Engineering Papers⌕ Search

SEARCH · Engineering Papers

Results for “TUNE”

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.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 631 records · Page 35
All sourcesNASA NTRSDOE OSTI
DOE OSTI2013

Extended electrical tuning of quantum cascade lasers with digital concatenated gratings

In this report, the sampled grating distributed feedback laser architecture is modified with digital concatenated gratings to partially compensate for the wavelength dependence of optical gain in a standard high efficiency quantum cascade laser core. This allows equalization of laser threshold over a wide wavelength range and demonstration of wide electrical tuning. With only two control currents, a full tuning range of 500 nm (236 cm{sup −1}) has been demonstrated. Emission is single mode, with a side mode suppression of >20 dB.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2014

Effect of frequency tuning on bremsstrahlung spectra, beam intensity, and shape in the 10 GHz NANOGAN electron cyclotron resonance ion source

Studies on the effect of the frequency tuning on the bremsstrahlung spectra, beam intensities, and beam shape of various ions have been carried out in the 10 GHz NANOGAN ECR ion source. The warm and cold components of the electrons were found to be directly correlated with beam intensity enhancement in case of Ar{sup 9+} but not so for O{sup 5+}. The warm electron component was, however, much smaller compared to the cold component. The effect of the fine tuning of the frequency on the bremsstrahlung spectrum, beam intensities and beam shape is presented.

43 PARTICLE ACCELERATORS↗
DOE OSTI2014

Tuning electronic and magnetic properties of armchair|zigzag hybrid graphene nanoribbons by the choice of supercell model of grain boundaries

Grain boundaries (GBs) attract much interest for its ability to tune the property of hybrid materials. Theoretically predicting the properties of hybrid graphene with GBs, even a linear GB remains challenging due to its inhomogeneous structure, which makes supercell model tough to choose in theoretic studies. For the first time, the applicability of supercells with different GBs and lattice-mismatches for describing armchair-zigzag hybrid graphene nanoribbons was validated by ab initio molecular dynamic simulations and first principles electronic structure calculations. And to what extent the electronic properties can be tuned by the strain effects resulting from the lattice-mismatch and the GBs distortion in supercells was demonstrated. This work showed that the intrinsic strain in such system plays a decisive role in determining the band structure and spin polarization properties. Hybrid graphene nanoribbon was found to be ferromagnetic in the ground state, especially for the case of using the supercell with nearly-perfect lattice match. Its high Curie temperature suggests the potential applications of this material in spintronics.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Tuning electron-electron correlation in noncentrosymmetric superconductor BiPd

In this work, we have successfully tuned the electron-electron correlation in new noncentrosymmertic superconductor (NCS) BiPd via Pb substitution on one of the inequivalent Bi sites present in BiPd and simultaneously keeping the parent noncentrosymmetric crystal structure intact. Heat capacity data is suggesting a fourfold increase in density of states at Fermi level via Pb substitution while superconducting transition temperature has been suppressed. This work will clearly open up a rare chance to study the effect of electron–electron correlation via selective tuning of DOS at Fermi level in NCS.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Enhancement of spin polarization via Fermi level tuning in Co{sub 2}MnSn{sub 1−x}Sb{sub x} (x = 0, 0.25. 0.5, 0.75, 1) Heusler alloys

Full potential approach has been employed to tune Fermi level in Co{sub 2}MnSn{sub 1−x}Sb{sub x} (x = 0, 0.25, 0.5, 0.75, 1) Heulser alloys for enhancement of spin polarization and finding signature of half metallicity. Present density functional theory (DFT) based calculation indicates that stoichoimetric Heusler alloy, Co{sub 2}MnSn is not a half-metallic ferromagnet but the doping of Sb in it results in the shifting of E{sup F} in well-defined energy gap which leads the 100% spin polarization in the resultant alloys. The magnetism in present alloys is governed by localized moment on Mn atom mainly. The tuning of half-metallicity using doping can be proved as an ideal technique to search the new materials which can accomplish the need of spintronics.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Tuned range-separated hybrid functionals in the symmetry-adapted perturbation theory

The aim of this study is to present a performance test of optimally tuned long-range corrected (LRC) functionals applied to the symmetry-adapted perturbation theory (SAPT). In the present variant, the second-order energy components are evaluated at the coupled level of theory. We demonstrate that the generalized Kohn-Sham (GKS) description of monomers with optimally tuned LRC functionals may be essential for the quality of SAPT interaction energy components. This is connected to the minimization of a many-electron self-interaction error and exemplified by two model systems: polyacetylenes of increasing length and stretching of He{sub 3}{sup +}. Next we provide a comparison of SAPT approaches based on Kohn-Sham and GKS description of the monomers. We show that LRC leads to results better or comparable with the hitherto prevailing asymptotically corrected functionals. Finally, we discuss the advantages and possible limitations of SAPT based on LRC functionals.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2015

Tuning of in-plane optical anisotropy by inserting ultra-thin InAs layer at interfaces in (001)-grown GaAs/AlGaAs quantum wells

The in-plane optical anisotropy (IPOA) in (001)-grown GaAs/AlGaAs quantum wells (QWs) with different well widths varying from 2 nm to 8 nm has been studied by reflectance difference spectroscopy. Ultra-thin InAs layers with thickness ranging from 0.5 monolayer (ML) to 1.5 ML have been inserted at GaAs/AlGaAs interfaces to tune the asymmetry in the QWs. It is demonstrated that the IPOA can be accurately tailored by the thickness of the inserted ultra-thin InAs layer at the interfaces. Strain-induced IPOA has also been extracted by using a stress apparatus. We find that the intensity of the strain-induced IPOA decreases with the thickness of the inserted InAs layer, while that of the interface-induced IPOA increases with the thickness of the InAs layer. Theoretical calculations based on 6 band k ⋅ p theory have been carried out, and good agreements with experimental results are obtained. Our results demonstrate that, the IPOA of the QWs can be greatly and effectively tuned by inserting an ultra-thin InAs layer with different thicknesses at the interfaces of QWs, which does not significantly influence the transition energies and the transition probability of QWs.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

PSO algorithm enhanced with Lozi Chaotic Map - Tuning experiment

In this paper it is investigated the effect of tuning of control parameters of the Lozi Chaotic Map employed as a chaotic pseudo-random number generator for the particle swarm optimization algorithm. Three different benchmark functions are selected from the IEEE CEC 2013 competition benchmark set. The Lozi map is extensively tuned and the performance of PSO is evaluated.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Thermal tuning of infrared resonant absorbers based on hybrid gold-VO{sub 2} nanostructures

Resonant absorbers based on plasmonic materials, metamaterials, and thin films enable spectrally selective absorption filters, where absorption is maximized at the resonance wavelength. By controlling the geometrical parameters of nano/microstructures and materials' refractive indices, resonant absorbers are designed to operate at wide range of wavelengths for applications including absorption filters, thermal emitters, thermophotovoltaic devices, and sensors. However, once resonant absorbers are fabricated, it is rather challenging to control and tune the spectral absorption response. Here, we propose and demonstrate thermally tunable infrared resonant absorbers using hybrid gold-vanadium dioxide (VO{sub 2}) nanostructure arrays. Absorption intensity is tuned from 90% to 20% and 96% to 32% using hybrid gold-VO{sub 2} nanowire and nanodisc arrays, respectively, by heating up the absorbers above the phase transition temperature of VO{sub 2} (68 °C). Phase change materials such as VO{sub 2} deliver useful means of altering optical properties as a function of temperature. Absorbers with tunable spectral response can find applications in sensor and detector applications, in which external stimulus such as heat, electrical signal, or light results in a change in the absorption spectrum and intensity.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Angular tuning of the magnetic birefringence in rippled cobalt films

