Haripriya Latheeshan/ Bahrain
What does the word ‘resistance’ suggest? An opposition? Something holding us back? Resistance can mean a lot of things. It is mostly associated with an element of avoidance, pushing away or not allowing something to happen. The resistance to happiness, the resistance to freedom, the resistance to free thought—all familiar types of resistance.
When we delve into the world of electricity and its phenomena, resistance means the opposition to the flow of electrons in a conductor. What are the causes of the resistance? What is its mechanism? What are the palpable effects of this resistance? Let’s start with the most basic of the questions raised, the source of the opposition. Electricity is the flow of free electrons across a conductor that has a potential difference across its terminals. Atoms and positive ions in a conductor are always in a state of vibration due to thermal energy available outside. Vibration levels increase with increasing thermal energy. When free electrons flow through a conductor, they undergo collisions with the positive ions in the conductor, hindering their free flow.Thus, electrons encounter an opposition, which we call resistance.
The higher the temperature of the conductor, the greater the vibration levels and, subsequently, the greater the resistance to the flow of electrons. A good analogy for the mechanics of this phenomenon would be a person walking or trying to walk through a crowded room in which people are constantly in motion. It would be extremely tiresome and frustrating to try to navigate in such an environment. What happens to electrons is something quite similar. When electrons bump into atoms or positive ions, some of their kinetic energy is transferred to the atoms and ions as thermal energy and the conducting wire gets heated up. This is why there is an energy loss associated with resistance in a conductor when current flows through it. This energy loss is the most important effect of resistance. Its importance arises from the fact that humankind loses a whole lot of energy in this manner and that prevention of this loss can literally change the world we live in. If the prevention of this loss can be achieved, the possibilities of the leaps in technological development are infinite.
What can be done to achieve this? Before getting into this question, we must first analyze how resistance changes with the conductor and environmental conditions. Does the resistance depend only on the atomic vibrations caused by the thermal energy? Well, it can be investigated by cooling metals to extremely low temperatures. Electrical resistance is naturally expected to gradually reduce with decreasing temperature and, at some point, resistance has to become zero, since at extremely low temperatures, vibrations of atoms and ions come to a standstill. However, experiments show this doesn’t happen for all the metals. Logically, since vibration levels reduce with reducing temperatures, one might expect that zero resistance will be attained at a certain temperature. Interestingly, that doesn’t always happen. Certain metals never reach zero resistance, the reason being the way atoms are structured in the metal.
Atoms are arranged in structures called lattices, and sometimes these lattices have imperfections due to atoms not lining up properly. This structure can be compared to a situation in which people are standing in rows within a room, but here and there a few people are out of place and not standing in the rows. If someone were to try to navigate through the room by walking in between the rows, they might end up getting blocked off by someone standing out of place. Even if all the people in the room were standing still, it wouldn’t be necessary that someone would be able to navigate the room without getting obstructed. The same is the case with electrons and metals with imperfect lattice structures. This meant that superconductivity would be restricted to certain materials and certain temperatures for a particular metal.
Since the early 20th century, scientists and researchers have been intensely working on finding which metals can become superconductors and, if they can, at what temperatures. In the year 1911, a Dutch physicist named Kamelingh Onnes, while carrying out experiments to study the behavior of metals when cooled to absolute zero temperature (-273 Deg Celsius), discovered that liquid mercury at a temperature of 4.15 kelvin (-268 Deg Celsius) attained zero resistance. Therefore, it conducts electricity to the fullest extent at that temperature. This particular temperature at which a conductor reaches this state, or in other words, “becomes a super conductor,” is called critical temperature. Onnes achieved these extremely low temperatures using liquid helium and carried out several experiments, discovering a whole new world of possibilities. In the year 1913, he was awarded the Nobel prize in physics for his discoveries in this domain. By the year 1957, a microscopic theory of superconductors was developed by three scientists: John Bardeen, Leon Cooper, and Robert Scherieffer. Their theory, called the BCS theory, explains, superconductivity in metal:
When an electron moves through a lattice, there is an attractive force between the electron and nearby positively charged atoms in the lattice. As the electron passes by, the attractive forces cause the lattice atoms to be pulled towards the electron.This results in the concentration of positive charges near the electron, and when a second electron is nearby, it can be attracted to the excess positive charge in the lattice. This phenomenon allows the two electrons to travel through the lattice as if they were a single particle.This particle pair is called a Cooper pair. These Cooper pairs are responsible for some of the unique properties of super conductivity.
A remarkable feature of superconductivity is that once the current has been established in a superconducting material at its critical temperature, the flow of electrons continues even if the applied potential difference is removed. Another effect of zero resistance is extremely high levels of current moving in the conductor, making superconductors sources of extremely powerful magnetic fields. Since 1960, superconductors have been employed for various magnetic applications, such as high speed magnetic levitation trains called maglev, magnetic resonance imaging (MRI), ultra high speed computer chips, and so on. In addition to being able to transform the way energy is distributed, exceptional qualities such as those mentioned make superconductors a very valuable asset, and more so for any superconductor whose critical temperature is high enough to avoid the need for cooling, thereby saving a lot of energy and reducing process complications. Just like in the case of any endeavor that could potentially yield unprecedented rewards, several groups of extraordinary individuals are striving to find such a material—a super conductor that can exist at room temperature.
In October 2020, the material with the highest accepted superconducting temperature was discovered. Powdered carbon and sulfur were squeezed together at an extremely high pressure of 267 Gpa (two and a half million times the pressure we normally experience under the atmosphere) and then exposed to hydrogen gas, after which the entire material mix was shot with a laser. This was followed by a few other processes to create a transparent superconducting crystal that exhibits superconductivity at a temperature of 15 °F. Could this mean that we are inching towards discovering a superconductor whose critical temperature is room temperature? The answer is yes!
