Enhancing Thermoelectric Performance of TiNiSn Half-Heusler

Subscriber access provided by UNIVERSITY OF LEEDS

Article

Enhancing Thermoelectric Performance of TiNiSn Half-Heusler Compound via Modulation Doping Tanya Berry, Chenguang Fu, Gudrun Auffermann, Gerhard H. Fecher, Walter Schnelle, Federico Serrano-Sanchez, Yuan Yue, Hong Liang, and Claudia Felser Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.7b02685 • Publication Date (Web): 31 Jul 2017 Downloaded from http://pubs.acs.org on July 31, 2017

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 free 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 accessible to all readers and 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.

Chemistry of Materials 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 8

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

Enhancing Thermoelectric Performance of TiNiSn Half-Heusler Compound via Modulation Doping Tanya Berry,† Chenguang Fu,*,† Gudrun Auffermann,† Gerhard H. Fecher,† Walter Schnelle,† Federico Serrano-Sanchez,† Yuan Yue,‡ Hong Liang,‡,§ and Claudia Felser† †

Max Planck Institute for Chemical Physics of Solids, Nöthnitzer Str. 40, 01187 Dresden, Germany Department of Materials Science and Engineering, Texas A&M University, 3003 TAMU, College Station, TX 778433003,USA § Department of Mechanical Engineering, Texas A&M University, 3123 TAMU, College Station, TX 77843-3123, USA ‡

ABSTRACT: MNiSn (M = Ti, Zr, and Hf) half-Heusler compounds are widely studied n-type thermoelectric materials for power generation. Most studies focus on Zr and Hf-based compounds due to their high thermoelectric performance. However, these kinds of compositions are not cost effective. Herein, the least expensive alloy in this half-Heusler family - TiNiSn - is investigated. Modulation doping of half-metallic MnNiSb in the TiNiSn system is realized by using spark plasma sintering. It is found that MnNiSb dissolves into TiNiSn matrix and forms a heavily doped Ti1-xMnxNiSn1-xSbx phase, which leads to largely enhanced carrier concentration and also slight increase of carrier mobility. As a result, the electrical conductivity and power factor of the modulation doped compounds are greatly improved. A maximum power factor of 45 × 10-4 WK-2m-1 is obtained at 750 K for the modulation doping system (TiNiSn)1-x+(MnNiSb)x with x = 0.05, which is one of the highest reported values in literature for TiNiSn systems. Furthermore, the lattice thermal conductivity is also suppressed due to the enhanced phonon scattering. Beneficial from the improved power factor and suppressed lattice thermal conductivity, a peak zT of 0.63 is obtained at 823 K for x = 0.05, which is a ~70% increase compared to the peak zT of TiNiSn. These results highlight the potential application of inexpensive TiNiSn-based TE materials and the effectiveness of modulation doping in enhancing the TE performance of HH compounds.

INTRODUCTION Presently, about 50% of the industrial energy is wasted as heat.1 If a small fraction of this waste energy is reduced, a huge impact to the world energy frontiers will be made. Thermoelectric (TE) materials facilitate this process by converting waste heat into electricity by using a temperature gradient. The performance of a TE material is usually gauged by the dimensionless figure of merit, zT = α2σT/κ, where α, σ, T and κ are Seebeck coefficient, electrical conductivity, absolute temperature, and thermal conductivity, respectively.2 In the past two decades, several strategies have been put forward to enhance the performance of TE materials, such as phonon-glass electron-crystal, band engineering, nanostructuring, all-scale hierarchical architectures, etc.3-6 Beneficial from these “big ideas”, both the traditional Bi2Te3-based, PbTebased and SiGe alloys as well as new compounds, such as filled skutterudites, half-Heusler (HH) compounds, Zintl phase compounds, selenides, tellurides, silicides, etc., have been developed to become high performance TE materials with high zT above unity, some even higher than 2.7-25 HH compounds are ternary intermetallic compounds with typical formula XYZ, where X and Y are usually the transition metals and Z can be a main group element.26,27 The adjustable compositions of HH compounds lead to their flexible electronic structure and thus abundant physical properties, such as magnetic, thermoelectric, superconducting, and topological insulating properties, etc.28-36 As for TE application, HH compounds have the advantages of good electrical and mechanical properties combined with thermal stability. 37-38 Up to now, the

typical HH compounds, i.e. n-type MNiSn, p-type MCoSb and FeNbSb, have been developed as promising TE materials with zT > 1.0.15,39-40 Based on these materials, HH-based TE modules have been successfully assembled, which show high power density and relatively high conversion efficiency.11,31-32 The results highlight this mechanically robust and thermally stable system as promising for power generation applications. As for the n-type MNiSn-based HH alloys, the widely reported compositions with zT > 1.0 are those typically with elements Hf and Zr.36,43-45 Considering that the prices of Hf and Zr are usually higher than that of Ti, it is necessary to further develop the TE performance of TiNiSn-based HH alloys from the view of practical application. For TiNiSn HH alloys, a main disadvantage that limits its TE performance is the relatively low power factor (PF = α2σ; usually in the range of 20-40 × 10-4 Wm-1K-2) as compared to its sister compounds HfNiSn and ZrNiSn, whose peak PFs reach 50 × 10-4 Wm-1K-2.33,46-48 Therefore, to improve the PF of TiNiSn could be an effective way to further enhance its TE performance. Modulation doping, which is a technique developed in the field of two dimensional electron gas thin film devices, aiming at improving the carrier concentration and mobility.49,50 This technique has recently been introduced into TE field to enhance the electrical conductivity and thus power factor of SiGe-based TE materials by Zebarjadi et al.9 Subsequently, Pei and Wu et al. also successfully employed this strategy to improve the TE performance of BiCuSeO and BiAgSeS, respectively.51,52 In the above mentioned works, a common way to realize modulation doping is first mixing the pristine compound and heavily doped counterpart and then condensing

