Photoluminescence behavior of Eu(3+) ion doped into gamma- and alpha-alumina systems prepared by combustion, ceramic and Pechini methods

Al(2)O(3):Eu(3+)(1%) samples were prepared by combustion, ceramic, and Pechini methods annealed from 400 to 1400 degrees C. XRD patterns indicate that samples heated up to 1000 degrees C present disordered character of activated alumina (gamma-Al(2)O(3)). However, alpha-Al(2)O(3) phase showed high crystallinity and thermostability at 1200-1400 degrees C. The sample characterizations were also carried out by means of infrared spectroscopy (IR), scanning electron microscopy (SEM) and specific surface areas analysis (BET method). Excitation spectra of Al(2)O(3):Eu(3+) samples present broaden bands attributed to defects of Al(2)O(3) matrices and to LMCT state of O -> Eu(3+), however, the narrow bands are assigned to (7)F(0) -> (5)D(J),(5)H(J) and (5)L(J) transitions of Eu(3+) ion. Emission spectra of samples calcined up to 1000 degrees C show broaden bands for (5)D(0) -> (7)F(J) transitions of Eu(3+) ion suggesting that the rare earth ion is in different symmetry sites showed by inhomogeneous line broadening of bands, confirming the predominance of the gamma-alumina phase. For all samples heated from 1200 to 1400 degrees C the spectra exhibit narrow (5)D(0) -> (7)F(J) transitions of Eu(3+) ion indicating the conversion of gamma to alpha-Al(2)O(3) phases, a high intensity narrow peak around 695 nm assigned to R lines of Cr(3+) ion is shown. Al(2)O(3):Eu(3+) heated up to 1100 degrees C presents an increase in the Omega(2) intensity parameter with the increase of temperatures enhancing the covalent character of metal-donor interaction. The disordered structural systems present the highest values of emission quantum efficiencies (eta). CIE coordinates of Al(2)O(3):Eu(3+) are also discussed. (C) 2007 Elsevier Inc. All rights reserved.

Synthesis of YAP nanopowder by a soft chemistry route

Polycrystalline fine powder of YAlO(3) (YAP) was synthesized by the modified polymeric precursor method. A preliminary gradual pyrolytic decomposition under nitrogen flux was crucial in the removal process of organic residues to avoid the formation of molecular level inhomogeneities. YAP single phase was crystallized at temperatures between 950 degrees C and 1000 degrees C using chemically homogeneous ball-milled amorphous particles and very fast heating rates, corresponding to the lowest synthesis temperature of pure YAP nanopowder by soft chemistry routes. (C) 2009 Elsevier Ltd. All rights reserved.

Phase-transition studies of Ba(0.90)Ca(0.10)(Ti(1-x)Zr(x))O(3) ferroelectric ceramic compounds

Perovskite-structured Ba(0.90)Ca(0.10)(Ti(1-x)Zr(x))O(3) ceramics were prepared in this work and subsequently studied in terms of composition-dependent dielectric and high-resolution long-range order structural properties from 30 to 450 K. The dielectric response of these materials was measured at several frequencies in the range from 1 kHz to 1 MHz. Combining both techniques, including Rietveld refinement of the X-ray diffraction data, allowed observing that, when increasing Zr(4+) content, the materials change from conventional to diffuse and relaxor ferroelectric compounds, the transition occurring spontaneously at the x = 0.18 composition. Interestingly, this spontaneous transition turned out to be prevented for a further increase of Zr(4+). On the basis of all the dielectric and structural results processed, a phase diagram of this system is presented. (C) 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Preparation of pyrochlore-free PMN by an alternative chemical method

This work reports the synthesis of the MgNb(2)O(6) and the ferroelectric lead magnesium niobate Pb(Mg(1/3)Nb(2/3))O(3) (PMN) using fine precursor powders obtained from a chemical Oxide Precursor Method (OPM). To obtain pyrochlore-free PMN ceramics, the synthesis of the precursor MgNb(2)O(6) powders was studied for 2-25 mol% excess of MgO and 10 mol% excess of PbO. Structural and microstructural properties of the sintered ceramics obtained by the cited method and by the classical columbite method were studied and compared. Results lead to good quality, pyrochlore-free PMN ceramic prepared by OPM, presenting greater grain size if compared with ceramic prepared by columbite method.

Features of dielectric response in PMN-PT ferroelectric ceramics

In this paper, electrical and structural properties were reported for pyrochlore free (1-x)[Pb(Mg1/3Nb2/3)O-3] - xPbTiO(3) (PMN-PT) (with 35 mol% PbTiO3) ceramics obtained from fine powders. Dielectric studies were focused on the investigation of the complex dielectric permittivity (epsilon '-i epsilon '') as a function of frequency and temperature. The effects of the dc applied electric field on dielectric response were also investigated. Results revealed a field dependence dielectric anomaly in the dielectric permittivity curves (epsilon(T)) in the low dc electric field region, which in turn prevails in the whole analysed frequency interval. To the best of our knowledge, these properties for the PMN-PT ceramic system have not been reported before as in this work. The results were analysed within the framework of the current models found in the literature.

