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Thermoelectric properties of phase separated Ti substituted Zr0.75Hf0.25NiSn0.985Sb0.015 half-Heuslers
Progress in Natural Science: Materials International xxx (xxxx) xxx–xxx HOSTED BY
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Original Research
Thermoelectric properties of phase separated Ti substituted Zr0.75Hf0.25NiSn0.985Sb0.015 half-Heuslers Rizwan Akrama,b,∗, Yonggao Yana, Mozaffar Hussainb, Xiaoyu Shea, Xinfeng Tanga a b
State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan, 430070, China Department of Physics, Air University, PAF Complex E-9, Islamabad, 44000, Pakistan
ARTICLE INFO
ABSTRACT
Keywords: Half-Heuslers Doping Sintering Thermoelectric Mobility
Sb is a very effective dopant for ZrNiSn based half-Heusler alloys. The effect of Ti substitution on Zr0.75Hf0.25NiSn0.985Sb0.015 half-Heusler (HH) semiconductor alloys has been investigated to explore the structural modifications and composition variation. TixHf0.25 Zr0.75-xNiSn0.985Sb0.015 (x = 0, 0.15, 0.30, 0.45) alloys were synthesized by induction melting. A set of samples was also annealed for comparative studies. The samples were then sintered using plasma activated sintering (PAS) technique. XRD results confirmed the existence of ZrNiSn type HH compounds. Backscattered electron (BSE) images showed phase separations in the samples. Ti substitution improved the carrier concentration and electrical conductivity of the alloys. Moreover, thermal conductivity was also significantly reduced due to the enhanced phonon scattering. Consequently, a ZT value of 1.11 at 873 K was obtained for 30% Ti substituted (annealed) sample.
1. Introduction Modern eminent attention in solid state physics based thermoelectric devices is driven by its noteworthy advantages, such as zero emissions, miniature size, stability, and non-moving parts [1–3]. For any thermoelectric device, conversion efficiency is generally evaluated in terms material performance, which can be presented in terms of the dimensionless figure of merit by the relation ZT = σα2T/κ, where α is the Seebeck coefficient, σ is the electrical conductivity, T is the absolute temperature and κ is the thermal conductivity, respectively. Low thermal conductivity κ and high power factor (σα2) are vital for achieving high performance. Half-Heuslers (HH) have been investigated by many scientists as probable candidate for advanced thermoelectric applications due to their promising thermoelectric performances, low toxicity and good chemical stability of the integral elements [4,5]. The valence electron count (VEC) of the HH alloys strongly affects their band structures. For instance, with valence electron count 8 or 18 HHs are narrow band gap semiconductors and any deviance from VEC = 18 can result in metallic conduction [6]. The most thoroughly investigated alloys for the HHs are MNiSn (M = Hf, Ti, Zr). The band gaps of these alloys are generally (0.1–0.2 eV) resulting in high power factors, i.e. good electrical properties [7]. Large effective masses are obtained from narrow band gaps, which in turn paves the way for high values of the Seebeck coefficient [8]. On the other hand,
∗
intrinsic high thermal conductivity of HHs makes them less favorable for thermoelectric applications. Besides, one of the main benefits of HH alloys is the possibility of substitution at each of the three occupied FCC sub-lattices independently. Disorders or mass fluctuations might be induced via substitution at the M and Ni position elements which may potentially reduce the thermal conductivity k. Moreover, the doping of Bi or Sb at the Sn position has been found to be effective in optimizing the electrical properties and tuning the band structure [9]. Also, phonon scattering can be enhanced by means of additional phonon scattering centers which will result in further reduction of thermal conductivity [10,11]. These scattering centers can be introduced by secondary phases. High power factors, i.e. 2–6×10-3 WK-2 m-1 are generally achieved by HH alloys [12]. Consequently, the primary hindrance in further enhancement of the thermoelectric performance of HHs is their comparatively high thermal conductivity. For further enhancement of the performance of Sb doped ZrNiSn HH compounds (our previous work) [13], this work is focused on introducing additional mass fluctuations at the Zr-site by substitution of Ti. The effects of heat treatment are also investigated. 1.1. Experimental setup At first, starting materials composed of chunks, rods and particles of
Corresponding author. Department of Physics, Air University, PAF Complex E-9, Islamabad, 44000, Pakistan. E-mail addresses: [email protected], [email protected] (R. Akram).
https://doi.org/10.1016/j.pnsc.2019.12.007 Received 6 January 2019; Received in revised form 26 December 2019; Accepted 31 December 2019 1002-0071/ © 2020 Chinese Materials Research Society. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/BY-NC-ND/4.0/).
