Phase Segregation and Superior Thermoelectric Properties of Mg2Si1

Jan 19, 2016 - Department of Physics, University of Michigan, Ann Arbor, Michigan ... SHS method can serve as an effective alternative fabrication rou...
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Phase Segregation and Superior Thermoelectric Properties of Mg2Si1−xSbx (0 ≤ x ≤ 0.025) Prepared by Ultrafast Self-Propagating High-Temperature Synthesis Qiang Zhang,†,‡ Xianli Su,*,† Yonggao Yan,† Hongyao Xie,† Tao Liang,† Yonghui You,† Xinfeng Tang,*,† and Ctirad Uher§ †

State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan 430070, China ‡ Key Laboratory of Interface Science and Engineering in Advanced Materials, Ministry of Education, Taiyuan University of Technology, Taiyuan 030024, China § Department of Physics, University of Michigan, Ann Arbor, Michigan 48109, United States ABSTRACT: A series of Sb-doped Mg2Si1−xSbx compounds with the Sb content x within 0 ≤ x ≤ 0.025 were prepared by self-propagating high-temperature synthesis (SHS) combined with plasma activated sintering (PAS) method in less than 20 min. Thermodynamic parameters of the SHS process, such as adiabatic temperature, ignition temperature, combustion temperature, and propagation speed of the combustion wave, were determined for the first time. Nanoprecipitates were observed for the samples doped with Sb. Thermoelectric properties were characterized in the temperature range of 300−875 K. With the increasing content of Sb, the electrical conductivity σ rises markedly while the Seebeck coefficient α decreases, which is attributed to the increase in carrier concentration. The carrier mobility μH decreases slightly with the increasing carrier concentration but remains larger than the Sb-doped samples prepared by other methods, which is ascribed to the self-purification process associated with the SHS synthesis. In spite of the increasing electrical conductivity with the increasing Sb content x, the overall thermal conductivity κ decreases on account of a significantly falled lattice thermal conductivity κL due to the strong point defect scattering on Sb impurities and possibly enhanced interface scattering on nanoprecipitates. As a result, the sample with x = 0.02 achieves the thermoelectric figure of merit ZT ∼ 0.65 at 873 K, one of the highest values for the Sb-doped binary Mg2Si compounds investigated so far. A subsequent annealing treatment on the sample with x = 0.02 at 773 K for 7 days has resulted in no noticeble changes in the thermoelectric transport properties, indicating an excellent thermal stability of the compounds prepared by the SHS method. Therefore, SHS method can serve as an effective alternative fabrication route to synthesize Mg−Si based themoelectrics and some other functional materials due to the resulting high performance, perfect thermal stability, and feasible production in large scale for commercial application. KEYWORDS: Mg2Si1−xSbx, self-propagating high-temperature synthesis, phase segregation, thermoelectric properties, thermal stability

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INTRODUCTION Great attention is being paid to the development of new efficient energy materials to mitigate the energy crisis and environmental pollution associated with the burning of fossil fuels. Thermoelectric materials, a component of new energy materials, are capable of converting waste heat into electricity and have been under intense worldwide development for many years.1,2 In general, the thermoelectric performance is evaluated by the dimensionless thermoelectric figure of merit ZT defined as ZT = σα2T/(κc + κL), where σ, α, T, κc, and κL are the electrical conductivity, the Seebeck coefficient, the absolute temperature, and the carrier and lattice components of the thermal conductivity, respectively.3−7 Due to the promising thermoelectric performance, high abundance in the earth’s crust, low cost and low density, as well as environmental friendliness, Mg2Si-based materials are viewed as one of the most promising candidates for applications as TE © 2016 American Chemical Society

power generators in the intermediate temperature range of 500−800 K.8−15 The thermoelectric properties of Sb doped Mg2 (Si,Sn) 1−xSb x solid solutions have got significantly improved with the highest figure of merit ZT above 1,13−18 resulting from the optimized carrier concentration,16 band convergence effect14 and great reduction of lattice thermal conductivity.17 Besides, the thermoelectric properties of Sb doped binary Mg2Si compounds have also been studied widely. Some groups have reported the perfect thermoelectric properties of Sb doped Mg2Si with the maximum figure of merit ZT ∼ 0.6−0.7.19−23 However, the synthesis of Mg2Sibased materials presents a considerable challenge due to a high vapor pressure and oxidization of Mg. So far, the Mg2Si-based Received: November 16, 2015 Accepted: January 19, 2016 Published: January 19, 2016 3268

