Research / 2020 / Article

Research Article | Open Access

Volume 2020 |Article ID 6591981 | https://doi.org/10.34133/2020/6591981

Jiasheng Liang, Pengfei Qiu, Yuan Zhu, Hui Huang, Zhiqiang Gao, Zhen Zhang, Xun Shi, Lidong Chen, "Crystalline Structure-Dependent Mechanical and Thermoelectric Performance in System", Research, vol. 2020, Article ID 6591981, 10 pages, 2020. https://doi.org/10.34133/2020/6591981

Crystalline Structure-Dependent Mechanical and Thermoelectric Performance in System

Received21 Apr 2020
Accepted17 Jun 2020
Published31 Jul 2020

Abstract

Self-powered wearable electronics require thermoelectric materials simultaneously with a high dimensionless figure of merit () and good flexibility to convert the heat discharged by the human body into electricity. Ag2(S,Se)-based semiconducting materials can well satisfy these requirements, and thus, they are attracting great attention in thermoelectric society recently. Ag2(S,Se) crystalizes in an orthorhombic structure or monoclinic structure, depending on the detailed S/Se atomic ratio, but the relationship between its crystalline structure and mechanical/thermoelectric performance is still unclear to date. In this study, a series of (, 0.1, 0.2, 0.3, 0.4, and 0.45) samples were prepared and their mechanical and thermoelectric performance dependence on the crystalline structure was systematically investigated. in the system was found to be the transition boundary between orthorhombic and monoclinic structures. Mechanical property measurement shows that the orthorhombic samples are brittle while the monoclinic Ag2Se1‐xSx samples are ductile and flexible. In addition, the orthorhombic samples show better electrical transport performance and higher than the monoclinic samples under a comparable carrier concentration, most likely due to their weaker electron-phonon interactions. This study sheds light on the further development of flexible inorganic TE materials.

1. Introduction

Recently, thermoelectric (TE) technology shows a great potential to be used as a sustainable power source in wearable electronics [13]. Via harvesting the heat discharged by the human body and converting it into electricity, the wearable electronics using TE technology can be self-powered without using any external batteries. The energy conversion efficiency of a TE material is determined by the dimensionless figure of merit, , where is the Seebeck coefficient, is the electrical conductivity, is the thermal conductivity, and is the absolute temperature. The TE material used in wearable electronics should possess high to maximize the energy conversion efficiency and good flexibility to match the curved surface of skin and endure repeated bending during service [46].

To date, Bi2Te3-based alloys are still the best room-temperature TE materials, but their application in wearable electronics is limited due to their inherent brittleness [714]. In contrast, the organic TE polymers show good flexibility, but their s are low [1520]. Recently, we reported a class of Ag2S-based flexible inorganic TE materials with excellent intrinsic flexibility and high s [21]. Monoclinic Ag2S shows surprisingly high ductility at room temperature due to its wrinkled layer structure (space group , with the sketch map shown in Figure 1(a), and weak Ag-S chemical bonds [22, 23]. However, its relatively wide band gap (around 1 eV) yields low only in the order of 10-1 S m-1. As a result, of the stoichiometric Ag2S around room temperature is very poor. Alloying Se or/and Te in Ag2S can significantly improve and enhance to 0.26 for Ag2S0.5Se0.5 and 0.44 for Ag2S0.5Se0.45Te0.05 at 300 K. Particularly, previous study showed that the good ductility can be well maintained when the Se alloying content in Ag2S reaches 50% or the Te alloying content reaches 20%, enabling these materials very suitable to be used in flexible wearable electronics. The above results arouse great interest on the Ag2S-based materials in TE society [21, 2325].

Being different with ductile Ag2S, Ag2Se is a brittle material. It adopts an orthorhombic structure (space group ) with the sketch map shown in Figure 1(b). The band gap of orthorhombic Ag2Se is around 0.2 eV, about one-fifth of that of monoclinic Ag2S. Orthorhombic Ag2Se has very high carrier mobility (~103 cm-2 V-1 S-1), excellent (~105 S m-1), and extremely low lattice thermal conductivity (~0.3-0.5 W m-1 K-1), resulting in high about 0.4~1.0 at 300 K [2632]. Despite the different crystalline structures of Ag2S and Ag2Se, Bindi and Pingitore [33] reported that Ag2S and Ag2Se can form a continuous solid solution. The room-temperature crystalline structure of solid solution is the same with the monoclinic Ag2S when , but the same with the orthorhombic Ag2Se when . Considering that the carrier concentration of Ag2Se is about several orders of magnitude higher than that of Ag2S [22], solid solution might process continuous adjustable carrier concentrations and TE properties. The mechanical properties and TE performance of () have been already reported previously [33], but those of () have not been investigated yet. Particularly, the relationship between crystalline structure and mechanical/TE performance for solid solution is still unclear so far.

