Download High temperature superconductors are the materials with T c value

Superconductivity
Superconductivity was discovered in 1911 by Heike Kamerlingh Onnes, who was
studying the resistance of solid mercury at cryogenic temperatures using the recently-discovered
liquid helium as a refrigerant. At the temperature of 4.2 K, he observed that the resistance abruptly
disappeared. Superconductivity is an electrical resistance of exactly zero which occurs in certain
materials below a characteristic temperature called critical temperature Tc. In subsequent decades,
superconductivity was found in several other materials. In 1913, lead was found to superconduct at
7 K, and in 1941 niobium nitride was found to superconduct at 16 K. Alloys of Nb3Sn, Nb3Ge, Nb3Al
etc show superconductivity & have Tc values of 18K, 20 K respectively.
Superconductors have some unique properties
•
They have zero electrical resistance.
•
They carry current with no energy loss.
•
Meissner effect.
•
Characteristic critical temperature Tc
Meissner effect
Meissner and Ochsenfeld found that superconducting materials will not permit a
magnetic field to penetrate their bulk. Superconducting materials expel all magnetic flux from their
interior & so are forced out of a magnetic field. This is called Meissner effect & gives rise to
levitation. Levitation occurs, when objects float on air. This can be achieved by the mutual
repulsion between a permanent magnet & a superconductor. Superconductor exhibit perfect
diamagnetism & it repels a magnet. Japanese have an experimental frictionless train ‘Maglev’ that
floats above magnetic rails & has achieved speed of 500km/h
.
A magnet levitating above a high-temperature superconductor, cooled with liquid nitrogen.
However until 1986, the highest temperature that a superconductor operated
was 23K & this temperature was achieved by cooling using liquid Helium. This made the use of
superconductor extremely expensive.
A high temperature super conductor levitating above a magnet
Critical temperature Tc- It is the temperature at which the resistance suddenly drops to zero .
superconductivity occurs. It is also found that Tc changes in presence of a magnetic field.
ie,
As the applied field increases, the critical temperature drops. ie, a
superconductor can be made non superconducting by the application of mag. field. The minimum value
of field strength required to bring about superconductivity is known as critical field strength. Similarly, if
the current in the superconductor exceeds a critical current, the superconductivity is destroyed. This is
known as Silsbee effect
An extremely high current can be passed through a very fine wire of
superconductor. Thus small coils with large number of turns can be used to make extremely powerful
high field electromagnet. Because of zero resistance the wire does not get hot. Because there is no
current loss, once the current is flowing in the coil, it continues indefinitely. The major obstacle to the
widespread use of low temperature superconductor has been the very low value of TC. The only way of
attaining this value was to use liquid helium which is very expensive.
Classification :
Based on their critical temperature super conductors can be classified as
Low temperature superconductor LTSC ( generally they need liquid Helium to be cooled under their
critical temperature). Eg: Hg, Pb, Nb,
High temp. superconductor HTSC ( having a Tc value > 77 K), - they reach the superconducting state
by just cooling them with liquid nitrogen.
Mechanism of low temperature superconductivity
BCS Theory (due to Bardeen, Cooper & Schrieffer)
The central concept of low temperature superconduction is the existence of
‘cooper pair’ i.e., a pair of electrons that exist on account of the two electron’s indirect interaction via
the nuclei of the atoms in the lattice .This pairing is caused by an attractive force between electrons
from the exchange of phonons & is slightly exceed the repulsion that they exert directly on one
another. These pairs were supposed to move cooperatively through the lattice in such a way that the
electron – phonon collisions were avoided. Because the cooper pair undergoes less scattering, it can
carry charge freely through the solid & hence give rise to super conduction. Low temperature favors
cooper pair formation & hence high temp superconductivity can not be predicted by BCS theory.
A cooper pair may be considered as a particle with twice the mass and charge of an
electron. In the super conducting state , the electric current is due to the motion of several cooper pairs
in the same direction with the same velocity.
Josephson Effect
In 1962, Josephson predicted that if two superconducting metals were placed next to each
other, separated only by a thin insulating layer, then a current would flow in the absence of any applied
voltage. This effect is observed because if the barrier is not too thick, then electron pairs can cross the
junction from one superconductor to the other without dissociating.
High temperature Superconductors
High temperature superconductors are the materials with Tc value well above 100+ 20K,
greater than the boiling point of nitrogen (77K), but much lower than room temp. In 1986, a new high
temperature superconductor with a Tc value of 35K was discovered. This compound is a mixed oxide
in the Ba – La – Cu –O system formulated as La2-xBax Cu O4-y where x is between 0.15 &0.20 and y is very
small. This compound has a perovskite structure, based on La2 Cu O4. Though La2 Cu O4 itself is
nonconducting, superconductivity can be made by replacing 7.5 – 10 % of La3+ ions by Ba2+.
