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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 721 records · Page 40
All sourcesNASA NTRSDOE OSTI
DOE OSTI2021

Metamorphic Tunnel Junctions Grown Via MOCVD Designed for GaAs 0.75 P 0.25 /Si Tandem Solar Cells

A high-performance metamorphic Al 0.2 Ga 0.8 As 0.75 P 0.25 /GaAs 0.75 P 0.25 heterojunction tunnel junction structure was developed for application to monolithic epitaxial GaAs 0.75 P 0.25 /Si 1.7 eV/1.1 eV bandgap tandem solar cells produced via metal-organic chemical vapor deposition. Doping optimization focused on both bulk doping concentrations and mitigating the transient incorporation effects that otherwise causes non-abrupt doping profiles in the Te doped layer. Furthermore, these efforts resulted in a fully-relaxed metamorphic tunnel diode at the target lattice constant having a peak tunneling current density (JP) of 279.1 A·cm –2 and a zero-bias resistance area product (RA) of 3.0×10 –4 Ω·cm 2 . After post-growth annealing to emulate the thermal load of the GaAs 0.75 P 0.25 top cell growth and elimination of the performance enhancing carbon activation anneal, the device achieved JP = 13.1 A·cm –2 and RA = 1.5×10 –3 Ω·cm 2 . These worst-case values are very promising as they enable the operation of GaAs 0.75 P 0.25 /Si tandems under one-sun AM1.5G illumination with negligible series resistance and at AM1.5D concentrations up to 240 suns with only 0.1% absolute efficiency loss.

14 SOLAR ENERGY↗
NASA NTRS1972

Radiation-induced extrinsic photoconductivity in Li-doped Si.

Investigation of the effects of lithium on radiation-produced complexes having long-time stability by examining the localized energy levels in the forbidden gap which give rise to extrinsic photoconductivity. The levels are found to disappear and in some cases shift with annealing in the 100-450 C temperature range. Due to the complexity of the system and the present lack of adequate theory, no complete analysis of the data obtained could be made.

Fenimore, E.↗
DOE OSTI2018

Effect of Metal Substitution on the Magnetic Properties of Spinel Ferrites Synthesized from Zinc-Bearing Dust

This paper provides a facile way for comprehensive utilization of multi-metal-associated and intractable zinc-bearing dust to synthesize multi-metal-doped spinel ferrites. The effects of preparation conditions including mass ratios of zinc-bearing dust to MgO addition and calcination temperature on the purity of as-prepared samples were investigated in detail, as well as the effect of metal substitution on the magnetic properties of as-synthesized spinel ferrites. The results show that single-phase spinel ferrite could be obtained when the mixture with the mass ratio of zinc-bearing dust to MgO addition of 1:0.20 was calcined at 1000 {sup ∘}C for 2 h. The saturation magnetization M{sub s} values for Mg-substituted, Ni-substituted, and Mn-substituted ferrites are 43.2, 47.6, and 54.3 emu g{sup − 1}, respectively, which can be explained by Neel’s two sub-lattice collinear models and the cation distribution between the tetrahedron (A) sites and octahedral (B) sites of the as-prepared samples. Notably, the heavy metals leaching from the as-prepared samples are very limited compared with zinc-bearing dust. In this way, hazardous zinc-bearing dust could be comprehensively utilized to synthesize spinel ferrites with excellent magnetic properties.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

An Investigation of Y{sub 3}Ba{sub 5}Cu{sub 8}O{sub 18} Doping with Ag Nanoparticles and Its Application as Superconductor

Y{sub 3}Ba{sub 5}Cu{sub 8}O{sub 18} superconductors were prepared through a standard solid-state reaction and the structural properties of the samples were studied through XRD and the Rietveld refinement method. The effect of silver nanoparticles doping on the Y{sub 3}Ba{sub 5}Cu{sub 8}O{sub 18} superconductors was studied as well. It is known that the size of nanoparticles is increased during aging. Therefore, two batches of samples with 1 and 2 wt% of Ag nanoparticles and the size range of 30, 200, 500, 700, 800, and 1000 nm were prepared. After preparing the samples and observing the Meissner effect, the crystallography, critical current density, critical temperature, magnetic susceptibility, SEM, and EDX experiments of the samples were carried out. The results of the critical current density measurements showed that the sample with 2 wt% Ag nanoparticles and the size of 700 nm has the maximum current density. In both batches of samples, increasing the nanoparticle size to 700 nm led to an increase in the critical current density. The crystallography studies showed that silver nanoparticles do not insert into the superconductor’s frame. Actually, they are placed in the samples as a distinct phase.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Structural and Magnetic Properties of the Series of Double-Perovskite Sr{sub 2−x}Bi{sub x}MnMoO{sub 6}

