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17-06-2010, 05:25 PM

.docx   HIGH TEMPERATURE SUPER CONDUCTORS.docx (Size: 134.53 KB / Downloads: 153)

A Paper Presentation

Presented By
Krishna district, AP .



A substantial fraction of electricity is lost in the form of heat through resistance associated with the traditional conductors such as copper or aluminum. More than 15% of the world's generated electricity is lost as heat-this corresponds to trillions of US dollars that could be saved. A.P.Telecom has announced that the electricity loss as heat is worth of 1000Crores.
The loss of electricity as heat is due to the resistance of transmission lines .Super conductors are those in which the electrical resistance is called equal to zero. But in earlier days, it exists at very low temperatures .But after sometime the temperature at which they act as superconductor has increased. Now it has crossed halfway from room temperature. Since these super conductors occur at high temperatures than 0ok, these are called High temperature super conductors.These have many applications in communications field (antennas..), electrical field (generators , trains..) etc.


Superconductivity is a phenomenon displayed by some materials when they are cooled below a certain temperature, These materials are known as the superconducting critical temperature, Tc. Below Tc,, superconducting materials exhibit two characteristic properties: Zero electrical resistance Perfect diamagnetism (the Meissner effect) Zero electrical resistance means that no electricity .This has many applications energy is lost as heat as the material conducts. Due to zero electrical resistance there is no wastage of energy.
The second of these properties, perfect diamagnetism, means that the superconducting material will exclude a magnetic field - this is known as the Meissner effect and can be used to display extraordinary physical effects Superconducting materials can be categorised into one of two types: Type I Superconductors - which totally exclude all applied magnetic fields. Most elemental superconductors are Type I. Type II Superconductors - which totally exclude low applied magnetic fields, but only partially exclude high applied magnetic fields; their diagmagnetism is not perfect but mixed in the presence of high fields. Niobium is an example of an elemental Type II superconductor.
Both types exhibit perfect electrical conductivity, and can be restored to 'normal' conductors in the presence of a sufficiently strong magnetic field.

The Meissner Effect :

A superconducting material cooled below its critical temperature in a magnetic field excluded the magnetic flux. This effect is known as the Meissner effect . Superconductors will not allow a magnetic field to penetrate into their interior. This finding is due to the generation of currents on the surface of the superconductor which exactly cancel the magnetic field in the superconductor's interior. This new property was called the Meissner Effect and it is this property that may someday allow the development of high speed levitating trains. Due to a limiting critical current density, Meissner discovered that if the magnetic field placed on a superconductor is increased beyond a critical value, the superconducting state suddenly disappears and resistance returns. This maximum magnetic field is now called the critical field. The limit of external magnetic field strength at which a superconductor can exclude the field is known as the critical field strength, Bc. Type II superconductors have two critical field strengths; Bc1, above which the field penetrates into the superconductor, and Bc2, above which superconductivity is destroyed, as per Bc for Type I superconductors.
Theory of Superconduction
BCS theory was proposed by J. Bardeen, L. Cooper and J. R. Schrieffer in 1957 .BCS suggests the formation of so-called 'Cooper pairs' .The BCS theory (1957) of Bardeen, Cooper and Schrieffer was a detailed microscopic theory that was quickly accepted as an explanation for the condensate in the superconductors that were known at the time. Such a condensate requires that the particles composing it be bosons, that is, have integral spin. Bosons obey Bose-Einstein statistics. Below the critical temperature the bosons in a superconductor can all gather together in the lowest possible energy state to form the condensate, and the greater the number that have accumulated, the harder it is for one of them to leave. Electrons are not bosons because they do not have integral spin. BCS theory explained how the interaction between the electrons and the phonons or lattice vibrations in the metal causes an electron-electron attraction. Some of the electrons form so-called Cooper pairs where the spins and momentum are opposite and therefore cancel out. Because the Cooper pairs have zero spin, they can participate in Bose condensation. It appeared that superconductivity was well explained and only possible at very low temperatures. However, BCS theory does not account well for high temperature superconduction, which is still not fully understood.


