The many applications of high-temperature superconductors
HIGH temperature (high Tc) superconductivity is one of the most revolutionary developments in the field of physics in the past decade. This phenomenon not only provides a new insight into the structure of matter but also opens up exciting possibilities for applications.
One such application is high Tc Superconducting Quantum Interference Devices (SQUIDs). These are instruments that provide an extremely sensitive measurement of magnetic fields. Although SQUIDs have been used for about 3 decades in a variety of magnetometers (instruments to measure magnetic fields), they could only operate at liquid helium temperatures (about -270 C). Liquid helium is extremely expensive to produce besides requiring very specialised equipment for handling. But with the discovery of high Tc superconducting materials, there is a possibility that one could produce SQUIDs that would operate at liquid nitrogen temperatures -- about -195 C (Nature, Vol 372, No 6506).
Liquid nitrogen is much easier and cheaper to produce and also boils away much slower than liquid helium making it easier to handle. Recently many groups have reported remarkable progress in the development of such high Tc SQUIDs. Most of these use the "wonder" material of high Tc superconductivity, YBCO (Yttrium Barium Cupric Oxides). Films of this compound are grown in the presence of oxygen using laser deposition or sputtering and then treated appropriately to get the desired properties. These devices are then, along with the required electronics, made into magnetometers.
The magnetometers with ordinary low temperature SQUIDS are usually extremely sensitive, highly reliable and have low noise levels. Although the high Tc SQUIDs are still not as sensitive as the older ones, there has been much progress in improving their efficiency. Along with high Tc superconductivity, advances in thin film technology and electronic noise reduction techniques have made possible these low noise devices.
The level of noise in liquid nitrogen SQUIDs has come down by a factor of almost 10,000 in the past 7 years. This is particularly useful in geophysics where many of the applications require very low noise levels because the magnetic field variations that need to be measured are extremely small.
Another promising area where these devices are being used is medical diagnostics. Magnetic imaging of the heart and the brain (magnetocardiology and magnetoencephalography, respectively) obviously demands sensitive and reliable equipment. Though the level of sensitivity required for these applications is yet to be reached, there is hope that in the near future we will see much progress.
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