Enhancement in Thermoelectric Figure-Of-Merit of an N-Type Half-Heusler Compound by the Nanocomposite Approach

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1 Enhancement in Thermoelectric Figure-Of-Merit of an N-Type Half-Heusler Compound by the Nanocomposite Approach Giri Joshi, Xiao Yan, Hengzhi Wang, Weishu Liu, Gang Chen,* and Zhifeng Ren* An enhancement in the dimensionless thermoelectric figure-of-merit ( ZT ) of an n-type half-heusler material is reported using a nanocomposite approach. A peak ZT value of 1.0 was achieved at 600 C 700 C, which is about 25% higher than the previously reported highest value. The samples were made by ball-milling ingots of composition Hf 0.75 Zr0.25NiSn0.99Sb 0.01 into nanopowders and hot-pressing the powders into dense bulk samples. The ingots were formed by arc-melting the elements. The ZT enhancement mainly comes from reduction of thermal conductivity due to increased phonon scattering at grain boundaries and crystal defects, and optimization of antimony doping. 1. Introduction Half-Heuslers (HHs) are intermetallic compounds which have great potential as high-temperature thermoelectric materials for power generation. [ 1 ] However, the dimensionless thermoelectric figure-of-merit (ZT) of HHs is lower than that of most stateof-the-art thermoelectric materials. [ 2 9 ] HHs are complex compounds: MCoSb and MNiSn, where M can be Ti, Zr, Hf, or a combination of two or three of these elements. The compounds form in a cubic crystal structure with an F4/3m (no. 216) space group. These phases are semiconductors with an 18 valenceelectron count (VEC) per unit cell and a narrow energy gap, and the Fermi level is slightly above the top of the valence band. [ ] So, HH phases have a decent Seebeck coefficient with moderate electrical conductivity. The performance of thermoelectric materials depends on ZT, defined by ZT = (S 2 σ /κ )T, [ 15 ] where σ is the electrical conductivity, S the Seebeck coefficient, κ the thermal conductivity, and T the absolute temperature. Half- Heusler compounds could be good thermoelectric materials due to their high power factor ( S 2 σ ). It has been reported that the MNiSn phases are promising n-type thermoelectric materials with exceptionally large power factors, [ 16 ] and MCoSb phases are promising p-type materials. [ 17 ] In recent years, Dr. G. Joshi, Dr. X. Yan, Dr. H. Wang, Dr. W. S. Liu, Prof. Z. F. Ren Department of Physics Boston College, Chestnut Hill Massachusetts 02467, USA renzh@bc.edu Prof. G. Chen Department of Mechanical Engineering Massachusetts Institute of Technology Cambridge, Massachusetts 02139, USA gchen2@mit.edu DOI: /aenm different approaches have been reported to improve the ZT of half-heusler compounds, mainly by optimizing the compositions. [ ] However, the observed peak ZT is only around 0.5 for p-type materials and 0.8 for n-type materials due to their relatively high thermal conductivity. [ 24, 25 ] By using the nanocomposite approach, we have achieved a significant ZT improvement from 0.5 to 0.8 in p-type HHs. [ 26 ] Here we show a 25% improvement in peak ZT, from 0.8 to 1.0 at 600 C 700 C, in n-type half-heusler compounds by the same nanocomposite approach. [ 2, 6, 7, 9, ] The ZT enhancement is mainly due to the reduction in thermal conductivity. These nanostructured HH samples are prepared by a direct current (dc) hot-pressing of ball-milled nanopowder from ingots, which are initially made by an arc-melting process. The hot-pressed dense bulk samples are nanostructured and consist of grains a couple of hundreds of nanometers in size with random orientations. 2. Results and Analyses In this work, we have used the nanostructure approach to reduce the lattice thermal conductivity, along with the optimization of antimony concentrations, to optimize the electrical conductivity for the highest power factor. Since an ingot of composition Hf 0.75 Zr 0.25 NiSn Sb0.025 has the previously reported best n-type HH with a peak ZT of 0.8, [ 25 ] nanostructured samples of compositions Hf 0.75 Zr 0.25 NiSn 1 x Sb x (x = 0.005, 0.01, and 0.025) were prepared and measured, and we observed that the best ZT values are obtained in the compound with composition Hf 0.75 Zr 0.25 NiSn 0.99 Sb 0.01 due to the nanostructuring process and the optimization of antimony concentration. Herein we present the temperature-dependent thermoelectric properties of n-type half-heusler samples of composition Hf 0.75 Zr 0.25 NiSn 0.99 Sb0.01. Figure 1 shows the X-ray diffraction (XRD) patterns and scanning electron microscope (SEM) image of the fractured surface of arc-melted ingot, SEM image of the ball-milled powder with a transmission electron microscope (TEM) image, and the SEM image of the fractured surface of the hot-pressed samples. The XRD patterns (Figure 1 a) clearly show that the sample is completely alloyed after arc-melting, and the peaks are well-matched with those of HH phases. [ 24 ] Figure 1b clearly shows that the ingot has large particles ranging from 10 micrometers upwards. These large particles are easily broken 643

