Facile synthesis of nanostructured n-type SiGe alloys with enhanced thermoelectric performance using rapid solidification employing melt spinning followed by spark plasma sintering

Accepted Manuscript Facile synthesis of nanostructured n-type SiGe alloys with enhanced thermoelectric performance using rapid solidification employing melt spinning followed by spark plasma sintering Avinash Vishwakarma, Sivaiah Bathula, Nagendra S. Chauhan, Ruchi Bhardwaj, Bhasker Gahtori, Avanish K. Srivastava, Ajay Dhar PII:

S1567-1739(18)30269-4

DOI:

10.1016/j.cap.2018.09.013

Reference:

CAP 4838

To appear in:

Current Applied Physics

Received Date: 20 July 2018 Revised Date:

21 August 2018

Accepted Date: 27 September 2018

Please cite this article as: A. Vishwakarma, S. Bathula, N.S. Chauhan, R. Bhardwaj, B. Gahtori, A.K. Srivastava, A. Dhar, Facile synthesis of nanostructured n-type SiGe alloys with enhanced thermoelectric performance using rapid solidification employing melt spinning followed by spark plasma sintering, Current Applied Physics (2018), doi: https://doi.org/10.1016/j.cap.2018.09.013. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Facile synthesis of nanostructured n-type SiGe alloys with enhanced Thermoelectric Performance using Rapid Solidification employing Melt Spinning followed by Spark Plasma Sintering

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Avinash Vishwakarma1,2, Sivaiah Bathula1,2,*, Nagendra S. Chauhan1,2, Ruchi Bhardwaj1,2 , Bhasker Gahtori1,2 Avanish K. Srivastava3 and Ajay Dhar1,∗

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applications. However, its high thermoelectric performance has been thus far realized only in alloys

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synthesized employing mechanical alloying techniques, which are time-consuming and employ

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several materials processing steps. In the current study, for the first time, we report an enhanced

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thermoelectric figure-of-merit (ZT) ~ 1.1 at 900°C in n-type Si80Ge20 nano-alloys, synthesized using

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a facile and up-scalable methodology consisting of rapid solidification at high optimized cooling

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rate ~ 3.4 x 107 K/s, employing melt spinning followed by spark plasma sintering of the resulting

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nano-crystalline melt-spun ribbons. This enhancement in ZT > 20% over its bulk counterpart, owes

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its origin to the nano-crystalline microstructure formed at high cooling rates, which results in

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crystallite size ~ 7 nm leading to high density of grain boundaries, which scatter heat-carrying

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phonons. This abundant scattering resulted in a very low thermal conductivity ~ 2.1 Wm-1K-1, which

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corresponds to ~ 50% reduction over its bulk counterpart and is amongst the lowest reported thus far

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in n-type SiGe alloys. The synthesized samples were characterized using X-ray diffraction, scanning

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electron microscopy and transmission electron microscopy, based on which the enhancement in their

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thermoelectric performance has been discussed.

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Keywords: SiGe alloys; rapid solidification; melt-spun; spark plasma sintering

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Academy of Scientific and Innovative Research (AcSIR), CSIR-National Physical Laboratory (CSIR-NPL) campus, New Delhi-110012, India. 2 Advanced Materials and Devices Metrology Division, CSIR-National Physical Laboratory, New Delhi-110012, India 3 CSIR - Advanced Materials and Processes Research Institute (AMPRI), Bhopal-462026, India

Abstract

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SiGe alloy is widely used thermoelectric materials for high temperature thermoelectric generator

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Corresponding author: [email protected]; [email protected] Tel.: +91 11 4560 9456; Fax: +91 11 4560 9310

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1. Introduction

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Thermoelectrics (TE) offers a direct conversion of heat into electricity and is a promising

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technology for efficient harvesting of waste-heat, which is available in abundance. TE research

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activities worldwide are mainly focused on the design and development of materials and devices for

