- Email: [email protected]
MnSi1.73
Materials Letters 265 (2020) 127388
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/mlblue
Significant enhancement in thermoelectric performance of bulk CrSi2 employing quasi-binary solid solution CrSi2/MnSi1.73 Naval Kishor Upadhyay a,b, L.A. Kumaraswamidhas b,⇑, Ajay Dhar c,⇑ a
Advanced Materials and Devices Metrology Division, CSIR-National Physical Laboratory, Dr. K.S. Krishnan Marg, New Delhi 110012, India Department of Mining Machinery Engineering, Indian Institute of Technology (ISM), Dhanbad, Jharkhand 826004, India c Academy of Scientific and Innovative Research (AcSIR) CSIR – Human Resource Development Centre, Ghaziabad, Uttar Pradesh 201 002, India b
a r t i c l e
i n f o
Article history: Received 9 December 2019 Received in revised form 17 January 2020 Accepted 19 January 2020 Available online 21 January 2020 Keywords: Semiconductors Chromium silicide (CrSi2) Electronic material Thermoelectric Solid solution Figure-of-merit (ZT)
a b s t r a c t Transition metal silicides, as thermoelectric (TE) materials, offer significant advantages necessary for commercialization of TE technology beyond their niche applications. In the present study, we examine the TE prospects of a quasi-binary solid solution of CrSi2 and MnSi1.73, which are both well-known chemically stable TE materials consisting of earth-abundant and non-toxic constituent elements, synthesized using spark plasma sintering at optimized processing parameters. A state-of-the-art TE figure of merit (ZT) ~ 0.29 at 673 K at an optimized solid solution composition of CrSi2/5 wt% MnSi1.73, which is ~140% higher compared to pristine CrSi2. Ó 2020 Elsevier B.V. All rights reserved.
1. Introduction Thermoelectric (TE) research in the past decade has majorly focused on the development of TE materials and devices for energy generation in the mid-temperature region, due to lack of commercial thermoelectric generators in this temperature regime, where most of the useful waste heat harnessing applications exist. The efforts to make the TE technology commercially viable requires serious considerations on vital parameters, including, figure-ofmerit (ZT), material and processing costs, thermal stability, earth-abundance and non-toxicity of its constituent elements. In this context, several silicides, such as, Mg2Si, MnSi1.73, b-FeSi2, CrSi2 etc. have already been identified, which exhibit promising prospects to fulfill these requirements. However, the relatively lower ZT exhibited by most of these silicide-based alloys often limits their applicability in TE devices. The ZT is quantified as figure-ofmerit ZT ¼ S jrT , where the S, r, T and j represents the Seebeck coefficient, electrical conductivity, absolute temperature and thermal conductivity respectively. Among the class of thermoelectric materials, transition metal silicides actively explored for TE application, CrSi2 offers significant advantages owing to their good electrical transport properties with 2
⇑ Corresponding authors. E-mail addresses: [email protected] [email protected] (A. Dhar). https://doi.org/10.1016/j.matlet.2020.127388 0167-577X/Ó 2020 Elsevier B.V. All rights reserved.
(L.A.
