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Improving thermoelectric performance of p-type Ag-doped Mg2Si0.4Sn0.6 prepared by unique melt spinning method
Accepted Manuscript Improving thermoelectric performance of p-type Ag-doped Mg2Si0.4Sn0.6 prepared by unique melt spinning method Xiaodan Tang, Yumeng Zhang, Yong Zheng, Kunling Peng, Tianyu Huang, Xu Lu, Guoyu Wang, Shuxia Wang, Xiaoyuan Zhou PII: DOI: Reference:
S1359-4311(16)30824-9 http://dx.doi.org/10.1016/j.applthermaleng.2016.05.146 ATE 8361
To appear in:
Applied Thermal Engineering
Received Date: Revised Date: Accepted Date:
28 December 2015 15 May 2016 24 May 2016
Please cite this article as: X. Tang, Y. Zhang, Y. Zheng, K. Peng, T. Huang, X. Lu, G. Wang, S. Wang, X. Zhou, Improving thermoelectric performance of p-type Ag-doped Mg2Si0.4Sn0.6 prepared by unique melt spinning method, Applied Thermal Engineering (2016), doi: http://dx.doi.org/10.1016/j.applthermaleng.2016.05.146
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 proof before it is published in its final 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.
Improving thermoelectric performance of p-type Ag-doped Mg2Si0.4Sn0.6 prepared by unique melt spinning method Xiaodan Tanga,b, Yumeng Zhanga, Yong Zhenga, Kunling Penga,b, Tianyu Huangb, Xu Lua, Guoyu Wangb, Shuxia Wanga*, Xiaoyuan Zhoua,* a
College of Physics, Chongqing University, Chongqing 401331, People’s Republic of China.
b
Chongqing Institute of Green and Intelligent Technology, Chinese Academy of Sciences, Chongqing 400714, People’s Republic of China.
*To whom correspondence should be addressed:
Abstract In our work, p-type Mg2-xAgxSi0.4Sn0.6 (x = 0.00, 0.01, 0.02, 0.05, 0.07) compounds with single-phase are successfully synthesized via a home-made melt spinning (MS) system followed by spark plasma sintering (SPS). This unique process not only largely shortens the time of sample preparation compared with the traditional methods, but also successfully prevents the presence of MgO impurity phase. All samples are single-phase solid solutions with an anti-fluorite structure as identified by XRD. The thermoelectric properties of Mg2-xAgxSi0.4Sn0.6 compounds were measured from room temperature to 773 K. The final results indicate that (1) Ag serves as effective holes donor; and (2) the electrical conductivity rises rapidly and the thermal conductivity decreases with increasing Ag content until x=0.05. As a result, a peak dimensionless figure of merit ZT value of 0.45 at 690 K is achieved when x=0.05. The enhanced thermoelectric performance coupled with the drastically reduced processing time will be of considerable significance to the commercial-scale production of Mg2X (X=Si, Ge, and Sn) thermoelectric material. Keywords: p-type Mg2Si0.4Sn0.6; Melt Spinning; Thermoelectric Properties.
1 Introduction In the last several decades, motivated by the pressure of the global energy crisis and environmental pollution, researchers are seeking new sources of energy that are sustainable and pollution-free. Among many solutions, thermoelectric (TE) materials have attracted much attentions as an alternative energy technology to improve the efficiency of the usage of fossil fuels and reduce the greenhouse gas emission. TE material can directly convert waste heat into useful electric power through solid state process, or inversely, electricity into temperature gradient
[1-4]
. The performance
of TE materials is usually evaluated by the dimensionless thermoelectric figure of merit ZT=σS2T/κ, where σ, S, T and κ are the electrical conductivity, the Seebeck coefficient, the absolute temperature and the total thermal conductivity, respectively
[5-7]
. In general, the term σS2, which is called the
power factor (PF), reflects the electrical property of the given material. The total thermal conductivity κ is composed of two components: the carrier contribution κe and the lattice part κL [8-9]. High thermoelectric performance can be achieved by means of enlarged Seebeck coefficient and electrical conductivity and minimized thermal conductivity
[10]
.
Mg2X (X=Si, Ge, and Sn) compounds, which have attracted considerable interest, are distinguished candidates for thermoelectric devices operating at medium temperature due to their outstanding characteristics, such as non-toxicity, earth-abundant and high-performance
[11-12]
.
Furthermore, the compounds consist of light elements metals, thus yielding low densities in the range of 2–3.6 g/cm3, which is of a particular advantage in the application of heat recovery in the automotive [13]
. In addition, researchers all over the world have made plentiful attempts to improve the figure of
merit of Mg2X (X=Si, Ge, and Sn) alloys. Khan et al.
