Simultaneous enhancement in thermoelectric performance and mechanical stability of p-type SiGe alloy doped with Boron prepared by mechanical alloying and spark plasma sintering

Accepted Manuscript Simultaneous enhancement in thermoelectric performance and mechanical stability of p-type SiGe alloy doped with Boron prepared by mechanical alloying and spark plasma sintering R. Murugasami, P. Vivekanandhan, S. Kumaran, R. Suresh Kumar, T. John Tharakan PII:

S0925-8388(18)33258-4

DOI:

10.1016/j.jallcom.2018.09.029

Reference:

JALCOM 47448

To appear in:

Journal of Alloys and Compounds

Received Date: 31 May 2018 Revised Date:

21 August 2018

Accepted Date: 2 September 2018

Please cite this article as: R. Murugasami, P. Vivekanandhan, S. Kumaran, R. Suresh Kumar, T. John Tharakan, Simultaneous enhancement in thermoelectric performance and mechanical stability of p-type SiGe alloy doped with Boron prepared by mechanical alloying and spark plasma sintering, Journal of Alloys and Compounds (2018), doi: 10.1016/j.jallcom.2018.09.029. 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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Simultaneous enhancement in thermoelectric performance and mechanical stability of p-type SiGe alloy doped with Boron prepared by mechanical alloying and spark plasma sintering Murugasami Ra, Vivekanandhan Pa, Kumaran Sa*, Suresh Kumar Rb, John Tharakan Tb Green Energy Materials and Manufacturing Research Group,

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a

Department of Metallurgical and Materials Engineering, National Institute of Technology, Tiruchirappalli - 620 015, India. b

Liquid Propulsion System Centre,

*

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Indian Space Research Organization, Thiruvananthapuram - 695 547, India.

Corresponding Author’s E mail: [email protected]

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Mobile: +91 99444 34705, Phone: +91 431 250 3482, Fax: +91 431 2500 133 Abstract

Silicon-Germanium (SiGe) based alloys are the promising and an attractive thermoelectric candidate for radioisotope thermoelectric generators (RTGs) at high temperature

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(>1000°C) application. Intensive efforts are being made to enhance the thermoelectric properties and mechanical stability of p-type SiGe alloy. The current work reports on synthesis of SiGe alloys doped with Boron (B) with varying concentration (0.5–2.0 at.%) through mechanical

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alloying followed by spark plasma sintering (SPS). The synthesized SiGe alloy doped with B1.5

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at.% showcase the superior figure of merit (ZT) ̴ 0.525 at 800°C which results in the significant enhancement of ~9.38 % than the p-type RTG SiGe alloy. The synthesized SiGe alloys exhibits an improvised thermoelectric compatibility factor of 0.968 V-1 (~16 % higher than RTG SiGe alloy). The SiGeB1.5 alloy demonstrated an ever-achieved physical robustness with the bench marked micro hardness of 9.93±0.12 GPa and fracture toughness of 2.36±0.173 MPa√m (48 % higher than SiGe alloy). Keywords: Silicon Germanium alloy, Thermoelectric materials, Mechanical alloying, Spark plasma sintering, Figure of merit, Mechanical properties.

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ACCEPTED MANUSCRIPT 1. Introduction Around the globe, the population and an industrialization growth are concurrently increasing in several folds for year to years which have made serious caution over the need of energy and sustainable greener environment [1]. The higher dependency and an exhaust usage of

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conventional fossil fuels have results in the huge scarcity of reserve to face the future energy demands. It is statistically reported that, the global energy requirement is estimated to 50 Tera Watt per year [2]. In order to meet this requirement, several countries are being made the various

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strategies and diplomatic policies towards the development of an alternative, promising and potential power generation technologies. Alongside, the environment pollution and green house

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effects have been seriously approaching to the critical points which alarming the entire world to seek for the potential alternative [3]. Considering the factual situations, the energy and its associated environment research have become ever important and it turns as global vision [4]. Therefore, it warrants an intensive research which urging the global researcher fraternity for

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finding the potential energy technologies to resolve the existing issues in tandem. It is been observed that, the huge amount more than ̴ 60% is being served as waste from the various thermal systems including automobiles, boilers, combustion chambers, nuclear decay reactors,