Polycrystalline samples of a series of double-perovskite Sr{sub 2−x}Bi{sub x}MnMoO{sub 6} (x = 0, 0.05, 0.10, 0.15, 0.20, and 0.25) were synthesized by solid-state reaction, and their structural and magnetic properties were investigated by means of X-ray power diffraction and magnetic measures. Rietveld analysis of room-temperature XRD data shows that all the samples are single phases and belong to the I4/m space group. The degree of Mn/Mo order (η) and the unit-cell volume decrease monotonically with the increase of Bi concentration. The field dependence of the magnetization suggests that all the samples are anti-ferromagnetism. With Bi doping, the magnetization increases at low Bi concentration (x ≤ 0.15), which indicates the formation of ferrimagnetic interaction between the Mo{sup 5+} and Mn{sup 2+} caused by the effect of the electron doping in the compounds; however, due to the decline of the Mn/Mo order (η), the magnetization tends to decrease upon the Bi concentration in the high doping levels (0.15 ≤ x ≤ 0.25). The non-monotonic variation of the Neel temperature (T{sub N}) is consistent with the variation tendency of magnetization upon Bi doping.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2024

Modeling and design of a separate effects irradiation test targeting fission gas release from Cr-doped UO 2

Fission gas release (FGR) from nuclear fuel during operation can diminish heat transfer properties across the pellet-cladding gap and increase the fuel rod internal pressure, thereby posing a concern to fuel reliability and safety during an accident. Enlarging the fuel grain size, which has been shown to improve fission gas retention, can be achieved by doping the fuel feedstock prior to sintering. In this work, the BISON fuel performance code was used to predict FGR from undoped and chromia-doped UO 2 (referred to as Cr-doped UO 2 ) fuel specimens with different grain sizes and across various temperatures. The BISON models identified the irradiation conditions for which FGR is most significant, and a separate effects irradiation experiment in the High Flux Isotope Reactor (HFIR) was then developed targeting those conditions. Further, the experiment leveraged the MiniFuel irradiation capability at Oak Ridge National Laboratory and consisted of 12 fuel specimens of varying grain size and Cr content. A coupling scheme between BISON FGR results and the ANSYS finite element thermal model used for experiment design was formulated to predict cumulative FGR from each fuel specimen based on expected irradiation temperature histories. The fuel samples were fabricated and characterized as a part of this work, and the fuel compositions modeled in BISON were representative of the specimens used in the experiment. This combined modeling and experimental effort aims to study the effect of fuel grain size and Cr content on FGR and to provide simulated BISON FGR results that can be used for future model validation activities.

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
DOE OSTI2018

The effect of Gd3+ ions on fabrication and luminescence properties of Nd3+-doped (Ca1-xGdx)F2+x transparent ceramics