[14]
synthesized n-type Bi- and Sb-doped
Mg2Si0.55 Sn0.4Ge0.05 compounds by the tradition solid state reaction method and achieved the ZT ~1.4
and ~1.2 for Bi and Sb members, respectively. Zhang et al.
[15]
reported that the highest ZT value ~
1.30 was obtained in n-type Mg2(Si0.4-xSbxSn0.6) (0≤x≤0.025) solid solutions at 773 K by using induction melting and SPS. However, the high thermoelectric performance was only found in n-type Mg2X (X=Si, Ge, and Sn) compounds. In contrast, the ZT values for p-type counterparts still remain far below unity. To produce high efficiency thermoelectric devices, the thermoelectric properties of p- and n-type compounds should be comparable. Thus, it is indeed significant to improve the thermoelectric performance of p-type Mg2X (X=Si, Ge, and Sn)-compounds via optimizing the synthesis procedure. Previous theoretical work indicated that Ag was an effective p-type dopant in Mg2X (X=Si, Ge, and Sn) system [16-17]. Several methods have been applied to investigate p-type Ag-doped Mg2Si1-xSnx solid solution. Bridgman method was employed by Chen et al. [18] to synthesize Ag-doped Mg2Sn. Isoda et al. [19] performed liquid-solid reaction and subsequent hot-pressing methods to synthesize Ag/Li double-doping p-type Mg2Si0.25Sn0.75. Unfortunately, the existing methods could not obtain great enhancement in thermoelectric performance in p-type Mg2X (X=Si, Ge, and Sn) compounds and the maximum ZT value is around 0.38
[20]
. In this work, we use MS associated with
SPS method to prepare Mg2-xAgxSi0.4 Sn0.6 (x = 0.00, 0.01, 0.02, 0.05, 0.07) compounds. The advantage of our experimental setup over existing methods is that the MS system is assembled inside the glove box, which can effectively avoid the potential exposure to oxygen. With this unique setup, we successfully obtain the pure Mg2-xAgxSi0.4Sn0.6 compounds without the presence of any second phase of MgO, which otherwise results in the high thermal conductivity caused by oxidation of Mg2Si as reported in other synthesis methods [21]. A figure of merit ZT up to ~0.45 is achieved at 690 K when x is equal 0.05, an improvement by 18% compared to the reported ZT of the Ag-doped p-type Mg2X (X=Si, Ge, and Sn) compounds [20].
2 Experiments High-purity bulk of Mg (99.5%), Sn (99.9%), Ag (99.9%), and powder of Si (99.99%) were weighed according to the stoichiometry of Mg2-xAgxSi0.4 Sn0.6 (x = 0.00, 0.01, 0.02, 0.05, 0.07). 5at.% excess of Mg was added to compensate its volatilization lost during melt spinning process. The raw materials were mixed in a graphite tube and molten under 1133-1173 K for 10 min. Then, the liquid was injected under a pressure of 0.07 MPa Ar gas onto the edge of a chilled copper roller rotating with linear speed of 25m/s, and rapidly solidified as thin ribbons. The obtained ribbons were subsequently collected and grinded into fine powders in an agate mortar. All the above operations were done inside a glove box to prevent any oxygen contamination. Afterwards, the fine powders were compacted by using Spark Plasma Sintering at 873 K for 5 min under the pressure of 40 MPa. The final products are cylindrical ingots with density more than 95% of theoretical density. Subsequently, the obtained cylinders were cut into appropriate shapes for the measurements of electrical and thermal transport properties. Crystal structure characterization was conducted using a PANalytical X’pert Pro type apparatus with Cu Kα radiation. The microstructure of the samples was investigated using a field emission scanning electron microscopy (JSM-7800F, JEOL). The room temperature carrier concentration n of all samples were measured by a home-made Hall apparatus under a magnetic field of 1 T and the carrier mobility μ were calculated according to the equation σ=nqμ. The electrical conductivity and the Seebeck coefficient of the samples were measured simultaneously using a commercial system (LSR-3, Linseis) under the protective atmosphere of Helium. The thermal diffusivity λ and heat capacity Cp were measured by the laser flash method (Netzsch, LFA 457) and scanning calorimeter (Netzsch, 404 F3), respectively. The densities d of samples were calculated from the sample dimensions and mass. Then, the thermal conductivity κ was calculated by the equation of κ= λρCP.
Both electrical and thermal transport properties measurements were carried out from room temperature to 773 K.