[The perovskite structure is formed by compounds with formula ABO3. Examples
include Ba TiO3, Ca Ti O3. The perovskite structure is cubic. A Ca2+ ion or Ba2+ is located at the
body centered position, the smaller Ti4+ ions are located at each corner & O2- ions are located
half way along each of the edges of the cube. Thus Ca2+ has a C.N of 12, since it is surrounded by
12 O atoms & Ti4+ are surrounded by 6 oxygen atoms ]
History & progress of HTSC
In 1986, a new high temperature superconductor. La2-xBax Cu O4-y
discovered.
.
with a Tc value of 35K was
LSCO (La2-xSrxCuO4) – i.e., when strontium was substituted for Ba, Tc value increases .
Discovered in the same year
•
January 1987 , yttrium based Y Ba2 Cu3 O7-X (YBCO ) was discovered to have a Tc of 90 K. This
is called 1-2-3 superconductor ( from the ratio of Y – Ba – Cu).This is prepared by
Y2(CO3)3 + 4 Ba CO3 + 6 CuCO3
→
2 Y Ba2Cu3O7-X + 13 CO2
This compound has a Tc of 90 K which was significant for practical reasons. It allows liquid N2 to
be used as coolant rather than liquid He.
•
1988 - BSCCO (Ba – Sr – Ca – Cu – O) discovered with Tc up to 108 K, and TBCCO (T=thallium – Ba
– Ca – Cu - O ) discovered to have Tc of 127 K. These are called warm super conductors.
•
The Bi–Sr–Ca–Cu–O system has three superconducting phases forming a homologous series as
Bi2Sr2Can-1CunO4+2n+x (n = 1, 2 and 3). These three phases are Bi-2201, Bi-2212 and Bi-2223,
having transition temperatures of 20, 85 and 110 K, respectively, where the numbering system
represent number of atoms for Bi, Sr, Ca and Cu respectively.[31]
•
As of 2009, the highest-temperature superconductor (at ambient pressure) is mercury barium
calcium copper oxide (HgBa2Ca2Cu3Ox), at 135 K and is held by a cuprate-perovskite material,
rising up to 164 K under high pressure.
•
Recently, iron-based superconductors with critical temperatures as high as 56 K have been
discovered. These are often also referred to as high-temperature superconductors.
A small sample of high temperature superconductor BSCCO-2223
Examples
Examples of high-Tc cuprate superconductors include La1.85Ba0.15CuO4, and YBCO (Yttrium-BariumCopper-Oxide), which is famous as the first material to achieve superconductivity above the boiling
point of liquid nitrogen.
Transition temperatures of well-known superconductors (Boiling point of liquid nitrogen for comparison)
4.2
Transition
temperature
(in kelvins)
Material
133
HgBa2Ca2Cu3Ox
110
Bi2Sr2Ca2Cu3O10(BSCCO)
90
YBa2Cu3O7 (YBCO)
77
Boiling point of liquid nitrogen
55
SmFeAs(O,F)
41
CeFeAs(O,F)
26
LaFeAs(O,F)
20
Boiling point of liquid hydrogen
18
Nb3Sn
10
NbTi
Hg (mercury)
The crystal structure of HTSC
Class
Copper-oxide superconductors
Iron-based superconductors
Metallic low-temperature
superconductors
1-2-3 super conductor has a perovskite structure .This comprises three cubic
perovskite units stacked one on top of the other giving an elongated (tetragonal) unit cell. The upper &
lower cubes have a Ba2+ ion at the body centered position and the smaller Cu2+ ions at each corner. The
middle cube is similar but has Y3+ ion at the body centre. An ideal perovskite has the formula ABO3 & the
stoichiometry of the compound is Y Ba2 Cu3 O9. O atoms are at the edge centre & hence in one unit
cell , number of O atoms = 12 x 1/4 = 3. For three units 3x 3 = 9 . Cu atoms are at the corners .Thus in
one unit cell, the number of Cu atoms = 8x 1/8 = 1. For the three units 3 x 1 = 3.
Thus the formula is
Y Ba2 Cu3 O9.
But in 1 – 2 – 3 super conductor, the actual formula found is Y Ba2 Cu3 O7- X . There is a
massive oxygen deficiency and about one quarter of oxygen sites in the crystal are vacant. These occur
between adjacent Cu atoms in the chain along the ‘c’axis. The vacancies are in the yttrium atom plane.
