Thermoelectric Performance of Rapidly Microwave Synthesized Î±

Subscriber access provided by WEBSTER UNIV

Article

Thermoelectric Performance of Rapidly Microwave Synthesized #-MgAgSb with SnTe Nanoinclusions Jiwu Xin, Junyou Yang, Sihui Li, Abdul Basit, Bingyang Sun, Suwei Li, Qiang Long, Xin Li, Ying Chen, and Qinghui Jiang Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b05014 • Publication Date (Web): 04 Feb 2019 Downloaded from http://pubs.acs.org on February 4, 2019

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 16 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Chemistry of Materials

Thermoelectric

Performance

of

Rapidly

Microwave Synthesized -MgAgSb with SnTe Nanoinclusions Jiwu Xin, Junyou Yang*, Sihui Li, Abdul Basit, Bingyang Sun, Suwei Li, Qiang Long, Xin Li, Ying Chen, Qinghui Jiang* *State Key Laboratory of Materials Processing and Die and Mould Technology, Huazhong University of Science and Technology, Wuhan 430074, PR China

ABSTRACT -MgAgSb (-MAS) has recently been discovered to be a promising p-type thermoelectric (TE) material owing to its earth-abundant and nontoxic nature. However, there are two main disadvantages hindered the large-scale application of -MAS, one is the single -MAS phase prepared by conventional method requires for an extended period of time, and the other is the severely bipolar effect thus poor electric properties. In this scenario, we have presented an effective approach typified by SnTe nanocompositing to significantly enhance the thermoelectric performance of rapidly microwave synthesized -MAS system. Specifically, the pure -MAS compound was first accessed by using a rapidly microwave synthesis. After the initial preparation, high-quality SnTe nanoparticles fabricated by a facile solvo-thermal method were incorporated into the microwave synthesized -MAS matrix. It is deserve to be mentioned that the rapidly microwave synthesis purifies single -MAS phase and allows the preparative time to be diminished from over two weeks to as little as five days. Moreover, the severe bipolar effect of pristine -MAS has been retarded effectively by the following step typified by compositing SnTe nanoinclusions, leading to a large Seebeck coefficient thus significantly enhanced power factor in -MAS/SnTe composited system. Concurrently, the lattice thermal conductivity has also been greatly reduced because of the extra phonons scattering due to the multiscale hierarchical architecture (e.g., SnTe nanostructures, high-density stacking faults and elastic strain fields). Eventually, an enhanced figure of merit ZT of ~1 at 548 K, which increases by ~ 53% compared with pristine -MAS, has been achieved in the 3 at% SnTe composited sample. This work impels the potential application of -MAS thermoelectric material as a robust candidate for waste heat recovery below 573 K.

INTRODUCTION Thermoelectric materials, which can directly convert heat energy to electricity or vice versa without moving parts,1-4 have been studying as an important alternative for waste heat recycling.

ACS Paragon Plus Environment

Chemistry of Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The performance of thermoelectric materials directly relate to the dimensionless figure of merit ZT= S2T/, where S is the Seebeck coefficient,  is the electrical conductivity, T is the absolute temperature, and  is the thermal conductivity, respectively. High ZT values require high , large S and low  simultaneously. However, it is a very challenging work to improve the ZT due to the strong interrelation among these thermoelectric parameters.5-7 As a promising candidate for thermoelectric application in low-mid temperature range, MgAgSb (MAS) compound has been paid increasing attention because of its eco-friendly nature since the discovery in 2012.8 MgAgSb is a half-Heusler alloy, and it exists in three different structures, as shown in Supplementary Figure S1, i.e., -MgAgSb (-MAS) of high temperature cubic phase (> ~ 633 K), The Cu2Sb-type -MgAgSb (-MAS) intermediate tetragonal phase (573 ~ 633 K), and tetragonal -MgAgSb (-MAS) room-temperature phase with a space group I4c2. Among them, -MAS is superior to the other two phases in thermoelectric performance. However, it is an arduous work to fabricate single phase -MAS compound. Concurrently, high-temperature melting9 and two-step mechanical alloying10, 11 (MA) methods are usually employed to obtain -MAS, and these two processes both need an additional vacuum annealing process of at least 2 weeks. Moreover, some impurities (Sb) or secondary phases like Ag3Sb always remain and thus degrade the thermoelectric performance. In addition, according to the previous works, 11-13 the intensive intrinsic excitation (~ 375 K) in -MAS system can inevitably results a substantial reduction in Seebeck coefficient thus relatively poor power factor. To date, the method of microwave synthesis has been employed into the fabrication of various materials, such as ceramic compound,14-16 metallic17, 18 and thermoelectric materials.19, 20 In this scenario, we have presented an effective approach typified by SnTe nanocompositing to significantly enhance the thermoelectric performance of rapidly microwave synthesized -MAS system. Specifically, this process is started from cold-pressed pallet and fabricated by a heating& annealing step using a rapid microwave synthesis to access the pure -MAS thermoelectric material. After the initial preparation, SnTe material is known as p-type thermoelectric compound with an ultrahigh carrier concentration ~1021 cm-3,21-24 higher than the pristine -MAS (~1019 cm-3). The carrier concentration of -MAS can be improved with the addition of SnTe. Besides, the intrinsic excitation temperature (~ 650 K) of SnTe is higher than that of the -MAS matrix (375 K), the introduction of SnTe may shift the intrinsic excitation of -MAS matrix to a higher temperature, suppressing the decrease of the Seebeck coefficient after the intrinsic excitation. Considering this, SnTe nanoparticles were obtained by a facile solvo-thermal method and incorporated into the microwave synthesized -MAS matrix. Subsequently, all the composites were sintered and consolidated at 543 K under the pressure of 240 MPa. It is deserve to be mentioned that the rapidly microwave synthesis of -MAS not only reduces impurity phases (e.g., Sb and Ag3Sb) and results in single phase but also allows the preparative time to be diminished from over two weeks to as little as five days. Moreover, the severe bipolar effect of pristine -MAS has been retarded effectively by the following step typified by compositing SnTe nanocrystals, leading to a large Seebeck coefficient thus significantly enhanced power factor in -MAS/SnTe composited system. Concurrently, the lattice thermal conductivity has also been greatly reduced because of the extra phonons scattering as a result from the multiscale hierarchical architecture (e.g., SnTe nanostructures, high-density stacking faults and elastic strain fields). Eventually, an enhanced ZT value of ~1 at 548 K, which increases by ~ 53% compared with pristine -MAS, has been achieved in the 3 at% SnTe composited sample.


