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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 1,549 records · Page 86
All sourcesNASA NTRSDOE OSTI
DOE OSTI2010

Grain growth and phase stability of nanocrystalline cubic zirconia under ion irradiation

Grain growth, oxygen stoichiometry and phase stability of nanostructurally-stabilized zirconia (NSZ) in pure cubic phase are investigated under 2 MeV Au ion bombardment at 160 and 400 K to doses up to 35 displacements per atom (dpa). The NSZ films are produced by ion-beam-assisted deposition technique at room temperature with an average grain size of 7.7 nm. The grain size increases with dose, and follows a power law (n=6) to a saturation value of ~30 nm that decreases with temperature. Slower grain growth is observed under 400 K irradiations, as compared to 160 K irradiations, indicating that thermal grain growth is not activated and defect-stimulated grain growth is the dominating mechanism. While cubic phase is perfectly retained and no new phases are identified after the high-dose irradiations, reduction of oxygen in the irradiated NSZ films is detected. The ratio of O to Zr decreases from ~2.0 for the as-deposited films to ~1.65 after irradiation to ~35 dpa. Significant increase of oxygen vacancies in nanocrystalline zirconia suggests substantially enhanced oxygen diffusion under ion irradiation, a materials behavior far from equilibrium. The oxygen deficiency may be essential in stabilizing cubic phase to larger grain sizes.

Environmental Molecular Sciences Laboratory↗
DOE OSTI2010

Colossal Electroresistive Properties Of CSD Grown Pr{sub 0.7}Ca{sub 0.3}MnO{sub 3} Films For Nonvolatile Memory Applications

Colossal electroresistance effects upon application of electric field in perovskite oxide Pr{sub 0.7}Ca{sub 0.3}MnO{sub 3}(PCMO) thin films, which is a promising candidate for resistance random access memory (RRAM) device have been investigated. Nanocrystalline PCMO films were grown on SiO{sub 2} substrates by chemical solution deposition and crystallized at 700 deg. C under different gas atmospheres. Four terminal current voltage characteristics of Ag/PCMO/Ag planar geometry exhibited a sharp transition from a low resistance state (LRS) to a high resistance state (HRS) with a resistance switching ratio of as high as 1100% at room temperature. Nonvolatility and high retention was confirmed by electric pulse induced resistive switching measurements. The resistance switching ratios were found to depend on the annealing conditions, suggesting an interaction between the nonlattice oxygen and oxygen vacancies and/or the cationic vacancy.

36 MATERIALS SCIENCE↗
DOE OSTI2011

Annealing behavior of the EB-centers and M-center in low-energy electron irradiated n-type 4H-SiC

After low-energy electron irradiation of epitaxial n-type 4H-SiC with a dose of 5x10{sup 16} cm{sup -2}, the bistable M-center, previously reported in high-energy proton implanted 4H-SiC, is detected in the deep level transient spectroscopy (DLTS) spectrum. The annealing behavior of the M-center is confirmed, and an enhanced recombination process is suggested. The annihilation process is coincidental with the evolvement of the bistable EB-centers in the low temperature range of the DLTS spectrum. The annealing energy of the M-center is similar to the generation energy of the EB-centers, thus partial transformation of the M-center to the EB-centers is suggested. The EB-centers completely disappeared after annealing temperatures higher than 700 deg. C without the formation of new defects in the observed DLTS scanning range. The threshold energy for moving Si atom in SiC is higher than the applied irradiation energy, and the annihilation temperatures are relatively low, therefore the M-center, EH1 and EH3, as well as the EB-centers are attributed to defects related to the C atom in SiC, most probably to carbon interstitials and their complexes.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2011

Mn incorporation into the GaAs lattice investigated by hard x-ray photoelectron spectroscopy and diffraction