Kumaraswamidhas),
a high value of S, excellent stability at higher temperature [1]. Similarly, MnSi1.73, which has also been widely studied [2] exhibits excellent TE properties and consists of earth-abundant and nontoxic constituent elements. To enhance the TE properties of CrSi2, several strategies, such as, substitutional doping [3,4], nanostructuring [5], nanocomposite approach [6,7] etc. have been actively explored. Using these approaches the ZT in the range of ~0.2–0.25 has been reported for CrSi2 by various researchers. Recently, higher values of ZT ~ 0.23 at 900 K have been reported for CrSi2 using the substitutional doping of Copper [4]. Nakasawa et al. reported ZT ~ 0.29 using double-doping approach [8]. In the present study, we report an enhanced ZT in intermetallic solid solution of well-known TE silicides, CrSi2 and MnSi1.73, synthesised employing spark plasma sintering at optimized processing parameters. An enhanced ZT ~ 0.29 at 673 K was realized in optimized solid solution composition of CrSi2/5 wt% MnSi1.73, thus making them prospective candidates for mid-temperature TE applications. 2. Experimental details Pristine CrSi2 was synthesized by melting of Cr (99.95%) and Si (99.99%) ingots under argon atmosphere in proper stoichiometric proportions using arc-melting (Edmund Buhler, MAM-1). The synthesis of single-phase MnSi1.73 was carried out using spark plasma
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N.K. Upadhyay et al. / Materials Letters 265 (2020) 127388
sintering (SPS) at optimized parameters, which are detailed in our group publication [9]. The pulverized and blended powders of MnSi1.73 and CrSi2 in proper stoichiometric proportions were sintered employing SPS at 1373 K for 10 min at 60 MPa in a highdensity graphite die. The characterization of phase, morphology and elemental composition of the sintered products was carried out using X-ray diffraction (XRD), Field Emission Scanning Electron Microscopy (FESEM) and Energy Dispersive X-ray Spectroscopy (EDS). The experimental details of electrical and thermal transport measurements are detailed in our earlier publication [7].
3. Results and discussions Fig. 1(a) shows the XRD patterns of the synthesized pristine CrSi2 and MnSi1.73, which suggest that both thermoelectric sili-
cides, synthesized at optimized process parameters with lattice parameters a = b = 4.428 Å and c = 6.369 Å for CrSi2 and a = 0.55, b = 0.55, and c = 6.55 nm corresponding to MnSi1.73. Fig. 1(b) depicts the XRD patterns of the synthesized solidsolutions of CrSi2/MnSi1.73 with varying wt% of MnSi1.73. This figure clearly indicates that the CrSi2/MnSi1.73 solid solutions exhibit a single-phase formation till about 10 wt% MnSi1.73 in CrSi2. However, solid-solution samples containing 15 wt% MnSi1.73 in CrSi2, a small peak corresponding to MnSi was noticed. Fig. 1(c) suggests that the lattice constants were found to decrease linearly with increasing wt% of MnSi1.73 in CrSi2, thereby suggesting a complete solubility between the two compounds. Fig. 2(a & d) shows the FE-SEM microstructure exhibiting the surface morphology of two typical solid solutions samples viz. CrSi2/5 wt% MnSi1.73 and CrSi2/15 wt% MnSi1.73. Fig. 2(a) indicates well distributed phases without any significant agglomeration
Fig. 1. X-ray pattern of: (a) Pristine CrSi2 and CrSi2/MnSi1.73 solid solution, (b) SPS sintered MnSi1.73 and (c) Change in lattice parameter in CrSi2/MnSi1.73 solid solution.
Fig. 2. FESEM images of: (a) CrSi2/5 wt%MnSi1.73, (b & C) EDS analysis of CrSi2/5 wt% MnSi1.73, (d) CrSi2/15 wt% MnSi1.73 and (b & C) EDS analysis of CrSi2/5 wt% MnSi1.73.
