Publications

Modeling thermal conductivity of graphene-based nanocomposites is analyzed from an effective medium approximation point of view, where interfacial thermal resistance and arbitrary shape of the fillers are considered. Two approximations, Maxwell-Garnett and coherent potential approximation, are used to show that the usual oblate spheroidal shape approximation of graphene nanodots is not satisfactory to model thermal conductivity in graphene-based composites.

Infrared imaging polarimetry with structured surfaces components allows performance enhancement for observation, supervision, identification devices, generally far-field detection in low visibility conditions (night, fog, smog, haze, smoke, heavy rainfall, underwater conditions), for civil applications, e.g. environmental monitoring or communications, which aims to reduce the risk of traffic accidents, as well as defense and security purposes, where a tactical advantage is needed. Other applications aim at improving the performance of sub-wavelength imaging systems: spatial scanning, optical flaw detection, microscopies, medical tomography or cancer diagnostics systems, generally near-field range investigation. We review many important polarization components from wire-grid structured surfaces to reconfigurable metasurfaces, that can be achieved in order to manage sub-wavelength near-field details and their transfer in far-field by polarimetry techniques without moving parts. Advanced research is needed to show which concept will prevail in the market for the achievement of ultracompact polarization optical components and their integration into a functional model of polarization state analyzer for imaging systems with high efficiency in long wave infrared.

Ceramic thin films belonging to SiO2-P2O5-CaO-MgO-ZnO-CaF2 system were obtained by combining the sol-gel approach with the spin coating technique. Titanium plates were employed as substrates. The deposited coatings were characterized in terms of composition, structure and morphology with the help of the following methods: X-ray diffraction, Fourier - transform infrared spectroscopy and scanning electron microscopy coupled with energy - dispersive X-ray spectroscopy. In order to assess the bioactivity of a potential metallic implant covered with such layers, the samples were immersed in simulated body fluid for 14 days and their surface was investigated. The results showed that the thin films calcined at a lower temperature have a better biological response due to the vitroceramic nature.

Ceramic compounds of ferroelectric lead zirconate titanate (PZT) and ferroelectric barium titanate (BT) were prepared from nanopowders corresponding to the formula (1-x)PZT.xBT with x = 0; 0.1; 0.2 ... 0.9; 1. The samples were investigated by X-ray diffraction and electron microscopy and the piezoelectric properties were determined by means of an impedance analyzer. The sintering process is complex since samples densify at different temperatures depending on the composition resulting in different morphostructures. Accordingly, the piezoelectric properties show unusual behavior discussed in terms of different grain size and composition. A remarkably high enhancement of the relative dielectric constant, epsilon(r) coupled with maxima in both, piezoelectric charge constant d(33) and the piezoelectric voltage constant g(33), is observed for x = 0.4 while the values of the coupling coefficient kp are still high similar to those of pure PZT.

We report on the formation of the Ti3SiC2 nanolaminate phase in the Ti-Si-C thin film system, using a UHV magnetron sputtering technique with top mounted sample holder, from elemental and compound targets. The formation of the Ti3SiC2 (or 312 phase) has been evidenced by detailed X-ray diffraction analysis followed by full-profile quantitative analysis of the obtained diffractograms. It has been proven that for deposition temperatures as low as 500 degrees C, there is a significant amount of 312 phase obtained in the deposited films, in co-existence with the majority TiC phase. This amount increased to about 21% when the deposition temperature was raised to 650 degrees C. The ternary 312 phase becomes predominant at around 60% relative abundance for slight off-stoichiometric, Si increased, content of the alloy, for temperatures as low as 650 degrees C. The conditions for improving the relative abundance of the 312 phase within our experimental setup are pointed out and explained in terms of a nucleation and growth model of nanostructure formation from the amorphous precursor (?).

Low carbon Fe-C steel surface has been studied before and after electrolytic galvanization. The corrosion products formed under atmospheric conditions on the Fe-C steel were identified and characterised by X-ray diffraction (XRD), transmission Mossbauer spectroscopy, conversion electron Mossbauer spectroscopy (CEMS), and conversion X-ray Mossbauer spectroscopy (CXMS). In decreasing order of abundance the found corrosion phases were magnetite, hematite, and goethite. After the surface preparation, an electrolytic galvanization process was applied. A magnetic anisotropy was evidenced after preparation, in the superficial layer of around 250 nm thickness. Scanning electron microscopy (SEM) analyses evidenced the formation of a uniform thin Zn film of 8 mu m at the surface of steel. No other Fe-Zn phases were identified at the steel-Zn interface.

Triglycine sulphate (TGS) is a crystal having a typical second order ferroelectric transition and the Curie point around 49 degrees C. It is quite easy to grow and to analyse its properties. Our TGS crystals were grown by slow solvent evaporation in paraelectric phase, around 54 degrees C, where no mechanical tensions due to ferroelectric domains were present. Dielectric properies of TGS crystals were continuously measured at a pace of 0.6 degrees C/min, starting from 65 degrees C, crossing down the Curie point until zero Celsius, on the frequency range 1 divided by 10(7) Hz. The two component of permittivity were measured and the parameters vs. temperature were estimated and analysed.

Polysaccharides irradiation is usually used for their decontamination of fungi, pathogenic bacteria, toxigenic molds, parasitic organisms. ESR studies are performed just to evidence their irradiation. Therefore the accent is put on radical stability in time and ESR signal intensity dependence on irradiation dose. Less attention is given to radical nature and their dependence on plant nature. In this report a study of gamma irradiated pectin is given. Dose dependence of double integrated ESR signal is analyzed in terms of a recombination reaction during the radiation period. Radical stability dependence on water content is put in evidence.

GeSi NCs / TiO2 multilayers with enhanced photocurrent properties were prepared and studied. Multilayers of TiO2 /(GeSi/TiO2)x2 /Si-p were deposited by magnetron sputtering and annealed by RTA at 700 degrees C for GeSi NCs formation. A post-annealing hydrogenation in plasma was performed on multilayers for healing of defects acting as traps and/or recombination centers and consequently producing the photocurrent enhancement. We studied the electrical and photoconductive properties of multilayers annealed by RTA and post-annealing hydrogenated. The current - temperature dependence reveals the conduction mechanisms in GeSi NCs / TiO2 multilayers RTA annealed, i.e. thermal activation of carriers to extended states (0.31 eV activation energy), the electron tunneling mechanism to nearest neighbors (T-1/2 behavior) and Mott variable range hopping (T-1/4 dependence). The photocurrent spectra made on multilayers structures hydrogenated for 10, 20 and 30 min evidence the photocurrent increasing up to 50%, showing that the hydrogenation is a suitable treatment for enhancing photocurrent. All photocurrent spectra present a dominant maximum (920 nm) and two shoulders (similar to 770 and similar to 1060 nm).

Investigations of barium strontium titanate (BST) layers deposited on MgO and Si substrates are presented. Since the Sr content determines the dielectric and optical properties of the BST layers at room temperature, accurate compositional analysis was performed by using Rutherford Backscattering technique at 3.041 Mev.