National Institute Of Materials Physics - Romania

In this work we design some atomic scale simulation methods as investigative tools in the study of the formation of compounds for the reversible storage of hydrogen in bulk materials. It was verified that the reaction between the LiBH4 and MgH2 is energetically favored for temperatures above 280 K and that this system can be used in the hydrogen storage and the fuel cell application. To identify the reaction mechanism at the interface of LiBH4, MgH2 and carbon layers we performed some Molecular Dynamics simulations and QM/MM calculations. The results show that the layers of ions formed at the interface with the graphite may assure the right arrangement of the atoms to start the formation of the crystals. Moreover, the presence of the hexagonal layers of graphite may play a role as a pattern template for the layers of boron atoms in the MgB2 lattice.

We present the preparation, structure and physical properties of CoNb2O6 ceramics using spark plasma sintering technique. Magnetic data reveal the two antiferromagnetic transitions at 2.9 and 2 K. Microwave measurements gives epsilon(r) = 23.5 and a loss tangent of 29x10(-3).

We have obtained by Spark Plasma Sintering (SPS), dense samples of MgB2 added with Ho2O3. Starting composition was (MgB2)(0.975)(HoO1.5)(0.025) and we used addition powders with an average particle size below and above 100 nm. For Mg, pristine and added MgB2 samples we measured potentiodynamic polarization curves in Phosphate Buffered Saline (PBS) solution media at room temperature. MgB2 based composites show corrosion/degradation effects. This behavior is in principle similar to Mg based alloys in the same media. Our work suggests that the different morphologies and phase compositions of the SPS-ed samples influence the interaction with corrosion medium; hence additions can play an important role in controlling the corrosion rate. Pristine MgB2 show a significant improvement of the corrosion resistance, if compared with Mg. The best corrosion resistance is obtained for pristine MgB2, followed by MgB2 with nano-Ho2O3 and mu-Ho2O3 additions.

The tunnel-diode resonator (TDR) technique for accurate measurements of the magnetic penetration depth is used to measure the London and Campbell penetration depths of polycrystalline SiC doped (10wt.%) MgB2. The Campbell length was used to investigate the field and temperature dependence of the critical current density. The as determined critical current density provides values as high as 6x10(6) A/cm(2) at 4.2K, 1T, which is higher than values estimated by Bean method.

The hydrogen storage material lithium borohydride was infiltrated from solvent solution into the pores of the carbon replica of MSU-H mesoporous silica by the incipient wetness method. Different amounts of lithium borohydride up to 40 % from the weight of the high surface area carbonic material were used. By this method was achieved a good dispersion of the hydride into the carbonaceous support. The hydrogen desorption starts at temperatures as low as 150 degrees C for the sample with 8 wt% LiBH4 dispersed onto the MSU-H carbon replica while pure LiBH4 does not release hydrogen below 300 degrees C. After rehydrogenation, a lower amount of hydrogen is desorbed due to the fact that increasing temperature up to 400 degrees C a part of hydride segregates outside the pores of the carbonaceous support.

We present the magnetic and transport properties of superconducting composites fabricated by admixing carbon-encapsulated Fe nanospheres and MgB2 powder. The addition of nanoparticles is expected to enhance the critical current density by carbon-doping the MgB2 matrix and by providing artificial pinning sites. Three samples with estimated amounts of 0.35, 0.6, and 1.0 wt. % metallic Fe were prepared using the spark plasma sintering technique. The average size of these nanoparticles is comparable to the superconducting coherence length of MgB2 at approximately 5 nm. We found that the additions do not significantly alter the critical temperature which is very high, close to that of the pure MgB2 samples. We have also observed improved current densities, as high as 1100 kA/cm(2) for the samples with 0.35 wt. % metallic Fe at 5 K and 1 T. A core-shell model for explaining the transport data is presented. The field and temperature dependence of the reduced pinning force is described in terms of pinning on grain boundaries and/or on point defects. (C) 2011 American Institute of Physics. [doi: 10.1063/1.3672820]

We report on the characterization of metallurgical phases and their magnetism at the interfaces of nanoscale MgB2/Fe layered structures. MgB2/Fe-57 multilayers with varying layer thicknesses were prepared by vacuum deposition and investigated, before and after annealing by electrical resistance measurements, x-ray diffraction and Fe-57 conversion-electron Mossbauer spectroscopy (CEMS) down to 5 K. Interfacial Fe-B phases, such as Fe2B, were identified by CEMS. A superparamagnetic-to-ferromagnetic transition is observed with increasing Fe-57 film thickness. Ultrahigh vacuum annealing at 500 degrees C of the multilayers leads to strong diffusion of Fe atoms into the boundary regions of the MgB2 layers. MgB2 in the as-grown multilayers is non-superconducting. Structural disorder and the effect of Fe interdiffusion contribute to the suppression of superconductivity in the MgB2 films of all the as-grown multilayers and the thinner annealed multilayers. However, an annealed MgB2/Fe-57/MgB2 trilayer with thicker (500 angstrom) MgB2 layers is observed to be superconducting with an onset temperature of 25 K. At 5 K, the annealed trilayer can be conceived as being strongly chemically modulated, consisting of two partially Fe-doped superconducting MgB2 layers separated by an interdiffused weakly magnetic Fe-B interlayer, which is characterized by a low hyperfine magnetic field B-hf of similar to 11 T. This chemically modulated layer structure of the trilayer after annealing was verified by Rutherford backscattering.

We have used scanning Hall probe microscopy to investigate vortex structures and vortex dynamics in Bi2Sr2CaCu2O8+delta thin films in very low perpendicular magnetic fields. After nominally zero field cooling in the Earth's field we find that the vortices appear to be in a stable glassy state in our highly disordered samples. After applying a cancellation field of a few Oersted at low temperature, however, the system enters a new regime at very low magnetic induction when the only image contrast is due to vortices that are intermittently trapped on strong pinning centres. This state shares many of the signatures of the re-entrant vortex liquid phase that has been theoretically predicted in these highly anisotropic materials at very low vortex densities. Analysing the trapping times for vortices in the fluctuating state we estimate that the pinning potential of typical strong pinning centres is about 900 K under our experimental conditions. To our knowledge, this is the first direct experimental evidence for the existence of a dynamic liquid-like vortex state in this highly anisotropic material at very low magnetic induction.

Multiband superconductors can have several types of domains that are inhibited in conventional single-band superconductors. These domains are phase domains and chiral domains and their domain wall are an interband phase difference soliton. In a superconductor with an odd number of electronic bands (five or more) and with positive interband Josephson interactions, we find other types of domains with different interband phase differences. We call these domains configuration domains because pseudo-order parameters for each band are dispersed in the complex plain and several configurations, which have several local minima. Fractional vortices serve as hubs for phase difference solitons (configuration domain walls). The divergence of the number of configurations with local minima would pose a serious problem for the stability of superconductivity. (C) 2011 Elsevier B.V. All rights reserved.