ACS Paragon Plus Environment


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

them to a heterogeneous composite, which is more likely a physical change process. In this study, we report a different way to realize modulation doping with chemical change. We start with independently synthesizing the pristine TiNiSn compound and halfmetallic HH alloy MnNiSb by using arc melting. With the help of ball milling, these two compounds are fully mixed according the designed ratio. After that, spark plasma sintering (SPS) is used to condense the composite. It is found that in the obtained bulk sample, MnNiSb is not kept as single phase but dissolves into the TiNiSn matrix and forms a heavily doped Ti1-xMnxNiSn1-xSbx phase. This process, which is in fact a chemical change process, is different from the above mentioned common way to realize modulation doping. In the case of this type of a modulation doping system, the dissolution of MnNiSb into the matrix causes an inflow of electrons as charge carriers and they act like an electron donor. It is found with increasing the dissolution content of MnNiSb, that the carrier concentration of the sample is largely increased while the carrier mobility is slightly enhanced, compared to the pristine TiNiSn. As consequence, this leads to improved electrical conductivity and optimized PF. A maximum PF of 45 × 10-4 Wm-1K-2 is achieved in the system with 5% MnNiSb, comparable to that of the best (Hf,Zr)NiSn HH system.

EXPERIMENTAL SECTION Synthesis. Shots of Ti (99.995%, Alfa Aesar), Ni (99.995%, Chempur), Sb (99.9999%, Alfa Aesar), and Sn (99.999%, Alfa Aesar), and Mn pieces (99.99%, Chempur) were used for the synthesis. Mn pieces were further cleaned to remove surface oxidation at 1273 K for 7 days. Stoichiometric elemental masses of Ti, Ni, and Sn and Mn, Ni, and Sb were arc melted three times to insure homogeneity. The arc-melted ingots of TiNiSn and MnNiSb were manually crushed to 300 microns under argon. The bulk powders were then mechanically pulverized by using a planetary ball-milling machine (Pulverisette 7, Fritsch) under argon. The powders were then individually mixed in different ratios to form desired compositions for the (TiNiSn)1-x+(MnNiSb)x series (x = 0, 0.01, 0.02, 0.03, 0.05, 0.075, and 0.1). These obtained powders were further mixed under argon using a Retsch MM200 mill machine to ensure homogenization. After that, the powders were placed in graphite dies with an inner diameter of 10.05 mm. Carbon foils were used to separate the sample from the die and punches. The dies were entered into the spark plasma sintering (SPS) instrument SPS-515 ET, Dr. Sinter setup [Fuji SDC (SPS Syntex), Japan] under inert conditions. The samples were heated to 1273 K with a heating rate of 200K/min under uniaxial pressure of 100 MPa under vacuum (≤10 Pa) and dwelling time of 1 min. The samples were afterwards sliced to disks with a diameter of 10 mm and a thickness of 1.25 mm. Characterization. Powder X-ray diffraction (XRD) measurements were performed with Cu Kα radiation at room temperature, using an image-plate Huber G670 Guinier camera equipped with a Ge(111) monochromator. The microstructure of the samples was examined by scanning electron microscopy (SEM) using a Philips X. Quantitative electron probe microanalysis (EPMA) of the phases was carried out by using an energy dispersive X-ray (EDX) spectroscopy analyzer (Phoenix V 5.29, EDAX) and a wavelength-dispersive spectrometer WDXS (Cameca SX 100), using the pure elements as stand-

Page 2 of 8

ards (the acceleration voltage was 25 kV, using the K- and Llines). The grain sizes and phase percentages were determined from back scattering electron (BSE) images which were obtained using SEM (JSM 7800F, JEOL). The Seebeck coefficient and resistivity were measured using a ULVAC ZEM-3 system. The thermal diffusivity was determined using laser flash analysis (LFA 457, Netzsch). The thermal conductivity was calculated using the equation κ = DρCP, where D is the thermal diffusivity, CP is the specific heat (measured using DSC, STA 449, Netzsch), and ρ is the density (AccuPyc 1330, Micromeritics, He-atmosphere). The estimated measurement uncertainties are 3% for electrical conductivity, 7% for Seebeck coefficient, 3% for thermal diffusivity, 5% for specific heat, and 1% for density, respectively. Considering that the reproducibility is smaller than the accuracy, the uncertainty of zT value is about 15%. The carrier concentration was determined based on the Hall Effect measurement using an IPMHT-Hall-900K system. Some basic parameters and room temperature thermal and electrical properties are listed in Table S1 (Supporting information, SI).

