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 (AF), pinning, percolation threshold (Pc), and anisotropy (γ) 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 (5 wt. %) and further 3 wt. % 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 (ρ) as a function of temperature (T) and field (B) in the range 5–300 K and 0–8 T are carried out in all the samples. The resistive superconducting onset Tcon ∼ 38.6 ± 0.3 K and offset (where ρ goes to zero) Tc0 ∼ 38.1 ± 0.3 K of the samples stay nearly unchanged. The inter-grain connectivity (AF) of the samples varies between 11%–20%. All the additives result in a critical current density (Jc) higher than the stoichiometric MgB2 sample, where the highest values (e.g., Jc(1 T, 5 K) ∼ 1.2 × 109 A/m2) are observed for the sample with 5 wt. % excess Mg. The major findings based on quantitative analysis of ρ (T, B) and Jc (B, T) data in all the samples are: (1) along with previously studied ex-situ samples, the Jc(AF) shows a significant increase at AF ∼ 7%; (2) the irreversibility lines lie lower than the characteristic Tc0(B) lines in the B-T phase diagram; (3) a universal core pinning (δl- and/or δTc- type) mechanism is revealed in the entire T range 5–30 K; and (4) typical values of Pc ∼ 0.57 ± 0.04 is indicative of weak link networks