E. R. Leite

Influence of Ca concentration on the electric, morphological, and structural properties of (Pb,Ca)TiO3 thin films

Pb1−xCaxTiO3 (0.10⩽x⩽0.40) thin films on Pt/Ti/SiO2/Si(100) substrates were prepared by the soft solution process and their characteristics were investigated as a function of the calcium content (x). The structural modifications in the films were studied using x-ray diffraction and micro-Raman scattering techniques. Lattice parameters calculated from x-ray data indicate a decrease in lattice tetragonality with the increasing content of calcium in these films. Raman spectra exhibited characteristic features of pure PbTiO3 thin films. Variations in the phonon mode wave numbers, especially those of lower wave numbers, of Pb1−xCaxTiO3 thin films as a function of the composition corroborate the decrease in tetragonality caused by the calcium doping. As the Ca content (x) increases from 0.10 to 0.40, the dielectric constant at room temperature abnormally increased at 1 kHz from 148 to 430. Also calcium substitution decreased the remanent polarization and coercive field from 28.0 to 5.3 μC/cm2 and 124 to 58 kV/c...

Semiconducting Sn3O4 nanobelts: Growth and electronic structure

The study of structures based on nonstoichiometric SnO2−x compounds, besides experimentally observed, is a challenging task taking into account their instabilities. In this paper, we report on single crystal Sn3O4 nanobelts, which were successfully grown by a carbothermal evaporation process of SnO2 powder in association with the well known vapor-solid mechanism. By combining the structural data and transport properties, the samples were investigated. The results showed a triclinic semiconductor structure with a fundamental gap of 2.9 eV. The semiconductor behavior was confirmed by the electron transport data, which pointed to the variable range hopping process as the main conduction mechanism, thus giving consistent support to the mechanisms underlying the observed semiconducting character.

A study of the SnO2·Nb2O5 system for an ethanol vapour sensor: a correlation between microstructure and sensor performance

Abstract Nanometric SnO 2 ·Nb 2 O 5 powders were studied with the purpose of developing an ethanol sensor. The powders were prepared by the Pechini method, morphologically characterised by X-ray diffraction, and their specific surface area determined by BET, after which they were subjected to ethanol vapour sensitivity tests. A correlation was established between the microstructure of the material, the effects of the dopant and the sensors response.

Effect of Cr2O3 in the varistor behaviour of TiO2

Laboratorio Interdisciplinar de Eletroquimica e Cerâmica Departamento de Quimica UFSCar, C.P. 676, 13565-905 Sao Carlos, SP

Analysis of the admittance-frequency and capacitance–voltage of dense SnO2⋅CoO-based varistor ceramics

This article describes the admittance-frequency feature of a class of SnO2⋅CoO-based polycrystalline ceramics with high nonlinear current–voltage characteristics (nonlinear coefficients above 50). Broad relaxation peaks caused by the presence of deep trap states were characterized based on the admittance response of different systems doped with La2O3, Pr2O3, and CeO2. The calculation of the energy of this deep trap level revealed not only that all the compositions share the same value but also that this value could be attributed to an oxygen vacancy or to CoSn▾ like defects. The values of barrier height and density of states obtained from a capacitance–voltage analysis are in good agreement with the nonlinear coefficients. The highest nonlinear coefficients are found in compositions with greater barrier height values and higher density of deep trap states at the grain boundary interface.

Improvement of the dielectric and ferroelectric properties in superlattice structure of Pb(Zr,Ti)O3 thin films grown by a chemical solution route

Making heterolayered perovskite materials constitutes an approach for the creation of better dielectric and ferroelectric properties. In the experiment reported here, heterolayered PZT40/PZT60 films were grown on Pt/Ti/SiO2/Si (100) by a chemical solution deposition. The dielectric constant of the heterolayered thin film was significantly enhanced compared with that of pure PZT40 and PZT60 thin films. A dielectric constant of 701 at 100 kHz was observed for a stacking periodicity of six layers having a total thickness of 150 nm. The heterolayered film exhibited greater remanent polarization than PZT60 and PZT40 films. The values of remanent polarization were 7.9, 18.5, and 31 μC/cm2, respectively, for pure PZT60, PZT40, and heterolayered thin films, suggesting that the superior dielectric and ferroelectric properties of the heterolayered thin film resulted from a cooperative interaction between the ferroelectric phases made from alternating tetragonal and rhombohedral phases of PZT, simulating the morphot...

Microstructural and ferroelectric properties of PbZr1−xTixO3 thin films prepared by the polymeric precursor method

Abstract The polymeric precursor method was employed in the preparation of PZT thin films on Pt(111)Ti/SiO 2 /Si(100) substrates. X-ray diffraction patterns revealed the polycrystalline nature of the PZT (53:47) thin films, which had a granular structure and a grain size of approximately 70 nm. A 350-nm thick film was obtained by running three cycles of the dip-coating/heating process. Atomic force microscopy (AFM) analyses showed the surface of these thin films to be smooth, dense and crack-free with low surface roughness (=2.0 nm). The PZT (53:47) thin films annealed at 700°C showed a well-saturated hysteresis loop. The C – V curves of perovskite thin film displayed normal ferroelectric behavior, while the remanent polarization (2 P r ) and coercive field ( E c ) of the film deposited and measured at room temperature were 40 μC/cm 2 and 110 kV/cm, respectively.

Influence of polymerization on the synthesis of SrTiO3: Part II. Particle and agglomerate morphologies

Abstract Morphologies of SrTiO3 particles and agglomerates synthesized by the traditional Pechini route and by the polymer precipitation route were characterized by the nitrogen adsorption/desorption technique and by transmission electron microscopy (TEM). A cluster structure of nanometric particles forming large agglomerates which are broken during pressing followed by cluster rearrangement was observed. The mean particle size is larger for SrTiO3 obtained by the Pechini route and is related to the precursor thermal decomposition and particle growth during calcination. The particle growth is controlled by neck growth among particles and further motion of the particle boundary.

Grain-boundary segregation and precipitates in La2O3 and Pr2O3 doped SnO2·CoO-based varistors

Structural heterogeneities in SnO2 .CoO-based varistors were analyzed by transmission electron microscopy. In SnO2 .CoO-based system doped with La2O3 and Pr2O3 two kinds of precipitate phases at grain boundary region were found. Using energy dispersive spectrometry they were found to be Co2SnO4 and Pr2Sn2O7, presenting a defined crystalline structure. It was also identified that such precipitate phases are mainly located in triple-junctions of the microstructure. HRTEM analysis revealed the existence of other two types of junctions, one as being homo-junctions of SnO2 grains and other due to twin grain boundaries inside the SnO2 .CoO grain. The role of these types of junction in the overall nonlinear electrical features is also discussed. # 2003 Elsevier Science Ltd. All rights reserved.

Influence of processing conditions on the crystal structure and magnetic behavior of La0.7Ca0.3MnO3±δ samples

Abstract In this work we report crystallographic structure variations and the related modifications on the magnetic behavior of La 0.7 Ca 0.3 MnO 3± δ introduced by heat-treatments in different synthesis atmospheric conditions. We have prepared polycrystalline ceramic samples using a modified polymeric precursors method, which produces highly homogeneous specimens. The use of argon atmosphere enlarges the crystalline c -axis as detected by Rietveld refinements. As a consequence, an improvement in the magnetic transition temperature T C of the samples was observed. Our results also indicate that different heat-treatment conditions change the magnetic interactions between the ferromagnetic (F) and antiferromagnetic (AF) structures of these systems. Our conclusions rely on the use of AC magnetic susceptibility measurements as the experimental tool for measuring these variations.