RESULTS AND DISCUSSION Realization of Modulation Doping. The method used in this work might be employed for the other TE system, at least for other HH compounds. Herein, a general process for the realization of modulation doping is illustrated schematically in Figure 1. Firstly, two compounds with different nominal compositions XYZ and MYN are synthesized independently. Then, the obtained XYZ and MYN samples are crushed and mixed according to the desired mass ratio. In the mixture, MYN powders randomly distribute and are surrounded by the matrix XYZ powders. The mixed composite powders are subsequently condensed by using SPS. The key step is the SPS process, in which a chemical displacement reaction between MYN and XYZ happens. The MYN powders dissolve into the adjacent matrix and form heavily doped X1-aMaYZ1-aNa phases, which could serve as effective carriers’ sources. In this way, the modulation doping is subtly realized. This process is different from the commonly used method in the other modulation doping works, in which no chemical displacement reaction takes place.

Figure 1. Schematic drawing of a general process for the realization of modulation doping. In this work, the final obtained bulk material after SPS contains the pristine TiNiSn, the host phase, and the heavily doped Ti1-xMnxNiSn1-xSbx, the modulation doping phase. This result is supported by the microstructure characterization, as displayed in Figure 2. The BSE images of two typical modulation doping systems with MnNiSb amount of 0.05 and 0.1 are shown in Figures 2b and 2c, respectively. It is obviously found that the matrix is still the pristine TiNiSn phase, while the


Page 3 of 8

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


heavily doped Ti1-xMnxNiSn1-xSbx phase can be seen randomly distributing in the matrix. This result justifies the realization of modulation doping in TiNiSn system. Furthermore, some other minority phases with typical compositions of TiNi2Sn and Ti6Sn5 are found from Figure 2, which can also be seen from the XRD patterns (SI, Figure S1). These minority phases are stable in the TiNiSn matrix and are repeatedly seen in different fabricating processes such as microwave synthesis, arc melting, and induction melting and even after long-term annealing. 53-58 The reason for these stable minority phases in TiNiSn might result from the fact that TiNiSn decomposes when cooling from its liquid phase.59 An interesting finding is that the undoped sample has a larger area/volume fraction of the minority phases whereas with increasing x the amount of minority phases seems to be reduced. This phenomenon can be observed from Figures 2b and 2c, in which the typical binary phase Ti6Sn5 was not as abundant as in the undoped sample. It is also seen from the XRD patterns and the Rietveld refinement results (Figures S1 and S2) that the phases of x = 0.05 and x = 0.1 are more pure compared to the undoped TiNiSn.

Figure 2. BSE images for the modulation doping system (TiNiSn)1-x+(MnNiSb)x: (a) x = 0, (b) x = 0.05, and (c) x = 0.1. In short, the most important finding in this part is that MnNiSb could dissolve into TiNiSn and form heavily doped phases after SPS. Thus, the modulation doping is successfully realized in the TiNiSn system through a chemical displacement reaction. This type of modulation doping has significant effect on the electrical properties, which is shown below. Electrical Properties. Figure 3 shows the electrical properties for the modulation doping system (TiNiSn)1-x+(MnNiSb)x. With the increased content of MnNiSb, it is found that the carrier concentration rapidly changes from 1.9 × 1020 cm-3 for x = 0 to 13 × 1020 cm-3 for x = 0.1, indicating that the halfmetallic MnNiSb dissolves into TiNiSn matrix and the obtained heavily doped Ti1-xMnxNiSn1-xSbx phase serves as an effective electron donor. Another important finding is the slightly higher carrier mobility of the modulation doped compounds (x > 0) than that of the undoped TiNiSn compound, as shown in Figure 3a. Usually, for a uniformly doped system, with increasing doping content, the enhanced ionized scattering will deteriorate its carrier mobility. In contrast, by using the modulation doping approach, the ionized impurity scattering rate can be suppressed, which consequently leads to improved carrier mobility compared to the uniformly doped counterpart.9 This might be an important reason for the slightly improved carrier mobility for the modulation doping system (TiNiSn)1-x+(MnNiSb)x. Furthermore, it is known from the above microstructure analysis that with increasing x, the amount of minority phases in the compound is reduced, especially the binary minority phases, which might also contribute to the slightly enhanced carrier mobility. Thus, due to the simultaneous increase of carrier concentration and mobility, the

electrical conductivity of the modulation doping system (TiNiSn)1-x+(MnNiSb)x is rapidly improved, as observed in Figure 3b. Deductively, the direct relation between the doping content and the electrical conductivity is due to the more metal-like behavior down in the series. For x = 0 and x = 0.01, the electrical conductivity shows a positive trending with temperature, which is typical semiconducting behavior. For the compound with higher doping content, the electrical conductivity starts to show metallic behavior. A maximum electrical conductivity of 35 × 104 Sm-1 is obtained at room temperature for x = 0.1.

