Enhancing Thermoelectric Performance of TiNiSn Half-Heusler

Article pubs.acs.org/cm

Enhancing Thermoelectric Performance of TiNiSn Half-Heusler Compounds 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, Texas 77843-3003, United States § Department of Mechanical Engineering, Texas A&M University, 3123 TAMU, College Station, Texas 77843-3123, United States ‡

S Supporting Information *

ABSTRACT: MNiSn (M = Ti, Zr, and Hf) half-Heusler (HH) compounds are widely studied n-type thermoelectric (TE) 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 the 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 W K−2 m−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 an ∼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.

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INTRODUCTION Presently, about 50% of 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 the 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”, the traditional Bi2Te3-based, PbTe-based, 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 © 2017 American Chemical Society

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, ptype MCoSb, and FeNbSb, have been developed as promising TE materials with zT > 1.0.15,39,40 On the basis of these Received: June 27, 2017 Revised: July 27, 2017 Published: July 31, 2017 7042

DOI: 10.1021/acs.chemmater.7b02685 Chem. Mater. 2017, 29, 7042−7048

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Chemistry of Materials

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 200 K/min under uniaxial pressure of 100 MPa under vacuum (≤10 Pa) and dwelling time of 1 min. The samples were afterward 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 standards (the acceleration voltage was 25 kV, using the K- and L-lines). 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 an 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, Heatmosphere). 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 the zT value is about 15%. The carrier concentration was determined based on the Hall effect measurement using an IPMHT-Hall-900 K system. Some basic parameters and room temperature thermal and electrical properties are listed in Table S1 (Supporting Information).

materials, HH-based TE modules have been successfully assembled, which show high power density and relatively high conversion efficiency.11,41,42 The results highlight this mechanically robust and thermally stable system as promising for power generation applications. As for the n-type MNiSnbased 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 W m−1 K−2) as compared to its sister compounds HfNiSn and ZrNiSn, whose peak PFs reach 50 × 10−4 W m−1 K−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, aims at improving the carrier concentration and mobility.49,50 This technique has recently been introduced into the 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 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 half-metallic 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 a consequence, this leads to improved electrical conductivity and optimized PF. A maximum PF of 45 × 10−4 W m−1 K−2 is achieved in the system with 5% MnNiSb, comparable to that of the best (Hf,Zr)NiSn HH system.

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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. First, two compounds with different nominal

EXPERIMENTAL SECTION

Figure 1. Schematic drawing of a general process for the realization of modulation doping.

Synthesis. Shots of Ti (99.995%, Alfa Aesar), Ni (99.995%, Chempur), Sb (99.9999%, Alfa Aesar), 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 ensure homogeneity. The arc-melted ingots of TiNiSn and MnNiSb were manually crushed to 300 μm 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

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 7043


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Chemistry of Materials 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. 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

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)1−x + (MnNiSb)x.

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 S m−1 is obtained at room temperature for x = 0.1. 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 the Goldsmid−Sharp

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.

modulation doping systems with MnNiSb amounts of 0.05 and 0.1 are shown in Figure 2b,c, respectively. It is obviously found that the matrix is still the pristine TiNiSn phase, while the heavily doped Ti1−xMnxNiSn1−xSbx phase can be seen randomly distributing in the matrix. This result justifies the realization of modulation doping in the 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 (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 Figure 2b,c, 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 those of the undoped TiNiSn. 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 half-metallic MnNiSb dissolves into the TiNiSn matrix and the obtained heavily doped Ti1−xMnxNiSn1−xSbx phase serves as an 7044


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Chemistry of Materials formula,60 which gives an average value of 0.2 eV, as shown in Table S1. Figure 3d shows the power factor for the 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 that of the undoped one. Especially, the highest power factor of 45 × 10−4 W K−2 m−1 is obtained at 750 K for x = 0.05, about 67% higher than that of the undoped TiNiSn, which is comparable to that of the state-of-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 300 K, which was

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) and rather independent of 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 hard to determine, 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 Figure 5. The lattice thermal conductivity κL obtained

Figure 4. (a) Pisarenko plot for the modulation doping system (TiNiSn)1−x + (MnNiSb)x; (b) electronic band structure for Ti1−xMnxNiSn1−xSbx (x = 0, 0.02, 0.05, 0.1). 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 versus x for the modulation doping system (TiNiSn)1−x + (MnNiSb)x.

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 states effective mass of 2.8me, generated from the electronic band structure of TiNiSn (Figure 4b and Figure S3), 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 To identify the possible reason, electronic band structure of Ti1−xMnxNiSn1−xSbx with x ranging 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

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 that 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 remains 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 7045


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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. 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

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.7b02685. Powder XRD patterns, Rietveld refinement, effective mass calculation, 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 (PDF)

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AUTHOR INFORMATION

Corresponding Author

*(C. Fu) E-mail: [email protected]. ORCID

Chenguang Fu: 0000-0002-9545-3277 Author Contributions

Figure 6. Thermoelectric figure of merit zT for the modulation doping system (TiNiSn)1−x + (MnNiSb)x.

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

The authors declare no competing financial interest.

823 K in the (TiNiSn)1−x + (MnNiSb)x system with x = 0.05, increasing 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 that of the undoped sample, indicating the high effectiveness of modulation doping in improving the TE performance of the 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), which is important for the practical application. The heating and cooling measurements of the electrical transport properties display good consistency (Figure S5), indicating that the samples are thermally stable in the investigated temperature range.

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ACKNOWLEDGMENTS 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, and 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”).

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REFERENCES

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CONCLUSIONS In summary, the modulation doping of MnNiSb in the TiNiSn system is realized by means of SPS. Microstructure analysis justifies the existence of the modulation doping phase, randomly distributing in the matrix. The carrier concentration of the modulation doping compounds is largely enhanced while carrier mobility also shows a slight increase. This consequently leads to improved electrical conductivity and optimized power factor. The maximum power factor of 45 × 10−4 W K−2 m−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 to 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. 7046


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Enhancing Thermoelectric Performance of TiNiSn Half-Heusler

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