THE FUNDAMENTAL SOLUTIONS OF DIFFERENTIAL OPERATOR WITH BLOCK - TRIANGULAR OPERATOR COEFFICIENTS

For the non-selfadjoint Sturm-Liouville equation with a block-triangular operator potential that increases at infinity, both increasing and decreasing at infinity operator solutions are found. The asymptotics of these solutions at infinity is defined

Room temperature surface piezoelectricity in SrTiO3 ceramics via piezoresponse force microscopy

SrTiO3 ceramics are investigated by piezoresponse force microscopy. Piezoelectric contrast is observed on polished surfaces in both vertical and lateral regimes and depends on the grain orientation varying in both sign (polarization direction) and amplitude. The observed contrast is attested to the surface piezoelectricity due to flexoelectric effect (strain gradient-induced polarization) caused by the surface relaxation. The estimated flexoelectric coefficient is approximately one order of magnitude smaller as compared to those recently measured in SrTiO3 single crystals. The observed enhancement of piezoresponse signal at the grain boundaries is explained by the dipole moments associated with inhomogeneous distribution of oxygen vacancies

Simulação de dano e fractura em ensaios de flexão em 3 pontos em provetes laminados

Mestrado em Engenharia MecânicaPolímeros reforçados por fibras (PRF) tem sido utilizados durante muito tempo para aplicações estruturais, particularmente com laminados. No projeto de laminados é importante caracterizar o seu comportamento mecânico. O comportamento mecânico é caracterizado por uma rigidez inicial e pela fractura. A estratégia de modelação utilizada nesta dissertação permite não só prever o “first-ply-failure”, como também modelar a sequência de eventos a seguir, como a delaminação e a fractura final da estrutura. Para isso é utilizado um modelo para a interface e para as camadas, permitindo a interação entre eles. A rigidez inicial também é avaliada com diferentes ratios comprimento para espessura, permitindo também diferentes contribuições do cisalhamento transversal e de flexão. Nesta dissertação com o método dos elementos finitos (FEM) é investigado o comportamento estrutural de uma viga compósita sobe ensaios de flexão em três pontos. Para este fim o programa comercial Abaqus é utilizado, permitindo simulações numéricas com elementos continuum casca e elementos convencionais casca. Também são realizadas simulações numéricas de modo a preparar a resultados para uma futura experiência, para o qual o tamanho do provete é escolhido com recomendações do EN ISO 14125Fiber-reinforced polymers (FRP) have been used for a long time for structural applications, particularly with laminates. It is important to characterize the mechanical behavior of laminates for their design. This behavior is characterized by an initial stiffness and by failure. The modeling strategy used in this thesis allows to predict not only first-ply-failure (FPF), but also to model the sequence of events afterwards, such as delamination and the final failure of the structure. This is achieved by using a damage model for the ply and for the interface, allowing the interaction between them. The initial stiffness is also evaluated with different length to thickness ratios, allowing different contributions from transverse shear and bending. In this thesis, with the finite element method (FEM) the structural behavior of a laminate composite beam under a three point bending configuration is investigated. For this purpose the commercial FEM package Abaqus is used, allowing numerical simulations with continuum shell and conventional shell elements. Also simulations are conducted, in order to prepare a future experiment for which the specimen size was chosen with recommendations from EN ISO 14125

Piezoresponse Force Microscopy: A Window into Electromechanical Behavior at the Nanoscale

Piezoresponse force microscopy (PFM) is a powerful method widely used for nanoscale studies of the electromechanical coupling effect in various materials systems. Here, we review recent progress in this field that demonstrates great potential of PFM for the investigation of static and dynamic properties of ferroelectric domains, nanofabrication and lithography, local functional control, and structural imaging in a variety of inorganic and organic materials, including piezoelectrics, semiconductors, polymers, biomolecules, and biological systems. Future pathways for PFM application in high-density data storage, nanofabrication, and spectroscopy are discussed

Domain Dynamics in Piezoresponse Force Microscopy: Quantitative Deconvolution and Hysteresis Loop Fine Structure