DOI: 10.1021/acsami.5b11063 ACS Appl. Mater. Interfaces 2016, 8, 3268−3276

ACS Applied Materials & Interfaces

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compounds have been synthesized by melting,19 by mechanical alloying (ball milling),24 by a solid state reaction,25 or by microwave heating26 methods combined with rapid sintering including hot-press sintering (HP),20 field-activated pressassisted sintering (FAPAS),24 spark plasma sintering (SPS),27 and plasma activated sintering (PAS).28 However, all the above synthesis methods actually have some drawbacks thereof. For example, it is very difficult to precisely control the composition of the melt due to the high vapor pressure of Mg; the solid state reaction synthesis consumes too much time and energy; microwave methods typically get the desirable phase with some amount of raw materials; and samples prepared by mechanical alloying are often contaminated and oxidized. The above drawbacks are not unique to just magnesium silicides but generally apply to most of the other thermoelectric materials. Hence, it is desirable to develop a new method for the synthesis of Mg2Si-based materials, which can save the time and energy, be scalable to large industrial production, and yield a high performance thermoelectric material. As an ultrafast and scalable method, the self-propagating high-temperature synthesis (SHS) has played a major role in the processing of refractory materials for many years. The SHS reaction is an exothermic reaction which can be self-sustaining provided it releases enough heat to react the neighboring regions of the sample. A certain minimum energy is required to ignite the reaction at a small part of the compacted elemental powders. Once the reaction is ignited, it will generate enough heat, become self-propagating, and pass through the entire pellet.29,30 One of the important parameters in the SHS process is the so-called adiabatic Tad, the highest temperature the pellet can reach within the adiabatic surroundings. The theoretical value of Tad can be calculated using the known thermodynamic parameters such as the standard enthalpy.31,32 Meanwhile, corresponding to the Tad is the actually measured value Tc which is somewhat smaller due to not exactly ideal adiabatic conditions (some heat escapes by radiation, conduction, and convestion). The ignited zone passes through the entire pellet in the time of seconds, making it an extremely fast synthesis process. While originally limited to the synthesis of high temperature refractory materials, ceramics, and intermetallics,30 we have shown recently that the SHS can also be exceptionally effective in the synthesis of compound thermoelectric materials such as Bi2Te3, skutterudites, and others.33,34 In this work, we show that single phase Sb-doped Mg2Si1−xSbx compounds can be successfully synthesized by the ultrafast self-propagating high-temperature synthesis (SHS) within several seconds. The reacted pellet of Mg2Si1−xSbx is not fully dense and is thus pulverized (consuming 12 min) and compacted (consuming 7 min) by plasma activated sintering (PAS), the whole process (including SHS reaction) taking less than 20 min. The obtained bulk sample exhibits a high carrier mobility μ and a superior figure of merit ZT. To test its thermal stability, we subsequently applied an annealing treatment at 773 K for 7 days in vacuum to one of the compounds (x = 0.02) and remeasured its thermoelectrical transport properties with no apparent changes noted. Thus, the ultrafast SHS process is, indeed, applicable for the synthesis of the high performance Mg2Si compounds and can be extended a large scale industrial production. Above all, this research lays the important foundation for the development of fast synthesis routes for other structural and functional materials.

