E. B. Araujo

Infrared spectroscopy: a tool for determination of the degree of conversion in dental composites.

Infrared spectroscopy is one of the most widely used techniques for measurement of conversion degree in dental composites. However, to obtain good quality spectra and quantitative analysis from spectral data, appropriate expertise and knowledge of the technique are mandatory. This paper presents important details to use infrared spectroscopy for determination of the conversion degree.

Ionized oxygen vacancy-related electrical conductivity in (Pb 1−x La x ) (Zr 0.90 Ti 0.10 ) 1−x/4 O 3 ceramics

The electrical conductivity behaviour in the paraelectric state has been investigated in lanthanum modified lead zirconate titanate (PLZT) ceramics, considering the universal relaxation law. The contribution of conductive processes to the dielectric response for the low-frequency range was discussed taking into account the oxygen vacancies mechanism. The activation energies for the thermally hopping and conductivity processes were obtained. The results for the frequency dependence of the conductivity suggest at the oxygen vacancy hopping processes, due to relaxations in oxygen vacancy-related dipoles, being mainly responsible for the conduction behaviour in the studied system.

Infrared studies of the monoclinic-tetragonal phase transition in Pb(Zr,Ti)O3 ceramics

Recently, the observation of a new monoclinic phase in the PbZr1−xTixO3 (PZT) system in the vicinity of the morphotropic phase boundary was reported. Investigations of this new phase were reported using different techniques such as high-resolution synchrotron x-ray powder diffraction and Raman spectroscopy. In this work, the monoclinic → tetragonal phase transition in PbZr0.50Ti0.50O3 ceramics was studied using infrared spectroscopy between 1000 and 400 cm−1. The four possible ν1-stretching modes (Ti–O and Zr–O stretch) in the BO6 octahedron in the ABO3 structure of PZT in this region were monitored as a function of temperature. The lower-frequency mode ν1-(Zr–O) remains practically unaltered, while both intermediate ν1-(Ti–O) modes decrease linearly as temperature increases from 89 to 263 K. In contrast, the higher-frequency ν1-(Ti–O) and ν1-(Zr–O) modes present anomalous behaviour around 178 K. The singularity observed at this mode was associated with the monoclinic → tetragonal phase transition in PbZr0.50Ti0.50O3 ceramics.

Setting time and thermal expansion of two endodontic cements

The purpose of this study was to evaluate the setting time and the thermal expansion coefficient of 2 endodontic cements, MTA-Angelus and a novel cement called CER. The setting time was determined in accordance to ANSI/ADA specifications no. 57. Three samples of 10 mm diameter and 2 mm thickness were prepared for each cement. The thermal expansion measurements were performed by strain gauge technique. Four samples of each cement were prepared using silicone rings of 5 mm diameter and 2 mm thickness. The data were analyzed statistically using the Student t test. The setting time obtained for the MTA-Angelus and CER cements was 15 (SD 1) min and 7 (SD 1) min, respectively. The linear coefficient of thermal expansion was 8.86 (SD 0.28) microstrain/ degrees C for MTA-Angelus and 11.76 (SD 1.20) microstrain/ degrees C for CER. The statistical analysis showed significant difference (P < .05) in the setting time and linear coefficient of thermal expansion between the 2 cements. The CER cement has a coefficient of expansion similar to dentin, which could contribute to a decrease of microleakage degree.

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.

Evidence for the monoclinic-tetragonal phase coexistence in Pb(Zr0.53Ti0.47)O3 thin films

The structure and ferroelectric properties of PbZr0.53Ti0.47O3 thin films were investigated in detail by using the x-ray diffraction technique. The surface morphology of the film was studied by using the atomic force microscopy technique, showing a film with a dense morphology and a smooth surface. Based on recent results reported by Pandey et al (2008 Acta Crystallogr. A64 192), Rietveld refinements of the structure were conducted considering different models proposed in the literature. Results suggested the monoclinic and tetragonal (M+T) phase coexistence, with P4mm and Cm space groups, respectively. The monoclinic phase (68 mol%) is dominant over the tetragonal phase (32 mol%) for this PZT film composition.

Monoclinic-tetragonal phase transition in Pb(Zr1-xTix)O3 studied by infrared spectroscopy

The discovery of a new monoclinic phase in the PbZr1-xTixO3 (PZT) system in the vicinity of the morphotropic phase boundary (MPB), previously considered as a region where the rhombohedral and tetragonal phases of PZT coexist, was recently reported. Investigations of this new phase were reported using different techniques such as high-resolution synchrotron x-ray powder diffraction and Raman spectroscopy. The main objective has been to define a new phase diagram of PZT. In this context, infrared spectroscopic studies were performed in the vicinity of the MPB and studies were initially centred on a PZT sample with x = 0.49 mol% Ti content. Results suggested that the monoclinic → tetragonal phase transition occurs at 237 K, confirming the use of IR as a useful technique to investigate this phase transition.

Domain wall contribution to dielectric and piezoelectric responses in 0.65Pb(Mg1/3Nb2/3)–0.35PbTiO3 ferroelectric ceramics

In this paper, the contribution of the domain walls motion to the dielectric and piezoelectric responses for the 0.65Pb(Mg1/3Nb2/3)O3–0.35PbTiO3 system is investigated. A monotonically increasing temperature dependence of the dielectric permittivity is observed from very low temperatures up to the ferroelectric–paraelectric phase transition temperature. It is verified that the major contribution to dielectric response at room temperature is from the extrinsic effect. A linear dependence of the permittivity with the amplitude of the ac applied electric field is verified from the nonlinear dielectric measurement. Rayleighs model is used to quantitatively evaluate the dielectric response, leading to similar behaviour of the typical soft PZT. On the other hand, direct piezoelectric response also displays a Rayleigh-type behaviour. Piezoelectric measurements show in the entire investigated dynamic stress range a decrease in the piezoelectric response with both increase in frequency of the applied dynamic stress and increase in the pre-stress on the samples.

Evidence of distinct phase transformations in lithium phosphate glass Li2O–P2O5

The purpose of this work is to study the Li2O–P2O5 glass using the differential scanning calorimetry (DSC) and x-ray diffraction (XRD) techniques to understand the crystallization process in this glass matrix. To study the glass by DSC, screened samples with different particle sizes to resolve the crystallization peaks were used. Both crystallization peaks were attributed to Li6P6O18 and LiPO3 phases. This evidence was corroborated by XRD analysis on glasses annealed at different temperatures in order to crystallize these phases.

Thickness effect on the structure, grain size, and local piezoresponse of self-polarized lead lanthanum zirconate titanate thin films

Polycrystalline lanthanum lead zirconate titanate (PLZT) thin films were deposited on Pt/TiO2/SiO2/Si substrates to study the effects of the thickness and grain size on their structural and piezoresponse properties at nanoscale. Thinner PLZT films show a slight (100)-orientation tendency that tends to random orientation for the thicker film, while microstrain and crystallite size increases almost linearly with increasing thickness. Piezoresponse force microscopy and autocorrelation function technique were used to demonstrate the existence of local self-polarization effect and to study the thickness dependence of correlation length. The obtained results ruled out the bulk mechanisms and suggest that Schottky barriers near the film-substrate are likely responsible for a build-in electric field in the films. Larger correlation length evidence that this build-in field increases the number of coexisting polarization directions in larger grains leading to an alignment of macrodomains in thinner films.