THERMOELECTRIC CONVERTERS OF ELECTRICAL AND OPTICAL SIGNALS-NEW CLASS OF THERMOELECTRIC DEVICES

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1 THERMOELECTRIC CONVERTERS OF ELECTRICAL AND OPTICAL SIGNALS-NEW CLASS OF THERMOELECTRIC DEVICES E.M. Sher E.M. Sher (Physics and Technical Institute, Saint-Petersburg, Russia) The new class of thermoelectric devices for conversion of various signals is offered. It is based on use of thermoelectric control in parameters of elements with phase transitions in narrow temperature area of phase transition. With decreasing width of working temperature area, the overall performance of thermoelements sharply increases, providing effective transformation of signals at the minimal expenses of energy. As a result, descriptions of several (developed on the basis of these two principles) thermoelectric converters of signals are obtained. The quantity of types of signal converters can be considerably increased by using elements with various phase transitions in various temperature areas. With reducing thermoelements to the micron and submicron size, possible applications of thermoelectric converters can be expanded to the area of high frequencies and integrated technologies. Introduction. Thermoelectric control of phase transitions Thermoelements usually operate under conditions when the temperature difference Δ T across them is close to its available maximum. However, in the case when you need to control some parameters of materials with phase transitions (for both st and nd types) the necessary value of ΔT is small. Under a small temperature difference and optimal current the maximum coefficients of cooling ε c and heating ε h of the thermoelement are approximately the same and determined by Ioffe equation [] T ε= f ( z ), () Δ T where f(z) is the function of the thermoelectric figure of merit z, T the temperature of the cold (or hot) junction of the thermoelement, Fig.. From the equation () we can see, that at ΔT of the order of temperature width of a phase transition (usually Δ T ~ K, Fig. ) the coefficients of cooling and heating are growing up to the values of 0 00 or more, Fig.. Fig.. Refrigerating (heating) coefficients ε for thermoelement working as heat pump at small ΔT. As the heat pumping of a thermoelement Q= ε W (W is the electric power), then in the case of phase transition control the necessary power of this process is less in ε times. This circumstance allows the control over the thermoelement temperature difference by means of a small electric signal, and, consequently, by the parameters (electrical, optical, magnetic, etc.) of phase transition elements, and permits electric signals to be converted. Variations in electric signals can also be converted and amplified into variations in electrical, optical or magnetic signals. We have termed such conversions as thermoelectric conversion of signals []. A number of devices such as amplifiers and rectifiers of small signals, generators, electrical-to-optical signal converters, memory elements of electrical and optical signals, etc. have been developed [, 5, 8, and 9]. ISSN Journal of Thermoelectricity, 007 8

2 . Thermoelements at the energy conversion at small temperature difference The power necessary to maintain a regime of small ΔТ for thermoelements is decrease sharply [, ]. When thermoelement is operating at conversion of the electrical energy in the thermal energy, it works as thermal pump. lg R T, C Fig.. Temperature dependence of resistance for element from vanadium oxide with Metal Semiconductor Phase Transition (MSPhT). From () we can see that when the necessary Т on the thermoelement decreases, the value ε will considerably increase (Fig. ). As the heat Q, pumped by a thermoelement, is equally to ε W (where W is the electrical power), then the necessary power W decreases by a factor of ε. At Т 0, the efficiency of conversion will increase unlimitedly. It is because the thermoelement operates as a thermal pump: it uses heat from the surrounding medium and the energy W is expend only on pumping heat Q from one junction to another with equal temperatures, i.e. W 0. For creation of such devices, also it is necessary to place thermoelement and element with phase transition in thermostat. Temperature thermostat should be to equal temperature of a working point of the nonlinear characteristic R(T) of an element with phase transition. Then it will be possible to use advantages of a mode small ΔT thermoelement for management of an element with phase transition. Fig. shows the elementary scheme of the thermoelectric converter, where is a thermoelement, is a working junction (commutation plate), the element with phase transition whose temperature should be monitored. R r E ΔU out 5 ΔU inp Fig.. Scheme of the thermoelectric conversion as amplifier or rectifier of signals. thermoelement, working junction, element with MSPhT (or posistors), thermostat, 5, electrically isolated thermal contacts, r load resistance, E direct-voltage source. The principle of an operation is illustrate schematically in Fig.. The input signal of the circuit ΔU inp evolves power in thermoelement, W = I U inp, where is a current through the thermoelement. This causes liberation or absorption (depending on current direction) of heat in junction, Q = ε W, where ε is the refrigerating ε r or the heating ε h factor accordingly. For state-of-the-art thermoelectric substances f(z) 0. and, for example, at ΔТ = K, from () and Fig. ε 0, i.e. the amount of heat selected on a junction (so, and on resistance R) is 0 times 8 Journal of Thermoelectricity, 007 ISSN