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solar energy etc., [5,6,7]. It not only results in the huge operational loss and also causes an

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inferior efficiency of the thermal systems. For the couple of decades the huge interest is on the greener mode of energy scavenging through thermoelectricity technology using thermoelectric (TE) materials. TE material involves the simple and direct conversion of heat into electricity that principally works on Seebeck effect and vice versa it could result in active cooling which named as Peltier effect [8]. Thermoelectric technology has received the tremendous and significant attention in various potential applications considering its technical and functional merits that includes compatibility, noise free operation, non-toxic, maintenance free, no emission of residuals, long functional life etc., [9]. 2

ACCEPTED MANUSCRIPT Furthermore, it significantly features with an unique and remarkable characteristics of energy harvesting in zero gravity. Thus, the thermoelectric materials have become the prime interest to supply electricity in space technology [10]. Among the thermoelectric materials family, silicongermanium (SiGe) alloy is one of the classical and potential candidate being used to harvest the

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electricity at high temperatures (>1000°C). SiGe alloy is highly appreciated for possessing the inherent merits such as abundant availability of Si in earth crust and non-toxicity of Ge. Since early 1960, SiGe alloy have been used in radioisotope thermoelectric generators (RTGs) for

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interplanetary missions [11]. The figure of merit of thermoelectric materials is mathematically


 =

 

.

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represented by [12]

(1)

where, S is the Seebeck coefficient, σ is the electrical conductivity and K is the total thermal conductivity which is contributed by the phonons (Kele) and lattice (Klat) at absolute temperature

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T [13]. The S2σ denotes the power factor of thermoelectric materials. Higher power factor is one among the favorable criteria to achieve higher ZT with remarkable reduction in the thermal conductivity. But it seems that the electrical and thermal conductivities are contradicted in the

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aspect of thermoelectric theory. Therefore, increase of thermoelectric ZT requires substantial

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increase in the electrical conductivity and simultaneous reduction in the thermal conductivity which is the practical challenge to achieve. In the prospectus of practical considerations, the higher ZT is attributed by the both n and

p type in a thermoelectric couple. Enhancement in ZT of p-type SiGe alloys, could eventually make these alloys more reliable and appreciable in the existing RTG application. Furthermore, it pave potential and diversified ways for successful implication in the other niche applications of semiconductor regime. In recent decades, notable research studies were carried out to enhance the ZT p-type SiGe alloys by several structural engineering strategies including nano structuring, 3

ACCEPTED MANUSCRIPT tuning the chemistry by multi phases, doping, second phase nano inclusions in semiconducting matrix etc., [7,14,15] On the other hand, the considerable importance must be given to thermoelectric compatibility factor and the mechanical stability of p-type SiGe alloys as these are the vital factors for functional deployment. Any thermoelectric materials possessing inferior

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on these contexts could leads to catastrophic failures which is undesirable [16]. All these functional needs lies on tailoring of material chemistry and processing designs. There are several liquid and solid-state processing methods that are employed to fabricate SiGe alloy [17, 18].

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However, liquid-based preparation of SiGe alloys results in non-homogeneity, contamination from crucible medium, evaporation of doping due to the difference in higher vapor pressure,

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wider grain distribution [19]. These are the key barrier in achieving excellent SiGe alloy with structural and chemical homogeneity. On the other hand, mechanical alloying (MA) is the potential solid-state synthesis process which demonstrates an effective and economically cheaper method to synthesize diversified thermoelectric alloy systems [20]. In spite, contamination from

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the milling medium has admitted as a serious factor that heading towards detrimental effects in thermoelectric properties. But, it can be overcome by optimizing the milling parameters such as ball to powder ratio, milling speed, milling time, milling medium etc., [21]. Among the powders

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consolidation technologies, spark plasma sintering (SPS) is appreciated for possessing high

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potential for consolidating wider material systems including hard to sinter material systems like ceramics with near theoretical density in a rapid fashion. This leads towards the retention of grain growth phenomena as compared to other counter processing methods like hot isostatic pressing (HIP) or hot pressing (HP) owing to longer time sintering [22]. The current work describes the synthesis of the SiGe alloys heavily doped with B by short duration milling (10h) and spark plasma sintering. The role of SPS mechanisms in the densification kinetics involved during SiGe alloy with varying B doping concentration elucidated in detail. The synthesized p-type SiGe-B1.5 alloys demonstrate the significant enhancement in 4

