A PRESENTATION ON SUPERCONDUCTIVITY

Prepared by-SOUNAK GUHA

UG-IVMETALLURGICAL & MATERIALS ENGINEERING

JADAVPUR UNIVERSITYguhasounak@gmail.comContact No-8981067720

CONTENT

• What is a Superconductor?• Discovery of Superconductivity• Theory of Superconductivity• Meissner Effect• Classification of superconductors• Applications of Superconductors• Limitations• Conclusion

What is superconductivity?

Superconductivity is a phenomenon of zero electrical resistance & expulsion of magnetic field occurring in certain materials when

cooled below a characteristic critical temperature (Tc)

DISCOVERY OF SUPERCONDUCTIVITY

Superconductivity was first discovered by H Kamerlingh Onnes in 1911. Just 3 years earlier, he had developed a method to liquefy Helium gas which he applied to cool a wire made of Mercury to the temperature 4.3K & found that it’s electrical resistance suddenly disappeared. He also discovered that a superconducting material can be returned to the normal (non superconducting) state either by passing a sufficiently large current through it or by applying a sufficiently strong magnetic field(Bc) to it. He was awarded the Noble prize for his contributions to Superconductivity.

THEORY OF SUPERCONDUCTIVITY

‘’Zero resistance’’- Is it possible?In a normal conductor, an electric current may be visualized as a flow

of electrons moving across a heavy ionic lattice. The electrons are constantly colliding with the ions in the lattice, and during each

collision some of the energy carried by the electrons is absorbed by the lattice and converted into heat, which is essentially the vibrational

kinetic energy of the lattice ions. As a result, the energy carried by the current is constantly being dissipated. This is the phenomenon of

electrical resistance. In superconductors the situation is completely different. WHY?

B.C.S THEORYIn 1957, J. Bardeen, L. Cooper and R. Schrieffer set forth their theory, known as the BCS theory which, for the first time, explained almost all the properties of superconductive materials. Their work was recognised with the Nobel Prize for Physics in 1972.According to L. Cooper, electrons

in superconductors can’t flow individually, they bond into Cooper pairs & flow through the lattice. The pairing is caused by attractive force between electrons from the exchange of Phonons. This pair of electron behaves like Bosons & are completely different from usual

electrons which are like Fermions. The electrons in Cooper pairs have opposite spin, so net spin of Cooper pair is zero. When a superconductor is cooled below its critical...........................temperature its electronic properties are altered appreciably, but no change in the crystal

structure is revealed by X-ray crystallographic studies...............................................................

PARAMETERS OF SUPERCONDUCTIVITY(1)Critical temperature(Tc) & (2) Critical magnetic field(Bc)

Below a critical temperature a material becomes superconductor & it expulses magnetic field. Conversely if a magnetic field is applied (which is higher than the critical value) to a superconductor, it’s superconductivity vanishes quickly.

Here Hc is the critical value of the applied magnetic field.

Band Gap in superconductorsTransition from normal to the superconductivity phase is very sharp. In pure metals it takes

place within temperature range as small as 10^-3 K. It has been seen experimentally that band gap in superconductors is dependent on their critical temperature(Tc)

ISOTOPE EFFECTAlthough B.C.S theory explained almost every properties of

superconductors , it couldn’t explain how do the atoms or their arrangement in a solid assist in the superconductivity. In 1950 scientist Frolich experimentally found that critical temperature Tc

Depends on the isotopic mass of the atoms comprising a sample. Many elements having different no of Neutrons can have different mass. He showed that Pb with an atomic mass 207 (from non radioactive ore) &

another Pb with M= 206( from Uranium ore) have different critical temperature. This effect in superconductors can be summarized as

(M^p*Tc)= constant , where p=1/2. But for some materials this value may vary. The different values of P depend on vibration spectrum of real

crystal lattices.

MEISSNER EFFECT

When a superconductor is placed in a weak external magnetic field H, and cooled below its transition temperature, the magnetic field is ejected. The Meissner effect does not cause the field to be completely ejected but instead the field penetrates

the superconductor but only to a very small distance, characterized by a parameter λ, called the London penetration depth, decaying exponentially to zero within the bulk of the material. The Meissner effect is a defining characteristic of

superconductivity. For most superconductors, the London penetration depth is on the order of 100 nm. Meissner effect must not be confused with Lenz’s law.

where H is the magnetic field and λ is the London penetration depth.This equation, which is known as the London equation, predicts that the magnetic field in a superconductor decays exponentially from whatever value it possesses at the surface. Meissner effect produces Quantum Levitation which is a remarkable

phenomenon of science.

