Cu2Se composite thermoelectric system

Journal Pre-proof Reduction in thermal conductivity and electrical resistivity in Cu2 SnSe3 /Cu2 Se composite thermoelectric system Riya Thomas, Ashok Rao, Ruchi Bhardwaj, Ling-Yu Wang, Yung-Kang Kuo

PII:

S0025-5408(19)31429-1

DOI:

https://doi.org/10.1016/j.materresbull.2019.110607

Reference:

MRB 110607

To appear in:

MRB

Received Date:

7 June 2019

Revised Date:

8 August 2019

Accepted Date:

30 August 2019

Please cite this article as: Thomas R, Rao A, Bhardwaj R, Wang L-Yu, Kuo Y-Kang, Reduction in thermal conductivity and electrical resistivity in Cu2 SnSe3 /Cu2 Se composite thermoelectric system, Materials Research Bulletin (2019), doi: https://doi.org/10.1016/j.materresbull.2019.110607

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Reduction in thermal conductivity and electrical resistivity in Cu2SnSe3/Cu2Se composite thermoelectric system Riya Thomasa, Ashok Raoa,*, Ruchi Bhardwajb, Ling-Yu Wangc, Yung-Kang Kuoc,** a. Department of Physics, Manipal Institute of Technology, Manipal Academy of Higher Education, Manipal 576104, India

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b. CSIR-Network of Institute for Solar Energy, CSIR-National Physical Laboratory, Dr. K. S. Krishnan Road, New Delhi 110012, India

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*Corresponding author: [email protected] ** Co-corresponding author: [email protected]

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Graphical abstract

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c. Department of Physics, National Dong Hwa University, Hualien 97401, Taiwan

Highlights    

The mole fraction of monoclinic phase decreases with increase in Cu2Se content. ρ(T) and S(T) obeys variable range hoping model. Electrical conductivity of composites are observed to be higher than that of the matrix. The systematic decrease in thermal conductivity with the addition of Cu2Se is attained.

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Abstract: A series of p-type Cu2SnSe3/xCu2Se (x = 0, 5, 10, 15, and 20 wt. %) composites were fabricated

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by spark plasma sintering, and the thermoelectric properties have been investigated in the temperature range 10 – 400 K. The crystal structure was evaluated by employing XRD analysis.

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Rietveld refinement was used to compute the amount of monoclinic and cubic phases in the composites, and the results revealed a decrease in the monoclinic phase with the increase in x wt.

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%. The measured thermoelectric properties of pristine as well as composites indicated that the introduction of Cu2Se leads to a simultaneous enhancement in electrical conductivity and reduction

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in thermal conductivity in Cu2SnSe3.

Keywords: Thermoelectric properties, Spark plasma sintering, composites, Thermal conductivity,

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Figure-of-merit.

1. Introduction

The ever-increasing energy demands and the management of waste heat from power plants, factories, automobiles, etc., still persist to be the key concerns of this century. Thus, it has become

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imperative to develop novel, clean, and renewable energy resources. Thermoelectrics is a technology by which one directly converts heat energy to electricity and vice-versa, without the requirement of steam-run turbines [1]. Apart from being environment-friendly, thermoelectric (TE) devices also hold additional advantages over conventional techniques, for example, they are noise-free, compact, have no moving parts, require no fuel/refrigerants, and have a longer lifespan. However, their applications are limited to niche areas due to their low efficiency and difficulty in

commercialization [2-5]. The thermoelectric conversion efficiency is evaluated employing a dimensionless quantity, ZT and is given by [1], 𝑍𝑇 =

𝜎𝑆 2 𝜅

𝑇

(1)

Here 𝜎 is the electrical conductivity, 𝑆 is the Seebeck coefficient, 𝜅 is the total thermal conductivity, and T is the absolute temperature. The numerator 𝜎𝑆 2 associated with electrical transport is called the power factor, which needs to be enhanced to attain a higher 𝑍𝑇 value. The

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total thermal conductivity 𝜅 consists of contributions from carrier 𝜅𝑐 and lattice 𝜅𝑙 components. For the past decades, achieving optimized ZT value has become a major challenge due to the

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reciprocally varying behavior of 𝜎 and 𝑆 and also due to the direct proportionality between 𝜎 and 𝜅𝑐 . The 𝜎 and 𝜅𝑐 are related by the Wiedemann-Franz law: 𝜅𝑐 = 𝐿0 𝜎𝑇, L is the Lorentz number

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[1]. Thus, many TE materials have been widely explored, such as Bi2Te3, half-Heusler, chalcogenides, skutterudites, etc., [1, 6-9] using a number of strategies like elemental doping [10,

higher thermoelectric efficiency.