3 Results and Discussion 3.1 Crystal structure Figure 1 shows the XRD patterns of the obtained Mg2-xAgxSi0.4Sn0.6 (x = 0.00, 0.01, 0.02, 0.05, 0.07) bulk samples after SPS. The results indicate that, the obtained bulk materials possess the same anti-fluorite structure (space group, Fm3m) and all diffraction peaks can be indexed to the Mg2Si0.4 Sn0.6 solid solution. In addition, all the peaks have no presence of any impurity phases. It is concluded that single phase Mg2-xAgxSi0.4Sn0.6 compounds have been successfully synthesized via our homemade melt spinning system followed by spark plasma sintering. 3.2 Morphology and microstructure Figures 2 shows the SEM photographs of ribbon sample and fractured surfaces of bulk sample after SPS. With the MS technique, two sides on the ribbons are different in terms of its microstructure: the contact face (shown in Figure 2(a)) which contacts with the edge of the copper wheel and the free face (shown in Figure 2(b)). As one can see, contact face is much smoother than free face. Such observed significant difference between the contact surface and the free surface is similar to what described in literature
[22-23]
, resulting from the ultrahigh but different cooling rates when the melted compounds
contact with copper wheel rotating at a high speed during the MS process. As shown in Figure 2(c), these samples are closely packed, consistent with the high density of our bulk samples using our synthesis route and the grain size of the sample is almost in the micron-scale range. In addition, there are much smaller particles with the size ranging from nano- to submicron scale in the bulk matrix, which is believed to be able to lower the thermal conductivity by scattering the heat-carrying phonons covering the broad phonon free paths [24].
3.3 Electrical transport properties Table 1 presents some physical parameters of Mg2-xAgxSi0.4 Sn0.6 at room temperature. Among them, the reduced Fermi level η and the carrier effective mass m* are obtained from Eqs(1)–(4) by employing a single parabolic band (SPB) model under the relaxation time approximation and the electrical transport dominated by acoustic phonon scattering (r=-1/2) [25-26].
(1)
(2) (3)
(4) Where η, Fi (ηF), r, and κB are the reduced Fermi energy, the Fermi integral, the scattering factor, and the Boltzmann constant, respectively. The introduction of Ag shifts the Seebeck coefficients of Mg2Si0.4 Sn0.6 to positive value and the measured Hall coefficients confirm the p-type conduction behavior, which implies that holes are the principal conduction carriers after doping. The carrier concentration increases with the increasing amount of Ag except for x=0.07 sample, which, we believe, is likely over the solubility limit of Ag in Mg2Si0.4Sn0.6 compounds. As seen in Table 1, it is obvious that carrier concentration n makes more contribution than carrier mobility μ to the electrical conductivity σ according to the equation of σ=nqμ. Figure 3(a)-(c) show the temperature dependences of electrical conductivity σ, Seebeck coefficient S and power factor PF for Mg2-xAgxSi0.4Sn0.6 (x = 0.00, 0.01, 0.02, 0.05, 0.07) between 300 and 773 K. In Figure 3(a), the undoped specimen (x=0.00) shows typical intrinsic semiconductor behavior as σ monotonically increases with increasing temperature. While the electrical conductivity σ of doped samples decreases from room temperature to 500 K and then increases afterwards, showing a typical
degenerate semiconductor behavior. In addition, electrical conductivity increases monotonously as the Ag content increases till up to x=0.05. The observed enhancement of electrical conductivity in Ag-doped samples may be caused by the increase in carrier concentration upon Ag doping as shown in Table 1. At lower doping level, there is little change in conductivity, as expected from the measured carrier concentration. However, larger amounts of doping (x=0.05) resulted in more than six times increase in σ. In Figure 3(b), all the doped samples shows the positive Seebeck coefficient in the temperature range studied, which indicates that the p-type conduction is established and Ag is an effective holes dopant for Mg2(Si,Sn) solid solutions. It is noted that Seebeck coefficient first increases then decreases with increasing temperature, opposite to the observed trend in electrical conductivity, governed by the change in carrier concentration. Furthermore, the absolute value of Seebeck coefficient decreases with the increase of Ag-doping content at the same temperature before it reaches the solid limit. In Figure 3(c), the PF of all the doped samples are much higher than that of the undoped specimen (x=0.00). The x=0.05 sample possesses the highest electrical conductivity compared to other samples, which leading to the highest power factor of 1.03 mW K-2m-1 at 690 K. 3.4 Thermal transport properties Figure 4(a)-(c) show the temperature dependence of total thermal conductivity κ, the carrier thermal conductivity κe and the term of (κ-κe) for Mg2-xAgxSi0.4Sn0.6 (x = 0.00, 0.01, 0.02, 0.05, 0.07) between 300 and 773 K. Among them, κe=LσT, a constant value of 2.0 ×10-8V2 K-2 is used. No matter what value is employed for the Lorenz constant, it does not affect the total thermal conductivity, final ZT, and the conclusions of this work