Please cite this article as: Rizwan Akram, et al., Progress in Natural Science: Materials International, https://doi.org/10.1016/j.pnsc.2019.12.007
Progress in Natural Science: Materials International xxx (xxxx) xxx–xxx
R. Akram, et al.
Zr, Hf, Ti, Ni, Sn and Sb were weighed in the stoichiometric proportions, TixHf0.25 Zr0.75-xNiSn0.985Sb0.015 (x = 0, 0.15, 0.30, 0.45), and put inside a quartz tube of 20 mm diameter. Then the elements were melted via high frequency induction melting under low pressure argon environment to obtain ingots. For ensuring homogeneity, induction melting was carried out at least three times for each sample. The parts of the ingots were further processed for heat treatment. The obtained ingots were then pulverized into powders and sintered by plasma activated sintering (PAS) at 1300 K for 7 min at 50 MPa to obtain high density disc shaped pellets. The phase compositions of the samples were determined by powder X-ray diffraction (XRD, Pro-PANalytical Empyrean, Netherlands). Microstructure characterizations of the bulk samples were done by field-emission scanning electron microscopy (FESEM, Hitachi SU-8020, Japan). The electrical conductivity and Seebeck coefficient were measured simultaneously by the standard four-probe method (ULVAC-RIKO ZEM-3) in Helium atmosphere. Densities of the samples were measured using the Archimedes method. Thermal diffusivity was determined using the laser flash method (NETZSCH LFA-457). The thermal conductivity was calculated from the measured thermal diffusivity D, specific heat Cp, and density d according to the relationship κ = DCpd. All measurements were performed in the temperature range from 300 K to 923 K. The errors in measurements of the electrical conductivity, Seebeck coefficient and thermal conductivity are estimated as ± 5%, ± 2% and ± 5%, respectively [13].
1.2.1. Microstructure analysis The composition and microstructure were inspected via the WDS, BSE and FESEM. Fig. 2 displays the recorded BSE images for x = 0.15 and x = 0.30. These images show the contrast contingent on the atomic element with two dissimilar sections identified: brighter area with heavier elements (higher concentration of Zr/Hf in the HH phase) and darker matrix areas with lighter elements (higher concentration of Ti in the HH phase) as identified. In contrast to the occupancy of the elements at the M-position, Tin and Nickel are uniformly dispersed. Small concentrations of Zr and Hf are found in the Ti rich parts. High concentrations of Hf and Zr are recorded in the Ti-poor matrix. Furthermore, a few tiny spots (white region) with a large amount of Hf can also be seen. Composition obtained by WDS for x = 0.30 is presented in Table 1. Michael Schwall et al. had recently conducted a comprehensive study for the phase separated HHs and our results are consistent with their findings [14]. Low-frequency phonons are responsible for heat transport. Moreover, low frequency phonons are particularly affected by the boundary scattering [15,16]. Therefore, it is very likely that the enhanced boundary scattering offered by the uneven grains (due to phase separation) can effectively limit the high thermal conductivity. Phonon transport may also be affected by the high Hf content as it may also act as additional phonon scattering spots. Fig. 3 displays the recorded BSE images of annealed samples for x = 0.30 and x = 0.45. These images show two different regions while the contrast contingent on the atomic element is identified: a brighter matrix region with heavier elements (higher concentration Zr/Hf in the HH phase) and darker regions with lighter elements (higher concentration Ti in the HH phase) as identified. Again, occupancy of the elements at the M-position, Sn and Ni are evenly dispersed over the entire sample.
1.2. Results and discussion Fig. 1 shows the XRD of TixHf0.25Zr0.75-xNiSn0.985Sn0.015 samples. All samples are well indexed to space group F4̅3 m, showing cubic crystal symmetry. However, a few traces of Hf content are also detected for some samples. Moreover, with the increase in Ti content the base of peaks was observed to be widening up.
1.2.2. Thermoelectric properties Electrical transport parameters at room temperature of the samples are given in Table 2. Carrier concentration and electrical conductivities increased with the rise in the Ti concentration. This substitution is isoelectronic, still it alters the electronic structure slightly and subsequently affects the electrical conductivities and carrier concentrations of the charge carriers. This impact is small as compared to doping with electrons by replacing Sn with Sb which can lead to an increase in carrier concentration with a factor 4 times the substitution level [17]. This trend of increase in electrical conductivity and carrier concentration is also consistent with the findings of Sakurada et al. [18]. The mobilities of samples were observed to decrease with rise in Ti content. This was due to the enhanced scattering caused by addition of a different species i.e., Ti in the matrix and by the increase in carrier concentration. Annealed samples have shown better electrical conductivities and carrier concentrations due to the increased homogeneity of the matrix. Single parabolic band (SPB) has been used for the approximation of effective mass and Lorenz number for the HHs (Hf, Zr) NiSn [19] and (Ti, Zr, Hf) NiSn [20,21] systems. The values of effective mass and Lorenz number (given in Table 2) were also calculated by single parabolic band model (SPB) assumption. The basic equations relevant to the SPB are:
=±
kB e
Fi ( F ) = 0
F
Fig. 1. XRD patterns of sintered TixHf0.25 Zr0.75-xNiSn0.985Sb0.015. XRD of annealed samples is presented at the bottom.
x i dx 1 + exp (x
F)
= EF (kB T )
n=
2
(r + 5/2) Fr + 3/2 ( F ) (r + 3/2) Fr + 1/2 ( F )
F
4 (2kB Tm )3/2 F1/2 ( F ) h3
(1)
(2) (3) (4)
Progress in Natural Science: Materials International xxx (xxxx) xxx–xxx
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Fig. 2. Typical backscattered electron images of the polished surfaces of TixHf0.25 Zr0.75-xNiSn0.985Sb0.015 (a) x = 0.15 (b) x = 0.30. Table 1 Distribution of elements as obtained by WDS for x = 0.30.
