Superconductors An exciting field of Physics!
General Objective To understand the nature of superconductivity Specific Objectives: You will be able to 1. Define Superconductivity 2. State the history of Superconductors 3. List the Properties, Types and Applications of Superconductors
Prerequisite knowledge Electrical Conductivity Thermal conductivity Conducting materials 1. Low resistive materials (Ex: Al, Cu, Ag) 2. high resistive materials (Ex: Tungsten, Platinum) 3. Zero resistive materials (Ex: Nichrome, Mercury, Tungsten, Platinum and Alloys)
Introduction Before the discovery of superconductivity, it was thought that the electrical resistance of a conductor becomes zero only at absolute zero. But it is found that, in some materials the electrical resistance becomes zero, when they are cooled to very low temperature. Ex: Electrical resistance of pure mercury suddenly drops to zero when it is cooled below 4.2 Kelvin and becomes a superconductor.
SO - 1 Superconductivity? The phenomenon of losing the resistivity absolutely, when cooled to sufficiently low temperature is called superconductivity. Superconductors are materials that conduct electricity with no resistance. This means that, unlike the more familiar conductors such as copper or steel, a superconductor can carry a current indefinitely without losing any energy at temperatures near absolute zero.
Comparisons of Temperatures Temperatures F C K water boils 212.0 100.0 373.2 body temp 98.6 37.0 310.2 room temp 77.0 25.0 298.2 water freezes 32.0 0.0 273.2 mercury freezes -37.8-38.8 234.4 dry ice -108.4-78.0 195.2 liquid Oxygen -297.4-183.0 90.2 liquid Nitrogen -320.8-196.0 77.2 liquid Helium -452.1-269.0 4.2 absolute zero -459.7-273.2 0.0
Resistance (Ω) HISTORY OF SUPERCONDUCTIVITY 1895 Helium - William Ramsay in England (Isolation of Helium). 1908 Liquid Helium ( 269 C) (about 4 K) - H. Kamerlingh Onnes 1911- Kamerlingh Onnes (Nobel prize in 1913) while studying the resistance of solid mercury at cryogenic temperatures using the recentlydiscovered liquid helium as a refrigerant. At 4.2 K, the resistance abruptly disappeared. 1913- lead (7 K), 1941 niobium nitride(16 K) 0.15 0.10 T c 0.0 4.0 4.1 4.2 4.3 4.4 Temperature (K)
Transition Temperature (or) Critical Temperature (T c ) The temperature at which a normal conductor loses its resistivity and becomes a super conductor is known as transition temperature or critical temperature. Low temperature superconductors ( > Tc ) i.e., low transition temperature superconductors. High temperature superconductors ( < Tc ) i.e., high transition temperature superconductors. Superconduction transition is reversible. i.e., above critical temperature (T c ), the superconductor again becomes normal conductor.
Critical Temperature (T c ) RESISTIVITY TEMPERATURE
Superconducting elements Ferromagnetic elements are not superconducting The best conductors (Ag, Cu, Au..) are not superconducting Nb has the highest T C = 9.2K from all the elements 11
Properties of Superconductors Electrical resistance, Magnetic property, Meissner effect, Effect of electric current and pressure, Isotopic effect
SO -3 Properties of Superconductors 1. Electrical Resistance The electrical resistance of a superconducting material is very less. It is of the order of 10-5 cm
2. Magnetic Property When super conducting materials are subjected to very large value of magnetic field, the super conducting property is destroyed. Critical magnetic field (Hc ): The minimum magnetic fields required to destroy the superconducting state is called the critical magnetic field (H c ) H c = H o [1- (T/T c ) 2 ] H 0 Normal H C H 0 Critical field at 0K Superconducting T - Temperature below T C T C - Transition Temperature T (K) T C
3. The Meissner Effect (1933 ) A complete expulsion of all magnetic field by a superconducting material is called Meissner effect Superconductors push out magnetic fields Act as perfect diamagnets Magnetic fields does not penetrate the sample Meissner Effect is reversible H H C Currents i appear, to cancel B. i x B on the superconductor produces repulsion.
4. Effect of electric current When a large value of A.C. current is applied to a super conducting material it induces some magnetic field in the material and because of this magnetic field, the super conducting property of the material is destroyed.
