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Mn0.7Zn0.3Fe2O4 composites
Author’s Accepted Manuscript Thermoelectric properties Graphene/Mn0.7Zn0.3Fe2O4 composites
of
Shupin Zhang, Aimin Li, Kangning Sun
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S0272-8842(17)30572-2 http://dx.doi.org/10.1016/j.ceramint.2017.03.183 CERI14952
To appear in: Ceramics International Received date: 3 January 2017 Revised date: 29 March 2017 Accepted date: 29 March 2017 Cite this article as: Shupin Zhang, Aimin Li and Kangning Sun, Thermoelectric properties of Graphene/Mn0.7Zn0.3Fe2O4 composites, Ceramics International, http://dx.doi.org/10.1016/j.ceramint.2017.03.183 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Thermoelectric properties of Graphene/Mn0.7Zn0.3Fe2O4 composites Shupin Zhanga,b, Aimin Lia,b, Kangning Suna,b,c a.Key Lab for Liquid Structure and Heredity of Materials, Ministry of Education, Shandong University, Jinan250061, China. b.Engineering Ceramics Key Lab. of Shandong Province, Shandong University, Jinan 250061,China. c. Corresponding author. E-mail: [email protected]. Abstract The Graphene/Mn0.7Zn0.3Fe2O4 composites were synthesized by coprecipitation and sintered by a spark-plasma-sintering (SPS) method. The thermoelectric properties of the sintered composites were evaluated in the temperature range of 343K~973K. The effect of graphene on the thermoelectrical properties of Mn0.7Zn0.3Fe2O4 was investigated. The dispersion of 2wt% graphene in Mn0.7Zn0.3Fe2O4 effectively enhanced the electrical conductivity and the absolute value of Seebeck coefficient, while thermal conductivity was decreased. The results showed that the maximum ZT value of 0.035 at 973K was obtained in the composite with 2wt% graphene. Keywords: Graphene, Mn0.7Zn0.3Fe2O4, Thermoelectric properties, SPS
1.Introduction Owing to the increase in the demand for renewable energy sources, thermoelectric materials have been steadily attracting scientific and technological interest over the past decade [1-3]. Various thermoelectric materials have been investigated over a wide range of temperatures, including tellurides [4-7], half-Heuslers [8,9], and silicides [10,11]. The need for oxide based thermoelectric materials is evident due to oxides being naturally abundant, non-toxic and excellent thermal stability [12]. Performance of thermoelectric materials depends on the dimensionless figure of merit
(ZT), defined as S2σT/κ, where S, σ, T and κ are Seebeck coefficient, electrical conductivity, absolute temperature and thermal conductivity, respectively [13]. Accordingly, the thermoelectric performance can be improved by increasing S and σ while simultaneously lowering κ. Since grapheme，a one-atom layer of grapheme, was isolated by Novoselov et al. in 2004 [14], it has been the focus of considerable research due to its excellent mechanical, electrical, thermal and magnetic properties [15]. The thermoelectric application of graphene has also been actively explored in spite of the low S and high κ [16]. As additives, graphene could be used to improve electrical properties of composites materials [17]. In recent years, some ferrites have been studied as thermoelectric materials, which have high σ and S [18-20]. T. Nozaki et al [21,22] have intensively investigated the ferrites, and found that the highest ZT values of 0.15 were obtained for CuFe0.5Cr0.5O2 at 1200K. MnZn ferrite (Mn1-xZnxFe2O4) is a kind of soft magnetic functional material. Fig.1 shows the crystal structure of MnZn ferrite, consisting of a tetrahedral A-site and an octahedral B-site. The conduction mechanism of the ferrites is attributed to the hopping of outer electrons from the different valence of ions (such as Fe2+ and Fe3+, Mn2+ and Mn3+, etc) [23,24]. As for the thermoelectric properties, Mn-Zn ferrite has low electrical conductivity (σ~10-3S/m) and high Seebeck coefficient (S~-530μV/K) [23, 25-28]. However, few papers have discussed κ and ZT of Mn-Zn fettite. In this sense, here, a thorough study of thermoelectric properties of Graphene/Mn0.7Zn0.3Fe2O4 composites is presented in this paper. 2. Experimental
2.1 Materials In this study, graphene (Thickness:4~20nm, Size: 5~10μm; Purity: >99.5wt%) were purchased from COCC (Chengdu Organic Chemicals Co. Ltd., Chinese Academy of Sciences). 2.2 Synthesis of Graphene/ Mn0.7Zn0.3Fe2O4 composites The Graphene/Mn0.7Zn0.3Fe2O4 powder was obtained by the coprecipitation method. Certain amounts in various weight percentages of graphene (1wt%, 2wt%, 3wt%) were ultrasonically dispersed in a aqueous solution of NaOH for 1h. Then a aqueous solution of MnCl2·4H2O, ZnCl2 and FeCl3·6H2O in their respective stoichiometry (the Mn:Zn:Fe molar ratio was maintained at 0.7:0.3:2) was added dropwise into the resulting mixture solution under constant stirring at 80℃. A temperature of 100℃ for 1h duration was sufficient for reaction. The product mixture was cooled to ambient temperature. The whole experimentation was carried out in an high purity nitrogen atmosphere. Then the obtained precipitates were filtered and rinsed several times with distilled water and ethanol until there were no chloride ions in the solution. The final product was dried in a vacuum oven at 80℃ for 12 h. The powders were charged to a column graphite die and finally sintered in vacuum using a SPS system (Sumitomo SPS-2050T). The ramp-up speed was 100℃/min, axial compressive stress was 40MPa and the holding time is 5min, while the holding temperature was 750℃. 2.3 Characterization XRD patterns of the Graphene/Mn0.7Zn0.3Fe2O4 composites were obtained by
X-Ray Diffraction (XRD, Rigaku DMAX- 2500PC, Japan), using Cu Kα radiation. The morphologies of the fractured surface were observed by Field Emission Scanning Electron
micropcopy
(FESEM,
SU-70
15kV).
