Journal of Nanomaterials
Volume 2016 (2016), Article ID 5923975, 7 pages
http://dx.doi.org/10.1155/2016/5923975

Research Article

Synthesis and Thermoelectric Properties of C₆₀/Cu₂GeSe₃ Composites

Degang Zhao, Jiai Ning, Shuyu Li, and Min Zuo

School of Materials Science and Engineering, University of Jinan, Jinan 250022, China

Received 10 March 2016; Accepted 12 April 2016

Academic Editor: Yalin Dong

Copyright © 2016 Degang Zhao et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract.

Nanosized C₆₀ powder was sufficiently incorporated with Cu₂GeSe₃ powder by ball milling and C₆₀/Cu₂GeSe₃ composites were prepared by spark plasma sintering. C₆₀ distributed uniformly in the form of clusters and the average size of cluster was lower than 1 μm. With the addition of C₆₀ increasing, the electrical resistivity and Seebeck coefficient of C₆₀/Cu₂GeSe₃ composites increased while the thermal conductivity decreased significantly which resulted from the phonon scattering by C₆₀ clusters locating on the grain boundaries of Cu₂GeSe₃ matrix. The maximum ZT of 0.20 was achieved at 700 K for 0.9% C₆₀/Cu₂GeSe₃ sample.

1. Introduction

Thermoelectric (TE) material which directly converts electricity to heat (and vice versa) has attracted increasing worldwide attention due to their potential applications in electronic cooling, waste heat recovery, and special power supplies [1, 2]. The conversion efficiency of TE material is determined by the dimensionless figure of merit, , where α is the Seebeck coefficient, ρ is the electrical resistivity, T is the absolute temperature, and κ is the total thermal conductivity. The total thermal conductivity consists of an electron part () and a phonon part (). Therefore, to maximize ZT value of TE material, low κ and ρ as well as large α are required. In recent years, several classes of bulk materials with high ZT have been developed, such as lead telluride, skutterudite, clathrates, and Cu-based chalcogenide semiconductors [3–5]. 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 systems, the Cu₂GeSe₃ structure has partial “phonon glass electron crystal” (PGEC) characteristic which is possible to achieve high TE performance. The Cu-Se bond network dominates the electron conduction while the contribution from the element Ge is very weak; thus the Ge site is suitable for optimizing TE property. Various attempts including doping by partial substitution have been made to improve the thermoelectric properties of Cu₂GeSe₃ compound [6–8]. Cho et al. synthesized the Cu₂GaxSe₃ samples and achieved the ZT value of 0.50 for Cu₂Ga0.07Sn0.93Se₃ sample [9]. Chetty et al. reported the maximum ZT value of in-doped Cu₂InxSe₃ was 0.23 at 700 K for Cu₂In0.1Ge0.9Se₃ sample [10]. Besides doping, the dispersion of nanostructure phases into thermoelectric matrix is also an attractive approach to improve the performance of TE materials. However, Cu₂GeSe₃-based thermoelectric composites are scarcely investigated because the enhancement of ZT is unapparent compared with the doping. Although significant reduction in can be achieved via enhanced phonon scattering at matrix/inclusion interfaces, the electrical resistivity of TE composites maybe also increases resulting in a marginal improvement of the overall ZT value. Therefore, effective enhancement of ZT for TE composites depends on the good selection of dispersed nanophase and the control of microstructure of composites [11].

Fullerene C₆₀ has ultra high elastic module and is a chemically stable nonpolar molecule. Nanosized C₆₀ maybe provide an effective phonon scattering which decreases . Meanwhile, the scattering of the charge carriers by C₆₀ molecules could be ineffective due to the large value of electron (hole) wavelength compared to a fullerene size. Blank et al. ever reported that the addition of 0.5 vol% C₆₀ improved the TE properties of Bi0.5Sb1.5Te₃ and the maximum ZT of 1.17 was obtained at 450 K [12, 13]. Shi et al. found that adding 6.5 mass% C₆₀ into the binary CoSb₃ can increase the ZT while adding amounts between 0.5% and 4.8% into CoSb₃ decreased the ZT [14]. Nandihalli et al. also reported the ZT value of C₆₀/Ni0.05Mo₃Sb5.4Te1.6 composites was enhanced in the whole temperature range due to the large decrease of [15]. In this contribution, we attempted to introduce C₆₀ into Cu₂GeSe₃ system and expect to achieve larger reduction in of C₆₀/Cu₂GeSe₃ composites.