In this study, a series of polycrystalline (, 0.1, 0.2, 0.3, 0.4, and 0.45) samples were prepared. Their crystalline structures, mechanical properties, and TE properties were systematically investigated. in the system was identified to be the transition boundary of the orthorhombic structure and monoclinic structure. Orthorhombic (, 0.1, and 0.2) samples are brittle, while monoclinic ( and 0.45) are ductile and flexible. Due to the stronger electron-phonon interaction, monoclinic Ag2Se0.6S0.4 has lower PF and than orthorhombic (, 0.1, and 0.2). However, the superior TE performance and thermal stability to the organic TE materials and the intrinsically good flexibility still promise a great potential for monoclinic to be used in wearable electronics.

2. Results and Discussion

Figure 1(c) shows the X-ray diffraction patterns for the prepared (, 0.1, 0.2, 0.3, 0.4, and 0.45) obtained at room temperature. When the content , the diffraction peaks are well indexed to the orthorhombic structure of Ag2Se with the space group of . This indicates that Ag2Se0.9S0.1 and Ag2Se0.8S0.2 still adopt the same crystalline structure with Ag2Se at room temperature. When and 0.45, the diffraction peaks agree well with the monoclinic structure of Ag2S phase with the space group of . Likewise, the () samples also crystalize in the same monoclinic structure of Ag2S [21]. Elemental energy dispersive spectroscopy (EDS) mappings confirm that all elements are homogeneously distributed in Ag2Se0.9S0.1, Ag2Se0.8S0.2, Ag2Se0.6S0.4, and Ag2Se0.55S0.45. No obvious secondary phase is observed. Thus, these samples are phase pure. Likewise, scanning electron microscopy (SEM) performed on the cross-section of Ag2Se0.6S0.4 indicates that it has an obvious layered structure (cf. Fig. S1).

One special composition in the system is Ag2Se0.7S0.3. As shown in Figure 1(c), its X-ray diffraction pattern looks like that of monoclinic Ag2Se0.6S0.4. However, besides those belonging to the monoclinic structure, some extra diffraction peaks with weak intensities are also observed, such as (102) at and (112) at (right panel in Figure 1(c)). These are indexed to the orthorhombic structure. Thus, Ag2Se0.7S0.3 is believed to be a mixture rather than a pure phase, in which the main phase crystalizes in the monoclinic structure while the secondary phase crystalizes in the orthorhombic structure. This result is different from that proposed by Bindi and Pingitore [33] that Ag2Se0.7S0.3 crystalizes in the same orthorhombic structure with Ag2Se. Figure 1(d) shows the EDS mappings performed on Ag2Se0.7S0.3. Surprisingly, all elements are still homogeneously distributed. Thus, it is concluded that the two different phases in Ag2Se0.7S0.3 have very similar chemical compositions but different crystalline structures. A similar polymorphic feature has been also observed in the system [34]. Combining the above result, it can be concluded that the transition boundary between the monoclinic and orthorhombic structures in solid solution should be around .

(, 0.1, 0.2, 0.3, 0.4, and 0.45) samples possess structure-dependent mechanical properties. The three-point bending test and Vickers hardness test were performed on . The material with larger bending deformation or small Vickers hardness is usually a benefit for the application in flexible electronics [2, 3, 6]. Figure 2(a) shows that orthorhombic Ag2Se can only endure a very small bending strain before cracking. The maximum bending deformation is about 0.56%. Likewise, orthorhombic Ag2Se0.9S0.1 also possesses a maximum bending deformation around 1.5%. The case is different for monoclinic Ag2Se0.6S0.4, which exhibits a bending deformation above 10% without cracking. Previously, large bending deformation was also observed for monoclinic (, 0.7, 0.9, and 1) in the bending test. It should be noted that such deformation is plastic, which is different from the elastic deformation of brittle materials with low dimension. Likewise, Vickers hardness for orthorhombic Ag2Se is 43.7 Hv at room temperature. It is monotonously increased to 51.5 Hv and 62.7 Hv for the orthorhombic Ag2Se0.9S0.1 and Ag2Se0.8S0.2, respectively. However, monoclinic Ag2Se0.6S0.4 has low Vickers hardness of only 33.8 Hv, about half of that for orthorhombic Ag2Se0.8S0.2. As shown in Figure 2(c), Ag2Se0.6S0.4 can be directly cut into thin strips with thickness about 0.1 mm like metal. Furthermore, these metal-like strips can be twisted into various shapes without cracking, confirming that Ag2Se0.6S0.4 has good ductility and flexibility.

