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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 595 records · Page 33
All sourcesNASA NTRSDOE OSTI
DOE OSTI2014

Change in magnetic and structural properties of FeRh thin films by gold cluster ion beam irradiation with the energy of 1.67 MeV/atom

The effect of energetic cluster ion beam irradiation on magnetic and structural properties of FeRh thin films have been investigated. The cluster ions used in the present studies consist of a few gold atoms with the energy of 1.67 MeV/gold atom. Saturation magnetization of the sample irradiated with Au3 cluster ion beam (280 emu/cc) is larger than that for the irradiated sample with Au1 ion beam (240 emu/cc) for the same irradiation ion fluence. These results can also be confirmed by the X-ray magnetic circular dichroism (XMCD) measurement; the XMCD signal for Au3 cluster ion irradiation is larger than that for Au1 ion irradiation. Since the ion beam irradiation induced magnetization of FeRh is significantly correlated with the amount of the lattice defects in the samples, cluster ion beam irradiation can be considered to effectively introduce the lattice defects in B2-type FeRh rather than the single ion beam. Consequently, cluster ion irradiation is better than single ion irradiation for the viewpoint of saturation magnetization, even if the same irradiation energy is deposited in the samples.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Should muffin tin radius vary in different structures of a material?: A case study

Quantum mechanical calculations based on density functional theory and a generalized gradient approximation (GGA) have been used to study the structural properties of YbN. Its predicted unit cell lattice parameter in NaCl (B1) structure is 4.7810Å and in CsCl (B2) structure it is 2.8685Å. In the determination of lattice parameter the muffin tin radius (R{sub MT}) of constituent atoms play important role. In both the structures the muffin tin radius for Yb and N converges to 2.3 and 1.4 a.u., respectively.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Covalent bonds are created by the drive of electron waves to lower their kinetic energy through expansion

An analysis based on the variation principle shows that in the molecules H{sub 2}{sup +}, H{sub 2}, B{sub 2}, C{sub 2}, N{sub 2}, O{sub 2}, F{sub 2}, covalent bonding is driven by the attenuation of the kinetic energy that results from the delocalization of the electronic wave function. For molecular geometries around the equilibrium distance, two features of the wave function contribute to this delocalization: (i) Superposition of atomic orbitals extends the electronic wave function from one atom to two or more atoms; (ii) intra-atomic contraction of the atomic orbitals further increases the inter-atomic delocalization. The inter-atomic kinetic energy lowering that (perhaps counter-intuitively) is a consequence of the intra-atomic contractions drives these contractions (which per se would increase the energy). Since the contractions necessarily encompass both, the intra-atomic kinetic and potential energy changes (which add to a positive total), the fact that the intra-atomic potential energy change renders the total potential binding energy negative does not alter the fact that it is the kinetic delocalization energy that drives the bond formation.

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

Molecular dynamics simulations of the melting curve of NiAl alloy under pressure

The melting curve of B2-NiAl alloy under pressure has been investigated using molecular dynamics technique and the embedded atom method (EAM) potential. The melting temperatures were determined with two approaches, the one-phase and the two-phase methods. The first one simulates a homogeneous melting, while the second one involves a heterogeneous melting of materials. Both approaches reduce the superheating effectively and their results are close to each other at the applied pressures. By fitting the well-known Simon equation to our melting data, we yielded the melting curves for NiAl: 1783(1 + P/9.801){sup 0.298} (one-phase approach), 1850(1 + P/12.806){sup 0.357} (two-phase approach). The good agreement of the resulting equation of states and the zero-pressure melting point (calc., 1850 ± 25 K, exp., 1911 K) with experiment proved the correctness of these results. These melting data complemented the absence of experimental high-pressure melting of NiAl. To check the transferability of this EAM potential, we have also predicted the melting curves of pure nickel and pure aluminum. Results show the calculated melting point of Nickel agrees well with experiment at zero pressure, while the melting point of aluminum is slightly higher than experiment.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Improvement of magnetic and structural stabilities in high-quality Co{sub 2}FeSi{sub 1−x}Al{sub x}/Si heterointerfaces

