Macroscopic elastic anisotropy in tough ceramics from the single crystal elastic behaviour

The modern techniques for engineering analysis are based on a deep understanding of the proportional relationship between stress and strain and the description in terms of isotropic elastic constants, in many cases, is enough. In anisotropic materials the knowledge of the elastic constants is important for the manufacturing process and for micro mechanical modelling behaviour of the material to develop the new composite materials. Theoretical calculations of elastics constants and anisotropies are applied to tough ceramics ZrO2, HfO2 and in rutile TiO2 and SnO2. Their dependence of the residual hydrostatic stress in the crystal are given. The microscopic elastic anisotropy determined in this work, could be applied in the macro scale in the case the constituent grains have a preferred orientation in the crystal lattice or the grains shape is not spherical (faceted) and it is aligned to a common crystal axis. To cover the case where the grains are distributed at randomly, the Voigt-Reuss-Hill polycrystalline approach is applied to obtain the average values of Young, bulk, shear modules, Poisson coefficient and sound velocity in the transversal and longitudinal modes. These theoretical results, could be useful in the interpretation of experimental results obtained with the method known as diffraction elastic constant (DEC).Fil: Caravaca, Maria de Los Angeles Dominga. Universidad Nacional del Nordeste. Facultad de Ingeniería; ArgentinaFil: Casali, Ricardo Antonio. Universidad Nacional del Nordeste. Facultad de Ciencias Exactas y Naturales y Agrimensura. Departamento de Física; ArgentinaFil: Ponce Altamirano, Claudio Ariel. Universidad Nacional del Nordeste. Facultad de Ciencias Exactas y Naturales y Agrimensura. Departamento de Física; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentin

Fast manipulation of a single electron along a quantum dot chain

We present a protocol based on shortcuts to adiabaticity for the manipulation of an electron in a linear quantum dot array of arbitrary size on an InSb heterostructure. We propose a model where the couplings between neighboring dots have a particular form, related to a spin operator that depends on the size of the chain. The system is driven by an electric field with a modified Landau-Zener time-dependence. We find that the transmission of the electron from the first site to the last one can be achieved with any desired fidelity (99.9% or even more) by choosing the appropriate evolution time of the system according to the length of the chain. Then, we consider Rashba spin-orbit (SO) interaction effects and possible chain defects and find that transmission probabilities are larger than 90% for moderate intensity of SO interactions. However, we can increase even more the fidelity by increasing the static magnetic field, since it would diminish the effect of the SO mechanism in the dynamics.Fil: Rus, María Estefanía. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Nordeste. Instituto de Modelado e Innovación Tecnológica. Universidad Nacional del Nordeste. Facultad de Ciencias Exactas Naturales y Agrimensura. Instituto de Modelado e Innovación Tecnológica; ArgentinaFil: Ponce Altamirano, Claudio Ariel. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Nordeste. Instituto de Modelado e Innovación Tecnológica. Universidad Nacional del Nordeste. Facultad de Ciencias Exactas Naturales y Agrimensura. Instituto de Modelado e Innovación Tecnológica; ArgentinaFil: Romero, Rodolfo Horacio. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Nordeste. Instituto de Modelado e Innovación Tecnológica. Universidad Nacional del Nordeste. Facultad de Ciencias Exactas Naturales y Agrimensura. Instituto de Modelado e Innovación Tecnológica; ArgentinaFil: Gomez, Sergio Santiago. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Nordeste. Instituto de Modelado e Innovación Tecnológica. Universidad Nacional del Nordeste. Facultad de Ciencias Exactas Naturales y Agrimensura. Instituto de Modelado e Innovación Tecnológica; Argentin

Ab initio study of electronic and structural properties of SnO2 nanoparticles under pressure

<p></p><p>ABSTRACT Tin oxide (SnO2) has important applications, including as an elective material in the development of gas sensors. These devices improve their sensitivity and decrease their work temperature when the nanoparticle size is in the nanoscale order. Therefore, the gas sensor raises its efficiency noticeably. For this reason, the research of behavior at the nanoscale is essential. The total and surface formation energies of SnO2 nanoparticles (NPs) were determined by means of ab initio calculations at different concentrations and size near to 2.5 nm. The analysis shows that at higher concentration, higher structural stability is reached, in the range of 0 to 10 GPa (applied external pressures). In addition, it was possible to define a crystalline core region and a distorted layer in the NP by way of the atomic displacement study. Taking into account the importance of detecting the NP size by X-ray diffraction (XRD), the crystalline core dimension was estimated at 0 GPa. We obtained crystalline cores of 1.46 nm in the case of 1.9-, 2.0- and 2.2- concentrations, while this core was not observed at pressures of 5 and 10 GPa. Observing different regions in the densities of electronic states (DOS), we confirmed the core sizes in each NP.</p><p></p

Ab initio study of the elastic properties of single and polycrystal TiO2, ZrO2 and HfO2 in the cotunnite structure