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efficient energy conversion. As a useful metric, that characterizes the TE energy conversion

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efficiency of a material, most studies are presently aimed towards enhancing the thermoelectric

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figure-of-merit (ZT = α2σT/κ) where α, σ, κ and T represent the Seebeck coefficient, electrical

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conductivity, total thermal conductivity and absolute temperature, respectively. All these physical

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parameters are inter-coupled, thus their favourable optimization for enhancing the ZT remains a key

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challenge [1, 2]. For high temperature TE application (> 850 K), currently the family of TE

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materials such as Zintl [3], Clathrates[4], half-Heuslers[5-7] and Oxides [8-10] were widely studied.

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However, most of these existing TE materials still exhibit a low TE performance (in terms of their

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magnitude of ZT) or lack a compatible n/p material, required for a TE power generating device.

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SiGe alloys have thus far remained the state-of-the-art TE material for high temperature TE power

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generation applications. Although possessing excellent thermal stability with insignificant

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degradation, conventional bulk SiGe alloys exhibit limited device efficiencies as an outcome of their

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lower ZT, especially for their p-type counterpart.

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Promising prospects of nanostructuring methodology have led to enhanced ZT in SiGe alloys

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leading to high device efficiencies. In both n and p-type SiGe (Si80Ge20) alloys, an enhanced ZT~1.5

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and 1.2 at 900°C has been reported earlier employing high-energy ball-milling[11, 12], which

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introduces nano-scale features, thus effectively reducing the lattice thermal conductivity owing to

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the scattering of heat-carrying phonons [13-15]. However, realizing nanostructuring in SiGe alloys

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using mechanical alloying is, in general, is a time-consuming process as it takes 60 to 90 hrs of

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milling time, which adds to the material processing costs. Added to this, there is an inherent risk of

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ACCEPTED MANUSCRIPT contamination of the nano-powders as a consequence of mechanical alloying at high milling speeds

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owing to the abrasive nature of Si particles.

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In the current study, a methodology consisting of arc melting and melt-spinning of SiGe alloys

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followed by spark plasma sintering of the resulting ribbons, which is both fast and up-scalable, has

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been employed for the first time for realizing nanostructuring in SiGe alloys, which resulted in a

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higher ZT ~ 1.1 at 900°C. The speed of the Cu-wheel, which determines the cooling rates, was

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optimized to obtain an optimum nanostructuring (~ 7 nm) leading to a very low thermal conductivity

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~ 2.1 Wm-1K-1, which corresponds to ~ 50% reduction over its bulk counterpart and is amongst the

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lowest reported thus far in n-type SiGe alloys. The observed enhancement in thermoelectric

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properties was corroborated with the detailed microstructural investigations employing XRD, FE-

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SEM and HR-TEM microscopy.

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2. Experimental details

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Si (Alfa Aesar, 99.98%), Ge (Alfa Aesar, 99.99%) in the stoichiometric composition of Si80Ge20

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were repeatedly arc melted with 2 at.% P (Merck, 99.5%) and further melt-spun (Melt Spinner HV,

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M/s. Edmund Bühler GmbH, Germany) at an optimized temperature of 1550°C under a vacuum of

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∼10−6 mbar . The melt was then ejected through a fine rectangular nozzle (0.2 mm × 10 mm) at an

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argon pneumatic pressure of 0.05 MPa on a rotating water-cooler Copper wheel (dia. 250 mm) at a

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nozzle-to-wheel distance of 0.4 mm. The melt-spun ribbons were subsequently consolidated and

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sintered under vacuum (~ 10 Pa) using spark plasma sintering (725, M/s. SPS Syntex, Japan) in a

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graphite die (dia. 12.7 mm) at 1150°C with a heating rate of 300°C/minute under a pressure of 60

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MPa with a holding time of 3 minutes. The phase identification and crystallite size determinations

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were carried out using X-ray powder diffraction (Miniflex, M/s. Rigaku Corporation, Japan).