The
densities
of
Graphene/Mn0.7Zn0.3Fe2O4 bulk samples were calculated using the Archimedes immersion method with deionized water as the immersion medium. Carrier mobility and carrier concentration were evaluated by Hall measurement (Acecnt HL5500PC). σ and S were measured by LSR-3 type thermoelectricity tester at the same time. κ is measured by laser thermal conductance instrument (Netzsch LFA 457 MicroFlash®). 3. Results and discussion Fig. 2 shows the XRD patterns of Graphene/Mn0.7Zn0.3Fe2O4 compounds with different contents of graphene. There is a diffraction peak at around 2θ≈26.48 corresponds to the indices of (002), which could be indexed to the disorderedly stacked graphene sheets [29]. Diffractions of other phases could be indexed as face-centered cubic spinel structure, and no impurity peaks were observed. It indicated that high purity crystalline Mn-Zn ferrite was synthesized. Fig.3 shows the FESEM photos of the fracture surface of the bulk samples with different contents of graphene. The graphene are dispersed in the Mn0.7Zn0.3Fe2O4 grains, and there are some holes (marked by arrow) in the composites. Besides, it is observed that the average grain size increases with increasing the content of graphene. As shown in Table 1, the relative densities of the bulk samples increase first and then decrease slightly as the contents of graphene increase. The results reveal that the existence of graphene increased the size of grains and the density of the samples. The
density increased makes the carrier concentration raise which is beneficial to the improvement of σ. However, at the same time, the increase of the grain size decreases the ability of grain boundary scattering phonons, which is detrimental to the decrease of κ. Fig.4 shows the temperature dependence of electrical conductivity (σ), Seebeck coefficient (S), thermal conductivity (κ) measured at 343~973K and the calculated power factor (PF= S2σ) of the Graphene/Mn0.7Zn0.3Fe2O4 composites with different contents of graphene. As shown in figure 4(a), σ of all the samples gradually increase with raising the measuring temperature, which indicate a classic intrinsic conduction behaviour of semiconductor. σ at 973K increases from 271 S/m to 2600S/m with increasing graphene content from 1wt% to 3wt%. σ at room temperature (see Table 1) are estimated by extrapolation from the 70℃ experimental data at the evaluated temperature, which are significantly higher than that in report [23]. It indicates that the adding of graphene could increase σ, which is desirable for higher value of ZT. Furthermore, the primary reason for the increased σ is ascribed to the variation of carrier concentration and carrier mobility after graphene adding. As listed in Table 1, the carrier concentration increases from 1.31×1016 cm-3 to 1.18×1019 cm-3 with increasing graphene content from 1wt% to 2wt%, while the carrier mobility correspondingly decreases from 19.35 cm2/Vs to 0.91cm2/Vs. Although the carrier concentration decreases to -1.67×1018 cm-3 with further increasing graphene content, the carrier mobility increases to 3.30 cm2/Vs. Therefore, the graphene addition can increase σ provided that the carrier concentration and carrier mobility are sufficiently