Investigation on dielectric response of PMN ceramics around paraelectric-ferroelectric diffuse phase transition

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

(1-x)PMN-xPT ceramics prepared by conventional and modified columbite route: Effect on electrical properties

This study aims to demonstrate how the chemical homogeneity of B cations affects the microstructure and electrical responses of (1-x) PMN-xPT ceramics. Two methodologies were employed to synthesize three different compositions, with x assuming the values 0.10, 0.28, and 0.35. If compared to conventional method, the Ti-modified columbite route, which is characterized by higher B cation homogeneity, leads to PMN-PT powders and ceramics with lower content of PNT pyrochlore phase and, for 0.65PMN-0.35PT composition, minor amount of tetragonal phase is found. Conclusively, PMN-PT ceramics obtained by modified route favors B cations homogeneity, enhancing the dielectric, ferroelectric and piezoelectric properties.

Color tunability of intense upconversion emission from Er3+-Yb3+ co-doped SiO2-Ta2O5 glass ceramic planar waveguides

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Mechanochemical synthesis of ceramic powders with perovskite structure

The mechanical activation is one of the most effective method for obtaining highly disperse system due to mechanical action stress fields form in solids during milling procedure. This effect results in changes of free energy, leading to release of heat, formation of a new surface, formation of different crystal lattice defects and initiation of solid-state chemical reaction. The accumulated deformation energy determines irreversible changes of crystal structure and consequently microstructure resulting in the change of their properties. Mechanochemical processing route has been developed recently for the production of intermetallic and alloy compounds. The intrinsic advantage of this process is that the solid-state reaction is activated due to mechanical energy instead of the temperature. It was shown that the chemical reactivity of starting materials could be improved significantly after mechanochemical activation and, subsequently, the calcination temperature was reduced. Besides, it was apparent that the mechanochemical treatment could enhance the reactivity of constituent oxides; however, the sintering process could not be avoided to develop the desired ceramics. A novel mechanochemical technique for synthesis of fine-grained perovskite structured powders has shown that it is possible to form perovskite at room temperature. The effect of milling on the formation of perovskite structure of barium titanate (BT), lead titanate (PT), PZT, PZN, magnesium niobate (PMN) and LM ceramic materials was analyzed. The dielectric properties of sintered ceramics are comparable with those prepared by other methods in the literature. (C) 2003 Elsevier B.V. B.V. All rights reserved.

Study of potassium additive on the phase formation and ferroelectric properties of 0.9PMN-0.1PT ceramics

A recent and innovative method to include Ti into the columbite precursor has permitted to synthesize 0.9PMN-0.1PT powders with high homogeneity. The present work describes this methodology, named modified columbite method, showing that the reaction between MN(T)and PbO at 800 degrees C for 2 h results in perovskite single-phase. The crystal structure alterations in the columbite and perovskite phases obtained by this methodology and the effects of potassium doping were investigated by the Rietveld method. Changes in the powder morphology, density and weight loss during the sintering process were also studied. Conclusively, potassium does not affect significantly the perovskite amount, but reduces the particle and grain sizes. This dopant also changes the relaxor behavior of 0.9PMN-0.1 PT ceramic, reducing the dielectric loss and enhancing the diffuseness of the phase transition. (C) 2005 Published by Elsevier Ltd and Techna Gronp S.r.l.

Dielectric spectroscopy on Bi3Zn2Sb3O14 ceramic: an approach based on the complex impedance

The dielectric properties and loss of Bi1.5ZnSb1.5O7 a poor-semiconducting ceramic were investigated by impedance spectroscopy, in the frequency range from 5 Hz to 13 MHz. Electric measurements were performed from 100 to 700 degreesC. Pyrochlore type phase was synthesized by the polymeric precursor method. Dense ceramic with 97% of the theoretical density was prepared by sintering via constant heating rate. The dielectric permittivity dependence as a function of frequency and temperature showed a strong dispersion at frequency lower than 10 kHz. The losses (tan delta) exhibit slight dependence with the frequency at low temperatures presenting a strong increase at temperatures higher than 400 degreesC. A decrease of the loss magnitude occurs with increasing frequency. Relaxation times were extracted using the dielectric functions Z(omega) and M(omega). The plots of the relaxation times tau(Z'), and tau(M) as a function of temperature follow the Arrhenius law, where a single slope is observed with activation energy values equal to 1.38 and 1.37 eV, respectively. (C) 2003 Elsevier Ltd. All rights reserved.

