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High pressure synthesis and thermoelectric performances of Cu2Se compounds
Physics Letters A 383 (2019) 125917
Contents lists available at ScienceDirect
Physics Letters A www.elsevier.com/locate/pla
High pressure synthesis and thermoelectric performances of Cu2 Se compounds Lisha Xue, Chao Fang, Weixia Shen, Manjie Shen, Wenting Ji, Yuewen Zhang ∗ , Zhuangfei Zhang ∗ , Xiaopeng Jia Key Laboratory of Material Physics of the Ministry of Education, School of Physics and Engineering, Zhengzhou University, Zhengzhou, 450052, China
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Article history: Received 15 May 2019 Received in revised form 19 August 2019 Accepted 23 August 2019 Available online 29 August 2019 Communicated by M. Wu Keywords: High pressure Thermoelectric property Cu2 Se
a b s t r a c t The Cu2 Se samples were synthesized by high pressure directly at room temperature in several minutes. The composition evolution under high pressure demonstrates that the critical conditions to synthesize Cu2 Se are the pressure of 1 GPa and the reaction time of 5 min. The synthetic pressure can effectively tune the morphology, carrier concentration and the electrical transport properties. The low lattice thermal conductivity less than 0.5 Wm−1 K−1 is obtained because of the intrinsic superionic character and the microstructures by high pressure including abundant micropores and lattice defects. A maximum zT of 0.92 at 783 K is achieved for Cu2 Se synthesized at 1 GPa. This work indicates the potentiality of high pressure technique to further enhance the thermoelectric properties of Cu2 Se materials. © 2019 Elsevier B.V. All rights reserved.
1. Introduction Thermoelectric (TE) materials exhibit great potential for solid state cooling and electricity generation [1–3]. The efficiency for a TE material is determined by zT value, zT = S2 σ T/κ , where S, σ , T and κ are the Seebeck coefficient, electrical conductivity, absolute temperature and the thermal conductivity, respectively [4]. The large magnitude of the electrical properties and low thermal conductivity are feasible for high TE performance. However, due to the coupling of thermoelectric transport parameters, it is challenging to achieve a large zT value. To maximize the power factor, the carrier concentration should be tuned to appropriate value by element doping and/or modifying the electronic band structure. Furthermore, reducing the κl is an effective way to enhance TE performance, because κl is the only parameter that is not explicitly related to the electrical properties if the electron-phonon scattering is weak compared to phonon-phonon or other scattering. Therefore, the materials intrinsically with low lattice thermal conductivity have received notable attention, such as SnSe and Cubased liquid-like materials. The conventional alloys with high TE performance have been exploited for commercial production, such as Bi2 Te3 and PbTe. However, the toxicity of Pb and Te along with high cost of Te could restrict the large-scale expansion of their applications. Copper se-
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Corresponding authors. E-mail addresses: [email protected] (Y. Zhang), [email protected] (Z. Zhang). https://doi.org/10.1016/j.physleta.2019.125917 0375-9601/© 2019 Elsevier B.V. All rights reserved.
lenium materials are one of the promising TE materials with advantages of low cost, environmental protection and high TE performance. Cu2 Se materials have quite complex atomic arrangements and extremely low lattice thermal conductivity, which are proposed as phonon-liquid and electron-crystal (PLEC) thermoelectric material [5,6]. Cu2 Se compounds exhibit α -phase below 400 K and β -phase above 400 K. The Cu atoms of stoichiometric Cu2 Se are localized in α -phase (space group: C2/c). In β -phase, the superionic Cu+ ions are kinetically disordered while the Se atoms constitute a face-centered cubic structure (space group: Fm-3m) [7–10]. Several approaches have been adopted to enhance the TE performance of Cu2 Se materials, including high-energy ball milling [11–13], chemical methods [14,15], melt-annealing and self-propagating hightemperature synthesis [16–18], which generate promising zT values ranging from 1.5 to 2.1. However, many synthetic procedures are time-consuming. Therefore, it is important to seek a simple and fast route to synthesize Cu2 Se materials. In the past decades, high-pressure technique has been applied for the synthesis of thermoelectric materials, such as Bi2 Te3 , PbTe, CoSb3 , etc. The crystal structure, electronic band structure and microstructures can be effectively modulated by applying high pressure to enhance thermoelectric performances. Interestingly, some materials originally without TE effect can exhibit noticeable TE properties after applying high-pressure, for example SmTe [19]. Therefore, high pressure technique is effective to synthesize TE materials and to tune the transport properties. However, high pressure technology has not been intensively researched. Further de-
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velopment of thermoelectric mechanism under high-pressure is urgent [20]. As a fundamental thermodynamic variable, pressure is an effective strategy to optimize the TE properties, but it has not been extensively utilized in tuning Cu2 Se TE performance. The in situ high-pressure measurements by diamond anvil cells indicated that pressure tuning can distinctly enhance the TE transport performances [21,22]. However, the sample dimension is too small (the magnitude of 10 μm) to be utilized in practical applications. The high-pressure and high-temperature (HPHT) method provides an alternative route to synthesize TE bulk materials and tune the transport properties, simultaneously. The excellent TE performances by pressure tuning can be partly maintained to ambient conditions even after the release of pressure [23–25]. Importantly, the sample by HPHT technique is >1 cm in size with favorable mechanical strength, which can be directly used on the device assembly after cutting. In our previous work, the effects of HPHT conditions on the microstructure and TE performances of Cu2 Se compounds were studied. HPHT could distinctly tune the microstructure, densification and carrier concentration of Cu2 Se [26]. To eliminate the drawback of Cu+ separation and Se volatility at high temperature during the synthesis process, the high-pressure synthesis method at room temperature (defined as HPRT) was introduced to synthesize Cu2 (S,Se) [27]. However, both the reaction mechanism by HPRT and the effect of various pressures on Cu2 Se properties have not been investigated. In this work, Cu2 Se samples were prepared by high pressure technique (1 GPa, 2 GPa and 3 GPa) at room temperature. The HPRT can realize the synthesis process directly from element powders to Cu2 Se bulks in several minutes, which is distinctly time-saving and energy-saving. The reaction mechanism and the pressure-tuning effects were explored. A maximum zT of 0.92 at 783 K was achieved for Cu2 Se at 1 GPa. 2. Experiment 2.1. Synthesis The sample synthesis included two procedures. (1) In highpressure synthesis process, the chemical reaction between Cu and Se powders was carried out to obtain Cu2 Se compounds by HPRT. Appropriate amounts of elements (Cu, 99.9%; Se, 99.99%) according to stoichiometry of Cu2 Se were weighted, uniformly mixed and pressed into disks (Φ 10.5 × 4 mm3 ). The disk was put into a pyrophyllite assembly as pressure transmitting medium for high pressure synthesis (1 GPa, 2 GPa and 3 GPa) for 5 minutes at room temperature on high-pressure apparatus (6 × 1200). After that, the pressure was released to ambient pressure when the experiment was over. The high pressure was generated by hydraulic driving WC anvils. The actual pressure at sample chamber was calibrated by the relation between resistance change of standard materials and the loading oil pressure. (2) In high-pressure retreatment process, the as-prepared bulks were ground into powders and densified at synthetic pressure for 5 minutes in order to achieve homogeneous materials. The operation was consistent with the above procedure. After the procedures, the Cu2 Se samples were obtained. The samples were annealed at 510 ◦ C for 30 minutes in vacuum before TE properties measurement to avoid the volatilization of Se. 2.2. Characterization The X-ray diffraction (XRD) was conducted by Cu Kα (λ = 1.5418 Å) radiation (SmartLab3KW). The morphology and microstructures were analyzed by field emission scanning electron microscopy (FESEM, Magellan 400, FEI Company, America) and high-resolution transmission electron microscopy (HRTEM,
Fig. 1. Room temperature powder XRD patterns of Cu2 Se samples by HPRT before and after annealing.
JEM-2200FS, JEOL Co., Japan). The composition analysis was performed by energy dispersive spectroscopy (EDS) and inductively coupled plasma optical emission spectrometry (ICP-OES) on Agilent 5110. Disks were cut into rectangular shapes (3 × 3 × 10 mm3 ) for electrical properties measurement (Namicro-3L, Wuhan Joule Yacht Science & Technology Co., China). The densities (D) were achieved by the Archimedes method. The specific heat (Cp ) was calculated by Dulong-petit law. Thermal diffusivity (λ) was measured by laser flash instrument (LFA 467, Netzsch Co., Germany). The thermal conductivity was calculated by κ = DCp λ. The carrier concentration (n) was measured using Hall effect measurement system (HMS-5500, Ecopia Co., Korea). 3. Results and discussion 3.1. phase composition and microstructure The XRD patterns confirmed that the mixtures of α -phase Cu2 Se (Monoclinic, C2/c No. 15) and β -phase Cu2 Se (Cubic, Fm-3m No. 225) were obtained for the samples by HPRT at different pressures (1 GPa, 2 GPa and 3 GPa), as indicated in Fig. 1. Before and after annealing, there is no significant change in phase composition. The fast synthesis is originated from the high pressure effects. As a fundamental thermodynamic variable, pressure can significantly lessen the interatomic distances and strengthen the interatomic interactions [28,29], and may further lower the activation energy between Cu and Se atoms, which makes it possible for the synthesis reaction to occur at room temperature [27]. Therefore, HPRT is a feasible and effective route to achieve Cu2 Se bulk materials. The fracture surface micrographs of Cu2 Se samples synthesized by HPRT at different pressures are shown in Fig. 2. Before annealing, the grain size decreased with the increasing synthetic pressure. The layered structures were randomly distributed throughout the samples (see Fig. 2 (a∼c)). After annealing, the micropores with size of 0.1∼0.5 μm appeared as shown in Fig. 2 (e∼f). These micropores were formed by the evaporation of Se during annealing. The Cu/Se atom ratios for the annealed samples confirm the deficiency of Se element, as shown in Table 1. The Se deficiency lightens as the synthetic pressure increases, which maybe benefits from the more sufficient reaction of Cu and Se at higher pressure. The architecture containing micropores and abundant grain boundaries can scatter phonon effectively, which is favorable for a low lattice thermal conductivity [30–32]. 3.2. HRTEM images HRTEM analyses were performed to survey microstructures of Cu2 Se samples by HPRT (Fig. 3). The interplanar distances of 0.334
L. Xue et al. / Physics Letters A 383 (2019) 125917
Fig. 2. FESEM micrographs of Cu2 Se samples synthesized by HPRT at 1 GPa, 2 GPa and 3 GPa before (a, b and c) and after annealing (d, e and f) respectively.
Table 1 Room temperature carrier concentration (n), density (D) and atom ratio of Cu:Se for annealed Cu2 Se samples. Samples
n [1019 cm−3 ]
D [g cm−3 ]
Cu:Se
1 GPa 2 GPa 3 GPa
4.36 2.93 2.16
6.28 6.59 6.38
2:0.82 2:0.85 2:0.96
Fig. 3. HRTEM images of Cu2 Se prepared at 1 GPa including lattice defects.
nm and 0.198 nm are in approximately agreement with that of (2 2 1) and (0 1 2) plane for Cu2 Se, respectively. Multiple defects including lattice disorder and fringes appeared, as marked by white delineated area. The defects could be originated from the non-stoichiometric ratio of Cu and Se atoms or the high-pressure compression. In addition to the liquid-like Cu ions, the thermal conductivity is also affected by lattice defects and strains. The presence of these microstructures may promote phonon scattering to decrease lattice thermal conductivity [33]. 3.3. TE performances The temperature dependent TE performances with error bars for Cu2 Se samples are shown in Fig. 4. The phase transition sections at about 400 K are not described [34,35]. The Seebeck coefficients are positive, revealing that the holes are dominant carriers. The electrical resistivity increases with the measured temperature, showing the degenerate semiconductor feature. High pressure had a remarkable influence on the electrical transport properties of Cu2 Se in the low-temperature range. Below 400 K, the Seebeck coefficient and electrical resistivity are distinctly enhanced with the increasing pressure. Because of the relatively high carrier concentration (Table 1) at room temperature, the electrical resistivity of the sample synthesized at 1 GPa was greatly reduced compared
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to that at 3 GPa. It manifested that the pressure can affect the electrical properties of the Cu2 Se samples by regulating the carrier concentration to some extent. However, the electrical properties of Cu2 Se at high temperature range change slightly with pressure, which reveals that the influence of high mobility of the liquidlike Cu+ ions predominated the electrical transport properties. The calculated power factor results are exhibited in Fig. 4 (c). The maximum value of 8.5 μWcm−1 K−2 at 483 K was achieved for the sample at 2 GPa. Fig. 4 (d) displays that the thermal conductivities of all Cu2 Se samples are less than 0.76 Wm−1 K−1 and decrease with the measured temperature. The lattice thermal conductivity (κl ) is calculated by κl = κ − κc , where the carrier thermal conductivity (κc ) is estimated by the Wiedemann-Franz law (κc = L0 T/ρ , where L0 = 1.68 × 10−8 V2 K−2 ) [4,22]. The results show that the lattice thermal conductivity dominates the thermal conductivity, as shown in Fig. 4 (d) and (e). The decrease in thermal conductivity with increasing temperature is originated from the remarkably reduced carrier thermal conductivity, because the lattice thermal conductivity almost exhibits temperature-independent behavior. All Cu2 Se samples have a relatively low lattice thermal conductivity, less than 0.5 Wm−1 K−1 . In addition to the intrinsic superionic feature, the low lattice thermal conductivity was also affected by the microstructures including numerous pores, lattice defects and strains produced by high pressure. In addition, the lattice thermal conductivities was reduced as the synthetic pressure decreased [36]. With the increase of synthetic pressure, the stoichiometry and synthesis reaction are more completed, and thus less lattice defects and strains may yield a higher lattice thermal conductivity. The excess liquid-like Cu+ ions due to the Se deficiency can interrupt the phonons’ transmission, and then contribute to the reduced lattice thermal conductivity. The sample synthesized at 1 GPa exhibits the lowest lattice thermal conductivity, less than 0.35 Wm−1 K−1 . With a moderate electrical property and a relatively low thermal conductivity, the maximum zT value of 0.92 at 783 K was achieved for the Cu2 Se sample synthesized at 1 GPa, which was 18% larger compared to that for the sample at 3 GPa. Therefore, it is promising for pressure tuning to further enhance the Cu2 Sebased performances. Since pressure tuning is a feasible and promising method to synthesize and to further optimize the TE performances of Cu2 Sebased materials, it is essential and urgent to clarify the reaction mechanism of the high pressure synthesis. Therefore, the composition evolution of the products under various synthetic conditions by HPRT was studied, as shown in Fig. 5. Because Cu2 Se cannot be synthesized at atmospheric pressure while it can be after high pressure of 1 GPa was applied for 5 min, it is clear that the pressure must be enough to activate the reaction. Therefore, the critical value of synthetic pressure for Cu2 Se should be determined. Fig. 5 (a) exhibits the XRD patterns of the products with the increasing synthetic pressures from 0.5 GPa to 1 GPa for 5 min. For the synthetic pressure of 0.5 GPa, the Cu3 Se2 phase is quite rich, while the peak intensity of the surplus Cu element is very strong, which means the products at 0.5 GPa are mixtures of Cu3 Se2 and Cu. At 0.67 GPa, the Cu3 Se2 phase is completely dissolved and only a little Cu peak can be detected. Above this pressure, the Cu3 Se2 phase disappear completely. It seems that the composition conversion of (Cu3 Se2 +Cu) to β -Cu2 Se must lie between 0.5 and 0.67 GPa. When the pressure increases to 0.83 GPa, the products become the mixtures of β+α Cu2 Se as little amount of α -phase Cu2 Se appears. With the pressure continuing to increase, the α -Cu2 Se is more competitive than β -Cu2 Se. The XRD intensity of the β -Cu2 Se phase is relatively weakened compared to that of α -Cu2 Se at 1 GPa, it is a sign that main product is α -Cu2 Se. The amount of α -Cu2 Se gradually increases with
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Fig. 4. Temperature dependent TE properties for Cu2 Se samples, (a) Seebeck coefficient, (b) electrical resistivity, (c) power factor, (d) thermal conductivity, (e) lattice thermal conductivity and (f) zT value.
the Cu content decreasing, which is consistent with the literature [16,17,37,38]. The XRD pattern results indicate that the pressuredriven composition evolution is from initial Cu+Se, Cu3 Se2 +Cu, β -Cu2 Se to β+α Cu2 Se. It can be concluded that the critical pressure for Cu2 Se synthesis is about 1 GPa. Namely, only when the pressure is above 1 GPa, the final product of Cu2 Se can be obtained. As for the high pressure above 1 GPa, obviously, the final product cannot be achieved instantly. Fig. 1 (b) shows the composition evolution process in 5 min at high pressure of 1 GPa. Once sufficient pressure is applied, the mixtures of β+α Cu2 Se exist. As the synthetic time increases, the diffraction peak intensities of β -Cu2 Se decrease gradually. This result is well consistent with the phase diagram proposed by Yu et al. [39]. The results show that the reaction of Cu with Se powders under high pressure have undergone a complicated phase evolution process. From the above, it can be concluded that the critical time for Cu2 Se synthesis is about 5 min. Meanwhile, the reaction rate is distinctly increased at higher pressure, which can be deduced from the almost same XRD patterns of 0.67 GPa for 5 min and 1 GPa for 30 s. In short, the critical conditions for Cu2 Se synthesis is the pressure of 1 GPa and the reaction time of 5 min. The final product of Cu2 Se cannot be achieved unless the synthetic conditions are satisfied.
4. Conclusion In summary, a simple and fast high pressure approach was employed to synthesize Cu2 Se liquid-like materials in the pressure range of 1∼3 GPa for 5 min. High pressure can distinctly tune the reaction process, microstructures and thermoelectric properties of Cu2 Se. Pressure-driven composition evolution shows the synthesis reaction is from Cu+Se element powders, Cu+Cu3 Se2 mixtures to α +β Cu2 Se bulks. Both the Seebeck coefficient and electrical resistivity of Cu2 Se were enhanced with the increasing synthetic pressure, due to the reduced carrier concentration. The micropores and multiple lattice defects of Cu2 Se samples produced by high pressure could contribute to the reduced lattice thermal conductivity. A maximum zT of 0.92 at 783 K was achieved for the Cu2 Se at 1 GPa. The results indicate that high pressure tuning is a feasible strategy to further optimize the TE performances of Cu2 Se-based materials. Acknowledgements This work was supported by the National Natural Science Foundation of China (No. 11704340 and 11804305), the China Postdoctoral Science Foundation (No. 2017M620303), the Key Project for Science and Technology Development of Henan Province (No.
L. Xue et al. / Physics Letters A 383 (2019) 125917
Fig. 5. Room temperature XRD patterns of the products after high-pressure process. (a) the products by various pressures for 5 min; (b) composition evolution in the synthesis reaction at 1 GPa.
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Contents lists available at ScienceDirect
Physics Letters A www.elsevier.com/locate/pla
High pressure synthesis and thermoelectric performances of Cu2 Se compounds Lisha Xue, Chao Fang, Weixia Shen, Manjie Shen, Wenting Ji, Yuewen Zhang ∗ , Zhuangfei Zhang ∗ , Xiaopeng Jia Key Laboratory of Material Physics of the Ministry of Education, School of Physics and Engineering, Zhengzhou University, Zhengzhou, 450052, China
a r t i c l e
i n f o
Article history: Received 15 May 2019 Received in revised form 19 August 2019 Accepted 23 August 2019 Available online 29 August 2019 Communicated by M. Wu Keywords: High pressure Thermoelectric property Cu2 Se
a b s t r a c t The Cu2 Se samples were synthesized by high pressure directly at room temperature in several minutes. The composition evolution under high pressure demonstrates that the critical conditions to synthesize Cu2 Se are the pressure of 1 GPa and the reaction time of 5 min. The synthetic pressure can effectively tune the morphology, carrier concentration and the electrical transport properties. The low lattice thermal conductivity less than 0.5 Wm−1 K−1 is obtained because of the intrinsic superionic character and the microstructures by high pressure including abundant micropores and lattice defects. A maximum zT of 0.92 at 783 K is achieved for Cu2 Se synthesized at 1 GPa. This work indicates the potentiality of high pressure technique to further enhance the thermoelectric properties of Cu2 Se materials. © 2019 Elsevier B.V. All rights reserved.
1. Introduction Thermoelectric (TE) materials exhibit great potential for solid state cooling and electricity generation [1–3]. The efficiency for a TE material is determined by zT value, zT = S2 σ T/κ , where S, σ , T and κ are the Seebeck coefficient, electrical conductivity, absolute temperature and the thermal conductivity, respectively [4]. The large magnitude of the electrical properties and low thermal conductivity are feasible for high TE performance. However, due to the coupling of thermoelectric transport parameters, it is challenging to achieve a large zT value. To maximize the power factor, the carrier concentration should be tuned to appropriate value by element doping and/or modifying the electronic band structure. Furthermore, reducing the κl is an effective way to enhance TE performance, because κl is the only parameter that is not explicitly related to the electrical properties if the electron-phonon scattering is weak compared to phonon-phonon or other scattering. Therefore, the materials intrinsically with low lattice thermal conductivity have received notable attention, such as SnSe and Cubased liquid-like materials. The conventional alloys with high TE performance have been exploited for commercial production, such as Bi2 Te3 and PbTe. However, the toxicity of Pb and Te along with high cost of Te could restrict the large-scale expansion of their applications. Copper se-
*
Corresponding authors. E-mail addresses: [email protected] (Y. Zhang), [email protected] (Z. Zhang). https://doi.org/10.1016/j.physleta.2019.125917 0375-9601/© 2019 Elsevier B.V. All rights reserved.
lenium materials are one of the promising TE materials with advantages of low cost, environmental protection and high TE performance. Cu2 Se materials have quite complex atomic arrangements and extremely low lattice thermal conductivity, which are proposed as phonon-liquid and electron-crystal (PLEC) thermoelectric material [5,6]. Cu2 Se compounds exhibit α -phase below 400 K and β -phase above 400 K. The Cu atoms of stoichiometric Cu2 Se are localized in α -phase (space group: C2/c). In β -phase, the superionic Cu+ ions are kinetically disordered while the Se atoms constitute a face-centered cubic structure (space group: Fm-3m) [7–10]. Several approaches have been adopted to enhance the TE performance of Cu2 Se materials, including high-energy ball milling [11–13], chemical methods [14,15], melt-annealing and self-propagating hightemperature synthesis [16–18], which generate promising zT values ranging from 1.5 to 2.1. However, many synthetic procedures are time-consuming. Therefore, it is important to seek a simple and fast route to synthesize Cu2 Se materials. In the past decades, high-pressure technique has been applied for the synthesis of thermoelectric materials, such as Bi2 Te3 , PbTe, CoSb3 , etc. The crystal structure, electronic band structure and microstructures can be effectively modulated by applying high pressure to enhance thermoelectric performances. Interestingly, some materials originally without TE effect can exhibit noticeable TE properties after applying high-pressure, for example SmTe [19]. Therefore, high pressure technique is effective to synthesize TE materials and to tune the transport properties. However, high pressure technology has not been intensively researched. Further de-
