Superconductivity
Superconductors are materials whose electrical resistance drops below the so-called transition temperature drops to zero. Superconductivity was discovered in 1911 by Heike Kamerlingh Onnes, a pioneer of low-temperature physics. In the superconducting state is or will the interior of the material field-free, because magnetic fields are displaced. This only quantum mechanically explicable Meissner effect, for example, levitate a superconducting sample in the magnetic field. The displacement of the internal magnetic field is achieved by the construction of appropriate shielding currents at the surface, which compensate for the magnetic field induced by them the internal magnetic field.
The transition temperature Tc is very low for most materials and sinks in an external magnetic field or when a current flows through the superconductor from further to 0 K at the "critical field strength" Hc. For H < Hc, the field penetrates only about 100 nm far into the material. This thin layer carries the shield and line currents. Deeper layers are field - and current-free.
Superconductor of the second kind have two critical field strengths, for a deeper penetration of the end of the start field and a higher breakdown of the superconductivity. The magnetic flux through the material focuses on so-called flux tubes, each with a flux quantum. It allows a higher current carrying capacity.
Technical applications of these metallic superconductors as well as so-called high-temperature superconductors made of ceramic material are the generation of strong magnetic fields - for accelerators, medical, Levitation - as well as measurement and energy technology.
- 2.1 superconductor first kind
- 2.2 superconductor second kind
- 2.3 Properties
- 4.2 microwaves in superconducting cavities
- 4.3 Measurement Technology
- 5.1 Energy transport and conversion
- 5.2 Mechanical bearings on the basis of superconductivity
- 5.3 Magnetic Energy Storage Systems based on superconductivity
- 5.4 Electronic circuits based on the superconductivity
Classification
Metallic superconductor
Superconductivity was discovered in 1911 by Heike Kamerlingh Onnes shortly after his discovery of helium liquefaction for measurements of the metal mercury. This novel effect at that time existed only at extremely low temperatures below 4.2 Kelvin. Even after a hundred years of research with many possible combinations of metals, no metallic superconductors was having a transition temperature above 23 K have been found. This limits the use of metallic superconductivity in relatively few applications, because that requires cooling liquid helium and is very time-consuming and expensive.
However, metallic superconductors have the great advantage that it can form wires, as they are necessary, for example for the construction of coils for the generation of very strong magnetic fields with respect to the following classes. The properties of metallic superconductors are explained by the BCS theory.
Ceramic high-temperature superconductors
As a high- temperature superconductor ( HTSC) materials are referred to, whose critical temperature is above 23 K, the highest transition temperature of the conventional metallic ( alloy ) superconductor. This class of ceramic superconductors ( cuprates ) with particularly high transition temperatures was discovered in 1986 by Bednorz and Müller, who were for 1987 awarded the Nobel Prize for Physics.
Are particularly interesting for the HTSC technology that allow having a transition temperature of about 77 K ( boiling point of nitrogen) inexpensive cooling. The best-known representative is the Yttriumbariumkupferoxid with the formula YBa2Cu3O7 - δ, which is also known as YBaCuO, YBCO or 123 oxide. Superconductivity is observed for δ = 0.05 to 0.65.
The possibilities are very limited, as can be from the brittle material not establish a flexible wires. In addition, the current management of these " ceramic tablets " takes place in separate current paths and is direction dependent. So far, there is speculation on which physical principles rests the power line in the HTSC.
Iron-containing high-temperature superconductors
A completely new and unexpected class of high temperature superconductors discovered the Japanese Hideo Hosono in 2008: compounds of iron, lanthanum, phosphorus and oxygen can be superconducting. After pnictogen phosphorus Eisenpnictide these superconductors are called.
What was surprising was the amount of iron atoms, because every other superconducting material becomes normal by sufficiently strong magnetic field. This strong internal magnetic fields could now even be a condition of superconductivity. The speculation about the fundamental physics has become even greater. So far, only it is clear that the current flow through pairs is carried by electrons, as described in the BCS theory. But of what effect combines these Cooper pairs is unclear. It seems certain that it is not - as in metallic superconductors - is an electron -phonon interaction.
