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

Abstract

Abstract:Promising results reported in our previous works led us to think that boron's production process plays a crucial role in MgBB 2 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 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 boron's crystallinity. To prevent excessive friction phenomena we synthesized non-stoichiometric MgB 2 and using magnesium as lubricant. Our goal is the J c improvement, but a global physical-chemical characterization was also made to analyze the improvement given by our treatments: this characterization includes X-ray diffraction, ρ(T) measurement, SEM image, besides magnetic and transport J c measurements. IntroductionAfter 12 years since the discovery of superconductivity in MgB 2, many efforts have been done in order to improve its performances and make it competitive with traditional superconductors. This is possible due to its relatively high critical temperature (T c ) that permits to use MgB 2 -based conductors at 20 K by using a cryocooler. Also compared to other superconducting materials MgB 2 tapes are easy to manufacture and of low cost. Despite the good results have been achieve so far, huge efforts are necessary especially on J c improvement, which is the main parameter from application point of view. This could be achieved by obtaining a complex balance between connectivity, upper critical field (H c2 ) and pinning force induced by grain boundaries, precipitates Experimental Boron powder preparation and characterization.Details of boron synthesis and purification have been given in [9]. Once boron is purified at the end of the process two amounts of powder were taken from the same batch to effect the heat treatment of crystallization. The first one (12LB) has been set in an iron boat and placed in a tubular furnace at 1200 °C for 120 minutes, under Ar flow. The second sample (17LB) was put in tantalum crucible, sealed in a quartz ampoule, and heat treated at 1700 °C through an inductive furnace, for 30 minutes. X-ray analysis was made on these powders at the end of the treatment, on pristine powder (LB), and on commercial boron (CB), to verify the improvement of crystal quality led by heat treatment. XRD measurements were performed using a PHILIPS (PW 1170/90 Cu Kα) diffractometer at 40 KV and 30 mA. MgB 2 bulk, powder and wire preparation and their characterization.After thermal treatment the powders are used to synthesize...

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