Page 2 of 16

Page 3 of 16 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


METHODS Synthesis of SnTe Nanocrystals. Tin telluride (SnTe) nanocrystals were prepared by a low-temperature solvo-thermal process. For the synthesis, Tellurium (10 mmol, Te, 99.999%, Aladdin) were chosen as “Te” source and first dissolved in 90 mL of N, N-Dimethylformamide (99.8%, Aladdin). After stirring for 5 min, 10 mmol of Tin chloride dehydrate (10 mmol, SnCl22H2O, 99.99%, Aladdin) was added as the “Sn” source, and stirred for 20 min for homogeneity. Finally, when all the added powders were completely dissolved, 80 mmol, 20 mmol of Potassium hydroxide (KOH, 85%, Aladdin) and Potassium borohydride (KBH4, 98%, Aladdin) were added as a reducing reagent to react with the Tellurium and also prevent the surface oxidation of synthesized nanocrystals.25, 26 The final mixture was sealed within a Teflon container (Wuhan Shenshi Instrument, 100 mL), inserting into a stainless steel chamber. The reaction was then performed at 150 C for 12h in an oven. After synthesis, the obtained nanocrystals were subsequently swilled by ethanol and DI water and they were treated within an ultrasonic bath to thoroughly rinse the nanocrystals, ensuring that the unreacted chemicals and solvent were cleaned and removed. The nanocrystals were collected by centrifugation and finally dried in a vacuum desiccator for 12h. Frabrication of -MAS +x at% SnTe Thermoelectric Material. Mg (99.99%, crumbs), Ag (99.99%, powders) and Sb (99.999%, powders) were first weighted according to nominal composition MgAg0.97Sb and cold pressed into cylindrical pellets, and then placed into polished graphite crucible, the crucible was surrounded with the microwave-susceptor SiC powders (99.0%, Aladdin) and placed in the large alumina crucible to minimize heat loss. The samples were fabricated by microwave heating using a commercial microwave furnace (Tangshan Nayuan) with rapidly (15 K min-1) raising to 1173 K and hold for 40 min, then cooled (15 K min-1) to 543 K and annealing for 5 days. The obtained ingots were crushed and ground into powders in a mortar-pestle. After the initial preparation, SnTe nanoparticles were obtained by a facile solvo-thermal method and then incorporated into the microwave synthesized -MAS matrix. All composited samples were weighted according to -MAS + x at% SnTe (x= 1 - 4) and mixed by ball-milling for 0.5h with high purity Ar gas. Subsequently, the -MAS + x at% SnTe (x= 1 - 4) were sintered and consolidated at 543 K for 0.5h in a 20 mm diameter steel mold with a pressure of 240 MPa for 1h. Structure Characterization. The room-temperature powder X-ray diffraction (PXRD) measurement was performed on a PANalytical Empyrean diffactometer with Cu K radiation. The fracture morphologies of bulk samples were observed with a Nano SEM 450 field-emission scanning electron microscope (FESEM) and a high-resolution transmission electron microscopy (HRTEM) observation of the selected specimens was carried out on a JEOL JEM-2100 microscope and the energy dispersive x-ray spectroscopy (EDS) equipped with TEM was performed in bright-field mode at an accelerating voltage of 200 KV. Thermoelectric Transport Properties Measurements. The electrical resistivity () as well as Seebeck coefficient (S) were measured by a commercial thermoelectric measurement system (Namicro-IV, China). The room-temperature Hall coefficient (RH) was measured using the Van der Pauw method under a magnetic field of 0.55 T.27 the Hall mobility (H) and Hall carrier concentration (nH) were obtained by the relation H = RH / and nH = 1/ (eRH), respectively.



