Electrons

Electrons are very small, charged particles. They form part of atoms which in turn make up molecules. In molecules, these electrons are often involved in holding the atoms together, however in some other materials, they are more free to move. In fact metals are often thought of as "positive ions in a sea of electrons". Since electrons are very small in comparison with ions this means that the ions are fairly fixed in their positions and the electrons can move freely.

The positive ions are represented by the red circles and the "Sea" of electrons is represented by the pale blue background. It is easy to see that the ions are fairly closely packed together and can't move much. The electrons however, being much smaller can "drift" around the ions.

Links: Metallic Bonding & Metals

Conductivity

Conductivity is a measure of how mobile the electrons are. This movement is called current and is measured in Amperes. If you put a large positive charge at one end of a lump of metal, all the electrons (negatively charged) will try to "drift" towards it. This is effectively what happens in batteries. A lead acid battery is a series of alternating lead and lead oxide plates in a solution of sulphuric acid, but a simple battery can be made at home using galvanised nails (zinc coated), iron nails, and a lemon.

A lemon "battery". The lemon behaves like the acid in a car battery, allowing the electrons to move from one nail to the other. |

The movement of electrons is governed by the presence of a charge or "potential difference" between the two ends of the material. The other thing that controls the current is the resistance which is measured in Ohms, which is dependent on the nature of the material. Current, potential difference (or voltage) and resistance are related:

V = IR, (I = current, V = voltage and R = resistance)

Materials that conduct are called "metallic conductors" and those that conduct very badly are called "insulators". Some materials don't conduct well, but are not as bad as insulators and are known as semiconductors. These are vital in the manufacture of computer chips and other electronic devices. They are characterised by the temperature dependence of their resistance.

On heating metallic conductors become worse conductors, while semiconductors become better conductors. The reason for this is that electrons can be thought of as being in "bands". For metallic conductors, the "conduction band" is the same as the "valence band" (or highest occupied band). This means that when they are heated and the electrons move about more there is effectively less space for them to move around in.

For Insulators, the conduction/valence band is already full, so there is virtually no space for the electrons to move around in any way. Semiconductors however have a full valence band, but the next level is very close by, so on heating the electrons can move up a level to this new conduction band, where they have lots of space to move around. This is idea is referred to as "Band Theory".

On heating, electrons are given extra energy, in metallic conductors this means there are more electrons in the highest energy band, in semiconductors this extra energy means the electrons have enough energy to jump into a higher energy level, making it conduct more.

Links: How batteries work..., Electricity and Magnetism & Band Theory

Piezoelectricity

A piezoelectric material is one that develops an electrical current when physical stress is applied to it. A typical example of this type of material is a-quartz, which is a distorted type of SiO2. This distortion allows each tetrahedron to have a small dipole, but the tetrahedra are arranged in such a way that the dipoles cancel out and the net dipole is zero. However, when the material is stretched, the Si-O-Si bond angle changes so that the dipoles not longer cancel and the crystal has a net electrical polarisation. This is very small, but the same is also true in reverse, i.e. if a current is applied to the crystal, it will expand slightly.

This is of particular use where a stable frequency oscillator is needed, for example, in time pieces. In a wrist watch for example, there is a small quartz crystal attached to a battery which applies and alternating current so that the crystal expands and contracts at a very regular, known rate, which can then be used to control the speed of the watch.

Although a-Quartz has a a lot of technological uses, there are other piezoelectric materials that have a much more efficient electrical output, for example Rochelle salt (potassium sodium tartrate). For some industrial applications like ultrasonic imaging these materials are very important.

Links: How watches work..., Morgan Electro Ceramics & Rochelle salt

Pyroelectricity

Pyroelectricity is similar piezoelectricity, in that an electrical current is generated when stress is applied to a material. In this case however, the stress thermal.

Tourmaline is an example material. Gem quality crystals of tourmaline are electrically polarised under ambient conditions, i.e. they one side of the crystal has a different charge to another. This is not usually apparent however as this small surface charge is neutralised by charged particles in the atmosphere. On heating, the crystals lose this charge. When these crystals are rapidly cooled however, the polarisation increases and the crystals will attract small charged particles.

Ferroelectric materials are a special case of pyroelectricity and the perovskites are classic examples. Perovskite is a mineral CaTiO3, which is simple material made up of TiO6 octahedra, joined at the corners. It is the parent of a large family of natural and synthetic materials, many of which have interesting properties, one of which in particular, BaTiO3, is ferroelectric.

The red atoms are the calcium atoms, the blue are titanium and the yellow are oxygen. The bonds show the TiO6 octahedra. Since it is not easy to see the octahedra clearly when drawn like this, it is common to show just the octahedra, without the bonds - Move your mouse over the picture to see how this works (You will need to have Java Script enabled).

At high temperature, the TiO6 octahedra are regular, with the titanium atom at the centre. When it is cooled however, perovskite undergoes a series of transitions, based around the movement of this titanium ion from the centre of the octahedra. As this happens, an electric dipole is introduced. However, as the material is cooled, domains grow each with a dipole, but overall these cancel out. If the cooling is done while a current is applied however, the domains all grow in one direction, resulting in a pyroelectric material.

Links: Ferroelectric Liquid Crystals

Ionic Conductivity

Some ions are very small and in some materials, it is possible for certain ions to move about. This is called ionic conductivity. It often occurs in materials that do not have a perfect structure. There are two broad categories of defect namely Schottky and Frenkel defects. Schottky defects consist of a vacancy in the lattice, while Frenkel defects are vacancies caused by the ion being squashed into another part of the lattice.

