Phonon drag effect in nanocomposite FeSb 2
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Phonon drag effect in nanocomposite FeSb2 Mani Pokharel, Huaizhou Zhao, Kevin Lukas, Zhifeng Ren, and Cyril Opeil, Department of Physics, Boston College, Chestnut Hill, Massachusetts 02467 Bogdan Mihaila, Los Alamos National Laboratory, Los Alamos, New Mexico 87545 Address all correspondence to Mani Pokharel at [email protected] (Received 15 November 2012; accepted 1 February 2013)
Abstract We study the temperature dependence of thermoelectric transport properties of four FeSb2 nanocomposite samples with different grain sizes. The comparison of the single crystals and nanocomposites of varying grain sizes indicates the presence of substantial phonon drag effects in this system contributing to a large Seebeck coefficient at low temperature. As the grain size decreases, the increased phonon scattering at the grain boundaries leads to a suppression of the phonon-drag effect, resulting in a much smaller peak value of the Seebeck coefficient in the nanostructured bulk materials. As a consequence, the ZT values are not improved significantly even though the thermal conductivity is drastically reduced.
Introduction Owing to its unusual magnetic and electronic transport properties, the narrow-gap semiconductor FeSb2 has been one of the extensively studied compounds in the past few decades.[1–4] The renewed interest in this compound came after Bentien et al.[5] reported a colossal value of the Seebeck coefficient of –45,000 µV/K with a record high value of the power factor (PF) of 2300 µW/K2/cm at around 10 K in single crystal samples, which may make this material a potential candidate for the Peltier cooling applications at very low temperature near 10 K. Despite the huge PF value, the dimensionless figure of merit (ZT) values for single crystal samples are limited by a very high thermal conductivity κ ∼ 500 W/m/ K at ∼10 K. In our earlier work,[6] we were able to reduce the thermal conductivity by three orders of magnitude to 0.5 W/m/K in achieving a peak ZT value of ∼0.013 at ∼50 K in nanocomposite samples. However, the Seebeck coefficient in the nanocomposites is severely degraded at low temperatures when compared with that of the single crystal counterpart. For optimized samples with κ = 0.40 W/m/K and ρ = 1.2 × 10−4 Ω m at 50 K, a Seebeck coefficient of –970 µV/K is required to achieve a ZT value of 1. Unfortunately, the measured value of the Seebeck coefficient at 50 K was only – 109 µV/K. Therefore, it is important to know the origin of the large Seebeck coefficient in this system to further improve ZT. The classical theory of thermoelectricity is based on the assumption that the flow of charge carriers and phonons can be treated independently. Under this assumption, the Seebeck coefficient arises due solely to spontaneous electron diffusion. However, when the two flows are linked, the effect of electron–phonon
scattering should be taken into account. Hence, in general, the Seebeck coefficient is given as the sum of two independent contributions[7] S = Sd + Sp ,
(1)
where Sd is the conventional electron-diffusi
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