Connecting conductors of different materials and introducing current will absorb (or release) heat at the contact point-node of different conductors. In 2008+0834, French physicist J.C.A.Peltier discovered the above thermoelectric effect. In 2008+0838, Russian physicist L.Lenz Ci discovered the above thermoelectric effect. If the direction of water flow is reversed, the ice that has just condensed at the node will immediately melt into water.
The thermoelectric effect itself is reversible. If the DC power supply in the experiment of Lengci is replaced by a light bulb, the light bulb will light up when we supply heat to the node. Although the scientific community at that time attached great importance to the discovery of Peletier and Lengci, this discovery did not quickly translate into application. This is because the thermoelectric conversion efficiency of metals is usually very low. It was not until 1950s that some semiconductor materials with excellent thermoelectric conversion performance were discovered, and the research of thermoelectric technology (thermoelectric refrigeration and thermoelectric power generation) became one.
Semiconductor refrigeration materials used near room temperature are based on bismuth telluride alloy. P-type and n-type semiconductors are made by doping. As mentioned above, the P-type column and the N-type column are connected by metal plates, which constitute the basic unit of the semiconductor refrigerator. If the current direction at the joint is from N-column to P-column, then the joint will become the "cold head" of the refrigeration unit (temperature is Tc) and be connected to the DC power supply.
Fermi level EF of N-type semiconductor is located in the upper part of the band gap, and P-type semiconductor is located in the lower part of the band gap. When they are connected together, their Fermi levels tend to be "flat". Therefore, when the current flows from N-type to P-type (i.e. holes from N to P; Electrons from p to n), the energy of carriers will increase. Therefore, as a cold head, the junction will absorb heat from the Tc end, resulting in refrigeration effect.
Peletier coefficient, where is the heat absorbed by the node in unit time, I is the current intensity, and the physical meaning of π is the energy difference when the unit charge passes through the node. In the study of thermoelectric materials, a relatively easy-to-measure related parameter is Zeebek coefficient α, where t is temperature. Obviously, α describes the entropy difference when the unit charge passes through the node.
For refrigeration applications, at first glance, the larger the current, the better, and the larger the Peletier coefficient (or Zeebek coefficient). Unfortunately, the nature of actual extrinsic semiconductors determines that you can't have both: high current requires high conductivity σ, σ and α are all functions of carrier concentration. With the increase of carrier concentration, σ shows an upward trend, while α decreases, resulting in α σ can only reach the maximum value at a specific carrier concentration.
The P-column and N-column of semiconductor refrigeration unit are bridged between Tc and th. This requires them to have large thermal resistance. Otherwise, the entropy of heat leakage between Tc and Th will increase, thus offsetting the refrigeration effect of heat absorption at Tc end and heat release at Th end. It is the combination parameters that ultimately determine the performance of thermoelectric materials, where κ is the thermal conductivity of the material. The product ZT of parameter z and temperature t is dimensionless, which is commonly used in evaluating materials and is the thermoelectric material with the best performance.
Glen Slack summarized the above requirements as "electron-crystal and phonon-glass". In other words, a good thermoelectric material should have high thermal conductivity like crystal and low thermal conductivity like glass. In long-range ordered crystals, electrons move in the form of Bloch waves. Rigid ionic lattices do not deflect the motion of conductive electrons. The resistance comes from the collision of electrons with impurities, lattice defects and thermal phonons. Therefore, in a perfect crystal, σ can be very large.
Thermal conductivity in semiconductors includes two contributions: one is caused by the directional motion of carriers (assuming electrons) (κ E); Secondly, this is due to the directional motion (κp) of the phonon equilibrium distribution group. According to Weidemann-Franz law, κe∝σ. It is impossible for people to require both large σ and small κe, and the potential to reduce thermal conductivity lies in reducing κp, which is closely related to the degree of lattice order: in long-range ordered crystals, thermal resistance can only come from umklapp process, defects and boundary scattering; In the amorphous glass structure, lattice disorder greatly limits the average free path of phonons, thus increasing the scattering mechanism of phonons. Therefore, the thermal conductivity κ of "phononic glass" can be very low.
The dimensionless figure of merit coefficient ZT is used to measure thermoelectric materials: BiSb series is suitable for the temperature range of 50- 150 K; Bi2Te3 series is suitable for 250—500k；; ; PbTe series is suitable for 500-800 k; SiGe series is suitable for1100-1300k. Low temperature thermoelectric devices (T≤220K) are mainly used to cool computer chips and infrared detectors. High-temperature thermoelectric devices can convert solar energy and nuclear energy into electric energy, which are mainly used for power supply of space probes and floating unmanned monitoring stations at sea. The ban of freon refrigerant provides a new opportunity for the development of semiconductor refrigeration. 48660.68868888666
Brian Sales and others have studied a new thermoelectric material called filled skutterudite antimonide. When the gap is not filled, the chemical formula of the material is CoSb3 (or Co4Sb 12). Each structural unit of Co4Sb 12 in the crystal contains a large-sized cage hole. If rare earth atoms (such as La) are filled in the cage holes, the chemical formula becomes LaCo4Sb 12. Because La atom is in a relatively loose space, its vibration amplitude is also large. Therefore, in LaCo4Sb 12, the rigid skeleton of Co4Sb 12 provides the foundation for the high thermal conductivity of the material, and the vibration of rare earth La in the cage enhances the phonon scattering-reduces the thermal conductivity of the material. The work of B.Sales has taken the first step in the direction of "electronic crystal and phonon glass".
High voltage (~ 2 GPA) technology has been used to improve the performance of thermoelectric materials. If the improvement of the properties of the parent material is observed under high pressure, people will be able to obtain similar structures through chemical doping and use them under normal pressure.
ZrNiSn has high σ and α, but its thermal conductivity κ is not low. Perhaps the fourth or fifth component can be added to enhance the "mass fluctuation scattering" of phonons, thus reducing the thermal conductivity.
Quasicrystals have complex and changeable structures and phonon glass properties. The focus of related research is to improve the conductivity of quasicrystals.
When current flows through this composite material, nano-metal (Ag) embedded in conductive polymer can produce a large temperature gradient. There is no theoretical explanation for this.
There are two kinds of low-dimensional thermoelectric materials with application prospects: CsBi4Te6 is actually Bi2Te3；; Selenium-doped hafnium pentafluoride, indium
In addition, thin films, artificial superlattices, carbon nanotubes, Bi nanowires and quantum well systems, cat-eye structures and so on all show potential in improving the performance of thermoelectric materials.