What destroys superconductivity? A current: produces magnetic field which in destroys superconductivity. Current density Magnetic field Magnetic field: the spins of the C-P will be directed parallel. (should be antiparallel in C-P) Temperature High temperatures: strong thermal vibration of the lattice predominate over the electron-phonon coupling. 17
Types of Super Conductors There are two types of super conductors based on their variation in magnetisation, due to external magnetic field applied. 1. Type I super conductor (or) Soft super conductor 2. Type II super conductor (or) Hard super conductor
Magnetisation The term magnetisation or the intensity of magnetisation is the process of converting a non magnetic material into a magnetic material. The magnetic moment (M=IA) per unit volume (I = M/V).
Type I (Soft) Super Conductor When the super conductor is kept in the magnetic field and if the field is increased the super conductor becomes a normal conductor abruptly at critical magnetic field. This type of materials are termed as Type I superconductors. Below Hc, the specimen excludes all the magnetic lines of force and exhibits perfect Meissner effect. Type I superconductors are perfect diamagnets. Super conductor Normal state
Type II (Hard) Super Conductor When the super conductor is kept in the magnetic field and if the field is increased, below the lower critical field H c1, the materials exhibits perfect diamagnetism (super conductor) and above H c1, the magnetisation decreases and hence magnetic flux starts penetrating through the material. The material is said to be in a mixed state between H c1 and H c2. Above H c2, it becomes normal conductor. The materials which loses its super conducting property gradually due to the increase in magnetic field are called type II super conductors. SC State Normal state
TYPE I SUPERCONDUCTORS Sudden loss of magnetisation Gradual loss of magnetisation TYPE I SUPERCONDUCTORS Exhibit Meissner Effect Does not exhibit complete Meissner Effect No mixed state One H C = 0.1 tesla, and the value is low Mixed state present - Gradual transition from Superconducting state to normal state Two H C s H C1 & H C2 ( 30 tesla) value is high Only one critical field T c Super current flows on material surface Perfect Diamagnetic and completely follows Meissner effct below H c1 Electrically superconductor between H c1 and H c2 Super current can flow over the bulk of the material Soft superconductor, Eg.s Pb, Sn, Hg Hard superconductor, Eg.s Nb-Sn, Nb-Ti Cannot carry large currents Can carry large currents when field is in between H c1 and H c2; Used to generate very high magnetic fields Can tolerate impurities without affecting the superconducting properties. Cannot tolerate impurities, i.e., the impurity affects the superconducting property -M Superconducting -M Superconducting Mixed Normal Normal H C H H C1 H C H C2
Nobel Prizes for superconductivity Kamerlingh Onnes (1913), Bardeen, Leon N. Cooper, and J. Robert Schrieffer (1972), Brian D. Josephson (1973), Georg Bednorz and Alex K. Muller (1987) Alexei A. Abrikosov, Vitaly L. Ginzburg, and Anthony J. Leggett (2003), "for pioneering contributions to the theory of superconductors and superfluids"
Important Factors to define a Superconducting State 1. critical temperature (Tc) 2. critical field (Hc) 3. critical current density (Jc).
BCS theory (1957) The Origin of Superconductivity Describes why materials are superconducting John Bardeen, Leon Cooper and Bob Schrieffer B. C. S. Nobel Prize in 1972 for their microscopic theory in 1957 nearly 50 years after their discovery by Kamerlingh Onnes! The theory describes superconductivity as a microscopic effect caused by a condensation of pairs of electrons into a boson-like state. (Bosons are one of the two fundamental classes of subatomic particles,)
Cooper pair The two electron interacting attractively in the phonon field are called cooper pair. Electrons pairs, called Cooper pairs, which propagate throughout the lattice
HIGH Tc SUPERCONDUCTORS Low (Tc) Superconductors Superconductors that require liquid helium coolant are called low temperature superconductors. Liquid helium temperature is 4.2K above absolute zero High (Tc) Superconductors Superconductors having their Tc values above the temperature of liquid nitrogen (77K) are called the high temperature superconductors.
Applications of Superconductivity Cost Saving and Cost Increase Zero resistance No energy lost, Novel uses Need refrigeration, fabrication costs.
