Another Nobel Prize-winning discovery, in 1986, of high transition temperature (HT _c) superconductors based on the so-called YBCO compounds, ceramic materials containing yttrium, barium, copper and oxygen, has led to a global research activity. Unlike conventional metal alloy superconductors that require cooling to liquid helium temperatures (4.2K) before they exhibit this effect, YBCO materials superconduct at far higher liquid nitrogen temperatures (77K) and above and are thus more likely candidates for practical applications. Whilst research groups from around the world are striving to develop room temperature superconductors, several are using existing HT _c compounds in sensors termed SQUIDs (superconducting quantum interference devices). These respond to minute changes in magnetic field strength with a resolution that can reach an almost unbelievable 10 ¹⁵Tesla.

A device developed at the University of Strathclyde in the UK is based on YBa ₂Cu ₃O ₇ and fabricated by pulsed laser deposition (PLD). This uses pulses from a 248nm excimer laser to ablate the YBCO from a solid target onto magnesium oxide substrates at 770 C. Argon ion beam etching is used after the film has been deposited to pattern it to the required structure. HT _c SQUIDs such as this are starting to find applications and one promising field is NDT (non-destructive testing). The Strathclyde group has used these sensors to locate cracks in the lower layers of multi-layer metal samples, a technique that could play an important role in detecting corrosion around rivets in aircraft...