Thermo-emf

The thermoelectric effect is due to the gradient in electrochemical potential caused by a temperature gradient in a conducting material. The Seebeck coefficient a is the constant of proportionality between the voltage and the temperature gradient which causes it when there is no current flow, and is defined as (A F/A7) as AT- 0 where A Fis the thermo-emf caused by the temperature gradient AT it is related to the entropy transported per charge carrier (a = — S /e). The Peltier coefficient n is the proportionality constant between the heat flux transported by electrons and the current density a and n are related as a = Tr/T. 

It can be seen from Table 12 that, in spite of a thermo-emf of up to 1 mV/ degree, the Gx factor of pure phthalocyanines is very low because of their low conductivity. However, the conductivity of some compounds can be increased by polymerization 13°) or doping 74-131> without an excessive fall in thermo-emf. For example, as can be seen from figure 11, although conductivity increases by several orders of magnitude, thermo-emfs of up to 1 mV/degree are still obtained in doped systems. 

Fig. 11. Relation between thermo-emf Q and conductivity. Plot Q = C — (kje log e) log a. iodine/phthalocyanine Q TCNE/phthalocyanine Q o-chloranil/poly-N-vinylcarbazolc...

Tabulated standard potentials — The - standard potential of various redox couples have been tabulated by using either the values obtained in electrochemical experiments or calculated from thermochemical data. The electrochemical determination of these values based on -> emf measurements is possible provided that -> diffusion potentials and thermo emf s are elimi-... 

Thermoelectric power measurements indicated that all the compounds are p-type semiconductors and thermo - emf increases with an increase in Zn2+ content (Table 1). In this system Zn + ions with stable oxidation state occupy A sites. With the introduction of Zn + in the lattice the concentration of Cu occupying octahedral site decreases, however the concentration of Mn ions at the octahedral sites of the series Cuj. Zn MnCr04 remains same. Therefore, with the increase in value of Zn2+, the number of ion pairs of Mn3+ - Mn + start decreasing thereby increasing the resistivity of Zn rich compositions. Seebeck coefficient ( a ) varied between +26 to +66 pV/K which shows with increase in zinc contents of the spinel, the Seebeck coefficient increases. 

Raman studies of bivalent Zn in fused ZnCl2-KCl show conclusively the formation of the [ZnCU] anion although thermo-EMF and thermal conductivity studies of the systems ZnCl2-CsCl and CdCl2-KCl show maxima at a 1 1 molar composition. Thermal analysis, however, shows the presence of Cs2ZnCl4 and CsZn2Cl5. 

ZnS thin films of different thicknesses were prepared by CBD using thiourea and zinc acetate as S " and Zn sources, respectively. Thermo-EMF measurement indicated that films prepared by this method are of n-type. ZnS thin films were prepared by decomposition of thiourea in an alkaline solution containing a zinc salt The reaction mechanism for deposition of ZnS films has been reported. In aqueous solution, zinc acetate dissociates to give Zn " ions. Hydrolysis of ammonia... 

This principle makes it possible to predict the sign of conduction of a mixed-valence phase, to explain the appearance of electrical conduction, and to select reliably ways of preparing materials with prescribed thermoelectric properties. The problem of the simultaneous introduction of several impurities into a semiconductor is considered. The role of cation and anion vacancies is determined. The thermo-emf power is estimated from the possibility of the appearance of ions of "abnormaT valence and from the role of the resultant polar bonds. The limits of the validity of Vetwey s principle are considered. 

The presence of a temperature gradient in thermoelectric materials produces a thermo-emf. Let us consider what happens in PbX with an excess or a deficiency of cations when this compound is subjected to a temperature gradient. We shall assume that the dissolved Pb2X or PbXjj pseudomolecules and the normal PbX molecules are bound by polar bonds PbX Pb2X and PbX PbX. ... 

Our method for predicting the sign of the thermo-emf from the presence of ions of "abnormal" valence can be used also to derive guiding principles for the doping of semiconducting compounds. The introduction of a dopant (an element, compound, or a more complex mixture) results in the formation of some cations of "abnormal" valence. 

In melts, an additional problem of thermo-emf arises, and the emf correction can be calculated from thermoelectric coefficients of phases in contact [35]. 

The universal definition of the standard potential of a redox couple Red/Ox is as follows the standard potential is the value of emf of an electrochemical cell, in which diffusion potential and thermo-emf are eliminated. This cell consists of an electrode, on which the Red/Ox equilibria establish under standard conditions, and a SHE. 

It has been shown that precipitation or dissolution potential = observed potential -Nernst potential - diffusion potential - phase potential - thermo-emf. 

