Low-Symmetry Monoclinic Phases and Polarization Rotation Path Mediated By Epitaxial Strain in Multiferroic BiFeO3 Thin Films

A morphotropic phase boundary driven by epitaxial strain has been observed in a lead-free multiferroic BiFeO3 thin films and the strain-driven phase transitions were widely reported to be iso-symmetric Cc-Cc ones by recent works. In this paper, we suggest that the tetragonal-like BiFeO3 phase identified in epitaxial films on (001) LaAlO3 single crystal substrates is monoclinic MC. This MC phase is different from MA type monoclinic phase reported in BiFeO3 films grown on low mismatch substrates, such as SrTiO3. This is confirmed not only by synchrotron x-ray studies but also by piezoresponse force microscopy measurements. The polarization vectors of the tetragonal-like phase lie in the (100) plane, not the (110) plane as previously reported. A phenomenological analysis was proposed to explain the formation of MC Phase. Such a low symmetry MC phase, with its linkage to MA phase and the multiphase coexistence open an avenue for large piezoelectric response in BiFeO3 films and shed light on a complete understanding towards possible polarization rotation paths and enhanced multiferroicity in BiFeO3 films mediated by epitaxial strain. This work may also aid the understanding of developing new lead-free strain-driven morphotropic phase boundary in other ferroic systems.Comment: 22 pages,Submitted to Advanced Functional Materials on Sep,7,2010, accepted on Oct,27,201

Comparative material study between PZT ceramic and newer crystalline PMN-PT and PZN-PT mateirals for composite bimorph actuators.

International audienceThe advent of commercially available giant piezoelectric coefficient monocrystalline materials such as PMN-PT (lead magnesium niobate - lead titanate) or PZN-PT (lead zinc niobate - lead titanate) broadens the gate for silicon-integrated applications (PiezoMEMS). Becoming more compatible with microtechnology batch processes, further advances are expected in terms of miniaturization, optimization, functionality or integration with electronics, all while reducing manufacturing costs. Subsequently, operating voltage will be lower and devices response time will improve dramatically. The paper compiles a base knowledge for composite bimorph actuators in line with a bottom-up approach for further more complex piezoelectric device designs such as "microrobots-on-chips". Material properties and constitutive equations of piezoelectric bimorph cantilevers are initially overviewed. Analytical and finite elements modeling (FEM) are afterwards performed on two designs : classical PZT on copper cantilevers and innovative PMN-PT and PZN-PT on silicon. Comparative results clearly report quantitative improvement of PMN-PT on Si design in terms of tip displacement and blocking force

Material selection for Micro-Electro-Mechanical-Systems (MEMS) using Ashby's approach

A key aspect in design optimization of a product or a system is the selection of materials that best meet the design needs, ensuring maximum performance and minimum cost. Ashby's approach, originally introduced for macro-systems and products, has been very successfully employed for Micro-Electro-Mechanical-Systems (MEMS)/micromachined sensors, actuators and devices. This paper presents a comprehensive review and critical analysis of MEMS material selection studies using Ashby's approach reported in the literature during the last two decades. Performance and Material Indices derived for various microsystems and MEMS devices have been summarized. Moreover, all MEMS materials reported in the literature and the most suitable materials proposed for a variety of MEMS systems and devices have also been consolidated. A material selection case study utilizing micro-scale properties of 51 MEMS compatible materials has been presented to demonstrate that the use of different materials' bulk properties is not the best choice for MEMS materials selection. This paper will serve as a reference guide and useful resource for researchers and engineers engaged in the design and fabrication of various microsystems and MEMS sensors, actuators and devices

Material characterization and modeling for piezoelectric actuation and power generation under high electromechanical driving levels

