Visual Analysis of Polarization Domains in Barium Titanate During Phase Transitions

WSCG 2014 Conference on Computer Graphics, Visualization and Computer Vision Visual Analysis of Polarization Domains in Barium Titanate during Phase Transitions Tobias Brix Florian Lindemann Jörg-Stefan Praßni Stefan Diepenbrock Klaus Hinrichs University of Münster Einsteinstraße 62 Germany 48149, Münster, NRW t.brix | lindemann | j-s.prassni | diepenbrock | khh @uni-muenster.de ABSTRACT In recent years, the characteristics of ferroelectric barium titanate (BaTiO3) have been studied extensively in materials science. Barium titanate has been widely used for building transducers, capacitors and, as of late, for memory devices. In this context, a precise understanding of the formation of polarization domains during phase transitions within the material is especially important. Therefore, we propose an application that uses a combination of proven visualization techniques in order to aid physicists in the visual analysis of molecular dynamic simulations of BaTiO3. A set of linked 2D and 3D views conveys an overview of the evolution of dipole moments over time by visualizing single time steps as well as combining multiple time steps in one single static image using flow radar glyphs. In addition, our system semi-automatically detects polarization domains, whose spatial relation can be interactively analyzed at different levels of detail on commodity hardware. The evolution of selected polarization domains over their lifetime can be observed by a combination of animated spatial and quantitative views. Keywords Visualization in Physical Sciences and Engineering, Glyph-based Techniques, Time-varying Data. 1 INTRODUCTION As computer simulations of molecular dynamics (MD) have become standard in physical research, the need for Since ferroelectricity in barium titanate (BaTiO ) has 3 a visualization of MD became apparent. Now there ex- been discovered in 1945 by Wul and Goldman [25], it ist many software packages for visualizing molecular has been one of the most exhaustively studied materi- structures. However, to our knowledge there is no dedi- als [15]. It has an industrial importance based on the cated tool to guide scientists in detecting and exploring common usage of BaTiO in thin films ([21],[5]) for 3 ferroelectric domains in a simulation dataset. memory devices like Fe-RAM. In 2012 the topic was even in the focus of the IEEE From a physical point of view, the material is interesting VisWeek SciVis contest [23]. In this paper we present because of its displacive phase transitions and its polar- our application to visualize polarization domains in bar- ization domain formation. The movement of positively ium titanate during phase transitions in combination and negatively charged ions during a phase transition with semi-automatic algorithms to classify domains. results in local dipole moments in the unit cells of the The tool provides numerical methods for dipole mo- barium titanate grid. Regions with similarly directed ment calculation and detection of resulting polarization dipole moments are called polarization domains. domains. The user can obtain a quick overview of the A visualization of the evolution of these polarization behavior of BaTiO during phase transitions by using domains during phase transitions in ferroelectric mate- 3 glyph techniques to reduce the potential clutter. A more rials can be useful to get deeper insights into the phys- in-depth study of spatial and temporal evolution of po- ical phenomenon of ferroelectricity. In 1963, first ap- larization domains is supported by several linked views proaches relied solely on microscopes [2] to catch a using proven visualization techniques for a visual anal- glimpse of domain formation. ysis, and by numerical parameters and histograms refer- ring to selected time steps or domains for a quantitative analysis. Our application has been implemented in the Permission to make digital or hard copies of all or part of framework Voreen (Volume Rendering Engine) [18] de- this work for personal or classroom use is granted without fee provided that copies are not made or distributed for profit veloped at the University of Münster. All statements ac- or commercial advantage and that copies bear this notice and cording to frames per seconds (fps) or processing times the full citation on the first page. To copy otherwise, or re- refer to a commodity computer with an Intel i7 proces- publish, to post on servers or to redistribute to lists, requires sor, 16 GB RAM and an AMD Radeon HD 7900 Series prior specific permission and/or a fee. graphics card. Full Papers Proceedings 9 ISBN 978-80-86943-70-1 WSCG 2014 Conference on Computer Graphics, Visualization and Computer Vision 2 RELATED WORK In the late 1950s theoretical physics conceived the idea of simulating