Synthesis and Electrical Properties of PZT/BaFe12O19 Multiferroic Ceramics

Multiferroic ceramics, based on the ferroelectric and ferrimagnetic phases of the Pb(Zr0.65Ti0.35)O3 (PZT) and BaFe12O19 (BaM) systems, respectively, were obtained from the conventional ceramic method. The electrical properties have been investigated in a wide temperature and frequency range. The influence of the magnetic phase on the ferroelectric and dielectric properties of the ferroelectrics phases have been taken into account. The phase transition characteristics shown to be strongly affected by the amount of the BaM phase, while the dielectric properties revealed to be directly dominated by the presence of conductive effects related to the charge transport mechanisms associated to the magnetic phase.FAPEMIG (00199-09 and 00342-10)CNPq (555496/2009-0 and 301730/2009-1)CAPES (4684/11-7

Quenching of chlorophyll fluorescence induced by silver nanoparticles

The interaction between chlorophyll (Chl) and silver nanoparticles (AgNPs) was evaluated by analyzing the optical behavior of Chl molecules surrounded by different concentrations of AgNPs (10, 60, and 100 nm of diameter). UV–Vis absorption, steady state and time-resolved fluorescence measurements were performed for Chl in the presence and absence of these nanoparticles. AgNPs strongly suppressed the Chl fluorescence intensity at 678 nm. The Stern-Volmer constant (KSV) showed that fluorescence suppression is driven by the dynamic quenching process. In particular, KSV was nanoparticle size-dependent with an exponential decrease as a function of the nanoparticle diameter. Finally, changes in the Chl fluorescence lifetime in the presence of nanoparticles demonstrated that the fluorescence quenching may be induced by the excited electron transfer from the Chl molecules to the metal nanoparticles

Impedance spectroscopy and dielectric properties of flash versus conventionally sintered yttria-doped zirconia electroceramics viewed at the microstructural level

The defect chemistry-modulated dielectric properties of dense yttria-doped zirconia ceramics prepared by conventional sintering (at 1350°C-1500°C) and electric field-assisted flash sintering (55 V/cm at 900°C) were studied by impedance spectroscopy. While the bulk dielectric properties from both sets of samples showed only small and insignificant changes in conductivity and permittivity, respectively, a huge increase of these properties was measured for the grain boundaries in the flash sintered specimens. A close analysis of these results suggests that flash sintering reduced grain-boundary thickness (by about 30%), while increasing the concentration of oxygen vacancies near these interfaces (by about 49%). The underlying mechanism proposed is electric field-assisted generation and accommodation of defects in the space-charge layers adjacent to the grain surface. The changes in measured permittivity are attributed to the boundary thickness effect on capacitance, while conductivity involved variations in its defect densitydependent intrinsic value, accounting for changes also observed in grain-boundary relaxation frequencies. Therefore, in terms of modifications to the specific dielectric properties of these materials, the overall consequence of flash sintering was to considerably lower the semi-blocking character of the grain boundaries.Office of Naval Research (ONR) (N00014-12-1-0710)FAPESP (12/06448-0

Proton mobility calculations in the presence of negative capacitances

During the last years the term “Negative Capacitance" (NC) has appeared in the literature. Until now there is not a satisfactory theory to explain NC. However, in this paper we develop a new method to calculate ionic mobility by using a simple quantitative model linked with the NC phenomenon. The other aim of this paper is to confirm the validity of our previous hypothesis about the origin of NC, and hence to contribute to a better understanding of the NC phenomenon

UV-enhanced ozone sensing response of ZnO-SnO2 heterojunctions at room temperature.

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UV-enhanced ozone gas sensing response of ZnO-SnO2 heterojunctions at room temperature

International audienceThe sensitivity of ZnO-SnO2 heterojunctions to ozone gas was investigated in this work, the two-phase materials of which were prepared via a hydrothermal route, resulting in nanocomposites in which the formation of heterojunctions was confirmed by microscopy analyses. While the sensing effectiveness of these materials is currently verified for application above 150 °C, these temperatures are here drastically reduced to room temperature by considering sensing activity under continuous UV irradiation, even for ozone concentrations as low as 20 ppb. This approach resulted in a fast sensing response, a short recovery time and a good selectivity compared to other gases, demonstrating a great potential of such heterojunctions for applications in environmental monitoring devices

Structural and optical properties of rare earth-doped (Ba(0.77)Ca(0.23))(1-x)(Sm, Nd, Pr, Yb)(x)TiO(3)

The structural, dielectric, and vibrational properties of pure and rare earth (RE)-doped Ba(0.77) Ca(0.23)TiO(3) (BCT23; RE = Nd, Sm, Pr, Yb) ceramics obtained via solid-state reaction were investigated. The pure and RE-doped BCT23 ceramics sintered at 1450 degrees C in air for 4 h showed a dense microstructure in all ceramics. The use of RE ions as dopants introduced lattice-parameter changes that manifested in the reduction of the volume of the unit cell. RE-doped BCT23 samples exhibit a more homogenous microstructure due to the absence of a Ti-rich phase in the grain boundaries as demonstrated by scanning electron microscopy imaging. The incorporation of REs led to perturbations of the local symmetry of TiO(6) octahedra and the creation of a new Raman mode. The results of Raman scattering measurements indicated that the Curie temperature of the ferroelectric phase transition depends on the RE ion and ion content, with the Curie temperature shifting toward lower values as the RE content increases, with the exception of Yb(3+) doping, which did not affect the ferroelectric phase transition temperature. The phase transition behavior is explained using the standard soft mode model. Electronic paramagnetic resonance measurements showed the existence of Ti vacancies in the structure of RE-doped BCT23. Defects are created via charge compensation mechanisms due to the incorporation of elements with a different valence state relative to the ions of the pure BCT23 host. It is concluded that the Ti vacancies are responsible for the activation of the Raman mode at 840 cm(-1), which is in agreement with lattice dynamics calculations. (c) 2011 American Institute of Physics. [doi:10.1063/1.3594710