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velopment of thermoelectric mechanism under high-pressure is urgent [20]. As a fundamental thermodynamic variable, pressure is an effective strategy to optimize the TE properties, but it has not been extensively utilized in tuning Cu2 Se TE performance. The in situ high-pressure measurements by diamond anvil cells indicated that pressure tuning can distinctly enhance the TE transport performances [21,22]. However, the sample dimension is too small (the magnitude of 10 μm) to be utilized in practical applications. The high-pressure and high-temperature (HPHT) method provides an alternative route to synthesize TE bulk materials and tune the transport properties, simultaneously. The excellent TE performances by pressure tuning can be partly maintained to ambient conditions even after the release of pressure [23–25]. Importantly, the sample by HPHT technique is >1 cm in size with favorable mechanical strength, which can be directly used on the device assembly after cutting. In our previous work, the effects of HPHT conditions on the microstructure and TE performances of Cu2 Se compounds were studied. HPHT could distinctly tune the microstructure, densification and carrier concentration of Cu2 Se [26]. To eliminate the drawback of Cu+ separation and Se volatility at high temperature during the synthesis process, the high-pressure synthesis method at room temperature (defined as HPRT) was introduced to synthesize Cu2 (S,Se) [27]. However, both the reaction mechanism by HPRT and the effect of various pressures on Cu2 Se properties have not been investigated. In this work, Cu2 Se samples were prepared by high pressure technique (1 GPa, 2 GPa and 3 GPa) at room temperature. The HPRT can realize the synthesis process directly from element powders to Cu2 Se bulks in several minutes, which is distinctly time-saving and energy-saving. The reaction mechanism and the pressure-tuning effects were explored. A maximum zT of 0.92 at 783 K was achieved for Cu2 Se at 1 GPa. 2. Experiment 2.1. Synthesis The sample synthesis included two procedures. (1) In highpressure synthesis process, the chemical reaction between Cu and Se powders was carried out to obtain Cu2 Se compounds by HPRT. Appropriate amounts of elements (Cu, 99.9%; Se, 99.99%) according to stoichiometry of Cu2 Se were weighted, uniformly mixed and pressed into disks (Φ 10.5 × 4 mm3 ). The disk was put into a pyrophyllite assembly as pressure transmitting medium for high pressure synthesis (1 GPa, 2 GPa and 3 GPa) for 5 minutes at room temperature on high-pressure apparatus (6 × 1200). After that, the pressure was released to ambient pressure when the experiment was over. The high pressure was generated by hydraulic driving WC anvils. The actual pressure at sample chamber was calibrated by the relation between resistance change of standard materials and the loading oil pressure. (2) In high-pressure retreatment process, the as-prepared bulks were ground into powders and densified at synthetic pressure for 5 minutes in order to achieve homogeneous materials. The operation was consistent with the above procedure. After the procedures, the Cu2 Se samples were obtained. The samples were annealed at 510 ◦ C for 30 minutes in vacuum before TE properties measurement to avoid the volatilization of Se. 2.2. Characterization The X-ray diffraction (XRD) was conducted by Cu Kα (λ = 1.5418 Å) radiation (SmartLab3KW). The morphology and microstructures were analyzed by field emission scanning electron microscopy (FESEM, Magellan 400, FEI Company, America) and high-resolution transmission electron microscopy (HRTEM,
Fig. 1. Room temperature powder XRD patterns of Cu2 Se samples by HPRT before and after annealing.
JEM-2200FS, JEOL Co., Japan). The composition analysis was performed by energy dispersive spectroscopy (EDS) and inductively coupled plasma optical emission spectrometry (ICP-OES) on Agilent 5110. Disks were cut into rectangular shapes (3 × 3 × 10 mm3 ) for electrical properties measurement (Namicro-3L, Wuhan Joule Yacht Science & Technology Co., China). The densities (D) were achieved by the Archimedes method. The specific heat (Cp ) was calculated by Dulong-petit law. Thermal diffusivity (λ) was measured by laser flash instrument (LFA 467, Netzsch Co., Germany). The thermal conductivity was calculated by κ = DCp λ. The carrier concentration (n) was measured using Hall effect measurement system (HMS-5500, Ecopia Co., Korea). 3. Results and discussion 3.1. phase composition and microstructure The XRD patterns confirmed that the mixtures of α -phase Cu2 Se (Monoclinic, C2/c No. 15) and β -phase Cu2 Se (Cubic, Fm-3m No. 225) were obtained for the samples by HPRT at different pressures (1 GPa, 2 GPa and 3 GPa), as indicated in Fig. 1. Before and after annealing, there is no significant change in phase composition. The fast synthesis is originated from the high pressure effects. As a fundamental thermodynamic variable, pressure can significantly lessen the interatomic distances and strengthen the interatomic interactions [28,29], and may further lower the activation energy between Cu and Se atoms, which makes it possible for the synthesis reaction to occur at room temperature [27]. Therefore, HPRT is a feasible and effective route to achieve Cu2 Se bulk materials. The fracture surface micrographs of Cu2 Se samples synthesized by HPRT at different pressures are shown in Fig. 2. Before annealing, the grain size decreased with the increasing synthetic pressure. The layered structures were randomly distributed throughout the samples (see Fig. 2 (a∼c)). After annealing, the micropores with size of 0.1∼0.5 μm appeared as shown in Fig. 2 (e∼f). These micropores were formed by the evaporation of Se during annealing. The Cu/Se atom ratios for the annealed samples confirm the deficiency of Se element, as shown in Table 1. The Se deficiency lightens as the synthetic pressure increases, which maybe benefits from the more sufficient reaction of Cu and Se at higher pressure. The architecture containing micropores and abundant grain boundaries can scatter phonon effectively, which is favorable for a low lattice thermal conductivity [30–32]. 3.2. HRTEM images HRTEM analyses were performed to survey microstructures of Cu2 Se samples by HPRT (Fig. 3). The interplanar distances of 0.334
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Fig. 2. FESEM micrographs of Cu2 Se samples synthesized by HPRT at 1 GPa, 2 GPa and 3 GPa before (a, b and c) and after annealing (d, e and f) respectively.
