Superconductivity, phenomenon, discovery, theory and applications

Superconductivity, phenomenon, discovery, theory, application, and temperature of superconductivity.

 

 

Superconductivity is a property certain materials possess absolutely zero electrical resistance when they reach the temperature below a certain value (called critical temperature).

 

Description. The phenomenon of superconductivity

The discovery of superconductivity

Nature, explanation and theory of superconductivity

Classification, types and uses of superconductors

The temperature of superconductivity of metals, alloys and other materials

Properties of superconductors

Application of superconductivity

 



Description. The phenomenon of superconductivity:

Superconductivity is a property certain materials possess absolutely zero electrical resistance when they reach the temperature below a certain value (called critical temperature).

Superconductivity have metals and their alloys, semiconductors, and ceramic materials and other substances. There are even superconducting alloys and materials with one of the elements or all the elementsentering into its composition, may not be superconductors. For example, the sulfide, the glory of mercury with gold and tin.

Superconducting state in the material occurs not gradually but in leaps and bounds – when the temperature is below the critical. Above this temperature the metal, alloy or other material is in the normal state, and below it – in the superconducting. For some substances the transition to the superconducting state becomes possible under certain external conditions, for example, upon reaching a certain pressure value.

Superconductivity as a phenomenon is accompanied by several effects. Very important are two of them: the disappearance of electrical resistance and expulsion of magnetic flux (field) from its scope. Therefore, it is paramount to not only the critical current, and critical magnetic field is a certain value of the magnetic field at which the superconductor loses its superconductivity.

The phenomenon of superconductivity can be demonstrated in practice. If you take a conductor, loop it, making a closed electrical circuit, cooling it to a temperature below the critical and to bring him an electric current, and then remove the source of electric current, the electric current in such a conductor will exist for an unlimited period of time.

Currently available superconductors having the property of superconductivity at room temperature.

 


The discovery of superconductivity:

The phenomenon of superconductivity was first discovered in 1911 by Dutch physicist Heike kamerlingh Onnes by investigating the dependence of electrical resistance of metals on temperature.

Ultra low temperatures he became interested in back in 1893, when he established the cryogenic laboratory.

In 1908 he managed to get liquid helium.

Cooling with it, metallic mercury, he was surprised to find that at a temperature close to absolute zero (4,15 K), electrical resistance (R) of mercury abruptly drops to zero.

In 1912 were discovered the two metals into superconducting state at low temperatures, lead and tin.

Was subsequently opened and other superconductors.

 

Nature, explanation and theory of superconductivity:

It should be noted that a fully satisfactory theory of superconductivity was currently missing.

In 1957, George. Bardin, L. Cooper and J. Sniffer proposed the so-called BCS theory (Bardeen – Cooper – shriffer).

Electric current is the movement of electrons. In a conventional conductor, electrons move singly and independently overcome various obstacles in its path. During movement, they collide with each other and with the crystal lattice and losing its energy. Thus, in the conductor due to different obstacles occurs the electrical resistance.

The electrons in normal conditions has spin taking the value of -1/2 or +1/2. But under certain conditions (when the temperature is below the critical), they form a pair. Electrons with opposite values of the spin are attracted to each other. These educated couples are also called Cooper pair. This pair has zero spin and twice the electron charge. Since the total spin of this pair is equal to zero, then it has the properties of a boson. The bosons form a condensate Bose-Einstein, joined all the free bosons, and are in the same quantum state. They become a single entity, able to move without colliding with the lattice and the remaining electrons, that is, without energy loss, without electrical resistance. So there is the effect of superconductivity.

However, this theory cannot explain superconductivity at high temperatures (high temperature superconductivity).

 

Classification, types of superconductors:

At the critical temperature superconductors are divided into low temperature, if the critical temperature below 77 K (-196 ° C) and high temperature.

Temperature of separation is the boiling point of nitrogen, which is 77.4 K (-195,75 °C).

This division has a practical value. In the first case, the cooling is made liquid or gaseous helium, and in the second case – the cheaper liquid or gaseous nitrogen.

The response of superconductors to a magnetic field they are superconductors type I and II superconductors.

Superconductors of the first kind to achieve only a certain value of the magnetic field (the so-called critical magnetic field Hc) lose their superconductivity. To this value of the magnetic field around the superconductor, and it over – penetrates and conductor loses its superconductivity.

Have type II superconductors have two critical magnetic field values Hc1 and Hc2. When the magnetic field of the first critical value Hc1 there is a partial penetration of magnetic field into the body of the superconductor, but superconductivity was retained. Above the second critical field Hc2, superconductivity is destroyed completely. The magnetic fields from the first to the second critical value in the superconductor there is a vortex structure of the magnetic field.

The material of the superconductors are divided into pure elements, alloys, ceramics, superconductors based on iron, organic superconductors, etc.