increased. Moreover, the sign of carrier is negative, suggesting that the dominate carrier is electron. As shown in Fig.4(b), S of all the samples is negative in the whole temperature range, indicating their n-type electrical transport property, and confirming the Hall effect data. The maximum | | of 1wt%, 2wt% and 3wt% samples are 288μV/K at 973K, 271μV/K at 343K and 159μV/K at 343K, respectively. The temperature of maximum | | decreases significantly with increasing the content of graphene, which may roughly be correlated with Curie temperature [30]. The temperature-dependent changes in S of the samples exhibit complicated behaviors. For the composite sample with 1wt% graphene, | | first increases and then decreases with raising the measuring temperature, which is the same trend as Mn-Zn ferrite [21]. It indicates that 1wt% graphene content is not sufficient enough to change the intrinsic conduction of the Mn0.7Zn0.3Fe2O4. With further increasing the contents of graphene, S shows a decreasing trend in the whole temperature range, and the varying trend gradually changes to be gentle. It is generally known that | | decreases if the electrical conductivity improves due to an increase in the carrier concentration by intrinsic conduction [31]. Interestingly, S increases although σ increases by adding 1wt%~2wt% graphene into Mn0.7Zn0.3Fe2O4. As a sort of spinel ferrites, the magnetic ordering has an influence on S of Mn0.7Zn0.3Fe2O4. According to Wu [32] and Mazen et al [33], S expressed in terms of Fe3+ and Fe2+ ions, considering the small polaron hopping conduction mechanism as follows: { [
]
[
] }
(1)
where [Fe3+]B and [Fe2+]B are the concentration of the Fe3+ and Fe2+ ions in the octahedral sites, respectively and β=1. Therefore the increase of graphene content could have an effect of reducing the number of Fe2+ ions on B-site. The power factor value of all the samples are calculated from the above value of electrical conductivity and Seebeck coefficient as shown in Fig.4(c). The values of PF all increase monotonically with increasing temperature, which is quite similar to the trend of electrical conductivity with temperature. Because of the high electrical conductivity and Seebeck coefficient, the maximum value of PF, 50.2μWm-1K-2, is obtained at 973K from the 2wt% Graphene/Mn0.7Zn0.3Fe2O4 composites. As shown in Fig.4(d), κ of the 1wt%~2wt% samples decrease significantly with increasing graphene content at high temperature. But κ of 3wt% sample increases sharply due to the high κ of graphene and the large grain size. κ of the 1wt% sample increases first and then decreases with increasing the temperature. The minimum (0.30Wm-1K-1) and the maximum (2.99Wm-1K-1) of κ are obtained from the 1wt% sample at 373K and 673K, respectively. At room temperature the smaller grain size of 1wt% sample results in the smaller κ than other samples. The increase in κ at higher temperatures might be attributed to an ambipolar contribution arising from the diffusion of electron-hole pairs with the onset of intrinsic contribution [34]. κ of the 2wt% and 3wt% samples decrease with increasing the temperature due to the strong phonon scattering caused by the lattice thermal vibration. Fig.5 shows ZT values of the Graphene/Mn0.7Zn0.3Fe2O4 composites with different contents of graphene. ZT values of all the samples increase with raising the measuring
temperature and the effect of graphene is obvious. The maximum ZT value of 0.035 is obtained from the 2wt% Graphene/Mn0.7Zn0.3Fe2O4 composites at 973K. As compared
to
other
thermoelectric
materials,
the
ZT
value
of
the
