A Peltier element is an electrical device that, under the influence of an electric current, generates a temperature difference on working sites. The principle of action is the opposite effect of Seebeck. It is noteworthy that it is accepted to call the junction terminals of the thermocouple, as well as the real metal junction in the sensitive place of the sensor. You should not be misled, the ends are usually connected to the measuring circuit and do not touch.
Effects of Thermoelectricity
July 21, 1820 is considered a turning point in the development of history: Oersted decided to publish his observations on the effect of the current-carrying wire on the orientation of the magnetic needle in space. Further discoveries follow a succession, we are interested in the invention of the first galvanometer. The manufacturer, Schweigger, called the device a multiplier for its ability to multiply the result of the action of several turns of wire carrying current on a magnetic needle. Due to this, a year later( 1821), a physicist of Estonian origin, Seebeck, discovered thermoelectricity. It is well known that what happened happened five years later to George Ohm to obtain a world-famous law.
The literature says that Seebeck used a solenoid with numerous turns of wire and a magnetic needle as a detector. The story is silent, as the bismuth-antimony spike hit the scientist, but it tells that the scientist connected the tandem as a power source and saw the compass oscillations constantly when he took the thermocouple in his hands. It was probably close to the discovery of his own supernormal abilities, but as a result it was concluded that the warmth of the hands was to blame. Great results, the scientist has achieved, using a lighting lamp as a source of heat.
Seebeck misinterpreted the result of the experiment, calling the discovery magnetic polarization: shifting the heating point to the other end changed the direction of the arrow deflection. As a result, the wrong theory was built. They began to assert that temperature makes it possible to directly obtain magnetic properties, and the earth's field is due to the activity of volcanoes. Georg Ohm, already shortly after the discovery described, applied thermo-emf to derive a known law, and in 1831 a similar source was used in electrolysis experiments.
The thermo-emf value is small. Usually tens of mV.If you want to find a specific value, use the tables. Platinum is the benchmark for temperatures in the Earth’s climate range. The tables contain the value of thermo-emf for thermocouples from the specified metal and the studied: chromel, alumel, copper, iron. Values are positive and negative. For example, for antimony it is +4.7 mV, and for bismuth - minus 6.5.The values add up and it becomes clear that when the temperature difference at the ends of a pair of 100 degrees EMF is formed at 12.2 mV.Georg Om tried to create similar conditions by immersing the first end in ice, and the second - in boiling water.
Reference tables sometimes contain many values. For example, for different temperatures in increments of 100 degrees. Then it is possible to calculate the values for each, but also with the substitution of zero for any of the specified temperatures. The difference between the larger and smaller value is taken. For individual thermocouples at a certain temperature, the direction of the thermo-emf changes to the opposite. For example, for copper and iron, the boundary point will be 540 degrees Celsius.
The Peltier Effect is called mirror reflection of thermoelectricity. In this case, the current transfers heat from the first end of the thermocouple to the second. And with a change in direction and the heated side turns to the opposite. The effect was discovered in 1834, having been misinterpreted. Only 4 years later, the “compatriot” Lenz managed to freeze and evaporate a drop of water using a thermocouple. In each case, the current showed its own direction.
The effect is simply explained in modern physics. Suppose there are two dissimilar semiconductors with the same type of conductivity. The electrons in each acquire a different value of energy, and the levels in both cases are close. Now imagine that the electric current began to transfer charges from one medium to another. What will happen? Electrons with high energy, being in the environment of low levels, will give an extra amount to the crystal lattice, producing heating. On the contrary, if the energy is not enough, it will be transferred from the crystal lattice, which will cause the cooling of the junction.
If the conductivity type of semiconductors in a thermocouple is not the same, the effect is explained differently. An electron entering the p-material takes the place of a hole( positive charge carrier) at the energy level. As a result, it loses the kinetic energy of motion and the difference between the current and past state. The released amount goes to the formation of free carriers on both sides of the pn-junction. The remainder is reported to the crystal lattice, from which the heating proceeds. If the energy at the initial moment is less, the cooling of the junction will begin. Recombination media are replenished by the power source.
The amount of heat released or absorbed is proportional to the charge passing through the conductor. The coefficient in the formula of linear dependence is named Peltier. A similar value is introduced for thermoelectricity, named after Seebeck. From the formula it follows that the amount of released heat, in contrast to the Joule-Lenz effect, is proportional to the first degree of electric current( determining the transferred charge).