In the present work, C₆₀ powder was incorporated into Cu₂GeSe₃ matrix using ball milling (BM) and C₆₀/Cu₂GeSe₃ composites were fabricated by spark plasma sintering (SPS). Effects of C₆₀ particles on the thermoelectric properties of C₆₀/Cu₂GeSe₃ composites were discussed.

2. Experimental Procedures

The polycrystalline samples Cu₂GeSe₃ were synthesized by melting method. The stoichiometric amounts of starting materials Cu (powder, 99.95%), Ge (powder, 99.999%), and Se (shot, 99.999%) were first placed in carbon crucible enclosed in evacuated fused-silica ampoules. The ampoules were slowly heated to 1223 K, held for 48 h in a vertical furnace, and then cooled to room temperature. To increase the homogeneity and crystallinity, the samples were annealed at 823 K for 72 h. The resulting ingots were initially ground into fine powder using mortar and pestle. Commercially available C₆₀ powder with average particle size of 500 nm (Figure 1, XFNANO, China) was chosen as the nanoinclusions. C₆₀ purity is 99.98% and the other 0.02% refers to impurities of C₇₀ and other carbon structures. C₆₀ particles were added to the Cu₂GeSe₃ powder at fractions of 0.3, 0.6, 0.9, and 1.2 vol%, respectively. Then C₆₀-added Cu₂GeSe₃ powders were mechanically ground with planetary ball milling equipment at 200 rpm for 300 min in an argon atmosphere. Steel balls of 5 mm were charged as the milling media. The as-milled powders were sintered by spark plasma sintering (SPS 2040) at around 850 K for about 8 min under uniaxial pressure of 50 MPa in vacuum.

Figure 1: SEM image of fullerene powder with average particle size of 500 nm.

The densities of the sintered C₆₀/Cu₂GeSe₃ composites were measured using the Archimedes method. The constituent phases of composites were determined by X-ray diffractometry (Cu , Rigaku, Rint2000). The chemical compositions of bulk samples were characterized using electron probe microanalysis (EPMA, JEOL, and JXA-8100) with a wavelength dispersive spectrometer (WDS). The compositions were calculated by averaging five spots. The microstructure of all C₆₀/Cu₂GeSe₃ composites was observed by high-resolution transmission electron microscopy (HREM, JEM2100F). The thermal diffusivity () was measured by laser flash method (Netzsch LFA427) in a flowing Ar atmosphere. The thermal conductivity was calculated from the relationship , where is the density of the sintered sample and is the specific heat capacity. (taken as 0.33 J/gK) was determined by the ratio method with a sapphire reference using a Netzsch Differential Scanning Calorimetry. The electrical resistivity and Seebeck coefficient were measured simultaneously using commercial equipment (ZEM-3, ULVAC-RIKO) on the bar-type sample with a dimension of 2 × 2 × 10 mm. The Hall coefficient () was measured using the van der Pauw’s method in vacuum with the magnetic field of 2 T. The carrier concentration () and mobility () were estimated from the relations of and based on the single band model, where is the electronic charge. All the measurements were performed in a temperature range of 300–700 K.

3. Results and Discussion

3.1. Microstructure and XRD Analysis

Figure 2 displays the SEM image of 0.9 vol% C₆₀-added Cu₂GeSe₃ powder after ball milling. It can be observed that the average size of milled C₆₀/Cu₂GeSe₃ powder was about 100 nm. Figure 3 shows the XRD patterns of xC₆₀/Cu₂GeSe₃ composites (x = 0, 0.3, 0.6, 0.9, and 1.2 vol%) after SPS. The measured relative densities for all C₆₀/Cu₂GeSe₃ composites after SPS are above 97% of the theoretical value. The diffraction peaks in Figure 3 are well indexed based on the JCPDS 65-2533 of Cu₂GeSe₃ (Imm2 group). However, no diffraction peak of C₆₀ is found in the XRD of all C₆₀/Cu₂GeSe₃ samples, which may be due to the low content of C₆₀ in the composites. Therefore, all C₆₀/Cu₂GeSe₃ samples show the same XRD patterns with the pure Cu₂GeSe₃ sample. The sintered pure Cu₂GeSe₃ sample and 1.2 vol% C₆₀/Cu₂GeSe₃ composite are listed in Figures 4(a) and 4(b), respectively. C₆₀ distributed uniformly in the form of clusters and the average size of cluster was lower than 1 μm. In Shi et al.’s research, most of C₆₀ agglomerate into irregular micrometer size clusters located at the grain boundaries in the CoSb₃ material [14]. The smaller size of clusters in this study should be due to the ball milling technology. The chemical composition of bulk C₆₀/Cu₂GeSe₃ composites was characterized by SEM and EDS, as shown in Figure 5. The results of EDS also confirm that the matrix was composed of 33.53 at.% Cu, 16.85 at.% Ge, and 49.62 at.% Se, indicating Cu₂GeSe₃ phase. The black phase only contains C element, corresponding to C₆₀ phase. Figure 6 is the HRTEM image of C₆₀ clusters in the composite. The area surrounded by the dashed lines in Figure 6 is a C₆₀ particle. The size of C₆₀ was about 80 nm, which means the ball milling process decreases the average size of C₆₀ powder significantly. According to the theory proposed by Faleev et al. [16, 17], nanophases that distribute in the thermoelectric matrix can result in strain fields, which could cause some changes in the band structure of material and greatly influence its thermoelectric properties.

