SuperconductivityINTRODUCTIONWe’ve all heard about superconductivity. But, do we all know what it is?How it works and what are its uses? To start talking about superconductivity, wemust try to understand the how “normal” conductivity works. This will make itmuch easier to understand how the “super” part functions. In the followingparagraphs, I will explain how superconductivity works, some of the currentproblems and some examples of its uses. CONDUCTIVITYConductivity is the ability of a substance to carry electricity.
Somesubstances like copper, aluminium, silver and gold do it very well. They arecalled conductors. Others conduct electricity partially and they are calledsemi-conductors. The concept of electric transmission is very simple tounderstand.
The wire that conducts the electric current is made of atoms whichhave equal numbers of protons and electrons making the atoms electricallyneutral. If this balance is disturbed by gain or loss of electrons, the atomswill become electrically charged and are called ions. Electrons occupy energystates. Each level requires a certain amount of energy.
For an electron to moveto a higher level, it will require the right amount of energy. Electrons canmove between different levels and between different materials but to do that,they require the right amount of energy and an “empty” slot in the band theyenter. The metallic conductors have a lot of these slots and this is where thefree electrons will head when voltage (energy) is applied. A simpler way to lookat this is to think of atoms aligned in a straight line (wire).
if we add anelectron to the first atom of the line, that atom would have an excess ofelectrons so it releases an other electron which will go to the second atom andthe process repeats again and again until an electron pops out from the end ofthe wire. We can then say that conduction of an electrical current is simplyelectrons moving from one empty slot to another in the atoms’ outer shells. The problem with these conductors is the fact that they do not let all thecurrent get through. Whenever an electric current flows, it encounters someresistance, which changes the electrical energy into heat.
This is what causesthe wires to heat. The conductors become themselves like a resistance but anunwanted one. This explains why only 95% of the power generated by an ACgenerator reaches consumers. The rest is converted into useless heat along theway. The conducting wire is made of vibrating atoms called lattice.
The higherthe temperature, the more the lattice shakes making it harder for the electronsto travel through that wire. It becomes like a jungle full of obstacles. Someof the electrons will bump with the vibrating atoms and impurities and fly offin all directions and lose energy in form of heat. This is known as friction. This is where superconductivity comes into work.
Inside a superconductor, thelattice and the impurities are still there, but their state is much differentfrom that of an ordinary conductor. SUPERCONDUCTIVITY (Theory / history)Superconductivity was discovered in 1911 by Heike Kamerlingh Onnes, a Dutchphysicist. It is the ability to conduct electricity without resistance andwithout loss. At that time, it took liquid helium to get extremely lowtemperatures to make a substance superconduct, around 4 kelvins. That wasn’tvery far from absolute Zero (The theoretical temperature at which the atoms andmolecules of a substance lose all of their frantic heat-dependent energy and atwhich all resistance stops short. ) Kelvin believed that electrons travelling ina conductor would come to a complete stop as the temperature got close toabsolute zero.
But others were not so sure. Kelvin was wrong. The colder it gets,the less the lattice shakes, making it easier for electrons to get through. There’s one theory that explains best what happens in a superconducting wire:When a conductor is cooled to super low temperatures, the electrons travellinginside it would join up in some way and move as a team. The problem with thisnotion was that electrons carry negative charges and like charges repel.
Thisrepulsion would prevent the electrons from forming their team. The answer tothat was phonons. It is believed that packets of sound waves (phonons) that areemitted by the vibrating lattice overcome the electrons natural repulsion makingit possible for them to travel in team. It’s as if they were all holding handstogether. If one of them falls in a hole or bumps into something, the precedingelectron would pull him and the following one would push.
There was no chanceof getting lost. Since the lattice was cooled, there was less vibration makingit easier for the paired electrons to go through. NEW MATERIALThat theory worked well for the conventional, metallic, low-temperaturesuperconducting materials. But later on, new materials were discovered.
