In this article, we will examine the working principle of superconducting materials, which are indispensable materials of new generation technology and which are still studied in depth.
Let's start at the beginning of the story. In 1908, Dutch physicist Heike Onnes succeeded in liquefying helium gas at 542.15 K (-269 °C), paving the way for his new discovery. In 1911, he observed that by cooling solid mercury with liquid helium, the electrical resistance of mercury dropped to zero at 4.2 K (-269.95 °C). In other words, there is no loss in the conductivity of mercury. This state is called superconductivity. We explain this phenomenon with quantum mechanics instead of classical mechanics.
When we think of a normal conductive material, in the outermost orbit of the metals, this orbit is also called the valence shell. While the electrons that provide the electric current are traveling through the material, they collide with other atoms, causing the material to heat up, that is, the resistance that creates an obstacle to the transmission of electrons.
In superconductivity, when we reduce the material to a certain critical temperature, the electrons create an electric current without encountering any resistance. So how does it achieve this physically?
The BCS theory, which best explains this phenomenon, was developed in 1957 by John Bardeen, Leon Cooper, and John Robert Schrieffer. This theory can be defined as follows;
Negatively charged electrons have the property of fermions. Fermion is that the spin of its particles (the nature of the particles shows the feature of rotation.) is in the form of half (½) and its multiples. For example, the electron spin is ± ½, in this case, the electrons show bipolar magnetism. Since the electron has half spin, it obeys the Pauli exclusion principle. This means, no two electrons with co-spin can occupy the same orbital. In order to rotate in the same orbit, they must have opposite spins. This results in solid materials. On the other hand, particles with a full spin that carry physical forces are also called bosons. For example, the photon carrying the electromagnetic force has a spin of ± 1.
Going back to the working principle of superconductivity, as electrons move through the crystal lattice, there is a repulsion of the electrons as well as a Coulomb attraction with a positive charge. During this attraction, electrons that come closer to each other form pairs with each other. This pair is called the Cooper pair. The attraction that makes up the Cooper couple is quite fragile. In particular, at high temperatures, for example at room temperature, it collides with these atoms and causes the Cooper pair to break down.
While Cooper electron pairs move in the crystal structure, it carries out the electrical flow without encountering any resistance, that is, no heating occurs. Materials that have reached the critical temperature varying from material to material show superconductivity. You can find some of them in the Table 1 below.
Table 1: The discovery of some superconductors and their critical temperature.
Among these materials, it is observed that the resistance of ordinary metals such as gold, silver, or copper decreases as the temperature increases. However, its electrical resistance is never zero, that is, it does not show superconductivity. The main reason for this is that the metal is not completely pure and the defects in its structure prevent this.
Figure 1: Resistance-temperature graph of a normal metal and a superconductor.
On the other hand, besides the fact that the resistance of superconductors drops to zero at a specific critical temperature, another important feature is that they show diamagnetism. The diamagnetic property creates a magnetic field opposite to the applied magnetic field by the formation of an electric current on the surface of a superconductor placed in an external magnetic field as a property of the spin of the material, that is, it excludes the externally applied magnetic field. Thus, the magnetic field inside the superconducting material becomes zero. After this simple application, the superconducting material remains suspended in the air. This effect is called the Meissner effect.
Figure 2: Diamagnetic representation of superconductor.
Figure 3: Meissner effect for superconductor.
As a result of the studies, two types of superconductivity are observed.
TYPE 1 SUPERCONDUCTORS:
Unalloyed (pure) metals, with the exception of niobium, vanadium, and technetium, show some conductivity at room temperature. When they reach the critical temperature specific to the material, they become type 1 superconductors, and their critical temperature is much higher than the absolute temperature.
TYPE 2 SUPERCONDUCTORS:
It is the type of superconductor that can be observed in alloy materials as well as pure metals. This type of superconducting material was first obtained from a lead-bismuth alloy in 1930. Unlike type 1, type 2 superconducting materials can be produced at higher temperatures. In addition, when examined physically, the transition from the normal state to the superconducting state does not occur in an instant by lowering the temperature. It is observed that there is a mixed situation between the two situations.
In the graphs below you can find the difference in the transition state of type1 and type2 superconducting materials.
Figure 4: Temperature-magnetic field graphs of type 1 and type 2 superconductors.
APPLICATION FİELDS OF SUPERCONDUCTORS
The most important contribution of technological developments is the improvement of efficiency. We now have the most effective superconducting materials that we can use. It does not create any heating problems, has high energy efficiency, and is relatively costly (unfortunately, we mentioned that liquid nitrogen or liquid helium is used to produce superconducting materials, although this creates energy efficiency, it is inconspicuous compared to the cost of liquid nitrogen and liquid helium. The good news is that many scientists continue to work day and night to overcome this) which is quickly incorporated into today's technologies.
We can examine these with the most well-known examples;
I) Maglev (Magnetic Levitation):
In order to minimize the losses and problems caused by friction between the train and the rails, strong electromagnetic magnets are used to take advantage of the floating property of superconductivity. The "Yamanashi MLX01 MagLev" train, which was put into operation in Japan in April 2015, has achieved a speed of 603 kilometers per hour.
II) Cables:
Although we have not yet been able to realize wireless electrical transmission, we were able to prevent the losses that occur in traditional cables thanks to cables made of superconducting materials.
III) Medical:
It is not surprising that superconductors use the area in medical technological devices. Magnetic resonance imaging (MRI) devices, which are used to examine the tissues and organs of patients, are also made of superconducting materials.
REFERENCES
Narlikar, Anant V. Superconductors. Oxford University Press, 2014.
Ford, Peter John, and George A. Saunders. The rise of the superconductors. CRC press, 2004.
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