E. Restrepo-Parra

Atomic-scale simulations of material behaviors and tribology properties for BCC metal film*

This work has two main purposes: (i) introducing the basic concepts of molecular dynamics analysis to material scientists and engineers, and (ii) providing a better understanding of instrumented indentation measurements, presenting an example of nanoindentation and scratch test simulations. To reach these purposes, three-dimensional molecular dynamics (MD) simulations of nanoindentation and scratch test technique were carried out for generic thin films that present BCC crystalline structures. Structures were oriented in the plane (100) and placed on FCC diamond substrates. A pair wise potential was employed to simulate the interaction between atoms of each layer and a repulsive radial potential was used to represent a spherical tip indenting the sample. Mechanical properties of this generic material were obtained by varying the indentation depth and dissociation energy. The load-unload curves and coefficient of friction were found for each test; on the other hand, dissociation energy was varied showing a better mechanical response for films that present grater dissociation energy. Structural change evolution was observed presenting vacancies and slips as the depth was varied.

Structural and morphological behavior of bismuth thin films grown through DC-magnetron sputtering

espanolEn este trabajo se depositaron peliculas delgadas de bismuto sobre sustratos de vidrio mediante la tecnica de sputtering con magnetron DC y se evaluaron los efectos de la temperatura del sustrato en la microestructura de las peliculas. El comportamiento estructural se analizo mediante difraccion de rayos X, mostrando una notable influencia de la temperatura del sustrato en el tamano de cristalito y microestres. La morfologia evaluada por medio de imagenes de la microsonda electronica, presento una leve diferencia en el tamano de grano con el incremento de la temperatura asi como un significativo incremento de la rugosidad superficial, segun medidas de perfilometria. EnglishBismuth thin films were grown onto glass substrates through the DC magnetron sputtering technique. The effects of substrate temperature on the microstructure of the films were evaluated. The structural behavior was analyzed via X-ray diffraction, and it showed a marked influence of the substrate temperature on the crystallite size and the micro-stress. The morphologies evaluated through Electron Probe Micro-analyzer images indicated a slight difference in the grain size with an increase in temperature as well as a significant increase in surface roughness, according to profilometer measurements.

Graphic User Interface for Monte Carlo Simulation of Ferromagnetic/Antiferromagnetic Manganite Bilayers

The manganites have been widely studied because of their important properties as colossal magnetoresistance and exchange bias that are important phenomena used in many technological applications. For this reason, in this work, a study of the exchange bias effect present in La2/3Ca1/3MnO3/La1/3Ca2/3MnO3. This study was carried out by using the Monte Carlo method and the Metropolis Algorithm. In order to make easy this study, a graphic user interface was built alloying a friendly interaction. The interface permits to control the thickness of Ferromagnetic and Antiferromagnetic layer, temperatures the magnetic field, the number of Monte Carlo steps and the exchange parameters. Results obtained reflected the influence of all of these parameters on the exchange bias and coercive fields.

Modelamiento y simulación de susceptibilidad magnética AC en manganitas La2/3Ca1/3MnO3

In this study, a model for obtaining the AC magnetic susceptibility of La 2/3 Ca 1/3 MnO 3 manganites was proposed. The samples were composed of a combination of Mn 4+ and Mn 3+ ions that influence the material stoichiometry. The model is based on the Hamiltonian, including the Heisenberg interaction and the Zeeman effect. For determining AC susceptibility, a time-dependent external magnetic field h ( t ) was applied to the sample. Several mathematical tools, such as Fourier transforms, were required for performing the simulations. The study was conducted for various angular frequencies and temperatures. The simulations were obtained using a graphic interface that allowed the online monitoring of the results. According to the results, the AC susceptibility decreases not only with the temperature, but also with the frequency

CuO thin films produced for improving the adhesion between Cu and Al2O3 foils in a direct bonded copper (DBC) process

ABSTRACT The direct bonded copper (DBC) process was carried out between Cu and Al2O3 foils and CuO thin films were grown on the surface of Cu foils to reduce the defects produced by the DBC on the surface. CuO thin films were synthesized using a magnetron sputtering system, employing a target of Cu with 99.99% of purity and substrates of Cu foils. The discharge atmosphere for the films growth was (Ar + O2). Once the coatings were grown, coated and uncoated Cu foils were joined at both sides (one on the top and the other at the button) of the alumina foil using the traditional direct bonded process. The Atomic concentration, chemical composition and bonding configuration of both cases were studied by X-ray photoelectron spectroscopy (XPS), finding Metallic Cu, Cu2O and Cu–O bonds; furthermore, the atomic concentration analysis showed that coated Cu foil exhibited lower oxygen percentage, compared with uncoated one. The study of the surface defects was carried out using scanning electron microscopy (SEM) showing that the Al2O3 ceramic was better pasted with the Cu foil including the CuO thin film.

Interfaz gráfica de usuario para la simulación por dinámica molecular de películas delgadas

En este trabajo se implemento el software de simulacion para la nanoindentacion de peliculas delgadas. El software desarrollado empleo la tecnica de Dinamica Molecular. El modelo con el cual opera la interfaz grafica (software), es un modelo aproximado, que reproduce las tendencias de las curvas caracteristicas en pruebas de dureza de materiales. Por otro lado, el programa se diseno y construyo con un entorno amigable, donde el usuario puede construir sus propias peliculas delgadas, tanto en monocapas como en bicapas. La interfaz permite controlar el espesor de la pelicula, el numero de capas y otras propiedades fisicas tales como energia de disociacion o parametros de energia en la esfera; ademas, se puede tener control de los pasos de tiempo de dinamica. Para comprobar el funcionamiento del software se realizaron una serie de simulaciones utilizando los parametros del cromo como material de referencia. Los resultados obtenidos, demostraron que el software puede reproducir las tendencias y curvas caracteristicas en pruebas de dureza de materiales. Se espera mejorar este software para que sea una herramienta que permita realizar comparaciones con futuras investigaciones experimentales.

Hysteresis loop behaviors of ferroelectric thin films： A Monte Carlo simulation study

The ferroelectric response of bismuth titanate Bi4Ti3O12 (BIT) thin film is studied through a Monte Carlo simulation of hysteresis loops. The ferroelectric system is described by using a Diffour Hamiltonian with three terms: the electric field applied in the z direction, the nearest dipole-dipole interaction in the transversal (x-y) direction, and the nearest dipole-dipole interaction in the direction perpendicular to the thin film (the z axis). In the sample construction, we take into consideration the dipole orientations of the monoclinic and orthorhombic structures that can appear in BIT at low temperature in the ferroelectric state. The effects of temperature, stress, and the concentration of pinned dipole defects are assessed by using the hysteresis loops. The results indicate the changes in the hysteresis area with temperature and stress, and the asymmetric hysteresis loops exhibit evidence of the imprint failure mechanism with the emergence of pinned dipolar defects. The simulated shift in the hysteresis loops conforms to the experimental ferroelectric response.