We report the measurement of magnetically induced birefringence in rippled Co films. For this purpose, the magneto-optical properties of ion beam eroded ferromagnetic films were studied using Kerr magnetometry and magnetic birefringence in the transmitted light intensity. Upon sufficient ion sculpting, these ripple surface nanostructures developed a defined uniaxial anisotropy in the in-plane magnetization, finely tuning the magnetic birefringence effect. We have studied its dependence on the relative orientation between the ripple direction and the magnetic field, and found this effect to be dramatically correlated with the capability to neatly distinguish the mechanisms for the in-plane magnetization reversal, i.e., rotation and nucleation. This double refraction corresponds univocally to the two magnetization axes, parallel and perpendicular to the ripples direction. We have also observed that tuned birefringence in stack assemblies of rippled Co films, which enables us to technically manipulate the number and direction of refraction axes.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Fluorescently tuned nitrogen-doped carbon dots from carbon source with different content of carboxyl groups

In this study, fluorescent nitrogen-doped carbon dots (NCDs) were tuned via varying the sources with different number of carboxyl groups. Owing to the interaction between amino and carboxyl, more amino groups conjugate the surface of the NCDs by the source with more carboxyl groups. Fluorescent NCDs were tuned via varying the sources with different content of carboxyl groups. Correspondingly, the nitrogen content, fluorescence quantum yields and lifetime of NCDs increases with the content of carboxyl groups from the source. Furthermore, cytotoxicity assay and cell imaging test indicate that the resultant NCDs possess low cytotoxicity and excellent biocompatibility.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Heterointerface design and strain tuning in epitaxial BiFeO 3 :CoFe 2 O 4 nanocomposite films

The ability to control the morphology of heterointerfaces with coupled functionalities is fascinating from both fundamental and technological perspectives. Here, using BiFeO 3 :CoFe 2 O 4 vertically aligned nanocomposite (VAN) films as a model system, we demonstrate a simple and effective method to modulate the heterointerface and its morphology in nanocomposite films with pulsed laser deposition. By tuning the deposition frequency through thickness during film growth, both vertically straight and gradient heterointerfaces have been achieved. The modulated heterointerface is strongly correlated with strain tuning and interface coupling, and thus modifies the magnetic anisotropy, coercive fields, and ferroelectric switching behavior. Furthermore, this study provides a viable approach for tailoring the interface strain and coupling in VAN and achieving tunable physical properties.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Tuning the band gap of silicene by functionalisation with naphthyl and anthracyl groups

Silicene is a relatively new material consisting of a two-dimensional sheet of silicon atoms. Functionalisation of silicene with different chemical groups has been suggested as a way to tune its electronic properties. In this work, density functional theory calculations and ab initio molecular dynamics simulations are used to examine the effects of functionalisation with naphthyl or anthracyl groups, which are two examples of small polycyclic aromatic hydrocarbons (PAHs). Different attachment positions on the naphthyl and anthracyl groups were compared, as well as different thicknesses of the silicene nanosheet. It was found that the carbon attachment position farthest from the bond fusing the aromatic rings gave the more stable structures for both functional groups. All structures showed direct band gaps, with tuning of the band gap being achievable by increasing the length of the PAH or the thickness of the silicene. Hence, modifying the functional group or thickness of the silicene can both be used to alter the electronic properties of silicene making it a highly promising material for use in future electronic devices and sensors.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2016

Cationic distribution assisted tuning of magnetic properties of Li{sub 0.5-x/2}Zn{sub x}Fe{sub 2.5-x/2}O{sub 4}

Zn addition induced modification of cationic distribution for tuning magnetic properties of Li{sub 0.5-x/2}Zn{sub x}Fe{sub 2.5-x/2}O{sub 4} (x = 0, 0.1, 0.25, 0.3, 0.5, 0.7, 0.9, 1.0) powders is studied by x-ray diffraction (XRD) and magnetic measurements. XRD shows the formation of cubic spinel structure, with Scherrer’s grain diameter ranging between 26.7 to 37.8 nm. With Zn addition, oxygen anions are displaced in such a way that the A-B interaction weakens, whereas A-A and B-B interaction strengthens; furthermore it also pushes Fe{sup 3+} and Li{sup 1+} ions to B site, leading to changes in magnetic properties. Highest saturation magnetization (M{sub s}) of 64.6 A m{sup 2} / kg was obtained for sample with x = 0.25 and lowest coercivity (H{sub c}) of 4949.9 A/m was obtained for the sample with x = 0.7. Calculated values of experimental magnetic moment ‘n{sub B}’ and Neel magnetic moment ‘n{sub N}’, display similar trend as that of M{sub s}. Present results very clearly show a strong correlation between Zn-addition induced changes in cation distribution and magnetic properties, which can be utilized effectively for tuning magnetic properties.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Tuning the photovoltaic effect of multiferroic CoFe{sub 2}O{sub 4}/Pb(Zr, Ti)O{sub 3} composite films by magnetic fields