3 as large as the power W brought to the circuit input. At a high thermal factor of the element with phase transition (for vanadium oxide this factor is changes on 5 orders, Fig. ), on a circuit exit, there will be a signal many times over exceeding a signal on an input. Let us obtain relations for main parameters of such converters. Fist we shall assume, that the Joule heat liberated in element (, Fig. ) with phase transition is negligible in comparison with the Peltier heat on thermoelement junctions. Then current I through a thermoelement will cause a change of the element temperature by the value: αit Δ TM =, () K where α is thermopower and K is thermal conductance of a thermoelement. Current I is express through input voltage U inp as: I = ( ΔUinp αδ TM ), () R where R T is thermoelement resistance. T From () and (), excluding I, we find ΔΤ as a function of the signal on an input: where Z is figure of merit: αtδuinp ΔUinp zt Δ TM = =, () Κ R +α T α + zt T α z = K R T. (5) Variation ΔU ou due to a change in ΔΤ M of the element temperature is: Er dr Δ Uout = ΔT ( R+ r) dt M. () Substituting for ΔΤ M from () in expression (), we obtain the voltage amplification factor: ΔU zt Er dr β M = =. (7) Δ α + + out Uinp zt ( R r) dt If the heat liberation in the element with resistance R can not be neglected, the modification of temperature ΔТ will be less or more than ΔΤ M, depending on heat, i.e. it is evolved (а) or absorb (b) on this junction. In this case β β M ΔTM. ΔT From (8) it follows, that in the case (а) we can obtain a considerable growth of the amplification coefficient due to positive feedback. Under certain conditions, in this case there can be a generation of oscillations. When coefficient amplification γ of the power is evaluate, it is impossible to neglect a heat release in element with resistance R. We shall suppose, that W R = W and the fluctuating power is W = / ΔW. ΔW is a load for the thermoelement: (8) ISSN Journal of Thermoelectricity,

4 According to (), αіτ = ΚΔΤ M and, hence: Transforming I in accordance with (): I = KΔTM α T and designating Θ = ΔT / ΔT M after simple transformations we have: ΔW = α ІΤ KТ. (9) ΔW = K (ΔΤ M ΔT), (0) ΔWout ( ΔTM ΔT) γ =. Δ W RI +αiδt () inp T Θ γ zt. () Δ T + zt Θ M Above we have considered cases when heats Peltier and Joule heats have opposite signs. In a case when these heats are summarized ΔТ > ΔΤ M and T Θ γ zt. () Δ T + zt Θ M The analysis of expressions (7) and () shows, that at the present it is quite real to obtain the voltage and power amplifications of about 0 0. The condition for an efficient conversion of signals is satisfied by materials whose optimum parameters differ from optimum value of the parameters of thermoelement materials used for the efficient conversion of energy. The voltage amplification factor β M from (7) is proportional to: where α is thermoemf, and z is figure of merit. δ= zt α + zt, Fig.. Dependences z = f (α ) for thermoelements (from Bi-Te-Se and from PbTe) and δ = f (α) for devices with thermoelectric control of elements with Phase Transitions. As seen from Fig., the optimum of δ (as a first approximation, not taking into account electron thermal conductivityand degeneration of the electron gas) lies in the region of lesser values of thermoemf than those for z: at α 80 0 μv/k rather than α ~ 00 μ V/K. 8 Journal of Thermoelectricity, 007 ISSN

