Publications

The aims of this study are to clarify the effect of different fast sintering techniques on densification behavior, microstructure and mechanical properties of a ZTA type composite obtained with 15 % vol. (Ca-Mg)-ZrO2 nanosized zirconia dispersed in 85% vol. Al2O3 matrix. Three kind of heat consolidation techniques were applied respectively on coprecipitated nanosized alumina-zirconia powders by: 1) conventional sintering; 2) microwaves field sintering at 2.45 GHz and 3) external pulsed electrical field (Spark Plasma Sintering), making possible very high heating rates. Microwave sintering of ceramics can offer certain advantages over conventional firing methods, including fast firing times and improved properties. Heat generated by the interaction of microwaves with the ceramic material results in volumetric heating of the ceramic, thus high heating rates is possible. On the other hand, a newly developed rapid sintering method, the Spark Plasma Sintering (SPS) which involves the application of electrical field and an external pressure is currently used to consolidate ceramic, metal and composite powders. SPSed high density zirconia toughened alumina (ZTA) ceramics show excellent mechanical strengths. Microstructural coarsening within ZTA is found to produce a fracture toughness increment, mainly associated with the effect of zirconia particles phase transformability. The obtained mechanical properties and grain growth kinetics are discussed in terms of microstructural features. The effects of heating technique on the far infrared and Raman spectra were investigated. Additional bands development was observed on spectra as the crystal symmetry is lowered, and certain of these bands are highly sensitive to strain-induced distortions of the crystal lattice.

Vanadium-cerium oxide gels and films obtained by the sot-gel method have been widely studied, due to their catalytic, electrochemical, electronic and ionic properties, a result of their ability to form combinations in more than one valence state. V-Ce oxide powders were synthesized by wet chemical methods, in the presence and absence of polyols. The preparation of an aqueous dispersion of mixed vanadium oxide/CeO2 started from a solution of the corresponding metal oxide and metal salt, V2O5 and Ce(NH4)(NO3)(6), respectively. The powders were characterized by IR-spectroscopy, X-ray photoelectronic spectroscopy, X-ray diffraction, electronic microscopy (TEM, SAED, HRTEM). The influence of the ratio of the components (V/Ce) and of the polyol complexation agent on the structure and morphology of the obtained binary oxide powders was established.

Two-dimensional photonic structure has been imprinted on the surface of arsenic sulphide glass using the pulses of a femtosecond laser. Due to the interaction of the laser beam with the glass, the laser traces were obtained as hillocks of around 150-200 nanometer in height, or holes of depth around 100-300 nm, without the need for a later etching stage as in the usual practice. The calculation of the band structure shows a photonic band gap between 0.9 and 1.0 c/a. By varying the energy of the laser pulse we have the possibility to choose between the two possibilities to produce a photonic structure made-up of hillocks or a photonic structure consisting of holes. The value of the threshold pulse power which controls the resulting surface morphology and the obtained photonic structure has found to be around 15 mW. The surface topography of the structures obtained has been investigated by atomic force microscopy (AFM).

The surface characterization of 304L stainless steel for orthopedic implant applications, unmodified and modified with poly (vinyl) alcohol (PVA) in physiological serum (PS) is discussed according to electrochemical measurements, Scanning Electron Microscopy with Energy Dispersive X-ray Spectroscopy (SEM / EDS) and X-ray Photoelectron Spectroscopy (XPS) techniques. The results obtained through polarization curves indicate that PVA reduces anodic dissolution and also decrease of the corrosion current (i(corr)); decrease of the corrosion current (i(corr)) was associated with a shift of corrosion potential (E-corr) to a less negative values; the polarization resistance(R-p) increases for the surfaces which were treated in physiological serum in the presence of PVA. SEM / EDS and XPS analysis showed that the surface layer which was formed on 304L stainless steel consists of PVA containing of small amount of other elements, such as: Na and Cl.

The calorimetric technique is a method used at the present time in several experiments in the search for different forms of dark matter, especially the existence of hypothetical Weakly Interacting Massive Particles or other candidates as neutrino, particles that satisfy the criteria to be non-baryonic, non-luminous and possibly non-relativistic Usually, in several experiments, the simultaneous measurements of the ionization and phonon or light from scintillation signals arc used in the identification of particles Silicon and germanium crystals are used in some experiments, but the energy used in the production of defects in these materials is usually not considered in the energy balance of the processes and could represent a source of errors in particle identification In this paper we investigate the kinetics of defects in cascade interaction processes at cryogenic temperatures, estimate the energetic cost of defect formation in the energy loss and predict the maximal errors in mass and energy identification for WIMPS and neutrinos respectively

A new asymmetric backward coupler is obtained by edge-coupling two types of composite right/left handed (CRLH) lines. This novel coupler is analyzed in a microwave range (1-10 GHz) and the magnitude of the scattering parameters reveals tight coupling effects (even 0 dB) in a dual frequency band and also very good directivity in the working band.

Strong, artificial pinning centres are required in superconducting films of large thickness for power applications in high magnetic fields. One of the methods for the introduction of pinning centres in such films is substrate decoration, i.e., growing nanoscale islands of certain materials on the substrate prior to the deposition of the superconducting film. Two other methods are building up a layered distribution of a second phase and homogeneous incorporation of second phase inclusions from a compositional target. In this paper, we compare the effectiveness of these methods in terms of the type of the self-assembly of nanoparticles. The comparison is made over a large set of YBa2Cu3O7 films of thickness up to 6.6 mu m deposited with Au, Ag, Pd, LaNiO3, PrBa2Cu3O7, YBCO, BaZrO3 and Gd2Ba4CuWOy nanoparticles. It is found that substrate-decoration self-assembly is able to provide higher critical current in low magnetic field than the incorporation of homogeneous second phase in the sample microstructure. By specific modification of substrate decoration we achieved the self-field critical current per centimetre of width of 896 A/cm at 77.3 K and 1620 A/cm at 65 K in a film of thickness of 4.8 mu m.

We present a Mossbauer (MS) analysis on the nanoparticle system xSnO(2)-(1-x)alpha-Fe2O3, (x is an element of [0.995 divided by 0.925]). The samples were prepared by a hydrothermal route and have been 50 % enriched in Fe-57. The MS spectra were recorded at room (RI) and liquid nitrogen temperature (LNT). Below x = 0.975 the MS consist in a paramagnetic pattern, while at x 0.975 magnetic hyperfine interactions were evidenced. The observed coexistence of paramagnetic and ordered magnetic phases is discussed by taking into account the role of oxygen vacancies (square), Sn-O and Sn-square distances together with the iron-oxygen configurations.

The electrical switching in solid compositions based on chalcogenide materials is discussed. Different mechanisms proposed for switching effect are reviewed. A new description of the switching phenomenon is done. The switching is regarded as due to formation and breaking of the links between the dendrites of crystalline nuclei in bulk materials, as a consequence of the energy pumped by an electrical field. This mechanism explains the very short switching time (<20ns), the possibility to get smart memories based on multisteps of resistivity and the high number of cycles supported by the cell (10(16)).

The influence of different crystallization conditions on the microstructure and magnetic hardening of RE-Fe-B amorphous ribbons with different Fe concentrations and Pr and Nd as rare earth elements was analyzed. The microalloying effects of Zr and Ti substitution on the evolution of crystallization process and the magnetic hardening was also discussed. The final aim of this experimental study was to obtain performing exchange spring magnets with as much as lower content of the expensive RE material.