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Enhanced thermoelectric power factor of Na1.2Co1.8Ag0.2O4 with reduced graphene oxide synthesized by the polymerized complex method and solid-state reaction
Materials Letters 249 (2019) 1–4
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/mlblue
Enhanced thermoelectric power factor of Na1.2Co1.8Ag0.2O4 with reduced graphene oxide synthesized by the polymerized complex method and solid-state reaction Thanongsak Phochai a,b, Romteera Chueachot b,c, Kunchit Singsoog d, Tosawat Seetawan d,e, Ronariddh Nakhowong a,b,⇑ a
Program of Physics, Faculty of Science, Ubon Ratchathani Rajabhat University, Ubon Ratchathani 34000, Thailand Functional Nanomaterials and Electrospinning Research Laboratory, Faculty of Science, Ubon Ratchathani Rajabhat University, Ubon Ratchathani 34000, Thailand c Program of Chemistry, Faculty of Science, Ubon Ratchathani Rajabhat University, Ubon Ratchathani 34000, Thailand d Center of Excellence on Alternative Energy, Research and Development Institution, Sakon Nakhon Rajabhat University, 680 Nittayo Rd., Mueang District, Sakon Nakhon 47000, Thailand e Program of Physics, Faculty of Science and Technology, Sakon Nakhon Rajabhat University, Sakon Nakhon 74000, Thailand b
a r t i c l e
i n f o
Article history: Received 20 March 2019 Received in revised form 11 April 2019 Accepted 13 April 2019 Available online 13 April 2019 Keywords: Thermoelectric property Sodium cobalt oxide Reduced graphene oxide Power factor
a b s t r a c t Thermoelectric oxide Na1.2Co1.8Ag0.2O4/reduced graphene oxide (rGO) composites were synthesized by polymerized complex method and solid-state reactions. Temperature dependence of electrical resistivity and the Seebeck coefficient were investigated from 312 K to 859 K. Lowest electrical resistivity was 30.2 lXm. Maximum values of the Seebeck coefficient and power factor of Na1.2Co1.8Ag0.2O4/rGO were 136 lVK 1, and 444 lW m 1 K 2, respectively. Results indicated that low content of incorporated rGO in Na1.2Co1.8Ag0.2O4 effectively enhanced the power factor (PF) of thermoelectric materials. Ó 2019 Published by Elsevier B.V.
1. Introduction Thermoelectric materials have recently attracted intense scientific and technological interest due to their capability of directly converting waste heat into electrical energy, a phenomenon known as the Seebeck effect. Oxide based thermoelectrics are popular since oxides are naturally abundant, non-toxic and have excellent thermal stability [1]. Efficiency of thermoelectric devices depends on the dimensionless figure of merit (ZT); ZT = S2rT/j, where S, r, T, and j denote the Seebeck coefficient, electrical conductivity, absolute temperature, and thermal conductivity, respectively [2]. High performance thermoelectric materials have large ZT values which require a large power factor (PF = rS2) while simultaneously lowering thermal conductivity. Materials with high Seebeck coefficients, high electrical conductivity and low thermal conductivity are potential candidates for application in thermoelectric devices [3]. Thermoelectric oxides including
⇑ Corresponding author. E-mail address: [email protected] (R. Nakhowong). https://doi.org/10.1016/j.matlet.2019.04.049 0167-577X/Ó 2019 Published by Elsevier B.V.
Ca3Co4O9, SrTiO3 and NaxCo2O4 have been extensively studied over the past decade [4]. However, their ZT values are low due to low electrical conductivity and high thermal conductivity because the ionic nature of oxides causes poor and limited application [4]. NaxCo2O4 shows high electrical conductivity, high Seebeck coefficient and low thermal conductivity [5]. Recently, metals and rare-earth elements have been used to enhance thermoelectric properties and control the nanostructure of bulk materials by introducing phonon-scattering interfaces which reduce lattice thermal conductivity [6,7]. Metal element doping enhances electronic correlations by tuning carrier concentrations and creating phonon-scattering sites [6]. Nowadays, graphene has attracted attention as it exhibits increased electrical and reduced thermal conductivity [8]. A small amount of graphene incorporated into thermoelectric materials can enhance the power factor and reduce thermal conductivity [8–10]. Here, the enhancing power factor properties of Na1.2Co1.8Ag0.2O4/rGO prepared by a polymerized complex method and a solid-state reaction were investigated, together with the effect of different rGO contents in Na1.2Co1.8Ag0.2O4.
