1,721,029 research outputs found
Structural Defects and Polytypism in Moissanite and Synthetic SiC Crystals
No full textIn recent years, defect characterisation of SiC polytypes has
been developed significantly, due to the increasing importance
of such polytypes for high temperature, high-power and highfrequency
electronic devices [1]. On the other side, moissanite-
SiC is a rare mineral that was discovered in 1905 within a
meteorite and only recently as a rock-forming mineral (8.4
vol%) in one unique specimen of a terrestrial volcanic rock [2].
Besides it has been found in kimberlitic pipes and can provide
some light on the redox conditions of the Earth Mantle. Finally
SiC is a particularly interesting species of presolar grain because
the polytype distribution can be related to different parameters
of the expanding stellar atmospheres of asymptotic giant
branch-AGB-stars.
In the frame of a research program in progress on SiC, growth
defects of bulky gem-crystals of 6H-SiC were studied by X-ray
Diffraction Topography-XRDT and it was found that the main
types of defects were dislocations parallel and perpendicular
to the growth directions, and micro-channels [3].
This piece of work is devoted to the study of structural defects
and degree of order of natural and synthetic moissanite.
Natural and synthetic samples have been investigated by XRD
Topography and TEM, focusing on the relationships between
defects and growth conditions. XRDT analyses of synthetic
6H-SiC allowed the characterization of dislocations and channels
to be made and other polytypes for further investigation by
TEM to be localized. All studied sample are characterized by
the presence of linear defects, dislocations and micro-channels,
uniformly distributed in the crystal. Moreover samples grown
by means of Physical Vapour Transport-PVT method, show the
same linear defects with different character, strictly related to
growth conditions.
TEM images and electron diffractions (EDs) strongly differentiate
natural from synthetic samples. ED patterns with [100]
incidence of natural crystals are consistent with the 6H polytype
and do not show streaks along the [001] stacking direction. This
result is confirmed by structure refinement from area detector
single crystal X-ray data [4]. Synthetic samples are comparatively
much more disordered. Conventional images show high
density of (001) faults, not observed in natural samples. Consistently,
ED patterns of the [100] zone are streaked along c*.
Atomic resolution imaging shows that synthetic samples mainly
consist of (001) stacking sequence described as (32)3 [5]. Locally
mixed stacking sequence described by notation 23(3233)5,
probably referred to a long period polytype, are present.
[1] Neudeck, P. G., Journal of Electronic Materials, 1995, 24, 283.
[2] Di Pierro S., Gnos E., Grobety B. H., Armbruster T., Bernasconi
S. M. and Ulmer P., Am. Mineral., 2003, 88, 1817.
[3] Agrosì G., Fregola R.A., Monno A., Scandale E., Tempesta G.,
Materials Science Forum, 2005, 483-485, 311.
[4] Capitani G.C.*, Tempesta G., Di Pierro S., Scandale E., 2006, This
congress.
[5] Zhdanov G.S., Minervina Z.V., Compt. Rend. Acad. Sci. URSS 1945,
48.
Industrial mineralogy
23rd European Crystallographic Meeting, ECM23, Leuven, 2006
Acta Cryst. (2006). A62, s64
Pag
Aree geografiche e distretti di successo.
No full textLe aree di eccellenza vitivinicola in Italia hanno una prerogativa: quella di essere state poste storicamente in zone protette pedecollinari caratterizzate da disponibilità idriche, assenza di eccessi termici, esposizioni a sud, protezione dai venti di bora e tramontana, da est e da nord, minor pressione parassitaria. È evidente che per il Nord Italia, il sistema montagnoso costituisce una protezione naturale ai venti freddi invernali prevalentemente orientati da nord e da est. Per le zone vitate dell’Italia centrale vale lo stesso schema, con la variante dell’area protetta dei principali vini di eccellenza toscana nord – sud – est – ovest, mentre per le Marche e Abruzzo i sistemi orografici sono di protezione nord ed ovest avendo la parte orientale calmierata dall’influenza marina. Le aree di eccellenza si sono localizzate storicamente nelle aree protette e rappresentano ancora oggi il nucleo iniziale della produzione vitivinicola italiana, vessillo del made in Italy nel mondo
GROWTH MARKS APPLIED TO THE ORIGIN OF GEMMOLOGICAL MATERIALS: GROWTH DEFECT CHARACTERIZATION OF YELLOW SAPPHIRES BY X-RAY DIFFRACTION TOPOGRAPHY
No full textCrystal chemical characterisation of red Beryl by ‘standardless’ laser-induced breakdown spectroscopy and single-crystal refinement by X-Ray diffraction. An example of validation of an innovative method for the chemical analysis of minerals
No full textLaser-induced breakdown spectroscopy (LIBS) is a valuable technique for performing qualitative and quantitative chemical determinations of all elements in one shot, including low atomic number elements such as Li and Be. This technique does not require any sample preparation to reveal the atomic species, even when present in trace amounts (< 0.01% m/m). In this study, for the first time, we provide an accurate mineral formula for a Cs-rich red beryl by combining crystallographic data obtained using the traditional single-crystal X-ray diffraction technique and quantitative chemical data obtained with an innovative ‘standardless’ method: Calibration-free-LIBS (CF-LIBS). In particular, a new LIBS prototype coupled with a petrographic microscope (CF-μLIBS) was used to analyse chemically homogeneous areas of about 10 μm spot size, causing minimal damage to the mineral. The results showed that calibration-free quantitative analysis is suitable for the quantification of major and minor low and high atomic number elements in beryl. The accuracy of quantification of low atomic number elements by CF-μLIBS led to the empirical formula: [12](Cs0.006Na0.019K0.017Ca0.019)Σ0.061[4](Be2.989Li0.011)Σ3.000[6](Ti0.053Mn0.051Mg0.007Al1.890)Σ2.000[4](Be0.116Fe0.024Si5.860)Σ6.000 O18. This formula is consistent with the crystal-structure refinement data and demonstrates the validity of CF-μLIBS for chemical analyses of minerals containing low atomic number elements
La diffusione della viticoltura delle eccellenze nelle aree di protezione in Italia.
No full textLe aree di eccellenza vitivinicola in Italia hanno una prerogativa: quella di essere state poste storicamente in zone protette pedecollinari caratterizzate da disponibilità idriche, assenza di eccessi termici, esposizioni a sud, protezione dai venti di bora e tramontana, da est e da nord, minor pressione parassitaria. È evidente che per il Nord Italia, il sistema montagnoso costituisce una protezione naturale ai venti freddi invernali prevalentemente orientati da nord e da est. Per le zone vitate dell’Italia centrale vale lo stesso schema, con la variante dell’area protetta dei principali vini di eccellenza Toscana nord – sud – est – ovest; mentre per le Marche e Abruzzo i sistemi orografici sono di protezione nord ed ovest avendo la parte orientale calmierata dall’influenza marina. Le aree di eccellenza si sono localizzate storicamente nelle aree protette e rappresentano ancora oggi il nucleo iniziale della produzione vitivinicola italiana, vessillo del made in Italy nel mondo
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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