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Influence of Ag doping on the thermoelectric properties of layered compound NdOZnSb
Author’s Accepted Manuscript Influence of Ag doping on the thermoelectric properties of layered compound NdOZnSb Meng Pan, Kai Guo, Wanyu Lv, Jiye Zhang, Dongli Hu, Xinxin Yang, Jun Luo, Jing-Tai Zhao www.elsevier.com
PII: DOI: Reference:
S0167-577X(16)31826-2 http://dx.doi.org/10.1016/j.matlet.2016.11.073 MLBLUE21767
To appear in: Materials Letters Received date: 15 October 2016 Accepted date: 21 November 2016 Cite this article as: Meng Pan, Kai Guo, Wanyu Lv, Jiye Zhang, Dongli Hu, Xinxin Yang, Jun Luo and Jing-Tai Zhao, Influence of Ag doping on the thermoelectric properties of layered compound NdOZnSb, Materials Letters, http://dx.doi.org/10.1016/j.matlet.2016.11.073 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.
Influence of Ag doping on the thermoelectric properties of layered compound NdOZnSb Meng Pana, Kai Guoa* , Wanyu Lva, Jiye Zhanga, Dongli Hua, Xinxin Yanga*, Jun Luo a a
, Jing-Tai Zhaoa,b
School of Materials Science and Engineering, Shanghai University, Shanghai 200444, China
b
State Key Laboratory of Advanced Special Steel, Shanghai University, Shanghai 200444, China
[email protected] [email protected] *
Corresponding author.
Abstract Quaternary layered compound NdOZnSb is viewed as a thermoelectric candidate owing to the stacked building blocks, e.g. conductive [ZnSb] layer and insulting [NdO] layer featuring narrow band-gap semiconductor behavior. However, the measured figure of merit ZT is quite low for pristine NdOZnSb resulted from poor electrical conductivity despite considerable Seebeck coefficient and low thermal conductivity. The present paper demonstrates that Ag doping can precisely tune the electrical transport properties and accordingly enhance the power factor. The electrical conductivity of NdOZnSb increases by an order of magnitude at 325 K via the substitution of Zn by Ag, and the maximal thermoelectric figure of merit ZT for NdOZn0.96Ag0.04Sb reaches 0.44 at 725 K.
1
Graphical abstract
Keywords: NdOZnSb; Semiconductors; Ceramics; Thermoelectric properties
1.
Introduction In order to alleviate the energy crisis, thermoelectric (TE) materials have received much
attention in the past decade, which are a key technology for converting waste heat into electricity. The conversion efficiency of TE materials is characterized by the dimensionless figure of merit ZT = S2σT/(κL+ κE), where S is the Seebeck coefficient, σ is the electrical conductivity, κL is the lattice thermal conductivity, κE is the electronic thermal conductivity, and T is the absolute temperature, respectively [1, 2]. These parameters determining the critical ZT depend on the electronic and 2
thermal transport process, while the close coupled behavior between electronic and thermal transport properties make the enhancement of ZT a challenge task [3]. In 1995, G. A. Slack proposed the concept of “phonon-glass electron-crystal” (PGEC), which contributed a new hint in exploring high-performance thermoelectric materials constructed by electron-transport and phonon-scattering building block [4]. The former guarantees good electronic transport properties while the latter is responsible for lowering thermal conductivity. This approach is the most achievable in materials with complex crystal structures, where p-type [5] or n-type doping ions [6], isoelectronic substitution atoms [7], filled elements [8], interstitials [9] or vacancies [10] can act as phonon scattering centers to reduce the lattice thermal conductivity. Layered compounds NdOZnSb has a tetragonal symmetry with the space group of P4/nmm, which consists of insulating [NdO] layers and conductive [ZnSb] layers alternatively stacked along the c axis [11]. The [NdO] layer acts as a charge reservoir, while the conductive [ZnSb] layer provides an ideal conduction pathway for carrier transport. In addition, it is reported that NdOZnSb is a narrow-band-gap semiconductor with considerably low thermal conductivity [12]. The above feathers of NdOZnSb well satisfy the concept of PGEC. However, the thermoelectric performance of the pristine NdOZnSb is seriously hindered by its poor electrical conductivity owing to the low carrier concentration. Fortunately, previous researches revealed that the electronic transport properties of isostructural compound REOZnSb could be well tuned and optimized by chemical modification in [REO] layers (RE = La, Ce, Nd) [13-15]. In this paper, we present the doping effects of Ag at Zn site in conductive layers of NdOZnSb and the further enhancement of thermoelectric properties has been obtained. 2.
