Papers by Mayra D Gonçalves
The Journal of Physical Chemistry C
Optical Materials, 2009
a b s t r a c t (Sr 0.995 Tm 0.005 )ZrO 3 (STZO) powders were prepared by the polymeric precursor... more a b s t r a c t (Sr 0.995 Tm 0.005 )ZrO 3 (STZO) powders were prepared by the polymeric precursor method and heat treated at different temperatures for 2 h under oxygen flow. These powders were analyzed by X-ray diffraction (XRD), Ultraviolet-visible (UV-vis) absorption spectroscopy, photoluminescence (PL) measurements, field-emission gun-scanning electron microscopy (FEG-SEM) and energy dispersive X-ray spectrometry (EDXS). XRD patterns revealed that the powders crystallize in an orthorhombic structure without the presence of secondary phases. UV-vis absorption spectra suggest that the STZO powders heat treated at low temperatures present intermediary energy levels within the band gap as consequence of structural defects in the lattice. PL measurements indicated the presence of broad, broad/narrow and narrow bands in STZO powders. The broad bands were associated to the asymmetric STZO structure and/or p-d transitions while, the narrow bands were ascribed to f-f transitions arising from thulium ions. A simple model was proposed in order to explain the PL behavior of these powders. FEG-SEM micrographs showed that these powders are composed by several microparticles with irregular morphologies and agglomerated nature. EDXS data were used for analyses of chemical compositional of powders.
A B S T R A C T Lanthanum-doped lead zirconate titanate (Pb 0.95 La 0.05 Zr 1−x Ti x O 3) with tw... more A B S T R A C T Lanthanum-doped lead zirconate titanate (Pb 0.95 La 0.05 Zr 1−x Ti x O 3) with two different compositions, x=0.20 (PLZT20) and 0.80 (PLZT80), were successfully obtained by the oxidant peroxide method (OPM). This environmental friendly synthetic route showed to be very attractive to prepare lead based ferroelectric powders since no carbon or halides compounds were used. Tetragonal perovskite structure was observed for PLZT80 and rhombohedral perovskite for PLZT20, with single phase identified in the powders calcined at temperatures higher than 700 °C. Powders crystallized at 700 °C were used to produce ceramic samples that were sintered at 1000 °C for 2 h by conventional and microwaves techniques. The composition of the fractured surface of ceramics were quite close to the nominal molar composition, indicating that the OPM promotes the formation of materials with high compositional homogeneity and similar microstructures regardless the sintering method used.
Highly reactive lead zirconate titanate powders (PZT) with different compositions were successful... more Highly reactive lead zirconate titanate powders (PZT) with different compositions were successfully synthesized by the oxidant-peroxo method (OPM) and used to prepare dense ceramic samples with composition near to the morphotropic phase boundary (MPB) sintered at 1000 °C for 2 h using a tubular conventional oven and a commercial microwave system. Crystalline phases were identified in the powder and ceramic samples by X-ray powder diffraction and FT-Raman spectroscopy at room temperature. The fractured surface of the ceramic sample showed a high degree of densification with fairly uniform grain sizes. Dielectric constants measured in the range from 30 to 500 °C at different frequencies (1, 10 and 100 kHz) indicated a normal ferroelectric behavior regardless of the sintering method. Samples sintered by a microwave radiation (MW) method and composition near to the MPB region showed a maximum dielectric constant of 17.911 and an anomalous high Curie temperature of 465 °C.
The oxidant peroxo method (OPM) exhibits several advantage and unique characteristics not found i... more The oxidant peroxo method (OPM) exhibits several advantage and unique characteristics not found in the traditional methods for the synthesis of lead-and bismuth-based oxides. First of all, it is a clean method based on hydrogen peroxide that matches perfectly with the green chemistry approach. Second, the oxidizing local atmosphere provided by the precursor during its crystallization is unique among all the wet chemical techniques of synthesis, which usually result in reducing environment. Besides these advantages, only a few studies have focused on the use of the OPM to obtain better materials, which makes this field of study an excellent opportunity for the development of materials with higher purity and controlled morphologies.
