Large-scale production of (GeTe)_x(AgSbTe₂)_100−x (x=75, 80, 85, 90) with enhanced thermoelectric properties via gas-atomization and spark plasma sintering

Abstract

(GeTe)_x(AgSbTe₂)_100−x: TAGS thermoelectrics are an attractive class of materials due to their combination of non-toxicity and good conversion efficiency at mid-temperature ranges. In the present work, we have utilized energy and time efficient high-pressure gas atomization and spark-plasma sintering techniques for large-scale preparation of samples with varying composition (i.e., (GeTe)_x(AgSbTe₂)_100−x where x = 75, 80, 85, and 90). High-temperature x-ray diffraction was used to understand the phase transformation mechanism of the as-atomized powders. Detailed high-resolution transmission electron microscopy of the sintered samples revealed the presence of nanoscale precipitates, antiphase, and twin boundaries. The nanoscale twins and antiphase boundaries serve as phonon scattering centers, leading to the reduction of total thermal conductivity in TAGS-80 and 90 samples. The maximum ZT obtained was 1.56 at 623 K for TAGS-90, which was ∼94% improvement compared to values previously reported. The presence of the twin boundaries also resulted in a high fracture toughness (K_IC) of the TAGS-90 sample due to inhibition of dislocation movement at the twin boundary.

Introduction

The demand for renewable energy has significantly increased due to growing world population, greenhouse gas emissions, and depletion of fossil fuel reserves. Thermoelectric (TE) energy conversion is one of the best alternative renewable energy technologies for recovery of the energy in waste heat. A thermoelectric module has several unique advantages that include low maintenance cost, absence of moving parts and environmental cleanliness [1]. Since the early 1960s, thermoelectric (TE) materials such as Bi₂Te₃, Sb₂Te₃, SiGe, SnTe, TAGS, and PbTe have been extensively investigated as possible alternatives to power generation and for refrigeration [2]. Among these, the compound (GeTe)_x(AgSbTe₂)_100−x (commonly called TAGS-x) is a promising source of p-type thermoelectric materials for power generation in the medium temperature range (500–800 K). This is the temperature range of automobile exhaust gases (i.e., from internal combustion engines) and industrial waste heat [3], [4], [5]. The performance of a TE material is governed by the thermoelectric figure of merit ZT (= (S² σ/κ)T, where S , σ, κ, and T are the Seebeck coefficient, electrical conductivity, thermal conductivity, and absolute temperature, respectively). Ultimately, a high-efficieny device requires a higher figure of merit. For this reason, great attention is needed to maximize the Seebeck coefficient while maintaining high electrical conductivity and low thermal conductivity. It is difficult to improve one of them without negatively affecting the other two. According to Eqs. (1), (2), the low carrier concentration semiconductors exhibited high Seebeck coefficient and low electrical conductivity. The Wiedemann-Franz law requires that the electronic part of thermal conductivity (κ) be proportional to electrical conductivity. The interrelationships between S, σ, and κ can be summarized as in Eqs. (1), (2), (3) [6].

$$S=\frac{8{\pi }^2{k}_B^2}{3e{h}^2}{m}^{\ast }T{\left(\frac{\pi }{3n}\right)}^{\raisebox{1ex}{$2$}\!\left/ \!\raisebox{-1ex}{$3$}\right.}$$

$$\sigma =ne\mu =\frac{n{e}^2\tau }{m^{\ast }}$$

$${\kappa }_{tot}={\kappa }_{lat}+{\kappa }_{ele}={\kappa }_{lat}+L\sigma T$$

where k_B is the Boltzmann constant, m* is the effective mass, n is the carrier concentration, τ is the relaxation time, μ is the mobility of the carriers, κ_tot is the total thermal conductivity, κ_lat is the lattice thermal conductivity, κ_ele is the electronic thermal conductivity and L is the Lorenz number.

Recently, Levin et al. reported a ZT of 1.5 in TAGS-85 by optimizing carrier concentration by doping with rare-earth atoms (Ce, Yb, and Dy) [7], [8]. There have also been many studies focused on minimizing the total thermal conductivity by introducing microstructure features such as nano-inclusions [9], grain boundaries [10], and nano/mesoscale structures [11]. The introduction of domain variants, twin inversion boundaries, defect layers and second phase precipitates into the matrix of TAGS-x thermoelectric material during the fabrication processes; have also been used to reduce the lattice contribution to total thermal conductivity [2], [12], [13]. These defects (and the nano/mesoscale structures) increase the Seebeck coefficient by enhancing carrier scattering and significantly reduce the thermal conductivity via enhanced phonon scattering. As a result, ZT values are significantly increased. Over the past years, ball milling, casting and annealing processes have been used to prepare TAGS-x materials that exhibit desirable TE properties [14]. Yang et al. reported a ZT of 1.53 in TAGS-75 and a ZT of 1.50 in TAGS-80 and TAGS-85 at 720 K. This was achieved by melting followed by quenching in liquid nitrogen and water to produce nano-domains with a size of 10 nm with different orientations. These nanostructures were responsible for achieving low lattice thermal conductivity and a high ZT [13]. However, after years of effort, the ZT values of TAGS-x TE materials have been limited to the range 1.0–1.6 [7], [8], [15]. Furthermore, the typical production techniques require long operating time and difficult control of impurities. Even worse, the results were of low yield and poor mechanical strength, which limit their broad application. Powder metallurgy has proven to be a cost-effective alternative way to prepare TE materials that exhibit an excellent combination of TE performance and mechanical properties; and thus, could overcome some of the processing drawbacks associated with TAGS-x TE. Gas-atomization (GA) with spark-plasma sintering offers significant advantages such as processing efficiency, less risk of contamination, excellent formability, grain refinement, and avoidance of segregation. Mass-production of powders with uniform microstructure is achievable with this technique due to rapid solidification and easy process control within a short period (2 kg/min) [16], [17], [18]. To the best of our knowledge, there has been no report on (GeTe)_x(AgSbTe₂)_100−x materials using GA and SPS processes. Most of the work reported for these materials has focused on phase transformation and thermoelectric properties [2], [12], with no comprehensive report on their mechanical properties.

Here we report on the synthesis of (GeTe)_x(AgSbTe₂)_100−x materials using a powder metallurgy process (shown in Fig. 1 ). The microstructures, which were systematically analyzed using high-resolution transmission electron microscopy, contain a high twin density. This microstructure resulted in a ZT value of ∼1.56 at 623 K (20% higher than most of the results reported), and in superior mechanical properties compared to other existing TE materials.