Thermal conductivity () was derived from  = DCp, where  is the relative densities of the bulk samples measured by the Archimedes method (Sartorius, YDK01). D is the thermal diffusivity measured using the laser flash method with a LFA-427 (NETZSCH) equipment, and Cp is the heat capacity were measured via differential scanning calorimetry (PerkinEmler Diamond DSC).28 The measurement uncertainties in Seebeck coefficient, electrical resistivity and thermal conductivity typically are 5%, 3% and 7%. Consequently, the deviation of ZT values was considered as 15%. Density Functional Theory Calculation. All the calculations were conducted using the Perdew-Burke-Ernzerhof29 generalized gradient approximation (GGA-PBE) and projector augmented wave30 potentials within the framework of density functional theory (DFT) as implemented in the Vienna ab initio simulation package (VASP).31 A plane wave cutoff energy of 400 eV was applied for all calculations. the geometry of MgAgSb system is fully relaxed until the magnitude of the force acting on all atom is less than 0.01 eV/Å, the accuracy of self-consistent was 5.010-7eV/atom, a Gamma centered k-point grid of 221 was employed for the Brillouin zone for the was adapted to calculate the total energy and DOS properties.

RESULTS AND DISCUSSION Structural and Morphological Characteristics. The cold-pressed pellets were first rapidly heated to 1173 K and then kept with different microwave-holding (MH) time (10, 20, 40 and 60 min) to obtain -MAS compounds by using microwave heating furnace. Supplementary Figure S2a shows the PXRD results of various MH samples and majority of peaks can be indexed to the half-heusler structure -MAS with a space group I4c2. Observably, a large amount of secondary phase can be identified to be Sb and Ag3Sb in MH samples. The low-magnification fracture morphologies of MH-40min sample with a typical overall densification as shown in Supplementary Figure S2b, c. The corresponding energy dispersive spectrometry (EDS) mapping and spots 1-3 results further indicates that there are some Mg-poor or Ag3Sb phase observed in the MH samples. In order to remove impurities and purify the -MAS phase of MH samples, the MH-40min sample was annealed at 543 K for 1, 3 and 5 days afterwards. As displayed in Figure 1a, the diffraction peaks of MH-40min + annealing 5 days sample exhibits good match with the calculated -MAS compared with conventional melting-annealing method.9 The Rietveld refinement result of microwave synthesized sample in Figure 1b suggests that single-phased -MAS compound has been successfully prepared through further microwave annealing, which allows the fabrication time to be reduced from over two weeks to as little as five days. Parallelly, high-quality SnTe nanocrystals obtained by a facile solvo-thermal method, with a space group of Fm3m ( Figure 1c) and size about 25 nm (Figure 1d, e), were continuously introduced to composite with microwave synthesized -MAS. All the -MAS + x at% SnTe composites were consolidated at 543 K with relative densities at least 95% (refer to Supplementary Table S1) by a hot pressed sintering (HPS). As shown in Figure 1f, the major XRD diffraction peaks of composites can be well indexed to the tetragonal -MAS structure and no peaks of SnTe phase can be observed within the detection limit. The DTA measurement (Figure 1g) reveals that there is no endothermic or exothermic peak can be observed between SnTe nanoinclusions and -MAS matrix within the working temperature in the composited system except the phase transformation


Page 4 of 16

Page 5 of 16 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


() of -MAS.

Figure 1. (a) Powder X-ray diffraction (PXRD) data of the hot pressed pallets with various microwave-annealing time, (b) a typical Rietveld refinement result using XRD data for -MAS (annealing 5 days) sample. (c) The XRD diffractions of pure SnTe synthesized by solvo-thermal method. (d, e) The typical TEM and HRTEM images for solvo-thermal synthesized SnTe and inset d is the corresponding SAED pattern. (f) The XRD patterns of -MAS + x at% SnTe composites and (g) the DSC measurement of -MAS/SnTe composited sample.

Characterization of Microstructures. Figure 2a shows the SEM fractographs of microwave synthesized -MAS matrix with excellent crystallinity. The inserted backscattered electron (BSE) image and corresponding EDS analysis (Figure 2b) further illustrates all the elements of Mg, Ag and Sb were very homogeneously distributed in the sample, in which we can hardly observe any second phase segregation or precipitate in the -MAS matrix. In comparison with the clean surface of the pristine -MAS sample, dispersed nanoinclusions embedded in the matrix can be readily detected in the -MAS + 3 at% SnTe composited sample as show in Figure 2c, the corresponding EDS analysis (Figure 2d) indicates these nanoscale particles should be SnTe, which is in line with the DTA results shown in Figure 1g.



Figure 2. (a) The fracture morphologies and inserted back scattered SEM images of -MAS matrix, (b) the corresponding EDS mapping and element analyses. (c) The SEM image of -MAS + 3 at% SnTe composites and (d) element analyses.