In this work, we study theoretically the elastic properties of the orthorhombic (Pnma) high-pressure phase of IV-B group oxides: titania, zirconia and hafnia. By means of the self-consistent SIESTA code, pseudopotentials, density functional theory in the LDA and GGA approximations, the total energies, hydrostatic pressures and stress tensor components are calculated. From the stress-strain relationships, in the linear regime, the elastic constants C ij are determined. Derived elastic constants, such as bulk, Young's and shear modulus, Poisson coefficient and brittle/ductile behavior are estimated with the polycrystalline approach, using Voigt-Reuss-Hill theories. We have found that C11, C22 and C33 elastic constants of hafnia and zirconia show increased strength with respect to the experimental values of the normal phase, P 21/c. A similar situation applies to titania if these constants are compared with its normal phase, rutile. However, shear elastic constants C44, C55 and C66 are similar to the values found in the normal phase. This fact increases the compound anisotropy as well as its ductile behavior. The dependence of unit-cell volumes under hydrostatic pressures is also analyzed. P-V data, fitted to third-order Birch-Murnaghan equations of state, provide the bulk modulus B0 and its pressure derivatives B′0. In this case, LDA estimations show good agreement with respect to recent measured bulk moduli of ZrO2 and HfO2. Thermo-acoustic properties, e.g. the propagation speed of transverse, longitudinal elastic waves together with associated Debye temperatures, are also estimated. © 2009 IOP Publishing Ltd.Fil: Caravaca, Maria de Los Angeles Dominga. Universidad Nacional del Nordeste. Facultad de Ingeniería; ArgentinaFil: Miño, Julio C.. Universidad Nacional del Nordeste. Facultad de Ingeniería; ArgentinaFil: Pérez, V.J.. Universidad Nacional del Nordeste. Facultad de Ingeniería; ArgentinaFil: Casali, Ricardo Antonio. Universidad Nacional del Nordeste. Facultad de Ciencias Exactas y Naturales y Agrimensura; ArgentinaFil: Ponce Altamirano, Claudio Ariel. Universidad Nacional del Nordeste. Facultad de Ciencias Exactas y Naturales y Agrimensura; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Nordeste; Argentin

Ab initio and shell model studies of structural, thermoelastic and vibrational properties of SnO 2 under pressure

The pressure dependences of the structural, thermoelastic and vibrational properties of SnO2 in its rutile phase are studied, as well as the pressure-induced transition to a CaCl2-type phase. These studies have been performed by means of ab initio (AI) density functional theory calculations using the localized basis code SIESTA. The results are employed to develop a shell model (SM) for application in future studies of nanostructured SnO2. A good agreement of the SM results for the pressure dependences of the above properties with the ones obtained from present and previous AI calculations as well as from experiments is achieved. The transition is characterized by a rotation of the Sn-centered oxygen octahedra around the tetragonal axis through the Sn. This rotation breaks the tetragonal symmetry of the lattice and an orthorhombic distortion appears above the critical pressure Pc. A zone-center phonon of B1g symmetry in the rutile phase involves such rotation and softens on approaching Pc. It becomes an Ag mode which stabilizes with increasing pressure in the CaCl2 phase. This behavior, together with the softening of the shear modulus (C11 −C12)/2 related to the orthorhombic distortion, allows a precise determination of a value for Pc. An additional determination is provided by the splitting of the basal plane lattice parameters. Both the AI and the experimentally observed softening of the B1g mode are incomplete, indicating a small discontinuity at the transition. However, all results show continuous changes in volume and lattice parameters, indicating a second-order transition. All these results indicate that there should be sufficient confidence for the future employment of the shell model.Fil: Casali, Ricardo Antonio. Universidad Nacional del Nordeste. Facultad de Ciencias Exactas Naturales y Agrimensura. Departamento de Física; ArgentinaFil: Lasave, Jorge Augusto. Universidad Nacional de Rosario. Facultad de Ciencias Exactas, Ingeniería y Agrimensura; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Rosario. Instituto de Física de Rosario (i); ArgentinaFil: Caravaca, M. A.. Universidad Nacional del Nordeste. Facultad de Ciencias Exactas Naturales y Agrimensura. Departamento de Física; ArgentinaFil: Koval, Sergio Fabian. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Rosario. Instituto de Física de Rosario (i); Argentina. Universidad Nacional de Rosario. Facultad de Cs.exactas Ingenieria y Agrimensura. Escuela de Cs.exactas y Naturales; ArgentinaFil: Ponce Altamirano, Claudio Ariel. Universidad Nacional del Nordeste. Facultad de Ciencias Exactas Naturales y Agrimensura. Departamento de Física; ArgentinaFil: Migoni, Ricardo Luis. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Rosario. Instituto de Física de Rosario (i); Argentina. Universidad Nacional de Rosario. Facultad de Cs.exactas Ingenieria y Agrimensura. Escuela de Cs.exactas y Naturales; Argentin