Graphene/Mn0.7Zn0.3Fe2O4 composites is too low. However higher ZT value could be expected by preferential distribution of divalent metal ions in the Mn1-xZnxFe2O4 or changing types of graphene. 4. Conclusions The thermoelectric properties of Graphene/Mn0.7Zn0.3Fe2O4 composites were investigated. It was found that, incorporating a small number of graphene into Mn0.7Zn0.3Fe2O4 can enhance its thermoelectric performance to a high ZT value over 0.035, demonstrating that 2wt% graphene in Mn0.7Zn0.3Fe2O4 can both increase σ and S in addition to reducing κ. The Graphene/Mn0.7Zn0.3Fe2O4 composites exibit thermoelectric properties, suggesting potential for use as a promising thermoelectric material. However, the thermoelectric property of Graphene/Mn0.7Zn0.3Fe2O4 composites is limited by their low σ as compared to other thermoelectric materials. If σ of the composites can be further improved by preferential distribution of divalent metal ions in the Mn1-xZnxFe2O4 or changing types of graphene, the higher ZT value of the composites would be attained. Acknowledgment This work was financially supported by the National Natural Science Foundation of China (Grant No. 81171463). References
[1] E.S. Toberer, S.R. Brown, T. Ikeda, S.M. Kauzlarich, G.J. Snyder, High thermoelectric efficiency in lanthanum doped Yb14MnSb11, Appl. Phys. Lett. 93 (2008) 062110. [2] K. Biswas, L.D. Zhao, M.G. Kanatzidis, Tellurium-Free Thermoelectric: The Anisotropic n-Type Semiconductor Bi2S3, Adv. Energy. Mater. 2 (2012) 634-638. [3] S. Kim, K.H. Lee, H.A. Mun, H.S. Kim, S.W. Hwang, J.W. Roh, D.J. Yang, W.H. Shin, X.S. Li, Y.H. Lee, G.J. Snyder, S.W. Kim, Dense dislocation arrays embedded in grain boundaries for high-performance bulk thermoelectrics, Science. 348 (2015) 109-114. [4] Y. Rosenberg, Y. Gelbstein, M.P. Dariel, Phase separation and thermoelectric properties of the Pb0.25Sn0.25Ge0.5Te compound, J. Alloy. Compd. 526 (2012) 31-28. [5] Y. Gelbstein, Z. Dashevsky, M.P. Dariel, In-doped Pb0.5Sn0.5Te p-type samples prepared by powder metallurgical processing for thermoelectric applications, Physica B. 396 (2007) 16-21. [6] Y. Gelbstein, J. Davidow, Highly efficient functional Ge xPb1-xTe based thermoelectric alloys, Phys. Chem. Chem. Phys. 16 (2014) 20120-20126. [7] Y. Gelbstein, Phase morphology effects on the thermoelectric properties of Pb 0.25Sn0.25Ge0.5Te, Acta Materialia. 61 (2013) 1499-1507. [8] K. Kirievsky, M. Shlimovich, D. Fuks, Y. Gelbstein, An ab-initio study of the thermoelectric enhancement potential in nano-grained TiNiSn, Phys. Chem. Chem. Phys. 16 (2014) 20023-20029. [9] K. Kirievsky, Y. Gelbstein, D. Fuks, Phase separation and antisite defects possibilities for enhancement the thermoelectric efficiency in TiNiSn half-Heusler alloys, J. Solid. State. Chem. 203 (2013) 247-254. [10] Y. Sadia, L. Dinnerman, Y. Gelbstein, Mechanical alloying and spark plasma sintering of higher manganese silicides for thermoelectric application, J. Electron. Mater. 42 (2013) 1926-1931.
[11] Y. Gelbstein, J. Tunbridge, R. Dixon, M.J. Reece, H.P. Ning, R. Gilchrist, R. Summers, I. Agote, M.A. Lagos, K. Simpson, C. Rouaud, P. Feulner, S. Rivera, R. Torrecillas, M. Husband, J. Crossley, I. Robinson, Physical, mechanical and structural properties of highly efficient nanostructured n- and psilicides for practical thermoelectric applications, J. Electron. Mater. 43 (2014) 1703-1711. [12] Y. Fujishiro, M. Miyata, M. Awano, K. Maeda, Characterization of thermoelectric metal oxide elements prepared by the pulse electric-current sintering method, J. Am. Ceram. Soc. 87 (2004) 1890-1894. [13] D. M. Rowe, Thermoelectrics handbook:macro to nano, Taylor & Francis Group, LLC (2006). [14] K.S. Novoselov, A.K. Geim, S.V. Morozov, D. Jiang, Y. Zhang, S.V. Dubonos, I.V. Grigorieva, A.A. Firsov, Electric field effect in atomically thin carbon films, Science. 306 (2004) 666-669. [15] Y, Zhao, G,S, Tang, Z,Z, Yu, J,S, Qi, The effect of graphite oxide on the thermoelectric properties of polyaniline, Carbon. 50 (2012 ) 3064-3073. [16] D. Suh, S. Lee, H. Mun, S.H. Park, K.H. Lee, S.W. Kim, J.Y. Choi, S. Baik, Enhanced thermoelectric performance of Bi0.5Sb1.5Te3-expanded grapheme composites by simultaneous modulation of electronic and thermal carrier transport, Nano. Energy. 