We introduce carbon onto boron sites in ${\mathrm{MgB}}_{2}.$ The resulting changes in crystal lattice constants and superconducting transition temperature ${T}_{c}$ are characterized by x-ray-diffraction, magnetic susceptibility, and electrical resistance measurements. The consequence of approximately 10% carbon doping of boron sites is a 1% contraction of the intraplane lattice dimension (with no appreciable change in the interplane dimension) and a lowering of ${T}_{c}$ by approximately 7 K. The relative contributions to the shift in ${T}_{c}$ from lattice contraction and charge transfer are evaluated.

Effects of Carbon-Doping on Superconductivity in Magnesium Diboride

The effect of nanometer sized NiFe2O4 (∼15 nm) addition in Bi1.6Pb0.4Sr2Ca2Cu3O10 (Bi2223) superconductor was studied. The (Bi1.6Pb0.4)Sr2Ca2Cu3O10 superconductor was prepared through the co-precipitation method and 0.01 wt% to 0.05 wt% of nano NiFe2O4 was added. The critical temperature (T
 c), critical current density (J
 c), phase formation and microstructure were investigated. All samples showed a major Bi2223 phase with (Bi,Pb)2Sr2CaCu2O8 (Bi2212) as the minor phase. The sample with 0.01 wt% nano NiFe2O4 showed the highest T
 c and J
 c: of more than 3 orders of magnitude higher than the non-added sample at 77 K. A further nano NiFe2O4 addition (>0.01 wt%) leads to degradation of T
 c and J
 c. These results indicate that addition of an optimum amount of NiFe2O4 nanoparticles can effectively enhance the transport critical current density in this system.

Enhanced Electrical Transport Properties of Nano NiFe2O4-added (Bi1.6Pb0.4)Sr2Ca2Cu3O10 Superconductor

Undoped and carbon-doped magnesium diboride (MgB2) samples were synthesized using two sets of mixtures prepared from the precursors, amorphous nanoboron, and as-received amorphous carbon-doped nanoboron. The microscopic defect structures of carbon-doped MgB2 samples were systematically investigated using X-ray powder diffraction, Raman and electron paramagnetic resonance spectroscopy. Mg vacancies and C-related dangling-bond active centers could be distinguished, and sp3-hybridized carbon radicals were detected. A strong reduction in the critical temperature Tc was observed due to defects and crystal distortion. The symmetry effect of the latter is also reflected on the vibrational modes in the Raman spectra.

/pdf/electron-paramagnetic-resonance-and-raman-spectroscopy-1r9ibtjskm.pdf

Electron paramagnetic resonance and Raman spectroscopy studies on carbon-doped MgB2 superconductor nanomaterials

Dc magnetization studies were carried out on a polycrystalline MgB2 superconductor, which was synthesized at 750°C in Fe tube encapsulation under a vacuum of 10−5Torr. Zero resistances at H=0 and 8T are obtained at 38 and 22K, respectively. The material possesses a sharp diamagnetic transition in the zero-field-cooled branch, whereas the field-cooled branch exhibits the paramagnetic-Meissner-effect-like phenomenon below Tc, indicating strong pinning in this compound. The critical current density at 20K and 2T, estimated by the Bean model is higher than 105A∕cm2. Due to the strong pinning, we observed the presence of flux avalanches below H=2T at temperatures below 20K. The magnetization M(H) grows (as usual) slowly with H and falls sharply to near zero moment value, and further grows again in a common way. The flux avalanches were seen quite symmetric in both increasing∕decreasing the field in all four quadrants of the M(H) loops. The dynamics behavior of sinusoidal-like symmetric reproducible flux avalan...

Avalanche of flux jumps in polycrystalline MgB2 superconductor

Iron-sheathed MgB2 wires doped with 0, 1.3 and 2.52 wt% carbon protected nickel superparamagnetic nanoparticles (the average diameter of the particles is 20 nm) and sintered at 650 °C were prepared. X-ray diffraction patterns and magnetization measurements showed that neither substitution of C for B nor substitution of Ni for Mg occurred during the synthesis process. Scanning electron microscopy imaging of the doped sample revealed a homogeneous distribution of nickel particles within the MgB2 matrix. Transport (magnetoresistivity R(T,B) and critical current density Jc(B) in the temperature range 1.5–40 K) and magnetic measurements (magnetic hysteresis loops at temperatures below and above the superconducting transition temperature) were performed on Fe-sheathed wires and the superconducting cores of these wires. A small enhancement of the irreversibility field Birr(t = Tirr(B)/Tirr(0)) of the doped wires was observed in the low field range. Significant enhancement of Jc(B), especially at low temperature (5 K), was observed: at 5 K and 10 T, for both doped wires, Jc is 2.5 times larger than that for the undoped wire.

