906 resultados para Intermetallic compounds
Designing Novel Sulphate-based Ceramic Materials as Insertion Host Compounds for Secondary Batteries
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Rechargeable batteries have propelled the wireless revolution and automobiles market over the past 25 years. Developing better batteries with improved energy density demands unveiling of new cathode ceramic materials with suitable diffusion channels and open framework structure. In this pursuit of achieving higher energy density, one approach is to realize enhanced redox voltage of insertion of ceramic compounds. This can be accomplished by incorporating highly electronegative anions in the cathode ceramics. Building on this idea, recently various sulphate- based compounds have been reported as high voltage cathode materials. The current article highlights the use of sulphate (SO4) based cathodes to realize the highest ever Fe3+/Fe2+ redox potentials in Li-ion batteries (LiFeSO4F fluorosulphate: 3.9V vs Li/Li+) and Na-ion batteries (Na2Fe2(SO4)(3) polysulphate: 3.8V vs Na/Na+). These sulphate-based cathode ceramic compounds pave way for newer avenues to design better batteries for future applications.
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An efficient azidation of 1,3-dicarbonyl compounds led to tertiary azides in the presence of tetrabutylammonium iodide (TBAI). TBAI is used as a pre-catalyst along with aq. tert-butyl hydroperoxide (TBHP) as an oxidant in aqueous medium. This operationally simple, practical, mild and green method provides an opportunity to synthesize a variety of azidated -keto esters, amides, and ketones in good yields.
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We have carried out dielectric and transport measurements in NdFe1-xMnxO3 (0 <= x <= 1) series of compounds and studied the variation of activation energy due to a change in Mn concentration. Despite similar ionic radii in Mn3+ and Fe3+, large variation is observed in the lattice parameters and a crossover from dynamic to static Jahn-Teller distortion is discernible. The Fe/Mn-O-Fe/Mn bond angle on the ab plane shows an anomalous change with doping. With an increase in the Mn content, the bond angle decreases until x = 0.6; beyond this, it starts rising until x = 0.8 and again falls after that. A similar trend is observed in activation energies estimated from both transport and dielectric relaxation by assuming a small polaron hopping (SPH) model. Impedance spectroscopy measurements delineate grain and grain boundary contributions separately both of which follow the SPH model. Frequency variation of the dielectric constant is in agreement with the modified Debye law from which relaxation dispersion is estimated.
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Significant research has been pursued to develop solar selective metallic coatings using a variety of coating deposition techniques, with limited attempts to assess the properties of bulk metallic materials for solar energy applications. In developing bulk solar reflectors with good reflectance in the entire solar range, we report a new class of reflector materials based on Cu-Sn intermetallics with tailored substitution of aluminium or zinc. Our experimental results suggest that the arc melted-suction cast Cu (78.8 at%)-Al (21.2 at%) alloy with nanoscale surface roughness can exhibit a combination of 89% bulk specular reflectance and 83% bulk solar reflectance, together with a hardness of 2 GPa. We show that the present alloy design approach paves the way for further opportunities of tuning the spectral properties of this new class of solar reflector material. (C) 2016 Elsevier B.V. All rights reserved.
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This study reports the synthesis and photophysical properties of a star-shaped, novel, fluoranthene-tetraphenylethene (TFPE) conjugated luminogen, which exhibits aggregation-induced blue-shifted emission (AIBSE). The bulky fluoranthene units at the periphery prevent intramolecular rotation (IMR) of phenyl rings and induces a blueshift with enhanced emission. The AIBSE phenomenon was investigated by solvatochromic and temperature-dependent emission studies. Nanoaggregates of TFPE, formed by varying the water/THF ratio, were investigated by SEM and TEM and correlated with optical properties. The TFPE conjugate was found to be a promising fluorescent probe towards the detection of nitroaromatic compounds (NACs), especially for 2,4,6-trinitrophenol (PA) with high sensitivity and a high Stern-Volmer quenching constant. The study reveals that nanoaggregates of TFPE formed at 30 and 70% water in THF showed unprecedented sensitivity with detection limits of 0.8 and 0.5ppb, respectively. The nanoaggregates formed at water fractions of 30 and 70% exhibit high Stern-Volmer constants (K-sv=79998 and 51120m(-1), respectively) towards PA. Fluorescence quenching is ascribed to photoinduced electron transfer between TFPE and NACs with a static quenching mechanism. Test strips coated with TFPE luminogen demonstrate fast and ultra-low-level detection of PA for real-time field analysis.
