Sorin Tascu

Ferroelectric and dielectric properties of ferrite-ferroelectric ceramic composites

Particulate composites of ferrite and ferroelectric phases with xNiFe2O4 (NF) and (1 − x)Pb0.988(Zr0.52Ti0.48)0.976Nb0.024O3 (where x = 2, 10, 20, 30, 50, 70, and 100 wt. %) were prepared in situ by sol-gel method. The presence of a diphase composition was confirmed by X-ray diffraction while the microstructure of the composites was studied by scanning electron microscopy revealing a good mixing of the two phases and a good densification of the bulk ceramics. The dielectric permittivity shows usual dielectric dispersion behavior with increasing frequency due to Maxwell-Wagner interfacial polarization. AC conductivity measurements made in frequency range 1 Hz-1 MHz suggest that the conduction process is due to mixed polaron hopping. The effect of NF phase concentration on the P-E and M-H hysteresis behavior and dielectric properties of the composites was investigated. At low NF concentration a sharp ferro-paraelectric transition peak can be observed at around 360 °C while for higher NF concentrations a tre...

Temperature dependence of tunability of Ba(SnxTi1−x)O3 ceramics

The electric field dependence of the dielectric constant (dc-tunability) as a function of temperature for a few compositions of Ba(Sn,Ti)O3 ceramics was investigated. Dense and homogeneous BaSnxTi1−xO3 ceramics (ferroelectric for x = 0.05 and x = 0.10, relaxor for x = 0.15 and 0.20) with high dielectric constant and low dielectric losses were prepared by solid state reaction. Their dc-tunability characteristics were investigated at a few temperatures from 20–120 °C, including the Curie range. The tunability data were discussed in terms of the Johnson model completed with a Langevin term that describes the “extrinsic” contribution to the nonlinear ɛ(E) dependences.

Using multi-walled carbon nanotubes in spark plasma sintered Pb(Zr0.47Ti0.53)O3 ceramics for tailoring dielectric and tunability properties

The addition of small amounts (below 0.1 wt. %) of multi-walled carbon nanotubes (MWCNTs) to Pb(Zr0.47Ti0.53)O3 (PZT) ceramics prepared by spark plasma sintering is proposed as a method of tailoring the electrical properties, which are expected to be modified with respect to the pure PZT, both as result of the presence of 1-D conductive fillers in the ceramic product and via the microstructural modifications of ceramics induced during the sintering. The addition of even small amounts of carbon nanotubes strongly reduced the sinterability of PZT ceramics and resulted in the porous and fine-grained microstructures (relative density of 73% for a MWCNT addition of 0.5 vol. % by comparison with 91% in the pure PZT, produced in the same conditions). A monotonous decrease of permittivity with increasing the MWCNT level from ∼830 in pure PZT to ∼627 for x = 0.5 vol. %, at a fixed frequency f = 1kHz, and low dielectric losses below 2% have been observed. Tunability increases with respect to the values of dense PZT...

Engineering magnetoelectric composites towards application as tunable microwave filters

Tunable microwave devices are widely used in radar, telecommunication and radio frequency measurement systems. In recent years, a promising technology for tunable microwave devices, which combines the advantages of using the magnetic and electrical properties of materials as well as their magnetoelectric coupling, has been developed. In the present work, xMnFe2O4-(1 − x)Pb0.988(Zr0.52Ti0.48)0.976Nb0.024O3 ferrite-ferroelectric composites have been prepared and their functional properties were investigated, in order to be designed as components in tunable microwave devices. Structural, microstructural, dielectric and magnetic investigations confirmed the formation of composite ceramics with valuable dielectric and magnetic properties at room temperature. The complex permittivity and permeability of the investigated magnetoelectric ceramics, measured in the frequency range of 1 MHz to 1 GHz, show as a combination of the properties of Pb(Zr,Ti)NbO3 ferroelectric and magnetic MnFe2O4 phases. Experimental measurements and the numerical simulations were done using a high frequency structure simulator (HFSS) which revealed that the investigated composite disk placed on a microstrip line acts as an electromagnetic resonator and it could be consider as a filter with tunable stop-band capabilities controlled by an external magnetic field. It was found that the central stop-band frequency of the designed filter can be tuned with 18 MHz by changing the magnetic field from 0 to 2 kOe.

