1,721,143 research outputs found
First-Principles Studies of Point Defects and Phase Transformations in Materials
Get PDFThe functional and mechanical properties of a material are often determined by the defects in them. A thorough understanding of the relationship between the defects and the properties allows for tailoring a material’s properties into the desired combinations. Amongst the different classes of defects, experimental identification of point defects is typically difficult and indirect, usually requiring an ingenious combination of different techniques. In this context, first-principles calculations, complemented with experiments, offer insights into the formation of defects and their role in properties. This was demonstrated in this thesis through investigations on the effect of calcium vacancies on structure, vibrational and elastic properties hydroxyapatite (HAp), and oxygen vacancies on elastic properties of zinc oxide (ZnO) using first-principles calculations based on density functional theory (DFT). Our results confirm a considerable reduction in the elastic constants of HAp—the inorganic constituent of bone—due to Ca-deficiency, which was experimentally reported earlier. Elastic anisotropic behavior of stoichiometric and Ca-deficient HAp is analyzed, which will be useful in understanding the effects of crystal orientation in designing synthetic bone. Local structural stability of HAp and Ca-deficient HAp structures is assessed with full phonon dispersion studies and the specific signatures in the computed vibrational spectra for Ca deficiency in HAp can be utilized in experimental characterization of different types of defected HAp. In ZnO, formation energies of oxygen vacancies in different types of oxygen deficient structures are analyzed to ascertain their stability. Our results show considerable degradation of some of the elastic moduli due to the presence of such vacancies. Further, the charge state of the defect structure is found to influence the shear elastic constants. Evaluation of elastic anisotropy of stoichiometric and oxygen deficient ZnO indicates the significant anisotropy in elastic properties and stiff c-axis orientation.
The second part of the thesis deals with developing some understanding of the pressure-induced phase transformations (PIPT) in an inorganic material, titanium nitride (TiN), and in a metal-organic framework (MOF), erbium formate crystal. PIPT, which is a common phenomenon in many materials, is of great interest in materials science as the properties of the transformation product can diverge significantly from those of the parent phase. Hence, it is important to understand the pressure induced changes so to improve the component reliability and to enhance service life of materials used in high pressure applications. TiN undergoes PIPT from NaCl to CsCl structure. On the basis of our DFT calculations, we propose a new transformation path, which shows that the stress required for this transformation is substantially lower when it is deviatoric in nature than that under hydrostatic pressure. Local stability of the structure is assessed with phonon dispersion determined at different pressures, and we find that CsCl structure of TiN is expected to distort after the transformation. Further, we provide a quantitative comparison of electronic structure of TiN in NaCl structure with that of high pressure phase with implication to electrical conduction properties. Next, we investigate the PIPT associated with bond rearrangement in erbium formate framework. Phase transition pressure is estimated and the corresponding changes in bonding characteristics are analyzed. Estimated lattice constants for both the phases agree well with the earlier experimental results. While the transformation pressure of the framework is overestimated with respect to experiment, our calculations confirm PIPT, and thus provide a theoretical evidence for the experimental finding
Transient Species Mediating Energy Transfer to Spin-Forbidden Mn d States in II-VI Semiconductor Quantum Dots
No full textThe nature of the transient species leading to emission from the spin/orbital-forbidden Mn d-d transition in doped semiconductor quantum dots has intrigued scientists for a long time. This understanding is important in the quest for energy efficiency as the energy from the conduction band is transferred efficiently to Mn in the femtosecond time scale overcoming other nonradiative recombination pathways. In this work, we have shown the presence of the transient species using materials with band gaps in resonance with the energy of the Mn emission to understand the nature of the absorbing, transient, and emitting species. Detailed studies lead to the emergence of a transient Mn 3+ state that is further corroborated with spin-dependent density functional theory calculations. This opens up a unique opportunity to realize a reversible photochemical reaction and high radiative efficiency in a semiconductor nanostructure by controlling the spin state of the magnetic ion by external illumination
Ab initio study of ferromagnetic La<SUB>0.5</SUB>Ba<SUB>0.5</SUB>CoO<SUB>3</SUB>
No full textWe study structure and magnetic properties of La0.5Ba0.5CoO3(LBGO) using ab initio density functional theory (DFT) method based on pseudopotentials and a plane-wave basis. We find the cubic structure of LBCO is ferromagnetic and lower in energy than the nonmagnetic rhombohedral structure. Through the calculation of G-point phonons of LBCO in the cubic structure, we determine its structural instabilities and find that they correspond to the structural phase transition accompanying a para-ferromagnetic transition observed recently
Theoretical and computational studies of nano-structures and nanomaterials
No full textTheoretical analysis and computer simulations have proven to be cost-effective and powerful tools in scientific studies of materials, particularly at nano-scale where synthesis of nano-structures, interpretation of their observed character and exploration of new structures are not always straightforward. We present here fundamental principles of techniques used today for computational simulations of materials, their capabilities and limitations. We then illustrate efficacy of such studies through review of their applications to nano-structures of oxide materials, carbon and boron nitride based nano-tubes and mechanical behavior of nano-structured materials. We finally present a wish-list of new tools and augmentation of existing tools that would allow expansion of the range of applications of computer simulations to nano-structures and materials
Theoretical and computational studies of nano-structures and nanomaterials
Get PDFTheoretical analysis and computer simulations have proven to be cost-effective and powerful tools in scientific studies of materials, particularly at nano-scale where synthesis of nano-structures, interpretation of their observed character and exploration of new structures are not always straightforward. We present here fundamental principles of techniques used today for computational simulations of materials, their capabilities and limitations. We then illustrate efficacy of such studies through review of their applications to nano-structures of oxide materials, carbon and boron nitride based nano-tubes and mechanical behavior of nano-structured materials. We finally present a wish-list of new tools and augmentation of existing tools that would allow expansion of the range of applications of computer simulations to nano-structures and materials
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
Localized orbital description of electronic structures of extended periodic metals, insulators, and confined systems: density functional theory calculations
Get PDFWe present a simple and general method for construction of localized orbitals to describe an electronic structure of extended periodic metals and insulators as well as confined systems. Spatial decay of these orbitals is found to exhibit exponential behavior for insulators and power law for metals. While these orbitals provide a clear description of bonding, they can be also used to determine polarization of insulators. Within density functional theory, we illustrate applications of this method to crystalline aluminium, copper, silicon, PbTiO3, and molecules, such as ethane and diborane
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