G. N. P. Oliveira

Resistive switching and activity-dependent modifications in Ni-doped graphene oxide thin films

The resistive switching (RS) mechanism in Ni-doped graphene oxide (GO) devices is studied. We found that RS depends strongly on the fabrication method of the GO sheet and on the electrode material. Resistive switching in GO-devices can be caused by the diffusion of ions from metallic electrode or by the migration of oxygen groups, depending on the fabrication process. We also show that GO-based structures possess activity-dependent modification capabilities, emphasized by the increase/decrease of device conductance after consecutive voltage sweeps of opposite polarity. Our results allow a better understanding of bipolar RS, towards future non-volatile memories and neuromorphic systems.

Characterization of laminar extrudates manufactured at room temperature in the absence of solvents for the delivery of drugs.

Extrudates with laminar shape for delivery of drugs were manufactured at room temperature and without solvents. Different lipidic materials, formulations and extrusion conditions were studied, as well as the influence of the size of particles of coumarin on the release from the extrudates. Materials were mixed prior to feeding an in-house built ram extruder with a rectangular shaped die. The process of extrusion was characterized for the force at steady state whereas extrudates were characterized immediately after production and over storage for density, porosity, bending strength, elasticity, stiffness, deformation, thermal behavior and release performance of coumarin, the model drug. The variety of lipid excipients and their proportions in the formulations directly influenced the properties of the extrudates, namely the bending strength, stiffness, deformation and elasticity and, the density and porosity, which changed over storage time: in general, the bending strength, stiffness and porosity increased over time, whereas deformation, elasticy and density decreased. The thermal analysis supported these observations as there was an increase in the enthalpies of fusion of the extrudates over time. The release of coumarin from the extrudates occured both by diffusion within the extrudates and by structural alterations of the extrudates and an increase of the coumarin particle size corresponded to a decrease of the release rate. The study has proved the ability to manufacture extrudates in a continuous fashion, with laminar shape using a green technology.

Local symmetry lowering in CdMn2O4 spinel

This work presents an atomic scale study of the electric field gradient (EFG) in the tetragonally distorted CdMn2O4 spinel manganite. The EFG temperature dependence at the Cd and Mn sites was followed via perturbed angular correlation measurements with the 111In and 111mCd probes, from 873 down to 12 K at Isolde-CERN. The results show that in the 12–600 K temperature range, a single Jahn-Teller distorted local phase exists. However above 100 K, a dynamic lattice distortion, evidenced by time dependent EFG fluctuations, sets in suggesting a structural instability. Above 600 K, a local MnO6 octahedra with relaxed Jahn-Teller distortions emerge and grow in the low temperature matrix, although no macroscopic tetragonal to cubic phase transition was observed.

Influence of Mn addition on magnetic and structural properties of barium hexaferrite

Two-phase nano-powders based on Ba hexaferrite with addition of Mn have been obtained by a sol-gel autocombustion method. Composition dependence of magnetic and structural properties has been studied using magnetic measurements, X-ray diffraction (XRD) and scanning electron microscopy (SEM). The observed decrease of saturation magnetization and coercivity with increase of Mn content is mainly attributed to the presence of Fe oxide as a result of Ba depletion. SEM images show chemically homogeneous, non-regular shaped agglomerated nano-particles (50-200 nm).

Delivery of Drugs from Laminar Co-Extrudates Manufactured by a Solvent-Free Process at Room Temperature

This work aims to design and manufacture laminar co-extrudates as a new dosage form for the delivery of drugs. Co-extrudates made of lipid-based materials with a laminar shape were manufactured at room temperature in the absence of solvents and assessed over time for their mechanical properties (bending strength, deformation, stiffness, and elasticity), density, porosity, thermal behavior and main mechanism of drug release. The study has shown that the extrusion force at steady state increased with the extrusion rate and with the number of layers. The bending strength and stiffness of extrudates increased over time. Laminar co-extrudates with higher number of layers presented a decreasing dissolution efficiency of 38.3 ± 0.6%, 23.0 ± 0.2%, and 12.3 ± 0.2%, for mono-, bi-, and trilayer, respectively. After 90 days, the density, the deformation, and elasticity decreased: trilayer extrudates were the denser and the ones to present the lowest ability to deform and the highest elasticity, whereas monolayer extrudates were the less dense presenting the highest ability to deform. Changes were more evident in the first days after manufacture leading to stabilization over time. Laminar (co-)extrudates have been confirmed as an innovative dosage form for tailored delivery of drugs made without solvents at room temperature.