Reduction in low-temperature thermal conductivity of Cu2Se via substitution of Se by Te atoms

https://doi.org/10.1016/j.jpcs.2021.110301Get rights and content

Highlights

  • The resistivity of the Cu2Se1-xTex compounds enhanced monotonically with increase in Te content.

  • XPS confirms that addition of Te atom in the Cu2Se matrix has less effect on binding energies of Cu, Se and Te atoms.

  • Thermal conductivity of the pure Cu2Se1-xTex compounds reduced systematically with addition of Te.

  • The pristine Cu2Se compound exhibited the highest figure of merit.

Abstract

We report the structural and thermoelectric properties of the Cu2Se compound with a systematic substitution of Se by Te atom. The bulk polycrystalline Cu2Se1-xTex samples for the Te concentrations of x = 0.00, 0.02, 0.04, 0.06, and 0.08 were prepared by the solid-state reaction method. The room-temperature XRD studies revealed that the studied samples possess a monoclinic crystal structure. The oxidation state and electronic structure are confirmed by employing X-ray photoemission spectroscopy (XPS). The hardness of the samples increased systematically with an increase in Te doping concentration. The electrical resistivity of the doped samples was found to be increased in comparison to the pure sample. The Seebeck coefficient was positive throughout the temperature range under investigation, indicating that holes are the majority charge carriers in the studied compounds. The thermal conductivity of the doped samples decreased, which is presumably attributed to the point defect scattering. The systematic evolution of thermoelectric properties via substitution of Se by Te over the range (0.00 ≤ x ≤ 0.08) concludes that the overall thermoelectric power factor (PF) and figure of merit (ZT) decrease with Te substitution compared to the pristine Cu2Se compound.

Introduction

With the growing world population, the energy demand is skyrocketing, which puts extraordinary pressure on energy resources. To meet the world's contemporary energy demands, the increasing combustion of the limited fossil fuel resource and their adverse environmental impact has forced policymakers and scientists to look for sustainable energy harvesting methods. Paradoxically, on the one hand, the world is facing a scarcity of clean energy resources. On the other hand, our building heating systems, automobile exhaust, and industrial processes generate an enormous amount of heat, which goes as a waste in the environment [1]. T The conversion of such waste heat to energy (electricity) can help us meet our energy demand and allow us to protect the environment [2]. The competence of thermoelectric (TE) materials to convert heat into electricity and vice versa provides potential solutions for the issues mentioned above. Thermoelectric devices are solid-state devices with no moving parts, and they have the advantages of low maintenance, excellent reliability, and superb scalability [3]. The TE devices are eco-friendly with zero emission of greenhouse gasses. Due to all these proprieties, thermoelectric materials have captivated the attention of the scientific community in the past decades. However, their efficiency remains low, making TE devices inadequate compared to their conventional counterparts. Designing TE materials with higher efficiency has remained a challenging task for the research community.

The efficiency of thermoelectric materials is defined by thermoelectric figure of merit (ZT), given by ZT=S2σTκ, where S is Seebeck coefficient, σ stands for electrical conductivity, T represents temperature in Kelvin, and κ corresponds to total thermal conductivity [4]. In this case, a good TE material should possess a high Seebeck coefficient S and electrical conductivity (σ), and low thermal conductivity (κ). However, the strong interdependence of these physical parameters (S, σ, κ) makes the enhancement of ZT value a very challenging task [5]. To improve the ZT of the TE materials, different strategies, like nanostructuring, alloying, introducing secondary phases, etc., have been employed. However, to achieve the efficiency to outperform the conventional counterpart still remains a challenge [[6], [7], [8]]. Recently, Cu2Se material has attracted tremendous attention among the thermoelectric research community due to its superionic conduction behavior. Cu2Se crystal structure is made up of two different sub-lattices in which Cu ions behave like a liquid within the rigid crystalline Se sub-lattice. The unusual sublattice structure of Cu2Se results in low thermal conductivity hence enhanced the ZT of about 1.5 at 1000 K [3,9,10]. Several strategies, such as doping and the formation of composites, have further improved the thermoelectric performance of the Cu2Se system. Peng et al. substituted Cu atoms with transition metals, Cu1.99A0.01Se (A = Fe, Ni, Mn, In, Zn, and Sm) and reported the highest ZT of 1.51 for Cu1.9925Ni0.0075Se and Cu1.99Ni0.01Se compounds at 823 K [6]. Although the ZT value for the Cu1.99A0.01Se system remains comparable to the pure Cu2Se, the lattice thermal conductivity is reduced significantly. Composites fabricated by incorporating 0.45 wt% of graphene into Cu2Se showed a high ZT of 2.44 ± 0.25 at 870 K [11]. A ZT value of 2.4 by incorporating 0.75 wt% of CNT into Cu2Se is achieved at 1000 K [12]. Zhu et al. synthesized and characterized Te doped Cu2Se1-xTex samples and attained the highest ZT of 1.25 for the compound with a doping concentration of x = 0.02 at 773 K [13].