Bright region (Ti-poor) Dark region (Ti-rich)
L=
kB e
2
(r + ) F (r + ) F 7
3
Ti(at%)
Zr(at%)
Hf(at%)
Ni(at%)
Sn(at%)
Sb(at%)
Detected Phase
4.95 22.65
15.07 5.97
12.97 4.71
33.45 32.91
32.73 32.19
0.54 0.33
Ti0.15Hf0.39Zr0.46NiSn0.98Sb0.017 Ti0.69Hf0.13Zr0.18 NiSn0.99Sb0.01
2
r+ 5 2 ( F )
2
r+ 1 2 ( F )
(r + ) F (r + ) F 5 3
2
r+ 3 2 ( F )
2
r+ 1 2 ( F )
Table 2 Carrier concentration,n, mobility, μH, electrical conductivity,σ, effective mass, m*/m, and Lorenz number of the TixHf0.25 Zr0.75-xNiSn0.985Sb0.015at room temperature.
2
(5)
where, Fi (ηF) is the Fermi integral, kB is the Boltzmann constant, ηF is the reduced Fermi level, m* is the effective mass, e is the electronic charge, h is the Plank constant, T is the absolute temperature and r is the scattering factor. The values of effective masses were in the range 3.3 ± 0.2m0. These values are very close to effective masses of (Hf, Zr) NiSn system in the literature and may suggest band structures with no major changes which can justify the applicability of SPB. The values of Lorenz number for any particular Ti concentration showed slight difference which was related to their slight dissimilar Seebeck coefficients (Fig. 4b), as Seebeck coefficient and the Lorenz number are correlated. Fig. 4a represents the electrical conductivities. Highly doped semiconductor trend is observed from these curves. Generally, with the rise in temperature the electrical conductivity decreases. This suggests stronger electron scattering at higher temperatures. Moreover, with further increase in temperature above 700 K, the decrease in electrical conductivity was comparatively less slow and for x = 0.15 and x = 0.45 there was a rise again in the electrical conductivity. This is
TixHf0.25Zr0.75-x NiSn0.985Sb0.015
n(1020cm-3)
σ(104S/m)
μH(cm2V-1s-1)
m*/m0
Lorenz number, L (108 2 -2 V K )
x x x x x x x x
4.70 4.92 5.39 6.63 4.95 5.71 6.05 6.77
19.80 20.15 22.01 24.98 20.49 23.13 23.97 24.81
26.55 25.76 25.35 23.51 25.93 25.16 23.83 23.13
3.23 3.39 3.45 3.37 3.28 3.43 3.46 3.45
1.88 1.87 1.86 1.96 1.87 1.92 1.95 1.96
= = = = = = = =
0 0.15 0.30 0.45 0-An 0.15-An 0.30-An 0.45-An
probably owed to thermal excitation which steadily increased carrier concentration. Fig. 4b displays the temperature dependence of the Seebeck coefficient. For x = 0, 0.15 and 0.30 Seebeck coefficient had shown slight increase in their respective values at 300 K. But for
Fig. 3. Typical backscattered electron images of the polished surfaces of annealed TixHf0.25 Zr0.75-xNiSn0.985Sb0.015 (a) x = 0.30 (b) x = 0.45. 3
Progress in Natural Science: Materials International xxx (xxxx) xxx–xxx
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Fig. 4. Temperature dependent (a) Seebeck coefficient (b) electrical conductivity (c) power factor and (d) thermal conductivity for TixHf0.25 Zr0.75-xNiSn0.985Sb0.015.
x = 0.45 the Seebeck coefficient value had decreased significantly. This could be due to the significant increase of carrier concentration for x = 0.45. Also, for x = 0.30 maximum Seebeck coefficient value was obtained i.e., -166.38 μV/K at 873 K. The temperature dependence of calculated power factor α2σ is presented in Fig. 4c x = 0.30 showed the highest power factor of 4.54×10-3 W/mK2 at 773 K. Thermal conductivities of the samples are presented in Fig. 4d. The samples of x = 0, 0.15 and 0.30 essentially showed the same trend but there was quite significant difference between their respective thermal conductivity values due to the extra alloying introduced by substitution of 15% and 30% Ti. The room temperature thermal conductivity for x = 0.45 was almost the same but it increased rapidly due to contributions from the electronic and bipolar thermal conductivity. Consequently, x = 0.30 exhibited the lowest thermal conductivity value of 3.66 W/mK recorded at 673 K, respectively. We further estimated the lattice and electronic thermal conductivity. The Lorenz number was calculated by employing the SPB model as discussed earlier. Estimated κL is thus shown in Fig. 5. Lattice thermal conductivities of the samples were observed to be significantly reduced with the lowest κL value of 2.41W/mK at 300 K for x = 0.45. Corresponding ZT values are plotted in Fig. 6. High ZT values are primarily attributed to the reduced thermal conductivity which was achieved through additional scattering centers created by the substitution of Ti, and improved power factor. Thus, x = 0.30 exhibited peak ZT value of 1.02 at a temperature of 923 K. TE properties of the annealed samples are discussed now. Fig. 7a represents the electrical conductivities of the samples. Behavior of highly doped semiconductors is apparent. Generally, with the rise in temperature the electrical conductivity decreases, which point towards stronger scattering of the electrons at higher temperatures. With further increase in temperature above 700 K, the decrease in electrical
Fig. 5. Temperature dependent lattice thermal conductivity.
