Superconductivity is the ability of certain materials to conduct electric current with practically zero resistance. This capacity produces interesting and potentially useful effects. For a material to behave as a superconductor, low temperatures are required.
2. INTRODUCTION TO SUPERCONDUCTIVITY
It was thought that the electrical resistance of a conductor
becomes zero only at absolute zero temperature. But in 1911,
H. Kammerlingh Onnes studied the properties of mercury at
very low temperature using liquid helium and is found that
the resistivity of mercury drops to zero at 4.2 K and changes
into a superconducting material.
3. SUPERCONDUCTIVITY
Definition
The ability of certain metals and alloys exhibit almost zero
electrical resistivity when they are cooled to low temperature
is known as superconducting. (ie., maximum conductivity with
zero resistance at zero Kelvin)
4. CRITICAL TEMPERATURE ( Tc ) (or)
TRANSITION TEMPERATURE
The temperature at which a normal conductor loses its resistivity and
becomes a superconductor is known as critical temperature (or) Transition
temperature. Every superconductor has its own critical temperature at
which it passes over into superconducting state. Depending on the
transition temperature, superconductors are classified into two groups are
• Low temperature superconductors (LTS):The superconductors
which have low transition temperature (below 30K) are known as
low temperature superconductors.
Example: Tin (3.2 K), Mercury (4.15 K).
• High temperature superconductors (HTS): The superconductors
which have high transition temperature (above 30K) is known as
high temperature superconductors.
Example: Barium - Lanthanum - Copper - Oxide (BLCO) - 35 K
Yttrium - Barium - Copper - Oxide - (Y Ba2 Cu3 O4) - 92 K
5. PROPERTIES OF SUPERCONDUCTORS
At Critical temperature, the following properties are observed.
1. The electrical resistivity drops to zero.
2. The magnetic flux lines are excluded (ejected out)
from the superconductors.
3. There is discontinuous change in the specific heat.
4. There are small changes in the thermal conductivity
and volume of the materials
7. SQUID
(Superconducting Quantum Interference Device)
(We know that a small charge in magnetic field produces
variation in the flux quantum.)
It consists of a superconducting ring which can have the magnetic
field of quantum values (1,2,3, ) of flux placed in between two Josephson
junctions as shown in figure.
When the magnetic field is applied perpendicular to the plane of the
ring, the current is induced at the two Josephson junctions.
The induced current produces the interference pattern and if flows
around the ring so that the magnetic flux in the ring can have the
quantum value of magnetic field applied.
Applications
1. SQUID can be used to defect the variation of very minute
magnetic signals in terms of quantum flux.
2. It is used as a storage device for magnetic flux.
3. It is used to study earth quakes and to remove
paramagnetic impurities
8. ISOTOPE EFFECT
The presence of isotopes in superconductor change the transition
temperature of the superconductor. The transition temperature is
found to be inversely proportional to the square root of the atomic
weight of the isotope (M).
The transition temperature of the heavier isotope is less than that of
the lighter isotope.
The atomic mass of mercury varies between 199.5 and 204.4. due to
variation in atomic mass, the transition temperature of Isotope of
mercury varies between 4.185 K and 4.146K
9. TYPES OF SUPER CONDUCTORS
Superconductors are classified as follows
Based on the value of Hc we have,
Type I (or) Soft superconductors
Type II (or) Hard superconductors
Based on the value of Tc we have,
High temperature superconductors
Low temperature superconductors
10. TYPE I SUPERCONDUCTOR
When the Super conductor is kept in the Magnetic field and if the field is increased
the super conductor becomes a normal conductor immediately at critical magnetic
field as shown in fig. This type of materials are termed as Type 1 super conductor.
Characteristics
1. They exhibit complete Meissner Effect.
2. They have only one critical magnetic field value.
3. Below the material behaves as superconductor and above the material
behaves as normal conductor.
These are called as Soft superconductors
11. TYPE II SUPERCONDUCTOR
In type II superconductor, the magnetic field is
excluded from the material and the material loses its
superconducting property gradually rather than
abruptly
Characteristics
1. They do not exhibit a complete Meissner Effect.
2. They have two critical magnetic field values. Lower critical magnetic filed [HCl] and
Higher critical magnetic field [HC2].
3. Below HC1 the material behaves as superconductor and above the material
behaves as normal conductor. The region in between [HCl] and [HC2] is called
mixed state or vortex region.
These are called as Hard superconductors
12. LOW TC SUPERCONDUCTORS AND HIGH TC
SUPERCONDUCTORS
Low Tc Superconductors
1. The superconductors having the critical temperature less than 30 K are known as
low TC Superconductors or elemental superconductors.
2. The Superconductors by BCS theory.
3. It is explained by BCS theory.
4. It is not so useful due to its low temperature maintenance.
5. It is called as N-type superconductor.
High Tc Superconductors
1. The superconductors having the critical temperature greater than 30 K are known
as high TC Superconductors or ceramic or oxide superconductors.
2. The Superconductors is due to hole states.
3. It is explained by RVB theory proposed by Anderson.
4. It is very useful for commercial and engineering purposes.
5. It is called as P-type superconductor
15. APPLICATIONS OF SUPERCONDUCTORS
General Applications
1. Electric generators can be made by using superconductors with smaller
size, less weight and low energy consumption.
2. Superconductors can be used for the transmission of power over very
long distances.
3. Superconductors can be used in switching Devices.
4. The superconductors can be used in sensitive electrical instruments.
5. It can be used as a memory or storage element in computers.
6. These are used to design Cryotron, Maglev, Josephson Devices and
SQUID.
7. DC superconducting motors are used in ship propulsion and in large
mills.
8. Superconducting magnetic field may be used to launch satellite into
orbit directly from the earth without use of rockets.
9. Ore separation can be done by using machines made of
superconducting magnets.
10. These are used in NMR (Nuclear Magnetic Resonance) imaging
equipments which is used for scanning purposes.
11. Superconductors are used for the detection of brain tumor, defective
cells, etc.,
12. Superconducting solenoids are used in magneto hydrodynamic power
generation to maintain the plasma in the body.
16. CRYOTRON
It is a magnetically operated current switch. The superconducting property
disappear when the magnetic field is greater than critical field (H >Hc).
It consists of a superconducting material [A] and it is surrounded by a super
conducting coil of wire [B].
When the critical magnetic field of wire B exceeds or less than that of a
Superconducting material A, the current in A can be controlled by the current in
the material B, it can act as relay or switching elements and it can be used as
memory or storage element in computers
17. MAGLEV (Magnetic Levitation)
Maglev is a magnetic levitated train, its works under the principal of
Electromagnetic induction. This train cannot move over the rail. Instead it floats
above the rails, so that it moves faster with speed of 500 Km/hr without any
frictional loss. It has two superconducting magnet on each side of the train and
there is guiding system consisting of ‘S” shaped coils on each side. Due to actions
of these magnets the train moves faster by levitation principle.
