Engineering Papers⌕ Search

SEARCH · Engineering Papers

Results for “Localized states”

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 163 records · Page 9
All sourcesNASA NTRSDOE OSTI
DOE OSTI2016

Current Practices in Efficiency Financing: An Overview for State and Local Governments

In recent years there has been significant growth in the size and sheer number of energy efficiency financing programs. The term “energy efficiency financing” refers to debt or debt-like products that support the installation of energy efficiency measures by allowing costs to be spread over time. The implementation of the American Recovery and Reinvestment Act (ARRA) led to a proliferation of energy efficiency financing programs, which was followed in subsequent years by the launch of green banks in several states and the ramp up of other ratepayer-supported financing initiatives in various jurisdictions. These activities have brought increased attention to energy efficiency financing as an area of programmatic interest. Yet the propagation of various types of financing in a growing number of markets may have also left some policymakers and program administrators with questions as to what categories of products and programs are best suited for their situation.

99 GENERAL AND MISCELLANEOUS↗
DOE OSTI2015

Solution Phase Exciton Diffusion Dynamics of a Charge-Transfer Copolymer PTB7 and a Homopolymer P3HT

Using ultrafast polarization-controlled transient absorption (TA) measurements, dynamics of the initial exciton states were investigated on the time scale of tens of femtoseconds to about 80 ps in two different types of conjugated polymers extensively used in active layers of organic photovoltaic devices. These polymers are poly(3-fluorothienothiophenebenzodithiophene) (PTB7) and poly-3-hexylthiophene (P3HT), which are charge-transfer polymers and homopolymers, respectively. In PTB7, the initial excitons with excess vibrational energy display two observable ultrafast time constants, corresponding to coherent exciton diffusion before the vibrational relaxation, and followed by incoherent exciton diffusion processes to a neighboring local state after the vibrational relaxation. In contrast, P3HT shows only one exciton diffusion or conformational motion time constant of 34 ps, even though its exciton decay kinetics are multiexponential. Based on the experimental results, an exciton dynamics mechanism is conceived taking into account the excitation energy and structural dependence in coherent and incoherent exciton diffusion processes, as well as other possible deactivation processes including the formation of the pseudo-charge-transfer and charge separate states, as well as interchain exciton hopping or coherent diffusion.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Thickness-dependent metal-insulator transition in epitaxial SrRuO{sub 3} ultrathin films

Transport characteristics of ultrathin SrRuO{sub 3} films, deposited epitaxially on TiO{sub 2}-terminated SrTiO{sub 3} (001) single-crystal substrates, were studied as a function of film thickness. Evolution from a metallic to an insulating behavior is observed as the film thickness decreases from 20 to 4 unit cells. In films thicker than 4 unit cells, the transport behavior obeys the Drude low temperature conductivity with quantum corrections, which can be attributed to weak localization. Fitting the data with 2-dimensional localization model indicates that electron-phonon collisions are the main inelastic relaxation mechanism. In the film of 4 unit cells in thickness, the transport behavior follows variable range hopping model, indicating a strongly localized state. Magnetoresistance measurements reveal a likely magnetic anisotropy with the magnetic easy axis along the out-of-plane direction.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2021

Improving molecular alignment and charge percolation in semiconducting polymer films with highly localized electronic states through tailored thermal annealing

Here, we have recently developed a strategy to improve charge percolation efficiency in a renowned family of naphthalene-diimide bithiophene (PNDI-T2) copolymers, based on the application of a tailored thermal treatment on pre-aligned films. Here we show that the accurate choice of thermal post-processing temperatures can induce a notable lamellar thickening and an improvement of molecular alignment, the latter being favored by alkyl chains with longer branches. Correspondingly, enhanced transport properties are observed. Interestingly, these are found to be independent of lamellar thickness. In fact, within this family of molecules, characterized by a locked non-planar conformation and strong intramolecular charge localization, the orientational order within charge percolation paths was found to be the only factor dictating charge transport.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2021

Broad Distribution of Local Polar States Generates Large Electrothermal Properties in Pb-Free Relaxor Ferroelectrics

