1,721,065 research outputs found
Effect of the composition and sintering process on mechanical properties and residual stresses in zirconia-alumina composites
No full textSamples of zirconia-toughened alumina (ZTA) with small amounts of chromia and magnetoplumbite-type crystalline phase (CeMgAl11 O19) have been prepared and processed under different conditions. Mechanical properties like hardness and fracture toughness were examined as a function of different parameters. As an example, fracture toughness was increased by the chromia addition, whereas platelets reinforcement addition suppressed the tetragonal zirconia (t-zirconia) - monoclinic zirconia (m-zirconia) transformation. In addition, transformability of the tetragonal zirconia and the residual stress in the alumina phase were examined by Raman and fluorescence piezo-spectroscopy, respectively. In particular, the extent to which t-zirconia transforms to m-zirconia was determined by Raman spectroscopy after Vickers indentation and the transformability was correlated to the fracture toughness. It was demonstrated that the monoclinic content and the toughness were correlated linearly and experimental results were compared with models already available for zirconia-based materials. On the other hand, residual stresses originated by transformation toughening mechanism were correlated to the transformability of the tetragonal phase
Terahertz Raman scattering as a probe for electron-phonon coupling, disorder and correlation length in molecular materials
Full text linkTerahertz (or low-frequency) Raman spectroscopy has been shown to be a quite useful tool to infer important information on some key properties of molecular materials, like polymorphism, phase purity and phase transitions. Based on some of our recent studies, we present promising new directions and possible development of the technique for the characterization of electron-lattice phonon coupling, disorder and correlation length in systems of low-dimensionality. The relative strength of electron-lattice phonon coupling can be extracted from the intensities of the Raman bands in the pre-resonance Raman regime, as exemplified in the charge-transfer (CT) crystal N,N-dimethylphenazine-tetracyanoquinodimethane (M2P-TCNQ). Disorder is instead reflected in the Raman bandwidth, which we analyze with polarized light for systems of reduced dimensionality. The sample system studied for the one-dimensional case is the tetramethylbenzidine-tetrafluoroTCNQ CT crystal. As an example of a quasi two-dimensional (2D) system we address pentacene, the classical case of a monomolecular material widely studied for its application in organic electronics. Here the discussion is mostly related to the dispersion of the phonon branches, eventually leading to peculiar spectral profiles depending on the 2D or 3D regime of the films grown under different deposition conditions
A New Approach to Polymorphism in Molecular Crystals: Substrate-Mediated Structures Revealed by Lattice Phonon Dynamics
Full text linkThe issue of polymorphism in molecular crystals is discussed, taking into account the substrate-mediated structures, that is, structures grown at the interface of different substrates. Bulk and thin films of a compound both share the potentiality to display different crystal forms. However, unlike bulk polymorphs, whose structures are determined by their different molecular packing, thin film structures depend very much on the molecular organization of the organic layers on the substrate, which may, or may not, lead to an ordered structure, depending on the nature of the interface and on the growth conditions. Based on large part in some of the authors' recent works, these thin film structures are classified as distorted bulk, substrate-selected and substrate-stabilized polymorphs, with some subtle differences which may yield a polymorph to belong not exclusively to a single one of these categories. Some experiments are then focused upon, involving charge transport at the interface, as well as how far the effect of the surface goes. Furthermore, the authors comment on how the surface-mediated structures evolve to the single crystal phase in the cases of pentacene and alpha-sexithiophene. Finally, the transition from a 3- to a 2D regime of growth is shortly discussed in terms of low-dimensional disorder
Bulk and Surface-Mediated Polymorphs of Bio-Inspired Dyes Organic Semiconductors: The Role of Lattice Phonons in their Investigation
Full text linkRaman spectroscopy of organic molecular materials in the low-wavenumber region gives access to lattice vibrational modes and to the wealth of information on solid state properties that these can provide. In the field of organic electronics a useful application concerns the discrimination of the crystalline forms i. e. polymorphism of the semiconductor. The capability of characterizing and identifying the polymorphs of a compound is in fact the prerequisite for an exhaustive study of the charge transport characteristics which arise from the relationship between molecular, electronic, and crystal structures. Thus, the need is felt of a non-invasive, non-destructive tool such as Raman, which probes the crystal phase by detecting the lattice modes which are sensitive even to subtle variations of the packing. Here we review the contribution of the technique to the study of organic pigments displaying promising semiconducting properties and characterized by polymorphism both in their bulk and thin film phases
What Changes in Topochemistry when Going from Small Molecule Dimerizations to Polymerizations in Single Crystals?
No full textThis publication promotes the increased necessity for strain management in topochemical reactions with an enlarged structural extension of the molecular products formed, i. e., when going from small molecule dimerizations, through linear polymerizations to the formation of 2D polymers. Further, it promotes to combine the trap model for photon absorption with concrete molecular scale consequences of this absorption on topochemical transformations and briefly discusses the expected consequences topological dimensionality of the forming (macro)molecular products has on trap location. The time appears ripe for going in this direction because local information concerning structural changes within single crystals is now accessible by the 3D-ΔPDF method. This method greatly facilitates the analysis of diffuse X-ray scattering providing access to concrete values of pair distribution functions and, thus, factual information on which and how distances change near a reaction site. Although only based on a first case where distance changes could be quantified in a lateral polymerization, the thoughts put forward may ignite more research towards a full understanding of all the action that occurs when a photochemically triggered topochemical reaction takes place. The 3D-ΔPDF method is so attractive for this purpose because it provides otherwise inaccessible local information in pair correlation functions rather than average structure information, which is used through the ubiquitous Bragg scattering
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
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