Abstract
The upper critical field sets the thermodynamic limit to superconductivity. A big gap is present between the upper-critical-field values measured in MgB2 polycrystalline bulk superconductors and those of thin films, where values as high as ~ 50 T have been achieved at 4.2 K. Filling this gap would unlock the potential of MgB2 for magnet applications. This work presents the results of an extensive experimental campaign on MgB2 bulk samples, which has been guided by a Design of Experiment. We modeled the dependence of the upper critical field on the main synthesis parameters and established a new record (~ 35 T at 4.2 K) preparing C-doped bulk samples by a non-conventional rapid-synthesis route. This value appears to be an upper boundary for the upper critical field in bulk samples. Structural disorder in films seems to act selectively on one of the two bands where superconductivity in MgB2 takes place: this enhances the upper critical field while reducing the critical temperature only by few Kelvins. On the other hand, the critical temperature in bulk samples decreases monotonically when structural disorder increases, and this imposes a limit to the maximum achievable upper critical field.
Highlights
The upper critical field sets the thermodynamic limit to superconductivity
The MgB2 bulk sample manufacturing process consists of a combination of the Internal Magnesium Diffusion (IMD) and the Powder-in-Closed-Tube (PiCT) techniques[26,27]
Based on a previous work of ours[29], we identified five synthesis parameters whose variation has a major effect on the samples’ superconducting properties, namely the heating ramp rate (HR), the dwell temperature (Td), the dwell time (t), the pressure of the Ar-gas quenching jet (ArP), and the pressure applied to the precursors before synthesis (P)
Summary
A big gap is present between the upper-critical-field values measured in MgB2 polycrystalline bulk superconductors and those of thin films, where values as high as ~ 50 T have been achieved at 4.2 K. We modeled the dependence of the upper critical field on the main synthesis parameters and established a new record (~ 35 T at 4.2 K) preparing C-doped bulk samples by a non-conventional rapid-synthesis route. In spite of an enhanced substitution rate of C in Mg(B1−xCx)[2], μ0HC2 appears bounded to maximum values of ~ 26 T and ~ 35 T at 10 K and 4.2 K, respectively These figures constitute new records for HC2 in bulk samples but remain far below what is achievable by the material in film form. The potential scalability for large volume productions of wires and bulk materials is another point of strength of this technique
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