13 (2015), 67-76. [17] L.Y. Wang, F.Z. Liu, C. Jin, T.R. Zhang, Q.J. Yin, Preparation of polypyrrole/Graphene nanosheet composites with enhanced thermoelectric properties, RSC. Adv. 4 (2014) 46187-46193. [18] A. Hirahara, T. Tamura, Y. Oikawa, Y. Kawamura, H. Ozaki, Thermoelectric Properties of Zn-Substituted Magnetite, J. Electron. Mater. 38 (2009) 1468-1471. [19] B.A. Griffiths, D. Elwell, R. Parker, The thermoelectric power of the system NiFe2O4-Fe304, Philos. Mag. 22 (1970) 163-174. [20] I.C. Nlebedim, E.M. Levin, R. Prozorov, K.W. Dennis, R.W. McCallum, D.C. Jiles, Magnetic and
thermoelectric properties of cobalt ferrite, IEEE. T. Magn. 49 (2013) 4269-4272. [21] T. Nozaki, K. Hayashi, Y. Miyazaki, T. Kajitani, Cation distribution dependence on thermoelectric properties of doped spinel M0.6Fe2.4O4, Mater. Trans. 53 (2012) 1164-1168. [22] T. Kajitani, T. Nozaki, K. Hayashi, Thermoelectric Iron Oxides, Adv. Sci. Technol. 74 (2010) 66-71. [23] D. Ravinder, K. Latha, Electrical conductivity of Mn-Zn ferrites, J. Appl. Phys. 75 (1994) 6118-6120. [24] Y.Y. Li, G.D. Li, Ferrite Physics, Beijing: Science Press (in Chinese). 1962. [25] D. Ravinder, K. Vijaya Kumar, Thermoelectric power studies of erbium substituted Mn-Zn ferrites, Mater. Lett. 49 (2001) 57-62. [26] B.R. Kumar, D. Ravinder, Thermoelectric power studies of gadolinium substituted Mn-Zn-Gd ferrites, Mater. Lett. 53 (2002) 441-445. [27] D. Ravinder, B. R. Kumar, Thermoelectric power studies of ceriu substituted Mn-Zn ferrites, Mate. Chem. Phys. 82 (2003) 321-326. [28] A.D.P. Rao, B. Ramesh, P.R.M. Rao, S.B. Raju, Thermoelectric power studies of Sn/Nb substituted Mn-Zn ferrites, J. Mater. Sci. 34 (1999) 621-623. [29] Y.J. Yao, J.C. Qin ,Y.M. Cai , F.Y. Wei ,F. Lu, S.B. Wang. Facile synthesis of magnetic ZnFe2O4-reduced graphene oxide hybrid and its photo-Fenton-like behavior under visible irradiation, Environ. Sci. Pollut. Res. 21 (2014) 7296-7306. [30] A.M. Shaikh, C.M. Kanamadi, B.K. Chougule, Electrical resistivity and thermoelectric power studies on Zn-substituted Li-Mg ferrites, Mater. Chem. Phys. 93 (2005) 548-551 [31] B.B. Liang, Z.J. Song, M.H. Wang, L.J. Wang, W. Jiang, Fabrication and thermoelectric
properties of Graphene/Bi2Te3 composite materials, J. Nanomater.Volume. (2013) 210767. [32] C.C. Wu, S. Kumarakrishnan, T.O. Mason, Thermopower composition dependence in ferrospinel, J. Solid. State. Chem. 37 (1981) 144-150. [33] S.A. Mazen, A. Elfalaky, Thermoelectric power and electrical conductivity of Cu-Ti ferrite, J. Magn. Magn. Mater. 195 (1999) 148-155. [34] H. Kaur, L. Sharma, S. Singh, B. Sivaiah, G.B. Reddy, T.D. Senguttuvan, Enhancement in figure of merit (ZT) by annealing of BiTe nanostructures synthesized by microwave-assisted flash combustion, J. Electron. Mater. 43 (2014) 1782-1789.
Table 1 The room-temperature actual density (d), relative density (dR), electrical conductivity (σ), Seebeck coefficient (S), carrier mobility (μ) and carrier concentration (n) of Graphene/Mn0.7Zn0.3Fe2O4 bulk samples with different contents of graphene.
Samples
d (g/cm3)
dR
σa
Sa
μ
n
a
(%)
(S/m)
(μV/K)
(cm2/Vs)
(1018/cm3)
1wt%
4.22
84.06
0.3
-240
19.35
-0.01
2wt%
4.70
94.76
30
-286
0.91
-11.80
3wt%
4.61
94.27
54
-170
3.30
-1.67
σ and S are estimated by extrapolation from the 70℃ experimental data at the evaluated temperature.
Fig. 1 Crystal structure of Mn1-xZnxFe2O4
Fig. 2 XRD patterns of the of the Graphene/Mn0.7Zn0.3Fe2O4 bulk samples with different contents of graphene.
Fig.3 FESEM micrographs of the Graphene/Mn0.7Zn0.3Fe2O4 bulk samples with different contents of graphene，(a)1wt%, (b)2wt%, (c)3wt%.
