Dr. Josip Horvat

Effect of processing temperature on high field critical current density and upper critical field of nanocarbon doped MgB2

W. K. Yeoh — Thu, 29 Jan 2009 21:09:33 PST

Correlation of upper critical field (Hc2) and critical current density (Jc) with processing temperature of nano-C doped MgB2 has been studied in comparison to SiC and pure MgB2. SiC and C doped MgB2 exhibit opposite trends in the dependence of Jc and Hc2 on sintering temperature. This is explained by different reactivities of carbon available upon creation of MgB2 for the two types of doping. Nanocarbon doped MgB2 requires sintering temperatures in excess of 900 �C to obtain high boron substitution for carbon, enhancing the vortex pinning and impurity scattering of charge carriers. However, carbon substitution in nano-SiC doped MgB2 occurs at less than 650 �C, allowing lower sintering temperature and high degree of carbon substitution. Both pure and SiC doped MgB2 benefit from low sintering temperature, which results in more grain boundary defects. Substantial carbon substitution can compensate for the disadvantage of sintering at high temperature of nano-C doped MgB2, giving the best Jc of 4.8x103 A/cm2 at 4.5 K and 12 T. This is comparable to the low-temperature sintered nano-SiC doped MgB2 wires.

Intrinsic nanostructural domains: Possible origin of weaklinkless superconductivity in the quenched reaction product of Mg and amorphous B

S. Li — Wed, 28 May 2008 16:04:41 PDT

Smooth modulation structure of Mg-B alloy in the quenched reaction product of Mg and amorphous B was studied. It indicates that the MgB2 formed possibly in spinodal decomposition, thus resulting in MgB2 nanodomains. It was found that the nanodomains with small angle boundaries of atomic-scale width were distributed within the subgrains that constitute the clusters in MgB2 grains. This nanostructural characteristic may be intrinsic in the quenched reaction product of Mg and amorphous B. It makes the nanodomain boundaries not act as barriers to the current percolation path, thus exhibiting no weak-link problem in the MgB2.

Order-disorder transition in Bi2.1Sr1.9CaCu2O8 + delta single crystals doped with Fe and Pb

K. K. Uprety — Wed, 28 May 2008 16:04:39 PDT

The magnetic field Hdis(T) where an order-disorder transition of the vortex lattice in high-Tc superconductors occurs, is investigated by measurements of the magnetization M(H) in Bi2.1Sr1.9CaCu2O8 + delta (Bi2212) single crystals doped with iron and lead. Comparative studies are made of the temperature dependences of the field Hpeak(T), where the second peak occurs in |M(H)|, and the fields Hmin(T), and Hinfl(T) where a minimum and an inflection point occur at the low-field side of this peak. It is proposed that Hdis(T) lies close to Hinfl. In Bi2.1Sr1.9Ca1.0(Cu1-yFey)2O8 + delta single crystals with Fe concentration y = 0, 0.005, 0.016, and 0.022, a pronounced peak in the derivative |dM/dH| is observed, whose position Hinfl(T) is independent of temperature T. We relate this peak to the field Hdis(T), which separates a weakly elastically disordered vortex lattice from a plastically disordered vortex solid. In heavily Pb-doped single Bi2212 crystals, Hinfl(T) decreases with increasing T. For the same crystals, a minimum in the normalized relaxation rate S(H) is observed at Hinfl, indicating two different flux-creep mechanisms above and below that field and two different solid vortex phases. It is argued that the negative slope of Hdis(T) in heavily-Pb-doped Bi2212 crystals is related to the enhanced c axis conductivity caused by the Pb sitting between the CuO2 layers and causing three-dimensional vortex lines, while in Fe-doped Bi2212 crystals the Fe ions sit on the CuO2 planes and thus do not enhance the coupling between pancake vortices.