Yttrium-doped barium zirconate ceramic powders were synthesized by the oxidant peroxide method in... more Yttrium-doped barium zirconate ceramic powders were synthesized by the oxidant peroxide method in air and under controlled atmosphere of nitrogen inside a glove box. The powders were characterized by thermogravimetry, X-ray diffraction, scanning electron microscopy and transmission electron microscopy. After uniaxial cold isostatic pressing, green pellets were sintered at 1600°C for 4 h. The electrical conductivity behavior was accessed by electrochemical impedance spectroscopy. The results show that specimens synthesized under controlled atmosphere achieved higher electrical conductivity, two orders of magnitude higher than specimens prepared in laboratory air. The enhancement in electrochemical properties and increase in sintering ability is attributed to the less carbonate contamination as a result lower grain boundary density in the samples prepared under controlled atmosphere.
The enthalpies of formation from binary oxide components at 25 oC of Ba(Zr1-xYx)O3-d, x ¼ 0.1 to ... more The enthalpies of formation from binary oxide components at 25 oC of Ba(Zr1-xYx)O3-d, x ¼ 0.1 to 0.5 solid
solutions are measured by high temperature oxide melt solution calorimetry in a molten solvent,
3Na2O.4MoO3 at 702 oC. The enthalpy of formation is exothermic for all the compositions and becomes
less negative when increasing yttrium content from undoped (-115.12 +- 3.69 kJ mol-1) to x = 0.5
(-77.09 +- 4.31 kJ mol-1). The endothermic contribution to the enthalpy of formation with doping
content can be attributed to lattice distortions related to the large ionic radius difference of yttrium and
zirconium and vacancy formation. For 0.3 < x > 0.5, the enthalpy of formation appears to level off,
consistent with an exothermic contribution from defect clustering. Raman spectra indicate changes in
short range structural features as a function of dopant content and, suggests that from x ¼ 0.3 to 0.5 the
defects begins to cluster significantly in the solid solution, which corroborates with the thermodynamic
data and the drop-off in proton conductivity from x > 0.3.
Materials Science Forum, Volumes 798 - 799, pages 407-412 , Jun 2014
Investigation on polycrystalline electroceramics involves the synthesis, the consolidation and th... more Investigation on polycrystalline electroceramics involves the synthesis, the consolidation and the analysis of the electrical behavior, along with careful evaluation of the final microstructure. The synthesis of ceramic powders with controlled characteristics is crucial in the study of materials with optimized properties. Distinct properties may be found in ceramic materials prepared by the several existing methods, due to chemical and phase homogeneities, and to the particle size distribution or medium particle size. In this work, yttrium-doped barium zirconate proton conductor was synthesized by spray pyrolysis, and characterized by several techniques aiming identifying the influence of some parameters of this method of synthesis with particle characteristics. Nanocrystalline powders synthesized at 600-700oC were found to be cubic and single phase. Moreover, depending on the gas flow and furnace temperature, spheroid and porous or cubic and solid particles may be obtained.
Journal of Materials Research, volume 29, pages 131-138, Jan 2014
The oxidant peroxo method (OPM) exhibits several advantage and unique characteristics not found i... more The oxidant peroxo method (OPM) exhibits several advantage and unique characteristics not found in the traditional methods for the synthesis of lead- and bismuth-based oxides. First of all, it is a clean method based on hydrogen peroxide that matches perfectly with the green chemistry approach. Second, the oxidizing local atmosphere provided by the precursor during its crystallization is unique among all the wet chemical techniques of synthesis, which usually result in reducing environment. Besides these advantages, only a few studies have focused on the use of the OPM to obtain better materials, which makes this field of study an excellent opportunity for the development of materials with higher purity and controlled morphologies.(Received May 17 2013)(Accepted September 19 2013)
Volume 40, Issue 1, Part A, Pages 911–917, Jan 2014
Polycrystalline powders of yttrium-doped barium zirconate with nominal composition BaZr0.9Y0.1O3−... more Polycrystalline powders of yttrium-doped barium zirconate with nominal composition BaZr0.9Y0.1O3−δ (BZY10) were synthesized by the oxidant-peroxo (OP) method using hexahydrated yttrium nitrate, barium nitrate and hydrated zirconyl nitrate. This is an environmental friendly method, free of common contaminants such as carbon graphite or halides, and promotes the formation of stoichiometric powders composed by sinteractive nanoparticles. The powders were amorphous and required a heat treatment to crystallize the perovskite phase. The powders were characterized by thermogravimetry and differential thermal analysis, X-ray fluorescence spectroscopy, X-ray diffraction, evaluation of specific surface area by inert gas adsorption, scanning electron microscopy and electrochemical impedance spectroscopy. Stoichiometric single perovskite BZY10 powders were successfully synthesized. Carbonate ions, adsorbed during the powder synthesis, are fully decomposed after calcination at 1200 °C. The powders consist of agglomerate reactive nanosized particles. Pressing the powders to pellets and sintering at 1600 °C yielded relative density higher than 91% of the theoretical density. Scanning electron microscopy images of chemically and thermally etched surfaces clearly defines two regions: one dense with grains of irregular shapes and average submicron size, and another porous. The impedance spectroscopy analysis shows that sintered pellets prepared with OP powders have higher electrical conductivity than pellets using powders obtained by solid state reaction.