conductivity was comparatively less slow and for all the samples. This is probably due to thermal excitation which may gradually increase carrier concentration. Fig. 7b displays the temperature dependence of the Seebeck coefficient. With the increase in Ti content Seebeck was observed to decrease at the room temperature range. The Seebeck coefficient increased continuously until a maximum value at around 773 K, after which it decreased due to the minority carrier contribution. The peak value of Seebeck coefficient was obtained for x = 0, which is -193.31 μV/K at 873 K. The temperature dependence of calculated power factor α2σ is presented in Fig. 7c. Here again x = 0 showed the highest power factor of 5.38×10-3 W/mK2 at 773 K. 4
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Fig. 8. Temperature dependent lattice thermal conductivity of annealed samples.
Fig. 6. Temperature dependent ZT for TixHf0.25Zr0.75-xNiSn0.985Sb0.015.
873 K). In contrast, un-substituted sample showed its lowest lattice thermal conductivity value of 2.57 W/mK at 923 K. This effective decrease in the lattice thermal conductivity of the Ti substituted samples is attributed to the establishment of an integrated network of second half-Heusler phase within the matrix. In conclusion, with x = 0.3 (i.e., Ti0.3Zr0.45Hf0.25) at the Zr-site, phonon scattering is most effective and annealing has caused the secondary phase to get well established throughout the matrix resulting in a reduction of 62% in the lattice thermal conductivity when compared with that of the single-phase Zr0.75Hf0.25NiSn0.985Sb0.015. Corresponding ZT values are plotted in Fig. 9. High ZT values are mainly ascribed to the reduced thermal conductivity which was achieved through additional scattering centers created by the substitution of Ti. Thus, the sample with x = 0.30 exhibited peak ZT value of 1.11 at 873 K.
Thermal conductivities vs temperature of the samples is presented in Fig. 7d. All the samples essentially showed the same trend but there was quite significant difference between their respective thermal conductivity values due to the extra alloying introduced by substitution of 15%, 30% and 45% Ti. The thermal conductivity for x = 0.45 increased rapidly due to contributions from the electronic and bipolar thermal conductivity. Consequently, x = 0.30 exhibited the lowest thermal conductivity value of 3.47 W/mK recorded at 673 K, respectively. The lattice and electronic thermal conductivity were also estimated further understanding. Estimated κL is thus shown in Fig. 8. A significant reduction in κL was observed with the lowest κL value of 2.28W/mK at 300K for x = 0.30 sample. The lowest κL values were reached by samples with x = 0.3 (1.49 W/mK at 923 K) and x = 0.3-annealed sample (0.97 W/mK at
Fig. 7. Temperature dependent, (a) electrical conductivity (b) Seebeck coefficient (c) power factor and (d) thermal conductivity of annealed samples. 5
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Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements This work was financially supported by National Key Research and Development program of China (Grant No. 2018YFB0703600), and the National Natural Science Foundation of China (No. 51772232) and the 111 Project of China (Grant No. B07040). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.pnsc.2019.12.007. References
Fig. 9. Temperature dependent ZT of annealed samples for TixHf0.25 Zr0.75xNiSn0.985Sb0.015.
[1] T.M. Tritt, Science 283 (5403) (1999) 804–805. [2] J.R. Sootsman, D.Y. Chung, M.G. Kanatzidis, Angew Chem. Int. Ed. Engl. 48 (46) (2009) 8616–8639. [3] X. Su, et al., Non-equilibrium preparation, and applications (Adv. Mater. 20/2017), Adv. Mater. 29 (20) (2017) (n/a-n/a). [4] S. Chen, Z. Ren, Mater. Today 16 (10) (2013) 387–395. [5] H. Alam, S. Ramakrishna, Nano Energy 2 (2) (2013) 190–212. [6] Q. Shen, et al., Appl. Phys. Lett. 79 (25) (2001) 4165–4167. [7] S.R. Culp, et al., Appl. Phys. Lett. 88 (4) (2006) 042106. [8] G.J. Snyder, E.S. Toberer, Nat. Mater. 7 (2) (2008) 105–114. [9] H. Hohl, et al., J. Phys. Condens. Matter 11 (7) (1999) 1697. [10] Y. Liu, P.F.P. Poudeu, J. Mater. Chem. 3 (23) (2015) 12507–12514. [11] P. Ying, et al., Adv. Funct. Mater. 27 (1) (2017) 1604145–n/a. [12] E. Rausch, et al., J. Mater. Chem. C 3 (40) (2015) 10409–10414. [13] R. Akram, et al., Intermetallics 74 (2016) 1–7. [14] M. Schwall, B. Balke, Materials 11 (4) (2018). [15] H. Goldsmid, A. Penn, Phys. Lett. A 27 (8) (1968) 523–524. [16] S. Populoh, et al., Scr. Mater. 66 (12) (2012) 1073–1076. [17] E. Rausch, et al., Apl. Mater. 3 (4) (2015) 041516. [18] S. Sakurada, N. Shutoh, Appl. Phys. Lett. 86 (8) (2005) 082105. [19] H. Xie, et al., Adv. Funct. Mater. 23 (41) (2013) 5123–5130. [20] M. Gürth, et al., Acta Mater. 104 (2016) 210–222. [21] R. Akram, et al., J. Electron. Mater. 44 (10) (2015) 3563–3570.