Electrothermal energy conversion provides attractive solutions for global energy management, such as energy harvesting from waste heat using pyroelectric energy conversion (PEC) and efficient cooling of portable electronics or data servers using the electrocaloric effect. Relaxor ferroelectrics are attractive for electrothermal energy conversion because of their large pyroelectric coefficients over a wide temperature range. Although Pb-based relaxors are well-known, toxicity concerns have mandated the intense search for Pb-free alternatives. Here, we engineered (Ba,Ca)TiO 3 -based relaxors based on a multisite doping strategy, which show promising electrothermal performance, viz. a maximum PEC efficiency of 14% and electrocaloric refrigeration capacity of 115 J/kg. Using local-scale structural analysis, we provide an atomistic model for large electrothermal properties in the newly designed Pb-free ferroelectrics, whereby a temperature-independent continuous distribution of cation displacement directions creates easy pathways for microscopic polarization reorientation. This research provides key structural insight for future atomic-scale engineering of environmentally sustainable ferroelectrics in energy applications.

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

Physical properties and electronic structure of a new barium titanate suboxide Ba{sub 1+δ}Ti{sub 13−δ}O{sub 12} (δ = 0.11)

The structure, transport, thermodynamic properties, x-ray absorption spectra (XAS), and electronic structure of a new barium titanate suboxide, Ba{sub 1+δ}Ti{sub 13−δ}O{sub 12} (δ = 0.11), are reported. It is a paramagnetic poor metal with hole carriers dominating the transport. Fermi liquid behavior appears at low temperature. The oxidization state of Ti obtained by the XAS is consistent with the metallic Ti{sup 2+} state. Local density approximation band structure calculations reveal the material is near the Van Hove singularity. The pseudogap behavior in the Ti-d band and the strong hybridization between the Ti-d and O-p orbitals reflect the characteristics of the building blocks of the Ti{sub 13} semi-cluster and the TiO{sub 4} quasi-squares, respectively.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Ultrafast X-ray scattering offers a structural view of excited-state charge transfer

Intramolecular charge transfer and the associated changes in molecular structure in N,N'-dimethylpiperazine are tracked using femtosecond gas-phase X-ray scattering. The molecules are optically excited to the 3p state at 200 nm. Following rapid relaxation to the 3s state, distinct charge-localized and charge-delocalized species related by charge transfer are observed. The experiment determines the molecular structure of the two species, with the redistribution of electron density accounted for by a scattering correction factor. The initially dominant charge-localized state has a weakened carbon–carbon bond and reorients one methyl group compared with the ground state. Subsequent charge transfer to the charge-delocalized state elongates the carbon–carbon bond further, creating an extended 1.634 Å bond, and also reorients the second methyl group. At the same time, the bond lengths between the nitrogen and the ring-carbon atoms contract from an average of 1.505 to 1.465 Å. The experiment determines the overall charge transfer time constant for approaching the equilibrium between charge-localized and charge-delocalized species to 3.0 ps.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
NASA NTRS1994

Deformation across the western United States: A local estimate of Pacific-North America transform deformation

We obtain a locally based estimate of Pacific-North America relative motion and an uncertainly in this estimate by integrating deformation rate along three different paths leading west across southwestern North America from east of the Rio Grande Rift to near the continental escarpment. Data are primarily Quatenary geologic slip rates estimates, and resulting deformation determinations therefore are 'instantaneous' in a geologic sense but 'long term' with respect to earthquake cycles. We deduce a rate of motion of the Pacific plane relative to North America that is 48 +/- 2 mm/yr, a rate indistinguishable from that predicted by the global kinematics models RM2 and NUVEL-1; however, we obtain an orientation that is 5-9 deg counterclockwise of these models. A more westerly motion of the Pacific plate, with respect to North America, is calculated from all three paths. The relatively westerly motion of the Pacific plate is accomodated by deformation in the North American continent that includes slip on relatively counterclockwise-oriented strike-slip faults (including the San Andreas fault), whic is especially relevant in and south of the Transverse Ranges, and a margin-normal component of net extension across the continent, which is especially relevant north of the Transverse Ranges. Deformation of the SW United States occurs in regionally coherent domains within the style of deformation is approximately uniform.

