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"tungsten"
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Niobium tungsten oxides for high-rate lithium-ion energy storage
The maximum power output and minimum charging time of a lithium-ion battery depend on both ionic and electronic transport. Ionic diffusion within the electrochemically active particles generally represents a fundamental limitation to the rate at which a battery can be charged and discharged. To compensate for the relatively slow solid-state ionic diffusion and to enable high power and rapid charging, the active particles are frequently reduced to nanometre dimensions, to the detriment of volumetric packing density, cost, stability and sustainability. As an alternative to nanoscaling, here we show that two complex niobium tungsten oxides—Nb
16
W
5
O
55
and Nb
18
W
16
O
93
, which adopt crystallographic shear and bronze-like structures, respectively—can intercalate large quantities of lithium at high rates, even when the sizes of the niobium tungsten oxide particles are of the order of micrometres. Measurements of lithium-ion diffusion coefficients in both structures reveal room-temperature values that are several orders of magnitude higher than those in typical electrode materials such as Li
4
Ti
5
O
12
and LiMn
2
O
4
. Multielectron redox, buffered volume expansion, topologically frustrated niobium/tungsten polyhedral arrangements and rapid solid-state lithium transport lead to extremely high volumetric capacities and rate performance. Unconventional materials and mechanisms that enable lithiation of micrometre-sized particles in minutes have implications for high-power applications, fast-charging devices, all-solid-state energy storage systems, electrode design and material discovery.
Micrometre-sized particles of two niobium tungsten oxides have high volumetric capacities and rate performances, enabled by very high lithium-ion diffusion coefficients.
Journal Article
Promotional Effect of Tungsten on the NH.sub.3-SCR Performance of a MnO.sub.x-TiO.sub.2 Catalyst: Balance of Surface Reducibility and Acidity
In this paper, MnO.sub.x/TiO.sub.2 catalysts with different manganese contents were prepared by using vacuum over-impregnation method to support manganese on TiO.sub.2 support. However, the activity temperature window was narrow and the denitrification efficiency was only 265-365 °C in the temperature zone above 80%. Therefore, doping W was used to improve the high temperature activity and broaden the activity temperature window. The experimental results show that the active temperature window of the catalyst is expanded to 260-550 °C when doping 20%WO.sub.3. The effects of tungsten doping on the structure and surface properties of the catalysts were investigated by XRD, TEM, Raman, XPS, H.sub.2-TPR and NH.sub.3-TPD. It is found that the increase of manganese content would increase the ratio of Mn.sup.4+ and O.sub.[alpha], and then improve the NH.sub.3-SCR activity at low temperature. Tungsten doping can provide more strong acid sites and improve the high temperature activity of the catalyst by inhibiting the oxidation of ammonia.
Journal Article
Compositional Optimization of Sputtered WOsub.3/MoOsub.3 Films for High Coloration Efficiency
Thin films of mixed MoO[sub.3] and WO[sub.3] were obtained using reactive magnetron sputtering onto ITO-covered glass, and the optimal composition was determined for the best electrochromic (EC) properties. A combinatorial material synthesis approach was applied throughout the deposition experiments, and the samples represented the full composition range of the binary MoO[sub.3]/WO[sub.3] system. The electrochromic characteristics of the mixed oxide films were determined with simultaneous measurement of layer transmittance and applied electric current through the using organic propylene carbonate electrolyte cells in a conventional three-electrode configuration. Coloration efficiency data evaluated from the primary data plotted against the composition displayed a characteristic maximum at around 60% MoO[sub.3]. Our combinatorial approach allows the localization of the maximum at 5% accuracy.
Journal Article
Tungsten Molecular Species in Deuterium Plasmas in Contact with Sputtered W Surfaces
We show that in plasmas generated in deuterium in the presence of sputtered W surfaces, various molecular tungsten species are formed, whose chemical composition depends on the presence of gaseous impurities, namely, nitrogen, oxygen, and hydrogen. A magnetron discharge was used for plasma sustaining, and the species were investigated by mass spectrometry and optical emission spectroscopy. The identified tungsten-containing molecules are described by the chemical formula WOxNyDzHt, where x = 0–4, y = 0–3, z = 0–3, t = 0–5. Presumptively, even higher mass tungsten molecular species are present in plasma, which were not detected because of the limitation of the spectrometer measurement range to 300 amu. The presence of these molecules will likely impact the W particle balance and dust formation mechanisms in fusion plasmas.
