National Institute Of Materials Physics - Romania

We investigated the influence of fullerene (C-60) addition on the superconducting parameters of MgB2 bulk samples obtained by spark plasma sintering (SPS). It was found that 1.5 wt% C-60 addition leads to the reduction of the critical temperature by a few K, whereas the magnetically determined critical current density increases by approximately one order of magnitude in an applied magnetic field H = 70 kOe at temperature T = 10 K. This indicates substitution of B by C in C-60 added MgB2 processed by SPS, contrary to some data reported in the literature obtained using conventional methods. For our relatively large specimens, the occurrence of macroscale flux jumps was observed over a wide (H, T) domain. The relaxation of the irreversible magnetization suggests that the macroscale flux jumps transform into microscale flux jumps with increasing H and T

Investigations on dielectric on compact resonators antennas with enhanced characteristics are described in this paper. The dielectric resonators were of (Zr(0.8)Sn(0.2))TiO(4) ceramic material (ZST). It is shown that the doping with Mg reduces the sintering temperature and provides a high dielectric constant, low loss and high thermal stability in microwaves. The coupling with an extra resonator can offer a substantial bandwidth enhancement. Moreover, antenna array with four antenna elements provides a higher gain and a more directive radiation pattern.

Nanocrystalline cubic ZnS doped with 0.2% mol manganese, exhibiting a stable mesoporous structure, was synthesized at room temperature by a non toxic surfactant-assisted liquid liquid reaction. The X-ray diffraction measurements demonstrate the formation of a sponge-like mesoporous material built from cubic ZnS nanocrystals of 1.8 nm average sizes, with a tight distribution of pores of 1.8 nm mean diameter. The transmission electron microscopy images confirm the formation of the mesoporous structure with walls of 3.1 nm mean thickness built from cubic ZnS nanocrystallites of 2.1 nm average size. The resulting tight distribution of crystallites and pores yields a well resolved Electron Paramagnetic Resonance spectrum, with the narrowest reported component lines attributed to three types of isolated Mn2+ centers, called Mn2+ (I), Mn2+ (II) and Mn2+ (III). From the analysis of the spin Hamiltonian parameters it is shown that in the Mn2+ (I) centers the paramagnetic ion is situated at substitutional Zn sites in the ZnS nanocrystals, being also subjected to a small axial distortion. The relative concentration changes under thermal treatment experiments strongly suggest that in both Mn2+ (II) and Mn2+ (III) centers the Mn2+ ion is localized on the surface of the ZnS nanocrystallites, being bond to an oxygen ion in the first case and to an additional water molecule in the second case.

We report on the synthesis of 700 nm thick bioglass thin films by magnetron sputtering from 45S5 bioglass targets in the argon atmosphere. The adhesion of films to Ti substrates increased 1 75 times when introducing an similar to 70 nm thick mixed glass-Ti buffer layer (BG(1-x)Ti(x) (x = 0-1)) with gradient of composition by co-sputtering. The morphological and structural properties of the films were studied by XRD, ETIR and microscopic techniques, showing an improvement after the two-hour thermal air treatment at 650 degrees C. We investigated in vitro the viability and proliferation of human osteoblast cells cultivated onto the surface of the films The osteoblasts spread over the surface adopting typical polyhedral shapes with numerous focal adhesion points and protrusions infiltrating deep into the films. We assigned this evolution to the improved mechanical properties and enhanced bioactivity due to the prevalent formation of combeite and wollastonite crystalline phases in the heat treated bioglass films (C) 2010 Elsevier B.V All rights reserved

Spin valve like AF/Fe/Cu/Fe (AF = Fe50Mn50 and Ir50Mn50) multilayer systems have been prepared by molecular beam epitaxy. Thin tracer layers enriched in the Fe-57 isotope were artificially grown at the AF/Fe and Fe/Cu interfaces and the interfacial atomic diffusion was observed via Fe-57 conversion electron Mossbauer spectroscopy. The results show that the atomic interdiffusion at all involved interfaces is lower in the IrMn based structures as compared to the FeMn based ones. (C) 2010 Elsevier B.V. All rights reserved

Short time spark plasma sintering at 1200 degrees C for 10 min and under a uniaxial pressure of 95 MPa followed by annealing at 1100 degrees C for 5 h is proposed for synthesis of high density (99%) Ba(Ti0.87Sn0.13)O-3 ceramic. Starting powder of 450 nm was obtained by co-precipitation, while the average grain size in the sintered ceramic was of 580 nm. X-ray diffraction analysis of the as prepared Ba(Ti0.87Sn0.13)O-3 samples indicated the presence of single phase BaTi0.87Sn0.13O3 developed in the BaTiO3 cubic structure. Samples have high relative dielectric constant of 5680 and low dissipation factor (tan delta = 0.009) at Curie temperature of 57 degrees C and 1 kHz, good tunability and ferroelectric properties. Remarkable is that these results are from some points of view different than for the conventionally processed samples with similar stoichiometry. Some ideas in this regard are presented and this suggests that SPS can be useful for expanded possibilities in the control of the properties. (C) 2010 Elsevier B.V. All rights reserved.

