Gianmarco Bovone

Large critical current density in MgB2 wire using MgB4 as precursor

A high-purity MgB4 phase has been synthesized and used as the precursor powder for the realization of in situ wires. Various final heat treatments, from 550 to 1100 ° C for 20 min each, have been carried out to convert the inner mixture to MgB2. Critical current densities up to 1.75 × 106 and 0.98 × 106 A cm−2 at 4.2 and 20 K, with 0.2 T, up to 0.76 × 105 A cm−2 at 4.2 K and 4 T, and up to 0.78 × 105 A cm−2 at 20 K and 2 T were measured. Critical temperatures up to 38.5 K were measured. These results suggest that this could be a good alternative to both ex situ and classical in situ routes.

High temperature heat treatment on boron precursor and PIT process optimization to improve the Jc performance of MgB2-based conductors

The promising results reported in our previous works led us to think that the production process of boron 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’s 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 the powder-in-tube (PIT) method for these nano-metric 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 manufacture, leading to failure. Packaging problems are related to the amorphous nature of boron synthesized in our laboratory, so a crystallization treatment was applied to improve the crystallinity of the boron. To prevent excessive friction phenomena we synthesized non-stoichiometric MgB2 and used magnesium as lubricant. Our goal is the Jc improvement, but a global physical–chemical characterization was also made to analyse the improvement given by our treatments: this characterization includes x-ray diffraction, ρ(T) measurement, and SEM imaging, besides magnetic and transport Jc measurements.

Size determination of superconducting MgB2 powder from magnetization curve, image analysis and surface area measurements

This paper reports on a method for estimating the average grain size of superconducting nanoparticles through their magnetic properties. The use of SQUID magnetometry to determine the average MgB2 particle size was investigated and the results were compared with those of several different techniques. In particular, the data obtained from zero field cooled magnetization measurements as a function of the temperature were compared with the results obtained by the scanning electron microscopy and Brunauer?Emmett?Teller techniques.The particle magnetization was measured by a commercial SQUID magnetometer in a magnetic field (10?G) and at temperatures ranging from 5 to 50?K by dispersing the powders in a grease medium. The grain size was obtained by fitting the data taking into account the Ginzburg?Landau temperature dependence of the London penetration depth. Variations in 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 though an SEM analysis coupled to the image analysis allows the extraction of more information about the grain-size distribution. Furthermore, a Matlab routine has been developed in order to automatically analyze several SEM images.

Manufacturing process influence on superconducting properties of MgB2 wires prepared using laboratory made boron

Here we report a systematic study of the superconductive properties of mono-filamentary MgB2-based wires, manufactured with four different techniques. A detailed comparison of the influence of manufacturing technique and final heat treatment on superconducting properties has been given. The boron used was synthesized in laboratory following magnesiothermic reduction of boron oxide, purified thanks to several acid leaching and heat treated at high temperature, to enhance crystalline degree and remove impurities. MgB2 conductors were manufactured using the same B precursor through four different techniques (ex situ, in situ, the MgB4 or ‘mixed’ technique (half ex situ and half in situ), and reactive liquid infiltration (Rli)). Transport critical current density was measured on the best wire for each technique, considering the literature data in order to identify the corresponding best final heat treatment. Magnetic critical current density and critical temperature were investigated at different synthesis/sintering temperatures in order to evaluate their dependence to the applied final heat treatment and the data were compared. Critical current density was evaluated on short wire pieces by magnetic measurement at 5 K in a MPMS 5.5 T Quantum Design SQUID, while critical temperature was measured with a four probe system by drop of resistivity during the cooling process of the sample in a liquid helium dewar. A detailed morphological analysis is given, with void percentage evaluation and analysis of elemental Mg diffusion across the transversal cross section. X-ray diffraction was performed on MgB2 powder extracted removing each metal sheath, in order to investigate the influence of manufacturing process on the MgB2 phase. This study shows that despite the presence of a wide void within the superconducting core (due to the Mg diffusion) in the Rli sample, this manufacturing technique allows wires with higher Jc (105 A cm−2 at 3 T when heat treatment at 700 °C is applied) than other powder in tube techniques, while the highest Tc (39.2 K at the onset) can be reached by following the mixed technique, which also shows low dependence to heat treatment for both Tc and Jc. In order to establish the presence of some impurities in the lab-made B precursor energy dispersion spectroscopy analysis, inductively coupled plasma atomic emission spectrometry and X-ray analysis were performed on B.

Improved Performances of

In this paper, we report about improved performances of amorphous and nano-structurated boron in MgB2 critical current density of wires and tapes. The boron produced by magnesiothermic reaction, using nano-structurated boron oxide, is used to synthesize nano-structurated MgB2 powders. These powders are used to produce several wires by ex-situ P.I.T. method. Here we applied only the ex-situ technique, in which our group is very well skilled. The same procedure can be applied to obtain in-situ P.I.T. tapes and wires as well. The boron and the MgB2 powders are characterized both from a physical-chemical point of view and from the superconductive point of view. We performed X-ray analysis to compare the home-made boron crystalline degree with respect to the commercial one. SQUID and transport measurements were done to compare the wires critical current density behavior with respect to our past results using different boron.

\hbox{MgB}_{2}

In this paper, we discuss the influence of heat treatment (applied to the wire as sintering for ex situ wire, or synthesis for in situ wire) on critical current density of MgB2-based wires. A comparison of this influence (changing the synthesis/sintering parameters, i.e., time and temperature, applied to wire samples) for different manufacturing techniques has been given. Boron synthesized in our laboratory by magnesiothermic reduction has been used as precursor. In this paper, we show its behavior with different PIT manufacturing techniques. Different heat treatments of synthesis/sintering have been performed on short wires, to evaluate the suitable one for each manufacturing technique. Ex situ wire shows its best results in terms of Jc (105 A/cm2 at 2 T) after heat treatment at 920°C, while for in situ wires a heat treatment at 800°C was observed to be the best. A sample prepared following a mixed (ex-situ-in-situ) technique shows an independent .Ic behavior from the final heat treatment. Here, we report the .Ic analysis performed from transport and magnetic measurements by using Beans model for wire samples. The transport .Ic of the mixed sample shows the best behavior at high magnetic field, one order of magnitude higher than the ex situ wire.

Conductor by Using of Innovative Amorphous and Nano-Structurated Boron

ABSTRACT In this study, we propose a new approach for the large-scale production of doped nanosized boron powder, which is also useful for manufacturing a wide choice of compounds. This work deals with synthesis of B with cryogenic freezing and lyophilization-based technique, evaluation of its structural properties, and study of superconducting properties of MgB2 wires manufactured with two types of B powder. This new process begins with dissolution of B2O3 precursor in hot water together with doping agents, which can be organic macromolecule or nanosized inorganic compound. This hot solution was then cryogenically frozen in liquid N2 and finally lyophilized. Nanostructured and doped boron powders were produced by magnesiothermic reaction of the lyophilized precursor (B2O3). After the structural and morphological characterization, these boron powders were used for preparation of MgB2 phase and MgB2-based conductors. Boron and MgB2 powders were morphologically analyzed by SEM technique to determine grain size distribution. Phase formation was studied and confirmed by XRD analysis. Short pieces (10 m long) of MgB2 superconducting wires were manufactured by ex situ PIT technique and characterized by magnetic and transport measurements. Results reported in this study confirm the suitability of the proposed method to produce nanostructured and doped boron powder. This nanosized boron can be used also in different fields where doping and nanosizing are fundamental and required characteristics.