916 resultados para thin-layer chromatography
Resumo:
The air-exposed surfaces of sintered and are-melted UC samples were examined by XPS and SIMS. XPS results indicate that the surface is covered with a very thin layer of UO2 mixed with free carbon, which would have formed along with the oxide during the reaction between UC and oxygen or moisture. From the SIMS depth profile of oxygen, the thickness of the oxide layer is found to be approximately 10 nm. The SIMS oxygen images of the surface as a function of etching time reveal that the surface of UC consists of a top layer of adsorbed moisture/oxygen; this contamination layer is followed by a layer containing uranium oxide, uranium hydroxide and free carbon and then grain boundary oxide and finally bulk UC. The behaviour of sintered and are-melted samples is similar.
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Nanoclusters of bimetallic Pt-Ru are electrochemically deposited on conductive polymer, poly(3,4-ethylenedioxythiophene)(PEDOT), which is also electrochemically deposited on a carbon paper substrate. The bimetallic deposition is carried out in an acidic electrolyte consisting of chloroplatinic acid and ruthenium chloride at 0.0 V versus saturated calomel electrode (SCE) on PEDOT coated carbon paper. A thin layer PEDOT on a carbon paper substrate facilitates the formation of uniform, well-dispersed, nano clusters of Pt-Ru of mean diameter of 123 nm, which consist of nanosize particles. In the absence of PEDOT, the size of the clusters is about 251 nm, which are unevenly distributed on carbon paper substrate. Cyclic voltammetry studies suggest that peak currents of methanol oxidation are several times greater on PtRu-PEDOT electrode than on Pt-Ru electrode in the absence of PEDOT. (C) 2011 Elsevier B.V. All rights reserved.
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Functionalized multiwalled carbon nanotubes (CNTs) are coated with a 4-5 nm thin layer of V(2)O(5) by controlled hydrolysis of vanadium alkoxide. The resulting V(2)O(5)/CNT composite has been investigated for electrochemical activity with lithium ion, and the capacity value shows both faradaic and capacitive (nonfaradaic) contributions. At high rate (1 C), the capacitive behavior dominates the intercalation as 2/3 of the overall capacity value out of 2700 C/g is capacitive, while the remaining is due to Li-ion intercalation. These numbers are in agreement with the Trasatti plots and are corroborated by X-ray photoelectron spectroscopy (XPS) studies on the V(2)O(5)/CNTs electrode, which show 85% of vanadium in the +4 oxidation state after the discharge at 1 C rate. The cumulative high-capacity value is attributed to the unique property of the nano V(2)O(5)/CNTs composite, which provides a short diffusion path for Lit-ions and an easy access to vanadium redox centers besides the high conductivity of CNTs. The composite architecture exhibits both high power density and high energy density, stressing the benefits of using carbon substrates to design high performance supercapacitor electrodes.
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The open circuit potentials of the galvanic cell,Pt (or Au)¦(Ar + H2S + H2)primeparCaS + ZrO2(CaO)par (Ar + H2S+ H2)Prime£t (or Au) has been measured in the temperature range 1000 to 1660 K and PH2S:PH 2 ratios from 1.73×10–5 to 2.65×10–1. The solid electrolyte consists of a dispersion of calcium sulphide in a matrix of calcia-stabilized zirconia. The surface of the electrolyte is coated with a thin layer of calcium sulphide to prevent the formation of water vapour by reaction of hydrogen sulphide with calcium oxide or zirconia present in the electrolyte. The use of a lsquopoint electrodersquo with a catalytically active tip was necessary to obtain steady emfs. At low temperatures and high sulphur potentials the emfs agreed with the Nernst equation. Deviations were observed at high temperatures and low sulphur potentials, probably due to the onset of significant electronic conduction in the oxide matrix of the electrolyte. The values of oxygen and sulphur potentials at which the electronic conductivity is equal to ionic conductivity in the two-phase electrolyte have been evaluated from the emf response of the cell. The sulphide-oxide electrolyte is unsuitable for sulphur potential measurements in atmospheres with high oxygen potentials, where oxidation of calcium sulphide may be expected.
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Through this paper we experimentally demonstrate the fabrication of a fiber Bragg grating (FBG) chemical sensor to detect and determine the manganese concentration in water and compare our results with sophisticated spectroscopic methods, such as atomic absorption spectrometry and the inductively coupled plasma method. Here we propose a simple method to develop a thin layer of gold nanoparticles above the etched grating region to enhance the sensitivity of the reflected spectrum of the FBG. By doing so, we achieve a sensitivity of 1.26 nm/parts per million in determining the trace level of Mn in water. Proper reagents are used to detect manganese in water. (C) 2011 Optical Society of America
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The relations for the growth and consumption rates of a layer with finite thickness as an end member and the product phases in the interdiffusion zone are developed. We have used two different methodologies, the diffusion based and the physico-chemical approach to develop the same relations. We have shown that the diffusion based approach is rather straightforward; however, the physico-chemical approach is much more versatile than the other method. It was found that the position of the marker plane becomes vague in the second stage of the interdiffusion process in pure A thin layer/B couple, where two phases grow simultaneously.
