On August 14, 2003, an electrical power outage shut down the northeastern USA and parts of Ontario in Canada. An antiquated electrical power grid in serious need of costly upgrades was blamed for this disruption, which was the worst in history. A breakthrough in materials may offer the eventual resolution of this problem.
In 1986, J. Georg Bednorz and K. Alex Müller at IBM’s Zürich Research Laboratory discovered a complex copper oxide perovskite with a superconducting transition temperature of 36 K. From a practical point of view, this meant that cryogenic refrigerators operating at 20 K could be used to cool the new materials — opening the door to applications beyond the physics lab. A few months later, Paul Chu at the University of Houston was on the cover of TIME Magazine as the inventor of another superconducting cuprate with a transition temperature of 93 K. (In fact, Robert J. Cava at Bell Labs is also credited with this invention.) The discovery got widespread attention because the transition temperature was above the boiling point of liquid nitrogen (77 K). In 1987, Bednorz and Müller were awarded the Nobel Prize in Physics for their discovery of high temperature superconductors (HTS). The headlines promised applications from levitating trains to high capacity, efficient electric power systems.
However, many technical problems needed to be solved before any commercialization could be realized. For instance, the early composition of YBa2Cu3O7−x required unique processing to enable a high current density to complement its high transition temperature. Likewise, oxides are inherently brittle, so a process was required to make pliable wires that could support the stresses and strains of manufacturing and subsequent applications. Furthermore, because current flow in the cuprates is anisotropic, with the maximum current flowing parallel to the Cu-O planes, texturing of the oxides is required to achieve optimum flow. The failure to solve these, and other, problems threatened to block the commercialization of HTS.
Nevertheless, in April 1987, Greg Yurek and three colleagues from MIT founded American Superconductor Corporation (AMSC) to manufacture HTS ceramic wires. The basis for the new company was a set of patents describing how to form composites of HTS with noble metals such as Ag. Such a composite would circumvent the inherent brittleness of the ceramic by using a metal that would not react with the HTS during heat treatments, but would readily diffuse oxygen to allow controlled changes in stoichiometry during the processing cycle. Yurek resigned from his faculty position in August 1988 to become President and CEO of AMSC. In 2002, AMSC opened the first HTS wire manufacturing plant, where ‘first generation’ wires up to 1 km in length are being produced. These wires, also known as multifilamentary composite wires, comprise very fine filaments of the ceramic superconductor embedded in a matrix of Ag. The ∼10 μm thick filaments are produced in a sequence of rolling and heat treatment steps that impart a high degree of crystallographic texture to optimize current-carrying capability. By March 2003, AMSC had received orders for a record 450 km of HTS wire from 11 different customers in four countries. For example, a power cable system will be installed underground in a live grid at transmission voltages to support the Long Island Power Authority. The superconductor cable, cooled by a liquid nitrogen cryogenic system, can typically carry three- to five-times more power than conventional Cu cables of the same size, and can be installed in existing rights-of-way. HTS wires are finding other applications, including the magnetically levitating trains that caused all the excitement back in 1987.
Today, ‘second generation’ (2G) HTS wires are being developed by AMSC, other companies, and research labs around the world. In these wires, a 1–2 μm thick coating of HTS on a flat substrate with an incredibly high degree of crystallographic texture creates, in essence, a coating of single crystal material along the entire length of the wire. Commercialization of 2G technology is expected to take three to four years, during which time the 1G wire will be the workhorse of the emerging HTS industry. Commercialization of HTS materials has taken only 15 years. Other materials, such as semiconductors, optical fibers, and Kevlar all took 15–20 years—so HTS is right on schedule. Incidentally, on Friday, August 15, 2003, the day following the ‘great power outage’, the shares of AMSC rose 42% in value.