ACCEPTED MANUSCRIPT ZT ̴ 0.525 at 800°C (~9.38 % higher) and thermoelectric compatibility factor (~16 % higher) as compared to the commercialized p-type RTG alloys. In addition, SiGeB1.5 demonstrated the ever-reported hardness (9.93±0.12 GPa) and fracture toughness of 2.36±0.17 MPa√m with the 48% enhancement was obtained. The obtained results were compared among the synthesized

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SiGe alloys and also with the start-of -art thermoelectric material systems. 2. Experimental methods

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2.1 Material selection

The preparation of SiGe alloy was synthesized by the sequential process involving

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mechanical alloying and SPS. The powder ingredients such as Si (particle size: -325 mesh, 99.50 % purity) and Ge (particle size: -325 mesh, 99.99% purity) were chosen to fabricate bulk SiGe alloy. Both the powers were supplied by M/s. Whole Win (Beijing) Materials Science and Technology Co., Ltd, China. Boron (B) (Purity. 99.5 %, Loba Chemie, Mumbai) was chosen as

2.2 Mechanical alloying

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doping element.

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Si-Ge is taken in the atomic stoichiometry of 80:20. B is taken as doping element at varying concentration from 0.5 – 2.0 at.% to synthesize the p-type SiGe alloy. The elemental powders

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were milled in high energy ball mill (Fritsch Pulverisette, Germany) using tungsten carbide vials (250 ml) and balls of diameter 10 mm. The vials were sealed with high purity argon (Ar) (99.999%) atmosphere. The ball to powder ratio (BPR) was maintained as 20:1. Steric acid of 0.25% of powder mass was used as process control agent (PCA). The mechanical alloying was carried out at 300 r.p.m for 10h with the milling cycle comprises of 10 minutes milling followed by 20 minutes cooling. 2.3 Spark plasma sintering

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ACCEPTED MANUSCRIPT The mechanically alloyed nanostructured SiGe alloy powder doped with B was sintered in high density graphite die upto 1150°C with the heating rate of 100°C at one step sintering cycle using spark plasma sintering (Dr.Sinter Lab, Japan). After reaching 1150°C the sample was soaked for 5 min followed by furnace cooling till room temperature. The sintering process

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was carried out with an applied pressure of 40 MPa in the vacuum atmosphere (<10 Pa). The sample temperature was continuously monitored using digital pyrometer integrated with SPS. The response of SPS processing parameters such as temperature, pressure, current, voltage and

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displacement during sintering were recorded for every successive 5 seconds using the data acquisition software. During SPS, the instantaneous reduction in height (∆L= L – Lf) of the SiGe

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powder was recorded from the digital Z axis displacement gauge of SPS. The corresponding instantaneous densification rate is estimated using the eqn.(2) [23]. 

=  

(2)

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Where, L and Lf are the instantaneous and initial height, respectively. The δ and δf are corresponds to the instantaneous and final density (i.e. theoretical density), respectively of the

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SiGe powder. The relative density of sintered SiGe alloy samples was estimated by Archimedes principle in density measurement kit (AY220, Shimadzu, Japan).

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2.4 X-ray diffraction and electron microscopy studies The phase analysis, crystallographic information such as lattice parameter (Å), crystallite

size (D) and lattice strain (ɛ) of milled powder and sintered SiGe samples were studied from Xray diffraction (XRD) patterns using X-ray diffractometer (ULTIMA-III, Rigagu, Japan). XRD patterns of the milled powder and bulk SiGe alloy were recorded in the 2ϴ range of 20° to 80°. The crystallite size (D) during mechanical alloying and the corresponding strain (ε) induced in

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=



 

+ 4ε

(3)

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Where, D is the average crystallite size, K is the shape factor (i.e. 0.9), λ is the wavelength of X-rays (λ = 1.541Å for Cu Kα radiation), βhkl is the peak broadening diffracted from hkl planes which measured at half of its maximum intensity (in radians), ε is the strain

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induced in the lattice during milling and θ is the diffraction angle.

