Superconductors: Types, Applications & Materials

Superconductors are materials that conduct electricity with zero resistance when cooled below a characteristic temperature. Also, these materials expel magnetic fields when transitioning to the superconducting state. A normal conductor can become a superconductor when the electrons are paired to “cooperate with a material’s vibrating atom”. The phenomenon of superconductivity was discovered in 1911 by Kamerlingh Onnes.

Superconductors find applications ranging from MRI machines, and particle accelerators to magnetically levitated trains. Ongoing research focuses on raising the operating temperature and developing new high-temperature superconductor materials for various objectives.

History of Superconductivity

One of the most intriguing quantum phenomena, superconductivity was first discovered in 1911. The major milestones in the scientific understanding of superconductors are as follows:

• 1911: Superconductivity was first experimentally observed in mercury by Dutch physicist Heike Kamerlingh Onnes. It vanished electrical resistance at 4.2K temperature.
• 1933: The Meissner effect explained the expulsion of magnetic fields from superconductors' interiors.
• 1957: Bardeen, Cooper and Schrieffer formulated the BCS theory successfully modelling superconductivity in conventional low-temperature superconductors through electron-phonon interactions.
• 1962: Brian Josephson predicted the tunnelling of electron pairs between superconductors separated by an insulating barrier. This Josephson effect enabled the design of ultra-sensitive magnetometers.
• 1986: Georg Bednorz and Alex Müller discovered new ceramic oxide superconductors with remarkably high critical temperatures exceeding liquid nitrogen temperatures.
• 1987: Yttrium barium copper oxide (YBCO) ceramic material was discovered with a superconducting transition temperature of 93K, which is above liquid nitrogen's 77K boiling point.
• 2008: Iron-based high-temperature superconductors were discovered further expanding the range of materials exhibiting this phenomenon.

Principle of Superconductivity

Conventional superconductivity is explained by the formation of electron pairs known as Cooper pairs combined with electron-phonon interactions as conceptualised in the BCS theory:

• Cooper pairs: At sufficiently low cryogenic temperatures, interactions between conducting electrons and crystal lattice vibrations (phonons) cause the lattice to distort slightly.
  • This causes some electrons near the Fermi level to become unstable and rearrange into pairs called Cooper Pairs.
  • These Cooper electron pairs can move through the metallic crystal lattice without resistance as they do not get scattered by the lattice vibrations.
  • Large-scale coherent movement of many such tightly bound Cooper pairs leads to exactly zero DC electrical resistance inside the material.
  • The Cooper pairs also induce a weak superconductivity in the rest of the material lattice further reinforcing the effect through a feedback loop.

• Meissner effect: The Meissner effect leads to the complete expulsion of external magnetic flux lines from a superconductor's interior when cooled below the transition temperature.
  • This distinguishes superconductors from other types of conductors.

Superconducting Materials

Different classes of materials exhibit superconductivity at different characteristic temperatures. Some that have achieved the highest known superconducting critical temperatures are:

Types of Superconductors

Superconductors can be broadly classified based on their transition temperature and material properties into low-temperature and high-temperature variants:

• Low-Temperature Superconductors (LTS): These include elemental superconducting metals like aluminium, lead, and mercury which exhibit superconductivity just a few degrees above absolute zero temperature.
• These also include weak/low-capacity superconducting alloys made using niobium-titanium, and niobium-tin with transition temperatures well below 30 K.
• LTS is explained well by the BCS theory which models the electron-phonon interactions.
• High-temperature superconductors (HTS): These are made using complex ceramic compounds such as YBCO, and BSCCO, which incredibly show superconducting transition at temperatures between 77-138 K.
• These do not require electron-phonon interactions and Cooper pairing and follow unconventional mechanisms arising from their crystalline lattice structure.
• They offer higher current carrying capacities and magnetic field tolerance compared to LTS. Also, they simplify cooling requirements drastically.
• Unconventional Superconductors: These are also being researched - carbon-based organic compounds, metallic hydrogen etc. that become superconducting upon specific doping or under extreme pressures.

Applications of Superconductors

Some major application areas and benefits of superconductors are:

Challenges with Superconductors

Some major technical and economic challenges exist in harnessing superconductors for practical applications:

• High operating temperatures: Superconductors face challenges in achieving high critical temperatures, as conventional superconductors require extremely low temperatures.
  • Researchers are developing high-temperature superconductors (HTS), like cuprate superconductors like YBCO, which can operate at higher temperatures, such as -181°C.
• Fabrication and scalability: Superconducting materials face challenges in fabrication and scalability due to complex manufacturing processes like.
  • Thin film deposition and wire fabrication, can be expensive and time-consuming, especially for high-quality wires using materials like REBCO.
• Material defects and stability: Superconductors are sensitive to defects, impurities, and structural instabilities, which can degrade their properties and reduce critical current density.
  • Researchers are exploring ways to mitigate these challenges through defect engineering and material stability improvement.
• Cost and commercialisation: Superconductors, particularly high-temperature ones, can be costly to produce and commercialize due to raw materials, fabrication techniques, and cryogenic cooling systems.
  • However, advancements in manufacturing processes and cost-effective production methods are being pursued to overcome this challenge.
• Integration and compatibility: Superconductors can be integrated into existing systems, requiring careful consideration of electrical insulation, thermal management, and mechanical stability to ensure compatibility with other materials and technologies.

Superconductors in India

India has proactively furthered superconductivity research and developments with noteworthy indigenous contributions:

• National Superconductivity Mission: It was launched in 2017 by the government to develop indigenous superconductors and their applications in various industries, 
  • Focusing on fundamental research, advanced materials, collaboration, technology transfer, and creating a skilled workforce.
• Local manufacturing of MRI machines: Indian scientists have successfully built the country's first MRI machine superconducting magnet domestically.
  • This aligns with national self-reliance goals in high-end medical equipment.
• Nuclear fusion progress: Sub-systems for fusion reactors relying on superconductivity are being developed locally via initiatives like the Steady State Superconducting Tokamak.
• High-temperature superconducting transformer: BHEL has indigenously developed India's first transformer using cutting-edge High-Temperature Superconductor technology through R&D efforts.
• International collaborations: Indian institutions are engaged with CERN for the Particle Physics project PIP-II, focusing on designing novel superconducting and room-temperature magnets.

Superconductors FAQs

Q1. What is a semiconductor?

Ans. Semiconductors are materials with electrical conductivity between that of metals and insulators. Their conductivity can be modulated by external electric fields.

Q2. How are semiconductors different from conductors and insulators?

Ans. Semiconductors have moderate bandgap allowing current flow under certain conditions, unlike insulators. They require doping, unlike intrinsic conductors.

Q3. What are the different types of semiconductors?

The main types are elemental semiconductors (silicon, germanium), compound semiconductors (GaAs, GaN, InP), organic semiconductors, liquid semiconductors etc.

Q4. What is doping in semiconductors?

Ans. Doping introduces controlled impurity atoms into the semiconductor crystal lattice to modulate its conductivity by either adding extra electrons (n-type) or electron holes (p-type).

Q5. How are p-n junctions formed in semiconductors?

Ans. Bringing a p-type and n-type doped semiconductor in contact creates a p-n junction across which voltage can modulate current flow.