Enhanced Thermoelectric Performance of Cu₂SnSe₃-Based Composites Incorporated with Nano-Fullerene

Abstract

In this study, nano-sized fullerene C₆₀ powder was sufficiently mixed with Cu₂SnSe₃ powder by ball milling method, and the C₆₀/Cu₂SnSe₃ composites were prepared by spark plasma sintering technology. The fullerene C₆₀ distributed uniformly in the form of clusters, and the average cluster size was less than 1 μm. With increasing C₆₀ content, the electrical conductivity of C₆₀/Cu₂SnSe₃ composites decreased, while the Seebeck coefficient was enhanced. The thermal conductivity of composites decreased significantly, which resulted from the phonon scattering by the C₆₀ clusters located on the grain boundaries of the Cu₂SnSe₃ matrix. The highest figure of merit ZT of 0.38 was achieved at 700 K for 0.8% C₆₀/Cu₂SnSe₃ composite.

1. Introduction

Thermoelectric (TE) materials have attracted increasing worldwide attention due to their potential application in electronic cooling, waste heat recovery, and power generation [1,2,3,4]. The conversion efficiency of TE materials is determined by the dimensionless figure of merit, ZT = α²σT/κ, where α is the Seebeck coefficient, σ is the electrical conductivity, T is the absolute temperature, and κ is the total thermal conductivity. The total thermal conductivity is composed of an electron part (κ_E) and a phonon part (κ_L). Therefore, to maximize the ZT value of TE materials, a large α and σ, as well as a low κ are required. In recent years, several classes of bulk materials with high ZT have been discovered and developed, such as skutterudites, clathrates, and Cu-based chalcogenide semiconductors.

Cu-based chalcogenide compounds with a diamond-like structure, such as ternary Cu₂MSe₃ (M = Sn, Ge) and Cu₃SbSe₄, have attracted a lot of attention recently, due to their quite low thermal conductivity. In several Cu-based chalcogenide compound systems, the Cu₂SnSe₃ structure has partial “phonon glass electron crystal” (PGEC) characteristic, which makes it possible to achieve high TE performance. The Cu-Se bond network dominates the electron conduction, while the contribution from the element Sn is very weak; thus, the Sn site is suitable for optimization of the TE property. Various attempts, including doping by partial substitution, have been made to improve the thermoelectric properties of Cu₂SnSe₃ compound [5,6,7]. Shi et al. have reported the In-doped Cu₂In_xSn_1-xSe₃ and the maximum ZT value reaches 1.14 at 850 K for Cu₂In_0.1Sn_0.9Se₃ sample [8]. Fan et al. fabricated the Cu₂Ga_xSn_1-xSe₃ samples using hot-press sintering technique and achieved the ZT value of 0.43 for Cu₂Ga_0.075Sn_0.925Se₃ sample [9]. In addition, Skoug, et al. also confirmed that doping with isoelectronic Ge on the Sn site is also effective in enhancing the ZT value [10]. Aside from doping, the dispersion of nanostructure phases into the thermoelectric matrix is also an attractive approach to improving the performance of TE materials. However, the Cu₂SnSe₃-based thermoelectric composites are scarcely investigated, because the enhancement of ZT is unapparent compared with the doping. Although significant reduction in the lattice conductivity can be achieved via enhanced phonon scattering at grain boundaries or matrix/inclusion interfaces, the electrical conductivity of TE composites also decreases, resulting in a marginal improvement of the overall ZT value. In addition, good selection of the dispersed phase and the control of microstructure are also required for TE composite [11,12,13,14]. Therefore, effective enhancement of ZT for TE composites depends on the microstructure of composites—i.e., the distribution or the shape of the component.