5 Based on these principles, we can develop rectifiers, converters of light, elements of memory, generators and other converters of signals demanding a small expense of power for management with big signals. Descriptions of some of them сan be found below.. The thermoelectric amplifier and the rectifier of small signals The peculiarities of thermoelements working at small differences of temperatures, specified in the previous paragraph, permit to construct the thermoelectric amplifier and the rectifier of small signals. The construction scheme of these devices used for measurements is show in Fig.. Thermoelement with branches made from alloys based on Вi Те is solder to an internal wall of thermostat through the electrical isolated thermal contact 5. Element with metal-semiconductor phase transition (MSPhT) or other element with the nonlinear characteristic is join through the electrical isolated thermal contact to the working junction of the thermoelement. 5 E ΔU inp R ΔU out R, Ohm Fig. 5. Construction principle of the thermoelectric amplifier and rectifier of small signals. thermoelement;, electrical isolated thermal contacts; thermostat; 5 controlled element; working junction; ΔU inp input signal; Е auxiliary voltage supply; R load resistance. The circuit for measurements of amplification and rectifying is shown on Fig. 5. A small input alternating signal ΔU inp is feeded to thermoelement with controlling element 5 on its working junction (). This resistor connects serially with power supply E and load resis- C tance r, from which the output signal ΔU out is taking off. A T, C Fig.. Dependences R (T) for:, posistors; element from VO with MSPhT. B The temperature dependences of the resistance of the used elements are show in Fig. : for posistors and for elements from vanadium oxides with MSPhT. For measurements under gain of small signals, the thermostat temperature was adjusted so that the resistance R of an element (Fig. ) was on the linear section of R(T) (Fig. ) for elements with MSPhT () or for posistors (, ). The thermoelement in this condition of small ΔT is pumping over heat which many times exceed the power of an input signal. That considerably reduces R of element with MSPhT, and, accordingly, leads to a considerable increase of the output signal ΔU out, i.e. to amplification of the input signal ΔU inp. ISSN Journal of Thermoelectricity,

6 The measured amplification coefficients of the device for the case when posistors used as controlled elements are shown in Fig. 7. For devices with MSPhT based on vanadium oxides, the measured amplification coefficients are shown in Fig. 8. In this manner, it is possible to obtain an amplification of an electrical signal of some ten and hundred or even more times. β β ΔU inp, mv Fig. 7. Amplification coefficient β in dependence from voltage changing ΔV inp at amplifier input - - controlled elements posistors. 0 0 ΔU inp, mv Fig. 8. Amplification coefficient β in dependence from voltage changing ΔV inp at amplifier input - controlled elements from VO with MSPhT. The amplification coefficient of a device with posistors as controlled element (Fig. 7) drops a little with growth of the ΔU inp. A drop in amplification coefficient β with growth of the input voltage ΔU inp is connected with the increase in temperature of the posistor that changing its performance. The behavior of the amplification coefficient of the arrangement when using a controlled element from vanadium oxides with MSPhT is shown in Fig. 8. Curves,, relate to elements with MSPhT, having a different steepness of a transitional section from semiconducting to metal state. The least steepness of this section has the element corresponding to a curve, the greatest to a curve. The falling of amplification coefficient of the arrangement with growth ΔU inp is bind with that the output signal goes out from limits of a transitional section of MSPhT element (Fig., curve) with a major steepness. It is possible to increase an amplification factor up to thousand and more times at using elements with MSPhT having more abrupt resistance dependence from temperature (as at Fig. ) than it is indicate on Fig. (curve ). The connection (see Fig. 5) between controlling thermoelement and an element is ensuring only by the expense of heat pumping. Because of that, the complete electrical decoupling of input and exit is one of essential advantages of such devices. The thermostat temperature for signal rectification must be adjust near the inflection point of dependence R(T) for element with MSPhT (, Fig. ) or near point of inflection of posistors dependence R(T) (point B, Fig. ). Here thermoelement also works in condition of small ΔT, temperature of its working junction oscillates from the input signal variables with ΔU inp near to an inflection point of dependence R(T). Because of the indicated raise of the transferred heat and because of the difference in declinations of curves R(T) on both sides from a point of inflection for element (Fig. ), on a load resistance r is separate out the rectified signal ΔU out. The rectification of an input signal happens with rectification ratio, approximately, equal to differences between amplification factors of curve sections with major 88 Journal of Thermoelectricity, 007 ISSN

