A study of the effects of the doping of yttrium barium copper oxide with graphene oxide and reduced graphene oxide.

Abstract

High temperature superconductors (HTSs) have a wide range of both small scale and large scale electronic and electrical applications. Therefore, research is constantly being done in order to improve the properties of existing superconductors and to find new superconductors with superior properties. Important characteristic properties, which are being investigated, range from the critical transition temperature (TC), critical current density (JC) to microhardness i.e. electrical to mechanical. An important system, within bulk polycrystalline HTSs material, is a network of weak links which exists at grain boundaries. This network of weak links in bulk superconductors, influences characteristic properties such as reducing the critical current density, increases the normal state resistivity and structurally weakens the bulk material. In present work, bulk high temperature superconductor Yttrium Barium Copper Oxide, the Y123 phase (Y1Ba2Cu3O7-δ), was doped with various concentrations of graphene oxide (GO) and reduced graphene oxide (rGO). Resistance vs. Temperature measurements were carried out in zero field and field cooling conditions to determine TC and the effect that the doping has on the weak links at grain boundaries. The weak links has been seen to improve with both rGO and GO doping. TC was seen to increase with an increase in graphene oxide and reduced graphene oxide doping in the range of 0.1 to 1 %wt. This was attributed to the increase in the nonstoichiometric oxygen content due to doping. This increase in non-stoichiometric oxygen content was measured by iodometric titration and X-ray diffraction. In certain GO doped samples TC increased above 93 K, the maximum TC of YBCO, which cannot be attributed to an increase in oxygen content. The increase in TC beyond 93 K led to investigations into possible GO doping pressure and impurity effects. The increase in TC was confirmed with magnetic measurements. It was discovered that the formation of manganese particles from impurities in the GO dopant led to an increase in TC = 101 K. The hypothesized mechanism responsible for this improvement in TC is the increase in the CuO2 planar buckling angle. Increase in the buckling angle results in the flattening of copper oxide planes which can be observed by the increase in surface area of the planes. These flatter planes have a more 2D structure thus resulting in an enhancement in TC. Micro-hardness of GO and rGO doped samples increased with increase in doping concentrations. This increase in micro-hardness was attributed to the reduction in motion of defects, which is attributed to a decrease in grain size and filling of voids and pores at grain boundaries.