Enhanced Connectivity and Percolation in Binary and Doped In Situ ${\rm MgB}_{2}$ Wires After Cold High Pressure Densification

Abstract

The cold high pressure densification technique (CHPD) was recently developed in Geneva for improving the in-field critical current density of in situ binary and alloyedMgB 2 wires and tapes [1], [2]. of CHPD treated square wires alloyed with malic acid (C 4 H 6 O 5 ) was enhanced by a factor 2 at 10 T and 4.2 K. In order to understand the fundamental mechanism behind this strong improvement of , the properties of binary and alloyed MgB 2 wires have been investigated without and with CHPD, using resistivity and specific heat measurements in the temperature range from 5 to 35 K in magnetic fields up to 15 T. In particular, a deconvolution of the specific heat data was used to determine the distribution of in the samples. We have found that the effect of the densification process on the electrical and transport properties is related to the improved grain connectivity and percolation. By combining the results arising from the analysis of the distribution and those from resistivity measurements, it is concluded that the minimum superconducting volume fraction needed for the percolation of a superconducting path is strongly reduced in samples treated by CHPD.

Technical aspects of cold high pressure densification (CHPD) on long lengths of In Situ MgB2 wires with enhanced Jc values

“…The results reported in the following have been obtained by densification at room temperature, with solid anvils acting simultaneously on all four sides of a square wire. As previously shown by electrical resistivity and specific heat measurements [2,3], the observed enhancement of J c in binary and alloyed MgB 2 wires is correlated to the enhancement of mass density [1], which is the cause of improved connectivity in the filament. In the present paper, the accent will be put on the technical aspect of densification of short and long wires, the physical aspect having been discussed in Refs.…”

“…The short sample prototype pressure cell used in the earlier publications [1,2,3,23,25] is schematically drawn in Fig. 1.…”

“…In FIGURE 1, an upper anvil transmits the force F exerted by a hydraulic press to the sample, which was previously square shaped by means of a turks head. For our experiments on wires, the horizontal anvil width was 1 mm, but for the preparation of samples for specific heat and electrical resistivity, a horizontal width of 2 mm was chosen [2]. Since little or no wire elongation was observed after densification, the conditions of pressure inside the pressure cell are supposed to be nearly homogeneous, at least for wires with an initial square shape.…”

“…In the present paper, the accent will be put on the technical aspect of densification of short and long wires, the physical aspect having been discussed in Refs. [2] and [3].…”

Effect of cold high pressure deformation on the properties ofex situMgB2wires

“…We found for the three types of ex situ wires that the CHP treatment had only a negligible effect on T c (see table 2). The T c value of ex situ wires treated at high pressure is expected to remain unchanged, which is in contrast to carbon doped in situ MgB 2 wires, where T c decreases after applying CHPD, making substitution of C more effective [19]. This was also confirmed by the change of the lattice parameter a and an enhancement of the upper critical field B c2 [17].…”