7 K ΔU inp, mv and small curvatures of temperature dependence R(T). The, curves on Fig. 9 is indicating the dependence of rectification ratio K from thermoelectric converter when as a controlled element are used: posistors (curves ) or elements with MSPhT (curves ). Increase of rectification ratio K for devices with posistors is connect with increase of a difference in the slope of the curve R(T) sections on both sides from a point of inflection. The same reason explains the K growth on initial section for element with MSPhT. Then the growth of K becomes less because of a small change of the slope far from the point of inflection R(T). Thus it is possible to obtain rectification of small signals from ~5 mv and higher. That is one of the advantages of such rectifiers: semiconducting diodes do not rectify an alternating voltage magnitude less than ~0.5 V. The estimated values of high-speed performance of the thermoelements in the device, working in regime of small ΔТ, depend on accessible sizes of extrusion branches (at an optimum current). For example, at small ΔΤ (up to 0. K) and at an altitude of thermoelement branches ~ mm, it is possible to ensure the functioning of such devices with frequency ~0 0 Hz.. The thermoelements at small temperature difference for an optical transparency control The optical method of the information processing make possible:. Parallel evaluation of the two-dimensional information;. A fast response time; 5 Fig. 9. Dependence of rectification coefficient k from changing input voltage ΔV inp - for posistors; - for elements with MSPhT.. Holding such operations, as a Fourier transformation and correlation analysis of the twodimensional information in real time. The method of the coherent correlative analysis can be carrying out with the help of the holographic coordinated filter. Thus for one of the basic elements of the optical correlator, the spatial light modulator, it is convenient to use a thermal chrome element PhTIROS []. The physical basis of operation of this substance also is the metal semiconductor phase transition, which occurs in vanadium oxides at a convenient for practice temperature ~0 С. Substance PhTIROS represents a thin-film structure of vanadium oxides applied onto a reflecting metal underlayer. Because of metal semiconductor phase transition the optical constants of the substance PhTIROS vary sharply and make it possible to note and map the optical information []. The substance PhTIROS has the necessary complex of properties for its use as a space light modulator: reversibility, possibility of operation as a fixed memory, high optical resolution and sensibility, fast response time (acting). However, realization of such optical correlator on PhTIROS basis meets with difficulties, mainly owing to its temperature efficient control. Using a functional mode of thermoelements with small temperature difference across them and reversing on of the current supply for the passage of element PhTIROS from cooling to heating and back, we can noticeably improve performances of this device. For that purpose the thermoelements, with element PhTIROS applied on their working junctions were located in thermostat with temperature ~ grades lower than the middle of PhTIROS hysteresis loop whose width was about ~ K. ISSN Journal of Thermoelectricity,

