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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 73 records · Page 4
All sourcesNASA NTRSDOE OSTI
DOE OSTI2015

Raman spectroscopy measurement of bilayer graphene's twist angle to boron nitride

When graphene is placed on hexagonal boron nitride with a twist angle, new properties develop due to the resulting moiré superlattice. Here, we report a method using Raman spectroscopy to make rapid, non-destructive measurements of the twist angle between bilayer graphene and hexagonal boron nitride. The lattice orientation is determined by using flakes with both bilayer and monolayer regions, and using the known Raman signature for the monolayer to measure the twist angle of the entire flake. The widths of the second order Raman peaks are found to vary linearly in the superlattice period and are used to determine the twist angle. The results are confirmed by using transport measurements to infer the superlattice period by the charge density required to reach the secondary resistance peaks. Small twist angles are also found to produce a significant modification of the first order Raman G band peak.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Nature of exciton transitions in hexagonal boron nitride

In contrast to other III-nitride semiconductors GaN and AlN, the intrinsic (or free) exciton transition in hexagonal boron nitride ( h -BN) consists of rather complex fine spectral features (resolved into six sharp emission peaks) and the origin of which is still unclear. In this work, the free exciton transition (FX) in h -BN bulk crystals synthesized by a solution method at atmospheric pressure has been probed by deep UV time-resolved photoluminescence (PL) spectroscopy. Based on the separations between the energy peak positions of the FX emission lines, the identical PL decay kinetics among different FX emission lines, and the known phonon modes in h -BN, we suggest that there is only one principal emission line corresponding to the direct intrinsic FX transition in h -BN, whereas all other fine features are a result of phonon-assisted transitions. The identified phonon modes are all associated with the center of the Brillouin zone. Our results offer a simple picture for the understanding of the fundamental exciton transitions in h -BN.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Search for giant magnetic anisotropy in transition-metal dimers on defected hexagonal boron nitride sheet

Structural and magnetic properties of many transition-metal dimers embedded in a defected hexagonal boron nitride monolayer are investigated through density functional calculations to search for systems with magnetic anisotropy energies (MAEs) larger than 30meV. In particular, Ir–Ir@Dh–BN is found to have both large MAE (∼126 meV) and high structural stability against dissociation and diffusion, and it hence can serve as magnetic unit in spintronics and quantum computing devices. This giant MAE mainly results from the spin orbit coupling and the magnetization of the upper Ir atom, which is in a rather isolated environment.

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

Thermal and thermoelectric transport measurements of an individual boron arsenide microstructure

Recent first principles calculations have predicted that boron arsenide (BAs) can possess an unexpectedly high thermal conductivity that depends sensitively on the crystal size and defect concentration. However, few experimental results have been obtained to verify these predictions. In the present work, we report four-probe thermal and thermoelectric transport measurements of an individual BAs microstructure that was synthesized via a vapor transport method. The measured thermal conductivity was found to decrease slightly with temperature in the range between 250K and 350 K. The temperature dependence suggests that the extrinsic phonon scattering processes play an important role in addition to intrinsic phonon-phonon scattering. The room temperature value of (186646) Wm -1 K -1 is higher than that of bulk silicon but still a factor of four lower than the calculated result for a defect-free, non-degenerate BAs rod with a similar diameter of 1.15 μm. The measured p-type Seebeck coefficient and thermoelectric power factor are comparable to those of bismuth telluride, which is a commonly used thermoelectric material. In conclusion, the foregoing results also suggest that it is necessary to not only reduce defect and boundary scatterings but also to better understand and control the electron scattering of phonons in order to achieve the predicted ultrahigh intrinsic lattice thermal conductivity of BAs.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Structure and energetics of nanotwins in cubic boron nitrides

Recently, nanotwinned cubic boron nitrides (NT c-BN) have demonstrated extraordinary leaps in hardness. However, an understanding of the underlying mechanisms that enable nanotwins to give orders of magnitude increases in material hardness is still lacking. Here, using transmission electron microscopy, we report that the defect density of twin boundaries depends on nanotwin thickness, becoming defect-free, and hence more stable, as it decreases below 5 nm. Using ab initio density functional theory calculations, we reveal that the Shockley partials, which may dominate plastic deformation in c-BNs, show a high energetic barrier. We also report that the c-BN twin boundary has an asymmetrically charged electronic structure that would resist migration of the twin boundary under stress. These results provide important insight into possible nanotwin hardening mechanisms in c-BN, as well as how to design these nanostructured materials to reach their full potential in hardness and strength.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Controllable Fabrication of Tungsten Oxide Nanoparticles Confined in Graphene-Analogous Boron Nitride as an Efficient Desulfurization Catalyst

Tungsten oxide nanoparticles (WOxNPs) are gaining increasing attention, but low stabiliity and poor dispersion of WOxNPs hinder their catalytic applications. Herein, WOxNPs were confined in graphene-analogous boron nitride (g-BN) by a one-step, in situ method at high temperature, which can enhance the interactions between WOxNPs and the support and control the sizes of WOxNPs in a range of about 4–5 nm. The as-prepared catalysts were applied in catalytic oxidation of aromatic sulfur compounds in which they showed high catalytic activity. A balance between the W loading and the size distribution of the WOxNPs could govern the catalytic activity. Furthermore, a synergistic effect between g-BN and WOxNPs also contributed to high catalytic activity. The reaction mechanism is discussed in detail and the catalytic scope was enlarged.

Wu, Peiwen↗
DOE OSTI2018

Hexagonal boron nitride catalyst in a fixed-bed reactor for exothermic propane oxidation dehydrogenation

Hexagonal boron nitride (h-BN) with high thermal conductivity is potentially an effective catalyst for highly exothermic propane oxidative dehydrogenation (ODH) reaction. Here, we report our experimental and theoretic studies of such a catalyst for propane ODH in a fixed-bed reactor. Based on the computational fluid dynamics calculation (CFD) results, the catalyst bed temperature increases by less than 1 degrees C in the h-BN catalyst bed which is much smaller than that (8 degrees C) in the VOx/gamma-Al2O3 catalyst bed at a similar propane conversion (25%) using a micro-tubular reactor with a diameter of 6 mm. Even in an industrially relevant reactor with an inner diameter of 60 mm, a uniform temperature profile can still be maintained using the h-BN catalyst bed due to its excellent thermal conductivity as opposed to a temperature gradient of 47 degrees C in the VOx/gamma-Al2O3 catalyst bed. The results reported here provide useful information for potential application of h-BN catalyst in propane ODH. (C) 2018 Elsevier Ltd. All rights reserved.

Tian, jinshu↗
DOE OSTI2016

Metallic Borides, La 2 Re 3 B 7 and La 3 Re 2 B 5 , Featuring Extensive Boron–Boron Bonding

We synthesized La 2 Re 3 B 7 and La 3 Re 2 B 5 in single-crystalline form from a molten La/Ni eutectic at 1000°C, in the first example of the flux crystal growth of ternary rare-earth rhenium borides. Both compounds crystallize in their own orthorhombic structure types, with La 2 Re 3 B 7 (space group Pcca) having lattice parameters a = 7.657(2) Å, b = 6.755(1) Å, and c = 11.617(2) Å, and La 3 Re 2 B 5 (space group Pmma) having lattice parameters a = 10.809(2) Å, b = 5.287(1) Å, and c = 5.747(1) Å. Furthermore, the compounds possess three-dimensional framework structures that are built up from rhenium boride polyhedra and boron-boron bonding. La 3 Re 2 B 5 features fairly common B 2 dumbbells, whereas La 2 Re 3 B 7 has unique one-dimensional subunits composed of alternating triangular B3 and trans-B4 zigzag chain fragments. Also observed in La 3 Re 2 B 5 is an unusual coordination of B by an octahedron of La atoms. Electronic band structure calculations predict that La 2 Re 3 B 7 is a semimetal, which is observed in the electrical resistivity data as measured on single crystals, with behavior obeying the Bloch-Grüneisen model and a room-temperature resistivity ρ300K of ~ 375 μΩ cm. The electronic band structure calculations also suggest that La 3 Re 2 B 5 is a regular metal.

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

Radiative properties of quantum emitters in boron nitride from excited state calculations and Bayesian analysis

Abstract Point defects in hexagonal boron nitride (hBN) have attracted growing attention as bright single-photon emitters. However, understanding of their atomic structure and radiative properties remains incomplete. Here we study the excited states and radiative lifetimes of over 20 native defects and carbon or oxygen impurities in hBN using ab initio density functional theory and GW plus Bethe-Salpeter equation calculations, generating a large data set of their emission energy, polarization and lifetime. We find a wide variability across quantum emitters, with exciton energies ranging from 0.3 to 4 eV and radiative lifetimes from ns to ms for different defect structures. Through a Bayesian statistical analysis, we identify various high-likelihood charge-neutral defect emitters, among which the native V N N B defect is predicted to possess emission energy and radiative lifetime in agreement with experiments. Our work advances the microscopic understanding of hBN single-photon emitters and introduces a computational framework to characterize and identify quantum emitters in 2D materials.

Chemistry↗
DOE OSTI2021

Printable hexagonal boron nitride ionogels

We describe aerosol jet-printable ionogels with high ionic conductivities and mechanical strengths using exfoliated hexagonal boron nitride (hBN) nanoplatelets as the solid matrix.

Hyun, Woo Jin↗
DOE OSTI2014

Ab-initio calculations of electronic, transport, and structural properties of boron phosphide

We present results from ab-initio, self-consistent density functional theory calculations of electronic and related properties of zinc blende boron phosphide (zb-BP). We employed a local density approximation potential and implemented the linear combination of atomic orbitals formalism. This technique follows the Bagayoko, Zhao, and Williams method, as enhanced by the work of Ekuma and Franklin. The results include electronic energy bands, densities of states, and effective masses. The calculated band gap of 2.02 eV, for the room temperature lattice constant of a=4.5383 Å, is in excellent agreement with the experimental value of 2.02±0.05 eV. Our result for the bulk modulus, 155.7 GPa, agrees with experiment (152–155 GPa). Our predictions for the equilibrium lattice constant and the corresponding band gap, for very low temperatures, are 4.5269 Å and 2.01 eV, respectively.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