Physical characterization of K-doped 0.9PMN-0.1PT synthesized by an association of the columbite route and the polymeric precursor method

The solid solution 0.9PbMg 1/3Nb 2/3O 3-0.1PbTiO 3 is one of the most widely investigated relaxor ceramic, because of its high dielectric constant and low sintering temperatures. PMN-PT powders containing single perovskite phase were prepared by using a Timodified columbite precursor obtained by the polymeric precursor method. Such precursor reacts directly with stoichiometric amount of PbO to obtain pyrochlore-free PMN-PT powders. The structural effects of K additive included in the columbite precursor and 0.9PMN-0.1PT powders were also studied. The phase formation at each processing step was verified by XRD analysis, being these results used for the structural refinement by the Rietveld method. It was verified the addition of K in the columbite precursor promotes a slight increasing in the powder crystallinity. There was not a decrease in the amount of perovskite phase PMN-PT for 1mol% of K, and the particle and grain size were reduced, making this additive a powerful tool for grain size control.

Metal-insulator transition in Nd1-xEuxNiO 3 compounds

Polycrystalline Nd1-xEuxNiO3 (0≤x≤0.5) compounds were synthesized in order to investigate the character of the metal-insulator (MI) phase transition in this series. Samples were prepared through the sol-gel route and subjected to heat treatments at ∼1000 °C under oxygen pressures as high as 80bar. X-ray diffraction (XRD) and neutron powder diffraction (NPD), electrical resistivity ρ(T), and magnetization M(T) measurements were performed on these compounds. The NPD and XRD results indicated that the samples crystallize in an orthorhombic distorted perovskite structure, space group Pbnm. The analysis of the structural parameters revealed a sudden and small expansion of ∼0.2% of the unit cell volume when electronic localization occurs. This expansion was attributed to a small increase of ∼0.003 of the average Ni-O distance and a simultaneous decrease of ∼-0.5° of the Ni-O-Ni superexchange angle. The ρ(T) measurements revealed a MI transition occurring at temperatures ranging from TMI∼193 to 336K for samples with x ≤ 0 and 0.50, respectively. These measurements also show a large thermal hysteresis in NdNiO3 during heating and cooling processes, suggesting a first-order character of the phase transition at TMI. The width of this thermal hysteresis was found to decrease appreciably for the sample Nd 0.7Eu0.3NiO3. The results indicate that cation disorder associated with increasing substitution of Nd by Eu is responsible for changing the first-order character of the transition in NdNiO3. © 2006 IOP Publishing Ltd.

Local electromethanical properties of the CaCu 3Ti 4O 12 ceramics

Scanning probe microscopy (SPM) was used to probe piezoelectric vibrations and local conductivity in CaCu 3Ti 4O 12 (CCTO) ceramics at room temperature. Piezoelectric contrast was observed on the polished surfaces of CCTO in both vertical (out-of-plane) and lateral (in-plane) modes and depended on the grain orientation varying in sign and amplitude. The piezoelectric contrast is shown to be controlled by the electrical bias (local poling) and displayed a ferroelectric-like reversible hysteresis accompanied with a change of the phase of piezoelectric signal. Flexoelectric effect (strain-gradient-induced polarization) due to surface relaxation was invoked to explain the observed contrast inside the grains. © 2010 Materials Research Society.

Structural refinement and photoluminescence properties of cube-like (Ca 1-xCu x)TiO 3 crystals synthesized by the microwave-hydrothermal method

In this work, (Ca 1-xCu x)TiO 3 crystals with (x = 0, 0.01 and 0.02), labeled as CTO, CCTO1 and CCTO2, were synthesized by the microwave-hydrothermal method at 140°C for 32 min. XRD patterns (Fig. 1), Rietveld refinement and FT-Raman spectroscopy indicated that these crystals present orthorhombic structure Pbnm. Micro-Raman and XANES spectra suggested that the substitution of Ca by Cu in A-site promoted a displacement of the [TiO6]-[TiO6] clusters adjacent from its symmetric center, which leads distortions on the [CaO 12] clusters neighboring and consequently cause the strains into the CaTiO3 lattice. FE-SEM images showed that these crystals have an irregular shape as cube like probably indicating an Ostwald-ripening and self-assemble as dominant mechanisms to crystals growth. The powders presented an intense PL blue-emission.