Photoelectron spectroscopy and diffraction have been used to investigate structural changes during the annealing process of Ga{sub 1-x}Mn{sub x}As samples. Hard x-ray radiation helped in observing photoelectron core-level spectra and electron diffraction from the bulk underlying the oxidized surface layer. High electron-energy resolution enabled us to separate the components due to substitutional and interstitial Mn atoms in the intrinsic Mn 2p{sub 3/2} photoemission profile, resulting in two peaks at 638.8 and 639.5 eV binding energy, respectively. The peaks display the known characteristic behavior after annealing, that is, an almost complete reduction of the interstitial component and preservation of the substitutional component. In the photoelectron diffraction, a sensitivity of high-energy polar plots to the incorporation sites of photoemitting atoms into the atomic lattice has been shown. As a consequence, the experimental polar plots from substitutional and interstitial Mn atoms, which are supported theoretically, show characteristic features that provide structural information. From the similarities and differences of the polar plots for Mn and Ga, we have confirmed the assignment of components within the intrinsic part of the photoemission Mn 2p{sub 3/2} signal suggested by photoelectron spectroscopy.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Primary radiation damage on displacement cascades in UO 2 , ThO 2 and (U 0.5 Th 0.5 )O 2

Primary radiation damage in UO 2 , ThO 2 and (U 0.5 Th 0.5 )O 2 has been investigated as function of temperature (300 to 1500 K) using classical molecular dynamics (MD) simulations. Displacement cascades of 5 keV are used for defect production. The number of defects produced from the cascade events in UO 2 is higher than that in ThO 2 at all temperatures. Our results indicate that the defect production in (U 0.5 Th 0.5 )O 2 is different from that in the pure oxides. The number of residual defects in the mixed oxide fuel is considerably higher than in UO 2 or ThO 2 . In all three systems studied, the fraction of anion defects is much higher than that of cations, in line with higher defect energies of the U sublattice compared to the O sublattice. It is also reported that for increasing temperature peak damage increases, while residual damage decreases. Furthermore, this is indicative of the greater recombination of defects due to enhanced mobility at high temperatures. Overall, all systems studied exhibit clustering in the residual damage with interstitial clusters tending to be smaller than voids.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Dielectric screening and vacancy formation for large neutral and charged Si n H m ( n > 1500 ) nanocrystals using real-space pseudopotentials

A commonly used procedure for computing the properties of defects in crystalline materials is to consider a large supercell that includes the defect of interest. This is a straightforward technique as standard energy band codes can be used for such computations. For neutral defects, the only impediment of such an approach is to avoid defect-defect interactions between adjoining cells. However, this procedure can be complex if the defect of interest is charged as the system at large contains Coulombic divergences. Moreover, some have recently argued that the conventional definition of formation energies for charged defects cannot be reconciled with statistical mechanics. Here, we focus on an alternative approach. We consider large nanocrystals wherein a charged defect can be placed. Since the system is confined, a charged defect within the nanocrystal does not result in a Coulombic divergence. The chief impediment is computational, i.e., while no defect-defect or Coulombic divergences are present, the nanocrystal must be sufficiently large to allow the system to properly replicate a bulklike configuration. With the development of new algorithms and hardware advances, computations for systems of sufficient size to address this issue are feasible. In particular, we solve the Kohn-Sham equation in real space using pseudopotential-density-functional theory for large silicon nanocrystals, which contain thousands of atoms. Further, we focus on (i) the screening of a point charge and (ii) the formation of a charged vacancy in hydrogen-terminated silicon nanocrystals. This approach allows us to examine the role of quantum confinement in addition to exploring the bulk limit. Comparisons to other methods confirm the viability of this approach.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Role of Lithium Codoping in Enhancing the Scintillation Yield of Aluminate Garnets

The aim of this work is to clarify the scintillation-yield enhancement in Lu YAG : Pr scintillators obtained by Li codoping via integrated study of the valence state of activators, the preferential site occupancy of Li codopants, and defect structures from experimental and theoretical insights. Here, with Li codoping, the light yield and energy resolution of 10 × 10 × 10 mm 3 Lu YAG : Pr samples are improved from 156000 to 248000 photons/MeV, and 5.3 to 4.3% at 662 keV,respectively.The optical absorption spectra indicate that Li codoping does not induce conversion of stable Pr 3 + to Pr 4 + in Lu YAG : Pr single crystals. Based on the formation energies of substitutional and interstitial Li sites using density-functional-theory (DFT) calculations and the 7 Li nuclear magnetic resonance results, it is shown that the Li ions prefer to dominantly occupy the fourfold coordinated interstitial sites and fourfold coordinated Al sites. The systematic analysis of thermoluminescence glow curves, positron annihilation lifetime spectroscopies, and defect formation energies derived from DFT calculations reveals that the concentration of isolated Lu and Al vacancies as dominant acceptor defects is reduced by Li codoping, whilst the shallow Li i interstitial defects and the deep V O oxygen vacancies are introduced simultaneously. We propose that the lowering of hole trapping at defects resulting from Li codoping contributes to the scintillation-yield enhancement.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2010