Experimental Section 3
NdOZn1-xAgxSb (x = 0, 0.01, 0.02, 0.03, 0.04 and 0.05) were synthesized by a two-step solid state reaction method including pre-reaction and annealing process. The starting materials were Nd (purity 99.9%), Nd2O3 (purity 99.99%), Zn (purity 99.99%), Sb (purity 99.999%) and Ag (purity 99.999%), which were weighed according to the stoichiometric ratio in a glove box filled with argon (O2 < 1 ppm, H2O < 1 ppm). The chemicals were subsequently put in graphite crucibles and sealed in an evacuated silica tube. Then, all samples were heated up to 1273 K for 3 days with a heating rate of 2 K min–1. A second annealing process occurred at 1173 K for 3 days after grinding the resulting products and pressing them into pellets of Φ 10 mm. Finally, the as-obtained powders were densified by hot-pressing sintering at 923 K under a uniaxial pressure of 60 MPa for 30 min in a vacuum. The phase purity of NdOZn1-xAgxSb samples were examined by X-ray powder diffraction (XRPD, Rigaku, Japan, Cu Kα radiation, λ = 1.541854 Å, 10° < 2θ < 80°, step width 0.02°). The microstructures and chemical composition were characterized by field-emission scanning electron microscopy (SEM, SUPRA 55 SAPPHIRE, ZEISS, Germany) equipped with energy dispersive X-ray spectroscopy (EDS). The thermal conductivity was calculated from the equation κ = DCpd, where D, Cp and d is thermal diffusivity coefficient, specific heat and density, respectively. D was measured using the laser flash diffusivity apparatus (LFA 447, Netzsch, Germany), and Cp was estimated by the model of Dulongand Petit as Cp = 3NR/M, where N is the number of atoms per formula unit, R is universal gas constant, and M is molar mass. The density d was acquired by Archimedes method. Seebeck coefficient and electrical conductivity were measured by ZEM-3 (ULVAC, Japan) on rectangular bars. 3.
Results and discussion 4
The main Bragg diffraction peaks of as-obtained samples match well with the simulated pattern (ICSD #419354), indicating that the main phases adopt NdOZnSb structure and the doping would not alter the symmetry but lattice parameters (Fig. 1a). Small amount of Nd2O3 impurity phase are found in all samples except the pristine one. With Ag concent increase, the lattice paramer a remains essentially unchaged but c linely increases when x < 0.3 (Fig. 1b), probably attributed to the layered feature. Higher Ag concent would not lead to signifiant varieties in the lattice paramer c, ascribed to the solution limit of Ag.
Fig. 1. (a) The XRPD patterns of NdOZn1-xAgxSb (x = 0, 0.01, 0.02, 0.03, 0.04 and 0.05). Bottom: the simulated pattern for NdOZnSb. (b) Lattice parameters of NdOZn1-xAgxSb depending on the Ag concent.
The morphologies of polished surface and cross section for typitcal samples NdOZnSb and NdOZn0.96Ag0.04Sb are observed by SEM. As is shown in Fig.2a and c, both of the samples exhibit dense surface since few pores are indentified, consist with the measured relative density (89.1% 5
for NdOZnSb and 94.7% for NdOZn0.96Ag0.04Sb). Fig. 2b and d reveal obvious prefered orientation texture, related to the laryered structure feature. Interestingly, these layered microstructures distribute randomly in the samples due to the absence of some induced mechnism during densitified process, which has insignificant influence on the anisotropy of the electrical and thermal transport properties of bulk NdOZn1-xAgxSb samples. Ag doping can improve the crystallization of NdOZnSb, as evidenced by the density values and cross-section morphologies. EDS
results
demonstrate
lower
Ag
content
for
nominal
NdOZn0.96Ag0.04Sb
(Nd0.87(3)O0.89(4)Zn0.97(5)Ag0.03Sb0.86(2)), indicating a solution limit for Ag in NdOZnSb.
Fig. 2.SEM images of typical NdOZn1-xAgxSb samples (a) x = 0, polished surfacemorphology, (b) x = 0, fractograph, (c) x = 0.04, polished surfacemorphology, (d) x = 0.04, fractograph.
