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.
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.
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.
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.
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)
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.
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.