Thermoelectric properties of p-type Mg₂Si_0.3Sn_0.7 doped with silver and gallium

Highlights

• Mg_2−xAg_x(Si_0.3Sn_0.7)_1−yGa_y is synthesized by melting in a closed crucible and SPS.

• Ag and Ga doping enhance carrier concentration and reduce mobility values.

• The rather high thermal conductivities cause low values of the ZT under 0.29.

• A small volume fraction of secondary phase benefits sample properties.

Abstract

Structure, composition, and transport properties of Mg_2−xAg_x(Si_0.3Sn_0.7)_1−yGa_y (x = {0, 0.02}, y = {0, 0.02, 0.04, 0.06}) solid solutions produced by melting followed by spark plasma sintering are investigated. The preparation method is adjusted to control sample stoichiometry and phase composition. Doping with two types of dopants at different sites, while employing synthesis methods which generate a small amount of secondary phase, is an uncommon approach in this materials, expected to enhance their thermoelectric performance. An enhanced carrier concentration but diminished mobility is observed in the samples with higher amounts of dopant, which leads to the highest values of the power factor, for Mg_1.98Ag_0.02Si_0.27Sn_0.67Ga_0.06 in a narrow temperature range (575–675 K) around the peak value, of 9.10⁻⁴ Wm⁻¹ K⁻² at 625 K. The two types of dopants have opposing effects on the thermal conductivity, with Ag promoting strong phonon scattering and decreasing its values while Ga increases them because of its enhanced carrier concentration. The rather high thermal conductivity values of the double doped compounds produce low values of the ZT without exceeding 0.29 at 627 K for Mg_1.98Ag_0.02Si_0.3Sn_0.7 sample.

Introduction

Nowadays, tremendous effort is being made in order to increase the thermoelectric figure of merit of materials, ZT = S²σTκ⁻¹, (where S is the Seebeck coefficient, σ is the electrical conductivity and κ is the thermal conductivity), which defines their heat-to-electric, or vice versa, energy conversion efficiency [1]. For this reason, various classes of thermoelectric (TE) materials showing high ZT values were investigated including half-Heusler [2], [3], [4], [5], oxides [6], [7], skutterudites [8], [9], [10], [11], chalcogenides [12], [13], [14] and silicides [15], [16]. Among them, magnesium silicide-stannide solid solutions, Mg₂Si_1−xSn_x, are remarkable due to the crust abundance and reduced costs of the constitutive elements, non-toxicity, low weight, and good thermoelectric performance, attributes that make them appealing for applications [17], [18], [19]. The values of ZT, resulting from reasonably low thermal conductivity and high power factor (PF=σS²) reach 1.7 for doped n-type Mg₂Si_1−xSn_x (0.4 <x < 0.7) [18], [20], [21], while their p-type homologues attain only 0.7 [17], [22], in a temperature range between 500 K and 800 K. For applications, however, in order to get an optimum device efficiency, both n-type and p-type materials are required and should have comparable thermoelectric and thermomechanical performance [23], [24], [25]. Therefore, finding appropriate methods to improve the p-type materials properties (e.g. by doping, substitutions, material synthesis) is essential.

Theoretical studies predict that a ZT_max∼0.8 at 700 K is possible in the p-type Mg₂(Si,Sn) solid solutions but at a high charge carrier concentration level (p) [18]. Experimentally, the optimization of carrier concentration is difficult because the synthesis of p-type samples often leads to low p and hence to low thermoelectric performance [25]. However, choosing the appropriate dopant and preparation method might alleviate the problem. To date, reports show that Li and Ag doping successfully enhance the carrier concentration when substituting Mg [26], [27], [28], while Ga preferentially substitutes Si/Sn atoms with similar effects [17], [29], [30], in various p-type Mg₂Si_1−xSn_x solid solutions. Whereas for In [31], Na[32], Cu [33], and Al [34], the doping effect, and the resulting conduction type, depends on which lattice site the doping atom will occupy. Similarly to Li, Na was expected to act as acceptor substituting Mg, however, it has been found that a much lower carrier concentration and hence reduced thermoelectric performance are achieved by Na doping.[22], [28]. The use of other elements, such as Al and Cu as dopants [35], was predicted to provide p-type conductivity, yet mostly n-type conduction was observed experimentally [33], [34], [36].

In Ag doped Mg₂Si_1−xSn_x solid solutions the preparation method is essential because it might determine the solubility limit of Ag [25], for example samples melted in ampules reach about 2% [37], but when employing melt-spinning with very high cooling rates that might ”freeze” a metastable state in the material it raise up to 5%. Ag single doping materials show promising thermoelectric properties and so far a peak figure of merit of 0.45 at 690 K has been reported for 5% Ag doping at a carrier concentration of ∼ 6 × 10¹⁹ cm⁻³ for melt-spun Mg₂Si_0.4Sn_0.6 [26].

Varying the Ga content of Mg₂(Si_0.3Sn_0.7)_1−yGa_y solid solutios, along with some over-stoichiometry of Mg, Liu et al. [29] were able to obtain high hole concentration of 3.10²⁰ cm⁻³ and carrier mobility of 20 cm²/Vs. However, the content of Ga did not have an appreciable influence on the lattice thermal conductivity, because the phonon scattering cannot be greatly enhanced by Ga doping, since Ga has a comparable radius and mass with Si. Therefore, they have achieved a figure of merit of 0.35 at 650 K.

The particular challenges related to p-type Mg₂Si_1−xSn_x, center mostly around the low charge carrier concentration and high thermal conductivity. A double doping approach, although common for the n-type analogues, is rare for p-type materials, for example Isoda et al. [27] show that Li and Ag double doping enhance considerably the carrier concentration of the materials compared to single Li or Ag doping. Therefore, in the present work, we report on Ag and Ga doping of Mg₂Si_0.3Sn_0.7, targeting substitution at both Mg and Si/Sn sites, in order to achieve higher charge carrier concentration, and concomitantly a reduction of the thermal conductivity due to the mass fluctuation introduced by Ag doping, hence improving the transport properties. Also, the role of the sample microstructure /morphology on sample properties is investigated. Depending on the thermal treatment recipe used, a reduced amount of Si rich secondary phase [38] is obtained, which by its intrinsic properties and by introducing new scattering centers will influence the transport properties.