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The induced strain (ε) is given by the relation as follows [24] 

 ε = ! "#$

(4)

High resolution transmission electron microscopy (HRTEM) were made on milled and

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sintered samples using JEOL electron microscope (2100-JEOL, Japan). Prior to the TEM analysis, the SiGe alloy powder was dispersed in ethanol and sonicated for 360s in ultrosonicator bath. The sonicated SiGe powder was spread over the Cu grid (200 mesh) and dried. In the case

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of sintered bulk SiGe sample, it was cross sectionally diced for ~300 micron thickness using slow speed diamond saw precision cutter using diamond wheel. The sliced sample was manually

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polished to the final thickness of 90µm adhering to the standard metallographic polishing protocol using emery sheets. Then the sample was cut in to the disk of dia of 3 mm with the through hole using focused ion beam (FIB) milling. 2.5 Thermoelectric properties of bulk SiGe alloys 2.5.1. Seebeck coefficient and electrical conductivity measurements The thermoelectric transport properties such as Seebeck coefficient (S) and electrical conductivity (σ) were simultaneously measured using Seebeck coefficient and electrical 7

ACCEPTED MANUSCRIPT resistance system (LSR-3 Seebeck, Linseis, Germany). The four-probe principle was employed for the measurement. The thermoelectric properties were studied in the temperature range of 100 – 800°C in the high purity helium (99.999%) at the constant pressure of 1 bar. The measurements were recorded at every successive 50°C with three values at each temperature.

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During measurement, the temperature gradient of 50°C was maintained between the top and bottom sides of the sample. The power factor or thermo power (S2σ) was estimated from the recorded S and σ values.

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2.5.2 Thermal conductivity measurement

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The thermal diffusivity (α) of SiGe alloys in the range 50°C to 800°C was measured using laser flash apparatus (LFA) in high purity Ar. (99.999%) atmosphere. The measurement was made at a successive temperature rise 50°C. The obtained thermal diffusivity of the bulk SiGe alloy was applied in the following empirical relation to estimate the thermal conductivity (k)

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[25].

%=

&

'.(

(5)

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where, ρ is the experimental density of the sample (measured from Archimedes method) and Cp

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is the specific heat capacity of the sample. 2.5.3 Thermoelectric compatibility factor Synder et. al suggested a relation to estimate the thermoelectric compatibility factor (s)

to understand the suitability of thermoelectric materials. This factor is the primary criterion for selecting the potential thermoelectric materials to design the excellent TEG device. The s is governed by variables such as figure of merit (ZT), Seebeck coefficient (S) measured at an absolute temperature (T). It is mathematically given by [26]

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s =

√+,-.+ .-

(6)

2.6 Mechanical properties of sintered SiGe alloys The fracture toughness of the bulk SiGe alloy was measured from the indentation technique

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using Vickers hardness tester (Vickers 402 MD, Wilson hardness, China). The load of 0.3 Kg was applied on the bulk sample with the dwell period of 10 sec. The palmqvist crack geometry

the fracture toughness (KIC) [27]. 6

8

#7

)

(7)

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/0 = 0.0319 (

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was observed on the sample and the corresponding mathematical relation was used to calculate

Where, P is the applied load (N), a is the mean indentation half-diagonal length (m) and l is the crack length (m).

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3. Result and discussion 3.1. X-ray diffraction analysis

XRD patterns corresponding to milled nano structured SiGe alloy powder and spark

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plasma sintered bulk SiGe alloy is shown in Fig.1. XRD patterns of 10h milled Si-Ge powder

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reveals the crystalline peaks corresponding to the primary SiGe alloy phase. It is well confessed by vanishing of peaks corresponding to the Ge after milling. It strongly infers that short duration milling effectively promote the solid-state reaction between the Si and Ge resulting SiGe alloying. It is effectively contributed by the higher activity between elemental species Si and Ge during milling. This leads to the diffusion of Ge atoms into Si lattice and form SiGe alloy. The diffraction from the planes (111), (220) and (311) at Bragg angles (2ϴ) respectively at the vicinities 28.44°, 47.31° and 56.13° are come in concurrence with the standard JCPDS card. In addition to alloying, the peaks of SiGe are widely broadened due to the crystallite size reduction

