National Institute Of Materials Physics - Romania

Iron-carbon nanocomposites have gained interest due to their new engineering and biomedical applications. Carbon coated iron nanoparticles (Fe@C) were obtained continuously and in a single step using the laser pyrolysis method. The continuous wave CO2 laser beam was used to continuously heat a sensitized (with ethylene) precursor gas mixture, in which iron pentacarbonyl (vapor) and acetylene were the iron and carbon donors, respectively. The effect of varying the residence time in the reaction zone through the variation of the internal nozzle diameter was explored in order to improve the particle size and the phase distributions. At increased nozzle diameter, (i) the particle mean diameter increases (from about 3.5 to 10.5 nm), (ii) higher ordering of the crystallographic network seems to occur, (iii) the dominance of the alpha-Fe and iron carbide phases is revealed. Onion-like graphenic layers often cover the buried iron cores. Magnetic measurements and temperature dependent Mossbauer spectroscopy were used in order to find correlations concerning the magnetic behavior and the Fe phase composition of samples. Preliminary experiments for obtaining stable water-based magnetic nanofluids are discussed. (C) 2011 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.

Superconducting (SC) MgB2 thin films were prepared in situ in one step by vacuum co-deposition at extremely low growth temperature (T-s similar to 110-150 degrees C) and at very low Mg deposition rate, far off from the MgB2 stability regime of the thermodynamic phase diagram. As-grown films are nanocrystalline and exhibit (002) texture, moderate surface roughness and SC onset up to T-c(on) = 18 K. This is the highest critical temperature ever observed in low-T-s as-grown MgB2 films. Our preparation method at very low T-s is favorable for the fabrication of layered SC hybrid electronic devices, where deposition and post-annealing at high temperature are detrimental to device functioning. Simple post-annealing of our films in ultrahigh vacuum enhances T-c(on) up to 22 K due to the improvement in structural properties.

We present an investigation of the field dependence of the pinning force of dense MgB2-based superconducting composites as obtained by spark plasma sintering with different ingredients which are designated to enhance the pinning force F. Generally, the latter quantity, scaled to the maximum pinning force F-max, obeys a scaling law as a function of the scaled field h when the irreversibility field B-irr is considered as scaling field. The scaled function is described in terms of a generalized scaled function h(p)(1-h)(q). However, in our samples the scaling is absent. In addition, the peak of the scaled function shifts to higher reduced fields h when the temperature increases. Depending on the level of doping and the nature of the nanosized particles used to built the superconducting composites, we found that the scaled force can be depicted either with a combination of generalized scaled functions with different exponents or with a single function but multiplied with an envelope, usually an exponential factor. The former dependence is present in the samples with a high amount of nanoparticles and mirrors two types of pinning of close weights. The presence of the exponential factor in the latter dependence is attributed to the combined effect of the intrinsic anisotropy of MgB2 and the random orientation of the grains relative to the magnetic field.

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]