[27]
. The term of (κ-κe) is usually called “lattice thermal conductivity”, which
is represented by the letter κL. In Figure 4(a), the total thermal conductivity of all the samples decreases from room temperature to 530 K and then increases at higher temperature, possibly due to the onset of
bipolar conduction. Besides, for all the Ag-doped samples, the thermal conductivity is lower than that of undoped sample, which results from the strong alloy scattering for phonons. The increased total thermal conductivity of doped samples (expect for x= 0.07) can be explained by the monotonously enhanced electrical conductivity before the presence of the intrinsic behavior. As shown in Figure 4(b), the carrier thermal conductivity κe of all the samples are negligibly small and do little contribution to the total thermal conductivity. According to the formula of κe= LσT, sample with x=0.05 possesses relatively large κe value due to the really high electrical conductivity σ. The Figure 4(c) shows that the term of (κ-κe) is a good approximation of the lattice thermal conductivity near room temperature where the bipolar contribution only plays a minor role. In addition, the (κ-κe) term follows T-1 relationship due to the Umklapp process [28-29] from room temperature to middle temperature range, then increases with increasing temperature since the onset of intrinsic excitations (bipolar effect) sets in. 3.5 Dimensionless thermoelectric figure of merit ZT Figure 5 presents the dimensionless thermoelectric figure of merit ZT for Mg 2-xAgxSi0.4 Sn0.6 (x = 0.00, 0.01, 0.02, 0.05, 0.07) samples as a function of temperature, showing ZT values monotonously increase as the content of Ag increases. The maximum ZT value of ~0.45 is obtained at 690 K for x=0.05, an 18% enhancement compared to the reported value in literature
[20]
. This method holds great promise for
commercial-scale production of p-type Mg2Si0.4 Sn0.6 based compounds. By optimizing solid solution composition, doping and generating nanocomposites, this technique should yield record-high performance Mg2Si0.4 Sn0.6 based thermoelectric materials with considerable savings in time and energy.
4 Conclusions In this study, we successfully synthesized the p-type Mg2-xAgxSi0.4Sn0.6 (x = 0.00, 0.01, 0.02, 0.05, 0.07) compounds via a home-made melt spinning system coupled with spark plasma sintering. This unique melt spinning system have two advantages: (1) shortening the time of sample preparation from several
days to less than 2 hours; and (2) complete prevention the formation of MgO impurity phase. The transport measurement results indicate that the introduction of Ag enlarge the hole concentration and thus enhances the electrical conductivity from 4.5×103 S/m to 3.2×104 S/m at room temperature. As a result, a peak dimensionless figure of merit ZT value of 0.45 at 690 K is achieved when x=0.05. The drastically reduced processing time coupled with the enhanced thermoelectric performance will be well for the commercial applications of this promising p-type Mg2Si0.4Sn0.6 thermoelectric material.
Acknowledgement The work was financially supported in part by the National Natural Science Foundation of China (Grant no. 11404044, 51472036, 51401202), the Fundamental Research Funds for the Central Universities (CQDXWL-2013-Z010). This work at the Chongqing Institute of Green and Intelligent Technology, Chinese Academy of Sciences is supported by the One Hundred Person Project of the Chinese Academy of Science, Grant No. 2013-46.
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Figure 1. XRD patterns of Mg2-xAgxSi0.4Sn0.6 compounds (x = 0.00, 0.01, 0.02, 0.05, 0.07).
Figure 2. Typical microstructure of (a) contact face; (b) free face of ribbon; (c) fractured surfaces of bulk and the inset is partial magnification.
Figure 3.Temperature dependent electrical transport properties of Mg 2-xAgxSi0.4Sn0.6 (x = 0.00, 0.01, 0.02, 0.05, 0.07) compounds. (a) Electrical conductivity, (b) Seebeck coefficient, (c) Power factor.
Figure 4.Temperature dependent thermal transport properties of Mg 2-xAgxSi0.4Sn0.6 (x = 0.00, 0.01, 0.02, 0.05, 0.07) compounds. (a) total thermal conductivity κ, (b) carrier thermal conductivity κe, (c) the term of (κ-κe).
Figure 5. Temperature dependent ZT values for Mg2-xAgxSi0.4Sn0.6 (x = 0.00, 0.01, 0.02, 0.05, 0.07) compounds.
Table 1. Electrical transport parameters of Mg2-xAgxSi0.4Sn0.6 bulks at room temperature.