Fig.3 presents the TE behaviors of pure and Ag-doped NdOZnSb by assessing S, σ and κ from 325 K to 725 K. The electrical conductivity σ for all samples increases with the rise of the temperature, suggesting semiconductor behaviors (Fig. 3a). As expected, the substitution of Zn by 6
Ag can significantly enhance the electrical properties, as evidenced by an order of magnitude increase in comparison with undoped sample at 325 K. It is found that for x = 0.04 and 0.05, the values of the electrical conductivity are close over the entire temperature range. The reason is that the solid solubility of Ag in NdOZnSb is below 0.04, agreed with the previous XRPD and EDX results. The Seebeck coefficient is negatively correlated to the electrical conductivity (Fig.3b). The positive signals indicate p-type electrical transport behaviors. For the pristine sample, the Seebeck coefficient decreases sharply upon 500 K, originating from the effect of bipolar diffusion. This parameter changes from 259 μV K-1 at 325 K to 174 μV K-1 at 725 K for NdOZnSb, and it decreases with increasing Ag doping fractions, down to 116 μV K-1 at 325 K and 126 μV K-1 at 725 K for NdOZn0.95Ag0.05Sb sample. The enhanced electrical conductivity and the positive temperature dependence of the Seebeck coefficient make the power factor increase at higher temperature, resulting in 5.8 μW cm-1 K-2 at 725 K for NdOZn0.96Ag0.04Sb sample, which grows threefold in comparison with pristine NdOZnSb (Fig. 3c). The electronic thermal conductivity is extracted by the Wiedemanne-Franz law: κE = LσT (L is the Lorenz number, Fig. 3d inset) [2]. Obviously, the electronic component of thermal conductivity increase with increasing Ag content, which is the main reason why doped samples have lager total thermal conductivity in Fig. 3e. The introduction of Ag lowers the lattice thermal conductivity κL at higher temperature as results of the scattering mechanism of point defect (Fig 3e insert). The combination of both two effects lead to the relative enhancement of total thermal conductivity for the doped samples. As is shown in Fig. 3f, the enlarged power factor gives rise to the increase of thermoelectric figure of merit and the maximum ZT reaches 0.44 for NdOZn0.96Ag0.04Sb at 725 K.
7
Fig. 3. Thermoelectric parameters as a function of temperature for NdOZn 1-xAgxSb (x = 0, 0.01, 0.02, 0.03, 0.04 and 0.05) samples. (a) Electrical conductivity σ, (b) Seebeck coefficient S, (c) Power factor PF, (d) Electronic thermal conductivity κE (inset: Lorenz constant), (e) Thermal conductivity κ (inset: lattice thermal conductivity κL) and (f) figure of merit ZT.
4.
Conclusion In summary, the thermoelectric properties of the Ag-doped NdOZnSb compounds have been
investigated systematically. Ag substitution Zn can increase the electrical conductivity and coresponding power factor due to the optimation of hole concentration. Thermoelectric performances with the maximum ZT value of 0.44 at 725 K is obtained for NdOZn0.96Ag0.04Sb, indicating that the Ag doping is an effective approach for improving the thermoelectric properties for this quartenary layered compounds. 8
Acknowledgement This work was supported by the National Natural Science Foundation of China under Project No. 21501118, the Shanghai Municipal Science and Technology Commission under Project No. 15DZ2260300. Kai Guo acknowledes the support by the Young Eastern Scholar Project of Shanghai Municipal Education Commission (QD2015031). References [1] D.M. Rowe, Thermoelectrics handbook : macro to nano, CRC/Taylor & Francis, Boca Raton, 2006. [2] G.J. Snyder, E.S. Toberer, Nat. Mater. 7 (2008) 105-114. [3] X. Shi, L.D. Chen, C. Uher, Int. Mater. Rev. 61 (2016) 379-415. [4] G.A. Slack, D.M. Rowe (Ed.), CRC Handbook of Thermoelectrics, CRC Press, Boca Raton,
