M.E.V. Costa

Dielectric relaxation in Ba-based layered perovskites

Ferroelectric materials with Bi-layered structure such as SrBi2Ta2O9 and SrBi2Nb2O9 are now intensively investigated in view of their applications in nonvolatile computer memories and high-temperature piezoelectric transducers. When Sr2+ is substituted with Ba2+, a significant disorder is induced and the material exhibits broadening of the phase transition. Such broadening is essential for applications since it allows achieving smooth temperature characteristics while maintaining high dielectric and piezoelectric properties. In this work, stoichiometric dense BaBi2Nb2O9 (BBN) ceramics are sintered using a mixed oxide route. Dielectric and ferroelectric properties are investigated in a broad range of temperatures and frequencies. Strong dispersion of the complex relative dielectric permittivity is observed including typical relaxor features such as shift of the permittivity maximum with frequency and broadening of the relaxation time spectrum with decreasing temperature. The dielectric relaxation obeys the...

Relaxor properties of Ba-based layered perovskites

Abstract Ferroelectric materials with Bi-layered structure such as SrBi 2 Ta 2 O 9 (SBT) and SrBi 2 Nb 2 O 9 (SBN) are now intensively investigated in view of their applications in non-volatile computer memories and high-temperature piezoelectric transducers. When Sr is substituted with Ba, significant disorder is induced and material exhibits broadening of the phase transition. In this work, BaBi 2 Ta 2 O 9 (BBT) and Ba 2 Bi 2 Nb 2 O 9 (BBN) ceramics were sintered using mixed oxide route. Dielectric properties were investigated in the temperature range 20–600°C at frequencies 25 Hz to 1 MHz. Strong dispersion of the dielectric permittivity is found indicating relaxor nature of the phase transition. It is shown that the dielectric relaxation in BBN ceramics is different from that of conventional relaxors such as PbMg 1/3 Nb 2/3 O 3 (PMN). Fitting with Vogel–Fulcher relationship is used to evaluate parameters of the dielectric relaxation such as freezing temperature and attempt frequency.

Transparent, conductive ZnO:Al thin film deposited on polymer substrates by RF magnetron sputtering

In this paper, we present the optical, electrical, structural and mechanical properties exhibited by aluminum-doped zinc oxide (ZnO:Al) thin films produced by RF magnetron sputtering on polymeric substrates (polyethylene terephthalate, PET; Mylar type D from Dupont®) with a standard thickness of 100 μm. The influence of the uniaxial tensile strain on the electrical resistance of these films was evaluated in situ for the first time during tensile elongation. In addition, the role of the thickness on the mechanical behavior of the films was also evaluated. The preliminary results reveal that the increase in electrical resistance is related to the number of cracks, as well as the crack width, which also depends on the film thickness.

Ferroelectric and dielectric anisotropy in high-quality SrBi2Ta2O9 single crystals

High-quality SrBi2Ta2O9 single crystals were obtained by a self-flux solution method. The crystals were naturally oriented with [001] direction perpendicular to the major face and edges parallel to [110] axes. The dielectric and ferroelectric properties of the crystals were investigated along the c axis and in the ab plane of the orthorhombic unit cell. The ferro–paraelectric phase transition was observed at TC=355°C. The anisotropy of dielectric permittivity, i.e., the ratio between average permittivity in the ab plane and along c axis was about 10 at TC and decreased to ∼2 at room temperature. Saturated hysteresis loops were observed for switching in the ab plane. The spontaneous polarization along ferroelectric a axis was estimated to be ≈20μC∕cm2.

Sulforaphane Induces Oxidative Stress and Death by p53-Independent Mechanism: Implication of Impaired Glutathione Recycling

Sulforaphane (SFN) is a naturally-occurring isothiocyanate best known for its role as an indirect antioxidant. Notwithstanding, in different cancer cell lines, SFN may promote the accumulation of reactive oxygen species (ROS) and cause cell death e.g. by apoptosis. Osteosarcoma often becomes chemoresistant, and new molecular targets to prevent drug resistance are needed. Here, we aimed to determine the effect of SFN on ROS levels and to identify key biomarkers leading to ROS unbalance and apoptosis in the p53-null MG-63 osteosarcoma cell line. MG-63 cells were exposed to SFN for up to 48 h. At 10 μM concentration or higher, SFN decreased cell viability, increased the%early apoptotic cells and increased caspase 3 activity. At these higher doses, SFN increased ROS levels, which correlated with apoptotic endpoints and cell viability decline. In exposed cells, gene expression analysis revealed only partial induction of phase-2 detoxification genes. More importantly, SFN inhibited ROS-scavenging enzymes and impaired glutathione recycling, as evidenced by inhibition of glutathione reductase (GR) activity and combined inhibition of glutathione peroxidase (GPx) gene expression and enzyme activity. In conclusion, SFN induced oxidative stress and apoptosis via a p53-independent mechanism. GPx expression and activity were found associated with ROS accumulation in MG-63 cells and are potential biomarkers for the efficacy of ROS-inducing agents e.g. as co-adjuvant drugs in osteosarcoma.