By choosing different admixtures such as arsenic, can the critical temperature of 4 K originally present on 56 K increase.
Metallic superconductors at extremely low temperatures
Depending on their behavior in a magnetic field, a distinction is superconductors of type I and type II superconductors also called type 1 and 2.
Superconductor first kind
Magnetic field lines are displaced in superconductors first kind down to a thin layer at the surface is completely out of the interior. The magnetic field decreases very rapidly exponentially on the surface of the superconductor; The characteristic dimension of about 100 nm of the surface layer is the so-called ( Londonsche ) penetration. We call this condition also as the Meissner phase. A superconductor first kind becomes normal for temperatures when either the external magnetic field a critical value or the current density exceeds a critical value through the superconductor. Most metallic elements exhibit this behavior and have a very low transition temperature in the range of a few Kelvin. Exceptions are the non-superconducting as well as alkali and alkaline earth metals, copper, silver and gold. The occurrence of a critical current density can be understood by bears in mind that for cranking a Abschirmstromes energy is needed. This energy must be supplied by the condensation energy in the phase transition of superconductivity by superconducting. Once the energy required exceeds the condensation energy, no longer superconductivity can exist.
In type -I superconductors, superconductivity is explained by a pairing of electrons ( Cooper pairs ) in the conductor.
In the normal electrical conduction of the electrical resistance through interaction of electrons with lattice defects in the crystal lattice and lattice vibrations is created. In addition, scattering processes of electrons with each other play an important role. The coupling of the electrons in the superconductor into Cooper pairs, the energy output is suppressed to the crystal lattice, allowing the flow of electric current without resistance. The two individual electrons are fermions, which join together to form a bosonic Cooper pair, while a macroscopic quantum state are taking ( see also superfluidity ).
The theory describing the type - I superconductors based on quantum physical effects, which are explained with the help of the BCS theory in the framework of many-body theory.
Superconductors second kind
Superconductors second type are up to so-called " lower critical magnetic field" in the Meissner phase, ie behave as Type I. At higher magnetic fields, magnetic field lines in the form of so-called flux tubes can be used in the material to penetrate ( Shubnikov - or mixed phase, and vortex or river tubing ) are observed before the superconducting state is completely destroyed in an " upper critical magnetic field". The magnetic flux in the flux tubes is always an integer multiple of the magnetic flux quantum:
When a current flows with the density J through the superconductor, so he exerts on the flux tubes a Lorentz force
Perpendicular to J and the magnetic field B from. This hike the flux tubes with velocity v across the material. Here, the tubes disappear at one edge and re-form on the opposite edge. This field motion in turn causes a Lorentz force that opposes the stream after the lenz 's rule. This counter force causes a voltage drop, so it is formed an electric resistance in the superconductor.
To prevent this, in the crystal lattice specific point defects ( pinning centers ) can be installed, which hold the flux tubes up to a certain limit force. Only when the Lorentz force exceeds this limit, it comes to drift and thus the so-called flux -flow resistance. Superconductor with a large interfacial force is called hard superconductors.
The type II superconductor are theoretically not as well understood as the superconductors of the first kind is true that the formation of Cooper pairs is assumed in these superconductors, however, a generally accepted model for its full description does not exist yet.
Examples of type II superconductors, the ceramic high-temperature superconductor. Two important groups are YBaCuO (yttrium -barium-copper oxide) and BiSrCaCuO ( bismuth -strontium- calcium - copper oxides). Furthermore, among the most superconducting alloys for type II, such as the niobium -aluminum alloys used for MR magnets. Since about 2008, a new class of materials has gained importance; the so-called iron pnictides. The basic building block of this superconductor is arsenic and iron and usually occurs in combination with a rare earth, oxygen and fluorine on.
