Enhanced thermoelectric figure of merit by composite effects and low thermal conductivity in distrontium silicide (Sr 2 Si) Farhan Mudasar a, * , Yukari Katsura a , Koichi Kitahara a, b , Kaoru Kimura a, b a Department of Advanced Materials Science, The University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8561, Japan b AIST-UTokyo Advanced Operand-Measurement Technology Open Innovation Laboratory (OPERAND-OIL), National Institute of Advanced Industrial,148 City Block 4, Kashiwanoha Campus,178-4 Wakashiba, Kashiwa, Chiba 277-0871, Japan article info Article history: Received 31 July 2018 Received in revised form 29 October 2018 Accepted 3 December 2018 Available online 6 December 2018 Keywords: Silicide Thermoelectric Thermal conductivity Composite Microstructure abstract The thermoelectric properties of Sr 2 Si, Sr 5 Si 3 , and their composites are reported. Sr 2 Si-Sr 5 Si 3 composites were synthesized by partial decomposition of Sr 2 Si to Sr 5 Si 3 during spark plasma sintering. Semi- conducting Sr 2 Si grains were surrounded by boundary regions composed of smaller grains of Sr 2 Si, metallic Sr 5 Si 3 , and semiconducting Sr 3 SiO. Addition of the Sr 5 Si 3 phase reduced the electrical resistivity from 1.69  10 3 Um to 6.45  10 5 Um at room temperature, which corresponded to the samples having no and the maximum amount of the Sr 5 Si 3 phase, respectively. The electrical resistivity decreased; however, the Seebeck coefficient slightly increased to 157 mV/K at 773 K, and the thermal conductivity was approximately constant. Therefore, the power factor and ZT monotonically increased with an in- crease in the amount of the Sr 5 Si 3 phase in the composite samples, which confirmed almost no effect resulted from the Sr 3 SiO phase. A low thermal conductivity was demonstrated by Sr 2 Si (ca. 1 W/mK at 776 K), which resulted from weaker atomic bonds, heavier atomic mass, greater atomic volume, and a greater number of atoms in the unit cell. A further enhancement of ZT can be achieved for higher amounts of Sr 5 Si 3 and at higher temperatures. © 2018 Elsevier B.V. All rights reserved. 1. Introduction Silicides are attractive semiconductors because of their envi- ronmental friendliness and their application in electronic [1] and thermoelectric (TE) devices [2]. New and less investigated silicides can replace the toxic and expensive transition metal silicides. Within the group of alkaline earth silicides, Mg 2 Si has been well studied. An excellent dimensionless figure of merit (ZT ¼ S 2 T rk ) of 1.1 at 870K has been reported in the n-type Mg 2 (Si,Sn) system [3], where S is the Seebeck coefficient, r is the electrical resistivity, k is the thermal conductivity, and T is the temperature. In comparison, p-type Mg 2 Si has a lower ZT than the n-type over the same tem- perature range, and ZT ¼ 0.7 has been reported for p-type Mg 2 Li 0.025 Si 0.4 Sn 0.6 [4]. A counter-doped p-type semiconductor should be used with n-type Mg 2 Si in TE modules. Furthermore, the k of Mg 2 Si remains high under these conditions (ca. 10 W/m K) [5]. Silicides of Ca, Sr, and Ba are less explored owing to difficulties in their synthesis and handling. However, the k of undoped Ca 2 Si, SrSi 2 , and BaSi 2 at room temperature are reported as 2.31 W/mK [6], 5.25 W/mK [5], and 1.56 W/mK [5], respectively. Wen et al. succeeded in reducing the electrical resistivity of Ca 2 Si through the addition of Na, where the addition of Na 2 CO 3 resulted in the r of Ca 2 Si decreasing from 56.32 Ucm to 3.74 Ucm at room temperature [7]. A suitable type, amount, and distribution of the nano- or microsize secondary phase can play an important role in improving the TE properties of composites. The effects of the composite are mainly derived from two phenomena. The first is the scattering of phonons by the grain boundary, which also exists in polycrystals of the single phase [8e11]. Although the mean free path of electrons is mostly shorter than several nanometers, which is usually smaller than the grain size, that of phonons is widely distributed to more than 1 mm. Therefore, by introducing a grain boundary, the lattice thermal conductivity, k lat , decreases much more markedly than does the electronic thermal conductivity, k el , and the electrical conductivity, s ¼ 1/r, where k ¼ k lat þk el . As a result, ZT can be increased. The second is the precipitation of more conductive phases, such as a metallic phase [12e17]. In such cases, both r and S decrease and S 2 /r can be optimized as in carrier doping. The second * Corresponding author. E-mail addresses: farhan@phys.mm.t.u-tokyo.ac.jp, farhan.mudasar@gmail.com (F. Mudasar). Contents lists available at ScienceDirect Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom https://doi.org/10.1016/j.jallcom.2018.12.037 0925-8388/© 2018 Elsevier B.V. All rights reserved. Journal of Alloys and Compounds 782 (2019) 1031e1040