The above test results prove that the monoclinic structure has significantly better ductility and flexibility in the system. The high cleavage energy and low slipping energy, related to the unique wrinkled layer structure, are the fundamental reason for the good ductility observed in monoclinic . Previous investigation found that the S atoms in monoclinic Ag2S always bonded to part of the surrounding Ag atoms during slipping [22, 35], which is benefit for preventing the material’s cleavage under mechanical stress. Likewise, it enables the bonding energy between Ag and S atoms to change fluently, yielding a low energy barrier for slipping. However, in the orthorhombic structure, such unique wrinkled layer structure does not exist; thus, the orthorhombic samples are brittle. Here, it should be noted that although the main phase in Ag2Se0.7S0.3 crystalizes in the monoclinic structure, Ag2Se0.7S0.3 still has low maximum bending deformation around 0.88%. The reason should be attributed to the presence of brittle orthorhombic grains among the ductile monoclinic grains. The second phase strengthening effect caused by these brittle orthorhombic grains will impede the movement of atoms, dislocations, or interfaces under mechanical stress like that in the pure orthorhombic phase, yielding poor ductility for Ag2Se0.7S0.3.

Figure 3 shows the measured and for (, 0.1, 0.2, 0.3, 0.4, and 0.45). All samples possess negative , indicating that they are -type semiconductors with electrons dominating the electrical transports. This scenario is the same with those () samples. Obvious and discontinuous jumps are observed between 360 K and 420 K, which are attributed to the orthorhombic-cubic or monoclinic-cubic phase transition. For example, Ag2Se experiences orthorhombic-cubic phase transition around 410 K. At 300 K, for Ag2Se is about  S·m-1. It increases with increasing temperature, reaching around  S·m-1 at 403 K, and then decreases to  S·m-1 after the phase transition. The value for Ag2Se0.55S0.45 at 300 K is only  S·m-1, almost one-fourth of the pristine Ag2Se. Likewise, the values vary from -146 μV·K-1 to -105 μV·K-1 at 300 K and from -45 μV·K-1 to -109 μV·K-1 at 420 K, without obvious chemical composition dependence. Furthermore, Fig. S2 shows that the prepared samples have good reproducibility. When the chemical composition and fabrication process are fixed, different batches of samples have comparable and data.

With the purpose to understand the electrical transport properties of , Hall measurements were performed. The Hall carrier concentration () and mobility () at 300 K are listed in Table SI. All samples possess in the order of 1018 cm-3. However, being similar with the above measured and , the values do not have monotonous variation with the sulfur content. The different contents of intrinsic defects inside the lattice, such as Ag interstitials, are expected to be the reason for this scenario. Likewise, the values for the samples are in the range of 309 cm2·V-1·s-1 to 1337 cm2·V-1·s-1. These values are quite high as compared with other state-of-the-art TE materials, such as 190 cm2·V-1·s-1 for -type Bi2Te3 [36] and 48 cm2·V-1·s-1 for -type filled skutterudites [37]. A single parabolic model (SPB) is used to further understand the electrical transports of . According to the SPB model, the Seebeck coefficient, carrier concentration, and carrier mobility can be correlated as follows [3840]: where is the Boltzmann constant, is the scattering factor, is the electron charge, is the density-of-state effective mass, is the Hall factor, is the sample density, is the velocity of longitudinal sound waves, is the drift mobility for acoustic phonon scattering in the nondegenerate limit, and Ξ is the deformation potential. is the Fermi integrals, and it is given by equation , where represents the reduced carrier energy and is the reduced Fermi energy.