Nominal composition
Seebeck coefficient α/μV K
-1
Carrier concentration 19
-3
nH/×10 cm
Carrier mobility 2 -1 -1
Reduced Fermi level
μ/cm V s
η= EF/kBT
-0.83
Effective mass
m*/m0
x=0.00
-260.3
-1.08
29.2
-0.60
x=0.01
205.5
2.12
31.3
-0.01
0.75
x=0.02
190.1
2.27
36.4
0.24
0.70
x=0.05
140.7
6.23
31.8
1.23
1.05
x=0.07
185.3
1.91
37.9
0.33
0.63
S1359-4311(16)30824-9 http://dx.doi.org/10.1016/j.applthermaleng.2016.05.146 ATE 8361
To appear in:
Applied Thermal Engineering
Received Date: Revised Date: Accepted Date:
28 December 2015 15 May 2016 24 May 2016
Please cite this article as: X. Tang, Y. Zhang, Y. Zheng, K. Peng, T. Huang, X. Lu, G. Wang, S. Wang, X. Zhou, Improving thermoelectric performance of p-type Ag-doped Mg2Si0.4Sn0.6 prepared by unique melt spinning method, Applied Thermal Engineering (2016), doi: http://dx.doi.org/10.1016/j.applthermaleng.2016.05.146
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 proof before it is published in its final 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.
Improving thermoelectric performance of p-type Ag-doped Mg2Si0.4Sn0.6 prepared by unique melt spinning method Xiaodan Tanga,b, Yumeng Zhanga, Yong Zhenga, Kunling Penga,b, Tianyu Huangb, Xu Lua, Guoyu Wangb, Shuxia Wanga*, Xiaoyuan Zhoua,* a
College of Physics, Chongqing University, Chongqing 401331, People’s Republic of China.
b
Chongqing Institute of Green and Intelligent Technology, Chinese Academy of Sciences, Chongqing 400714, People’s Republic of China.
*To whom correspondence should be addressed:
Abstract In our work, p-type Mg2-xAgxSi0.4Sn0.6 (x = 0.00, 0.01, 0.02, 0.05, 0.07) compounds with single-phase are successfully synthesized via a home-made melt spinning (MS) system followed by spark plasma sintering (SPS). This unique process not only largely shortens the time of sample preparation compared with the traditional methods, but also successfully prevents the presence of MgO impurity phase. All samples are single-phase solid solutions with an anti-fluorite structure as identified by XRD. The thermoelectric properties of Mg2-xAgxSi0.4Sn0.6 compounds were measured from room temperature to 773 K. The final results indicate that (1) Ag serves as effective holes donor; and (2) the electrical conductivity rises rapidly and the thermal conductivity decreases with increasing Ag content until x=0.05. As a result, a peak dimensionless figure of merit ZT value of 0.45 at 690 K is achieved when x=0.05. The enhanced thermoelectric performance coupled with the drastically reduced processing time will be of considerable significance to the commercial-scale production of Mg2X (X=Si, Ge, and Sn) thermoelectric material. Keywords: p-type Mg2Si0.4Sn0.6; Melt Spinning; Thermoelectric Properties.
1 Introduction In the last several decades, motivated by the pressure of the global energy crisis and environmental pollution, researchers are seeking new sources of energy that are sustainable and pollution-free. Among many solutions, thermoelectric (TE) materials have attracted much attentions as an alternative energy technology to improve the efficiency of the usage of fossil fuels and reduce the greenhouse gas emission. TE material can directly convert waste heat into useful electric power through solid state process, or inversely, electricity into temperature gradient
[1-4]
. The performance
of TE materials is usually evaluated by the dimensionless thermoelectric figure of merit ZT=σS2T/κ, where σ, S, T and κ are the electrical conductivity, the Seebeck coefficient, the absolute temperature and the total thermal conductivity, respectively
[5-7]
. In general, the term σS2, which is called the
power factor (PF), reflects the electrical property of the given material. The total thermal conductivity κ is composed of two components: the carrier contribution κe and the lattice part κL [8-9]. High thermoelectric performance can be achieved by means of enlarged Seebeck coefficient and electrical conductivity and minimized thermal conductivity
[10]
.
Mg2X (X=Si, Ge, and Sn) compounds, which have attracted considerable interest, are distinguished candidates for thermoelectric devices operating at medium temperature due to their outstanding characteristics, such as non-toxicity, earth-abundant and high-performance
[11-12]
.
Furthermore, the compounds consist of light elements metals, thus yielding low densities in the range of 2–3.6 g/cm3, which is of a particular advantage in the application of heat recovery in the automotive [13]
. In addition, researchers all over the world have made plentiful attempts to improve the figure of
merit of Mg2X (X=Si, Ge, and Sn) alloys. Khan et al.
[14]
synthesized n-type Bi- and Sb-doped
Mg2Si0.55 Sn0.4Ge0.05 compounds by the tradition solid state reaction method and achieved the ZT ~1.4
and ~1.2 for Bi and Sb members, respectively. Zhang et al.