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Aeronautics and Astronautics, 2002.Includes bibliographical references (p. 257-262).High electromechanical loads parallel to piezoelectric polarization might result in depolarization of the material, depending on the material property itself and the external excitations such as electrical field, electrical driving frequency, stress and stress duration. In this work, material properties under these effects were first characterized experimentally. The experiments included monitoring general piezoelectric responses of PZT-5H and PZT-5A subjected to large electric excitations (butterfly curves) under various static compressions and measuring generalized piezoelectric constants under short and open circuit conditions for actuation of PZT-5A and power generation of PZT-5H, single crystals PZN-PT, and single crystals PMN-PT. To model these observed material behaviors, one- and three-dimensional rate dependent nonlinear constitutive models based on thermodynamic potentials for PZT-5H and PZT-5A piezoelectric materials were then developed. An internal variable, net remnant polarization D*, was used to simulate the hysteric behaviors of piezoelectric materials. An evolution law of D* was derived to specify the rate dependent responses of the materials. The parameters of the material models were determined by minimizing the error between the data and the models. The material models were capable of describing the responses subjected to large electric excitations under static compression, but incapable of predicting accurate piezoelectric constants under dynamic compression. This flaw was believed due to the absence of stress rate dependency in the models. It was also found that the PZT-5A model performed worse than the PZT-5H model because of its highly hysteretic strain-polarization relation.(cont.) This hysteresis could be explained by the slow switching rate of 90-degree domain movement. Finally, to simulate devices under non-uniform field or with irregular geometries using these material models, differential algebraic equations for mixed finite element analysis of 3-D nonlinear rate dependent piezoelectric materials were formulated and solved numerically by DASPK solver. Using 4-node tetrahedral elements, this formulation was demonstrated by examples with uniform and skewed electric excitations. The combination of the nonlinear mixed FEM model and the material model provided a useful tool for modeling the response of active devices with complicated geometries and irregular boundary conditions.by Ching-Yu Lin.Ph.D

Application of a phase-field model to ferroelectrics

The current work uses a phase-field model to design nanoscale ferroelectric device concepts and to explore nanoscale ferroelectric behaviour. The results are expected to serve initial steps for nanoscale experimentation and to assist in the industrial application of ferroelectrics. The use of ferroelectrics in nanoscale devices such as sensors or energy harvesters has increased in recent times. This has advanced research interests in nanoscale ferroelectric properties. In the current study, a phase-field model is used as a research tool to explore nanoscale behaviour of barium titanate. The model is first used to design and optimize nano-actuator concepts that generate actuation strains ~0.45%. This strain is four times as large as the strain offered by piezoceramic actuators in market. Next, the model is used to describe how scaling and surface conditions affect the formation of nanoscale polarization patterns, such as vortices. The results demonstrate the dominant effect of surface conditions and explain experimental observations of phase, tetragonality and polarization patterns in ferroelectric nanoparticles. The phase-field model is next extended to three dimensions and is used to test the stability of nanoscale periodic polarization patterns. The results describe the internal stress and electric fields developed in these patterns, and illustrate a mechanism of how complex polarization patterns can be formed under external strain or electric fields. Here, a conceptual design of an energy harvester is also explored. The current work explores the application of a phase-field model as a design tool to develop nano-actuator and energy harvester concepts. The results on actuation/harvester cycle and the design parameter optimization would assist in developing device prototypes and eventually benefit in industrial fabrication. The phase-field model extended to three dimensions contributes to the study of domain patterns with out-of-plane polarizations and will assist in engineering a more realistic ferroelectric device. The results on the effects of scaling, surface energy and external loads on nanoscale polarization patterns, explains prior experimental observations of polarization patterns and provide useful insights on how to engineer domain configurations for nanoscale applications. Finally, the results motivate research towards developing prospective nanoscale applications from other smart materials, such as relaxors and ferromagnets

