Larry Scipioni, PVD Products, Wilmington, MA
At a scientific meeting in 1911, Kamerlingh Onnes described the resistance behavior of mercury near liquid helium temperatures. He reported that “the resistance diminished very rapidly and disappeared at 4°,19K.” The abrupt transition was not in accord with any of the existing theories of low-temperature electrical properties of metals, and ongoing efforts to explain this in the 10,000+ materials which exhibit it has led to much interesting 20^th century physics.
At a scientific meeting in 1987, Mueller, Tanaka and others reported the observation of high temperature (high-T_c) superconductivity in Lanthanum Barium Copper Oxide at a temperature 13°C above the liquid nitrogen boiling point. One attendee reported to us the excitement in the air and the overflowing crowd pouring out into the hallway, staying until 3 AM to hear the results (you can experience this “Woodstock of Physics” yourself on YouTube!) These perovskites have sparked great imaginations ever since, with thoughts of applications ranging from lossless utility grids to levitating cars. Although Bednorz and Mueller used liquid phase methods to prepare polycrystalline films, high-T_c superconductivity was demonstrated that same year in Yttrium Barium Copper Oxide (YBCO) films deposited epitaxially by pulsed laser deposition, which actually produced higher quality material.
At this tutorial we will continue the story, discussing the physics, technology, and applications of thin film, high-T_c superconductor electrical carriers, grown entirely or partly by physical vapor deposition. Since YBCO is a ceramic, it cannot be worked or drawn into wires nor can it be flexed. However, in thin film form on a flexible metal tape, it can be bundled into cables or wrapped into magnet coils. And reel-to-reel coating processes can be utilized for scale-up, similar to web coating applications. Several buffer layers are required to obtain ordered YBCO films, making for multiple process steps. There are also a variety of techniques employed to deposit the actual superconducting layer, each method bringing its own advantages and issues. All these technical issues are interesting in their own right, and they also influence the suitability of each manufacturer’s product for various applications. Large current capacity cables, high efficiency motors, and high strength magnets are all developed with this technology. And although flying cars are not on the high-T_c technology roadmap, applications such as compact fusion reactors are poised to drive this industry toward high volume scale-up.