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The morphology of the samples was studied using a Field Emission Scanning Electron Microscope

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(Supra 40VP, M/s. Zeiss, UK) and the structural analysis was carried out employing a Transmission

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ACCEPTED MANUSCRIPT Electron Microscope (G2 F30 STWIN, M/s. FEI Company, USA) operated at the electron

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accelerating voltage of 300 kV using field emission gun as an electron source. The thermal

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diffusivity measurements were made on circular disc specimens of 12.7 mm diameter using a laser

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flash system (LFA 1000, M/s. Laser Flash Analyzer, Germany) under a vacuum of 10-3 mbar and the

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specific heat capacity was measured employing a Differential Scanning Calorimeter (DSC 404F3,

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M/s. Netzsch, Germany). The thermal conductivity was calculated as a product of diffusivity,

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specific heat capacity and density. The Seebeck coefficient and electrical conductivity were

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measured on rectangular specimens of dimensions 12 x 3 x 3 mm3 using a four-probe DC method

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(ZEM-3, M/s. Ulvac Inc., Japan) in helium atmosphere. The density, which was measured using the

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conventional Archimedes principle, was found to be > 98.6% of the theoretical density, for all the

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samples. The accuracies in transport measurement are: ± 6% for thermal diffusivity, ± 5% for

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electrical conductivity, ± 7% for Seebeck coefficient, ± 2% for specific heat and ± 0.5% for density.

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The rapid solidification process was employed to synthesize nano-crystalline SiGe alloys by

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optimizing the high cooling rate of the melt during melt-spinning, which was controlled by the

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speed of water-cooled copper wheel. The average cooling rates were estimated using the equation

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dT/dt = (h × ∆T)/ (t × ρ × Cp), as shown in Table 1, where h is the heat transfer coefficient[16], ∆T

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is the temperature difference between the melt and the water-cooled copper substrate, ρ is the

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density, Cp is the specific heat of the material and t is the thickness of the melt-spun ribbons. The

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cooling rates at the wheel speeds of 23, 27 and 30 m/s, estimated from thickness of the as-melt spun

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ribbons (15 - 25 µm), were found to be ~ 106 - 107 K/s as shown in Table 2. It may be noted that

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higher wheel speeds (> 30 m/s) during melt spinning although resulted in enhanced cooling rates of

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the melt, but resulted the formation of an amorphous SiGe phase, which is detrimental for its

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thermoelectric properties. On the other hand, at lower wheel speeds (< 23 m/s), resulted in higher

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thickness of the melt-spun ribbon with relatively higher grain size, which is not favourable for the

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ACCEPTED MANUSCRIPT enhancement of the thermoelectric performance[17]. Hence, in the current study, we have only

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analysed the SiGe nanoalloy samples melt-spun at the wheel speeds in the range of 23 – 30 m/s.

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3. Results and Discussion

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3.1 Microstructural analysis

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Figure 1 shows the XRD patterns of the melt-spun followed by spark plasma sintering (SPS)

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Si80Ge20 nanostructured alloys synthesized at different cooling rates, which indicates the formation

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of a crystalline single-phase with a lattice constant of 5.47 Å, which is in perfect agreement with that

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calculated using modified Vegard’s law [18], thus suggesting a complete solubility of Ge in the Si

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matrix. It is apparent from Fig. 1 that XRD peaks exhibit a significant broadening, which could be

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attributed to nano-scale features as well as a considerable amount of strain in the melt-spun ribbons,

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due to the high cooling rates of melt. The average crystallite size of melt-spun SiGe alloy has been

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calculated using Williamson-Hall method [19] and was found to be ~ 7 nm, which coarsened to 12

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nm after SPS for the melt spun sample (cooling rate of 3.4 x 107 K/s), depending on the cooling rate

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of the melt (Table 2). Expectedly, Table 2 suggests a finer crystallite size with increasing cooling

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rate.

