Abstract—The cold high pressure densification technique (CHPD) was recently developed in Geneva for improving the in- field critical current density J c of in situ binary and alloyed MgB 2 wires and tapes [1, 2]. J c 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, the behavior being almost isotropic. In order to understand the fundamental mechanism behind this strong improvement of J c , the properties of binary and alloyed MgB 2 wires have been investigated before and after CHPD, using resistivity and specific heat measurements in the temperature range between 5 and 35 K at magnetic fields up to 15 T. In particular, a deconvolution of the specific heat data was used to determine the distribution of T c 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 T c distribution and those from resistivity measurements, it follows that the minimum superconducting volume fraction needed for the percolation of a superconducting path is strongly reduced in samples treated by CHPD. Index Terms—MgB 2 , cold densification, connectivity, percolation, specific heat, T c distribution. I. INTRODUCTION HE relatively high T c , the absence of weak-links at grain boundaries and the abundance of starting materials render MgB 2 a promising material for industrial applications, not only in view of cryogen-free devices operating between 20 and 30K, but also at 4.2K as a replacement for the more expensive Nb 3 Sn wires in the field region between 9 and 12T. At present, powder-in-tube (PIT) MgB 2 wires are produced in km lengths either by the ex situ or the in situ technique. Further developments focus on the improvement of the current carrying capability at increasing temperatures and magnetic fields. The improvement of the transport critical current of powder based MgB 2 wires can be achieved: Manuscript received 3 August 2010. This work was supported by the Swiss National Science Foundation through the National Centre of Competence in Research - Materials with Novel Electronic Properties (MaNEP/NCCR). C. Senatore, M. S. A. Hossain and R. Flükiger are with the Département de Physique de la Matière Condensée (DPMC) and the Département de Physique Appliquée (GAP), Université de Genève, Geneva, CH-1211, Switzerland (corresponding author phone: +41 22 37 96669; fax: +41 22 37 93980; e-mail: carmine.senatore@unige.ch ). . (i) enhancing B c2 , by alloying MgB 2 with Carbon in elemental form [3] or by decomposition of C-based compounds, e.g. SiC, B 4 C [4,5] or of various carbohydrates, e.g. malic acid [6]; (ii) improving the grain connectivity and thus increasing the effective cross section for the conduction of transport current [7]. The improvement of connectivity is accompanied by an enhanced contact area of neighbouring grains and thus by an increased number of flux pinning centres. Low connectivity and porosity are an inherent problem of MgB 2 wires prepared by the in situ technique, the typical filament mass density being as low as 45% of the theoretical density (2.62 g/cm 3 ) due to the reduction of volume during the reaction between magnesium and boron [1]. Recently, cold high pressure densification (CHPD) was introduced at GAP in Geneva as a new route for enhancing the filament mass density of in situ MgB 2 wires [1,2], alternative to the high temperature densification techniques [8]. A high pressure step (p > 1.5 GPa) is performed on square wires at the end of the deformation process, in order to densify the Mg+B mixture before the reaction heat treatment. After reaction, this corresponds to an enhancement of the MgB 2 filament mass density up to 73% of the theoretical value. Using this technique, a considerable enhancement of the critical current density was reported [1,2]. After applying 1.5 GPa to malic acid (C 4 H 6 O 5 ) added wires, the highest J c values so far reported for in situ MgB 2 wires were obtained, as J c (4.2K)=10 4 A/cm 2 at 13.8 and 13.4 T (1 μV/cm criterion) for parallel and perpendicular field, respectively [9]. The aim of the present work is the understanding of the fundamental mechanisms behind this improvement of J c . We report on the effects of CHPD on the fundamental superconducting properties, T c distribution, B c2 and B irr , as well as on the parameters influencing the effective cross- sectional area for the current transport, i.e. percolation threshold and grain connectivity. Resistivity and specific heat measurements were performed on binary and malic acid added MgB 2 wires, non-densified and after CHPD, in magnetic fields up to 15 T. Variations in connectivity and critical fields were evaluated from the ρ(B,T) data. A deconvolution of the specific heat data was used to determine the distribution of T c in the samples. This analysis, in combination with the results of the resistivity measurements, allowed us to estimate the minimum Enhanced Connectivity and Percolation in Binary and Doped in situ MgB 2 Wires after Cold High Pressure Densification Carmine Senatore, Md. Shahriar Al Hossain, and René Flükiger T