Revisiting some chalcogenides for thermoelectricity

Abstract

Thermoelectric materials that are efficient well above ambient temperature are needed to convert waste-heat into electricity. Many thermoelectric oxides were investigated for this purpose, but their power factor (PF) values were too small (∼10^-4 W m^-1 K^-2) to yield a satisfactory figure of merit zT. Changing the anions from O^2- to S^2- and then to Se^2- is a way to increase the covalency. In this review, some examples of sulfides (binary Cr-S or derived from layered TiS₂) and an example of selenides, AgCrSe₂, have been selected to illustrate the characteristic features of their physical properties. The comparison of the only two semiconducting binary chromium sulfides and of a layered AgCrSe₂ selenide shows that the PF values are also in the same order of magnitude as those of transition metal oxides. In contrast, the PF values of the layered sulfides TiS₂ and Cu_0.1TiS₂ are higher, reaching ∼10^-3 W m^-1 K^-2. Apparently the magnetism related to the Cr-S network is detrimental for the PF when compared to the d⁰ character of the Ti⁴⁺ based sulfides. Finally, the very low PF in AgCrSe₂ (PF = 2.25 × 10^-4 W m¹ K^-2 at 700 K) is compensated by a very low thermal conductivity (κ = 0.2 W m^-1 K^-1 from the measured C_p) leading to the highest zT value among the reviewed compounds (zT_700K = 0.8). The existence of a glassy-like state for the Ag⁺ cations above 475 K is believed to be responsible for this result. This result demonstrates that the phonon engineering in open frameworks is a very interesting way to generate efficient thermoelectric materials.

Keywords: chromium selenide; chromium sulfide; thermoelectricity; titanium sulfide.

Figure 1

Structural models for Cr₂S₃ (left) and Cr₅S₆ (right). The stacking of S layers creates octahedral voids which are fully occupied to form a dense CrS layer of CrS₆ edge-shared octahedra which alternates with a partially filled layer, Cr_1/3S (left) and Cr_2/3S (right) for Cr₂S₃ and Cr₅S₆, respectively.

Figure 2

Structural models for Cr₂S₃. Two sequences, 1–2–3 and 1–2–1–2, are obtained along the stacking direction c, corresponding to the rhombohedral and trigonal forms, respectively.

Figure 3

Structures of Cu_xTiS₂ (left) and AgCrSe₂ (right). Both structures can be described as a stacking of CdI₂-type layers, TiS₂ or CrSe₂ (Ti or Cr cations are at the centers of the green octahedra), with Cu cation in the van der Waals spacing (blue octahedra) or Ag in tetrahedral coordination (blue tetrahedra), respectively.

Figure 4

Temperature-dependent magnetic susceptibility (χ = M/H) for Cr₂S₃ before (rhombohedral R-Cr₂S₃, left y-axis) and after SPS (trigonal T-Cr₂S_2.8, right y-axis). The measurements were carried out in the zero-field cooling mode.

Figure 5

(a) Normalized electrical resistivity (ρ) as a function of temperature for both Cr₂S₃ samples, rhombohedral R-Cr₂S₃ and trigonal T-Cr₂S_2.8. The data collected upon cooling in H = 0 and 7 T reveal the existence of a negative magnetoresistance in the T_N region for the rhombohedral precursor R-Cr₂S₃. Inset: magnetic field driven magnetization (M) for both Cr₂S₃ samples (T = 100 K). (b) Magnetic field (H) driven normalized resistivity collected at 100 K for both Cr₂S₃ samples.

Figure 6

Temperature dependence of the ac magnetic susceptibility (χ′, real part) for Cr₅S₆, before and after SPS (μ₀H_ac = 10 Oe, f = 100 Hz).

Figure 7

Normalized electrical resistivity for Cr₅S₆ (left y-axis). The corresponding dρ/dT = f(T) curve is also given for the SPS sample (right y-axis).

Figure 8

Top panel: magnetic field (H) driven resistance ratio for Cr₅S₆. The temperatures are indicated in the bottom panel where the corresponding M(H) curves are given. The measurements were performed with current perpendicular to H.

Figure 9

Temperature-dependent specific heat C of the Cr₅S₆ precursor measured upon warming (right y-axis) and corresponding χ(T) curves measured upon cooling (c) and warming (w) after zero-field-cooling (zfc) and field-cooling (fc) processes (left y-axis).

Figure 10

Temperature-dependent Seebeck coefficient (S) for the precursor and SPS-treated Cr₂S₃ (a) and Cr₅S₆ (b).

Figure 11

Temperature dependences of ρ and S for SPS-treated Cr₅S₆.

Figure12

SPS-densified Cr₅S₆: PF and κ versus temperature.

Figure 13

Trigonal densified Cr₂S₃ sample T-Cr₂S_2.8. Thermal conductivity (right y-axis), and zT (left y-axis) versus temperature.

Figure 14

Cu_xTiS₂ (x = 0.00 and x = 0.10). Top panel: S(T) (solid symbols), ρ(T) (open symbols). Lower panel: PF(T), κ(T) [total and lattice part κ_ph].

Figure 15

AgCrSe₂: ρ(T), S(T), zT(T) and κ(T).

Figure 16

zT curves for Cu_xTiS₂ with x = 0 and 0.10.

Figure 17

Temperature dependence of specific heat for AgCrSe₂. The horizontal line indicates the Dulong Petit value 3Nk_B.