The discovery of superconductivity soon generated interest in practical applications, mainly because of its potential to save energy. Indeed the replacement of copper or other normal conductors by superconducting materials avoids heat dissipation and other energy losses due to finite resisitance. In some types of equipment such as magnetic separators, these losses may account for most of the energy consumed in the device. Early prototypes for motors, transmission lines and energy storage magnets were developed, but they were never widely accepted.
There were important reasons for this, apart from the tremendous investment in existing technology. In most superconducting metals and alloys the superconductivity tends to fail in self- generated magnetic fields when the current densities through them are increased to practical levels. A second problem was the cost and complexity of operating refrigeration equipment near liquid helium temperatures (4 K, -269°C). Removing one watt of heat generated at 4 K demands about 1000 W of refrigeration power at room temperature.

High temperature superconductor


The recent discovery of high-temperature superconductivity at liquid nitrogen temperatures (77 Kelvinâ„¢s) brings us a giant step closer to the vision of early scientists. Applications currently being pursued are mostly extensions of current technology used with the low-temperature superconductors such as powerful magnets used in MRI scanners. Additional applications include magnetic shielding devices, extremely sensitive medical imaging systems, infrared sensors, analog signal devices, and microwave communication devices, and waveguides. As our knowledge of the properties of high- temperature superconducting materials increase, more efficient power transmission lines, smaller and more efficient generators, energy storage devices, particle accelerators, and levitating trains will become more practical.

High Temperature Superconduction :

The highest known temperature at which a material went superconducting increased slowly as scientists found new materials with higher values of Tc, but it was in 1986 that a Ba-La-Cu-O system was found to superconductor at 35K - by far the highest then found. This was interesting as BCS theory had predicted a theoretical limit of about 30-40K to Tc (due to thermal vibrations). Soon, materials were found that would superconduct above 77K - the melting point of liquid nitrogen, which is far safer and much less expensive than liquid helium as a refrigerant. Although high temperature superconductors are more useful above 77K, the term technically refers to those materials that superconduct above 30-40K. In 1994, the record for Tc was 164K, under 30GPa of pressure, for HgBa2Ca2Cu3O8+x. In February 1987, a still higher superconducting record of 92 K was made by scientists at the University of Houston and the University of Alabama in Huntsville who substituted yttrium for lanthanum bringing superconductivity into the liquid nitrogen range. This increase in temperature is significant because liquid nitrogen (which boils at 77 K) is as cheap as coffee. Because of this large increase in operating temperature, these new materials are now called "High Temperature Superconductors". Since then scientists have found additional materials that superconduct at temperatures exceeding 133 K-nearly half way to room temperature (or 290 K)! Currently, many governments, corporations and universities are investing huge sums of money in the study of High Temperature Superconductivity, particularly in the development of commercial applications. The higher operating range of these new materials has influenced vast efforts in the development of these compounds, and changing the theory of the behavior of superconductors at these relatively higher temperatures.

Disadvantages :

These grandiose expectations inevitably led to disappointment. Room temperature superconductivity has remained a dream. Critical current densities in HTS materials also tend to be naturally too low for technological applications, while there are persistent problems with poor mechanical properties. These problems are both related to the ceramic, granular, anisotropic nature of the HTS materials (other than MgB2, which behaves as a brittle metal). They need to be formed at high temperatures in the presence of oxygen. Like all ceramics, HTS materials are very brittle and very difficult to shape and handle, while long, flexible, superconducting wires are necessary for many large-scale applications. Large supercurrents can only flow along the CuO2 planes, and only a small fraction of the material in a completed device is likely to be correctly oriented. The grain boundaries attract impurities, leading to weak links, which reduce the inter- grain current density and provide an easy path for flux vortices to enter the material. Flux creep or vortex penetration into HTS material is unusually rapid. The coherence length or diameter of a vortex core tends to be very small. This is a problem because pinning is most effective if the defect or impurity is of the same 'size' as the coherence length.