Sulforaphane induces DNA damage and mitotic abnormalities in human osteosarcoma MG-63 cells: correlation with cell cycle arrest and apoptosis.

Osteosarcoma is a recalcitrant bone malignancy with poor responsiveness to treatments; therefore, new chemotherapeutic compounds are needed. Sulforaphane (SFN) has been considered a promising chemotherapeutic compound for several types of tumors by inducing apoptosis and cytostasis, but its effects (e.g., genotoxicity) in osteosarcoma cells remains exploratory. In this work, the MG-63 osteosarcoma cell line was exposed to SFN up to 20 μM for 24 and 48 h. SFN induced G2/M phase arrest and decreased nuclear division index, associated with disruption of cytoskeletal organization. Noteworthy, SFN induced a transcriptome response supportive of G2/M phase arrest, namely a decrease in Chk1- and Cdc25C-encoding transcripts, and an increase in Cdk1-encoding transcripts. After 48-h exposure, SFN at a dietary concentration (5 μM) contributed to genomic instability in the MG-63 cells as confirmed by increased number of DNA breaks, clastogenicity, and nuclear and mitotic abnormalities. The increased formation of nucleoplasmic bridges, micronuclei, and apoptotic cells positively correlated with loss of viability. These results suggest that genotoxic damage is an important step for SFN-induced cytotoxicity in MG-63 cells. In conclusion, SFN shows potential to induce genotoxic damage at low concentrations and such potential deserves further investigation in other tumor cell types.

Relevance of the sterilization-induced effects on the properties of different hydroxyapatite nanoparticles and assessment of the osteoblastic cell response

Hydroxyapatite (Hap) is a calcium phosphate with a chemical formula that closely resembles that of the mineral constituents found in hard tissues, thereby explaining its natural biocompatibility and wide biomedical use. Nanostructured Hap materials appear to present a good performance in bone tissue applications because of their ability to mimic the dimensions of bone components. However, bone cell response to individual nanoparticles and/or nanoparticle aggregates lost from these materials is largely unknown and shows great variability. This work addresses the preparation and characterization of two different Hap nanoparticles and their interaction with osteoblastic cells. Hap particles were produced by a wet chemical synthesis (WCS) at 37°C and by hydrothermal synthesis (HS) at 180°C. As the ultimate in vivo applications require a sterilization step, the synthesized particles were characterized ‘as prepared’ and after sterilization (autoclaving, 120°C, 20 min). WCS and HS particles differ in their morphological (size and shape) and physicochemical properties. The sterilization modified markedly the shape, size and aggregation state of WCS nanoparticles. Both particles were readily internalized by osteoblastic cells by endocytosis, and showed a low intracellular dissolution rate. Concentrations of WCS and HS particles less than 500 μg ml−1 did not affect cell proliferation, F-actin cytoskeleton organization and apoptosis rate and increased the gene expression of alkaline phosphatase and BMP-2. The two particles presented some differences in the elicited cell response. In conclusion, WCS and HS particles might exhibit an interesting profile for bone tissue applications. Results suggest the relevance of a proper particle characterization, and the interest of an individual nanoparticle targeted research.

Influence of the Strain on the Electrical Resistance of Zinc Oxide Doped Thin Film Deposited on Polymer Substrates

Tensile tests were performed on PET films coated with Al doped zinc oxide films by RF magnetron sputtering. During the tensile elongation, the electrical resistanceof the oxide was evaluated in situ. The results indicate that the increase in the electrical resistance is related to the crack debsity and crack width, which also depends on the film thickness.

Effect of electrode alterations on the a.c. behaviour of Li2OZnO humidity sensors

The dielectric behaviour of humidity sensors based on Li-doped ZnO has been investigated at frequencies from 1 Hz to 1 MHz, at room temperature. Sputtered gold electrodes and Teflon® film electrodes were used to obtain the dielectric response of the system electrodes/sensor to humidity changes. It is suggested that the main dielectric relaxation observed when Teflon electrodes are used corresponds to one of the three different dielectric relaxations observed for the system with gold electrodes. Current theories are used to discuss the origin of the observed relaxations.

Silicon carbide alloys produced by hot wire, hot wire plasma-assisted and plasma-enhanced CVD techniques

In this work, we report the optical and compositional properties of hydrogenated amorphous silicon carbide (a-SiC:H) thin films produced by plasma-enhanced chemical vapor deposition (PE-CVD), hot wire CVD (HW-CVD) and hot wire plasma-assisted CVD (HWPA-CVD) processes. The optical band gap of a-SiC:H films was controlled from 1.85 to 3.5 eV by varying the percentage of ethylene in the silane gas mixture from 3 to 100%. Adding a rf plasma to the hot wire process the carbon gas source dissociation is implemented leading to an increase in bulk carbon incorporation. This evidence is proved by the enhancement of the peak ascribed to the SiC stretching vibration mode, the reduction of the peak related to the SiH wagging modes, the decrease in the refractive index and the increase of optical band gap. The influence of hydrogen gas dilution on the properties of the films obtained by the different methods is also reported.