RACA Journal March 2020 | Page 59

Getting Technical CHARLES NICOLSON Charles Nicolson has a physics and chemistry degree from Natal University which he subsequently put to good use by applying speciality chemicals in mining and industrial processes where water is a major factor. This created an enduring interest in water technology, a passion that expanded to the HVAC industry in 1984 when he joined BHT Water Treatment. Since then, water technology in HVAC water circuits has continued to be an abiding interest. COMPARING X-RAYS AND MRIS By Charles Nicolson The feature article in the January 2020 issue of RACA Journal introduced the lower regions of very cold and even colder – from a now generally accepted cryogenic upper limit of -50°C down to absolute zero temperature of -273.15°C. T HISTORY OF SUPERCONDUCTIVITY The discovery of superconductivity was made in 1911 by Kamerlingh Onnes using solid mercury as the conductor, supercooled by liquid helium down to close to -270°C. Subsequently, Onnes attempted to make electromagnets with various types of superconducting material windings but found that even relatively low magnetic fields destroyed the superconductivity within the materials he was investigating. www.hvacronline.co.za A section of a hand X-ray plate showing the bones and knuckle joints. he article went on to describe the first widely-used cryogenic product – dry ice or solid carbon dioxide – which conveniently sublimes or changes directly from solid to vapour phase at -78.15°C at normal atmospheric pressure. The fact that dry ice is still in common use since the 1920s is a good example of how substances which have to be made and stored for use at cryogenic temperatures continue to provide useful benefits. Many other useful products have been created in controlled cryogenic conditions including those which become electrically superconducting below specific temperatures. The electrical resistance of ordinary metallic conductors decreases gradually as temperatures are lowered down to levels only a few degrees above absolute zero. However, certain other conducting materials which can acquire an ability known as ‘superconductivity’ suddenly lose all of their electrical resistance when reducing temperatures reach specific levels. Electric currents in superconducting circuits can persist indefinitely with no power source. Promising future applications of superconductivity include magnetic refrigeration at potentially higher coefficient of performance (COP) levels and far more efficient refrigeration with added benefits of greatly reduced carbon emissions. Further progress of research during the first half of the 20th century into this new and complex field of superconducting tended to be drawn out by practical difficulties of running experiments at temperatures below -200°C. In addition, two world wars relegated most of the purely scientific research programmes to back burners although some specific metallurgical research concerning machining of metals at cryogenic temperatures was done in the USA. During the 1950s the physicist G B Yntema determined that superconducting wire made from niobium-based alloys could be used for windings which generated stable magnetic fields. Further developments incorporating niobium/tin combinations showed that these alloys could carry currents up to 100 000 amperes per square centimetre (A/cm 2 ) in superconducting conditions controlled within a 5°C approach to absolute zero. Despite being brittle and difficult to fabricate, niobium-tin coils have proved to be practical for generating magnetic fields RACA Journal I March 2020 57