Magnetically Leviated Train:

The magnetically levitated (Maglev) train is a super-high-speed nonadhesive transport system with a combination of superconducting magnets (SCMs) and linear motor technology. The concept was developed at the Railway Technical Research Institute of the Japanese National Railways in 1970. In 1990, construction of the Maglev test line in the Yamanashi prefecture started, for the final confirmation of the Maglev train for practical use; running tests have been carried out since April 1997.
The Maglev system applies the superconducting technology of low-temperature superconductors, Nb-Ti wires, and SCMs that require liquid helium as a coolant. In addition to these well-developed technologies, high-critical temperature superconductors that show superconductivity at liquid nitrogen are also prospective components for the Maglev system. Rare-earth barium-copper-oxide (REBCO) bulk superconductors are being considered for a superconducting magnetic bearing, a flywheel, a motor, high- field magnetic shielding, and a superconducting bulk magnet.3 The magnetization of rare earth (RE)Ba2Cu3O7-x superconductors with a high critical-current density (Jc) results in a strong bulk magnet with liquid nitrogen refrigeration.4 The trapped magnetic field of the superconducting bulk magnet with a large single domain has been reported to be superior to that of a conventional permanent magnet. The superconducting bulk magnet can generate a higher magnetic field with increasing Jc and volume. Further, a large light rare earth (LRE)Ba2Cu3O7-x (e.g., Nd, Sm, Eu, or Gd) bulk superconducting magnet is believed to trap very high magnetic fields” more than 5 T at 77 K. Therefore, a superconducting bulk magnet or the superconducting quasi-permanent magnet for the Maglev train is possible.
MAGLEV Train :

Superconducting Antennas :

By now, you are probably wondering how can these new high- temperature superconductors could improve the communications industry. Only the mind can set limits to the potential number of improvements that can be envisioned. The telecommunications industry already uses high-temperature superconducting films to coat the inside of their microwave waveguides to reduce losses in their system. Furthermore, as superconducting transistors are developed, perhaps longer lasting and smaller "finals" could be developed for transceivers.
A more immediate application could perhaps be in the antenna system. Theoretically, superconductors could be employed to reduce the resistive losses in an antenna. However, since one "S" unit of signal strength corresponds to a change of 6 dB, a substantial increase in efficiency will be required for a target station to notice any improvement. Although less likely at the short wavelengths used by many world wide broadcast stations, dramatic improvements are more likely at very long wavelengths because of the severe space limitations of the antenna. It is well known that an antenna needs to be a minimum of 1/8 wavelength in length to be reasonably efficient. Unlike the short wave frequencies employed in most world wide communications, this constraint is not severe. Due to salt water penetrating ability, submarines utilize 40 km wavelengths; therefore, an efficient antenna needs to be several miles long in order to have a reasonable efficiency. These long wires do pose obvious difficulties in the operation of submarines; it will be shown below how superconductivity could provide significant reductions in the antenna length while keeping nearly a 100% radiation efficiency.

World's First Superconducting Antennas

In the spring of 1995, the Fusion Energy Division of the Oak Ridge National Laboratory built a 2m VHF BSCCO antenna. Using a Hewlett-Packard 8753A Network Analyzer, the principle investigators, E.C. Jones and D.O. Sparks, discovered that the resonance frequency dropped by approximately 5% as the superconducting tape was cooled below the superconducting transition temperature.
In addition, the Q- factor increased only slightly as already discussed above. The change in resonance frequency was believed to be the result of the rf current redistributing from the silver matrix in the normal state to the superconducting filaments as the tapes were cooled to their superconducting state with liquid nitrogen. Since these tapes had twisted filaments, the current had a 5% longer conduction path, i.e., "longer effective wavelength", at these superconducting temperatures. To the best of our knowledge, this was the first VHF antenna of its kind to have ever been built and to my disappointment, the Oak Ridge National Laboratory (who owned my superconductor patent rights) and the Department of Energy decided not to pursue this line of work or file any patents. * In 1997, on my personal time, I built a 2 foot tall 160m superconducting antenna for use near 1.86 MHz. I found that as the antenna was cooled with liquid nitrogen, the signal strength meter of the transceiver used to test the antenna increased from an S1 to S9 indicating the clear feasibility of these materials for long-wavelength communications. Also, to the best of my knowledge, I am unaware of any one else who has ever built a superconducting antenna for long-wave communications. In contrary, other research groups have used superconducting antennas to reduce the ac-losses found at the higher microwave frequencies. The first microwave superconducting antenna is credited to the Electronic Materials and Devices Research Group at the University of Birmingham (United Kingdom) and this group was recently presented with the International IEE Premium Award for their work.