Research Article

EXPERIMENTAL METHODS

Synthesis. High purity powders of Mg (2.5 N, 150 mesh), Si (4 N, 300 mesh), and Sb (4 N, 200 mesh) were weighed according to stoichiometric compositions Mg2Si1−xSbx (0 ≤ x ≤ 0.025) but with an excess of Mg (2 mol % to compensate for the loss of Mg during the synthesis). The weighed powders were thoroughly mixed in an agate mortar in a glovebox filled with high purity Ar, cold-pressed into a cylindrical pellet and sealed under vacuum in a quartz tube. Afterward, one side of the cylinder was ignited with torch to initiate the selfpropagating high-temperature synthesis (SHS) reaction. A sketch of the SHS process is presented in Figure 1. Once ignited at a local point,

Figure 1. Sketch map of self-propagating high-temperature synthesis. the combustion wave passes through the remaining body of the cylinder. At a particular time, the entire cylinder can be viewed as divided into four parts: the reacted zone (a zone where the combustion wave has passed through), the combustion zone (a location where the combustion wave is situated right now), the preheating zone (a zone which is adjacent to the combustion wave and will be passed through subsequently), and the unreacted zone (a zone that is further away from the combustion wave). The reacted cylinder was pulverized and ground for 12 min and then compacted to its near theoretical density by the plasma activated sintering (PAS) method (consuming 7 min). Subsequently, samples were cut into appropriate shapes and sizes with a diamond disk for measurements of electrical and thermal transport properties. After the measurements were done, the sample with x = 0.02 was annealed at 773 K for 7 days in vacuum and its thermoelectric properties measured again to check the thermal stability. Stucture and Oxygen Content Characterization. The phase structure of samples was determined using an X-ray diffractometer (XRD, PANalytical X’pert Pro type, Netherlands) with Cu Kα radiation. The microstructure of the sintered bulk fresh fractured surfaces was characterized by field emission scanning electron microscopy (FESEM, Hitachi SU8020, Japan). Backscattered electron images (BEI), secondary electron images (SEI) and the chemical composition on the polished surfaces were obtained with an Electron Probe Micro Analyzer (EPMA, JXA-8230, Japan) equipped with wavelength dispersive X-ray spectroscopy (WDS). Nanophases were detected by high-resolution transmission electron microscopy (HRTEM, JEM-2100F, JEOL, Japan). The oxygen content in units of wt % of all samples was determined using a TC600 Nitrogen/ Oxygen Determinator (LECO Co.). Thermoelectric Transport Properties Measurements. The electrical conductivity σ and the Seebeck coefficient α of the samples were measured simultaneously using a commercial instrument (ZEM3, Ulvac Sinku-Riko) by the standard four-probe method under He atmosphere. The thermal diffusivity λ was obtained by a laser flash technique on a Netzsch LFA-457 apparatus. The heat capacity Cp at constant pressure was determined by a comparison method with DSC (Q20) from the TA Company. The obtained heat flow was used to determine the ignition temperature Tig. The experimental density d was evaluated by the Archimedes method. The thermal conductivity κ 3269

DOI: 10.1021/acsami.5b11063 ACS Appl. Mater. Interfaces 2016, 8, 3268−3276

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Figure 2. (a) DSC curves of mixed pure Mg and Si powders for different heating rates (2, 5, 10, 20, and 40 K/min) and the corresponding initial reaction temperature. (b) Schematic description about how to determine the velocity of the combustion wave v and the actural combustion temperature Tc.

Table 1. Parameters of SHS Relevant to Mg2Si compd

ΔHf,298 °

Tad

Tig

Tc

v

Mg2Si

−79.49 kJ mol−1

1282 K

850 K

1007 K

3.9 mms−1

was calculated using the formula κ = λCpd. The Hall coefficient of all samples was obtained under a magnetic field of 1 T at room temperature on a PPMS system from Quantum Design. Errors in the measurements of the electrical conductivity, Seebeck coefficient, and thermal conductivity are estimated as ±5%, ±2%, and ±5%, respectively.

