Seebeck effect - is the formation of a potential difference at the boundary interface of two different materials by heating the art.
From the history
History is silent, he wanted to get the Seebeck in 1822, when warmed contact from antimony and bismuth. Perhaps the effect was the result of random coincidences, as often happens, and how it happened in Oersted of the compass arrow. The galvanometer Seebeck recorded when a hand was holding the thermocouple junction. It is considered a happy coincidence, he is obliged to own a successful compass design. The device consisted of two halves: one metal frame, the rim of another glass cover. Resting his hand on the property, Seebeck said deviation of the magnetic needle from its initial position. Surely the difference was not too noticeable, but the scientist patiently repeating the experience of watching the result.
The figure shows that the display lamp causes significant deflection of the magnetic meridian. This is due to the field current flowing. Staple, adhere arrow above, carries charges (positive sign) in said direction. Creates a circular magnetic field that changes the compass readings. It is not known for certain whether the compass was made from antimony and bismuth or Seebeck found materials later in private, but still thermocouples are often made of these metals. The combination is selected for its high efficiency.
The use of thermoelectric generators
Experimenting, we found that the efficiency of the thermocouple reaches almost 3%. At the beginning of the XIX century it is pretty decent, able to compete with any steam engine. In Soviet literature provides information that the thermoelectric efficiency did not reach 0.5%. Firstly, it is not always regards separate thermocouples, and, secondly, is considered to be communist propaganda. In Soviet times, branded personal computers (a concept developed in the USSR), and now any official boasts a brand new laptop from Korea or the United States. The authors are more likely to rely on foreign sources, leading figures in the region of 3%.
Georg Ohm, using a thermocouple, opened the well-known law, Faraday used them for electrolysis research. Scientists quickly acquired a taste for, and by the middle of the XIX century were already present thermoelectric generators big enough performance - and for cladding of metal parts. With report Seebeck thermocouple become an integral part of the experimental facilities where required to obtain stability. And by the beginning of the XX century we have created an array of structures.
The Russian magazine wrote about lighting thermoelectric furnace, Gyulhera battery is used to charge the batteries. Interest in this area slightly weakened after the invention of the internal combustion engine and electric motors, but in the modern world thermocouples are considered promising power sources for the development of view. However, the prospect of using the sun rays seemed attractive, even at the beginning of the XX century. The first experimental data published in 1922 were: "The apparatus 105 of thermocouples (Copper-Constantan), an area of 1 sq. cm each, demonstrating the efficiency of 0.008%, at noon giving energy of about 61 mW. "
At the same time the Seebeck effect began to be used to power portable radios. The topic of thermoelectric generators shown reklamka that time. In plain English readers are given to understand that the new power supply is good to listen to the latest news. No wonder that in a short time and appeared in the USSR Journal notes lovers, it was reported that the heat kerosene lamp may sensibly be used to power electronics circuits. Czeczik in the journal "The poor" (1928) reported a generator of its own design of the iron-nickel thermocouples. The same kind of article appeared in at number 13, "Radifront" magazine in 1937.
Postwar generators Seebeck effect gave a beneficial effect in the amount of 1 Watt power for 1 kg weight. But the efficiency is still low. In one Russian literature right - Soviet articles did not go ahead of the rest. Already during the Second World War, the Seebeck effect supplied the quiet energy of radio operators, warming themselves at a fire in a troubled time. Considered the possibility of using again, many physics textbooks contain information about the generator Joffe, created in the early 50s (see. Fig.). By the time it was believed that it is possible to achieve efficiency of 5-7%, were issued generator TGK-3 for Radio. solar cell area of 360 square was created. see that gave 0.175 watts at an efficiency of 0.59%. One can see that it has increased significantly.
For example, kerosene lamp generator Ioffe sample 50s radio allow feed at a temperature in the internal junctions of 300-350 degrees Celsius and external - in the range of 60. Then we have proven ability to create devices with an efficiency of 8%. History of the development of thermoelectric generators to the latest time possible to contemplate in the relevant section, and now look at the physical processes occurring in the conductors.
By the beginning of the 80s to 40% of all industrial measurements necessary on the temperature and 2/3 of that number of sensors working on the Seebeck effect. Scientists quickly came to the conclusion that the low efficiency is justified by high precision. In the USSR, it would have learned sooner bothering to translate into Russian the work of Georg Ohm's early 20-ies of the XIX century. Shaking breadth of application devices - from 0.5 to 3000 K.
