In physics and electric engineering, there are three main types of materials according to flow of charges: conductors, insulators and semiconductors. Conductors are those materials which have plenty of free electrons, thus allowing electric current to easily pass through them. Their valance and conduction band overlap each other, so there is no difficulty for electrons to keep moving. 2ndly, insulators have no free electrons and there is a large forbidden energy gap b/w valence band and conduction band. Thus insulators don’t allow electric current to pass through them. Lastly, semiconductors are those materials, which have properties of both conductors and insulators and their forbidden energy gap is b/w 1 eV and 4 eV. Usually, at room temperature, semiconductors are insulators and they become conductors by increasing temperature. Besides these 3 types of materials, there is an exceptional case, to which we name as superconductors.
What is a superconductor?
A superconductor is defined as a material which can conduct electricity, offering zero resistance, when it is cooled below a threshold temperature. At some specific temperature (critical temperature Tc), this material offers zero resistance. In other words, a superconductor material is that which allows electric current to flow without any energy dissipation. Clearly, if there is no resistance for electrons then definitely there will be no loss of energy in any form like sound or heat, when that material has reached up to its critical temperature. Not all the materials become superconductive at critical temperature, only some specific do and all of such materials have their own Tc.
No wastage of energy, this is the advantage of superconductors but the problem is that most of materials become superconductive when they are too cold, and putting such materials to very low temperature needs excessive amounts of energy.
Before moving towards the discussion about types of superconductors, we need to understand some basic terms.
When a superconducting material is cooled below its critical temperature then its resistance falls to zero value which indirectly means that material now have infinite conductivity. E.g. mercury becomes superconducting below 4 K.
A superconductor does not allow magnetic field to penetrate through it. This property of superconductors is known as meissner effect.
Critical magnetic field
If the applied magnetic field value is increased beyond certain limit then that material behaves as a common conductor. Thus, the value of magnetic field beyond which superconductor changes its identity to a simple conductor is known as critical magnetic field. Critical magnetic field value is dependent on temperature. If temperature of material is reduced then its critical magnetic field value increases.
Current passing through a superconductor causes the production of magnetic field. If value of current increases, magnetic field will also rise to such a value at which superconductor returns to its initial state. This value of current is critical current.
If a superconducting ring is cooled below its critical temperature in a magnetic field and then magnetic field is removed then due to self-inductance, current is induced in ring. Lenz’s law tells the direction of thus current i.e. current direction is opposite to the change of flux passing through ring. This president current continues to flow through superconductor.
Types of superconductors
There are 2 types of superconductors:
- Type – I Superconductor
- Type – II Superconductor
Type – I superconductor
Type 1 involves pure metals and metalloids that have ability to show some electric conductivity at normal temperature i.e. They act as conductors at room temperature. As they get cooled below critical temperature, electrons slow down as their molecular vibrations reduce. Type 1 superconductors are named as “soft” superconductors or low temperature superconductors. They display a very sharp transition to superconducting state as there is no mixed state in type – I. As critical temperature reaches, they abruptly change their state and easily lose their superconductivity, as clear from graph.
Type – I shows “perfect” diamagnetism (the ability to repel magnetic field). Since Magnetic field can’t penetrate through type – I so they completely obey meissner’s law.
Type – I superconductors are those which consist of basic conductive elements which are used everywhere like electric wiring and micro-chips. The critical temperature range of type – I is 0.000325 K to 7.8 K. There are some superconductors from type – I which need very high pressure to access superconductivity e.g. sulfur needs 9.3 million atm pressure and 17 K temperature to be a superconductor. Slight impurities can’t affect the superconductivity of type – I. About half elements of periodic table show superconductivity, some of them are given below:
- Mercury at – 4.15 K
- Aluminum at – 1.17 K
- Zinc at – 0.85 K
- Lead at – 7.2 K
Type – II superconductor
As compared to type – I, type – II superconductors need higher temperature to show superconductivity. Critical temperature of type – II is greater than that of type – I. Why does type – II needs higher temperature than Type –I, is still not understood by the experts. Clearly, critical magnetic field of Type – II is also greater, which is greater than 1 T usually. They are formed with metallic compounds and alloys like lead or copper.
Since Magnetic field can penetrate through type – II so they partly obey the meissner effect. They are not completely diamagnetic i.e. they show paramagnetic behavior. They cannot conduct electricity at normal temperature. They are named as hard superconductors as they differ in their transition from normal state to superconducting state from type – I. Their transition to superconductive state is much gradual than type – I i.e. they don’t lose superconducting state easily.
Type – II is useful to make electromagnets, which are used to produce strong magnetic fields, as they have high critical magnetic field. They are also known as high temperature superconductors. There exists a mixed state in type-II Superconductors b/w low critical magnetic field (at which material starts to loose superconducting state) and high magnetic field ( at which it completely loses superconducting behavior), as this process is gradual in type – II. Even very minute amount of impurity can affect superconductivity of type – II. NbTi and Nb3Sn are examples of type – II.
Uses of superconductors
Superconductors are used in variety of applications, some of which are given as follows:
- Superconductors exhibit Meissner effect: all magnetic flux inside material is cancelled thus making the material, fully diamagnetic.
- They are used to produce an intense magnetic field by electromagnetic induction, which is then used to direct and accelerate the beam wherever it is desired.
- Magnetic-levitation is a superconductor application. Transport vehicles, like trains, can be ‘floated’ on solid superconducting magnets, eliminating the friction b/w train and its tracks. In addition, traditional electromagnets lose a huge amount of electrical power in the form of heat and are physically much larger than superconducting magnets.
- In bio-magnetism, superconductors are also used. To identify what’s going on inside the human body, doctors need a non-invasive procedure. Because of a strong superconductor-derived magnetic field, hydrogen atoms in the water and fat molecules in the body are compelled to draw energy from the magnetic field. They then release this energy at a frequency which a computer can detect and view graphically: Magnetic Resonance Imaging (MRI).
- Electric generators are made of superconducting wires, wound with copper wires, making them more powerful than traditional generators. Their performance is currently above 99% and their volume is around half that of traditional generators. These facts make them very profitable in power utility projects.
- Another commercial power project on which the superconductor also operates is energy storage, to increase power stability.