N.K. Upadhyay et al. / Materials Letters 265 (2020) 127388
3
magnitude of S was found to decrease with increasing concentration of MnSi1.73 in CrSi2 upto 5 wt% MnSi1.73 and thereafter it tends to increase in lower temperature region with further increase in the concentration of MnSi1.73. This variation in S is in-line with the in the changes in the value of n with increasing concentration of MnSi1.73 in CrSi2 (Fig. 3(a)). The temperature dependence of the power factor (S2r) for all the synthesized pristine CrSi2 and CrSi2/ MnSi1.73 solid solution samples is shown in Fig. 3(d), which suggests a highest power factor ~2.2 W/mK2 at 473 K for CrSi2/5 wt % MnSi1.73 solid solution sample. The temperature dependence of the j of the synthesized CrSi2 and CrSi2/MnSi1.73 solid solutions is shown in Fig. 3(e). It is apparent from this figure that among all the synthesized samples, the pristine CrSi2 sample exhibits highest j ~ 9.5 W/mK at 323 K, which is comparable with values reported earlier [3,7]. Further, a reduction in j was observed with the addition of MnSi1.73 in CrSi2. The increase in j at higher temperatures can be attributed to the bipolar conduction due to both majority and minority carriers [12]. Furthermore, the electronic (je) and lattice contribution (jL) of j were calculated using the Widemann-Franz law employing the temperature dependent Lorentz number, using Seebeck data [13,14] and employing j = je + jL and their temperature variation is depicted as Fig. 3(f). A comparison of Fig. 3(e) and (f) clearly suggests that the major contribution to the total j arises from jL for all the synthesized samples. Fig. 3(g) depicts the temperature dependence of ZT of the synthesized pristine CrSi2 and CrSi2/MnSi1.73 solid solutions. The pristine CrSi2 shows the maximum ZT ~ 0.12 at 623 K, which is close to the values reported earlier [7], while a state-of-the-art ZT ~ 0.29 at 673 K was realized in the optimized solid solution composition CrSi2/5 wt% MnSi1.73 which corresponds to ~140% enhancement over its pristine CrSi2 and comparable to earlier reports [6,8]. 4. Conclusions
Fig. 3. Electronic and thermoelectric transport properties of CrSi2 and CrSi2/ MnSi1.73: (a) Carrier concentration and mobility, (b) Electrical conductivity, (c) Seebeck Coefficient, (d) Power factor (e) Thermal conductivity (f) Lattice and electronic thermal conductivity, (g) Figure of merit.
(confirmed by EDS analysis (Fig. 2b & c) suggesting good solubility of MnSi1.73 in CrSi2 [10,11]. The FESEM image of CrSi2/15 wt% (Fig. 1(d)), clearly exhibits a phase contrast region due to Mn rich micron sized secondary phase, which is confirmed by EDS analysis as shown on Fig. 2(f). These results are also in agreement with the XRD studies of these two samples (Fig. 1(b)), which clearly suggest the appearance of an additional MnSi phase for the extreme composition CrSi2/15 wt% MnSi1.73. Fig. 3(a) shows the carriers concentration (n) and mobility (l) data of all the synthesized samples at 323 K (near room temperature), which suggests an increase in n was observed upto 5 wt% MnSi1.73 in CrSi2 followed by a decrease at higher concentration. In contrast, l was found to vary only marginally with MnSi1.73 concentration in the CrSi2/MnSi1.73 solid solution. Fig. 3(b), which depicts the temperature variation of r of the pristine CrSi2 and CrSi2/MnSi1.73 solid solutions, suggest that the magnitude of the r at 323 K for all the synthesized samples is in-line with their corresponding n and l data, as shown in Fig. 1(a). The Fig. 3(b) clearly suggests that the sample CrSi2/5 wt% MnSi1.73 exhibits the highest r among all the synthesized solid solution samples in the time temperature range of measurement. Fig. 3(c) illustrates the S of the pristine CrSi2 and CrSi2/MnSi1.73 solid solutions, which shows the maximum value of S ~ 179 mV/K at 573 K (for pristine CrSi2), amongst all the synthesized solid solution samples. Further the
The present work reports a state-of-the-art ZT ~ 0.29 at 673 K in an optimized solid solution composition of CrSi2/5 wt% MnSi1.73, which was synthesized employing, spark plasma sintering at optimized process parameters. This enhancement, which corresponds to ~140% over its pristine counterpart CrSi2, was primarily owing to an increase in the electrical conductivity with a simultaneous decrease in the thermal conductivity. Thus, the solid solution approach, employed in the present studies, presents a promising prospect for the enhancement of the thermoelectric performance. Author contribution The contributions of all authors mentioned below: Naval Kishor Upadhyay: Synthesis, characterization, data analysis and original drafting of paper. L. A. Kumaraswamidhas: Data Analysis and drafting of paper. Ajay Dhar: Conceptualization, data analysis and review/editing of manuscript. 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 The authors sincerely acknowledge the Director, National Physical laboratory for continuous support to carry out this work. We also extremely thankful to our research team Dr. Bhasker Gahtori,
4
N.K. Upadhyay et al. / Materials Letters 265 (2020) 127388
Dr. Bathula Sivaiah, Dr. M Saravanan, Mr. Radhey Shyam, Ms. Ruchi Bhardwaj, Mr. Kishor Kumar Johari for their continuous support.