FL 1995 [5] C. Barreteau, D. Bérardan, E. Amzallag, L.D. Zhao, N. Dragoe, Chem. Mater. 24 (2012) 3168-3178. [6] Y.Z. Pei, J. Lensch-Falk, E.S Toberer, D.L. Medlin, G.J. Snyder, Adv. Funct. Mater. 21 (2011) 241-249. [7] H. Zhang, M. Baitinger, M.B. Tang, Z.Y. Man, H.H. Chen, X.X. Yang, Y. Liu, L. Chen, Y. Grin, J.T. Zhao, Dalton Trans. 39 (2010) 1101-1104 [8] G.S. Nolas, M. Kaeser, R.T. Littleton, T.M. Tritt, Appl. Phys. Lett. 77 (2000) 1855. [9] Y.Z. Pei, L.L. Zheng, W. Li, S.Q. Lin, Z.W. Chen, Y.Y. Wang, X.F. Xu, H.L. Yu, Y. Chen, B.H. Ge, Adv. Electron. Mater. 2 (2016) 1600019. [10] Y. Liu, L.D. Zhao, Y.C. Liu, J.L. Lan, W. Xu, F. Li, B.P. Zhang, D. Berardan, Y.H. Lin, C.W. Nan, J.F. Li, H.M. Zhu, J. Am. Chem. Soc. 134 (2012) 3312. [11] K. Guo, Z.Y. Man, Q.G. Cao, H.H. Chen, X. Guo, J.T. Zhao, Chem. Phys. 380 (2011) 54-60. [12] K. Guo, Z.Y. Man, X.J.Wang, H.H. Chen, M.B. Tang, Z.J. Zhang, Y. Grin, J.T. Zhao, Dalton trans. 40 (2011) 10007-10013. [13] T. Suzuki, M.S. Bahramy, R. Arita, Y. Taguchi, Y. Tokura, Phys. Rev. B 83 (2011) 035204. [14] J. Liu, J. Wang, C.L.Wang, S.Q. Xia, J. Alloy. Compd. 688 (2016) 849-853. [15] M. Pan, K. Wang, W.Y. Lv, D.L. Hu, K. Guo, X.X. Yang, J.T. Zhao, J. Alloy. Compd. 688 (2016) 153-157. 9
Highlights
Ploycrystalline samples NdOZn1-xAgxSb were successfully synthesized by a two-step solid state reaction method.
The thermoelectric perfomance of NdOZn1-xAgxSb were firstly studied by measuring
electrical
conductivity,
Seebeck
coefficient
and
thermal
conductivity.
Ag doping increases the hole concentration, which leads to the enhancement of electrical conductivity and thermoelectric figure of merit ZT, indicating that Ag is an effective dopant for layered compounds NdOZnSb.
10
PII: DOI: Reference:
S0167-577X(16)31826-2 http://dx.doi.org/10.1016/j.matlet.2016.11.073 MLBLUE21767
To appear in: Materials Letters Received date: 15 October 2016 Accepted date: 21 November 2016 Cite this article as: Meng Pan, Kai Guo, Wanyu Lv, Jiye Zhang, Dongli Hu, Xinxin Yang, Jun Luo and Jing-Tai Zhao, Influence of Ag doping on the thermoelectric properties of layered compound NdOZnSb, Materials Letters, http://dx.doi.org/10.1016/j.matlet.2016.11.073 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.
Influence of Ag doping on the thermoelectric properties of layered compound NdOZnSb Meng Pana, Kai Guoa* , Wanyu Lva, Jiye Zhanga, Dongli Hua, Xinxin Yanga*, Jun Luo a a
, Jing-Tai Zhaoa,b
School of Materials Science and Engineering, Shanghai University, Shanghai 200444, China
b
State Key Laboratory of Advanced Special Steel, Shanghai University, Shanghai 200444, China
[email protected] [email protected] *
Corresponding author.
Abstract Quaternary layered compound NdOZnSb is viewed as a thermoelectric candidate owing to the stacked building blocks, e.g. conductive [ZnSb] layer and insulting [NdO] layer featuring narrow band-gap semiconductor behavior. However, the measured figure of merit ZT is quite low for pristine NdOZnSb resulted from poor electrical conductivity despite considerable Seebeck coefficient and low thermal conductivity. The present paper demonstrates that Ag doping can precisely tune the electrical transport properties and accordingly enhance the power factor. The electrical conductivity of NdOZnSb increases by an order of magnitude at 325 K via the substitution of Zn by Ag, and the maximal thermoelectric figure of merit ZT for NdOZn0.96Ag0.04Sb reaches 0.44 at 725 K.
1
Graphical abstract
Keywords: NdOZnSb; Semiconductors; Ceramics; Thermoelectric properties
1.
