Carbon solubility and superconductivity in MgB2

Abstract

Successful replacement of B by C in the series MgB 2-x C x for values of x upto 0.3 are reported. Resistivity and ac susceptibility measurements have been carried out in the samples. Solubility of carbon, inferred from the observed change in the lattice parameter with carbon content indicates that carbon substitutes upto x=0.30 into the MgB 2 lattice. The superconducting transition temperature, T C measured both by zero resistivity and the onset of the diamagnetic signal shows a systematic decrease with increase in carbon content upto x=0.30, beyond which the volume fraction decreases drastically. The temperature dependence of resistivity in the normal state fits to the Bloch-Gruneisen formula for all the carbon compositions studied. The Debye temperatures, θ D , extracted from the fit is seen to decrease with carbon content from 900K to 525K, whereas the electron-phonon interaction parameter, λ, obtained from the McMillan equation using the measured T C and θ D , is seen to increase monotonically from 0.8 in MgB 2 to 0.9 in the x=0.50 sample. The ratio of the resistivities between 300K and 40K versus T C is seen to follow the Testardi correlation for the C substituted samples. The decrease in T C is argued to mainly arise due to large decrease in θ D with C concentration and a decrease in the hole density of states at N(E F ).

Enhanced critical parameters of nanocarbon doped MgB2 superconductor

“…The exact carbon content in Mg͑B 1−y C y ͒ 2 is evaluated indirectly using the equation y = 7.5ϫ⌬c / a, where ⌬c / a is the change in c / a value as compared to the pure sample and y is the exact content by atomic wt % of nanocarbon substituted at the boron site. [17][18][19] The exact value calculated in this way is found to be quite less than expected. The net maximum substitution level is just 6% by atomic The net carbon, which exactly goes at the boron site, creates disorder in the sigma band and causes intrinsic pinning to enhance the critical parameters.…”

“…The above trend of change in RRR values of our samples is in confirmation with the literature. 17,18 The critical field is determined for all the samples using the criterion that H c2 = H at which =90% N and N is the normal resistivity or resistivity at about 40 K. The transition temperature with this criterion of =90% N instead of =0 are also determined for all the samples and are tabulated in Table III. The value of applied field in a column directly corresponds to the H c2 value at the temperature given below in that column for corresponding samples.…”

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

“…Resistivity of carbon doped MgB 2 was reported to increase with the amount of carbon added to the sample. [30][31][32] This high resistivity may be due to the presence of unreacted carbon between the grain boundaries. The drop of the normal state resistivity with sintering temperature in the present work can be ascribed to an enhancement of A F , 19 where the A F value increases from 0.15 to 0.22 as the sintering temperature increases from 700 to 900°C ͑Table I͒.…”

Retention of two-band superconductivity in highly carbon-dopedMgB2

“…Several groups 11,12,13,14,15,16,17,18 have investigated the doping of MgB 2 with C with conflicting results concerning the C solubility limit, the composition dependence of T c and the possible presence of phase separation. Much of the discrepancy is possibly a synthesis problem due to the slow kinetics of C incorporation.…”