13,123 research outputs found
Effect of nearest neighbor Pb-O divacancy pairs on the ferroelectric-relaxor transition in Pb(Sc1/2Nb1/2)O3
No full textMolecular dynamics simulations were performed on a frst-principles-based effective Hamiltonian for Pb(Sc1/2Nb1/2)O3 (PSN) with nearest neighbor Pb-O divacancy pairs. Simulations were performed for PSN with ideal chemical order, chemical short-range order, and random chemical disorder. Divacancy concentration vs temperature, X[Pb-O] vs T, phase diagrams were calculated, and they are shown to be topologically equivalent to the previously calculated hydrostatic pressure vs temperature diagram. In PSN with ideal chemical order, the Burns temperature defnes an isotherm, which is analogous to the "Griffiths phase" in the magnetic dilution problem.Fil: Burton, B.. National Institute Of Standards And Technology; Estados UnidosFil: Cockayne, E.. National Institute Of Standards And Technology; Estados UnidosFil: Tinte, Silvia Noemi. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Santa Fe. Instituto de Desarrollo Tecnológico para la Industria Química. Universidad Nacional del Litoral. Instituto de Desarrollo Tecnológico para la Industria Química; ArgentinaFil: Waghmare, U.. J. Nehru Theoretical Sciences Unit; Indi
Universal Statistical Properties of Inertial-particle Trajectories in Three-dimensional, Homogeneous, Isotropic, Fluid Turbulence
No full textWe obtain new universal statistical properties of heavy-particle trajectories in three-dimensional, statistically steady, homogeneous, and isotropic turbulent flows by direct numerical simulations. We show that the probability distribution functions (PDFs) P(Φ), of the angle Φ between the Eulerian velocity u and the particle velocity v, at a point and time, scales as P(Φ) ∼Φ−, with a new universal exponent ≃ 4
Mean flow generation by Görtler Vortices in a rotating annulus with librating side walls
No full textLongitudinal libration of the cylinder side walls of a rotating annulus in the supercritical regime induces a centrifugally unstable Stokes boundary layer which generates Görtler vortices only in a portion of a libration cycle. We show for the first time that these vortices propagate into the fluid bulk and generate an azimuthal mean flow which is retrograde (prograde) over the outer (inner) cylinder side wall. Direct numerical simulations (DNS) are carried out and Reynolds-averaged equations and kinetic energy budget of mean and fluctuating flow are used as diagnostic equations to discuss the generation mechanism and scaling behavior of the azimuthal mean flow in the fluid bulk
Magneto-Optical Stark Effect in Fe-Doped CdS Nanocrystals
No full textFe2+ doping in II-VI semiconductors, due to the absence of energetically accessible multiple spin state configurations, has not given rise to interesting spintronic applications. In this work, we demonstrate for the first time that the interaction of homogeneously doped Fe2+ ions with the host CdS nanocrystal with no clustering is different for the two spin states and produces two magnetically inequivalent excitonic states upon optical perturbation. We combine ultrafast transient absorption spectroscopy and density functional theoretical analysis within the ground and excited states to demonstrate the presence of the magneto-optical Stark effect (MOSE). The energy gap between the spin states arising due to MOSE does not decay within the time frame of observation, unlike optical and electrical Stark shifts. This demonstration provides a stepping-stone for spin-dependent applications
First Principles Study Of Structure And Stacking Fault Energies In Some Metallic Systems
No full textPlastic deformation in crystalline materials largely depends on the properties of dislocations, in particular their mobility. While continuum description of deformation of a crystalline metal can be made reasonably well by considering the elastic properties of dislocations and neglecting the core, crystallographic aspects of dislocation motion require precise understanding of the core effects. The concept of the generalized stacking fault (GSF) energy was proposed as means to describe this. GSF energy, a fundamental property of a given material, can be determined using first principles total energy calculations. In this thesis, we use GSF to understand some of the intriguing mechanical responses recently observed in some metallic systems.
First, we examine the structures and stacking fault energies in Mg-Zn-Y alloy system. This system is unique in the sense that trace additions of Zn and or Y result in long period stacking sequences such as 6l and 14l, as reported in recent literature. Further, these alloys exhibit extraordinary mechanical properties. We attempt to rationalize these experimental observations through first principles calculations of energies of periodic structures with different stacking sequences and stacking faults. For pure Mg, we find that the 6-layer structure with the ABACAB stacking is most stable after the lowest energy hcp structure with ABAB stacking. Charge density analysis shows that the 2l and 6l structures are electronically similar, which might be a cause for better stability of 6l structure over a 4l sequence or other periodic structures. Addition of 2 atomic% Y leads to stabilization of the structure to 6l sequence whereas the addition of 2 atomic% Zn makes the 6l energetically comparable to that of the hcp. Stacking fault (SF) on the basal plane of 6l structure is higher in energy than that of the hcp 2l Mg, which further increases upon Y doping and decreases significantly with Zn doping. SF energy surface for the prismatic slip indicates dissociation of dislocations in alloys with a 6l structure. Thus, in an Mg-Zn-Y alloy, Y stabilizes the long periodicity, while Zn doping provides a synergistic effect in improving the mechanical properties alongwith strengthening due to long periodic phases.
