1,721,123 research outputs found
Laser-Inducing Extreme Thermodynamic Conditions in Condensed Matter to Produce Nanomaterials for Catalysis and the Photocatalysis
No full textVaporization from the aluminum target surface, under nanosecond laser irradiation, was evaluated in the framework of the unsteady adiabatic expansion model, while the homogeneous nucleation of vapor bubbles in the metastable liquid (phase explosion) was simulated in the framework of the classical nucleation theory. The size distribution of the liquid nanodroplets produced in the phase explosion process was found to obey a power law in agreement with the few available experimental data when it is assumed that nanoparticles formation comes from solidification of liquid nanodroplets. Some experimental examples are reported to show that pulsed-laser deposition technique is able to synthesize nanoparticles in a single step with the required features for catalysis and photocatalysis applications
Development of Cobalt based Nanocatalysts for Energy and Environment
No full textThere is a rising concern about energy and environment for future. Transition from current fossil fuels to green fuels and building of cleaner environment to lead sustainable life is at enormous task. Hydrogen gas is recognized as a clean fuel and may be a sustainable solution. Hydrogen can be directly used as clean fuel in fuel cells with no harmful by-products. Chemical hydrides with high hydrogen storage capacity in terms of gravimetric and volumetric efficiencies are the most promising candidates to supply pure hydrogen at room temperature. Among them, Ammonia Borane (NH3BH3, AB) and Sodium borohydride (NaBH4, SBH) have drawn a lot of interest as they are stable, non-flammable, and nontoxic. Large amount of pure hydrogen gas is released during the hydrolysis of these hydrides in presence of certain catalysts and the by-products are non-toxic, environmentally safe and can be recycled. Co based catalysts are considered as good candidates for catalyzed hydrolysis owing to their good catalytic activity, low cost and effortless synthesis. In favor of environmental concern, especially the air pollution (conversion of CO to CO2) and water pollutions (organic pollutants) are vital problems and there is a serious need to mitigate these problems. Cobalt (Co) based materials are with high catalytic activity for hydrolysis, organic pollutants degradation and CO oxidation. So, a single Co based catalysts as powders and as immobilized coatings prepared by chemical reduction method and pulsed laser deposition (PLD) were studied for hydrogen production by hydrolysis of AB and SBH and thin film coatings Co3O4 were studied for CO oxidation and organic pollutants degradation. On the basis of characterization results, the role of catalyst to enhance catalytic activity is discussed in hydrolysis, CO oxidation and pollutants degradation reactions. The stability and re-usability of these catalysts have also been investigated
Synthesis and characterization of nanostructures for catalysis
No full textCatalysts are of great importance in many different fields, including the energy and the environmental sectors. It is important to produce them with simple preparation technique and to enhance the catalysts surface-to-volume ratio. The work undertaken in this thesis concerns the synthesis of nanostructures by Pulsed Laser Deposition (PLD) and R.F. sputtering deposition and the tailoring of their structures by varying deposition parameters.
We synthesized Cobalt oxide nanoparticles (NPs) by PLD and
studied the influence of the deposition parameters (i.e. substrate temperature, target-to-substrate distance and partial pressure of Oxygen in the chamber) on the final structure and crystalline phase of the NPs. The deposited NPs can be divided in two main categories: small NPs having a diameter of about 5 nm, and big NPs of size ranging from 50 to 400 nm. Depending on the value chosen for the deposition parameters, small NPs have CoO- or Co3O4 crystalline phase, and NPs can have a core/shell structure. The phase composition of the core and of the shell also vary according to the deposition conditions.
We synthesized thin film of Co-B NPs by PLD. Depending on the energy density, the laser process is able to produce well-dispersed spherical Co NPs partially embedded within B-based film matrix in a single-step deposition. The small size, the polycrystalline nature of Co NPs, and the presence of Boron matrix is important for catalytic performance of the Co-B film. The catalytic activity of the Co-B has been tested in hydrolysis of chemical hydrides (ammonia borane and sodium borohydride). PLD deposition of C-film, to serve as support for Co-B NPs, was performed at different Ar pressures (from 10 to 50 Pa) to tailor film roughness in order to investigate the role of porous and irregular C- surface on supporting Co-B NPs acting as catalysts. The measured hydrogen generation rate attained with C-supported Co-B catalyst film is higher than both unsupported-Co-B film and conventional Co-B powder.
