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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 19 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2016

Improvement of charge separation in TiO{sub 2} by its modification with different tungsten compounds

Highlights: • Ammonium m-tungstate doped to TiO{sub 2} highly improved charge separation in TiO{sub 2}. • Negative electrokinetic potential of TiO{sub 2} facilitates holes migration to its surface. • Fast migration of holes to TiO{sub 2} surfaces increased yield of OH radicals formation. • Adsorption of dyes on photocatalyst increased its decomposition under visible light. - Abstract: Three different tungsten precursors were used for TiO{sub 2} modification: H{sub 2}WO{sub 4}, WO{sub 2}, and ammonium m-tungstate. It was proved that modification of TiO{sub 2} with tungsten compounds enhanced its photocatalytic activity through the improvement of charge separation. This effect was obtained by coating of TiO{sub 2} particles with tungsten compound, which changed their surficial electrokinetical potential from positive onto negative. The most efficient tungsten compound, which caused enhanced separation of free carriers was ammonium m-tungstate (AMT). Two dyes with different ionic potential were used for the photocatalytic decomposition. It appeared that cationic dye—Methylene Blue was highly adsorbed on the negatively charged surface of TiO{sub 2} modified by AMT and decomposed, however this photocatalyst was quickly deactivated whereas anionic dye—acid red was better adsorbed on the less acidic surface of TiO{sub 2} and was rapidly decomposed with almost the same rate in the five following cycles.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Electronic Structure of Tungsten-Doped β -Ga 2 O 3 Compounds

Tungsten (W) doped gallium oxide (Ga2O3) (Ga2-2xWxO3, 0.00=x=0.30, GWO) polycrystalline ceramic compounds were synthesized via conventional, high-temperature solid-state reaction method. The effect of W-doping on the crystal structure and electronic structure of the resulting GWO materials is studied in detail. The GWO compounds were single-phase, crystallized in ß-Ga2O3 for x=0.15, at which point the Ga2O3-WO3 composite formation occurs. The average crystallite size increases with increasing W-content; however, the effect is predominant only in the single phase GWO compounds. Corroborating with structural analyses, the X-ray photoelectron spectroscopy (XPS) measurements reveal the chemical state of W ions vary in GWO compounds as a function of W concentration. The mixed chemical valence states of W (W4+ and W6+) were evident in single-phase GWO compounds where the W-concentration is lower. However, W ions exhibit the highest chemical valence state (W6+) for higher x values, which resulted in the Ga2O3-WO3 composite formation. The Ga ions exists in their highest chemical valence state (Ga3+) in all of the GWO compounds. The scientific understanding of the electronic structure of the GWO materials derived as function of W concentration could be useful while considering the W-doped Ga2O3 materials and/or W-Ga2O3 contacts for electronic and optoelectronic device applications.

W-doping, Ga2O3-WO3 Composite, Electronic Structur↗
DOE OSTI2014

Phonon transport in single-layer transition metal dichalcogenides: A first-principles study

Two-dimensional transition metal dichalcogenides (TMDCs) are finding promising electronic and optical applications due to their unique properties. In this letter, we systematically study the phonon transport and thermal conductivity of eight semiconducting single-layer TMDCs, MX{sub 2} (M = Mo, W, Zr, and Hf, X = S and Se), by using the first-principles-driven phonon Boltzmann transport equation approach. The validity of the single-mode relaxation time approximation to predict the thermal conductivity of TMDCs is assessed by comparing the results with the iterative solution of the phonon Boltzmann transport equation. We find that the phononic thermal conductivities of 2H-type TMDCs are above 50 W/mK at room temperature while the thermal conductivity values of the 1T-type TMDCs are much lower, when the size of the sample is 1 μm. A very high thermal conductivity value of 142 W/mK was found in single-layer WS{sub 2}. The large atomic weight difference between W and S leads to a very large phonon bandgap which in turn forbids the scattering between acoustic and optical phonon modes and thus resulting in very long phonon relaxation time.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Interface structure in nanoscale multilayers near continuous-to-discontinuous regime