The fist of publications [1-50] covers the period from 1958 to 1990, i.e., up to the very last years of the united Soviet Union. It includes only a small part of the publications and stiU reflects the wide variety of research on solid electrolytes at the IE US AS/IE UD AS and the journals publishing these results. There were many theoretical and experimental studies on electrochemical cells with solid electrolytes [1, 4, 5, 21, 39] and an extensive research on the phase composition of oxide ionic conductors [2] and their electric properties [3, 6, 8, 10, 13, 14, 17, 18, 20, 30, 34, 48]. Many papers were related to practical applications like sohd electrolyte degradation [33, 47] or application limits related to the electronic conductivity of the solid electrolytes [5, 30, 40, 43]. There were many publications on the implementation of different electrodes and on the kinetics of electrode processes [23,27, 31, 35, 36, 45], on investigations of the electrode overvoltage [7, 12, 25, 28], on impedance spectroscopy of solid electrolytes [19, 27], and on isotope exchange research [15,16]. The double layer and electrocapUlarity of solid electrolytes were studied in detail [9, 11, 19, 32, 44]. Systematic studies were performed on the thermo-EMF of different solid electrolytes [22,24,29], the EMF of electrochemical cells with solid electrolytes [26, 39], and the thermodynamics of oxygen in molten copper [41]. Applied research was focused on electrochemical oxygen pumps... 

A brilliant researcher, Perfil yev constantly paid attention to the development of the research methodology in the sohd electrolytes. The results of such activities have already been reported on the impedance method of the polycrystalline sohd electrolyte conductivity investigation and development of the electrochemical ceU with diffusion channel for the analytical chemistry and instnnnentation. To define the lower temperature hmit for the electrode eqiuhbriiun of the electrochemical cells, he has proposed a very original method based on the measrrring the thermo-EMF of the electrochemical cells. 

Chebotin s scientific interests were characterized by a variety of topics and covered nearly all aspects of solid electrolytes electrochemistry. He made a significant contribution to the theory of electron conductivity of ionic crystals in equilibrium with a gas phase and solved a number of important problems related to the statistical-thermodynamic description of defect formation in solid electrolytes and mixed ionic-electronic conductors. Vital results were obtained in the theory of ion transport in solid electrolytes (chemical diffusion and interdiffusion, correlation effects, thermo-EMF of ionic crystals, and others). Chebotin paid great attention to the solution of actual electrochemical problem—first of all to the theory of the double layer and issues related to the nature of the polarization at the interface of the solid electrol34e and gas electrode. 

Chebotin VN, Fridman SL, Pal guyev SF (1970) Thermo-emf in Zr02-l-CaO solid electrolyte. Elektrokhimiya (Rus) 6 1300-1304... 

In melts, problem of TLJP (thermo-emf) always arises, and the emf correction is typically calculated from thermoelectric coefficients of phases in contact [83]. However, no detailed research of TUP contribution to this correction is known. The problem is probably close to solvent/solvent junctions. 

Fadeev GI, Perfilyev MV (1988) Studying equilibrium potentials of gas electrodes in the cells with Zr02-03 electrolyte at reduced temperatures by the thermo-emf method. In Perfilyev MV (ed) Electrode processes in solid-electrolyte systems. Ural Branch of the Academy of Sciences of USSR, Sverdlovsk, pp 85-95... 

The sample temperature was set using a special thermostat that could control the temperature in the interval 0-40 °C. In order to prevent the formation of condensate on the sample and electrodes, the entire test fixture (Agilent Model 1645IB) was arranged in a vacuum chamber (Figure 7.4) evacuated by forevacuum pump. The sample temperature was measured by a chromel-alumel thermocouple, one junction of which was fixed with an epoxy glue on the lower electrode, in the immediate vicinity of the sample. The second junction was placed into a water-ice mixture at 0 ° C. The thermo emf in the thermocouple was measured by a digital millivoltmeter. 

Phase Coefficient of thermo-emf (abs. values), V/deg Accu- racy ( ). fiV/deg Ref. Year Remarks... 

Structural and physical properties of solid solutions Ce Sm B were studied by Aivazov et al. (1980) on a series of samples prepared by borothermal reduction of the mixed oxides at 1700°C in vacuum. X-ray and chemical analysis revealed a continuous solid solubility of SmBj, to CeB i, CaB -type, Pm3m (table 4). Magnetic susceptibilities (80-1000 K 3, 5, 10 kOe) revealed Sm-ions to be present in different valence states. Hall emf, electrical conductivity and thermo-emf were also measured on specimens obtained by hotpressing at 1900-2000°C, 500 kg/cm in vacuum. 

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