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Reduced graphene oxide-lead dioxide composite is formed when EGO coated surface is electrochemically reduced along with lead ions in the solution. This composite has been shown to be an excellent material for low level detection of arsenic. Various functional groups present on EGO, in a wide pH range of 2-11, are responsible for the favorable interaction between metal ion and the modified electrode surface and subsequent trace level detection. X-ray photoelectron spectroscopy and Raman spectroscopic techniques confirm the formation of composite and its composition. Thin layer of lead dioxide along with reduced exfoliated graphene oxide has been shown to be responsible for the enhanced activity of the surface. The detection limit of arsenic is found to be 10 nM. This study opens up the possibility of using the composites for sensing applications and possibly simultaneous detection of arsenic and lead. (C) 2011 Elsevier B.V. All rights reserved.
Resumo:
In this paper, the low temperature electrical conductivity and microwave absorption properties of carbon coated iron nanoparticles-polyvinyl chloride composite films are investigated for different filler fractions. The filler particles are prepared by the pyrolysis of ferrocene at 980 degrees C and embedded in polyvinyl chloride matrix. The high resolution transmission electron micrographs of the filler material have shown a 5 nm thin layer graphitic carbon covering over iron particles. The room temperature electrical conductivity of the composite film changes by 10 orders of magnitude with the increase of filler concentration. A percolation threshold of 2.2 and an electromagnetic interference shielding efficiency (EMI SE) of similar to 18.6 dB in 26.5-40 GHz range are observed for 50 wt% loading. The charge transport follows three dimensional variable range hopping conduction. (C) 2012 Elsevier Ltd. All rights reserved.
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Herein we report a novel fluoranthene-based fluorescent fluorophore 7,10-bis(4-bromophenyl)-8,9-bis4-(hexyloxy)phenyl]fluoranthene (S-3) and its remarkable properties in applications of explosive detection. The sensitivity towards the detection of nitroaromatics (NACs) was evaluated through fluorescence quenching in solution, vapor, and contact mode approaches. The contact mode approach using thin-layer silica chromatograp- hic plates exhibited a femtogram (1.15 fg cm(-2)) detection limit for trinitrotoluene (TNT) and picric acid (PA), whereas the solution-phase quenching showed PA detection at the 2-20 ppb level. Fluorescence lifetime measurements revealed that the quenching is static in nature and the quenching process is fully reversible. Binding energies between model binding sites of the S-3 and analyte compounds reveal that analyte molecules enter into the cavity created by substituted phenyl rings of fluoranthene and are stabilized by strong intermolecular interactions with alkyl chains. It is anticipated that the sensor S-3 could be a promising material for the construction of portable optical devices for the detection of onsite explosive nitroaromatics.
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A series of fluoranthene derivatives (F1-F5) varied with nature and type of substituents were synthesized via Diels-Alder reaction followed by in situ decarbonylation. The solid state structures have been established through single crystal X-ray diffraction (XRD). The presence of extended conjugation and having two alkyloxy chains on phenyl rings induces flexibility to orient opposite to each other and interacts with another fluoranthene unit with weak pi-pi interactions and show unique supramolecular arrangements. The envisaged photophysical and DFT studies demonstrated that HOMO-LUMO levels were effectively tuned by different substituents with an optical band gap from 3.44 to 3.88 eV provoked to examine as sensitive fluorescent chemosensors for the detection of nitroaromatic compounds (NACs). The sensitivity toward the detection of NACs was evaluated through fluorescence quenching in solution (aqueous and non-aqueous) and solid state (vapor and contact mode). Fluorescence studies demonstrated that electron transfer occurs from the electron rich fluoranthene fluorophores to the electron deficient NACs by the dominant static quenching mechanism and the quenching process is reversible. It was found that the detection sensitivity increases with extent of conjugation on fluoranthene unit. The contact mode approach using thin layer silica chromatographic plates exhibits a femtogram (1.15 fg/cm(2)) detection limit for trinitrotoluene (TNT) and picric acid (PA), while the solution state fluorescence quenching shows for PA detection at the 2-20 ppb level. The sensing performance of fluoranthene thin films to NACs in aqueous solution reveals that fluorophores are highly selective towards the detection of PA. The smart performances of thin film fluorophores with high photostability have great advantage than those of conjugated polymers with superior sensitive detection of PA in groundwater.
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Micro-blast waves emerging from the open end of a detonation transmission tube were experimentally visualized in this study. A commercially available detonation transmission tube was used (Nonel tube, M/s Dyno Nobel, Sweden), which is a small diameter tube coated with a thin layer of explosive mixture (HMX traces of Al) on its inner side. The typical explosive loading for this tube is of the order of 18 mg/m of tube length. The blast wave was visualized using a high speed digital camera (frame rate 1 MHz) to acquire time-resolved schlieren images of the resulting flow field. The visualization studies were complemented by computational fluid dynamic simulations. An analysis of the schlieren images showed that although the blast wave appears to be spherical, it propagates faster along the tube axis than along a direction perpendicular to the tube axis. Additionally, CFD analysis revealed the presence of a barrel shock and Mach disc, showing structures that are typical of an underexpanded jet. A theory in use for centered large-scale explosions of intermediate strength gave good agreement with the blast trajectory along the tube axis. The energy of these micro-blast waves was found to be J and the average TNT equivalent was found to be . The repeatability in generating these micro-blast waves using the Nonel tube was very good and this opens up the possibility of using this device for studying some of the phenomena associated with muzzle blasts in the near future.