We study high-quality Co{sub 2}FeSi{sub 1−x}Al{sub x} Heusler compound/Si (0 ≤ x ≤ 1) heterointerfaces for silicon (Si)-based spintronic applications. In thermal treatment conditions, the magnetic and structural stabilities of the Co{sub 2}FeSi{sub 1−x}Al{sub x}/Si heterointerfaces are improved with increasing x in Co{sub 2}FeSi{sub 1−x}Al{sub x}. Compared with L2{sub 1}-ordered Co{sub 2}FeSi/Si, B2-ordered Co{sub 2}FeAl/Si can suppress the diffusion of Si atoms into the Heusler-compound structure. This experimental study will provide an important knowledge for applications in Si-based spin transistors with metallic source/drain contacts.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Theoretical investigation of La monopnictides: Electronic properties and pressure-induced phase transition

The NaCl-type La monopnictides are proper reference materials for the study of strongly correlated rare-earth pnictides. Yet, despite the simple crystal structure of this system, traditional density functional theory (DFT) calculations have dramatic failures in describing their electronic properties: DFT severely underestimates the band gaps and thus predicts incorrect transport characters of them. Here, we perform a corrected DFT calculation to rectify this failure. Our results show that LaN, LaP, and LaAs are semiconductor with band gaps of 0.82, 0.25, and 0.12 eV, respectively, and LaSb is semimetallic with an overlap of conduction and valence bands approximately 0.28 eV, in agreement with the available experiments. Additionally, under high-pressure, we find that LaN displays a new sequence of phase-transition, B1 → anti-B10 → B2, which is different from the previous theoretical predictions but consistent with the recent experiment.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2014

Nonequilibrium structural condition in the medical TiNi-based alloy surface layer treated by electron beam

The research is devoted to study the structural condition and their evolution from the surface to the depth of TiNi specimens treated by low-energy high-current electron beams with surface melting at a beam energy density E = 10 J/cm{sup 2}, number of pulses N = 10, and pulse duration τ = 50 μs. Determined thickness of the remelted layer, found that it has a layered structure in which each layer differs in phase composition and structural phase state. Refinement B2 phase lattice parameters in local areas showed the presence of strong inhomogeneous lattice strain.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Phase composition in NiTi near-surface layers after electron beam treatment and its variation depending on beam energy density

In the work, we study the mechanisms of structural phase state formation in NiTi surface layers after low-energy pulsed electron beam irradiation depending on the electron beam energy density. It is revealed that after electron beam treatment of the NiTi specimens at energy densities E{sub 1} = 15 J/cm{sup 2}, E{sub 2} = 20 J/cm{sup 2}, and E{sub 3} = 30 J/cm{sup 2}, a series of effects is observed: the absence of the Ti2Ni phase and the presence of new peaks correspond to the B19′ martensite phase with monoclinic structure. Estimation of the relative volume content of the B2 and B19′ phases from the total intensity of their peaks shows that the percentage of the martensite phase increases from ∼5 vol.% in the NiTi specimen irradiated at E{sub 1} = 15 J/cm{sup 2} to ∼80 vol.% in the NiTi specimen irradiated at E{sub 3} = 30 J/cm{sup 2}. It is found that in the NiTi specimens irradiated at E ≤ 20 J/cm{sup 2}, the layer that contains a martensite phase resides not on the surface but at some depth from it.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Structural evolution in Ti-Cu-Ni metallic glasses during heating

The structural evolution of Ti{sub 50}Cu{sub 43}Ni{sub 7} and Ti{sub 55}Cu{sub 35}Ni{sub 10} metallic glasses during heating was investigated by in-situ synchrotron X-ray diffraction. The width of the most intense diffraction maximum of the glassy phase decreases slightly during relaxation below the glass transition temperature. Significant structural changes only occur above the glass transition manifesting in a change in the respective peak positions. At even higher temperatures, nanocrystals of the shape memory B2-Ti(Cu,Ni) phase precipitate, and their small size hampers the occurrence of a martensitic transformation.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Capping layer-tailored interface magnetic anisotropy in ultrathin Co{sub 2}FeAl films

