Effect of processing route on the phase formation and properties of Bi4Ti3O12 ceramics

Our efforts were directed to the preparation of bismuth titanate-Bi4Ti3O12 (BIT) by two procedures: mechanically assisted synthesis and polymeric precursor method to display a variety of their advantages. To follow the nucleation and phase formation of BIT, XRD and Rietveld refinement analysis were used and it was shown that Bi4Ti3O12 ceramic can been successfully prepared from nano-sized powders obtained by both methods. The ferroelectric properties were determined and the loops from BIT obtained by polymeric precursor method were not fully saturated with a remnant polarization of 20 mu C/cm(2) and coercitive field of 1500 kV/cm. BIT obtained from powders prepared by mechanically assisted synthesis shows a remnant polarization of 0.65 mu C/cm(2) and coercitive field of 1050 kV/cm. The grain morphology may be the factor causing the observed differences. (C) 2005 Published by Elsevier Ltd and Techna Group S.r.l.

Influence of strains and defects on ferroelectric and dielectric properties of thin-film barium-strontium titanates

Pristine, W and Mn 1% doped Ba(0.6)Sr(0.4)TiO(3) epitaxial thin films grown on the LaAlO(3) substrate were deposited by pulsed laser deposition (PLD). Dielectric and ferroelectric properties were determined by the capacitance measurements and X-ray diffraction was used to determine both residual elastic strains and defect-related inhomogeneous strains-by analyzing diffraction line shifts and line broadening, respectively. We found that both elastic and inhomogeneous strains are affected by doping. This strain correlates with the change in Curie-Weiss temperature and can qualitatively explain changes in dielectric loss. To explain the experimental findings, we model the dielectric and ferroelectric properties of interest in the framework of the Landau-Ginzburg-Devonshire thermodynamic theory. As expected, an, elastic-strain contribution due to the epilayer-substrate misfit has an important influence on the free-energy. However, additional terms that correspond to the defect-related inhomogeneous strain had to be introduced to fully explain the measurements.

Photoluminescence at room temperature in amorphous SrTiO3 thin films obtained by chemical solution deposition

Intense photoluminescence in highly disordered strontium titanate amorphous thin films prepared by the polymeric precursor method was observed at room temperature (300 K). The luminescence spectra of SrTiO3 amorphous thin films at room temperature revealed an intense single-emission band in the visible region. X-ray absorption near edge structure was used to probe the local atomic structure of SrTiO3 amorphous and crystalline thin films. Photoluminescence intensity in the 535 nm range was found to be correlated with the presence of non-bridging oxygen defects. A discussion is presented of the nature of this photoluminescence, which may be related to the disordered structure in SrTiO3 amorphous thin films. (C) 2002 Elsevier B.V. B.V. All rights reserved.

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.

Hyperfine interaction measurements on ceramics: PZT revisited

The solid solution of PbZr1-xTixO3, known as lead-zirconate titanate (PZT), was probably one of the most studied ferroelectric materials, especially due to its excellent dielectric, ferroelectric and piezoelectric properties. The highest piezoelectric coefficients of the PZT are found near the morphotropic phase boundary (MPB) (0.46 <= x <= 0.49), between the tetragonal and rhombohedral regions of the composition-temperature phase diagram. Recently, a new monoclinic phase near the MPB was observed, which can be considered as a bridge between PZT's tetragonal and rhombohedral phases. This work is concerned with the study of the structural properties of the ferroelectric PZT (Zr/Ti = 52/48, 53/47) by hypertine interaction (HI) measurements obtained from experiments performed by using the nuclear spectroscopy time differential perturbed angular correlation (TDPAC) in a wide temperature range. (c) 2006 Elsevier B.V. All rights reserved.

Niobium doped Bi4Ti3O12 ceramics obtained by the polymeric precursor method

Pure and niobium doped bismuth titanate ceramics (Bi4Ti3-xNbxO12 (BTN)), with x ranging from 0 to 0.4 were prepared by the polymeric precursor method. X-ray diffraction showed no secondary phases. Increasing niobium content leads to more resistive ceramics. The shape and size of the grains are strongly influenced by the niobiurn added to the system. The dielectric constant is not influenced by the niobium addition while hysteresis loops are significantly narrowed. (c) 2006 Elsevier B.V All rights reserved.

