PhD defense Trapping magnetic flux-lines leads to better superconducting materials

When electrical currents are moved from the energy producers to the customers, some percentages of the energy is lost to thermal dissipation due to the electrical resistance of the electrical transmission lines. On a global scale, it is estimated that 2-4% of the electricity produced is wasted as transmission losses. In countries with weak or old power infrastructures the losses can be even higher.

Superconductors are materials that demonstrate the remarkable features of zero electrical resistivity and the partial (or complete) expulsion of magnetic field from the sample interior. The employment of such materials could drastically decrease the transmission losses of power lines.

Three years ago Yuri Aparecido Opata came to Denmark, through the Brazilian “Government Science Without Borders”-program, to study superconducting materials at DTU Energy. Recently, Yuri defended the results in his PhD thesis “Studies of doping effect on the superconducting properties of DyBa₂Cu₃O_7-δ and its possible manufacture as thin film in view of potential use for power applications.”

In the lecture given at his defence, Yuri used the hydroelectric dam at Itaipu, between Brazil and Paraguay, as an example where superconducting materials could be utilized. The installation has a total production capacity of 14 GW, but around 9% of this electrical power, a total of 1.26 GW, is lost to thermal dissipation in the power grid.

“So, within 45 minutes the energy lost in that system will have a cost of 417.000 Real or around 100.000 euro”, said Yuri, using numbers from a report made by the Brazilian Government in 2012.

Using defects to trap flux-lines

When cooled below a characteristic temperature, superconductor materials turn into the superconducting state, characterized by the absence of electrical resistivity. Another effect manifested by these materials is observed when applying magnetic fields to the materials. Initially the system will initially expel all magnetic fields from the material interior, but with continuous increase of the applied field, lines of magnetic fields, also called flux-lines, start to penetrate the material.

At this point, the application of a large electrical current to the superconductor will then result in the movement of these flux-lines, leading to energy dissipation. So Yuri and other researchers at DTU Energy are incorporating small defects in the material to “trap” the flux-lines and, thereby, stop their movement and also the energy dissipation (up to some extent).

A huge effort to counter this massive loss of energy due to dissipation has been initiated on a worldwide basis, and many investigations in the so called high temperature superconducting materials, based on a family of materials named REBa₂Cu₃O_7-d (where RE is a rare earth material) in thin film samples/tapes, has been performed.

Yuri looked into the material DyBa₂Cu₃O_7-δ, trying to improve its performance by doping it to allow stronger magnetic fields to be applied and larger electrical current to be able to move through the material.

By adding defects in the DyBa₂Cu₃O_7-δ material, Yuri was able to trap the flux-lines and, thus, to increase the inherent value of electrical current that the system could carry without energy dissipation., not a lot but a step in the right direction.

“I am not solving the problem of the world but I am improving the properties of this material, making it better. But in order to make it better you have to add defects (make it worse), which is kind of funny”, says Yuri Aparecido Opata, who has now returned to Brazil after successfully defending his thesis.