Multilayer ITO/Cr-doped-TiO2 thin films have been synthesized by radiofrequency magnetron sputtering in order to sensitize TiO2 in visible light and to lower the charge recombination rate in the Cr-doped-TiO2. When the multilayer films were exposed to visible light, we observed that the photocurrent increases as function of the number of bilayers by reaching the maximum with 6-bilayers of ITO/Cr-doped- TiO2. The superior photocatalytic efficiency of the 6-bilayers film implies higher hydrogen production rate through water-splitting.
Spontaneous growth of Lead nanowires (NWs) have been observed in composite Al-Pb film deposited by R.F. sputtering deposition. The parameters of deposition and the storage of the Al-Pb films after deposition has been changed in order to understand the growth process of NWs. Evolution of NWs growth was also observed inside a SEM chamber. We propose that a stress-driven mechanism and the corrosion occurring on the films surface in environment atmosphere are the cause of NWs growth
Development of Solar Sensitive Thin Film for Water Splitting and Water Heating using Solar Concentrator
No full textPhotocatalytic water splitting using solar energy could contribute to the solution of environmental and energy issues related to the hydrogen production. Key research area in this field is the development of photo-catalyst able to provide high energy conversion efficiency. TiO2 has been mostly preferred material as the photo-electrode due to many advantages, mainly related to the cost factor and stability. We have studied on hydrogen production by water splitting in photo-electrochemical cells prepared by using photoanodes made by two different kinds of TiO2: one deposited by RF sputtering and the other one by sol-gel method. Depositions were performed on electrical conducting ITO whose electrical properties plays vital role to reduce the photon energy loss. The photoanodes have been characterised by several techniques to infer on their optical and compositional properties. The observed differences in hydrogen production have been attributed to the peculiarities in absorption properties of the two TiO2 films that in the case of sputter-deposited films are more prone to absorb radiation also because of the produced defects during the deposition process. Metals like Cr and Fe were doped in TiO2 by RF magnetron sputtering and sol-gel methods to increase the efficiency of hydrogen production by water splitting by sensitizing the doped-TiO2 in visible light spectrum. The doping method, dopant concentration, charge transfer from metal dopants to TiO2, and type of dopants used for modification of TiO2 were investigated for their ability to enhance photocatalytic activity. UV-Visible spectra show that the sputter-metaldoped- TiO2 films are much more efficient than the chemically-prepared samples to induce red shift of the absorption edge for absorbing visible light. In addition, we proved that dopant atoms must be located, at low concentration, near the ITO-TiO2 interface to avoid the formation of recombination centers for photo-generated electron-hole pairs. H2 production rate is higher with Fe-doped TiO2 (15.5 μmole/h) than with Cr-doped TiO2 (5.3 μmole/h) because Fe ions trap both electrons and holes thus avoiding recombination.
On the other hand, Cr can only trap one type of charge carrier. To increase the light conversion efficiency and reduce the recombination processes of Cr-doped TiO2, a multilayer structure of ITO/Cr-doped-TiO2 (9 at.%) was developed. When the multilayer films were exposed to visible light, we observed that the photocurrent increases as function of the number of bilayers by reaching the maximum with 6-bilayers of ITO/Crdoped-TiO2. The enhanced photocurrent is attributed to: 1) higher absorption of visible light by Cr-doped-TiO2, 2) number of space-charge layers in form of ITO/TiO2 interfaces in multilayer films, and 3) generation of photoelectrons just in/or near to the spacecharge layer by decreasing the Cr-doped-TiO2 layer thickness. The superior photocatalytic efficiency of the 6-bilayers film implies higher hydrogen production rate through water splitting: we obtained indeed 24.4 μmole/h of H2 production rate, a value about two times higher than that of pure TiO2 (12.5 μmole/h). Similar experiment we performed by doing TiO2 with vanadium metal. With 6-ilayers vanadium doped TiO2 film Shows higher hydrogen production rate of about 31.2 μmole/h. This rate is higher than that of CR doped and pure TiO2. A constant H2 generation rate is obtained for long periods of time by all the investigated TiO2 films because of the separate evolution of H2 and O2 gas, thus eliminating the back-reaction effect. Even Ar+ or N+ ion implantation of energy 30 keV was adopted to vary the energy band gap of TiO2 film in order to absorb visible light.The original anatase phase was not changed by implantation. Increase in full visible absorption range was observed for both kinds of ion implanted-TiO2 films which further increases with the ion fluencies, while N+ ion implantation also causes the shift of the absorption edge from UV to visible light range. N+ implanted TiO2 showed narrowing of band gap from 3.2 eV for untreated anatase TiO2 to 2.78 eV for maximum implantation dose. The Ar+ and N+ implantation creates oxygen vacancies related defect energy level in the band gap. In case of N+ implantation, nitrogen also substitutionally replaces the oxygen atoms thus forming an energy level just above the valence band which further interacts with O 2p states resulting in the narrowing of band gap.