Quantitative determination of site occupancy of multi-rare-earth elements doped into Ca{sub 2}SnO{sub 4} phosphor by electron channeling microanalysis

X-ray fluorescence analysis based on electron channeling effects in transmission electron microscopy (TEM) was performed on Ca{sub 2}SnO{sub 4} phosphor materials doped with Eu{sup 3+}/Y{sup 3+} at various concentrations, which showed red photoluminescence associated with the {sup 5}D{sub 0}-{sup 7}F{sub 2} electric dipole transition of Eu{sup 3+} ions. The method provided direct information on which host element site dopant elements occupy, the results of which were compared with those of X-ray diffraction (XRD)-Rietveld analysis. The obtained results indicated that while it is not favorable for a part of Eu{sup 3+} to occupy the smaller Sn{sup 4+} site, this is still energetically better than creating Ca vacancies or any other of the possible charge balance mechanisms. The local lattice distortions associated with dopant impurities with different ionic radii were also examined by TEM-electron energy-loss spectroscopy (TEM-EELS). The change in PL intensity as a function of dopant concentration is discussed based on the experimental results, although the general concept of concentration quenching applies. - Abstract: The composition of Ca{sub 1.8}Eu{sub 0.2}Y{sub 0.2}Sn{sub 0.8}O{sub 4} determined by electron channeling microanalysis is graphically shown, where the rare-earth dopants, Eu and Y preferentially occupy the Ca and Sn sites, respectively, to maintain the local charge neutrality.

36 MATERIALS SCIENCE↗
DOE OSTI2012

Crystal structure and non-stoichiometry of cerium brannerite: Ce{sub 0.975}Ti{sub 2}O{sub 5.95}

The crystal structure of cerium brannerite, Ce{sub 0.975}Ti{sub 2}O{sub 5.95}, was determined from Rietveld analysis of high resolution neutron powder diffraction data (space group C2/m, a=9.8320(1) A, b=3.75287(6) A, c=6.8852(1) A, {beta}=119.230(1) Degree-Sign ). The presence of both cerium and oxygen vacancies was confirmed from Rietveld analysis and defect energy calculations, combined with careful investigation of the phase diagram. Formation of oxygen vacancies at the O1 site, charge compensated by Ce vacancies, relieves considerable Coulombic repulsion and hence structural strain associated with short O1-O1 contacts forming the shared edge of neighbouring TiO{sub 6} polyhedra. Consideration of the bond valence sum of Ce{sup 4+} in Ce{sub 0.975}Ti{sub 2}O{sub 5.95} afforded a revised value of the r{sub 0}=2.098{+-}0.039 A for the reference bond length, based on analysis of 21 crystal structures mined from the ICSD. - Graphical abstract: Neutron diffraction and atomistic defect energy calculations reveal the mechanism of nonstochiometry in cerium brannerite, Ce{sub 0.975}Ti{sub 2}O{sub 5.95}. Highlights: Black-Right-Pointing-Pointer The defect brannerite Ce{sub 0.975}Ti{sub 2}O{sub 5.95} was prepared by solid state synthesis. Black-Right-Pointing-Pointer Neutron diffraction confirmed the presence of Ce vacancies charge compensated by O vacancies. Black-Right-Pointing-Pointer Defect energy calculations show oxygen vacancies relieve structural strain associated with short O-O contact distances.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2011

Mechanism for transient migration of xenon in UO{sub 2}

In this letter, we report recent work on atomistic modeling of diffusion migration events of the fission gas product xenon in UO{sub 2} nuclear fuel. Under nonequilibrium conditions, Xe atoms can occupy the octahedral interstitial site, in contrast to the thermodynamically most stable uranium substitutional site. A transient migration mechanism involving Xe and two oxygen atoms is identified using basin constrained molecular dynamics employing a Buckingham type interatomic potential. This mechanism is then validated using density functional theory calculations using the nudged elastic band method. An overall reduction in the migration barrier of 1.6-2.7 eV is obtained compared to vacancy-mediated diffusion on the uranium sublattice.