Properties
Superconductors, with minor differences 1 to 2 kinds, in addition to the practical loss of the electrical resistance and the displacement of magnetic fields from their structure have several other properties. Most can be personalized with the BCS theory of superconductivity, or used for the free energy (also called " Gibbs function" referred to ) explain. The free energy of each phase can be through various monitoring parameters ( eg pressure, temperature, magnetic field ) directions. Gibbs function is defined in this case by a minimum, i.e., the superconducting phase is stable in comparison to the normal conductive phase, when the free energy of the superconducting phase is greater than that of the normal-conducting (and vice versa). A rotating superconductor generates a magnetic field whose orientation is coincident with the rotational axis of the superconductor, which is denoted by London moment.
A so-called critical magnetic field in which the superconductivity breaks down, can be considered as a function of ambient temperature T. Near absolute zero must be expended to destroy the superconducting phase. Upon reaching the transition temperature of the superconducting phase interrupts together without an external magnetic field. The function of the external magnetic field can be critical to a good approximation by
Are described. The explanation for the collapse of superconductivity at sufficiently high magnetic fields is the binding energy of the Cooper pairs. If the Cooper pairs an energy is supplied that is greater than their binding energy, then they break up - which describes the transition to the normal conducting phase. The ambient temperature must be correspondingly lower, to compensate for this process with the condensation of Cooper pairs. The critical power can be generated not only by magnetic fields. Also functions with the pressure ( 1 ) and electric fields ( 2 ) were found to ambient temperature. Since the breaking Cooper pairs is endothermic, it can be cooled by a magnetic field and a substance contained therein is in the superconducting state, the vicinity of the superconductor. Being a technical application, however, this cooling process is uninteresting by demagnetization.
The volume of a substance in the normal conductive phase (for temperature) is smaller than the volume of the superconducting phase (). If so, the two values is approximately ( ) correspond. This is interesting because both phases S and N coexist during the transition phase in the conductor. To explain this phenomenon, however, increasing consideration is necessary.
The specific heat capacity of the electrons increases during the transition from normal to superconducting state at for Typ-I/II-Supraleiter leaps and bounds ( Rutgers formula). In classical superconductors, it decreases in the superconducting state exponentially with temperature, as Cooper pairs can not absorb heat and thus only contribute electrons to the heat capacity, which are excited about the energy gap (see also Boltzmann factor ). The heat capacity of phonons ( lattice vibrations ) remains unchanged during the transition to the superconducting state.
The superconducting state, has little effect on the thermal conductivity. One must consider this effect for two types of substances. Firstly substances, in which heat is transferred mainly through the grid, which makes up a major part of conductors. This heat conduction is inhibited in the vicinity of the strong interference at the transitions between S- and N-type layers, however, by the lack of a better interaction with the electrons in comparison to the normal conductive phase. For substances where the electrons have a large proportion of the heat conduction, it is logically worse. It was thought in this respect also to use superconductors as controllable via a critical field switch for heat flows.
Theories
Realized Applications
An important application is the generation of strong, constant or slowly varying magnetic fields. The ohmic resistance of the coil windings of conventional electromagnets generate large amounts of heat and thus a large energy loss.
For this application only classic superconductors ( SL ) may be used, especially alloys of niobium. They reach a higher magnetic field strengths. For high-temperature superconductors ( HTSC) is currently still lacking the required manufacturing techniques. The production of strong superconducting coil requires the drawing of miles long, only a few micrometers thin conductor strands. SL are made of conventional metal alloys, to which this is possible. HTSC ceramics are rather similar, and from them one can not make such threads so far.
Superconductivity allows it to close by a high current -carrying field coils in itself, after which the current in principle can be maintained indefinitely without any loss in the coil. To "Load" the self-contained coil a short section of the coil above the critical temperature is heated. Wherein the coil is open and can be supplied via supply lines with current. When the desired current is reached, the heater is turned off. The coil is thereby again closed in itself. In permanent operation, the electrical connections can be mechanically removed and the container of the coil are sealed after loading the coil. To maintain the field just a regular refilling of the cooling medium liquid helium and liquid nitrogen is then required. A good example of this is an NMR apparatus.
