Publications

The wettability of 18 dental impression materials, commercially available, during their working time was investigated using the contact angle method, as a good wettability is desired to obtain a high fidelity impression. Polyetheric materials, A-silicones with added surfactants and C-silicones were found to have a good wettability. Alginate was found to be the most hydrophilic material, but this quality decreases in time. In the paper an explanation for the time dependence of the contact angle is advanced.

The two crystallographic modifications of AuSe, alpha-AuSe and beta-AuSe, were studied by Au-197 Mossbauer spectroscopy. Despite the different crystal structures, the Mossbauer spectra of the two phases are nearly the same, and reveal the presence of two gold sites with Mossbauer parameters typical for Au-I and Au-III. The Mossbauer-Lamb factor of the Au-I is less than half of that of the Au-III. Calculations using the WIEN2k program reproduce the experimental results quite well and also yield the signs of the electric field gradients.

Carbon nanotubes are attractive materials with numerous possibilities of applications in various domains. Nevertheless, pertinent characterization studies are required in order to insure reproducibility in preparation, handling and incorporation in devices. Several spectroscopic tools have been developed so far, such as transmission electron microscopy and near-field electrical microscopy. Optical techniques have also been improved with the use of Raman Scattering, Surface Enhanced Raman Scattering (SERS) and near-field optical spectroscopy (SNOM). These non-destructive techniques have been used for nanotubes after synthesis and exploited to follow the modifications of their spectroscopic features when they are functionalized or embedded in host matrices. In this paper, we show that, by using SERS conditions, a new effect is observed: a "single-beam pumped'' Coherent anti-Stokes Raman Scattering (CARS). We demonstrate that under a tight-focusing of the excitation light, this emission, resulting from a wave mixing process between the incident laser light (omega(l)) and Stokes Raman light (omega(s)), is generated by a Surface Enhanced Raman Scattering (SERS) mechanism. Since abnormal anti-Stokes/Stokes intensity ratios are also observed for carbon nanotubes in powders, we have investigated in details their behaviour as a function of several parameters which include the excitation wavelengths, the laser power, the sample support, as well as the presence of nanotubes in isolated or in bundled form. It appears that resonance phenomena explain the anti-Stokes/Stokes intensity ratios in the case of powders and that CARS is only observed when SERS conditions are used. In addition, we have investigated other materials in which anomalies are also observed. This is the case of poly(bithiophene) (PBTh) electrochemically polymerized on carbon nanotubes for which its main Raman line at 1450 cm(-1) is enhanced, this enhancement being presumably generated by the plasmon excitation of metallic tubes. In the case of PEDOT, the analysis of the anti-Stokes branch provides additional information on the functionalization of carbon nanotubes with this polymer.

Different phases of boron nitride (BN) nano-structures are synthesized from an hBN ceramic target immersed in acetone, by ablation with a high power pulsed Nd: YAG laser. Transmission electron microscopy (TEM) and electron diffraction (ED) are used to identify the morphology and structure of the prepared colloidal suspensions. It is revealed that by varying solely a single experimental parameter, i.e. the laser pulse fluency, a large variety of BN nanostructures can be produced: nanotubes, very thin graphene-like foils, nano-curls and nanoparticles, all with the hexagonal graphite-like hBN structure, as well as high pressure BN phases: orthorhombic explosion E-BN nano-rods, or cubic diamond-like cBN nano-particles.

Calcium phosphate ceramic powders were synthesized via sol-gel method. The powders were sintered at 600 and 1000 degrees C and characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), electron microscopy (SEM and TEM), elemental microanalysis (EDS), infrared spectroscopy (FTIR), and thermal analysis (TGA and DTA). The XRD analysis revealed a well crystallized hydroxyapatite (HAp) structure at all temperatures. At 1000 degrees C a small amount of CaO (about 1.2 %) was detected. The average crystallite size increased from 19 nm at 80 degrees C to about 125 nm at 1000 degrees C. FTIR spectra showed the presence of various PO43- and OH- groups present in the powders. In order to verify the biocompatibility of HAP powders with hFOB 1.19 osteoblasts cells, an 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) test was applied. It was noticed that the ceramics obtained at 1000 degrees C, increased the cell viability with 10%, 40% and 55% after 6, 12 respectively 24 h, whereas that obtained at 600 degrees C was less effective. Our results proved the high biocompatibility of calcium phosphate ceramic powders obtained by sol-gel and sintered at 1000 degrees C.