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The elemental mapping of a typical synthesized SiGe nanoalloy sample shown in Fig. 2, of the melt-

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spun and sintered sample confirms the uniform distribution of Si, Ge, P elements, while EDS

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spectrum (inset of Si, Fig. 2) indicates the close agreement of sample’s stoichiometry with its initial

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chemical composition.

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HR-TEM was employed to explore the microstructural features of the sintered SiGe nanostructured

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alloy, the images of which are displayed in Fig. 3 and 4. Fig. 3(a), which shows the TEM images of

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the melt-spun and sintered SiGe nanoalloys, suggests that an extremely fine and uniform

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microstructure. A region with ultrafine microstructure (marked as A) magnified to an atomic scale

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(inset B) clearly indicates a set of atomic planes of Si (crystal structure: cubic, space group 3,

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lattice parameter: a=0.543 nm, reference: JCPDS 27-1402) stacked along hkl:111 with inter-planar

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ACCEPTED MANUSCRIPT spacing of 0.314 nm. Another notable feature in this micrograph was the presence of Moiré fringes

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(marked as C) due to an overlap of very tiny crystallites. In an enlarged view, such Moiré patterns

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with an inter-separation of about 0.52 nm are displayed in inset D of Fig. 3(a), which reveals the

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presence of atomic planes of Si (hkl:220) with an inter-planar spacing of 0.192 nm. A set of selected

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area electron diffraction patterns (SAEDPs) along [011] and [123] zone axes of cubic crystal

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structure of Si are displayed in Figs. 3(b) and 3(c), respectively, along with their corresponding

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atomic planes of Si, hkl: 200 and 111 in Fig. 3(b) and hkl: 111, 420 and 331 in Fig. 3(c). Figure

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4(a), which shows an enlarged image of the ultrafine grains depicted in the inset A of Fig. 3(a),

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indicates a very narrow crystallite size distribution between 7 - 10 nm. A few of these ultrafine

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crystallites with inter-planar spacing’s of 0.314, 0.192, and 0.163 are marked in Fig. 4(a). The inset

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B of Fig. 4(a) reveals a lattice scale image with a hexagonal pattern having constituted by a set of

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planes of inter-planar spacing of 0.314 nm, corresponding to hkl: 111. It was further noted that the

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ultrafine crystallites with random orientation, as depicted in Fig. 4(b), results polycrystalline

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behavior in the microstructure. The grains with different orientations and inter-planar spacing’s (Fig.

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4(b)) result in a pattern consisting of planes with hkl: 111, 220, 311, 400 and 331, respectively (inset

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C in Fig. 4(b)). Further, the width of ring pattern shown in Fig. 4(b) exhibits the very fine nano-

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sized grains overlapped/stacked on each other.

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3.2 Thermoelectric transport properties

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The temperature dependence of the thermoelectric properties of melt-spun and sintered SiGe

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nanoalloys samples are shown in Fig 5. Figure 5(a) show the temperature dependence of the

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electrical conductivity of melt-spun and sintered Si80Ge20 nanoalloys synthesized at different cooling

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rates, which indicate a monotonic decrease in the electrical conductivity with increasing temperature

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till ~ 800°C for all the samples, suggesting a typical degenerate semiconducting behavior. However,

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at higher temperature > 800°C, there is a slight increase in the σ, which can be attributed to the

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thermal excitation of charge carriers across the gap above 800°C, in the finite band gap synthesized

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ACCEPTED MANUSCRIPT SiGe alloys. The increase in intrinsic electrical conduction above 800°C was observed in all the

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synthesized samples and is found to be anomalous and marginally affected by the cooling rate in

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melt-spun and sintered nanoalloy samples. These results indicating the increase in the carrier density

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above 800°C are qualitatively similar to those reported earlier for SiGe alloys[20-22]. As a

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consequence of nanostructuring, lower magnitude of σ was observed in the entire measured

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temperature range, which can be attributed to the enhanced scattering of charge carriers by nano-

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scale grain boundaries of crystallites, which are comparable to the wavelength of charge carriers in

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SiGe[20-22]. The HR-TEM images in Fig. 4 clearly indicate that that the crystallite size of the

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synthesized melt-spun and sintered SiGe (typically for a cooling rate of ~ 3.4 x 107 K/s) was

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observed in the range of 7 - 10 nm (Fig. 3(a)), which is also close to the average crystallite size, as

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estimated by the XRD data. It has been also reported earlier that lower dimensional nano-scale

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crystallites would scatter the more charge carriers that leads to drastic reduction in the σ[23, 24].

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Further, Fig. 5(a) suggests an inverse dependence of σ on the cooling rate[25, 26] in melt-spun and

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sintered nanoalloy samples.

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mobility with increasing cooling rate as observed from room temperature Hall measurements (Table

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2). With increasing cooling rate, the crystallite size in SiGe decreases (Table 2), which suggest

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increasing participation of nano-crystallite grain boundaries in the scattering of electrons thereby,

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leading to decrease in the σ.

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Figure 5(b) shows the temperature dependence of the Seebeck coefficient (α) in synthesized melt-

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spun and sintered SiGe nano-alloys, which suggests a nearly-linear increase in α with increasing

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temperature up to a temperature of ~ 750 - 800ºC for all the samples. Above 800 ºC, the thermal

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excitation of carriers, causes a negative contribution to α which arises from the valence band carriers

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resulting in net reduction in α.

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sample melt-spun at a cooling rate ~ 3.4 x 107 K/s and this is the highest value reported thus far for

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SiGe alloys with similar chemical composition, which is ~ 25% higher than its bulk counterpart[20].

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This can be attributed to decrease in charge carriers and carrier

A maximum value ~ 357 µV/K at 800ºC has been realized for

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ACCEPTED MANUSCRIPT Further, the effective mass (m*) was estimated based on energy-independent relaxation time

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approximation and assuming a single parabolic band near room temperature, where the bipolar

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effect is insignificant (Table 2). This table suggests the m* is maximum for the sample melt-spun at

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a cooling rate ~ 3.4 x 107 K/s and the variation in m* was found to be consistent with the

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corresponding changes in α. However, it may be noted from Fig. 5(b) that at high temperatures α

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shows a decrease with increasing temperature, which may be attributed to the thermal excitation of

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charge carriers across the band gap[20, 27].

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The temperature dependence of thermal conductivity (κ) of the synthesized SiGe nano-alloys, melt-

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spun at different cooling rates, is shown in Fig. 5(c). This figure suggests that κ decreases with

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increasing cooling rates employed during melt-spinning and a lowest thermal conductivity of ~ 2.1

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Wm-1K-1 at 900°C is realized for SiGe nanoalloy melt-spun at an optimized cooling rate ~ 3.4 x 107

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K/s. This values of the κ is ~ 50% lower than its bulk counterpart and is amongst the lowest

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reported thus far in similar Si80Ge20 alloys of similar composition[11, 20-22]. A comparison of

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thermal transport observed in the current study with the previous reports on SiGe alloys at 1073 K,

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suggest that the value of κ obtained at optimal cooling rate are significantly lower than that reported

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previously[11, 20-22] and also stipulates the future possibility of attaining κ <1 Wm-1K-1[21] by

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further optimization of processing conditions. Further, the electronic contribution (κe) to the total

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thermal conductivity was evaluated using the Wiedemann-Franz law as κe = LσT (where L

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represents the Lorenz number which is calculated using the temperature dependent Seebeck

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coefficient data (Fig. 5(b)) in the expression L = 1.5 + exp −  ∗ 10 where the units of L is

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in WΩK-2 and α is in µV/K [28]. The Lorenz number was found to lie to be in the range of (1.5-1.7)

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× 10-8 WΩ K-2. The temperature dependent κL was then calculated by subtracting κe from the κ and