2. Conclusions Ti is introduced as substituting element at the Zr-site for Hf0.25 Zr0.75NiSn0.985Sb0.015 compound. It is perceived that their thermal conductivity decreases due to the substitution, which eventually lead to better ZT. For the Ti substituted samples mainly single phase ZrNiSn type cubic crystal structure has been observed. With the rise in Ti content the lattice parameter has been observed to decrease, indicating the formation of a solid solution. The BSE images confirm the presence of phase separated sample which have further identified as Ti-rich and Zr-rich phases. The induction melted Ti substituted samples show a relatively high power factor of 4.54×10-3 W/mK2 at 773 K for x = 0.30. x = 0.30 also exhibits the lowest thermal conductivity, 3.66 W/mK at 673 K. The obtained ZT value is 1.02 at 923 K. For the annealed samples, a ZT value of 1.11 at 923 K is achieved for x = 0.30. The improved ZT for the annealed samples is due to the reduced thermal conductivity due to the enhanced phonon scattering induced by the inclusion of Ti in the matrix, enhanced power factor, which result from the uniformity of the grains and increase the electrical conductivities.
6
Contents lists available at ScienceDirect
Progress in Natural Science: Materials International journal homepage: www.elsevier.com/locate/pnsmi
Original Research
Thermoelectric properties of phase separated Ti substituted Zr0.75Hf0.25NiSn0.985Sb0.015 half-Heuslers Rizwan Akrama,b,∗, Yonggao Yana, Mozaffar Hussainb, Xiaoyu Shea, Xinfeng Tanga a b
State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan, 430070, China Department of Physics, Air University, PAF Complex E-9, Islamabad, 44000, Pakistan
ARTICLE INFO
ABSTRACT
Keywords: Half-Heuslers Doping Sintering Thermoelectric Mobility
Sb is a very effective dopant for ZrNiSn based half-Heusler alloys. The effect of Ti substitution on Zr0.75Hf0.25NiSn0.985Sb0.015 half-Heusler (HH) semiconductor alloys has been investigated to explore the structural modifications and composition variation. TixHf0.25 Zr0.75-xNiSn0.985Sb0.015 (x = 0, 0.15, 0.30, 0.45) alloys were synthesized by induction melting. A set of samples was also annealed for comparative studies. The samples were then sintered using plasma activated sintering (PAS) technique. XRD results confirmed the existence of ZrNiSn type HH compounds. Backscattered electron (BSE) images showed phase separations in the samples. Ti substitution improved the carrier concentration and electrical conductivity of the alloys. Moreover, thermal conductivity was also significantly reduced due to the enhanced phonon scattering. Consequently, a ZT value of 1.11 at 873 K was obtained for 30% Ti substituted (annealed) sample.
1. Introduction Modern eminent attention in solid state physics based thermoelectric devices is driven by its noteworthy advantages, such as zero emissions, miniature size, stability, and non-moving parts [1–3]. For any thermoelectric device, conversion efficiency is generally evaluated in terms material performance, which can be presented in terms of the dimensionless figure of merit by the relation ZT = σα2T/κ, where α is the Seebeck coefficient, σ is the electrical conductivity, T is the absolute temperature and κ is the thermal conductivity, respectively. Low thermal conductivity κ and high power factor (σα2) are vital for achieving high performance. Half-Heuslers (HH) have been investigated by many scientists as probable candidate for advanced thermoelectric applications due to their promising thermoelectric performances, low toxicity and good chemical stability of the integral elements [4,5]. The valence electron count (VEC) of the HH alloys strongly affects their band structures. For instance, with valence electron count 8 or 18 HHs are narrow band gap semiconductors and any deviance from VEC = 18 can result in metallic conduction [6]. The most thoroughly investigated alloys for the HHs are MNiSn (M = Hf, Ti, Zr). The band gaps of these alloys are generally (0.1–0.2 eV) resulting in high power factors, i.e. good electrical properties [7]. Large effective masses are obtained from narrow band gaps, which in turn paves the way for high values of the Seebeck coefficient [8]. On the other hand,
∗
intrinsic high thermal conductivity of HHs makes them less favorable for thermoelectric applications. Besides, one of the main benefits of HH alloys is the possibility of substitution at each of the three occupied FCC sub-lattices independently. Disorders or mass fluctuations might be induced via substitution at the M and Ni position elements which may potentially reduce the thermal conductivity k. Moreover, the doping of Bi or Sb at the Sn position has been found to be effective in optimizing the electrical properties and tuning the band structure [9]. Also, phonon scattering can be enhanced by means of additional phonon scattering centers which will result in further reduction of thermal conductivity [10,11]. These scattering centers can be introduced by secondary phases. High power factors, i.e. 2–6×10-3 WK-2 m-1 are generally achieved by HH alloys [12]. Consequently, the primary hindrance in further enhancement of the thermoelectric performance of HHs is their comparatively high thermal conductivity. For further enhancement of the performance of Sb doped ZrNiSn HH compounds (our previous work) [13], this work is focused on introducing additional mass fluctuations at the Zr-site by substitution of Ti. The effects of heat treatment are also investigated. 1.1. Experimental setup At first, starting materials composed of chunks, rods and particles of
Corresponding author. Department of Physics, Air University, PAF Complex E-9, Islamabad, 44000, Pakistan. E-mail addresses: [email protected], [email protected] (R. Akram).
https://doi.org/10.1016/j.pnsc.2019.12.007 Received 6 January 2019; Received in revised form 26 December 2019; Accepted 31 December 2019 1002-0071/ © 2020 Chinese Materials Research Society. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/BY-NC-ND/4.0/).