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In this paper we report a novel hydrogel functionalized optical Fiber Bragg Grating (FBG) sensor based on chemo-mechanical-optical sensing, and demonstrate its specific application in pH activated process monitoring. The sensing mechanism is based on the stress due to ion diffusion and polymer phase transition which produce strain in the FBG. This results in shift in the Bragg wavelength which is detected by an interrogator system. A simple dip coating method to coat a thin layer of hydrogel on the FBG has been established. The gel consists of sodium alginate and calcium chloride. Gel formation is observed in real-time by continuously monitoring the Bragg wavelength shift. We have demonstrated pH sensing in the range of pH of 2 to 10. Another interesting phenomenon is observed by swelling and deswelling of FBG functionalized with hydrogel by a sequence of alternate dipping between acidic and base solutions. It is observed that the Bragg wavelength undergoes reversible and repeatable pH dependent switching.
Resumo:
A cost-effective 12 V substrate-integrated lead-carbon hybrid ultracapacitor is developed and performance tested. These hybrid ultracapacitors employ flexible-graphite sheets as negative plate current-collectors that are coated amperometrically with a thin layer of conducting polymer, namely poly-aniline to provide good adhesivity to activated-carbon layer. The positive plate of the hybrid ultracapacitors comprise conventional lead-sheet that is converted electrochemically into a substrate-integrated lead-dioxide electrode. 12 V substrate-integrated lead-carbon hybrid ultracapacitors both in absorbent-glass-mat and polymeric silica-gel electrolyte configurations are fabricated and characterized. It is possible to realize 12 V configurations with capacitance values of similar to 200 F and similar to 300 F, energy densities of similar to 1.9 Wh kg(-1) and similar to 2.5 Wh kg(-1) and power densities of similar to 2 kW kg(-1) and similar to 0.8 kW kg(-1), respectively, having faradaic-efficiency values of similar to 90 % with cycle-life in excess of 100,000 cycles. The effective cost of the mentioned hybrid ultracapacitors is estimated to be about similar to 4 US$/Wh as compared to similar to 20 US$/Wh for commercially available ultracapacitors.
Resumo:
This study investigated the influence of soil properties on the density and shape of epigeous fungus-growing termite nests in a dry deciduous forest in Karnataka, India. In this environment, Odontotermes obesus produces cathedral shaped mounds. Their density, shape (height and volume) and soil physicochemical properties were analyzed in ferralsol and vertisol environments. No significant difference was observed in O. obesus mound density (n = 2.7 mound ha(-1) on average in the vertisol and ferralsol areas). This study also showed that O. obesus has a limited effect on soil physical properties. No differences in soil particle size, pH, or the C:N ratio and base saturation were measured whereas the C and N contents were reduced and CEC was higher in termite nest soils in both environments. Clay mineralogical composition was also measured, and showed the presence of higher amounts of smectite clays in termite nest soils, which thus explained the increasing CEC despite the reduced C and N content. However, the main difference was the shape of the termite mounds. The degradation of the nests created a hillock of eroded soil at the base of termite mounds in the vertisol while only a thin layer of eroded soil was observed in the ferralsol. The increased degradation of termite mounds in the vertisol is explained by the presence of smectites (2:1 swelling clays), which confer macroscopic swelling and shrinking characteristics to the soil. Soil shrinkage during the dry season leads to the formation of deep cracks in the termite mounds that allow rain to rapidly penetrate inside the mound wall and then breakdown unstable aggregates. In conclusion, it appears that despite a similar abundance, termite mound properties depend to a large extent on the soil properties of their environments. (C) 2015 Elsevier B.V. All rights reserved.
Resumo:
A gray, fine-grained arkosic sandstone tablet bearing an inscription in ancient Hebrew from the First Temple Period contains a rich assemblage of particles accumulated in the covering patina. Two types of patina cover the tablet: a thin layer of black to orange iron-oxide-rich layer, a product of micro-biogenic processes, and a light beige patina that contains feldspars, carbonate, iron oxide, subangular quartz grains, carbon ash particles and gold globules (1 to 4 microns [1 micron = 0.001 millimeter] in diameter). The patina covers the rock surface as well as the engraved lettering grooves and blankets and thus post-dates the incised inscription as well as a crack that runs across the stone and several of the engraved letters. Radiocarbon analyses of the carbon particles in the patina yield a calibrated radiocarbon age of 2340 to 2150 Cal BP. The presence of microcolonial fungi and associated pitting in the patina indicates slow growth over many years. The occurrence of pure gold globules and carbon ash particles is evidence of a thermal event in close proximity to the tablet (above 1000 degrees Celsius). This study supports the antiquity of the patina, which in turn, strengthens the contention that the inscription is authentic.