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T. Phochai et al. / Materials Letters 249 (2019) 1–4
2. Experimental 2.1. Preparation of Na1.2Co1.8Ag0.2O4/rGO composites Reduced graphene oxide (rGO) was synthesized from graphite powder (particle size < 20 mm, Sigma) using a modified Hummer’s method with reduction to rGO by hydrazine hydrate [11,12]. Na1.2Co1.8Ag0.2O4 was prepared by a polymerized complex method. Firstly, citric acid and ethylene glycol were mixed in proportions of 4 mol and 180 mol for each mole of metal cation, respectively and then NaNO3 (QREC), Co(NO3)26H2O (QREC), and AgNO3 (Carlo Erba) were dissolved in the mixture solution at a mole ratio corresponding to the nominal composition of Na1.2Co1.8Ag0.2O4 and stirried at 300 °C for 1 h to obtain a viscous gel. This viscous precursor was decomposed to a dark mass at 400 °C in air. The mass product was then ground and calcined at 800 °C for 5 h. The resultant Na1.2Co1.8Ag0.2O4 powder was mixed with different rGO contents (x = 0, 0.02. 0.04, 0.06, and 0.085 wt%) and ground for 2 h before pressing the Na1.2Co1.8Ag0.2O4/x-wt% rGO mixture into square pellets of 25.4 25.4 mm2 and 4.0 mm thickness under a pressure of 17 MPa. The pellets were sintered at 900 °C for 24 h to obtain Na1.2Co1.8Ag0.2O4/rGO. 2.2. Characterization Crystalline phases of the samples were performed with an X-ray diffractometer (XRD, 6100, Shimadzu) using CuKa radiation (k = 1.5406 Å) and morphologies were investigated using a field emission scanning electron microscope (FE-SEM, JSM-7610FPlus, JOEL) attached with an energy dispersive X-ray spectrometer (EDS) and transmission electron microscope (TEM, FEI Tecnai G2 F20 FE-TEM). The Seebeck coefficient and electrical resistivity were measured by ZEM-3 (ULVAC-RIKO) from 312 K to 859 K. Carrier concentrations and mobilities were calculated following the Hall effect measurement system (7561, Yokogawa, Japan). Degree of rGO crystallinity was investigated by a Raman spectroscope (T64000, Jobin Yvon Horiba, Japan) with laser excitation at 532 nm. 3. Results and discussion The XRD patterns Fig. 1(a) showed that all the diffraction peaks matched well with c-NaCo2O4 (JCPDS No. 27-0682), Ag2O, Na2O2, and Na2Co3. Intensity of the diffraction peaks increased as rGO content increased, indicating that rGO incorporated Na1.2Co1.8Ag0.2O4 can play a significant role in improving grain growth during the sintering process. As shown in Fig. 1(b), TEM image of rGO revealed single or few graphene layers with randomly wrinkled and transparent ultrathin sheets. Raman spectrum exhibited the characteristic Raman mode D (1335 cm 1) and G (1581 cm 1) belonging to rGO as shown in the inset figure. The G band corresponded to inplane vibration of sp2-hybridized carbon atoms while the D band appeared due to the presence of disorder in the graphene and C sp2 structure as symmetry mode defects such as vacancies and oxide functional groups [13,14]. An intensity ratio (ID/IG) value of 1.16 indicated a high degree of disorder and sp2 domain size, suggesting the presence of large numbers of rGO defects [15]. FE-SEM images (Fig. 2(a–e)) revealed dense structure and finer grain size of Na1.2Co1.8Ag0.2O4/rGO after incorporation of rGO content, indicating that rGO incorporated in Na1.2Co1.8Ag0.2O4 improved crystal grain growth. Ag2O (light gray) was observed as dispersed in the composites due to AgNO3 precipitated as coarse grains after the sintering process while some holes appeared in the bulk samples. Fig. 2(f and g) display the EDS spectrum and element mapping of Na1.2Co1.8Ag0.2O4/0.085 wt% rGO and show the constituent
Fig. 1. (a) XRD patterns of Na1.2Co1.8Ag0.2O4 with different rGO contents. (b) TEM image and Raman spectrum of rGO (inset figure).