The 0–3 type CoFe{sub 2}O{sub 4}-Pb(Zr,Ti)O{sub 3} (CFO-PZT) multiferroic composite films have been prepared by a sol-gel process and spin-coating technique. A confirmable photovoltaic effect is observed under ultraviolet light irradiation. Moreover, this photovoltaic effect can be tuned by external magnetic fields. The maximum magnetic modulation ratios of short-circuit current density and open-circuit voltage can reach as high as 13.7% and 12.8% upon the application of 6 kOe DC magnetic field. Through remnant polarization measurements under various magnetic fields and detailed analysis of the energy band structures, we elucidate the mechanism of tuning photovoltaic effect by magnetic fields and attribute it to the combination of two factors. One is the decreased ferroelectric-polarization-induced depolarization electric field and another is the band structure reconstruction at CFO-PZT interfaces, both of which are dominated by the magnetoelectric coupling via interfacial stress transferring at nanoscale. This work makes some attempts of coupling photo-induced effects with magnetoelectric effect in multiferroic materials and will widen the practical ranges of multiferroic-based applications.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Asymmetric-shell ignition capsule design to tune the low-mode asymmetry during the peak drive

The low-mode radiation flux asymmetry in the hohlraum is a main source of performance degradation in the National Ignition Facility (NIF) implosion experiments. To counteract the deleterious effects of the large positive P2 flux asymmetry during the peak drive, this paper develops a new tuning method called asymmetric-shell ignition capsule design which adopts the intentionally asymmetric CH ablator layer or deuterium-tritium (DT) ice layer. A series of two-dimensional implosion simulations have been performed, and the results show that the intentionally asymmetric DT ice layer can significantly improve the fuel ρR symmetry, hot spot shape, hot spot internal energy, and the final neutron yield compared to the spherical capsule. This indicates that the DT asymmetric-shell capsule design is an effective tuning method, while the CH ablator asymmetric-shell capsule could not correct the fuel ρR asymmetry, and it is not as effective as the DT asymmetric-shell capsule design.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2016

Tune-out wavelengths for helium atom in plasma environments

We investigate the effect of plasma screening on the tune-out wavelengths for helium atom using correlated exponential wave function within the framework of Debye shielding approach. The pseudostate summation technique has been used to calculate the dynamic dipole polarizability for the states (2 {sup 1}S, 3 {sup 1}S, 2 {sup 3}S, 3 {sup 3}S) of helium embedded in plasma environments. In a free-atomic system, our calculated results are in agreement with available theoretical and experimental predictions. The tune-out wavelengths show interesting behavior as functions of screening parameter.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2017

Energy level alignment at molecule-metal interfaces from an optimally tuned range-separated hybrid functional

The alignment of the frontier orbital energies of an adsorbed molecule with the substrate Fermi level at metal-organic interfaces is a fundamental observable of significant practical importance in nanoscience and beyond. Typical density functional theory calculations, especially those using local and semi-local functionals, often underestimate level alignment leading to inaccurate electronic structure and charge transport properties. Here, we develop a new fully self-consistent predictive scheme to accurately compute level alignment at certain classes of complex heterogeneous molecule-metal interfaces based on optimally tuned range-separated hybrid functionals. Starting from a highly accurate description of the gas-phase electronic structure, our method by construction captures important nonlocal surface polarization effects via tuning of the long-range screened exchange in a range-separated hybrid in a non-empirical and system-specific manner. We implement this functional in a plane-wave code and apply it to several physisorbed and chemisorbed molecule-metal interface systems. Our results are in quantitative agreement with experiments, the both the level alignment and work function changes. This approach constitutes a new practical scheme for accurate and efficient calculations of the electronic structure of molecule-metal interfaces.

36 MATERIALS SCIENCE↗