phonon coupling on photoluminescence and ultrafast response of electron doped monolayer MoS2, using a combination of density functional theory, time dependent density functional theory, and many-body theory. For small doping (~1–3%) of interest here, the electron–phonon coupling parameter is modest (~0.1–0.2) but its effect on the emissive properties and response of the system to femtosecond (fs) laser pulses is striking. We find an ultrafast (fs) relaxation of the electronic subsystem as well as a high fluence of visible light emission induced by electron phonon interaction. Together with high carrier mobility, these features of monolayer MoS2 may be relevant for optoelectronic technologies.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2018

Effect of Yb Substitution on Microstructure and Superconducting Properties of Y{sub 1−x}Yb{sub x}Ba{sub 2}Cu{sub 3}O{sub 7−δ} Films

The purpose of this study was to improve the critical current density (J{sub c}) of YBa{sub 2}Cu{sub 3}O{sub 7−δ} (YBCO) superconducting films at high magnetic fields. Thin Y{sub 1−x}Yb{sub x}Ba{sub 2}Cu{sub 3}O{sub 7−δ} (Y{sub 1−x}Yb{sub x}BCO) superconducting films on LaAlO{sub 3} substrates were prepared by modified low-fluorine metal organic deposition method (LF-MOD) in order to investigate the effect of Yb substitution on the microstructure and superconducting properties of a YBCO-coated conductor. In this study, single-phased, c-axial-aligned Y{sub 1−x}Yb{sub x}BCO films were obtained. Yb doping had a little effect on the critical temperature (T{sub c}) and transition width (ΔT) values of Y{sub 1−x}Yb{sub x}BCO films. An abnormal phenomenon was observed that the J{sub c} of Y{sub 1−x}Yb{sub x}BCO films at low field was smaller than that of the pure YBCO film, but the J{sub c} of Y{sub 1−x}Yb{sub x}BCO films at high field was much greater than that of the undoped YBCO film. Compared to pure YBCO film, the J{sub c} values of Y{sub 0.75}Yb{sub 0.25}BCO and Y{sub 0.25}Yb{sub 0.75}BCO films were enhanced by a factor of 8 at 8 T, 65 K, which meant that chemical substitution of Yb into the Y site could improve flux pinning significantly under the high magnetic field in Y{sub 1−x}Yb{sub x}BCO coated conductors.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
NASA NTRS1997

Growth, Spectroscopy and Photorefractive Investigation of Vanadium Doped CdSSe

We present two-wave mixing results obtained with a CdS(0.8)Se(0.2):V crystal. The CdS(0.8)Se(0.2):V crystal was grown by physical vapor transport (PVT) along with a concentration of 150 ppm (nominal) vanadium for creating trap centers. The as-grown crystal has a large crystal size, good optical quality, and a medium resistivity of 10(exp 6) - 108 ohms-cm. A large photorefractive gain coefficient of ().24 cm-' was observed at 633 nm with an optical intensity of 60 mW/cm(exp 2) and a grating period of 1.6 microns. To our knowledge, this is the first observation of the photorefractive effect in a vanadium doped CdSSe crystal. Room temperature absorption and low temperature photoluminescence spectroscopy measurements are also discussed. With a significant photorefractive effect, the CdSSe:V crystals are promising for device applications based on photorefractive effect, in the wavelength range of 600-700 nm.

Davis, Michelle↗
DOE OSTI2020

Influence of substrate misorientation on the optical properties of Mg-doped GaN

The GaN substrate miscut angle has a significant effect on the optical properties of Mg-doped GaN grown by metal-organic chemical vapor deposition. We have studied the optical properties of epilayers grown on c-plane GaN wafers with nominal miscut angles of 0.3° and 4° toward the [11¯00] direction. Periodic surface steps are observed in thin films grown on 0.3° miscut substrates. The step separation and height, as well as the surface roughness, increase with layer thickness, leading to a decrease in Mg doping efficiency. For films grown on 4° miscut substrates, step bunching causes the formation of visually observable wavy features on the film surface with strong spatial variations in cathodoluminescence characteristics, indicating a significant effect of surface morphology on the nature of Mg incorporation and p-type characteristics.

Liu, Hanxiao (ORCID:0000000331067542)↗
DOE OSTI2017

Doped-carbon composites, synthesizing methods and applications of the same

A method of synthesizing a doped carbon composite includes preparing a solution having a carbon source material and a heteroatom containing additive, evaporating the solution to yield a plurality of powders, and subjecting the plurality of powders to a heat treatment for a duration of time effective to produce the doped carbon composite.

36 MATERIALS SCIENCE↗