The low- and high- resolution transmission electron microscopy technologies were also carried out on the -MAS/SnTe composites for further characterizing the microstructure. As illustrated in Figure 3a, b, it is noteworthy that large amounts of stacking faults with a typical spacing of a few nanometers (insert Figure 3b) have been observed in the composites, which should be induced by the low temperature (543 K) and high pressure condition (240 MPa) of the hot-pressed process.27 The enlarged HRTEM taken from a typical distorted area was shown in Figure 3c, from which, various and abundant distortions, such as atomic-scale lattice distortions and high density of edge dislocations (marked with white T in IFFT image) can be observed. Moreover, the geometric phase analysis (GPA), which is a semi-quantitative lattice image-processing approach for revealing spatial distributed elastic strain fields,32 was performed on the high-quality HRTEM images as shown in Figure 3d. The distribution of strain state can be reflected in the strain tensor map, indicating that the elastic strain (marked with white dashed circle) can provide an extensive scattering for phonons moving along different directions. The low-magnification bright-field TEM (BF-TEM) image in Figure 3e presents a large number of nanoinclusions typified by spherical (Figure 3f) and square (Figure 3g) nanocrystals are well dispersed in the host material with a disconnected heterojunction interface, which can realize a synergistic regulation of the thermoelectric properties of -MAS as will be discussed later.


Page 6 of 16

Page 7 of 16 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 3. The low- (a) and high- (b) magnification TEM images for -MAS reveals high-density stacking faults. (c) The HRTEM image focusing on stacking faults (marked with white T) and insets are the FFT and inversed FFT patterns, respectively. (d) HRTEM images taken from a typical distorted area and insert is the corresponding strain analysis. (e) The dispersed nanoinclusions typified like by spherical (f) and square (g) nanocrystals are embedded in the -MAS matrix, (h) HRTEM image shows the interplanar spacing of SnTe from the direction (200).

Electronic Transport Properties. As shown in Figure 4a, c, the room-temperature electrical resistivity () and Seebeck coefficient (S) for the SnTe composited samples decrease as a result of the increase of hole concentration (Figure 4b). As for the pristine -MAS, the temperature-dependent  increase first and then decrease, suggesting a semiconductor behavior. As expected, the Seebeck coefficient (S) shows an opposite trend with a peaked value about 375 K, which can be attributed to the intrinsic carrier excitation of -MAS (stage 1). The -MAS/SnTe composited samples decrease substantially on the  and  with elevating temperature due to the bipolar effects (T > 475 K), leading to an intensive supplement of hole concentration.33 It is worth noting that the intrinsic excitation temperature (stage 2) for all the composites are delayed to a higher region, closing to the phase change temperature ~ 573 K (-MAS-MAS), which demonstrates that compositing SnTe can effectively suppress the minority carrier excitation, thus in favor of obtaining large Seebeck coefficient and high power factor. Moreover, Figure 4d shows the Pisarenko relation of all samples at 300 K. all the experimental data of -MAS + x at% SnTe composites lies between the theoretical Pisarenko plot of -MAS and intrinsic SnTe, suggesting a composite character. Thereby, as shown in Supplementary Figure S3, the power factor of -MAS + x at% SnTe composites is enhanced and the maximum value of 1930 Wm-1K-2 was achieved due to the large Seebeck coefficient induced by the retardation of carrier excitation.



Figure 4. (a) The temperature-dependent electrical resistivity, (b) the carrier concentration and Hall mobility of the samples, (c) the temperature dependence of the Seebeck coefficient and (d) room-temperature Seebeck coefficient as a function of carrier concentration plot for -MAS + x at% SnTe (x= 1 - 4) composites.

Thermal Conductivities. The thermal transport properties of -MAS + x at% SnTe (x= 1 - 4) composites were plotted in Figure 5. The total thermal conductivity total (Figure 5a) first decrease at low temperature as a result of the Umklapp scattering process and then increases at higher temperature owing to the bipolar effect.10 Generally, the total thermal conductivity (total= e + lat) consists of two parts, including electronic thermal conductivity (e), lattice thermal conductivity (lat). The total increases gradually with increasing SnTe-compositing concentration from x = 0 to 4, mainly due to the largely enhanced contribution of e. The e is estimated by Widemann-Franz relationship e = LT/ (refer to Supplementary Figure S4). Since the Lorenz number L was assumed by the single parabolic band structure and the main scattering mechanism is acoustic phonon scattering before the intrinsic excitation for -MAS, therefore we focus on the properties of e from RT to 450 K for reasonable assessment of lat. As shown in Figure 5b, the lat of pristine -MAS is only 0.71 Wm-1K-1 and the tot is almost lower than 1 Wm-1K-1 throughout the temperature range. Therefore, the DFT calculations were performed to explore the origin of intrinsic low lat of -MAS system. As shown in Figure 5c, the 4d-orbital electrons of Ag atoms are far from the Fermi level between -6.5 and -4.5 eV, indicating that the 4d10 electrons form into a closed shell structure and have a weak interaction between each other. Therefore, the valence electron count (VEC=8) of -MAS is mainly dominated by the s- and p-orbital electrons near the Fermi level, which is unlike with the half-heulser compounds (VEC=18)34-36 and result in the formation of weak chemical bonds.37 Figure 5d shows that the formation of Mg-Ag-Sb three-center bond will increase the interatomic distance along the direction of other twisted structural units, which promotes to the vibration of Ag atoms.38 In addition, the charge density difference (Figure 5e, f) of -MAS is constrained in the Mg-Ag-Sb triangle. These results indicate


Page 8 of 16

Page 9 of 16 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


that the weak chemical bond induced by the Mg-Ag-Sb three-center bond and reinforced lattice vibration, are beneficial to reduce the lat of intrinsic -MAS system.

Figure 5. The temperature-dependent (a) total thermal conductivity total, (b) lattice thermal conductivity lat for the -MAS + x at% SnTe (x= 0 - 4) samples, respectively. (c) The partial density of states, (d) structure unit and (e, f) the calculated charge density difference of -MAS system.

Moreover, The lat of composites are significantly reduced by strong scattering of phonons across multiscale SnTe nanostructures and high-density stacking faults.39, 40 the lat at 450 K for pristine -MAS decreases from 0.71 Wm-1K-1 to 0.54 Wm-1K-1 for 4 at% SnTe composites, which is still higher than the theoretical minimum value of min (~0.47) and can be suppressed for further reduced. To evaluate the possible mechanism for the variation of thermal transport properties, we have estimated the theoretical values for the composite by employing self-consistent Effective Medium Theory (SCEMT, details refer to Supplementary Note 1) 41-44 for lat. As shown in Figure 6a and 6b, the experimental of -MAS + x at% SnTe is below the calculated values based on SCEMT and the deviation of lat at 300 K (~14.5%) is much higher than that at 450 K (~4.7%). It indicates that the high-temperature case is more close to the theoretical estimation of SCEMT, which is mainly attributed by the week interface scattering, plays more significantly roles at low-temperature than high-temperature. In addition, the experimental lat lower than the theoretical values assumed by the Klemens-Drabble model and were displayed in Figure 6c, which



in accordance with the analysis of SCEMT and both reveals that there must be additional phonon-scattering mechanisms to account for such deviation. Therefore, to well understand the effect of the observed multiscale phonon scattering centers (TEM, Figure 3) on reducing the lat, we can clarify which type of phonons contributes to by evaluating the spectral lattice thermal conductivity (s, refer to Supplementary Note 2) due to the integral of s with respect to the phonon frequency equals to lat as shown in Figure 6d. The Umklapp-process (U) can scatter the whole frequency phonons and the electrons-phonons (EP) can effectively scatter low frequency phonons, while the middle + high frequency phonons are scattered by point defects (PD) + grain-boundaries (B) and stacking faults (SF) + nanoparticles (NP).

Figure 6. The lat at (a) 300 K and (b) 450 K as a function of SnTe contents compared with the calculated values based on SCEMT for composites. (c) The room-temperature lat as a function of SnTe fraction for composites and (d) calculated spectral lattice thermal conductivity s for 0.03 at% SnTe composites with various phonon scattering centers.

Thermoelectric Performance. Consequently, the resultant ZT value of SnTe-composited samples realizes a significant improvement in the whole temperature as illustrated in Figure 7a, and the maximal ZT~1 at 548 K was achieved in the 3 at% SnTe composited sample due to the successful optimization in both the power factor and lattice thermal conductivity. Compared with the previously reported -MAS literature10-13, 38, 45 as shown in Figure 7b, we can easily find that the effective strategy of SnTe nanocompositing contributes ZT performance of the rapidly microwave synthesized -MAS and makes it to be competitive with conventional element-doping via mechanical alloying or melting method, impelling the potential application of -MAS thermoelectric material as a robust candidate for waste heat recovery below 573 K.


Page 10 of 16

Page 11 of 16 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 7. (a) ZT values as a function of temperature for -MAS + x at% SnTe (x=0 - 4) samples, (b) the current state-of-the-art ZT comparison of -MAS thermoelectric materials obtained from available literature data.

CONCLUSIONS In this work, we have presented an effective approach typified by SnTe nanocompositing to significantly enhance the thermoelectric performance of rapidly microwave synthesized -MAS system. Specifically, this process is started from cold-pressed pallet and fabricated by a heating& annealing step using a rapid microwave synthesis to access the pure -MAS thermoelectric material. After the initial preparation, the high-quality SnTe nanocrystals obtained by a solvo-thermal method were composited with rapidly microwave synthesized -MAS matrix. Subsequently, all the composites were sintered and consolidated at 543 K under the pressure of 240 MPa. It is deserve to be mentioned that the rapidly microwave synthesis of -MAS not only reduces impurity phases (e.g., Sb and Ag3Sb) and results in single phase but also allows the preparative time to be diminished from over two weeks to as little as five days. Moreover, the severe bipolar effect of pristine -MAS has been retarded effectively by the following step typified by compositing SnTe nanocrystals, leading to a large Seebeck coefficient thus significantly enhanced power factor in -MAS/SnTe composited system. Concurrently, the lattice thermal conductivity has also been greatly reduced because of the extensive phonons scattering (e.g., SnTe nanostructures, high-density stacking faults and elastic strain fields). Eventually, sharply enhanced ZT value of ~1.0 at 548 K, which increases by ~ 53% compared with pristine -MAS, was achieved in the 3 at% SnTe composited sample.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS publications website at DOI: Calculated diffraction patterns and crystal structure of different MgAgSb phases; XRD diffraction patterns and the EDS spectra for MH-40 min sample; temperature-dependent power factors; thermal diffusivities, Lorenz number and carrier thermal conductivities for -MAS + x at% SnTe (x= 0 - 4) composites; sample densities measured by Archimedes method; parameters used to calculate L and min based on various phonon scattering processes; details of modeling study



based on Self-consistent Effective Medium Theory (SCEMT); the calculation for phonon modeling studies.

AUTHOR INFORMATION Corresponding Authors *Email: [email protected] *Email: [email protected] ORCID Junyou Yang: 0000-0003-0849-1492 Qinghui Jiang: 0000-0001-7851-2653 Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS This work is co-financed by National Natural Science Foundation of China (Grant No. 51572098, 51632006, 51772109 and 51872102), the Fundamental Research Funds for the Central Universities (No. 2018KFYXKJC002) ， Open Fund of State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology (No. 2016-KF-5), Graduates' Innovation Fund, Huazhong University of Science and Technology (No. 5003110006). The technical assistance from the Analytical and Testing Center of HUST is likewise gratefully acknowledged.

REFERENCES (1). Xiao, Y.; Wu, H.; Cui, J.; Wang, D.; Fu, L.; Zhang, Y.; Chen, Y.; He, J.; Pennycook, S. J.; Zhao, L.-D., Realizing High Performance N-Type Pbte by Synergistically Optimizing Effective Mass and Carrier Mobility and Suppressing Bipolar Thermal Conductivity. Energy Environ. Sci. 2018, 11, (9), 2486-2495. (2). Zhu, T.; Liu, Y.; Fu, C.; Heremans, J. P.; Snyder, J. G.; Zhao, X., Compromise and Synergy in High‐Efficiency Thermoelectric Materials. Adv. Mater. 2017, 29, (14), 1605884. (3). Chen, Z.-G.; Shi, X.; Zhao, L.-D.; Zou, J., High-Performance Snse Thermoelectric Materials: Progress and Future Challenge. Prog. Mater Sci. 2018, 97, 283-346. (4). Luo, Y.; Yang, J.; Liu, M.; Xiao, Y.; Fu, L.; Li, W.; Zhang, D.; Zhang, M.; Cheng, Y., Multiple Heteroatom Induced Carrier Engineering and Hierarchical Nanostructures for High Thermoelectric Performance of Polycrystalline in 4 Se 2.5. J. Mater. Chem. A 2015, 3, (3), 1251-1257. (5). Xin, J.; Jiang, Q.; Wen, Y.; Li, S.; Zhang, J.; Basit, A.; Shu, L.; Li, X.; Yang, J., An in Situ Eutectic Remelting and Oxide Replacement Reaction for Superior Thermoelectric Performance of Insb. J. Mater. Chem. A 2018, 6, (35), 17049-17056. (6). Hong, M.; Wang, Y.; Liu, W.; Matsumura, S.; Wang, H.; Zou, J.; Chen, Z. G., Arrays of Planar Vacancies in Superior Thermoelectric Ge1− X− Ycdxbiyte with Band Convergence. Adv. Energy Mater. 2018, 8, (30), 1801837.