Li-N-H and Li-Si-N-H composites have been synthesized by ball milling starting from lithium amide, lithium hydride and silicon powders. Hydrogen sorption behaviour was investigated for the Si-containing composites and a Li-N-H reference sample. The as-milled Si-containing sample showed the presence of the constituent powders but after few absorption-desorption cycles an important amount of Li(2)SiN(2) appeared in both de-hydrogenated and re-hydrogenated composites. This phase was present in higher amount in the de-hydrogenated sample than in the re-hydrogenated one. The presence of this phase was also confirmed by XPS measurements. Li-N-H and Li-Si-N-H composites in de-hydrogenated state also contained lithium imide. X-ray diffraction data indicated a reversible hydrogen generating reaction between lithium amide, lithium hydride and silicon to produce Li(2)SiN(2). lithium imide and hydrogen. The plateau pressure of Li-N-Si-H system is twice the one for Li-N-H at the same temperature (about 265 degrees C), as shown by pressure-composition isotherms in desorption mode. For the first time it has been observed a thermodynamic destabilization of Li-N-H by mixing it with Si. (C) 2010 Elsevier B.V. All rights reserved.

The characteristic magnetic fields for the second peak (SP) on the dc magnetization curves of high-temperature superconductors with random quenched disorder increase with decreasing temperature T in the low-T range. It was argued that this aspect rules out the existence of an order-disorder transition in the vortex system as the primary cause for the occurrence of the SP, and a model based on a thermally induced square-to-rhombic vortex lattice transition was recently proposed. We investigated the T variation of the field H-on for the onset of the SP in the case of underdoped La2-xSrxCuO4 single crystals (x=0.08) with the external magnetic field oriented along the c axis. It was found that in the low-T domain H-on(T) proportional to 1/T-2, and an inflectionlike point in the H-on(T) dependence is still present, similar to that reported for overdoped specimens. We show that the observed behavior and the strong variation in the characteristic fields for the SP with the doping level are in agreement with a dynamic energy balance relation for the order-disorder transition in the vortex system at the SP.

Silver nanoparticles were prepared by the chemical reduction of silver nitrate, AgNO3, with excess of sodium borohydride, NaBH4, in dilute aqueous solutions containing triblock copolymers poly(ethylene oxide) (PEO)-poly(propylene oxide) (PPO)-poly(ethylene oxide) (PEO) Pluronic L64. The synthesis was completed in similar to 24h and produced highly stable particle dispersion. The growth of nanoparticles was assessed by combining Mie theory, UV-vis spectroscopy and dynamic light scattering, and the average particle size was determined from transmission electron microscopy. The silver nanoparticles were sterically stabilised as a result of the block copolymer adsorption onto the nanoparticle surface. Besides the stabilizing character, the copolymer affected the kinetics of particle growth, the size and optical properties of as-synthesized Ag nanoparticles. It was revealed that the particle formation took place in two distinct stages. The first step was dominated by Ag ion reduction and growth driven by accretion, while in the second stage coagulated clusters of nanoparticles were obtained obtained. The as-synthesized particles were polydisperse, the individual ones had a size of similar to 7.6 nm as given by transmission electron microscopy, and the hydrodynamic radius of clusters amounted to 60 nm. The silver nanoparticles exhibited broad and structured emission bands centered at 300 and 330 nm for excitation wavelengths in two different ranges, 220-240 nm and 260-290 nm, respectively. This process may be assigned to two emission components corresponding to two different excitation wavelength domains. (C) 2010 Elsevier B.V. All rights reserved.

A single ferroelectric Schottky diode was obtained on a SrRuO3-Pb(Zr0.2Ti0.8)O-3-Ta (SRO-PZT20/80-Ta) structure in which the SRO-PZT20/80 interface is the rectifying contact and the PZT20/80-Ta interface behaves as a quasiohmic contact. Both the capacitance-voltage (C-V) and the current-voltage (I-V) characteristics show the memory effect due to the ferroelectric polarization. However, retention studies had revealed that only the "down" orientation of ferroelectric polarization is stable in time (polarization oriented from top to bottom contact). The analysis of the experimental results suggests that the PZT20/80 is n type and that the stable orientation of polarization is related to the presence of a depletion region at the SRO-PZT20/80 Schottky interface.