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strain of SiGe doped with B is summarized in the Table 1. Remarkably, B doping strongly influence in the crystallographic evolution in SiGe lattice. This can be understood by the shift of SiGe principal plane (111) from its mean vicinity of ~28° with the increase in B doping

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concentration. This is due to the synergetic contribution of processing strain and corresponding structural defects including lattice disorder and distortion. Accordingly, increase in B from 0.5

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at% to 2.0 at% significantly increases the lattice parameter of Si (Fig.1b). It provides the practical agreement with the empirical relation for the binary compound (i.e. Si1-XGeX, which has the form of a0(X) = ASi(1-X) + aGeX, where a0(X) is the lattice constant of Si1-XGeX) [28]. Figure 1c shows the XRD patterns of spark plasma sintered bulk SiGe alloy peaks are same as SiGe

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alloy phase observed after mechanical alloying. After SPS, the peaks are observed narrow and sharper that denotes the growth of SiGe grain during sintering. The increased crystallite size and

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its lattice strain of bulk SiGe are given in Table 1.

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Fig.1. XRD patterns of a) SiGe alloy power doped with B after 10h milling, b) Vegard’s plot of SiGe with varying B concentration (at.%) and c) sintered bulk SiGe doped with B (0.5-2.0

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at.%)

Table 1. Crystallographic quantification of milled and sintered SiGe alloy After SPS (at 1150°C)

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After ball milling (10 h)

As mixed B0

-

137

-

-

Crystallite size (nm) -

0.0

16

0.002

100

80

0.004

B1

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Sample Code

B doping(at. %)

0.5

13

0.007

99

96

0.001

B2

1.0

15

0.006

98

119

0.002

B3

1.5

17

0.007

97.5

118

0.001

B4

2.0

24

0.009

97

117

0.001

Crystallite size (nm)

Lattice strain (%)

Relative density (%)

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Lattice strain (%) -

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3.2 Transmission electron microscopic characterization TEM image of milled SiGe alloy powder doped with B2.0 at% is shown in Fig.2.ia. It

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exhibits finer and homogenous nanostructured SiGe particles. The corresponding selective area electron diffraction (SAED) depicts the polycrystalline rings with the finer white spots of planes (111), (220) and (311) corresponding to SiGe phase (Fig.2i.a inset). This confirms the

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nanostructure phenomena of the milled SiGe alloy powder. The high resolution TEM image (Fig.2.ib) clearly exhibits the detailed information of the milled SiGe alloy powder. The short

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duration milling significantly affect the Si and Ge particles by mechanical collision that increases the defect concentration. The corresponding lattice distortion and lattice misorientation are marked as 1 and 2, respectively (Fig.2.i.b). The interplanar distance (d) of SiGe-B2.0 at% alloy powder is measured as 0.317 nm in the SiGe (111) plane. This measured value is observed

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higher than the d value of Si (0.313 Å). This observation is due to the solid state diffusion of Ge with the substantial doping of B atoms (2.0 at.%) and the concurrent stress field in the lattice of SiGe. The TEM discussion on the nanostructure and the variation in d spacing provides strong

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attestation to the XRD discussion (Fig.1a).

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Fig.2. TEM images (i) a.SiGeB2.0 at% powder (inset: SAED pattern), (b) HRTEM image and

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(ii) a. sintered SiGe alloy with B2.0 at.% and b. EDX elemental mapping of bulk SiGeB2.0

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alloy and corresponding spectrum

Figure 2.ii.a shows the TEM observation of sintered SiGe alloy. The sintered sample

possess the excellent density of (> 97) with polycrystalline grains. The bulk SiGe possess equiaxed grain of size ~500 nm. Figure 2.ii.b shows the elemental mapping corresponding to bulk SiGe-B2.0 at.%. It is understood that, the bulk SiGe alloy possess the uniform distribution of constituent elements viz. Si, Ge and B. Hence, it confirms the chemical and structural homogeneity. Furthermore, EDS spectrum conceded the presence of constituent elements Si, Ge and B. 13