Enhancement of the critical current density and flux pinning of MgB2 superconductor by nanoparticle SiC doping

S. X. Dou — Wed, 28 May 2008 16:04:36 PDT

Doping of MgB2 by nano-SiC and its potential for the improvement of flux pinning were studied for MgB2-x(SiC)x/2 with x = 0, 0.2, and 0.3 and for 10 wt % nano-SiC-doped MgB2 samples. Cosubstitution of B by Si and C counterbalanced the effects of single-element doping, decreasing Tc by only 1.5 K, introducing intragrain pinning centers effective at high fields and temperatures, and significantly enhancing Jc and Hirr. Compared to the undoped sample, Jc for the 10 wt % doped sample increased by a factor of 32 at 5 K and 8 T, 42 at 20 K and 5 T, and 14 at 30 K and 2 T. At 20 K and 2 T, the Jc for the doped sample was 2.4�105 A/cm2, which is comparable to Jc values for the best Ag/Bi-2223 tapes. At 20 K and 4 T, Jc was twice as high as for the best MgB2 thin films and an order of magnitude higher than for the best Fe/MgB2 tapes. The magnetic Jc is consistent with the transport Jc which remains at 20 000 A/cm2 even at 10 T and 5 K for the doped sample, an order of magnitude higher than the undoped one. Because of such high performance, it is anticipated that the future MgB2 conductors will be made using a formula of MgBxSiyCz instead of pure MgB2.

Effect of carbon nanotube doping on critical current density of MgB2 superconductor

S. X. Dou — Wed, 28 May 2008 16:04:33 PDT

The effect of doping MgB2 with carbon nanotubes on transition temperature, lattice parameters, critical current density and flux pinning was studied for MgB2-xCx with x = 0, 0.05, 0.1, 0.2, and 0.3. The carbon substitution for B was found to enhance Jc in magnetic fields but depress Tc. The depression of Tc, which is caused by the carbon substitution for B, increases with an increasing doping level, sintering temperature, and duration. By controlling the extent of the substitution and addition of carbon nanotubes we can achieve the optimal improvement on critical current density and flux pinning in magnetic fields while maintaining the minimum reduction in Tc. Under these conditions, Jc was enhanced by two orders of magnitude at 8 T and 5 K and 7 T and 10 K. Jc was more than 10 000 A/cm2 at 20 K and 4 T and 5 K and 8.5 T, respectively.

Superconducting screening on different length scales in high-quality bulk MgB2 superconductor

J. Horvat — Wed, 28 May 2008 16:04:31 PDT

High-quality bulk MgB2 exhibits a structure of voids and agglomeration of crystals on different length-scales. Because of this, the superconducting currents percolate between the voids in the ensuing structure. Magnetic measurements reveal that the superconducting currents circulate on at least three different length-scales, of ~1 micrometre, ~10 micrometre and whole of the sample (~millimetre). Each of these screenings contributes to the measured irreversible magnetic moment (m). The analysis of the field dependence of m for samples of subsequently decreasing size showed that the critical current obtained using the simple critical state model is erroneous. This leads to the artefact of the sample size-dependent critical current density Jc and irreversibility field. Our data analysis enables the separation of the contribution of each of the screening currents to m. The field dependence of each of the currents follows a stretched exponential form. The currents flowing around whole of the sample give a dominant contribution to m in the intermediate fields (1T < H < 4T at 20K) and they can be used to obtain the value of Jc from critical state model, which corresponds to the transport Jc. The stretched exponential field dependence of these currents is similar to the one obtained for high-temperature superconductors, and it seems to be connected with the percolation of the currents.

