The Hall coefficient, electrical resistivity, and Seebeck coefficient of n-type specimens of β-Ag2Se, the low temperature polymorph of silver selenide, were measured over the temperature range from 70 to 300 K. The results showed maxima in both Hall coefficient and the electrical resistivity just below the onset of the intrinsic conduction range. This anomaly was qualitatively explained by the deviation of the Coulomb scattering from the usual assumption of independence due to the degenerate nature of the samples. The estimated energy gap for different samples of about 160 meV seems to confirm the existence of the second low temperature phase β2. This second phase is a probable reason for the relatively high thermoelectric figure of merit observed.

Thermoelectric and transport properties of β-Ag2Se compounds

Lattice thermal conductivity of semiconductors: A chemical bond approach

Ag_(2)Te simultaneously shows high mobility and low thermal conductivity, however the relatively low band gap of ~0.2 eV prevents it from achieving high thermoelectric figure of merit, zT, in the high temperature phase. In this study, the band gap of Ag_(2)Te has been increased enabling a zT of unity by forming alloys and composites with PbTe, thereby demonstrating the importance of exploiting potentially good thermoelectrics among these small band gap semiconductors and similar materials.

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Alloying to increase the band gap for improving thermoelectric properties of Ag2Te

The study of electrical properties of β‐Ag2Te and β‐Ag2Se has been extended to 4.2°K. The former compound was zone refined without decomposition. Both n‐ and p‐type samples of β‐Ag2Te were studied; all samples of β‐Ag2Se prepared were n type to 4.2°K. Neither semiconductor showed any indication of extrinsic carrier freeze‐out or of impurity banding. Study of oxygen as a possible acceptor in β‐Ag2Te showed no significant effect; a selenium‐doped sample of β‐Ag2Se was n‐type to 4.2°K.

Electrical Properties of β‐Ag2Te and β‐Ag2Se from 4.2° to 300°K

To design superior thermoelectric materials the minority carrier blocking effect in which the unwanted bipolar transport is prevented by the interfacial energy barriers in the heterogeneous nanostructures has been theoretically proposed recently. The theory predicts an enhanced power factor and a reduced bipolar thermal conductivity for materials with a relatively low doping level, which could lead to an improvement in the thermoelectric figure of merit (ZT). Here we show the first experimental demonstration of the minority carrier blocking in lead telluride-silver telluride (PbTe-Ag2Te) nanowire heterostructure-based nanocomposites. The nanocomposites are made by sintering PbTe-Ag2Te nanowire heterostructures produced in a highly scalable solution-phase synthesis. Compared with Ag2Te nanowire-based nanocomposite produced in similar method, the PbTe-Ag2Te nanocomposite containing ∼5 atomic % PbTe exhibits enhanced Seebeck coefficient, reduced thermal conductivity, and ∼40% improved ZT, which can be well explained by the theoretical modeling based on the Boltzmann transport equations when energy barriers for both electrons and holes at the heterostructure interfaces are considered in the calculations. For this p-type PbTe-Ag2Te nanocomposite, the barriers for electrons, that is, minority carriers, are primarily responsible for the ZT enhancement. By extending this approach to other nanostructured systems, it represents a key step toward low-cost solution-processable nanomaterials without heavy doping level for high-performance thermoelectric energy harvesting.

Enhanced thermoelectric properties in bulk nanowire heterostructure-based nanocomposites through minority carrier blocking.

The Seebeck coefficient α, electrical conductivity σ, and thermal conductivity K data are given for a number of Ag2Te specimens measured at room temperature. The maximum value observed for the figure of merit α2σ/K was 1.3×10−3°C−1. The relationship between K and σ was linear, and Kel could be expressed approximately by (π2/3) (k/e)2σT. Kph was found to be 0.72×10−2 w cm−1°C−1. There was no evidence of an ambipolar diffusion contribution to K. It is suggested that the small energy gap (0.02 ev) previously determined in this compound could account for the absence of a measurable ambipolar diffusion effect, the degeneracylike behavior of Kel, and the relatively low values of the Seebeck coefficient.

C. Wood

Papers

Thermoelectric Properties of Ag2Te