Grain size determination of superconducting MgB2 powders from magnetization curve, image analysis and surface area measurement

The present article reports a method for the average grain size evaluation of superconducting nano-particles through their magnetic properties. The use of SQUID magnetometry to determine the average MgB2 particle size was investigated and the results compared with those achieved through other techniques. In particular the data obtained from zero field cooled magnetization measurement as function of the temperature were compared with the results obtained by scanning electron microscopy and Brunauer-Emmett-Teller techniques. The particle magnetization was measured by a commercial SQUID magnetometer in magnetic field (1 mT) and temperatures ranging from 5 to 50 K dispersing the powders in a grease medium. The grain size is obtained by fitting the data taking into account the Ginzburg-Landau temperature dependence of the London penetration depth. Variations on typical modeling parameters were explored in order to gain a better picture of the average grain size and the effectiveness of various measurement techniques. We find that it is possible to use the magnetization measurements to determine the average grain size even if the SEM image analysis allows extracting more information about the grain size distribution. Furthermore a Matlab routine has been developed in order to get automatic analysis of SEM images.

In-situ high energy synchrotron X-ray diffraction investigation of the MgB2 phase formation and MgB2 tapes sintering

In the present paper we report an in-situ high-energy X-ray diffraction analysis of MgB2 tapes during the preparation process. The experiment was performed in a specifically designed furnace working in reducing atmosphere, compatible with the Laue diffraction condition. The MgB2 synthesis was realized starting from MgH2 and amorphous B in powder form as precursors, varying reaction temperature and testing different cooling processes. We analyzed both the MgB2 synthesis and the sintering process of tapes prepared with these powders. Phase evolution, micro and crystallographic structure were monitored during the different thermal treatments. Among the main results we observed the formation of MgB2 at an extraordinary low temperature (300C), probably as a result of a solid-state reaction between MgH2 and B. Furthermore, we studied the dependence of the micro-structure upon the thermal treatment and its effect on the critical current performance of the superconducting tapes.

Superconducting performance of ex-situ SiC-doped MgB2 mono-filamentary tapes

We report on the superconducting performance of the ex-situ SiC doped MgB2 monofilamentary tapes. Polycrystalline powders of MgB2 doped with 5 and 10 wt% SiC were synthesized by conventional solid-state reaction route and characterized for their superconducting performance. It is found that superconducting parameters i.e. upper critical field (Hc2), irreversibility field (Hirr) and critical current density (Jc) are all improved significantly with SiC addition. Also it was found that relatively lower synthesis temperature (700 C) resulted in further improved superconducting parameters. As synthesized powders are used for ex-situ powder-in-tube (PIT) monofilamentary tapes and superconducting parameters are determined. Albeit the superconducting transition temperature (Tc) is decreased slightly (2K) for SiC doped tapes, the superconducting performance in terms of critical current density (Jc), being determined from both magnetization and transport measurements, is improved significantly. In particular the SiC doped and 700 {deg}C synthesized MgB2 tapes exhibited the transport Jc of nearly 10^4 A/cm2 under applied fields of as high as 7 Tesla. Further it is found that the Jc anisotropy decreases significantly for SiC doped tapes. Disorder due to substitution of C at B site being created from broken SiC and the presence of nano SiC respectively in SiC added ex-situ MgB2 tapes are responsible for decreased anisotropy and improved Jc(H) performance.

A novel process to produce amorphous nano-sized B useful for MgB2 phase synthesis

In this paper we report a new synthesis route to produce boron powders characterized as being amorphous and having very fine particle size. This route has been developed to improve the performances of superconducting MgB2 powders, which can be directly synthesized from this nano-structured boron precursor by following the ex-situ or the in-situ P.I.T. method during the manufacturing of tapes, wires and cables. All the procedure steps are explained and the chemical-physical characterization of the boron powder, using x-ray diffraction (Xrd), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) techniques, is reported. Furthermore, a comparison with commercial boron is given. Preliminary results of the magnetic and electrical characterization, such as the critical temperature (TC) and the transport critical current density (JC t), for the MgB2 tape are reported and compared with the tape prepared with commercial boron.

High temperature heat treatment of B precursor and P.I.T. process optimization to increase Jc performances of MgB2-based conductor

Promising results reported in our previous works led us to think that production of B powder plays a crucial role in MgB2 synthesis. A new method for boron preparation has been developed in our laboratory. This particular process is based on magnesiothermic reaction (Moissan process) with the addition of an initial step that gives boron powder with nano-metric grain size. In this paper we report our efforts regarding optimization of PIT method for these nanometric powders and the resolution of problems previously highlighted such as the difficulty in powder packaging and the high friction phenomena occurring during cold working. This increases cracking during the tape and wire manufacturing leading to its failure. Packaging problems are related to the amorphous nature of boron synthesized in our laboratory, so a crystallization treatment was applied to improve crystallinity of B powder. To prevent excessive friction phenomena we synthesized non-stoichiometric MgB2 and using magnesium as lubricant. Our goal is the Jc improvement, but a global physical-chemical characterization was also made to analyze the improvement given by our treatments: this characterization includes X-ray diffraction, resistivity vs. temperature measurement, SEM image, besides magnetic and transport Jc measurements.