1 Introduction

Many IV–VI compounds have good thermoelectric (TE) performances, such as PbTe [1], PbSe [2], and SnTe [3]. However, the expensiveness of Te and the toxicity of Pb restrict their scale-up commercial applications. SnSe with earth-abundant and environmentally friendly elements has attracted wide attention. As the most potential candidate for next generation TE, p-type single crystal SnSe has a world-record peak ZT of about 2.6 at 923 K along the b-axis [4]. Unlike its analogs with a NaCl structure, SnSe crystallizes in a layered structure in the orthorhombic Pnma space group below 810 K [5, 6]. The different Se–Sn layered substructures are linked with weaker Sn–Se bonding, which creates a path that slip easily between Se–Sn layered substructures and results in poor mechanical properties [4, 7]. The ideal strength (0.59 GPa) [7] of SnSe is much lower than that of other high-performance thermoelectric materials such as Mg2Si (4.54 GPa) [8], CoSb3 (7.17 GPa) [9], and TiNiSn (10.52 GPa) [10].

Mechanical properties are vital for the commercial use of the thermoelectric materials since the TE modules are subjected to various mechanical and thermal stresses during the operation. So far, there are few experimental reports on the mechanical properties of polycrystalline SnSe [11,12,13]. Previous research has shown that incorporating additives with high strength and high elastic modulus, such as nano-particles or nano-wires, is a good method to improve the mechanical properties and is widely used in TE [14, 15], ceramics [16, 17], and alloys [18, 19]. These significant enhancements can be attributed to the pinning effect, fiber bridging and fiber pull-out mechanisms in the matrix particles, which creates mechanically interlocked interfaces between the additives and the matrix. The crack may deflect where the crack tip meets the additives, thus increasing the ability to resist crack growth [16, 17, 20]. Potassium titanate whiskers (K2Ti6O13) are promising reinforcement materials due to their excellent mechanical properties (Mohs hardness: 4; strength: 7 GPa; modulus: 280 GPa) and other properties (ultralow thermal conductivity: 0.054 W/mK at room temperature; thermal stability: melting point 1643 K; lower coefficient of thermal expansion: 6.8 × 10−6 K−1; smaller mass density: 3.3 g/cm3; lower cost and good dispersibility). K2Ti6O13 has been widely used as a reinforcement material in polymers [21], metals [18], insulation materials [22], and friction materials [23].

In this work, K2Ti6O13 whiskers were selected as the reinforcement phase. Different fractions of K2Ti6O13 whiskers were introduced into the polycrystalline SnSe. High energy ball milling and ultrasonic dispersion were used to disperse the whiskers uniformly in the matrix. The influences of K2Ti6O13 whiskers on microstructure, and TE and mechanical properties of the composites were investigated systematically.

2 Experimental section

2.1 Sample preparation

Polycrystalline SnSe were synthesized by conventional melting of stoichiometric amounts of tin (Sn, 99.9%) and selenium (Se, 99.999%). The raw materials were sealed into evacuated quartz tubes and kept at 1223 K for 10 h. The obtained ingots were ground into powders with an agate mortar. Commercially available K2Ti6O13 whiskers (diameter: ∼50 nm; length: 0.5–3 µm) were selected as the additives. After ultrasound dispersed in alcohol, the x vol% K2Ti6O13 whiskers (x = 0, 1, 3, 5) were mixed with the SnSe powders using a vario-planetary ball mill at 200 rpm for 2 h. The obtained mixtures were consolidated by HPS at 623 K for 7 min under 2.5 GPa. Subsequently, samples were cut into appropriate sizes for measurements of thermoelectric and mechanical properties.

2.2 Characterization

The densities (d) of the samples were measured by the Archimedes method. The phases were checked by X-ray diffractometry (XRD, Bruker D8 Advance). The micro-structures of the fresh fractured surface were characterized by field-emission scanning electron microscopy (FE-SEM, Hitachi SU8020). The electrical conductivities (σ) and Seebeck coefficients (S) were measured by the standard four-probe method (CTA instrument; CRY-ALL) under a He atmosphere from 300 to 830 K. The thermal diffusivity (λ) was measured by the laser flash method (Netzsch, LFA457). The total thermal conductivity κtol of the composites was calculated by the formula κtol = Cpλd, where Cp = 0.25 is specific heat [4]. Due to the anisotropic characteristic of SnSe alloys, the thermal and electrical properties are measured in the same direction. The Vickers hardness test was conducted with an indentation force of 2.94 N maintained for 5 s on a hardness tester (HV-1000A). Three-point flexural tests were carried out at room temperature on samples for measurement of the flexural strength using a Z-wick testing system (the bending span was fixed at 8 mm, the strain rate was 2 × 10E-4/s), with size 1.5 × 3 × 12 mm. Compression tests were also carried out using the Z-wick testing system with size 3 × 3 × 6 mm. The fracture toughness of the specimen was measured using the three-points flexural specimen by the single-edge notch beam test. The starting notch with a length of half the width was created at the midspan of the specimen using a diamond-wire saw instrument, and the width of the notch was less than 0.3 mm. The fracture toughness can be calculated with the following equation [24]:

$${K_{IC}}=\frac{{3PL\sqrt {a \times {{10}^{ - 3}}} }}{{2b{w^2}}}\left[ {1.93 - 3.07\left( {\frac{a}{w}} \right)+14.53{{\left( {\frac{a}{w}} \right)}^2} - 25.07{{\left( {\frac{a}{w}} \right)}^3}+25.08{{\left( {\frac{a}{w}} \right)}^4}} \right]$$

where KIC is fracture toughness, P is critical applied load, L is the bending span, b is the thickness, w is the height, and a is the depth of starting notch. At least 12 valid samples were used for each set of tests.

3 Results and discussion

3.1 Phase and microstructure

Powder X-ray diffraction patterns of the SnSe + x vol% K2Ti6O13 whiskers (x = 0, 1, 3, 5) samples are shown in Fig. 1. All diffraction peaks can be well indexed to the orthorhombic SnSe phase with a Pnma symmetry (PDF #48-1224). Since the K2Ti6O13 content is very low, the peaks of K2Ti6O13 were not found in the XRD patterns even for the sample dispersed with 5 vol% K2Ti6O13. In order to obtain the arbitrary section and reflect the state of K2Ti6O13 in the matrix more intuitively, the samples used for fracture scanning electron microscopy testing were soaked in liquid nitrogen for 30 min before breaking them. It can be seen from Fig. 2a that there are no cracks or voids in the bulk K2Ti6O13-free sample. The relative densities of the samples are 97.2%, 96.7%, 96.5% and 96.2%, for x = 0, 1, 3 and 5, respectively, which decrease slightly with increasing K2Ti6O13 fraction. The theoretical densities, dth of the composites are calculated using dth = (1-x)dSnSe + xdK2Ti6O13 in which x, dSnSe and dK2Ti6O13, are the volume fraction, theoretical density of SnSe (6.18 g/cm3) and theoretical density of K2Ti6O13 whiskers (3.3 g/cm3), respectively. Combined with ball milling and sintered by cubic high pressure apparatus with lower temperature and shorter time, no obvious orientation preference were obtained as shown in Fig. 2a, compared with spark plasma sintering and hot pressure methods [25, 26]. As shown in Fig. 2b–f, it was found that the K2Ti6O13 whiskers are uniformly dispersed and tightly incorporated throughout the matrix, significantly improving the mechanical properties [27,28,29].

Fig. 1
figure 1

XRD patterns of SnSe with different amounts of K2Ti6O13 whiskers. The inset shows the microstructure of K2Ti6O13 whiskers

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Fig. 2
figure 2

FE-SEM images of freshly fracture surfaces of SnSe + K2Ti6O13 whiskers composites: ax = 0 vol%, bx = 1 vol%, c, dx = 3 vol%, and e, fx = 5 vol%

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3.2 Thermoelectric properties

Figure 3 shows the temperature dependences of TE properties for the samples. As shown in Fig. 3a, the σ of all samples increase monotonically with increasing temperature, indicative of intrinsic semiconductor transport behavior. As the temperature increases from 300 to 700 K, the σ is very low due to the intrinsically low carrier concentration [4], then increases rapidly to the maximum at a certain temperature. The upturn of the σ should result from the thermal excitation of carriers [4, 30]. The σ of SnSe sintered by HPS is higher than those by other processed routes in literatures [31, 32], which may be attributed to the decreased band gap under high pressure [33, 34], or difference in composition due to the loss and oxidation under different technological conditions. In addition, the σ of the composites is slightly reduced by the addition of K2Ti6O13, due to the higher electrical resistivity of K2Ti6O13 (3.3 × 1013 Ω m). The S values for all the samples are positive, as shown in Fig. 3b, indicating that they belong to typical p-type semiconductor. With increasing temperature S decreases in the temperature range of 400–573 K and 673–810 K and increases from 573 to 673 K. A similar temperature dependence trend was observed for the reports [35], but the values were higher. This may be attributed to the increased carrier effective mass induced by high pressure in this work [36].