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is shown as an inset in Fig 5(c), for all the melt-spun and sintered SiGe nanoalloy samples. A direct

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comparison of the temperature dependences of κ and κL in Fig. 5(c) clearly suggests that the major

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contribution to the observed κ owe their origin to its lattice counterpart [1, 13, 29]. The low value of

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ACCEPTED MANUSCRIPT κL is primarily due to scattering of heat-carrying phonons by the high density of nano-scale grain

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boundaries, owing to nanostructuring[27]. The high cooing rates employed during melt-spinning

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lead to rapid solidification thereby resulting a nano-scale microstructure due to the high under-

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cooling of the melt, which leads to substantial grain refinement. The HRTEM images in Fig. 4 also

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suggest that the crystallite size of the melt-spun SiGe nanoalloys is in the range of 7 - 10 nm, which

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is also supported by the XRD analysis (Fig. 1(a)). Thus, the low value of κ in the melt-spun and

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sintered SiGe nanoalloys, owes its origins to the nano-scale microstructure, resulting from rapid

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solidification during melt-spinning, which results in abundant phonon scattering by nano-scale grain

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boundaries.

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4. Conclusions

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In summary, an enhanced ZT ~ 1.1 at 900°C has been realized in SiGe nanoalloys employing a

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methodology consisting of melt-spinning followed by spark plasma sintering, which is both fast and

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up-scalable. The high cooling rates during melt-spinning were optimized by controlling the speed of

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the water-cooled Cu-wheel, which resulted in a nano-scale microstructure, owing to the rapid

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solidification of the melt. The resulting nano-scale grain boundaries lead to extensive phonon

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scattering, which resulted in very low lattice thermal conductivity ~2.1 Wm-1K-1 at 900ºC, which is

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primarily responsible for the enhanced thermoelectric performance. This facile methodology could

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also be extended other thermoelectric alloys for enhancing their thermoelectric performance.

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Acknowledgements

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The authors sincerely acknowledge the Board of Research in Nuclear Sciences, Department of

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Atomic Energy and Sanction No: 37(3)/14/22/2016-BRNS with BSC, India for the financial

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support. Avinash Vishwakarma would like to acknowledge the financial support from DST-India in

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form of INSPIRE-JRF. The technical support rendered by Mr. Radhey Shyam, and Mr. Naval

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Kishor Upadhyay are also gratefully acknowledged.

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ACCEPTED MANUSCRIPT Table 1: Estimation of average cooling rates Heat Th Tw Th-Tc transfer (Hot (Cool (K) coefficients temperatur temperatur (h) e) K e) K 2 W/(m K) 2.4x105 1773 293 1480

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3.3 x105

1773

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1480

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4.0 x105

1773

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1480

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Cp(kJ/(kg K))

dT/dt Cooling rate (K/s)

2.94

0.70

6.8 x106

2.99

0.57

1.3 x107

2.96

0.39

3.4 x107

Table 2: Average crystallite sizes and electrical transport parameters of spark plasma sintered n-type SiGe alloys synthesized at different wheel speed

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Thickness of melt-spun ribbons (µm) 25

Cooling rate (K/s) 6.8 x106

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1.3 x107

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3.4 x107

Average crystallite size(nm) melt-spun melt-spun and SPS 12 34

Carrier concentration (1019 cm-3)

Mobility (cm2/V s)

Effective mass

4.6

66.1

1.17me

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4.5

65.5

1.20 me

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4.2

62.3

1.13 me

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density (ρ) (g/cc)

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Thickness of meltspun ribbons (µm) 25

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Figure 1: (a) X-ray diffraction pattern of melt-spun and sintered n-type Si80Ge20 nanoalloy and (b d) FE-SEM micrographs of melt-spun Si80Ge20 ribbons at different wheel speed of 23, 27, 30 m/s.