On the basis of the Seebeck and Peltier coefficients, Lord Kelvin( Thomson) predicted a new effect in 1856: the conductor heated in the center cools on one side and passes hot on the other. The theoretical data are confirmed empirically, opening the way for the creation of climate technology and other things.
The idea of Lord Thomson: if there is a temperature gradient along the conductor( see Electric field), heat will begin to flow when current flows. This device works on the principle of heat pump. The power carried is proportional to the gradient: the steeper the graph of temperature changes along the conductor length, the greater the thermal effect is manifested.
The proportionality coefficient in the formula is named after Thomson and is associated with thermoelectricity and Peltier coefficients. Above, the authors gave explanations according to the kinetic( microscopic) theory, which operates with levels of energy states of charge carriers. Lord Kelvin adhered to the thermodynamic( macroscopic) concept, where global flows and forces are taken into account. This distinction applies to many branches of physics. For example, Ohm’s law for a chain section can be considered as a variant of a thermodynamic view of things.
Called and similarities. In the thermodynamic concept, the following constants are massively applied: speech on the thermal conductivity coefficient( Fourier law) and isothermal conductivity( Ohm's law).
A number of useful laws related to the topic under discussion:
- In a closed circuit of a homogeneous material due to temperature, the electric current cannot be maintained. This statement bears the name of the German physicist Magnus. Sometimes referred to as the law of a homogeneous chain.
- The law of intermediate metals states that the algebraic sum of the thermo-emf of a closed loop consisting of any number of segments of heterogeneous conductive materials is zero provided that the temperature of the sections is the same.
Using Thermoelectric and Electrothermal Effects
For a long time, the direct and inverse thermoelectric effect did not find application, the useful value turned out to be too small. Gradually, physicists have created alloys whose properties overlap the pure metals used by Peltier and Lenz by two orders of magnitude. Now thermoelectricity is applied. Recall the refrigerator thermostat or thermoelectric refrigerators without moving parts. The space industry is much more interesting, where the phenomenon is used to cool photoresistors: when the temperature drops only 10 degrees, the sensitivity of such sensors increases by an order of magnitude.
An additional advantage of the described technical solutions is compactness and low energy consumption: with a weight of 150 g, the unit cools the thermistor by 50-60 degrees. In consumer electronics, the Peltier effect supports the normal mode of processors in the system unit of personal computers. Yes, it is worth the technical solution is not cheap, but noiselessness is guaranteed. For example, enthusiasts from the 2010s design refrigerators at home. High efficiency can not be achieved due to large losses through the body. But with the advent of new insulating building materials, the situation will improve.
Interestingly, when the direction of the electric current changes, the effect starts to work in the opposite direction. Heating is possible. On the basis of the effects described, thermostats are created that monitor the temperature to thousandths of a degree. Among the promising areas celebrate domestic air conditioners and other cooling systems. The most noticeable disadvantage is the price. And we must not forget that the efficiency of the air conditioner, as a rule, is greater than 1, this unit works on the principle of a heat pump. Let the efficiency drop sharply with increasing ambient temperature, while thermocouples are far behind traditional cooling methods with their 10%.
Express other opinions. Academician Ioffe, some maxims of which are used in the above topic, proposed to create systems for heating and cooling rooms as split-systems. In this case, a complication arises, as with typical conditioners, but the efficiency reaches 200%.Meaning: during heating, for example, a heat absorbing junction is placed outside, and the evolving junction is placed indoors. It is not easy to swing the heat out of the cold, because the technique has limitations. However, it is not forbidden to create heat pumps on the basis of this methodology.
The unconditional advantages of climate systems using the Peltier element include the ability to work in the opposite direction. In summer, the stove will become air-conditioned. It is only necessary to change the direction of current flow. Known opposite developments, designed to convert solar heat into electrical energy. But while such designs are made on the basis of silicon, and there is no place for thermocouples.
Materials for creating thermocouples
Obviously, ordinary metals are not suitable for creating powerful systems. Requires a pair of power from 100 µV to 1 degree. In the latter case, high efficiency is achieved. The materials are alloys of bismuth, antimony, tellurium, silicon, selenium. The disadvantages of the components include the fragility and relatively low operating temperature. Low efficiency adds limitations, but with the introduction of nanotechnology there is hope that the usual framework will be overcome. Scientists among promising areas called the development of a fundamentally new semiconductor base with truly unique properties, including the exact value of the energy levels of materials.