Figure 3. (a) Hall carrier concentration and Hall mobility versus x, (b) electrical conductivity, (c) Seebeck coefficient, and (d) power factor for the modulation doping system (TiNiSn)1x+(MnNiSb)x. Contrary to the changing trend of electrical conductivity, the Seebeck coefficient gradually decreases as the enhanced modulation doping percentage, as seen in Figure 3c. In the temperature range from 300 to 700 K, the absolute Seebeck coefficient increases with temperature. However, from 700 to 880 K, the absolute Seebeck coefficient for x = 0.01, 0.02, 0.03, and 0.05 reduces due to the bipolar effect. The bipolar effect occurs at a lower temperature for undoped TiNiSn compared to the modulation doped samples because of its lower carrier concentration. For x = 0.075 and 0.1, the samples have a much higher bipolar temperature than what was measured and show a monotonic increase in the Seebeck coefficient, indicating a strong degenerate semiconductor behavior. The band gap of the samples is roughly estimated by Goldsmid-Sharp formula,60 which gives an average value of 0.2eV, as shown in Table S1 (SI). Figure 3d shows the power factor for modulation doping system (TiNiSn)1-x+(MnNiSb)x. Due to the largely enhanced electrical conductivity, the power factor for all the modulation doped samples is larger than the undoped one. Especially, the highest power factor of 45 × 10-4 WK-2m-1 is obtained at 750K for x = 0.05, about 67% higher than that of the undoped TiNiSn, which is comparable to that of the stateof-the-art n-type (Hf,Zr)NiSn-based HH compounds. Figure 4a shows the Pisarenko plot for the modulation doping (TiNiSn)1-x+(MnNiSb)x system at 300K, which was calculated using the single parabolic band (SPB) model under the assumption of that acoustic phonon and alloying scattering dominate the carrier transport, which give the same scattering factor of -0.5.33 A constant density of state effective mass of 2.8me, generated from the electronic band structure of TiNiSn



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 4b and Figure S3, SI), is used to calculate the theoretical curve. It is found that the experimental data for the samples with low carrier concentration agree well with the theoretical curve whereas at the high carrier concentration the experimental data deviate. Usually, the deviation might result from two reasons: one is that the effective mass of the samples with higher carrier concentration increases,61 and the other one is the change of scattering mechanism.62

Page 4 of 8

ure 5. The lattice thermal conductivity κL is obtained by subtracting the electronic component κe from the total κ and κe is calculated via the Wiedemann-Franz law, κe = LσT, where L is the Lorentz number was computed using the SPB approximation.23 The total thermal conductivity κ at room temperature shows an obvious increase with increasing modulation doping content. This is mainly due to the largely enhanced electronic thermal conductivity κe as shown in Figure 5b. The κ of all the modulation doping samples show decreasing trend with temperature, mainly resulting from the reduced κL (Figure 5c). It is worth noting that the κL of the undoped sample keeps almost constant in the investigated temperature range. The abnormal temperature independent κL of the undoped sample might relate to its complex microstructure, as shown in Figure 2a, in which several binary phases are found in the matrix. Besides the undoped sample, all the modulation doped samples show reduced κL with increasing temperature and doping content. This reduction of κL might result from the enhanced phonon scattering by the modulation doping phase. Typically at 600 K (Figure 5d), the κL reduces and the κe rises with the increase of x. This increasing rate of κe is larger than the decreasing rate of κL, which results in a larger κ for the compound with higher doping content. This explains why the κ of the samples converges at about 800 K.

Figure 4. (a) Pisarenko plot for the modulation doping system (TiNiSn)1-x+(MnNiSb)x; (b) Electronic band structure for Ti1xMnxNiSn1-xSbx (x = 0, 0.02, 0.05, 0.1). In order to identify the possible reason, electronic band structure of Ti1-xMnxNiSn1-xSbx with x ranges from 0 to 0.1 is calculated by means of SPRKKR,63 as shown in Figure 4b. Making use of the coherent potential approximation to model the mixed side occupations by Ti and Mn or Sn and Sb. The details of the calculations are described by Fecher et al.27,64 The compounds exhibit an indirect band gap with the top of the valence band at G and the bottom of the conduction band at X. The Fermi energy is shifted into the conduction band when x (corresponding to the MnSb content) becomes larger, that is, when electrons are doped into the system (note: due to the difference in the valence electrons by 4 comparing TiSn and MnSb, the number of electrons is changed by 4x). Further, the introduction of MnSb into the TiNiSn system causes a broadening of the conduction band and the appearance of impurity states. The latter are located at the bottom of the conduction band and become clearly visible for x above 5%. The broadening is due to chemical disorder scattering. The effective mass of the electrons in the conduction band at X is ~2.8me (see Figure S3, SI) and rather independent on x in the considered range below 10%. Thus, the deviation between the experimental data and theoretical curve in Figure 4a might result from the change of scattering mechanism. However, considering that this system has complex microstructure and several minority phases, the scattering mechanism is thus hardly to be determined and the exact reason for this deviation cannot be concluded here. Thermal properties. The thermal properties for the modulation doping (TiNiSn)1-x+(MnNiSb)x system are shown in Fig-

Figure 5. (a) total thermal conductivity, (b) electronic thermal conductivity, and (c) lattice thermal conductivity as a function of temperature and (d) total, lattice, and electronic thermal conductivity verses x for the modulation doping system (TiNiSn)1-x+(MnNiSb)x. TE performance. The overall figure of merit zT increases as the modulation doping concentration is improved for all the samples, as seen in Figure 6. A highest zT of 0.63 is obtained at 823K in (TiNiSn)1-x+(MnNiSb)x system with x = 0.05, increases by 70% compared to the peak zT of TiNiSn. The key contribution to this elevated zT is the largely enhanced electrical conductivity and thus optimized power factor, resulting from the modulation doping. The overall zT for all the (TiNiSn)1-x+(MnNiSb)x system with x > 0 is higher than the undoped sample, indicating the high effectiveness of modulation doping in improving the TE performance of TiNiSn system. Besides the good zT, the modulation doping samples also show good mechanical properties. The Vickers hardness is about an average of 8 GPa for all the samples (details can be seen in Figure S4, SI), which is important for the practical


Page 5 of 8

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


application. The heating and cooling measurements of the electrical transport properties display good consistency (Figure S5, SI), indicating that the samples are thermally stable in the investigated temperature range.

