Thermoelectric properties of the incommensurate layered semiconductor Ge x NbTe 2
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The compounds GexNbTe2 (0.39 艋 x 艋 0.53) have been studied for their thermoelectric properties. By changing x, the carrier concentration can be adjusted so that the material changes from a p-type metal to a p-type semiconductor. The maximum germanium concentration at about Ge0.5NbTe2 is also the most semiconducting composition. High- and low-temperature electrical resistivity, Hall effect, Seebeck coefficient, and thermal conductivity were measured. Evidence of electronic ordering was found in some samples. The thermal conductivity is reasonably low and glasslike with room temperature values around 20–25 mW/cm K. However, the power factor is too low to compete with state-of-the-art materials. The maximum thermoelectric figure of merit, ZT found in these compounds is about 0.12. The low ZT can be traced to the low carrier mobility of about 10 cm2/Vs. The related compounds Si0.5NbTe2 and Ge0.5TaTe2 were also studied.
I. INTRODUCTION
The growth of commercial applications of thermoelectric devices depends primarily on increasing the figure of merit, ZT, for thermoelectric materials. The figure of merit is defined as ZT ⳱ ␣2T/, where ␣ is the Seebeck coefficient, the electrical conductivity, the thermal conductivity, and T is the absolute temperature. Materials with a large ␣2 value, or power factor, are usually heavily doped semiconductors, such as Bi2Te3. The thermal conductivity of semiconductors is usually dominated by phonon or lattice thermal conductivity. Thus, one method for finding new, advanced thermoelectric materials is to search for semiconductors with low lattice thermal conductivity. Disordered solids such as glasses tend to have low lattice thermal conductivity due to increased phonon scattering. However, in order to have a high carrier mobility a crystalline material is desired. Slack1 has introduced the concept of an ideal thermoelectric being an electron crystal, phonon glass. With this in mind, we have studied Gex NbTe2, which can have an incommensurately modulated structure and is calculated to be a low band gap semiconductor (for references see Ref. 2). When x in Gex NbTe2 is irrational an incommensurate structure is found where ordered, crystalline layers of NbTe2 are incommensurately modulated from one layer to the next. It has been found that the x ⳱ 0.5 compound a)
Address all correspondence to the author. e-mail: [email protected] J. Mater. Res., Vol. 15, No. 12, Dec 2000
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is a (commensurate) semiconductor. For use as a thermoelectric x can be changed (1⁄3 艋 x 艋 1⁄2 are known) both to adjust the carrier concentration to the optimal level and to provide an incommensurate structure. II. EXPERIMENTAL
Polycrystalline samples were prepared by mixing and reacting elemental powders in evacuated silica ampoules for several days at 950 °C. The powders were then hot pressed in graphite dies into dense samples, 3 mm long and 12 mm in diameter. The hot pressing was conducted at a pressure of 1400 kg/cm2 and 850 °C for about 2 h unde
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