Amador C. Caballero

Temperature Sensing with Up-Converting Submicron-Sized LiNbO3:Er3+/Yb3+ Particles

We present the temperature dependence of the green up-converted luminescence in submicron-sized LiNbO3:Er3+/Yb3+ particles in the range between 12 and 180 °C. It is shown that LiNbO3:Er3+/Yb3+ exhibits a very high sensitivity (S = 0.007/°C at physiological temperature and a maximum value of S = 0.014/°C at 350 °C) that surpasses those previously found for other materials based on Er3+/Yb3+ luminescence, indicating that LiNbO3:Er3+/Yb3+ is a suitable probe for optical temperature sensing through the fluorescence intensity ratio technique. This high sensitivity can be explained using the intrinsic spectroscopic parameters of Er3+ ions in this host.

Ceramic-electrode interaction in PZT and PNN-PZT multilayer piezoelectric ceramics with AG/PD 70/30 inner electrode

Pb(Zr0.53Ti0.47)O3(PZT) and PbNi1/3Nb2/3O3-PZT (PNN–PZT) multilayers with Ag/Pd 70/30 inner electrodes have been studied from the point of view of the ceramic–electrode interaction. PZT multilayers have shown silver free ceramic layers, and no lead has been detected in the electrode after sintering. Silver transport through the ceramic is avoided because of low open porosity–close porosity transition temperature. This transition seems to occur prior to the strong increase of the silver vapour pressure.For PNN–PZT multilayers, similar behaviour to that commonly reported in the literature for PZT is observed. A constant amount of silver, independent of the distance from the electrode, is detected in the ceramic layer. Lead is also detected in the electrode. On the other hand, no interaction between ceramic and electrode has been observed for the other cations.

Relaxor behavior of PbxBi4Ti3+xO12+3x (x=2,3) Aurivillius ceramics

The relaxor response of the PbxBi4Ti3+xO12+3x (x=2,3) Aurivillius ceramics is reported. The relaxor behavior appears with an increase of the symmetry in the ab plane of the structure and a decrease of the transition temperature. By the application of a bias field on nonpoled samples, it is possible to induce the piezoelectric response. The piezoelectric coefficient in the planar mode of −72 pC/N was measured at 210 °C for Pb2Bi4Ti5O18 ceramic disks. These results represent a step in the search of high-temperature piezoelectric ceramics.

New Features of the Morphotropic Phase Boundary in the Pb(Zr1-xTix)O3 System

Abstract Recently a new monoclinic phase in the Pb(Zr1−xTix)O3 ceramic system has been reported by Noheda et al.[1] for a composition of x=0.48. In this work, samples with Ti contents x=0.47 and 0.50, which are both tetragonal below their Curie point, have been investigated. In the sample with x=0.50, the tetragonal phase was found to transform to a monoclinic phase at about 200 K as the temperature was lowered. The sample with x=0.47 showed a complicated region of phase coexistence between ∼440K–320K, becoming rhombohedral at around 300 K. No further symmetry change was found down to 20K. Dielectric measurements for these two samples are also reported. On the basis of these results, a preliminary phase diagram is presented. Optimum compositional homogeneity is needed to properly characterize the new monoclinic region.

Processing and dielectric properties of the mixed-layer bismuth titanate niobate Bi7Ti4NbO21 by the metal-organic precursor synthesis method

Abstract The preparation of homogeneous Bi 7 Ti 4 NbO 21 single phase nanosized ceramic powders at temperatures as low as 400–500°C using a metal-citrate complexes method, based on the Pechini-type reaction route, is described. The thermal decomposition/oxidation of the polymerized resin, as investigated by TG/DTA, XRD, Raman spectroscopy and SEM, led to the formation of a well defined orthorhombic Bi 7 Ti 4 NbO 21 compound with lattice parameters a =0.544, b =0.540 and c =2.905 ± 0.0005 nm. Reaction formation takes place through an intermediate binary phase with a stoichiometry close to Bi 20 TiO 32 which forms between 300 and 375°C. The metal-organic precursor synthesis method, allows the control of the Bi/Ti/Nb stoichiometric ratio leading to the rapid formation of the nanosized bismuth titanate niobate, Bi 7 Ti 4 NbO 21 , ceramic powders, at temperatures much lower than usually needed in the conventional route. Sintering of the as-prepared ceramic powders led to near full density samples in the temperature range 1100–1130°C. The mixed-layer structure Bi 7 Ti 4 NbO 21 ceramics showed two phase transitions at about 650 and 850 ± 5°C, and a piezoelectric modulus, d 33 , as high as 20×10 −12 C/N.