Interfacial atomic diffusion, reaction, and formation of microstructure in nanoscale level are investigated in W/B{sub 4}C multilayer (ML) system as functions of thickness in ultrathin limit. Hard x-ray reflectivity (XRR) and x-ray diffuse scattering in conjunction with x-ray absorption near edge spectroscopy (XANES) in soft x-ray and hard x-ray regimes and depth profiling x-ray photoelectron spectroscopy (XPS) have been used to precisely evaluate detailed interfacial structure by systematically varying the individual layer thickness from continuous-to-discontinuous regime. It is observed that the interfacial morphology undergoes an unexpected significant modification as the layer thickness varies from continuous-to-discontinuous regime. The interfacial atomic diffusion increases, the physical density of W layer decreases and that of B{sub 4}C layer increases, and further more interestingly the in-plane correlation length decreases substantially as the layer thickness varies from continuous-to-discontinuous regime. This is corroborated using combined XRR and x-ray diffused scattering analysis. XANES and XPS results show formation of more and more tungsten compounds at the interfaces as the layer thickness decreases below the percolation threshold due to increase in the contact area between the elements. The formation of compound enhances to minimize certain degree of disorder at the interfaces in the discontinuous region that enables to maintain the periodic structure in ML. The degree of interfacial atomic diffusion, interlayer interaction, and microstructure is correlated as a function of layer thickness during early stage of film growth.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

A Ta/W mixed addenda heteropolyacid with excellent acid catalytic activity and proton-conducting property

A new HPAs H{sub 20}[P{sub 8}W{sub 60}Ta{sub 12}(H{sub 2}O){sub 4}(OH){sub 8}O{sub 236}]·125H{sub 2}O (H-1) which comprises a Ta/W mixed addenda heteropolyanion, 20 protons, and 125 crystalline water molecules has been prepared through ion-exchange method. The structure and properties of H-1 have been explored in detail. AC impedance measurements indicate that H-1 is a good solid state proton conducting material at room temperature with a conductivity value of 7.2×10{sup −3} S cm{sup −1} (25 °C, 30% RH). Cyclic voltammograms of H-1 indicate the electrocatalytic activity towards the reduction of nitrite. Hammett acidity constant H{sub 0} of H-1 in CH{sub 3}CN is −2.91, which is the strongest among the present known HPAs. Relatively, H-1 exhibits excellent catalytic activities toward acetal reaction. - Highlights: • A Ta/W mixed addenda Heteropolyacid (H-1) was isolated. • Hammett acidity constant H{sub 0} of H-1 is the strongest among the present known HPAs. • H-1 exhibits excellent catalytic activities toward acetal reaction. • H-1 is a good solid state proton conducting material at room temperature.

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

Characterization of phase transformation during hot compressive deformation in a β-stabilized Ti–45Al–7Nb–0.4W–0.15B alloy

A β-stabilized Ti–45Al–7Nb–0.4W–0.15B (at.%) alloy was hot deformed by uniaxial compression and the phase evolution during the compression was characterized with X-ray diffraction, scanning electron microscopy and electron probe microanalysis. The results show that modest deformation stress during hot compression restrains the γ → α transformation and the decomposition of β phase. The restrained γ → α transformation is a result of a modified equilibrium of the γ and α phases due to the applied stress, and the restrained β decomposition is a kinetic effect due to the decelerated diffusion of β-stabilizing elements caused by the compressive stress. - Highlights: • In β-stabilized TiAl alloys, hot deformation has great influence on the equilibrium of the γ, α, and β phases. • Deformation restrains the γ → α transformation. • Deformation inhibits the decomposition of the β phase.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Modification of microstructure and properties of Ti-47Al-2Cr-4Nb-0.3W alloys fabricated by SPS with trace multilayer graphene addition

Highlights: • The TiAl alloys with different content addition of multilayer graphene were fabricated. • When the graphene content is 0.8 at.%, the mean grain size decreases to 8.2 μm. • The bending and fracture strength increase to 1000 MPa and 2347 MPa, respectively. • The friction coefficient decreased from 0.6 to 0.4, with a decrease by 1/3. - Abstract: In this paper, Ti-47Al-2Cr-4Nb-0.3W alloys with trace multilayer graphene addition were fabricated by spark plasma sintering to enhance both mechanical properties and tribological property simultaneously. With the addition of multilayer graphene, bending strength, compressive strength and tribological property of TiAl alloys are all improved. When graphene content increases from zero to 0.8 at.%, the mean grain size decreases from 14.8 ± 1.4 μm to 8.2 ± 0.8 μm. The bending strength reaches almost 1000 MPa and the fracture strength increases to 2347 ± 12 MPa, an increase of 15% compared with TiAl alloy. The friction coefficient also decreases from about 0.6 to 0.4, a decrease by 1/3 compared with TiAl alloy. The reasons for the improved mechanical properties and tribological property are the in-situ reaction plate-shape Ti{sub 2}AlC phase, fine needle-like Ti{sub 3}AlC phase as well as fine and homogeneous microstructure. The wear mechanism changes from abrasive wear to abrasive wear plus adhesive wear with the increased graphene content.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Effect of cooling rate on microstructure evolution of Ti-45Al-8.5Nb-0.2W-0.2B-0.02Y alloy during multi-step heat treatment

Highlights: • Novel ideas to control cooling rate during different phase transformation are proposed. • Cooling rates induced microstructure evolutions during the whole phase transformation are investigated. • Phase transformation texture and the characteristics of lamellar structures are related to cooling rate. - Abstract: Microstructure evolution of Ti-45Al-8.5Nb-0.2W-0.2B-0.02Y alloy (at.%) resulted from different cooling rates during the whole phase transformation process has been thoroughly investigated. The results show that the cooling rate during β → α transformation plays a crucial role in the modification of the crystallographic orientation relationship between the α grains and their parent β phase. The decrease of cooling rate can reduce the volume fraction of Burgers α grains and avoid the preferential distribution of lamellar interface traces. This could be attributed to the different nucleation and growth kinetics of α phase affected by the cooling rate. Moreover, the lamellar structure characteristics are closely related to the cooling rate during α → α{sub 2} + γ transformation and their responses to stabilization treatment shall vary accordingly. A fast cooling rate retains a high quantity of high temperature α/α{sub 2} phase. These α/α{sub 2} phase could decompose into ultrafine lamellar structure during subsequent stabilization treatment. While the average lamellar spacing of the slow cooled microstructure has a small increase by limited continuous growth of the pre-existing γ lamellae during stabilization treatment.

36 MATERIALS SCIENCE↗
DOE OSTI2013

Cross-plane thermal properties of transition metal dichalcogenides

In this work, we explore the thermal properties of hexagonal transition metal dichalcogenide compounds with different average atomic masses but equivalent microstructures. Thermal conductivity values of sputtered thin films were compared to bulk crystals. The comparison revealed a >10 fold reduction in thin film thermal conductivity. Structural analysis of the films revealed a turbostratic structure with domain sizes on the order of 5-10 nm. Estimates of phonon scattering lengths at domain boundaries based on computationally derived group velocities were consistent with the observed film microstructure, and accounted for the reduction in thermal conductivity compared to values for bulk crystals.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Mechanical and thermal properties of h-MX{sub 2} (M = Cr, Mo, W; X = O, S, Se, Te) monolayers: A comparative study

Using density functional theory, we obtain the mechanical and thermal properties of MX{sub 2} monolayers (where M = Cr, Mo, W and X = O, S, Se, Te). The Γ-centered phonon frequencies (i.e., A{sub 1}, A{sub 2}{sup ″}, E′, and E″), relative frequency values of A{sub 1}, and E′ modes, and mechanical properties (i.e., elastic constants, Young modulus, and Poisson's ratio) display a strong dependence on the type of metal and chalcogenide atoms. In each chalcogenide (metal) group, transition-metal dichalcogenides (TMDCs) with W (O) atom are found to be much stiffer. Consistent with their stability, the thermal expansion of lattice constants for TMDCs with O (Te) is much slower (faster). Furthermore, in a heterostructure of these materials, the difference of the thermal expansion of lattice constants between the individual components becomes quite tiny over the whole temperature range. The calculated mechanical and thermal properties show that TMDCs are promising materials for heterostructures.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Magnetic, dielectric properties, and scaling behaviors of Aurivillius compounds Bi{sub 6−x∕3}Fe{sub 2}Ti{sub 3−2x}(WCo){sub x}O{sub 18} (0 ≤ x ≤ 0.15)

