Structural investigations of nitrided Nb2O5 and Nb2O5–SiO2 sol–gel derived films (original) (raw)
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XAFS investigations of nitrided NbN–SiO2 sol–gel derived films
Journal of Non-Crystalline Solids, 2012
This work presents the results of the structural analysis of xNbN-(100-x)SiO 2 (x = 100, 80, 60 mol%) thin films by X-ray absorption spectroscopy (XAS). To prepare the films, thermal nitridation of sol-gel derived coatings have been performed. The resulting films have a granular structure with NbN grains distributed in the SiO 2 matrix. The size of the grains depends on the NbN/SiO 2 molar ratio. A detailed X-ray absorption fine structure (XAFS) data analysis shows that in all the samples both nitrogen and oxygen atoms are present as nearest neighbours of Nb. The intra-granular phase is an ordered NbN phase, whereas the shells around the grains are formed mainly by an oxide phase and, possibly, by other niobium nitride phases (probably with low nitrogen content). Two possible origins of the inter-granular oxide phase were considered: incomplete nitridation of Nb 2 O 5 and addition of SiO 2 . Both of them are connected with the sample preparation method. The obtained XAS results allowed us to correlate the thickness and stoichiometry of the films under study with the electronic structure of the Nb ions and with the local geometric structure in their environment.
A multinuclear solid state NMR study of the sol–gel formation of amorphous Nb2O5–SiO2 materials
Solid State Nuclear Magnetic Resonance, 2005
Multinuclear 1 H, 13 C, 17 O, 29 Si MAS and 93 Nb static NMR is reported from a series of sol-gel prepared (Nb 2 O 5 ) x (SiO 2 ) 1Àx materials with x ¼ 0:03; 0.075 or 0.30. 13 C NMR shows that by 500 1C the organic precursor fragments have been removed although some residual carbon remains as a separate phase. The 29 Si NMR typically shows three Q-species (Q 2,3,4 ) in the initial gels, and that with increasing heat treatment the average n of the Q n -species increases as the organic fragments and hydroxyl groups are removed. O shows unequivocally that the x ¼ 0:03 and 0.075 samples are not phase separated, while at the much higher niobia-content of x ¼ 0:30 Nb-O-Nb signals are readily detected, a definite indication of the atomic scale phase separation of Nb 2 O 5 . The x ¼ 0:03 and 0.075 samples heated to 750 1C are thus representative of amorphous niobium silicates. Comparison is made to other sol-gel prepared metal silicates especially with another Group Va metal tantalum. The effects of tantalum and niobium on the silica network are very different and it is suggested here that most of the niobium is present as NbO 4 , forming part of the silicate network. r 2004 Elsevier Inc. All rights reserved.
SiO2Nb2O5 thin films using sol-gel technology
Journal of Non-Crystalline Solids, 1992
Xerogel and aerogel SiO2-Nb205 thin films were obtained and characterized by UV, visible and IR microscopy. The aerogel sheet shows a h-4 evolution close to monolithic one. Water and silanol contents are also analyzed for each type of thin sheets. Glass surface treatment before dip-coating was investigated using SEM, ellipsometry, reflectance measurement and contact angle. The best washing schedule was deduced. One to eight successive coatings were usually done and analyzed. Refractive indexes and thickness of the coatings are presented.
Sol–gel synthesis and characterization of Nb2O5 powders
Materials Letters, 2004
Nb 2 O 5 powders were prepared by the sol -gel procedure. Nb(OC 2 H 5 ) 5 dissolved in ethanol was (a) rapidly hydrolysed by adding a concentrated NH 4 OH solution, or (b) slowly hydrolysed by adding a small amount of H 2 O. The Nb 2 O 5 powders were characterized by XRD, DTA/TGA, FT-IR spectroscopy and SEM. The initial Nb 2 O 5 powders were amorphous. Upon heating at 300 jC of the amorphous powder obtained by the method (a) there was no change as shown by XRD, whereas the amorphous powder obtained by the method (b) and also heated at 300 jC contained Nb 2 O 5 (TT-phase) plus an amorphous fraction. The amorphous powders heated at 500 jC contained Nb 2 O 5 (TT-phase), whereas at 650 jC the Nb 2 O 5 (T-phase) was obtained. Crystallite sizes were determined using the Scherrer method. The agreggates of nanosized Nb 2 O 5 were present in amorphous Nb 2 O 5 precursors and upon their heating at 300 jC, whereas upon heating at 650 jC a porous interconnected microstructure was formed in both preparations, (a) and (b), as shown by SEM. D
Comparative investigation of NbN and Nb-Si-N films: Experiment and theory
Journal of Superhard Materials
NbN and Nb-SiN films have been deposited by magnetron sputtering of the Nb and Si tar gets on silicon wafers at various powers supplied to the Nb target. The films have been investigated by an atomic force microscope, X ray diffraction, X ray photoelectron spectroscopy, nanoindentaion and microindentation. The NbN films were nanostructured, and the Nb-SiN films represented an aggrega tion of δ NbN x nanocrystallites embedded into the amorphous Si 3 N 4 matrix (nc δ NbN x /a Si 3 N 4). The annealing of the films in vacuum showed that their intensive oxidation occurred at annealing temperature higher than 600°C. To explain the experimental results on the Nb-SiN films, first principles molecular dynamics simulations of the NbN(001)/Si 3 N 4 heterostructures have been carried out.