Co{sub 2}FeAl (CFA) thin films of various thicknesses (2 nm ≤ d ≤ 50 nm) have been grown on (001) MgO single crystal substrates and then capped with Cr, V, and Ta. Their magnetic and structural properties have been studied by x-ray diffraction (XRD), vibrating sample magnetometry, and broadband microstrip ferromagnetic resonance (MS-FMR). The XRD revealed that the films are epitaxial with the cubic [001] CFA axis normal to the substrate plane and that the chemical order varies from the B2 phase to the A2 phase when decreasing the thickness. The deduced lattice parameters showed that the Cr-capped films exhibit a larger tetragonal distortion, as compared with the films capped with V or Ta. The presence of magnetic dead layers has been observed in CFA samples capped with V and Ta but not in the case of the Cr-capped ones. The effective magnetization, deduced from the fit of MS-FMR measurements, increases (decreases) linearly with the CFA inverse thickness (1/d) for the Cr-capped (Ta-capped) films while it is constant for the V-capped ones. This allows quantifying the perpendicular surface anisotropy coefficients of −0.46 erg/cm{sup 2} and 0.74 erg/cm{sup 2} for Cr and Ta-capped films, respectively. Moreover, the fourfold and the uniaxial anisotropy fields, measured in these films, showed different trends with a respect to the CFA inverse thickness. This allows inferring that a non-negligible part of the fourfold magnetocrystalline term is of interfacial origin.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Wavelength-dependent magnetic transitions of self-organized iron–aluminum stripes induced by pulsed laser irradiation

We investigate the laser wavelength dependence of structural and magnetic transitions on the surface of an iron–aluminum (FeAl) alloy induced by nanosecond pulsed laser irradiation. The formation of self-organized FeAl stripes with a wavelength-dependent period is observed in a local area on the (111)-oriented plane. Focused magneto-optical Kerr effect measurements reveal that the coercivity reaches up to 1.2 kOe with increasing the magnetic field rotation angle, which is estimated from the stripe direction, in FeAl stripes irradiated at 355 nm, and its magnetization reversal can be explained by the domain-wall motion model. On the other hand, the magnetization reversal agrees with the Stoner–Wohlfarth model in FeAl stripes irradiated at 1064 nm. This magnetic transition originates from the B2-to-A2 phase transition in stripe structures and bulk regions. These results indicate that the magnetic transition from the incoherent to coherent mode as well as the structural transformation of stripe patterns can be controlled by the incident laser wavelength.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Herschel/HIFI line surveys: Discovery of interstellar chloronium (H{sub 2}Cl{sup +})

Prior to the launch of Herschel, HCl was the only chlorine-containing molecule detected in the interstellar medium (ISM). However, chemical models have identified chloronium, H{sub 2}Cl{sup +}, as a relatively abundant species that is potentially detectable. H{sub 2}Cl{sup +} was predicted to be most abundant in the environments where the ultraviolet radiation is strong: in diffuse clouds, or near the surfaces of dense clouds illuminated by nearby O and B stars. The HIFI instrument on Herschel provided the first detection of H{sub 2}Cl{sup +} in the ISM. The 2{sub 12}-1{sub 01} lines of ortho-H{sub 2}{sup 35}Cl{sup +} and ortho-H{sub 2}{sup 37}Cl{sup +} were detected in absorption toward NGC 6334I, and the 1{sub 11}-0{sub 00} transition of para-H{sub 2}{sup 35}Cl{sup +} was detected in absorption toward NGC 6334I and Sgr B2(S). The derived HCl/H{sub 2}Cl{sup +} column density ratios, ∼1-10, are within the range predicted by models of diffuse and dense Photon Dominated Regions (PDRs). However, the observed H{sub 2}Cl{sup +} column densities, in excess of 10{sup 13} cm{sup −2}, are significantly higher than the model predictions. These observations demonstrate the outstanding spectroscopic capabilities of HIFI for detecting new interstellar molecules and providing key constraints for astrochemical models.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2015

Linear and nonlinear optical absorption coefficients and refractive index changes in GaN/Al{sub x}Ga{sub (1−x)}N double quantum wells operating at 1.55 μm

