In this article we will discuss about:- 1. Introduction to Superconductivity 2. Condition for Super Conductivity 3. Types 4. Silsbee Rule 5. Meissner Effect 6. Modern Ceramic 7. Superconducting Magnet.
Introduction to Superconductivity:
It was discovered in 1911 by Kamerlingh Onnes in Leiden when he observed that the electrical resistivity of mercury disappeared completely at temperature below approximate 4.2°K.
A state of material in which it has zero resistivity is called super conductivity.
These elements compounds and alloys of metal exhibit extraordinary magnetic and electrical behaviour at extremely low temperature.
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Transition Temperature:
The temperature at which the transition from the ‘normal’ state to the super conducting state occurs is called transition temperature (TC).
Note:
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1. The material having number of valance electron (Z) from 2 to 8 generally show super conductivity.
2. The transition temperature all lie below 10°K.
3. The elements which at room temperature are good conductors (Cu, Ag, Au and the alkali metal) are absent from the list of super conducting materials. In fact, the superconducting elements are relatively poor conductors at room temperature.
4. Fe, Ni, Co, are also not superconducting materials.
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5. The super conducting compounds and alloys do not necessarily have super conducting components such as- Pb2Au, Pb2Tl2, Snsb, CuS, NbN, MoN, NbB, ZrC.
Conditions for Super Conductivity:
1. Resistivity should be zero.
2. Perfect dia-magnetism (µr = 0).
3. Applied magnetic field should be less than critical magnetic field.
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4. Power loss (copper) l2R = 0.
5. Specific resistance, P = 0.
6. Magnetic susceptibility x = -1.
Critical Magnetic Field:
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It is a minimum amount of magnetic field required at a given temperature to H^ destroy super conductivity.
Its value is given by:
Where,
H0 = Critical field at 0°K
HC = Critical field at T°K
TC = Transition or critical temperature
Types of Super-Conductors:
Type -1 Super-Conductor:
Low temperature superconductor- Critical Temp, less than 25K. Mostly LTS are metallic e.g. Nb-Ti, Nb-Zr, Nb3Sn, Nb3AI, V3Ga etc.
High temperature super conductor (HTS)- Critical temperature > 25K. These are ceramics type e.g.: Perovskite oxide.
Certain materials, particularly those, that have low melting points and are mechanically soft and easily obtained in pure, strain free condition display similarities in their superconductive behavior. They are distinguished as Ideal or soft super-conductors.
Their critical field and transition temperature are low.
They exhibit complete meissner’s effect and silsbee rule.
The change of state from normal to super-conducting and vice versa is abrupt or sharp.
These are unsuitable as magnets because of their low HC the values.
Example: Zn, Pb, Hg, Al, In etc.
Type-II Super-Conductor:
They are also called hard super-conductor or non-ideal super-conductor.
The exhibit incomplete meissner’s effect and break down of silsbee rule.
There critical field and transition temperature are high.
The change of state from normal to super-conductor and vice versa is gradual.
These super conductor exhibit incomplete meissner’s effect in vortex region.
Example:
Nb3Al, NbTi, Nb3Sn, (BiPbSrCaCuO) bismuth lead strontium calcium copper oxide.
Application of Super-Conductors:
i. Magnetic Resonance Imaging (MRI)
ii. Generators and Motors
iii. Switching elements like “Cryotrons”
iv. Magnets for Nuclear Fission.
Silsbee Rule of Superconductivity:
The magnetic field which causes a super-conductor to become normal is not necessarily an externally applied field; it may also arise as a result of electric current flow in the conductor.
Thus superconductivity in a long circular wire of radius R may be destroyed when the current exceeds the value lC which at the surface of the wire would produce the critical magnetic field HC.
If a current I is flowing in long wire of super conducting materials, according to ampere’s law-
This rule prevents the use of super conductors as coils for the production of strong magnetic fields.
The disappearance of super conductivity for fields above the critical field is the principle on which a cryotron operates.
As we know that in super conductor resistively is zero and also the magnetic flux density in a super conductor also vanishes i.e., B = 0.
Also we know that B = µ0(H + M)
but B = 0 => M = -H
Therefore, M = (µr – 1)H => -H = (µr – 1)H
-1 = µr – 1
µr = 0
Relationship between Transition Temperature TC and Isotopic Mass M-
M1/2TC = constant
Meissner Effect of Superconductivity:
The repulsion of magnetic flux from the interior of a piece of superconducting material, as the material undergoes to the transition to the super conducting phase is known as meissner’s effect.
It has been observed that when a long super conductor is cooled in a longitudinal magnetic field from above transition temperature, the lines of induction are pushed out. Then inside the specimen, B = 0
Also B = m0 (H + M), for B = 0, H = – M, consequently since Xm = -M/H = -1, we may state that magnetic susceptibility in a superconductor is negative. This referred to as perfect diamagnetism. This phenomenon is called Meissner Effect.