8 The thermobattery consisted of six extruded elements ~ mm in height. Control of a PhTIROS element ~ 0. cm square was taking place by temperature oscillations of working junctions about K. At the given frequency of the entry and erasing of optical images on PhTIROS device of 8 Hz a ~0 mv voltage was required at a current of ~ 0. A. The expended power of control was only ~ mw. For comparison we would point out, that when a condition of small ΔТ and reversion of the supply current is not used, the corresponding power will increase more than by an order of magnitude. Thus, the offered functional mode of the thermoelements for control of an optical transparency gives more than a ten-fold gain in power. The frequency of change of a recorded image on the element PhTIROS by thermoelectric device can be increase to the value 5 0 Hz [, 7] and it is enough for effective control of the optical transparent.. The thermoelectric generator of electrical and optical signals An experimental model of such arrangement is shown in Fig. 0, where thermoelement, element from VO with MSPhT with a jump on resistance,,, 5 the interference PhTIROS-type element with MSPhT, thermostat, 8 power supply. The operation of this device consists in the following. The temperature of the thermostat is set above the transition points of the devices and 5 from the semiconducting to the metal state. Originally element is in the metal (low-ohmic) state. When connecting a power supply to a thermoelement the current flows through the thermoelement and the Peltier-heat is absorbed at working junction. The device is cooling and passes into the semiconducting (high-ohmic) state. Then the current through the thermoelement sharply drops, cooling stops and the device is refund in the metallic state, all process iterates, and thus undamped electrical oscillations (which are taking off from the thermoelement) are ensure E Fig. 0. Construction principle of the thermoelectric generator thermoelement; element with MSPhT; -5 element PhTIROS; thermostat; 7 thermostat window; 8 power supply E; 9, 0 electrically isolated thermal contacts; working junction The temperature oscillations of the thermoelement working junction are transmitting to interferential element PhTIROS, which is in good thermal contact with the working junction. Under illumination of its surface by a monochromatic radiation, there are beginning oscillations of an output optical signal. These oscillations are due to the fluttering of reflection coefficient between the direct and the opposite path transition of the element PhTIROS with MSPhT. Both types of elements: with jump in resistance and 5 element PhTIROS with equal temperature of MSPhT were select. 90 Journal of Thermoelectricity, 007 ISSN

9 In the tested experimental model were used the extruded thermoelements and element with MSPhT (a plate: cm and 0 - cm thick). The specific resistances of a device in metal state was ρ = 0 - Ohm m. The interferential element PhTIROS 5 by size cm was bond on to working junctions. The electrical oscillations of frequency ~ 5 Hz originated when power supply was turned on. Modulation of surface reflection coefficient of element PhTIROS 5 with the same frequency observed at its illumination by He-Ne-laser ray. Thus, such oscillator based on overlapping of the heat pump (Peltier element) and elements with MSPhT, ensures a low frequency of oscillations at the efficient simplicity of construction. 5. The electrical signal control over optical signal The thermoelements working in a condition of small ΔΤ are also convenient for controlling the intensity of light beams. A small in power and voltage electrical signal can control over a wide range of optical signals with the help of element PhTIROS. In the interference element PhTIROS in the region of its MSPhT the optical signal can create strong and reversible changes of the reflection coefficient []. The spectral dependence of element PhTIROS reflection coefficient on temperatures lowers and higher of the metal semiconductor phase transition points (TP) is show in Fig.. By changing slightly the composition of oxides in element PhTIROS it is possible to shift the curve minimum on both sides of TP. In so doing, the qualitative pattern (picture) of spectral dependence of the reflection coefficient will remain the same. In the composition of oxides in element PhTIROS, used in measurements, the wavelength of the minimum on the reflection coefficient after MSPhT was equal to the wavelength of the He-Ne laser (0. μm). R, % 80 T = 5 C before PhTr 0 T = 80 C after PhTr λ, μm Fig.. Dependence of reflection coefficient from wavelength for element PhTIROS. The arrangement of an experimental model of the thermoelectric device for controlling the reflection coefficient is show on Fig.. Here: thermoelement, 5 element PhTIROS, the temperature-controlled thermostat, 7 radiation detector, 8 He-Ne laser, 9 working junction, E controlling signal. The temperature of thermostat is adjust at the operating temperature near the phase transition point. The transition of the element PhTIROS through the temperature of MSPhT and back is carrying out by applying to the thermoelement small voltages of direct (heat) or indirect (cooling) polarities. The coefficients of reflection were ~ 0 % for temperatures lower than MSPhT and ~% for temperatures higher than MSPhT (Fig. ). I.e. the reflection coefficients of the interference element PhTI- ROS were changing by a factor of ~ 0. Such control of reflection coefficients was carried out at an input electrical signal about 5 mv and the power consumption was about 0. mwatt. The thermoelectric control of the transmittance coefficient of elements with MSPhT (as it was already mentioned) can be conduct within considerably wider limits (0. 0 ) by consumption of the same power. ISSN Journal of Thermoelectricity, 007 9