The ability of superconductors to conduct electricity with zero resistance can be exploited in the use of many electronic applications.High temperature super conductors are used in many fields .These have many applications in many sectors . Sceintists are making many experiments to take the high temperature super conductors to room temperature which make many revolutions . We hope that High temperature super conductors will come to room temperature and serve our application.

References :

1. Material Science
by M.S.Vijaya
2. Principles of Material Science
and Engineering
3. The Nature and properties of
Engineering materials
by William F.Smith
by Ebiqmiew D.Jastrzebski
4. http://www.ecjones.org/hightc.html
5. http://www.o-keating.com/hrs/maglev.html

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17-06-2010, 06:05 PM

.docx   HIGH TEMPERATURE SUPER CONDUCTOR.docx (Size: 220.63 KB / Downloads: 82)

This paper describes about high temperature superconductors and their applications in
the elite electronic world. It also explains the reasons behind this theory along with recent
breakthroughs in high temperature superconductors.
The discovery of high-temperature superconducting materials in 1986 sparked a dream
of an amazing new electrical & electronics world , a world of loss- free power
transmission from coast to coast, of enormously powerful computers, and of levitated
trains passing in a blur of speed.
High-temperature superconductors are generally considered to be those that demonstrate
superconductivity at or above the temperature of liquid hydrogen, or - 196 °C (77 K),
since this is the most easily attainable cryogenic
temperature. Conventional
superconductors, by contrast, require temperatures no higher than a few degrees above
absolute zero (- 273.15 °C or - 459.67 °F). Though it is extremely cold by everyday
standards, in the field of superconductivity, 77 K is considered high temperature.
Recently, other unconventional superconductors have been discovered. Some of them
also have unusually high values of the critical temperature Tc, and hence they are
sometimes also called high-temperature superconductors, although the record is still held
by a cuprate perovskite material (Tc=138 K, that is - 135 °C) (although slightly higher
transition temperatures have been achieved under pressure).
High-Tc superconductivity is believed to originate from strongly interacting or "paired"
electrons moving through copper oxide layers. A single atom of zinc, a strong scatterer of
electrons, substituted for an atom of copper, which would be the source of any paired
electrons, proved to be an ideal probe for studying the underlying physics of high-Tc
Superconductors have also been used to make digital circuits (e.g. based on the Rapid
Single Flux Quantum technology) and microwave filters for mobile phone base stations.
Promising future applications include high-performance transformers, power storage
devices, electric power transmission, electric motors (e.g. for vehicle propulsion, as in
Victorians or maglev trains), magnetic levitation devices, and Fault Current Limiters.
However superconductivity is sensitive to moving magnetic fields so applications that
use alternating current (e.g. transformers) will be more difficult to develop than those that
rely upon direct current
Scientist believe that an entire periodic table will have to be put together to make a room
temperature superconductor .Nevertheless it is believed by some researchers that if room
temperature superconductivity is ever achieved it will be in a different family of
With the devlopment of new oxide superconductors having Tc of 125k or above, there
has been a tremendous excitement in scientific world. This opens a new age of high
temperatures superconducting dervices,which have widespread commercial applications.


