Superconductors are different from electrical conductors like copper and silver. These materials allow the flow of charge in one or more directions like metals and other common conductors but without electrical resistance. This defining characteristic gives them a superior advantage. Furthermore, aside from being a more efficient conductor of electricity, superconductors specifically exhibit a set of physical properties called superconductivity.
Understanding the Basics of Superconductors: Properties, Types, and Applications
Conductors indeed allow the flow of charge. However, in considering known conductors like copper used in wires or other metals inside electronic devices, they do not conduct electricity properly despite being considered excellent conductors. The structures of these materials still resist the flow of electrons. This is called electrical resistance. This resistance results in a loss of electrical energy and manifests itself as heat and sometimes light.
Some degree of electrical resistance is acceptable in certain applications. This phenomenon is the foundational technology behind common electronic devices or products such as electric stoves and electric convection ovens, flat iron, and heaters. However, in several cases, the loss of energy from this resistance becomes substantial and troublesome in large-scale applications such as the transmission of electricity through power grids. The generated heat also becomes problematic in applications such as computing and energy storage.
Dutch physicist and Nobel laureate Heike Kamerlingh Onnes discovered in 1911 that certain materials can conduct electricity with zero resistance if cooled below a certain temperature called the critical temperature. He specifically observed this phenomenon while working on the resistance of solid mercury at cryogenic temperatures using the recently produced liquid helium as a refrigerant. This phenomenon is called superconductivity.
Further research on superconductivity revealed that superconductors also exhibit what is now called the Meissner effect. German physicists Fritz Walter Meissner and Robert Ochsenfeld discovered in 1933 that superconducting materials expelled applied magnetic fields. The sibling tandem of German-British physicists Fritz Wolfgang London and Heinz London explained in 1935 that the Meissner effect is a consequence of the minimization of the electromagnetic free energy carried by superconducting current.
Welsh theoretical physicist Brian David Josephson made a theoretical prediction in 1962 that a supercurrent can flow between two superconductors separated by a thin layer of insulator. This phenomenon is now called the Josephson effect. It specifically occurs when two superconducting materials are placed in proximity and in addition of some barrier or restriction between them. This phenomenon is an example of a macroscopic quantum phenomenon.
Nevertheless, based on the aforementioned, the following are the properties of superconductors:
• Zero Electric Resistance or Infinite Conductivity: A material in its superconducting state has no electrical resistance. It carries an electric current without any loss of energy. This property is achieved when the superconductor is cooled below its critical temperature. Reaching the desired temperature results in electrons forming pairs and moving without being scattered by the atoms in the semiconductor.
• Critical Temperature: The critical temperature or transition temperature of a superconductor is the temperature below which the material changes from a normal conductor to a superconductor. The transition is sudden and complete. It also depends on the type of material and the applied magnetic field.
• Meissner Effect or Expulsion of Magnetic Field: A superconducting material expels external magnetic fields from its interior. This phenomenon is called the Meissner effect. Perfect diamagnetism is another property of semiconductors. The effect can be demonstrated by levitating a superconductor above a magnet.
• Critical Magnetic Field: The critical magnetic field of a superconductor is the maximum magnetic field that the material can withstand in the superconducting state. It depends on the temperature and the type of material. It increases as the temperature decreases and as the material becomes more resistant to magnetic flux penetration. A superconductor will revert to a normal conductor if the external magnetic field exceeds this value.
• Josephson Effect or Tunneling Current: Another property of superconductors is the demonstration of the Josephson effect. It occurs when two superconductors are separated by a thin layer of insulating material. A current can flow across the intersection without an applied voltage due to the quantum tunneling of Cooper pairs.
• Persistent Current: A persistent current is an ability of a superconducting material to sustain an electric current flowing continuously in a closed loop without an external source of power. This is possible because there is no electrical resistance to dissipate the energy and weaken the motion of the current. The unique property gives rise to a constant and unchanging magnetic field inside the closed loop.
2. Types and Examples
It is important to underscore the fact that superconductors are not rare. In fact, based on the periodic table of elements alone, they are collectively abundant. There are more than 50 elements known to enter a superconducting state when cooled below their respective critical temperatures. Most work as superconductors under ambient pressure while some enter a superconducting state with the addition of high pressure.
