Publications

Ba(Zn1/3Ta2/3)O-3 dielectric materials were prepared by, solid state reaction. The samples were sintered at temperatures in the range 1400 divided by. 1600 degrees C for 4 h. Morphostructural characterization was performed by using SE l and XRD. The dielectric properties were measured in the microwave range (6 divided by 7 GHz). An additional annealing at 1400 degrees C for 10 hours was performed in order to improve the dielectric parameters. The best parameters were achieved for the samples sintered at 1600 degrees C with additional thermal treatment.

In this paper a study of some planar microwave band pass filters composed of defected ground resonators is presented. Different kinds of couplings between two defected ground resonators are investigated: electric coupling, magnetic-coupling different mixed couplings. The values of the coupling and two coefficients are extracted from the simulation results, obtained by using a full-wave EM-field simulation software. On the basis of this study, some second-order coupled planar microwave bandpass Chebyshev of Chebyshev filters are designed. The simulated performances of these novel filter structures are very close to the filler requirements, validating this way the design method and its results.

d The effect induced by the calcination temperature on gas sensing properties of thin SnSe2 layers exposed to reducing gas species, respectively CH4, CO and H2O, has been investigated. The sensor signal defined as the ratio between the electrical resistance in air and that in the test gas atmosphere has been used as monitoring parameter.

Carboxymethyl-cellu lose has been labelled with a stable free radical by reaction with 4-aminotempo in the presence of a coupling agent (EEDQ). The spin-labelled cellulose is highly stable, no leaking being noticed after months at room temperature. EPR spectroscopy was used as a main tool to study the wettability of such a material. The EPR spectrum of the dry spin-labelled cellulose shows the expected anisotropy, while addition of several solvents (acetone, ethyl acetate, DCM, methanol, toluene, PEG 200) induces the splitting of the spectrum into a two component system. Thus, the EPR spectrum is composed of a mobile component superimposed onto an immobilized one. Addition of water leads to a monocomponent isotropic spectrum. These data clearly indicate the presence of two types of sites to which the spin-label is attached. Variable temperature EPR spectra showed that an increase of temperature results in an increase in the mobility of the spin-label. Deposition of plain or spin-labelled gold nanoparticles on the cellulose fibres also affects the structure of the polymeric chain, as seen by changes in the EPR spectra.

Research results focussed on the combined influence of iron and boron, in proportions of 0.5 and up to 10 wt.% respectively, in complex alloyed Ni3Al synthesised by Self Propagating High Temperature Synthesis (SHS) in the thermo-explosion mode at two ignition temperatures, 950 and 1150 degrees C, are presented. By XRD and Mossbauer spectroscopy it was established that for 950 degrees C ignition temperature, the evolved heat is not high enough for the added Fe to be fully incorporated into the synthesised Ni3Al phase, a temperature of 1150 degrees C being required. For this temperature, the density of the synthesised alloys, their capacity to be cold deformed by re-pressing, hardness and bending strength variations as a function of B and Fe contents, proved their cumulative effects of the ductility and mechanical properties of complex alloyed Ni3Al enhancing.

Ba1-xSrxTiO3 solid solutions with x = 0.25, 0.5, 0.75 and 0.9 were prepared by solid-state reaction from powder oxides with high purity. In order to improve the granular growth 1 wt.% MgO and 1 wt.% MnO2 was added to the calcined powders. Samples were sintered for 2 h at 1230 and 1260 degrees C. Structural parameters, grain sizes, bulk densities and microwave dielectric parameters were determined. A monotonic decrease of the Curie temperature with the increase of strontium content was determined by low-frequency dielectric measurements. Microwave measurements around 1 GHz showed substantial decrease of the permittivity with the x increase from about 1080 to 200. Also, the variation of losses from 11 % to less than 0.2% was observed. The results indicate the BST ceramic as dielectric material, which can be utilized in paraelectric state for microwave devices. (c) 2006 Elsevier Ltd. All rights reserved.

New luminescent centers with a large near-infrared emissions were obtained by unusual electrolytic coloration method (low temperature and high electric field) in the KBr:Pb2+ crystals co-doped with alkali-earth ions (Ca2+, Sr2+, Ba2+). These so-called Tx centers involve Pb--negative metal ions and the presence of at least one alkali-earth ion, which enhances the photoluminescence properties of those centers. The formation of these new centers is due to the presence of F-centers created in the process of electrolytical coloration. The photoluminescence properties, together with the magnetic circular dichroism spectra, prove the existence of at least one paramagnetic luminescent center. (c) 2006 Elsevier B.V. All rights reserved.

Barium strontium titanate (BST) bulk ceramic c was used as target for PLD thin film deposition. In order to avoid stoichiometry modification during the deposition, a more reactive oxidant ambient in the chamber was produced by, using a radiofrequency discharge 13.56 MHz, 150 W. Thin films of stoichiometric BST was deposited on alumina substrate with the thickness between 400 and 500 nm heated at 650 degrees C. An aditional anealing was made at 800 degrees C for 6 hours. XRD and SEM were used,for sample characterization, Capacity measurements tit 100 kHz were performed versus temperature.

The paper presents an analysis of the electrical transport, phototransport and photoluminescence measurements performed on silicon-based nanocrystalline systems. The experimental results are discussed within the frame of a quantum confinement model. It is proved that the differences between the energy levels identified in the measurements are related to the quantum selection rules specific to each process. It is also proved that, at nanometric scale, the nature of the atoms composing the nanocrystal represents a first order correction, while the size represents a zero order factor.

This paper presents the manufacturing and the characterization of AlN membrane supported F-BAR structures. The 2 mu m thin AlN layer was grown using sputtering techniques, on a high resistivity (111) oriented silicon substrate. Conventional contact lithography, e-gun Ti/Au evaporation and lift-off techniques were used to define top-side metallization of the the FBAR structures. Bulk micromachinig techniques were used for the release of the AlN membrane. The bottom side metallization of the micromachined structure was obtained by means of sputtered gold. S-parameter measurements have shown a resonance around 1.6 GHz. The extracted value of acoustic velocity is in good agreement with that reported by other authors on materials fabricated by other methods.