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Synthesis and Electronic Transport Characteristics of Superconducting MgB2+MgO Composite Near Continuous Percolation

Author: ZhangYiZuo
Tutor: ZhouShiPing
School: Shanghai University
Course: Radio Physics
Keywords: MgB2 Superconducting composite Electric transport Differential resistance Hysteresis effect
CLC: O511
Type: PhD thesis
Year: 2007
Downloads: 83
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Abstract


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 （Ⅰ-Ⅴ） 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 Tc. 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:ρSPT=ρ0+αnTn+αe4T4+αmT6.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 （Tc）, 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 Tc, （?） 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 Tc. 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 （Ⅰ-Ⅴ） curves of superconducting MgB2+MgO composites were measured and hysteresis effects inⅠ-Ⅴcurves were observed. The hysteresis goes anticlockwise and is enlarged with the current sweep speed,△Ⅰ/△t, 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,βc=ωcCR. The applications of superconductive fault current limiter （SCFCL） of MgB2+MgO materials were discussed also by the HE properties.

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CLC: > Mathematical sciences and chemical > Physics > Low Temperature Physics > Superconductivity
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