The Optimization of J c , B irr , n value, Wires Diameter, and Research of the Dominant Pinning Mechanism for Applicatio
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ORIGINAL PAPER
The Optimization of Jc, Birr, n value, Wires Diameter, and Research of the Dominant Pinning Mechanism for Applications of Undoped MgB2 in Superconducting Multi-section Coils D. Gajda 1 & A. J. Zaleski 1 & A. Morawski 2 & T. Cetner 2 & M. Małecka 1 & M. Rindfleisch 3 Received: 16 July 2020 / Accepted: 28 August 2020 # The Author(s) 2020
Abstract This article discusses the influence of MgB2 wire diameter on critical parameters, dominant pinning mechanism, and n value. Studies were conducted on multifilament MgB2 wires of the 0.84 mm and 0.64 mm diameter. The wires were annealed under 1 GPa isostatic pressure, between 650 and 730 °C. Studies show that the best results (Jc, Birr, n value, and point pinning mechanism) for a 0.64-mm wire were obtained with annealing temperature of 650 °C. However, the best results for wires with a 0.84-mm diameter were obtained with annealing temperature of 730 °C. This research was conducted for exploring the use of MgB2 wires in multi-section superconducting coils, e.g., main coils generating magnetic fields and compensation coils— increasing the homogeneity of the magnetic field. Our research indicates that the main coil made with larger diameter wires should be heated at higher temperature (730 °C). In contrast, compensating coils with smaller diameter wires should be heated at a lower temperature (650 °C). Keywords Critical current density . n value . Irreversible magnetic field
1 Introduction Research presented by Uchiyama et al. [1], Wang et al. [2], and Gajda et al. [3] showed that the reduction of the diameter of a superconductor wire as the result of cold drawing led to an increase of critical current density, irreversible magnetic field, pinning force, n value, and dominant pinning mechanism for samples heat treated at a constant temperature, due to increased MgB2 density, Mg elongation, formation of a layered structure, and increased density of pinning centers [1–5]. In contrast, Xu et al. indicated that a significant reduction of wire diameter causes a decrease of Jc [6]. The critical current density, irreversible magnetic field, and dominant pinning
* D. Gajda [email protected] 1
Institute of Low Temperature and Structure Research PAS, Okólna 2, 50-422 Wroclaw, Poland
2
Institute of High Pressure Physics PAS, Unipress, Sokołowska 29/37, 01-142 Warszawa, Poland
3
Hyper Tech Research, Inc., 539 Industrial Mile Road, Columbus, OH 43228, USA
mechanism in MgB2 material depend on many factors, e.g., connections between grains, nano-additives [7, 8], the purity of the starting materials [9, 10], milling time [9–11], type of pinning centers [12–14], heat treatment temperature [15], defects, pores, and voids [16]. The n value, which qualifies the transition from the superconducting state to the normal state, is an important factor that influences the selection of the operating current for a superconducting device, coil, magnet, etc. [17]. The n value of a superconductor has a dependency on both intrinsic (connections between grains, grain microstructure, pin
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