Please cite this article as: Rizwan Akram, et al., Progress in Natural Science: Materials International, https://doi.org/10.1016/j.pnsc.2019.12.007
Progress in Natural Science: Materials International xxx (xxxx) xxx–xxx
R. Akram, et al.
Zr, Hf, Ti, Ni, Sn and Sb were weighed in the stoichiometric proportions, TixHf0.25 Zr0.75-xNiSn0.985Sb0.015 (x = 0, 0.15, 0.30, 0.45), and put inside a quartz tube of 20 mm diameter. Then the elements were melted via high frequency induction melting under low pressure argon environment to obtain ingots. For ensuring homogeneity, induction melting was carried out at least three times for each sample. The parts of the ingots were further processed for heat treatment. The obtained ingots were then pulverized into powders and sintered by plasma activated sintering (PAS) at 1300 K for 7 min at 50 MPa to obtain high density disc shaped pellets. The phase compositions of the samples were determined by powder X-ray diffraction (XRD, Pro-PANalytical Empyrean, Netherlands). Microstructure characterizations of the bulk samples were done by field-emission scanning electron microscopy (FESEM, Hitachi SU-8020, Japan). The electrical conductivity and Seebeck coefficient were measured simultaneously by the standard four-probe method (ULVAC-RIKO ZEM-3) in Helium atmosphere. Densities of the samples were measured using the Archimedes method. Thermal diffusivity was determined using the laser flash method (NETZSCH LFA-457). The thermal conductivity was calculated from the measured thermal diffusivity D, specific heat Cp, and density d according to the relationship κ = DCpd. All measurements were performed in the temperature range from 300 K to 923 K. The errors in measurements of the electrical conductivity, Seebeck coefficient and thermal conductivity are estimated as ± 5%, ± 2% and ± 5%, respectively [13].
1.2.1. Microstructure analysis The composition and microstructure were inspected via the WDS, BSE and FESEM. Fig. 2 displays the recorded BSE images for x = 0.15 and x = 0.30. These images show the contrast contingent on the atomic element with two dissimilar sections identified: brighter area with heavier elements (higher concentration of Zr/Hf in the HH phase) and darker matrix areas with lighter elements (higher concentration of Ti in the HH phase) as identified. In contrast to the occupancy of the elements at the M-position, Tin and Nickel are uniformly dispersed. Small concentrations of Zr and Hf are found in the Ti rich parts. High concentrations of Hf and Zr are recorded in the Ti-poor matrix. Furthermore, a few tiny spots (white region) with a large amount of Hf can also be seen. Composition obtained by WDS for x = 0.30 is presented in Table 1. Michael Schwall et al. had recently conducted a comprehensive study for the phase separated HHs and our results are consistent with their findings [14]. Low-frequency phonons are responsible for heat transport. Moreover, low frequency phonons are particularly affected by the boundary scattering [15,16]. Therefore, it is very likely that the enhanced boundary scattering offered by the uneven grains (due to phase separation) can effectively limit the high thermal conductivity. Phonon transport may also be affected by the high Hf content as it may also act as additional phonon scattering spots. Fig. 3 displays the recorded BSE images of annealed samples for x = 0.30 and x = 0.45. These images show two different regions while the contrast contingent on the atomic element is identified: a brighter matrix region with heavier elements (higher concentration Zr/Hf in the HH phase) and darker regions with lighter elements (higher concentration Ti in the HH phase) as identified. Again, occupancy of the elements at the M-position, Sn and Ni are evenly dispersed over the entire sample.
1.2. Results and discussion Fig. 1 shows the XRD of TixHf0.25Zr0.75-xNiSn0.985Sn0.015 samples. All samples are well indexed to space group F4̅3 m, showing cubic crystal symmetry. However, a few traces of Hf content are also detected for some samples. Moreover, with the increase in Ti content the base of peaks was observed to be widening up.
1.2.2. Thermoelectric properties Electrical transport parameters at room temperature of the samples are given in Table 2. Carrier concentration and electrical conductivities increased with the rise in the Ti concentration. This substitution is isoelectronic, still it alters the electronic structure slightly and subsequently affects the electrical conductivities and carrier concentrations of the charge carriers. This impact is small as compared to doping with electrons by replacing Sn with Sb which can lead to an increase in carrier concentration with a factor 4 times the substitution level [17]. This trend of increase in electrical conductivity and carrier concentration is also consistent with the findings of Sakurada et al. [18]. The mobilities of samples were observed to decrease with rise in Ti content. This was due to the enhanced scattering caused by addition of a different species i.e., Ti in the matrix and by the increase in carrier concentration. Annealed samples have shown better electrical conductivities and carrier concentrations due to the increased homogeneity of the matrix. Single parabolic band (SPB) has been used for the approximation of effective mass and Lorenz number for the HHs (Hf, Zr) NiSn [19] and (Ti, Zr, Hf) NiSn [20,21] systems. The values of effective mass and Lorenz number (given in Table 2) were also calculated by single parabolic band model (SPB) assumption. The basic equations relevant to the SPB are:
=±
kB e
Fi ( F ) = 0
F
Fig. 1. XRD patterns of sintered TixHf0.25 Zr0.75-xNiSn0.985Sb0.015. XRD of annealed samples is presented at the bottom.