Table 1 Room temperature carrier concentration (n), density (D) and atom ratio of Cu:Se for annealed Cu2 Se samples. Samples
n [1019 cm−3 ]
D [g cm−3 ]
Cu:Se
1 GPa 2 GPa 3 GPa
4.36 2.93 2.16
6.28 6.59 6.38
2:0.82 2:0.85 2:0.96
Fig. 3. HRTEM images of Cu2 Se prepared at 1 GPa including lattice defects.
nm and 0.198 nm are in approximately agreement with that of (2 2 1) and (0 1 2) plane for Cu2 Se, respectively. Multiple defects including lattice disorder and fringes appeared, as marked by white delineated area. The defects could be originated from the non-stoichiometric ratio of Cu and Se atoms or the high-pressure compression. In addition to the liquid-like Cu ions, the thermal conductivity is also affected by lattice defects and strains. The presence of these microstructures may promote phonon scattering to decrease lattice thermal conductivity [33]. 3.3. TE performances The temperature dependent TE performances with error bars for Cu2 Se samples are shown in Fig. 4. The phase transition sections at about 400 K are not described [34,35]. The Seebeck coefficients are positive, revealing that the holes are dominant carriers. The electrical resistivity increases with the measured temperature, showing the degenerate semiconductor feature. High pressure had a remarkable influence on the electrical transport properties of Cu2 Se in the low-temperature range. Below 400 K, the Seebeck coefficient and electrical resistivity are distinctly enhanced with the increasing pressure. Because of the relatively high carrier concentration (Table 1) at room temperature, the electrical resistivity of the sample synthesized at 1 GPa was greatly reduced compared
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to that at 3 GPa. It manifested that the pressure can affect the electrical properties of the Cu2 Se samples by regulating the carrier concentration to some extent. However, the electrical properties of Cu2 Se at high temperature range change slightly with pressure, which reveals that the influence of high mobility of the liquidlike Cu+ ions predominated the electrical transport properties. The calculated power factor results are exhibited in Fig. 4 (c). The maximum value of 8.5 μWcm−1 K−2 at 483 K was achieved for the sample at 2 GPa. Fig. 4 (d) displays that the thermal conductivities of all Cu2 Se samples are less than 0.76 Wm−1 K−1 and decrease with the measured temperature. The lattice thermal conductivity (κl ) is calculated by κl = κ − κc , where the carrier thermal conductivity (κc ) is estimated by the Wiedemann-Franz law (κc = L0 T/ρ , where L0 = 1.68 × 10−8 V2 K−2 ) [4,22]. The results show that the lattice thermal conductivity dominates the thermal conductivity, as shown in Fig. 4 (d) and (e). The decrease in thermal conductivity with increasing temperature is originated from the remarkably reduced carrier thermal conductivity, because the lattice thermal conductivity almost exhibits temperature-independent behavior. All Cu2 Se samples have a relatively low lattice thermal conductivity, less than 0.5 Wm−1 K−1 . In addition to the intrinsic superionic feature, the low lattice thermal conductivity was also affected by the microstructures including numerous pores, lattice defects and strains produced by high pressure. In addition, the lattice thermal conductivities was reduced as the synthetic pressure decreased [36]. With the increase of synthetic pressure, the stoichiometry and synthesis reaction are more completed, and thus less lattice defects and strains may yield a higher lattice thermal conductivity. The excess liquid-like Cu+ ions due to the Se deficiency can interrupt the phonons’ transmission, and then contribute to the reduced lattice thermal conductivity. The sample synthesized at 1 GPa exhibits the lowest lattice thermal conductivity, less than 0.35 Wm−1 K−1 . With a moderate electrical property and a relatively low thermal conductivity, the maximum zT value of 0.92 at 783 K was achieved for the Cu2 Se sample synthesized at 1 GPa, which was 18% larger compared to that for the sample at 3 GPa. Therefore, it is promising for pressure tuning to further enhance the Cu2 Sebased performances. Since pressure tuning is a feasible and promising method to synthesize and to further optimize the TE performances of Cu2 Sebased materials, it is essential and urgent to clarify the reaction mechanism of the high pressure synthesis. Therefore, the composition evolution of the products under various synthetic conditions by HPRT was studied, as