[15]
reported that the highest ZT value ~
1.30 was obtained in n-type Mg2(Si0.4-xSbxSn0.6) (0≤x≤0.025) solid solutions at 773 K by using induction melting and SPS. However, the high thermoelectric performance was only found in n-type Mg2X (X=Si, Ge, and Sn) compounds. In contrast, the ZT values for p-type counterparts still remain far below unity. To produce high efficiency thermoelectric devices, the thermoelectric properties of p- and n-type compounds should be comparable. Thus, it is indeed significant to improve the thermoelectric performance of p-type Mg2X (X=Si, Ge, and Sn)-compounds via optimizing the synthesis procedure. Previous theoretical work indicated that Ag was an effective p-type dopant in Mg2X (X=Si, Ge, and Sn) system [16-17]. Several methods have been applied to investigate p-type Ag-doped Mg2Si1-xSnx solid solution. Bridgman method was employed by Chen et al. [18] to synthesize Ag-doped Mg2Sn. Isoda et al. [19] performed liquid-solid reaction and subsequent hot-pressing methods to synthesize Ag/Li double-doping p-type Mg2Si0.25Sn0.75. Unfortunately, the existing methods could not obtain great enhancement in thermoelectric performance in p-type Mg2X (X=Si, Ge, and Sn) compounds and the maximum ZT value is around 0.38
[20]
. In this work, we use MS associated with
SPS method to prepare Mg2-xAgxSi0.4 Sn0.6 (x = 0.00, 0.01, 0.02, 0.05, 0.07) compounds. The advantage of our experimental setup over existing methods is that the MS system is assembled inside the glove box, which can effectively avoid the potential exposure to oxygen. With this unique setup, we successfully obtain the pure Mg2-xAgxSi0.4Sn0.6 compounds without the presence of any second phase of MgO, which otherwise results in the high thermal conductivity caused by oxidation of Mg2Si as reported in other synthesis methods [21]. A figure of merit ZT up to ~0.45 is achieved at 690 K when x is equal 0.05, an improvement by 18% compared to the reported ZT of the Ag-doped p-type Mg2X (X=Si, Ge, and Sn) compounds [20].
2 Experiments High-purity bulk of Mg (99.5%), Sn (99.9%), Ag (99.9%), and powder of Si (99.99%) were weighed according to the stoichiometry of Mg2-xAgxSi0.4 Sn0.6 (x = 0.00, 0.01, 0.02, 0.05, 0.07). 5at.% excess of Mg was added to compensate its volatilization lost during melt spinning process. The raw materials were mixed in a graphite tube and molten under 1133-1173 K for 10 min. Then, the liquid was injected under a pressure of 0.07 MPa Ar gas onto the edge of a chilled copper roller rotating with linear speed of 25m/s, and rapidly solidified as thin ribbons. The obtained ribbons were subsequently collected and grinded into fine powders in an agate mortar. All the above operations were done inside a glove box to prevent any oxygen contamination. Afterwards, the fine powders were compacted by using Spark Plasma Sintering at 873 K for 5 min under the pressure of 40 MPa. The final products are cylindrical ingots with density more than 95% of theoretical density. Subsequently, the obtained cylinders were cut into appropriate shapes for the measurements of electrical and thermal transport properties. Crystal structure characterization was conducted using a PANalytical X’pert Pro type apparatus with Cu Kα radiation. The microstructure of the samples was investigated using a field emission scanning electron microscopy (JSM-7800F, JEOL). The room temperature carrier concentration n of all samples were measured by a home-made Hall apparatus under a magnetic field of 1 T and the carrier mobility μ were calculated according to the equation σ=nqμ. The electrical conductivity and the Seebeck coefficient of the samples were measured simultaneously using a commercial system (LSR-3, Linseis) under the protective atmosphere of Helium. The thermal diffusivity λ and heat capacity Cp were measured by the laser flash method (Netzsch, LFA 457) and scanning calorimeter (Netzsch, 404 F3), respectively. The densities d of samples were calculated from the sample dimensions and mass. Then, the thermal conductivity κ was calculated by the equation of κ= λρCP.
Both electrical and thermal transport properties measurements were carried out from room temperature to 773 K.