Issues in the design of shape memory alloy actuators

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Aeronautics and Astronautics, 2002."June 2002."Includes bibliographical references (p. 93-96).This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.This thesis considers the application of shape memory alloy (SMA) actuators for shape control of the undertray of a sports car. By deforming the shape of the structure that provides aerodynamic stability to the car, we expect to improve the overall performance of the vehicle by adapting its aerodynamics according to the vehicle speed. We then develop a methodology for designing SMA actuators in this application. The methodology is based on the integration of the different models involved: mechanical, thermal, and electrical. The constraints imposed on the device are also incorporated. Unfortunately, the analysis predicts an actuation time that is too slow for this particular application. Still, we use our assembled model to sketch the expected characteristics of SMA actuators. A significant result is that the actuation time is a function of the amount of energy the active material has to provide, and that there is a necessary trade-off between the mass of actuators and the actuation time. In particular, the expected energy density may have to be decreased to achieve acceptable actuation times. Finally, we propose a way to estimate a priori the suitability of SMA actuators for a particular application.by Stéphane Lederlé.S.M

International Conference on Nanomaterials Science and Mechanical Engineering: book of abstracts

The conference (International Conference on Nanomaterials Science and Mechanical Engineering, University of Aveiro, Portugal, July 16-18, 2018) looks for significant Modern Problems of Nanomaterials Science and Mechanical Engineering, to provide a platform to the global researchers and practitioners from both academia as well as industry to meet and share cutting-edge development in the fields, to give possibility for young scientists and students present results and find your place in the future world.publishe

Synthèse d'oligomères et de polymères enrichis en porphyrines pour la conversion de l'énergie solaire

Le projet de cette thèse consistait à élaborer de nouveaux matériaux donneurs d’électrons pour les cellules solaires organiques. Cette technologie photovoltaïque émergente en plein essor a d’ores et déjà atteint la limite d’efficacité lui permettant d’être industrialisée et commercialisée à grande échelle. Le faible coût de production des dispositifs photovoltaïques organiques les rendent compétitives vis-à-vis des technologies inorganiques déjà bien implantées. Mais leur plus gros avantage est surement leur légèreté et leurs propriétés mécaniques qui les rendent très souples. Elles devraient donc certainement avoir un rôle majeur à jouer dans le futur en complément des cellules solaires classiques, avec une utilisation pour des applications spécifiques. Nous avons ainsi développé des polymères en utilisant des chromophores réputés pour leurs propriétés photophysiques : les porphyrines, les BODIPY et les dicétopyrrolopyrroles. Ces différentes unités absorbent intensément la lumière, ce qui les rend adéquates pour être utilisées pour la conversion de l’énergie solaire en électricité. En concevant un design original et adapté à cette application, nous avons ainsi obtenu plusieurs nouveaux polymères prometteurs. Nous avons ensuite pu étudier leurs propriétés électrochimiques et électroniques, ainsi que leurs caractéristiques photophysiques. Pour cela nous avons utilisé de nombreux outils (caméra streak, absorption transitoire femtoseconde, etc.) afin de comprendre en détails leur propriétés d’absorption et de luminescence. Ces informations nous ont permis de pouvoir ensuite comprendre leur comportement une fois intégrés dans la couche active des dispositifs photovoltaïques. En effet, le mécanisme de fonctionnement pour la création d’un courant électrique met en jeu des transferts d’électrons ultrarapides (∼50 fs) vers un accepteur d’électron. Il est alors crucial de pouvoir comprendre et contrôler les paramètres pouvant influencer l’efficacité de ces transferts et la stabilisation des charges qui en résultent, pour pouvoir finalement mener à des rendements de conversion de l’énergie lumineuse élevés

Novel Multiferroic Magnetoelectric Materials from the Bix+yPb1-x-yFexMnyTi1-x-yO3 Ternary System along the Line of Morphotropic Phase Boundaries

Tesis Doctoral inédita leída en la Universidad Autónoma de Madrid, Facultad de Ciencias, Departamento de Química-Física Aplicada. Fecha de lectura: 29-06-2016Esta tesis tiene embargado el acceso al texto completo hasta el 29-12-201