is much lower, yet the combustion wave propagates. In fact, we have shown recently34 that many compound thermoelectric materials, among them CoSb3, Cu2Se, Bi2Te3 and all with the adiabatic temperature much lower than 1800 K, can be synthesized by SHS. Reference 34 has provided a new, empirically based criterion for the applicability of the SHS process. The new criterion states that the SHS process will be self-propagating provided the adiabatic temperature exceeds the melting point of the lower melting point component, that is, Tad/Tm,L > 1. Physically, this means that the exothermic reaction must generate enough heat to melt the lower melting point component in the neighboring layers in which the other component rapidly dissolves. Figure 2a shows heat flow curves of mixed Mg and Si powders for different heating rates (2, 5, 10, 20, and 40 K/ min). One notes that the exothermic peak gradually becomes sharper and narrower as the heating rate increases which indicates transformation from a solid state reaction process to SHS. With the increase of heating rate, the initial reaction temperature increases from 787.4 K with the heating rate of 2 K/min to 849 K with the heating rate of 20 K/min. With further increase of the heating rate, the initial reaction temperature almost unchanged. Since the heating rate is much larger than 40 K/min during the SHS process, the ignition temperature in this case, Tig ∼ 850 K. Figure 2b presents a schematic view illustrating how the actual combustion temperature Tc and the propagation velocity of the combustion wave v are measured. With the aid of a pair of thermocouples inserted into the wall of the cylinder at two points along its length separated by distance l, the combustion speed follows from v = l/t, where t is the time the combustion wave takes to pass between the two thermocouples, and meanwhile each records its own maximum temperature. The measured combustion speed is 3.9 mm s−1, within the range of combustion speeds (1−150 mm s−1) reported in the literature.37 The highest temperature recorded by the thermocouples is the combustion temperature Tc which, in this case, is recorded as 1007 K. The main reason why Tc < Tad

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RESULTS AND DISCUSSION Thermodynamic Parameters. Regarding the SHS process, one needs to specify some key parameters: the enthalpy ΔH°f,298 is the heat released during the reaction, the adiabatic temperaure Tad is the highest theoretical temperature reached during the reaction, the actural combustion temperature Tc is the highest temperature actually measured, the ignition temperature Tig is the temperature needed to initiate the reaction (see Figure 2a), and the propagation velocity v is the velocity with which the combustion wavefront moves through the cylindrical sample. All parameters are listed in Table 1. Generally, Tad can be obtained by using the following equations: ° + ΔH ° = ΔHf,298

Tad

∫298 ΔCp(product)dT

C P = a + b × 10−3T +

c × 105 + d × 10−6T 2 T2

(1)

(2)

For the SHS reaction 2Mg + Si → Mg2Si, the enthalpy ΔH°f,298 = −79.49 kJmol−1 and values of constants a, b, c, and d related to the molar specific capacity Cp of Mg2Si are available in ref 35. Due to a very rapid reaction when the combustion wave passes through, the temperature of the cylindrical compact is raised immediately with little heat exchange to the surroundings. Assuming an adiabatic environment, we obtain the adiabatic temperature Tad = 1282 K with ΔH° = 0. According to the traditional criterion of Merzhanov et al.36 for high-temperature intermetallics and ceramics, the combustion wave will propagate provided the adiabatic temperature reaches at least 1800 K. Clearly, our addiabatic temperature of 1282 K 3270

DOI: 10.1021/acsami.5b11063 ACS Appl. Mater. Interfaces 2016, 8, 3268−3276

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prepared fracture surfaces are shown in Figure 4a and b. From Figure 4a, it follows that the sample comprises grains of different sizes decorated with many nanodots. In order to see more details, Figure 4b provides a magnified image which depicts a transgranular fracture mode with tightly bonded grains after PAS. Furthermore, the figure shows that the nanophase appears not only at grain boundaries, but is also present inside the grains. The composition of the nanoprecipitates was determined by EDS-equipped HRTEM. Figure 4c shows in situ formed nanosized grains with the diameter of around 100 nm embedded in the matrix material. An enlarged image of a nanograin numbered 8 is presented in Figure 4d and the inset depicts the fast Fourier transform (FFT) of a region delineated by a square in Figure 4d which belongs to the nanograin 8. The interplanar spacings of 3.00 and 2.34 Å are in perfect agreement with the spacings of crystal planes (200) and (220) of Mg2Si according to ICDD# 03-065-0690. Combined with the EDS compositional analysis of grains 7, 8 and the matrix 9 shown in Figure 4g, it can be inferred that the nanoprecipitates crystallize with the same cubic antifluorite structure as the matrix but their Sb content is significantly larger than in the matrix. In addition, backscattered electron images (BSI) obtained on a polished surface (see Figure 4e), show that the sample is a multiphase structure and the phases are differentiated by their varied content of Sb determined using EPMA with WDS, which are listed in Figure 4f. Thus, we can conclude that the phase segeragation is observed for the first time in the Mg2Si1−xSbx which is induced by inhomogeneous distribution of Sb in the system. Electrical Transport Properties. Table 3 lists some physical properties of all samples at room temperature. Experimental values of the electrical conductivity, Hall coefficient and the Seebeck coefficient served as input parameters to calculate the carrier concentration, carrier mobility, the reduced Fermi level, and the Lorenz number under an assumption of a single parabolic band model and the dominant acoustic phonon scattering (r = −1/2).38−41The relevant relations in terms of the Fermi integrals are given in eq 3-7.

