Search results for "GST"
showing 10 items of 874 documents
Thermal stability and enhanced thermoelectric properties of the tetragonal tungsten bronzes Nb8-xW9+xO47 (0 <x <5)
2017
Thermoelectric materials are believed to play a fundamental role in the energy field over the next years thanks to their ability of directly converting heat into usable electric energy. To increase their integration in the commercial markets, improvements of the efficiencies are needed. At the same time, cheap and non-toxic materials are required along with easily upscalable production cycles. Compounds of the tetragonal tungsten bronze (TTB) series Nb8-xW9+xO47 fulfill all these requirements and are promising materials. Their adaptive structure ensures glass-like values of the thermal conductivity, and the substitution on the cation side allows a controlled manipulation of the electronic p…
Improvement in the performance of low temperature H2-O2 fuel cell with chitosanephosphotungstic acid composite membranes
2016
Abstract Free-standing chitosan/phosphotungstic acid polyelectrolyte membranes, prepared by ionotropic gelation on alumina porous supports, were employed as proton conductor in low temperature H 2 –O 2 fuel cell. A drying step on glass substrate was introduced in the fabrication procedure to reduce shrinkage and consequent corrugation. Membranes were tested with electrodes prepared according to different procedures and with two different Pt loadings, namely 0.5 and 1 mg cm −2 . Both the investigated kinds of electrodes allowed to get very promising power peaks of 550 mW cm −2 in spite of the different Pt content. The polarization curves and the electrochemical impedance spectra suggest that…
Molecular dynamics and reverse Monte Carlo modeling of scheelite-type AWO4(A = Ca, Sr, Ba) WL3-edge EXAFS spectra
2016
Classical molecular dynamics (MD) and reverse Monte Carlo methods coupled with ab initio multiple-scattering extended x-ray absorption fine structure (EXAFS) calculations were used for modeling of scheelite-type AWO4 (A = Ca, Sr, Ba) W L 3-edge EXAFS spectra. The two theoretical approaches are complementary and allowed us to perform analysis of full EXAFS spectra. Both methods reproduce well the structure and dynamics of tungstates in the outer coordination shells, however the classical MD simulations underestimate the W–O bond MSRD due to a neglect of quantum zero-point-motion. The thermal vibration amplitudes, correlation effects and anisotropy of the tungstate structure were also estimat…
Local structure and vibrational dynamics in NiWO4
2001
Abstract Systematic studies of nickel tungstate thin film, amorphous and polycrystalline powders were performed by X-ray absorption spectroscopy at the Ni K and W L1,3 edges, X-ray diffraction and Raman spectroscopy. We found that in spite of the similarity of the local environment around nickel and tungsten ions in all three materials, there is strong difference in the Ni-O and W-O interactions for thin film/amorphous powder and polycrystalline powder. The nickel-oxygen bonding becomes stronger by going from thin film or amorphous powder to polycrystalline tungstate at the expense of the tungsten-oxygen bonding strength. Besides, in thin film and amorphous NiWO4, nickel and tungsten ions h…
Analysis of extended x-ray absorption fine structure data from copper tungstate by the reverse Monte Carlo method
2014
The static disorder and lattice dynamics of crystalline materials can be efficiently studied using reverse Monte Carlo simulations of extended x-ray absorption fine structure spectra (EXAFS). In this work we demonstrate the potentiality of this method on an example of copper tungstate CuWO4. The simultaneous analysis of the Cu K and W L3 edges EXAFS spectra allowed us to follow local structure distortion as a function of temperature.
Performance of H2-fed fuel cell with chitosan/silicotungstic acid membrane as proton conductor
2020
Composite organic–inorganic proton exchange membranes for H2–O2 fuel cells were fabricated by ionotropic gelation process combining a biopolymer (chitosan) with a heteropolyacid (silicotungstic acid). According to scanning electron microscopy analysis, compact, homogeneous and free-standing thin layers were synthesized. X-ray diffraction proved the crystallinity of the fabricated membranes and showed the presence of Chitosan Form I polymorph soon after the reticulation step and of the Form II polymorph after the functionalization step. Fourier-transform infrared spectroscopy demonstrated that the Keggin structure of the heteropolyacid is maintained inside the membrane even after the fabrica…
Evaluation of surface roughness of enamel after various bonding and clean-up procedures on enamel bonded with three different bonding agents : an in-…
2016
Background The purpose of this study was to analyze and compare the enamel surface roughness before bonding and after debonding, to find correlation between the adhesive remnant index and its effect on enamel surface roughness and to evaluate which clean-up method is most efficient to provide a smoother enamel surface. Material and methods 135 premolars were divided into 3 groups containing 45 premolars in each group. Group I was bonded by using moisture insensitive primer, Group II by using conventional orthodontic adhesive and Group III by using self-etching primer. Each group was divided into 3 sub-groups on the basis of type of clean-up method applied i,e scaling followed by polishing, …
Evidence of hexagonal WO3 structure stabilization on mica substrate
2009
International audience; WO3 nanorods are grown by a simple vapor deposition method on a mica substrate and characterized by Selected Area Electron Diffraction and Energy Dispersive X-rays Spectroscopy. Experimental results show the clear evidence of an unexpected WO3 hexagonal structure as well as an epitaxial growth on the mica substrate. Besides, potassium is evidenced inside the nanorods. It is thus deduced that a metastable WO3 hexagonal phase is stabilized by epitaxy through a tungsten bronze interlayer having same hexagonal structure.
Phosphomolybdic acid and mixed phosphotungstic/phosphomolybdic acid chitosan membranes as polymer electrolyte for H2/O2 fuel cells
2017
Abstract Flat, free-standing phosphomolybdic acid and mixed phosphotungstic/phosphomolybdic acid chitosan membranes were prepared by in-situ ionotropic gelation process at room temperature on porous alumina support firstly impregnated by heteropolyacid. Scanning electron microscopy revealed the formation of compact and homogenous polymeric membranes, whose thickness resulted to be dependent on reticulation time, and almost independent on the employed heteropolyacid nature and concentration. X-ray diffraction and Fourier transform infrared spectroscopy evidenced the formation of crystalline membranes without appreciable concentration of unprotonated NH 2 groups and heteropolyacid ions with p…
Nanoscale membrane electrode assemblies based on porous anodic alumina for hydrogen–oxygen fuel cell
2007
In this paper, we demonstrate that nanoscale membrane electrode assemblies, functioning in a H 2/O 2 fuel cell, can be fabricated by impregnation of anodic alumina porous membranes with Nafion® and phosphotungstic acid. Porous anodic alumina is potentially a promising material for thin-film micro power sources because of its ability to be manipulated in micro-machining operations. Alumina membranes (Whatman, 50 μm thick, and pore diameters of 200 nm) impregnated with the proton conductor were characterized by means of scanning electron microscopy, X-ray diffraction, and thermal analysis. The electrochemical characterization of the membrane electrode assemblies was carried out by recording t…