Effects of Nb doping on thermoelectric properties of Zn 4 Sb 3 at high temperatures
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X.Y. Qin Key Laboratory of Materials Physics, Institute of Solid State Physics, Chinese Academy of Sciences, 230031 Hefei, People’s Republic of China (Received 17 June 2008; accepted 8 October 2008)
The thermoelectric properties of Nb-doped Zn4Sb3 compounds, (Zn1–xNbx)4Sb3 (x = 0, 0.005, and 0.01), were investigated at temperatures ranging from 300 to 685 K. The results showed that by substituting Zn with Nb, the thermal conductivities of all the Nbdoped compounds were lower than that of the pristine b-Zn4Sb3. Among the compounds studied, the lightly substituted (Zn0.995Nb0.005)4Sb3 compound exhibited the best thermoelectric performance due to the improvement in both its electrical resistivity and thermal conductivity. Its figure of merit, ZT, was greater than the undoped Zn4Sb3 compound for the temperature range investigated. In particular, the ZT of (Zn0.995Nb0.005)4Sb3 reached a value of 1.1 at 680 K, which was 69% greater than that of the undoped Zn4Sb3 obtained in this study.
I. INTRODUCTION
Zn4Sb3 is known to have three structural phases, namely a-, b- and g-Zn4Sb3, which are stable below 263 K, between 263 and 765 K, and above 765 K, respectively.1,2 Among them, b-Zn4Sb3 is a structurally disordered narrow-gap semiconductor, which has attracted a lot of interest as a thermoelectric material for its possible application in energy conversion and power generation.3,4 The efficiency of a thermoelectric material is expressed by its dimensionless figure-of-merit ZT [= S2T/rl, where S, T, r, and l are the Seebeck coefficient (or thermopower), the absolute temperature, the electrical resistivity, and the total thermal conductivity, respectively], which indicates that a good thermoelectric material should combine high values of S, and low r, with low l. The figure of merit of b-Zn4Sb3 is the highest among known thermoelectric materials in the temperature range of 473–673 K3,4 due to its extremely low thermal conductivity originating from the structural disorder. To further improve the thermoelectric performance of Zn4Sb3, one approach is by partially substituting Zn with metals such as Cd, In, Ga, and Al.5–8 Such a doping approach can be used to optimize the thermoelectric properties by reducing its thermal conductivity and adjusting its carrier concentration. Nakamoto et al.6 reported the maximum formal stoichiometry of Zn2.4Cd1.6Sb3 and a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2009.0058
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J. Mater. Res., Vol. 24, No. 2, Feb 2009
found that Cd substitution led to an increase in both the electrical resistivity and Seebeck coefficient. As a result, the power factor for (Zn0.8Cd0.2)4Sb3 at 300 K reaches a maximum of 6 106 W cm1K2. In a different report, a maximum ZT value of 1.4 was obtained at 523 K for Zn3.2Cd0.8Sb3 solid solutions.8 Liu et al.5 investigated the thermoelectric properties of (Zn0.98M0.02)4Sb3 (M = Al, Ga, and In) at low temperatures ranging from 5 to 310 K. Their results indicated that the substitution of M (M = Al, Ga, and In) for Zn in Z
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