rests in a not ideal adiabatic condition of the sample which is allowed to exchange heat with the surroundings. Oxygen Content and Phase Composition. The content of oxygen is measured for the reactants (Mg and Si powders) and for powders prepared following the SHS and solid state reaction (SSR) products, respectively. The results, including the theoretically calculated value based on the measured oxygen content of reactants, are given in Table 2. Interestingly, the data Table 2. Oxygen Content of Mg Powder, Si Powder, Mg2Si Powder Synthesized by SHS, Mg2Si Powder Synthesized by Solid State Reaction (SSR), and Theoretically Calculated Oxygen Content of Mg2Si powder

Mg

Si

O2/wt %

0.090

0.902

Mg2Si (SHS) 0.332

Mg2Si (SSR) 0.510

Mg2Si (theory) 0.384

clearly show that the powder obtained by SHS has the lowest oxygen content 0.332 wt % compared with the powder from SSR (0.510 wt %) and the calculated oxygen content (0.384 wt %). In general, oxygen in Mg2Si is always present in the form of MgO, which might exert a detrimental effect on thermoelectric tranport properties due to its relatively high lattice thermal conductivity and low electrical conductivity. It seems that the SHS synthesis employed here actually purifies the final product as the combustion wave passes through.32 We speculate that as the combustion wave is approaching, a substantial amount of heat released activates the volatile contaminants (moisture and oxygen) and forces them to vaporize away from the surface of powders leading to a purification of the reacted material. The higher oxygen content in the powder prepared from the SSRreacted product is likely due to a much longer time of synthesis which provides a good opportunity for powders to react with oxygen. XRD patterns in Figure 3 show that all Mg2Si1−xSbx (0 ≤ x ≤ 0.025) compounds prepared by SHS or SHS combined with PAS are almost single phase materials accompanied by some barely detectable tiny MgO peaks. The main Bragg peaks match perfectly the peaks of Mg2Si (ICDD# 03-065-0690) which crystallizes in the cubic antifluorite structure (space group Fm− 3m). This confirms that the SHS synthesis is applicable to Mg2Si. Microstructure. Figure 4a−e presents the microstructure of the bulk sample with x = 0.02. FESEM images of freshly

α=±

(r + 5/2)Fr + 3/2(ηF) ⎤ kB ⎡ ⎢ηF − ⎥ e ⎢⎣ (r + 3/2)Fr + 1/2(ηF) ⎥⎦

(3)

Figure 3. Powder XRD patterns of Mg2Si1−xSbx (0 ≤ x ≤ 0.025) prepared by (a) SHS and (b) SHS combined with PAS. 3271

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Figure 4. Sample with x = 0.02: (a,b) FESEM images of the fractured surfaces; (c,d) HRTEM images of the bulk material; (e) backscattered electron image (BSI) of a polished surface with the actual composition determined by (f) wavelength dispersive spectrum (WDS) on EPMA and (g) energy dispersive spectrum (EDS) on HRTEM.