Itconducted at temperatures never before dreamed possible. That material wasceramic. What was believed to be an insulator became a superconductor. Thelatest Ceramic material discovered superconducts at 125 Kelvin. This is stillfar away from room temperature but now, liquid nitrogen could be used.
It ismuch cheaper than the rare, expensive liquid Helium. Scientists still don’t knowhow the new superconductivity works. Some scientists have suggested that the newceramics are new kinds of metals that carry electrical charges, not viaelectrons, but through other charged particles. PROBLEMS / SOLUTIONSThroughout the time, scientists have succeeded in increasing the transitiontemperature which is the temperature required by a material to superconduct. Although they have reached temperatures much higher than 4k, it is stilldifficult to use superconductors in the industry because it is well below roomtemperature. Another problem is the fact that the new ceramic conductors are toofragile.
They cannot be bent, twisted, stretched and machined. This makes themreally useless. Scientists are attempting to find a solution to that by tryingto develop composite wires. This means that the superconducting material wouldbe covered by a coating of copper. If the ceramic loses its superconductivity,the copper would take over until the superconductor bounced back. The oldsuperconductors have no problem with being flexible but the required very lowtemperatures remain to be a problem.
One good thing about ceramics is the factthat they generate extremely high magnetic fields. The old superconductors useto fail under low magnetic fields but the new ones seem to do well even withextremely high magnetic field applied on them. POSSIBLE USESThe characteristics of a superconductor (low resistance and strong magneticfields) seemed to have many uses. Highly efficient power generators;superpowerful magnets; computers that process data in a flash; supersensitiveelectronic devices for geophysical exploration and military surveillance;economic energy-storage units; memory devices like centimetre-long video tapeswith super conducting memory loops; high definition satellite television; highlyaccurate medical diagnostic equipment; smaller electric motors for shippropulsion; magnetically levitated trains; more efficient particle accelerators;fusion reactors that would generate cheap, clean power; and even electromagneticlaunch vehicles and magnetic tunnels that could accelerate spacecraft to escapevelocity.
THE MAGNETICALLY LEVITATED TRAINIn my research, I had the chance to learn how two of these applicationswork: the magnetically levitated train and magnetically propelled ships. First, the magnetically levitated train, a fairly simple but brilliantconcept. That train can reach great speeds since it had no friction with it’strack. The guideway has thousands of electromagnets for levitation set in thefloor along the way. More electromagnets for propulsion are set on the sides ofthe U-shaped track.
The superconducting magnets on the train have the samepolarity of the electromagnets of the track, so they push against each other andmake the train float about 4 inches above ground. The interesting concept comeswith propulsion. The operator sends and AC current through the electromagnets onthe sides and can control the speed of the train by changing the frequency ofthe pulses. Supposing that the positive peak reaches the first electromagnet onthe side of the track. That magnet will push the magnet making the train moveforward.
When the negative peak reaches that same magnet, the magnet on thetrain would have moved forward so it will be pushed by that same magnet on thetrack and pulled by the following electromagnet on the track, which now has thepositive voltage across it. So the first would be pushing and the second wouldbe pulling. It takes some time to clearly understand what is going on but itbecomes so obvious afterwards. It’s as if the train was “surfing” on waves ofvoltage. THE MAGSHIPAnother interesting application is what is referred to as the magship.
Thisship has no engine, no propellers and no rudder. It has a unique power sourcewhich is electromagnetism. The generator on the boat creates a current whichtravels from one electrode to another which go underwater on each side of theship. This makes the water electrically charged.
This only works in salt waterbecause pure water would not conduct the current. The magnets which are locatedon the bottom of the ship would produce a magnetic field which will push thewater away making the ship move forward. There are a lot of problems relatedwith that. The magnetic field could attract metallic objects and even otherships causing many accidents.
CONCLUSIONAs time goes by, transition temperature, critical field (maximum magneticfield intensity that a superconductor can support before failing), currentcapacity and all other problems are improving slowly. But, at least they showthat we are moving in the right direction. A lot of people are gettinginterested in that field since it promises a lot for the future.Science