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11], band engineering [12, 13], nanostructuring [14] and alloying [15], with an aim to arrive at

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Recently, Cu-based TE materials have attracted a lot of interest and have been extensively investigated primarily due to their eco-friendly and earth-abundant constituents, especially the readily available copper [16]. Cu2SnSe3 (CTSe) belonging to the category of ternary Cu-based

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compound with diamond-like structure, is one such promising TE material. CTSe follows the concept of phonon-glass-electron-crystal (PGEC), introduced by G. Slack, which suggests the coexistence of crystal-like electron transport and glass-like thermal conductivity as a condition to attain high TE performance [17]. CTSe has a complex crystal structure which leads to low thermal conductivity. It also has good electrical conductivity and doping at Sn-sites provides tunable

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electrical properties [18]. A maximum ZT of 1.14 was obtained for In-doped Cu2SnSe3 at 850 K, which is comparable to that of other high-performance TE materials [19]. Several works have been reported to increase the TE performance of CTSe using dopants like Zn [10], Sb [20], Mn [21], Pb [22], Co [23], Ga [24], Si, and Ti [11]. In addition to elemental substitution, fabricating composites by combining two materials with good TE performance has recently gained attention as an effective way to increase ZT. The introduction of secondary phase into the matrix either improves the electronic transport properties or reduces

the thermal conductivity by enhancing the interface and boundary scattering of phonons. Li et al. reported a maximum ZT of 1.26 at 880 K and achieved this by incorporating nanophase PbTe particles in the matrix of polycrystalline SnSe [25]. This enhancement is attributed to the increase in electrical conductivity due to the increase in carrier concentration as well as the reduction in thermal conductivity owing to the increased interface scattering of phonons. There have been quite a few works reported on composites with Cu-based materials as the matrix. Zhang et al. incorporated graphite nanosheets into CuGaTe2 and attained a high ZT value of about 0.93 at 873

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K [26]. The introduction of graphene nanosheets into Cu2SnSe3 had led to an increase in electrical conductivity and phonon scattering by graphene interfaces; thus, a maximum ZT of 0.44 at 700 K was obtained [27]. The thermoelectric properties of ZnO/Cu2SnSe3, TiO2/Cu2SnSe3, and

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WO3/Cu2SnSe3 nanocomposites have recently been investigated with maximum ZT values of 0.35,

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0.30, and 0.177 at 700 K, respectively [28-30].

In the present work, a secondary phase of Cu2Se has been introduced into the matrix of Cu2SnSe3.

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The reason for choosing Cu2Se is that it falls in the category of superionic conductors having two sub-lattices: sub-lattice of Se atoms facilitating the conduction of carriers and Cu sub-lattice with mobile Cu ions resulting in lower thermal conductivity [31, 32]. Cu2Se exhibits a good TE

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performance with ZT ~1.5 at 1000 K [31, 33]. Liu et al. reported an enhancement in the thermoelectric performance by combining Cu2Se with BiCuSeO matrix, which has a high Seebeck

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coefficient [34]. Zhang et al. also reported a decrease in thermal conductivity by introducing nanophase Cu2Se in CuGaTe2, resulting in a remarkably high ZT value of 1.2 at 859 K [35]. To the best of our knowledge, no work has been done on combining Cu2Se with Cu2SnSe3 system. The incorporation of Cu2Se into the matrix of Cu2SnSe3 is expected to create more interfaces, which might cause a decrease in thermal conductivity due to increased phonon scattering. Taking this into consideration, in this communication, we investigate the thermoelectric properties of