Study of Ti-modified columbite route and the effect of lithium doping in 0.9PMN-0.1PT ceramics

The influence of lithium on the structural characteristics of PMN-PT ceramic was studied. The synthesis of PMN-PT powders using this precursor leads to the formation of high amount of perovskite phase. The insertion of Li(+) ions in B-site affects the microstructure because the rise in mass transport changes the mechanical characteristics of sintered ceramic. Higher values of K(m) and T(m) were gotten when lithium is inserted into perovskite phase. Secondary phase was found when lithium content increase beyond 1 mol%, besides the occurrence of transgranular fractures in sintered ceramic. Also, the additive acts increasing the relaxor behavior.

Preparation of ceramic membranes from surface modified tin oxide nanoparticles

The preparation of crack-free SnO2 supported membranes requires the development of new strategies of synthesis capable to allow controlled changes of surface chemistry and to improve the processability of supported layers. In this way, the controlled modification of the SnO2 nanoparticle surface by adding capping molecules like Tiron(R) ((OH)(2)C6H2(SO3Na)(2)) during the sol-gel process was studied, aiming to obtain high performance membranes. Colloidal suspensions were prepared by hydrolyzing SnCl4.5H(2)O aqueous solution with NH4OH in presence of Tiron(R). The effect of the amount of Tiro(R) (from I to 20 wt.%) on the structural features of nanoparticles, powder redispersability and particle-solution interface properties was investigated by X-ray powder diffraction (XRPD), extended X-ray absorption fine structure (EXAFS), quasi-elastic light scattering and electrophoretic mobility measurements. XRPD and EXAFS results showed that the addition of Tiron(R) up to 20 wt.% to colloidal suspensions does not affect the crystallite size of SnO2 primary particles, determined around 2-3 nm. This value is comparable to the hydrodynamic size measured after redispersion of powder prepared with amount of Tiro(R) higher than 7.5 wt.%, indicating the absence of condensation reactions between primary particles after the initial precipitation step. As a consequence the powder with amount of Tiron(R) > 7.5 wt.%, can be fully redispersed in aqueous solution at pH greater than or equal to I I until a nanoparticle concentration of 6 vol.%. The electrophoresis measurements showed a decrease of the isoelectric point by increasing the amount of grafted Tiron(R) at the SnO2 nanoparticle surface, resulting in negatively charged particle-solution interface in all the studied pH range (2-11). These features govern the gelation process favoring the preparation of crack-free SnO2 supported membranes. The control exercised by Tiron(R) modifying agent in the aggregation process allows the fine-tuning of the porosity, from 0.124 to 0.065 cm(3) g(-1), and mean pore size, from 6.4 to 1.9 nm, as the amount of grafted molecules increases from 0 to 10 wt.%. In consequence, the membrane cut-off determined by filtration of polyethylene glycol standard solutions can be screened from 1500 to 3500 g mol(-1). (C) 2002 Elsevier B.V. B.V. All rights reserved.

Seeding of PMN powders made by the Pechini method

The partial oxalate method, with the columbite route, associated with the Pechini method, was used to obtain a very fine ceramic powder at low temperature. Using this route it was possible to obtain a very reactive powder and to synthesize a lead magnesium niobate (PMN) powder with up to 100% of perovskite phase at a lower temperatures than the PbO melting point. The influence of the BaTiO3 and PbTiO3 seeds in the phase formation, along with time and temperature during the PMN calcination, was also investigated. The amount of 2% of BaTiO3 seeds promoted PMN powder formation with 100% of perovskite phase and a very narrow particle size distribution. (C) 2001 Elsevier B.V. Ltd and Techna S.r.l. All rights reserved.

Upconversion luminescence and thermal effects in terbium-ytterbium codoped fluorogermanate glass-ceramic

Cooperative energy transfer upconversion luminescence is investigated in Tb(3+)/Yb(3+)-codoped PbGeO(3)-PbF(2)-CdF(2) glass-ceramic and its precursor glass under resonant and off resonance infrared excitation. Bright UV-visible emission signals around 384, 415, 438 nm, and 473-490, 545, 587, and 623 nm are identified as due to the (5)D(3)((5)G(6))->(7)F(1) (J=6,5,4) and (5)D(4)->(7)F(1) (J=6,5,4,3) transitions, respectively, and readily observed. The results indicate that cooperative energy transfer between ytterbium and terbium. ions followed by excited state absorption are the dominant upconversion excitation mechanisms involved. Comparison of the upconversion process in a glass-ceramic sample and its glassy precursor revealed that the former present much higher upconversion efficiency. The dependence of the upconversion emission upon pump power, temperature, and doping content is also examined.