Figure 3(d) presents that the and data for the present (, 0.1, 0.2, 0.3, 0.4, and 0.45) samples roughly fall around the calculated theoretical Pisarenko curve with ( is the mass of free electron) and (acoustic phonon scattering). For comparison, the previously reported data for Ag2Se and Ag2Se0.5S0.5 are also summarized in Figure 3(d). These data also fall around the theoretical curve. This scenario suggests that all these samples might possess a similar band structure near the Fermi level, despite the fact that some of them crystalize in the monoclinic structure while others in the orthorhombic structure. This is possible because the conduction band minimums of Ag2S and Ag2Se are at the same gamma point and both of them are mainly dominated by Ag-5s electrons [21, 41]. Alloying S at the Se sites would mainly alter the valence band maximum instead of the conduction band minimum, yielding the similar mentioned above. In addition, the small , about for these Ag2(S,Se) samples, are responsible for their higher than those for -type Bi2Te3 [36] and filled skutterudites [37] mentioned above.

Figure 3(e) plots the and relationship at 300 K for (, 0.1, 0.2, 0.4, 0.45, and 0.5). The data for Ag2Se0.7S0.3 are not included because its polymorphic feature might introduce extra boundary scattering to electrons and interrupt the understanding on the carrier mobility. As shown in Figure 3(e), under the similar , the (, 0.1, and 0.2) samples with an orthorhombic structure have higher than the (, 0.45, and 0.5) samples with a monoclinic structure. By fitting the experimental data of the and relationship using Equations (1)–(5), deformation potential values of 11 eV and 19 eV can be extracted for the orthorhombic (, 0.1, and 0.2) and the monoclinic (0.4, 0.45, and 0.5), respectively. The thermal vibrations of the crystal lattice can influence the energy-band structure and perturb the carrier transports. The intensity of such interaction between electrons and phonons can be reflected by the magnitude of Ξ [42]. The lower Ξ value for the orthorhombic suggests that the electron-phonon interaction wherein is weaker than that in the monoclinic . This is responsible for the higher observed for the orthorhombic .

The power factors (PF) for (, 0.1, 0.2, 0.3, 0.4, and 0.45) samples calculated from the formula are shown in Figure 3(c). At 300 K, the PF value for Ag2Se reaches 22.5 μW·cm-1·K-2. It gradually decreases with increasing the S-alloying content. At 300 K, the PF for Ag2Se0.55S0.45 is just 5.2 μW·cm-1·K-2, about one-fourth of that for Ag2Se. The greatly reduced is responsible for the lowered PF. The PF value for Ag2Se0.6S0.4 is comparable with that for Ag2Se0.5S0.5 reported before [21, 24], about 4.8 μW·cm-1·K-2 at 300 K. Based on the SPB model, the theoretical PF vs. curves at 300 K can be obtained for the orthorhombic and monoclinic samples, respectively. As shown in Figure 3(f), the experimental PF and data basically fall around these lines. Under the comparable , the PF for the orthorhombic samples are much higher than those of the monoclinic samples. The reason is that the orthorhombic samples have weaker electron-phonon interaction than the monoclinic samples, which yields lower Ξ value for higher and PF. In addition, based on the SPB model, the optimal carrier concentration () corresponding to the peak PF is around  cm-3 for both orthorhombic and monoclinic . for the present orthorhombic (, 0.1, and 0.2) and monoclinic () samples are already close to this value.

Figure 4(a) shows as a function of temperature for (, 0.1, 0.2, 0.3, 0.4, and 0.45). The discontinuous jumps on the curves are attributed to the orthorhombic-cubic or monoclinic-cubic phase transitions. ranges from 0.7 W·m-1·K-1 to 1.4 W·m-1·K-1 for all samples. Ag2Se0.6S0.4 exhibits the lowest among all samples. for Ag2Se0.6S0.4 at 300 K is around 0.7 W·m-1·K-1. This value is about 36% decrement as compared with that for Ag2Se0.8S0.2. Generally, is composed of two parts: the electron part and the lattice part . can be calculated according to the Wiedemann-Franz law [43]. Figure 4(b) shows the calculated for (, 0.1, 0.2, 0.3, 0.4, and 0.45). It can be seen that the values are in the range of 48% to 80%, indicating that the carriers’ contribution is very important in the thermal transports. By subtracting from , can be obtained. The results at 300 K are listed in Table SI. They range from 0.20 W·m-1·K-1 to 0.48 W·m-1·K-1, which are very low values as compared with those for the state-of-the-art TE materials [4449]. The proximity to the cubic superionic phase is responsible for the low values observed in these orthorhombic and monoclinic phases [23].