3 Results and Discussion 3.1 Crystal structure Figure 1 shows the XRD patterns of the obtained Mg2-xAgxSi0.4Sn0.6 (x = 0.00, 0.01, 0.02, 0.05, 0.07) bulk samples after SPS. The results indicate that, the obtained bulk materials possess the same anti-fluorite structure (space group, Fm3m) and all diffraction peaks can be indexed to the Mg2Si0.4 Sn0.6 solid solution. In addition, all the peaks have no presence of any impurity phases. It is concluded that single phase Mg2-xAgxSi0.4Sn0.6 compounds have been successfully synthesized via our homemade melt spinning system followed by spark plasma sintering. 3.2 Morphology and microstructure Figures 2 shows the SEM photographs of ribbon sample and fractured surfaces of bulk sample after SPS. With the MS technique, two sides on the ribbons are different in terms of its microstructure: the contact face (shown in Figure 2(a)) which contacts with the edge of the copper wheel and the free face (shown in Figure 2(b)). As one can see, contact face is much smoother than free face. Such observed significant difference between the contact surface and the free surface is similar to what described in literature
[22-23]
, resulting from the ultrahigh but different cooling rates when the melted compounds
contact with copper wheel rotating at a high speed during the MS process. As shown in Figure 2(c), these samples are closely packed, consistent with the high density of our bulk samples using our synthesis route and the grain size of the sample is almost in the micron-scale range. In addition, there are much smaller particles with the size ranging from nano- to submicron scale in the bulk matrix, which is believed to be able to lower the thermal conductivity by scattering the heat-carrying phonons covering the broad phonon free paths [24].
3.3 Electrical transport properties Table 1 presents some physical parameters of Mg2-xAgxSi0.4 Sn0.6 at room temperature. Among them, the reduced Fermi level η and the carrier effective mass m* are obtained from Eqs(1)–(4) by employing a single parabolic band (SPB) model under the relaxation time approximation and the electrical transport dominated by acoustic phonon scattering (r=-1/2) [25-26].
(1)
(2) (3)
(4) Where η, Fi (ηF), r, and κB are the reduced Fermi energy, the Fermi integral, the scattering factor, and the Boltzmann constant, respectively. The introduction of Ag shifts the Seebeck coefficients of Mg2Si0.4 Sn0.6 to positive value and the measured Hall coefficients confirm the p-type conduction behavior, which implies that holes are the principal conduction carriers after doping. The carrier concentration increases with the increasing amount of Ag except for x=0.07 sample, which, we believe, is likely over the solubility limit of Ag in Mg2Si0.4Sn0.6 compounds. As seen in Table 1, it is obvious that carrier concentration n makes more contribution than carrier mobility μ to the electrical conductivity σ according to the equation of σ=nqμ. Figure 3(a)-(c) show the temperature dependences of electrical conductivity σ, Seebeck coefficient S and power factor PF for Mg2-xAgxSi0.4Sn0.6 (x = 0.00, 0.01, 0.02, 0.05, 0.07) between 300 and 773 K. In Figure 3(a), the undoped specimen (x=0.00) shows typical intrinsic semiconductor behavior as σ monotonically increases with increasing temperature. While the electrical conductivity σ of doped samples decreases from room temperature to 500 K and then increases afterwards, showing a typical
degenerate semiconductor behavior. In addition, electrical conductivity increases monotonously as the Ag content increases till up to x=0.05. The observed enhancement of electrical conductivity in Ag-doped samples may be caused by the increase in carrier concentration upon Ag doping as shown in Table 1. At lower doping level, there is little change in conductivity, as expected from the measured carrier concentration. However, larger amounts of doping (x=0.05) resulted in more than six times increase in σ. In Figure 3(b), all the doped samples shows the positive Seebeck coefficient in the temperature range studied, which indicates that the p-type conduction is established and Ag is an effective holes dopant for Mg2(Si,Sn) solid solutions. It is noted that Seebeck coefficient first increases then decreases with increasing temperature, opposite to the observed trend in electrical conductivity, governed by the change in carrier concentration. Furthermore, the absolute value of Seebeck coefficient decreases with the increase of Ag-doping content at the same temperature before it reaches the solid limit. In Figure 3(c), the PF of all the doped samples are much higher than that of the undoped specimen (x=0.00). The x=0.05 sample possesses the highest electrical conductivity compared to other samples, which leading to the highest power factor of 1.03 mW K-2m-1 at 690 K. 3.4 Thermal transport properties Figure 4(a)-(c) show the temperature dependence of total thermal conductivity κ, the carrier thermal conductivity κe and the term of (κ-κe) for Mg2-xAgxSi0.4Sn0.6 (x = 0.00, 0.01, 0.02, 0.05, 0.07) between 300 and 773 K. Among them, κe=LσT, a constant value of 2.0 ×10-8V2 K-2 is used. No matter what value is employed for the Lorenz constant, it does not affect the total thermal conductivity, final ZT, and the conclusions of this work