Applications of Superconductivity Magnetic levitation, SQUIDS Cryotron,
1. MAGLEV or MAGNETIC LEVITATION The levitation coils are installed on the sidewalls of the guide way. When the on board superconducting magnets pass through the coils, an electric current is induced (electromagnets temporarily). As a result, the forces push the superconducting magnet upwards and ones which pull them upwards simultaneously, thereby levitating the Maglev vehicle. Highest recorded speed, 603 km / h, Japan, April 2015 Potential to exceed 6,400 km/h if deployed in an evacuated tunnel.
2. SQUID (Superconductor Quantum Interference Device) Superconducting Quantum Interference Devices can measure tiny fields such as those due to currents flowing in your heart muscle The most sensitive type of detector known to science to measure very small magnetic fields. Invented in 1964 by Robert Jaklevic, John Lambe, Arnold Silver, and James Mercereau of Ford Research Labs Principle : Small change in magnetic field, produces variation in the flux quantum. Construction: The superconducting quantum interference device (SQUID) consists of two superconductors separated by thin insulating layers to form two parallel Josephson junctions.
Types Two main types of SQUID: 1) RF SQUIDs has only one Josephson junction 2)DC SQUIDs have two or more junctions. Thereby, more difficult and expensive to produce. much more sensitive.
How it works Phase change due to external magnetic field Current flow Voltage change Due to B field Due to junctions Must be quantized
3. CRYOTRON It is a magnetically operated current switch. Principle: The super conducting property disappears when the magnetic field is grater than critical field (H c )
PROBLEMS The transition temperature of Pb is 7.2K. But at 5 K it loses the superconducting property If subjected to a magnetic field of 3.3 x10 4 A/m. Find the maximum value of H which will allow the metal to retain its superconductivity at 0K Solution: H c = H o [1-(T/Tc) 2 ] H o = H c / [1-(T/Tc) 2 ] = 3.3 x10 4 A/m /1-(25/51.28) Ans: 6.37x10 4 A/m The transition temperature of lead is 7.26K. The maximum critical field for the material is x10 5 A/m. Lead has to be used as a superconductor subjected to a magnetic field of 4 x10 4 A/m. What precaution will have to be taken? T = Tc [1- H c (T)/ H c (o)] 1/2 = 7.08K The temp of the metal should be held below 7.08K
The dream - Tomorrow s Superconducting World Energy Saving: power lines electric motors transformers Medical Diagnostics: Magnetic Resonance Imaging SQUID: Brain activity Heart function Computing: 1000 times faster supercomputers Information Technology: much faster, wider band communications magnetically launched space shuttle Cargocarrying submarines, all-electric US Navy 350 mph levitated Intercity trains Underground rapid transit: Heathrow to Gatwick in 10 minutes
http://www.bestofjesse.com/projects /indust/project1.html Some of these dreams are already reality SQUID measurement of neuromagnetic signals Japanese levitating train has superconducting magnets onboard Superconducting power cable installed in Denmark (nuclear) magnetic resonance imaging of the brain, in the field from a superconducting magnet
Formative Assessment 1. In superconductivity, the electrical resistance of material becomes Zero Infinite Finite All of the above Zero
2. The superconducting state is perfectly in nature. Diamagnetic Paramagnetic Ferromagnetic Ferromagnetic Diamagnetic
3. Which of the following are the properties of superconductors? They are diamagnetic in nature They have zero resistivity They have infinite conductivity All of the above All of the above
4. Superconductivity was first observed by 1 : Ohm 2 : Ampere 3 : H.K. Onnes 4 : Schrieffer 3 : H.K. Onnes
5. The first successful theory on superconductivity was due to 1 : Schrieffer 2 : Onnes 3 : Ampere and Schrieffer 4 : Bardeen Cooper and Schrieffer 4 : Bardeen Cooper and Schrieffer
Stimulating question 1. Even though Nb 3 Sn has produced high magnetic field than Nb-Ti, why it is not used in the MRI?
Stimulating question 2. How does a magnetic levitation train stops? A linear motor (propulsion coils) mounted in the track. This linear motor operates to propel the train forward, and when it is necessary to stop the train, the linear motor acts in reverse.
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