Synthesis And Electronic Transport Characteristics Of Superconducting MgB₂+MgO Composite Near Continuous Percolation

Basing on vacuum sintering techniques and solid replacement reactions, a new method for fabricating superconducting MgB₂+MgO composites was put forward and realized. DC resistances, AC impedances, current-voltage (â… -â…¤) curves of the superconductive compounds on the normal state and in the vicinity of critical transition were studied in this thesis. The compound superconductors, MgB₂+MgO, exhibit excellently superconductive properties comparing with other superconducting composites then suggest latent applications. The experiments show a new technique of sintering MgB₂ superconductors with simple crafts and low costs.Polycrystal superconductors of MgB₂ were fabricated also with the similar vacuum technique (VT). By comparing experimental data between pure-phase MgB₂ and compound MgB₂+MgO, we found that superconducting MgB₂+MgO composites have well metal transport characteristics on the normal state and excellently superconductive properties near T_c. It shows that matrix phaseof MgO has no much influence on superconducting connection and critical transition temperature of MgB₂. The resistivities of superconducting MgB₂+MgO composites had almost same-order quantity of pure-phase MgB₂. The residual resistance ratio, RRR=R(3OOK)/R(4OK)=2.4. An onset superconductive transition temperature of about 38 K and a zero-resistance temperature of over 36 K were shown by resistance-temperature (RT) and susceptibility-temperature (xT) curves.That the matrix phase of MgO had little depression on electric transport of MgB₂ was explained by the continuous percolation model of metal-insulator mixtures, and an expression of percolation resistivity with temperature of superconducting MgB₂+MgO composites on the normal state was given:ρ^SPT=ρ₀+α_nTⁿ+α_e4T⁴+α_mT⁶.In the normal-superconductive (N-S) transition, two peaks of the DC differential resistance (dR/dT) of compound MgB₂+MgO superconductor were detected. Experiments suggested that DC resistance experienced two transitions at the above and below of the critical temperature (T_c), respectively, when temperature decreased. The first transition was caused by the N-S transitions of an amount of MgB₂ grains, and the second one was resulted from the tunneling of Cooper pairs in the weak links among MgB₂ grains. Considering the grain distribution and microstructure of MgB₂+MgO composites, a formula of resistivity vs temperature in the vicinity of T_c, (?) was derived by metal-superconductive percolation model. This resistivity function with temperature fitted very well with experimental results. This two-peak phenomenon of DC differential resistance was also observed by measuring AC impedance near T_c. Imaginary parts of AC impedance of MgB₂+MgO composites showed that the second peak increased evidently with the AC source frequency. It confirmed that the second transition was contributed by the weak links of MgB₂ intergrains.On the constant-current source, current-voltage (â… -â…¤) curves of superconducting MgB₂+MgO composites were measured and hysteresis effects inâ… -â…¤curves were observed. The hysteresis goes anticlockwise and is enlarged with the current sweep speed,△Ⅰ/â–³t, increasing. This history effect (HE) could be explained according to SM model of Josephson junction, and the larger hysteresis was considered to a large of effective resistance ratio,β_c=ω_cCR. The applications of superconductive fault current limiter (SCFCL) of MgB₂+MgO materials were discussed also by the HE properties.