Year

2012

Degree Name

Doctor of Philosophy

Department

Institute for Superconducting and Electronic Materials

Abstract

The discovery of superconductivity in MgB2 has attracted enormous interest among the superconducting materials community. MgB2 has emerged as a potential candidate for many applications, replacing conventional low critical temperature (Tc) superconductors, not only due to its high Tc of 39 K, but also to its intrinsically “weaklink” free grain boundaries, its rich multiple-band structure, and its low fabrication cost. Its rapid drop in critical current density (Jc) under applied magnetic field, and low upper critical field (Hc2), however, exclude it from many industrial applications where a high Jc under high magnetic field is required.

Chemical doping can be identified as the simplest and cheapest way to improve the electronic structures of superconductors and their superconducting properties. Various types of carbon sources have been used as dopants for MgB2 so far, and Jc and Hc2 have been significantly enhanced due to charge carrier scattering, thanks to the two-band nature of MgB2. The carbon doping, however, comes with its own drawback of reducing Tc, which limits the application temperature of MgB2. Furthermore, many carbon dopants are detrimental to the performance of the low field Jc.

Therefore, the objective of the this work is to improve the critical current density of MgB2 superconductor through chemical doping using graphene (G) and graphene oxide (GO) as the carbon sources, while addressing the common disadvantages of carbon doping. The work in this thesis is focussed on the processing and characterization of G-and GO- doped MgB2 with the main objective being the enhancement of the critical current density at both low and high magnetic field. Therefore, the effects of G- and GO- doping in MgB2 on the electromagnetic properties, as well as on the microstructural changes were studied systematically.

Graphene is becoming recognized as a novel dopant for MgB2 due to its specific way of improving Jc, as it improves the intergrain connectivity, and at the same time, leaves micro-strains in the MgB2 matrix, which are beneficial for improving the flux pinning. The effects of graphene doping on the superconducting properties of MgB2 were studied using bulk samples made by the diffusion method. MgB2 was chemically doped with graphene according to the formula MgB2-xCx, where x = 0, 1, 3, and 5 mol % graphene. It has been found that a small amount of graphene can significantly improve the Jc and the intergrain connectivity, with only a small depression in the Tc.

The optimally doped sample (x = 1%), showed a Jc that was 43 times higher compared to the undoped sample at 5 K, 8 T, with a drop in Tc of slightly less than 1 K, together with an enhancement in the zero-field performance of Jc. This is a significant improvement as most carbon sources adversely affect the Jc performance at low field. Low resistivity and comparatively improved critical fields were observed for the optimally doped sample. The improvement in grain-to-grain connectivity has been identified as one of the major factors responsible for such enhancement of the superconducting properties, especially for the low field performance of Jc. Characterization of the samples revealed improved intergrain connectivity and induced strain due to doping, while a noticeable increase in the flux-flow activation energy in graphene doped MgB2 samples was observed at low fields. Furthermore, it was found that spatial fluctuation in the transition temperature (δTc pinning) is the flux pinning mechanism in graphene doped MgB2, although this is uncommon for carbon doped MgB2 samples.

Inspired by the improvements gained due to graphene doping, the effects of the chemical synthesis on the quality of the end product were investigated, along with the effects of doping the different end products into MgB2. There have been different procedures reported for the reduction of graphene oxide (GO), including chemical reduction, thermal annealing, and microwave irradiation. The presence of oxygen is inevitable in graphene synthesis. The carbon to oxygen ratio of the end product, however, totally depends on the method of preparation and the degree of the reduction process. Therefore, the effectiveness of a two-stage reduction process on reducing GO and the effects of these carbon additives on improving the superconducting properties of MgB2 were systematically studied.

Reduced graphene oxide (rGO) and highly reduced chemically converted graphene (rCCG) samples were prepared under different processing conditions and were doped into MgB2 by a diffusion process at 800 ºC for 10 hours. Both graphene types showed positive effects on the superconducting properties of MgB2, however, addition of the rCCG type showed better improvement compared to the rGO addition. This is owing to the effective reduction that occurred during its synthesis process, which is evidenced by the X-ray photoelectron spectroscopy (XPS) analysis. rCCG addition had a considerable effect on the intergrain connectivity of MgB2 samples, with positive consequences for the superconducting properties. Doping of MgB2 with 1 mol% rCCG resulted in nearly 32% improvement of Jc at 5 K, 6 T over that of the rGO sample.

Although there is a improvement in the performance of Jc at high magnetic field in the graphene doped samples, it is not that pronounced compared to the performances of the other reported carbon sources such as SiC, carbon nanotubes (CNTs) or malic acid. Those dopants, however, always come with their own disadvantages in terms of degradation of the Jc in zero field.

Although SiC remains one of the best dopants, similar to any other carbon source, this too displays degradation of the critical current desnity (Jc) in low field. On the other hand, graphene has been recognized as a new dopant for MgB2 which can improve the zero field Jc through improving the intergrain connectivity. Therefore, both graphene and SiC were used as co-dopants to investigate possible improvements in the superconducting performance of MgB2. The superconducting properties characterized by Jc, intergrain connectivity, and critical fields were significantly improved due to co-doping.

The sample co-doped with graphene and 5 wt. % SiC showed improvements of 15% and 40% of Jc at 20 K and zero field, compared to the 5 wt. % SiC doped and undoped samples respectively. Low resistivity and an apparent improvement in intergrain connectivity characterized the sample with both 5 wt. % SiC and graphene. This finding indicates that co-doping of MgB2 in the way described can result in complementary beneficial effects on the superconducting properties.

Although significant property enhancement can be attained through graphene doping, the high cost involved in its synthesis renders the use of graphene impractical in largescale applications. On top of that, restacking of graphene sheets during the synthesis process results in an inevitable reduction of the MgB2 performance in applications. On the other hand, GO, which is typically monolayer in nature, can be used as a dopant and sintering process, which prevents the aggregation of graphene sheets in the matrix. In addition to that, the high possibility of attaining homogeneous dispersions of GO in organic solvent would result in good dispersion of the dopant in the matrix, which would improve the effectiveness of the dopant even more.

Based on above considerations, the effects of graphene oxide (GO) doping on the superconducting properties of MgB2 were studied using bulk samples made by the diffusion method. Homogeneous dispersions of GO in tetrahydrofuran (THF) were obtained through a novel synthesis method. MgB2 was then chemically doped according to the formula MgB2-xCx, where x = 0, 1, 2, 3, and 4 at% GO in THF. It was found that GO doping significantly improves the critical current density, both at low and at high magnetic fields, which distinguishes GO from all the other elements doped into MgB2 so far. This type of doping results in significant improvements in grain-to-grain connectivity, and to the irreversibility and upper critical fields of MgB2, with only a small depression in the superconducting transition temperature. Furthermore, a noticeable increase in the flux-flow activation energy in graphene doped MgB2 samples was also observed. Microstructural investigations revealed the improved intergrain connectivity and induced strain due to doping.

The sample with 2 at% GO showed a Jc that was 27 times higher compared to the undoped sample at 5 K, 8 T, with a slight drop in Tc of just 1.2 K. At the same time, this doping level resulted in a 50% enhancement of the Jc performance at 20 K at zero field, over that of the undoped sample. This improved Jc performance at both zero field and high field can be attributed to the improved intergrain connectivity and increased Hc2, respectively.

Overall, the work presented in this thesis is mainly based on the processing and characterization of graphene, graphene oxide, and bulk MgB2 samples. These results are important for future MgB2 fabrication.

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