Platinum role in hydrophilicity enhancement of Cr-doped TiO2 thin films

Abstract In this paper, we have investigated the hydrophilic properties of the titania films doped with increasing chromium percentages (from 2.1 at.% till 4.0 at.%). Cr-doping induces an increase in the rutile weight %, a more compact structure, and a significant red shift of the TiO2 absorption edge, the last property being very important in the self cleaning applications. For the chosen Cr concentrations, the films did not show promising hydrophilic properties. To improve them, we have applied a novel surface modification method, reported in literature mainly for powders, namely, surface metallisation. We have observed that, by depositing Pt islands on the film with the highest Cr content, its hydrophilic properties improve for a certain metal coverage area. The explanation was based on FT-IR and X-ray photoelectron spectroscopy analysis, performed on the UV irradiated and non-irradiated films, which gives information on the relationship between hydrophilicity and the amount of the adsorbed hydroxyl groups.

Combined effects of p–n heterojunctions and active surface areas in a composite material dedicated to gas sensing applications

In this study, we analyze the unexplored field of combined effects of active surface area and p–n heterojunctions of a composite material dedicated to gas sensing applications. The used materials are thin films of mixtures of copper and zinc oxides. Individually, both copper oxide and zinc oxide shows interesting sensing performances toward a broad category of gases. The investigated films are obtained by thermal oxidation of Cu/Zn metallic multilayers. Cu/Zn metallic multilayers were obtained by using physical vapor deposition technique. The sensitivity of the composite materials, at ethanol, LPG and CO respectively, was investigated and it was observed that the highest sensitivity is obtained for ethanol. Complementary investigations (X-ray Diffraction, X-ray Photoelectron Spectroscopy and Scanning Electron Microscopy respectively) were conducted in order to understand, explain and confirm the relationship between the oxidation conditions, structure, surface morphology and the exhibit sensing properties. The experimental results indicate that the sensitivity depend on the oxidation duration.

Nb-doped TiO2 thin films as photocatalytic materials

Amorphous undoped and Nb-doped films were obtained by the spin coating method. The films have a compact structure, as revealed by scanning electron microscopy, and are very thin, with thickness values under 100 nm. The photocatalytic activity of the films was evaluated by observing the decomposition of an oleic acid solution under UV irradiation, and by studying the change in the optical transmittance of an aqueous solution containing methylene blue, in the presence of the UV-irradiated films. More than 30 h, depending on doping, are needed to recover their initial contact angles before applying oleic acid. The increase of the optical transmittance of the methylene blue solution confirms the photocatalytic degradation of methylene blue on the Nb-doped TiO2 films. X-ray photoelectron spectroscopy studies, performed to detect the presence of the carbon on the irradiated surface of the films, drive to the conclusion that at the surface of the films, even for contact angles close to 0 ∘, the presence of carbon still can be detected, which demonstrates that hydrophilicity is ruled by a different mechanism than photocatalysis.

Broadband integrated beam splitter using spatial adiabatic passage

Light routing and manipulation are important aspects of integrated optics. They essentially rely on beam splitters which are at the heart of interferometric setups and active routing. The most common implementations of beam splitters suffer either from strong dispersive response (directional couplers) or tight fabrication tolerances (multimode interference couplers). In this paper we fabricate a robust and simple broadband integrated beam splitter based on lithium niobate with a splitting ratio achromatic over more than 130 nm. Our architecture is based on spatial adiabatic passage, a technique originally used to transfer entirely an optical beam from a waveguide to another one that has been shown to be remarkably robust against fabrication imperfections and wavelength dispersion. Our device shows a splitting ratio of 0.52±0.03 and 0.48±0.03 from 1500 nm up to 1630 nm. Furthermore, we show that suitable design enables the splitting in output beams with relative phase 0 or π. Thanks to their independence to material dispersion, these devices represent simple, elementary components to create achromatic and versatile photonic circuits.

Anomalous angular dispersion in lithium niobate one-dimensional waveguide array in the near-infrared wavelength range

Knowing the dispersion properties of a device is important in many applications (e.g., wavelength separation). For an isolated waveguide, besides the material dispersion, one must consider the waveguide influence as well, through waveguide dispersion and mode dispersion. For a waveguide array, one must consider the influence of evanescent coupling between adjacent waveguides as well. We investigate by the Finite Element Method the angular dispersion of a LiNbO3 waveguide array using two techniques. The first one assumes the Coupled Mode Theory in a 2-waveguide system. The other one uses the actual diffraction curve determined in a 7-waveguide system. In both approaches, we find that by decreasing the array period, one passes from normal angular dispersion by an achromatic point to anomalous angular dispersion. We then illustrate the wavelength separation by the waveguide array by doing Runge-Kutta light propagation simulations. As all the values of parameters are technologically feasible, this opens new p...