The High Quarterly

A superconductor is a material that can conduct electric current with no resistance. Appropriate conditions are needed for this to happen; mainly the temperature has to be below a transition temperature. Currently, the transition temperatures of known superconducting materials are still very low, the highest being the common compound Hydrogen Sulphide, which was discovered to be superconductive very recently, at -70oC when subjected to very high pressures.

It was discovered in 1911 by Heike Kamerlingh Onnes, after he had discovered the boiling point of helium was in fact lower than the melting point of hydrogen and had managed to isolate liquid helium. It had previously been believed that hydrogen would have the lowest boiling point as it is the lightest element. After this feat, which he won a Nobel Prize for in 1913, he decided to research into what would happen to the resistance of a metal if it was cooled to absolute zero.  He discovered if mercury was cooled below the boiling point of liquid helium (4.2K/-269oC), the resistance disappeared suddenly and completely, however it took a long time for scientists to understand what exactly was happening in the metal. In a crystalline solid, the atoms are arranged in a regular pattern. The atoms are always vibrating but at low temperatures they vibrate less. Normally, as a current flows through the wire, electrons collide with these atoms creating resistance. Overcoming this resistance means energy is wasted and given off as heat, meaning the wire warms up. In a superconducting material, the electrons act together, in pairs called Cooper Pairs. As an electron passes through the superconducting wire, they attract the positive ions. The positive ions are distorted and because they are a lot bigger and slower than the electrons, the distortion remains after the electron has moved on, creating a region of positive charge. This causes a second electron to be attracted to the first and its surrounding distortion of positively charged ions. The electron pairs move through the structure without any collisions.

In 1933 Walther Meissner and Robert Oschenfeld discovered that magnetic fields were expelled from the superconductor once the temperature reached below the transition temperature. This is because the electrical currents which run across the surface of the superconductor, which are called screening currents, produce an exterior magnetic field, which acts in opposition to the magnetic field applied. The exterior magnetic field is a repulsive force which can outweigh gravity, this is how superconductors can float. The expulsion of a magnetic field from a superconductor is called the Meissner Effect.

So, what can superconductors do for us?

That is quite an exciting and expensive question. Magnetic Levitation, or MagLev, trains are a particularly stirring development which use superconductors to make a carriage float above a guideway. The only running train is the Japanese Maglev Train, which currently holds the land speed record for a railed vehicle at 375mph. As there is no friction they can travel faster, and are a lot more efficient to run, however the rails are very expensive to build and liquid helium needs to be available to keep the superconductors cold enough.  Lots of ideas have come apparent in power transmission. If superconducting cables could be used, there would be no power wasted as heat when overcoming resistance, so power cables would be very efficient which is excellent both economically and environmentally. There have even been suggestions that we could put solar panels in North Africa and connect Europe to them using hundreds of miles of superconductive cables without losing any power. The problem is the transition temperatures are too low for this to be beneficial. Most applications of superconductivity are just too expensive or impractical to get off the ground, due to the low temperatures needed. This is why funding and research is mostly for finding high temperature superconductors (HTS).  

By Emily Coleman

 www.chemistryexplained.com 

cooper-pairs.html paged_r.html 

interactive.guim.co.uk