elements and homogeneous element distribution throughout all the samples. The Seebeck coefficient, electrical resistivity, carrier concentration, carrier mobility, and power factor of Na1.2Co1.8Ag0.2O4/rGO are displayed in Fig. 3. Electrical resistivity of Na1.2Co1.8Ag0.2O4/ rGO slightly increased and then decreased with increasing temperature (Fig. 3(a)). Na1.2Co1.8Ag0.2O4/rGO exhibited lower electrical resistivity than Na1.2Co1.8Ag0.2O4 with lowest value of 30.2 lXm for 0.06% rGO at 765 K. Electrical resistivity or conductivity of materials can be estimated using the following equation: r = nel, where n, e, and l are charge-carrier concentration, charge per carrier, and charge-carrier mobility, respectively [8]. Electrical conductivity is directly proportional to n and l, indicating that electrical conductivity increases due to attributed carrier concentration and mobility. As shown in Fig. 3(b), the maximum carrier concentration of Na1.2Co1.8Ag0.2O4/rGO was 25.97 1017 cm 3 at 0.085 wt% rGO, while carrier mobility decreased from 112.89 cm2/Vs to 10.09 cm2/Vs as rGO content increased. Carrier mobility is inversely dependent on carrier concentration, indicating that decrease of carrier mobility depends on increasing rGO
T. Phochai et al. / Materials Letters 249 (2019) 1–4
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Fig. 2. FE-SEM images of Na1.2Co1.8Ag0.2O4 with rGO contents (a) unmixed, (b) 0.02, (c) 0.04, (d) 0.06, and (e) 0.085 wt%. The EDS spectrum (f) and element mapping images (g) of Na, Co, O, and Ag in Na1.2Co1.8Ag0.2O4/0.085 wt% rGO.
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Fig. 3. (a) Electrical resistivity of Na1.2Co1.8Ag0.2O4 with different rGO contents. (b) Carrier concentration and mobility of Na1.2Co1.8Ag0.2O4/rGO at room temperature. (c) The Seebeck coefficients, and (d) the power factor of Na1.2Co1.8Ag0.2O4/rGO composites.
content and creating impurity scattering in the samples. However, poor distribution of rGO content in Na1.2Co1.8Ag0.2O4 might lead to lower carrier concentration and mobility. As shown in Fig. 3(c), the Seebeck coefficient of all samples was positive over the entire temperature range, indicating that holes are the majority carriers of the samples. All samples exhibited a similar trend with the Seebeck coefficient increasing significantly as temperature increased. Maximum value of the Seebeck coefficient was 136 mVK 1 with 0.04 wt% rGO at 859 K. Power factor (Fig. 3(d)) of Na1.2Co1.8Ag0.2O4/rGO was higher than Na1.2Co1.8Ag0.2O4 with maximum power factor of 444 lW m 1 K 2 at 0.04 wt% rGO and 859 K.
4. Conclusions Thermoelectric properties of Na1.2Co1.8Ag0.2O4/rGO composites were successfully enhanced by polymerized complex method and solid-state reactions. Electrical resistivity, the Seebeck coefficient, and power factor of Na1.2Co1.8Ag0.2O4/rGO were enhanced with addition of a small amount of rGO. Carrier concentration tended to increase as rGO content increased while mobility decreased.
Conflicts of interest None.