Schottky Defects have holes in both the positive and negative lattices, so that the charges balance. Consequently, a key characteristic of Schottky materials is that their density is lower than would be expected. Examples include LiF, LiI, NaCl. KCl, MgO etc..
Frenkel Defects have holes, usually only in one lattice, with the missing ions located elsewhere in the structure. For this reason, the defect atoms must be small in Frenkel materials, so that they can fit in without too much distortion. Examples include CaF2, SrF2, AgCl, AgBr, UO2 etc..

Ions that are small enough to fit in gaps within the lattice include lithium, silver and fluorine, although where large atoms are present, the size is relative.

These defects are called "Intrinsic Defects", it is also possible to insert an extra atom into the lattice, which is how semiconductors are made. These materials are said to have "Extrinsic Defects".

Links: Measuring Ionic Conductivity & Defects in Solids

Superconductivity

Superconductivity is a phenomenon that is still not fully understood, but as it has many potential uses it is widely studied. It was discovered in 1911, when Kamerlingh Onnes was measuring resistance with the temperature variation in liquid mercury. He found that while it decreased as it was cooled, at 4.2K, the resistance suddenly disappeared completely! He called this "Superconductivity" and the temperature which it occurs the "Superconducting Critical Temperature", TC. He realised that this is a major discovery; a circuit made from a superconductor NEVER loses power and doesn't heat up like a normal wire does.

Resistance plots with a metallic conductor in red and the superconductor in blue.

Tests have shown that many metallic elements are superconductors at low temperatures (it is interesting to note however that the best conductors, copper, silver and gold are not included in this). All these are at temperatures too low to be of any real practical use though, so the search was started for a room temperature superconductor.

Periodic Table of the elements, showing magnetic elements in sky blue, known superconductors outlined in red and elements that superconduct under pressure or in thin films outlined in green.

Although there have been minor improvements since, the most dramatic improvement so far has been the discovery of the YBa2Cu3O7 or YBCO family of High TC superconductors. The first material was developed in 1987, based on the mineral perovskite, it is a layered material, made up of copper oxide layers. In between these layers there are atoms (like perovskite), but they alternate barium-yttrium-barium-barium-yttrium-barium.

Polyhedral representation of the perovskite related YBa2Cu3O9 (left) and the superconducting phase YBa2Cu3O6.8 (right). Yttrium is shown in turquoise, barium in red, copper in the centre of the polyhedra, in blue and oxygen in yellow. Removing the oxygens from the yttrium layer of YBa2Cu3O9 gives YBa2Cu3O7. The superconducting phase has more oxygens missing, from the middle layer. To see the perovskite YBa2Cu3O7, move the mouse onto either of the images (you will need to have Java Script enabled). Compare the three pictures to see the differences.

Superconductors also have important magnetic properties . In the same way as an applied field is partially expelled by a ferromagnet (reducing the density of field lines), the field in a superconducting material is completely expelled making it a "perfect ferromagnet", this is called the "Meissner Effect".

| Schematic of a super conductor in a magnetic field (lines shown in red). Above, the superconductor is above it's Tc and below it is in it's superconducting phase, showing the magnetic field excluded from the sample.

This has many possible applications, for example a superconductor will levitate when placed over a magnet, so it could be used anywhere where friction is a problem, from trains or trams that float on magnetic rails to clocks that never need winding - the possibilities are endless!

Quite why some materials superconduct at certain temperatures is still not fully understood. However, it is thought to result from the movement of "Cooper Pairs" of electrons through the lattice. The first of these electrons moves through the structure causing the lattice to vibrate, attracting positive ions towards it. Since the electron moves faster than the lattice of atoms, it leaves in it's wake a "trail of positiveness", which the second electron is attracted to. In this way, the electrons are paired and their movement is related, but the electrons themselves may be relatively far apart. This is known as the Bardeen, Cooper and Schriefer (or BCS) Theory.

Links: Oak Ridge National Laboratory Teaching Guide, Superconductors, Uses for Superconductors & Organic Superconductors

Ferroelectricity

Ferroelectricity is a special case of pyroelectricity. Some materials have the ability to store an electric field in a similar way to ferromagnetic materials that store a magnetic field. In a strong electric field, a permanent electric dipole is created, i.e. the one end has a positive charge and the other has a negative charge. This is also called polarisation and is an important requirement for Non-Linear Optics. Striking a ferroelectric will cause the generation of an extremely high voltage (but low current) two-peaked current (one for the compression and one for the rebound).

One of the best known ferroelectric materials is BaTiO3. It has a perovskite structure which can become ferroelectric because the titanium atom is slightly too small to come into bonding contact with the surrounding oxygen atoms. Because of this, the titanium moves so that it touches only three of them, creating a net dipole.

A picture of BaTiO3. If you move your mouse onto the image, you will see the distortion to the ferroelectric phase (you will need to have Java Script enabled).

Antiferroelectrics are distorted in a similar way to ferroelectric structures, except that neighbouring ions are displaced in opposite directions.

The red circles represent negatively charged atoms or groups, the turquoise circles represent positively charged ions, with the unit shown in green. This schematic shows the difference between an undistorted structure, ferroelectric structures and antiferroelectric structures. Ferroelectric structures are polarised in one direction, while with antiferroelectrics, adjacent cells are polarised in opposite directions, so the polarisations cancel out (like antiferromagnetic structures).

Links: Fundamentals of Ferroelectrics & Ferroelectric Ceramics

Last updated 31 October, 2002