References [1] T. Dasgupta, J. Etourneau, B. Chevalier, S.F. Matar, A. Umarji, Structural, thermal, and electrical properties of CrSi2, J. Appl. Phys. 103 (11) (2008) 113516. [2] S. Muthiah, R. Singh, B. Pathak, P.K. Avasthi, R. Kumar, A. Kumar, A. Srivastava, A. Dhar, Significant enhancement in thermoelectric performance of nanostructured higher manganese silicides synthesized employing a melt spinning technique, Nanoscale 10 (4) (2018) 1970–1977. [3] H. Nagai, T. Takamatsu, Y. Iijima, K. Hayashi, Y. Miyazaki, Effects of Ge substitution on thermoelectric properties of CrSi2, Jpn. J. Appl. Phys. 55 (11) (2016) 111801. [4] H. Nagai, T. Takamatsu, Y. Iijima, K. Hayashi, Y. Miyazaki, Improved thermoelectric performance from CrSi2 by Cu substitution into Si sites, Jpn. J. Appl. Phys. 57 (12) (2018) 121801. [5] C.-H. Lee, Fabrication and characterization of thermoelectric CrSi2 compound by mechanical alloying and spark plasma sintering, J. Nanosci. Nanotechnol. 15 (7) (2015) 5070–5073. [6] M. Mikami, Y. Kinemuchi, Microstructure and thermoelectric properties of WSi2-added CrSi2 composite, J. Alloys Compd. 690 (2017) 652–657.
[7] N.K. Upadhyay, L. Kumaraswamidhas, B. Gahtori, S. Bathula, S. Muthiah, R. Shyam, N.S. Chauhan, R. Bhardwaj, A. Dhar, Enhancement in thermoelectric performance of bulk CrSi2 dispersed with nanostructured SiGe nanoinclusions, J. Alloys Compd. 765 (2018) 412–417. [8] H. Nakasawa, T. Takamatsu, Y. Iijima, K. Hayashi, Y. Miyazaki, Thermoelectric properties of Mo and Ge co-substituted CrSi2, Trans. Mater. Res. Soc. Jpn. 43 (2) (2018) 85–91. [9] S. Muthiah, R. Singh, B. Pathak, A. Dhar, Facile synthesis of higher manganese silicide employing spark plasma assisted reaction sintering with enhanced thermoelectric performance, Scr. Mater. 119 (2016) 60–64. [10] I. Saltykova, K.I. Gol’dberg, F. Sidorenko, P. Gel’d, The electron structures of solid solutions in the quasibinary system CrSi2-MnSi2, Powder Metall. Met. Ceram. 7 (6) (1968) 483–487. [11] A. Wittmann, K. Burger, H. Nowotny, Untersuchungen im Dreistoff: Ni–Al–Si sowie von Mono-und Disilicidsystemen einiger Übergangsmetalle, Monatshefte für Chemie und verwandte Teile anderer Wissenschaften 93 (3) (1962) 674–680. [12] K. Biswas, J. He, I.D. Blum, C.-I. Wu, T.P. Hogan, D.N. Seidman, V.P. Dravid, M.G. Kanatzidis, High-performance bulk thermoelectrics with all-scale hierarchical architectures, Nature 489 (7416) (2012) 414. [13] G. Mahan, M. Bartkowiak, Wiedemann-Franz law at boundaries, Appl. Phys. Lett. 74 (7) (1999) 953–954. [14] H.-S. Kim, Z.M. Gibbs, Y. Tang, H. Wang, G.J. Snyder, Characterization of Lorenz number with Seebeck coefficient measurement, Appl. Mater. 3 (4) (2015) 041506.
