Introduction
Once the superconductivity of MgB2 was discovered, MgB2 becomes a promising superconductor for applications in cryogenic environment due to its higher transition temperature (Tc = 39 K) \cite{28439448}. Up until now, powder-in-tube (PIT) ex-situ\cite{g2007} and in-situ \cite{al2001}\cite{al2007}MgB2 strands have been fabricated for practical applications. Over the years, many routes have been tried to enhance the critical current density of MgB2 wire. One of these routes is to enhance the density of the MgB2 layer in wire. The PIT in-situ wire is manufactured by mixing Mg and B powder into a metallic tube and the reacted Mg powder induced void formation in MgB2 phases. Therefore, the PIT in-situ wire is limited by its high porosity. In order to solve this problem, Giunchi et al. firstly utilized "Reactive Liquid Mg Infiltration" (RLI) in MgB2 strand\cite{al2003}\cite{e2007}. For RLI strand, a stainless steel tube internally lined with a Nb tube was filled with a coaxial Mg rod which was embedded in amorphous, fine-grained B powder and then this billet was heat-treated at 750 - 950 oC for 1 - 3 h after cold-working \cite{al2003}. In this case, liquid Mg reactively diffused into B layer and a very compact, dense MgB2 layer formed in wire \cite{al2003}. Subsequently, Kumakura et al enhance the Jc of this type MgB2 wire by using HT temperature slightly higher than melting point of Mg and SiC doping \cite{k2008}. This high-performance MgB2 wire is called "Internal-Mg-diffusion" (IMD) processed wire. Li et al also obtained high Jc value through IMD process as well as by optimizing B particles, C doping level, and strand architecture \cite{ew2012} \cite{ew2013}.
However, all IMD-processed strands have a same problem that the reacted MgB2 layer is very thin and this layer is surrounded by unreacted B particles. Once a dense MgB2 layer formed in an IMD-processed strand, it will inhibit further infiltration of liquid Mg into B powder and therefore B particles cannot fully react with Mg . Based on the architecture of IMD-processed wire, three Jcs can be defined. (i) layer Jc is defined as critical current Ic divided by the area of reacted MgB2 layer. (ii) non-barrier Jc is calculated as Ic divided by whole area inside Nb barrier. (iii) engineering Je is obtained by Ic divided by whole area of wire. A IMD-processed wire usually has a high layer Jc due to dense MgB2 layer. Nevertheless, the non-barrier Jc and Je of the IMD wire are limited by its thin superconducting layer.
In order to expand the superconducting layer in IMD wire, Ye et al attempted to add 6 - 12 mol% Mg powder into amorphous , 325 mesh B powder (~44 micron) and obtained the best 4.2 K, 10 T layer Jc of 1.2 x 104 A/cm2 for undoped wire and of 5 x 104 A/cm2 for SiC-doped wire \cite{t2012}. In our study, we utilized much more fine C-doped B powder (< 250 nm) mixed with 15 mol% Mg powder on IMD-processed MgB2 strands to produce high non-barrier Jcs and Jes. In addition, the influence of HT condition on the Jcs and microstructure of the IMD wire with 15 mol% Mg addition was systematically investigated in this paper.
Experimenal
A series of precursor monofilamentary strands was fabricated by Hyper Tech Research, Inc. (HTR). A Mg rod was positioned at the central axis of a double tube of Nb and monel. The space between the Mg rod and the double tube was filled with a mixture of 2 mol% C-doped amorphous B powder (10 - 100 nm, Specialty Materials Inc.) and Mg powder (325 mesh, Alfa Aesar). The concentration of Mg powder in the mixture is 15 mol%. The composite billets were cold-worked into wires and then were heat-treated in a tube furnace with a flowing Ar gas. The HT conditions were varied as 650 oC/2 h, 650 oC/4 h, 650 oC/6 h, and 675 oC/1 h. An IMD-processed MgB2 without Mg powder addition was also manufactured for comparison and this wire was processed at 675 oC for 1 h. In this paper, the IMD-processed MgB2 strands with 15 mol% Mg addition were named as "MA"-series sample. As the pure IMD-processed MgB2 strands manufactured by HTR are also called "advanced-internal-Mg-infiltration" (AIMI) strands, "AIMI" was used to refer the pure IMD-processed wire in this paper.