Thickness dependence of structure and piezoelectric properties at nanoscale of polycrystalline PZT thin films

Lead zirconate titanate Pb(Zr 0.50Ti 0.50)O 3 (PZT) thin films were deposited by a polymeric chemical method on Pt(111)/Ti/SiO2/Si substrates to understand the mechanisms of phase transformations and the effect of film thickness on the structure, dielectric and piezoelectric properties in these films. PZT films pyrolyzed at temperatures higher than 350 °C present a coexistence of pyrochlore and perovskite phases, while only perovskite phase grows in films pyrolyzed at temperatures lower than 300 °C. For pyrochlore-free PZT thin films, a small (100) orientation tendency near the film-substrate interface was observed. Finally, we demonstrate the existence of a self-polarization effect in the studied PZT thin films. Results suggest that Schottky barriers and/or mechanical coupling near the filmsubstrate interface are not primarily responsible for the observed self-polarization effect in our films. © 2012 IEEE.

Low-temperature synthesis of nanosized bismuth ferrite by the soft chemical method

This paper describes research on a simple low-temperature synthesis route to prepare bismuth ferrite nanopowders by the polymeric precursor method using bismuth and iron nitrates. BiFeO 3 (BFO) nanopowders were characterized by means of X-ray diffraction analyses, (XRD), Fourier transform infrared (FT-IR) spectroscopy, Raman spectroscopy (Raman), thermogravimnetric analyses (TG-DTA), ultra-violet/vis (UV/Vis) and field emission scanning electron microscopy (FE-SEM). XRD patterns confirmed that a pure perovskite BiFeO 3 structure with a rhombohedral distorted perovskite structure was obtained by heating at 850 °C for 4 hours. Typical FT-IR spectra for BFO powders revealed the formation of a perovskite structure at high temperatures due to a metal-oxygen bond while Raman modes indicated oxygen octahedral tilts induced by structural distortion. A homogeneous size distribution of BFO powders obtained at 850 °C for 4 hours was verified by FE-SEM analyses. © 2012 Elsevier Ltd and Techna Group S.r.l.

Chemical stability and electrical properties of Nb doped BaCe 0.9Y 0.1O 3-δ as a high temperature proton conducting electrolyte for IT-SOFC

BaCe 0.9-xNb xY 0.1O 3-δ (where x=0, 0.01, 0.03 and 0.05) powders were synthesized by solid-state reaction to investigate the influence of Nb concentration on chemical stability and electrical properties of the sintered samples. The dense electrolyte pellets were formed from the powders after being uniaxially pressed and sintered at 1550 °C. The electrical conductivities determined by impedance measurements in temperature range of 550-750 °C in different atmospheres (dry argon and wet hydrogen) showed a decreasing trend with an increase of Nb content. For all samples higher conductivities were observed in the wet hydrogen than in dry argon atmosphere. The chemical stability was enhanced with increasing of Nb concentration. It was found that BaCe 0.87Nb 0.03Y 0.1O 3-δ is the optimal composition that satisfies the opposite demands for electrical conductivity and chemical stability, reaching 0.8×10 -2 S cm -1 in wet hydrogen at 650 °C compared to 1.01×10 -2 S cm -1 for undoped electrolyte. © 2012 Elsevier Ltd and Techna Group S.r.l.