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

ACKNOWLEDGMENT The authors thank Horst Borrmann, Steffen Hückmann, and Yurii Prots for XRD measurement, Sylvia Kostmann, Monika Eckert, Petra Scheppan, and Ulrich Burkhardt for SEM and EDX measurements, Igor Veremchuk for his guidance in using the SPS instrumentation, Ralf Koban for transport measurements. C. Fu acknowledges financial support from the Alexander von Humboldt Foundation. This work was financially supported by the European Research Council (ERC Advanced Grant No. 291472 “Idea Heusler”).

REFERENCES

Figure 6. Thermoelectric figure of merit zT for the modulation doping system (TiNiSn)1-x+(MnNiSb)x. CONCLUSIONS In summary, the modulation doping of MnNiSb in the TiNiSn system is realized by means of SPS. Microstructure analysis justifies the existence of modulation doping phase, randomly distributing in the matrix. The carrier concentration of the modulation doping compounds is largely enhanced while carrier mobility also shows slight increase. This consequently leads to improved electrical conductivity and optimized power factor. The maximum power factor of 45 × 10-4 WK-2m-1 is obtained at 750 K for (TiNiSn)1-x+(MnNiSb)x with x = 0.05, which is one of the highest reported in literature for TiNiSn systems. Also, at high temperature, the lattice thermal conductivity is reduced due to the enhanced phonon scattering. The enhanced power factor and reduced lattice thermal conductivity contribute a large improvement of zT. A highest zT of 0.63 is obtained at 823 K for x = 0.05, which is ~70% increase compared to the peak zT of TiNiSn. These results highlight the potential application of TiNiSn-based TE materials and the effectiveness of modulation doping in enhancing the TE performance of HH compounds.

ASSOCIATED CONTENT Supporting Information on the powder XRD patterns, Rietveld refinement, effective mass calculation, and Vickers hardness, heating and cooling measurements, and table containing some properties such as density, average grain size, lattice parameter, carrier concentration, Hall mobility, effective mass, and Goldsmid-Sharp band gap of the materials are available.

AUTHOR INFORMATION Corresponding Author *[email protected]

Author Contributions

(1) Beaton, J. B. Thermoelectricity at the gate. Nat. Nanotech. 2014, 9, 876-877. (2) Snyder, G. J.; Toberer, E. S. Complex thermoelectric materials. Nat. Mater. 2008, 7, 105-114. (3) Dresselhaus, M. S.; Chen, G.; Tang, M. Y.; Yang, R.; Lee, H.; Wang, D.; Ren, Z.; Fleurial, J.-P.; Gogna, P. New directions for low-dimensional thermoelectric materials. Adv. Mater. 2007, 19, 1043-1053. (4) Yang, J.; Xi, L.; Qiu, W.; Wu, L.; Shi, X.; Chen, L.; Yang, J.; Zhang, W.; Uher, C.; Singh, D. J. On the tuning of electrical and thermal transport in thermoelectrics: an integrated theory experiment perspective. npj Comput. Mater. 2016, 2, 15015. (5) Pei, Y.; Wang, H.; Snyder, G. J. Band engineering of thermoelectric materials. Adv. Mater. 2012, 24, 6125-6135. (6) Biswas, K.; He, J.; Blum, I. D.; Wu, C. I.; Hogan, T. P.; Seidman, D. N.; Dravid, V. P.; Kanatzidis, M. G. Highperformance bulk thermoelectrics with all-scale hierarchical architectures. Nature 2012, 489, 414-418. (7) Poudel, B.; Hao, Q.; Ma, Y.; Lan, Y.; Minnich, A.; Yu, B.; Yan, X.; Wang, D.; Muto, A.; Vashaee, D.; Chen, X.; Liu, J.; Dresselhaus, M. S.; Chen, G.; Ren, Z. High-thermoelectric performance of nanostructured bismuth antimony telluride bulk alloys. Science 2008, 320, 634-638. (8) Pei, Y. Z.; Shi, X. Y.; LaLonde, A.; Wang, H.; Chen, L. D.; Snyder, G. J. Convergence of electronic bands for high performance bulk thermoelectrics. Nature 2011, 473, 66-69. (9) Zebarjadi, M.; Joshi, G.; Zhu, G. H.; Yu, B.; Minnich, A.; Lan, Y. C.; Wang, X. W.; Dresselhaus, M.; Ren, Z. F.; Chen, G. Power Factor Enhancement by Modulation Doping in Bulk Nanocomposites. Nano Lett. 2011, 11, 2225-2230. (10) Shi, X.; Yang, J.; Salvador, J. R.; Chi, M.; Cho, J. Y.; Wang, H.; Bai, S.; Yang, J.; Zhang, W.; Chen, L. Multiplefilled skutterudites high thermoelectric figure of merit through separately optimizing electrical and thermal transports. J. Am. Chem. Soc. 2011, 133, 7837-7846. (11) Fu, C.; Bai, S.; Liu, Y.; Tang, Y.; Chen, L.; Zhao, X.; Zhu, T. Realizing high figure of merit in heavy-band p-type half-Heusler thermoelectric materials. Nat. Commun. 2015, 6, 8144. (12) Brown, S. R.; Kauzlarich, S. M.; Gascoin, F.; Snyder, G. J. Yb14MnSb11: New high efficiency thermoelectric material for power generation. Chem. Mater. 2006, 18, 1873-1877. (13) Chen, X.; Weathers, A.; Carrete, J.; Mukhopadhyay, S.; Delaire, O.; Stewart, D. A.; Mingo, N.; Girard, S. N.; Ma, J.; Abernathy, D. L.; Yan, J.; Sheshka, R.; Sellan, D. P.; Meng, F.; Jin, S.; Zhou, J.; Shi, L. Twisting phonons in complex crys-



