Transport Properties of Ag‐doped ZnSb

Abstract

The intermetallic compound ZnSb is a (II‐V) narrow gap semiconductor with interesting thermoelectric properties. Electrical resistivity, Hall coefficient, thermopower and thermal conductivity were measured up to 400 K on Ag‐doped samples with concentrations 0.2, 0.5, 1, 2, and 3 at.%, which were consolidated to densities in excess of 99.5 % by spark plasma sintering. The work confirms a huge improvement of the thermoelectric Figure‐of‐merit, ZT, upon Ag doping. The optimum doping level is near 0.5 at.% Ag and results in ZT values around 1.05 at 390 K. The improvement stems from a largely decreased resistivity, which in turn relates to an increase of the hole charge carrier concentration by two orders of magnitude. It is argued that Ag can replace minute concentrations of Zn (on the order of 0.2 at.%) in the crystal structure which enhances the intrinsic impurity band of ZnSb. Excess Ag was found to segregate in grain boundaries. So the best performing material may be considered as a composite Zn~0.998Ag~0.002Sb/Ag~0.003.

High thermoelectric performance in p-type ZnSb upon increasing Zn vacancies: an experimental and theoretical study

“…In all of the samples, the high-intensity diffraction peaks correspond to (112) and (121) planes. When synthesising ZnSb using stoichiometric reaction mixtures with a Zn : Sb ratio of 1 : 1, a minor amount of Sb is usually observed which is unavoidable upon increasing Zn vacancies, which is consistent with prior studies, 34,50,51 as seen in Fig. 1b.…”

“…46 In particular, Ueda et al 47 reported that n-type ZnSb exhibits the maximum S 2 s of ∼0.84 mW m −1 K −2 at 573 K upon 2.06 at% Te doping, whereas p-type ZnSb has shown a S 2 s of ∼1.12 mW m −1 K −2 at 573 K. Moreover, 0.1 at% Sn-doped ZnSb increased the carrier concentration to 1.4 × 10 19 cm −3 , leading to a zT of ∼0.85 at 660 K. 48 On the other hand, a 0.375% Cd doped Zn 1−x Sb material has shown a very high zT of ∼0.95 at 523 K because of its ultralow k latt of ∼0.67 W m −1 K −1 due to the phonon scattering by mass strain disorder of the system. 49 Recently published work by Eklöf et al 50 suggests that excess of Zn in Zn 1.0195 Ag 0.005 Sb alloy leads to an elevated zT of about ∼1.05 at 390 K and the presence of Ag segregation at grain boundaries resulted in increased phonon scattering causing a huge thermal conductivity reduction. Further, the scattering of phonons via Ag 3 Sb nanoparticles, point defects, and nanometric voids has paved the pathway to a zT of ∼1.15 at 673 K for 0.2% Ag-doped ZnSb.…”

Enhanced Thermoelectric Performance in Zn1–xCdxSb (x = 0–0.375) Solid Solutions by Dynamic Optimization of Charge Carrier Concentration

“…Furthermore, heavy acceptor doping of ZnSb had remained a challenge until recently 14,15 when it was found that group 11 (Ag, Cu) impurities can be used to achieve the desired charge carrier concentration range, which is in the order of 10 19 cm −3 . 12,16−18 This resulted in a decent TE performance (zT ≈ 1.1 in doped ZnSb 16,17,19 scattering centers 20−22 ) and, coupled with other attractive features such as low cost and chemical stability, led to a renewed interest in this material system for midtemperature power generation applications. 12,14−19,23−35 However, challenges remain in improving the thermoelectric performance of ZnSb-based materials.…”

“…Thus, obtaining optimal doping in Zn 1– x Cd x Sb compounds is challenging as it necessitates a temperature-dependent charge carrier concentration tuning. Furthermore, heavy acceptor doping of ZnSb had remained a challenge until recently , when it was found that group 11 (Ag, Cu) impurities can be used to achieve the desired charge carrier concentration range, which is in the order of 10 19 cm –3 . ,− This resulted in a decent TE performance ( zT ≈ 1.1 in doped ZnSb ,, and zT ≈ 1.2 with additional phonon scattering centers − ) and, coupled with other attractive features such as low cost and chemical stability, led to a renewed interest in this material system for midtemperature power generation applications. ,− ,− …”

“…However, challenges remain in improving the thermoelectric performance of ZnSb-based materials. First, tuning the charge carrier concentration in ZnSb with group 11 impurities is difficult as even a minute amount of doping with these impurities results in n near the optimum value ( n opt ≈ 2 × 10 19 cm –3 ) at room temperature. ,,− , Also, the temperature dependence of n requires a dynamic optimization of the n value which has not been attempted until now.…”