Highlights: • Nd:(Ca{sub 1-x}Gd{sub x})F{sub 2+x} transparent ceramics were fabricated by hot-pressing method. • Concentration effect of Gd{sup 3+} ion on the optical properties are discussed. • The emission properties have been greatly enhanced by codoping with Gd{sup 3+}ions. • The fluorescence parameters were compared to other Nd{sup 3+}-doped optical materials. - Abstract: Nd:(Ca{sub 1-x}Gd{sub x})F{sub 2+x} (x = 0.01–0.1) transparent ceramics were fabricated by hot-pressing the co-precipitated synthetic nanopowders in vacuum. The as-fabricated Nd:(Ca{sub 1-x}Gd{sub x})F{sub 2+x} transparent ceramics exhibit nearly pore-free and uniform microstructure, the in-line transmittance of all samples is beyond 86% at 1064 nm. The effect of Gd{sup 3+} ions on the transparency and grain size of Nd:(Ca{sub 1-x}Gd{sub x})F{sub 2+x} transparent ceramics has been demonstrated. The dependence of emission intensity and lifetime on Gd{sup 3+} concentration for Nd:(Ca{sub 1-x}Gd{sub x})F{sub 2+x} transparent ceramics is investigated and discussed. The introducing of Gd{sup 3+} is found to increase emission intensity and lifetime dramatically. The fitting results of fluorescence decay curves show the lifetime of 1 at.% Nd:(Ca{sub 0.9}Gd{sub 0.1})F{sub 2.1} transparent ceramic is to be 373.80 μs which is greatly larger than that of 1 at.% Nd:CaF{sub 2} transparent ceramic. The spectroscopic parameters of Nd:(Ca{sub 1-x}Gd{sub x})F{sub 2+x} transparent ceramics and other Nd{sup 3+}-doped optical materials have been compared and discussed.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Effect of Heat Treatment on Microstructure and Magnetic Properties of Ce-Doped NdFeB Ribbons

The substitution for Nd by abundant element cerium (Ce) is a practical way for the comprehensive utilization of rare earth resources in NdFeB permanent magnets. In this letter, we have prepared the Ce-doped NdFeB ribbons and conventional NdFeB ribbons by melt quenching method and investigated the effects of heat treatment on the crystallization behavior, microstructure, and magnetic properties of the alloy. The results show that: (1) The crystallization behavior and the microstructural changes of the (Nd,Ce)FeB magnets are similar to the conventional NdFeB magnet when heat treatment. In addition, the Ce{sub 2}Fe{sub 14}B phase has a significant effect on the properties of the whole magnets. (2)The NdFeB phase and CeFeB phase are relatively close to each other after being precipitated from the amorphous phase. The coupling effect between the two phases is strong enough to weaken the effect of the addition of Ce and making the properties of the NdFeB magnets to not reduce too much after adding Ce.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

Gradient Doping in Sn–Pb Perovskites by Barium Ions for Efficient Single-Junction and Tandem Solar Cells

Narrow bandgap tin-lead (NBG Sn-Pb) perovskites generally have a high density of unintentional p-type self-doping, which reduces the charge carrier lifetimes, diffusion lengths and device efficiencies. Here, we demonstrate a p-n homojunction across the Sn-Pb perovskite resulted from a gradient doping by Ba 2+ . We report that 0.1 mol% Ba 2+ can effectively compensate the p-doping of Sn-Pb perovskites or even turns it to n-type without changing its bandgap. Ba 2+ cations are found to stay at the interstitial sites and work as shallow electron donor. In addition, Ba 2+ cations show a unique heterogeneous distribution in perovskite film. Most of the barium ions stay in the top 600 nanometer region of the perovskite films and turn it into weakly n-type, while the bottom portion of the film remains to be p-type. The gradient doping forms of a homojunction from top to bottom of the perovskite films with a built-in field that facilitates extraction of photogenerated carriers, resulting an increased carrier extraction length. Furthermore, this strategy enhanced the efficiency of Sn-Pb perovskite single-junction solar cells to over 21.0% and boosted the efficiencies of monolithic perovskite-perovskite tandem solar cells to 25.3% and 24.1%, for active areas of 5.9 mm2 and 0.94 cm 2 , respectively.

14 SOLAR ENERGY↗
DOE OSTI2019

The Importance of p-Doping for Quantum Dot Laser on Silicon Performance

p-type modulation doping of the quantum dot active region is known to improve high temperature and dynamic performance of quantum dot lasers. These improvements are critical to realizing commercially relevant quantum dot devices on silicon and are shown to enable continuous wave operation over 100°C, nearly complete insensitivity to optical feedback, and orders of magnitude improvement in device reliability relative to unintentionally doped active regions. We also describe a spectrally resolved analysis of the effect of p-modulation doping on the optical gain revealing anomalous behavior that explains the high characteristic temperatures commonly observed in literature for similar devices on native substrate.