Tin chalcogenides SnX2 and SnX, where X = S, Se and Te present a particularly interest for their electronic properties and applications in gas sensors. The state of tin in these materials is important for understanding of the sensing effect and improvement of the sensor performances. Mossbauer spectroscopy is a widely used technique for the analysis of the local electronic structure or chemical bonding in solids. In this paper we applied Mossbauer technique for the investigation of bulk and thin films of SnSe2 chalcogenide. The films of SnSe2 chalcogenide were obtained by the methods: PLD ("Pulsed laser deposition") and PED ("Pulsed electron deposition"). Mossbauer measurements were performed by transmission (TMS), respectively conversion electron spectroscopy (CEMS). By CEMS spectroscopy surfaces, coatings and thin films containing Sn can be studied on substrates and to various depths up to 1000 nanometers.

The precipitation of calcium carbonate by liquid-liquid reaction usually provides a mixture of polymorphs: rhombohedra calcite, orthorhombic aragonite with whiskers like particles and hexagonal vaterite with crystallites agglomerated as spheres. Calcite is the stable polymorph at ambient conditions. Recently the interest for aragonite, the high pressure polymorph is increasing because new fields, like biomaterials, are developing. We investigated the possibility of developing an aragonite type calcium carbonate with uniform particle size and submicron dimensions by precipitation in liquid-liquid reaction using double-jet precipitation method in the presence of ethanol, at the temperature of 50 degrees C, with a flow rate of the reagents of 10 ml/min. Two agitation modes were used: magnetic stirrer and an ultrasonic device. The ultrasonic field was applied either at the beginning of the reaction period or at the end of it. Intensity of the ultrasounds was also varied from 50 % to 100 %. All samples were examined by FTIR spectroscopy, scanning electron microscopy, and particle size analyzer. FTIR spectra proved that calcium carbonate synthesized in these conditions is almost pure aragonite. As example, the SEM image shows uniform product structure with fine rods with 2 divided by 3 mu m lengths and submicron diameters.

We present the fabrication and electric properties of MgB2 ceramic samples doped with nanosized spheres, 4-8 nm, of graphite with a metallic core. The samples were prepared using the spark plasma sintering technique. The size of the additive is comparable to the superconducting coherence length. The short processing time limits the diffusion of the carbon while keeping the core intact. Therefore, in addition to the doping with carbon, the metallic core, which has the size smaller than the superconducting coherence length, create pinning centers which might improve the dissipationless electric transport. The results are analyzed in the framework of different pinning models.

The most promising media for rewritable applications are the phase change materials [ 1]. For the modelling of the phase transition processes in phase change materials it has been considered the switching unit called "commuton" [2]. A commuton is a minimum cluster of atoms that supports the reversible change from OFF to ON state under the influence of an external applied energy pulse. It was developed a bidimensional model based on the cellular automata approach [3] to predict switching behavior in a Ge2Sb2Te5 phase change chalcogenide film. We suppose that the formation of the percolation paths as a function of phase change induced in commutons can explain the switching phenomenon. The percolation thresholds of the switching were calculated for different sizes of the simulated system and the percolation kinetics was investigated.

LiF:Pb doped crystals were successfully grown by Kyropoulos method, starting with drying powders. The presence of Pb2+ ions in the LiF crystals were evidenced by the absorption band at 278 nm and by 375 nm photoluminescence. The presence of some other Pb structures with oxygen compounds in the as made samples was evidenced, decreasing after some annealing procedures. The local environment and valence state of Pb in LiF were studied by X-ray Absorption Spectroscopy at the Pb L-III and L-I edges. XANES data reveal that Pb is present as Pb2+ whereas EXAFS data show that it is incorporated in the crystal and not forming PbF2 precipitates. Identical spectra are obtained for samples as prepared and after thermal annealing up to 650 degrees C demonstrating the stability of the incorporation site. Also the concentration of Pb in the crystal has no effect on the location site of the metal as the same spectrum is obtained for specimens with different dopant concentrations.