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Motivated by recent experimental work, we use first-principles density functional theory methods to conduct an extensive search for low enthalpy structures of C$_6$Ca under pressure. As well as a range of buckled structures, which are energetically competitive over an intermediate range of pressures, we show that the high pressure system ($\gtrsim 18$ GPa) is unstable towards the formation of a novel class of layered structures, with the most stable compound involving carbon sheets containing five- and eight-membered rings. As well as discussing the energetics of the different classes of low enthalpy structures, we comment on the electronic structure of the high pressure compound and its implications for superconductivity.
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In order to improve the wear resistance of the gamma-TiAl intermetallic alloy, microstructure, room- and high-temperature (600 degrees C) wear behaviors of laser clad gamma/Cr7C3/TiC composite coatings with different constitution of NiCr-Cr3C2 precursor-mixed powders have been investigated by optical microscopy (OM), scanning electron microscopy (SEM), X-ray diffraction (XRD), energy-dispersive spectrometer (EDS), block-on-ring (room-temperature) and pin-on-disk (high-temperature) wear tests. The responding wear mechanisms are discussed in detail. Results show that microstructures of the laser clad composite coatings have non-equilibrium solidified microstructures consisting of primary hard Cr7C3 and TiC carbides and the inter-primary gamma/Cr7C3 eutectic matrix, about three to five times higher average microhardness compared with the TiAl alloy substrate. Higher wear resistance than the original TiAl alloy is achieved in the clad composite coatings under dry sliding wear conditions, which is closely related to the formation of non-equilibrium solidified reinforced Cr7C3 and TiC carbides and the positive contribution of the relatively ductile and tough gamma/Cr7C3 eutectics matrix and their stability under high-temperature exposure.
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By means of a surface plastic deformation method a nanocrystalline (NC) intermetallic compound was in situ synthesized on the surface layer of bulk zirconium (Zr). Hardened steel shots (composition: 1.0C, 1.5Cr, base Fe in wt.%) were used to conduct repetitive and multidirectional peening on the surface layer of Zr. The microstructure evolution of the surface layer was investigated by X-ray diffraction and scanning and transmission electron microscopy observations. The NC intermetallic layer of about 25 gm thick was observed and confirmed by concentration profiles of Zr, Fe and Cr, and was found to consist of the Fe100-xCrx compound with an average grain size of 22 nm. The NC surface layer exhibited an extremely high average hardness of 10.2 GPa. The Zr base immediately next to the compound/Zr interface has a grain size of similar to 250 nm, and a hardness of similar to 3.4 GPa. The Fe100-xCrx layer was found to securely adhere to the Zr base. (c) 2007 Elsevier B.V All rights reserved.
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The NiAl intermetallic layers and NiAl matrix composite layers with TiC particulate reinforcement were successfully synthesized by laser cladding with coaxial powder feeding of Ni/Al clad powder and Ni/Al + TiC powder mixture, respectively. With optimized processing parameters and powder mixture compositions, the synthesized layers were free of cracks and metallurgical bond with the substrate. The microstructure of the laser-synthesized layers was composed of 6-NiAl phase and a few gamma phases for NiAl intermetallic; unmelted TiC, dispersive fine precipitated TiC particles and refined beta-NiAl phase matrix for TiC reinforced NiAl intermetallic composite. The average microhardness was 355 HV0.1 and 538 HV0.1, respectively. Laser synthesizing and direct metal depositing offer promising approaches for producing NiAl intermetallic and TiC-reinforced NiAl metal matrix composite coatings and for fabricating NiAl intermetallic bulk structure. (C) 2004 Laser Institute of America.