Optical Materials, Volume 31, Issue 8, Pages 1134–1143, Jun 2009
(Sr0.995Tm0.005)ZrO3 (STZO) powders were prepared by the polymeric precursor method and heat trea... more (Sr0.995Tm0.005)ZrO3 (STZO) powders were prepared by the polymeric precursor method and heat treated at different temperatures for 2 h under oxygen flow. These powders were analyzed by X-ray diffraction (XRD), Ultraviolet–visible (UV–vis) absorption spectroscopy, photoluminescence (PL) measurements, field-emission gun-scanning electron microscopy (FEG-SEM) and energy dispersive X-ray spectrometry (EDXS). XRD patterns revealed that the powders crystallize in an orthorhombic structure without the presence of secondary phases. UV–vis absorption spectra suggest that the STZO powders heat treated at low temperatures present intermediary energy levels within the band gap as consequence of structural defects in the lattice. PL measurements indicated the presence of broad, broad/narrow and narrow bands in STZO powders. The broad bands were associated to the asymmetric STZO structure and/or p–d transitions while, the narrow bands were ascribed to f–f transitions arising from thulium ions. A simple model was proposed in order to explain the PL behavior of these powders. FEG-SEM micrographs showed that these powders are composed by several microparticles with irregular morphologies and agglomerated nature. EDXS data were used for analyses of chemical compositional of powders.
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Papers by Mayra D Gonçalves
solutions are measured by high temperature oxide melt solution calorimetry in a molten solvent,
3Na2O.4MoO3 at 702 oC. The enthalpy of formation is exothermic for all the compositions and becomes
less negative when increasing yttrium content from undoped (-115.12 +- 3.69 kJ mol-1) to x = 0.5
(-77.09 +- 4.31 kJ mol-1). The endothermic contribution to the enthalpy of formation with doping
content can be attributed to lattice distortions related to the large ionic radius difference of yttrium and
zirconium and vacancy formation. For 0.3 < x > 0.5, the enthalpy of formation appears to level off,
consistent with an exothermic contribution from defect clustering. Raman spectra indicate changes in
short range structural features as a function of dopant content and, suggests that from x ¼ 0.3 to 0.5 the
defects begins to cluster significantly in the solid solution, which corroborates with the thermodynamic
data and the drop-off in proton conductivity from x > 0.3.
solutions are measured by high temperature oxide melt solution calorimetry in a molten solvent,
3Na2O.4MoO3 at 702 oC. The enthalpy of formation is exothermic for all the compositions and becomes
less negative when increasing yttrium content from undoped (-115.12 +- 3.69 kJ mol-1) to x = 0.5
(-77.09 +- 4.31 kJ mol-1). The endothermic contribution to the enthalpy of formation with doping
content can be attributed to lattice distortions related to the large ionic radius difference of yttrium and
zirconium and vacancy formation. For 0.3 < x > 0.5, the enthalpy of formation appears to level off,
consistent with an exothermic contribution from defect clustering. Raman spectra indicate changes in
short range structural features as a function of dopant content and, suggests that from x ¼ 0.3 to 0.5 the
defects begins to cluster significantly in the solid solution, which corroborates with the thermodynamic
data and the drop-off in proton conductivity from x > 0.3.