Ednan Joanni

A polaronic stacking fault defect model for CaCu3Ti4O12 material: an approach for the origin of the huge dielectric constant and semiconducting coexistent features

This paper proposes a polaronic stacking fault defect model as the origin of the huge dielectric properties in CaCu3Ti4O12 (CCTO) materials. The model reconciles the opposing views of researchers on both sides of the intrinsic versus extrinsic debate about the origin of the unusually high values of the dielectric constant measured for CCTO in its various forms. Therefore, by considering stacking fault as the origin of the high dielectric constant phenomena, it was shown that the internal barrier layer capacitance mechanism is enhanced by another similar, but different in nature, mechanism that operates in the nanoscale range due to polaron defects associated with stacking fault, a mechanism that was referred to as nanoscale barrier layer capacitance (NBLC). The NBLC approach explains the origin of the CCTOs huge dielectric constant coexisting with semiconducting features.

Thickness effect on the dielectric, ferroelectric, and piezoelectric properties of ferroelectric lead zirconate titanate thin films

Lead zirconate titanate (PbZr0.52Ti0.48O3−PZT) thin films with different thicknesses were deposited on Pt(111)/Ti/SiO2/Si substrates by a sol-gel method. Single perovskite phase with (111)-texture was obtained in the thinnest films, whereas with the increase in thickness the films changed to a highly (100)-oriented state. An increase in the mean grain size as the film thickness increased was also observed. Dielectric, ferroelectric, and piezoelectric properties were analyzed as a function of the film thickness and explained based on film orientation, grain size, domain structure, domain wall motion, and nonswitching interface layers. Both serial and parallel capacitor models were used to analyze the influence of the nonswitching interface layer in the dielectric properties and the effect of substrate clamping in the microscopic piezoelectric response as the film thickness decreased. The scanning force microscopy technique was used to study the effect of thickness on the microscopic piezoresponse. Signific...

Deposition of bioactive glass-ceramic thin-films by RF magnetron sputtering

Abstract Thin films of bioactive glass-ceramics have been deposited on titanium and silicon substrates by RF magnetron sputtering. The crystalline phases and the microstructure of the films have been characterized using XRD and SEM analysis; the main phases present were calcium–magnesium phosphates, enstatite and forsterite. The adhesion of the films on titanium has been examined by pull-off testing; the adhesion strength for as-deposited films was around 40 MPa, but after crystallization the strength dropped to about half this value due to the presence of cracks. Samples kept in simulated body fluid showed an apatite-like layer, suggesting that the films are bioactive.

P-type semiconducting gas sensing behavior of nanoporous rf sputtered CaCu3Ti4O12 thin films

The fabrication of nanoporous sputtered CaCu3Ti4O12 thin films with high gas sensitivity is reported in this work. The porous microstructure and the nanocrystalline nature of the material promoted the diffusion of the atmosphere into the film, shortening the response time of the samples. Behaving as p-type semiconductor, the material presents enhanced sensitivity even at low working temperatures. Impedance spectroscopy measurements were performed in order to investigate the mechanisms responsible for the performance of the devices.

Fabrication of interdigitated micro-supercapacitor devices by direct laser writing onto ultra-thin, flexible and free-standing graphite oxide films

In this work we present graphene-based in-plane flexible interdigitated micro-supercapacitor devices fabricated through direct laser writing onto ultra-thin graphite oxide (GO) films. The fabrication route is simple, fast, additive-free, mask-free and cost effective. This involves direct micro-writing of reduced graphene oxide (rGO) by a pulsed UV laser on a very small area (1.14 cm2). The fabricated micro-supercapacitor contains nineteen pairs of rGO electrodes separated by the unreduced portion of the GO film. The single laser patterned rGO electrode presents low resistivity, while the unpatterned portion is non-conducting. Under the optimized laser parameters the 2.2 μm ultra-thin GO films were completely and uniformly reduced. The electrochemical measurements showed that the micro-supercapacitor, packed in a glass cavity, and in the presence of a liquid electrolyte have a capacitance nearly 288% higher (288.7 mF cm−3) compared to the as-fabricated device (0.36 mF cm−3). The as-fabricated micro-supercapacitor without electrolyte also shows some capacitance due to the presence of free ions in the unreduced portion of GO which plays a crucial role. Furthermore, the cycling stability of the as-fabricated micro-supercapacitor is robust, with not much performance degradation for more that 5000 cycles.