10 E Fig.. Construction principle of the thermoelectric device for control of reflection coefficient thermoelement;, electrically isolated thermal contacts; thermostat; 5 element PhTIROS; window in thermostat; 7 radiation detector; 8 He-Ne laser; 9 working junction; E controlling signal. It should be emphasized, that here the control energy of optical signals (also owing to application of the indicated functional mode of thermoelements) was in 0 0 times less than the energy expended at a traditional mode of the controlling optical properties.. Thermoelectric control on ferroelectric and magnetic phase transition ε 0 8 C A T, C Fig. 5. Dependence of dielectric constant from temperature for ferroelectric. Some additional opportunities can be realize by thermoelectric control of elements with ferroelectric and magnetic phase transitions (FMPhT). Based on these types of control and by means of the Peltier effect at small temperature differences, it is possible to create the analogous devices as those based on elements with MSPhT. To this, it should be add, that the temperature range in which ferroelectric and magnetic phase transitions can be used, is much wider, than for MSPhT. Such properties as steepness of FMPhT, the width of a hysteresis loop, etc., also give additional potential capability. Control of magnetic phase transition allows producing a low-frequency modulation of a magnetic flow. Fig., illustrate an elementary scheme, based on control by means of the Peltier effect of ferroelectric or magnetic phase transition elements, which can work accordingly as amplifier, rectifier and frequency converter. On Fig. : thermoelement, thermostat, the capacitor with ferroelectric material (0.5 PbMg / Nb / O PbSc / Nb / O / ) located in it. The dependence of dielectric constant ε on temperature T for this compound is shown on Fig. 5. If the thermostat temperature is selected in such a manner that the operating point (OP) is on a linear section, the circuit works as the amplifier. If the temperature of the thermostat is selected in such a manner, that OP correspond to points A or С of the dependence ε (T), the device will work as a frequency converter or a rectifier. The employment of the magnetic phase transition for conversion of signals is illustrated on Fig.. Shown here is the scheme for controlling the magnetic phase transition in nickel by means of the Peltier effect: () П shaped permalloy cores; () nickel wire; () constantan wire; () thermostat maintaining a temperature close to the point of the magnetic PT in nickel. A constantan wire in contact with a 9 Journal of Thermoelectricity, 007 ISSN

11 nickel wire forms a controlling thermoelement. Depending on the magnitude and direction of the input signal changing the temperature near the Curie point in nickel, either the right or the left magnetic circuit is disrupt, and, hence, the balance of the bridge is upset; thus, a signal appears at the output. 5 V E A r U out Fig.. Thermoelectric converter of signals (amplifier, rectifier, and frequency transformer) with ferroelectric controlled element thermoelement; thermostat; controlled element- ferroelectric;, 5 electrical isolated thermal contacts; oscillator of audio frequencies. U inp + 5 R R U out Fig.. Thermoelectric amplifier and a frequency converter with magnetic controlled element. iron cores; thermostat; nickel wire; constantan wire; 5 oscillator of audio frequencies. The circuit in Fig. behaves as an amplifier if the operating point is located on the linear portion of the temperature dependence of the magnetic susceptibility of nickel, or else as a rectifier or frequency converter if the operating point is located on a curved portion of the dependence. This type of conversion can occur in the high temperature region, since the Curie point for nickel is 70 C (and if nickel is substitute by iron, it equals 770 C). As is known, the extending time of thermal process is proportional to quadrate of thermoelement branches length and is in inverse proportion to thermal diffusivity of material. The thermal diffusivity in semiconducting thermoelements used at present is a =0 - cm /s (length of branches l = mm, and time constant τ ~ s). In metals and their alloys a = cm /s, the length of thermoelectric branches can be made up to ~0 - cm and thus it is possible to obtain τ = s. So, at the size of thermoelement branches equal to a grain of micron size the working frequency of converters can be increase up to 0 0 Hz. It was shown in a number of studies [, 5, 8, 9], that such reduction in grain size does not reduce the thermoelectric efficiency, but, on the contrary, improves it. Conclusion. A new field of application for thermoelectrics is offer.. Thermoelements in the suggested functional modes of small temperatures difference work with high values of cooling ε c or heating ε h coefficients providing effective control of devices based on several types of phase transitions elements with non-linear characteristics and ensuring high factors of conversion.. Thermoelectric devices belonging to a new type of thermoelectric converters of information were realized on the suggested principles. Such devices are: amplifiers and rectifiers of small electrical signals, converters of an electrical signal into optical signal, converters of an optical signal into electrical signal, thermoelectric elements of memory, a device permitting to control an optical transparent by means of small electrical signals on a thermoelement, the devices based on the superposition of two signals, thermoelectric frequency converters, the analyzer and the comparator of small signals, and many others. ISSN Journal of Thermoelectricity, 007 9

