Written by Dustin Stelzer
Now that we’ve built a base understanding of how atoms work, the charges that make them up, and the forces that interact with these particles, we can start to discuss the difference between conductors and insulators. One of the most important things we deal with as electricians are conductors that allow an electric current to flow, and insulators that do not allow this flow.
Conductors are objects that have atoms with an excess of electrons, therefore give them up easily when a force is applied. In a perfect conductor, every atom has an equal number of electrons and protons. However, in real life, this isn’t the case and only a portion of the atoms on the surface of the material are conductors while others are insulators. There are tons of different types of atoms in a typical conductor, and with impurities, there is no truly “perfect” conductor. Because of this, all electrical current flows through the conductor’s outer layer of atoms. Atoms in this “conduction band” of a conductor have higher energy than those electrons that remain attached to their parent nuclei.
The outer valance shell of an atom can contain up to eight valence electrons. Since valence electrons share any energy applied to them, the atoms that have fewer valence electrons in them are more likely to give up their electrons when a force is applied. Materials with only one or two valence electrons are typically the best conductors.
Conductivity is rated in two ways: by the resistance through the material, which determines how much voltage is required to apply a certain current, and by the thermal flow of electrons across the material when a temperature gradient exists along its length. To lower the resistance, it is best to make the material as long and thin as possible, because a long object offers more surface area over which electrons can pass. The higher the temperature of the material, the easier it becomes for electrons to jump between atoms while still maintaining stability. This is why utility companies run thin conductors over hundreds of miles, rather than thick ones.
Most metals are good conductors. The ones you are probably most familiar with are copper (27 electrons), silver (47 electrons), and gold (79 electrons). The more electrons a material has, the better conductor it is–but also the higher its melting temperature and the less ductile (easier to bend) it becomes. Most conductors have more electrons than protons as well. And when you move from one conductor to another, the net charge on the conductor changes by the addition or subtraction of electrons only. In other words, all metals are charged and are considered to have a “positive charge.”
Insulators are materials from which electrons are very difficult to free. The atoms that are more likely to receive an electron or energy are called insulators. The atoms of insulators have their valence shells filled with eight electrons or more than half-filled. Any energy applied to such an atom will be distributed amongst a relatively large number of electrons. But in addition to this, these atoms resist giving up their electrons because of a phenomenon known as chemical stability.
An atom is completely stable when its outer shell is completely filled or when it has eight valence electrons. A stable atom resists any sort of activity. In fact, when it is stable like this, it will not combine with any other atoms to form compounds. Some examples of insulators are the gases helium and neon, the covalent nonmetals carbon and silicon, as well as most of the elements in groups 13 to 16 of the periodic table. In addition, there are six naturally stable elements: helium, neon, argon, krypton, xenon, and radon. These are known as inert gases.
All atoms that have less than eight valence electrons tend to attain a stable state. Those that are less than half-filled, the conductors, tend to release their electrons to empty the unstable shell. But those that are more than half-filled, the insulators, strive to collect electrons to fill up their valence shell. So, not only is it difficult to free their electrons, but these Adams of the insulators also oppose the production of electricity with their tenancy to catch any electrons that may be freed. Those Adams that have seven valence electrons most actively try to be filled, and are excellent electrical insulators.
Semiconductors are those materials that are neither good conductors nor good insulators. In other words, they can conduct electricity better than insulators can, but not as well as conductors can. Semiconductors do not conduct electricity as well as good conductors because they have more than one or two valence electrons, and so do not give up the electrons so easily. At the same time, semi-conductors have less than seven or eight valence electrons, which is what insulators have. Thus they do not resist giving up electrons as much as the insulators do, and therefore will allow some conduction.
Modern electronics use semiconductors like silicon, which has 14 electrons, but low-temperature superconductors have only 7 electrons, and yet allow electricity to flow with no ohmic resistance.
In a semiconductor, the valence electrons (which are free to move around) can be donated into empty energy levels in the conduction band or taken from levels in the valence band. This action of donating and withdrawing electrons is called charge transfer, which we have discussed earlier when we talked about semiconductors. However as you can see semiconductor conductivity depends on how many electrons it has, unlike metals where conductivity depends on how easily they can give up their free electrons to electricity. As semiconductors lack some of their outermost shell’s orbitals’ capacity for bonding with other atoms (such as losing an electron for bonding), they exhibit semiconductivity when doped – when the semiconductor gains electrons to “fill up” its valence shell.
Semiconductor conductivity depends on how many electrons it has. The semiconductor industry controls semiconductivity by controlling the number of electron vacancies in semiconductors, called dopants. Semiconductor conductivity increases with an increased concentration of dopants (such as silicon’s donor atoms of phosphorus). However, even pure semiconductors are not ideally suited for semiconductor electronics, because they suffer from a phenomenon known as band-gap energy that creates a certain amount of resistance. Band-gap energy occurs due to the semiconductor’s electronic energy levels only allowing electrons at a higher energy level to move freely within the material. This means an inefficiency of energy flow from semiconductor to semiconductor and thus wasted electricity.
However, there is a class of semiconductors known as superconductors whose conductivity can approach zero resistance under specific conditions (theoretically achieving absolute zero). Additionally, these semiconductors have no electronic band-gap energy; instead, they maintain nearly perfect energy flow between semiconducting elements. These two properties are ideal for semiconductor electronics but are not achieved by any natural semiconductors in the universe today. However, artificial superconducting semiconductor materials such as metallic hydrogen do exist in laboratory settings.
Superconductors are a special type of conductor which has no resistance to the flow of electrical current and therefore achieves close to 100 percent conduction. Due to this fact, superconductors do not get hot when they conduct because they give off no kinetic energy from the flow of electrons. They are thus better than semiconductors for use in electronics and electric motors, where it is important that semiconductors stay cool. When semiconductors become very hot, semiconductor junctions will break down or semiconductor components will melt and fuse together.
Superconducting materials can be made into wires, using coils of superconductor wire to make what are called Josephson junctions (after the man who first discovered them), which can generate electrical current at frequencies hundreds of times faster than normal silicon semiconductors in circuits and switches without any loss. Superconductors are used in magnetic resonance imaging (MRI) as the material for a very powerful magnet and were crucial to the development of the MRI scanner. In an MRI machine, the superconductor is cooled with liquid nitrogen to keep it cold enough to be superconducting; this is done because if we could not do so, then there would be heating effects caused by the magnetic field from its own huge current flowing through it.
Superconductors have even been proposed as a form of energy storage device called a “super battery” which could store vast amounts of electrical power generated by solar cells or wind turbines all day long until it was needed at night when demand for electricity peaks. It would be able to do this because very little power would be lost in the superconductor during storage.