The biggest fault is the so-called quenching ( engl. to quench = delete ). This locally breaks the superconductivity. The now normal conductive body acts as an electrical resistor. It heats up, whereby the resistance still increased. The normal conducting area increases by heat conduction. So collapses the current and thus the magnetic field in a short time. Since the stored energy in the magnetic field is quite large, this process can lead to the destruction of the coil.
Superconductors are ideal diamagnetic. Therefore can only flow at the surface, a current and for large currents without exceeding the limiting current density has to turn many thin SL threads in parallel. By embedding the filaments in the copper is now achieved in that in the quenching of the power is absorbed by the normally conducting copper and the heating is low, so that the normal-conducting portion does not increase too quickly (stabilized superconductor ). This destruction of the conductor is avoided. Such coils of stabilized superconductors are used for example in magnetic resonance imaging, particle accelerators and thermonuclear fusion reactors.
Microwaves in superconducting cavities
For particle accelerators, there are high-frequency fields to accelerate particles. Here, too, superconductors are used, although the critical field strength decreases significantly with frequency. Above a critical frequency, the Cooper pairs are broken directly by photon absorption. Then, the critical field strength is reduced to zero. The only way to move this limit further, deeper cooling.
For example, in the TESLA project superconducting cavities made of pure niobium to be developed. Advantage and disadvantage of the system is the low damping. Thus, the efficiency is very high, but parasitic modes are not attenuated at the same time.
Measurement
The Josephson effect and SQUIDs, allow highly accurate measurement of magnetic fields, eg for the determination of brain and cardiac magnetic fields in non-destructive material testing, or geological and archaeological prospecting.
Planned applications
The previously known superconducting materials must either be very costly cooled to extremely low temperatures, or they can be difficult to process. The following applications are only economical if material combinations are found, bringing their usability by none of these disadvantages more problems than benefits. The ideal would be a superconductor at ambient temperature.
Energy transport and conversion
In superconductors of the second kind for the transport of higher electrical currents, there is the difficulty that these materials during the transition to the normal state can not be like the metals to normal, good electrical conductors, but also - to a good approximation - to insulators. If such a current-carrying superconductor enters the normal state (for example, if the maximum current density ), the short continues flowing through the line inductance current will heat the material according to the Joule 's law, which can lead to the complete destruction of the superconductor. Therefore, it is necessary to such materials, for example as a microscopically thin strands to embed into a normal conductor. The difficulty of drawing from these ceramic-like materials thin threads, is one of the main obstacles for use at higher currents.
It already made his first investment in energy distribution networks, in which high-temperature superconductors are used as fault current limiters. This causes an increased current density in the case of short circuit, that the superconductor changes first in the mixing area and then into the normal conducting region. The advantage over short-circuit current limiting reactors is that a voltage drop during normal operation occurs only greatly reduced. Furthermore, it can be stated as an advantage over fuses and KS- limiters with detonators that the superconducting state without exchange of resources is reached again and a normal operation shortly after the error occurs again possible.
As on classic lines electric power using high voltages can be transferred efficiently, superconductors are hardly competitive as an energy carrier. The higher, compared to conventional cables achievable current density, however, more electric power can be transferred in the same space. Therefore, superconducting cables are used where enhancements must be made by increasing the demand for limited construction space.
It can be prepared low-loss transformers with the same performance significantly reduced dimensions and weight have and thus, for example in mobile operation (locomotives ) provide benefits. In addition, can be dispensed with an environmentally hazardous oil cooling. With good thermal insulation, it is possible to operate the transformer with a refrigerating machine.
Almost lossless electric motors with high-temperature superconductors could lead to a significant volume and weight savings compared to traditional motors. A possible increase in the already very good efficiency of 98% ( in large engines ) would, however, almost meaningless.