Our investigation of surface properties and magnetism in Ni revealed that, the universal binding energy relation (UBER) derived earlier to describe the cohesion between two rigid atomic planes, does not accurately capture the cohesive properties when the cleavage cracked surfaces are allowed to relax through atomic displacements. We find that two characteristic length-scales are involved in the cleavage of a crystal accompanied by structural relaxation at the cleaved surface. Based on that, we suggest a modified functional form of UBER that is analytical and at the same time accurately models the properties of relaxed surfaces upon cleavage. We demonstrate the generality as well as the validity of this modified UBER through first-principles density functional theory calculations of cleavage in fcc, bcc, and hcp metals, as well as covalently bonded materials. We also found that the cohesive law (stress-displacement relation) differs significantly in the case where cracked surfaces are allowed to relax, with lower peak stresses occuring at higher displacements. We have attempted understanding these ideas through images obtained from electronic densities and eigen states. Our work should be useful in providing inputs to multi-scale simulations of fracture in materials.
The third phase of the work reports the stacking fault energy and twinning in Ni with a particular emphasis on the size effect. Experimental and computational research on Nan crystalline metals (mostly on Ni) indicates unique facets of dislocation activity (extended partial dislocations) and modes of deformation (twinning). In order to capture the intrinsic scaling eject in the nano-regime, it is imperative to account for the complex electronic structure of the metal in question. The stacking fault (SF) and twinning fault (TF) energies in nano-thin elm of Ni with 7, 13, 19, and 25 layers of (111) planes were determined using rest-principles density functional theory (DFT) total energy calculations. Generalized planar fault (GPF) energy curves of the nano-thin alms show higher extreme vis-a-vis the bulk, indicating that creation of SFs in nano-Ni is relatively difficult. In contrast, the ratios of energy barriers relevant to nucleation of dislocations and twinning support the observed enhanced tendency for extended partial dislocation formation and twinning in the nano-thin films in comparison with bulk. Our results should be useful in understanding deformation behavior of nano-structured Ni-based alloys used as advanced structural materials
First-Principles Studies of Point Defects and Phase Transformations in Materials
No full textThe 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
Wave turbulence of a rotating array of quantized vortices in the T → 0 temperature limit
No full textThe dynamics of quantized vortices in the zero temperature limit is currently of great interest, particularly in the case of the Fermi superfluid He-B. Here we study wave turbulence, generated by the librating motion of a rotating cylindrical container filled with He-B, in the limit of vanishing viscous forces at temperatures . The polarization of the quantized vortices with respect to the axis of rotation is measured using non-invasive NMR techniques. We observe a decrease of the polarization when the librating motion is started, and a two-stage relaxation process when the modulation of the rotation velocity is stopped. The first relaxation process is associated with the dissipation of large-scale flow stored in inertial waves and the solid body rotation of the vortex array. From the decay of these energy reservoirs we determine the rate of energy dissipation of large-scale flow. The later second process is related to the relaxation of Kelvin waves on individual vortices. This process is monitored by the recovery of the polarization. The existence of a Kelvin wave cascade at the lowest temperatures is currently a central open question. We supply some evidence for the cascade
Dynamics of finite-sized light spheres in turbulence
No full textWe report experimental results on the Lagrangian dynamics of finite-size light particles in turbulence. Using an orthogonal camera setup and 3D particle tracking, we study the velocity and acceleration statistics of rigid light spheres in a water tunnel with nearly homogeneous and isotropic turbulence. The Reynolds number (ReY) is varied from 180 to 300, and the study covers a range of size ratios (4 < D/η < 16) for marginally light spheres. We find that the normalised acceleration PDF decreases in intermittency with increasing size ratio - in qualitative agreement with the predictions of the Faxén corrected model. We also present preliminary results on the rotational dynamics of large light spheres in turbulence
Numerical investigation of localized exact solutions of the Navier-Stokes equations in pipe flow
No full textThe edge state solution in pipe flow at Re=2200 is calculated numerically. The solution has the form of spatially localized puff-like structure drifting downstream. In the moving frame it is represented by a steady average flow and time-periodic pulsation flow. It is shown, that the Kelvin-Helmholtz instability mechanism is not valid for pulsation generation in the edge state flow
- …