Acknowledgments This work was financially supported by the Program of Physics, Faculty of Science, Ubon Ratchathani Rajabhat University, Thailand. References [1] N. Habibi, Y. Wang, H. Arandiyan, M. Rezaei, Adv. Powder Technol. 28 (2017) 1249–1257. [2] S. Zhang, A. Li, K. Sun, Ceram. Int. 43 (2017) 8643–8647. [3] K. Miyazawa, F. Amaral, A.V. Kovalevsky, M.P.F. Graça, Ceram. Int. 42 (2016) 9482–9487. [4] B. Feng, G. Li, Z. Pan, H. Xiaoming, L. Peihai, H. Zhu, L. Yawei, X. Fan, J. Alloys Compd. 754 (2018) 131–138. [5] W. Wang, Y. Jiang, M. Niu, L. Wang, B. Cao, Powder Technol. 184 (2008) 25–30. [6] Y. Yin, B. Tudu, A. Tiwari, Vacuum 146 (2017) 356–374. [7] O.J. Dura, R. Andujar, M. Falmbigl, P. Rogl, M.A. López de la Torre, E. Bauer, J. Alloys Compd. 711 (2017) 381–386. [8] N. Daichakomphu, R. Sakdanuphab, A. Harnwunggmoung, S. Pinitsoontorn, A. Sakulkalavek, J. Alloys Compd. 753 (2018) 630–635. [9] W.H. Shin, K. Ahn, M. Jeong, J.S. Yoon, J.M. Song, S. Lee, W.S. Seo, Y.S. Lim, J. Alloys Comp. 78 (2017) 342–348. [10] X. Feng, Y. Fan, N. Nomura, K. Kikuchi, L. Wang, W. Jiang, A. Kawasaki, Carbon 112 (2017) 169–176. [11] J. Chen, B. Yao, C. Li, G. Shi, Carbon 64 (2013) 225–229. [12] A.K. Dizaji, H.R. Mortaheb, B. Mokhtarani, Mater. Chem. Phys. 199 (2017) 424– 434. [13] S. Yadav, S. Chaudhary, D.K. Pandya, Ceram. Int. 44 (2018) 10628–10634. [14] C. Mallada, J.L. Menéndez, O.J. Dura, M.A. López de la Torre, R. Menéndez, R. Santamaría, J. Eur. Ceram. Soc. 37 (2017) 3741–3746. [15] N. Sharma, V. Sharma, S.K. Sharma, K. Sachdev, Mater. Lett. 236 (2019) 444– 447.
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/mlblue
Enhanced thermoelectric power factor of Na1.2Co1.8Ag0.2O4 with reduced graphene oxide synthesized by the polymerized complex method and solid-state reaction Thanongsak Phochai a,b, Romteera Chueachot b,c, Kunchit Singsoog d, Tosawat Seetawan d,e, Ronariddh Nakhowong a,b,⇑ a
Program of Physics, Faculty of Science, Ubon Ratchathani Rajabhat University, Ubon Ratchathani 34000, Thailand Functional Nanomaterials and Electrospinning Research Laboratory, Faculty of Science, Ubon Ratchathani Rajabhat University, Ubon Ratchathani 34000, Thailand c Program of Chemistry, Faculty of Science, Ubon Ratchathani Rajabhat University, Ubon Ratchathani 34000, Thailand d Center of Excellence on Alternative Energy, Research and Development Institution, Sakon Nakhon Rajabhat University, 680 Nittayo Rd., Mueang District, Sakon Nakhon 47000, Thailand e Program of Physics, Faculty of Science and Technology, Sakon Nakhon Rajabhat University, Sakon Nakhon 74000, Thailand b
a r t i c l e
i n f o
Article history: Received 20 March 2019 Received in revised form 11 April 2019 Accepted 13 April 2019 Available online 13 April 2019 Keywords: Thermoelectric property Sodium cobalt oxide Reduced graphene oxide Power factor
a b s t r a c t Thermoelectric oxide Na1.2Co1.8Ag0.2O4/reduced graphene oxide (rGO) composites were synthesized by polymerized complex method and solid-state reactions. Temperature dependence of electrical resistivity and the Seebeck coefficient were investigated from 312 K to 859 K. Lowest electrical resistivity was 30.2 lXm. Maximum values of the Seebeck coefficient and power factor of Na1.2Co1.8Ag0.2O4/rGO were 136 lVK 1, and 444 lW m 1 K 2, respectively. Results indicated that low content of incorporated rGO in Na1.2Co1.8Ag0.2O4 effectively enhanced the power factor (PF) of thermoelectric materials. Ó 2019 Published by Elsevier B.V.