2 Figure 1. a) XRD patterns: lower curve: arc-melted ingot; middle curve: ball-milled powder; and upper curve: hot-pressed sample. b) SEM images of the fractured surface of arc-melted ingot, c) SEM image of ball-milled nanopowder with TEM image as inset, and d) SEM image of the fractured surface of hot-pressed Hf 0.75 Zr0.25NiSn 0.99 Sb 0.01 samples. samples (runs 1, 2, and 3, which are prepared by the same procedure) with a composition of Hf 0.75 Zr 0.25 NiSn 0.99 Sb 0.01, in comparison with the values for a reference sample which is the previously reported best n-type half- Heusler ingots with a composition of Hf 0.7 5Zr 0.25 NiSn Sb [ 25 ] These nanostructured and ingot samples are measured by the same measurement systems. Figure 3 a clearly shows that the electrical conductivity of the nanostructured Hf 0.75 Zr 0.25 NiSn 0.99 Sb 0.01 samples is much lower than that of the ingot sample, which is desirable for lower electronic contribution to the thermal conductivity. The Seebeck coefficient of the nanostructured Hf 0.75 Zr 0.25 NiSn 0.99 Sb 0.01 samples is higher than that of the ingot sample (Figure 3 b). This fact could be due to the lower doping (antimony) concentration. As a result, the power factor of the nanostructured Hf 0.75 Zr 0.25 NiSn 0.99 Sb 0.01 samples is almost the same as that of the Hf 0.75 Zr 0.25 NiSn Sb ingot sample (Figure 3 c). However, the thermal conductivity of the nanostructured Hf 0.75 Zr 0.25 NiSn 0.99 Sb 0.01 sample is significantly lower than that of the Hf 0.75 Zr 0.25 NiSn Sb ingot sample (Figure 3 d). The lower thermal conductivity of nanostructured Hf 0.75 Zr 0.25 NiSn 0.99 Sb 0.01 samples is due to both the lower electrical into nanoparticles by ball-milling (Figure 1 c) with a grain size of around 50 nm (inset of Figure 1 c), and significant grain growth takes place during the hot-pressing process (Figure 1 d). Moreover, TEM has been carried out to study the microstructures of the hot-pressed samples. Figure 2 shows low-, and high-magnification TEM images of the hot-pressed Hf 0.75 Zr 0.25 NiSn 0.99 Sb 0.01 samples. The TEM images (Figure 2 a) confirm the grain size observed from the SEM image, which is in the range of around nm, the existence of clear crystalline grain boundaries (Figure 2 b, the inset shows grain 1 is also crystalline even though it looks amorphous due to a different orientation when the image was taken), some precipitates or aggregates in the matrix (Figure 2 c), and the discontinuous heavily distorted crystal lattice, marked by arrows (Figure 2 d). The small grains, precipitates, and lattice distortions are desirable for lower thermal conductivity due to a possible increase in phonon scattering. Figure 3 shows the temperature-dependent electrical conductivity, Seebeck coefficient, power factor, thermal conductivity, lattice thermal conductivity, and ZT values of the three ball-milled and hot-pressed nanostructured Figure 2. a) Low- and b d) high-magnification TEM images of nanostructured Hf 0.75 Zr 0.25 NiSn 0.99 Sb 0.01 samples made by ball-milling and hot-pressing. The inset in (b) shows the crystalline nature of grain 1 with a rotation. The inset in d) shows the grains as good crystalline structures. 644

3 Figure 3. a) Temperature-dependent electrical conductivity, b) Seebeck coeffi cient, c) power factor, d) total thermal conductivity, e) lattice thermal conductivity, and f) ZT values of three nanostructured Hf 0.75 Zr 0.25 NiSn 0.99 Sb 0.01 samples, and the sample annealed at 800 C for 12 hours in air; the line is for viewing guidance only, for comparison with the ingot sample (open circles) which matches the previously reported best n-type half-heusler composition. [ 25 ] conductivity and the expected stronger grain-boundary scattering that results in lower lattice thermal conductivity (Figure 3 e). The lattice thermal conductivity is calculated by subtracting the carrier and bipolar contributions from the total thermal conductivity, [ 30 ] where the carrier contribution is obtained from Wiedemann Franz law by using the temperature-dependent Lorenz number, [ 31 ] and the bipolar contribution is taken into account by assuming κ lattice T 1. [ 30 ] Since both the ingot and nanostructured samples are heavily doped (degenerate semiconductors), a single band approximation is used to calculate the Lorenz number. As a result, a peak ZT of around 1.0 at 600 C 700 C (Figure 3 f) is observed, which is about 25% higher than that of the ingot sample. This enhancement in ZT by ballmilling and hot-pressing is mainly due to the reduction in lattice and electronic thermal conductivities. Figure 3 shows that the results of nanostructured samples are reproducible 645

