Effects of Preparation Methods on the Thermoelectric Performance of SWCNT/Bi2Te3 Bulk Composites

Single-walled carbon nanotube (SWCNT)/Bi2Te3 composite powders were fabricated via a one-step in situ reductive method, and their corresponding bulk composites were prepared by a cold-pressing combing pressureless sintering process or a hot-pressing process. The influences of the preparation methods on the thermoelectric properties of the SWCNT/Bi2Te3 bulk composites were investigated. All the bulk composites showed negative Seebeck coefficients, indicating n-type conduction. A maximum power factor of 891.6 μWm−1K−2 at 340 K was achieved for the SWCNT/Bi2Te3 bulk composites with 0.5 wt % SWCNTs prepared by a hot-pressing process, which was ~5 times higher than that of the bulk composites (167.7 μWm−1K−2 at 300 K) prepared by a cold-pressing combing pressureless sintering process, and ~23 times higher than that of the bulk composites (38.6 μWm−1K−2 at 300 K) prepared by a cold-pressing process, mainly due to the enhanced density of the hot-pressed bulk composites.


Introduction
According to the Seebeck coefficient, thermoelectric (TE) materials can directly convert waste heat into useful electrical energy [1,2]. The TE properties of a material are judged by the figure of merit, ZT = S 2 σT/κ, where S is the Seebeck coefficient, σ is the electrical conductivity, T is the absolute temperature, and κ is the thermal conductivity [3][4][5][6]. So far, the highest ZT value of 2.6 has been reported for SnSe [7]. The actual applied TE devices near room temperature (RT) are mainly made of inorganic bulk materials, e.g., Bi-Te-based alloys due to them being one of the most widely studied TE materials and having exhibited a high ZT value at RT [8][9][10]; however, the energy conversion efficiency of TE devices is still much lower than that of the maximum possible Carnot efficiency [2], which has limited the wide application of TE devices. In order to scale out the application of TE devices, the TE performance of inorganic bulk materials should be improved (e.g., optimizing their preparation technologies and compositions), and the conversion efficiency of thermoelectric devices should also be enhanced [11] (e.g., optimizing their geometric structures [12]).
Carbon nanotubes (CNTs) have extraordinary electrical and mechanical properties [13,14] and can form conductive network structures in a relatively low threshold value when used as fillers for preparation of composite materials; therefore, introducing CNTs into inorganic TE matrixes is an effective method to further improve the TE properties of inorganic materials [15][16][17][18][19][20]. For instance, Choi et al. [15] prepared Te nanowires using a chemical method and then synthesized flexible singlewalled carbon nanotube (SWCNT)/Te nanowire films via a vacuum filtering method, and a power factor of 3.40 μWm −1 K −2 was obtained for the composite film with 2 wt % SWCNTs at RT. Zhang et al. [16] first fabricated multi-walled carbon nanotube (MWCNT)/Bi2Te3 nanocomposite powders via a cryogenic grinding method and then prepared bulk composites through a spark plasma sintering process, and a power factor (PF = S 2 σ) of ~800 μWm −1 K −2 (ZT value of ~0.3) at 300 K was obtained for the composite with 3 vol% MWCNTs. Ahmad et al. [17] first fabricated SWCNT/Bi2Te3 nanocomposite powders via a mechanical mixing method and then prepared bulk composites through a high-frequency induction-heated sintering furnace, and a PF of ~900 μWm −1 K −2 (ZT value of ~0.32) at 300 K was obtained for the composite with 0.5 wt % SWCNTs. Kim et al. [18] prepared MWCNT/Bi2Te3 nanocomposite powders via a chemical route combing ball milling method and then fabricated corresponding bulk composites via a spark plasma sintering process, and a PF of 1060 μWm −1 K −2 (ZT value of 0.48) at 300 K was achieved for the bulk composite with 0.7 wt % MWCNTs. Hosokawa et al. [19] firstly synthesized Bi2Te3 nanoplates via a solvothermal method and then prepared Bi2Te3 nanoplate/SWCNT composite film through a drop-casting technique, and a PF of 91 μWm −1 K −2 for the composite film at 300 K was obtained after an annealing process was applied. Jin et al. [20] fabricated highly ordered Bi2Te3 nanocrystals anchored on an SWCNT network using a magnetron sputtering technique, and a power factor of ~1600 μWm −1 K −2 (ZT value of 0.89) at RT was obtained. However, so far, systematic research of SWCNT/Bi2Te3 bulk composites using the hydrothermal synthesized SWCNT/Bi2Te3 nanopowders as raw materials, followed by a coldpressing combing pressureless sintering process or a hot-pressing process, is limited. Herein, SWCNT/Bi2Te3 composite powders were fabricated via a one-step in situ reductive method, and their corresponding bulk composites were prepared by a cold-pressing combing pressureless sintering process or a hot-pressing process. The effects of the preparation methods on the composition, microstructure, and TE properties of the bulk composites were investigated.

Preparation of the SWCNT/Bi2Te3 Composite Powders
SWCNT/Bi2Te3 composite powders were prepared via a one-step in situ reductive method [21]. It mainly contained the following three steps. (1) An appropriate amount of SWCNTs was added in deionized water (80 mL) and then ultrasonicated for 1 h to form Solution I. C4H4Na2O6·2H2O (2 g), TeO2 (0.48 g), and KOH (4.5 g) were added to Solution I and then stirred for 0.5 h to form Solution II. Bi(NO3)3·5H2O (0.97 g) was added into Solution II and then stirred for 2 h to form Solution III. KBH4 (1.2 g) was added into Solution III and then stirred for 0.5 h to form Solution IV. (2) Solution IV was transferred to the sealed Teflon-lined autoclave and then heated for 24 h at 180 °C before naturally being cooled to RT. (3) The reaction products were washed several times by deionized water and absolute ethyl alcohol, and then dried at 60 °C for 6 h in a vacuum. The weight percentages (nominal compositions) of SWCNTs in the SWCNT/Bi2Te3 composite powders were 0.25, 0.5, and 0.75 wt %, respectively. Figure 1a illustrates the preparation procedures of the SWCNT/Bi2Te3 composite powders.

Preparation of the SWCNT/Bi2Te3 Bulk Composites
Cold-pressing combing pressureless sintering process: SWCNT/Bi2Te3 composite powders with different contents of SWCNTs from 0.25 to 0.75 wt % were cold-pressed into pellets at 30 MPa for 0.5 h, and then the bulk composites with 0.5 wt % SWCNTs were pressurelessly sintered at different temperatures (598, 648, or 698 K) for 1 h in an argon (Ar) atmosphere, respectively, before naturally being cooled to RT. The resulting bulk composites are denoted as CNT/BT-PS-598, CNT/BT-PS-648, and CNT/BT-PS-698, respectively. The thicknesses of the as-prepared samples were ~1.3 mm. Figure  1b illustrates the preparation procedures of the SWCNT/Bi2Te3 bulk composites by a cold-pressing combing pressureless sintering process.
Hot-pressing process: SWCNT/Bi2Te3 composite powders with 0.5 wt % SWCNTs were hotpressed into pellets in a graphite die (diameter: 12.7 mm) under vacuum at 80 MPa (648 K) for 1 h, and then naturally cooled to RT. The resulting bulk composites are denoted as CNT/BT-HP-648. The thicknesses of the hot-pressed SWCNT/Bi2Te3 bulk composites were ~1.55 mm. Figure 1c illustrates the preparation procedures of SWCNT/Bi2Te3 bulk composites by a hot-pressing process.

Sample Characterization
The compositions of the as-prepared SWCNT/Bi2Te3 composites were characterized by X-ray powder diffraction (XRD, D/max 2200PC, Rigaku, Tokyo, Japan). The morphologies of the samples were characterized by high-resolution transmission electron microscopy (HRTEM, Technai G2 F20, FEI, Waltham, MA, USA) and scanning electron microscopy (SEM, Philips PW6800/70, Philips, Amsterdam, The Netherlands). The Seebeck coefficient and electrical conductivity were measured simultaneously from 300 to 360 K by an MRS-3L thin-film thermoelectric test system (Wuhan Giant Instrument Technology Co., Ltd., Wuhan, China) in a low-vacuum atmosphere (≤40 Pa). The instrument test errors are 6% and 5% for the Seebeck coefficient and electrical conductivity, respectively. The thicknesses of the samples were measured by a helical micrometer (Links 150-0.01, Links, Harbin, China). By calculating the ratios of measured density/theoretical density of the SWCNT/Bi2Te3 bulk materials, the corresponding relative density of the as-prepared sample was obtained. The densities of Bi2Te3 and SWCNTs used for calculating the theoretical density of the SWCNT/Bi2Te3 bulk materials were 7.86 [22] and 2.1 g/cm 3 (from the manufacturer), respectively.  [23,24]. No obvious characteristic peak of SWCNTs is observed as the content of SWCNTs increases from 0.25 to 0.75 wt %, mainly due to the low content of SWCNTs, which agrees with a previous report in Reference [17].   Figure 3f, it can be seen that the lattice spacing is ~0.32 nm, which corresponds to the (015) lattice plane of Bi2Te3, indicating good crystallinity of the Bi2Te3 particles in the composite powders [26]. At 300 K, as the content of SWCNTs increases from 0.25 to 0.75 wt %, the σ of the cold-pressed SWCNT/Bi2Te3 bulk composites enhances from 39.5 to 62.9 S/cm, while the absolute values of Seebeck coefficients (|S|) are in the range of 70-90 μV/K (the Seebeck coefficients are negative, indicating ntype conduction), and a highest power factor of 38.6 μWm −1 K −2 is obtained for the bulk composites with 0.5 wt % SWCNTs. In order to further enhance the TE properties of the cold-pressed samples, the pressureless sintering and hot-pressing processes were applied for the SWCNT/Bi2Te3 bulk composites with 0.5 wt % SWCNTs, respectively. Figure 4a-c shows the fracture surface SEM images of CNT/BT-PS-598, CNT/BT-PS-648, and CNT/BT-PS-698, respectively. Figure 4d shows the fracture surface SEM image of CNT/BT-HP-648. It can be seen that the SWCNTs are partly embedded into the Bi2Te3 matrix, and there are some voids both in the bulk composites prepared by the pressureless sintering process and the hot-pressing process. The content of voids in CNT/BT-HP-648 is much lower than those of CNT/BT-PS-598, CNT/BT-PS-648, and CNT/BT-PS-698, mainly because CNT/BT-HP-648 has undergone a high pressure (80 MPa). An obviously oriented microstructure was observed in CNT/BT-HP-648 (Figure 4d), mainly due to the anisotropy of Bi2Te3, which agrees with the results reported in Reference [24]. The peak intensity of the (0015) plane (~44.5°) significantly increases in CNT/BT-HP-648 ( Figure S1) when compared with the as-prepared composite powders (Figure 2), which also confirms the oriented microstructure of CNT/BT-HP-648.  , as a function of temperature from 300 to 360 K. At 300 K, as the pressureless sintering temperature rises from 598 to 698 K, the σ increases from 117.8 to 325.8 S/cm, while the |S| decreases from 101.4 to 71.8 μV/K, and a highest power factor of 167.7 μWm −1 K −2 is obtained for the CNT/BT-PS-698, which is ~4 times higher than that of the cold-pressed samples (38.6 μWm −1 K −2 at 300 K) without having undergone pressureless sintering treatment, indicating that pressureless sintering treatment is an effective method to enhance the TE properties of the cold-pressed samples. As the measurement temperature rises from 300 to 360 K, the σ of CNT/BT-PS-598, CNT/BT-PS-648, and CNT/BT-PS-698 remains basically unchanged, while the absolute values of the Seebeck coefficient first reduce and then enhance. As a result, the power factors of