Rampant industrialization has caused the need for new methods for the control of technological processes. For growing horizons not keep up with the working class and the people in need of rest and leisure. In the words of a writer, scientific discoveries have become commonplace in the United States, when it was possible to establish life and to calm the militant Indians. Without science, the country does not develop and does not see the profit, leisure and free time is considered as a valuable resource. The advantages of the sensors on the Seebeck effect are:
- Low inertia. If required by the first product stability, making them deliberately cumbersome, slow response to the external environment, modern thermocouple (see. Fig.) is small in size and is included in the composition and consumer devices (e.g., refrigerators).
- Ease of installation. The man who is faced with the need to replace the barometric refrigerator thermostat knows how it is difficult and time-consuming process. The thermocouple is connected a cut of the wire, quickly and easily.
- An extensive range of temperature measurement has underlined. Today, even the testers sold consumer thermocouple as a sensor. The range is dependent on the design features, it is easy to select affordable and advanced features.
- The technological process is characterized by repeatable parameters from batch to batch uniformity, ease of manufacture, the possibility of miniaturization, suitable for automated assembly.
These features make it possible to flexibly, accurately and quickly track temperature changes. The sensitivity of the product is determined by the Seebeck coefficient, reaching 100 mV / K. The main characteristic of thermocouples - stability parameter depending on the mechanical, thermal, magnetic, and so forth. impacts. Therefore it is not always considered to be the main characteristic of stability. It is sometimes to the detriment of efficiency by selecting an alloy with maximum resistance to certain external factors.
How does a thermocouple
The Seebeck effect is long remained without explanation. Today distinguish two theories describing the processes:
- kinetic (microscopic);
- thermodynamic (macroscopic).
This suggests that accurate data on the mechanism of action of the thermocouple in today's science is not there.
First, rather than plunge into the complex theory, it is proposed to consider the simple explanation given by the students of various universities. Professors interpret an event, based on the thermionic emission phenomenon and electronegativity of metals and alloys. The first is known to be floated on the light when the light bulb filament developed. Preconditions became Edison's research. Electronic lamp works because in vacuo preheated electrode begins to emit charge carriers from the surface. Of course, were soon established leaders in the field, the surface is now covered by the electrolysis of the right materials.
The essence of the effect of the thermionic emissions: charge carriers exhibit zero work function of the crystal lattice. It is believed that at normal temperature hovers above the metal surface a thin electron cloud. But on the body image of the positive charge on the case is not. As a result, heating output electron gets the energy and able to leave the metal. Much intensity of the process is observed at a temperature of 1000 K. The work function is not the same for metals, scientists believe that this is partly due to their electronegativity.
When the two samples are brought into contact, the redistribution process begins. This occurs until a dense electron cloud of metal balances the other. The process seems to be complete. But... just Seebeck discovered that heat makes up charges. Recombination occur, fusion and disintegration, as a result of thermocouple formed at the ends of the potential difference. The effect is enhanced by using two or more junctions. What to do physics in the first half of the XIX century. Then the first thermocouple junction is heated and the other cooled.
When heated, warp density of the electron clouds of the two metals grows stronger. Consequently, the potential difference increases. Energy removes heat energy is compensated current source. Seebeck effect is manifested at any temperature, strongly increases with its increase.
The thermodynamic theory of the Seebeck effect
Thermodynamic theory operates with common values: flow, gradients forces. By solving equations obtained Ohm's law on the relationship between current, voltage, resistance and Fourier - heat flow of communication and temperature gradient. Introduced special factors with specific names:
- Insulated conductivity (inverse of resistivity);
- coefficient of thermal conductivity.
The resulting equation is a consequence of the presence of once three effects: Seebeck, Peltier and Lord Kelvin. They are set for the most part experimental, without a theory. The Seebeck effect is already fairly considered, Peltier discovered the formation of reverse junction temperature difference by the current. Thomson effect intricate. He argues that when there along (difference) the conductor temperature gradient begins transferred (released or absorbed) heat. examined and proved in law in the thermodynamic theory:
- Intermediate metals in a closed circuit made of dissimilar metals at the same temperature EMF zero sum. This is considered an expression of the second law of thermodynamics. The work is not done without the expenditure of energy. What happens at the same temperature junctions "Proof: heat transfer due to the current impossible because of the Peltier effect. This would cause a heating of some areas and other cooling. That would mean the transfer of heat from the colder places in the absence of an external power source. Air conditioning could not operate on electricity, but due to a special wiring connections. "
- Magnus in the closed loop of the same material is not supported by current temperature difference. Consequence of the law was the EMF dependent only on the difference between junction temperatures. Do not worry about the heating or cooling of the external conditions of the conductors themselves.
