12-01-2013, 05:02 PM
Superconductors
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What Are Superconductors?
As most people know the component of an electrical circuit that causes energy loss is called "resistance," which can be defined as a materials opposition to current being passed through it. Usually, this resistance results in the production of heat, sound, or another form of energy. In many cases, this transformation of energy is useful in such applications as toasters, heaters, and light bulbs. Even though it is a useful property, resistance often gets in the way of performance in such cases as high voltage transmission wires, electric motor output, and other cases where internal system energy losses are unwanted. This is where the phenomenon of superconducting materials comes into play and may present the solution to this energy loss problem.
Superconductors are materials that display zero resistance under certain conditions. These conditions are called the "critical temperature" and "critical field," denoted Tc and Hc respectively. The Tc is the highest temperature state the material can attain and remain superconductive. The Hc is the highest magnetic field the material can be exposed to before reverting to its normal magnetic state. Within the substances currently known to superconduct, there is a divide between what has come to be called type I and type II superconductors. Type I are composed of pure substances, usually metals, and type II are composite compounds, usually some sort of ceramic.
Additional differences between type I and type II exist, mainly that type II display superconducting qualities at much higher temperatures and can remain superconductive in the presence of much higher magnetic fields. While type I have Tc's that hover just a few degrees from absolute zero, type II can have Tc's of over 130 K. The graph below shows how type I and type II superconductors compare as related to Tc and Hc:
history
In the early 1900's a duch physicist by the name of Heike Kammerlingh Onnes (pictured above), discovered superconductivity. Before his discovery, Onnes had spent most of his scientific career studying extreme cold. The first step he took toward superconductivity was on July 10, 1908 when he liquified helium and cooled it to an astonishing 4 K, which is roughly the temperature of the background radiation in open space. Using this liquid helium, Onnes began experimenting with other materials and their properties when subjected to intense cold. In 1911, he began his research on the electrical properties of these same materials. It was known to Onnes that as materials, particularly metals, cooled, they exhibited less and less resistance. Bringing a mercury wire to as close to absolute zero as possible, Onnes observed that as the temperature dropped, so to did the resistance, until 4.2 K was reached. There resistance vanished and current flowed through the wire unhindered. Below is an approximate graph displaying resistance as a function of temperature for the experiment Onnes conducted with mercury: