Thermoelectric properties of bismuth-doped magnesium silicide obtained by the self-propagating high-temperature synthesis

BULLETIN OF THE POLISH ACADEMY OF SCIENCES TECHNICAL SCIENCES, Vol. 70(3), 2022, Article number: e141007 DOI: 10.24425/bpasts.2022.141007 MATERIAL SCIENCE AND NANOTECHNOLOGY Thermoelectric properties of bismuth-doped magnesium silicide obtained by the self-propagating high-temperature synthesis Bartosz BUCHOLC 1 , Kamil KASZYCA Mirosław J. KRUSZEWSKI 2 , Łukasz CIUPIŃSKI 1 2 , Piotr ŚPIEWAK 2 , Krzysztof MARS 3 , , Krystian KOWIORSKI 1 , and Rafał ZYBAŁA 1,2 ∗ 1 Łukasiewicz Research Network – Institute of Microelectronics and Photonics, Aleja Lotników 32/46, 02-668 Warsaw, Poland 2 Faculty of Materials Science and Engineering, Warsaw University of Technology, Wołoska 141, 02-507 Warsaw, Poland 3 Faculty of Materials Science and Ceramic, AGH University of Science and Technology, Kraków, Al. Mickiewicza 30, 30-059, Poland Abstract. Doping is one of the possible ways to significantly increase the thermoelectric properties of many different materials. It has been confirmed that by introducing bismuth atoms into Mg sites in the Mg2 Si compound, it is possible to increase career concentration and intensify the effect of phonon scattering, which results in remarkable enhancement in the figure of merit (ZT) value. Magnesium silicide has gained scientists’ attention due to its nontoxicity, low density, and inexpensiveness. This paper reports on our latest attempt to employ ultrafast selfpropagating high-temperature synthesis (SHS) followed by the spark plasma sintering (SPS) as a synthesis process of doped Mg2 Si. Materials with varied bismuth doping were fabricated and then thoroughly analyzed with the laser flash method (LFA), X-ray diffraction (XRD), scanning electron microscopy (SEM) with an integrated energy-dispersive spectrometer (EDS). For density measurement, the Archimedes method was used. The electrical conductivity was measured using a standard four-probe method. The Seebeck coefficient was calculated from measured Seebeck voltage in the sample subjected to a temperature gradient. The structural analyses showed the Mg2 Si phase as dominant and Bi2 Mg3 located at grain boundaries. Bismuth doping enhanced ZT for every dopant concentration. ZT = 0.44 and ZT=0.38 were obtained for 3wt% and 2wt% at 770 K, respectively. Key words: thermoelectric materials; magnesium silicide; bismuth doping; SHS; spark plasma sintering. 1. INTRODUCTION As one of many consequences of global climate change, great attention has been paid to enhancing the energy efficiency of industrial power plants and all other types of devices and installations using or generating energy. Thermoelectric generators (TEG), composed of thermoelectric materials with n-type and p-type conduction types, might be a solution when commonly used for efficient waste heat recovery. They facilitate the conversion of temperature gradient into electricity by the Seebeck effect. So far, several obstacles have prevented their widespread use, such as high production costs, the toxicity of processing byproducts, fabrication scalability challenges, and relatively low energy conversion efficiency. The last one is in positive correlation with the so-called dimensionless figure of merit ZT, which can be written as ZT = α 2 · σ · T · λ −1 , where α is the Seebeck coefficient, T is the temperature σ and λ stands for the electrical and thermal conductivity, respectively [1]. Among many different approaches to improving the aforementioned parameter, one can indicate alloying [2, 3], doping [4, 5], nanostructuring [6–8], nanocomposites fabrication [9, 10], or obtain∗ e-mail: Manuscript submitted 2022-02-17, revised 2022-02-179, initially accepted for publication 2023-03-03, published in June 2022. ing them in the form of thin films and superlattices [11–13]. The purpose of those treatments is usually to increase the power factor, which is the product of α 2 · σ , or reduce thermal conductivity λ . Attempts were made to combine the effects of both these factors. Compared to other popular thermoelectric materials, Mg2 Sibased exhibit a unique combination of properties. Apart from the promising thermoelectric performance [14], low density [15], nontoxicity of its processing byproducts, and relatively low price of its primary constituents should be pointed out. To enable the use of Mg2 Si-based materials in practical applications, it would be advisable to produce equally efficient materials, with both types of conduction p and n, by an easily scalable method. Since the first investigations of its thermoelectric properties in the 1960s [16], many dopants and manufacturing techniques have been tested. Aside from the conventional ceramic method, some novel advanced procedures have been employed. Berthebaud and Gascoin used the microwaved assisted fast synthesis to fabricate n and p-doped Mg2 Si [17]. For the n-type material, the highest value of ZT was achieved for antimony and tin doping (0.7 at 770 K), and in the case of p-type addition of silver resulted in ZT of 0.35 at 770 K. Nakagawa et al. synthesized Mg2 Si by the liquid encapsulated vertical gradient freezing method [18]. The vertical Bridgman method was used for bulk crystal growth by Yoshinaga et al. to Bull. Pol. Acad. Sci. Tech. Sci., vol. 70, no. 3, p. e141007, 2022 © 2022 The Author(s). This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/) 1 B. Bucholc et al. achieve a power factor of 7.8 · 10−6 Wcm−1 K−2 at 373 K [19]. Fu et al. suggested the vacuum plasma thermal spray (VPS) as a potential deposition technique for thermoelectric materials [20]. They obtained samples of Mg2 Si with maximum ZT = 0.16 at 700 K. In addition to Ag, Sb and Sn doping also reports of Cu [21], Li [22], Al [21], Co [17] and Bi [14,15,21–24] should be noted. Especially the last one has gained a lot of attention since its first investigation in 2005 by Tani and Kido [14], who fabricated Mg2 Si0.98 Bi0.02 exhibiting ZT = 0.86 at 862 K. Since then, attempts have been made to increase that value, and synthesize such material with various methods. Choi et al. utilized the vacuum melting method followed by Spark Plasma Sintering (SPS) [23]. The same method for consolidation was used by Nieroda et al. to densify material obtained by direct synthesis [15]. In their work, they annealed bulk samples for seven days to improve homogeneity resulting in ZT = 0.55 at 720 K. Li et al. in their paper presented Bi-doped Mg2 Si with one of the highest values of ZT equal to 0.98 at 883 K [25]. The preparation route included high-pressure synthesis and SPS. Each processing route mentioned is energy and time-consuming or very difficult to scale. To ensure the economic viability of the thermoelectric materials manufacturing process, it is necessary to use adaptable techniques for large-scale production, such as fast self-propagating high-temperature synthesis (SHS) [26]. This technique was successfully employed by Zhang et al. for the synthesis of Sb doped (...truncated)