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.
thermoelectrics of n-type conductivity have been prepared by the microwave-solvothermal method and spark plasma sintering. These compounds behave as degenerate semiconductors from room temperature up to temperature T d % 470 K. Within this temperature range the temperature behavior of the specific electrical resistivity is due to the temperature changes of electron mobility determined by acoustic and optical phonon scattering. Above T d , an onset of intrinsic conductivity takes place when electrons and holes are present. At the Lu and Tm doping, the Seebeck coefficient increases, while the specific electrical resistivity and total thermal conductivity decrease within the temperature 290-630 K range. The increase of the electrical resistivity is related to the increase of electron concentration since the Tm and Lu atoms are donor centres in the Bi 2 Te 3 lattice. The increase of the density-of-state effective mass for conduction band can be responsible for the increase of the Seebeck coefficient. The decrease of the total thermal conductivity in doped Bi 2 Te 3 is attributed to point defects like the antisite defects and Lu or Tm atoms substituting for the Bi sites. In addition, reducing the electron thermal conductivity due to forming a narrow impurity (Lu or Tm) band having high and sharp density-ofstates near the Fermi level can effectively decrease the total thermal conductivity. The thermoelectric figure-of-merit is enhanced from $ 0.4 for undoped Bi 2 Te 3 up to $ 0.7 for Bi 1.
The Lu x Bi 2−x Te 3 thermoelectrics with x=0, 0.05, 0.1 and 0.2 have been prepared by microwavesolvothermal method and spark plasma sintering. All the compositions are semiconductors of n-type conductivity. It was found that electron concentration increases and electron mobility decreases with x increasing. The Lu doping results in remarkable increase of the thermoelectric figure-of-merit from ∼0.4 for undoped Bi 2 Te 3 up to ∼0.9 for Bi 1.9 Lu 0.1 Te 3 . Enhancing the thermoelectric efficiency at the doping is originated from: (i) increase of the electron concentration since the Lu atoms behave as donors in the Bi 2 Te 3 lattice that decreases the specific electrical resistance, (ii) increase of the Seebeck coefficient via increase of the density-of-states effective mass for conduction band, (iii) decrease of the total thermal conductivity via forming the point defects like antisite defects and Lu atoms substituting for the Bi sites. Formation of narrow non-parabolic impurity band lying near the Fermi energy with sharp density of states is believed to be responsible for increasing the density-of-states effective mass and decreasing the electron contribution to the total thermal conductivity.
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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