Microstructural Control and Thermoelectric Properties of Misfit Layered Sulfides (LaS)_1+mTS₂ (T = Cr, Nb): The Natural Superlattice Systems

Abstract

We investigated the high-temperature thermoelectric properties of misfit layered n-type (LaS) 1.20 CrS 2 and p-type (LaS) 1.14 NbS 2 . The samples were prepared by CS 2 sulfurization of 6 or 12 h duration and then consolidated using pressure-assisted sintering to produce randomly and highly oriented samples whose microstructures were tunable. Transmission electron microscopy analysis showed that perfectly layered structures containing some stacking faults had formed. The randomly and highly oriented natural superlattices provided ultralow lattice thermal conductivities (as low as ∼0.9 and ∼0.5 W K −1 m −1 , respectively, at 950 K) perpendicular to the pressing axis. The improved electrical conductivities of the oriented CrS 2 and NbS 2 samples resulted in high power factors of 170 and 410 μW K −2 m −1 , respectively. The highly oriented texture produced the highest thermoelectric figure of merit ZT of 0.14 at 950 K among the (LaS) 1.20 CrS 2 system, whereas the weakly/randomly oriented texture produced the highest ZT of 0.15 at 950 K among the (LaS) 1.14 NbS 2 system. These misfit layered sulfides exhibit phonon glass−electron crystal behavior and provide tremendous opportunities for further enhancing ZT by optimizing the thermoelectric properties.

Hierarchical Architecturing for Layered Thermoelectric Sulfides and Chalcogenides

“…A mixture of CS 2 and Ar gases was introduced into the quartz tube as soon as the powders reached 573 K. The CS 2 gas was obtained by bubbling Ar carrier gas through CS 2 liquid. Single phases of TiS 2 , [LaS] 1.20 CrS 2 , and [LaS] 1.14 NbS 2 were successfully prepared through CS 2 sulfurization at 1073 K for 4–12 h [29,32]. …”

“…The micrographs reveal a dense structure and well-organized microtexture, indicating that the pressurized sintering promotes densification and grain orientation. The XRD patterns show that the crystalline c -axis is preferentially oriented along the pressing (out-of-plane) direction [29,32,34]. A centrifugal heating technique also provides the dense samples with highly oriented microtexture [56].…”

“…Scanning electron micrographs of the fractured section of sintered compacts of ( a ) Ti 1.008 S 2 [29]; ( b ) [LaS] 1.20 CrS 2 [32]; and ( c ) Pb 5 Bi 6 Se 14 [34]. …”

“…Crystal structures of the thermoelectric sulfides and chalcogenides addressed in this article: ( a ) TiS 2 [29,30]; ( b ) [LaS] 1.14 NbS 2 [31,32]; ( c ) Pb 5 Bi 6 Se 14 [33,34]; ( d ) SnSe [35]; and ( e ) Cu 12 Sb 4 S 13 [36,37]. Sizes of atoms in this figure are arbitrary.…”

High performance thermoelectrics for power generation using earth-abundant and low toxicity elements

“…In this system, the charge carrier type (p or n) is tunable through the change in the chemical composition. [19][21] p-type Chevrel-phase sulfides exhibit high potential for high-temperature applications. [22][23] However, as summarized in Fig.…”