Table 3. Physical Parameters of All Samples at Room Temperature: Hall Coefficient RH (cm3 C−1), Carrier Concentration nH (×1019 cm−3, nH = 1/(eRH)), Carrier Mobility μH (cm2 V−1 s−1, μH = σRH), Seebeck Coefficient α (μV K−1), Reduced Fermi Level ηF = EF/kBT, and Lorenz Number L (×10−8 V2 K−2) sample x x x x x x

= = = = = =

0.00 0.005 0.010 0.015 0.020 0.025

Fi(ηF) =

∫0

∞

RH

nH

μH

α

ηF

m*/me

L

−0.65 −0.15 −0.079 −0.064 −0.054 −0.053

0.96 4.19 7.91 9.83 11.4 11.8

198 126 101 96.1 95.8 95.3

−223.6 −128.9 −105.1 −97.5 −90.1 −85.2

−0.45 1.38 2.09 2.36 2.82 2.88

0.78 0.87 1.02 1.08 1.04 1.05

1.58 1.78 1.87 1.91 1.97 1.97

x i dx 1 + exp(x − ηF)

(4)

ηF = E F /(kBT )

(5)

⎤ ⎛ k ⎞2 ⎡ (r + 7/2)Fr + 5/2(ηF) − δ 2(ηF)⎥ L = ⎜ B⎟ ⎢ ⎝ e ⎠ ⎢⎣ (r + 3/2)Fr + 1/2(ηF) ⎥⎦

(6)

δ(ηF) =

stronger character of a highly doped semiconductor. The highest room temperature value of about 1.8 × 105 Sm−1 is obtained with samples x = 0.02 and x = 0.025. As expected (see eq 8), as the carrier concentration increases, the magnitude of the Seebeck coefficient decreases. Nevertheless, the degradation of the Seebeck coefficient at increasing levels of Sb doping does not fully compensate the rising electrical conductivity. And as a consequence, the power factor PF is marginally enhanced at higher Sb content and reaches 2.6 × 10−3 W m−1 K−2 at 820 K for the sample with x = 0.02. In particular, the higher degeneracy of Sb doped heavily samples prevents an early onset of bipolar conduction and effectively extends the operational range of the compound to temperatures of at least 800 K.

(r + 5/2)Fr + 3/2(ηF) (r + 3/2)Fr + 1/2(ηF)

(7)

Here, ηF, Fi(ηF), r, and kB stand for the reduced Fermi level, the Fermi integral, the scattering factor, and the Boltzmann constant, respectively. Doping by Sb on the site of Si enhances electron carrier concentration effectively, in accord with the previous studies.8,21,22,42 Concurrently, the Fermi level EF is lifted from the band gap (ηF = −0.45) into the conduction band (ηF increases from 1.38 to 2.88) with the increasing content of Sb. From Table 3 we also find that the effective mass m* remains almost constant since the Fermi level EF is located above the conduction band edge, consistent with the parabolic nature of the band. Temperature dependent electronic transport properties are shown in Figure 5a−c. As the content of Sb increases, the electrical conductivity rises rapidly and attains a progressively

α=

8π 2kB 2T 3qh2

⎛ π ⎞2/3 m* ⎜ ⎟ ⎝ 3n ⎠

(8)

Figure 5d presents the dependence of the carrier concentration and the carrier mobility on the Sb content x. Clearly, the carrier concentration increases while the carrier mobility is strongly degraded by the increasing level of Sb impurity. At the highest Sb content used in our Mg2Si compounds, both the charge carrier concentration and the electron mobility just saturate. Coincidently, at these highest levels of Sb doping there seems to be an abrupt drop in the lattice parameter as shown in the inset of Figure 5d. The distinct change in the lattice parameter that takes place for 3272

DOI: 10.1021/acsami.5b11063 ACS Appl. Mater. Interfaces 2016, 8, 3268−3276

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Figure 5. Temperature dependent electrical transport properties of Mg2Si1−xSbx (0 ≤ x ≤ 0.025). (a) electrical conductivity, (b) Seebeck coefficient, (c) power factor, and (d) Sb content dependent carrier concentration, carrier mobility, and lattice constant (in the inset).