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

tals with quasi-one-dimensional substructures. Nat. Commun. 2015, 6, 6723. (14) Zhao, L.-D.; Tan, G.; Hao, S.; He, J.; Pei, Y.; Chi, H.; Wang, H.; Gong, S.; Xu, H.; Dravid, V. P.; Uher, C.; Snyder, G. J.; Wolverton, C.; Kanatzidis, M. G. Ultrahigh power factor and thermoelectric performance in hole-doped single-crystal SnSe. Science 2016, 351, 141-144. (15) 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, 1605884. (16) Gelbstein, Y.; Tunbridge, J.; Dixon, R.; Reece, M. J.; Ning, H.;Gilchrist, R.; Summers, R.; Agote, I.; Lagos, M. A.; Simpson, K.; et al.Physical, Mechanical, and Structural Properties of Highly Efficient Nanostructured N- and P-Silicides for Practical Thermoelectric Applications. J. Electron. Mater. 2014, 43, 1703−1711 (17) Appel, O.; Zilber, T.; Kalabukhov, S.; Beeri, O.; Gelbstein, Y.; Morpholoical effects on the thermoelectric properties of Ti0.3Zr0.35Hf0.35Ni1+Sn alloys following phase separation, J. Mater. Chem. C 2015, 3, 11653-11659. (18) Gelbstein, Y. Phase Morphology Effects on the Thermoelectric Properties of Pb0.25Sn0.25Ge0.5Te. Acta Mater. 2013, 61, 1499−1507. (19) Hazan, E.; Ben-Yehuda, O.; Madar, N.; Gelbstein, Y. Functional Graded Germanium-Lead Chalcogenide-Based Thermoelectric Module for Renewable Energy Applications. Adv. Energy Mater. 2015, 5,1500272. (20) Samanta, M.; Biswas, K. Low thermal conductivity and high thermoelectric performance in (GeTe)1-2x(GeSe)x(GeS)x: competition between solid solution and phase separation. J. Am. Chem. Soc. 2017, 139, 9382-9391 (21) Banik, A.; Shenoy, U. S.; Saha, S.; Waghmare, U. V.; Biswas, K. High power factor and enhanced thermoelectric performance of SnTe-AgInTe2: synergistic effect of resonance level and valence band convergence. J. Am. Chem. Soc. 2016, 138, 13068−13075. (22) Hu, Y.; Wang, J.; Kawamura, A.; Kovnir, K.; Kauzlarich, S. M. Yb14MgSb11 and Ca14MgSb11—New Mg-containing Zintl compounds and their structures, bonding, and thermoelectric properties. Chem. Mater. 2015, 27, 343-351. (23) Grebenkemper, J. H.; Hu, Y.; Barrett, D.; Gogna, P.; Huang, C.; Bux, S. K.; Kauzlarich, S. M. High temperature thermoelectric properties of Yb14MnSb11 prepared from reaction of MnSb with the elements. Chem. Mater. 2015, 27, 5791-5798. (24) Fu, C.; Wu, H.; Liu, Y.; He, J.; Zhao, X.; Zhu, T. Enhancing the figure of merit of heavy-band thermoelectric materials through hierarchical phonon scattering. Adv. Sci., 2016, 3, 1600035. (25) Fu, C.; Zhu, T.; Pei, Y.; Xie, H.; Wang, H.; Snyder, G. J.; Liu, Y.; Liu, Y.; Zhao, X. High Band degeneracy contributes to high thermoelectric performance in p-type half-Heusler compounds. Adv. Energy Mater. 2014, 4, 1400600. (26) Graf, T.; Felser, C.; Parkin, S. S. P. Simple rules for the understanding of Heusler compounds. Prog. Solid State Chem. 2011, 39, 1-50. (27) Ouardi, S.; Fecher, G.H.; Balke, B.;, Kozina, X.; Stryganyuk, G.; Felser, C.; Lowitzer, S.; Ködderitzsch, D.; Ebert, H.; Ikenaga, E. Electronic transport properties of electron-and hole-doped semiconducting C1b Heusler compounds: NiTi1−xMxSn (M= Sc, V). Phys. Rev. B. 2010, 82, 085108.