There are also vacancies between copper atoms along ‘a’ axis in the Cu – O planes of Ba atoms.
(Deficiency=8x1/4=2. Ie, 2 O atoms lost ). )
Cu Ba O7-X
Cu Y O7-X
Cu Ba O7-X
The following generalizations can be done.
1.
All these super conductors are related to Perovskite structure
2.
Superconductivity occurs in the Cu O2layer.
3. Many, but not all warm superconductors contain Cu. Two features of copper are that it can exist in
three oxidation states (+1), (+2) & (+3) and Cu II forms many tetragonally distorted octahedral
complexes. Both these factors are important.
In the La2Cu O4 compounds some Ba2+ ions substituted for La3+. To balance the charge, some
Cu II atoms changes into Cu III. Another point to note is that if we assign the usual oxidation number
(Y3+, Ba2+ , O2- ) ,then the average oxidation number of Cu turns out to be 2.33. Thus it is inferred that
Y Ba2 Cu3 O7 is a mixed oxidation state material that contains Cu II & Cu III. Super conductivity in this
system is thought to involve the transfer of electrons from Cu II to Cu III. But if the process involves
two electrons, it could involve electron transfer from Cu I to Cu III
4. Oxygen deficiency seems to be critical. In ideal perovskite, Cu is octahedrally surrounded by 6 O
atoms . But when an oxygen deficiency occurs, then the two Cu atoms may interact directly with each
other. Interactions such as Cu II - Cu III occur by transferring an electron between the two Cu atoms.
5. In the 1 – 2- 3 super conductor, if all three sections were based exactly on perovskite unit cells, we
expect O atoms in the middle of each cube edge. This give a formula of Y Ba2 Cu3 O9. This formula is
improbable because it gives an average oxdn state for the three copper atoms of 11/3 implying that
the unit cell contains both Cu III & Cu IV, which is unlikely as Cu IV complexes are extremely rare . The
unit cell in fact contains only seven O atoms Y Ba2 Cu3 O7 . The O atoms on the vertical edges of central
cube are not there & there are also two, missing from both top and bottom faces. A unit cell containing
7 O atoms has an average oxidation state of 2.33 indicating the presence of Cu II & Cu III in the unit
cell.
6. La2Cu O4 has no superconductivity, because it is antiferromagnetic. This crystal contains Cu2+ ions &
the unpaired electrons on Cu ions align anti parallel through out the structure. This interaction locks the
electron to the lattice stopping both conductivity & superconductivity. When some Ba2+ ions are
introduced in the lattice replacing La3+, some charge compensation has to be made. Thus for every Ba 2+
ion ,one Cu2+ must be oxidized to Cu3+ & this break up the alignment of unpaired spins . When the
average oxidation state of Cu reaches a critical value of 2.2, the anti ferromagnetism disappears & super
conductivity appears.
If the super conductor 1 – 2- 3 is made more deficient in oxygen(Y – Ba2 – Cu3 O6.5), the super
conductivity drops to 60 K & at Y – Ba2 – Cu3 O6 the superconductivity disappears.
Applications
The uses of superconductors are based on four main properties
1. Meissner effect
2. Lack of electrical resistance
3. The ability to create superconducting magnets
4. The Josephson effect
Due to Meissner effect, Levitation is readily induced in super conducting material, which
have a variety of applications including transport systems with ‘Maglev’ trains
Superconducting magnets are some of the most powerful electromagnets known. They are
used in MRI and NMR machines, mass spectrometers, and the beam-steering magnets used in particle
accelerators. They can also be used for magnetic separation, where weakly magnetic particles are
extracted from a background of less or non-magnetic particles, as in the pigment industries.
In the 1950s and 1960s, superconductors were used to build experimental digital
computers using cryotron switches. More recently, superconductors have been used to make digital
circuits based on rapid single flux quantum technology and RF and microwave filters for mobile phone
base stations.
Superconductors are used to build Josephson junctions which are the building blocks of
SQUIDs (superconducting quantum interference devices), the most sensitive magnetometers known.
SQUIDs are used in scanning SQUID microscopes and Magnetoencephalography. Series of Josephson
devices are used to realize the SI volt. Depending on the particular mode of operation, a Josephson
junction can be used as a photon detector or as a mixer.
Promising future applications include high-performance smart grid, electric power
transmission, transformers, power storage devices, electric motors (e.g. for vehicle propulsion, as in
vactrains or maglev trains), magnetic levitation devices, fault current limiters, nanoscopic materials such
as buckyballs, nanotubes, composite materials and superconducting magnetic refrigeration