molecular behavior (molecular dynamics) [1] in addition to the observation of molecular phe- nomena in laboratories with technical devices. With more powerful computers these simulations became more feasible and cost-efficient than laboratory studies. In order to handle the generated simulation data effi- ciently, numerous molecule representations have been implemented in different software frameworks. Exam- ples of such software frameworks are Visual Molecule Dynamics (VMD) [9] and the Open Visualization Tool OVITO [22] both of which inspired our design. Although many different representations of molecule structures are frequently employed, like ribbons [3], which usually represent the backbone of protein struc- Figure 1: Schematic representation of the application tures, or Van der Waals surfaces [24], which visualize workflow, which is subdivided in a preprocessing stage the surface of a molecule based on the van der Waals and an interactive analysis stage. forces, we simply use the classic ball-and-stick model by A. Hofmann [10]. Since our application is intended in a single static image by these glyphs. for exploring ferroelectric domains, we do not see a The application by Scharnowski et al. ([20]) for finding benefit of supporting further molecular representations. polarization domains and exploring the data in several If a need for further representations arises we refer to visualizations was the winning entry in the VisContest the above mentioned visualization frameworks. 2012 [23]. They used surface representations based on The biggest challenge in molecular visualization is the Krone et al. [13] for the polarization domains and vec- large quantity of atoms. To visualize millions of atoms tor fields for representing the dipole moments for each at interactive frame rates, advanced rendering algo- unit cell. We are using the same vector visualization rithms and storage techniques have to be used. Op- for dipole moments, but differ in the domain represen- timized rendering techniques have been presented for tation. Furthermore, instead of having only one view to example by Lampe et al. [16], who use billboards, i. e., see temporal evolution in the detected polarization do- quads facing towards the camera, instead of rendering mains, which can be tracked and identified over time, geometrical spheres for representing the atoms, and by we use several linked views to provide an application Krone et al. [12], who use a ray casting approach. In for supporting physical research in ferroelectrics. One our application we employ the billboards as proposed of the latest publications in the field of dipol moment by Lampe et al. visualizations is from 2012 by Grottel et al.[6], who In the past, most visualizations in the physical research also use an arrow glyph based representation of the of polarization domains in ferroelectric materials have dipol moments. Like Scharnowski et al. they use the been generated without applying computer rendering framework MegaMol [17] developed at the University techniques. In 1963 Amelinckx and Blank [2] have ob- of Stuttgart. served polarization domains in BaTiO3 by an electron microscope and used simple gray scale slices for visualization. Later in 1995, Kolosov et al. [11] visualized 3 APPLICATION DESIGN ferroelectric domains by an atomic force microscope. Our application workflow can be subdivided into two Although it has been the first observation with a resolu- stages as shown in Fig. 1. In the data processing stage tion of 10nm they also used coarse pixelated gray scale the dipole moments of each unit cell are computed, and slices for visualization. In 2010, Pasciak´ et al. [19] em- the resulting polarization domains are detected (see ployed a 3D approach using an algorithm to automati- Sec. 4). Since these calculations are quite expensive, cally detect polarization domains in barium titanate and a diagnosis on the fly is not possible. The subsequent visualizing the results in a binary volumetric manner interactive visual analysis stage encompasses the by distinguishing unit cells belonging to domains from visualizations for exploring the data and for analyzing those which lie outside. In our application we use glyph the pre-detected domains (see Sec. 5). techniques to represent the domains. We want to stress The complete dataset, the precomputed dipole mo- our use of the flow radar glyph of Hlawatsch et al. [8], ments and the detected polarization domains are stored which is a glyph based flow visualization applied to a in main memory in order to achieve interactive frame molecular problem. All dipole moments of multiple rates during the visual analysis stage. Therefore, it time steps of the molecular dynamics can be visualized is necessary to convert the data obtained from the Full Papers Proceedings

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