Another field of application generators come into question. This could be by means of superconductors even at high power levels significantly lighter and more compact design than conventional generator concepts. Generators of this type would cause lower tower head mass and thus a reduction in cost of the entire system in wind power plants. First research projects are already underway.
Mechanical bearings on the basis of superconductivity
Under the use of superconducting bearings, energy storage for short-term storage of electrical energy can be constructed. These stores serve in particular to compensation faster load variations of interconnected networks. Using the stock flywheels are mounted without friction, which store the energy.
Magnetic Energy Storage Systems based on superconductivity
In a superconducting magnetic energy storage (SMES ), energy is stored by the superconducting coil. The energy is quickly available and is therefore used for the compensation of fast load fluctuations in electricity grids ( Flickerkompensator ) or a pulse generator for short, intense pulses.
Electronic circuits based on the superconductivity
Since the 1970s there have been attempts to develop a superconducting electronics. Among other research projects have been conducted by IBM. In these projects has been attempted to apply the methods based on voltage level on the semiconductor electronics superconductivity. From fundamental physical reasons, the clock speed can thus be achieved is limited to a few GHz, and thus faster than current semiconductor processors.
1985 was supported by a research group at the University of Moscow, an alternative approach proposed, the ( Josephson effect and flux quantization in superconducting loops) is adapted to the special properties of superconductivity. This approach is based on the exchange of individual flux quanta between superconducting loops and is therefore referred to as rapid single flux quantum electronics ( RSFQ electronics, engl. Rapid Single Flux Quantum). These electronics family is characterized by extremely low power dissipation and extremely high clock frequencies (above 100 GHz).
In RSFQ electronics niobium used. The operating temperature is T = 4.2 K and is achieved in most cases by the use of liquid helium. Unlike the aforementioned applications, the superconducting electronics would not be able to benefit from the development of a room -temperature superconductor. The superconducting electronics based on extremely low signal levels. At increasing temperatures, the power of the thermal noise increases linearly so that at temperatures above 30 K, the low signal -to-noise preventing function of a complex circuit.
History
Before experiments could be carried out at temperatures close to absolute zero, there were various theories as to how the electrical resistance would behave in this temperature range, such as that the resistance would increase significantly or that he would not fall below a certain level.
The effect of superconductivity was first discovered on April 8, 1911 by Dutch Heike Kamerlingh Onnes in experiments with liquid helium. He observed that mercury suddenly lost its electrical resistance below 4.19 Kelvin. Although quantum mechanics was still new, it already postulated that the superconductivity could only be explained by quantum mechanics.
The first phenomenological interpretation of superconductivity came from the German physicists Fritz and Heinz London London in the 1930s.
In 1950, the successful phenomenological Ginzburg -Landau theory arose. A quantum mechanical theory of superconductivity was given by the American physicists John Bardeen, Leon N. Cooper and John R. Schrieffer ( BCS theory ) only in 1957, for which them the Nobel Prize for Physics in 1972.
In 1986 the German physicist Johannes Georg Bednorz and Karl Alex Müller of the Swiss published (both were employed at the IBM Research in Zurich ) their discovery of high temperature superconductivity, for which they received the Nobel Prize in 1987. One theory about the genesis of this type superconductivity is still pending.
The Russian physicist Vitaly Ginzburg and Alexei Alexeyevich Lasarevich Abrikossow received the 2003 Nobel Prize for their research on the different types of superconductors ( superconductor 1st and 2nd kind ).
In August 2005 the world's first generator with high temperature superconducting (HTS ) has been successfully placed in the system test for large drives of Siemens AG in Nuremberg in operation. The generator delivers around 4 MW at 3600 rpm.
In Essen, a 110 kV connection between two substations in 2013 in a pilot project, " Ampacity " RWE Germany, replaced by a 10 -kV HTS cable. Located 1 km long, designed for 40 MW cable is manufactured by Nexans in Hanover.