Domain dynamics in the Piezoresponse Force Spectroscopy (PFS) experiment is studied using the combination of local hysteresis loop acquisition with simultaneous domain imaging. The analytical theory for PFS signal from domain of arbitrary cross-section is developed and used for the analysis of experimental data on Pb(Zr,Ti)O3 polycrystalline films. The results suggest formation of oblate domain at early stage of the domain nucleation and growth, consistent with efficient screening of depolarization field within the material. The fine structure of the hysteresis loop is shown to be related to the observed jumps in the domain geometry during domain wall propagation (nanoscale Barkhausen jumps), indicative of strong domain-defect interactions.Comment: 17 pages, 3 figures, 2 Appendices, to be submmited to Appl. Phys. Let

Piezoresponse in ferroelectric materials under uniform electric field of electrodes

The analytical solution for the displacements of an anisotropic piezoelectric material in the uniform electric field is presented for practical use in the “global excitation mode” of piezore-sponse force microscopy. The solution is given in the Wolfram Mathematica interactive program code, allowing the derivation of the expression of the piezoresponse both in cases of the anisotropic and isotropic elastic properties. The piezoresponse’s angular dependencies are analyzed using model lithium niobate and barium titanate single crystals as examples. The validity of the isotropic approximation is verified in comparison to the fully anisotropic solution. The approach developed in the paper is important for the quantitative measurements of the piezoelectric response in nano-materials as well as for the development of novel piezoelectric materials for the sensors/actuators applications. © 2021 by the author. Licensee MDPI, Basel, Switzerland.This research was funded by the Russian Science Foundation, grant number 19-72-10076

Natural and Eco-Friendly Materials for Triboelectric Energy Harvesting

Triboelectric nanogenerators (TENGs) are promising electric energy harvesting devices as they can produce renewable clean energy using mechanical excitations from the environment. Several designs of triboelectric energy harvesters relying on biocompatible and eco-friendly natural materials have been introduced in recent years. Their ability to provide customizable self-powering for a wide range of applications, including biomedical devices, pressure and chemical sensors, and battery charging appliances, has been demonstrated. This review summarizes major advances already achieved in the field of triboelectric energy harvesting using biocompatible and eco-friendly natural materials. A rigorous, comparative, and critical analysis of preparation and testing methods is also presented. Electric power up to 14 mW was already achieved for the dry leaf/polyvinylidene fluoride-based TENG devices. These findings highlight the potential of eco-friendly self-powering systems and demonstrate the unique properties of the plants to generate electric energy for multiple applications.[Figure not available: see fulltext.]. © 2020, © 2020, The Author(s).Instituto Nacional de Ciência e Tecnologia para Excitotoxicidade e Neuroproteção, INCT-ENMinistry of Education and Science of the Russian Federation, MinobrnaukaFundação para a Ciência e a Tecnologia, FCT: SFRH/BPD/117475/2016, CENTRO-01-0145-FEDER-031679, POCI-01-0145-FEDER-031132This work was developed within the scope of the project CICECO-Aveiro Institute of Materials, refs. UIDB/50011/2020 & UIDP/50011/2020, financed by national funds through the FCT/MEC. S.K. and A.K. were partly supported by FCT (Portugal) through the project “BioPiezo”- PTDC/CTM–CTM/31679/2017 (CENTRO-01-0145-FEDER-031679). M. Soares dos Santos was also supported by FCT, through the grant reference SFRH/BPD/117475/2016. All authors were partly supported by FCT through the project “SelfMED” (POCI-01-0145-FEDER-031132). Part of this work was funded by national funds (OE), through FCT—Fundação para a Ciência e a Tecnologia, I.P., in the scope of the framework contract foreseen in the numbers 4, 5, and 6 of the article 23, of the Decree-Law 57/2016, of August 29, changed by Law 57/2017, of July 19. The research was also supported by the Ministry of Education and Science of the Russian Federation in the framework of the Increase Competitiveness Program of NUST « MISiS » (No. K2-2019-015)