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Nanocrystalline intermetallic Co3Fe7 was produced on the surface of cobalt via surface mechanical attrition (SMA). Deformationinduced diffusion entailed the formation of a series of solid solutions. Phase transitions occurred depending on the atomic fraction of Fe in the surface solid solutions: from hexagonal close-packed (<4% Fe) to face-centered cubic (fcc) (4-11% Fe), and from fcc to body-centered cubic (>11% Fe). Nanoscale compositional probing suggested significantly higher Fe contents at grain boundaries and triple junctions than grain interiors. Short-circuit diffusion along grain boundaries and triple junctions dominate in the nanocrystalline intermetallic compound. Stacking faults contribute significantly to diffusion. Diffusion enhancement due to high-rate deformation in SMA was analyzed by regarding dislocations as solute-pumping channels, and the creation of excess vacancies. Non-equilibrium, atomic level alloying can then be ascribed to deformation-induced intermixing of constituent species. The formation mechanism of nanocrystalline intermetallic grains on the SMA surface can be thought of as a consequence of numerous nucleation events and limited growth. (C) 2007 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
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Salvinia (Salvinia minima Willd.) is a water fern found in Florida waters, usually associated with Lemna and other small free-floating species. Due to its buoyancy and mat-forming abilities, it is spread by moving waters. In 1994, salvinia was reported to be present in 247 water bodies in the state (out of 451 surveyed public waters, Schardt 1997). It is a small, rapidly growing species that can become a nuisance due to its explosive growth rates and its ability to shade underwater life (Oliver 1993). Any efforts toward management of salvinia populations must consider that, in reasonable amounts, its presence is desirable since it plays an important role in the overall ecosystem balance. New management alternatives need to be explored besides the conventional herbicide treatments; for example, it has been shown that the growth of S. molesta can be inhibited by extracts of the tropical weed parthenium (Parthenium hysterophorus) and its purified toxin parthenin (Pande 1994, 1996). We believe that cattail, Typha spp. may be a candidate for control of S. minima infestations. Cattail is an aggressive aquatic plant, and has the ability to expand over areas that weren't previously occupied by other species (Gallardo et al. 1998a and references cited there). In South Florida, T. domingensis is a natural component of the Everglades ecosystem, but in many cases it has become the dominant marsh species, outcompeting other native plants. In Florida public waters, this cattail species is the most dominant emergent species of aquatic plants (Schardt 1997). Several factors enable it to accomplish opportunistic expansion, including size, growth habits, adaptability to changes in the surroundings, and the release of compounds that can prevent the growth and development of other species. We have been concerned in the past with the inhibitory effects of the T. domingensis extracts, and the phenolic compounds mentioned before, towards the growth and propagation of S. minima (Gallardo et al. 1998b). This investigation deals with the impact of cattail materials on the rates of oxygen production of salvinia, as determined through a series of Warburg experiments (Martin et al. 1987, Prindle and Martin 1996).
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The surface mechanical attrition treatment (SMAT) technique was developed to synthesize a nanocrystalline (NC) layer on the surface of metallic materials for upgrading their overall properties and performance. In this paper, by means of SMAT to a pure zirconium plate at the room temperature, repetitive multidirectional peening of steel shots (composition (wt%): 1C, 1.5Cr, base Fe) severely deformed the surface layer. A NC surface layer consisting of the intermetallic compound FeCr was fabricated on the surface of the zirconium. The microstructure characterization of the surface layer was performed by using X-ray diffraction analysis, optical microscopy, scanning and transmission electron microscopy observations. The NC surface layer was about 25 mu m thick and consisted of the intermetallic compound FeCr with an average grain size of 25 +/- 10 nm. The deformation-induced fast diffusion of Fe and Cr from the steel shots into Zr occurred during SMAT, leading to the formation of intermetallic compound. In addition, the NC surface layer exhibited an ultrahigh nanohardness of 10.2 GPa.
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The effects of constitution of precursor mixed powders and scan speed on microstructure and wear properties were designed and investigated during laser clad gamma/Cr7C3/TiC composite coatings on gamma-TiAl intermetallic alloy substrates with NiCr-Cr3C2 precursor mixed powders. The results indicate that both the constitution of the precursor mixed powders and the beam scan rate have remarkable influence on microstructure and attendant hardness as well as wear resistance of the formed composite coatings. The wear mechanisms of the original TiAl alloy and laser clad composite coatings were investigated. The composite coating with an optimum compromise between constitution of NiCr-Cr3C2 precursor mixed powders as well as being processed under moderate scan speed exhibits the best wear resistance under dry sliding wear test conditions. (C) 2008 Elsevier Ltd. All rights reserved.