Transport Jc, Je and MgB2 Fill Factor

Table 2 shows the MgB2 area and MgB2 fill factor of each sample. For MA-series samples with HT at 650 oC, the area and fill factor of MgB2 layer increase as HT time was expanded from 2 h through 4 h to 6 h.  In the initial composite billet, the volume of 15 mol% Mg + B mixture can be calculated through the equation:
To evaluate the volume occupied by reacted MgB2, the following equation should be used:
It can be seen that the space filled by the reacted MgB2 is larger than that by the mixture of Mg powder and B. Longer HT time induces more MgB2 grains form and grow and therefore increases the fill factor of MgB2.  Fig. 7 demonstrates the field dependency of layer Jc for each sample.  For MA-series wires, the highest 4.2 K, 10 T layer Jc value of about 5 x 104 A/cm2 was attained by the MA2 sample . The layer Jcs of the AIMI sample are two times higher than those of MA-series wires. This indicates that the AIMI sample should have a much better electrical connectivity for reacted MgB2 layer than the MA-series wires. Fig. 8 shows the transport  Je versus magnetic field at 4.2 K.  The best 4.2 K, 10 T Je of the MA-series samples was also obtained by sample MA2 with 9.7 x 103 A/cm2.  The MA-series samples still have lower Je values at all applied fields than the AIMI sample, but the Je values of the MA-series samples become much close to those of the AIMI sample because of large MgB2 fill factor for the MA-series samples.  
Although the added Mg powder was fully reacted with B in the MA4 sample processed at 675 oC for 1 h, its layer Jcs and Jes are the lowest at 9 - 12 T possibly due to its high porosity. Ye et al suggest that the Jc of IMD-processed wire will be enhanced as the concentration of Mg addition is lower than 6 mol%, even though the Mg addition below 6 mol% will induce a high density of B-rich phases in MgB2 layer.   \cite{t2012}.  Therefore, compared to the B-rich phases, the voids in MgB2 layer is likely to play a more significant role on impairing the electrical connectivity and Jcs of IMD-processed wires.