Fig. 4 Temperature dependence of electrical conductivity (a), Seebeck coefficient (b), thermoelectric power factor (c), and thermal conductivity (d) of the Graphene/Mn0.7Zn0.3Fe2O4 bulk samples with different contents of graphene
Fig. 5 Temperature dependence of ZT of the Graphene/Mn0.7Zn0.3Fe2O4 bulk samples with different contents of graphene
of
Shupin Zhang, Aimin Li, Kangning Sun
www.elsevier.com/locate/ceri
PII: DOI: Reference:
S0272-8842(17)30572-2 http://dx.doi.org/10.1016/j.ceramint.2017.03.183 CERI14952
To appear in: Ceramics International Received date: 3 January 2017 Revised date: 29 March 2017 Accepted date: 29 March 2017 Cite this article as: Shupin Zhang, Aimin Li and Kangning Sun, Thermoelectric properties of Graphene/Mn0.7Zn0.3Fe2O4 composites, Ceramics International, http://dx.doi.org/10.1016/j.ceramint.2017.03.183 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Thermoelectric properties of Graphene/Mn0.7Zn0.3Fe2O4 composites Shupin Zhanga,b, Aimin Lia,b, Kangning Suna,b,c a.Key Lab for Liquid Structure and Heredity of Materials, Ministry of Education, Shandong University, Jinan250061, China. b.Engineering Ceramics Key Lab. of Shandong Province, Shandong University, Jinan 250061,China. c. Corresponding author. E-mail: [email protected]. Abstract The Graphene/Mn0.7Zn0.3Fe2O4 composites were synthesized by coprecipitation and sintered by a spark-plasma-sintering (SPS) method. The thermoelectric properties of the sintered composites were evaluated in the temperature range of 343K~973K. The effect of graphene on the thermoelectrical properties of Mn0.7Zn0.3Fe2O4 was investigated. The dispersion of 2wt% graphene in Mn0.7Zn0.3Fe2O4 effectively enhanced the electrical conductivity and the absolute value of Seebeck coefficient, while thermal conductivity was decreased. The results showed that the maximum ZT value of 0.035 at 973K was obtained in the composite with 2wt% graphene. Keywords: Graphene, Mn0.7Zn0.3Fe2O4, Thermoelectric properties, SPS
1.Introduction Owing to the increase in the demand for renewable energy sources, thermoelectric materials have been steadily attracting scientific and technological interest over the past decade [1-3]. Various thermoelectric materials have been investigated over a wide range of temperatures, including tellurides [4-7], half-Heuslers [8,9], and silicides [10,11]. The need for oxide based thermoelectric materials is evident due to oxides being naturally abundant, non-toxic and excellent thermal stability [12]. Performance of thermoelectric materials depends on the dimensionless figure of merit
(ZT), defined as S2σT/κ, where S, σ, T and κ are Seebeck coefficient, electrical conductivity, absolute temperature and thermal conductivity, respectively [13]. Accordingly, the thermoelectric performance can be improved by increasing S and σ while simultaneously lowering κ. Since grapheme，a one-atom layer of grapheme, was isolated by Novoselov et al. in 2004 [14], it has been the focus of considerable research due to its excellent mechanical, electrical, thermal and magnetic properties [15]. The thermoelectric application of graphene has also been actively explored in spite of the low S and high κ [16]. As additives, graphene could be used to improve electrical properties of composites materials [17]. In recent years, some ferrites have been studied as thermoelectric materials, which have high σ and S [18-20]. T. Nozaki et al [21,22] have intensively investigated the ferrites, and found that the highest ZT values of 0.15 were obtained for CuFe0.5Cr0.5O2 at 1200K. MnZn ferrite (Mn1-xZnxFe2O4) is a kind of soft magnetic functional material. Fig.1 shows the crystal structure of MnZn ferrite, consisting of a tetrahedral A-site and an octahedral B-site. The conduction mechanism of the ferrites is attributed to the hopping of outer electrons from the different valence of ions (such as Fe2+ and Fe3+, Mn2+ and Mn3+, etc) [23,24]. As for the thermoelectric properties, Mn-Zn ferrite has low electrical conductivity (σ~10-3S/m) and high Seebeck coefficient (S~-530μV/K) [23, 25-28]. However, few papers have discussed κ and ZT of Mn-Zn fettite. In this sense, here, a thorough study of thermoelectric properties of Graphene/Mn0.7Zn0.3Fe2O4 composites is presented in this paper. 2. Experimental
2.1 Materials In this study, graphene (Thickness:4~20nm, Size: 5~10μm; Purity: >99.5wt%) were purchased from COCC (Chengdu Organic Chemicals Co. Ltd., Chinese Academy of Sciences). 2.2 Synthesis of Graphene/ Mn0.7Zn0.3Fe2O4 composites The Graphene/Mn0.7Zn0.3Fe2O4 powder was obtained by the coprecipitation method. Certain amounts in various weight percentages of graphene (1wt%, 2wt%, 3wt%) were ultrasonically dispersed in a aqueous solution of NaOH for 1h. Then a aqueous solution of MnCl2·4H2O, ZnCl2 and FeCl3·6H2O in their respective stoichiometry (the Mn:Zn:Fe molar ratio was maintained at 0.7:0.3:2) was added dropwise into the resulting mixture solution under constant stirring at 80℃. A temperature of 100℃ for 1h duration was sufficient for reaction. The product mixture was cooled to ambient temperature. The whole experimentation was carried out in an high purity nitrogen atmosphere. Then the obtained precipitates were filtered and rinsed several times with distilled water and ethanol until there were no chloride ions in the solution. The final product was dried in a vacuum oven at 80℃ for 12 h. The powders were charged to a column graphite die and finally sintered in vacuum using a SPS system (Sumitomo SPS-2050T). The ramp-up speed was 100℃/min, axial compressive stress was 40MPa and the holding time is 5min, while the holding temperature was 750℃. 2.3 Characterization XRD patterns of the Graphene/Mn0.7Zn0.3Fe2O4 composites were obtained by
X-Ray Diffraction (XRD, Rigaku DMAX- 2500PC, Japan), using Cu Kα radiation. The morphologies of the fractured surface were observed by Field Emission Scanning Electron
micropcopy
(FESEM,
SU-70
15kV).