42 ENGINEERING↗
DOE OSTI2020

Tunable Dopants with Intrinsic Counterion Separation Reveal the Effects of Electron Affinity on Dopant Intercalation and Free Carrier Production in Sequentially Doped Conjugated Polymer Films

Carrier mobility in doped conjugated polymers is limited by Coulomb interactions with dopant counterions. This complicates studying the effect of the dopant’s oxidation potential on carrier generation because different dopants have different Coulomb interactions with polarons on the polymer backbone. In this work, dodecaborane (DDB)-based dopants are used, which electrostatically shield counterions from carriers and have tunable redox potentials at constant size and shape. DDB dopants produce mobile carriers due to spatial separation of the counterion, and those with greater energetic offsets produce more carriers. Neutron reflectometry indicates that dopant infiltration into conjugated polymer films is redox-potential-driven. Remarkably, X-ray scattering shows that despite their large 2-nm size, DDBs intercalate into the crystalline polymer lamellae like small molecules, indicating that this is the preferred location for dopants of any size. These findings elucidate why doping conjugated polymers usually produces integer, rather than partial charge transfer: dopant counterions effectively intercalate into the lamellae, far from the polarons on the polymer backbone. Finally, it is shown that the IR spectrum provides a simple way to determine polaron mobility. Overall, higher oxidation potentials lead to higher doping efficiencies, with values reaching 100% for driving forces sufficient to dope poorly crystalline regions of the film.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2017

High-resolution structural characterization and magnetic properties of epitaxial Ce-doped yttrium iron garnet thin films

Thin films of magnetic garnet materials, e.g. yttrium iron garnet (Y 3 Fe 5 O 12 , YIG), are useful for a variety of applications including microwave integrated circuits and spintronics. Moreover, substitution of rare earth ions, such as cerium, is known to enhance the magneto-optic Kerr effect (MOKE) as compared to pure YIG. Thin films of Ce 0.75 Y 2.25 Fe 5 O 12 (Ce:YIG) have been grown using the pulsed laser deposition (PLD) technique and their crystal structure examined using high resolution scanning transmission electron microscopy. Homogeneous substitution of Ce in YIG, without oxidation to form a separate CeO 2 phase, can be realized in a narrow process window with resulting enhancement of the MOKE signal. We investigated the thermally generated signal due to spin Seebeck effect for the optimally doped Ce:YIG films.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Thickness dependent itinerant ferromagnetism in ultrathin Ba-doped SrRu O 3 films

The electronic and magnetic properties in AB⁢O 3 perovskite oxides are extremely sensitive to lattice structure, but also to dimensionality, such as the thickness in thin film-form. Here, we report the thickness-dependent electromagnetic properties of ultrathin epitaxially stabilized Sr 1-x ⁢Ba x ⁢Ru⁢O 3 (x = 0.08, 0.2) thin films on a SrTi⁢O 3 (001) substrate. The results reveal that the barium doping (0.08 ≤ x ≤ 0.20) reduces Ru⁢O 6 orthorhombic distortions existing in SrRu⁢O 3 and induces a tetragonal distortion, as evidenced by out-of-plane lattice expansion. A metal-to-insulator transition, accompanied by a ferromagnetic to nonmagnetic transition occurs with reducing film thickness from 10 to 3 unit cells for both x = 0.08 and 0.2, regardless of the doping level. The results suggest that the effects of compositional vacancies and surface/interface contributions introduced via dimensional confinement are more dominant than A-site chemical disorder or structural distortion for the loss of metallicity and ferromagnetism in ultra-thin epitaxial films.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Quasiparticle and Optical Properties of Carrier-Doped Monolayer MoTe 2 from First Principles

The intrinsic weak and highly nonlocal dielectric screening of two-dimensional materials is well-known to lead to high sensitivity of their optoelectronic properties to environment. Less studied theoretically is the role of free carriers in those properties. In this report we use ab initio GW and Bethe-Salpeter equation calculations, with a rigorous treatment of dynamical screening and local-field effects, to study the doping dependence of the quasiparticle and optical properties of a monolayer transition-metal dichalcogenide, 2H MoTe 2 . Here we predict a quasiparticle band gap renormalization of several hundreds of meV for experimentally attainable carrier densities and a similarly sizable decrease in the exciton binding energy. This results in an almost constant excitation energy for the lowest-energy exciton resonance with an increasing doping density. Using a newly developed and generally applicable plasmon-pole model and a self-consistent solution of the Bethe-Salpeter equation, we reveal the importance of accurately capturing both dynamical and local-field effects to understand detailed photoluminescence measurements.