Based on the measured , , and , can be calculated according to for orthorhombic Ag2Se is around 0.6 in the whole measured temperature range, which is comparable with those reported before [2830]. Orthorhombic Ag2Se0.9S0.1 and Ag2Se0.8S0.2 possess comparable with Ag2Se. However, for monoclinic Ag2Se0.6S0.4 and Ag2Se0.55S0.45 are much lower than those for orthorhombic Ag2Se. At 300 K, is just 0.26 for Ag2Se0.6S0.4 and 0.23 for Ag2Se0.55S0.45.

As shown in Figure 4(d), the theoretical vs. curves for orthorhombic and monoclinic samples at 300 K are given by assuming that all samples possess the same  W m-1 K-1. The experimental and data basically fall around these lines. Under the comparable , for the orthorhombic samples is much higher than that of the monoclinic samples because their weaker electron-phonon interaction yields lower Ξ value and larger for higher PF. The optimal carrier concentration () corresponding to the peak is around  cm-3 for orthorhombic , while  cm-3 for monoclinic . for the present orthorhombic (, 0.1, and 0.2) and monoclinic (, 0.45, and 0.5) samples are still higher than these values. Thus, if their carrier concentration can be further reduced, higher can be expected.

Although the TE performances for ductile monoclinic are lower than those for brittle orthorhombic , their performances are still much higher than those for the organic TE materials reported before. This can be clearly reflected by the scenario shown in Figure 5. The PF values for most of the -type TE organic TE materials are lower than 1 μW·cm-1·K-2 at 300 K. Even for the best -type TE organic TE materials reported so far, poly(Ni-ett) [19], its PF, 3.6 μW·cm-1·K-2 at 300 K, is still lower than the present monoclinic . The superior PF values achieved in the monoclinic samples would promise high power output when they are fabricated into TE devices. In addition, most -type organic TE materials are unstable in the air because their electron affinity is too low to stabilize the -type dopants [2, 50]. In contrast, the present ductile monoclinic samples are inert to oxygen or water; thus, they can realize good service stability in real application. Combining the intrinsically good flexibility and ductility, these monoclinic samples show great potential to be used in wearable electronics.

3. Conclusion

In summary, this study systematically studied the crystalline structure, mechanical properties, and TE properties of (, 0.1, 0.2, 0.3, 0.4, and 0.45). samples crystalize in the orthorhombic structure when , while in the monoclinic structure when . Only the monoclinic samples possess good ductility and flexibility. Under comparable carrier concentration range, the orthorhombic samples have higher carrier mobility, larger power factor, and better than the monoclinic samples because they have weaker electron-phonon interaction. This leads to lower values of monoclinic ( and 0.45) samples than those of the orthorhombic ( and 0.2) samples. However, the higher PF and better thermal stability promise a great potential for monoclinic to be used in wearable electronics.

Conflicts of Interest

The authors declare no competing financial interests.

Authors’ Contributions

P.Q., X.S., and L.C. conceived the idea. J.L. conducted the synthesis, TE transport property measurement, and bending test. Z.G. and H.H. conducted the Hall measurement and Vickers hardness measurements. J.L, P.Q., Y.Z. and Z.Z. contributed to data analysis, model interpretation, and paper drafting. J.L., P.Q., and X.S. completed the writing of the manuscript. All authors discussed the results and gave comments regarding the manuscript.

Acknowledgments

This work is supported by the National Key Research and Development Program of China (2018YFB0703600) and the National Natural Science Foundation of China (51625205, 91963208, and 5181101519). X.S. thanks the support by the CAS-DOE Program of Chinese Academy of Sciences under Grant No. 121631KYSB20180060. P.Q. thanks the support by the Youth Innovation Promotion Association of CAS under Grant No. 2016232 and Shanghai Rising-Star Program under Grant No. 19QA1410200. This work is also partially supported by the Swedish Research Council (VR) via the Sweden-China collaborative project 2018-06030.

Supplementary Materials

Materials and methods. Fig. S1: cross-section image of Ag2Se0.6S0.4 bulk sample. Fig. S2: electrical properties for three batches of Ag2Se0.8S0.2 and Ag2Se0.6S0.4. Table SI: room-temperature thermoelectric properties of samples. (Supplementary Materials)

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