1. Introduction Thermoelectric materials have recently attracted intense scientific and technological interest due to their capability of directly converting waste heat into electrical energy, a phenomenon known as the Seebeck effect. Oxide based thermoelectrics are popular since oxides are naturally abundant, non-toxic and have excellent thermal stability [1]. Efficiency of thermoelectric devices depends on the dimensionless figure of merit (ZT); ZT = S2rT/j, where S, r, T, and j denote the Seebeck coefficient, electrical conductivity, absolute temperature, and thermal conductivity, respectively [2]. High performance thermoelectric materials have large ZT values which require a large power factor (PF = rS2) while simultaneously lowering thermal conductivity. Materials with high Seebeck coefficients, high electrical conductivity and low thermal conductivity are potential candidates for application in thermoelectric devices [3]. Thermoelectric oxides including
⇑ Corresponding author. E-mail address: [email protected] (R. Nakhowong). https://doi.org/10.1016/j.matlet.2019.04.049 0167-577X/Ó 2019 Published by Elsevier B.V.
Ca3Co4O9, SrTiO3 and NaxCo2O4 have been extensively studied over the past decade [4]. However, their ZT values are low due to low electrical conductivity and high thermal conductivity because the ionic nature of oxides causes poor and limited application [4]. NaxCo2O4 shows high electrical conductivity, high Seebeck coefficient and low thermal conductivity [5]. Recently, metals and rare-earth elements have been used to enhance thermoelectric properties and control the nanostructure of bulk materials by introducing phonon-scattering interfaces which reduce lattice thermal conductivity [6,7]. Metal element doping enhances electronic correlations by tuning carrier concentrations and creating phonon-scattering sites [6]. Nowadays, graphene has attracted attention as it exhibits increased electrical and reduced thermal conductivity [8]. A small amount of graphene incorporated into thermoelectric materials can enhance the power factor and reduce thermal conductivity [8–10]. Here, the enhancing power factor properties of Na1.2Co1.8Ag0.2O4/rGO prepared by a polymerized complex method and a solid-state reaction were investigated, together with the effect of different rGO contents in Na1.2Co1.8Ag0.2O4.
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T. Phochai et al. / Materials Letters 249 (2019) 1–4
2. Experimental 2.1. Preparation of Na1.2Co1.8Ag0.2O4/rGO composites Reduced graphene oxide (rGO) was synthesized from graphite powder (particle size < 20 mm, Sigma) using a modified Hummer’s method with reduction to rGO by hydrazine hydrate [11,12]. Na1.2Co1.8Ag0.2O4 was prepared by a polymerized complex method. Firstly, citric acid and ethylene glycol were mixed in proportions of 4 mol and 180 mol for each mole of metal cation, respectively and then NaNO3 (QREC), Co(NO3)26H2O (QREC), and AgNO3 (Carlo Erba) were dissolved in the mixture solution at a mole ratio corresponding to the nominal composition of Na1.2Co1.8Ag0.2O4 and stirried at 300 °C for 1 h to obtain a viscous gel. This viscous precursor was decomposed to a dark mass at 400 °C in air. The mass product was then ground and calcined at 800 °C for 5 h. The resultant Na1.2Co1.8Ag0.2O4 powder was mixed with different rGO contents (x = 0, 0.02. 