Characterization and application of PZT/PU and graphite doped PZT/PU composite

Composites, made of lead zirconate titanate (PZT) ceramic powder and castor oil-based polyurethane (PU), were prepared in the film form. The films were obtained in the thickness range 100-300 mum using up to 50/50 vol.% of ceramic. Another composite (PZT/C/PU) was obtained by adding a small amount (1.0 vol.%) of graphite (C) to the PZT/PU composite. By increasing the conductivity of PU-containing graphite, polarization of PZT could be carried out with better efficiency. A comparison of piezo- and pyroelectric activities and spatial distribution of polarization between graphite doped and undoped composites reveal the advantages of using semiconductor filler. These composites were used as sensors to detect acoustic emission (AE). The detection was made using two simulated sources of AE, i.e., ball bearing drop and pencil lead break. PZT/C/PU composite was able to detect both flexural and extensional components of wave vibration. (C) 2002 Elsevier B.V. B.V. All rights reserved.

The role of defect states in the creation of photoluminescence in SrTiO3

We discuss the nature of visible photoluminescence at room temperature in amorphous strontium titanate in the light of the results of a recent experimental and quantum mechanical theoretical study. Our calculation of the electronic structure involves the use of first-principles molecular calculations to simulate the variation of the electronic structure in the strontium titanate crystalline phase, which is known to have a direct band gap, and we also make an in-depth examination of amorphous strontium titanate. The results of our simulations of amorphous strontium titanate indicate that the formation of five-fold coordination in the amorphous system may introduce delocalized electronic levels in the highest occupied molecular orbital and the lowest unoccupied molecular orbital. These delocalized electronic levels are ascribed to the formation of a tail in the absorbance-spectrum curve. Optical absorption measurements experimentally showed the presence of a tail. The results are interpreted by the nature of these exponential optical edges and tails associated with defects promoted by the disordered structure of the amorphous material. We associate them with localized states in the band gap.

Ferroelectric and optical properties of Ba0.8Sr0.2TiO3 thin film

Barium strontium titanate (Ba0.8Sr0.2TiO3) thin films have been prepared on Pt/Ti/SiO2/Si substrates using a soft solution processing. X-ray diffraction and also micro-Raman spectroscopy showed that the Ba0.8Sr0.2TiO3 thin films exhibited a tetragonal structure at room temperature. The presence of Raman active modes was clearly shown at the 299 and 725 cm(-1) peaks. The tetragonal-to-cubic phase transition in the Ba0.8Sr0.2TiO3 thin films is broadened, and suppressed at about 35 degreesC, with a maximum dielectric constant of 948 (100 kHz). Electrical measurements for the prepared Ba0.8Sr0.2TiO3 thin films showed a remnant polarization (P-r) of 6.5 muC/cm(2), a coercive field (E-c) of 41 kV/cm, and good insulating properties. The dispersion of the refractive index is interpreted in terms of a single electronic oscillator at 6.97 eV. The direct band gap energy (E-g) and the refractive index (n) are estimated to be 3.3 eV and n = 2.27-2.10, respectively. (C) 2002 American Institute of Physics.

Synthesis and characterization of Li2ZnTi3O8 spinel using the modified polymeric precursor method

In this work we report the synthesis procedure, crystallographic, structural and magnetic properties of the Li2ZnTi3O8 spinel obtained using a modified polymeric precursor method. This synthesis method generates very reactive and property-controlled nanoparticles. The samples were characterized using X-ray powder diffraction (XRD) associated to the Rietveld refinement method, thermogravimetric analysis (TG), specific surface area, scanning electron microscopy (SEM) and magnetic susceptibility measurements.The phase formation temperature of the lithium zinc titanate spinel was observed to decrease due to the homogeneity and highly controlled nanometric particle size. (C) 2003 Elsevier B.V. All rights reserved.

Electrical properties of screen printed BaTiO3 thick films

The deposition of thick film pastes by screen-printing is a relatively simple and convenient method to produce thicker layers with thickness up to 100 mum. In the present work, the barium titanate thick films were prepared from mechanically activated powders based on BaC03 and TiO2. After mixing, the powders were calcined at low temperature by slow heating and cooling rates. The thick films were deposited on to Al2O3 substrates through hybrid technology. The obtained films were fired at 850 degreesC together with electrode material (silver/palladium). The electrical properties of thick films: dielectric permittivity, dielectric losses, Curie temperature, hysteresis loop were reported. The obtained BT thick films can be applied in as multilayer capacitors or in gas sensor application. (C) 2003 Elsevier Ltd. All rights reserved.