Factors Affecting Super Conductivity and Transition Temperature:
1. Frequency:
Super conductivity decreases with increase in frequency and it is observed upto radio frequency (10 MHz). Above 10 MHz resistivity increases. At infrared frequency (1013 Hz) the resistivity of material is same as normal state.
2. Entropy:
Entropy increases on going from super conducting state to normal state.
3. Thermal Conductivity:
It decreases on going from normal state to super conducting state.
4. Isotope Mass (M):
TC ∝ 1/√M.
Modern Ceramic Superconductors:
La Ba CuO superconductor, super conducting at 70K.
The Yttrium barium copper oxide i.e. YBCO (Y Ba2 Cu3 O7-x) behave as superconductors at 90 K. Critical current densities in excess of 108 A/cm2 was achieved in it.
Bismuth and thallium superconducting compounds has characteristics same order line with yttrium compounds.
BSCCO (Bi Sr Ca CuO) showing TC = 106 K. and Zero resistance at 85 K.
Thallium barium calcium copper oxide i.e. TBCCO (Th Ba Ca CuO) superconducting at 125 K, but practical application is doubtful due to highly poisonous nature of thallium.
In 2005, the value of TC = 115 K has been reported in Sn2 Ba2 (Ca05 Tm0.5) Cu3Ox.
The value of Tc = 127-128 K has been found with material system TI2Ba2Ca2Cu3O10.
The highest value of critical temperature Tc = 138 K reported so far is in Hq0.8 TIP.2 Ba2 Ca2 Cu3 08.33.
Josephson Junction is a device having two superconducting plates separated by an oxide layer. These junctions are used in SQUID, (superconducting quantum interference device) computer memories and other microelectronic devices. It works on the principle of Josephson Effect. This quantum effect is characterized by the tunneling of supper electrons through the insulator of Josephson junction, and is found that flow of current without any driving potential.
Maglev or Magnetic Levitation. This is a levitation phenomenon caused by the opposing magnetic fluxes. The use of this is in wheel-less high speed trains that are equipped with superconducting magnets. The Nb3 Sn and Nb-Ti are used as magnet grade superconductors.
Magneto hydrodynamics (MHD): MHD deals with the motion of an electrically conducting fluid under the influence of magnetic field.
This principle is employed in:
a. MHD power generation
b. MHD ship propulsion, and
c. MHD magnets
In MHD power generation, the thermal energy of hot ionized gas is converted into d.c. electric power. In MHD ship propulsion, the sea water is accelerated by magnetic field. In both these applications, the superconducting magnets are required. The Japanese experimental ship Yamato sailed in 1992 utilized MHD principle.
Magnetic resonance imaging (MRI)- Here, the visual images of the brain and other body parts can be created. Superconducting magnets above 2T are suitable for this purpose.
Suspension systems and motors. Frictionless suspension systems may be found by the interaction between a magnetic flux produced externally and the currents flowing in a superconductor. If the superconductor is pressed downwards it tries to oppose and come out of the magnetic field, hence the magnetic flux on which it rests is compressed and the repelling force is increased.
Since it is possible to allow high speed rotation to a suspended superconducting body, and that all the conductors in the motor are free of resistance, it is obvious that the ideal of a 100% efficient motor can be closely approximated.
Radiation Detectors:
The operation of these is based on the heat provided by the incident radiation. The superconductor is kept just above its critical temperature, where the resistance is a rapidly varying as a function of temperature. The change in resistance is then calibrated as a function of the incident radiation.
Superconducting Magnet:
Superconducting magnet which can generate magnetic flux densities upto B = 15 tesla (H = 12 × 106 Am-1 in free space). The wires used to make the coils of the superconducting solenoid can carry much higher current densities than conventional conductors.
A cryotron is an assembly of a superconducting wire (core material) ‘α’ surrounded by a coil of another superconducting material β. When temperature of a system in which the cryotron is used, is below the transition temperature of two materials, both ‘α’ and ‘β’ are superconducting; otherwise they attain a normal state. Based on this fact, a cryotron is used as an element in control devices such as flip-flop in a computer.
Its working may be understood as follows:
(i) The current lα in the wire is controlled by the current lβ in the coil. It is due to magnetic field produced by the coil normally exceeds the critical field of the wire (core material) at operating temperature. The intensity of controlling current lβ required to make the core in normal state, depends upon d.c. current flowing through the core. Hence, the core current also produces a magnetic field.
(ii) Tantalum is a suitable material for making wire (or core), if the operating temperature is extremely low (≈ 4.2 K which is liquefaction temperature of helium). Since the coil has to be superconducting even if the control current flows, hence niobium or lead are suitable coil materials.
Supercooled Coils:
Supercooled coils are such electrical systems which can produce a flux density of 10 tesla or more in superconducting state. This flux density is ‘ten’ times more than the flux density produced by normal coils at room temperature.
Note:
115.5 K (-157.5°C) is known as cryogenic temperature. Those industries in which the operations are performed below this temperature are called cryogenic industries. Industries involved fertilizer industry (at -190°C) etc. are such examples. Engines using liquid fuels at so low temperatures are called cryogenic engines.