Progetto comune di ricerca

Effetto della densificazione su fili superconduttori a base MgB2 tramite processo HIP

Responsabili di progetto
Carlo Ferdeghini, Andrzej Morawski
POLONIA - PAS (NUOVO ACCORDO) - Polish Academy of Sciences/Polska Akademia Nauk
CNR-PAN 2017-2019
Scienze fisiche e tecnologie della materia
Area tematica
Scienze fisiche e tecnologie della materia
Stato del progetto

Proposta di ricerca

Superconducting materials are very important for several application fields, but large amount of liquid helium are required for the operating service. Actually helium is getting extremely expensive with possible limitations to operability of devices based on Low Temperature Superconductors (LTS). One of the best candidates for replacement of LTS is MgB2, discovered as superconductor in 2001, with a critical temperature of 40 K that allows this material to operate at temperature far above liquid helium. In addition availability and low cost of raw materials and wire fabrication process makes MgB2 much more interesting for applications with respect other new superconductors (Cuprates and Iron-based). Despite the interest of scientific community during the last 15 years, a great work of optimization is still to be done, especially on availability of "high-performance" boron precursor and on reaching high powder densification inside the wire. SPIN Institute is owner of a patent for boron synthesis with an innovative procedure [1, 2] that permits to obtain nano metrical boron with nano-clusters insides, useful as pinning centres. This is a key point in order to enhance superconducting properties of final MgB2 powder. Densification of MgB2 core is another crucial operation for enhancing performances of wires. Ex-situ wires (metal sheath filled with pre-reacted MgB2) shows high dense core, but due difficulties in powder sintering after cold deformation process critical current density (Jc) is low. On the other hand in-situ wires (metal sheath filled with MgB2 precursors) shows high Jc thanks to good grain connectivity. Due to volume reduction, central core is not dense and full of voids. To overcome this problem it has been developed a new manufacture process based on the use of MgB4 [3] despite boron, as MgB2 precursor. Reaction between magnesium (Mg) and MgB4 has lower volume reduction and results in higher densification and so critical current densities. A certain amount of voids still remains and to enhance Jc a densification process is required. Morawsky's group at the Institute of High Pressure of Polish Academy of Science in Warsaw is equipped with a performing Hot Isostatic Pressure process (HIP) capable to heat treat wire samples at high temperature and high pressure. Under these conditions, during MgB2 synthesis/sintering it is possible to largely compact powder inside wire and prevent voids formation.
Italian group from SPIN will focus on powders preparation and wires manufacturing. Synthesis of purified nano-metrical boron powder is essential for the project and this activity will take large part of time dedicated. Purified powders of pure nano-boron will be used for MgB4 synthesis. Key point of this part will be the optimization of synthesis parameters (time and temperature) to reach highest Jc and Tc, as MgB4 process is newly discovered and need further investigation. Also Polish group will work on MgB4 with a synthesis route based on mechanical alloying as a possible alternative method. Synthesised powders will be used for wire manufacturing through mixed technique in SPIN laboratory. A first reaction and characterization of short pieces of wire will be made in Italy (in CNR SPIN Labs) using available apparatus for measurements under applied magnetic field (i.e. SQUID magnetometer up to 5 T, and PPMS up to 9 T and, the special apparatus for Jc measurements up to 7 T). Short pieces (15 cm) of unreacted wires will be dedicated to HIP process made by polish group. Their role will be to optimize the synthesis parameters of mixed manufactured wires using HIP process and evaluate their influence on superconducting properties, with respect to un-HIPed wires.
Another interesting route for fabrication of high performance MgB2 wires is Reacting Liquid Infiltration (RLI) route, firstly developed by Giunchi [4] for bulk and recently largely investigated also for wires [5]. It is based on the use of a Mg rod centred in metal sheath, covered by boron powders. During final heat treatment Mg liquefy and infiltrates through B layer synthesising a thin and very dense MgB2 layer with a large single void in the centre. This technique permits to achieve and reach very high Jc values due to high densification, but the presence of a large void, compromises wire stability, homogeneity and strength on long lengths. During collaboration between Italian and Polish group we will explore a new manufacturing procedure based on RLI, in which MgB4 will be used instead of B. This technique will permit to synthesize a denser MgB2 core during final heat treatment thanks to Mg infiltration, but lower volume reduction allows enhancing the filling factor, reducing central void and enhancing mechanical stability of the wire. In addition HIP will be largely effective on these wires: traditional RLI wires will collapse under high pressure, but this configuration permits to enhance wire resistance to the process thanks to a reduced central void.

[1] European Patent: EP2199258 - A process for producing optionally doped elemental boron
[2] M. Vignolo et al, "A Novel Process to Produce Amorphous Nanosized Boron Useful for MgB2 Synthesis" 2012 IEEE trans. on Appl. Supercond. 22 6200606
[3] A Nardelli D, Matera D, Vignolo M, Bovone G, Palenzona A, Siri A S and Grasso G 2013 Supercond. Sci. Technol. 26 075010
[4] Giunchi G, Ceresara S, Ripamonti G, Di Zenobio A, Rossi S, Chiarelli S, Spadoni M, Wesche R and Bruzzone P L 2002 Supercond. Sci. Technol. 16 285
[5] Shujun YE and Hiroaki Kumakura 2016 Supercond. Sci. Technol. 29 113004

Obiettivi della ricerca

- Optimization of B and MgB4 synthesis parameters;
- Synthesis of MgB4 through mechanical alloying;
- Study of effects of HIP process and modification of parameters (e.g. temperature, pressure, heating speed) on mixed manufactured wires;
- Comparison between HIPed and un-HIPed wires for comprehension of its benefits;
- Developing of an innovative fabrication route for MgB2-based wire and evaluation of benefits led by HIP process.

Ultimo aggiornamento: 12/08/2020