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Документ Effect of aging on thermoelectric properties of the Bi2Te3 polycrystals and thin films(Науково-технологічний комплекс "Інститут монокристалів", 2021) Rogacheva, E. I.; Doroshenko, A. N.; Novak, K. V.; Sipatov, A. Yu.; Khramova, T. I.; Saenko, S. A.The temperature dependences (77-300 K) of the thermoelectric (TE) properties (the Seebeck coefficient S, electrical conductivity σ, Hall coefficient RH, Hall charge mobility μH>, and TE power factor P) were studied for freshly prepared and for exposed to air at room temperature during 5 years p-Bi2Te3 (60.0 at.% Te) and n-Bi2Te3 (62.8 at.% Te) polycrystals and thin films grown from them by thermal evaporation in vacuum. It was found that after aging, in the p- and n-Bi2Te3 bulk crystals and in the n-type film obtained from the n-Bi2Te3 crystal, type of conductivity is reserved but the p-type film obtained from the p-Bi2Te3 crystal, change the type of conductivity from hole to electronic. The activation energies of possible defect states were determined using the RH(T) dependences. After aging, at the temperatures close to room temperature, the p values of n-Bi2Te3 and p-Bi2Te3 polycrystals decreases by ~ 20 %, but p values of the n-type film grown from n-Bi2Te3 crystal increases by 20-30 %. In the p-type film obtained from p-Bi2Te3 polycrystal, and having changed the type of conductivity after aging, the p values exceed the p values of a film obtained from n-Bi2Te3 polycrystal by ~ 35 % at 250 K and by 25 % at 300 K, remaining at these temperatures below the p values for n-Bi2Te3 polycrystal after aging by ~ 15 %.Документ Effect of deviation from stoichiometry on transport and mechanical properties of Bi2Se3 polycrystals(AIP Publishing, 2021) Menshikova, S. I.; Rogacheva, E. I.The dependences of electrical conductivity, the Hall coefficient, the Seebeck coefficient, thermoelectric power factor and microhardness of Bi2Se3 polycrystals on the degree of deviation from stoichiometry 59.9–60.0 at. % Se and temperature (77–300 K) were obtained. The samples exhibited n-type conductivity in the studied ranges of compositions and temperatures. The boundaries of the Bi2Se3 homogeneity region were estimated. A non-monotonic behavior of the concentration dependences of the properties in the studied composition range, associated with a change in the phase composition and defect structure under the deviation from stoichiometry, was observed. The calculation of the power coefficient in the temperature dependence of electron mobility showed that in a stoichiometric Bi2Se3, electrons are predominantly scattered by acoustic phonons, and under the deviation from stoichiometry the contribution of impurity scattering increases. Based on the experimental data, the Fermi energy EF was calculated in the approximation of the relaxation time and within the framework of the single-band model with a quadratic dispersion law. The obtained values of EF showed that both in stoichiometric and non- stoichiometric Bi2Se3, the conduction is mainly due to electrons in the lower conduction subband.Публікація Percolation transition and physical properties of Bi1-xSbx solid solutions at low Bi concentration(Elsevier Ltd, 2020) Rogacheva, E. I.; Doroshenko, A. N.; Khramova, T. I.; Nashchekina, O. N.; Fedorov, A. G.; Mateychenko, P. V.The dependences of microhardness H, electrical conductivity σ, charge carrier mobility μH, the Seebeck coefficient S, and thermoelectric power factor P = S2σ on the composition of Bi1-xSbx solid solutions in the vicinity of pure Sb (x = 1.0–0.975) were obtained. In the range of x = 0.9925–0.9875, an anomalous decrease in H and S and increase in σ and μH with increasing Bi concentration were observed. For all the alloys, the dependences of H on the load on an indenter G were plotted. It was found that the H(G) dependences for samples with x smaller than ~ 0.99 and for samples with x exceeding 0.99, exhibit different behavior. The results obtained are interpreted on the basis of our assumption about the existence of a percolation-type phase transition from impurity discontinuum to impurity continuum that occurs in any solid solution.