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Cu2SnSe3/xCu2Se (x = 0, 5, 10, 15 and 20 wt. %) composites in the temperature range 10 – 400 K. 2. Experimental details Elemental Cu (99.7%, Loba Chemie), Sn (99.999%, Alfa Aesar), and Se (99.999%, Alfa Aesar) were accurately weighed to the atomic ratios of both Cu2SnSe3 and Cu2Se, and separately ground in agate motor. The powders were pressed into pellets of Cu2Se and Cu2SnSe3. The pellets were sealed in evacuated (10−3 Torr) quartz ampoules which were then heat-treated for 72 h at

temperatures of 500 °C and 600 °C for Cu2SnSe3 and Cu2Se, respectively. After the ampoules had slowly cooled to room temperature, the obtained ingots were ground into a fine powder. The Cu2Se powder was mixed well into Cu2SnSe3 at fractions of 5, 10, 15, and 20 wt. %, respectively. The so formed composites were then densified using spark plasma sintering (SPS) at 500 °C for 5 min under axial compressive stress of 60 MPa, resulting in disk-shaped samples. The crystallinity, phase structure, and purity of the samples were determined by the X-ray diffraction (XRD) analysis carried out by Rigaku Miniflex using monochromatic Cu-Kα radiation.

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The surface morphology of the pure as well composite samples were imaged by a JEOL JSM7000F field emission scanning electron microscope (FESEM). The samples were cut into

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rectangular parallelepiped shapes for the low-temperature measurements of transport properties. The temperature-dependent resistivity measurements for all the samples were performed using the

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conventional dc four-probe technique with a closed cycle refrigerator (CCR) in the temperature range 10 – 400 K. The Seebeck coefficient and thermal conductivity were measured using a direct

techniques are given elsewhere [36].

3.1. Structural analysis

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3. Results and discussion:

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heat-pulse technique for the same temperature range. Further details of the thermal measurement

The crystal structure and phase purity of Cu2SnSe3 with/without Cu2Se addition have been

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identified by employing X-ray diffraction, and the XRD patterns are given in Fig. 1(a). There are several reports available on the various allotropic forms of Cu2SnSe3 having different crystalline structures such as orthorhombic, monoclinic, and cubic [22-24]. Since these allotropes are derived from the cubic sphalerite structure, their XRD patterns have the major peaks at almost the same 2θ values. The high-intensity peaks in Fig. 1(a) appear at 2θ values of around 26.9°, 44.8°, 53.2°,

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65.3°, and 72.1° which can be indexed to a cubic structure (space group: 𝐹4̅3𝑚) of Cu2SnSe3. However, the monoclinic phase (space group: Cc) of Cu2SnSe3 can be distinguished by the presence of a few very low-intensity peaks, as shown in Fig. 1(b). The dominant phase in the asprepared composite samples is that of Cu2SnSe3 matrix, and the presence of Cu2Se phase is probably compromised due to the peak overlapping with the dominant phase. However, there are few peaks of Cu3Se2 observed in the spectra of composites with a higher concentration of Cu2Se.

The absence of a systematic peak shift suggests that the crystal structure of the matrix is not affected by the addition of Cu2Se. Quantitative structural refinement for the XRD patterns of the composites was investigated by employing Rietveld analysis using FULLPROF program, and the refinements were carried out using monoclinic and cubic phases of Cu2SnSe3. The results of refinement are tabulated in table 1, and it can be seen that the mole fraction of monoclinic phase has decreased from ~92% (for x = 5%) to ~61% (for x = 20%). The average crystallite size has been calculated using Scherrer 0.9 𝜆

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formula, 𝐷 = 𝛽𝐶𝑂𝑆𝜃, where, 𝜆 = 1.54 Å, β is the line broadening in radians, θ is the Bragg angle, and the results are also listed in table 1. An overall decrease in the average crystallite size and an

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growth is inhibited with the addition of the secondary phase.