[27]
. The term of (κ-κe) is usually called “lattice thermal conductivity”, which
is represented by the letter κL. In Figure 4(a), the total thermal conductivity of all the samples decreases from room temperature to 530 K and then increases at higher temperature, possibly due to the onset of
bipolar conduction. Besides, for all the Ag-doped samples, the thermal conductivity is lower than that of undoped sample, which results from the strong alloy scattering for phonons. The increased total thermal conductivity of doped samples (expect for x= 0.07) can be explained by the monotonously enhanced electrical conductivity before the presence of the intrinsic behavior. As shown in Figure 4(b), the carrier thermal conductivity κe of all the samples are negligibly small and do little contribution to the total thermal conductivity. According to the formula of κe= LσT, sample with x=0.05 possesses relatively large κe value due to the really high electrical conductivity σ. The Figure 4(c) shows that the term of (κ-κe) is a good approximation of the lattice thermal conductivity near room temperature where the bipolar contribution only plays a minor role. In addition, the (κ-κe) term follows T-1 relationship due to the Umklapp process [28-29] from room temperature to middle temperature range, then increases with increasing temperature since the onset of intrinsic excitations (bipolar effect) sets in. 3.5 Dimensionless thermoelectric figure of merit ZT Figure 5 presents the dimensionless thermoelectric figure of merit ZT for Mg 2-xAgxSi0.4 Sn0.6 (x = 0.00, 0.01, 0.02, 0.05, 0.07) samples as a function of temperature, showing ZT values monotonously increase as the content of Ag increases. The maximum ZT value of ~0.45 is obtained at 690 K for x=0.05, an 18% enhancement compared to the reported value in literature
[20]
. This method holds great promise for
commercial-scale production of p-type Mg2Si0.4 Sn0.6 based compounds. By optimizing solid solution composition, doping and generating nanocomposites, this technique should yield record-high performance Mg2Si0.4 Sn0.6 based thermoelectric materials with considerable savings in time and energy.
4 Conclusions In this study, we successfully synthesized the p-type Mg2-xAgxSi0.4Sn0.6 (x = 0.00, 0.01, 0.02, 0.05, 0.07) compounds via a home-made melt spinning system coupled with spark plasma sintering. This unique melt spinning system have two advantages: (1) shortening the time of sample preparation from several
days to less than 2 hours; and (2) complete prevention the formation of MgO impurity phase. The transport measurement results indicate that the introduction of Ag enlarge the hole concentration and thus enhances the electrical conductivity from 4.5×103 S/m to 3.2×104 S/m at room temperature. As a result, a peak dimensionless figure of merit ZT value of 0.45 at 690 K is achieved when x=0.05. The drastically reduced processing time coupled with the enhanced thermoelectric performance will be well for the commercial applications of this promising p-type Mg2Si0.4Sn0.6 thermoelectric material.
Acknowledgement The work was financially supported in part by the National Natural Science Foundation of China (Grant no. 11404044, 51472036, 51401202), the Fundamental Research Funds for the Central Universities (CQDXWL-2013-Z010). This work at the Chongqing Institute of Green and Intelligent Technology, Chinese Academy of Sciences is supported by the One Hundred Person Project of the Chinese Academy of Science, Grant No. 2013-46.
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Figure 1. XRD patterns of Mg2-xAgxSi0.4Sn0.6 compounds (x = 0.00, 0.01, 0.02, 0.05, 0.07).
Figure 2. Typical microstructure of (a) contact face; (b) free face of ribbon; (c) fractured surfaces of bulk and the inset is partial magnification.
Figure 3.Temperature dependent electrical transport properties of Mg 2-xAgxSi0.4Sn0.6 (x = 0.00, 0.01, 0.02, 0.05, 0.07) compounds. (a) Electrical conductivity, (b) Seebeck coefficient, (c) Power factor.
Figure 4.Temperature dependent thermal transport properties of Mg 2-xAgxSi0.4Sn0.6 (x = 0.00, 0.01, 0.02, 0.05, 0.07) compounds. (a) total thermal conductivity κ, (b) carrier thermal conductivity κe, (c) the term of (κ-κe).
Figure 5. Temperature dependent ZT values for Mg2-xAgxSi0.4Sn0.6 (x = 0.00, 0.01, 0.02, 0.05, 0.07) compounds.
Table 1. Electrical transport parameters of Mg2-xAgxSi0.4Sn0.6 bulks at room temperature.
Nominal composition
Seebeck coefficient α/μV K
-1
Carrier concentration 19
-3
nH/×10 cm
Carrier mobility 2 -1 -1
Reduced Fermi level
μ/cm V s
η= EF/kBT
-0.83
Effective mass
m*/m0
x=0.00
-260.3
-1.08
29.2
-0.60
x=0.01
205.5
2.12
31.3
-0.01
0.75
x=0.02
190.1
2.27
36.4
0.24
0.70
x=0.05
140.7
6.23
31.8
1.23
1.05
x=0.07
185.3
1.91
37.9
0.33
0.63