Fullerene C₆₀ has very high elastic modulus and is a chemically-stable nonpolar fullerene molecule. C₆₀ and C₆₀-decorated grain boundaries may provide an effective phonon scattering, which could decrease the lattice thermal conductivity. Meanwhile, the scattering of charge carriers (electrons or holes) by the C₆₀ could be ineffective due to the large value of electron (hole) wavelength compared to a fullerene molecule size. Blank and Kulbachinskii, et al. reported that the addition of 0.5 vol % C₆₀ improved the TE properties of Bi_0.5Sb_1.5Te₃ material, and the ZT value obtained was 1.17 at 450 K [15,16]. Shi, et al. found that adding 6.5 mass% C₆₀ into pure CoSb₃ can increase the ZT value, while adding amounts between 0.5% and 4.8% into CoSb₃ decreased the ZT value [17]. The similar results of Itoh, et al. showed that the maximum ZT value for the 1% C₆₀/Co_0.92Ni_0.08Sb_2.96Te_0.04 composite was 0.62 at 800 K, which was evidently higher than that of C₆₀-free sample [18]. Nandihalli, et al. also reported that the ZT value of C₆₀/Ni_0.05Mo₃Sb_5.4Te_1.6 composites was enhanced in the whole temperature range due to the large decrease of κ_L [19]. In this contribution, we attempted to introduce the C₆₀ into a Cu₂SnSe₃ system and expected to achieve a larger reduction in the thermal conductivity of C₆₀/Cu₂SnSe₃ composites.

In the present work, the fullerene C₆₀ powder was incorporated into Cu₂SnSe₃ matrix using ball milling (BM), and C₆₀/Cu₂SnSe₃ composites were fabricated by spark plasma sintering (SPS) technology. Effects of C₆₀ particles on the thermoelectric properties of C₆₀/Cu₂SnSe₃ composites were discussed, and the results are beneficial to the development of Cu₂SnSe₃-based composites with high performance using BM-SPS technology.

2. Experimental Procedures

The polycrystalline Cu₂SnSe₃ samples were synthesized by melting method. The stoichiometric amount of starting materials Cu (powder, 99.95%), Sn (powder, 99.999%), and Se (shot, 99.999%) were first placed in a carbon crucible enclosed in evacuated fused-silica ampoules. The ampoules were slowly heated to 1173 K and held for 12 h in a vertical furnace. Then, the ampoules were slowly cooled to 873 K in 24 h, followed by annealing at this temperature for 2 days. Finally, the obtained ingots were ground into fine powder. Commercially available fullerene powder with average particle size of 500 nm (XFNANO, Nanjing, China) was chosen as the nanoinclusion, as shown in Figure 1. The fullerene C₆₀ purity is 99.98%, and the other 0.02% refers to impurities of C₇₀ and other carbon structures. The fullerene C₆₀ powder was added into the Cu₂SnSe₃ powder at fractions of 0.4, 0.8, 1.2, and 1.6 vol %, respectively. Then, the C₆₀-added Cu₂SnSe₃ powders were mechanically ground with planetary ball milling equipment at 150 rpm for 240 min. The as-milled powders were sintered by spark plasma sintering (SPS 2040) at around 860 K for about 8 min under uniaxial pressure of 50 MPa in vacuum.

The density of the sintered C₆₀/Cu₂SnSe₃ composites was measured using the Archimedes method. The constituent phases of the samples were determined by X-ray diffractometry (Cu K_α, Rigaku, Rint2000, Tokyo, Japan). The chemical composition of bulk samples was characterized using electron probe micro-analysis (EPMA, JEOL, JXA-8100, Tokyo, Japan) with a wavelength dispersive spectrometer (WDS). The composition was calculated by averaging five spots. The microstructure of all C₆₀/Cu₂SnSe₃ composites was observed by high-resolution transmission electron microscopy (HRTEM, JEM2100F, JEOL, Tokyo, Japan). The thermal diffusivity (λ) was measured by laser flash method (Netzsch LFA427) in a flowing Ar atmosphere between 300 and 700 K. The thermal conductivity was calculated from the relationship κ = ρλC_p, where ρ is the density of the sintered sample and C_p is the specific heat capacity. The electrical conductivity and Seebeck coefficient were measured simultaneously using commercial equipment (ZEM-3, ULVAC-RIKO, Tokyo, Japan) on a bar-type sample with a dimension of 2 × 2 × 10 mm³. The Hall coefficient (R_H) was measured using the van der Pauw’s method in vacuum with a magnetic field of 2 T. The carrier concentration (p_H) and mobility (μ_H) were estimated from the relations of p_H = 1/(eR_H) and μ_H = σR_H, based on the assumption of single band model, where e is the electronic charge. All measurements were performed in a temperature range of 300–700 K.

3. Results and Discussion

3.1. Microstructure and XRD Analysis

Figure 2 shows the SEM image of the 1.6 vol % C₆₀-added Cu₂SnSe₃ powder after ball milling. It can be observed that the average particle size of milled C₆₀/Cu₂SnSe₃ powder was about 100 nm. Figure 3 shows the X-ray diffraction patterns of xC₆₀/Cu₂SnSe₃ composites (x = 0, 0.4, 0.8, 1.2, and 1.6 vol %) after SPS. The measured relative densities for all C₆₀/Cu₂SnSe₃ composites after SPS are above 97% of the theoretical value. The diffraction peaks in Figure 3 are well-indexed based on the JCPDS 65-4145 (Joint Committee on Powder Diffraction Standards) of Cu₂SnSe₃. As the content of C₆₀ in the composites is very low, the diffraction peak of C₆₀ is not found in the XRD pattern of all C₆₀/Cu₂SnSe₃ samples. Therefore, all C₆₀/Cu₂SnSe₃ samples show the same XRD patterns with the pure Cu₂SnSe₃ sample.

Figure 4a,b show the SEM microstructure of the sintered pure Cu₂SnSe₃ sample and 1.6 vol % C₆₀/Cu₂SnSe₃ composite, respectively. The fullerene C₆₀ distributed uniformly in the form of clusters, and the average cluster size was lower than 1 μm. Shi, et al. reported that in the CoSb₃ material, most of the C₆₀ molecules agglomerate into irregular micrometer-size clusters located at the grain boundaries [17]. The smaller size of clusters in this study should be due to the ball milling technology. The chemical composition of C₆₀/Cu₂SnSe₃ composites was characterized by SEM and energy-dispersive X-ray spectroscopy (EDS), as shown in Figure 5. The results of EDS also confirm that the matrix was analyzed to be composed of 33.53 at. %; Cu, 16.85 at. %; Sn, and 49.62 at. %; Se, corresponding to the Cu₂SnSe₃ phase. The black phase only contains C element, indicating C₆₀ phase. To further analyze the C₆₀ clusters in the C₆₀/Cu₂SnSe₃ composite, HRTEM of C₆₀/Cu₂SnSe₃ composite was carried out, as shown in Figure 6. The size of C₆₀ is about 80 nm, which means the ball milling process decreases the average size of C₆₀ particles. According to the theory proposed by Faleev and Zebardaji, et al. [20,21], nano-phases that distribute in the thermoelectric matrix can result in strain fields, which could cause some changes in the band structure of the material and then greatly influence its thermoelectric properties.

3.2. Electrical Transport Properties

Figure 7 shows the temperature dependence of electrical conductivity (σ) for C₆₀/Cu₂SnSe₃ composites with different vol % C₆₀. It can be seen that the σ of the Cu₂SnSe₃ matrix decreases approximately linearly with rising temperature over the measured temperature range, indicating a typical behavior of a heavily-doped semiconductor. The similar tendency of σ was also observed in C₆₀/Cu₂SnSe₃ composites. In addition, the σ of C₆₀/Cu₂SnSe₃ composites decreases with increasing C₆₀ content, which should be attributed to the enhanced carrier scattering at the incoherent interfaces between well-dispersed C₆₀ clusters and the Cu₂SnSe₃ matrix. Generally, in the case of carriers primarily scattered by grain barriers or interfaces between the second phase and matrix in the composites, the carrier mobility can be written as [22]

$${\mu }_H=\frac{\text{eb}}{\sqrt{8k_{B}T{m}^{*}}}{\mathrm{e}}^{-\frac{E_B}{k_{B}T}}$$

(1)

where b is the average grain size, k_B the Boltzmann constant, m* the carrier effective mass, and E_B the activation energy characterizing the barrier height between the matrix and the second phase. As the relative density of the xC₆₀/Cu₂SnSe₃ composite is higher than 97%, the porosity effect can be eliminated.

Table 1. Chemical composition, some physical and structural parameters of xC₆₀/Cu₂SnSe₃ composites at room temperature.

x (vol %)	Relative Density	σ (Ω−1·cm−1)	p (1019cm−3)	μH (cm2/Vs)	α (μV/K)	κL (Wm−1·K−1)	m* (m0)
0	98.7%	127	3.74	21.2	131	2.77	2.6
0.4	97.2%	110	4.81	14.3	165	2.22	2.8
0.8	98.9%	90	5.01	11.2	188	2.06	2.9
1.2	98.2%	75	4.98	9.4	225	1.96	3.1
1.6	97.9%	62	5.09	7.6	252	1.81	3.2

lists some physical and structural parameters of xC₆₀/Cu₂SnSe₃ composites at room temperature. As the C₆₀ could act as an electron acceptor in the p-type C₆₀/Cu₂SnSe₃ composite, the carrier concentration increases with increasing C₆₀ content, which is consistent with the results of Blank [15]. In addition, it can be noted that the carrier mobility decreases with increasing C₆₀ content. Therefore, the σ of xC₆₀/Cu₂SnSe₃ composites decreases compared with the σ of the Cu₂SnSe₃ matrix.