x i dx 1 + exp (x
F)
= EF (kB T )
n=
2
(r + 5/2) Fr + 3/2 ( F ) (r + 3/2) Fr + 1/2 ( F )
F
4 (2kB Tm )3/2 F1/2 ( F ) h3
(1)
(2) (3) (4)
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Fig. 2. Typical backscattered electron images of the polished surfaces of TixHf0.25 Zr0.75-xNiSn0.985Sb0.015 (a) x = 0.15 (b) x = 0.30. Table 1 Distribution of elements as obtained by WDS for x = 0.30.
Bright region (Ti-poor) Dark region (Ti-rich)
L=
kB e
2
(r + ) F (r + ) F 7
3
Ti(at%)
Zr(at%)
Hf(at%)
Ni(at%)
Sn(at%)
Sb(at%)
Detected Phase
4.95 22.65
15.07 5.97
12.97 4.71
33.45 32.91
32.73 32.19
0.54 0.33
Ti0.15Hf0.39Zr0.46NiSn0.98Sb0.017 Ti0.69Hf0.13Zr0.18 NiSn0.99Sb0.01
2
r+ 5 2 ( F )
2
r+ 1 2 ( F )
(r + ) F (r + ) F 5 3
2
r+ 3 2 ( F )
2
r+ 1 2 ( F )
Table 2 Carrier concentration,n, mobility, μH, electrical conductivity,σ, effective mass, m*/m, and Lorenz number of the TixHf0.25 Zr0.75-xNiSn0.985Sb0.015at room temperature.
2
(5)
where, Fi (ηF) is the Fermi integral, kB is the Boltzmann constant, ηF is the reduced Fermi level, m* is the effective mass, e is the electronic charge, h is the Plank constant, T is the absolute temperature and r is the scattering factor. The values of effective masses were in the range 3.3 ± 0.2m0. These values are very close to effective masses of (Hf, Zr) NiSn system in the literature and may suggest band structures with no major changes which can justify the applicability of SPB. The values of Lorenz number for any particular Ti concentration showed slight difference which was related to their slight dissimilar Seebeck coefficients (Fig. 4b), as Seebeck coefficient and the Lorenz number are correlated. Fig. 4a represents the electrical conductivities. Highly doped semiconductor trend is observed from these curves. Generally, with the rise in temperature the electrical conductivity decreases. This suggests stronger electron scattering at higher temperatures. Moreover, with further increase in temperature above 700 K, the decrease in electrical conductivity was comparatively less slow and for x = 0.15 and x = 0.45 there was a rise again in the electrical conductivity. This is
TixHf0.25Zr0.75-x NiSn0.985Sb0.015
n(1020cm-3)
σ(104S/m)
μH(cm2V-1s-1)
m*/m0
Lorenz number, L (108 2 -2 V K )
x x x x x x x x
4.70 4.92 5.39 6.63 4.95 5.71 6.05 6.77
19.80 20.15 22.01 24.98 20.49 23.13 23.97 24.81
26.55 25.76 25.35 23.51 25.93 25.16 23.83 23.13
3.23 3.39 3.45 3.37 3.28 3.43 3.46 3.45
1.88 1.87 1.86 1.96 1.87 1.92 1.95 1.96
= = = = = = = =
0 0.15 0.30 0.45 0-An 0.15-An 0.30-An 0.45-An
probably owed to thermal excitation which steadily increased carrier concentration. Fig. 4b displays the temperature dependence of the Seebeck coefficient. For x = 0, 0.15 and 0.30 Seebeck coefficient had shown slight increase in their respective values at 300 K. But for
Fig. 3. Typical backscattered electron images of the polished surfaces of annealed TixHf0.25 Zr0.75-xNiSn0.985Sb0.015 (a) x = 0.30 (b) x = 0.45. 3
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Fig. 4. Temperature dependent (a) Seebeck coefficient (b) electrical conductivity (c) power factor and (d) thermal conductivity for TixHf0.25 Zr0.75-xNiSn0.985Sb0.015.
