SUPERCONDUCTORS EXPLAINED
April 8th, 1911. Heike Kamerlingh Onnes, a Dutch Physicist, was investigating how resistance varies with temperature in a conductor. He cooled mercury to 4 degrees kelvin when he noticed that its resistance disappeared and hence the “superconductor” was discovered. Superconductivity is essentially a set of physical properties observed in certain materials whereby electrical resistance vanishes and magnetic flux fields are expelled from the material. Any material displaying these properties is known as a “superconductor”.
In normal conductors, as electrons collide with the vibrating lattice ions, energy is lost as thermal energy thus increasing the temperature. At higher temperatures the lattice ions will vibrate with a greater frequency increasing the likelihood of collisions between them and the electrons, therefore, there will be more resistance. However, in superconductors, after the critical temperature, the resistance drops to 0.
Looking at Ohms Law: V=IR, when resistance is 0 no potential difference is required to carry a current. Additionally, Power loss = current² x resistance. In superconductors, resistance is 0 therefore no energy must be lost. Hence, a current will flow indefinitely once set in motion making superconductors one of the closest things to perpetual motion in nature. This would make them ideal for long-distance, low-voltage electric grids as no transmission loss would occur.
But why does this happen? As you cool down a conductor the thermal vibrations in the ions in the lattice decrease as you approach the critical temperature these vibrations and eventually become negligible. As an electron moves through the conductor, the attraction between the negatively charged electron and positively charged ions of the lattice causes the structure to distort; the structure is drawn towards the electron creating an area with a higher density of positive charge. When another electron enters the region you would expect it to repel the original electrons as like charges repel however many atoms are part of this disturbance and this attraction occurs over many atoms so the electron becomes attracted to this distorted area and they bind together forming a “cooper pair”. Cooper pairs are very weak bonds and even thermal vibrations would break them; this is why extremely low temperatures are required.
All these cooper pairs behave as one big group of particles — when they are in the same state. The explanation for this goes into quantum mechanics. Briefly, electrons are fermions which means they cannot be in the same state. However, when two electrons are in a cooper pair they act together as bosons (particles which are allowed to be in the same state), therefore all the cooper pairs go into their ground state. These cooper pairs are bonded over large distances therefore they all become entangled and overlap to form one large network of interactions. This behavior prevents collisions from occurring thus leading to no resistance.
This theory is called the BCS theory; the name originating from the 3 people that helped form it — John Bardeen, Leon Cooper, and John Schrieffer. This theory, however, only explains why Type 1 superconductors work. Yes, there are two types of superconductors. Type 1 superconductors have relatively low critical temperatures and a more abrupt transitioning state. They are generally pure metals such as mercury and lead. Type 2 superconductors have comparatively higher critical temperatures and have a more gradual transition into the superconducting state; during the transition, they enter a mixed state where they exhibit properties of both conductors and superconductors. They are generally alloys and complex oxides of ceramic such as niobium-titanium. Currently, there is no perfect explanation for how type 2 superconductors work, however, scientists are working towards finding an explanation.
One significant property of superconductors is that when they are making the transition from normal to the superconducting state, they expel magnetic fields from their interiors.
A normal conductor lets magnetic field lines pass through it, however, when a magnet is brought near a superconductor, (a potential difference is induced) cause electrons flow in the superconductor producing Eddie currents. They produce magnetics fields that mirror the magnetic field outside the superconductor; this causes all field lines in the superconductor to cancel out and repels the magnet upwards. Therefore, the magnetic field forms around the superconductor. This is called “the Meissner effect”.
Type 1 superconductors obey the Meissner effect completely (making them diamagnetic), but type 2 superconductors contain impurities, thus they are not perfectly diamagnetic which means magnetics fields can still penetrate the material. The magnetic field passes through columns called “flux tubes” subsequently creating magnetic vortices. At lower temperatures, the flux tubes are pinned in place and cannot move, which allows for a phenomenon known as “flux pinning” to occur. This holds the superconductor in position, and due to gravity, there comes an equilibrium position where the forces are balanced allowing the object to levitate. This property is used in MAGLEV trains where the train is levitated using superconductors to eliminate friction so high speeds can be attained.
Superconductors have great potential for practical applications, however, a major limitation is the low critical temperature required. This is difficult, very expensive, and energy-intensive to maintain. Moreover, the superconducting materials, in addition to being chemically unstable in some environments, are usually brittle and hard to shape. Nevertheless, superconductors are among the most exciting materials yet discovered and scientists are continuing to research them with room temperature superconductivity a key goal.
Video:
Related articles:
