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The year was 1908, and the Dutch physicist Heike Kamerlingh Onnes had won the race for ultimate cold; he had liquefied helium, the most difficult gas to liquefy. In the decades prior to his victory, physicists had been working extensively with the liquefactions of gases, such as oxygen, nitrogen, and the more challenging hydrogen. While many of these physicists were interested in gas liquefaction simply to attain temperatures closer and closer to absolute zero, Onnes was interested in obtaining large quantities of these liquids to test their properties. When helium, the most volatile gas in existence, was discovered in 1895, Onnes was determined to liquefy it. By accomplishing this, he would be able to reach the coldest temperature yet attained and see if he could discover any unusual properties at such an extreme temperature. In fact, Onnes discovered superconductivity, the disappearance of resistance from a conductor. This was a new concept at the time, and from that point to the present day, many innovations have been created through the help of it. Currently, scientists are looking for the presence of superconductivity at higher temperatures with the hopes that it could eventually be used in everyday life. Onnes’ investigations in low temperatures stemmed from the work of Diderik van der Waals and an interest in the behavior of gases. Onnes needed exceptionally cold conditions in order to test the truth of van der Waals’ laws in the extremes. In 1877, physicists Louis Cailletet and Raoul Pictet independently shattered the myth of permanent gases by liquefying oxygen and nitrogen. Although the droplets of liquid produced would be too small for Onnes to examine, this would prove to be a crucial step in cryogenics; Cailletet and Pictet demonstrated the possibility that any gas could be liquefied. Because they were the pioneers of cryogenics, their methods would be improved upon by many scientists to achieve even colder temperatures. The Joule-Thomson effect is a phenomenon that allows cooled gases to reach even lower temperatures without heat transfer. A gas at regular temperature that is expanded yields a temperature increase. However, when the gas falls below a certain “inversion temperature”, its temperature will decrease as a result of expansion. Cailletet obtained his oxygen droplets by compressing the gas to around 300 atm with a hydraulic press and then rapidly reducing the pressure and expanding the gas. Pictet, on the other hand used the cascade process, in which a collection of circulating gas cycles is arranged in order of decreasing condensation temperatures. The least volatile gas is first cooled and condensed, and then travels to a refrigerator where the evaporating vapor is forced off. This vapor is used to cool the next-least volatile gas, and the cascade continues until each gas’s vapors have cooled the next gas (a more difficult to condense gas) to a colder temperature. Carl von Linde used a throttling process to make use of the Joule-Thomson effect in his liquefaction of air. He simply compressed and cooled the air, and throttled it to a lower pressure to gain the extra temperature decrease. In 1898, after James Dewar had invented the vacuum flask for retaining coldness, and Olszewki had found the inversion temperature for hydrogen, Dewar became the first person to liquefy hydrogen (in doing it, he reached the lowest temperature attained at his time). While it seemed Dewar had succeeded in liquefying the most volatile gas, William Ramsay had discovered helium just three years before, and this new gas was indeed more volatile than hydrogen. In order for Onnes to conduct his studies at low temperature, he needed to produce large amounts of liquid hydrogen (this would be a necessary step before the liquefaction of helium). In 1906, Onnes had finished work on a machine that used a five-step cascade process, the Joule-Thomson effect, and a device to recycle hydrogen that failed to liquefy. The vapor from condensed methyl chloride, the first cycle gas, chills ethylene, whose vapor in turn chills oxygen. All of the first three cycles contained mechanisms to return the vapors to their condensers (whereas Pictet only had two cycles with recycling vapors and three cycles in total ). Although Linde obtained liquid air without these intermediate steps, Onnes used them for efficiency purposes. Nevertheless, he would still need the Linde process (throttling) for the liquefactions of hydrogen and helium, since their relative temperature drops are comparable to the drop from room temperature to liquid air temperatures. In the final step, compressed hydrogen would pass through a refrigerator cooled by liquid air, then through a throttle valve for its expansion. The hydrogen that failed to liquefy after expansion would be caught and recondensed for another round of throttling, while the liquid hydrogen would be collected in a vacuum glass. Soon, Onnes had set up a cascade of six gases in order to liquefy helium. He added a sixth cycle to the hydrogen liquefier, in which compressed helium was cooled by hydrogen, run through a regenerator spiral, and expanded at a throttle valve. The liquid helium was caught in a vacuum glass, but this glass was protected by several layers of other liquids in vacuum vessels: liquid hydrogen, air, and alcohol (in order from inner to outer). On July 10, 1908, Onnes ran his apparatus, producing a full vessel of liquid helium. One other major factor contributing to Onnes’ success was the large quantity of helium he was able to obtain. Onnes’s brother, who had the necessary connections, helped Onnes obtain a large amount of monazite sand to be shipped to Holland from gravel pits in North Carolina. He proceeded to extract 300 liters of helium from the sand, more than any of his competitors were able to obtain. After winning the race to liquefy helium and reaching the lowest temperature then recorded, Onnes wanted to simply the liquefaction process, and more importantly, make measurements and observations. When the liquid helium product was occupying the experimental chamber, the only passage of entry was through the regenerator spiral. However, by placing instruments inside the chamber (namely a low-temperature thermometer and a dilometer to measure the effect of temperature on density) before liquefaction, he could make measurements. Because he was unable to accomplish much more while confined to this chamber, Onnes developed a helium cryostat, a vessel for sustaining helium at a constant temperature. The liquid was siphoned through a vacuum tube chilled with liquid air to the cryostat. In his experiments with the helium in cryostats, Onnes found that metals could completely lose their resistances at low temperatures. It was already known that the resistance of a metal decreased with temperature, but many scientists believed that after reaching a minimum, it would re-rise as a result of electrons being frozen in place. Prior to Onnes’ experiments, temperatures low enough to test this had not been attained, but Onnes soon disproved the conjecture. He first tested a platinum wire for conductivity, and found that liquid helium temperature, the resistance stopped decreasing and “became independent of temperature”. He deduced that if the wire were made of pure platinum, the resistance would disappear completely at the boiling point of helium, and thus by eliminating impurities from a metal, he would be able to drive it to a state of superconductivity. Onnes proceeded to distill liquid mercury to an extremely pure form, and then poured it into a connected series of U-shaped capillary tubes with electrodes placed at the ends (for conducting and measuring current). While cooling the mercury, he noticed that the resistance decreased steadily until, at around helium’s boiling point, it abruptly disappeared. The experiment was retested several times to make sure that the resistance jump was real, which it indeed was. The one major disappointment he discovered was that only a small current could experience transit along a superconductor without resistance. The superconductive property of the metal coils was so sensitive that could be destroyed by the magnetic fields associated with larger currents. For many decades after Onnes’ research and his death in 1926, very little progress was made in raising the maximum temperature at which superconductivity can exist (the critical temperature. However, his work had great influence on many scientists; the year Onnes died, his pupil, Willem Keeson solidified helium. It was discovered in the 1930s that superconductors lack an interior magnetic flux, and that an exterior magnetic field bounces off the superconductor, causing the magnet to levitate. These superconducting magnets could replace the giant resistive electromagnets previously used in labs. Several Nobel prizes were also given to physicists for work that developed off of Onnes’ discoveries. John Bardeen, Leon Cooper, and Robert Schrieffer won in 1972 for their theory explaining superconductivity (the BCS-theory). Pyotr Kapitsa won in 1978 for his discovery of superfluidity, a property in which the viscosity of helium disappears below 2.2K. By 1973, the highest temperature a metallic superconductor could exist at was still only 23.3K. It was not until 1986 that superconductors were created using non-metals. Johannes Bednorz and Karl Mueller produced a ceramic oxide combination that exhibited superconductivity at 30K. Laboratories all over the world suddenly began producing superconductors at higher temperatures. A group of American physicists led by C.W. Chu and M.K. Wu raised the critical temperature to 52.5K by adding more than 10,000 atm of pressure to a ceramic superconductor made from a yttrium-barium-copper-oxide compound. Since then, some more advanced superconductors have been created including neodymium-barium-copper-oxide, which can exclude a magnetic field, and thulium-barium-copper-oxide, which can superconduct under much stronger magnetic fields than other superconductors. These two compounds are able to resist, to an extent, the magnetic fields that destroy the superconductivity of most other compounds, thus they are capable of carrying larger currents. The most advanced ceramic can superconduct at 77K; however, ceramics are too brittle to be made into practical wires. This is one of the many challenges still left over from Onnes’ experiments. Though a room-temperature superconductor would be extremely useful, many applications of superconductors already exist. Because of the absence of magnetic flux on the interior of a superconductor, they are used to make extremely powerful superconducting magnets. These magnets are very important parts of MRI machines, particle accelerators, and SQUIDs (superconducting quantum interference devices). MRI uses a superconducting coil to impose a powerful magnetic field on a patient and produce a map of the entire body. Particle physicists were very enthusiastic about the use of superconducting magnets to accelerate protons to nearly the speed of light. SQUIDs, used to sense magnetic fields, appealed to a wide range of people; they could be used to detect submarines and mines, to find mineral deposits, and even to study magnetic fields inside the brain. These emerging applications of superconductors will all be enhanced with the improvement of superconductors. In the future, superconductors will most likely be used in power storage devices, electric motors, and even in magnetic levitation (Japan has planned to put levitating trains into use). If durable wires can be built from ceramics, the world may soon see a room-temperature superconductor, which would be extremely useful in creating frictionless electrical transmission lines. Superconducting films used in computer chips would greatly speed up computers by eliminating the resistance that limits them. Superconducting generators and storage units would have enormous benefits on the world, decreasing the size, and increasing the efficiency of their non-superconducting counterparts. Automobile manufacturers are even considering using superconductors to power cars. In short, the discovery of a room-temperature superconductor, would cause highly efficient superconducting appliances to become commonplace, and the existing costly low-temperature superconducting devices would be greatly improved. Onnes’ discoveries encouraged scientists from all over the world to improve upon his work to explore further in the new field of low-temperature physics. Onnes himself saw the potential existing in the field, and his breakthrough made this potential particularly noticeable. If the liquefaction of helium had not been accomplished by someone so meticulous and interested as Onnes, superconductivity may have not been discovered, and the possibilities for application might have not been as numerous as they are today. Works Cited |
Physics, 1st Place