Figure 8 displays the Seebeck coefficient (α) of xC₆₀/Cu₂SnSe₃ composites as a function of temperature. All composites have a positive α across the whole temperature range, indicating that the holes are major carriers. With rising temperature, the α of all xC₆₀/Cu₂SnSe₃ composites increases approximately linearly and the α of 1.6% C₆₀/Cu₂SnSe₃ composite reaches 314 μV/K at 700 K. Moreover, the α of xC₆₀/Cu₂SnSe₃ composites significantly increases with the increasing content of C₆₀. At room temperature, the α increases from 130 μV/K for the Cu₂SnSe₃ matrix to 252 μV/K for the 1.6 vol % C₆₀/Cu₂SnSe₃ composite. The enhancement of α of xC₆₀/Cu₂SnSe₃ composites should be related to the “energy filter” effect. The Seebeck coefficient can be expressed as [23],

$$\mathsf{\alpha }=-\frac{\pi }{3}\frac{k_B}{e}{k}_{B}T\frac{d\ln \sigma \left(E\right)}{dE}{|}_{E=E_F}-\frac{T}{n\left(E\right)}\frac{dn\left(E\right)}{dE}{|}_{E=E_F}$$

(2)

where k_B, σ(E), and n(E) are Boltzmann constant, electrical conductivity, and value of density of states (DOS), respectively. Many studies have confirmed that when nano-phases or nano-inclusions are incorporated into a semiconducting matrix material, the band bending at the inclusion/matrix interface will produce a potential energy barrier which could effectively block low energy electrons, while transmitting high energy electrons [24]. This “electron energy filter” could evidently increase the local density of states near the Fermi level (E_F) and enhance the Seebeck coefficient.

The μ_H of xC₆₀/Cu₂SnSe₃ composites is shown in Figure 9. The μ_H of xC₆₀/Cu₂SnSe₃ composites decreases with increasing C₆₀ content. In addition, the μ_H of xC₆₀/Cu₂SnSe₃ composites was in the order of 10 cm²·V⁻¹·s⁻¹ at room temperature, which was close to that of skutterudites [25,26]. It may be caused by the similar carrier effective mass of Cu₂SnSe₃ and CoSb₃ compounds. The m* can be estimated in the single parabolic band model using the following equations,

$$m^{*}=\frac{h^2}{2k_{B}T}{\left(\frac{n}{4\pi F_{\frac{1}{2}}\left(\eta \right)}\right)}^{2/3}$$

(3)

$$\alpha =-\frac{k_B}{e}\left[\frac{\left(r+2\right)F_{r+1}\left(\eta \right)}{\left(r+1\right)F_r\left(\eta \right)}-\eta \right]$$

(4)

where F_r, h, and r are Fermi integral, Planck’s constant, and scattering parameter of relaxation time, respectively. The evaluated equivalent carrier effective mass of xC₆₀/Cu₂SnSe₃ composites at room temperature is listed in Table 1. It can also be seen from Figure 9 that the μ_H of pure Cu₂SnSe₃ shows a temperature dependence of T^−1.5 above 500 K, indicating that the acoustic phonon scattering is dominant in the temperature range from 500 to 700 K. Below 500 K, the μ_H of pure Cu₂SnSe₃ has a weak temperature dependence relationship and the relationship of μ_H∝T^−0.5 is observed, suggesting that a dominative mechanism is alloy scattering. However, the μ_H of xC₆₀/Cu₂SnSe₃ composites deviates from the T^−1.5 or T^−0.5 dependence over the entire temperature range, indicating that a mixed scattering mechanism dominates these samples.

