Highly textured Bi2Te3-based materials for thermoelectric energy conversion

Abstract

This work is concerned with Bi2Te3-based compounds known as being highly effective materials for thermoelectric applications near room temperature. These compounds are characterized by a remarkable anisotropy linked to their R3¯m crystal structure. Two textured p-type Bi0.4Sb1.6Te3 samples were prepared using a powder metallurgy approach, with the c axis parallel to the pressing direction. One sample was undoped while the second was doped with Pb which acts as an acceptor. The electrical conductivity, Hall coefficient, and magnetoresistivity were measured from room temperature down to 6K. The Seebeck coefficient α and electrical conductivity σ were measured along and perpendicular to the c axis from 300 up to 550K, and the thermal conductivity κ was measured at 300K. Different values of Seebeck coefficient were observed along and perpendicular to the c axis at temperatures above Ti, the beginning of intrinsic region in which the influence of the minority carriers becomes significant. Below Ti, the Seebeck coefficient was isotropic. The maximal power factor P=α2σ, calculated on the basis of the experimental results, was about 40μWcm−1K−2 for the direction perpendicular to the c axis. The thermal conductivity values for the temperature domain above 300K were calculated on the basis of a physical model and the measured values at 300K. The calculated values of the figure of merit Z=P∕κ were 3×10−3 and 2×10−3K−1 at 300 and 400K for the two samples, respectively. These values are comparable to those observed in Bi2Te3-based single crystals, thus making the powder metallurgy approach appropriate for thermoelectric conversion applications.

Simulation of Morphological Effects on Thermoelectric Power, Thermal and Electrical Conductivity in Multi‐Phase Thermoelectric Materials

“…This is due to the fact, that in transverse to crystallographic c axis, TE power factor (numerator in ZT expression) is considerably higher, than in parallel to this direction, mainly due to higher electrical conductivity values. For powder metallurgy synthesized Bi 2 Te 3 -based materials, it was shown that moderate powder grinding pressures, might retain some of the crystallographic anisotropy, due to the weaker van der Waals bonding of atoms located in adjacent layers along c-axis, compared to ionic/covalent bonding between atoms located in each of the layers [31]. In this example, higher ZT values in transverse to powder pressing direction are expected as in single crystals.…”

Thermoelectric and thermomechanical properties of the hot pressed polycrystalline Bi0.5Sb1.5Te3 alloys

“…Since the rhombohedral Bi 2 Te 3 based alloys have a quasi-layered crystal structure, it is important to determine whether all these hot pressed samples are textured or not. The Orientation Factor F, which was used to quantitatively investigate the degree of preferred orientation of (0 0 l)-planes from the XRD results, is defined by the following formulae [22]:…”

Functionally Graded Bi<inf>2</inf>Te<inf>3</inf> based material for above ambient temperature application

“…It is noteworthy that due to the difference in the hole to electron mobility ratio parallel and perpendicular to the c-axis, [14] the influence of the minority carriers on the Seebeck coefficient in a pseudo-isotropic sample is deprived in comparison to a highly textured sample along the conversion direction. [7] The thermal conductivity κ of the pseudo-isotropic sample was deduced from the thermal conductivities of a highly textured sample along κ || and perpendicular κ ⊥ to the c-axis [7] as described in Airapetiants and Bresler model, [15] ) (1)…”

“…[5, 6] One must keep in mind, however, that samples prepared by powder metallurgy might not be perfectly oriented, and consequently their figure of merit may be reduced. On the other hand, by providing a strong preferred orientation (texture) [7] or taking advantage of the Functionally Graded (FG) concept, [8,9], the figure of merit values may be improved significantly, reaching levels comparable to those of single crystals.…”