Fig. 3
figure 3

Temperature dependence of a the electrical conductivity σ, b the Seebeck coefficient S, c lattice thermal conductivity κL, and d the dimensionless figure of merit ZT. The inset in c shows total thermal conductivity κtol. The data of SnSe-SPS samples are shown for comparison [35]

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The lattice thermal conductivity κL (κL = κtol−κe, κe is electrical thermal conductivity κe = LσT, L = 1.5 × 10−8 V2/K2 is Lorentz number [4]) is shown in Fig. 3c. The κL constitutes the main factor of κtol as shown in the inset of Fig. 3c. The electronic contribution is less than 5% because of the low σ. Owing to the ultralow thermal conductivity (0.017 W/mK at 1033 K) of K2Ti6O13 whiskers, the κtol of the composites is lower than pure SnSe and a minimum ∼0.5 W/mK is obtained with 5 vol% K2Ti6O13 introduced. The thermoelectric figure of merit ZT (ZT = S2σT/κ, T is absolute temperature), for all samples is illustrated in Fig. 3d. Upon introducing K2Ti6O13, the ZT values show a slight decrease at elevated temperatures, but still achieves a comparable ZT ∼ 0.5 at 810 K when 5 vol% K2Ti6O13 is introduced [13, 37].

3.3 Mechanical properties

Mechanical properties, such as Vickers hardness, flexural strength, compressive strength and fracture toughness of the TE materials must be sufficiently robust to resist the thermal fatigue and stress as well as the potential for fracture, during the TE module in service. All the above mechanical parameters of the composites were improved significantly due to the excellent mechanical properties of K2Ti6O13, as shown in Fig. 4a–d.

Fig. 4
figure 4

Room temperature mechanical properties: a the Vickers hardness; b the flexural strength; c the compressive strength; d the fracture toughness

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Figure 4a shows that the Vickers hardness increases linearly from 669.3 to 876.6 MPa with K2Ti6O13 whiskers added, which is much higher than that of SnSe prepared by other methods [12, 13]. A 31% improvement in hardness is obtained by the addition of 5% K2Ti6O13 whiskers, further evidence that whiskers super strength (Mohs hardness: 4; modulus: 280 GPa) [38] has transferred to the matrix. The tight interface interaction between whiskers and matrix as shown in Fig. 2b–f may be another reason for the enhanced hardness. As shown in Fig. 4b, c, the flexural and compressive strengths of the composites are reached at 41.5 and 78.9 MPa, after introducing 5 vol% K2Ti6O13 whiskers, representing 98% and 86% improvements, respectively. Figure 4d depicts that the fracture toughness increases from 0.63 to 1.08 MPa m1/2 for the samples containing 5 vol% K2Ti6O13 whiskers, which represents 71% improvement. Compared with other literatures [13], the fracture toughness in this work is much higher. These remarkable enhancements may be owed to the increased plastic deformation of the matrix along the matrix-reinforcement interface, crack branching due to hindrance by reinforcements, and fiber bridging of the crack [39]. This bridging is accompanied by pull-outs of whiskers during fracture of the composites as shown in Fig. 2. The toughening mechanism for the composites containing K2Ti6O13 whiskers is crack deflection due to the load transfer between K2Ti6O13 and the matrix while a crack is propagating. All the results indicate that the dispersion of K2Ti6O13 whiskers is a promising strategy for enhancing the mechanical properties of polycrystalline SnSe and other TE materials.

4 Conclusion

In the present study, a novel reinforcement whisker K2Ti6O13, with low-cost, high strength, and good dispersibility were firstly introduced to TE materials as the reinforcement phase. These results indicate it is a promising strategy for enhancing the mechanical properties of the SnSe matrix via incorporation of K2Ti6O13. All the mechanical parameters of the composites have a substantial enhancement. In particular, the flexural strength, compressive strength and fracture toughness were improved by 98%, 86%, and 71% respectively, upon the addition of 5 vol% K2Ti6O13 whiskers. These whiskers embedded and incorporated with the matrixes which contribute to the significant enhancement of mechanical properties. A further enhancement might be expected if the damage to whiskers is reduced during ball milling and sintering, by using longer whiskers, or by adding whiskers and nanoparticles simultaneously.