In the present paper, the linear and nonlinear optical absorption coefficients and refractive index changes between the ground and the first excited states in double GaN/Al{sub x}Ga{sub (1−x)}N quantum wells are studied theoretically. The electronic energy levels and their corresponding wave functions are obtained by solving Schrödinger-Poisson equations self-consistently within the effective mass approximation. The obtained results show that the optical absorption coefficients and refractive index changes can be red- and blue-shifted through varying the left quantum well width and the aluminum concentration x{sub b2} of the central barrier, respectively. These structural parameters are found to present optimum values for carrying out the transition of 0.8 eV (1.55 μm). Furthermore, we show that the desired transition can also be achieved by replacing the GaN in the left quantum well with Al{sub y}Ga{sub (1−y)}N and by varying the aluminum concentration y{sub Al}. The obtained results give a new degree of freedom in optoelectronic device applications such as optical fiber telecommunications operating at (1.55 μm)

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Structural, electronic and mechanical properties of rare earth nitride-ErN: A first principles study

The structural, electronic and mechanical properties of rare earth nitride ErN is investigated by the first principles calculations based on density functional theory using the Vienna ab-initio simulation package. At ambient pressure ErN is stable in the ferromagnetic state with NaCl structure. The calculated lattice parameters are in good agreement with the available results. The electronic structure reveals that ErN is half metallic at normal pressure. A pressure-induced structural phase transition from NaCl (B1) to CsCl (B2) phase is observed in ErN. Ferromagnetic to non magnetic phase transition is predicted in ErN at high pressure.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Open volume defects and magnetic phase transition in Fe{sub 60}Al{sub 40} transition metal aluminide

Magnetic phase transition in the Fe{sub 60}Al{sub 40} transition metal aluminide from the ferromagnetic disordered A2-phase to the paramagnetic ordered B2-phase as a function of annealing up to 1000 °C has been investigated by means of magneto-optical and spectroscopy techniques, i.e., Kerr effect, positron annihilation, and Mössbauer spectroscopy. The positron annihilation spectroscopy has been performed in-situ sequentially after each annealing step at the Apparatus for In-situ Defect Analysis that is a unique tool combining positron annihilation spectroscopy with temperature treatment, material evaporation, ion irradiation, and sheet resistance measurement techniques. The overall goal was to investigate the importance of the open volume defects onto the magnetic phase transition. No evidence of variation in the vacancy concentration in matching the magnetic phase transition temperature range (400–600 °C) has been found, whereas higher temperatures showed an increase in the vacancy concentration.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Structural disorder and magnetism in the spin-gapless semiconductor CoFeCrAl

Disordered CoFeCrAl and CoFeCrSi{sub 0.5}Al{sub 0.5} alloys have been investigated experimentally and by first-principle calculations. The melt-spun and annealed samples all exhibit Heusler-type superlattice peaks, but the peak intensities indicate a substantial degree of B2-type chemical disorder. Si substitution reduces the degree of this disorder. Our theoretical analysis also considers several types of antisite disorder (Fe-Co, Fe-Cr, Co-Cr) in Y-ordered CoFeCrAl and partial substitution of Si for Al. The substitution transforms the spin-gapless semiconductor CoFeCrAl into a half-metallic ferrimagnet and increases the half-metallic band gap by 0.12 eV. Compared CoFeCrAl, the moment of CoFeCrSi{sub 0.5}Al{sub 0.5} is predicted to increase from 2.01 μ{sub B} to 2.50 μ{sub B} per formula unit, in good agreement with experiment.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

DFT calculations of electronic and optical properties of SrS with LDA, GGA and mGGA functionals

The theoretical investigations of electronic and optical properties of SrS are made using the first principle DFT calculations. The calculations are performed for the local-density approximation (LDA), generalized gradient approximation (GGA) and for an alternative form of GGA i.e. metaGGA for both rock salt type (B1, Fm3m) and cesium chloride (B2, Pm3m) structures. The band structure, density of states and optical spectra are calculated under various available functional. The calculations with LDA and GGA functional underestimate the values of band gaps with all functional, however the values with mGGA show reasonably good agreement with experimental and those calculated by using other methods.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Electronic and ground state properties of ThTe

The electronic properties of ThTe in cesium chloride (CsCl, B2) structure are investigated in the present paper. To study the ground state properties of thorium chalcogenide, the first principle calculations have been calculated. The bulk properties, including lattice constant, bulk modulus and its pressure derivative are obtained. The calculated equilibrium structural parameters are in good agreement with the available experimental and theoretical results.

36 MATERIALS SCIENCE↗