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

Abstract

We report on the high thermoelectric performance of p-type polycrystalline BiCuSeO, a layered oxyselenide composed of alternating conductive (Cu 2 Se 2 ) 2 À and insulating (Bi 2 O 2 ) 2 þ layers. The electrical transport properties of BiCuSeO materials can be significantly improved by substituting Bi 3 þ with Ca 2 þ . The resulting materials exhibit a large positive Seebeck coefficient of B þ 330 lV K À1 at 300 K, which may be due to the 'natural superlattice' layered structure and the moderate effective mass suggested by both electronic density of states and carrier concentration calculations. After doping with Ca, enhanced electrical conductivity coupled with a moderate Seebeck coefficient leads to a power factor of B4.74 lW cm À1 K À2 at 923 K. Moreover, BiCuSeO shows very low thermal conductivity in the temperature range of 300 (B0.9 W m À1 K À1 ) to 923 K (B0.45 W m À1 K À1 ). Such low thermal conductivity values are most likely a result of the weak chemical bonds (Young's modulus, EB76.5 GPa) and the strong anharmonicity of the bonding arrangement (Gruneisen parameter, cB1.5). In addition to increasing the power factor, Ca doping reduces the thermal conductivity of the lattice, as confirmed by both experimental results and Callaway model calculations. The combination of optimized power factor and intrinsically low thermal conductivity results in a high ZT of B0.9 at 923 K for Bi 0.925 Ca 0.075 CuSeO.

Promising Thermoelectric Bulk Materials with 2D Structures

“…The top of the valence band mainly consists of O 2p, Cu 3d, and Se 4p states. [48] Additionally, in the vicinity of the Fermi level, whose electrons dominate transport, the DOS is mainly attributed to contributions from Cu and Se, confirming the existence of conductive and insulting layers. As an indi rect band material, the conduction band minimum is located at the Z point, while the first valence band maximum (VBM) lies along the Γ M direction.…”

“…[80] According to solid state physical principles, the ZT value for bulk mate rials with a single parabolic band (SPB) along a certain direc tion can be given as: [4] The timeline for thermoelectric bulk materials with 2D structures, showing data from a superlattice, [8] SnSe, [11,[19][20][21] Bi 2 Te 3 , [12,14,16,17,[22][23][24][25][26][27][28][29][30][31][32][33][34][35][36][37][38][39][40][41] BiCuSeO, [12,[42][43][44][45][46][47][48][49][50][51][52][53][54][55][56] Na x CoO 2 , [57,58] Ca 3 Co 4 O 9 , [5...…”

“…Reproduced with permission. [48] Copyright 2013, Nature Publishing Group. www.advmat.de www.advancedsciencenews.com shown in Figure 8a demonstrates the superiority of the thermal resistance of BiCuSeO even without optimization.…”

Metal oxides for thermoelectric power generation and beyond

“…This is mainly due to the low electrical conductivity caused by the intrinsic low carrier concentration of 1 [50][51][52][53]. To enhance the TE properties of BiCuSeO, the alkaline-earth metals (Mg, Ca, Ba, and Sr) with 2 + valence have been reported to be good p-type dopants replacing the Bi 3+ in BiCuSeO [49,[54][55][56]. Doping these elements can improve the electrical conductivity by increasing the carrier concentration, which leads to an enhancement of PF.…”

Intrinsically low thermal conductivity from a quasi-one-dimensional crystal structure and enhanced electrical conductivity network via Pb doping in SbCrSe3

“…14 Weak chemical bonding generally results in the slow transport of phonons and thus yields lower values of the lattice thermal conductivity. 15 Moreover, the asymmetrical local atomic coordination with a large distortion structure, which possess both strong and weak bond types, is beneficial for increasing the bond anharmonicity, which is another important aspect that affects the lattice thermal conductivity. 16 The bond anharmonicity serves as a measure of the deviation in lattice vibrations from a perfect harmonic motion and is characterized by the effective Grüneisen parameter.…”