Bi2Te3-Sb2Te3 Solid Solutions Research Articles

Features in the behavior of temperature dependences of magnetic susceptibility of crystals of Bi-=SUB=-2-=/SUB=-Te-=SUB=-3-=/SUB=--Sb-=SUB=-2-=/SUB=-Te-=SUB=-3-=/SUB=- solid solutions containing from 25 to 70 percent antimony telluride

Temperature dependences of the magnetic susceptibility of p-type crystals of Bi2Te3-Sb2Te3 solid solutions containing from 25 to 70 mol percent antimony telluride were examined in the range from 2 to 400 K. A correlation was found between the features observed in the behavior of temperature dependences of the magnetic susceptibility of crystals containing 60 and 70 mol percent Sb2Te3 and the ratio of the plasmon energy to the energy of the gap between the chemical potential level and the heavy hole subband. An assumption is formulated that in the case of convergence of these energies, the increasing electron--plasmon interaction affects the electron system state and, consequently, the magnitude of magnetic susceptibility. Keywords: bismuth and antimony tellurides, magnetic susceptibility, plasmon, interband transitions, electron--plasmon interaction.

Revisiting the Nature of the Anomalous Temperature Dependence of the Hall Coefficient Observed for Semiconductor Crystals of Bi2Te3–Sb2Te3 Solid Solutions

Revisiting the Nature of the Anomalous Temperature Dependence of the Hall Coefficient Observed for Semiconductor Crystals of Bi2Te3–Sb2Te3 Solid Solutions

Magnetic susceptibility of Bi2 − x Sb x Te3 (0 < x < 1) solid solution crystals in the range from 2 to 400 K

The magnetic susceptibility of single crystals of Bi2Te3-Sb2Te3 solid solutions containing 10, 25, and 50 mol % Sb2Te3 has been measured using a superconducting Josephson interferometer in the temperature range from 2 to 400 K at 3-K intervals in magnetic fields of up to 2.38 × 106 A/m at two orientations of the magnetic field H with respect to the trigonal axis C3 of the crystals (H ‖ C3 and H ⊥ C3) with a relative uncertainty within 2%. A new approach has been proposed for separately assessing the contributions of the ionic core and free charge carriers to the magnetic susceptibility, which uses experimental data on magnetic susceptibility and effective mass anisotropy. Analysis of the data obtained in this study demonstrates that the temperature variation of carrier susceptibility in the region of degenerate and nondegenerate states of the electronic system can be described in terms of the Pauli and Landau-Peierls approaches if one takes into account the complex structure of the valence band and “nonparabolicity” of the light hole energy spectrum.

Process controls for Bi2Te3-Sb2Te3 prepared by mechanical alloying and hot pressing

p-Type Bi2Te3-Sb2Te3 solid solutions were prepared by mechanical alloying (MA) and hot pressing (HP) under different process conditions, after which the transport and the thermoelectric properties were evaluated. The relative densities of all hot-pressed specimens were over 98%, and the microstructure and crystal orientation were independent of the HP direction. All specimens exhibited p-type conduction, and the electrical resistivity was observed to increase slightly with increasing temperature, indicating a degenerate semiconductor behavior. The carrier concentration decreased with increasing HP temperature while the mobility increased. The maximum figure of merit obtained was 0.86 at 323 K for Bi0.5Sb1.5Te3 hot-pressed at 648 K.

Plasma screening of the fundamental absorption edge in crystals of Bi2Te3-Sb2Te3 solid solutions with more than 80 mol % Sb2Te3

The regularities of changes in the optical properties of crystals of Bi2Te3-Sb2Te3 solid solutions in the range of the effects caused by free-carrier plasma oscillations at a variation in the ratio of Bi2Te3 and Sb2Te3 components are investigated. It is established that, when the Sb2Te3 content in a solid solution exceeds 80 mol %, the fundamental absorption edge undergoes plasma screening. Doping a Sb2Te3 crystal with tin increases the free-carrier concentration and plasma frequencies, thus enhancing the screening effect, whereas introduction of 0.1 mol % selenium reduced the plasma frequency and thus removes the fundamental-absorption-edge screening.

Influence of grain size distribution on the lattice thermal conductivity of Bi2Te3-Sb2Te3-based nanostructured materials

The change of the lattice thermal conductivity of bulk nanostructured materials based on Bi2Te3-Sb2Te3 solid solutions with grain size distribution has been studied. These materials have a polycrystalline structure with grain sizes ranging from a few tens of a nanometer to a few micrometers. Large grains may contain inclusions or consist of several smaller parts which can be identified with coherent scattering regions seen in X-ray diffraction. The change of the lattice thermal conductivity mediated by additional scattering by inclusions and grain boundaries has been calculated. This calculation allows for the effect of nanoparticle size distribution. The calculated estimates are compared with the available experimental data.

Investigation of the possibilities for increasing the thermoelectric figure of merit of nanostructured materials based on Bi2Te3-Sb2Te3 solid solutions

The transport coefficients and thermoelectric figure of merit ZT for bulk nanostructured materials based on Bi2Te3-Sb2Te3 solid solutions have been investigated theoretically. Similar materials prepared by rapid quenching of the melt with the subsequent grinding and sintering contain amorphous and nanocrystalline regions with different sizes of particles. According to the performed estimations, the thermoelectric figure of merit of the amorphous phase can exceed the value of ZT for the initial solid solution by a factor of 2–3 primarily due to the significant decrease in the thermal conductivity. The effective transport coefficients of the medium as a whole have also been investigated as a function of the parameters of each phase, and the concentration range of the amorphous phase, which corresponds to the effective values ZT > 1, has been determined.