x = 0.45 the Seebeck coefficient value had decreased significantly. This could be due to the significant increase of carrier concentration for x = 0.45. Also, for x = 0.30 maximum Seebeck coefficient value was obtained i.e., -166.38 μV/K at 873 K. The temperature dependence of calculated power factor α2σ is presented in Fig. 4c x = 0.30 showed the highest power factor of 4.54×10-3 W/mK2 at 773 K. Thermal conductivities of the samples are presented in Fig. 4d. The samples of x = 0, 0.15 and 0.30 essentially showed the same trend but there was quite significant difference between their respective thermal conductivity values due to the extra alloying introduced by substitution of 15% and 30% Ti. The room temperature thermal conductivity for x = 0.45 was almost the same but it increased rapidly due to contributions from the electronic and bipolar thermal conductivity. Consequently, x = 0.30 exhibited the lowest thermal conductivity value of 3.66 W/mK recorded at 673 K, respectively. We further estimated the lattice and electronic thermal conductivity. The Lorenz number was calculated by employing the SPB model as discussed earlier. Estimated κL is thus shown in Fig. 5. Lattice thermal conductivities of the samples were observed to be significantly reduced with the lowest κL value of 2.41W/mK at 300 K for x = 0.45. Corresponding ZT values are plotted in Fig. 6. High ZT values are primarily attributed to the reduced thermal conductivity which was achieved through additional scattering centers created by the substitution of Ti, and improved power factor. Thus, x = 0.30 exhibited peak ZT value of 1.02 at a temperature of 923 K. TE properties of the annealed samples are discussed now. Fig. 7a represents the electrical conductivities of the samples. Behavior of highly doped semiconductors is apparent. Generally, with the rise in temperature the electrical conductivity decreases, which point towards stronger scattering of the electrons at higher temperatures. With further increase in temperature above 700 K, the decrease in electrical
Fig. 5. Temperature dependent lattice thermal conductivity.
conductivity was comparatively less slow and for all the samples. This is probably due to thermal excitation which may gradually increase carrier concentration. Fig. 7b displays the temperature dependence of the Seebeck coefficient. With the increase in Ti content Seebeck was observed to decrease at the room temperature range. The Seebeck coefficient increased continuously until a maximum value at around 773 K, after which it decreased due to the minority carrier contribution. The peak value of Seebeck coefficient was obtained for x = 0, which is -193.31 μV/K at 873 K. The temperature dependence of calculated power factor α2σ is presented in Fig. 7c. Here again x = 0 showed the highest power factor of 5.38×10-3 W/mK2 at 773 K. 4
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Fig. 8. Temperature dependent lattice thermal conductivity of annealed samples.
Fig. 6. Temperature dependent ZT for TixHf0.25Zr0.75-xNiSn0.985Sb0.015.
873 K). In contrast, un-substituted sample showed its lowest lattice thermal conductivity value of 2.57 W/mK at 923 K. This effective decrease in the lattice thermal conductivity of the Ti substituted samples is attributed to the establishment of an integrated network of second half-Heusler phase within the matrix. In conclusion, with x = 0.3 (i.e., Ti0.3Zr0.45Hf0.25) at the Zr-site, phonon scattering is most effective and annealing has caused the secondary phase to get well established throughout the matrix resulting in a reduction of 62% in the lattice thermal conductivity when compared with that of the single-phase Zr0.75Hf0.25NiSn0.985Sb0.015. Corresponding ZT values are plotted in Fig. 9. High ZT values are mainly ascribed to the reduced thermal conductivity which was achieved through additional scattering centers created by the substitution of Ti. Thus, the sample with x = 0.30 exhibited peak ZT value of 1.11 at 873 K.
Thermal conductivities vs temperature of the samples is presented in Fig. 7d. All the samples essentially showed the same trend but there was quite significant difference between their respective thermal conductivity values due to the extra alloying introduced by substitution of 15%, 30% and 45% Ti. The thermal conductivity for x = 0.45 increased rapidly due to contributions from the electronic and bipolar thermal conductivity. Consequently, x = 0.30 exhibited the lowest thermal conductivity value of 3.47 W/mK recorded at 673 K, respectively. The lattice and electronic thermal conductivity were also estimated further understanding. Estimated κL is thus shown in Fig. 8. A significant reduction in κL was observed with the lowest κL value of 2.28W/mK at 300K for x = 0.30 sample. The lowest κL values were reached by samples with x = 0.3 (1.49 W/mK at 923 K) and x = 0.3-annealed sample (0.97 W/mK at
Fig. 7. Temperature dependent, (a) electrical conductivity (b) Seebeck coefficient (c) power factor and (d) thermal conductivity of annealed samples. 5
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Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements This work was financially supported by National Key Research and Development program of China (Grant No. 2018YFB0703600), and the National Natural Science Foundation of China (No. 51772232) and the 111 Project of China (Grant No. B07040). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.pnsc.2019.12.007. References
Fig. 9. Temperature dependent ZT of annealed samples for TixHf0.25 Zr0.75xNiSn0.985Sb0.015.
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2. Conclusions Ti is introduced as substituting element at the Zr-site for Hf0.25 Zr0.75NiSn0.985Sb0.015 compound. It is perceived that their thermal conductivity decreases due to the substitution, which eventually lead to better ZT. For the Ti substituted samples mainly single phase ZrNiSn type cubic crystal structure has been observed. With the rise in Ti content the lattice parameter has been observed to decrease, indicating the formation of a solid solution. The BSE images confirm the presence of phase separated sample which have further identified as Ti-rich and Zr-rich phases. The induction melted Ti substituted samples show a relatively high power factor of 4.54×10-3 W/mK2 at 773 K for x = 0.30. x = 0.30 also exhibits the lowest thermal conductivity, 3.66 W/mK at 673 K. The obtained ZT value is 1.02 at 923 K. For the annealed samples, a ZT value of 1.11 at 923 K is achieved for x = 0.30. The improved ZT for the annealed samples is due to the reduced thermal conductivity due to the enhanced phonon scattering induced by the inclusion of Ti in the matrix, enhanced power factor, which result from the uniformity of the grains and increase the electrical conductivities.
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