Navegando por Palavras-chave "Ferroelectrics"
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- ItemSomente MetadadadosInfluence of Domain Switching and Domain Wall Bending on the Electrical Permittivity of PZT Thin Films(Taylor & Francis Ltd, 2011-01-01) Lente, Manuel Henrique [UNIFESP]; Bacichetti, Antonio Leondino; Mendes, Renata Gonçalves; Eiras, Jose Antonio; Universidade Federal de São Paulo (UNIFESP); Universidade Federal de São Carlos (UFSCar)The electrical permittivity dependence on the electric bias field was investigated in Pb(Zr-0.53, Ti-0.47)O-3 films. the results revealed the existence of two mechanisms contributing to the permittivity. the first one was related to the domain switching, which was responsible for a strong nonlinear dielectric response. the second mechanism was associated with the non-180 degrees domain wall vibrations, which presented a reasonable linear electrical behavior with the applied field, contributing always to the permittivity independently of the poling state of the sample. the reduction of the electrical permittivity with increasing the bias field was related to the bending of non-180 degrees domain walls.
- ItemSomente MetadadadosPhase transitions and magnetoelectroelastic instabilities in Pb(Fe1/2Nb1/2)O-3 multiferroic ceramics(Elsevier B.V., 2013-03-01) Fraygola, B.; Frizon, N.; Lente, M. H. [UNIFESP]; Coelho, A. A.; Garcia, D.; Eiras, J. A.; Universidade Federal de São Carlos (UFSCar); Universidade Federal de São Paulo (UNIFESP); Universidade Estadual de Campinas (UNICAMP)Magnetoelectric coupling can be observed in multiferroic materials in the regions where magnetic and ferroelectric ordering coexist and in many cases is mediated via lattice strain. Pb(Fe1/2Nb1/2)O-3 (PFN) is a well-known multiferroic, presenting ferroelectric ordering (below T-C = 380 K) and a Neel temperature (antiferromagnetic ordering) T-N = 143 K. However, recent experimental results suggest that PFN can display ferromagnetic ordering at higher temperatures and a still unclear sequence of phase transitions. in this work, high-density PFN ceramics were prepared by a conventional ceramic method. Dielectric, magnetic and anelastic responses were measured as a function of temperature. the experimental results present complementary evidence of anomalies (additional to those which can be directly associated with the reported transitions at T-C and T-N) which cold be associated with a structural phase transition (T similar to 315 K), an antiferromagnetic ferromagnetic phase transition (110 K) and magnetoelectroelastic instabilities around 250 K. (C) 2012 Acta Materialia Inc. Published by Elsevier B.V. All rights reserved.
- ItemSomente MetadadadosPropriedades físicas de fibras monocristalinas de niobato de sódio e potássio puras e dopadas com cobre obtidas por Micro-Pulling-Down(Universidade Federal de São Paulo (UNIFESP), 2020-02-28) Silva, Marcus Vinicius De Siqueira [UNIFESP]; Lente, Manuel Henrique [UNIFESP]; Universidade Federal de São PauloDue to the toxicity of lead and its large application in electronic devices, the current work aimed to study the matrix ((Na0.52,K0.48) NbO3 and (Na0.52,K0.48) (Nb0.985,Cu0.015)O3 in single crystal fibers as an alternative to replace lead-based ferroelectric (Pb) materials, such as lead zirconate titanate. Additionally, single crystal ferroelectric materials, in general, show enhanced electromechanical properties compared to polycrystalline ceramics of same composition. The experimental growth proccess of pure and copper-doped single crystal fibers was carried out evaluating the effect of the atmosphere (argon, synthetic air and oxygen) on the dielectric properties of the grown fibers. The influence of the growth atmosphere on the microstructural, structural, chemical and dilatometric properties was also evaluated. The results reveled that synthetic air is the best atmosphere to grow single crystal fibers.
- ItemSomente MetadadadosSíntese e estudo das transições de fases estruturais no sistema (1−x)Ba(Zr0.2Ti0.8)O3–x(Ba0.7Ca0.3)TiO3 (BCZT) utilizando a associação de diferentes técnicas de caracterização(Universidade Federal de São Paulo (UNIFESP), 2019-07-31) Aredes, Rangel Graudiston [UNIFESP]; Antonelli, Eduardo [UNIFESP]; Universidade Federal de São Paulo (UNIFESP)Lead-free piezoelectric ceramics can demonstrate excellent properties when their compositions are projected with sequences of specific phase transitions and / or the coexistence of phases. It has been reported that some compositions of (1−x)Ba(Zr0.2Ti0.8)O3–x(Ba0.7Ca0.3)TiO3 (xBCZT) may present phases coexistence (rhombohedral “R” and orthorhombic “O”) at room temperature. Besides, recent studies have observed the O phase as an intermediate phase before the tetragonal “T”-cubic “C” phase transitions. Herein, we study the xBCZT to verify if the experimental characterizations can detect R-O, O-T and T-C phase transitions and identify the physical anomalies that confirm the phases coexistence. The phase transitions were studied during a heating program for xBCZT and compared with pure BaTiO3 by dielectric spectroscopy, pyroelectric coefficient analyses, thermal analyses, linear expansion and powder X-ray diffraction. The interpretation of different responses allows us to conclude that the substitutions of Ca2+ for Ba2+ or Zr4+ for Ti4+ in the formulation xBCZT can result in two or more different solid solutions, belonging to the perovskite family, which coexist in the ceramic. The transitions of each one of the phases happen in a cooperative effect and, in some specific interval of temperatures, the break of the cooperative effect happens and make clear the phase coexistence.