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/mlblue
Significant enhancement in thermoelectric performance of bulk CrSi2 employing quasi-binary solid solution CrSi2/MnSi1.73 Naval Kishor Upadhyay a,b, L.A. Kumaraswamidhas b,⇑, Ajay Dhar c,⇑ a
Advanced Materials and Devices Metrology Division, CSIR-National Physical Laboratory, Dr. K.S. Krishnan Marg, New Delhi 110012, India Department of Mining Machinery Engineering, Indian Institute of Technology (ISM), Dhanbad, Jharkhand 826004, India c Academy of Scientific and Innovative Research (AcSIR) CSIR – Human Resource Development Centre, Ghaziabad, Uttar Pradesh 201 002, India b
a r t i c l e
i n f o
Article history: Received 9 December 2019 Received in revised form 17 January 2020 Accepted 19 January 2020 Available online 21 January 2020 Keywords: Semiconductors Chromium silicide (CrSi2) Electronic material Thermoelectric Solid solution Figure-of-merit (ZT)
a b s t r a c t Transition metal silicides, as thermoelectric (TE) materials, offer significant advantages necessary for commercialization of TE technology beyond their niche applications. In the present study, we examine the TE prospects of a quasi-binary solid solution of CrSi2 and MnSi1.73, which are both well-known chemically stable TE materials consisting of earth-abundant and non-toxic constituent elements, synthesized using spark plasma sintering at optimized processing parameters. A state-of-the-art TE figure of merit (ZT) ~ 0.29 at 673 K at an optimized solid solution composition of CrSi2/5 wt% MnSi1.73, which is ~140% higher compared to pristine CrSi2. Ó 2020 Elsevier B.V. All rights reserved.
1. Introduction Thermoelectric (TE) research in the past decade has majorly focused on the development of TE materials and devices for energy generation in the mid-temperature region, due to lack of commercial thermoelectric generators in this temperature regime, where most of the useful waste heat harnessing applications exist. The efforts to make the TE technology commercially viable requires serious considerations on vital parameters, including, figure-ofmerit (ZT), material and processing costs, thermal stability, earth-abundance and non-toxicity of its constituent elements. In this context, several silicides, such as, Mg2Si, MnSi1.73, b-FeSi2, CrSi2 etc. have already been identified, which exhibit promising prospects to fulfill these requirements. However, the relatively lower ZT exhibited by most of these silicide-based alloys often limits their applicability in TE devices. The ZT is quantified as figure-ofmerit ZT ¼ S jrT , where the S, r, T and j represents the Seebeck coefficient, electrical conductivity, absolute temperature and thermal conductivity respectively. Among the class of thermoelectric materials, transition metal silicides actively explored for TE application, CrSi2 offers significant advantages owing to their good electrical transport properties with 2
⇑ Corresponding authors. E-mail addresses: [email protected] [email protected] (A. Dhar). https://doi.org/10.1016/j.matlet.2020.127388 0167-577X/Ó 2020 Elsevier B.V. All rights reserved.
(L.A.