Page 12 of 16

Page 13 of 16 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(7). Zhang, D.; Yang, J.; Jiang, Q.; Fu, L.; Xiao, Y.; Luo, Y.; Zhou, Z., Ternary Cusbse 2 Chalcostibite: Facile Synthesis, Electronic-Structure and Thermoelectric Performance Enhancement. J. Mater. Chem. A 2016, 4, (11), 4188-4193. (8). Kirkham, M. J.; dos Santos, A. M.; Rawn, C. J.; Lara-Curzio, E.; Sharp, J. W.; Thompson, A. J., Abinitio Determination of Crystal Structures of the Thermoelectric Material Mgagsb. Phys. Rev. B 2012, 85, (14), 144120. (9). Ying, P.; Liu, X.; Fu, C.; Yue, X.; Xie, H.; Zhao, X.; Zhang, W.; Zhu, T., High Performance Α-Mgagsb Thermoelectric Materials for Low Temperature Power Generation. Chem. Mater. 2015, 27, (3), 909-913. (10). Liu, Z.; Wang, Y.; Mao, J.; Geng, H.; Shuai, J.; Wang, Y.; He, R.; Cai, W.; Sui, J.; Ren, Z., Lithium Doping to Enhance Thermoelectric Performance of Mgagsb with Weak Electron–Phonon Coupling. Adv. Energy Mater. 2016, 6, (7), 1502269. (11). Shuai, J.; Kim, H. S.; Lan, Y.; Chen, S.; Liu, Y.; Zhao, H.; Sui, J.; Ren, Z., Study on Thermoelectric Performance by Na Doping in Nanostructured Mg 1-X Na X Ag 0.97 Sb 0.99. Nano Energy 2015, 11, 640-646. (12). Sui, J.; Shuai, J.; Lan, Y.; Liu, Y.; He, R.; Wang, D.; Jie, Q.; Ren, Z., Effect of Cu Concentration on Thermoelectric Properties of Nanostructured P-Type Mgag 0.97− Xcuxsb 0.99. Acta Mater. 2015, 87, 266-272. (13). Liu, Z.; Zhang, Y.; Mao, J.; Gao, W.; Wang, Y.; Shuai, J.; Cai, W.; Sui, J.; Ren, Z., The Microscopic Origin of Low Thermal Conductivity for Enhanced Thermoelectric Performance of Yb Doped Mgagsb. Acta Mater. 2017, 128, 227-234. (14). Tun, K. S.; Gupta, M., Improving Mechanical Properties of Magnesium Using Nano-Yttria Reinforcement and Microwave Assisted Powder Metallurgy Method. Compos. Sci. Technol. 2007, 67, (13), 2657-2664. (15). Ramesh, P. D.; Brandon, D.; Schächter, L., Use of Partially Oxidized Sic Particle Bed for Microwave Sintering of Low Loss Ceramics. Materials Science and Engineering: A 1999, 266, (1-2), 211-220. (16). Agrawal, D. K., Microwave Processing of Ceramics. Curr. Opin. Solid State Mater. Sci. 1998, 3, (5), 480-485. (17). Mastrovito, C.; Lekse, J. W.; Aitken, J. A., Rapid Solid-State Synthesis of Binary Group 15 Chalcogenides Using Microwave Irradiation. J. Solid State Chem. 2007, 180, (11), 3262-3270. (18). GavináWhittaker, A., Microwave-Assisted Solid-State Reactions Involving Metal Powders and Gases. Journal of the Chemical Society, Dalton Transactions 1993, (16), 2541-2543. (19). Biswas, K.; Muir, S.; Subramanian, M., Rapid Microwave Synthesis of Indium Filled Skutterudites: An Energy Efficient Route to High Performance Thermoelectric Materials. Mater. Res. Bull. 2011, 46, (12), 2288-2290. (20). Birkel, C. S.; Zeier, W. G.; Douglas, J. E.; Lettiere, B. R.; Mills, C. E.; Seward, G.; Birkel, A.; Snedaker, M. L.; Zhang, Y.; Snyder, G. J., Rapid Microwave Preparation of Thermoelectric Tinisn and Ticosb Half-Heusler Compounds. Chem. Mater. 2012, 24, (13), 2558-2565. (21). Zhao, L. D.; Zhang, X.; Wu, H.; Tan, G.; Pei, Y.; Xiao, Y.; Chang, C.; Wu, D.; Chi, H.; Zheng, L., Enhanced Thermoelectric Properties in the Counter-Doped Snte System with Strained Endotaxial Srte. J. Am. Chem. Soc. 2016, 138, (7), 2366-2373. (22). Zhang, Q.; Liao, B.; Lan, Y.; Lukas, K.; Liu, W.; Esfarjani, K.; Opeil, C.; Broido, D.; Chen, G.; Ren, Z., High Thermoelectric Performance by Resonant Dopant Indium in Nanostructured Snte. PNAS