Figure 6. Temperature dependent (a) thermal conductivity and lattice thermal conductivity combined with bipolar thermal conductivity (in the inset), and (b) dimensionless thermoelectric figure of merit ZT.

samples with x = 0.02 and x = 0.025 is also likely the reason why a trend in the electrical conductivity and the Seebeck coefficient as a function of x seems to be interrupted at these Sb concentration in Figure 5a and c. Thermal Transport Properties and Dimensionless Figure of Merit ZT. Figure 6 displays the temperature dependent thermal conductivity κ and the dimensionless figure of merit ZT. The lattice thermal conductivity κL augmented by a contribution of the bipolar effect κBi is presented in the inset of Figure 6a. The term κL + κBi is obtained from the total thermal conductivity κ by subtracting the electronic thermal conductivity LσT (using the Wiedemann−Franz law) where L is the Lorenz number. In spite of a rapidly rising electrical conductivity with the increasing content of Sb, the overall thermal conductivity decreases. This is due to a much suppressed lattice thermal conductivity κL on account of heat

conducting phonons being strongly scattered by Sb impurity centers. In general, κL is described in terms of the specific heat Cv, the phonon velocity v, the phonon mean free path l, or, alternatively, the phonon relaxation time τ as κL = (1/3)Cvvl = (1/3)Cvv2τ.43 The overall relaxation time contains contributions from various scattering processes and is expressed as 1/τ = 1/τB + 1/τD + 1/τU where τB, τD, and τU represent phonon relaxation time concerning grain boundary scattering, point defect scattering, and phonon−phonon U-process scattering. Low lattice thermal conductivity is achieved with materials having soft lattice bonds which yield low phonon velocities and where phonons are effectively scattered over a broad range of frequencies of phonons that carry heat. Highfrequency phonon can be very effectively taken care of by point defect scattering of phonons such as taking place at sites where Sb substitutes for Si. Scattering of this type is particularly 3273

DOI: 10.1021/acsami.5b11063 ACS Appl. Mater. Interfaces 2016, 8, 3268−3276

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Figure 7. Microstructure of the polished surface of sample with x = 0.02: secondary electron image (SEI) (a) before and (b) after annealing; backscattered electron image (c) before and (d) after annealing.

Figure 8. Temperature dependent transport parameters of the sample with x = 0.02 before and after annealing: (a) electrical conductivity, (b) Seebeck coefficient, and (c) thermal diffusivity λ.

ZT values based on the experimental data are presented in Figure 6b where, for comparative reasons, we also include some of the representative data from the literature for Sb-doped Mg2Si.19,21−23 As the content of Sb increases, the dimensionless figure of merit ZT rises monotonously and reaches the maximum value of ZTmax ∼ 0.65 at 873 K for the sample x = 0.02. The reader recalls that at this Sb content the power factor was maximized and the lattice thermal conductivity reached its lowest value. ZT values achieved with our SHS synthesized Sbdoped Mg2Si structures are comparable or better than the most

effective when the mass and the size of the impurity atom differ greatly from the native atom. The resulting large mass defect fluctuation and elastic strain scatter phonons strongly. To scatter low frequency phonons requires the presence of nanometer scale structures and this is where nanoinclusions become particularly useful.44−46 In our SHS-synthesized Sb doped Mg2Si samples, we have both of these key structural features that impede the flow of phonons and this is the reason for a significant reduction of the lattice thermal conductivity. 3274