Resistive-Switching Behavior in Polycrystalline CaCu3Ti4O12 Nanorods

Highly aligned CaCu(3)Ti(4)O(12) nanorod arrays were grown on Si/SiO(2)/Ti/Pt substrates by radio-frequency sputtering at a low deposition temperature of 300 °C and room temperature. Structural and morphological studies have shown that the nanostructures have a polycrystalline nature and are oriented perpendicular to the substrate. The high density of grain boundaries in the nanorods is responsible for the nonlinear current behavior observed in these arrays. The current-voltage (I-V) characteristics observed in nanorods were attributed to the resistive memory phenomenon. The electrical resistance of microcapacitors composed of CaCu(3)Ti(4)O(12) nanorods could be reversibly switched between two stable resistance states by varying the applied electric field. In order to explain this switching mechanism, a model based on the increase/decrease of electrical conduction controlled by grain boundary polarization has been proposed.

Direct laser writing of micro-supercapacitors on thick graphite oxide films and their electrochemical properties in different liquid inorganic electrolytes

In this article we demonstrate a simple approach to fabricate interdigitated in-plane electrodes for flexible micro-supercapacitors (MSCs). A nanosecond ultraviolet laser treatment is used to reduce and pattern the electrodes on thick graphite oxide (GO) freestanding films. These laser-treated regions obtained by direct writing provide the conducting channels for electrons in the capacitors. The electrochemical performance of the MSCs was evaluated in the presence of two different electrolytes and they exhibit characteristics of nearly electrical double layer capacitors. The MSCs have areal capacitances as 2.40, 2.23 and 1.62μF/cm2 for NaOH, Na2SO4 and KCl electrolytes respectively, for measurements performed at the scan rate of 50mV/s. They retain ∼93.1% of their initial capacitances after 3500 cycles (scan rate=80mV/s) in NaOH electrolyte. The proposed laser treatment approach enables facile and fast fabrication of flexible MSCs without the need for tedious processing methods such as photolithographic micro-patterning and deposition of porous carbon or metallic current collectors.

Study of the oxygen vacancies changes in SnO2 polycrystalline thick films using impedance and photoemission spectroscopies

Changes in the concentration of oxygen vacancies within the grains of polycrystalline SnO2, due to different atmosphere exposures, were detected using impedance and photoemission spectroscopies. From measured capacitance values, variations of the potential barrier widths could be determined. It is shown that under the presence of an oxygen rich atmosphere, at relatively low temperature, the width of intergranular potential barriers increase to the point that grains become completely depleted of carriers. With subsequent exposure to vacuum, capacitance adopts a higher value, indicative of intergranular barriers and quasi-neutral regions at the center of the grains. X-ray and ultraviolet photoemission spectroscopy measurements showed that SnO2 samples treated in oxidizing or reducing environments have similar barrier heights and different work functions. Results are especially relevant in the study of mechanisms responsible for metal oxide gas sensing.

Grain size effect on the electrical response of SnO2 thin and thick film gas sensors

Porous nano and micro crystalline tin oxide films were deposited by RF Magnetron Sputtering and doctor blade techniques, respectively. Electrical resistance and impedance spectroscopy measurements, as a function of temperature and atmosphere, were performed in order to determine the influence of the microstructure and working conditions over the electrical response of the sensors. The conductivity of all samples increases with the temperature and decreases in oxygen, as expected for an n-type semiconducting material. The impedance plots indicated the existence of two time constants related to the grains and the grain boundaries. The Nyquist diagrams at low frequencies revealed the changes that took place in the grain boundary region, with the contribution of the grains being indicated by the formation of a second semicircle at high frequencies. The better sensing performance of the doctor bladed samples can be explained by their lower initial resistance values, bigger grain sizes and higher porosity.

Pulsed laser deposition of SiO2 - P2O5 - CaO - MgO glass coatings on titanium substrates

Thin films of bioactive glass-ceramic have been deposited on titanium substrates by the Pulsed Laser Deposition (PLD) technique under different experimental conditions. The effect of parameters such as deposition pressure and temperature of heat treatments was studied. The microstructure and the crystalline phases of the coatings were characterized using SEM, EDX and XRD analysis; the phases present were titanium oxides, calcium magnesium silicates and phosphates. The adhesion of the as-deposited films has been examined by scratch tests. The interfacial adhesion of the coatings was better when the deposition was performed at low pressure. Samples were immersed in simulated body fluid (SBF), and a calcium-phosphate precipitate was observed on the surface of less crystallized samples, suggesting that there is some relationship between surface reactivity and crystallinity.