Superconductivity ��

Superconductivity is an exciting field of physics!

Definition

• Superconductivity is the flow of electric current without resistance in certain metals, alloys, and ceramics at temperatures near absolute zero, and in some cases at temperatures hundreds of degrees above absolute zero = -273ºK.

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Comparisons of Temperatures

Comparisons of Temperatures

Discoverer of Superconductivity

• Superconductivity was first discovered in 1911 by the Dutch physicist,Heike Kammerlingh Onnes.

• Source: Nobel Foundation

The Discovery

• Onnes, felt that a cold wire's resistance would dissipate. This suggested that there would be a steady decrease in electrical resistance, allowing for better conduction of electricity.
• At some very low temperature point, scientists felt that there would be a leveling off as the resistance reached some ill-defined minimum value allowing the current to flow with little or no resistance.
• Onnes passed a current through a very pure mercury wire and measured its resistance as he steadily lowered the temperature. Much to his surprise there was no resistance at 4.2K.

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At 4.2K, the Electrical Resistance (opposition of a material to the flow of electrical current through it)Vanished, Meaning Extremely Good Conduction of Electricity-Superconductivity

Superconductivity �

• Today however, superconductivity is being applied to many diverse areas such as: medicine, theoretical and experimental science, the military, transportation, power production, electronics, as well as many other areas.
• Superconductors have the ability to conduct electricity without the loss of energy. When current flows in an ordinary conductor, for example copper wire, some energy is lost.

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The Science of Superconductivity, cont.

• The behavior of electrons inside a superconductor is vastly different.
• The impurities and lattice framework are still there, but the movement of the superconducting electrons through the obstacle course is quite different.
• As the superconducting electrons travel through the conductor they pass unobstructed through the complex lattice.
• Because they bump into nothing and create no friction they can transmit electricity with no appreciable loss in the current and no loss of energy.

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BCS Theory…..

• The understanding of superconductivity was advanced in 1957 by three American physicists-John Bardeen, Leon Cooper, and John Schrieffer, through their Theories of Superconductivity, know as the BCS Theory.
• Pictures of Bardeen, Cooper, and Schrieffer, respectively.

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(Source: Nobel

Foundation)

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• The BCS theory explains superconductivity at temperatures close to absolute zero.
• Cooper realized that atomic lattice vibrations were directly responsible for unifying the entire current.
• They forced the electrons to pair up into teams that could pass all of the obstacles which caused resistance in the conductor.

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BCS Theory…..

• The BCS theory successfully shows that electrons can be attracted to one another through interactions with the crystalline lattice. This occurs despite the fact that electrons have the same charge.
• When the atoms of the lattice oscillate as positive and negative regions, the electron pair is alternatively pulled together and pushed apart without a collision.
• The electron pairing is favorable because it has the effect of putting the material into a lower energy state.
• When electrons are linked together in pairs, they move through the superconductor in an orderly fashion.

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BCS Theory…..

• One can imagine a metal as a lattice of positive ions, which can move as if attached by stiff springs. Single electrons moving through the lattice constitute an electric current.
• Normally, the electrons repel each other and are scattered by the lattice, creating resistance.
• A second electron passing by is attracted toward this positive region and in a superconductor it follows the first electron and they travel bond together through the lattice.

In Simpler Terms…

• When atoms join to form a solid, they create what is called a lattice. A lattice is like a jungle gym that links all of the atoms together. Electricity can move through a lattice by using the outer parts of the atoms ‑ the electrons. But imagine the jungle gym is shaking. This would make it very difficult for a person to climb through it. Especially if he's in a hurry. So, it is with electrons. They are constantly colliding with vibrating atoms because of the heat within the lattice.

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• The first electron convinces the next atom that you deserve special treatment. Once the process starts, everyone joins in and you begin moving forward effortlessly. The person‑to‑person exchange represents the 2 electrons. And, your body represents the electrical charge.

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Cooper Pair:

• Two electrons that appear to "team up" in accordance with theory - BCS or other - despite the fact that they both have a negative charge and normally repel each other. Below the superconducting transition temperature, paired electrons form a condensate - a macroscopically occupied single quantum state - which flows without resistance

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Animation of Cooper pairs:

The Science….

• An electrical current in a wire creates a magnetic field around the wire.
• The strength of the magnetic field increases as the current in the wire increases.
• Because superconductors are able to carry large currents without loss of energy, they are well suited for making strong electromagnets.

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The Science….

• Soon after Kamerlingh Onnes discovered superconductivity, scientists began dreaming up practical applications for this strange new phenomenon.
• Powerful new superconducting magnets could be made much smaller than a resistive magnet,because the windings could carry large currents with no energy loss.

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Factors Describing Superconductivity……

• The superconducting state is defined by three very important factors: critical temperature (Tc), critical field (Hc), and critical current density (Jc).
• critical temperature (T ) The highest temperature at which superconductivity occurs in a material. Below this transition temperature T the resistivity of the material is equal to zero.
• critical magnetic field (Hc ) Above this value of an externally applied magnetic field a superconductor becomes nonsuperconducting
• critical current density (Jc) The maximum value of electrical current per unit of cross-sectional area that a superconductor can carry without resistance.

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Applications of Superconductors

• magnetic shielding devices
• medical imaging systems, e.g. MRI’s
• superconducting quantum interference devices (SQUIDS) used to detect extremely small changes in magnetic fields, electric currents, and voltages.
• infrared sensors
• analog signal processing devices
• microwave devices

SQUIDS

Source: Superconductors.org

Emerging Applications

• power transmission
• superconducting magnets in generators
• energy storage devices
• particle accelerators
• levitated vehicle transportation
• rotating machinery
• magnetic separators

What Types of Superconducting Power Systems Equipment Can Help Us?

• Underground transmission cables
• Fault current limiters
• Transformers
• Motors
• SMES, Generators, etc.

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Another Nobel Prize for Superconductivity Researchers

• The awards committee honored the trio--Vitaly Ginzburg, Alexei Abrikosov and Anthony Leggett (shown below)--for "decisive contributions concerning two phenomena in quantum physics: superconductivity and superfluidity.”

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Thanks