5 cm - length samples were used for measuring 4.2 K transport current. The two ends of each sample was soldered on two current leads and a pair of voltage taps was soldered on the middle of the sample with a separation of 5 mm. The 4.2 K transport measurement was conducted by immersing the samples into a pool of liquid helium with applied transverse magnetic fields up to 12 T. Optical microscopy (OM) were used to observe the microstructures of the MgB2 strands and energy dispersive x-ray spectroscopy (EDS) was utilized to identify the compositions of phases in the strands. 

Results

Microstructure  and HT condition

Fig. 1 shows the transverse cross-section of AIMI wire. It can seen that a light annulus of reacted MgB2 layer was surrounded by a dark annulus and the central hole was left behind by melt Mg rod. As shown in Fig. 2, three layers co-exist in some parts of the AIMI wire. According to the EDS analysis, the inner layer is reacted MgB2 layer. The middle layer and outer layer are B-rich layer with Mg/B ratio of 1:4 and 1:7, respectively. This result confirms that B cannot fully reacted with Mg for IMD-processed wire with HT at 675 oC for 1 h. The inner MgB2 layer is very compact and dense, but the outer B-rich layer is porous. For AIMI wire, the thickness of MgB2 layer ranges from 10 to 50 micron.