4 Figure 4. a) Temperature-dependent specifi c heat capacity and b) thermal diffusivity of arcmelted and then ball-milled and hot-pressed Hf 0.75 Zr 0.25 NiSn 0.99 Sb 0.01 samples, and the sample annealed at 800 C for 12 hours in air, in comparison with the ingot sample. within the experimental errors, and also includes the results of an annealed nanostructured sample (run 1), which does not show any significant degradation in thermoelectric properties after annealing. The sample was annealed at 800 C for 12 hours in air, which is an accelerating condition since the application temperature is expected to be at or below 700 C. Here, we also show the temperature-dependent specific heat capacity and thermal diffusivity of nanostructured Hf 0.75 Zr 0.25 NiSn 0.99 Sb 0.01 samples in comparison with the ingot sample. Figure 4 clearly shows that the specific heat capacity of nanostructured Hf 0.75 Zr 0.25 NiSn 0.99 Sb 0.01 samples is almost the same as that of the ingot sample (Figure 4 a), and these values agree fairly well with the Dulong and Petit value of specific heat capacity (solid line). However, the thermal diffusivity of nanostructured Hf 0.75 Zr 0.25 NiSn 0.99 Sb 0.01 samples is significantly lower (Figure 4 b) than that of the ingot sample due to the small grain-size effect and lower electronic contribution. Since the size of the nanoparticles is essential in reducing the thermal conductivity to achieve higher ZT values, it is possible to further increase the ZT value of the n-type half-heusler compounds by making the grains even smaller. In this report, we made grains of 200 nm and greater (Figure 2 a). It would be possible to achieve a grain size of less than 100 nm by preventing grain growth during hot-pressing if the correct grain growth inhibitor were found. 4. Experimental Section Half-Heusler phases were prepared by melting hafnium (Hf; 99.99%, Alfa Aesar), zirconium (Zr; 99.99%, Alfa Aesar) chunks, nickel (Ni; 99.99%, Alfa Aesar), tin (Sn; 99.99%, Alfa Aesar), and antimony (Sb; 99.99%, Alfa Aesar) pieces according to a composition Hf 0.75 Zr 0.25 NiSn 0.99 Sb 0.01 using an arc-melting process. The melted ingot was then milled for 1 50 hours to get the desired nanopowders by using a commercially available ball-milling machine (SPEX 8000M Mixer/Mill). The mechanically prepared nanopowders were then pressed at temperatures of C using a dc hot-press method in graphite dies with a 12.7-mm central cylindrical opening diameter to get nanostructured bulk half-heusler samples. The samples were characterized by XRD, SEM, and TEM to study their crystallinity, homogeneity, average grain size, and the grain-size distribution of the nanoparticles. These parameters signifi cantly affect the thermoelectric properties of the fi nal dense bulk samples. The volume densities of these samples were measured using an Archimedes kit. The nanostructured bulk samples were then cut into mm bars for electrical conductivity and Seebeck coeffi cient measurements on commercial equipment (Ulvac, ZEM-3), 12.7-mm-diameter discs with appropriate thickness for thermal diffusivity measurements on a laser flash system (Netzsch LFA 457) from 100 C to 700 C, and 6-mm-diameter discs with appropriate thickness for specifi c heat capacity measurements on a differential scanning calorimeter (200-F3, Netzsch Instruments, Inc.) from room temperature to 600 C (the data point at 700 C is extrapolated). The thermal conductivity was calculated as the product of the thermal diffusivity, specifi c heat capacity, and volume density of the samples. To confi rm the reproducibility of the sample preparation process and reliability of the measurements of nanocrystalline bulk samples, the same experimental conditions were repeated at least three to six times for each composition and it was found that the thermoelectric properties are reproducible within 5% under the same experimental conditions. The volume densities of three measured nanostructured samples (runs 1, 2, and 3) are 9.73, 9.70, and 9.65 g cm 3, respectively. Acknowledgements The work is funded by the Solid State Solar-Thermal Energy Conversion Center (S 3 TEC), an Energy Frontier Research Center funded by the U.S. Department of Energy, Offi ce of Science, Offi ce of Basic Energy Sciences under Award Number: DE-SC /DE-FG02 09ER46577 (ZFR and GC). 3. Conclusions A cost-effective ball-milling and hot-pressing technique has been applied to n-type HHs to improve the ZT value. 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