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increase in strain are observed with increase in the Cu2Se content, suggesting that the crystal

Figure 2 shows the FESEM images of the finely polished surface of the pristine and composite bulk samples. It is observed that the increase in Cu2Se addition leads to the formation of smaller

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crystal particles. Large crystal particles of Cu2SnSe3 become fewer in the samples with more Cu2Se content. Thus, the addition of Cu2Se in Cu2SnSe3 matrix is considered to be effective in

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suppressing the growth of grain boundary. 3.2. Thermoelectric performance:

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3.2.1. Electrical transport

The temperature-dependence of electrical resistivity ρ(T) measured in the temperature range 10 – 400 K for pristine as well as composite samples are presented in Fig. 3(a). It is found that ρ(T) for 𝑑𝜌

all the samples show a semiconductor-like behavior, i.e., 𝑑𝑇 < 0. Moreover, one can see that there

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is an overall decrease in resistivity for the composite samples as compared to that of the pristine (~ 0.096 Ω cm at 300 K). The lowest value of electrical resistivity is about 0.029 Ω cm for the x = 15 wt. % composite sample at room temperature. This decrease in ρ(T) for composite samples is attributed to the higher hole concentration as a result of introducing the secondary phase of Cu2Se which contains Cu deficiency defects and has a higher hole concentration [35, 37]. Meanwhile, the increased ρ(T) for the x = 20 wt. % composite sample could be a result of an increased boundary scattering of the carriers leading to a decrease in mobility [37].

The Seebeck coefficient as a function of temperature S(T) for Cu2SnSe3/xCu2Se (x = 0, 5, 10, 15 and 20 wt. %) is given in Fig. 3(b). It is seen that all the samples have positive values of S(T), suggesting that they belong to p-type semiconductors. The pristine sample shows the highest value of S(T) throughout the temperature range of measurement. The decreased S(T) for the composite samples is presumably attributed to the increased carrier concentration, as the value of S is inversely proportional to carrier concentration according to Mott’s equation [20, 22]. This result is also in agreement with ρ(T) data. A similar trend is seen in the case of CuGaTe2/xCu2Se

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composite materials where S(T) monotonously decreases with an increase in Cu2Se as a result of the increase in hole concentration [35]. The Seebeck coefficient of Cu2SnSe3/xCu2Se (x = 0, 5, 10, 15, and 20 wt. %) increases with increasing temperature, while the electrical resistivity is observed

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to decrease with temperature. This implies that the semiconductor-like temperature dependence of ρ(T) is not reconciled with the metal-like temperature dependence of S(T). To facilitate the

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understanding of the transport process of carriers, we have analyzed the charge transport in the studied samples using a hopping model. According to the Mott’s variable-range hopping (VRH)

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𝑆(𝑇) = 𝑇

(𝑑−1) (𝑑+1)

(2)

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𝜌(𝑇) =

𝑇 (𝑑+1) 𝜌0 exp [ 0 ] 𝑇

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transport mechanism, ρ(T) and S(T) can be expressed as follows [38]:

(3)

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where 𝜌0 is a prefactor, 𝑇0 is the characteristic Mott temperature, and d is the dimensionality. Therefore, for a 3-dimensional system, ln and S should vary as 𝑇 −1/4 (Fig. 4(a)) and 𝑇 1/2 (Fig. 4(b)), respectively [20, 38]. As all the curves in Fig. 4 show a rather linear relationship, this indicates that the transport mechanism follows the VRH process for all the samples.

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3.2.2. Thermal transport

The measured total thermal conductivity κ(T) of pristine and composite samples in the temperature range 10 – 400 K is illustrated in Fig. 5. The behavior of κ(T) for all the samples is similar to that of typical solids [39]. Firstly, κ(T) increases rapidly with temperature, and a peak is observed at around 30 K, which is a typical feature for the reduction of thermal scattering in solids at low temperatures. With further increase in temperature, κ decreases with temperature due to the domination of phonon-phonon scattering (Umklapp processes) [39, 40]. It is seen that the peak

value of κ(T) systematically decreases with the increase in x wt. % of Cu2Se, while the composites saturate to nearly the same value of κ(T) at room temperature. This reduction can be ascribed to a stronger phonon scattering by the introduction of interfaces between Cu2SnSe3 and Cu2Se. Ning J. et al. have studied the effect of ZnO and TiO2 inclusions in the matrix of Cu2SnSe3 and a reduction in thermal conductivity in composites was obtained [28, 29]. A significant reduction in κ(T) was also reported for CuGaTe2/xCu2Se, which led to the enhancement of thermoelectric performance in this system [35].