Kumaraswamidhas),
a high value of S, excellent stability at higher temperature [1]. Similarly, MnSi1.73, which has also been widely studied [2] exhibits excellent TE properties and consists of earth-abundant and nontoxic constituent elements. To enhance the TE properties of CrSi2, several strategies, such as, substitutional doping [3,4], nanostructuring [5], nanocomposite approach [6,7] etc. have been actively explored. Using these approaches the ZT in the range of ~0.2–0.25 has been reported for CrSi2 by various researchers. Recently, higher values of ZT ~ 0.23 at 900 K have been reported for CrSi2 using the substitutional doping of Copper [4]. Nakasawa et al. reported ZT ~ 0.29 using double-doping approach [8]. In the present study, we report an enhanced ZT in intermetallic solid solution of well-known TE silicides, CrSi2 and MnSi1.73, synthesised employing spark plasma sintering at optimized processing parameters. An enhanced ZT ~ 0.29 at 673 K was realized in optimized solid solution composition of CrSi2/5 wt% MnSi1.73, thus making them prospective candidates for mid-temperature TE applications. 2. Experimental details Pristine CrSi2 was synthesized by melting of Cr (99.95%) and Si (99.99%) ingots under argon atmosphere in proper stoichiometric proportions using arc-melting (Edmund Buhler, MAM-1). The synthesis of single-phase MnSi1.73 was carried out using spark plasma
2
N.K. Upadhyay et al. / Materials Letters 265 (2020) 127388
sintering (SPS) at optimized parameters, which are detailed in our group publication [9]. The pulverized and blended powders of MnSi1.73 and CrSi2 in proper stoichiometric proportions were sintered employing SPS at 1373 K for 10 min at 60 MPa in a highdensity graphite die. The characterization of phase, morphology and elemental composition of the sintered products was carried out using X-ray diffraction (XRD), Field Emission Scanning Electron Microscopy (FESEM) and Energy Dispersive X-ray Spectroscopy (EDS). The experimental details of electrical and thermal transport measurements are detailed in our earlier publication [7].
3. Results and discussions Fig. 1(a) shows the XRD patterns of the synthesized pristine CrSi2 and MnSi1.73, which suggest that both thermoelectric sili-
cides, synthesized at optimized process parameters with lattice parameters a = b = 4.428 Å and c = 6.369 Å for CrSi2 and a = 0.55, b = 0.55, and c = 6.55 nm corresponding to MnSi1.73. Fig. 1(b) depicts the XRD patterns of the synthesized solidsolutions of CrSi2/MnSi1.73 with varying wt% of MnSi1.73. This figure clearly indicates that the CrSi2/MnSi1.73 solid solutions exhibit a single-phase formation till about 10 wt% MnSi1.73 in CrSi2. However, solid-solution samples containing 15 wt% MnSi1.73 in CrSi2, a small peak corresponding to MnSi was noticed. Fig. 1(c) suggests that the lattice constants were found to decrease linearly with increasing wt% of MnSi1.73 in CrSi2, thereby suggesting a complete solubility between the two compounds. Fig. 2(a & d) shows the FE-SEM microstructure exhibiting the surface morphology of two typical solid solutions samples viz. CrSi2/5 wt% MnSi1.73 and CrSi2/15 wt% MnSi1.73. Fig. 2(a) indicates well distributed phases without any significant agglomeration
Fig. 1. X-ray pattern of: (a) Pristine CrSi2 and CrSi2/MnSi1.73 solid solution, (b) SPS sintered MnSi1.73 and (c) Change in lattice parameter in CrSi2/MnSi1.73 solid solution.
Fig. 2. FESEM images of: (a) CrSi2/5 wt%MnSi1.73, (b & C) EDS analysis of CrSi2/5 wt% MnSi1.73, (d) CrSi2/15 wt% MnSi1.73 and (b & C) EDS analysis of CrSi2/5 wt% MnSi1.73.