Page 6 of 8

(28) Chadov, S.; Qi, X.; Kübler, J.; Fecher, G. H.; Felser, C.; Zhang S. C. Tunable multifunctional topological insulators in ternary Heusler compounds. Nat. Mater. 2010, 9, 541-545. (29) Yan, B.; Visser, A. Half-Heusler topological insulators. MRS Bulletin 2014, 39, 859-866. (30) Liu, E.; Wang, W.; Feng, L.; Zhu, W.; Li, G.; Chen, J.; Zhang, H.; Wu, G.; Jiang, C.; Xu, H.; Boer, F. Stable magnetostructural coupling with tunable magnetoresponsive effects in hexagonal ferromagnets. Nat. Commun. 2012, 3, 873. (31) Singh, S.; D'Souza, S. W.; Nayak, J.; Caron, L.; Suard, E., Chadov, S.; Felser, C. Effect of platinum substitution on the structural and magnetic properties of Ni2MnGa ferromagnetic shape memory alloy. Phys. Rev. B 2016, 93, 134102. (32) Yu, C.; Zhu, T. J.; Shi, R. Z.; Zhang, Y.; Zhao, X. B.; He, J. High-performance half-Heusler thermoelectric materials. Acta Mater. 2009, 57, 2757-2764. (33) Xie, H.; Wang, H.; Pei, Y.; Fu, C.; Liu, X.; Snyder, G. J.; Zhao, X.; Zhu, T. Beneficial contribution of alloy disorder to electron and phonon transport in half-Heusler thermoelectric materials. Adv. Funct. Mater. 2013, 23, 5123-5130. (34) Shen, Q.; Chen, L.; Goto, T.; Hirai, T.; Yang, J.; Meisner, G.; Uher, C. Effects of partial substitution of Ni by Pd on the thermoelectric properties of ZrNiSn-based half-Heusler compounds. Appl. Phys. Lett. 2001, 79, 4165-4167. (35) Fu, C.; Zhu, T.; Liu, Y.; Xie, H.; Zhao, X. Band engineering of high performance p-type FeNbSb based half-Heusler thermoelectric materials for figure of merit zT > 1. Energy Environ. Sci. 2015, 8, 216-220. (36) Culp, S. R.; Poon, S. J.; Hickman, N.; Tritt, T. M.; Blumm, J. Effect of substitutions on the thermoelectric figure of merit of half Heusler phases at 800°C. Appl. Phys. Lett. 2006, 88, 042106. (37) Yang, J.; Li, H. M.; Wu, T.; Zhang, W. Q.; Chen, L. D.; Yang, J. H. Evaluation of half-Heusler compounds as thermoelectric materials based on the calculated electrical transport properties. Adv. Funct. Mater. 2008, 18, 2880-2888. (38) Rogl, G.; Grytsiv, A.; Gü rth, M.; Tavassoli, A.; Ebner, C.; Wü nschek, A.; Puchegger, S.; Soprunyuk, V.; Schranz, W.; Bauer, E.; Müller, H.; Zehetbauer, M.; Rogl, P. Mechanical properties of half-Heusler alloys. Acta Mater. 2016, 107, 178−195. (39) Chen, S.; Ren, Z. F. Recent progress of half-Heusler for moderate temperature thermoelectric applications. Mater. Today 2013, 16, 387-395. (40) Xie, W.; Weidenkaff, A.; Tang, X.; Zhang, Q.; Poon, J.; Tritt, T. Recent advances in nanostructured thermoelectric half-Heusler compounds. Nanomaterials 2012, 2, 379-412. (41) He, R.; Kraemer, D.; Mao, J.; Zeng, L.; Jie, Q.; Lan, Y.; Li, C.; Shuai, J.; Kim, H. S.; Liu, Y.; Broido, D.; Chu, C.-W.; Chen, G.; Ren, Z. Achieving high power factor and output power density in p-type half-Heuslers Nb1-xTixFeSb. Proc. Natl. Acad. Sci. U. S. A. 2016, 113, 13576-13581. (42) Populoh, S.; Brunko, O. C.; Galanzka, K.; Xie, W.; Weidenkaff, A. Half-Heusler (Ti,Zr,Hf)NiSn unileg module with high powder density. Materials 2013, 6, 1326-1332. (43) Joshi, G.; Yan, X.; Wang, H.; Liu, W.; Chen, G.; Ren, Z. Enhancement in thermoelectric figure-of-merit of an n-type half-Heusler compound by the nanocomposite approach. Adv. Energy Mater. 2011, 1, 643-647. (44) Liu, Y.; Xie, H.; Fu, C.; Snyder, G. J.; Zhao, X.; Zhu, T. Demonstration of a phonon-glass electron-crystal strategy in