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As the total thermal conductivity is the combination of phononic contribution κl(T) and contribution from mobile charge carriers κc(T), κl(T) can be estimated by subtracting κc(T) from 𝐿0 𝑇 𝜌

, L0 is the Lorenz number

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κ(T). κc(T) is determined from the Wiedemann-Franz law: 𝜅𝑐 (𝑇) =

obtained S(T) data by employing Eq. (4) [22]. −|𝑆|

𝐿0 = 1.5 + 𝑒𝑥𝑝 [ 116 ]

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(4)

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and ρ the electrical resistivity. A simple estimation of L0 for semiconductors can be done using the

Here L0 has the units of 10-8 W Ω K-2 and S is in μV/K. The obtained L0 values are used to estimate

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the lattice thermal conductivity. From this estimation, we can conclude that the total thermal conductivity is mainly associated with the lattice phonons rather than the charge carriers, due to the high electrical resistivity of these composites. As a result, the main contribution to the total

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thermal conductivity comes from the phononic contribution κl with a negligibly small contribution of κc. It is also evident from the plot of κl(T) vs. T-1 (as shown in the insets of Fig. 5) that all the composites exhibit a rather linear relationship with T-1, which according to the phononic heat transfer theory suggests that phonon-phonon scattering is the primary source of thermal resistance.

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3.2.3. Power factor and Figure of merit The power factor (PF) reflects the electronic characteristics of the material which is contributed by its Seebeck coefficient (S) and electrical resistivity (ρ), and is determined by Eq (5): 𝑃𝐹 =

𝑆2 𝜌

(5)

Fig. 6(a) demonstrates the PF vs. temperature for pristine as well as composite samples, and the PF of all studied samples increases with increasing temperature. The higher values of S(T) for the

pristine sample contributes to its significantly high PF values as compared to that of the composite samples. The dimensionless figure-of-merit ZT (= S2T/ρκ) is estimated employing the measured values of S, ρ, and κ, and is given in Fig. 6(b). An overall decreasing trend in the ZT value is seen with the addition of Cu2Se, due to the adverse effect of S(T). Although both the thermal conductivity and electrical resistivity decrease with the increase in Cu2Se concentration, the predominantly high Seebeck coefficient of the pristine sample leads to the lower ZT in composite

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samples.

4. Conclusions

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In summary, the pure Cu2SnSe3 and its composites with different weight percent of Cu2Se were synthesized by solid-state reaction followed by spark plasma sintering. The XRD analysis reveals

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a cubic structure (space group: 𝐹4̅3𝑚 ) along with distinguished low-intensity peaks of the monoclinic phase (space group: Cc) of Cu2SnSe3. Quantitative refinements were carried out using

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Rietveld analysis to estimate the amount of monoclinic and cubic phase with the increase in Cu2Se content. The mole fraction of monoclinic phase was observed to decrease from ~92% (at x = 5%) to ~ 61% (at x = 20%). The electrical transport properties, i.e., the temperature-dependent

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resistivity and Seebeck coefficient were investigated in the temperature range 10 – 400 K, and the carrier transport mechanism can be satisfactorily described by the Mott’s variable range hopping

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model. The decreased ρ(T) and S(T) of composites as compared to the pristine could be attributed to the increase in hole concentration, due to the introduction of Cu2Se. The temperaturedependence of thermal conductivity was studied in the temperature range 10 – 400 K. A systematic decrease in the peak height of κ(T) was observed with increase in x wt. % of Cu2Se, which is attributed to the stronger phonon scattering by the interfaces formed between Cu2SnSe3 and Cu2Se.

