Introduction:
In 20-th century a major breakthrough in science came with the discovery of new state of matter, called the superconducting state. Generally materials are divided into three different classes based on their ability to conduct current. They are metals, insulators and semiconductors. Metals (Example: Copper, silver, gold) are good conductors, electrons are moving freely and carry electrical charge inside a conductor. Insulators (Example: rubber, glass, plastics) are bad conductors. Semiconductors are those whose conductivity lies in between metals and insulators. Superconductor are not belongs to any three above categories.
Metallic wires conduct current offer resistance to the flow of electrons. Inside conductor electrons suffer scattering from lattice defects, vibration of atoms in the crystals, and free paths of those electrons become truncated. This is what is called resistance of a wire and it depends on the metal itself. Due to this resistance we see heating effect in wires and energy loss. If one can make a zero resistance coil then we can forget about this heating effect. This dream came into reality in 1911 with the discovery of superconductivity. Professor Kamerlingh Onnes observed that on cooling mercury below 4.2 K its electrical resistance suddenly drops down to zero. The zero resistance state is named as superconducting state. The temperature at which the transition from normal state to superconducting state took place is called critical temperature Tc.
History behind the discovery of superconductors:
Professor Kamerlingh Onnes in his laboratory at University of Leiden had developed technology to liquefy gases like; oxygen, hydrogen to produce low temperature. He wants to study how conductivity of elements like mercury, lead would behave at this extreme low temperature. Finally in 1908 he succeeded in liquefying helium at temperature of 4.2 K, the lowest among all other gases. Discovery of liquid helium promotes the discovery of superconductivity in mercury in 1911. Even today many of us could not imagine how much cold 4.2 K is. In the beginning of 20-th century Professor Onnes made his laboratory equipped with unique cryogenic facility to investigate properties of matter at such extreme condition. He received the Nobel Prize in 1913 for his striking discovery.
Why should we learn Superconductivity?
Discovery of superconductors creates revolutions in science. It changes our understanding behind the microscopic source of electrical resistance in materials below TC. So many unexpected observations associated with superconductivity create tremendous research interest in this field. Here I am mentioning few of them.
- In 1914, Professor Onnes discovered once current induced in a superconducting ring persists for long time never degrading or dissipating.
- Superconductivity observed in many metals, in alloys, in ceramics, allotropes of carbon etc., but it has not been observed in some good normal conductors like; cooper, silver, gold except at high pressure.
- Superconductivity can be destroyed by application of strong magnetic field, called the critical field HC even if the temperature of the sample is less than T
- External magnetic field can never penetrate inside a superconductor. This is Meissner effect, discovered in 1933. Flux exclusion was also seen when a material first cooled to TC and then the field is switched on.
- When two superconductors are joined by a very thin insulating layer, tunnelling of superconducting electron pairs across the thin layer happens. This is called Josephson Effect, discovered in 1962.
- Levitation is the magneto-mechanical phenomenon. Being a perfect diamagnet bulk of the superconductor is shielded from external magnetic field by appropriate surface current that causes a counter reacting magnetic field and superconductors are found to be levitate over a strong magnet.
Superconductors are neither conductors nor insulators. It is a unique state of a material. Zero electrical resistance and perfect diamagnetism defines the superconducting state.Theoretical understanding of superconducting phase has been reached after several years from the discovery. Meissner Effect describes that magnetic flux cannot penetrate inside the bulk of specimen was explained by two British Physicists; H. London and F. London in 1935. London equation described that magnetic flux lines does not abruptly stops at the surface rather enters into superconductor and exponentially die out within 50 nm to 500 nm. Finally quantum mechanical description of superconductivity came in 1957 by Barden, Cooper and Schrieffer. This is so called the BCS theory of superconductivity. BCS theory describes that supercurrent is not carried by a single electron rather by pair of electrons with opposite spin and momentum, called cooper pairs.
Applications of superconductors
Science from the discovery in 1911 thousands of material found as superconductors. Zero electrical resistance, perfect diamagnetism, persistent current are some of the key features that make them suitable for application in science and technology. Superconducting electromagnet can produce stable high magnetic field, used in particle accelerators, magnetic resonance imaging (MRI) and nuclear magnetic resonance (NMR) machine. Josephson Junctions are used in sensitive magnetometers based on SQUID (superconducting quantum interference device). But perhaps MRI is the best successful commercial application of superconductors in 20-th century.
Magnetic Resonance Imaging (MRI)
Application of superconductor in medical science was started around 1970 in Great Britain and United States of America. Once current is stored in superconducting coli it stays for ever and produces a steady strong magnetic field used in MRI machines. The MRI body scanner became an essential diagnosis tool for the doctors across the world.
How does it work???
Inside human body soft tissues, blood vessels, fat molecules contains a large percentage of water molecules that contains protons behaving as tiny magnet or like a compass needle. When a person is placed inside a MRI scanner those protons orient them in the direction of magnetic field. The typical field strength needed is 1 Tesla or more. However some of those protons going out of alignment by absorbing energy from radio waves at just right frequency generated by MRI machine. Radio frequency field is composed of short pulses. At the end of a short pulse those protons gradually realign with the static constant field. You may consider this as deflection in compass needle occurs every time when a weak magnet passes nearby. As proton in various parts of the body realign at different rates, different structure of the tissues can be identified and captured. In addition MRI can produce cross sectional view therefore various tissues and their associated disordered could be identified.
Advantages and Disadvantages of MRI Scanner:
- MRI can take images of every parts of the body. It is less harm full because human cells are not exposed to any kind of ionizing radiation.
- Perhaps the best possible technique to look inside our body without having any cut or open. There are other techniques also like; CT scans but MRI produces better image quality than others, hence most widely used for medical diagnostic.
- MRI usually takes longer time for imaging. If a patient have pacemaker in his body then MRI scan is not advisable. Strong magnetic field and powerful radio frequency source causes some harmful effects in this case.
Future Prospects:
The initial problems of cryostat design were overcome and MRI technology is quite mature now. Long solenoid type magnets with uniform winding density are used to produce strong field. The strength of the magnetic field could be found from Ampere’s Law. Most of the high resolution MRI scanners are operated at field strength of 1.5 Tesla. Although 3 Tesla MRI are regularly used for diagnosis and 7 Tesla systems are mainly used for medical research. High field MRI scanner gives better signal to noise ratio and better resolution. With the advancement of superconducting technology we expect that more functionality could be incorporated and cost of the system could be reduced to make it more accessible for us.
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