Evaluation of Piezoelectric Properties of Bi4Ti3O12—Based Ceramics at High Temperature

Nowadays many industries as aeronautic, aerospace, etc, are claiming for new sensors and actuators that can operate in hostile environments. PZT ceramics are the common materials used in piezoelectric devices, however their low Curie temperature prevents their use at high temperatures. Bi4Ti3O12-based ceramics show a great potential for high temperature applications. Although piezoelectric parameters of BIT ceramics are well described at room temperature, its behaviour at high temperatures is almost lacking. This paper describes an experimental set-up to measure the piezoelectric properties of high temperature piezoceramics up to 700°C. The variation of piezoelectric parameters of W-doped BIT ceramics as a function of temperature indicates that their operating temperature extends up to 500°C, surpassing the expected one (½ Tc) in more than 200°C.

Phosphor-doped BaTiO3 : microstructure development and dielectric properties

The phosphor cation retards BaTiO3 formation during the first reaction stage, and favours Ba2TiO4 formation. As a consequence, the end of the reaction is only accomplished at 1150 °C. The average particle size of the synthesized powder, after attrition milling was <1.0 μm. Isopressed bars were sintered from 1200–1400 °C. Dilatometric measurements showed the existence of two sintering mechanisms; these were confirmed by means of isothermal sintering experiments. Microstructural development at low sintering temperature differs from that corresponding to higher ones. Very high permittivity values were measured in the 1325–1350 °C sintered samples. This anomalous dielectric behaviour is associated with the presence of phosphor.

Highly photoactive anatase nanoparticles obtained using trifluoroacetic acid as an electron scavenger and morphological control agent

Fluorine species adsorbed on the surface of TiO2 anatase nanoparticles improve their photocatalytic performance by reducing the recombination rate of photogenerated electrons and holes. Trifluoroacetic acid (TFAA) has been recently proposed as a promising harmless substitute for hydrofluoric acid (HF) in preparing fluorine-modified anatase with good scavenger properties. However the photocatalytic performance of the TiO2 nanoparticles also depends on their specific morphology, by which the number and type of crystal facets, which remain exposed, are determined. A comprehensive study is presented in this contribution which for the first time describes the role of TFAA in stabilizing the highly photoactive {001} TiO2 facets. Furthermore, by reducing the amount of water incorporated into the reaction medium, a simple one-step synthesis method is also proposed that allows the preparation of TFAA-modified anatase nanoparticles with specific morphology and selected stabilized facets, eventually yielding a semiconductor material with excellent photocatalytic properties.

Microwave-induced fast crystallization of amorphous hierarchical anatase microspheres

The fabrication of hierarchical anatase microspheres with potential photocatalytic properties eventually comprises a consolidation step in which a high degree of crystalline order is typically achieved through conventional electric heating treatments. This however entails a substantial reduction in the specific surface area and porosity of the powders, with the consequent deterioration in their photocatalytic response. Here, we have tested the employ of microwave heating as an alternative energy-saving sintering method to promote fast crystallization. The results obtained suggest that under the microwave radiation, the TiO2 hierarchical structures can effectively crystallize in a drastically reduced heating time, allowing the specific surface area and the porosity to be kept in the high values required for an improved photocatalytic performance.

Preparation of Ca0.5Zr2(PO4)3 and Ca0.45Eu0.05Zr2(PO4)3 nanopowders: structural characterization and luminescence emission study

Ca0.5Zr2(PO4)3 and Ca0.45Eu0.05Zr2(PO4)3 nanophosphors have been synthesized by a sol–gel process under acid and basic conditions. In order to achieve the reduction of Eu3+ to Eu2+, europium-doped samples were treated in a reducing atmosphere flow. The effects of the different synthesis conditions and the partial substitution of calcium by europium ions on the structure of the samples were analyzed by x-ray diffraction (XRD) transmission electron microscopy (TEM) and micro-Raman spectroscopy. Luminescence and magnetic properties were investigated by photoluminescence (PL) spectroscopy and magnetic susceptibility measurements. XRD patterns can be indexed to a rhombohedral symmetry of space group R-3 with Z = 6, consistent with a NASICON-type structure. A higher crystallinity was found in the samples prepared under basic conditions. TEM images of all the synthesized samples show spherically shaped particles. A broadening of the Raman bands as a result of non-equivalent vibrations of the orthophosphate groups is observed for samples prepared under acid conditions. The same effect was found when calcium is substituted by europium into the Ca0.5Zr2(PO4)3 host. PL spectra exhibit strong emission in the blue-green spectral range due to Eu2+ 4f65d1-4f7 transitions and no evidence of Eu3+ emission. Magnetic measurements confirm the 2+ oxidation state of europium ions in all samples.