N.K. Upadhyay et al. / Materials Letters 265 (2020) 127388
3
magnitude of S was found to decrease with increasing concentration of MnSi1.73 in CrSi2 upto 5 wt% MnSi1.73 and thereafter it tends to increase in lower temperature region with further increase in the concentration of MnSi1.73. This variation in S is in-line with the in the changes in the value of n with increasing concentration of MnSi1.73 in CrSi2 (Fig. 3(a)). The temperature dependence of the power factor (S2r) for all the synthesized pristine CrSi2 and CrSi2/ MnSi1.73 solid solution samples is shown in Fig. 3(d), which suggests a highest power factor ~2.2 W/mK2 at 473 K for CrSi2/5 wt % MnSi1.73 solid solution sample. The temperature dependence of the j of the synthesized CrSi2 and CrSi2/MnSi1.73 solid solutions is shown in Fig. 3(e). It is apparent from this figure that among all the synthesized samples, the pristine CrSi2 sample exhibits highest j ~ 9.5 W/mK at 323 K, which is comparable with values reported earlier [3,7]. Further, a reduction in j was observed with the addition of MnSi1.73 in CrSi2. The increase in j at higher temperatures can be attributed to the bipolar conduction due to both majority and minority carriers [12]. Furthermore, the electronic (je) and lattice contribution (jL) of j were calculated using the Widemann-Franz law employing the temperature dependent Lorentz number, using Seebeck data [13,14] and employing j = je + jL and their temperature variation is depicted as Fig. 3(f). A comparison of Fig. 3(e) and (f) clearly suggests that the major contribution to the total j arises from jL for all the synthesized samples. Fig. 3(g) depicts the temperature dependence of ZT of the synthesized pristine CrSi2 and CrSi2/MnSi1.73 solid solutions. The pristine CrSi2 shows the maximum ZT ~ 0.12 at 623 K, which is close to the values reported earlier [7], while a state-of-the-art ZT ~ 0.29 at 673 K was realized in the optimized solid solution composition CrSi2/5 wt% MnSi1.73 which corresponds to ~140% enhancement over its pristine CrSi2 and comparable to earlier reports [6,8]. 4. Conclusions
Fig. 3. Electronic and thermoelectric transport properties of CrSi2 and CrSi2/ MnSi1.73: (a) Carrier concentration and mobility, (b) Electrical conductivity, (c) Seebeck Coefficient, (d) Power factor (e) Thermal conductivity (f) Lattice and electronic thermal conductivity, (g) Figure of merit.
(confirmed by EDS analysis (Fig. 2b & c) suggesting good solubility of MnSi1.73 in CrSi2 [10,11]. The FESEM image of CrSi2/15 wt% (Fig. 1(d)), clearly exhibits a phase contrast region due to Mn rich micron sized secondary phase, which is confirmed by EDS analysis as shown on Fig. 2(f). These results are also in agreement with the XRD studies of these two samples (Fig. 1(b)), which clearly suggest the appearance of an additional MnSi phase for the extreme composition CrSi2/15 wt% MnSi1.73. Fig. 3(a) shows the carriers concentration (n) and mobility (l) data of all the synthesized samples at 323 K (near room temperature), which suggests an increase in n was observed upto 5 wt% MnSi1.73 in CrSi2 followed by a decrease at higher concentration. In contrast, l was found to vary only marginally with MnSi1.73 concentration in the CrSi2/MnSi1.73 solid solution. Fig. 3(b), which depicts the temperature variation of r of the pristine CrSi2 and CrSi2/MnSi1.73 solid solutions, suggest that the magnitude of the r at 323 K for all the synthesized samples is in-line with their corresponding n and l data, as shown in Fig. 1(a). The Fig. 3(b) clearly suggests that the sample CrSi2/5 wt% MnSi1.73 exhibits the highest r among all the synthesized solid solution samples in the time temperature range of measurement. Fig. 3(c) illustrates the S of the pristine CrSi2 and CrSi2/MnSi1.73 solid solutions, which shows the maximum value of S ~ 179 mV/K at 573 K (for pristine CrSi2), amongst all the synthesized solid solution samples. Further the
The present work reports a state-of-the-art ZT ~ 0.29 at 673 K in an optimized solid solution composition of CrSi2/5 wt% MnSi1.73, which was synthesized employing, spark plasma sintering at optimized process parameters. This enhancement, which corresponds to ~140% over its pristine counterpart CrSi2, was primarily owing to an increase in the electrical conductivity with a simultaneous decrease in the thermal conductivity. Thus, the solid solution approach, employed in the present studies, presents a promising prospect for the enhancement of the thermoelectric performance. Author contribution The contributions of all authors mentioned below: Naval Kishor Upadhyay: Synthesis, characterization, data analysis and original drafting of paper. L. A. Kumaraswamidhas: Data Analysis and drafting of paper. Ajay Dhar: Conceptualization, data analysis and review/editing of manuscript. 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 The authors sincerely acknowledge the Director, National Physical laboratory for continuous support to carry out this work. We also extremely thankful to our research team Dr. Bhasker Gahtori,
4
N.K. Upadhyay et al. / Materials Letters 265 (2020) 127388
Dr. Bathula Sivaiah, Dr. M Saravanan, Mr. Radhey Shyam, Ms. Ruchi Bhardwaj, Mr. Kishor Kumar Johari for their continuous support.