2013, 110, (33), 13261-13266. (23). Tan, G.; Zhao, L. D.; Shi, F.; Doak, J. W.; Lo, S. H.; Sun, H.; Wolverton, C.; Dravid, V. P.; Uher, C.; Kanatzidis, M. G., High Thermoelectric Performance of P-Type Snte Via a Synergistic Band Engineering and Nanostructuring Approach. J. Am. Chem. Soc. 2014, 136, (19), 7006-17. (24). Moshwan, R.; Yang, L.; Zou, J.; Chen, Z. G., Eco ‐ Friendly Snte Thermoelectric Materials: Progress and Future Challenges. Adv. Funct. Mater. 2017, 27, (43), 1703278. (25). Datta, A.; Paul, J.; Kar, A.; Patra, A.; Sun, Z.; Chen, L.; Martin, J.; Nolas, G. S., Facile Chemical Synthesis of Nanocrystalline Thermoelectric Alloys Based on Bi− Sb− Te− Se. Crystal Growth & Design 2010, 10, (9), 3983-3989. (26). Hong, M.; Chasapis, T. C.; Chen, Z.-G.; Yang, L.; Kanatzidis, M. G.; Snyder, G. J.; Zou, J., N-Type Bi2te3–X Se X Nanoplates with Enhanced Thermoelectric Efficiency Driven by Wide-Frequency Phonon Scatterings and Synergistic Carrier Scatterings. ACS Nano 2016, 10, (4), 4719-4727. (27). Luo, Y.; Jiang, Q.; Yang, J.; Li, W.; Zhang, D.; Zhou, Z.; Cheng, Y.; Ren, Y.; He, X.; Li, X., Simultaneous Regulation of Electrical and Thermal Transport Properties in Cuinte 2 by Directly Incorporating Excess Znx (X=S, Se). Nano Energy 2017, 32, 80-87. (28). Ren, Y.; Yang, J.; Jiang, Q.; Zhang, D.; Zhou, Z.; Li, X.; Xin, J.; He, X., Synergistic Effect by Na Doping and S Substitution to High Thermoelectric Performance of P-Type Mnte. J. Mater. Chem. C 2017, 5, 5076-5082. (29). Perdew, J. P.; Burke, K.; Ernzerhof, M., Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1996, 77, (18), 3865. (30). Blöchl, P. E., Projector Augmented-Wave Method. Phys. Rev. B 1994, 50, (24), 17953. (31). Kresse, G.; Hafner, J., Norm-Conserving and Ultrasoft Pseudopotentials for First-Row and Transition Elements. J. Phys.: Condens. Matter 1994, 6, (40), 8245. (32). Hÿtch, M.; Snoeck, E.; Kilaas, R., Quantitative Measurement of Displacement and Strain Fields from Hrem Micrographs. Ultramicroscopy 1998, 74, (3), 131-146. (33). Luo, Y.; Yang, J.; Jiang, Q.; Li, W.; Zhang, D.; Zhou, Z.; Cheng, Y.; Ren, Y.; He, X., Progressive Regulation of Electrical and Thermal Transport Properties to High‐Performance Cuinte2 Thermoelectric Materials. Adv. Energy Mater. 2016, 6, (12), 1600007. (34). Xin, J.; Tang, Y.; Liu, Y.; Zhao, X.; Pan, H.; Zhu, T., Valleytronics in Thermoelectric Materials. npj Quantum Mater. 2018, 3, (1), 9. (35). Xia, K.; Liu, Y.; Anand, S.; Snyder, G. J.; Xin, J.; Yu, J.; Zhao, X.; Zhu, T., Enhanced Thermoelectric Performance in 18-Electron Nb0.8cosb Half-Heusler Compound with Intrinsic Nb Vacancies. Adv. Funct. Mater. 2018, 28, (9), 1705845. (36). Anand, S.; Xia, K.; I. Hegde, V.; Aydemir, U.; Kocevski, V.; Zhu, T.; Wolverton, C.; Snyder, G. J., A Valence Balanced Rule for Discovery of 18-Electron Half-Heuslers with Defects. Energy Environ. Sci. 2018, 11, (6), 1480-1488. (37). Ying, P.; Li, X.; Wang, Y.; Yang, J.; Fu, C.; Zhang, W.; Zhao, X.; Zhu, T., Hierarchical Chemical Bonds Contributing to the Intrinsically Low Thermal Conductivity in Α ‐ Mgagsb Thermoelectric Materials. Adv. Funct. Mater. 2017, 27, (1), 1604145. (38). Li, D.; Zhao, H.; Li, S.; Wei, B.; Shuai, J.; Shi, C.; Xi, X.; Sun, P.; Meng, S.; Gu, L., Atomic Disorders Induced by Silver and Magnesium Ion Migrations Favor High Thermoelectric Performance in Α‐Mgagsb‐Based Materials. Adv. Funct. Mater. 2015, 25, (41), 6478-6488. (39). Pan, Y.; Aydemir, U.; Sun, F.; Wu, C.; Chasapis, T. C.; Snyder, G. J.; Li, J., Self‐Tuning N‐


Page 14 of 16

Page 15 of 16 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Type Bi2(Te,Se)3/Sic Thermoelectric Nanocomposites to Realize High Performances up to 300 °C. Adv. Sci. 2017, 4, (11), 1700259. (40). Xin, J.; Yang, J.; Jiang, Q.; Li, S.; Basit, A.; Hu, H.; Long, Q.; Li, S.; Li, X., Reinforced Bond Covalency and Multiscale Hierarchical Architecture to High Performance Eco-Friendly Mnte-Based Thermoelectric Materials. Nano Energy 2019, 57, 703-710. (41). Zebarjadi, M.; Joshi, G.; Zhu, G.; Yu, B.; Minnich, A.; Lan, Y.; Wang, X.; Dresselhaus, M.; Ren, Z.; Chen, G., Power Factor Enhancement by Modulation Doping in Bulk Nanocomposites. Nano Lett. 2011, 11, (6), 2225. (42). Wu, D.; Pei, Y.; Wang, Z.; Wu, H.; Huang, L.; Zhao, L. D.; He, J., Significantly Enhanced Thermoelectric Performance in N‐Type Heterogeneous Biagses Composites. Adv. Funct. Mater. 2015, 24, (48), 7763-7771. (43). Pei, Y.-L.; Wu, H.; Wu, D.; Zheng, F.; He, J., High Thermoelectric Performance Realized in a Bicuseo System by Improving Carrier Mobility through 3d Modulation Doping. J. Am. Chem. Soc. 2014, 136, (39), 13902-13908. (44). Nan, C. W., Physics of Inhomogeneous Inorganic Materials. Prog. Mater Sci. 1993, 37, (1), 1-116. (45). Liu, Z.; Wang, Y.; Gao, W.; Mao, J.; Geng, H.; Shuai, J.; Cai, W.; Sui, J.; Ren, Z., The Influence of Doping Sites on Achieving Higher Thermoelectric Performance for Nanostructured Α-Mgagsb. Nano Energy 2017, 31, 194-200.



An effective approach typified by SnTe nanocompositing to significantly enhance the thermoelectric performance of rapidly microwave synthesized α-MAS system. 192x117mm (300 x 300 DPI)


Page 16 of 16

Thermoelectric Performance of Rapidly Microwave Synthesized Î±

Recommend Documents