Thermoelectric effect
Under thermoelectricity one understands each other the mutual influence of temperature and electricity and its implementation. Seebeck effect (also thermoelectric effect), Peltier effect, and Thomson effect each describe a reversible interaction between the two physical quantities.
Effects
Seebeck effect
According to the Seebeck effect, named after Thomas Johann Seebeck, produced in a circuit from two different electrical conductors at a temperature difference between the contact points of an electrical voltage of magnitude
In this case, and the temperatures of the contact between the materials A and B. The Seebeck coefficient and material constants that depend on the temperature. Alternatively, it is also used as a symbol a. The Seebeck coefficient has the dimension of an electric potential per temperature difference (Volts / Kelvin). The typical range for metals at room temperature at 10 uV / K.
For small temperature differences and constant values for the Seebeck coefficient, the formula simplifies to
In the thermal voltage always occurs only the difference of the Seebeck coefficient. Since individual ( absolute ) Seebeck coefficients are difficult to determine (see Thomson effect ), platinum ( Thermoelectric voltage range see ) was chosen as a reference element for tabulated numerical values.
The Seebeck effect only describes the origin of this voltage. Any resulting from external wiring of current flow is not part of this effect and merely follows from Ohm's law.
Historical
Thomas Johann Seebeck accidentally discovered that in a circuit of two dissimilar metals ( for example, in rod form), an electric voltage is produced when there is a temperature difference between the two connecting points of the rods. The current flowing in the formed of the two rods circuit electric current, he was able to demonstrate through its magnetic field with a compass needle. Seebeck used this effect in 1821 in a first thermocouple from.
Explanation
The voltage produced by the thermal diffusion currents in a material. The consideration of only one material with temperature gradient thus provides a satisfactory explanation. For measurement purposes, one needs two different metals. At the hot end of the conductor, there is more high-energy electrons, and few electrons of low energy ( below the chemical potential ). By diffusion move in accordance with high-energy electrons to the cold end and electrons with little energy in the opposite direction. This describes the heat conduction through electrons. A possible imbalance of the currents is compensated by an electric field, as in the open circuit, no current can flow. The resulting voltage ( integral of the electric field) is the Seebeck voltage.
It is determined by the dependence of the mobility and the number ( density ) of the electron energy. The dependence of the mobility of the energy depends sensitively on the nature of the scattering of the electrons. Accordingly, relatively small impurities can affect quite strongly the thermopower. The driving force for diffusion is proportional to the temperature. As a rough trend can therefore expect an increase in the Seebeck coefficient is approximately proportional to temperature for metals.
A special case is the so-called electron drag. At low temperatures of about 1 /5 of the Debye temperature, the phonons are due mainly to collisions with electrons. The phonons thus drag the electrons with lower temperatures in direction. Thus, the thermoelectric effects can be somewhat larger than one would otherwise be expected in this temperature range. At higher temperatures win Umklappprozesse for the scattering of phonons in importance and the effect is smaller.
Numerical values
Seebeck coefficients of some metals and alloys relative to platinum (see Thermoelectric voltage range ):
To obtain the absolute Seebeck coefficient of the materials specified herein the absolute Seebeck coefficient of platinum (from -4.04 uV / K at 273 K and about -5 uV / K at 300 K) has to be added.
Peltier effect
When Peltier effect, applied to the Peltier element, are inverse relationships compared to the Seebeck effect - an electric current flow causes a change in the heat transport. However, during the Seebeck effect describes the formation of a voltage, the Peltier effect occurs exclusively by the flow of electric current. In a current-carrying thermocouple always occur both effects in metallic thermocouples, however, the Peltier effect is difficult to prove. The discovery was made by Jean Peltier therefore until 1834, thirteen years after the discovery of the Seebeck effect.
When an electric current I flows to a contact of a material A into material B, results in a heat source of the size:
Peltier coefficients are material constants, which generally depend upon the temperature.
Depending on the sign of the current can heat released or heat be withdrawn. As a rule, ie the one contact is warm and the other cold.
Explanation
To explain the Peltier effect, the link to the Seebeck effect on the Thomson relation is sufficient. There are for the Peltier effect but also a relatively intuitive direct explanation: Moving electron transport in addition to the charge e Whatever energy. How much is in the middle, depends, among other things, on the number of charge carriers, and the spreading rate of the energy dependent. Higher energy electrons carry a larger contribution to electricity, transport but also more energy. In the transition from one material to another, the transported with the electron energy changes. The difference is released at the point of contact as a heat or added (Peltier effect). The transported with the electron energy just corresponds to the Peltier coefficient. In semiconductors, the distance between the chemical potential and the band edge is a significant proportion. In particular, as will be explained that, in the semiconductor thermoelectric effects are often much larger than in metals.