0.04, 0.06, and 0.085 wt%) and ground for 2 h before pressing the Na1.2Co1.8Ag0.2O4/x-wt% rGO mixture into square pellets of 25.4 25.4 mm2 and 4.0 mm thickness under a pressure of 17 MPa. The pellets were sintered at 900 °C for 24 h to obtain Na1.2Co1.8Ag0.2O4/rGO. 2.2. Characterization Crystalline phases of the samples were performed with an X-ray diffractometer (XRD, 6100, Shimadzu) using CuKa radiation (k = 1.5406 Å) and morphologies were investigated using a field emission scanning electron microscope (FE-SEM, JSM-7610FPlus, JOEL) attached with an energy dispersive X-ray spectrometer (EDS) and transmission electron microscope (TEM, FEI Tecnai G2 F20 FE-TEM). The Seebeck coefficient and electrical resistivity were measured by ZEM-3 (ULVAC-RIKO) from 312 K to 859 K. Carrier concentrations and mobilities were calculated following the Hall effect measurement system (7561, Yokogawa, Japan). Degree of rGO crystallinity was investigated by a Raman spectroscope (T64000, Jobin Yvon Horiba, Japan) with laser excitation at 532 nm. 3. Results and discussion The XRD patterns Fig. 1(a) showed that all the diffraction peaks matched well with c-NaCo2O4 (JCPDS No. 27-0682), Ag2O, Na2O2, and Na2Co3. Intensity of the diffraction peaks increased as rGO content increased, indicating that rGO incorporated Na1.2Co1.8Ag0.2O4 can play a significant role in improving grain growth during the sintering process. As shown in Fig. 1(b), TEM image of rGO revealed single or few graphene layers with randomly wrinkled and transparent ultrathin sheets. Raman spectrum exhibited the characteristic Raman mode D (1335 cm 1) and G (1581 cm 1) belonging to rGO as shown in the inset figure. The G band corresponded to inplane vibration of sp2-hybridized carbon atoms while the D band appeared due to the presence of disorder in the graphene and C sp2 structure as symmetry mode defects such as vacancies and oxide functional groups [13,14]. An intensity ratio (ID/IG) value of 1.16 indicated a high degree of disorder and sp2 domain size, suggesting the presence of large numbers of rGO defects [15]. FE-SEM images (Fig. 2(a–e)) revealed dense structure and finer grain size of Na1.2Co1.8Ag0.2O4/rGO after incorporation of rGO content, indicating that rGO incorporated in Na1.2Co1.8Ag0.2O4 improved crystal grain growth. Ag2O (light gray) was observed as dispersed in the composites due to AgNO3 precipitated as coarse grains after the sintering process while some holes appeared in the bulk samples. Fig. 2(f and g) display the EDS spectrum and element mapping of Na1.2Co1.8Ag0.2O4/0.085 wt% rGO and show the constituent
Fig. 1. (a) XRD patterns of Na1.2Co1.8Ag0.2O4 with different rGO contents. (b) TEM image and Raman spectrum of rGO (inset figure).
elements and homogeneous element distribution throughout all the samples. The Seebeck coefficient, electrical resistivity, carrier concentration, carrier mobility, and power factor of Na1.2Co1.8Ag0.2O4/rGO are displayed in Fig. 3. Electrical resistivity of Na1.2Co1.8Ag0.2O4/ rGO slightly increased and then decreased with increasing temperature (Fig. 3(a)). Na1.2Co1.8Ag0.2O4/rGO exhibited lower electrical resistivity than Na1.2Co1.8Ag0.2O4 with lowest value of 30.2 lXm for 0.06% rGO at 765 K. Electrical resistivity or conductivity of materials can be estimated using the following equation: r = nel, where n, e, and l are charge-carrier concentration, charge per carrier, and charge-carrier mobility, respectively [8]. Electrical conductivity is directly proportional to n and l, indicating that electrical conductivity increases due to attributed carrier concentration and mobility. As shown in Fig. 3(b), the maximum carrier concentration of Na1.2Co1.8Ag0.2O4/rGO was 25.97 1017 cm 3 at 0.085 wt% rGO, while carrier mobility decreased from 112.89 cm2/Vs to 10.09 cm2/Vs as rGO content increased. Carrier mobility is inversely dependent on carrier concentration, indicating that decrease of carrier mobility depends on increasing rGO
T. Phochai et al. / Materials Letters 249 (2019) 1–4
3
Fig. 2. FE-SEM images of Na1.2Co1.8Ag0.2O4 with rGO contents (a) unmixed, (b) 0.02, (c) 0.04, (d) 0.06, and (e) 0.085 wt%. The EDS spectrum (f) and element mapping images (g) of Na, Co, O, and Ag in Na1.2Co1.8Ag0.2O4/0.085 wt% rGO.