Influence of the network modifier on the characteristics of MSnO 3 (M=Sr and Ca) thin films synthesized by chemical solution deposition

CaSnO3 and SrSnO3 alkaline earth stannate thin films were prepared by chemical solution deposition using the polymeric precursor method on various single crystal substrates (R- and C-sapphire and 100-SrTiO3) at different temperatures. The films were characterized by X-ray diffraction (θ-2θ, ω- and φ-scans), field emission scanning electron microscopy, atomic force microscopy, micro-Raman spectroscopy and photoluminescence. Epitaxial SrSnO3 and CaSnO 3 thin films were obtained on SrTiO3 with a high crystalline quality. The long-range symmetry promoted a short-range disorder which led to photoluminescence in the epitaxial films. In contrast, the films deposited on sapphire exhibited a random polycrystalline growth with no meaningful emission regardless of the substrate orientation. The network modifier (Ca or Sr) and the substrate (sapphire or SrTiO3) influenced the crystallization process and/or the microstructure. Higher is the tilts of the SnO6 octahedra, as in CaSnO3, higher is the crystallization temperature, which changed also the nucleation/grain growth process. © 2012 Elsevier Inc. All rights reserved.

Effect of the composition on the thermal behaviour of the SrSn1-xTixO3 precursor prepared by the polymeric precursor method

Strontium stannate titanate Sr(Sn, Ti)O3 is a solid solution between strontium stannate (SrSnO3) and strontium titanate (SrTiO3). In the present study, it was synthesized at low temperature by the polymeric precursor method, derived from the Pechini process. The powders were calcined in oxygen atmosphere in order to eliminate organic matter and to decrease the amount of SrCO3 formed during the synthesis. The powders were annealed at different temperatures to crystallize the samples into perovskites-type structures. All the compositions were studied by thermogravimetry (TG) and differential thermal analysis (DTA), infrared spectroscopy (IR) and X-ray diffraction (XRD). The lattice former, Ti4+ and Sn4+, had a meaningful influence in the mass loss, without changing the profile of the TG curves. On the other hand, DTA curves were strongly modified with the Ti4+:Sn4+ proportion in the system indicating that intermediate compounds may be formed during the synthesis being eliminated at different temperature ranges, while SrCO3 elimination occurs at higher temperature as shown by XRD and IR spectra. © 2013 Akadémiai Kiadó, Budapest, Hungary.

Enhanced stability in CO2 of Ta doped BaCe0.9Y 0.1O3-δ electrolyte for intermediate temperature SOFCs

The influence of Ta concentration on the stability of BaCe 0.9-xTaxY0.1O3-δ (where x=0.01, 0.03 and 0.05) powders and sintered samples in CO2, their microstructure and electrical properties were investigated. The ceramic powders were synthesized by the method of solid state reaction, uniaxially pressed and sintered at 1550 °C to form dense electrolyte pellets. A significant stability in CO2 indicated by the X-ray analysis performed was observed for the samples with x≥0.03. The electrical conductivities determined by impedance measurements in the temperature range of 550-750 °C and in various atmospheres (dry argon, wet argon and wet hydrogen) increased with temperature but decreased with Ta concentration. The highest conductivities were observed in the wet hydrogen atmosphere, followed by those in wet argon, while the lowest were obtained in the dry argon atmosphere for each dopant concentration. The composition with Ta content of 3 mol% showed satisfactory characteristics: good resistance to CO2 in extreme testing conditions, while a somewhat reduced electrical conductivity is still comparable with that of BaCe0.9Y0.1O3-δ. © 2012 Elsevier Ltd and Techna Group S.r.l.

New methodology for a faster synthesis of SrSnO3 by the modified Pechini method

SrSnO3, a perovskite-type complex oxide, was synthesized by the modified Pechini method using two different precursors, tin chloride and metallic tin. The first one is already traditional in the literature and it claims about 30 days, only for the cleaning of tin citrate aiming at the elimination of the chloride. The second route was developed by our research group and saves time, taking 6 h to complete the synthesis of the resin. The results show that SrSnO3 obtained from the metallic tin show a higher short range order, leading to a band gap value higher than those reported in the literature, besides a meaningful reduction in the formation of SrCO 3, as compared to the one obtained from tin chloride.

