PHD → Inorganic chemistry → Solid state chemistry ↓
Superconductivity
Superconductivity is a fascinating and complex phenomenon that occurs in certain materials when they are cooled to critical temperatures, usually close to absolute zero. In simple terms, superconductivity involves the complete absence of electrical resistance and the expulsion of magnetic fields in a material.
Basics of superconductivity
The cornerstone of superconductivity is a property discovered by Dutch physicist Heike Kamerlingh Onnes in 1911. Onnes found that the resistance of mercury drops abruptly to zero when cooled below a critical temperature of about 4.2 Kelvin. This discovery led to the understanding that some materials exhibit perfect conductivity at low temperatures.
Understanding resistance and conductivity
To understand superconductivity, one must first understand basic electrical resistance and conductivity. Resistance is the resistance a material offers to the flow of electrical current. It is measured in ohms (Ω
). Conductivity, on the other hand, is a measure of how easily electricity flows through a material.
Ohm's law
Ohm's law describes the relationship between voltage (V
), current (I
) and resistance (R
) in an electric circuit. It is expressed as:
V = I * R
When a material becomes a superconductor, R = 0
, which means that for any current, the voltage drop across the material is zero.
Subtle understanding: The Cooper pairs
The microscopic theory behind superconductivity was explained in 1957 by the BCS (Bardeen-Cooper-Schrieffer) theory. According to the BCS theory, electrons in a superconductor form Cooper pairs - pairs of electrons with opposite momentum and spin. These pairs move through the lattice structure of the material without scattering, causing no resistance.
Role of phonons
Phonons, or quantized lattice vibrations, play a key role in the formation of Cooper pairs. The interaction between electrons and phonons provides the attractive force necessary for pairing, even though individual electrons repel each other because of their similar charges. This pairing mechanism greatly reduces the energy required for electrical conduction.
Types of superconductors
Superconductors can be broadly classified into two categories:
Type I superconductor
Type I superconductors are materials that exhibit superconductivity only below the critical magnetic field they are exposed to. They have a single critical temperature and suddenly lose their superconducting properties when exposed to a magnetic field stronger than their critical field.
Type II superconductor
On the other hand, type II superconductors have two critical magnetic fields: Hc1
and Hc2
. Between these two fields, the material allows the magnetic fields to partially penetrate through it by allowing vortices to form, thereby retaining some superconducting properties. This makes them more resilient to external magnetic fields than type I superconductors.
Meissner effect
The Meissner effect is a special property of superconductors. It describes the expulsion of magnetic field lines from the interior of a superconducting material as it drops below the critical temperature. This leads to phenomena such as magnetic levitation.
In the above example, imagine a superconductor at the center. The magnetic field lines try to penetrate, but due to the Meissner effect, they are repelled, resulting in levitation over the superconductor.
Applications of superconductivity
Superconductivity offers many applications in technology and industry:
Magnetic resonance imaging (MRI)
MRI machines typically use superconducting magnets. These magnets provide strong magnetic fields while maintaining energy efficiency due to their zero electrical resistance.
Maglev trains
Magnetically levitated trains use superconducting magnets to achieve frictionless travel on tracks. The lack of friction due to levitation results in extremely high speeds and increased energy efficiency.
Challenges and future prospects
Although superconductivity has incredible potential, there are many challenges to overcome. The requirement for extremely low temperatures makes the implementation of superconductors expensive and energy-intensive.
High temperature superconductor
The discovery of high-temperature superconductors (HTS) has opened up new possibilities. Unlike conventional superconductors, which require liquid helium cooling, HTS materials, such as some copper oxides and iron niktides, can be cooled using liquid nitrogen, which is less expensive. Understanding the detailed mechanism of HTS remains a challenge, as it differs from the BCS theory.
Conclusion
Superconductivity is one of the most interesting phenomena in solid state chemistry, highlighting the rich interplay between quantum mechanics and materials science. Ongoing research aims to uncover materials that superconduct at higher temperatures and with greater efficiency, promising a future where superconductivity could revolutionize energy transmission, medical imaging, transportation, and beyond.