Figure 2: SEM image of the 1.2 vol% C₆₀/Cu₂GeSe₃ powder after ball milling.

Figure 3: XRD patterns of sintered C₆₀/Cu₂GeSe₃ composites.

Figure 4: (a) SEM microstructure of the sintered pure Cu₂GeSe₃; (b) SEM microstructure of sintered 1.2 vol% C₆₀/Cu₂GeSe₃ composite.

Figure 5: (a) SEM image of the sintered 1.2% C₆₀/Cu₂GeSe₃ composite; (b) EDS results of Cu₂GeSe₃ matrix and C₆₀ phase.

Figure 6: HRTEM image of C₆₀ particle in C₆₀/Cu₂GeSe₃ composite.

3.2. Electrical Transport Properties

The electrical resistivity () as a function of temperature for C₆₀/Cu₂GeSe₃ composites with different vol% C₆₀ is shown in Figure 7. ρ of Cu₂GeSe₃ matrix increases with rising temperature over the whole temperature range, indicating a typical behavior of a heavily doped semiconductor. The similar tendency of ρ was also present in C₆₀/Cu₂GeSe₃ composites. In addition, ρ of C₆₀/Cu₂GeSe₃ composites increases with the content of C₆₀ increasing, which should be due to the enhanced carrier scattering at the incoherent interfaces between well dispersed C₆₀ clusters and the Cu₂GeSe₃ matrix. Generally, in the case of carriers dominantly scattered by grain barriers or interfaces between the second phase and matrix in the composites, the carrier mobility can be expressed as [18]

where is the Boltzmann constant, is the carrier effective mass, is the average grain size, and is the activation energy characterizing the barrier height between the matrix and second phase. As the relative density of xC₆₀/Cu₂GeSe₃ composite is higher than 97%, the porosity effect could be eliminated. Table 1 lists some physical parameters of C₆₀/Cu₂GeSe₃ composites at room temperature. It can be noted that the carrier mobility () decreases with the content of C₆₀ increasing. Therefore, ρ of C₆₀/Cu₂GeSe₃ composites increases compared with ρ of Cu₂GeSe₃ matrix.

Table 1: Chemical composition and some physical parameters of C₆₀/Cu₂GeSe₃ composites at room temperature.

(vol%)
Relative density
(mΩcm)
(10¹⁹ cm⁻³)
(cm²/Vs)
(μV/K)
(Wm⁻¹K⁻¹)

0
97.6%
2.85
13.8
15.7
58
2.37

0.3
98.1%
3.53
12.4
14.2
67
1.49

0.6
97.5%
4.26
12.1
12.0
80
1.46

0.9
98.3%
6.37
10.7
9.1
93
1.40

1.2
97.8%
8.02
9.5
8.2
101
1.36

Figure 7: The electrical resistivity () as a function of temperature for C₆₀/Cu₂GeSe₃ composites.

Figure 8 displays the Seebeck coefficient (α) of xC₆₀/Cu₂GeSe₃ composites as a function of temperature. All composites have a positive α over the whole temperature range, indicating the holes are major carriers. With rising temperature, α of all xC₆₀/Cu₂GeSe₃ composites increases and α of 1.2% C₆₀/Cu₂GeSe₃ composite reaches 183 μV/K at 700 K. In addition, α of C₆₀/Cu₂GeSe₃ composites increases with the content of C₆₀ increasing. At room temperature, α increases from 58 μV/K for Cu₂GeSe₃ matrix to 101 μV/K for 1.2% C₆₀/Cu₂GeSe₃ composite. The enhancement of α of xC₆₀/Cu₂GeSe₃ composites should be related with the “energy filter” effect. The Seebeck coefficient can be expressed as [19]

where , , and are Boltzmann constant, value of density of states (DOS), and electrical conductivity, respectively. When nanophases or nanoinclusions are incorporated into a semiconducting matrix, the band bending at inclusion/matrix interface will produce potential energy barrier which could effectively block low energy electrons, while transmitting high energy electrons [20]. This “electron energy filter” could enhance the Seebeck coefficient by moving the Fermi level () to an energy level with larger local DOS.