DOI: 10.1021/acsami.5b11063 ACS Appl. Mater. Interfaces 2016, 8, 3268−3276

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ACS Applied Materials & Interfaces results in the literature.19,21−23 This attests to an excellent prospect for the use of the SHS synthesis in fabrication of Mg2Si-based materials as not only the process is exceptionally rapid and energy saving, but also the resulting structures cause the superior thermoelectric performance. Thermal Stability of Mg2Si1−xSbxwith x = 0.02. In order to explore the thermal stability of samples prepared by SHS combined with PAS, we selected the sample x = 0.02 with the best thermoelectric performance and annealed it at 773 K for 7 days in vacuum. Figure 7 displays secondary electron images (SEI) and backscattered electron images (BEI) of the sample before and after the heat treatment. As is known, SEI can provide detailed information about the surface topography and morphology. The contrast in SEI image is strongly affected by the so-called edge effect which can give rise to regions with different brightness primarily caused by humps and pits on the surface. We note that the images in Figure 7a and b show no contrast, indicating very flat surfaces. BEI images in Figure 7 c and d, however, show clearly contrasting patterns which are attributed to compositional fluctuations as there is obviously no topographical difference. Subsequent compositional analysis with WDS on EPMA verified that Sb-rich and Sb-poor regions correspond to bright and dark zones, respectively, perfectly consistent with the data in Figure 4e and f. Hence, annealing at 773 K for 7 days does not cause any detrimental effect on the microstructure and compositional distribution of SHS synthesized samples. To explore the effect of annealing on the thermoelectric transport properties of the sample with x = 0.02, the temperature dependence of electrical conductivity, Seebeck coefficient and thermal diffusivity were remeasured and are displayed in Figure 8. The data in Figure 8 imply that the above three transport parameters have not obvious changes as a result of annealing. Therefore, it follows that samples prepared by SHS combined with PAS show an excellent thermal stability to temperature of 773 K, a highly reassuring and significant point from the perspective of large scale industrial use of these compounds.

result of dramatically decreased lattice thermal conductivity where the strong point defect scattering of phonons on Sb impurities and interface scattering on nanometer scale precipitates combine effectively to impede the flow of heat. As a result, the maximum thermoelectric figure of merit ZT ∼ 0.65 is achieved at 873 K for the sample with x = 0.02. This represents one of the highest figures of merit for Sb-doped Mg2Si compounds to this day. Subsequent annealing studies performed on the highest ZT structure (x = 0.02) indicates great thermal stability of the sample in terms of both its microstructure and thermoelectric properties. Thus, we have shown that the ultrafast SHS method combined with PAS compaction is very attractive not only for its time and energy savings but it also actually results in a superior thermoelectric material, and is easily scalable for large industrial production. With this ultrafast synthesis method, Sb doped Mg2Si1−xSnx solid solutions should be prepared as the next-step research considering their higher thermoelectric properties with ZT above 1, which would be of much more significance for the future application.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is financially supported by the National Basic Research Program of China (973 program) under Project 2013CB632502, the Natural Science Foundation of China (Grant Nos. 51172174, 51402222, and 51521001), the Fundamental Research Funds for the Central Universities (WUT: 2014-II-016 and 2014-III-046) and the 111 Project of China (Grant No. B07040), and by the joint U.S.-China CERC−CVC Project. Besides, Q. Zhang is grateful to R. Jiang and T. T. Luo for their help with HRTEM, W. Y. Chen and J. M. An for their help with DSC, as well as M. J. Yang and X. L. Nie for their help with EPMA in Materials Research and Test Center of WUT.

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CONCLUSION An alternative ultrafast method called the self-propagating hightemperature synthesis (SHS) was employed to successfully fabricate Sb-doped Mg2Si thermoelectric materials. The key parameters underpinning the SHS process were determined. Following the SHS synthesis, the reacted product was pulverized and then compacted to a near theoretical density using plasma activated sintering. Detailed structural evaluation was carried out to establish the composition, oxygen content, and microstructure. The results indicate that Sb doping leads to phase segregation with nanostructural regions rich in Sb and the matrix somewhat poorer in Sb. An analysis of the oxygen content suggests that the passage of the combustion wave helps to minimize oxidation and purifies the resulting reacted product. Regarding the thermoelectrical properties, the results show that doping by Sb dramatically increases the carrier concentration which, in turn, gives rise to a significantly increased electrical conductivity σ and monotonously decreased magnitude of the Seebeck coefficient with the overall modest gain in the power factor. The highest power factor of 2.6 × 10−3 W m−1 K−2 at 820 K was achieved with the sample x = 0.02. As the content of Sb increases, the thermal conductivity κ falls notably in spite of a rapidly increasing electronic thermal conductivity. The overall degradation of heat transport is the

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DOI: 10.1021/acsami.5b11063 ACS Appl. Mater. Interfaces 2016, 8, 3268−3276