Features in thermoelectric properties of non-textured and textured Bi 1.9 Gd 0.1 Te 3 compounds are analysed. Cold isostatic pressuring was applied to prepare non-textured samples, whereas textured samples were fabricated via spark plasma sintering. The same starting powder was used for both preparation methods. Texturing [001] axis coincided with direction of spark plasma sintering pressuring. Thermoelectric properties of non-textured sample are isotropic, that is due to random grains orientation. Strong anisotropy in electrical resistivity and thermal conductivity, measured in directions parallel and perpendicular to direction of spark plasma sintering pressuring was found for textured sample. Texturing is partially recovering anisotropy inherent to single-crystalline bismuth telluride via redistributing anisotropic contributions from crystal a-b plane and c-axis into thermoelectric properties. Electrical resistivity decreases and thermal conductivity increases for parallel measuring orientation as compared to these properties for perpendicular measuring orientation. Highest thermoelectric figure-of-merit (~0.75 at~420 K) was observed for textured sample for perpendicular measuring orientation.

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Elemental composition, crystal and grain structures, specific electrical resistivity, Seebeck coefficient, thermal conductivity, and thermoelectric figure-of-merit of n-type grained Bi 1.9 Gd 0.1 Te 3 compounds, spark-plasma-sintered at T S = 690, 720, 750, 780 and 810 K, have been studied. All the samples are highly textured along the 001 direction parallel to the pressing direction. The average grain size measured along the pressing direction is much less as compared to the average grain size measured in the perpendicular direction. A strong anisotropy in the transport properties measured along directions parallel and perpendicular to the pressing direction was found within the 290 ÷ 630 K interval. Electrical resistivity decreases and thermal conductivity increases for parallel orientation as compared to these properties for perpendicular orientation. The T S -effect on thermoelectric figure-of-merit of textured Bi 1.9 Gd 0.1 Te 3 compounds has been found and analyzed. Highest thermoelectric figure-of-merit (∼0.75) was observed for sample with T S = 750 K at perpendicular orientation.

Spark plasma sintering method was applied to prepare bulk n-type Bi 1.9 Lu 0.1 Te 2.7 Se 0.3 samples highly textured along the 001 direction parallel to the pressing direction. The texture development is confirmed by X-ray diffraction analysis and scanning electron microscopy. The grains in the textured samples form ordered lamellar structure and lamellar sheets lie in plane perpendicular to the pressing direction. The average grain size measured along the pressing direction is much less as compared to the average grain size measured in the perpendicular direction (∼50 nm against ∼400 nm). A strong anisotropy in the transport properties measured along directions parallel and perpendicular to the pressing direction within the 290 ÷ 650 K interval was found. The specific electrical resistivity increases and the thermal conductivity decreases for the parallel orientation as compared to these properties for the perpendicular orientation. The Seebeck coefficient for both orientations is almost equal. Increase of the electrical resistivity is stronger than decrease of the thermal conductivity resulting in almost three-fold enhancement of the thermoelectric figure-of-merit coefficient for the perpendicular orientation (∼0.68 against ∼0.24 at ∼420 K). The texturing effect can be attributed to (i) recovery of crystal structure anisotropy typical for the single crystal Bi 2 Te 3 -based alloys and (ii) grain boundary scattering of electrons and phonons. An onset of intrinsic conductivity observed above T d ≈ 410 K results in appearance of maxima in the temperature dependences of the specific electrical resistivity, the Seebeck coefficient and the thermoelectric figure-of-merit coefficient and minimum in the temperature dependence of the total thermal conductivity. The intrinsic conductivity is harmful for the thermoelectric efficiency enhancement since thermal excitation of the electron-hole pairs reduces the Seebeck coefficient and increases the thermal conductivity.

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Medium-entropy BiSbTe 1.5 Se 1.5 alloy has been prepared by self-propagating high-temperature synthesis (to prepare a starting powder with desired composition and structure) and spark plasma sintering (to prepare block-textured samples). Under texturing, a partial ordering of grains, which is typical for Bi 2 Te 3 -based alloys, takes place resulting in forming a lamellar grain structure. Lamellar sheets are not continuous for whole volume of the textured sample. There are blocks with continuous lamellar sheets of some definite orientation, but the orientations of the sheets in neighboring blocks are different from each other. Forming the block-textured structure can be related to specific features of the starting powder, applied to sinter the bulk samples. The starting powder was strongly inhomogeneous and particles in the starting powder were rather big and shape-isotropic. As result, the texturing can be initiated in local domains of volume independently from each other resulting in forming the blocks with different preferential grains orientation. Due to the block texturing, the thermoelectric properties of the BiSbTe 1.5 Se 1.5 alloy, measured perpendicularly or parallel to a texturing axis, are different. Many features, found in these properties, are typical for textured Bi 2 Te 3 -based alloys. The thermoelectric properties of the medium-entropy block-textured BiSbTe 1.5 Se 1.5 alloy can be believed to be promising enough. The highest thermoelectric figure-of-merit equal to ~0.43 was observed for the perpendicular measuring orientation. This alloy can be next applied as a precursor for developing five-or six-element high-entropy alloys with enhanced thermoelectric efficiency.