Introduction In order to alleviate the energy crisis, thermoelectric (TE) materials have received much
attention in the past decade, which are a key technology for converting waste heat into electricity. The conversion efficiency of TE materials is characterized by the dimensionless figure of merit ZT = S2σT/(κL+ κE), where S is the Seebeck coefficient, σ is the electrical conductivity, κL is the lattice thermal conductivity, κE is the electronic thermal conductivity, and T is the absolute temperature, respectively [1, 2]. These parameters determining the critical ZT depend on the electronic and 2
thermal transport process, while the close coupled behavior between electronic and thermal transport properties make the enhancement of ZT a challenge task [3]. In 1995, G. A. Slack proposed the concept of “phonon-glass electron-crystal” (PGEC), which contributed a new hint in exploring high-performance thermoelectric materials constructed by electron-transport and phonon-scattering building block [4]. The former guarantees good electronic transport properties while the latter is responsible for lowering thermal conductivity. This approach is the most achievable in materials with complex crystal structures, where p-type [5] or n-type doping ions [6], isoelectronic substitution atoms [7], filled elements [8], interstitials [9] or vacancies [10] can act as phonon scattering centers to reduce the lattice thermal conductivity. Layered compounds NdOZnSb has a tetragonal symmetry with the space group of P4/nmm, which consists of insulating [NdO] layers and conductive [ZnSb] layers alternatively stacked along the c axis [11]. The [NdO] layer acts as a charge reservoir, while the conductive [ZnSb] layer provides an ideal conduction pathway for carrier transport. In addition, it is reported that NdOZnSb is a narrow-band-gap semiconductor with considerably low thermal conductivity [12]. The above feathers of NdOZnSb well satisfy the concept of PGEC. However, the thermoelectric performance of the pristine NdOZnSb is seriously hindered by its poor electrical conductivity owing to the low carrier concentration. Fortunately, previous researches revealed that the electronic transport properties of isostructural compound REOZnSb could be well tuned and optimized by chemical modification in [REO] layers (RE = La, Ce, Nd) [13-15]. In this paper, we present the doping effects of Ag at Zn site in conductive layers of NdOZnSb and the further enhancement of thermoelectric properties has been obtained. 2.
Experimental Section 3
NdOZn1-xAgxSb (x = 0, 0.01, 0.02, 0.03, 0.04 and 0.05) were synthesized by a two-step solid state reaction method including pre-reaction and annealing process. The starting materials were Nd (purity 99.9%), Nd2O3 (purity 99.99%), Zn (purity 99.99%), Sb (purity 99.999%) and Ag (purity 99.999%), which were weighed according to the stoichiometric ratio in a glove box filled with argon (O2 < 1 ppm, H2O < 1 ppm). The chemicals were subsequently put in graphite crucibles and sealed in an evacuated silica tube. Then, all samples were heated up to 1273 K for 3 days with a heating rate of 2 K min–1. A second annealing process occurred at 1173 K for 3 days after grinding the resulting products and pressing them into pellets of Φ 10 mm. Finally, the as-obtained powders were densified by hot-pressing sintering at 923 K under a uniaxial pressure of 60 MPa for 30 min in a vacuum. The phase purity of NdOZn1-xAgxSb samples were examined by X-ray powder diffraction (XRPD, Rigaku, Japan, Cu Kα radiation, λ = 1.541854 Å, 10° < 2θ < 80°, step width 0.02°). The microstructures and chemical composition were characterized by field-emission scanning electron microscopy (SEM, SUPRA 55 SAPPHIRE, ZEISS, Germany) equipped with energy dispersive X-ray spectroscopy (EDS). The thermal conductivity was calculated from the equation κ = DCpd, where D, Cp and d is thermal diffusivity coefficient, specific heat and density, respectively. D was measured using the laser flash diffusivity apparatus (LFA 447, Netzsch, Germany), and Cp was estimated by the model of Dulongand Petit as Cp = 3NR/M, where N is the number of atoms per formula unit, R is universal gas constant, and M is molar mass. The density d was acquired by Archimedes method. Seebeck coefficient and electrical conductivity were measured by ZEM-3 (ULVAC, Japan) on rectangular bars. 3.