Chemistry of Materials, 2005
Supported niobium pentoxide materials have been effective catalysts for a variety of acid and redox reactions (e.g., dehydration of alcohols, esterifications, etc.). To the best of our knowledge, there are no reports about Nb 2 O 5 supported on silica-alumina. Catalysts of Nb 2 O 5 /SiO 2 -Al 2 O 3 were prepared with 2, 5, 10, 15, 20, and 25 wt % of Nb 2 O 5 by aqueous solution impregnation using ammonium niobium oxalate on silica-alumina. The materials, after being dried at 100°C, were calcined at 800°C and characterized by several methods. Investigation through X-ray diffraction showed the typical patterns of crystalline Nb 2 O 5 , which were composed of mixtures of orthorhombic and monoclinic phases (T, M, and H, respectively) present in the materials with content higher than 10 wt %. DTA curves displayed an exothermic peak at 1356°C (average) without mass loss (confirmed by TG), which may be ascribed to a phase transition (H phase formation) of Nb 2 O 5 supported on silica-alumina. Pure Nb 2 O 5 ‚nH 2 O showed a transition from amorphous to hexagonal or orthorhombic phase (TT or T, respectively) at 567°C. FTIR and DRIFTS results confirmed the reaction of the niobium oxide with the hydroxyl functionality of silica-alumina. The formation of surface niobium pentoxide species over the support through selective and progressive consumption of hydroxyl groups from the support and the appearance of characteristic niobium hydroxylated species on the surface were demonstrated. Raman spectra attested a two-dimensional overlayer of niobium pentoxide on silica-alumina at contents below 10 wt %. At higher concentrations, the absorptions characterize the formation of phases T and H of bulk Nb 2 O 5 . * Corresponding authors.
Effect of processing method on physical properties of Nb2O5
Journal of the European Ceramic Society, 2011
Samples of Nb 2 O 5 were prepared by laser floating zone (LFZ) technique and by solid-state reaction in order to study some of their physical properties as a function of synthesis conditions. Single crystals fibres were obtained by LFZ, while a structural orthorhombic to monoclinic phase transition was observed in samples sintered at temperature higher than 800 • C. Transmission optical spectroscopy and photoconductivity measurements allowed identifying a ∼3.2 eV bandgap energy for the H-Nb 2 O 5 monoclinic crystalline phase. Band gap shrinkage of ∼100 meV was observed from 14 K to RT. For the orthorhombic phase (T-Nb 2 O 5 ), the photoconductivity measurements evidence a higher energy bandgap. The sintered samples have shown a broad recombination luminescence band at the orange/red spectral region while no luminescence was detected from the LFZ grown fibres. A dielectric constant of ∼40 was found for the 800 • C and 1200 • C sintered pellets while that of 1000 • C reached four times that value.
Synthesis and characterization of niobium oxide layers on silica and the interaction with nickel
Applied Catalysis A: General, 2000
In present work, polyindole-Nb2O5 nanocomposite was synthesized and characterized by various analytical methods. Niobium oxide nanoparticles were prepared by sol gel method. Polyindole and Nb2O5 nanocomposites was prepared by chemical polymerization method and the morphology of Nb2O5 nanoparticles, polyindole and the nanocomposite was studied by SEM. The chemical structure of Nb2O5 nanoparticle, polyindole and the nanocomposite was characterized by UV-visible, FTIR and NMR spectroscopic techniques. Elemental composition and chemical character was analyzed by the use of EDAX technique. The X-ray powder diffraction technique was used to determine the degree of crystallinity and crystallite sizes. Thermal analysis such as TGA and DSC showed that the conducting polymers in the nanocomposites were stable even at high temperatures.