2D material↗
DOE OSTI2026

Synergistic Doping and Stabilization of Magnetically Tunable LnTi3(Sb,Sn)4 (Ln: Ce–Gd) Kagome Metals

Here, we present our synthesis and characterization of the LnTi3(Sb,Sn)4 (Ln: Ce, Pr, Nd, Sm, Gd) family of cleavable kagome metals. While these materials are isostructural to the LnTi3Bi4 family, they only form as (Sb,Sn) solid-solutions with no corresponding LnTi3Sb4 or LnTi3Sn4 phases. We use a combination of first-principles density functional theory (DFT) and crystal orbital Hamilton population (COHP) calculations to show that (Sb,Sn) alloying has a stabilizing effect on the structure by adjusting the Fermi level, filling bonding states, depopulating antibonding states, and adjusting the density of states (DOS) toward local minima, an effect we call “synergistic doping.” Through a detailed characterization of the SmTi3(Sb,Sn)4 series, we further demonstrate that the tunable Fermi level has a profound effect on the physical properties. We observe multiple magnetic phases that stem from a competition between antiferromagnetic (AFM) and ferromagnetic-like (FM) ground states. Furthermore, the (Sb,Sn) ratio allows us to tune from direct AFM-FM competition toward a complex admixture with properties reminiscent of the recently discovered TbTi3Bi4. Ultimately, our work demonstrates how the concept of synergistic doping provides a means to stabilize new structures while developing systems with intrinsically tunable chemical, magnetic, and electronic properties.

Ortiz, Brenden [ORNL] (ORCID:0000000213337003)↗
DOE OSTI2024

Ion diffusion retarded by diverging chemical susceptibility

Abstract For first-order phase transitions, the second derivatives of Gibbs free energy (specific heat and compressibility) diverge at the transition point, resulting in an effect known as super-elasticity along the pressure axis, or super-thermicity along the temperature axis. Here we report a chemical analogy of these singularity effects along the atomic doping axis, where the second derivative of Gibbs free energy (chemical susceptibility) diverges at the transition point, leading to an anomalously high energy barrier for dopant diffusion in co-existing phases, an effect we coin as super-susceptibility. The effect is realized in hydrogen diffusion in vanadium dioxide (VO 2 ) with a metal-insulator transition (MIT). We show that hydrogen faces three times higher energy barrier and over one order of magnitude lower diffusivity when it diffuses across a metal-insulator domain wall in VO 2 . The additional energy barrier is attributed to a volumetric energy penalty that the diffusers need to pay for the reduction of latent heat. The super-susceptibility and resultant retarded atomic diffusion are expected to exist universally in all phase transformations where the transformation temperature is coupled to chemical composition, and inspires new ways to engineer dopant diffusion in phase-coexisting material systems.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
NASA NTRS1971

Dynamic Nuclear Polarization in Samarium Doped Lanthanum Magnesium Nitrate

The dynamic nuclear polarization of hydrogen nuclei by the solid effect in single crystals of samarium doped lanthanum magnesium nitrate (Sm:LMN) was studied theoretically and experimentally. The equations of evolution governing the dynamic nuclear polarization by the solid effect were derived in detail using the spin temperature theory and the complete expression for the steady state enhancement of the nuclear polarization was calculated. Experimental enhancements of the proton polarization were obtained for eight crystals at 9.2 GHz and liquid helium temperatures. The samarium concentration ranged from 0.1 percent to 1.1 percent as determined by X-ray fluorescence. A peak enhancement of 181 was measured for a 1.1 percent Sm:LMN crystal at 3.0 K. The maximum enhancements extrapolated with the theory using the experimental data for peak enhancement versus microwave power and correcting for leakage, agree with the ideal enhancement (240 in this experiment) within experimental error for three of the crystals.

Byvik, C. E.↗