Critical current densities andn-values of MgB₂strands over a wide range of temperatures and fields

Abstract

Transport measurements of critical current density, J ct , in monocore powder-in-tube MgB 2 strands have been carried out at temperatures, T, of from 4.2 K to 40 K, and in transverse fields, B, of up to 14 T. Processing methods used were conventional continuous-tube-forming-filling (CTFF) and internal-magnesium-diffusion (IMD).Strands with several powder compositions were measured, including binary (undoped) MgB 2 , 2% carbon doped MgB 2 , and 3% carbon doped MgB 2 . Magnetization loops (M-B) were also measured, and magnetic critical current density, J cm , values extracted from them. The transport, J ct (B) and magnetic, J cm (B), critical current densities were compared.Also studied was the influence of doping on the resistively measured irreversibility field, B irr and upper critical field B c2 . Critical current densities, J ct , and n-values were extracted from transport measurements and were found to be universally related (for all B and T) according to n∝ J ct m in which m = 0.52 ± 0.11. Likewise n was found to be related to B according to n ∝ B -p with a T-dependent p in the range of about 0.08~0.21. Further analysis of the field (B) and temperature (T) dependencies of n-value resulted in an expression that enabled n(B,T), for all B and T, to be estimated for a given strand based on the results of transport J ct (B) measurements made at one arbitrarily chosen temperature. Symbols B Magnetic field strength B irrResistively determined irreversibility field B irr,trans Apparent irreversibility field for magnetically induced current paths which must always include a portion transverse to the strand axis B c2Upper critical field B 0 (t)Temperature-dependent field-normalization quotient in the exponential form of J c (B,t) -see below B 00 Zero-temperature value of B 0 (t) CTFF Continuous tube forming/filling (process for strand fabrication) d 0 Inner diameter of the Nb chemical barrier (both CTFF and IMD strands) d i Inner diameter of the annular MgB 2 reaction layer (IMD strands) ∆M Full height of the magnetization loop normalized to the actual SC volume F p Bulk pinning force density F p,max Maximum value of F p vs B or B/B irr IMD Internal magnesium diffusion (process for strand fabrication) J c Abbreviation for the words "critical current density" J ct Transport-measured critical current density J cm Magnetically measured critical current density J cte Transport critical current density found by normalized I ct to the whole area inside the Nb chemical barrier J c2Critical current density transverse to the strand axis (magnetically induced) J c (B,t) Field-and temperature dependent J c J ct (B,t) Transport-measured value of field-and temperature dependent J c J c0 (t)Fitted zero-field temperature-dependent J c J ct0 (t)Fitted zero-field temperature-dependent J c based on transport measured field and temperature dependent J c equation J c00Fitted value of J c at zero field and zero temperature

Fabrication of FeSe 0 . 5 Te 0 . 5 Superconducting Wires by an Ex Situ Powder-in-Tube Method

“…The transport measurements of critical current (I c ) using a standard four-probe method were carried out in transverse fields up to 0.75 T in pool-boiling liquid He at 4.2 K on 50-mm-long samples with a gauge length of 5 mm. The electric field criterion for the transport measurements is 1 μV/cm [31][32][33]. The transport J c was calculated by dividing the I c by the area of the superconducting core.…”

Enhanced higher temperature (20–30 K) transport properties and irreversibility field in nano-Dy2O3 doped advanced internal Mg infiltration processed MgB2 composites

“…19 As compared in Table II, using same pre-C-doped boron, D3 shows higher B irr s at all temperatures than the best C-doped PIT or C-doped infiltration-processed strands. This is important because, as is well known, C or C-related doping substantially increases B irr and J c s of MgB 2 at 4.2 K, but is not effective at higher temperatures (20 K and above) 7,20 There is a clear improvement in both J c and B irr attributable to Dy 2 O 3 doping, but calculations based on the magnetic J c s of Figure 4 indicates that the maximum pinning strength …”

Superior critical current density obtained in Mg11B2 low activation superconductor by using reactive amorphous 11B and optimizing sintering temperature

“…MgB 2 attracted lots of attention because of its simple crystal structure, large coherence length, relatively high critical current density, and transparency of the grain boundaries. During the development for more than 10 years, MgB 2 has been fabricated [2][3][4][5][6][7][8][9][10]into bulks, single crystals, thin films, tapes and wires.…”