1,720,959 research outputs found
Etching of SiOC ceramic foams
No full textPolymer derived SiOC microcellular foams were etched by means of a 20 vol.-%HF solution. An increase in one order of magnitude in specific surface area (SSA) values compared to the unetched samples was observed. This SSA increase was accompanied by micro- and mesopores formation. The limited increase in the specific surface area was attributed to several factors: the smaller dimension of the etchable SiO2 nanodomains and the more amorphous nanostructure of these polymer derived ceramics compared to the sol–gel derived ones. Moreover a possible role of the carbon residue deriving from polymethylmetacrylate (PMMA) microbeads used as porosity source was supposed. Higher SSA values (up to 65 m2 g−1) were reached by inducing a slight phase separation, accompanied by a growth of the nanodomains size, and by an oxidative treatment that partly removed the residual carbon. Microcellular ceramic foams with a bimodal pore size distribution were produced
Thermal Annealing and Phase Transformation of Serpentine-Like Garnierite
No full textThis study is focused on the vibrational and microstructural aspects of the thermally induced transformation of serpentine-like garnierite into quartz, forsterite, and enstatite occurring at about 620 degrees C. Powder specimens of garnierite were annealed in static air between room temperature and 1000 degrees C. The kinetic of the transformation was investigated by means of thermogravimetric and differential thermal analysis, and the final product was extensively characterized via micro-Raman spectroscopy and X-ray diffraction. Our study shows that serpentine-like garnierite consists of a mixture of different mineral species. Furthermore, these garnierites and their composition can provide details based on the mineralogy and the crystalline phases resulting from the thermal treatment
Polymer-Derived Ceramics: 40 Years of Research and Innovation in Advanced Ceramics
No full textPreceramic polymers were proposed over 30 years ago as precursors
for the fabrication of mainly Si-based advanced ceramics,
generally denoted as polymer-derived ceramics (PDCs). The
polymer to ceramic transformation process enabled significant
technological breakthroughs in ceramic science and technology,
such as the development of ceramic fibers, coatings, or ceramics
stable at ultrahigh temperatures (up to 20001C) with respect to
decomposition, crystallization, phase separation, and creep. In
recent years, several important advances have been achieved
such as the discovery of a variety of functional properties associated
with PDCs. Moreover, novel insights into their structure
at the nanoscale level have contributed to the fundamental understanding
of the various useful and unique features of PDCs
related to their high chemical durability or high creep resistance
or semiconducting behavior. From the processing point of view,
preceramic polymers have been used as reactive binders to produce
technical ceramics, they have been manipulated to allow for
the formation of ordered pores in the meso-range, they have
been tested for joining advanced ceramic components, and have
been processed into bulk or macroporous components. Consequently,
possible fields of applications of PDCs have been extended
significantly by the recent research and development
activities. Several key engineering fields suitable for application
of PDCs include high-temperature-resistant materials (energy
materials, automotive, aerospace, etc.), hard materials, chemical engineering (catalyst support, food- and biotechnology, etc.), or
functional materials in electrical engineering as well as in micro/
nanoelectronics. The science and technological development of
PDCs are highly interdisciplinary, at the forefront of micro- and
nanoscience and technology, with expertise provided by chemists,
physicists, mineralogists, and materials scientists, and engineers.
Moreover, several specialized industries have already
commercialized components based on PDCs, and the production
and availability of the precursors used has dramatically increased
over the past few years. In this feature article, we highlight
the following scientific issues related to advanced PDCs
research:
(1) General synthesis procedures to produce silicon-based preceramic
polymers.
(2) Special microstructural features of PDCs.
(3) Unusual materials properties of PDCs, that are related to
their unique nanosized microstructure that makes preceramic
polymers of great and topical interest to researchers across a
wide spectrum of disciplines.
(4) Processing strategies to fabricate ceramic components from
preceramic polymers.
(5) Discussion and presentation of several examples of possible
real-life applications that take advantage of the special characteristics
of preceramic polymers.
Note: In the past, a wide range of specialized international symposia
have been devoted to PDCs, in particular organized by the
American Ceramic Society, the European Materials Society,
and the Materials Research Society. Most of the reviews available
on PDCs are either not up to date or deal with only a subset
of preceramic polymers and ceramics (e.g., silazanes to produce
SiCN-based ceramics). Thus, this review is focused on a large
number of novel data and developments, and contains materials
from the literature but also from sources that are not widely
available
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
Photoluminescence spectroscopy of Er3+/Yb3+ co-activated silica-alumina monolithic xerogels
No full textTwo Er3+/Yb3+ monolithic silica-alumina xerogels doped with different Yb3+ concentrations were
prepared by sol–gel route. The samples were thermally treated in air at 950◦C for 120 h, and Raman and UVvisible-
NIR spectroscopy were used to monitor the degree of densification of the glasses and the residual OH content. Back energy transfer from Er3+ to Yb3+ was demonstrated by measurement of Yb3+ emission upon
Er3+ excitation at 514.5 nm. Photoluminescence excitation spectroscopy over the 880–1010 nm wavelength range,
spanning the energy range of the Yb3+–2F5/2 and Er3+–4I11/2 excited states,was used to obtain information about the
effective excitation efficiency of Er3+ ions by co-doping with Yb ions. The emission of 4I13/2→4I15/2 of Er3+ ion
transition with a 47 nm bandwidth was observed upon excitation at 514.5 nm and a lifetime of the 4I13/2 metastable
state of 8.7 ms was measured
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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