However, it is also worth mentioning that not all elements or compounds and materials that are considered superconductors enter their respective superconducting states and demonstrate all properties of a superconductor equally. This fact has led researchers to group superconductors into two types. These are Type I superconductors and Type II superconductors.
The following are the two types of superconductors and their respective descriptions and examples:
• Type I Superconductors: These are usually pure metals. Common examples include mercury, lead, and aluminum. They have a single critical magnetic field. They also exhibit a strong Meissner effect. This means that they expel almost all external magnetic fields from their interior during their superconducting state. They also have relatively low critical temperatures that range from 0.000325 Kelvin to 7.8 Kelvin. They are characterized by a clear and well-defined transition from the normal state to the superconducting state at their respective and specific critical temperatures.
• Type II Superconductors: These are usually alloys or compounds. Examples include yttrium-barium-copper-oxide and iron-pnictides. They have higher critical magnetic field values on top of two critical magnetic fields that permit the partial penetration of magnetic fields in between. These superconductors have a more complex response to magnetic fields and are capable of sustaining higher magnetic fields within their superconducting state. They exhibit the Meissner effect below the lower critical magnetic field and have higher critical temperatures than type I superconductors.
Remember that cooling these superconductors below their critical temperatures is needed for them to demonstrate the known properties of a superconductor. However, throughout the years, researchers have worked on discovering hypothetical materials that can exhibit superconductivity in higher temperatures or room temperatures. These are known as room-temperature superconductors. A material called LK-99 is a potential candidate.
Superconductivity is an important phenomenon that has several applications in science technology, and different industries or sectors. Remember that at the heart of this phenomenon is zero electrical resistance. Adding to this is the exhibition of the Meissner effect or diamagnetism and the Josephson effect. The discovery of superconductors has brought forth newer technologies and has even helped advance or improve existing ones.
The following are the general areas describing the applications of superconductors:
• Superconductors can create powerful electromagnets that can be used for medical imaging, magnetic levitation trains, quantum computers, and particle accelerators. The Large Hadron Collider at CERN uses superconducting magnets to accelerate protons to near the speed of light and collide them to study the origin of mass.
• They can enable efficient electrical transmission and storage, as they can carry large currents without any energy loss or heating. This can reduce the environmental impact and cost of electricity generation and distribution. Superconducting cables can transmit power from renewable sources like wind and solar farms to distant cities.
• Superconductors can generate novel phenomena and applications. These include the Josephson effect, the London moment, and high-temperature superconductivity. These phenomena can reveal new insights into the nature of matter and quantum physics or open up new possibilities for technological innovation.
The following are more specific and actual real-world examples of applications:
• Medical Diagnosis and Medical Imaging: These materials are used to create powerful electromagnets that can be used for medical imaging. An example is magnetic resonance imaging or MRI scanner. This medical diagnostic machine uses superconducting magnets to generate a strong magnetic field that can produce detailed images of the internal organs and tissues of the human body.
• Particle Accelerators and Research: Superconductors are used to create electromagnets that can generate very high magnetic fields for high-energy physics research. Particle accelerators like the Large Hadron Collider use these materials to enable the creation of strong magnetic fields to steer and focus charged particles.
• Supercomputers and Quantum Computing: Quantum computers isolate and protect qubits from the environment to maintain data integrity. Superconductors help in achieving these goals by providing a stable and scalable platform for qubits. These materials could create magnetic fields for isolation, enable fast and precise measurement and manipulation of qubits, and reduce heat generation.
• Nuclear Fusion Energy and Research: These materials also enable the construction of powerful magnetic confinement systems for experimental nuclear fusion reactors. They help contain and control plasma at extremely high temperatures. This is a crucial step in overcoming the challenges in electricity generation via fusion and in achieving sustainable nuclear fusion for clean energy production.
• Superconducting Quantum Interference Devices: A superconducting quantum interface device or SQUID is a very sensitive magnetometer used to measure extremely weak magnetic fields based on superconducting loops containing Josephson junctions. SQUIDs are used in superparamagnetic relaxometry. There is also a potential application in anti-submarine warfare as a magnetic anomaly detector.
It is important to note that the widespread applications of these materials remain limited. The properties of superconductors mentioned above mean that cooling processes below critical temperatures are essential to maintain the superconducting state. Coolants are expensive to produce and use. This means that superconductors are impractical in applications like electricity transmission, commercial batteries, and consumer electronic devices.