1,721,006 research outputs found
Molecular Crystals
No full textInternational audienceMolecular crystals are the basis of numerous advanced materials used nowadays in many field of applications such as pharmacy (active pharmaceutical ingredients, excipients), electronics and opto-electronics (organic semiconductors) or energy storage. The properties of the materials are crucially dependent on the packing organization of the molecules in the crystalline state. Hence, the development of materials based on molecular crystals lie on a profound knowledge of their crystal arrangement and on the crystallization routes affording the desired crystal forms. In particular, polymorphism, which is defined as the ability of a compound to crystallize in more than one crystal structure, is a phenomenon which has to be understood and hopefully controlled. This lecture will, after a brief introduction on the practical interest of molecular crystals, review the fundamentals of crystallography, phase stability and crystallization that are needed to understand the challenge of designing molecular crystals. Then, a description of the main characterization techniques used to decipher the (often complex) phase behaviour of molecular compounds will be performed. Lastly, the author will select some case studies from his own research to highlight the wealth of crystal forms that even simple molecules can exhibit and the difficulties encountered to obtain one particular crystal form in certain cases
Etude du polymorphisme et de la miscibilité à l'état solide dans la série des acides gras saturés
Full text linkCe travail concerne l'étude du polymorphisme et de la miscibilité à l'état solide d'acides gras saturés (CnH2n-1O2) avec un nombre d'atomes de carbone compris entre 13 et 23. Ces matériaux présentent de nombreuses phases solides à température ambiante apparaissant, soit seules soit en co-existence, selon les méthodes de préparation des échantillons (recristallisation après fusion ou dissolution, broyage). La stabilité relative des diverses phases solides observées à température ambiante a été établie pour la majorité des acides considérés. Quelque soit la phase "basse température", celle-ci transite en une phase dite "haute température" juste avant la fusion. Les structures des phases stables basse et haute température, résolues principalement à partir de diagrammes de diffraction X de poudre, montrent que les acides gras se présentent sous forme de bicouches, des dimères étant formés par liaisons hydrogène entre les groupes carboxyles des molécules voisines. Deux phases haute température différentes ont été mises en évidence pour les acides gras avec un nombre impair d'atomes de carbone, l'une de groupe d'espace A2/a pour les acides gras avec 13 et 15 atomes de carbone et l'autre de groupe d'espace P21/a pour les acides plus longs. L'analyse comparative de diagrammes de phase de mélanges binaires impair-impair (Δn=2), impair-pair et pair-impair (Δn=1) montre que la miscibilité à l'état solide est faible dans cette famille de composés. De plus, plusieurs phases solides intermédiaires sont stabilisées par la mixité. Finalement, des domaines d'alliages plus étendus sont observés lorsque le degré d'homéomorphisme est élevé (Δn=1) et que les mailles cristallines des phases solides des constituants se ressemblent.This work deals with the study of the polymorphism and the solid state miscibility of saturated fatty acids (CnH2n-1O2) with a number of carbon atoms comprised between 13 and 23. These materials present a great number of solid phases at room temperature which appear, either alone or in co-existence, according to the methods of preparation of the samples (recrystallization from the melt or from solution, crushing). The relative stability of the various solid phases observed at room temperature have been established for the major part of the acids considered. Whatever the "lowtemperature" phase is, this one transits into a "high-temperature" phase just below the melting. The structures of the low and high-temperature stable phases, solved mainly from X-ray powder diffraction patterns, show that fatty acids consist in bi-layers, dimmers being created by hydrogen bonding between the carboxyl groups of neighbouring molecules. Two different high-temperature phases have been put forward for the odd-numbered fatty acids, one with A2/a space group for the fatty acids with 13 and 15 carbon atoms and the other with P21/a space group for the longer acids. The comparative analysis of phase diagrams of odd-odd (Δn=2), odd-even and even-odd (Δn=1) binary systems shows that the solid state miscibility is weak in this family of compounds. Moreover, various intermediary solid phases are stabilized by mixing. Finally, wider domains of alloys are observed when the degree of homeomorphism is high (Δn=1) and the unit cells of the solid phases of the constituents are similar
Polymorphism of dioctyl-terthiophene within thin films: the role of the first monolayer
No full textThe origins of specific polymorphic phases within thin films are still not well understood. The polymorphism of the molecule dioctyl-terthiophene is investigated during the presence of a silicon-oxide surface during the crystallisation process. It is found that a monolayer of molecules forms two-dimensional crystals on the surface. In the case of thicker films crystalline islands are formed, A comparison of the three polymorphic phases observed within thin films and the thermodynamically more stable single crystal phases reveals distinct differences which can be related to an adaption of the molecular packing with the flat surface of the substrate
Etude du polymorphisme et de la miscibilité à l'état solide dans la série des acides gras saturés
No full textCe travail concerne l'étude du polymorphisme et de la miscibilité à l'état solide d'acides gras saturés (CnH2n-1O2) avec un nombre d'atomes de carbone compris entre 13 et 23. Ces matériaux présentent de nombreuses phases solides à température ambiante apparaissant, soit seules soit en co-existence, selon les méthodes de préparation des échantillons (recristallisation après fusion ou dissolution, broyage). La stabilité relative des diverses phases solides observées à température ambiante a été établie pour la majorité des acides condisérés. Quelque soit la phase "basse température", celle-ci transite en une phase dite "haute température" juste avant la fusion. Les structures des phases stables basse et haute température, résolues principalement à partir de diagrammes de diffraction X de poudre, montrent que les acides gras se présentent sous forme de bi-couches, de dimères étant formés par liaisons hydrogène entre les groupes carboxyles des molécules voisines. Deux phases haute température différentes ont été mises en évidence pour les acides gras avec un nombre impair d'atomes de carbone, l'une de groupe d'espace A2/apour les acides gras avec 13 et 15 atomes de carbone et l'autre de groupe d'espace P21/a pour les acides plus longs. L'analyse comparative de diagrammes de phase de mélanges binaires impair-impair (n=2), impair-pair et pair-impair (n=1) montre que la miscibilité à l'état solide est faible dans cette famille de composés. De plus, plusieurs phases solides intermédiaires sont stabilitées par la mixité. Finalement, des domaines d'alliages plus étendus sont observés lorsque le degré d'homéomorphisme est élévé (n=1) et que les mailles cristallines des phases solides des constituants se ressemblent.BORDEAUX1-BU Sciences-Talence (335222101) / SudocSudocFranceF
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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