100 years of Superconductivity
In 1911, the later Nobel Prize Winner Heike Kamerlingh Onnes noticed that cooling mercury below 4K leads to the resistance drop down to the value too small to be measurable, essentially zero. The phenomenon of zero electric resistance occuring for some materials below a special value of temperature (so called critical temperature, Tc) is called superconductivity.
Superconductivity is registered for 27 elements of the Periodic Table (niobium has the highest critical temperature of 9.2 K) and more than 1000 alloys (the highest Tc=23.2 K is known for Nb3Ge). Superconductivity is also observed for a wide class of compounds which are usually called high temperature superconductors (HTS, the highest Tc=138 K).
Superconductors are characterized by a number of critical parameters. The main of them are critical temperature (Tc), critical current density (jc) – the value of current above which the material turns to its normal (non-superconducting) state and critical magnetic field (Bc) – the value of magnetic field over which the material becomes non-superconducting. These parameters depend on each other, so the boundary between superconducting and non-superconducting states is described by the surface in 3D space (T, j, B).The lower is one of parameters (for example, temperature), the higher are another two (i.e., current density and magnetic field).
The benefits of superconductors were evident just upon their discovery. However, wide usage of superconductivity in 20th century was obstructed by underdevelopment of cryogenic technologies and rather low critical characteristics of superconductors known. The breakthrough came in 1960s after the discovery of superconductors with high value of critical magnetic field, and especially Nb-Ti and Nb3Sn materials. It lead to building of devices providing high magnetic fields for tomography, accelerators and colliders, tokamaks, equipment for magnetic separation, etc. These applications contribute now for the multi-billion-dollar device market.
The discovery of HTS in 1986 lead to another broadening of scope of potential application for superconductors, since requirements to cryogenic systems are much softer in case of HTS. It took 20 years to develop technologies to make flexible long length HTS wires from these materials, however.
First advances in development of HTS wires are connected with tapes of Bi-Sr-Ca-Cu-O (BSCCO) superconductor inside silver matrix, so called 1G HTS tapes. Later, the technology of 2G HTS tapes based on R-Ba-Cu-O superconductor (RBCO, R is rare-earth element) was developed. In both cases superconducting material is obtained in high-textured state that is very important for high current carrying capability. The cross-section of superconductor material in the HTS wire is low: for 1G HTS tapes it does not exceed 40%, and for second generation it is even lower, typically below 5%.
Very complex technologies are employed for industrial production of HTS wires. The technology requires knowledge of chemistry, physics and materials science. Architecture of 1G HTS wire includes multi-filamentary superconducting composite embedded in silver alloy matrix. A coated conductor approach is applied for 2G HTS wire. It uses a metal tape as a substrate (typically made of nickel alloy) which is sequentially covered by very thin layers, including buffer layer, superconducting layer, and protective metal layer. Buffer layers are needed for isolation of metal substrate and superconductor layer to prevent chemical reaction between them. Even though they are very thin (below 0.5 micrometer), buffer layers are the key problem in 2G HTS wire technology. The protecting metal layer (typically 1 micrometer thick silver) prevents superconductor from water vapor, carbon dioxide of the air. Superconductor layer protection from mechanical impacts and direct contact with shunt material (copper or stainless steel) are also performed with this silver layer.
At liquid nitrogen temperature, 77K, 2G HTS wire possess a critical advantage against 1G HTS wire in terms of higher critical current in strong magnetic field. There are some fundamental reasons restricting critical current in BSCCO in strong magnetic field, such as high anisotropy and poor flux pinning. It makes RBCO the superconducting system with the best critical characteristics at liquid nitrogen temperature. Moreover, with the future technology development, 2G HTS wire is expected to become cheaper than 1G HTS wire, which cost is limited by high content of silver metal.
There are many different ways to manufacture the 2G HTS wire. Either Ion Beam Assisted Deposition (IBAD) or Inclined Substrate Deposition (ISD), or Rolling Assisted Biaxially Textured Structure (RABiTS) technique can be used for textured metal substrate fabrication. Functional layers can be obtained with different techniques, which can be divided into two groups, notably chemical and physical. Former group is usually better in terms of deposition rate, throughput, low cost and scaling up. Better performance and simplicity in tailoring process parameters are the common signs of the physical deposition methods.
High transmission capability of electrical current accompanied by low losses, compact size, reliability and ecological benefits makes HTS technology so special. Fault current limiters, motors and generators, transformers, power cables, synchronous compensators, magnetic energy systems are the key devices making electricity infrastructure more efficient and resistant to failures such as blackouts. 2G HTS wire can give new dimension to the novel transport technologies such as naval low-speed engine and magnetic suspension for high-speed trains. Highly effective induction heaters, compact medical noninvasive analyzers are also fabricated using HTS materials as a key element of such devices. Thus, equipment based on HTS tapes are improving current achievements of conventional technologies or creating new records in various fields of human activities.
Today, SuperOx is the only company that develops 2G HTS wire technology in Russia.