ACCEPTED MANUSCRIPT 3.3 Densification kinetics of nano SiGe alloy powder during SPS The instantaneous densification rate and relative density kinetics of nano structured SiGe alloy powder doped with varying B concentration is portrayed in Fig.3. Interestingly, the

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densification profiles exhibits the flat line profiles till some point and ramped rapidly during the higher densification. This leads to an argument on the nanostructured SiGe alloy powder densification and thermal kinetics in the flat line profile regions and the drastic thermodynamical phenomena at higher temperature. The densification mechanism of the SiGe powder progressed

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with three sequential steps namely as primary (RT to 200°C), secondary (200 - 600°C) and final

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densification (600 - 1150°C). Prior to the sintering, the load of 40 MPa is applied to the highdensity graphite die filled with SiGe powder. The imposed pressure helps in attaining the cold compaction of the powder mixture. The finer nanocrystalline powder attributed the cold compaction in the range of 65 – 70 % with varying B concentration. During SPS, in the primary stage (RT to 200°C), there is no thermodynamical contribution. However, the applied uniaxial

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pressure helps in the rearrangement of nano structured SiGe alloy particles. The rearrangement enhances the inter-particle contacts network path with neighbor particle [29]. Increase in the SPS

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temperature beyond till 600°C the electron migration density is increases and results in the higher spark plasma generation. This generated plasma encapsulates each SiGe particles and

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triggers the joule heating (self-heating) in the localized contacts [29]. The resistance between the oxide layer and the powder particles causes the capacitor gaps and effects in the breaking of inherent oxide layers and ensure the uniform heating of particles. This refers to the surface cleaning of the particles [30]. The plasma assisted surface cleaning results in newer surface evolution that enhances the higher surface energy. After crossing the temperature 900°C, the crucial thermodynamical phenomena has taken place. This promotes the surface melting of the powders and the formation of necking between the particles and initiates the surface diffusion process. The thin amorphous layer results in the plastic flow in the particles. It further assisted in 14

ACCEPTED MANUSCRIPT self alignment and plastic flow into intergranular pores thereby the densification is increased. Therefore, the increase in the ramp is observed. The SPS temperature promotes the higher volume transportation due to the diffusion process. The occurrence of diffusion rate is very rapid due to the increase in the SPS temperature. In addition, the pores are eliminated which

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substantially increases the density. This can be seen by the rapidly increasing densification curves. This regime is marked as the final densification stage (Fig.3a). At this region the densified SiGe alloy undergoes the steady creep phenomena due to the imposed constant

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pressure under temperature. The corresponding instantaneous densification (%) is shown in Fig.3b. The superior densification more than 97% is observed. The SiGe without doping exhibits

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the excellent density equals to theoretical value (100%). Wherein, in SiGe with doping concentration of B from 0.5 to 2.0 at.% significantly alters the densification and diffusion kinetics. This is due to the chemical potential which results in the varying diffusion velocity. It seems increase in B concentration reduces the density values. The higher concentration of B2.0

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at.% offers the lowest density value of 97%.

Fig.3. SPS plots of SiGe powder doped with B at 1150°C a) instantaneous densification rate and b) instantaneous relative density 3.4 Thermoelectric properties of bulk SiGe alloy

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The temperature dependent transport properties of sintered bulk SiGe alloy doped with varying B con concentration of holes in SiGe semiconductor generated by the extrinsic doping of B. Thus, it acts as a primary charge carriers by acceptor phenomena. Notably, It can be seen that the charge carriers are strongly depends on the carrier density of SiGe with varying B concentrataion when

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varying from 0.5 to 2.0 at.%. Thereby, an increase in temperature provides the activation energy to the charge carriers to drift from valence band to conduction band. Figure 4a depicts that, S values decrease monotonically with the increase in B concentration in the temperature range

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100°C - 800°C. The lower and higher S values of 145 µV/K and 294 µV/K are recorded for B0.5 addition at temperatures 100°C and 800°C, respectively. On the other hand, the S values are

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significantly reduced with an increase in the B concentrations (>0.5 at.%). The lowest S values are recorded at temperatures 100°C and 800°C are 101 µV/K and 229 µV/K, respectively, for the SiGe heavy doped with B of 2.0 at % concentration. It is understood that, the increase in the extrinsic doping of B beyond 0.5 at.% in SiGe attributes in the significant reduction in the