shown in Fig. 5. Because Cu2 Se cannot be synthesized at atmospheric pressure while it can be after high pressure of 1 GPa was applied for 5 min, it is clear that the pressure must be enough to activate the reaction. Therefore, the critical value of synthetic pressure for Cu2 Se should be determined. Fig. 5 (a) exhibits the XRD patterns of the products with the increasing synthetic pressures from 0.5 GPa to 1 GPa for 5 min. For the synthetic pressure of 0.5 GPa, the Cu3 Se2 phase is quite rich, while the peak intensity of the surplus Cu element is very strong, which means the products at 0.5 GPa are mixtures of Cu3 Se2 and Cu. At 0.67 GPa, the Cu3 Se2 phase is completely dissolved and only a little Cu peak can be detected. Above this pressure, the Cu3 Se2 phase disappear completely. It seems that the composition conversion of (Cu3 Se2 +Cu) to β -Cu2 Se must lie between 0.5 and 0.67 GPa. When the pressure increases to 0.83 GPa, the products become the mixtures of β+α Cu2 Se as little amount of α -phase Cu2 Se appears. With the pressure continuing to increase, the α -Cu2 Se is more competitive than β -Cu2 Se. The XRD intensity of the β -Cu2 Se phase is relatively weakened compared to that of α -Cu2 Se at 1 GPa, it is a sign that main product is α -Cu2 Se. The amount of α -Cu2 Se gradually increases with
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Fig. 4. Temperature dependent TE properties for Cu2 Se samples, (a) Seebeck coefficient, (b) electrical resistivity, (c) power factor, (d) thermal conductivity, (e) lattice thermal conductivity and (f) zT value.
the Cu content decreasing, which is consistent with the literature [16,17,37,38]. The XRD pattern results indicate that the pressuredriven composition evolution is from initial Cu+Se, Cu3 Se2 +Cu, β -Cu2 Se to β+α Cu2 Se. It can be concluded that the critical pressure for Cu2 Se synthesis is about 1 GPa. Namely, only when the pressure is above 1 GPa, the final product of Cu2 Se can be obtained. As for the high pressure above 1 GPa, obviously, the final product cannot be achieved instantly. Fig. 1 (b) shows the composition evolution process in 5 min at high pressure of 1 GPa. Once sufficient pressure is applied, the mixtures of β+α Cu2 Se exist. As the synthetic time increases, the diffraction peak intensities of β -Cu2 Se decrease gradually. This result is well consistent with the phase diagram proposed by Yu et al. [39]. The results show that the reaction of Cu with Se powders under high pressure have undergone a complicated phase evolution process. From the above, it can be concluded that the critical time for Cu2 Se synthesis is about 5 min. Meanwhile, the reaction rate is distinctly increased at higher pressure, which can be deduced from the almost same XRD patterns of 0.67 GPa for 5 min and 1 GPa for 30 s. In short, the critical conditions for Cu2 Se synthesis is the pressure of 1 GPa and the reaction time of 5 min. The final product of Cu2 Se cannot be achieved unless the synthetic conditions are satisfied.
4. Conclusion In summary, a simple and fast high pressure approach was employed to synthesize Cu2 Se liquid-like materials in the pressure range of 1∼3 GPa for 5 min. High pressure can distinctly tune the reaction process, microstructures and thermoelectric properties of Cu2 Se. Pressure-driven composition evolution shows the synthesis reaction is from Cu+Se element powders, Cu+Cu3 Se2 mixtures to α +β Cu2 Se bulks. Both the Seebeck coefficient and electrical resistivity of Cu2 Se were enhanced with the increasing synthetic pressure, due to the reduced carrier concentration. The micropores and multiple lattice defects of Cu2 Se samples produced by high pressure could contribute to the reduced lattice thermal conductivity. A maximum zT of 0.92 at 783 K was achieved for the Cu2 Se at 1 GPa. The results indicate that high pressure tuning is a feasible strategy to further optimize the TE performances of Cu2 Se-based materials. Acknowledgements This work was supported by the National Natural Science Foundation of China (No. 11704340 and 11804305), the China Postdoctoral Science Foundation (No. 2017M620303), the Key Project for Science and Technology Development of Henan Province (No.
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Fig. 5. Room temperature XRD patterns of the products after high-pressure process. (a) the products by various pressures for 5 min; (b) composition evolution in the synthesis reaction at 1 GPa.
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