CONDUCTORS

When a voltage is applied across a material some electrons flow. The ratio of the current that flows and the voltage is called electrical resistance (i.e. the lower the resistance the greater the current that flows).

In a solid electrons are gaining and losing energy all the time from acceleration by electric or magnetic fields and from collisions with atoms. Quantum theory tells us that an electron can only have particular amount of energy. Therefore, any change in energy occurs in jumps of predefined units. This is called the Band model, where the electrons in a solid occupy an almost continuous range of discrete energy levels. The low energy bands (valence bands) the electrons are assumed to be bound to atoms, whilst the higher energy bands (conduction band) electrons are free to move through the solid under the action of an electromagnetic field.

Electrical insulators and conductors can be explained with the band model. In metals, which are good conductors, electrons are already in the conduction band. So all that is needed for electrical conduction is an electric field to move them. However, in Insulators, there are no electrons in the conduction band. Although the valence band is almost full the gap upto the conduction band is too large for electrons at room temperature to jump up and be available for conduction. Insulators will conduct if the electrons gain enough energy.

SEMICONDUCTORS

Semiconductors have a very special role in modern society. In fact, without them we would not have much of the technology that we take for granted. It is the heart of the computer industry, and most household appliances have some kind of semiconductor chip inside them.

Above, we have used the band model to explain good conductors and bad conductors (insulators). With a perfect semi-conductor (called Intrinsic) there is also a large band gap, so at very low temperatures there are no electrons that have enough energy to jump to the conduction band. It is an insulator. But, as the temperature rises some electrons do gain enough energy from atomic collisions to reach the conduction band, and the material starts to conduct electricity.

In a regular lattice each atom of semiconductor is bonded to other atoms by a certain number of electrons (with Silicon it is four). If an atom of the semiconductor material is replaced by an atom of a different material (with a different number of outer electrons for bonding) then there will either be an excess or deficit of electrons. This creates what is known as a Negative-type or Positive-Type forms of an Extrinsic semiconductor. These added impurities are essential for the semiconductor industry, which is based largely junctions formed between an N-type and P-type semiconductors.

SUPERCONDUCTORS

Electrical resistance can be envisaged as electrons interacting with atoms. The negatively charged electrons are attracted to the positive charge of the protons at the dense nucleus of the atom. The atoms vibrate and move near to the travelling electrons, capturing them. The energy of the electron is transferred to the atom and ultimately creates heat. So, forcing too much current down a wire can sometimes produce so much heat through its resistance that the wire (or the plastic insulation) melts.

When we cool a material the atoms vibrate to a lesser extent, and thus there is less chance that an electron will be captured. This causes a decrease in electrical resistance. In 1911 the Dutch Scientist Heike Onnes successfully cooled some materials and measured their resistance. As expected, he found that the resistance of some metals decrease uniformly with temp.

Discovery of Superconductors

Lord Kelvin had predicted that the resistance of all metals would increase near zero (-273 ° C). But, when Onnes cooled metals down to these ultra-low temeperatures the resistance decreased sharply. In other words the electricity flowed without much loss of energy. However, Lord Kelvins prediction was the case for Platinum and Copper. There was obviously a new phenomenon that the theorists had not expected. Onnes had discovered Superconductivity, and was awarded the Nobel prize in 1913 - just two years afterwards!

Meissner effect

In 1933 German physicists Meissner and Oscenfeld discovered a phenomenon (now called the Meissner Effect) that a magnetic field is repelled by a Superconducting material. The magnetic field causes circulating vortices of current, which generate a magnetic field that opposes the applied field. So, a magnet experiences a repulsive force from the material, which if strong enough causes the magnet to counter the force of gravity and float above the material. But, if the magnetic field is too strong this destroys the superconductivity.

Two fluid model

In 1935 F and A London developed a theory to explain Superconductivity. This assumed that there were two electrical currents flowing, one normal and one superconducting.

BCS theory

John Bardeen won the 1956 Nobel prize Physics for his part in the development of the transistor. The following year he and two colleagues (Leon Cooper and Robert Schrieffer) developed a theory for superconductivity for which they won the 1972 Nobel prize for Physics.

The BCS theory is based on the fact that the electrons interact with vibrating atoms to create a bound pair of electrons, called a Cooper pair. A magnetic field tends to align all the electrons in the same direction, hence destroying the Cooper pairs, and thus any Superconductivity.

So, what are these Cooper pairs? Well, the lack of resistance of a Superconductor necessitates that the negatively charged electrons are no longer attracted to the positively charged atomic nucleus. The idea is that the attraction of an electron to an atom is negated by another electron moving in the opposite direction on the other side of the atom.

High Temperature Superconductors

Another Nobel prize was due, this time in 1987, to Alexander Muller/Johannes Bednorz for creating a Superconductor that worked at the 'high temp' of 30K!. This material was a ceramic oxide of lanthanum, barium and copper (LaBaCuO). The structure of the material is called a Perovskite. One of the main characteristics of these materials is imperfections in the crystal structure (e.g. a lack of oxygen atoms produced by annealing)

To achieve ultra-low temperatures liquid Helium is used, but a material that is supercondicting above a temperature of -211 ° C (62K) could use Liquid Nitrogen, the price of which is much lower. This was achieved in 1986 by IBM researchers who reached -179 ° C. In 1992 Japanese researchers reached -103 ° C. In 1995 Los Alamos researchers created a flexible film which was superconducting at -196 ° C.

Here is a web page that shows you how to make your own High Temperature Superconductor.

Type I, Type II Superconductors

There is a lack of a magnetic field in the superconducting material (a situation called perfect diamagnetism). In the Meissner effect the applied magnetic field induces vortices of current on the surface, resulting in no penetration of the magnetic field inside the material.

For most superconductors this situation suddenly disappears at a critical magnetic field strength. These materials are called a Type I superconductors. Some pure metals exhibit this type of Superconductivity.

However there are some materials, Type II, in which the magnetic field drops off steadily after a critical field. This is thought to be due to imperfections in the material trapping (or pinning) the vortices. Some of the High Temperature Ceramic superconductors are Type II.

Recent discoveries have shown that the two electrons in a Cooper pair do not have to have opposite spins, as previously thought. Cooper pairs with electrons of the same spin have been found, creating a superconductor that is less prone to magnetic fields.

For more detailed information on this subject CLICK HERE.