We investigate the structural, magnetic, dielectric properties, and scaling behaviors of Aurivillius compounds Bi{sub 6−x∕3}Fe{sub 2}Ti{sub 3−2x}(WCo){sub x}O{sub 18} (0 ≤ x ≤ 0.15). The room-temperature weak ferromagnetism is observed for the W/Co co-doped samples. The results of the dielectric constant ε{sub r}, complex impedance Z″, the dc conductivity σ{sub dc}, and hopping frequency f{sub H} manifest that the dielectric relaxation of the x = 0 sample and the doped samples in the dielectric anomaly region (450–750 K) can be ascribed to the trap-controlled ac conduction around the doubly ionized oxygen vacancies and the localized hopping process of oxygen vacancies, respectively. The scaling behaviors reveal that the dynamic process of both electrons in the x = 0 sample and oxygen vacancies in the doped samples is temperature independent. The ferroelectric Curie-temperature T{sub c} decreases slightly from 973 K to 947 K with increasing the doping level of W/Co. In addition, the dielectric loss exhibits a dielectric relaxation above 800 K with the rather large activation energies (1.95 eV ≤ E{sub a} ≤ 2.72 eV)

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Growth and surface topography of WSe{sub 2} single crystal

Tungsten Di-Selenide belongs to the family of TMDCs showing their potential applications in the fields of Optoelectronics and PEC solar cells. Here in the present investigation single crystals of WSe{sub 2} were grown by Direct Vapour Transport Technique in a dual zone furnace having temperature difference of 50 K between the two zones. These single crystals were characterized by EDAX which confirms the stiochiometry of the grown crystals. Surface topography of the crystal was studied by optical micrograph showing the left handed spirals on the surface of WSe{sub 2} crystals. Single crystalline nature of the crystals was confirmed by SAED.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Fabrication and independent control of patterned polymer gate for a few-layer WSe{sub 2} field-effect transistor

We report the fabrication of a patterned polymer electrolyte for a two-dimensional (2D) semiconductor, few-layer tungsten diselenide (WSe{sub 2}) field-effect transistor (FET). We expose an electron-beam in a desirable region to form the patterned structure. The WSe{sub 2} FET acts as a p-type semiconductor in both bare and polymer-covered devices. We observe a highly efficient gating effect in the polymer-patterned device with independent gate control. The patterned polymer gate operates successfully in a molybdenum disulfide (MoS{sub 2}) FET, indicating the potential for general applications to 2D semiconductors. The results of this study can contribute to large-scale integration and better flexibility in transition metal dichalcogenide (TMD)-based electronics.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2017

Insulator to metal transition in WO 3 induced by electrolyte gating

Tungsten oxide and its associated bronzes (compounds of tungsten oxide and an alkali metal) are well known for their interesting optical and electrical characteristics. We have modified the transport properties of thin WO 3 films by electrolyte gating using both ionic liquids and polymer electrolytes. We are able to tune the resistivity of the gated film by more than five orders of magnitude, and a clear insulator-to-metal transition is observed. To clarify the doping mechanism, we have performed a series of incisive operando experiments, ruling out both a purely electronic effect (charge accumulation near the interface) and oxygen-related mechanisms. We propose instead that hydrogen intercalation is responsible for doping WO 3 into a highly conductive ground state and provide evidence that it can be described as a dense polaronic gas.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Synthesis and Photodriven Hydrogenation of Tungsten Nitride Complexes Prepared from Dinitrogen Cleavage

Oxidation of the Chatt-type tungsten dinitrogen compound, trans-(depe) 2 W(N 2 ) 2 (depe = Et 2 PCH 2 CH 2 PEt 2 ), with [(η 5 -C 5 H 5 ) 2 Fe][BAr F 4 ] (BAr F 4 = B(3,5-(CF 3 ) 2 C 6 H 3 ) 4 ) resulted in isolation of [(depe) 2 WN][BAr F 4 ], a rare example of a tungsten(IV) nitride prepared from N 2 cleavage. A bimetallic μ-N 2 ditungsten intermediate supported by terminal N 2 ligands was identified, and irradiation with visible light promoted dinitrogen cleavage and formation of [(depe) 2 WN][BAr F 4 ]. Performing the analogous one-electron oxidation of the related tungsten dinitrogen compound, trans-(dppe) 2 W(N 2 ) 2 (dppe = Ph 2 PCH 2 CH 2 PPh 2 ), furnished the corresponding cationic, 17-electron tungsten dinitrogen complex, [(dppe) 2 W(N 2 ) 2 ][BAr F 4 ], that was characterized by X-ray diffraction and vibrational and EPR spectroscopies. The generation of [(dppe) 2 W(N)][BAr F 4 ] was observed in low yield from the in situ formed mixed N 2 -bridged compound, [(N 2 )(depe) 2 W(μ-N 2 )W(dppe) 2 (N 2 )][BAr F 4 ] 2 , and was confirmed by independent synthesis using 1-azidoadamantane. Addition of ammonia or water to [(depe) 2 WN][BAr F 4 ] resulted in formation of the cationic imide and hydroxide complexes, [(depe) 2 W(NH)(X)][BAr F 4 ] (X = NH 2 , OH). Irradiation of [(depe) 2 WN][BAr F 4 ] with 440 nm visible light in the presence of Ir(ppy) 3 (ppy = 2-phenylpyridine) under 4 atm of dihydrogen resulted in hydrogenation of the tungsten nitride to the cationic tungsten pentahydride, [(depe) 2 WH 5 ][BAr F 4 ], with the release of free ammonia in 21% yield, a rare example of ammonia generation from dinitrogen and dihydrogen from a well-defined tungsten nitride.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2018

