Publications

Barium tetratitanate (BaTiO9) powder, synthesized by coprecipitation method, was used to prepare BaTi4O9-Nd2O3, BaTi4O9-Sm2O3 and BaTi4O9-WO3 ceramics with microwave dielectric properties. The structure and microwave dielectric properties were studied. BaNd2Ti5O14 and BaSm2Ti4O12 single-phase ceramics were obtained from BaTi4O9:Nd2O3=1:1 and BaTi4O9:Sm2O3=1:1, respectively. A multiple phases ceramic containing mainly BaWO4 and small amounts of Ba2Ti9O20 and BaTi4O9 was obtained from BaTi4O9:WO3=1:1. BaTi4O9-Nd2O3, BaTi4O9-Sm2O3 and BaTi4O9-WO3, ceramics sintered at 1375 degrees C for 8.5 h in air, shown good microwave dielectric properties.

The PbS/SiO2/Si structures show a clockwise hysteresis of capacitance-voltage (C-V) characteristics. The loop width increases with temperature and sweep time. The conductance-voltage (G-V) characteristics also present a hysteresis effect with two bumps. Both characteristics show a strong shift with frequency. We show that these effects are determined by the positive mobile ions injected in the oxide region during the chemical bath deposition of PbS and by the high concentration Of Si/SiO2 interface states. The corresponding C-V and G-V characteristics are simulated and the main peculiarities of the experimental results are well reproduced by the modeled curves.

Field Activated Sintering Techniques (FAST) are increasingly used to consolidate metal and ceramic powders. They combine application of a pulsed electrical current with external pressure to activate the densification process. The nature of electrical field/current interaction with powder systems is not clear yet. In this regard, in-situ transport measurements and microstructural analyses were carried out during pulsed current application in the FAST processing of Ni powders. Our experimental results were compared with existing electrical conduction theories and observed effects were relatively well explained qualitatively within these theories. Under electrical field it was confirmed that no voltage-current phase-shift occurs at elevated operating interfacial temperatures. For the field situation a higher applied pressure is decreasing the contact instability among particles contacts and of the sintering onset temperature. Microstructural investigations of the FAST-sintered samples indicate rather distribution of the higher density regions and a considerable increase of neck formation above 340 degrees C.

We studied the chemical growth of calcium phosphate nanostructured coatings onto silicon wafers pre-covered with carbonated polycrystalline hydroxyapatite films. Silicon wafers were covered with hydroxyapatite thin layers by radio frequency magnetron sputtering and then immersed in 370 degrees C simulated body fluids for up to 20 days. Immersed structures were extracted every 2 days for studies by Fourier transform IR spectrometry. The chemically grown layers were further analyzed by X-ray diffraction and scanning electron microscopy. The growth kinetics of calcium phosphate deposits was monitored by estimating the area of phosphate, carbonate and water stretching bands of the recorded infra-red vibrational spectra. Sequential annealing in vacuum up to 950 degrees C was applied to elucidate the nature of incorporated water. Our studies revealed that the layers growing in simulated body fluid are rougher than initial hydroxyapatite interlayers, contain carbonates, include water in interlayer's voids and have therefore an increased biological activity for promoting faster implants integration with human bone tissues.

Fine lanthanum zirconate powder was prepared by thermally decomposing a nitrate-alkoxide-based precursor derived from dehydrated lanthanum nitrate, zirconium n-butoxide and 2-methoxyethanol. Upon heating, the decomposition of the organic groups was promoted by the nitrate groups, yielding a porous powder that crystallized into a pyrochlore phase at 800 degrees C. The powder that was heat treated at 900 degrees C for 1 h was composed of friable agglomerates of approximately 60-nm-sized nanoparticles. The ceramics obtained from the powder heat treated at 900 degrees C and milled for 30 min reached a relative density of 97.9 % after sintering at 1,400 degrees C for 10 h, which is at least 100 degrees C lower than the typically reported temperatures for this material.

Co-MgO granular films presenting TMR effects were prepared by thermo-ionic vacuum arc method with the simultaneous ignition of plasma in Co and MgO vapors. Morphologic, structural and magnetic behaviors were analyzed in as prepared and annealed samples. The influence of the Co content on the magnetic properties of the prepared films was analyzed, in correlation with tunneling magneto-resistance effects. The tunneling magneto-resistance effect is maximal for certain Co content. This behavior was interpreted by the contrary effects of decreasing the average size of the magnetic grains, and hence the average inter-grains distance at higher Co relative content, and the enhanced magnetic disorder in very fine grains dispersed in the insulating matrix.

A simple electrical resistance sensor for monitoring of the coastal marine waters has been developed. The good sensitivity and reproducibility of the measurements make this sensor attractive for using in the research of the mixing of sweet (river) waters with the marine water, as well as the variation in the depth of the sea of the water salinity. Another advantage of the sensor is the possibility to monitor the water salinity by simply tracing the electrical resistance when the sensor is drawn at the surface or vertically in the sea and ocean waters.

We have studied the multiharmonic ac susceptibility response of HgBa2Ca4Cu5Oy (Hg:1245), a multilayered high-temperature superconductor (HTS) having two crystallographically inequivalent CuO2 planes in a unit cell with very imbalanced carrier concentrations. Vortex melting lines are well described by the commonly accepted model of melting with moderate anisotropy factors of 40-50, depending on the doping level. The diamagnetic response with applied fields parallel to superconducting (a,b) planes also shows a quite robust supercurrent along the c axis. Our results are also discussed in connection with contradictory models and experiments regarding the coexistence of superconductivity and antiferromagnetism in HTSs.

The dc electrical conductivity of the bulk amorphous GeSb2Te4 material has been investigated. Pure and samples doped by 10 at. % SnSe2 have been measured. The conductivity in the samples has been compared with that of SnSe2 bulk sample. The activation energy of the doped sample is 0.165 eV. During heating the conductivity of doped material increases, reaches a maximum and then decreases. The comparison with the pure SnSe2 samples allows to explain this behavior by the release above 148 degrees C of a small amount of selenium not bonded in the network.

Water soluble gold nanoparticles, obtained by the reduction of the gold (III) chloride with sodium borohydride in the presence of citric acid or thioctic acid, were covered with a paramagnetic silica layer using the Stober method, yielding a hybrid metallic-inorganic nanomaterial (gold nanoparticles, with an average size of 5 nm, embedded into silica nanoparticles, with an average size of 100 nm). The paramagnetic silica layer was formed by copolymerization of a paramagnetic silica precursor (derived from 3-aminopropyltrimethoxysilane) with tetramethyorthosilicate. The paramagnetic silica precursor was obtained by coupling 3-aminopropyltrimethoxysilane with 3-carboxy-proxyl free radical. TEM pictures show that each silica nanoparticle of about 100 nm in size embedded about 10 gold nanoparticles. These hybrid nanoparticles are quite stable and exhibit the expected paramagnetic characteristics, as seen by electron paramagnetic resonance. The accessibility of methanol through the silica layer was also studied. Depending on the capping ligands of the gold nanoparticles (citric or thioctic acid), different silica networks are formed, as seen by the mobility of the spin-label inside the silica layer. The EPR spectra showed that the paramagnetic silica layer is very robust and the mobility of the spin-probe inside the silica layer is very little affected by methanol. However, if spin-labeled thioctic acid protected gold nanoparticles were used in the material synthesis, the mobility of the spins attached to the gold surface is quite high in the presence of methanol, while the spins embedded into the silica layer remains immobilized.