Enhanced thermoelectric properties of pristine CrSi2 synthesized using a facile single-step spark plasma assisted reaction sintering
Introduction
Thermoelectric (TE) materials convert waste heat into electricity and their performance is determined by a dimensionless figure-of-merit,, where S, σ, T and κ are the Seebeck coefficient, electrical conductivity, absolute temperature and thermal conductivity, respectively [1]. The total thermal conductivity consists electronic and lattice contributions, which are termed as electronic thermal conductivity (κe) and lattice thermal conductivity (κl) respectively. Ideally, a high TE figure-of-merit (ZT), which is a pre-requisite of a good TE material, is generally a consequence of a high Seebeck coefficient with large electrical conductivity and low thermal conductivity.
Apart from the ZT, there are some other issues and parameters, which finally decide the practical applicability of a TE material for commercial TE generator applications, such as, materials and processing costs, toxicity etc. In order to harness waste-heat in the mid-temperature regime, where most of the useful TE application exist, there are various classes of TE materials available, which include, filled Skutturdites [[2], [3], [4]], half-Heuslers [[5], [6], [7]], Clathrates [8] etc. However, most of the above TE materials are either not stable at high operating temperatures or difficult to synthesize in a single-phase. Added to this, most of these TE materials consist of elements, which are either toxic and/or expensive. The transition metal silicides (MnSi1.73, Mg2Si, FeSi2, CrSi2 etc) could be potential materials for TE device applications for mid-temperature thermoelectric generator applications, which apart from consisting of earth-abundant, non-toxic elements are also thermally stable at high operating temperature. Among these, CrSi2 inter-metallic compound is a promising candidate for medium temperature range TE power generation application sowing to the natural abundance and non-toxicity of its constituent elements [1] and high temperature stability in air up to 1000K [9].
Pristine CrSi2 is a p-type semiconductor with narrow indirect band gap of 0.35eV [10,11] possessing C-40 type hexagonal crystal structure and having space group P6222 [[12], [13], [14]] with lattice parameter a=b=4.428 Å and c=6.369 Å [15]. Although, the reported electrical conductivity and Seebeck coefficient of single-phase CrSi2 is quite promising but its high thermal conductivity is mainly responsible for the low reported values of ZT. Several groups have reported the thermoelectric properties of pristine CrSi2, employing different processing techniques. Despite this, the practical TE device applications of CrSi2 are rather limited due to low values of its reported ZT ~ 0.08 to 0.16 at ~700K [[16], [17], [18]]. The main reason of its low thermoelectric performance, apart from its high thermal conductivity, is the formation of an undesired CrSi phase, which is metallic in nature and hence detrimental to its semiconducting properties.
In order to enhance the ZT of CrSi2, various groups have used different strategies, including, doping [[18], [19], [20], [21], [22], [23]] and nanostructuring to reduce the thermal conductivity [[24], [25], [26]]. Nagai et. al. [18,20]. reported a ZT ~ 0.20 and 0.25 at 700K by substitution doping of Nb and Ge in CrSi2, respectively. Ohishi et. al. [11]. reported a thermal conductivity ~ 5.7 Wm-1K-1 at 673K employing Mo as dopant, which resulted in a ZT ~ 0.23 at 800K. Recently, Nagai et al. [27] reported ZT ~ 0.23 at 900K by substitutional doping of Cu at Si site in CrSi2 by reducing the thermal conductivity ~ 5W/mK at 900K. The co-substitution of Mo and Ge in CrSi2 resulted in an enhanced ZT ~ 0.29 at 700K owing to the reduction of the thermal conductivity ~ 4.0W/mK [21]. The contamination of Fe in CrSi2 during ball milling has also been reported resulting in a ZT ~ 0.2 at 600 K [4,14]. Nanostructuring of CrSi2 has also been employed by several groups [[24], [25], [26]]. The composite approach to enhance thermoelectric performance of CrSi2 led to an enhanced ZT ~ 0.29 at 700K was reported by the addition of WSi2 into the CrSi2 reduced thermal conductivity [28]. Recently, our group has also reported an enhanced ZT~0.32 at 673K by nanoinclusions of nanostructured boron doped SiGe in the CrSi2 matrix [29].
However, in all of these reported studies, CrSi2 was synthesized using arc-melting and/or mechanical alloying followed by consolidation employing hot pressing [13,16,23] or spark plasma sintering [18,20,21,27]. It may be noted that the practical applicability of TE material for commercial device applications is generally marred due to its high costs of TE material processing. The processing techniques, reported earlier for the synthesis of CrSi2, apart from being time-consuming and involving multi-step processing, are also prone to contamination and other stoichiometry related issues. The mechanical alloying is known to result in contamination [30,31] while arc-melting leads to non-stoichiometry due to evaporation of constituent's elemental powders at high temperatures [32,33].
Thus, in the present study, we report a single-step rapid synthesis of CrSi2 using reaction sintering [[34], [35], [36], [37]] employing spark plasma assisted sintering at optimized processing parameters. This facile synthesis technique substantially reduces the processing time as well as cost [38], which is pre-requisite for low cost and commercial manufacturing of TE materials. Despite using the single-step processing a single-phase CrSi2was realized with a state-of-the-art ZT ~ 0.19 at 673K.
Section snippets
Experimental
High purity Cr (99.5%, -100 mesh) and Si (99.999%, -325 mesh) powders were taken in proper stoichiometric proportion and hand-blended in agate mortar pestle for ~30min. The resulting hand-blended elemental powders were subjected to spark plasma sintering (SPS 725, Fuji electric co., Japan) under vacuum at an optimized temperature 1423K for 5 min at 60 MPa pressure with heating rate of 100 K/min in high-strength graphite die of diameter 12.7 mm. The phase identification of sintered samples was
Result and discussion
In the present study, the series of samples (S1 to S3) were synthesized employing the single-step reaction sintering employing spark plasma sintering with varying at.% of Si, which are tabulated in Table 1. Fig. 1(a-c) depict the X-ray diffraction patterns of samples S1, S2 and S3 synthesized using the single-step reaction sintering with varying amount of excess Si. The PDF 4+ (ICDD) data of CrSi2, CrSi and Si are also shown in this figure for ease of phase identification. As, it is evident
Conclusion
We report a single-step material processing technique for the synthesis of single-phase CrSi2 employing reaction sintering using spark plasma assisted sintering which substantially reduces the processing time as well as costs. This single-step technique is both facile and fast, in contrast to the conventional processing of CrSi2, which involves several processing steps, including arc-melting and/or mechanical alloying followed by spark plasma sintering, thereby making the processing both
CRediT authorship contribution statement
Naval Kishor Upadhyay: Writing - original draft, Formal analysis. L.A. Kumaraswamidhas: Writing - original draft, Formal analysis. Bhasker Gahtori: Writing - original draft, Formal analysis. S.R. Dhakate: Formal analysis. Ajay Dhar: Writing - review & editing, Conceptualization, Formal analysis.
Declaration of competing interest
There is no conflict of interest to declare.
Acknowledgement
The authors sincerely acknowledge the Director, National Physical laboratory for continuous support to carry out this work. We also thankful to Dr. Sudheer Hausale for microstructural evaluation. We also extremely thankful to our research team Dr. Bathula Sivaiah, Dr. M Saravanan, Mr. RadheyShyam, Dr. Nagendra Singh Chauhan, Ms. Ruchi Bhardwaj, Mr. Kishor Kumar Johari for their constant technical support
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