Jan Očenášek

Bio: Jan Očenášek is an academic researcher from University of West Bohemia. The author has contributed to research in topics: Raman spectroscopy & Plasticity. The author has an hindex of 10, co-authored 24 publications receiving 276 citations. Previous affiliations of Jan Očenášek include Technische Universität München.

Micromechanics of pyramidal indentation in fcc metals: Single crystal plasticity finite element analysis

Abstract: This work concerns analysis of Vickers and Berkovich indentation experiments through extensive crystal plasticity finite element simulations. The simulations are performed by recourse to the Bassani and Wu hardening model for pure fcc crystals undergoing both easy-glide stage I and stage II deformations, as well as with the model proposed by Pierce, Asaro and Needleman for precipitation hardened fcc crystals, deforming initially under stage II which also undergo strong hardening saturation in stage III. Simulations are also conducted with a model based on the hardening description by Bassani and Wu, whose physical basis and predictive capability have been enhanced for pure copper crystals. Based upon the activity of the slip systems and the strength of dislocation interactions, this work provides a fundamental insight into the influence of prior work hardening in single crystal indentation. Discussions are given on the role of the latent hardening description upon the development of material pileup and sinking-in at the contact boundary as well as on the correlation between the single crystal and polycrystalline contact responses. The present investigation further illustrates on the influence of the orientation of the slip systems with respect to the pyramidal tip upon the formation of irregular imprint morphologies. Extraction of the single crystal hardening parameters from instrumented indentation P – h s curves is also briefly addressed. Finally, the contact deformation regimes ruling the response of isotropic strain hardening media are examined in light of the simulations for single crystal indentation.

Nanomechanics of flexoelectric switching

Abstract: We examine the phenomenon of flexoelectric switching of polarization in ultrathin films of barium titanate induced by a tip of an atomic force microscope (AFM). The spatial distribution of the tip-induced flexoelectricity is computationally modeled both for perpendicular mechanical load (point measurements) and for sliding load (scanning measurements), and compared with experiments. We find that (i) perpendicular load does not lead to stable ferroelectric switching in contrast to the load applied in the sliding contact load regime, due to nontrivial differences between the strain distributions in both regimes: ferroelectric switching for the perpendicular load mode is impaired by a strain gradient inversion layer immediately underneath the AFM tip; while for the sliding load regime, domain inversion is unimpaired within a greater material volume subjected to larger values of the mechanically induced electric field that includes the region behind the sliding tip; (ii) beyond a relatively small value of an applied force, increasing mechanical pressure does not increase the flexoelectric field inside the film, but results instead in a growing volume of the region subjected to such field that aids domain nucleation processes; and (iii) the flexoelectric coefficients of the films are of the order of few nC/m, which is much smaller than for bulk BaTiO3 ceramics, indicating that there is a “flexoelectric size effect” that mirrors the ferroelectric one

Fast parallel algorithms for short-range molecular dynamics

Abstract: Three parallel algorithms for classical molecular dynamics are presented. The first assigns each processor a fixed subset of atoms; the second assigns each a fixed subset of inter-atomic forces to compute; the third assigns each a fixed spatial region. The algorithms are suitable for molecular dynamics models which can be difficult to parallelize efficiently—those with short-range forces where the neighbors of each atom change rapidly. They can be implemented on any distributed-memory parallel machine which allows for message-passing of data between independently executing processors. The algorithms are tested on a standard Lennard-Jones benchmark problem for system sizes ranging from 500 to 100,000,000 atoms on several parallel supercomputers--the nCUBE 2, Intel iPSC/860 and Paragon, and Cray T3D. Comparing the results to the fastest reported vectorized Cray Y-MP and C90 algorithm shows that the current generation of parallel machines is competitive with conventional vector supercomputers even for small problems. For large problems, the spatial algorithm achieves parallel efficiencies of 90% and a 1840-node Intel Paragon performs up to 165 faster than a single Cray C9O processor. Trade-offs between the three algorithms and guidelines for adapting them to more complex molecular dynamics simulations are also discussed.

Giant Flexoelectric Effect in Ferroelectric Epitaxial Thin Films

Abstract: We report on nanoscale strain gradients in ferroelectric HoMnO3 epitaxial thin films, resulting in a giant flexoelectric effect. Using grazing-incidence in-plane x-ray diffraction, we measured strain gradients in the films, which were 6 or 7 orders of magnitude larger than typical values reported for bulk oxides. The combination of transmission electron microscopy, electrical measurements, and electrostatic calculations showed that flexoelectricity provides a means of tuning the physical properties of ferroelectric epitaxial thin films, such as domain configurations and hysteresis curves.

Flexo-photovoltaic effect

Abstract: It is highly desirable to discover photovoltaic mechanisms that enable enhanced efficiency of solar cells. Here we report that the bulk photovoltaic effect, which is free from the thermodynamic Shockley-Queisser limit but usually manifested only in noncentrosymmetric (piezoelectric or ferroelectric) materials, can be realized in any semiconductor, including silicon, by mediation of flexoelectric effect. We used either an atomic force microscope or a micrometer-scale indentation system to introduce strain gradients, thus creating very large photovoltaic currents from centrosymmetric single crystals of strontium titanate, titanium dioxide, and silicon. This strain gradient–induced bulk photovoltaic effect, which we call the flexo-photovoltaic effect, functions in the absence of a p-n junction. This finding may extend present solar cell technologies by boosting the solar energy conversion efficiency from a wide pool of established semiconductors.