Figure 8: The Seebeck coefficient () as a function of temperature for C₆₀/Cu₂GeSe₃ composites.

3.3. Thermal Transport Properties

Figure 9 displays the temperature dependence of total thermal conductivity (κ) and lattice thermal conductivity () for C₆₀/Cu₂GeSe₃ composites. is estimated by subtracting the electronic contribution via the Wiedemann-Franz law (, where the Lorenz number is taken as a constant of 2.0 × 10⁻⁸ V²/K²) from the total thermal conductivity. κ for all samples declines with increasing temperature. Moreover, of xC₆₀/Cu₂GeSe₃ composites decreases with the content of C₆₀ increasing. The achieved κ of 1.2% C₆₀/Cu₂GeSe₃ composite at room temperature is 1.43 W/mK, which is 45% lower than that of Cu₂GeSe₃ matrix. The minimal of 1.2% C₆₀/Cu₂GeSe₃ composite is 0.65 W/mK at 700 K. As known to all, the grain boundary, wide or point defects, porosity, and impurity could contribute to the decrease of κ. Owing to the high relative density of C₆₀/Cu₂GeSe₃ composites, the reduction of κ originated from the porosity is negligible. Meanwhile, the calculation of shows that the reduction of has a limited contribution to the decrease of κ. Therefore, the decrease of κ for C₆₀/Cu₂GeSe₃ composites mainly originates from the depression of due to the enhancement of phonon scattering by C₆₀ inclusions or nanoparticles in the composite. Figure 9(b) is the temperature dependence of of C₆₀/Cu₂GeSe₃ composites. of C₆₀/Cu₂GeSe₃ composites drastically decreases with the content of C₆₀ increasing. The minimal achieved in the present work is 0.58 W/mK at 700 K for 1.2% C₆₀/Cu₂GeSe₃, which is 59% lower than that of pure Cu₂GeSe₃.

(a)

(b)

Figure 9: Temperature dependence of total thermal conductivity (κ) of C₆₀/Cu₂GeSe₃ composites; (b) temperature dependence of lattice thermal conductivity () of C₆₀/Cu₂GeSe₃ composites.

3.4. Figure of Merit

Figure 10 shows the dimensionless figure of merit (ZT) of C₆₀/Cu₂GeSe₃ composites as a function of temperature. Like other doped Cu₂GeSe₃ investigated before [9, 10, 21], the ZT value of C₆₀/Cu₂GeSe₃ composites increases approximately linearly with increasing temperature. Compared with the ZT of Cu₂GeSe₃ sample, the ZT value of C₆₀/Cu₂GeSe₃ composites is enhanced. The inset in Figure 10 shows the ZT value as a function of volume fraction of C₆₀ at different temperature. For 0.9% C₆₀/Cu₂GeSe₃ composite, the maximum ZT is 0.20 at 700 K which is almost three times higher than that of pure Cu₂GeSe₃ sample. The enhancement of ZT for C₆₀/Cu₂GeSe₃ composites is mainly attributed to the reduced and the enhanced α. The addition of nano-C₆₀ into the Cu₂GeSe₃ matrix through BM method improves the TE properties, which provides a novel process to 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 obtained.

Figure 10: Temperature dependence of dimensionless figure of merit of (ZT) of C₆₀/Cu₂GeSe₃ composites. The inset shows the ZT value as a function of volume fraction of C₆₀.

4. Conclusions

In this study, C₆₀ was incorporated into Cu₂GeSe₃ matrix and C₆₀/Cu₂GeSe₃ composites were fabricated using BM-SPS method. C₆₀ phase distributed uniformly in the form of clusters and the average size of cluster was lower than 1 μm. With the content of C₆₀ increasing, the electrical resistivity and Seebeck coefficient of C₆₀/Cu₂GeSe₃ composites increased. The thermal conductivity of C₆₀/Cu₂GeSe₃ composites decreased significantly which originated from the phonon scattering by C₆₀ clusters locating on the grain boundaries of Cu₂GeSe₃ matrix. The highest ZT value for 0.9% C₆₀/Cu₂GeSe₃ composite was 0.20 at 700 K.

Competing Interests

The authors declare that they have no competing interests.

Acknowledgments

The authors would like to thank the help for the measurement of TE properties in California Institute of Technology and the discussion with Dr. Jeffrey. G. Snyder. Financial support from the National Natural Science Foundations of China (51202088) is acknowledged.

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