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The lattice thermal conductivity κl(T) estimated using the Wiedemann-Franz law exhibits a linear relationship with T-1, suggesting that the dominance of the Umklapp phonon-phonon scattering is the primary source of thermal resistance. With the introduction of Cu2Se, a reduction in thermal conductivity in the low-temperature regime as well as electrical resistivity has been achieved. However, the addition of Cu2Se results in a significant decrease in the value of the Seebeck coefficient which, in turn, has an adverse effect on the overall thermoelectric performance for the studied composites.

Acknowledgements: The authors (AR and RT) acknowledge Council of Scientific and Industrial Research Grant (sanction no.: 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 Nos. MOST-106-2112-M-312 259-002-MY3 and MOST-108-2112-M-

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259-001 (YKK).

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Plenum Publishers, New York, 2004.

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-p

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Jo

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Fig. 1 (a): XRD patterns of Cu2SnSe3/xCu2Se (x= 0, 5, 10, 15 and 20 wt. %) (b): Low intensity peaks of monoclinic Cu2SnSe3 phase (c): Rietveld refinement of Cu2SnSe3/xCu2Se (x= 0, 5, 10, 15 and 20 wt. %) system.

of ro -p re lP ur na Jo Fig. 2: FESEM images of finely polished surface of (a) Cu2SnSe3 and composites of Cu2SnSe3 with (b) 5 wt. % Cu2Se (c) 10 wt.% Cu2Se (d) 15 wt. % Cu2Se (e) 20 wt. % Cu2Se.

of ro -p re lP ur na Jo Fig. 3 (a): Temperature dependent resistivity and (b) Seebeck coefficient plot for Cu2SnSe3/xCu2Se (x= 0, 5, 10, 15 and 20 wt. %) system.

of ro -p re lP ur na Jo

Fig. 4: VRH model validation of (a) ρ(T) as a function of T-1/4 and (b) S(T) as a function of T1/2 for Cu2SnSe3/xCu2Se (x= 0, 5, 10, 15 and 20 wt. %) system.

of ro -p

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Fig. 5: Temperature dependent total thermal conductivity for Cu2SnSe3/xCu2Se (x= 5, 10, 15 and 20 wt. %) with insets showing the linear behavior of 𝜅𝑙 with T-1.

20

of ro -p

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Fig. 6 (a): PF and (b) ZT of Cu2SnSe3/xCu2Se (x= 0, 5, 10, 15 and 20 wt. %) system.

21

Table 1: Crystal structure parameters of Cu2SnSe3/xCu2Se (x= 0, 5, 10, 15 and 20 wt. %) composites obtained from refinement of XRD.

x wt. % a

b

c

Cc (67.19%)

6.9931

12.0691

6.9630

(0.0001)

(0.0008)

(0.0001)

F-43m (32.81%)

5.6913

5.6913

5.6913

15

(0.0001)

(0.0001)

(0.0001)

Cc (91.94%)

6.9969

12.1036

6.9757

(0.0001)

(0.0006)

(0.0001)

F-43m (8.06%)

5.6914

5.6914

5.6914

(0.0002)

(0.0002)

(0.0002)

Cc (74.67%)

6.9983

12.1044

6.9701

(0.0002)

(0.0001)

(0.0002)

F-43m (25.33%)

5.6902

5.6902

5.6902

(0.0003)

(0.0003)

Cc (70.47%)

7.0269

12.1173

(0.0004)

(0.0003)

F-43m (29.53%)

5.7043

5.7043

(0.0003)

F-43m (38.82%)

6.9967 (0.0001)

(0.0002)

5.7043

12.0767

6.9708

(0.0009)

(0.0001)

5.7032

5.7032

5.7032

(0.0003)

(0.0003)

(0.0003)

Jo 22

37.68

1.9

9.37

1.7

35.19

11.6

33.38

1.7

11.1

32.21

1.9

11.8

30.54

(0.0003)

(0.0003)

lP

Cc (61.18%)

11.0

6.9935

(0.0003)

ur na

20

1.9

ro

10

D (nm)

-p

5

Rwp %

of

Cu2Se 0

χ2

Mole mass ratio

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of

Lattice parameter (Å)