Study of Electronic, Thermodynamic and Vibrational Properties of SnO2 Nanoparticles with Different Stoichiometries and Effect of Nonhydrostatic Pressure

High pressure behavior of SnO2 nanoparticles of 3nm is studied up to 19 GPa using ab initio method. First, elecgtronic strcuture equilibrium configuration, elastic and vibrational properties are performed on nanoparticles with different stoichiometries and at normal external pressure. The results are used to evaluate the contribution of the stoichiometry on the surface energy and on the relative stability for different operation condition of temperatura and oxygen partial pressure. Secondly, in order to get microscopic description of the shell size and nature of the disorder found with Raman Spectrocsopy (RS) in NP of 2.8 nm under high pressure, we introduce in the calculation external non hydrostatic pressure as applied in the RS experiment. The change in the structural properties is analyzed. We calculated for the first time the vibrational density of state of the nanoparticle for different stoiciometries and compared with the Raman spectrum obtained for the same size. The purpose is to know if it is possible to determine the probable stoiciometry of the nanoparticles studied experimentally. In addition, the frequencies of vibration of a free nanoparticle in one spheroidal mode are calculated in the frozen phonon approach.Fil: Caravaca, Maria de Los Angeles Dominga. Universidad Nacional del Nordeste. Facultad de Ingeniería; ArgentinaFil: Casali, Ricardo Antonio. Universidad Nacional del Nordeste. Facultad de Ciencias Exactas y Naturales y Agrimensura. Departamento de Física; ArgentinaFil: Girao, Helainne T.. Université Claude Bernard Lyon 1; FranciaFil: Cornier, Thibaut. Université Claude Bernard Lyon 1; FranciaFil: Daniele, Stephane. Université Claude Bernard Lyon 1; FranciaFil: Debord, Regis. Université Claude Bernard Lyon 1; FranciaFil: Melinon, Patrice. Université Claude Bernard Lyon 1; FranciaFil: Ponce Altamirano, Claudio Ariel. Universidad Nacional del Nordeste. Facultad de Ciencias Exactas y Naturales y Agrimensura. Departamento de Física; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Nordeste. Instituto de Modelado e Innovación Tecnológica. Universidad Nacional del Nordeste. Facultad de Ciencias Exactas Naturales y Agrimensura. Instituto de Modelado e Innovación Tecnológica; Argentina23rd International Conference on Nanomaterials and NanotechnologyLondresReino UnidoConference Series LL

Magnetic properties for the Mn2GeTe4 compound

Measurements of magnetic susceptibility χ, in the temperature range from 2 to 300 K, and of magnetization M vs. applied magnetic field B, up to 5 T, at various temperatures were made on polycrystalline samples of the Mn2GeTe4 compound. It was found that Mn2GeTe4 has a Néel temperature TN of about 135 K, shows mainly antiferromagnetic behavior with a very weak superimposed ferromagnetic component that is attributed to spin canting. Also, the magnetic results suggest that a possible spin-glass transition takes place at Tf≈45 K. The spin-glass order parameter q(T), determined from the susceptibility data, was found to be in agreement with the prediction of conventional spin-glass theory. The M vs. B results indicated that bound magnetic polarons (BMPs) occur in the compound, and that the effects from BMPs disappear at approximately 80 K. The M vs. B curves were well fitted by a Langevin type of equation, and the variation of the fitting parameters determined as a function of temperature. Using a simple spherical model, the radius of the BMP in the material was found to be about 27 Å; this value is similar to the effective Bohr radius for an acceptor in the II-IV-V2 and I-III-VI2 ternary semiconductor compounds. © 2008 Elsevier B.V. All rights reserved.Fil: Quintero, M.. Universidad de Los Andes Facultad de Ciencias; VenezuelaFil: Quintero, E.. Universidad de Los Andes Facultad de Ciencias; VenezuelaFil: Caldera, D.. Universidad de Los Andes Facultad de Ciencias; VenezuelaFil: Moreno, E.. Universidad de Los Andes Facultad de Ciencias; VenezuelaFil: Morocoima, M.. Universidad de Los Andes Facultad de Ciencias; VenezuelaFil: Grima, P.. Universidad de Los Andes Facultad de Ciencias; VenezuelaFil: Ferrer, D.. Universidad de Los Andes Facultad de Ciencias; VenezuelaFil: Marchan, N.. Universidad de Los Andes Facultad de Ciencias; VenezuelaFil: Bocaranda, P.. Universidad de Los Andes Facultad de Ciencias; VenezuelaFil: Delgado, G. E.. Universidad de Los Andes Facultad de Ciencias; VenezuelaFil: Henao, J. A.. Facultad de Ciencias; ColombiaFil: Macías, M. A.. Facultad de Ciencias; ColombiaFil: Pinto, J. L.. Facultad de Ciencias; ColombiaFil: Ponce Altamirano, Claudio Ariel. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Universidad Nacional del Nordeste. Facultad de Ciencias Exactas y Naturales y Agrimensura; Argentin