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Fig. 3. (a) Electrical resistivity of Na1.2Co1.8Ag0.2O4 with different rGO contents. (b) Carrier concentration and mobility of Na1.2Co1.8Ag0.2O4/rGO at room temperature. (c) The Seebeck coefficients, and (d) the power factor of Na1.2Co1.8Ag0.2O4/rGO composites.
content and creating impurity scattering in the samples. However, poor distribution of rGO content in Na1.2Co1.8Ag0.2O4 might lead to lower carrier concentration and mobility. As shown in Fig. 3(c), the Seebeck coefficient of all samples was positive over the entire temperature range, indicating that holes are the majority carriers of the samples. All samples exhibited a similar trend with the Seebeck coefficient increasing significantly as temperature increased. Maximum value of the Seebeck coefficient was 136 mVK 1 with 0.04 wt% rGO at 859 K. Power factor (Fig. 3(d)) of Na1.2Co1.8Ag0.2O4/rGO was higher than Na1.2Co1.8Ag0.2O4 with maximum power factor of 444 lW m 1 K 2 at 0.04 wt% rGO and 859 K.
4. Conclusions Thermoelectric properties of Na1.2Co1.8Ag0.2O4/rGO composites were successfully enhanced by polymerized complex method and solid-state reactions. Electrical resistivity, the Seebeck coefficient, and power factor of Na1.2Co1.8Ag0.2O4/rGO were enhanced with addition of a small amount of rGO. Carrier concentration tended to increase as rGO content increased while mobility decreased.
Conflicts of interest None.
Acknowledgments This work was financially supported by the Program of Physics, Faculty of Science, Ubon Ratchathani Rajabhat University, Thailand. References [1] N. Habibi, Y. Wang, H. Arandiyan, M. Rezaei, Adv. Powder Technol. 28 (2017) 1249–1257. [2] S. Zhang, A. Li, K. Sun, Ceram. Int. 43 (2017) 8643–8647. [3] K. Miyazawa, F. Amaral, A.V. Kovalevsky, M.P.F. Graça, Ceram. Int. 42 (2016) 9482–9487. [4] B. Feng, G. Li, Z. Pan, H. Xiaoming, L. Peihai, H. Zhu, L. Yawei, X. Fan, J. Alloys Compd. 754 (2018) 131–138. [5] W. Wang, Y. Jiang, M. Niu, L. Wang, B. Cao, Powder Technol. 184 (2008) 25–30. [6] Y. Yin, B. Tudu, A. Tiwari, Vacuum 146 (2017) 356–374. [7] O.J. Dura, R. Andujar, M. Falmbigl, P. Rogl, M.A. López de la Torre, E. Bauer, J. Alloys Compd. 711 (2017) 381–386. [8] N. Daichakomphu, R. Sakdanuphab, A. Harnwunggmoung, S. Pinitsoontorn, A. Sakulkalavek, J. Alloys Compd. 753 (2018) 630–635. [9] W.H. Shin, K. Ahn, M. Jeong, J.S. Yoon, J.M. Song, S. Lee, W.S. Seo, Y.S. Lim, J. Alloys Comp. 78 (2017) 342–348. [10] X. Feng, Y. Fan, N. Nomura, K. Kikuchi, L. Wang, W. Jiang, A. Kawasaki, Carbon 112 (2017) 169–176. [11] J. Chen, B. Yao, C. Li, G. Shi, Carbon 64 (2013) 225–229. [12] A.K. Dizaji, H.R. Mortaheb, B. Mokhtarani, Mater. Chem. Phys. 199 (2017) 424– 434. [13] S. Yadav, S. Chaudhary, D.K. Pandya, Ceram. Int. 44 (2018) 10628–10634. [14] C. Mallada, J.L. Menéndez, O.J. Dura, M.A. López de la Torre, R. Menéndez, R. Santamaría, J. Eur. Ceram. Soc. 37 (2017) 3741–3746. [15] N. Sharma, V. Sharma, S.K. Sharma, K. Sachdev, Mater. Lett. 236 (2019) 444– 447.