Effect of uranium doping and thermal neutron irradiation on the flux-pinning of silver-clad Bi-Sr-Ca-Cu-O tapes

D. Marinaro — Wed, 28 May 2008 16:04:28 PDT

Ag/Bi-2223 tapes doped with small quantities of 235UO 4 powder were prepared by the powder-in-tube process and irradiated in a thermal neutron environment. Substantial improvements in critical current density (Jc)-applied field (H) performance and anisotropy have been previously reported. However, the radioactivity of the silver sheath is a limiting factor for commercial and industrial applications of this technique. Here we report the performance of the technique using various doping levels (from 0.15 to 2 percent by weight (wt.%) UO4) and thermal neutron fluences (Φn), in order to further reduce the silver radioactivity. Optimum fluence levels are identified and an optimum combination in terms of Jc - H performance is discussed. At a doping level of 2% 235 UO4, a normalised Jc enhancement of 250 times is observed for an 0.8 T field aligned along the c-axis, and 25 times at 3 T along the ab-plane at 77 K, compared to pre-irradiation values. At 0.6%, these figures are 500 and 10 times, respectively. The effects of the uranium doping and thermal neutron irradiation on the flux pinning strengths are also directly probed using dynamic relaxation techniques. The results show an increase in the effective pinning potential after doping and irradiation.

Effect of grain size and doping level of SiC on the superconductivity and critical current density in MgB/sub 2/ superconductor

S. Soltanian — Wed, 28 May 2008 16:04:25 PDT

SiC doped MgB/sub 2/ polycrystalline samples were fabricated by in-situ reaction using different grain sizes (20 nm, 100 nm, and 37 /spl mu/m) of SiC and different doping levels (0, 8, 10, 12, 15 wt%). Phases, microstructures, superconductivity, critical current density and flux pinning have been systematically investigated using XRD, SEM, TEM, and magnetic measurements. Results show that grain sizes of the starting precursors of SiC have a strong effect on the critical current density and its field dependence. The smaller the SiC grains are, the better the J/sub c/ field performance is. Significant enhancement of J/sub c/ and the irreversibility field H/sub irr/ were revealed for all the SiC doped MgB/sub 2/ with additions up to 15 wt%. A J/sub c/ as high as 20,000 A/cm/sup 2/ in 8 Tesla at 5 K was achieved for the sample doped with 10 wt% SiC with a grain size of 20 nm. Results indicate that the nano-inclusions and substitution inside MgB/sub 2/ are responsible for the enhancement of flux pinning.

Improvement of critical current in MgB2 superconducting wires by a ferromagnetic sheath

J. Horvat — Wed, 28 May 2008 16:04:22 PDT

Transport critical current (Ic) was measured for Fe-sheathed MgB2 round wires. A critical current density of 5.3 x 10^4 A/cm^2 was obtained at 32K. Strong magnetic shielding by the iron sheath was observed, resulting in a decrease in Ic by only 15% in a field of 0.6T at 32K. In addition to shielding, interaction between the iron sheath and the superconductor resulted in a constant Ic between 0.2 and 0.6T. This was well beyond the maximum field for effective shielding of 0.2T. This effect can be used to substantially improve the field performance of MgB2/Fe wires at fields at least 3 times higher than the range allowed by mere magnetic shielding by the iron sheath. The dependence of Ic on the angle between field and current showed that the transport current does not flow straight across the wire, but meanders between the grains.

A comparison of Ag and Ag-alloy sheathed Bi-2223 tapes

H. K. Liu — Wed, 28 May 2008 16:04:19 PDT

Ag and Ag-alloy sheathed Bi-2223 tapes were fabricated by a powder-in-tube technique with different configurations of the precursor and restack sheath materials: Ag, AgAu7 wt%, AgSb0.6 wt%, AgMg0.2 wt%. Analysis of the I/sub c/ and volume fractions of the Bi-2223, Bi-2212, Bi-2201 and Bi-3221 phases indicated that volume fractions of Bi-2223>90%, Bi-2212/spl sim/5%, Bi-2201/spl sim/0% and Bi-3221<2%, normally result in tapes with the highest I/sub c/. The mechanical properties of the tapes revealed consistent results. Generally, the harder the sheath material, the higher tolerance to the bending strain and higher the tensile strength of the tape. The sequence of the alloys' hardness from highest to lowest was AgMg0.2 wt%, AgSb0.6 wt%, AgAu7 wt% or Ag.