3.3. Thermal Transport Properties

Figure 10 displays the temperature dependence of total thermal conductivity (κ) and lattice thermal conductivity (κ_L) for C₆₀/Cu₂SnSe₃ composites. The κ_L is estimated by subtracting the electronic contribution via the Wiedmann–Franz law (κ_E = L₀σT, where the Lorenz number L₀ is taken as a constant of 2.0 × 10⁻⁸ V²/K²) from the total thermal conductivity. The κ for all samples declines with increasing temperature. Moreover, the κ of xC₆₀/Cu₂SnSe₃ composites decreases with increasing C₆₀ content. The achieved κ of 1.6 vol % C₆₀/Cu₂SnSe₃ composite at room temperature is 1.85 W/mK, which is 34% lower than that of pure Cu₂SnSe₃. The minimal κ of 1.6 vol % C₆₀/Cu₂SnSe₃ composite is 0.71 W/mK at 700 K. It is well-known that the grain boundary, wide or point defects, porosity, and impurity could contribute to the decrease of κ. Owing to high relative density of C₆₀/Cu₂SnSe₃ composites, the reduction of κ originating from the porosity is negligible. Meanwhile, the calculation of κ_E shows that the reduction of κ_E has a limited contribution to the decrease of κ. Therefore, the decrease of κ for C₆₀/Cu₂SnSe₃ composites mainly originates from the depression of κ_L due to the enhancement of phonon scattering by the C₆₀ inclusions or nano-particles in the composite. Just as shown in Figure 10b, the κ_L of C₆₀/Cu₂SnSe₃ composites drastically decreases with the content of C₆₀ increasing. The minimal κ_L achieved in the present work is 0.68 W/mK at 700 K for the 1.6 vol % C₆₀/Cu₂SnSe₃ sample, which is 43% lower than that of pure Cu₂SnSe₃. According to the kinetic theory [27], the minimum lattice thermal conductivity κ_Lmin can be obtained when the phonon mean free path reaches the shortest interatomic distance. The κ_Lmin can be estimated from the formula κ_L = 1/3C_vν_ml, where C_v is heat capacity per unit volume of the system using Dulong and Petit value, ν_m the mean sound velocity, and l the mean free path of phonon. The ν_m comes from the data in reference [28]. If we assume the minimum l to be the interatomic distance for Cu₂SnSe₃ (0.238 nm), the κ_Lmin is calculated as 0.52 Wm⁻¹·K⁻¹, just as shown by dashed line in Figure 10b. The κ_L of 1.6 vol % C₆₀/Cu₂SnSe₃ composites approaches the κ_Lmin of Cu₂SnSe₃ at high temperature.

3.4. Figure of Merit

The conversion efficiency of TE materials depends on the maximum dimensionless figure of merit (ZT). Figure 11 shows the dimensionless figure of merit (ZT) of C₆₀/Cu₂SnSe₃ composites as a function of temperature. Like other doped Cu₂SnSe₃ investigated before [8,29,30], the ZT value of C₆₀/Cu₂SnSe₃ composites increases approximately linearly with increasing temperature. Compared with the ZT of the Cu₂SnSe₃ sample, the ZT value of C₆₀/Cu₂SnSe₃ composites is enhanced. For the 0.8 vol % C₆₀/Cu₂SnSe₃sample, the maximum ZT value is 0.38 at 700 K, which is 45% higher than that of the pure Cu₂SnSe₃ sample. The enhancement of ZT for C₆₀/Cu₂SnSe₃ composites is mainly attributed to the reduced κ_L and the enhanced α. The addition of C₆₀ into the Cu₂SnSe₃ matrix could improve the TE properties, which is a promising process to the design Cu-based chalcogenide compounds with high TE performance. When the material with optimized carrier concentration is selected as the matrix, the higher ZT value of TE composite could be achieved.

4. Conclusions

In this study, C₆₀ was incorporated into a Cu₂SnSe₃ matrix, and C₆₀/Cu₂SnSe₃ composites were fabricated using BM-SPS method. The C₆₀ phase distributed uniformly in the form of clusters, and the average cluster size was less than 1 μm. With increasing C₆₀ content, the electrical conductivity of C₆₀/Cu₂SnSe₃ composites decreased, while the Seebeck coefficient of C₆₀/Cu₂SnSe₃ composites increased due to the “electron energy filter” of the C₆₀ nano-phase. The thermal conductivity of C₆₀/Cu₂SnSe₃ composites decreased significantly, which originated from the phonon scattering by the C₆₀ clusters located on the grain boundaries of the Cu₂SnSe₃ matrix. The maximum ZT of 0.38 was obtained at 700 K for 0.8 vol % C₆₀/Cu₂SnSe₃ composite.