Thickness dependence of structure and piezoelectric properties at nanoscale of polycrystalline lead zirconate titanate thin films

Lead zirconate titanate Pb(Zr0.50Ti0.50)O3 (PZT) thin films were deposited by a polymeric chemical method on Pt(111)/Ti/SiO2/Si substrates to understand the mechanisms of phase transformations and the effect of film thickness on the structure, dielectric, and piezoelectric properties in these films. PZT films pyrolyzed at temperatures higher than 350 °C present a coexistence of pyrochlore and perovskite phases, while only perovskite phase grows in films pyrolyzed at temperatures lower than 300 °C. For pyrochlore-free PZT thin films, a small (100)-orientation tendency near the film-substrate interface was observed. Finally, we demonstrate the existence of a self-polarization effect in the studied PZT thin films. The increase of self-polarization with the film thickness increasing from 200 nm to 710 nm suggests that Schottky barriers and/or mechanical coupling near the film-substrate interface are not primarily responsible for the observed self-polarization effect in our films. © 2013 AIP Publishing LLC.

Effect of yttrium doping in barium zirconium titanate ceramics: A structural, impedance, modulus spectroscopy study

In the current article, we studied the effect of yttrium [Y3+] ions' substitution on the structure and electric behavior of barium zirconate titanate (BZT) ceramics with a general formula [Ba1-x Y 2x/3](Zr0.25Ti0.75)O3 (BYZT) with [x = 0, 0.025, 0.05] which were prepared by the solid-state reaction method. X-ray diffraction patterns indicate that these ceramics have a single phase with a perovskite-type cubic structure. Rietveld refinement data confirmed [BaO 12], [ZrO6], [TiO6], [YO6] clusters in the cubic lattice. The Y3+ ions' effects on the electric conductivity behavior of BZT ceramics as a function of temperature and frequency are described, which are based on impedance spectroscopy analyses. The complex impedance plots display a double semicircle which highlights the influences of grain and grain boundary on the ceramics. Impedance analyses showed that the resistance decreased with the increasing temperature and resulted in a negative temperature coefficient of the resistance property in all compositions. Modulus plots represent a non-Debye-type dielectric relaxation which is related to the grain and grain boundary as well as temperature-dependent electric relaxation phenomenon and an enhancement in the mobility barrier by Y3+ ions. Moreover, the electric conductivity increases with the replacement of Ba 2+ by Y3+ ions may be due to the rise in oxygen vacancies. © 2013 The Minerals, Metals & Materials Society and ASM International.

Syntheses of bismuth titanate templates obtained by the molten salt method

Bismuth titanate templates (Bi4Ti3O12) were synthesized by the molten salt method in Na2SO4 and K2SO4 fluxes, using an amorphous Bi4Ti 3O12 precursor and a mechanically mixed Bi 2O3+TiO2 mixture as the starting materials. The templates were characterized by means of X-Ray Diffraction, FT-IR, FT-Raman, FEG-SEM and TEM. The templates are free of secondary phases and present orthorhombic structure with orientation in the c-plane. FT-IR suggests no traces of sulfate groups revealing that the molten salt synthesis was beneficial for elimination of inorganic species and for the arrangement of individual nanocrystals into ordered lattices. FEG-SEM analyses of BIT templates revealed that most of the grains were homogeneous with a length of 3.1 μm and a width of 0.3 μm and had plate-like morphology. TEM investigations show that the c-axis of the perovskite units is parallel to the thickness direction of the grains and no liquid-phase was formed during BIT phase formation. © 2013 Elsevier Ltd and Techna Group S.r.l.

Fases magnéticas em compostos Er'Co IND. x''Mn IND. 1-x''O IND> 3+ou - δ'

Pós-graduação em Ciência e Tecnologia de Materiais - FC