http://truthufo.blogspot.com/: Superconductors and magnetic levitation

Friday, 22 August 2008

Superconductors and magnetic levitation

In 1911, the Dutch physicist Heike Kamerlingh Onnes-discovered when cooled mercury to 4.11 K, its electrical resistance drops abruptly to a value extremely low: it becomes superconducting. The resistance of a material to the superconducting state is zero, within a margin of error at least 1012 times smaller than the resistivity of a typical metal. The temperature below which a superconducting material is called temperature is critical and noted Tc. Since 1911, superconducting properties were discovered on several other materials, each with its own critical temperature. The simple with the critical temperature is the highest niobium, which goes to the state superconducting at 9.26 K. Before 1986, the Nb3Ge was recognized as the superconducting material with the highest Tc, or 23.3 K.

The superconductors can lead perfectly electric current without loss of energy. Many practical applications of superconductivity based on this property, indeed all those who rely on the use of superconducting electromagnets that can generate strong magnetic fields without constant supply of energy (with the exception of that necessary to maintain the material in its critical temperature). But that is not essential. What characterizes most fundamentally a superconductor is its ability to exclude the magnetic field lines: if we plunge an object in a superconducting magnetic field, a current surface appears against which produces a magnetic field as the total magnetic field is zero to Inside the convention. In 1939 that W. Meissner and R. Ochsenfeld have observed this effect (called Meissner effect) on the lead. It is on the Meissner effect that is based magnetic levitation.

Several years passed before a microscopic explanation of superconductivity is born. In 1956, Leon Cooper has shown that in a solid, electrons can attract each other to form what is called a pair of Cooper.

But this is possible only if they are quite distant from one another so that their interaction with phonons (vibrations of the crystal, see the issue article 9 of the Attractor on this subject) premium on repulsion coulombienne (electric). The amplitude of the attractive force between electrons generated a pair of Cooper by phonons decreases with increasing temperature. The following year, John Bardeen, Leon Cooper and Robert Schrieffer developed the BCS theory, based on the Cooper pairs. They found that electrons, to combine must have quantities of movement and spins opposed. Moreover, they showed that there was only 0 K and in the absence of external magnetic field and electric current internal electrons are all matched. Otherwise, a non-zero electrons are single. Note that the electrons in a pair of Cooper are remote at least 10-6 m, or nearly 200 times the distance interatomique. In a regular driver, the voltage applied to the electron communicates a quantity of movement which is transmitted to the crystal through collisions between electrons and phonons. Thus, the energy supplied by the electric field is dissipated by thermal vibrations. The Cooper pairs, they can not dispose of energy phonons, as this would require the electrons pair a passage to another quantum state, quantum state fortunately inaccessible because of its excessive energy compared to that of the Cooper pair. Until the sixties, scientists were convinced that all superconducting acted the same way in the presence of a magnetic field. They knew that superconductivity disappears if it is subject to a more intense magnetic field that a certain critical field Bc, and that the value of this critical field depends on the temperature. Thus, the object could be located in its normal state, either in the superconducting state, depending on the value of temperature and applied magnetic field. Today, it describes the materials behaving this way of superconducting first case or the first type. However, this is not the case for all superconductors. In 1962 a second type of superconductors was discovered. These materials second species have two magnetic fields reviews (BC1 and BC2) dependent on temperature. Thus, they may find themselves in three states: the normal state, the superconducting state and the state mixed. Sub BC1, the material is completely in the superconducting state. When it crosses the critical magnetic field, he finds himself in the mixed, ie that the magnetic flux begins to enter the object through thin beams called vortex. The center of each vortex is characterized by normal conductivity, and the flow through the east kept constant by persistent current loops formed on their circumference. The density of vortex increases in proportion to the applied field. If this field exceeds BC2, the material reaches its normal state, the same way as if it had exceeded the critical temperature without being subject to any external magnetic field.

For 15 years, the BCS theory has enabled scientists to understand the world of superconductivity, and thus able to predict properties of superconductors and develop new experiences. But this story does not end there. With the emergence of high-temperature superconducting critical in 1986, the study of superconductivity has grown fast. It began with oxide Ba-La-Cu-O who had a critical temperature of 34 K. But this record was quickly beaten a few months later by the Y-Ba-Cu-O to 92 K then by the Tl-Sr-Ca-Cu-O with its 125 K in 1988. With these new superconducting compounds, the most important thing is that we no longer need to cool with liquid helium (4 K), because liquid nitrogen (77 K) is enough! This new cooling mode has several advantages. Present in large quantities in the atmosphere, nitrogen, unlike helium, costs nothing. Liquifié, nitrogen handles more easily than helium. However, warmer does not necessarily mean better. Indeed, the critical current that can move in this type of superconductor is disappointing. The current criticism is the current beyond which the force induced by him is greater than the force of attachment vortex, so they begin to move by creating some resistance in the material, which loses its superconducting properties ( high-temperature superconductors are all critical of the second case). In addition, the electronic noise associated with these superconductors may be an obstacle to the achievement of certain devices, which merely a critical temperature lowest in favour of greater accuracy. Thus, even if high-temperature superconductors are available, their use remains limited. So far, the highest temperature associated with a reproducible superconducting reached 164 K (-109 C), and using mercury highly pressurized. Finally, there is still no satisfactory explanation of superconductivity in these materials, but as we had to wait 40 years before the birth of the BCS theory to explain superconductivity Typically, 15 years since the discovery of high-temperature superconductors should not discourage us!

Finally, even if the high-temperature superconductors are not as good quality and precision than conventional superconductors, their discovery was of great importance in the history of superconductivity. Before it, in the 1970's, several said that advances had become impossible in the field of superconductivity. But they were wrong. Today, the same speech is included, but who knows if a superconducting at room temperature does happen?