The Superconducting Revolution

Phase 3

Development of superconductivity

Records are being broken one after another by improving deposition speed and critical current density, as well as by approaching longer lengths.

When the IBAD method was first discovered, its principle was verified using a small stationary device. Due to the limitations of the device, the intermediate layer wire that could be produced here was only a few centimeters long. After that, we designed and produced a device that could deposit the film while moving the initial base tape. In order to show the world that it could function as a wire, we considered the selection and arrangement of the ion source so that we could produce a meter-class wire. We asked a certain heavy electrical equipment manufacturer that is skilled in ion implantation technology to produce the device, but they initially refused due to budgetary constraints. However, we ended up paying them a promotion and they produced it for a very low price. Thus, the device was completed at the end of fiscal year 1992. Because this device requires a long time for orientation, the wire production speed was only 10 cm per hour. Therefore, it took several days and nights to produce an intermediate layer wire of several meters.

After further improvements, the equipment was developed to be capable of producing 1m per hour. Eventually, it became possible to produce an intermediate layer 100m long. Furthermore, in the Y-based wire development project that began in 2003, the ion source was enlarged four times from 60cm x 8cm to 110cm x 15cm, so that a 500m-long intermediate layer could be produced at 5m/h. With this equipment, a superconducting wire 504m long and with an average critical current of 440A was realized at the end of 2007.

Furthermore, in order to improve the deposition speed, we are researching and developing intermediate layer materials and ion irradiation methods, and we will soon be able to achieve deposition speeds of several tens of m/h to 100 m/h.

The superconducting layer can be produced most effectively by using a gas phase method. Currently, the main gas phase processes are Pulsed Laser Deposition (PLD), Chemical Vapor Deposition, and Metal Organic Deposition (MOD). The PLD method is a method in which a laser is irradiated onto a disk called a target, which is made of sintered superconductor powder, and the particles that fly out from the disk are deposited on a substrate. This method allows high-speed film formation and the cost of raw materials is low because the primary raw materials such as yttrium oxide can be used as the target directly. By selecting a temperature of 800 to 900 degrees Celsius, appropriate oxygen gas, and laser irradiation conditions for film formation, a superconducting wire with a critical current of several hundred A can be produced. If the film formation temperature is too low, crystals called a-axis particles that do not contribute to the superconducting current will grow, and if the temperature is too high, the formed superconductor will decompose, so in either case only a low critical current film can be produced. To address this issue, efforts have been made to increase the output of lasers and improve temperature control methods, and while the critical current density was around 500,000 A/cm2 in the mid-1990s, it became possible to obtain long lengths of wire with a critical current density of over 1 million A/cm2 by around 2005. This was further improved to 2 million A/cm2 in 2007, and currently wires with a current density of over 3 million A/cm2 can be obtained.