Journal Article
Tungsten Material Behavior under Hsub.2, Dsub.2, and He Plasma Interaction Conditions in the Framework of Fusion-Relevant Studies
In the current study, bulk tungsten material surfaces are exposed to hydrogen, deuterium, and helium plasmas in the radiofrequency domain (13.56 MHz) at an input power of 250 W using the hollow-cathode configuration. The ejected material is collected on titanium substrates at various distances (from 6 mm up to 40 mm). Therefore, the exposed tungsten materials are investigated for surface changes (blister occurrence, dust formation, or nano-structuration), along with the crystallinity, depending on the plasma’s exposure times (from 30 min up to 120 min for each plasma type). Also, the collected materials are analyzed (morphological, structural, and statistical investigations) for dust and dust film-like appearance. Plasma discharges are analyzed using two methods: optical emission spectroscopy, and single Langmuir probes, to emphasize the nature of the used plasmas (cold discharges, ~2 eV), along with the presence of tungsten emission (e.g., WI 406.31 nm, WI 421.31 nm) during the plasma lifetime. By using a dedicated protocol, a method was established for obtaining fusion-relevant tungsten surfaces in the hydrogen and deuterium plasma discharges. By using the implemented method, the current paper introduces the possibility of obtaining a new tungsten morphology, i.e., the dandelion-like shape, by using helium plasma, in which the W[sub.18]O[sub.49] compound can be found.
Journal Article
Observation of moiré excitons in WSe2/WS2 heterostructure superlattices
Moiré superlattices enable the generation of new quantum phenomena in two-dimensional heterostructures, in which the interactions between the atomically thin layers qualitatively change the electronic band structure of the superlattice. For example, mini-Dirac points, tunable Mott insulator states and the Hofstadter butterfly pattern can emerge in different types of graphene/boron nitride moiré superlattices, whereas correlated insulating states and superconductivity have been reported in twisted bilayer graphene moiré superlattices
1
–
12
. In addition to their pronounced effects on single-particle states, moiré superlattices have recently been predicted to host excited states such as moiré exciton bands
13
–
15
. Here we report the observation of moiré superlattice exciton states in tungsten diselenide/tungsten disulfide (WSe
2
/WS
2
) heterostructures in which the layers are closely aligned. These moiré exciton states manifest as multiple emergent peaks around the original WSe
2
A exciton resonance in the absorption spectra, and they exhibit gate dependences that are distinct from that of the A exciton in WSe
2
monolayers and in WSe
2
/WS
2
heterostructures with large twist angles. These phenomena can be described by a theoretical model in which the periodic moiré potential is much stronger than the exciton kinetic energy and generates multiple flat exciton minibands. The moiré exciton bands provide an attractive platform from which to explore and control excited states of matter, such as topological excitons and a correlated exciton Hubbard model, in transition-metal dichalcogenides.
Moiré superlattice exciton states are observed in WSe
2
/WS
2
heterostructures with closely aligned layers.
Journal Article
Enhanced interactions of interlayer excitons in free-standing heterobilayers
Strong, long-range dipole–dipole interactions between interlayer excitons (IXs) can lead to new multiparticle correlation regimes
1
,
2
, which drive the system into distinct quantum and classical phases
2
–
5
, including dipolar liquids, crystals and superfluids. Both repulsive and attractive dipole–dipole interactions have been theoretically predicted between IXs in a semiconductor bilayer
2
,
6
–
8
, but only repulsive interactions have been reported experimentally so far
3
,
9
–
16
. This study investigated free-standing, twisted (51°, 53°, 45°) tungsten diselenide/tungsten disulfide (WSe
2
/WS
2
) heterobilayers, in which we observed a transition in the nature of dipolar interactions among IXs, from repulsive to attractive. This was caused by quantum-exchange-correlation effects, leading to the appearance of a robust interlayer biexciton phase (formed by two IXs), which has been theoretically predicted
6
–
8
but never observed before in experiments. The reduced dielectric screening in a free-standing heterobilayer not only resulted in a much higher formation efficiency of IXs, but also led to strongly enhanced dipole–dipole interactions, which enabled us to observe the many-body correlations of pristine IXs at the two-dimensional quantum limit. In addition, we firstly observed several emission peaks from moiré-trapped IXs at room temperature in a well-aligned, free-standing WSe
2
/WS
2
heterobilayer. Our findings open avenues for exploring new quantum phases with potential for applications in non-linear optics.
Reduced dielectric screening in a free-standing heterobilayer results in higher formation efficiency of interlayer excitons and leads to strongly enhanced dipole–dipole interactions, enabling the observation of many-body correlations at the quantum limit.
Journal Article