CNT/BT-PS-598, CNT/BT-PS-648, and CNT/BT-PS-698 first decrease and then increase-for instance, the power factor of CNT/BT-PS-698 decreases from 167.7 (300 K) to 151.3 μWm −1 K −2 (320 K), then it increases to 168.0 μWm −1 K −2 at 360 K. Figure 5d presents the temperature dependence of TE properties of CNT/BT-HP-648 from 300 to 360 K. The σ of CNT/BT-HP-648 is 782.6 S/cm at 300 K, which is higher than that of a Bi2Te3 nanoplate/SWCNT film (111.9 S/cm at 300 K) [19] and that of an SWCNT/Bi2Te3 bulk material (~700 S/cm at 300 K) [17], while close to that of an MWCNT/Bi2Te3 bulk (~790 S/cm at 300 K) [18]. The |S| of CNT/BT-HP-648 is 101.4 μV/K at 300 K, which is higher than that of a Bi2Te3 nanoplate/SWCNT composite film (90.1 μV/K at 300 K) [19]; however, it is lower than those of an MWCNT/Bi2Te3 bulk material (113 μV/K at 300 K) [18] and an SWCNT/Bi2Te3 bulk material (~115 μV/K at 300 K) [17]. As the measurement temperature rises from 300 to 360 K, the σ of CNT/BT-HP-648 shows a decreasing tendency, which suggests the degenerate semiconductor behavior of the hot-pressed bulk composites [27]. The |S| of CNT/BT-HP-648 first increases and then decreases in the measurement temperature ranges from 300 to 360 K, and the |S| achieves 109.2 μV/K at 340 K. The power factor of CNT/BT-HP-648 first increases and then decreases, due to the same trend of |S|, and a highest power factor of 891.6 μWm −1 K −2 is obtained at 340 K. This value (891.6 μWm −1 K −2 ) is higher than those of the bulk composites prepared by a cold-pressing combing pressureless sintering process (168.0 μWm −1 K −2 at 360 K) and the Bi2Te3 nanoplate/SWCNT composite film (91 μWm −1 K −2 at 300 K) [19]; however, it is lower than those of the SWCNT/Bi2Te3 bulk material (~980 μWm −1 K −2 at 400 K) [17] and the MWCNT/Bi2Te3 bulk material (~1180 μWm −1 K −2 at 423 K) [18].  Table 1. As the pressureless sintering temperature rises from 598 to 698 K, the relative densities increase slowly from 78.19% to 81.72%. The relative density of CNT/BT-HP-648 (91.51%) is much higher than those of CNT/BT-PS-598, CNT/BT-PS-648, and CNT/BT-PS-698, which is beneficial to improve the σ, as well as the power factor of the as-prepared samples, and it might be the main reason for the higher TE properties of the hot-pressed bulk composites, when compared with the bulk composites prepared by the cold-pressing combing pressureless sintering process. Additionally, it is noted there are still a lot of voids in the bulk composites, which could scatter the phonons and charge carriers, decrease the thermal conductivity, and then improve the ZT value of the bulk composites [28].

Conclusions
SWCNT/Bi2Te3 bulk composites were prepared by a cold-pressing combing pressureless sintering process or a hot-pressing process, using the hydrothermal synthesized SWCNT/Bi2Te3 nanopowders as raw materials. For the bulk composites with 0.5 wt % SWCNTs prepared by a coldpressing combing pressureless sintering process, as the pressureless sintering temperature increased from 598 to 698 K, the electrical conductivity was enhanced, while the absolute value of Seebeck coefficient reduced, and a power factor of 168.0 μWm −1 K −2 at 360 K was obtained, which is much higher than that of the bulk composites (38.6 μWm −1 K −2 at 300 K) prepared by a cold-pressing process; however, it is much lower than that of the bulk composites prepared by a hot-pressing process (891.6 μWm −1 K −2 at 340 K); the main reason is that the hot-pressed SWCNT/Bi2Te3 bulk composites have much higher relative densities. To this end, the hot-pressing process is a better choice than the coldpressing or cold-pressing combing pressureless sintering processes, although it needs expensive equipment. The TE performance of the SWCNT/Bi2Te3 bulk composites could be further improved by enhancing their relative densities via optimizing the parameters for the hot-pressing process, or controlling the parameters of the gradient cooling process for the cold-pressing combing pressureless sintering process. The as-prepared SWCNT/Bi2Te3 bulk composites have a potential for thermoelectric devices.