- Successive (intermediate) Temperature: the algebraic sum of the emf on the contour in the range from T1 to T3 is The algebraic sum of the emf folded along the contour at intervals from T1 to T2 and from T2 to T3, with any values of T1, T2 and T3.
All three of the law argue that the resulting electromotive force becomes a function of only the junction temperature. These postulates are recognized as the basis of measurements, including what is happening in domestic refrigerators. Other treatment: thermocouple is not necessarily contains two metals. If you want to measure the temperature gradient along the thermoelectrode, it is enough for one occurrence of EMF. A second material will contact the findings. This is the case of a degenerate and quite efficient thermocouple, stemming from the fundamental equations of the thermodynamic theory. Consequently, the effect predicted analytically.
Analytical calculations using formulas
It is made in the image below in view of the complexity of recording mathematical formulas Internet imposition. It is seen that in the absence of electric current the first equation of thermodynamic theory is simplified. It follows that removing the voltage analog-to-digital converter with a smaller current limit supply, refrigerator Bosch costs "thermocouple" of a single metal.
Distinguish between absolute and relative EMF. The second refers to a pair of materials, and the first - characterizes only a single. Absolute emf is measured using standard, for which other methods already measured indicated value (oil oil). From the experiments is a differential EMF, which allows to calculate the value. Standards currently recognized:
- For relatively high temperatures (above 100 K): platinum, gold, copper, tungsten.
- Lead in other cases.
At very low temperatures below 20 K the absolute EMF is determined directly. In some materials becomes zero and, combining the test sample with a pair of immediately come to the desired value. Most metals absolute EMF in the range from 0 to 80 mV / K.
The kinetic theory has to do with the non-equilibrium state of the environment. He studies them in motion. It is based on the Bardeen-Cooper-Schrieffer, is not so well-known to the public. In consideration of probability accepted theory, each particle is considered individually, without regard to the overall system performance. For that theory called microscopic.
In consideration of the concepts introduced: Cooper pair of electrons the Fermi surface Debye frequency and so forth. Theory operates probabilistic equations positions of the particles, the Boltzmann function. According to representations of science at the beginning of the XX century in each metal there is a certain concentration of electrons scattered at random, but obey the Boltzmann model. These diverse theories were named:
According to the Boltzmann model, the average translational energy of motion of the particles is 2/3 kT, where k - is Boltzmann's constant. According to this interpretation thermoelectric emf is a function of the concentration of particles in the two metals of the thermocouple and temperature (see. Fig.). It is easy to verify that the formula presented for the metal does not correspond to real observations. This is performed by simply counting the emf values on dividing the temperature range and find the Seebeck coefficient. He clearly overestimated.
Calculations of the kinetic theory
formula for calculating
Controversy formula beginning of XX century was eliminated Frenkel and Sommerfeld's theory (1927). Last electrons are placed in the model of quantum statistics of Fermi-Dirac. Sommerfeld Seebeck coefficient is obtained a very small quantity. This is easily explained by the fact that the formula of the kinetic theory operate with electron concentrations directly, but they are difficult to measure and track.
Semiconductors better obey the kinetic theory. Electrons from a material with a higher density diffuse and reach for the interface. The process continues as long as the counter field "emigrants" counterbalances the movement counter flow. In terms of explaining the process of the kinetic theory is not different from the arguments, cast the two headings above, but there are nuances:
- With increased concentration of carriers of a particular sign stream carries them to the cold end, but there are already accumulated charge prevents during the process. The carriers of the opposite sign, on the contrary, accelerated by the field. As a result, counter flows at the boundary between the media will be equal, and the potential difference is determined temperature value.
- The diffusion coefficient is closely associated with the charge mobility. This relation is established by Einstein. The uneven concentration is created, thus the temperature gradient. Less mobile charges form a sort of cork in its path because of the low speed of movement. The field of this accumulation of charge carriers of opposite sign wings particles. As a result, the process reaches equilibrium.
The presence of two carrier symbols due to the high Seebeck coefficients in the semiconductor. In metals have an increased concentration of electrons reaches 10 sextillions per cubic centimeter. Consequently, the temperature fluctuations of the figures can not be great, which explains the low Seebeck coefficient of metals.