Anisotropic thermoelectric properties of Bi1.9Lu0.1Te2.7Se0.3 textured via spark plasma sintering

Abstract

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.

Comparative analysis of the thermoelectric properties of the non-textured and textured Bi1.9Gd0.1Te3 compounds

“…In the grained thermoelectrics based on bismuth telluride, Bi 2 Te 3 , the grain boundaries are usually acting as n-type dopants, enhancing electron concentration [24][25][26]. Moreover, some thermoelectrics having a layered crystal structure, like Bi 2 Te 3 and Bi 2 Te 3 -based alloys [27][28][29][30][31][32][33][34][35][36][37][38], SnSe [39][40][41], BiCuSeO [42], Ca 3 Co 4 O 9 [43,44], are readily textured via technological processes based on uniaxial pressuring a starting powder. For the textured samples, a preferential orientation of the grains takes place.…”

Термоэлектрические свойства твердых растворов Bi-=SUB=-2-x-=/SUB=-Lu-=SUB=-x-=/SUB=-Te-=SUB=-2.7-=/SUB=-Se-=SUB=-0.3-=/SUB=-

“…В соответствии с выражением (3) изменение S образцов Bi 2−x Lu x Te 2.7 Se 0.3 с различными x может быть связано с изменением концентрации электронов, фактора рассеяния, а также эффективной массы плотности состояний для электронов (например, при формировании примесной, связанной с Lu зоны с резким изменением плотности состояний вблизи уровня Ферми). Широкие максимумы, наблюдаемые на кривых S(T ) при температуре ∼ 480 K, связаны с наступлением при этой температуре в исследуемых образцах собственной проводимости [18]. В этом случае имеет место термическая генерация электронно-дырочных пар, причем коэффициент Зеебека для дырочной проводимости будет уже положительный.…”

Microstructure and thermoelectric properties of the medium-entropy block-textured BiSbTe1.5Se1.5 alloy

“…The bismuth telluride itself is widely applied to prepare n-Bi 2 Te 3-x Se x and p-Bi 2-x Sb x Te 3 alloys for low-temperature thermoelectric applications [17,18]. It is known that the Bi 2 Te 3 -based alloys are readily texturing under uniaxial pressuring [19][20][21][22][23][24][25]. Moreover, the texturing is one of key parameters affecting the thermoelectric properties of these materials, since it results in a partial recovering of natural crystal anisotropy in the properties inherent to single-crystalline Bi 2 Te 3 [26].…”

“…To texture the alloy being studied, spark plasma sintering (SPS) was applied. The SPS method is often applied to prepare various Bi 2 Te 3 -based materials with improved and governed thermoelectric properties [19,20,22,31,32].…”