One of the most interesting and unusual properties of solids is that certain metals and
alloys exhibit almost zero resistivity when they are cooled to sufficiently low
temperatures .This phenomenon is called superconductivity.
Superconductivity occurs in a wide variety of materials, including simple elements like
tin and aluminium, various metallic alloys, some heavily-doped semiconductors and a
family of cuprate-perovskite ceramic materials known as high- temperature
superconductors. Superconductivity does not occur in noble metals like gold and silver,
nor in most ferromagnetic metals.
In 1986 the discovery of high- temperature superconductors, with critical temperatures in
excess of 90 kelvin, spurred renewed interest and research in superconductivity for
several reasons. As a topic of pure research, these materials represented a new
phenomenon not explained by the current theory. And, because the superconducting state
persists up to more manageable temperatures, more commercial applications are feasible,
especially if materials with even higher critical temperatures could be discovered.
In a class of superconductors known as type II superconductors (including all known
high-temperature superconductors), an extremely small amount of resistivity appears at
temperatures not too far below the nominal superconducting transition when an electrical
current is applied in conjunction with a strong magnetic field (which may be caused by
the electrical current). This is due to the motion of vortices in the electronic superfluid,
which dissipates some of the energy carried by the current.
If the current is sufficiently small, the vortices are stationary, and the resistivity vanishes.
The resistance due to this effect is tiny compared with that of non-superconducting
materials, but must be taken into account in sensitive experiments. However, as the
temperature decreases far enough below the nominal superconducting transition.
Most prominent materials in the high- Tc range are the so-called cuprates, such as
La1.85Ba0.15CuO4, YBCO (Yttrium-Barium-Copper-Oxide) and related substances.All
known high-Tc(critical temperature) superconductors are so-called Type-II
superconductors. A Type-II superconductor allows magnetic field to penerate its interior
in the units of flux quanta, creating 'holes' (or tubes) of normal metallic regions in the
superconducting bulk. This property makes high-Tc superconductors capable of
sustaining much higher magnetic fields.
According to BCS theory, super electrons are responsible for superconductivity. They
exist as copper pairs. They form a bound single system. Their motions are correlated.
Electron-electron interaction via lattice deformation, copper pair formation, flux
quantisation ..etc are underlining principles for high temperature superconductors.


The radical idea that high temperature superconductivity and related phenomena occur in
certain materials is because of quantum- mechanical fluctuations in these materials
increase as temperature decreases. Usually such fluctuations, which determine the
properties of all matter in the universe, decrease as temperature decreases.
Varma's theory did not explain the nature of the fluctuations; he accomplished this in a
theory he proposed in 1996, while still at Bell Labs, in which he noted that in copper
oxide materials, also known as cuprates, superconductivity is associated with the
formation of a new state of matter in which electric current loops form spontaneously,
going from copper to oxygen atoms and back to copper. His theory concluded that the
quantum- mechanical fluctuations are the fluctuations of these current loops. Physicists
consider these fluctuations in the current loops to be fluctuations of time.
In a normal conductor, an electrical current may be visualized as a fluid of electrons
moving across a heavy ionic lattice. The electrons are constantly colliding with the ions
in the lattice, and during each collision some of the energy carried by the current is
absorbed by the lattice and converted into heat (which is essentially the vibrational
kinetic energy of the lattice ions.) As a result, the energy carried by the current is
constantly being dissipated. This is the phenomenon of electrical resistance.
The situation is different in a superconductor. In a conventional superconductor, the
electronic fluid cannot be resolved into individual electrons. Instead, it consists of bound
pairs of electrons known as Cooper pairs. This pairing is caused by an attractive force
between electrons from the exchange of phonons. Due to quantum mechanics, the energy
spectrum of this Cooper pair fluid possesses an energy gap, meaning there is a minimum
amount of energy E that must be supplied in order to excite the fluid. Therefore, if E is
larger than the thermal energy of the lattice (given by kT, where k is Boltzmann's constant
and T is the temperature), the fluid will not be scattered by the lattice. The Cooper pair
fluid is thus a superfluid, meaning it can flow without energy dissipation.
(Behavior of heat capacity (cv) and resistivity () at the superconducting phase
The onset of superconductivity is accompanied by abrupt changes in various physical
properties, which is the hallmark of a phase transition. For example, the electronic heat
capacity is proportional to the temperature in the normal (non-superconducting) regime.
At the superconducting transition, it suffers a discontinuous jump and thereafter ceases to
be linear. At low temperatures, it varies instead as e- a/T for some constant a. (This
exponential behavior is one of the pieces of evidence for the existence of the energy gap.)
The order of the superconducting phase transition was long a matter of debate.
Experiments indicate that the transition is second-order, meaning there is no latent heat.
In the seventies calculations suggested that it may actually be weakly first-order due to
the effect of long-range fluctuations in the electromagnetic field. Only recently it was
shown theoretically with the help of a disorder field theory, in which the vortex lines of
the superconductor play a major role, that the transition is of second order in the type II
and of first order (i.e., latent heat) with in the type I regime, and the two regions are
separated by a tricritical point.
The Meissner effect was explained by London and London, who showed that the
electromagnetic free energy in a superconductor is minimized provided
where H is the magnetic field and is the penetration depth.
This equation, which is known as the London equation, predicts that the magnetic field in
a superconductor decays exponentially from whatever value it possesses at the surface.
The Meissner effect breaks down when the applied magnetic field is too large.
Superconductors can be divided into two classes according to how this breakdown
occurs. In high temperature superconductors, raising the applied field past a critical value
Hc1 leads to a mixed state in which an increasing amount of magnetic flux penetrates the
material, but there remains no resistance to the flow of electrical current as long as the
current is not too large. At a second critical field strength Hc2, superconductivity is
destroyed. The mixed state is actually caused by vortices in the electronic superfluid,
sometimes called fluxons. Most pure elemental superconductors (except niobium,
technetium, vanadium and carbon nanotubes) are Type I, while almost all impure and
compound superconductors are Type II(High temperature superconductors).