Page 7 of 8

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


(Hf,Zr)NiSn half-Heusler thermoelectric materials by alloying. J. Mater. Chem. A 2015, 3, 22716-22722. (45) Makongo, J. P. A.; Misra, D. K.; Zhou, X. Y.; Pant, A.; Shabetai, M. R.; Su, X. L.; Uher, C.; Stokes, K. L.; Poudeu, P. F. P. Simultaneous large enhancements in thermopower and electrical conductivity of bulk nanostructured half-Heusler alloys. J. Am. Chem. Soc. 2011, 133, 18843-18852. (46) Bhattacharya, S.; Pope, A. L.; Littleton, R. T.; Tritt, T. M.; Ponnambalam, V.; Xia, Y.; Poon, S. Effect of Sb doping on the thermoelectric properties of Ti-based half-Heusler compounds, TiNiSn1-xSbx. Appl. Phys. Lett. 2000, 77, 24762478. (47) Kim, S.-W.; Kimura, Y.; Mishima, Y. High temperature thermoelectric properties of TiNiSn-based half-Heusler compounds. Intermetallics 2007, 15, 349-356. (48) Chen, S.; Lukas, K. C.; Liu, W.; Opeil, C. P.; Chen, G.; Ren, Z. Effect of Hf concentration on thermoelectric properties of nanostructured n-type half-Heusler materials HfxZr1-xNiSn0.99Sb0.01. Adv. Energy Mater. 2013, 3, 1210-1214. (49) People,R.; Bean, J. C.; Lang, D. V.; Sergent, A. M.; Störmer, H. L.; Wecht, K. W.; Lynch, R. T.; Baldwin, K. Modulation doping in GexSi1−x/Si strained layer heterostructures. Appl. Phys. Lett. 1984 , 45, 1231-1233. (50) Neophytou, N.; Thesberg, M. Modulation doping and energy filtering as effective ways to improve the thermoelectric power factor. J. Comput. Electron. 2016, 15, 16-26. (51) Pei, Y. L.; Wu, H. J.; Wu, D.; Zheng, F. S.; He, J. Q. High thermoelectric performance realized in a BiCuSeO system by improving carrier mobility through 3D modulation doping. J. Am. Chem. Soc. 2014, 136, 13902-13908. (52) Wu, D.; Pei, Y. L.; Wang, Z.; Wu, H. J.; Huang, L.; Zhao, L. D.; He, J. Q. Significantly enhanced thermoelectric performance in n-type heterogeneous BiAgSeS composites. Adv. Funct. Mater. 2014, 24, 7763-7771. (53) Berry, T.; Ouardi, S.; Fecher, G.H.; Balke, B.; Kreiner, G.; Auffermann, G.; Schnelle, W; Felser, C. Improving thermoelectric performance of TiNiSn by mixing MnNiSb in the half-Heusler structure. Phys. Chem. Chem. Phys., 2017, 19, 1543-1550. (54) 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.; Pollock, T. M.; Seshadri, R.; Stucky, G. D. Rapid microwave preparation of thermoelectric TiNiSn and TiCoSb half-Heusler compounds. Chem. Mater. 2012, 24, 2558-2565. (55) Douglas, J. E.; Birkel, C. S.; Miao, M.-S.; Torbet, C. J.; Stucky, G. D.; Pollock, T. M.; Seshadri, R. Enhanced thermoelectric properties of bulk TiNiSn via formation of a TiNi2Sn second phase. Appl. Phys. Lett. 2012, 101, 183902. (56) Gelbstein, Y.; Tal, N.; Yarmek, A.; Rosenberg, Y.; Dariel, M. P.; Ouardi, S.; Balke, B.; Felser, C.; Kohne, M. Thermoelectric properties of spark plasma sintered composites based on TiNiSn half-Heusler alloys. J. Mater. Res. 2011, 26, 1919-1924. (57) Schwall, M.; Balke, B. Phase separation as a key to a thermoelectric high efficiency. Phys. Chem. Chem. Phys. 2013, 15, 1868-1872. (58) Dowine, R.A.; Smith, R.I.; MacLaren, D.A.; Bos, J-W. G. Metal distributions, efficient n-type doping and evidence for in-gap states in TiNiMySn (M = Co, Ni, Cu) half-Heusler nanocomposites. Chem. Mater. 2015, 27, 2449-2459. (59) Jung, D.-Y.; Kurosaki, K.; Kim, C.-E.; Muta, H.; Yamanaka, S. Thermal expansion and melting temperature of the

half-Heusler compounds: MNiSn (M = Ti, Zr, Hf). J. Alloys Compd. 2010, 489, 328-331. (60) Goldsmid, H.J.; Sharp, J.W. Estimation of the thermal band gap of a semiconductor from Seebeck measurements. J. Electron. Mater. 1999, 28, 869-872. (61) Heremans, J. P.; Jovovic, V.; Toberer, E. S.; Saramat, A.; Kurosaki, K.; Charoenphakdee, A.; Yamanaka, S.; Snyder, G. J. Enhancement of thermoelectric efficiency in PbTe by distortion of the electronic density of states. Science 2008, 321, 554557. (62) Suh, J.; Yu, K. M.; Fu, D.; Liu, X.; Yang, F.; Fan, J.; Smith, D. J.; Zhang, Y.H.; Furdyna, J. K.; Dames, C.; Walukiewicz, W. Simultaneous enhancement of electrical conductivity and thermopower of Bi2Te3 by multifunctionality of native defects. Adv. Mater., 2015, 27, 3681-3686. (63) Ebert, H.; Koedderitzsch, D.; Minar, J. Calculating condensed matter properties using the KKR-Green's function method—recent developments and applications. Rep. Prog. Phys. 2011, 74, 096501. (64) Fecher, G.H.; Rausch, E.; Balke, B.; Weidenkaff, A.; Felser, C. Half-Heusler materials as model systems for phase-separated thermoelectrics. Phys. Status Solidi A 2015, 213, 716-731.



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

TOC graphic


Page 8 of 8

Enhancing Thermoelectric Performance of TiNiSn Half-Heusler

Recommend Documents