High superionic conduction arising from aligned large lamellae and large figure of merit in bulk Cu1.94Al0.02Se

Abstract

Good thermoelectric materials should have low thermal conductivity, high electrical conductivity, and Seebeck coefficient, which cannot be easily balanced in bulk materials. Exceptionally, the super-ionics in β-Cu2Se can favorably contribute large ionic electrical conductivity and a liquid-like thermal conductivity by Cu+ ions. In the previous work, the superionic mechanism was found to be enhanced by small and randomly orientated lamellae with alternating ordered Se ion monolayer and disordered Cu ion bilayers. Here, we further enhance the superionic mechanism by increasing and better aligning lamellae in bulk Cu1.94Al0.02Se, resulting in a large thermoelectric figure of merit of 2.62 at 756 °C.

High-performance thermoelectric Cu2Se nanoplates through nanostructure engineering

“…As can be seen, our κ L of β-Cu 2 Se is relatively independent on the temperature ranging from 400 K to 850 K, and is much lower than that (0.4-0.6 Wm -1 K -1 ) of bulk β-Cu 2 Se [13]. Such a low κ L is not only attributed by the liquid-like behavior from the superionic Cu + ions, which strongly scatter the phonons [13,15,22,23], but also contributed by the nanostructure engineering (discuss later). Such an improved S and the very low κ found in our sintered Cu 2 Se pellets finally lead a high ZT of 1.82±0.05 at 850 K, as shown in Figure 2d and S1).…”

High Performance Thermoelectric Materials: Progress and Their Applications

“…Through rational design, all-scale hierarchical architectures [67,85,98,[198][199][200][201] can be realized to provide full-spectrum phonon scattering which can dramatically improve the TE performance ( Figure 6b). The principle of all-scale hierarchical architectures includes (1) using point defects [149] (including substitution atoms) to scatter phonons with short wavelengths; (2) using grain boundaries, [202] dislocations (dislocation arrays), [85,203] and lamellar/multilayer [204] structures to scatter phonons with intermediate wavelengths; and (3) using nanomesoscale grains [70,72,193,195,196] or inclusions to scatter phonons with long wavelengths, so that the overall κ L can be reduced closer to the amorphous limit. [48,70,71,85,118] By providing all-scale phonon scattering, an ultrahigh ZT of 2.2 at 915 K in Na-doped p-type PbTe (endotaxially nanostructured with SrTe) [67] was obtained due to the low κ L .…”

“…Cu 2 Se adopts polymorphic αand β-phases at different temperature ranges, among which α-Cu 2 Se is the thermodynamically favored phase at room temperature with a monoclinic crystal structure (a = 0.7138 nm, b = 1.238 nm, c = 2.739 nm, β = 94.308°) while β-Cu 2 Se is the kinetically stable phase with a lattice parameter a = 0.58 nm for bulk Cu 2 Se. [103] Bulk Cu 2 Se is an intrinsic p-type semiconductor with a band gap of ≈1.23 eV, [42,69,103,204,228,235] and shows a peak ZT of ≈1.5 with a high S 2 σ up to 12 μW cm −1 K −1 at 1000 K. [42] The phase transition from α-Cu 2 Se to β-Cu 2 Se can result in a very high ZT (>2) in I-doped Cu 2 Se. During the phase transformation, Se ions formed a face-center-cubic frame and Cu + ions partially occupied the interstitial sites, these Cu + ions are highly mobile and exhibit superionic liquid-like behavior, which is crucial for its intrinsically low κ L because the phonons can be strongly scattered by such liquid-like ions and finally lead to a high ZT of Cu 2 Se.…”

“…[224] The phase transformation takes place at ≈400 K, which is reversible through cooling or heating. [69] Zhong et al [204] reported an peak ZT ≈2.6 of bulk Cu 1.94 Al 0.02 Se at around 1000 K due to the aligned lamellae structure. [48] Up to now, Cu 2 Se-based materials have been fabricated by various methods including solid state reaction method, [42,69,[225][226][227][228] ultrasonic chemical method, [229] solvothermal method, [48,230] wet chemistry method, [231,232] Schlenk line techniques, [233,234] and self-propagating high-temperature synthesis.…”

Improvement of thermoelectric properties and their correlations with electron effective mass in Cu1.98SxSe1−x

“…As for the Cu 2− x Se system, the doping approach, using such elements as Ag, Sb, Al, and Sn for the Cu sites232425262728 and Te and I for the Se sites2129, has been chosen to modify its electronic structures and thermal conductivity up to now. The results illustrate, however, that only a small amount doping with one of these elements could lead to limited improvements to the thermoelectric performance in this system.…”