Connectivity and critical current density of in-situ processed MgB2 superconductors: Effect of excess Mg and non-carbon based additives

In a sequel to our previous paper (J. Appl. Phys. 113, 036908 (2013)), where we reported comprehensive analysis of inter-grain connectivity (A(F)), pinning, percolation threshold (P-c), and anisotropy (gamma) in a series of ex-situ processed MgB2, we address the same issues in in-situ processed samples. MgB2 samples with stoichiometric composition, excess Mg (5wt. %) and further 3wt. % addition of various non-carbon based additives like nano-Ag, nano-Ni, and YBCO are synthesised by the in-situ route. Detailed investigations of X-ray diffraction, magnetization (M), and resistivity (rho) as a function of temperature (T) and field (B) in the range 5-300K and 0-8 T are carried out in all the samples. The resistive superconducting onset T-con similar to 38.6 +/- 0.3K and offset (where rho goes to zero) T-c0 similar to 38.1 +/- 0.3K of the samples stay nearly unchanged. The inter-grain connectivity (A(F)) of the samples varies between 11%-20%. All the additives result in a critical current density (J(c)) higher than the stoichiometric MgB2 sample, where the highest values (e.g., J(c)(1 T, 5K) similar to 1.2 x 10(9) A/m(2)) are observed for the sample with 5 wt. % excess Mg. The major findings based on quantitative analysis of rho (T, B) and J(c) (B, T) data in all the samples are: (1) along with previously studied ex-situ samples, the J(c)(A(F)) shows a significant increase at A(F)similar to 7%; (2) the irreversibility lines lie lower than the characteristic T-c0(B) lines in the B-T phase diagram; (3) a universal core pinning (delta l- and/or delta T-c- type) mechanism is revealed in the entire T range 5-30 K; and (4) typical values of P-c similar to 0.57 +/- 0.04 is indicative of weak link networks