1,721,019 research outputs found
Sloshing Behaviors of Rectangular and Cylindrical Liquid Tanks Subjected to Harmonic and Seismic Excitations
No full textImaging the distribution of framework aluminum in mesoporous molecular sieve MCM-41
No full textThe distribution of framework aluminum inside micrometer-size particles of the mesoporous molecular sieve MCM-41 has been investigated using transmission electron microscopic (TEM) images, after supporting platinum clusters inside the aluminum-containing region selectively via the ion exchange of Pt(NH3)(4)(2+). The TEM images show that the framework aluminum becomes distributed homogeneously inside the resultant MCM-41 particles if sodium aluminate is added to the reaction mixture, consisting of an aqueous solution of hexadecyltrimethylammonium (HTA) chloride and sodium silicate, before the formation of the surfactant-silicate mesostructure. On the other hand, the framework aluminum can be incorporated into the outer region within MCM-41 particles if the aluminum source is added after completing the formation of the mesostructure. The thickness of the aluminum-containing region, which results from the aluminum diffusion into the surfactant-silicate mesostructure, can be controlled by the diffusion time and the concentration of the aluminum source. In addition, powder X-ray diffraction indicates that the structural order is unproved when the aluminum source is added after the formation of the surfactant-silicate mesostructure, rather than before
Block-copolymer-templated ordered mesoporous silica: Array of uniform mesopores or mesopore-micropore network?
No full textMicroporosity and connectivity of ordered mesopores of SEA-15 silica were studied using nitrogen adsorption and novel methods based on selective pore blocking via organosilane modification, and on the imaging of inverse platinum replica of ordered mesoporous structure. It was found that SEA-15 exhibits a relation between the pore size, pore volume and specific surface area which is significantly different from that for cylindrical or hexagonal pores, which suggests that the SEA-15 structure is more complex than an array of hexagonally ordered channels, even if they are corrugated. Nitrogen and argon adsorption measurements provided evidence that large mesopores are accompanied by a certain amount of significantly smaller pores (of the size below about 3.4 nm) with a broad distribution primarily in the micropore/small-mesopore range. The modification of SEA-15 via chemical bonding of small trimethylsilyl ligands partially blocked the complementary pores, and the bonding of larger octyldimethylsilyl groups made them essentially: fully inaccessible to nitrogen molecules, which manifested itself in dramatic changes in the relation between the pore size, pore volume, and specific surface area. After dissolution of the SEA-15 framework, platinum wires grown inside the porous structure formed bundles, as seen from transmission electron microscopy. These results provided strong and unambiguous evidence that large ordered mesopores of SEA-15 are accompanied by much smaller disordered pores and that an appreciable fraction of the latter is located in the pore walls, providing connectivity between the ordered large-pore channels. The complementary pores are suggested to form as'. result of penetration of poly(ethylene oxide) chains of the triblock copolymer template within the silica framework of as-synthesized SBA-15. We also studied thermal stability of SEA-15 structure and its complementary porosity. As inferred from nitrogen adsorption data, the complementary porosity was retained to a significant extent even after calcination at 1173 K, but most likely completely disappeared at 1273 K. The heat treatment was accompanied not only by a significant decrease in the specific surface area and pore volume but also by narrowing the pore size distribution at temperatures up to 1173 K. Thus, we were able to demonstrate for the first time that the SBA-15 sample with nitrogen adsorption properties similar to those of MCM-41 can be obtained via calcination at 1273 K, although the pore volume and specific surface area of such SEA-15 material is relatively low.The donors of the Petroleum Research
Fund administrated by the American Chemical Society are gratefully acknowledged for partial support of this research. One
of the Reviewers of this paper is acknowledged for his/her suggestion to study the thermal stability of SBA-15 structure
Mesocrystal engineering using non-bonded interaction to obtain optically transparent mesoporous silica films and plates with uniform orientation
No full textA sol-gel process which achieves self-organization between surfactant and silicate through van der Waals type, weak, multiple, non-bonded interaction has been developed in order to synthesize mesoporous silica in the form of optically transparent films, fibers and plates. The synthesis process consists of the following two steps: an organic silica precursor such as tetraethylorthosilicate is hydrolyzed and simultaneously condensed to a suitable degree of polymerization in non-aqueous solutions containing substoichiometric amounts of water, hydrochloric acid and surfactants. The solvent is subsequently evaporated so that the surfactant-silicate organization is achieved by the weak enthalpy effect without the interference of the mixing entropy effect. The synthesis can be controlled so that mesoporous channels with uniform diameter are either hexagonally packed, parallel to the flat external surface or randomly oriented, depending on the reactant ratios and degree of the silicate polymerization in the first step. The optically transparent mesoporous silica plates, which are crack-free up to centimeters in size and 0.5 mm in thickness, exhibit uniform birefringence throughout the entire plate like a single crystal. The mesoporous silica materials show possibilities of the application as advanced materials, direct measurement of transport properties through channels, investigation of the order-disorder effects, and spectroscopic investigation of adsorbed species without using the diffuse reflectance. (C) 1998 Elsevier Science B.V. All rights reserved
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe 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
Get PDF“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
Get PDFWe 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
Get PDFWe 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
- …