Results and discussion 4
The main Bragg diffraction peaks of as-obtained samples match well with the simulated pattern (ICSD #419354), indicating that the main phases adopt NdOZnSb structure and the doping would not alter the symmetry but lattice parameters (Fig. 1a). Small amount of Nd2O3 impurity phase are found in all samples except the pristine one. With Ag concent increase, the lattice paramer a remains essentially unchaged but c linely increases when x < 0.3 (Fig. 1b), probably attributed to the layered feature. Higher Ag concent would not lead to signifiant varieties in the lattice paramer c, ascribed to the solution limit of Ag.
Fig. 1. (a) The XRPD patterns of NdOZn1-xAgxSb (x = 0, 0.01, 0.02, 0.03, 0.04 and 0.05). Bottom: the simulated pattern for NdOZnSb. (b) Lattice parameters of NdOZn1-xAgxSb depending on the Ag concent.
The morphologies of polished surface and cross section for typitcal samples NdOZnSb and NdOZn0.96Ag0.04Sb are observed by SEM. As is shown in Fig.2a and c, both of the samples exhibit dense surface since few pores are indentified, consist with the measured relative density (89.1% 5
for NdOZnSb and 94.7% for NdOZn0.96Ag0.04Sb). Fig. 2b and d reveal obvious prefered orientation texture, related to the laryered structure feature. Interestingly, these layered microstructures distribute randomly in the samples due to the absence of some induced mechnism during densitified process, which has insignificant influence on the anisotropy of the electrical and thermal transport properties of bulk NdOZn1-xAgxSb samples. Ag doping can improve the crystallization of NdOZnSb, as evidenced by the density values and cross-section morphologies. EDS
results
demonstrate
lower
Ag
content
for
nominal
NdOZn0.96Ag0.04Sb
(Nd0.87(3)O0.89(4)Zn0.97(5)Ag0.03Sb0.86(2)), indicating a solution limit for Ag in NdOZnSb.
Fig. 2.SEM images of typical NdOZn1-xAgxSb samples (a) x = 0, polished surfacemorphology, (b) x = 0, fractograph, (c) x = 0.04, polished surfacemorphology, (d) x = 0.04, fractograph.
Fig.3 presents the TE behaviors of pure and Ag-doped NdOZnSb by assessing S, σ and κ from 325 K to 725 K. The electrical conductivity σ for all samples increases with the rise of the temperature, suggesting semiconductor behaviors (Fig. 3a). As expected, the substitution of Zn by 6
Ag can significantly enhance the electrical properties, as evidenced by an order of magnitude increase in comparison with undoped sample at 325 K. It is found that for x = 0.04 and 0.05, the values of the electrical conductivity are close over the entire temperature range. The reason is that the solid solubility of Ag in NdOZnSb is below 0.04, agreed with the previous XRPD and EDX results. The Seebeck coefficient is negatively correlated to the electrical conductivity (Fig.3b). The positive signals indicate p-type electrical transport behaviors. For the pristine sample, the Seebeck coefficient decreases sharply upon 500 K, originating from the effect of bipolar diffusion. This parameter changes from 259 μV K-1 at 325 K to 174 μV K-1 at 725 K for NdOZnSb, and it decreases with increasing Ag doping fractions, down to 116 μV K-1 at 325 K and 126 μV K-1 at 725 K for NdOZn0.95Ag0.05Sb sample. The enhanced electrical conductivity and the positive temperature dependence of the Seebeck coefficient make the power factor increase at higher temperature, resulting in 5.8 μW cm-1 K-2 at 725 K for NdOZn0.96Ag0.04Sb sample, which grows threefold in comparison with pristine NdOZnSb (Fig. 3c). The electronic thermal conductivity is extracted by the Wiedemanne-Franz law: κE = LσT (L is the Lorenz number, Fig. 3d inset) [2]. Obviously, the electronic component of thermal conductivity increase with increasing Ag content, which is the main reason why doped samples have lager total thermal conductivity in Fig. 3e. The introduction of Ag lowers the lattice thermal conductivity κL at higher temperature as results of the scattering mechanism of point defect (Fig 3e insert). The combination of both two effects lead to the relative enhancement of total thermal conductivity for the doped samples. As is shown in Fig. 3f, the enlarged power factor gives rise to the increase of thermoelectric figure of merit and the maximum ZT reaches 0.44 for NdOZn0.96Ag0.04Sb at 725 K.