CLASSIFICATION OF SUPERCONDUCTORS

There are many criteria by which superconductors are classified. The most common are:

Response to a magnetic field : A superconductor can be Type I, meaning it has single critical field, above which all superconductivity is lost; or Type II,

meaning it has two critical fields, between which it allows partial penetration of the magnetic field.

By theory of operation: It is conventional if it can be explained by the BCS theory or its derivatives, or unconventional, otherwise.

By critical temperature: A superconductor is generally considered high temperature if it reaches a superconducting state when cooled using

liquid nitrogen – that is, at only Tc > 77 K) – or low temperature if more aggressive cooling techniques are required to reach its critical temperature.

By material: Superconductor material classes include chemical elements (e.g. mercury or lead), alloys (such as niobium-titanium,

and niobium nitride),ceramics (YBCO and magnesium diboride), or organic superconductors (fullerenes and carbon nanotubes; though perhaps

these examples should be included among the chemical elements, as they are composed entirely of carbon.

APPLICATIONS OF SUPERCONDUCTORS

MEDICAL FIELDM.R.I: Superconducting magnets are some of the most powerful electromagnets known. They

are used in M.R.I machines. MRI machines are incredibly expensive, largely because they require exotic substances like liquid helium to cool metal coils to the point that they can conduct enough electricity to create the strengths of magnetic field of 3-5 tesla (20 times stronger than Earth’s magnetic field) required to bulk-reorient the hydrogen atoms and fat molecules within the human body. These atoms and molecules then release energy that is detected and formed

into a graphic image. MRI is a widely used radiographic method for medical diagnosis.

MODERN TECHNOLOGYSQUID: Superconducting Quantum Interference Device, it is used in

MEG, Particle accelerators and many other devices.Superconducting wires: They are the most important part of modern

technology. Starting from data storage to L.H.C , superconductors have been an indispensable part of modern technology. Maglev train is the greatest example of that. It’s based on the magnetic levitation.

LIMITATIONS

The biggest problem of the superconductors is

their ‘’COOL’’ nature . Creating a temperature near 0k is not a easy job. Cooling arrangements involve a huge cost which is why superconductors have yet to make a really big

impact on the world, despite being discovered a century ago.

SOLUTION? Scientists are researching on how to create

superconductors at room temperature. So they are basically dealing with high temperature superconductors.

ACHIEVEMENTIn 2009 Scientists created the highest temperature

superconductor (at ambient pressure) named-Mercury Thallium Barium Calcium Copper Oxide

Hg12Tl3Ba30Ca30Cu45O125

having critical temperature 135-166K.

RECENT RESEARCH WORK

In 2013, an international team working with Max Planck researcher Andrea Cavalleri discovered that

when YBCO(Yttrium Barium Copper Oxide)is irradiated with infrared laser pulses it briefly

becomes superconducting at room temperature. That superconductivity property was stable for

milliseconds.

CONCLUSION

Yes, the biggest advantage of all superconductors is their capacity of carrying enormous amount of current without

power loss under critical temperature. So, from this information one may think superconductors can save lots of energy. But if you go for practical purpose , you’ll see that you would probably waste far more energy cooling

transmission wires than you would ever save from having no resistance in the wires. If superconductors could exist at room temperature, the world energy crisis would be

solved. Many scientists are putting their efforts together to create superconductors at high temperature.

SUMMARY (1)Superconductivity depends on 2 variables (1)temp (2)magnetic field.(2)In superconductors electrons exist as cooper pairs which is the main

reason behind superconductivity. (3)Band gap in superconductors is dependent on critical temperature

which is related to the isotopic mass of elements comprising the sample.

(4)Superconductors experience zero electrical resistance & expulsion of magnetic field below a critical temperature.

(5) Meissner effect is the expulsion of magnetic field in superconductors below Tc. Magnitude of magnetic field as a function of distance( depth of penetration)on superconductors is expressed in London equation.

(6) Superconductivity is one of those concepts — like electron spin or time dilation — that seems somewhat esoteric, but which, if mastered

through technology, could truly revolutionize the world.

THANK YOU