1,720,972 research outputs found
Uniform fast growth of hydrotalcite-like compounds
No full textSmall size, monodispersed hexagonal particles of Mg,Al-CO3 hydrotalcite-like compounds have been synthesized in short periods of time by the microwave-hydrothermal method. An in-depth study of the influence of the irradiation time and the process temperature has been carried out. Well-crystalized materials with relatively high specific surface area values have been prepared in 30 min at 150 °C. © 2006 American Chemical Society
Microwaves and layered double hydroxides: A smooth understanding
No full textMicrowave-hydrothermal treatment (MWHT), a modification of conventionalhydrothermal treatment, has been used during post-treatment of different layered doublehydroxides (LDHs). In some cases, microwaves (MWs) have been used simultaneously withurea hydrolysis or for reconstruction of the LDH structure. The main advantages of replacingthe conventional furnaces by MW ovens are a noticeable reduction in the time required tocomplete the process to obtain well-crystallized materials, and modification of their particlesize distribution and textural and thermal properties. MW radiation leads to an increase inthe rate of urea hydrolysis and consequently to fast precipitation of LDHs. Finally, thememory effect of Ni,Al-based LDHs is also improved. © 2009 IUPAC
Microwave-treated layered double hydroxides containing Ni2+ and Al3+: The effect of added Zn2+
No full textNi containing layered double hydroxides (LDHs) have been prepared by precipitation and hydrothermally treated under microwave irradiation for different periods of time. The solids have been calcined at three temperatures corresponding to stable phases formed during thermal decomposition of LDHs. The properties of the irradiated samples and of the calcined products were studied in order to ascertain whether the ageing treatment under microwave irradiation modifies not only the properties of the layered materials, but also the properties of the calcined products. A structural and textural study was carried out by PXRD, FT-IR and Vis-UV spectroscopy, thermal analyses (DTA and TG), N2 adsorption/desorption at -196 °C and TEM microscopy; the reducibility of the nickel species was studied as well by TPR. The results show that the microwave treatment leads to better crystallized LDHs with modified thermal stability and reducibility. In addition, the degree of crystallinity of the layered precursors and their textural properties determine the properties of their thermal decomposition products. © 2006 Elsevier Inc. All rights reserved
Production of carbon nanotubes from methane. Use of Co-Zn-Al catalysts prepared by microwave-assisted synthesis
No full textCo-Zn-Al mixed oxides obtained from layered double hydroxide (LDH) precursors were synthesized and tested in the formation of multi-walled carbon nanotubes (MWNTs) by catalytic chemical vapour deposition (CCVD) of methane. The precursors were synthesized by co-precipitation from Co, Zn and Al nitrates with carbonate and submitted to microwave-hydrothermal treatment, in order to study the effect of time treatment on the catalyst performance. The microwave ageing treatment does not only play a role on some features of the precursors, such as crystallinity and textural properties, but also on the properties of the catalysts obtained by controlled calcination. The microwave-hydrothermal treatment affects to the distribution of the cations within the layers because of an improved ordering and this effect leads to better dispersed active species in the catalyst. In this sense, it was found that the aged catalysts reach better activity and stability levels during the methane decomposition reaction. Furthermore, the duration of microwave-hydrothermal treatment also produces a change of the kind of carbon nanofilaments produced. When the catalyst is not aged, herringbone carbon nanofibers (diameter ∼14 nm) were found. On the other hand, multi-walled carbon nanotubes (diameter ∼20-30 nm) could be observed in the sample after reaction with the 300 min aged catalyst. Finally, the application of a kinetic model based on the growing mechanism of nanocarbonaceous materials (NCMs), allows determining the influence of the microwave-hydrothermal treatment time on the kinetic parameters. © 2009 Elsevier B.V. All rights reserved
Incidence of microwave hydrothermal treatments on the crystallinity properties of hydrotalcite-like compounds
No full textLayered double hydroxides with different cations in the layers, MgAl, ZnAl and CoAl, have been prepared. The solids were obtained by the coprecipitation method and then they were submitted to the microwave radiation in order to improve the crystallinity and minimize the particle size of the solids. The samples were characterized by element chemical analysis, powder X-ray diffraction, FT-IR spectroscopy, particle size distribution, and transmission electron microscopy. © 2007 WILEY-VCH Verlag GmbH & Co. KGaA
Microwave-assisted homogeneous precipitation of hydrotalcites by urea hydrolysis
No full textThe use of urea as a precipitating agent in the synthesis of Ni-Al and Zn-Al layered double hydroxides having a hydrotalcite-like structure via a microwave-hydrothermal method is reported. For comparison purposes, the samples were also prepared by a conventional hydrothermal method. Ni-Al compounds with the hydrotalcite-like structure were obtained in shorter periods of time by the microwave method than by the conventional method, whereas when zinc cations were involved, no successful synthesis was achieved regardless of the method used. In order to find the best synthesis conditions for the Ni-Al solids, samples were submitted to microwave-hydrothermal treatment at different temperatures for increasing periods of time, and the structural, thermal, and textural properties of the synthesized materials were evaluated. All of the solids were fully characterized by chemical elemental analysis, powder X-ray diffraction (PXRD), FT-IR spectroscopy, and transmission and scanning electron microscopy as well as by N2 adsorption/desorption at -196°C for assessment of specific surface area and porosity. The PXRD patterns showed that the layered structure appeared after merely 10 min when the synthesis was carried out at 125°C; however, the FT-IR spectra showed the presence of some cyanate groups that were formed during urea hydrolysis and were quite difficult to remove completely. When the conventional hydrothermal treatment was used, longer periods of time were required in order to develop the hydrotalcite-like structure, but increasing the aging time improved the crystallinity of the compounds and yielded large particles. © 2008 American Chemical Society
Microwave-hydrothermally aged Zn,Al hydrotalcite-like compounds: Influence of the composition and the irradiation conditions
No full textZn,Al-CO3 compounds with the hydrotalcite-like (layered double hydroxide, LDH) structure were prepared by a co-precipitation method followed by hydrothermal treatment under microwave irradiation. The influence of the ageing treatment was studied in two series of samples with different Zn2+/Al3+ ratios, namely, 3/1 and 2/1. Moreover, the effects of the heating temperature and of the irradiation time were studied in order to select the optimum preparation conditions. The solids were fully characterised by powder X-ray diffraction (PXRD), 27Al MAS-NMR, FT-IR spectroscopy, thermal analyses (DTA and TG), N2 adsorption/desorption at -196 °C and TEM. The results showed that whatever the chemical composition of the starting mixture, HTlcs with Zn2+/Al3+ ratio equal to 2 were obtained, and it was impossible to overcome the ZnO segregation when the molar ratio was >2. These compounds were stable and their crystallinity could be quickly enhanced when the temperature treatment was 100 °C, whereas at 125 °C the ZnO segregation was not prevented. © 2007 Elsevier Inc. All rights reserved
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
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