 

The temperature of superconductivity of metals, alloys and other materials:

Materials Critical temperature, K Critical fields (at 0 K), GS (e*)
The superconductors of the 1st kind   Hc
Rhodium 0,000325 0,049
Tungsten 0,012 1*
Hafnium 0,37 —**
Titan 0,39 60
Ruthenium 0,47 46*
Cadmium 0,52 28
Cubic Zirconia 0,55 65*
Osmium 0,71 46,6*
Uranium 0,8 —**
Zinc 0,85 53
Rhodium 0,9 —**
Gallium Of 1.08 59
Aluminium 1,2 100*
Rhenium 1,7 188*
Double-layer graphene ~1,7 500
The Alloy AI-Bi 1,84 —**
Thallium 2,37 180
Indium 3,41 280
Tin 3,72 305
Mercury 4,15 411
Tantalum 4,5 830*
Vanadium 4,89 1340*
Lead 7,19 803
Technetium 11,2 —**
H2S (hydrogen sulfide) 203 at a pressure of 150 GPA 720 000
The superconductors of the 2nd kind   Hc1 Hc2
Niobium 9,25 1735 4040
Nb3Sn 18,1 220 000
Nb3Ge 23,2 400 000
Pb1Mo5,1S6 14.4 V 600 000
YBa2Cu3O7 93 1000*** 1 000 000***
HgBa2Ca2Cu3O8+x 135 —** —**

Note to table:

* for materials that are marked * critical value of the field specified in OE (Oersted), to the rest of GS (Gauss).

** – no data.

*** Extrapolated to absolute zero.

 


Properties of superconductors:

1. Zero electrical resistance.

Strictly speaking, the resistance of superconductors is zero only for a constant electric current. Resistance in superconductors while passing through them an alternating current is zero and increases with increasing temperature.

2. The critical temperature of superconductors.

3. Critical magnetic field superconductors.

This value of magnetic field above which the superconductor loses its superconductivity and becomes a normal state characteristic of the normal conductor.

The value of the critical magnetic field varies depending on the material of the superconductor and may vary from several tens of Gauss to several hundred thousand Gauss. In the values table of the superconductivity of the materials indicates the critical magnetic field at a temperature of absolute zero (0 K).

Critical magnetic critical temperature are interrelated. When the temperature of the superconductor critical magnetic field decreases. The transition temperature from superconducting state to normal state critical magnetic field is zero, and at absolute zero it is possible.

The dependence of the critical field on temperature with good precision is described by the expression:

NS(T) = FNL · (1 – T2 / Tc2)

where NS(T), the critical magnetic field at a given temperature, NSO – critical field at zero temperature, T is the specified temperature, TC – critical temperature.

For type II superconductors, we specify two values of the magnetic field. Also it is easy to see what a giant field capable of withstanding the type-II superconductors without destroying superconductivity.

4. The critical current in superconductors.

This value is the maximum DC current that can withstand the superconductor without loss of the superconducting state. When this value is exceeded, the superconductor loses its superconductivity.

As the critical magnetic field, critical current is inversely proportional to temperature dependent, decreasing with its increase.

5. The expulsion of magnetic field by a superconductor from its volume.

This phenomenon was called the Meissner effect after the name of discoverer.

The Meissner effect means the complete expulsion of magnetic fields from the volume of the conductor in its transition to the superconducting state. Inside the superconductor the magnetization is equal to zero. For the first time the phenomenon was observed in 1933, the German physicists W. Meissner and R. Ochsenfeld.

However, not all superconductors there is a complete Meissner effect. Substances exhibiting a complete Meissner effect are called superconductors of the first kind and fractional – superconductors of the second kind. For superconductors the magnetic field in the range of values Hc1 – Hc2 penetrates and acts in the form of Abrikosov vortices. However, it should be noted that in low magnetic fields (lower values of Hc and Hc1 ) complete the Meissner effect have all types of superconductors.

The absence of a magnetic field in the volume of the superconductor means that electric current flows only in the surface layer of the superconductor.

6. The depth of penetration.

This is the distance at which the magnetic flux penetrates the superconductor. Typically, this value is called andonovski penetration depth (after the London brothers).

The depth of penetration is a function of temperature, is directly proportional to her and different in different materials.

Based on the actions of the Meissner effect, the magnetic field is expelled from the superconductor by currents circulating in its surface layer, the thickness of which is approximately equal to the depth of penetration. These currents create a magnetic field, which kompensiruet field applied from the outside, not allowing him to get inside.

Upon reaching the magnetic field the critical value it fully penetrates the depth of penetration and captures the entire superconductor.

7. Coherence length.

This is the distance at which electrons interact with each other, creating a superconducting state. The electrons within the coherence length move in concert – coherently (as if “up”).

8. Specific heat.

This value shows the amount of heat required to raise the temperature of 1 gram of a substance 1 K.

The specific heat of a superconductor abruptly (abruptly) increases near the transition temperature to the superconducting state, and quickly (abruptly) decreases with decreasing temperature. In other words, in the transition region to raise the temperature of a substance in the superconducting state requires more heat than normal, and at very low temperatures – on the contrary.

 

Application of superconductivity:

– to obtain strong magnetic fields. Since the passage of the superconductor to strong currents, creating a strong magnetic field, no thermal losses. For obtaining strong magnetic fields are used in type II superconductors because the critical magnetic field HC2 for them is very great,

in electric cables and power lines (power lines). So, one thin electrical cable from the superconductor capable of transmitting electrical current for transmission is normally a conductor must have a considerable size (diameter),

– in high-current generators and motors,

– in measuring instruments,

in Maglie (the magnetic levitation train).

 

© Photo ,