References [1] T. Dasgupta, J. Etourneau, B. Chevalier, S.F. Matar, A. Umarji, Structural, thermal, and electrical properties of CrSi2, J. Appl. Phys. 103 (11) (2008) 113516. [2] S. Muthiah, R. Singh, B. Pathak, P.K. Avasthi, R. Kumar, A. Kumar, A. Srivastava, A. Dhar, Significant enhancement in thermoelectric performance of nanostructured higher manganese silicides synthesized employing a melt spinning technique, Nanoscale 10 (4) (2018) 1970–1977. [3] H. Nagai, T. Takamatsu, Y. Iijima, K. Hayashi, Y. Miyazaki, Effects of Ge substitution on thermoelectric properties of CrSi2, Jpn. J. Appl. Phys. 55 (11) (2016) 111801. [4] H. Nagai, T. Takamatsu, Y. Iijima, K. Hayashi, Y. Miyazaki, Improved thermoelectric performance from CrSi2 by Cu substitution into Si sites, Jpn. J. Appl. Phys. 57 (12) (2018) 121801. [5] C.-H. Lee, Fabrication and characterization of thermoelectric CrSi2 compound by mechanical alloying and spark plasma sintering, J. Nanosci. Nanotechnol. 15 (7) (2015) 5070–5073. [6] M. Mikami, Y. Kinemuchi, Microstructure and thermoelectric properties of WSi2-added CrSi2 composite, J. Alloys Compd. 690 (2017) 652–657.
[7] N.K. Upadhyay, L. Kumaraswamidhas, B. Gahtori, S. Bathula, S. Muthiah, R. Shyam, N.S. Chauhan, R. Bhardwaj, A. Dhar, Enhancement in thermoelectric performance of bulk CrSi2 dispersed with nanostructured SiGe nanoinclusions, J. Alloys Compd. 765 (2018) 412–417. [8] H. Nakasawa, T. Takamatsu, Y. Iijima, K. Hayashi, Y. Miyazaki, Thermoelectric properties of Mo and Ge co-substituted CrSi2, Trans. Mater. Res. Soc. Jpn. 43 (2) (2018) 85–91. [9] S. Muthiah, R. Singh, B. Pathak, A. Dhar, Facile synthesis of higher manganese silicide employing spark plasma assisted reaction sintering with enhanced thermoelectric performance, Scr. Mater. 119 (2016) 60–64. [10] I. Saltykova, K.I. Gol’dberg, F. Sidorenko, P. Gel’d, The electron structures of solid solutions in the quasibinary system CrSi2-MnSi2, Powder Metall. Met. Ceram. 7 (6) (1968) 483–487. [11] A. Wittmann, K. Burger, H. Nowotny, Untersuchungen im Dreistoff: Ni–Al–Si sowie von Mono-und Disilicidsystemen einiger Übergangsmetalle, Monatshefte für Chemie und verwandte Teile anderer Wissenschaften 93 (3) (1962) 674–680. [12] K. Biswas, J. He, I.D. Blum, C.-I. Wu, T.P. Hogan, D.N. Seidman, V.P. Dravid, M.G. Kanatzidis, High-performance bulk thermoelectrics with all-scale hierarchical architectures, Nature 489 (7416) (2012) 414. [13] G. Mahan, M. Bartkowiak, Wiedemann-Franz law at boundaries, Appl. Phys. Lett. 74 (7) (1999) 953–954. [14] H.-S. Kim, Z.M. Gibbs, Y. Tang, H. Wang, G.J. Snyder, Characterization of Lorenz number with Seebeck coefficient measurement, Appl. Mater. 3 (4) (2015) 041506.