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
DOE OSTI2011

Hole capture in PbWO{sub 4}:Mo,La(Y) scintillator crystals

The processes of hole localization in PbWO{sub 4}:Mo,La(Y) single crystals were investigated by electron spin resonance. It was found that the holes created by uv irradiation are trapped at the regular oxygen ions in the vicinity of perturbing defects such as lead vacancies, impurity ions (La,Y), and other lattice imperfections. This leads to a variety of O{sup -} centers which differ in both the thermal stability (from about 160 K up to 240 K) and spectroscopic parameters. The thermal release of such trapped holes and their subsequent recombination with electrons stored at different traps, including (MoO{sub 4}){sup 3-} complexes, are accompanied by the appearance of the thermally stimulated luminescence peaks located at around 190 K and in the 225-250 K range.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2010

Coupled anion and cation ordering in Sr{sub 3}RFe{sub 4}O{sub 10.5} (R=Y, Ho, Dy) anion-deficientperovskites

The Sr{sub 3}RFe{sub 4}O{sub 10.5} (R=Y, Ho, Dy) anion-deficient perovskites were prepared using a solid-state reaction in evacuated sealed silica tubes. Transmission electron microscopy and {sup 57}Fe Moessbauer spectroscopy evidenced a complete A-cations and oxygen vacancies ordering. The structure model was further refined by ab initio structure relaxation, based on density functional theory calculations. The compounds crystallize in a tetragonal a{approx}2{radical}2a{sub p{approx}}11.3 A, c{approx}4c{sub p{approx}}16 A unit cell (a{sub p}: parameter of the perovskite subcell) with the P4{sub 2}/mnm space group. Oxygen vacancies reside in the (FeO{sub 5/4{open_square}3/4}) layers, comprising corner-sharing FeO{sub 4} tetrahedra and FeO{sub 5} tetragonal pyramids, which are sandwiched between the layers of the FeO{sub 6} octahedra. Smaller R atoms occupy the 9-fold coordinated position, whereas the 10-fold coordinated positions are occupied by larger Sr atoms. The Fe sublattice is ordered aniferromagnetically up to at least 500 K, while the rare-earth sublattice remains disordered down to 2 K. -- Graphical abstract: The Sr{sub 3}RFe{sub 4}O{sub 10.5} (R=Y, Ho, Dy) anion-deficient perovskites with a complete ordering of the A-cations and oxygen vacancies have been prepared using a solid-state reaction in evacuated sealed silica tubes. Oxygen vacancies reside in the (FeO{sub 5/43/4}) layers, comprising corner-sharing FeO{sub 4} tetrahedra and FeO{sub 5} tetragonal pyramids, which are sandwiched between the layers of the FeO{sub 6} octahedra. Smaller R atoms occupy the 9-fold coordinated position, whereas the 10-fold coordinated positions are occupied by larger Sr atoms.

36 MATERIALS SCIENCE↗
DOE OSTI2010

Remarkable enhancement in crystalline perfection, second harmonic generation efficiency, optical transparency, and laser damage threshold in potassium dihydrogen phosphate crystals by L-threonine doping

Effect of L-threonine (LT) doping on crystalline perfection, second harmonic generation (SHG) efficiency, optical transparency, and laser damage threshold (LDT) in potassium dihydrogen phosphate (KDP) crystals grown by slow evaporation solution technique (SEST) has been investigated. The influence of doping on growth rate and morphology of the grown crystals has also been studied. Powder x-ray diffraction data confirms the crystal structure of KDP and shows a systematic variation in intensity of diffraction peaks in correlation with morphology due to varying LT concentration. No extra phase formation was observed which is further confirmed by Fourier transform Raman (FT-Raman) studies. High-resolution x-ray diffraction curves indicate that crystalline perfection has been improved to a great extent at low concentrations with a maximum perfection at 1 mol % doping. At higher concentrations (5 to 10 mol %), it is slightly reduced due to excess incorporation of dopants at the interstitial sites of the crystalline matrix. LDT has been increased considerably with increase in doping concentration, whereas SHG efficiency was found to be maximum at 1 mol % in correlation with crystalline. The optical transparency for doped crystals has been increased as compared to that of pure KDP with a maximum value at 1 mol % doping.