Humphreys, Eugene D.↗
DOE OSTI2016

Material properties of perovskites in the quasi-ternary system LaFeO{sub 3}–LaCoO{sub 3}–LaNiO{sub 3}

An overview is presented on the variation of electrical conductivity, oxygen permeation, and thermal expansion coefficient as a function of the composition of perovskites in the quasi-ternary system LaFeO{sub 3}–LaCoO{sub 3}–LaNiO{sub 3}. Powders of thirteen nominal perovskite compositions were synthesized under identical conditions by the Pechini method. The powder X-ray diffraction data of two series, namely La(Ni{sub 0.5}Fe{sub 0.5}){sub 1−x}Co{sub x}O{sub 3} and LaNi{sub 0.5-x}Fe{sub x}Co{sub 0.5}O{sub 3}, are presented after the powders had been sintered at 1100 °C for 6 h in air. The measurements revealed a rhombohedral structure for all compositions except LaNi{sub 0.5}Fe{sub 0.5}O{sub 3} for which 60% rhombohedral and 40% orthorhombic phase was found. The maximum DC electrical conductivity value of the perovskites at 800 °C was 1229 S cm{sup −1} for the composition LaCoO{sub 3} and the minimum was 91 S cm{sup −1} for the composition LaCo{sub 0.5}Fe{sub 0.5}O{sub 3}. The oxygen permeation of samples with promising conductivities at 800 °C was one order of magnitude lower than that of La{sub 0.6}Sr{sub 0.4}Co{sub 0.8}Fe{sub 0.2}O{sub 3} (LSCF). The highest value of 0.017 ml cm{sup −2} min{sup −1} at 950 °C was obtained with LaNi{sub 0.5}Co{sub 0.5}O{sub 3}. The coefficients of thermal expansion varied in the range of 13.2×10{sup −6} K{sup −1} and 21.9×10{sup −6} K{sup −1} for LaNi{sub 0.5}Fe{sub 0.5}O{sub 3} and LaCoO{sub 3}, respectively. {sup 57}Fe Mössbauer spectroscopy was used as probe for the oxidation states, local environment and magnetic properties of iron ions as a function of chemical composition. The substitution had a great influence on the chemical properties of the materials. - Graphical abstract: Compositional dependence of isothermal electrical conductivity at 800 °C and thermal expansion coefficient in the series La(Fe{sub 0.5}Ni{sub 0.5}){sub (1−x)}Co{sub x}O{sub 3} and LaNi{sub 0.5−x}Fe{sub x}Co{sub 0.5}O{sub 3}.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2025

Evaluating the gradients of localized diabatic state energies and couplings at minimum cost

We calculate the gradients of Boys diabatic state energies and couplings when the electronic vector space is generated by configuration interaction singles. Our approach follows the Lagrangian approach of Paz and Glover (rather than direct differentiation of the adiabatic-to-diabatic approaches that have been published previously). The result is that we achieve a dramatic increase in savings over previous approaches, and the present approach should be immediately useful to scientists focused on electronic relaxation, especially chemists studying electron transfer who wish to go beyond the Condon approximation. Here, a future extension to time-dependent density functional theory in the Tamm–Dancoff approximation is clear.

Chemical physics↗
DOE OSTI2021

Point-Defect-Localized Bound States in the Continuum in Photonic Crystals and Structured Fibers

In this work, we show that point defects in two-dimensional photonic crystals can support bound states in the continuum (BICs). The mechanism of confinement is a symmetry mismatch between the defect mode and the Bloch modes of the photonic crystal. These BICs occur in the absence of band gaps and therefore provide an alternative mechanism to confine light. Furthermore, we show that such BICs can propagate in a fiber geometry and exhibit arbitrarily small group velocity which could serve as a platform for enhancing nonlinear effects and light-matter interactions in structured fibers.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2011

Locally deformed states of the crystal structure of double semiconductors ZnSe and ZnTe doped with vanadium ions

Specific features of the crystal structure of a lightly doped Zn{sub 1-x}V{sub x}Te crystal (x = 0.0002) were investigated for the first time by thermal neutron diffraction. The experimental results are discussed together with the data of neutron diffraction measurements that were performed previously on Zn{sub 1-x}V{sub x}Se single crystals (x = 0.0018). An enhanced response of the ZnTe lattice (in comparison with ZnSe) to a perturbation introduced by an incorporated 3d ion is found.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