Thermoelectric properties and investigations of low thermal conductivity in Ga-doped Cu2GeSe3

Abstract

In this study, we have synthesized a series of low thermal conductivity diamond-like materials with the general formula Cu 2 Ga x Ge 1-x Se 3 for 0 ≤ x ≤ 0.1, and their transport properties were evaluated to establish their suitability for TE based waste heat recovery applications. We report results for the Seebeck coefficient (S), electrical resistivity (ρ), thermal conductivity (κ), Hall coefficient (R H ), crystal structure, and elastic properties of Cu 2 Ga x Ge 1-x Se 3 for x = 0.01, 0.03, 0.05, 0.07 and 0.1. Powder x-ray diffraction revealed that a small amount of a related cubic polymorph appeared along with the orthorhombic parent phase at high Ga concentrations. This cubic phase is related to the parent phase in that both contain three-dimensional tetrahedral diamond-like substructures. All samples showed positive values of S and R H over the entire temperature range studied, indicative of p-type charge carriers. The largest value of S = 446 μVK -1 was observed at 745 K for undoped Cu 2 GeSe 3 . With increasing Ga content, both S and ρ decreased. Low values of κ were observed for all samples with the lowest value of κ = 0.67 W m -1 K -1 at 745 K for undoped Cu 2 GeSe 3 . This value approaches the theoretical minimum thermal conductivity for these materials at high temperatures. Although this diamondlike material has highly symmetric, lower coordination number tetrahedral bonding, an unusually large Grüneisen parameter (γ), a measure of bonding anharmonicity, was observed for Cu 2 Ga 0.1 Ge 0.9 Se 3 . A value of γ = 1.7 was calculated from the measured values of the elastic properties, heat capacity, and volume thermal expansion. Given the fact that all materials investigated have similar elastic property values and likely comparable coefficients of thermal expansion we surmise that this large Grüneisen parameter is a general feature for this material system. We conclude that this high level of anharmonicity gives rise to enhanced phononphonon scattering that is, in addition to the scattering brought about by the disordered structure, resulting in very low values of thermal conductivity.

High thermoelectric performance of oxyselenides: intrinsically low thermal conductivity of Ca-doped BiCuSeO

“…22 The ZT value of Bi 0.925 Ca 0.075 CuSeO at 873 K is also higher than that of 0.7 for Bi 0.90 Sr 0.10 CuSeO at 873 K. 21 The present ZT of 0.9 is higher than most thermoelectric materials with intrinsically low thermal conductivity reported so far. [13][14][15][16][17][18][19][20] Therefore, it can be concluded that the higher thermoelectric performance of BiCuSeO is mainly due to the intrinsically low thermal conductivity of the material.…”

“…44 Indeed, BiCuSeO exhibits a lower sound velocity, 23 Young's modulus and lattice thermal conductivity at 300 K than other oxides, 40 In addition to the analysis of Young's modulus (E), which is related to the chemical bonding strength, the Gruneisen parameter (g) is also relevant in evaluating the intrinsically low thermal conductivity. 11,18 The Gruneisen parameter (g) is often referred to as a temperaturedependent anharmonicity parameter that reflects how much phonon vibrations in a crystal lattice deviate from harmonic oscillations. Anharmonicity of the chemical bond drives the phonon-phonon umklapp and normal processes that limit the lattice thermal conductivity.…”

“…As the thermal conductivity values are very low, it is worth comparing them with the calculated minimum lattice thermal conductivity k lat,min for BiCuSeO to assess if there is still room for improvement. The lattice thermal conductivity k lat can be estimated by the following relationship 11,14,18 :…”

“…The minimal lattice thermal conductivity k lat has been estimated by the minimum l to be the interatomic distance 11,14,18 of B3.0 Å for the BiCuSeO system. Using equation (6), k lat,min of 0.47 Wm À1 K À1 is obtained at 300 K, it is much lower than the value of 0.88 Wm À1 K À1 obtained in this study, suggesting a possibility for further reduction of k lat in BiCuSeO.…”

“…Alternatively, high-performance single-phase thermoelectric materials can be obtained using materials with intrinsically low lattice thermal conductivities. Typical materials included in this class are as follows: AgSbTe 2 (kB0.39 Wm À1 K À1 , ZTB1.6 at 673 K), 10,11 Ag 9 TlTe 5 (kB0.22 Wm À1 K À1 , ZTB1.2 at 700 K), 12 Ag 0.95 GaTe 2 (kB0.20 Wm À1 K À1 , ZTB0.7 at 850 K), 13 Ag 3.9 Mo 9 Se 11 (kB0.75 Wm À1 K À1 , ZTB0.6 at 800 K), 14 Cu 3 SbSe 4 (kB0.50 Wm À1 K À1 , ZTB0.8 at 650 K), 15 Cu 2 Ga 4 Te 7 (kB0.67 Wm À1 K À1 , ZTB0.6 at 940 K), 16 Cu 2.1 Zn 0.9 SnSe 4 (kB0.50 Wm À1 K À1 , ZTB0.9 at 860 K), 17 Cu 2 Ga 0.07 Ge 0.93 Se 3 (kB0.67 Wm À1 K À1 , ZTB0.5 at 745 K), 18 CsBi 4 Te 6 (kB0.50 Wm À1 K À1 , ZTB0.8 at 275 K), 19 and K 2 Bi 8 Se 13 (kB0.80 Wm À1 K À1 , ZTB0.4 at 400 K). 20 Recently, we reported a quaternary BiCuSeO compound that exhibits very low thermal conductivity.…”