12 . The possible range of operating temperature for this type of information conversion is from temperature of liquid nitrogen up to ~ 500 C. It involves, firstly, the available substances (semiconducting and metal) for the thermoelements working in the corresponding interval of temperatures: for Bi-Sb-up to 00 С, for Bi Te up to 00 С, for PbTe, for GeTe-up to 00 С, for SiGe-up to 900 С, for boron and borides up to 500 С. Secondly, there is a great many of substances, the phase transition in which occures at one of the temperatures, lying in the indicated range. Semiconductor devices, based on application of transistors and integrated circuits, can work at temperatures, usually, not higher than 0 С. 5. Thermoelements and elements with phase transition are much more stable to the action of different radiation than usual semiconductor elements. It is because in semimetal thermoelements and devices with phase transition the density of current carriers is ~ cm -, in metal thermoelements it is ~0 cm -. I.e. the density of current carriers here is in some orders (up to six-seven) of magnitude higher than in usual semiconducting devices where this density is about 0 0 cm -. Because of that, these devices can work in extreme conditions of high temperatures and at respectively high levels of radiation.. Complete electrical decoupling of input and exit is one of advantages of devices, considered in this chapter. 7. At present, the frequency conversion of electrical signals in the described devices with application of extruded thermoelements does not exceed a value of ~0 5 Hz. This frequency can be increased up to ~ 0 0 Hz using phase transition elements with a hysteresis loop < and by application of thermoelements of micron sizes. References. Ioffe A. F. Thermoelements and Thermoelectric Cooling. London, Sher E.M. Thermoelectric Conversion of Signals / The First European Conference on Thermoelectrics. Edited by D.M.Rowe. Cardiff, UK, 987. London: P. Peregrinus, 988. P Bugayev A.A., Zakharchenya F.A., Chudnovsky B.P. Metal-semiconductor phase transition and its application. St. Petersburg: Nauka, 979 (in Russian). Sher E.M. Electrical Properties of Dispersed Porous Oxides of Alkaline-Earth Metals with High Thermoelectric Efficiency / Proceedings of the 5 International Conference on Thermoelectrics, pp8-7.pasadena, CA, USA, Bulat L.P., Sher E.M. Increase of Thermoelectric Efficiency in Macroscopically Inhomogeneous Medium / Proceedings of the 9 International Conference on Thermoelectrics, p.09, Cardiff, UK, 000. Galperin V.L. Non-steady Cooling Processes in Pulsed-Mode Thermoelectric Devices / Proceedings of the International Conference on Thermoelectrics, p. -8. St. Petersburg, Russia, Galperin V.L., Khahaev I.A., Chudnovskii F.A., Shadrin E.B. Control of metal semiconductor phase transition by a fast-acting thermoelectric cooler, part III // Zh. Tekh. Fiz V. 8.. p Sher E.M. Thermoelectric Properties of the Transition Metal Oxides in Finely Dispersed State / Proceedings of the 0th International Conference on Thermoelectrics, p. 8-8, Beijing, China, Bulat L.P., Sher E.M. Some aspects of phase transitions control by thermoelectric method / Proceedings of the st International Conference on Thermoelectrics, p Long Beach, USA, 00. Submitted Journal of Thermoelectricity, 007 ISSN

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