T. de Lacerda-Arôso

Ancient Portuguese Ceramic Wall Tiles (“Azulejos”): Characterization of the Glaze and Ceramic Pigments

Ancient ceramic wall tiles, called “azulejo”, firstly used on Portuguese churches, monasteries and palaces (15-18th century) have progressively been used in particular houses till the last century. These tiles and its use in huge decorative panels can be considered as a precious but fragile cultural heritage from Brazil to India, in several countries influenced by Portuguese culture. Morphologically, these tiles are composed by a porous clay-based ceramic body, the terracotta, covered by a protective glassy phase, the glaze. As artistic paintings, these murals incorporated various kinds of pigments in the glaze layer to create a pictorial impact on the walls of rich palaces or churches, real and durable monumental works-of-art. In the 21st century, degradation marks are visible on these ceramic tiles because of their use under corrosive conditions (moisture, atmospheric cycles…) along centuries. In order to promote their conservation and enhance their restoration, the physical-chemical characterization of the azulejos is performed in the present work, using mainly non-destructive processes like micro-Raman spectroscopy or X-Ray diffraction. In particular, Raman spectroscopy allows the detection of some nano/microcrystals present in the amorphous glaze due to pigments or opacifying agents or related to the elaboration process of the azulejo. Based on the observation of various selected fragments, one states that very few pigments have been used as colouring agents in this ceramic art during 17-18th centuries. Thus, the relationship between the different colours, the introduced pigments and the structural aspects of the glass will be focused. Some features related with the ancient ceramic technology will also be discussed.

Frequency dependence of aging, rejuvenation and memory in a disordered ferroelectric

Abstract:We characterize in details the aging properties of the ferroelectric phase of KTa1-xNbx O3 (KTN), where both rejuvenation and (partial) memory are observed. In particular, we carefully examine the frequency dependence of several quantities that characterize aging, rejuvenation and memory. We find a marked subaging behaviour, with an a.c. dielectric susceptiblity scaling as ω, where tw is the waiting time. We suggest an interpretation in terms of pinned domain walls, much along the lines proposed for aging in a disordered ferromagnet, where both domain wall reconformations and overall (cumulative) domain growth are needed to rationalize the experimental findings.

Search for memory and return to disorder in potassium-lithium tantalate crystals

Return to disorder, and less frequently memory, have been demonstrated in various disordered materials. In order to find the conditions necessary for the observation of these effects, the evolution of the real part ´ of the dielectric constant of two disordered paraelectric crystals K1-x Lix TaO3 (KLT) has been extensively studied by the means of the capacitance C (T , t ) around the beginning and the end of a temperature plateau. The return to disorder and memory effects have not been seen in KTL, in contrast to what was recently observed in disordered ferroelectric crystals KTa1-y Nby O3 of a similar family. The variations dC = P (T 0 , t 0 ) dT + Q (T 0 , t 0 ) dt in the vicinity of the point (T 0 , t 0 ) are split into a contribution depending on temperature only and an isothermal contribution only depending on time. All the results of such an analysis of the features observed in KLT can be explained by the domain wall model in its original form.

Structural and Mechanical Properties of AZOY Thin Films Deposited on Flexible Substrates

Transparent conducting Al doped ZnO films have been deposited by dc magnetron sputtering on glass and polymer substrates at room temperature. Depositions have been carried out from an AZOY (contains a small amount of Y2O3 in addition to Al2O3 and ZnO) target under different conditions such as working pressure, substrate bias voltage and oxygen flow rate. The crystallinity of the Al doped ZnO films has been improved by using low-energy-ion bombardment. Likewise, the use of either the rotation or the static mode of the substrate during deposition influences the crystallinity and therefore the optical parameters and the electrical resistance of the films. Increasing the thickness of the films reduces the threshold strain at which the films can be deformed without provoking significant changes on their electrical properties.

Aging and other related phenomena in potassium niobo-tantalate crystals studied through the frequency dependence of their dielectric constant

We report on dielectric measurements in the frequency range from 1 to 1000 kHz in two disordered ferroelectric KTa1-yNbyO3 (KTN) crystals (y0.02). We have studied the frequency dependence of the dielectric constant (real and imaginary parts) during several thermal evolutions: isothermal aging, rejuvenation by cooling or heating after a temperature plateau, memory. In every case the frequency dependence is well described by a power law. The temperature and plateau duration dependences of the exponents have been studied. We give a description of our results within a model where the variations of the dielectric constant are attributed to the motion (growth and reconformation) of ferroelectric domain walls. In particular we are able to explain the identity of the exponents of the frequency power law dependence of memory and isothermal aging.

Aging in strontium - calcium titanate crystals

The complex dielectric constant of the orientational glass (SCT) with x = 0.0055 was measured as a function of time at 4.2 K, for frequencies ranging from 1 kHz to 1 MHz, after different thermal histories. Both the in-phase component and the out-of-phase component clearly exhibit aging as shown by the cooling rate dependence of their evolution. Moreover, a weak dependence has been seen for the asymptotic values of both components at very long time. Hence, evidence exists for ergodicity breaking. This is in contrast to what was seen for spin glasses, and similar to what was observed for the series (KLT) of orientational glasses studied previously. However, when temperature jumps are imposed on the SCT sample, the response differs both from that of spin glasses and from that of the KLT series.