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Figure 2: Elemental distribution maps in melt-spun and sintered n-type Si80Ge20 nanoalloy (a) Si, (b) Ge, and (c) P. Inset of (a) shows the corresponding EDS pattern.

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Figure 3: HR-TEM image of melt-spun and sintered n-type Si80Ge20 nanoalloy showing, (a) uniform  23] microstructure, (b) SAEDP along [011] zone axis cubic crystal of Si, and (c) SAEDP along [1 zone axis cubic crystal of Si. Insets: (A) atomic scale image, (B) moiré pattern in the microstructure.

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Figure 4: HR-TEM results of n-type melt-spun and sintered n-type Si80Ge20 nanoalloy showing (a) atomic scale image with different size distribution of Si, (b) atomic scale image region used for electron diffraction pattern. Insets: (A) ultrafine nanocrystallites, (B) atomic scale image, (C) SAEDP of ultrafine crystallites.

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Figure 5: Temperature dependence of thermoelectric properties (a) Electrical conductivity (b) Seebeck coefficient (c) Thermal conductivity, (d) ZT of n-type Si80Ge20 alloys synthesized employing melt-spun and SPS.

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[21] R. Basu, S. Bhattacharya, R. Bhatt, M. Roy, S. Ahmad, A. Singh, M. Navaneethan, Y. Hayakawa, D. Aswal, S. Gupta, Improved thermoelectric performance of hot pressed nanostructured n-type SiGe bulk alloys, Journal of Materials Chemistry A, 2 (2014) 6922-6930. [22] X. Wang, H. Lee, Y. Lan, G. Zhu, G. Joshi, D. Wang, J. Yang, A. Muto, M. Tang, J. Klatsky, Enhanced thermoelectric figure of merit in nanostructured n-type silicon germanium bulk alloy, Applied Physics Letters, 93 (2008) 193121. [23] L.-D. Zhao, V.P. Dravid, M.G. Kanatzidis, The panoscopic approach to high performance thermoelectrics, Energy & Environmental Science, 7 (2014) 251-268. [24] 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 (2012) 414. [25] Z. Zamanipour, D. Vashaee, Comparison of thermoelectric properties of p-type nanostructured bulk Si0. 8Ge0. 2 alloy with Si0. 8Ge0. 2 composites embedded with CrSi2 nano-inclusisons, Journal of applied physics, 112 (2012) 093714. [26] Z. Zamanipour, X. Shi, A.M. Dehkordi, J.S. Krasinski, D. Vashaee, The effect of synthesis parameters on transport properties of nanostructured bulk thermoelectric p‐type silicon germanium alloy, physica status solidi (a), 209 (2012) 2049-2058. [27] B. Khasimsaheb, N.K. Singh, S. Bathula, B. Gahtori, D. Haranath, S. Neeleshwar, The effect of carbon nanotubes (CNT) on thermoelectric properties of lead telluride (PbTe) nanocubes, Current Applied Physics, 17 (2017) 306-313. [28] H.-S. Kim, Z.M. Gibbs, Y. Tang, H. Wang, G.J. Snyder, Characterization of Lorenz number with Seebeck coefficient measurement, APL materials, 3 (2015) 041506. [29] P.F. Poudeu, J. D'Angelo, H. Kong, A. Downey, J.L. Short, R. Pcionek, T.P. Hogan, C. Uher, M.G. Kanatzidis, Nanostructures versus Solid Solutions: Low Lattice Thermal Conductivity and Enhanced Thermoelectric Figure of Merit in Pb9. 6Sb0. 2Te10-x Se x Bulk Materials, Journal of the American Chemical Society, 128 (2006) 14347-14355.

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High cooling rate of melt-spun ~ 3.4 x 107 K/s resulted in a crystallite size ~ 7 nm

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Lower crystallite size drastically reduced the thermal conductivity ~ 2.1 Wm-1K-1

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Figure of merit 1. 1 at 900°C and this enhancement is > 20% over its bulk counterpart

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