Most of the studies of doping in Cu2Se has been done at high temperatures [[14], [15], [16]]. However, the low-temperature study of these compounds is equally crucial to understanding the electrical and thermal transport mechanisms. It could also be essential for the fabrication of devices like Peltier coolers and Peltier heat pumps. The low-temperature studies of Ni, Zn, and Te doped Cu2Se have been carried for a single doping concentration of x = 0.01 for Ni, Te, and x = 0.02 for Zn by Yao et al., and the highest ZT value of 0.2, 0.18, 0.15, respectively, were obtained [17]. To the best of our knowledge, no study has been carried out to investigate the systematic evolution of the thermoelectric properties of Cu2Se at low temperature. In this regard, we have synthesized a series of Cu2Se1-xTex (0 ≤ x ≤ 0.08) compounds and studied the structural and low-temperature thermoelectric properties. We also examined the hardness of the synthesized materials.

Section snippets

Experimental details

The pure and Te-doped series of Cu2Se1-xTex (0 ≤ x ≤ 0.08) bulk polycrystalline samples were prepared by employing the conventional solid-state reaction method. The starting materials in powder form Cu (99.7%, Loba Chemie), Se, and Te (99.999%, Alfa aesar) were taken in stoichiometric ratio and ground using a mortar pestle. The obtained powders were compressed into rectangular pellets with a pressure of 5 MPa using a hydraulic press and sealed in a quartz tube under a high vacuum (10−6 mBar).

Structural properties

Rietveld fitted and observed room-temperature powder X-ray diffraction (PXRD) patterns of representative samples of Cu2Se1-xTex (0 ≤ x ≤ 0.08) series are shown in Fig. 1. All the permitted peaks are represented by vertical lines, and their differences between the observed and fitted patterns are shown as blue lines. The Rietveld analysis of the observed PXRD data confirmed that all the Cu2Se1-xTex (0 ≤ x ≤ 0.08) samples crystallize in a monoclinic structure with a C1m1 space group. It is seen

Conclusions

In this study, a series of bulk polycrystalline Cu2Se1-xTex (0 ≤ x ≤ 0.08) samples were synthesized using the solid-state reaction technique with a conventional sintering method. The room-temperature X-ray diffraction analysis of the prepared samples shows that all the studied compounds possess a monoclinic structure with a C1m1 space group. The XRD data confirms the successful incorporation of Te atoms at the Se sites in the Cu2Se material. A comparative analysis of the X-ray peak broadening

Author statement

  • 1.Mr. Suraj Mangavati: This work is his PhD work and he has prepared all the samples and has done the XRD analysis.

  • 2.

    Dr. Anand Pal: He has helped Mr. Suraj in the manuscript and analysis of XPS

  • 3.

    Dr. Ashok Rao: He is the supervisor of Mr. Suraj Mangavati and has contributed to the manuscript

  • 4.

    Zhao-Ze Jiang: Has done the thermoelectric measurements

  • 5.

    Dr. Yung-Kang Kuo: He has done the co-supervision and has contributed to some thermoelectric calculations and contributed in the manuscript

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

We are thankful to Parashurama Salunkhe for his help in Hall measurements and Glass processing lab MIT, MAHE, Manipal for density measurement. AR would like to acknowledge the Council of Scientific and Industrial Research (CSIR), Government of India (Grant number: 03(1409)/17/E MR-II), for the financial support required for this work. The electrical and thermal transport measurements were supported by the Ministry of Science and Technology of Taiwan under Grant No. MOST-109-2112-M-259-007-MY3

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