The
densities
of
Graphene/Mn0.7Zn0.3Fe2O4 bulk samples were calculated using the Archimedes immersion method with deionized water as the immersion medium. Carrier mobility and carrier concentration were evaluated by Hall measurement (Acecnt HL5500PC). σ and S were measured by LSR-3 type thermoelectricity tester at the same time. κ is measured by laser thermal conductance instrument (Netzsch LFA 457 MicroFlash®). 3. Results and discussion Fig. 2 shows the XRD patterns of Graphene/Mn0.7Zn0.3Fe2O4 compounds with different contents of graphene. There is a diffraction peak at around 2θ≈26.48 corresponds to the indices of (002), which could be indexed to the disorderedly stacked graphene sheets [29]. Diffractions of other phases could be indexed as face-centered cubic spinel structure, and no impurity peaks were observed. It indicated that high purity crystalline Mn-Zn ferrite was synthesized. Fig.3 shows the FESEM photos of the fracture surface of the bulk samples with different contents of graphene. The graphene are dispersed in the Mn0.7Zn0.3Fe2O4 grains, and there are some holes (marked by arrow) in the composites. Besides, it is observed that the average grain size increases with increasing the content of graphene. As shown in Table 1, the relative densities of the bulk samples increase first and then decrease slightly as the contents of graphene increase. The results reveal that the existence of graphene increased the size of grains and the density of the samples. The
density increased makes the carrier concentration raise which is beneficial to the improvement of σ. However, at the same time, the increase of the grain size decreases the ability of grain boundary scattering phonons, which is detrimental to the decrease of κ. Fig.4 shows the temperature dependence of electrical conductivity (σ), Seebeck coefficient (S), thermal conductivity (κ) measured at 343~973K and the calculated power factor (PF= S2σ) of the Graphene/Mn0.7Zn0.3Fe2O4 composites with different contents of graphene. As shown in figure 4(a), σ of all the samples gradually increase with raising the measuring temperature, which indicate a classic intrinsic conduction behaviour of semiconductor. σ at 973K increases from 271 S/m to 2600S/m with increasing graphene content from 1wt% to 3wt%. σ at room temperature (see Table 1) are estimated by extrapolation from the 70℃ experimental data at the evaluated temperature, which are significantly higher than that in report [23]. It indicates that the adding of graphene could increase σ, which is desirable for higher value of ZT. Furthermore, the primary reason for the increased σ is ascribed to the variation of carrier concentration and carrier mobility after graphene adding. As listed in Table 1, the carrier concentration increases from 1.31×1016 cm-3 to 1.18×1019 cm-3 with increasing graphene content from 1wt% to 2wt%, while the carrier mobility correspondingly decreases from 19.35 cm2/Vs to 0.91cm2/Vs. Although the carrier concentration decreases to -1.67×1018 cm-3 with further increasing graphene content, the carrier mobility increases to 3.30 cm2/Vs. Therefore, the graphene addition can increase σ provided that the carrier concentration and carrier mobility are sufficiently
increased. Moreover, the sign of carrier is negative, suggesting that the dominate carrier is electron. As shown in Fig.4(b), S of all the samples is negative in the whole temperature range, indicating their n-type electrical transport property, and confirming the Hall effect data. The maximum | | of 1wt%, 2wt% and 3wt% samples are 288μV/K at 973K, 271μV/K at 343K and 159μV/K at 343K, respectively. The temperature of maximum | | decreases significantly with increasing the content of graphene, which may roughly be correlated with Curie temperature [30]. The temperature-dependent changes in S of the samples exhibit complicated behaviors. For the composite sample with 1wt% graphene, | | first increases and then decreases with raising the measuring temperature, which is the same trend as Mn-Zn ferrite [21]. It indicates that 1wt% graphene content is not sufficient enough to change the intrinsic conduction of the Mn0.7Zn0.3Fe2O4. With further increasing the contents of graphene, S shows a decreasing trend in the whole temperature range, and the varying trend gradually changes to be gentle. It is generally known that | | decreases if the electrical conductivity improves due to an increase in the carrier concentration by intrinsic conduction [31]. Interestingly, S increases although σ increases by adding 1wt%~2wt% graphene into Mn0.7Zn0.3Fe2O4. As a sort of spinel ferrites, the magnetic ordering has an influence on S of Mn0.7Zn0.3Fe2O4. According to Wu [32] and Mazen et al [33], S expressed in terms of Fe3+ and Fe2+ ions, considering the small polaron hopping conduction mechanism as follows: { [