Microstructure, Texture, and Mechanical Properties of Ni-W Alloy After Accumulative Roll Bonding

In this study, the microstructure, texture, and mechanical properties evolution of Ni-14W (wt.%) alloy processed up to four cycles of accumulative roll-bonding (ARB) were investigated using electron backscatter diffraction, microhardness measurements, and tensile tests. The initial equiaxed grains, with an average size of 10 μm, underwent a strong refinement after ARB processing. The elongated ultrafine grains were parallel to the rolling direction, with a grain thickness of 0.2 µm. The texture after ARB processing was characterized by the typical rolling components (Copper, S and Brass), which showed a tendency toward stabilization after four cycles. The microhardness increased substantially (+ 86%) and seemed to saturate after three cycles. The tensile tests demonstrated that Ni-14W samples subjected to ARB processing exhibited high strength (> 1200 MPa after three ARB cycles) and very poor ductility.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Characterisation of ODS Fe-14Cr-2W-0.3Ti before and after high temperature triple and low temperature single ion irradiations

Highlights: • Nanoparticles stable after high temperature triple-beam and low temperature single-beam irradiation • Samples appear to have maintained the composition after both irradiations. • Slight decrease of the nanoparticle sizes below 5 nm after both irradiations • There appears to be incomplete dissolution of smaller nanoparticles after both irradiations. - Abstract: Oxide dispersion strengthened reduced activation ferritic steels are being considered as structural materials for future fusion reactors and Gen IV fission reactors. In this work, the stability of the nanometric oxides after being irradiated at different conditions and temperatures has been investigated. A Fe-14Cr-2W-0.3Ti-0.3Y{sub 2}O{sub 3} alloy was simultaneously triple-beam irradiated at 600 °C with Fe, He and H ions. The same alloy was irradiated with Fe ions at − 80 °C. The microstructure of this alloy was investigated using Transmission Electron Microscopy, Atom Probe Tomography and nanoindentation before and after the irradiations. There were no significant changes in the morphology and chemical composition of the nanoparticles after the irradiations, although the size of the smaller ones tends to decrease for both irradiation conditions. Small irradiation-induced bubbles were present after the triple simultaneous irradiation at high temperature. Nanoindentation also shows no significant differences with a slight increase in hardness for the sample irradiated at low temperature, while softening appears in the high temperature irradiation.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Tungsten Biscorroles: New Chiral Sandwich Compounds

Abstract The oxidative metalation method, involving the interaction of free‐base meso ‐triarylcorroles and W(CO) 6 in refluxing decalin, led to a set of three tungsten(VI) biscorroles, the first homoleptic sandwich compounds involving corroles. Single‐crystal X‐ray structures of two of the complexes revealed square‐antiprismatic coordination and strongly domed corroles with long W−N distances of 2.15–2.22 Å and a substantial displacement of ∼1.17 Å of the metal relative to the mean N 4 planes of the ligands. The structures correspond to approximate C 2 symmetry and are thus chiral. DFT calculations strongly indicate that the enantiomers are configurationally stable and hence amenable to chiral resolution. Their other notable properties include a strongly blueshifted Soret band at (357±2) nm, a relatively intense π→W(d ) near‐IR feature at (781±3) nm, and a low electrochemical HOMO–LUMO gap of approximately 1.3 V. The results obtained herein suggest that metallobiscorroles may emerge as a new class of inherently chiral chromophores with novel optical and electrochemical properties.

Alemayehu, Abraham B.↗