Superconductors

Superconductors.

 

 

Superconductors are materials the electrical resistance which when reaching the temperature below a certain value (called critical temperature) becomes equal to the absolute zero. In such cases we say that the material acquires superconductivity, superconducting properties of or goes into the superconducting state.

 

Description. Materials superconductors

The discovery of superconductors

Low and high temperature superconductors

Classification, types and uses of superconductors

Properties of superconductors, effects

The the Meissner effect, quantum levitation

The superconductors 1 kind and 2 kinds of superconductors, metals and other materials critical temperature and critical magnetic field

The application and use of superconductors

 



Description. The materials are superconductors.

Superconductors are materials the electrical resistance which when reaching the temperature below a certain value (called critical temperature) becomes equal to the absolute zero. In such cases we say that the material acquires superconductivity, superconducting properties of or goes into the superconducting state.

Superconductors are completely different materials, which in the normal state are not even conductors. In addition to metals and their alloys, for superconductors include certain semiconductors, ceramic materials, superconductors based on iron, organic superconductors and other substances, such as hydrogen sulfide.

The superconductor becomes superconducting state, 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 (conductor, semiconductor or dielectric), and below it – a superconductor. Some substances superconducting properties occur when certain external conditions, for example, upon reaching a certain pressure value.

As a rule, the critical temperature is very low, which limits the use of superconductors. However, in recent years (in 2017) was opened superconductors having superconductivity at room temperature.

 


The discovery of superconductors:

The first superconductor was opened in 1911 by the Dutch physicist Heike kamerlingh Onnes in mercury. He conducted experiments for verification of the electrical properties of this metal with decreasing temperature. At that time it was assumed that with decrease in the temperature is gradually lowered and the electric resistance of the conductor, and at too low temperatures allegedly electrons almost stop and the metal does no longer conduct current.

However, in the experiment, were obtained the opposite effect. First – at low temperatures the electrical resistance of mercury fell gradually, and then after overcoming the temperature of 4.15 Kelvin – disappeared entirely. This effect was called superconductivity.

The following year, was discovered two metal-superconductor: lead and tin.

Was subsequently opened and other superconductors.

 

Low and high temperature superconductors:

Depending on the values of the critical temperature of all superconductors are divided into low and high temperature. As a starting point the adopted temperature of 77 K (-196 ° C), which is approximate equal to the boiling point of liquid nitrogen 77,4 K (-95,75 °C).

The division has clearly practical value. So, for cooling material using liquid gases. To cool the material below 77 K (-196 ° C) using liquid helium. The boiling point of liquid helium is 4,222 K (-268,928 °C). For cooling high-temperature superconductors, the critical temperature in which more than 77 K, using liquid nitrogen, which is easier and cheaper to obtain.

 


Classification, types of superconductors:

The response of superconductors on the magnetic field they share superconductors 1st (first) kind and superconductors 2 (second) kind.

Superconductors 1st (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.

Superconductors have the 2 (second) kind, there are two critical magnetic field values Hc1 and Hc2. When a 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.

At the critical temperature superconductors are divided into low-temperature superconductors (TC < 77 K) and high temperature superconductors (TC > 77K).

 

Properties of superconductors, effects:

1. Zero electrical resistance.

The resistance of superconductors is zero when it is passed a constant electric current. If you pass an alternating electric current, it is nonzero and increases with increasing temperature.

2. The critical temperature of superconductors.

The critical temperature divides the superconductors in two States: normal and superconducting.

3. Critical magnetic field superconductors.

If a superconductor placed in an external magnetic field, the latter will bend around it. However, under certain critical values of magnetic field the material will lose its superconducting properties and becomes normal material. This value of the magnetic field is considered to be a critical field.

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

This phenomenon was called the Meissner effect after the name of discoverer. For the first time the phenomenon was experimentally observed in 1933 by German physicists W. Meissner and R. Ochsenfeld.

The Meissner effect means the complete expulsion of external magnetic fields from the volume of the conductor in its transition to the superconducting state. Inside the superconductor the magnetization is equal to zero. Inside the superconductor are continuous currents, which create an internal magnetic field directed opposite to external, applied magnetic field and compensating for it.

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 an external magnetic field in the volume of the superconductor means that electric current flows only in the surface layer of the superconductor.

6. Quantum levitation.

If you take a superconductor of the second kind (pre-chilled), and then to bring him to a powerful magnet, the superconductor forms its own magnetic field, similar in strength with the magnetic field. In the result, the magnetic field of the superconductor and the magnet push each other and quietly levitates the magnet – hovering over a superconductor. This effect is also referred to as the Meissner effect.

And Vice versa, respectively, if you place a superconductor over a magnet, the superconductor due to the action of the Meissner effect will also hover – levitate above the magnet.

The magnetic field is literally “missing” superconductor and tenacious “holds” in any position in which he was initially over or under the magnet. Therefore it is possible not only to hold the superconductor or the magnet in the desired position in the air, but to force the superconductor to move over and even under the magnetic “rails” at high speed.

7. The transition of a substance in the superconducting state is accompanied by changes in its thermal properties – specific heat capacity.

The specific heat capacity refers to a physical quantity that is numerically equal to the quantity of heat required to raise the temperature of a substance of mass 1 kg by 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.

8. The critical current.

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.

 

The superconductors 1 kind and 2 kinds of superconductors, metals and other materials critical temperature and critical magnetic field:

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*
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
Pb1Mo5,1S6 14.4 V 600 000
Nb3Sn 18,1 220 000
(Nb3Al)4Ge 20 —** —**
Nb3Ge 23,2 400 000
MgB2 39 —** —**
Yb0,9Ca0,1Ba1,8Sr0,2Cu4O8 86 —** —**
YBa2Cu3O7 93 1000*** 1 000 000***
Bi1,6Pb0,6Sr2Ca2Sb0,1Си3Ох 115 —** —**
HgBa2Ca2Cu3O8+x 135 —** —**

Note:

* 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.

 

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