Thomson effect
(not to be confused with the Joule- Thomson effect or the Gibbs -Thomson effect )
The Thomson effect, named after William Thomson, 1st Baron Kelvin in 1856, describes the modified heat transport along a current-carrying conductor in which there is a temperature gradient.
Each current-carrying conductor with a temperature difference between two points, depending on the metal, either carry more or less heat than would be the case without the current flow due to the thermal conductivity. However, this effect is superimposed on the warming of the electrical conductor by the current due to its resistance and is therefore difficult to detect.
A current density J in a homogeneous conductor causes a heat capacity per unit volume of
Wherein the resistivity of the material, the temperature gradient in the conductor and the Thomson coefficient is.
The first term is the irreversible Joule heating. The second term is the Thomson heat, which changes sign with the direction of the current.
For the Thomson effect, there is no technical application. About the Thomson effect can be determined by the absolute thermoelectric coefficients by integration over the temperature.
Thomson relations
Seebeck, Peltier and Thomson effect are not independent of each other but have a common cause. By 1854 Thomson found between the corresponding coefficients of two contexts, the (sometimes Kelvin relations) are called Thomson - relations:
Here are
- The Peltier - coefficient
- S is the Seebeck coefficient
- T is the absolute temperature
- The Thomson coefficient.
The second equation told the Thomson effect ahead.
Anisotropic materials
In the general case of an anisotropic material, the electrical and thermal conductivity may be tensor. The same equally applies to the thermoelectric coefficient. It is thus possible, for example that heat is released when the current to a grain boundary changes direction relative to the crystal axes. This will Bridgman effect, named after the American physicist PW Bridgman.
In general, the directional dependence is neglected. Many materials have been due to cubic symmetry actually isotropic in terms of conductivity.
Applications and implications
Thermocouples made of metals convert thermal energy only very inefficiently into electrical energy and are therefore almost only used as a thermocouple for temperature measurement. For the measurement of small temperature differences, many thermocouples are electrically connected in series, such as in the thermopile to measure radiation.
The thermal stresses occur as a disruptive effect on when measuring low DC voltages. Care must be taken according to small temperature gradients and an appropriate choice of materials.
Through the use of semiconductor materials (materials and structure see at Peltier element ) can increase the conversion efficiency of 3-8%. So you can build thermoelectric generators. Such generators or converters are, inter alia, in nuclear batteries application and convert wear without moving parts thermal energy into electrical energy. The efficiency but also with the new materials still well below the Carnot efficiency.
Currently, one seeks such thermoelectric generators also reinforces the use of waste heat, such as in cars, cogeneration plants, waste water treatment plants or waste incineration plants to use.
The Peltier effect can be used in Peltier elements for cooling and temperature control. Because of the relatively poor efficiency but this remains limited to rather small applications. Other advantages are the good scalability, controllability and reliability. From the construction and the required material properties Peltier elements and thermoelectric generators are similar.
The technical application for cooling is limited by the phonon heat conduction, it causes especially for wide temperature ranges an opposing heat flux that cancels the current flow caused by the heat flow from about 70K. For the same reason, thermoelectric generators have only a low efficiency.
For the Thomson effect, there is no technical application. The effect is so small that is not already the proof simple.
Recent Developments
The efficiencies of Peltier and Seebeck elements have remained low despite all the research programs. The poor efficiency is due to the unwanted heat conduction between the metals or semiconductors. A more recent approach to prevent this, pursuing the Thermotunneling method: Two metals are separated by a minimum vacuum gap. The heat conduction through the lattice vibrations is completely suppressed. The vacuum gap is only so wide that it can "tunnel " single electrons quantum mechanically.
At first glance, this interruption of phonon heat conduction, that is, the heat conduction through the lattice vibrations to be extremely efficient. With a gap size which allows quantum mechanical tunneling, the electromagnetic forces, however, are so large that a virtually unimpeded transmission of the lattice vibration occurs due to electromagnetic coupling.
An effective decoupling of the lattice vibrations takes place only when the gap size is in the range of wavelengths. At ordinary temperatures at which such elements are to be used, the wavelengths of electromagnetic emissions in the range of a few hundred nanometers to a few micrometers. These sizes, however, a quantum- mechanical tunneling is virtually no longer possible.