Researchers at the Physics Institute of the State University of Campinas (Unicamp) were the first to measure and describe an important characteristic of magnesium diboride (MgB2), identified in January of this year as a superconductor and now with good prospects of becoming the dominant material in this sector over the next few years. Led by Oscar Ferreira de Lima, the team from São Paulo has shown that this material exhibits a relatively low value for a parameter technically defined as anisotropy.
In other words, the physicists have proven that the superconductor properties of the diboride, including its capacity to transmit electric current with zero resistance, without loss of energy in the form of heat, are almost totally uniform, varying only a little according to the spatial direction in which it is directed.
This is to say that the electrons flow in a manner more or less similar in any direction within a crystal of MgB2, both along its vertical axis and its horizontal axis. Actually, there is a difference of close to 70% in the capacity to transmit electrical current between the two axes, the horizontal plane being the more efficient. Among superconductors, a disparity of this order is not considered to be high. “In ceramic superconductors, the anisotropy is normally a lot higher,” says Lima. “In some compounds, the current created in one direction could be up to 200 times larger than in the other direction.”
The measurements are of important value to companies and universities interested in designing products using the new superconductor, as they show that the anisotropy of MgB2 doesn’t jeopardize its potential to conduct high currents. The results of the study were described at the end of June in Physical Review Letters, one of the most prestigious scientific magazines for physics.
Right from the beginning of the year, when the Japanese researcher Jun Akimitsu, of the Aoyama Gakuin University, announced that the almost forgotten magnesium diboride, discovered in 1953, behaves like a superconductor when it is cooled to -234 degrees Celsius, physicists throughout the world have been studying the material in detail. Their interest is due to two reasons. Magnesium diboride, MgB2, is the semi-metallic compound with the highest critical temperature (Tc) – the limit below which a material begins to conduct electric current with zero resistance – that is known, and its production cost is smaller than the two superconductors based on niobium, today the most widely used in applications such as the magnetic resonance tomography apparatus.
“Magnesium diboride is not a revolutionary material”, ponders Lima “but it is possible that it will come to substitute with some advantages the superconductors that are in use today.”Until the entrance on the scene of the diboride, physicists believed that there was not very much more to be learned in terms of superconductors made exclusively with metals. The studies with this type of material had seemed to have reached their limit. Since they had not managed to discover superconducting compounds or alloys with a higher Tc, since the middle of the decade of the 80s, their efforts had returned to ceramic superconductors, non-metal compounds capable of transmitting current with zero resistance at higher temperatures.
The arrival of magnesium diboride changed the scene. Nobody imagined that the hottest new development would be hidden in a compound as simple as MgB2: its boron atoms, in the same manner as carbon in graphite, form hexagonal structures, separated by a layer of magnesium atoms. All of a sudden laboratories began to regard favorably again, with clear sight, the studies done with material made exclusively of metals. As soon as he was aware of the discovery of the superconductivity of the compound based on magnesium and boron, Lima gathered three doctorate students and a laboratory technician and explained his plan of action. If they worked quickly, they could be the first to prove if the MgB2 was isotropic or anisotropic, a doubt which hung in the air at that moment.
Magnesium diboride is sold commercially in powder, but, to obtain crystals of high purity, the Unicamp researchers preferred to produce the compound themselves. They bought pure grains of boron and magnesium, put them in a closed tube and placed them in a furnace at 1,200 degrees Celsius. They synthesized the MgB2 in solid form and ground it to a very fine powder, and filtered it so that only small crystals of between 5 to 20 microns passed (a micron is equivalent to one thousandth part of a millimeter). Finally, the crystalline powder was mechanically spread over the two sides of a sheet of textured paper (Canson), generally used by artists.
It was exactly on this minute piece of dressed paper that the measurements were done which verified a smooth anisotropy for MgB2. By painting the paper with the diboride powder, the researchers managed to produce a perfect alignment of the MgB2 crystals. All of them fitted into the pores of the paper in the same manner, creating ideal conditions for the application of magnetic fields over the sample and for the determination of the value of the current in various directions of the compound.
“A material is anisotropic because, on the microscopic scale, it is not homogenous”, states Lima “Looking at different directions in the interior of the crystal, we can see distinct sceneries.” The image generated of the sample by an electron microscope allows a view of each grain of material on the paper and the spatial interval between them. “We carried out the measurements during Carnival”, recalls Lima. The physicists began to study the MgB2 on the 10th of February as an extension to a thematic project coordinated by José Antonio Sanjurjo. Twenty days later, they had the results on hand.
Still small filaments
Even with a good potential for current transmission, magnesium diboride will have to overcome some restrictions in order to establish itself as an alternative to the superconductors already in commercial use. “Like the other metallic compounds, magnesium diboride has to be cooled using liquid helium, in a relatively expensive process in order to demonstrate its superconducting properties”, Lima. Another disadvantage is that, different from other metallic superconductors, the diboride is brittle, a typical defect of ceramics, which makes the production of conducting filaments difficult.
Difficult, but not impossible, to judge by the results obtained by physicists at the Ames Laboratory in the United States. The researchers at this center have already managed to make filaments of MgB2 of up to 5 centimeters in length. It’s a modest result. The scientists, nevertheless, are hoping to shortly find means of manufacturing filaments of the diboride that are much longer.
A Study of Superconductor Materials (nº 95/04721-4); Modality Thematic project; Coordinator Dr. José Antonio Sanjurjo – Physics Institute of Unicamp; Investment R$ 221,250.00 and US$ 530,900.00