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MOST IMPORTANT SCIENTIFIC DISCOVERIES
Superconductivity. History and essence of scientific discovery
Directory / The most important scientific discoveries Even in antiquity it was noted that the state of aggregation of matter depends on external conditions. The most striking and illustrative example is the transformation of water into ice and steam. Gas (ammonia) was first liquefied in 1792 by the Dutch physicist M. van Marum. Michael Faraday, starting in 1823, converted several gases into a liquid state at once: chlorine, sulfur dioxide and carbon dioxide. The process was not difficult, because the intermediate gases liquefy at a fairly high temperature. True gases are another matter. It took more than fifty years until they managed to convert them into a liquid state. In 1877, R. Pictet and L. Calete obtained liquid oxygen and liquid nitrogen. On an industrial scale, the liquefaction of air was carried out by the German engineer K. Linde only in 1895. Now, it seemed, according to the already worked out scheme, it would be easy to transfer any other gas to a liquid state. But it was not there. Indeed, the vast majority of gases cool during expansion. However, obstinate hydrogen, neon and helium behave "dishonestly" - they heat up when expanding. A way out was found towards the end of the nineteenth century. It turned out that in order to obtain liquid hydrogen and helium, you only need to pre-cool them to a relatively low temperature. Olshevsky in Krakow, Kamerling-Onnes in Holland, and Dewar in England simultaneously tried to obtain liquid hydrogen. Dewar won this competition: on May 10, 1898, he received 20 cubic centimeters of liquid hydrogen. A few months later, he managed to obtain solid hydrogen. Only 14 degrees separated it from absolute zero. Brilliant mind, excellent art of the experimenter and excellent erudition helped James Dewar to become one of the pioneers of cryogenic technology. It is noteworthy that both the term itself (from the Greek "kryos" - cold), and the famous "Dewar vessel" belong to him. But helium stubbornly refused to submit. It was not until July 9, 1908 that the news arrived that Dr. Heike Kamerling-Onnes (1853–1926) from the University of Leiden had liquefied helium. He countered Dewar's intuition and mastery with a system, with the abilities of an excellent organizer. The famous Kamerling-Onnes laboratory in Leiden, of which he became director at the age of 29, is called the first model of a research institute of the XNUMXth century. “At the end of the experiment, Kamerling-Onnes made an attempt to get solid helium,” writes R. Bakhtamov. “He failed. He failed later, when he reached a temperature of 1,38, and then 1,04 degrees Kelvin. reasons for this strange phenomenon, he, however, forced himself to step back and proceeded to the next point of the planned program - to the study of the properties of metals at helium temperature. Onnes measured the electrical resistance of gold, platinum and took up mercury. And then the surprises began. On April 28, 1911, he reported to the Royal Netherlands Academy that the resistance of mercury had reached such a low value that "the instruments did not detect it." On May 27, the message was clarified: the resistance of mercury does not fall gradually, but sharply, abruptly, and decreases so much that one can speak of the "disappearance of resistance." In an article published in March 1913, Onnes used the term "superconductivity" for the first time. After another 11 years, he will begin to understand something in this strange phenomenon. In 50 years, the phenomenon will be explained, although by no means completely. Several times Onnes observed another rather strange phenomenon - an unusually high mobility of helium. But it was already so unnatural that Onnes did not even try to understand something. He continued his line, moving closer and closer to absolute zero. He used, in essence, one method: to reduce the vapor pressure of liquid helium, he installed more and more powerful pumps. In the end, Onnes reached 0,83 degrees Kelvin. It seemed to be the limit. However, in April 1926 - two months after the death of Kamerling-Onnes - the American professor Latimer, having developed the idea of the Canadian William Gioka, proposed a new method of cooling - magnetic. In 1956, Francis Simon of Oxford obtained a temperature of 0,00001 degrees Kelvin, only one hundred-thousandth of a degree above absolute zero." Surprisingly, only thirty years after the liquefaction of helium, its most exotic property, superfluidity, was discovered, although thousands of experiments were carried out. But one day a group of Canadian scientists still dared to give a description, resolutely refusing to draw conclusions. “The correct conclusion about a new phenomenon,” they noted, “is not difficult to make even for a first-year student. But only mature and experienced physicists would take it upon themselves to quite seriously assume that the thermal conductivity of a liquid suddenly increases millions of times.” Early in 1938, Nature published two articles. One of them belonged to a Soviet scientist P.L. Kapitsa, and the other to Allen and Mizenar of the University of Cambridge. Their results and conclusions coincided: the flow of liquid helium is almost completely devoid of viscosity. It is Kapitsa who owns the term "superfluidity", which has become generally accepted. Strikingly, helium atoms and free electrons of a metal behave in the same way. This discovery made it possible to connect both phenomena: superconductivity and superfluidity of an electron flow in a conductor. Superconductivity was discovered at the beginning of the century, but it was only in 1957 that Bardeen, Cooper and Schriefer were able to give a satisfactory explanation for the phenomenon of superconductivity by constructing a theory that bears their name (the BCS theory). “What happens in a superconductor?” Regge asks in his book. “The full answer to this question is long and complicated. Normally, two electrons repel each other in a vacuum, but in a metal, the positive charges of the nuclei shield the negative charges of the electrons, and the repulsion can almost completely disappear. In many cases the screening turns out to be incomplete, and then superconductivity is not observed. In some cases, the lattice shrinks around an electron, thus creating a cloud of positive charges that wraps around that electron and attracts other electrons. The result is a slight attraction between the electrons. Since this attraction is weak, it only causes the electrons to move in pairs; thus, a bond arises, similar to a chemical one, but thousands of times weaker. Consequently, a Cooper pair is similar to a "two-electron" molecule, and the transition to the state of superconductivity can be considered as the transformation of an electron gas into a gas consisting of such "molecules". A similar phenomenon occurs in chemistry: for example, if diatomic oxygen is heated, it breaks up into single atoms that can recombine when cooled. The electron gas moving in the metal condenses into a liquid of Cooper pairs, which we will call "condensate". The radius of such a pair is approximately 300 angstroms, which is much larger than the distance between neighboring atoms (several angstroms). In a sea of Cooper pairs, it is hard to imagine ripples or waves shorter than the pairs themselves. Therefore, lattice inhomogeneities with dimensions of no more than ten angstroms do not represent obstacles for the condensate flow, and no energy loss occurs. This is the main cause of superconductivity." It is still difficult to imagine all the consequences of this discovery. The effect of superconductivity has already been successfully used in high-speed Japanese Maglev trains. “Superconducting magnetic systems with unique characteristics have been created and are working,” writes R. Bakhtamov. “Lockheed, for example, has built an electromagnet that weighs 85 kilograms and produces a magnetic field of 15 oersteds. The largest superconducting magnets with a field of 30-40 thousand oersteds and a size of about 4 meters are already working in a number of accelerator laboratories in Europe and America, magnets with a field of up to 170 thousand oersteds have been created. Work is underway to create the largest electric machines - turbo- and hydro-generators with superconducting excitation systems. Superconductors open up completely new possibilities in the creation of computers. The current in superconducting systems is an ideal storage device capable of storing an enormous amount of data and issuing it at a fantastic speed... Alloys have already been obtained that retain superconductivity at 18–20 degrees Kelvin. The creation of a substance that would have properties at a temperature of at least 100 degrees Kelvin would lead to a revolution in electrical engineering. Modern science believes that the task is real, and the consequences of its solution will be defined in one word - fantastic. Author: Samin D.K.
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