Energy filtration of charge carriers in a nanostructured material based on bismuth telluride

The dependences of the electrical conductivity and thermopower on the size of grains in a nanocrystalline material based on Bi2Te3-Sb2Te3 solid solutions of the p type have been investigated theoretically and experimentally. The relaxation time in the case of hole scattering by nanograin boundaries in an isotropic polycrystal has been calculated taking into account the energy dependence of the probability of tunneling of charge carriers and the dependence of the scattering intensity on the nanograin size L n . A decrease in the probability of boundary scattering with an increase in the energy of charge carriers leads to an increase in the thermopower. The dependences of the thermopower and electrical conductivity on the nanograin size, which have been obtained taking into account the boundary scattering and scattering by acoustic phonons, are in good agreement with experimental data. For the material under consideration, the thermopower coefficient increases by 10–20% compared to the initial solid solution at L n = 20–30 nm. This can lead to an increase in the thermoelectric figure of merit by 20–40%, provided that the decrease in the electrical conductivity and the decrease in the lattice thermal conductivity compensate each other. Despite the absence of a complete compensation, it has been possible to increase the thermoelectric figure of merit for the samples under investigation to ZT = 1.10–1.12.

Optical functions of Bi2Te3-Sb2Te3 solid solutions in the range of plasmon excitation and interband transitions

The spectra of the optical functions of Bi2Te3-Sb2Te3 solid solutions recorded in the range of 350–4000 cm−1 exhibited features caused by the response of free-carrier plasma and interband transitions forming the fundamental absorption edge in these materials. An increase in the Sb2Te3 percentage in the solid solution to 80% is accompanied by discontinuities of the functional dependences of the parameters characterizing plasma oscillations and a sharp increase in the optical band gap Eg opt. The set of optical functions has been simulated in the adiabatic approximation taking into account the effect of plasmons and interband transitions and the general polarization background of the crystal. The results obtained make it possible to detail the pattern of change in the physical properties of Bi2Te3-Sb2Te3 solid solutions, which are of great practical importance for thermoelectric materials science.

Features of reflection spectra of single crystals of Bi2Te3-Sb2Te3 solid solutions in the region of plasma effects

Reflectance spectra of single crystals of Bi2Te3-Sb2Te3 solid solutions containing 0, 10, 25, 40, 50, 60, 65, 70, 80, 90, 99.5, and 100 mol % of Sb2Te3 have been studied in the range of 400–4000 cm−1 at the temperature T = 291 K and with orientation of the vector of the electric-field strength E perpendicular to the trigonal axis of the crystal C3 (E ⊥ C3). The shape of the spectra is characteristic of plasma reflection; the spectra include the features in the range 1250–3000 cm−1 corresponding to the optical band gap Eg opt. The features become more pronounced as the content of Bi2Te3 is increased to 80 mol % in the composition of the Bi2Te3-Sb2Te3 solid solution. A further increase in the content of Sb2Te3 is accompanied by discontinuities in the functional dependences of the parameters characterizing the plasma oscillations of free charge carriers on the solid-solution composition and also by a sharp increase in Eg opt.

Transport properties of Sn and SbI3 doped single crystal p-Bi2Te3

Bi2Te3-Sb2Te3 solid solutions, the most commonly used room temperature thremoelectrics, are characterized by inhomogeneities and defects of various types, and doping aggravates the situation with crystal homogeneity. However, several publications reported on unusual behaviour of Sn in Bi2Te3, specifically, on improvement of spatial distribution of Seebeck coefficient over a cleavage face. This work reports on measurements done on single crystals of Bi2Te3 doped with 0.5% Sn as grown by the Czochralski method using a floating crucible. To shift the Fermi level, further doping with the traditional iodine impurity, was introduced as SbI3. X-ray diffraction, ρ⊥(T) (4-point, low freq. A.C.), ρ⊥(4K, 0 < H < 8T || C3-axis), and Rhall measurements were carried out. Two different behaviours are observed: metallic-like resistivity vs. temperature and magnetoresistivity vs. magnetic field for 0.05%SbI3 and non-monotonic resistivity temperature dependence and quantum oscillations in the magnetoresistivity for 0.1%SbI3. The X-ray results and the quantum oscillations indicate the high quality of the single crystals despite the high content of 2 impurities. The observed results are explained by the existence of a band of quasi-local impurity states of Sn located on the background of Bi2Te3 valence band, whose energy levels are estimated.

Thermoelectric Properties of n-Type 90% Bi&lt;sub&gt;2&lt;/sub&gt;Te&lt;sub&gt;3&lt;/sub&gt;+10%Bi&lt;sub&gt;2&lt;/sub&gt;Se&lt;sub&gt;3&lt;/sub&gt; Thermoelectric Materials Produced by Melt Spinning Method and Sintering

N-type Bi2Te3-Sb2Te3 solid solutions doped with CdCl2 was prepared by melt spinning, crushing and vacuum sintering processes. Microstructure, bending strength and thermoelectric property were investigated as a function of the doping quantity from 0.03wt.% to 0.10wt.% and sintering temperature from 400oC to 500oC, and finally compared with those of conventionally fabricated alloys. The alloy showed a good structural homogeneity as well as bending strength of 3.88Kgf/mm2. The highest thermoelectric figure of merit was obtained by doping 0.03wt.% and sintering at 500oC.