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Register a new userCorrelated vortex pinning in Si-nanoparticle doped MgB2
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The magnetoresistivity and critical current density of well characterized Si-nanoparticle doped and undoped Cu-sheathed MgB$_{2}$ tapes have been measured at temperatures $Tgeq 28$ K in magnetic fields $Bleq 0.9$ T. The irreversibility line $B_{irr}(T)$ for doped tape shows a stepwise variation with a kink around 0.3 T. Such $B_{irr}(T)$ variation is typical for high-temperature superconductors with columnar defects (a kink occurs near the matching field $% B_{phi}$) and is very different from a smooth $B_{irr}(T)$ variation in undoped MgB$_{2}$ samples. The microstructure studies of nanoparticle doped MgB$_{2}$ samples show uniformly dispersed nanoprecipitates, which probably act as a correlated disorder. The observed difference between the field variations of the critical current density and pinning force density of the doped and undoped tape supports the above findings.
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We discuss pinning properties of MgB2 thin films grown by pulsed-laser deposition (PLD) and by electron-beam (EB) evaporation. Two mechanisms are identified that contribute most effectively to the pinning of vortices in randomly oriented films. The EB process produces low defected crystallites with small grain size providing enhanced pinning at grain boundaries without degradation of Tc. The PLD process produces films with structural disorder on a scale less that the coherence length that further improves pinning, but also depresses Tc.
0.5 to 5.0 wt.% Dy2O3 was in-situ reacted with Mg + B to form pinned MgB2. While Tc remained largely unchanged, Jc was strongly enhanced. The best sample (only 0.5 wt.% Dy2O3) had a Jc of 6.5 x 10^5 A/cm^2 at 6K, 1T and 3.5 x 10^5 A/cm^2 at 20K, 1T, around a factor of 4 higher compared to the pure sample, and equivalent to hot-pressed or nano-Si added MgB2 at below 1T. Even distributions of nano-scale precipitates of DyB4 and MgO were observed within the grains. The room temperature resistivity decreased with Dy2O3 indicative of improved grain connectivity.
The recently-discovered MgB2 super-conductor has a transition temperature Tc approaching 40K, placing it intermediate between the families of low and high temperature super-conductors (LTS and HTS). In practical applications, super-conductors are permeated by quantised magnetic flux vortices, and when a current flows there is dissipation unless the vortices are pinned in some way, and so inhibited from moving under the influence of the Lorentz force. This vortex motion sets the limiting critical current density Jc in the super-conductor. Vortex behaviour has proved to be more complicated in the HTS than in LTS materials. While this has stimulated extensive theoretical and experimental research, it has impeded applications. Clearly it is important to explore vortex behaviour in MgB2; here we report on Jc, and also on the creep rate S, which is a measure of how fast the persistent currents decay. Our results show that naturally-occurring grain boundaries are highly transparent to supercurrent, and suggest that the steep decline in Jc with increasing magnetic field H reflects a weakening of the vortex pinning energy, possibly because this compound forms naturally with a high degree of crystalline perfection.
MoSi2 doped MgB2 tapes with different doping levels were prepared through the in-situ powder-in-tube method using Fe as the sheath material. Effect of MoSi2 doping on the MgB2/Fe tapes was investigated. It is found that the highest JC value was achieved in the 2.5 at.% doped samples, more than a factor of 4 higher compared to the undoped tapes at 4.2 K, 10 T, then further increasing the doping ratio caused a reduction of JC. Moreover, all doped tapes exhibited improved magnetic field dependence of Jc. The enhancement of JC-B properties in MoSi2 doped MgB2 tapes is attributed to good grain linkage and the introduction of effective flux pining centers with the doping.
We report the synthesis and variation of superconductivity parameters such as transition temperature Tc, upper critical field Hc, critical current density Jc, irreversibility field Hirr and flux pinning parameter (Fp) for the MgB2-xCx system with nano-Carbon doping up to x=0.20. Carbon substitutes successfully on boron site and results in significant enhancement of Hirr and Jc(H). Resistivity measurements reveal a continuous decrease in Tc under zero applied field, while the same improves remarkably at higher fields with an increase in nano-C content for MgB2-xCx system. The irreversibility field value (Hirr) is 7.6 & 6.6 Tesla at 5 and 10K respectively for the pristine sample, which is enhanced to 13.4 and 11.0 Tesla for x = .08 sample at same temperatures. Compared to undoped sample, critical current density (Jc) for the x=0.08 nano-Carbon doped sample is increased by a factor of 24 at 10K at 6 Tesla field.
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