Between 1986 and 1994 most advances in the field of superconductivity related to the
discovery of new superconductor "systems" and compounds. In recent years, except for
the discovery of additional elements that will superconduct under extreme high pressure,
superconductor news has been mainly about novel ways to employ the new ceramic
superconductors, innovative fabricating techniques and atypical superconductors


Silicon Becomes A Superconductor
Silicon -- the archetypal semiconductor -- has at long last been shown to demonstrate
superconductivity. By substituting 9% of the silicon atoms with boron atoms
Superconducting Qubits May Enable Quantum Computing
Boson-Mediated Electron Pairing Observed in HT Superconductors
Superconductors to Facilitate World's First Artificial Sun
Multi-walled Carbon Nanotubes Superconduct at 12K
DNA Nanowires Exhibit SQUID- like Behavior
Superconductor Makes Nano-Refrigerator Possible
High Tc, Low Toxicity: 115K Superconductivity in Sn-3212-Tm
Larger, non-spherical pure carbon fullerenes that superconduct have recently been
discovered .superconductivity is found in single-walled carbon nanotubes at around 15
Kelvin. Silicon-based fullerides like Na2Ba6Si46 will also superconduct. However, they
are structured as infinite networks, rather than discrete molecules. Fullerenes are
technically part of a larger family of organic conductors,


Superconductors are used to make some of the most powerful electromagnets
known to man, including those used in MRI machines and the beam-steering
magnets used in particle accelerators
Superconductors have also been used to make digital circuits (e.g. based on the
Rapid Single Flux Quantum technology) 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.
Series of Josephson devices are used to define the SI volt. Depending on the
particular mode of operation, a Josephson junction can be used as photon detector
or as mixer
The large resistance change at the transition from the normal- to the
superconducting state is used to build thermometers in cryogenic micro-
calorimeter photon detectors.


There are some potential problems when using these higher temperature superconductors.
The material will come out of the furnace much harder and more difficult to re-
Higher temperatures induce a risk of melting the materials, especially if the
temperature indicator is inaccurate.
Moisture may slowly destroy the superconductivity of the material.


Supercomputers, SQUIDS, electric power transmission, motors, and magnetically
levitated trains are just some of the things superconductors can do; without wasting any
Are superconductors the future After understanding what superconductivity is and what
it is doing today I am convinced they are the future of us or the generations to come.
Some day superconductors will replace the conductors of electricity we use today.
Superconductors will save billions of dollars for countries of the world, and make life
easier for us all. I believe superconductors are the future.


With the devlopment of new oxide superconductors having Tc of 125k or above, there
has been a tremendous excitement in scientific world. This opens a new age of high
temperatures superconducting dervices,which have widespread commercial
applications.Many scientist believe that an entire periodic table will have to be put
together to make a room temperature superconductor.


Tinkham, Michael (2004). Introduction to Superconductivity, 2nd ed.,
Tipler, Paul; Llewellyn, Ralph (2002). Modern Physics, 4th ed.,
P.K Paliniswamy ,Solid state physics
S.O Pillai , solid state physics
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24-12-2010, 02:44 PM

.doc   ARSALAAN chem.doc (Size: 292.5 KB / Downloads: 63)

Prepared By:Arsalaan Sahil Dar


Superconductivity, which is defined as the absence of resistance in a conducting material to a continuously flowing electric current, is a special property that a sizable number of substances attain suddenly at very low temperatures. The substances (called superconductors) include elements, alloys , compounds, and nonstoichiometric ceramic materials. Superconductors also exhibit perfect diamagnetism; that is, magnetic fields cannot penetrate them (the Meissner effect), and small powerful magnets actually float (levitate) above flat superconductor surfaces. A superconductor's critical transition temperature, T C , is the temperature above which no super-conductivity can be obtained. For elements, alloys, and simple compounds, very low critical transition temperatures ( T C 23 K) mean that the cooling effects of liquid helium ( B.P. = 4 K) are needed to bring about and to maintain their superconductivity. The discovery in 1986 that nonstoichiometric ceramics containing copper and oxygen can have much higher T C values has provided a new impetus for developing superconducting materials.

