Publications

The Sr2FeMo1-xMxO6 perovskites with M = W or Ta and x <= 0.3 were studied by x-ray and electron microscopy. The magnetic and magnetoresistive properties were correlated with their microstructures. The effect of sintering time on their physical properties was analysed.

Polystyrene-CrO2 composite films with extrinsic tunneling magnetoresistance have been obtained. The electric conduction is 2D Mott variable range hopping in the temperature range 77-300 K. The magnetoresistance is negative when the field is aligned parallel to the current (in-plane geometry) and is due to the spin dependent intergrain tunneling of the charge carriers. It reaches a value MR = -10.6 % at room temperature and a magnetic field of 1T. When the field is applied in-plane but perpendicular to the current the magnetoresistivity is positive and has the average value MR = 3.56 %.

The xZnO-(1-x)alpha-Fe2O3 nanoparticles system has been obtained by mechanochemical activation for x=0.1. 0.3 and 0.5 and for ball milling times ranging from 2 to 24 hours. Structural and morphological characteristics of the zinc-doped hematite system were investigated by X-ray diffraction (XRD) and Mossbauer spectroscopy. As ZnO is not soluble in hematite in the bulk form, the present study clearly demonstrates that the solubility limits of an immiscible system can be extended beyond the limits in the solid state by mechanochemical activation. Moreover, this synthesis route allowed us to reach nanometric particle dimensions, which would make the materials very important for gas sensing applications.

Design, simulation fabrication and measurement of prefractal antennas for wireless communications are presented in this paper Two types of geometries, i.e. Sierpinski gasket and Koch curve, were investigated. The proposed antennas are compact, can operate for multiple frequency bands and exhibit small mutual coupling.

A study was made on the temperature behavior of the main piezoelectric parameters, namely the electromechanical coupling factor k(p), the mechanical quality factor Q, the dielectric permittivity (real and imaginary part), the loss tangent t(g) and the piezoelectric charge constants 6133 and d(31) of a soft type piezoelectric material. It was evidenced that temperatures under 150 degrees C do not influence these parameters, which means that every transducer made from this type of material may be successfully used up to this temperature. Between 150 degrees C and 250 degrees C, the piezoelectric properties undergo more or less important changes, mainly due to the depoling effect. After that, at temperatures over 250 degrees C, they degrade very rapidly, tending to zero.

Functionally graded materials represent a new class of composite type materials characterized by a continuous variation of properties through the thickness direction. Due to this they found numerous applications in aerospace, automobile, biomedical or nuclear industries. Bilayered structures made from typically hard and typically soft PZT type materials as well as plain hard and soft and intimately mixed hard and soft samples were prepared by green pressed powdersfollowed by sintering at different temperatures between 1000 and 1300 degrees C. The dielectric and piezoelectric properties of sintered compacts were determined in relation with the nature of the interdiffusion layer involved. Electron microscopy as well as X-Ray diffractometry was used to get an insight into the interfacial diffusion process. The composition and the width of the interdiffusion layer depend on temperature. The mechanism of the interdiffusion layer and materials properties were discussed in terms of the ionic diffusion coefficients and activation energies of the species which take place into the diffusion process.

Ba1-xSrxTiO3 solid solutions were prepared by solid-state reaction from raw materials. Four compositions with x = 0.25, 0.50, 0.75 and 0.90 have been investigated. The perovskite type and polycrystalline structure of the BST samples were revealed by X-ray diffraction data. The morphology, grain size distribution, porous structure and elemental composition of the sintered ceramics were analyzed by using scanning electron microscopy (SEM) and energy-dispersive X-ray (EDX) microanalysis. The temperature dependence of permittivity and of dielectric loss tangent at low frequency (1 kHz) showed decrease of Curie temperature with increase of Sr content. Microwave measurements (1 GHz) showed substantial decrease of the dielectric constant from about 1600 to 200 and also of the losses from 12% to less than 2% with the Sr concentration increase from x = 0.25 to x = 0.90. Moreover, the addition of MgO and MnO2 1 wt.% each, improved sintering process and lowered the microwave losses up to 0.2%. (c) 2007 Elsevier Ltd. All rights reserved.

Thermoluminescence (TL) manifested by gamma-irradiated mineral zircon has shown a strong TL peak at about 165 degrees C which is due to recombination of electrons and Dy3+ related shallow hole traps. After they have been removed by a short preheat we have observed two TL peaks at 300-320 degrees C and approximate to 420 degrees C, which are mainly due to recombination of electrons and Tb3+ related hole traps centres yielding its characteristic luminescence. The experimental results indicate that optically stimulated luminescence (OSL) is due to luminescent emission of Tb3+ ions and [SiO4](4-) groups. The deep traps related to the 420 degrees C TL peak contribute to the Tb3+ related OSL. The deep traps related to the 300-320 degrees C TL peak contribute to OSL associated with the luminescent emission of [SiO4](4-) groups. (c) 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinhem.

Barium titanate was prepared by the advanced procedure of mechanochemical synthesis from barium and titanium oxides after 50 hours of milling. The X-ray examination of the milled powders showed the phase evolution during synthesis and the perovskite phase was initiated after 5 hours and was completed after 50 hours. Sintered samples from these nanopowders show different dielectric properties for fine and coarse grained ceramics. Samples with an average grain size of 2 mu m showed the highest dielectric constant of 5800 at room temperature. A sharp maximum of the dielectric constant with values of about 16000 was recorded at the Curie temperature.

In order to fulfil the European task for market survey in food irradiation the first Romanian laboratory for detection of irradiated foodstuffs was established at IRASM Irradiation Centre. In this preliminary study, a wide range of Romanian food samples (spices, vegetables and meat) gamma irradiated at IRASM have been studied using different detection methods: (1) DNA comet assay, (2) thermoluminescence (TL) and (3) electron spin resonance (ESR) for foodstuffs containing bone or cellulose. The results suggest that there is no general available detection method and there is no perfect detection method. In conclusion, in order to carry out a correct identification of radiation treatment of a food sample it is recommended to use at least two standardised detection methods. (c) 2007 Elsevier Ltd. All rights reserved.