Composites PVDF-TrFE/BT used as bioactive membranes for enhancing bone regeneration

In this paper a piezoelectric composite membranes were developed for charge generator to promoter bone regeneration on defects sites. Is known that the osteogenesis process is induced by interactions between biological mechanisms and electrical phenomena. The membranes were prepared by mixing Barium Titanate (BT) powders and PVDF-TrFE (PVDF:TrFE = 60:40 mol%) on dimethylformamide medium. This precursor solution was dried and crystallized at 100degreesC for 12 hours. Composites membranes were obtained by following methods: solvent casting (SC), spincoating (SP), solvent extraction by water addition (WS) and hot pressing (HP).The microstructural analysis performed by SEM showed connectivity type 3-0 and 3-1 with high homogeneity for samples of ceramic volume fraction major than 0.50. Powder agglomerates within the polymer matrix was evidenced were observed for composites with the BT volume fraction major than 40%. The composite of ceramic fraction of 0.55 presented the best values of remanent polarization (similar to33 muC/cm(2)), but the flexibility of these composites with the larger ceramic fraction was significantly affected.For in vivo evaluation PVDF-TrFE/BT 90/10 membranes with 3cm larger were longitudinally implanted under tibiae of male rabbit. After 21 days the animals were sacrificed. By histological analyses were observed neo formed bone with a high mitotic activity. In the interface bone-membrane was evidenced a pronounced callus formation. These results encourage further applications of these membranes in bone-repair process.

Investigation of piezoelectricity in perovskite (LaFeO3): A theoretical study

In a previous report we studied theoretically the piezoelectric effect in barium titanate (BaTiO3) [O. Treu Filho, J.C. Pinheiro, R.T. Kondo, J. Mol. Struct. (THEOCHEM), 671 (2004) 71]. In this article we applied the Hartree-Fock (HF) theory in the investigation of piezoelectricity in LaFeO3. Initially, the generator coordinate HF (GCHF) method was used to build 22s14p, 30s19p13d, and 32s24p17d Gaussian basis sets for O(3p), Fe(D-5), and La(D-2) atoms. Then those basis sets were contracted to [7s6p], [13s8p6d], and [18s13p7d], respectively. The quality of the contracted basis sets in polyatomic calculations was evaluated through calculations of total and orbital energies (HOMO and HOMO-1) of (FeO1+)-Fe-2 and (LaO1+)-La-1. Finally, the contracted basis sets were supplemented with polarization and diffuse functions and used to investigate the piezoelectricity in LaFeO3. The calculated properties were dipole moment, total energy, and atomic charges and the analysis of those properties showed that covalent bonds constitute the electronic structure of [LaFCO3](2) fragment. Therefore, it is reasonable to believe that LaFeO3 does not present piezoelectric properties. (C) 2006 Elsevier B.V. All rights reserved.

Cracks developed during SrTiO3 thin-film preparation from polymeric precursors

Strontium titanate (SrTiO3) thin films were prepared by dip-coating Si(111) single-crystal substrates in citrate solutions of ethylene glycol, considering several citric acid/ethylene glycol (CA/EG) ratios. Measurements of intrinsic viscosity indicate that increasing the amount of EG increases the precursors' polymeric chains and increases the weight loss. After deposition the substrates were dried on a hotplate (approximate to 150 degrees C); this was followed by heat treatment at temperatures ranging from 500 to 700 degrees C using heating and cooling rates of 1 degrees C min(-1). SEM and optical microscopy investigations of the sintered films obtained from different CA/EG ratios indicate that there is a critical thickness above which the films present cracks. This critical thickness for SrTiO3 films deposited on the Si(111) substrate is about 150 nm, Measurements of crack spacing as a function of film thickness indicate that the origin of cracks cannot be explained by the elastic behavior of the film but rather by the viscoelastic relaxation of the film during pyrolysis and sintering. Copyright (C) 1999 John Wiley & Sons, Ltd.

Photoluminescence in disordered Zn2TiO4

In this work, the polymeric precursor method Was used to obtain disordered Zn2TiO4 powders, either Undoped or doped with Sn4+, Cr3+ and V5+, to be applied its photoluininescent material. The characterization was undertaken by means of thermal analysis (TG and DTA), X-ray diffraction (XRD), infrared spectroscopy (IR) and photoluminescence (PL). Previous works stated that titanate octahedra containing a short Ti-O distance show efficient luminescence at roorn temperature if these octahedra are isolated from each other. In the present work, the phenomenon was observed in condensed octahedra, sharing edges. The room temperature PL noticed in undoped Zn2TiO4 had its intensity increased by the dopant addition-the increase was of about 300% for V5+ doping 400% for Cr3+ and 800% for Sn4+. (c) 2005 Elsevier B.V. All rights reserved.