Thermoelectric (Te) materials have attracted wide attention in recent decades because of their excellent residual heat generation capacity and wide application prospects in solid-state refrigeration [1-4]. The ZT value of SnSe single crystal reaches 2.6. As another member of IV-VI semiconductor, SnTe is not only a lead-free material, but also has similar crystal and band structure to PbTe crystal. SnTe is considered as one of the most promising thermoelectric materials. However, recent studies have shown that the ZT values of SnTe crystals are still very low because of their high thermal conductivity (usually larger than 2.5Wm).
In this study, the researchers designed a simple, ultra-fast, green, high-yield microwave hydrothermal synthesis of nanoparticles (NPs). The growth mechanism and morphology control technique of directional adhesion are discussed systematically. In order to further verify and understand the nanometer size effect, SnTe reference samples were prepared by ball milling combined with spark plasma sintering (SPS).
Compared with the thermoelectric properties of pure SnTe bulk materials, the ultra-low thermal conductivity (1.5 W m). 1 K1 to 0.60 SMI 1 K1323-800 K) and the relatively high Seebeck coefficient (58-90 μ V K) were found in NPs sintered samples with an average diameter of 165 nm. The ZT value is relatively high (about 0. 49 at 803K), which is related to the enhancement of phonon scattering and the enhancement of energy filtering effect.
Fig. 1： (a) Microwave hydrothermal synthesis system：(b)The temperature curve is a function of the reaction time.(c)XRD patterns of SnTe nanoparticles,(d) and (e) are SEM images at different magnifications,(f) is an octahedral structure simulating SnTe nanoparticles;(g), (h), and (i) are overall TEM, SEAD, and HRTEM images.
Fig. 2： (a), (b) and (c) are SEM images of SnTe NPs synthesized from 1.0 g ~ (-1) g NaOH.; (d) is the XRD map of samples (a) and (c). (e) and (h), (f) and (i), (g) and (j) are SEM images of SnTe particles synthesized from 1.50 g / 2. 50 g NaOH and 5.50 g NaOH.
Fig. 3： (a)The overall morphology of the sample after SPS, the inlays are rectangular columns and square wafers, used to test thermoelectric properties and Hall measurements; (b)is XRD pattern of undoped SnTe reference sample； (c), (d), (e) and (f) are detailed scanning electron microscopic images of dense samples sintered from mechanically alloyed powder (c) or particles of different diameters (165 nm (d), 550 nm (e) and 8.2 mm (f).
Fig. 4： the properties of dense samples sintered from 165 nm,550 nm nanocrystalline (NPs), 8.2mm micron (MPS) and mechanically alloyed (MA) powders are the total thermal conductivity of: (a), (b) lattice thermal conductivity. (c) conductivity, (d) and (e) transport mechanism, (f) hole mobility carrier concentration and mmx*/0* ratio are used as function, (g) Seebeck coefficients to reduce grain size. (h) power factor and (i) ZT value.