https://iopscience.iop.org/article/10.1088/0953-2048/25/9/095018/pdf

Enhancement of the critical current density in MgB2 wires doped with Ni nanoparticles

Polycrystalline MgB2 samples, with added 0, 2, 4 and 6% nano-Co3O4, synthesized by vacuum (10−5 Torr) annealing at 750 °C for two and a half hours each, are found to be nearly single phase with the presence of only a small quantity of Mg/MgO in the pristine sample in addition to the Co2O3 in the doped compounds. All the samples exhibited clear and sharp diamagnetic transitions at around 38 K, in zero-field-cooled (ZFC) magnetic susceptibility measurements with a sizeable signal. The field-cooled (FC) measurements, though having sharp transitions, showed a very small signal, indicating a high level of pinning centres in these samples. Further, some of the doped samples exhibited the paramagnetic Meissner effect (PME) in an applied field of 5 Oe. The critical current density (Jc), estimated by invoking Bean's model for the pristine compound, increases by nearly an order of magnitude for 2% and 4% nano-Co3O4 doping and then decreases sharply for the 6% sample at nearly all studied temperatures and applied fields. Further, the increased Jc (~108 A cm−2) is coupled with fluxoid jumps (T≤20 K and H≤1 T). Fluxoid jumps are not seen in the relatively low Jc pristine or 6% sample. This means that the fluxoid jumps are intrinsic only to the high-Jc samples.

Fluxoid jump coupled high critical current density of nano-Co3O4 doped MgB2

Nano particle of Fe3O4 (nFe3O4) up to 6 at% were doped in the superconducting MgB2 samples. Despite the strong ferromagnetic nature of Fe3O4, both the ac susceptibility and the resistivity measurements show that up to 4 at% of Fe3O4, T
 c
 =38 K is not changed, whereas for 6% T
 c
 decreases by 6 K. This indicates that a low concentration of Fe does not substitute either the Mg or B sites and probably occupies the intergrain spaces. For 0.5% doped Fe3O4, an increase in J
 c
 with respect to the pure MgB2 samples is observed in the lower field and temperature regions (H 2 T, J
 c
 of the doped samples is always less than that of MgB2, and the activation is dominant in comparison with the effective pinning potential provided by the doping. Flux jumps are observed in lower T and H regions for the samples doped up to 1% nFe3O4 only. Magnetization plots of higher Fe content samples exhibited clear paramagnetic background. Mossbauer measurements for the higher (4, 6 at%) nFe3O4 doped MgB2 samples show that at RT, the hyperfine field for both samples is ∼100 kOe and ∼120 kOe at 90 K. This means that the nFe3O4 particles decompose and form possibly an intermetallic Fe-B phase in the matrix.

Nano Fe3O4 Induced Fluxoid Jumps and Low Field Enhanced Critical Current Density in MgB2 Superconductor

We report optimization of the synthesis parameters viz. heating temperature (TH), and hold time (thold) for vacuum-annealed (10-5 Torr) and LN2 (liquid nitrogen) quenched MgB2 compound. These are single-phase compounds crystallizing in the hexagonal structure (space group P6/mmm) at room temperature. Our XRD results indicated that for phase-pure MgB2, the TH for 10-5 Torr annealed and LN2-quenched samples is 750°C. The right stoichiometry i.e., MgB2 of the compound corresponding to 10-5 Torr and TH of 750°C is found for the hold time (thold) of 2.30 hours. With varying thold from 1–4 hours at fixed TH (750°C) and vacuum (10-5 Torr), the c-lattice parameter decreases first and later increases with thold (hours) before a near saturation, while the a-lattice parameter first increases and later decreases beyond a thold of 2.30 hours. The c/a ratio versus thold plot showed an inverted bell-shaped curve, touching the lowest value of 1.141, which is the reported value for perfect stoichiometry of MgB2. The optimized stoichimetric MgB2 compound exhibited superconductivity at 39.2 K with a transition width of 0.6 K. In conclusion, the synthesis parameters for phase pure stoichimetric vacuum-annealed MgB2 compound are optimized and are compared with widely-reported Ta tube encapsulated samples.

PHASE FORMATION AND SUPERCONDUCTIVITY OF Fe-TUBE ENCAPSULATED AND VACUUM-ANNEALED MgB2

Comparisons for the resistivity behaviors of different encapsulated MgB2 samples

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Papers

Fluxoid jump coupled high critical current density of nano-Co3O4 doped MgB2

Nano Fe3O4 Induced Fluxoid Jumps and Low Field Enhanced Critical Current Density in MgB2 Superconductor

PHASE FORMATION AND SUPERCONDUCTIVITY OF Fe-TUBE ENCAPSULATED AND VACUUM-ANNEALED MgB2

Comparisons for the resistivity behaviors of different encapsulated MgB2 samples