36 MATERIALS SCIENCE↗
DOE OSTI2011

Defects at nitrogen site in electron-irradiated AlN

In high resistance AlN irradiated with 2 MeV electrons, an electron paramagnetic resonance (EPR) spectrum, labeled EI-1, with an electron spin S=1/2 and a clear hyperfine (hf) structure was observed. The hf structure was shown to be due the interaction between the electron spin and the nuclear spins of four {sup 27}A nuclei with the hf splitting varying between {approx}6.0 and {approx}7.2 mT. Comparing the hf data obtained from EPR and ab initio supercell calculations we suggest the EI-1 defect to be the best candidate for the neutral nitrogen vacancy in AlN.

36 MATERIALS SCIENCE↗
DOE OSTI2010

Evidence for an anomalous like-sign dimuon charge asymmetry

We measure the charge asymmetry A of like-sign dimuon events in 6.1 fb{sup -1} of pp collisions recorded with the D0 detector at a center-of-mass energy {radical}(s)=1.96 TeV at the Fermilab Tevatron collider. From A, we extract the like-sign dimuon charge asymmetry in semileptonic b-hadron decays: A{sub sl}{sup b}=-0.00957{+-}0.00251 (stat){+-}0.00146 (syst). This result differs by 3.2 standard deviations from the standard model prediction A{sub sl}{sup b}(SM)=(-2.3{sub -0.6}{sup +0.5})x10{sup -4} and provides first evidence of anomalous CP violation in the mixing of neutral B mesons.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2011

Search for Neutral Minimal Supersymmetric Standard Model Higgs Bosons Decaying to Tau Pairs in pp Collisions at {radical}(s)=7 TeV

A search for neutral minimal supersymmetric standard model (MSSM) Higgs bosons in pp collisions at the LHC at a center-of-mass energy of 7 TeV is presented. The results are based on a data sample corresponding to an integrated luminosity of 36 pb{sup -1} recorded by the CMS experiment. The search uses decays of the Higgs bosons to tau pairs. No excess is observed in the tau-pair invariant-mass spectrum. The resulting upper limits on the Higgs boson production cross section times branching fraction to tau pairs, as a function of the pseudoscalar Higgs boson mass, yield stringent new bounds in the MSSM parameter space.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2010

Incident photon energy and Z dependence of L X-ray relative intensities

The intensity ratios, I{sub Lk}/I{sub L{alpha}1} (k = l, {eta}, {alpha}{sub 2}, {beta}{sub 1}, {beta}{sub 2,15}, {beta}{sub 3}, {beta}{sub 4}, {beta}{sub 5,7}, {beta}{sub 6}, {beta}{sub 9,10}, {gamma}{sub 1,5}, {gamma}{sub 6,8}, {gamma}{sub 2,3}, {gamma}{sub 4}), have been evaluated for elements with atomic number 36 {<=} Z {<=} 92 at incident photon energies ranging E{sub L1} < E{sub inc} {<=} 200 keV using currently considered to be more reliable theoretical data sets of different physical parameters, namely, L{sub i} (i = 1-3) subshell photoionization cross sections based on the relativistic Hartree-Fock-Slater model, the X-ray emission rates based on the Dirac-Fock model, and the fluorescence and Coster-Kronig yields based on the Dirac-Hartree-Slater model. At incident photon energies above the K-shell ionization threshold, the contribution to the production of different L X-ray lines due to the additional L{sub i} (i = 1-3) subshell vacancies created following decay of the primary K-shell vacancies have also been included in the present calculations. The important features pertaining to dependence of the tabulated intensity ratios on the incident photon energy and atomic number have been discussed.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