Search results for "Tungsten"

Mismatch strain versus dangling bonds: formation of "coin-roll nanowires" by stacking nanosheets.

2010

Magnéli oxides as promising <em>n</em>-type thermoelectrics

2014

The discovery of a large thermopower in cobalt oxides in 1997 lead to a surge of interest in oxides for thermoelectric application. Whereas conversion efficiencies of p-type oxides can compete with non-oxide materials, n-type oxides show significantly lower thermoelectric performances. In this context so-called Magneli oxides have recently gained attention as promising n-type thermoelectrics. A combination of crystallographic shear and intrinsic disorder lead to relatively low thermal conductivities and metallic-like electrical conductivities in Magneli oxides. Current peak-zT values of 0.3 around 1100 K for titanium and tungsten Magneli oxides are encouraging for future research. Here, we …

Facet-Independent Electric-Field-Induced Volume Metallization of Tungsten Trioxide Films.

2015

Reversible metallization of band and Mott insulators by ionic-liquid gating is accompanied by significant structural changes. A change in conductivity of seven orders of magnitude at room temperature is found in epitaxial films of WO3 with an associated monoclinic-to-cubic structural reorganization. The migration of oxygen ions along open volume channels is the underlying mechanism.

Gram-scale selective synthesis of WO3−x nanorods and (NH4)xWO3 ammonium tungsten bronzes with tunable plasmonic properties

2021

Localized surface plasmon resonance properties in unconventional materials like metal oxides or chalcogenide semiconductors have been studied for use in signal detection and analysis in biomedicine and photocatalysis. We devised a selective synthesis of the tungsten oxides WO3-x and (NH4)xWO3 with tunable plasmonic properties. We selectively synthesized WO3-x nanorods with different aspect ratios and hexagonal tungsten bronzes (NH4)xWO3 as truncated nanocubes starting from ammonium metatungstate (NH4)6H2W12O40·xH2O. Both particles form from the same nuclei at temperatures >200 °C; monomer concentration and surfactant ratio are essential variables for phase selection. (NH4)xWO3 was the prefe…

Refractory metal reactivity towards oxide surface : W/TiO2(1 1 0) case.

2006

International audience; Reactivity of deposited tungsten towards TiO2(1 1 0) surface was studied using synchrotron radiation photoemission spectroscopy (both core levels, valence band and resonant photoemission) on Materials Science Beamline at ELETTRA. W depositions carried out at room temperature on TiO2(1 1 0) surface give rise to an interfacial reaction which leads to a metastable situation due to kinetic limitations. Annealing induces chemical changes which are function of the initial coverage; for fractional coverage, annealing induces completion of oxidation of deposit whereas reduction to metallic tungsten occurs for highest coverage. These results demonstrate that interaction of W …

Temperature dependence of slow-positron production and of positronium formation on untreated surfaces

1987

Low-energy positron emission from tungsten moderators, placed at a electron accelerator beam stop slows down with increasing moderator temperature. Efficient positronium formation is reported on untreated and unoriented metal surfaces at higher target temperatures.

Light transmittance of zirconia as a function of thickness and microhardness of resin cements under different thicknesses of zirconia

2012

Objective: The objective of this study was to compare microhardness of resin cements under different thicknesses of zirconia and the light transmittance of zirconia as a function of thickness. Study design: A total of 126 disc-shaped specimens (2 mm in height and 5 mm in diameter) were prepared from dual-cured resin cements (RelyX Unicem, Panavia F and Clearfil SA cement). Photoactivation was performed by using quartz tungsten halogen and light emitting diode light curing units under different thicknesses of zirconia. Then the specimens (n=7/per group) were stored in dry conditions in total dark at 37°C for 24 h. The Vicker's hardness test was performed on the resin cement layer with a micr…

Smart optically active VO2 nanostructured layers applied in roof-type ceramic tiles for energy efficiency

2016

Abstract The capability to control in an smart way the infrared reflectance to environmental temperature variations can be achieved with thermochromic materials like VO2. In this paper, we report the application of VO2 on ceramic tiles aiming to control the reflected infrared radiation on smart roofs and thus improving the energy efficiency, which is associated to the reduction of the carbon dioxide emissions. The VO2 nanoparticles have been produced by hydrothermal synthesis assisted by microwave irradiation, providing a new, quicker and cleaner production route. Afterwards, the VO2 nanoparticles were transferred to the surface of ceramic glassy tiles, which were prepared through dispersin…

Enhanced thermoelectric properties of the n-type Magnéli phase WO2.90: reduced thermal conductivity through microstructure engineering

2014

The thermoelectric properties of the Magneli phase WO2.90 were investigated, with special attention to how the thermoelectric performance can be altered by changing its microstructure. Spark plasma sintering (SPS) allowed the direct preparation of large amounts of consolidated material. Adding Ta2O5 to the reaction mixture lead to the formation of solid solutions W1−xTaxO2.90via a concurrent reaction between WO3 and Ta2O5 during the SPS treatment. In addition, micron-sized inclusions containing tungsten surrounded by WOx embedded in a WO2.90 matrix were formed, which act as additional scattering centers. As a result, the thermal conductivity of the Ta-containing samples was reduced by ≈30% …

Effects of pulse current mode on plasma electrolytic oxidation of 7075 Al in Na2WO4 containing solution: From unipolar to soft-sparking regime

2018

Abstract Plasma electrolytic oxidation coatings were produced on 7075 Al alloy in a silicate based solution containing sodium tungstate using unipolar, usual bipolar and soft-sparking bipolar pulsed current regimes. X-ray diffraction proved that the coatings contain gamma alumina and metallic tungsten. EDS results showed that the tungsten content decreases with increasing the negative half cycle. Scanning electron microscopy showed that the pancake and volcano-like were dominant morphologies for the coatings produced by the unipolar and bipolar current regimes, respectively. Under unipolar current regime, a band of large pores is observed at the metal/coating interface, while they became di…