Публікація Percolation effects in semiconductor IV-VI based solid solutions and thermoelectric materials science(Scrivener Publishing LLC, 2019) Rogacheva, E. I.; Nashchekina, O. N.The chapter reviews the present status of our experimental studies aimed at con firming the existence of anomalies in the dependences of thermoelectric (TE) and other properties on composition in semiconductor IV-VI–based solid solutions in the region of small impurity concentration. For all studied solid solutions, we observed an anomalous behavior of the isotherms of electrical conductivity, phonon thermal conductivity, heat capacity, charge carrier mobility, microhardness, the Seebeck coefficient, and TE figure of merit in the vicinity of (0.5–1) at. % of impurity. We believe that the observed phenomenon is connected with the existence of percolation-type phase transitions corresponding to the transition from dilute solid solutions to the impurity continuum and is accompanied by self-organization processes. The non-monotonic behavior of the concentration dependences of the TE properties and the underlying percolation phenomena should be taken into consideration when developing efficient TE materials and optimizing their properties. To get additional confirmation of the assumption regarding the universal character of phase transitions connected with the formation of the impurity continuum, it is necessary to expand the range of objects to be studied as well as spectrum of properties to be measured.Публікація Percolation effects and self-organization processes in cold-pressed Bi2(Te1−xSex)3 solid solutions(Elsevier Ltd, 2021) Rogacheva, E. I.; Martynova, E. V.; Shelest, T. N.; Doroshenko, A. N.; Nashchekina, O. N.It was established that the dependences of thermoelectric and mechanical properties of cold-pressed Bi2(Te1−xSex)3 alloys on composition (x = 0–0.07) exhibit a non-monotonic behavior in certain concentration ranges: an anomalous decrease in the Seebeck coefficient, thermoelectric power factor, and microhardness, and increase in electrical conductivity with increasing x. We observed similar anomalies earlier for cast Bi2(Te1−xSex)3 alloys and explained them by percolation and self-organization phenomena. Thus, the existence of the anomalies does not depend on the method of sample preparation. However, in pressed samples as compared to cast ones conductivity type changes from p to n and thermoelectric power factor increases.Публікація Temperature and magnetic field dependences of thermoelectric properties of Bi1-xSbx solid solutions in the range x = 0-0.25(Elsevier Ltd, 2021) Rogacheva, E. I.; Doroshenko, A. N.; Nashchekina, O. N.Bi1–xSbx solid solutions are the best n-type thermoelectric materials for use at temperatures ≤200 K. An important parameter determining material’s figure of merit is charge carrier concentration. To determine it correctly, one should carry out measurements in a weak magnetic field. On the basis of the magnetic field dependences of the Hall coefficient and magnetoresistance, the dependences of the weak magnetic field boundary Bc on composition (x = 0–0.25) and temperature (T = 77–300 K) for polycrystalline Bi1–xSbx alloys were plotted. It was established that the Bc(x) dependences exhibit a non-monotonic behavior which is attributed to the existence of electronic phase transitions.Документ Quantum size effects and thermoelectric properties of IV-VI-based 2D-nanostructures(Національна академія наук України, 2019) Rogacheva, E. I.Публікація Concentration dependences of galvanomagnetic and thermoelectric properties of Bi1-xSbx thin films in the range x = 0 – 0.25(Національна академія наук України, 2019) Rogacheva, E. I.; Doroshenko, A. N.; Sipatov, A. Yu.; Nashchekina, O. N.Публікація Percolation effects and self-organization processes in Bi₂(Te₁₋ₓSeₓ)₃ solid solutions(Easy Conferences Ltd, 2019) Rogacheva, E. I.; Shelest, T. N.; Martynova, E. V.; Doroshenko, A. N.; Nashchekina, O. N.Публікація Temperature and magnetic field dependences of thermoelectric properties of Bi1-xSbx solid solutions in the range x = 0 – 0.25(Easy Conferences Ltd, 2019) Rogacheva, E. I.; Doroshenko, A. N.; Nashchekina, O. N.