The theoretical aspects of evaluating the electrical resistance of a thermoelectric leg–metal contact are considered. A physical model of such a contact and methods for calculating the main components of the contact resistivity, namely, the resistivity of the interfacial layer and the resistivity related to the transfer of charge carriers through a potential barrier at the boundary between a material of the thermoelectric leg and a metal, are proposed. The contact resistivity for thermoelectric legs made of Bi2Te3 based materials with deposited antidiffusion nickel layers is calculated. It was established that the contact resistivity in such thermoelements reaches a value from 0.25 × 10−6 to 2.5 × 10−6 Ω cm2 and depends on the temperature and interfacial layer thickness. It is demonstrated that the findings are in good agreement with the known experimental values of contact resistivity.
The field dependence of the power factor for a layered thermoelectric material with a closed Fermi surface in a quantizing magnetic field and at helium temperatures has been studied in the geometry where the temperature gradient and the magnetic field are perpendicular to the material layers. The calculations are carried out in the constant relaxation time approximation. In weak magnetic fields, the layered-structure effects are shown to manifest themselves in a phase retardation of power factor oscillations, increase of their relative contribution, and certain reduction of the power factor in whole. In high magnetic fields, there exists an optimal range, where the power factor reaches its maximum, with the corresponding value calculated for the chosen parameters of the problem in the effective mass approximation being by 12% higher than that for real layered crystals. Despite low temperatures, the power factor maximum obtained with those parameters in a magnetic field of 1 T has a value characteristic of cuprate thermoelectric materials at 1000 K. For this phenomenon to take place, it is necessary that the ratio between the free path of charge carriers and the interlayer distance should be equal to or larger than 30,000. However, in ultraquantum magnetic fields, the power factor drastically decreases following the dependence P ∝ T −3 B −6 . The main reason for this reduction is a squeeze of the Fermi surface along the magnetic field in the ultraquantum limit owing to the condensation of charge carriers on the bottom of a single filled Landau subband. K e y w o r d s: power factor, thermoelectric coefficient, quantizing magnetic field, Landau's subband, Fermi surface squeeze.
The longitudinal Seebeck coefficient of the charge-ordered layered crystals in a strong quantizing magnetic field normal to layers plane has been determined.The conditions whereby charge ordering parameter and chemical potential of charge carriers are the oscillating functions of the magnetic field induction are considered. The longitudinal Seebeck coefficient has been calculated for two models of the relaxation time: i) constant relaxation time and ii) the relaxation time proportional to the longitudinal velocity. It has been shown that in a quasi-classical region of magnetic fields for the case of the relaxation time proportional to the longitudinal velocity the longitudinal Seebeck coefficient is less than for the case of constant relaxation time. In this region, for selected problem parameters it does not exceed 4.37μV/K. In the strong quantizing magnetic fields for both models of the relaxation time the longitudinal Seebeck coefficient is virtually the same. For selected problem parameters its maximal modulus is 2033μV/K. At the same time, in the disordered layered crystals, in a quasiclassical region, the Seebeck coefficient is approximately one order of magnitude less than for the charge ordered crystals. In the strong magnetic fields, the Seebeck coefficient for the disordered layered crystals is factor of 7 to 9 less than for the charge-ordered crystals. However, in super strong magnetic fields, under current carriers concentration in the only filled Landau sub-band, for both models of the relaxation time the modulus of the Seebeck coefficient tends to zero according to asymptotic law 2 B zz . IntroductionAt the present time much attention is given to the development and study of the properties of new thermoelectric materials. The objects of experimental investigation are metals, alloys, semiconductors [1, 2], fullerenes [3], composites [4], including biomorphic [5], etc.Theory of thermoelectric properties of materials, including nanosystems, is being actively developed as well [6,7]. One of the first works on the theory of transverse Seebeck coefficients of metals in quantizing magnetic fields was performed by Kosevich and Andreyev [8].Many of the investigated materials, for instance, semiconductor systems of A II B VI C VII class, intercalated graphite compounds, synthetic metals, graphene, etc. belong in their crystal structure to layered materials. At the same time, the overwhelming majority of theoretical works dedicated to behaviour of such layered systems in quantizing magnetic fields are mainly concerned with the transverse galvanomagnetic effects. The author of this paper is aware of only one work which deals with the thermal conductivity of graphene in a quantizing magnetic field [9]. In so doing, its Fermi surface is considered to be open, that is, such which occupies the entire one-dimensional Brillouin zone and, with a periodic continuation, is a connected one, that is, represents a continuous corrugated cylinder.
A physical model of the sublimation of a volatile component from a thermoelectric material has been developed. On its basis, two versions of the mathematical description of the degradation process of thermoelectric material are presented. The first of them takes into account only the diffusion of tellurium as a volatile impurity to the evaporation surface, on which the pressure and, consequently, the concentration of atoms of the volatile component are considered to be known. The second explicitly takes into account the volatility of the evaporating component and, hence, the boundary flux on the evaporation surface. In both cases, an analytical solution of the one-dimensional diffusion equation is obtained taking into account the presence of a temperature gradient along the length of the leg. Further, by computer methods in the Mathcad environment, the time dependence of the thickness of the layer with a reduced concentration of the volatile component and the nature of the distribution of the concentration of this component in it was determined. On this basis, the degradation time of thermoelectric material due to the loss of volatile components is estimated and the requirements for the protective coating of thermoelectric branches are established.
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