7
Fig. 3. Thermoelectric parameters as a function of temperature for NdOZn 1-xAgxSb (x = 0, 0.01, 0.02, 0.03, 0.04 and 0.05) samples. (a) Electrical conductivity σ, (b) Seebeck coefficient S, (c) Power factor PF, (d) Electronic thermal conductivity κE (inset: Lorenz constant), (e) Thermal conductivity κ (inset: lattice thermal conductivity κL) and (f) figure of merit ZT.
4.
Conclusion In summary, the thermoelectric properties of the Ag-doped NdOZnSb compounds have been
investigated systematically. Ag substitution Zn can increase the electrical conductivity and coresponding power factor due to the optimation of hole concentration. Thermoelectric performances with the maximum ZT value of 0.44 at 725 K is obtained for NdOZn0.96Ag0.04Sb, indicating that the Ag doping is an effective approach for improving the thermoelectric properties for this quartenary layered compounds. 8
Acknowledgement This work was supported by the National Natural Science Foundation of China under Project No. 21501118, the Shanghai Municipal Science and Technology Commission under Project No. 15DZ2260300. Kai Guo acknowledes the support by the Young Eastern Scholar Project of Shanghai Municipal Education Commission (QD2015031). References [1] D.M. Rowe, Thermoelectrics handbook : macro to nano, CRC/Taylor & Francis, Boca Raton, 2006. [2] G.J. Snyder, E.S. Toberer, Nat. Mater. 7 (2008) 105-114. [3] X. Shi, L.D. Chen, C. Uher, Int. Mater. Rev. 61 (2016) 379-415. [4] G.A. Slack, D.M. Rowe (Ed.), CRC Handbook of Thermoelectrics, CRC Press, Boca Raton,
FL 1995 [5] C. Barreteau, D. Bérardan, E. Amzallag, L.D. Zhao, N. Dragoe, Chem. Mater. 24 (2012) 3168-3178. [6] Y.Z. Pei, J. Lensch-Falk, E.S Toberer, D.L. Medlin, G.J. Snyder, Adv. Funct. Mater. 21 (2011) 241-249. [7] H. Zhang, M. Baitinger, M.B. Tang, Z.Y. Man, H.H. Chen, X.X. Yang, Y. Liu, L. Chen, Y. Grin, J.T. Zhao, Dalton Trans. 39 (2010) 1101-1104 [8] G.S. Nolas, M. Kaeser, R.T. Littleton, T.M. Tritt, Appl. Phys. Lett. 77 (2000) 1855. [9] Y.Z. Pei, L.L. Zheng, W. Li, S.Q. Lin, Z.W. Chen, Y.Y. Wang, X.F. Xu, H.L. Yu, Y. Chen, B.H. Ge, Adv. Electron. Mater. 2 (2016) 1600019. [10] Y. Liu, L.D. Zhao, Y.C. Liu, J.L. Lan, W. Xu, F. Li, B.P. Zhang, D. Berardan, Y.H. Lin, C.W. Nan, J.F. Li, H.M. Zhu, J. Am. Chem. Soc. 134 (2012) 3312. [11] K. Guo, Z.Y. Man, Q.G. Cao, H.H. Chen, X. Guo, J.T. Zhao, Chem. Phys. 380 (2011) 54-60. [12] K. Guo, Z.Y. Man, X.J.Wang, H.H. Chen, M.B. Tang, Z.J. Zhang, Y. Grin, J.T. Zhao, Dalton trans. 40 (2011) 10007-10013. [13] T. Suzuki, M.S. Bahramy, R. Arita, Y. Taguchi, Y. Tokura, Phys. Rev. B 83 (2011) 035204. [14] J. Liu, J. Wang, C.L.Wang, S.Q. Xia, J. Alloy. Compd. 688 (2016) 849-853. [15] M. Pan, K. Wang, W.Y. Lv, D.L. Hu, K. Guo, X.X. Yang, J.T. Zhao, J. Alloy. Compd. 688 (2016) 153-157. 9
Highlights
Ploycrystalline samples NdOZn1-xAgxSb were successfully synthesized by a two-step solid state reaction method.
The thermoelectric perfomance of NdOZn1-xAgxSb were firstly studied by measuring
electrical
conductivity,
Seebeck
coefficient
and
thermal
conductivity.
Ag doping increases the hole concentration, which leads to the enhancement of electrical conductivity and thermoelectric figure of merit ZT, indicating that Ag is an effective dopant for layered compounds NdOZnSb.
10