]
[
] }
(1)
where [Fe3+]B and [Fe2+]B are the concentration of the Fe3+ and Fe2+ ions in the octahedral sites, respectively and β=1. Therefore the increase of graphene content could have an effect of reducing the number of Fe2+ ions on B-site. The power factor value of all the samples are calculated from the above value of electrical conductivity and Seebeck coefficient as shown in Fig.4(c). The values of PF all increase monotonically with increasing temperature, which is quite similar to the trend of electrical conductivity with temperature. Because of the high electrical conductivity and Seebeck coefficient, the maximum value of PF, 50.2μWm-1K-2, is obtained at 973K from the 2wt% Graphene/Mn0.7Zn0.3Fe2O4 composites. As shown in Fig.4(d), κ of the 1wt%~2wt% samples decrease significantly with increasing graphene content at high temperature. But κ of 3wt% sample increases sharply due to the high κ of graphene and the large grain size. κ of the 1wt% sample increases first and then decreases with increasing the temperature. The minimum (0.30Wm-1K-1) and the maximum (2.99Wm-1K-1) of κ are obtained from the 1wt% sample at 373K and 673K, respectively. At room temperature the smaller grain size of 1wt% sample results in the smaller κ than other samples. The increase in κ at higher temperatures might be attributed to an ambipolar contribution arising from the diffusion of electron-hole pairs with the onset of intrinsic contribution [34]. κ of the 2wt% and 3wt% samples decrease with increasing the temperature due to the strong phonon scattering caused by the lattice thermal vibration. Fig.5 shows ZT values of the Graphene/Mn0.7Zn0.3Fe2O4 composites with different contents of graphene. ZT values of all the samples increase with raising the measuring
temperature and the effect of graphene is obvious. The maximum ZT value of 0.035 is obtained from the 2wt% Graphene/Mn0.7Zn0.3Fe2O4 composites at 973K. As compared
to
other
thermoelectric
materials,
the
ZT
value
of
the
Graphene/Mn0.7Zn0.3Fe2O4 composites is too low. However higher ZT value could be expected by preferential distribution of divalent metal ions in the Mn1-xZnxFe2O4 or changing types of graphene. 4. Conclusions The thermoelectric properties of Graphene/Mn0.7Zn0.3Fe2O4 composites were investigated. It was found that, incorporating a small number of graphene into Mn0.7Zn0.3Fe2O4 can enhance its thermoelectric performance to a high ZT value over 0.035, demonstrating that 2wt% graphene in Mn0.7Zn0.3Fe2O4 can both increase σ and S in addition to reducing κ. The Graphene/Mn0.7Zn0.3Fe2O4 composites exibit thermoelectric properties, suggesting potential for use as a promising thermoelectric material. However, the thermoelectric property of Graphene/Mn0.7Zn0.3Fe2O4 composites is limited by their low σ as compared to other thermoelectric materials. If σ of the composites can be further improved by preferential distribution of divalent metal ions in the Mn1-xZnxFe2O4 or changing types of graphene, the higher ZT value of the composites would be attained. Acknowledgment This work was financially supported by the National Natural Science Foundation of China (Grant No. 81171463). References
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Table 1 The room-temperature actual density (d), relative density (dR), electrical conductivity (σ), Seebeck coefficient (S), carrier mobility (μ) and carrier concentration (n) of Graphene/Mn0.7Zn0.3Fe2O4 bulk samples with different contents of graphene.
Samples
d (g/cm3)
dR
σa
Sa
μ
n
a
(%)
(S/m)
(μV/K)
(cm2/Vs)
(1018/cm3)
1wt%
4.22
84.06
0.3
-240
19.35
-0.01
2wt%
4.70
94.76
30
-286
0.91
-11.80
3wt%
4.61
94.27
54
-170
3.30
-1.67
σ and S are estimated by extrapolation from the 70℃ experimental data at the evaluated temperature.
Fig. 1 Crystal structure of Mn1-xZnxFe2O4
Fig. 2 XRD patterns of the of the Graphene/Mn0.7Zn0.3Fe2O4 bulk samples with different contents of graphene.
Fig.3 FESEM micrographs of the Graphene/Mn0.7Zn0.3Fe2O4 bulk samples with different contents of graphene，(a)1wt%, (b)2wt%, (c)3wt%.
Fig. 4 Temperature dependence of electrical conductivity (a), Seebeck coefficient (b), thermoelectric power factor (c), and thermal conductivity (d) of the Graphene/Mn0.7Zn0.3Fe2O4 bulk samples with different contents of graphene
Fig. 5 Temperature dependence of ZT of the Graphene/Mn0.7Zn0.3Fe2O4 bulk samples with different contents of graphene