High Temperature Superconductors:-
In April 1986, K. Alex Müller and J. Georg Bednorz (with IBM in Switzerland) reported the superconductivity of a nonstoichiometric ceramic oxide of lanthanum, barium, and copper, La 2− x Ba x CuO 4− y , with the then record high T C of 35 K. Further experiments conducted by Müller, Bednorz, and others showed that slight modifications made to La 2− x Ba x − CuO 4− y ( x 0.2 and y is even smaller) could yield materials having T C s of 50 K. By early 1987, Paul C. W. Chu (at the University of Houston), Maw-Kuen Wu (at the University of Alabama), and their coworkers synthesized another ceramic oxide material, YBa 2 Cu 3 O 7− y , and observed that super-conductivity in the material was attainable by cooling it with liquid nitrogen ( B.P. = 77 K). This "high temperature superconductor" made possible.
A magnet is hovering over a superconductor, demonstrating that magnetic fields cannot penetrate the superconductor, known as the Meissner effect.
superconductor applications that were impractical with the low temperature superconductors.
Other nonstoichiometric ceramic oxides that contain copper in nonintegral oxidation states have been synthesized and evaluated. Several of these materials have even higher T C 's than that of YBa 2 Cu 3 O 7− y . The BSCCO series Bi 2 Sr 2 Ca n − 1Cu n O 2 n +4+ y (for n = 1 to 4) reaches a T C maximum of 110 K for Bi 2 Sr 2 Ca 2 Cu 3O 10+ y ; a similar Tl 2 Ba 2 Ca n −1Cu n O 2 n +4+ y series reaches a maximum of 122 K for Tl 2 Ba 2 Ca 2 Cu 3 O 10+ y ; HgBa 2 Ca 2 Cu 3 O 8+ y has a T C of 135 K at ambient pressure; and Hg 0.8 Tl 0.2 Ba 2 Ca 2 Cu 3 O 8.33 has a T C of 138 K. Also, the T C of HgBa 2 Ca 2 Cu 3 O 8+ y has been reported to increase to 153 K at a pressure of 150,000 atmospheres and to 160 K at 280,000 atmospheres. Even higher T C values have been claimed for portions of multiparticle ceramics, but no macroscopic material has shown unambiguous superconductivity at these higher temperatures (above 160 K).
Other new classes of superconductors that are being investigated include intermediate temperature range superconductors, such as magnesium diboride ( T C = 39 K), alkali-doped C 60 (M 3 C 60 has a T C of 33 K), and hole-doped C 60 ( T C = 52 K). The latter result led Jan Hendrik Schon, Christian Kloc, and Bertram Batlogg (of Bell Labs) to the newer haloform-intercalated, high temperature C 60 superconductors C 60 • 2CHCl 3 and C 60 • 2CHBr 3 , with T C values of 80 K and 117 K, respectively.
The theoretical interpretation of the high temperature superconductors is still under development. The copper oxide ceramic superconductors obtain their paired conducting electrons from copper in mixed oxidation states of I and II or II and III, depending on the particular system. The paired conducting electrons are called Cooper pairs, after Leon N. Cooper. Cooper's name also gives us the C of BCS; the BCS theory is an interpretation of superconductivity for low temperature superconductors (having T C 's of less than 40 K).
The Meissner effect is the repulsion of a magnetic field from the interior of a super-conductor below its critical temperature. Whereas a weak magnetic field is totally excluded from the interior of a superconductor, a very strong magnetic field will penetrate the material and concurrently lower the critical transition temperature of the superconductor. W. Meissner and R. Ochsenfeld discovered the Meissner effect in 1933.

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