The black solar absorber material develop over the copper target to absorb concentrated solar radiation and supply heat to the surrounding water. A black copper oxide layer was synthesized over copper substrate by using chemical oxidation treatment. We varied several treatment parameters and optimized the best condition to obtain a black textured layer which has the properties to absorb total solar radiation.
The untreated polished copper showed 50 to 60 % reflectance (R) (incidence angle of 15o) and this value decreases to almost zero for whole wavelength range after formation of black copper oxide. The percentage absorption decreases by negligible amount as the angle of incidence increases. The SEM images of the copper oxide layer at high magnification showed a nano-petal like structure which causes the surface texture effect for higher absorption where surface irregularities such as grooves and pores with dimensions similar to the wavelength of the incident radiation simply increase the solar absorptance by multiple reflections. Long time thermal stability and corrosion resistance in hot water was also studied for the copper oxide film. The results revealed that the copper oxide was very stable and showed no changes in optical properties after the test.
For the same water heating system a quartz window is used through which the solar radiation is transmitted on the copper target. Thus to acquire high power conversion efficiency it is necessary for quartz window to transmit the entire solar radiation incident on it without much lost due to the reflection on the surface. In general quartz window is able to transmit 90-91 % of the solar radiation while 1-2 % is absorbed and 7-8 % is reflected from the surface. Thus to have nearly complete transmittance it is necessary to cover the surface of quartz window with anti-reflecting (AR) coating: this was the part of my work. We developed single-layer and multi-layer AR coating for single specific wavelength and broad-band wavelength range respectively. Low reflective index material like MgF2 is deposited by e-beam technique to obtain single-layer AR coating. While Al2O3 and ZrO2 layers deposited, by RF-magnetron sputtering, on top of MgF2 forms multi-layer AR coating. The combination of MgF2/ZrO2/Al2O3/MgF2 deposited on both side of quartz showed excellent results with reflectance value of around 0.8% in broad spectral range.
The heat exchanger efficiency obtained after using these developed black copper oxide absorber material and AR coating is around 83 % which seems to be significantly higher than the other commercially available water heating system.
Concentrating solar power (CSP) systems are utilized to convert sunlight to thermal electric power by using solar absorber. However, the solar absorber are operated at elevated temperature (700-800 oC) and should be spectrally selective to act as perfect absorbers over the solar spectrum (high solar absorptance (α)) and perfect reflectors in the thermal infrared (IR) (low thermal emittance (ε)). Cermet composite solar absorber shows such selective properties at high temperatures. In the present work, we developed Al-AlN based multilayer cermet films by RF magnetron sputtering. We choose combination of Ni/AlxN(1-x)/AlN layers as a solar absorber due to its stability at elevated temperature and high corrosion resistance. In this combination, Ni layer, deposited near to substrate, act as the IR light reflector to provide high thermal emittance. While AlxN(1-x) layer act as an absorber layer for UV-Vis spectrum of solar radiation and transparent AlN layer on top functions as AR coating. To improve absorptance, 3 or 4 layers of AlxN(1-x) film with grading of metal content was synthesized by varying N2 flow during deposition. The optical measurement for these multilayer selective absorber films showed high solar absorptance of 0.92-0.96 and low thermal emittance of around 0.1-0.07. To test the stability of our multilayer coating at high temperature, we annealed these samples at 700 oC with holding time of 2 hrs in air, low vacuum and high vacuum. We observed a slight decrease in solar absorptance value (0.90) for the annealed samples but the results showed that overall performance was not hindered by heat treatment thus proving the thermal stability of our multilayer cermet coating
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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