Thermoelectric materials are substances that produce electrical power from temperature differences or heat. They are employed to convert wasted heat into electricity, create refrigeration without moving parts, or reap thermal energy for electronic devices, such as sensors.

Semiconductor alloys such as bismuth telluride, lead telluride, or silicon-germanium are examples of the most commonly used thermoelectric materials. These materials have a high Seebeck coefficient, which enables them to produce a substantial voltage per unit temperature difference. Additionally, they have low thermal conductivity, meaning that the heat generated by the voltage difference is not easily dissipated.

Researchers continually seek out new thermoelectric materials that are more efficient, long-lasting, and environmentally friendly. Recent studies have investigated organic molecules, complex oxides, or topological materials as potential thermoelectric candidates.

Thermoelectric materials have a wide range of applications in various industries, transportation, and energy production. They can be utilized to recover energy from power plants, increase the effectiveness of vehicles, operate wearable devices, or measure the temperature of medical implants.

Thermoelectricity:

Thermoelectricity is the phenomenon in which a temperature difference between two dissimilar materials results in electrical current flow. A thermoelectric generator (TEG) is a device that generates electrical energy through thermoelectricity. It uses a temperature gradient created by a temperature difference to create an electrical voltage, making it well-suited for high-temperature environments, such as power generation or waste heat recovery in engines. The efficiency of a TEG is determined by its thermoelectric figure of merit, or ZT value, which is influenced by its material properties, its size and geometry, and the temperature difference across it. Recent breakthroughs in nanomaterial science and engineering have led to the development of high-ZT thermoelectric materials for better TEG efficiency. TEGs have the potential to provide clean and renewable energy, especially in areas with abundant waste heat, such as industrial and transportation sectors.

Thermoelectric Refrigeration:

Thermoelectric refrigeration, also referred to as Peltier cooling, is a method of cooling that utilizes the thermoelectric effect to convert electrical energy into a temperature gradient. The process requires passing an electric current through a thermoelectric module, which contains two dissimilar materials joined together. One side of the module becomes hot, while the other side becomes cold.

The thermoelectric effect is a result of the behavior of electrons when they travel from one material to another. When electrons flow from a conductor with a high electron density (a type of metal) to a conductor with a lower electron density (a type of semiconductor), it leads to heat transfer and creates a temperature gradient. The Peltier effect works in the opposite direction by inducing an electric current flow with a temperature gradient.

Thermoelectric refrigeration applications include small cooling units, like beverage coolers and camping refrigerators, as well as medical and scientific applications that require temperature control. However, thermoelectric refrigeration is generally less efficient and less powerful than other cooling methods, like traditional compression cooling, and is therefore not typically employed for large-scale applications.

Degenerate Thermoelectric Materials:

Degenerate thermoelectric materials are unique types of semiconductors that possess electrical conductivity due to high concentrations of mobile charge carriers. These materials have a distinct property that allows the conversion of wasted heat into usable electrical energy. The Seebeck coefficient, which determines how much voltage is produced by a substance in reaction to a temperature gradient, is proportional to the concentration of charge carriers present within the material.

In degenerate thermoelectric materials, the concentration of charge carriers is high enough that the Seebeck coefficient becomes negligible, but the electrical conductivity remains high. This property enables these materials to generate electricity even when the temperature gradient is small.

The development of degenerate thermoelectric materials is a challenge due to the compromise between high electrical conductivity and low thermal conductivity. Most materials that are good conductors of electricity are also excellent heat conductors, requiring a large temperature gradient to generate a significant amount of electrical energy.

Non-degenerate thermoelectric materials refer to those with a lower carrier density and a Fermi level that is in close proximity to the band edges. Such materials provide a superior advantage in thermoelectric applications due to their higher Seebeck coefficient, indicating that the voltage generated per unit temperature difference is greater, and their lower electronic thermal conductivity, which showcases how efficiently electrons can carry heat when compared to their degenerate counterparts.

Some examples of non-degenerate thermoelectric materials include semiconductors, specifically bismuth telluride (Bi2Te3) with its layered crystal structure that provides a natural barrier to heat flow. Silicon germanium (SiGe) is also utilized in thermoelectric generators, especially for space applications due to its high efficiency at low temperatures. There are also other promising materials that are currently under research for thermoelectric applications. These include tin selenide (SnSe), lead telluride (PbTe) and magnesium silicide (Mg₂Si).