Engineering Properties of Superconducting Materials (Second Volume)

Dear Colleagues,

The search for clean energy sources has been a fundamental key in materials research. The development of superconducting materials attracts significant scientific and technological resources toward achieving low costs, as well as suitable and profitable power generation, storage, distribution, and transmission. In addition, superconducting electronics can provide devices and circuits with properties not obtainable by any other known technology; i.e., very low loss, zero frequency-dispersion signal transmission lines, very high Q-value resonators and filters, and quantum limited electromagnetic sensors.

All of these advances require high-quality superconducting materials, and in recent years, great strides have been made to improve the properties of existing materials, as well as the continuing discovery of new systems and materials, such as the Pnictides.

In 1911, Heike Kamerlingh Onnes discovered superconductivity in mercury by cooling it down to a frosty 4.2 K (–268.95 °C). Since then, it has been the Holy Grail of material scientists to achieve this transition—from a normal to a superconducting state—at room temperature (above 273.15 K or 0 °C). The hope of finding a room-temperature superconductor (RTS) bloomed after physicists discovered high-temperature superconductivity (HTS) in the 1980s and 1990s in a class of ceramic materials called cuprates. These are characterized by the presence of interleaving copper-oxide layers. Their transition temperature—also known as critical temperature (Tc)—was significantly higher than those of conventional metallic superconductors discovered decades earlier.

From 1911 and until the discovery of superconductivity in Lanthanum Barium Cuprate in 1986, there was a steady rate of discovery of new materials including Nb3Sn and NbTi (important in NMR, MRI and high field magnets). However, the discovery that really opened the R&D floodgates was of superconductivity in an yttrium–barium–copper–oxide (YBCO) system, in which Tc was 93 K. Soon, scientists were investigating a wide variety of such systems, including bismuth- and mercury-based compounds. More recently, a range of materials which are distinct from the cuprates, such as MgB2 and iron-based superconductors, have been discovered.

There is a continuous drive toward higher and higher transition temperatures, and to date, the highest superconducting Tc achieved and confirmed is 203 K in 2015. However, this was not in a high-Tc cuprate system but in hydrogen–sulfide (H₂S) subjected to very high pressure: about 1.5 million atmospheres. The highest Tc achieved in a cuprate material was in 1993 at 138 K, in a mercury–barium–calcium–copper–oxide system at atmospheric pressure. The Tc increased to 164 K when the pressure was increased to ~296,000 atmospheres.

From an engineering point of view, although higher transition temperatures are desirable, of greater interest is the development of the engineering properties of the materials. Consequently, this Special Issue aims to focus on the development of superconductors, in a materials relationship framework, and specifically to collate their engineering properties. Topics of interest include but are not limited to the following topics:

• Coated conductors, especially critical current versus field and temperature;
• Iron-based superconductors;
• Superconductivity in unconventional materials (e.g., graphene);
• Flux pinning mechanisms;
• AC losses;
• Normal zone propagation velocity;
• Materials and process for high-throughput fabrication.

Dr. Tim Coombs
Guest Editor

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