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temperature difference (∆T) between the hot (Th) and cold (Tc) junction. Notably, it reduces the change in voltage (∆V) generation across the junction. Therefore, the change in Seebeck values (∆S) are significantly reduced (where, ∆S = ∆V/∆T) [31]. It is further observed that, SiGe doped

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with B0.5 at.% and B1.0 at.% shows the significant improvement comparing to S value of p-type

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RTG SiGe alloy that is denoted in dashed straight line (Fig.4a) [10, 21, 32]. The enhancement attributed by the SiGe due to doping of B0.5 at.% and B1.0 at.% are 22% and 1.09%, respectively than the p-type RTG SiGe alloy. The electrical conductivity performance of the SiGe alloy significantly increases with the increase in B concentration (0.5 to 2.0 at.%) (Fig.4b). On the other hand, the electrical conductivity characteristics of SiGe alloys doped with B is exhibiting the higher electrical conductivity at 100°C and gradually reducing with the increase in the temperature up to 800°C. The higher electrical conductivity (σ) value of 44 x 103 s/m is obtained for SiGe doped with B0.5 16

ACCEPTED MANUSCRIPT at.% at 100°C. This is drastically reduced with further increase in the temperature till 800°C. The lower σ value of 21x103 s/m is observed at the maximum temperature of 800°C. At lower temperature (100°C), superior electrical conductivity of 61.53 x 103 s/m is observed for SiGe highly doped with B2.0 at.%. This is gradually reduces to 37 x 103 s/m at 800°C. However, these

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observations are significantly higher than the p-type SiGe RTG which demonstrates higher σ value of 88 x 103 s/m at 100°C and lower σ value of 32 x 103 s/m at 800°C. One among the primary fact is that, the higher B doping of 2.0 at.% increases the free charge carriers in the bulk

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SiGe alloy which potentially exhibits the higher mobility that is excited with the temperature. On the other hand, the higher B concentration from 0.5 at.% to 2.0 at.% remarkably increases the

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crystallographic defect concentration. The Vegard’s plot strongly depicts the increasing of lattice parameter (Å) (Fig.1b). This leads towards the higher order lattice distortion and dislocation leading towards the lattice mismatch in the SiGe. In addition to that, the instantaneous density plot (Fig.3b), reveals the availability of porosity in SiGe with increasing B doping concentration.

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The lattice mismatch and porosities notably acted as scattering centers and scatters the charge carriers with the increase in the temperature. It is furthermore noticed that, SiGe highly doped with B2.0 at.% shows the drastic increment in the σ values beyond 300°C that reports the higher σ

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values at 800°C which exhibits the closer value to SiGe doped with B1.5 at.%. This is due to the

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degeneration phenomena occurrence in the charge carriers in the bulk SiGe alloy. This occurrence possibly expected by the reduction of band gap of SiGe. The SiGe with higher doping of B2.0 at.% showcased the enhanced σ values of 62 x 103 s/m and 38 x 103 s/m at 100°C and 800°C respectively. The significant enhancement contributed by the SiGe alloys doped with B1.5 at.% and B2.0 at.% are 19% and 16% respectively than the SiGe RTG (p-type) alloys (maximum value marked as straight dashed line in Fig.4b).

Figure 4c shows the power factor (S2σ) plots at varying temperature from 100°C to 800°C. This plo considerable reduction in thermal conductivity (Ref. Eq.1). The power factor values of SiGe 17

ACCEPTED MANUSCRIPT doped with B (0.5 – 2.0 at.%) seems increasing and almost close in performance at 600°C. However, SiGe alloys doped with B1.5 and B2.0 at.% are surpassing the other synthesized compositions. The superior power factor is observed for SiGe alloys with B1.5 at.% and B2.0 at.% doping pertaining 19 µW/cm-K2 and 20 µW/cm-K2 respectively, at 800°C. It may be due to the

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higher activation kinetics involved to accelerate the mobility of high concentration holes after 600°C. In the case of p-type RTG SiGe, the power factor increasing to 19.7 µW/cm-K2 at 600°C. It gradually reducing and obtained 18.58 µW/cm-K2 at 800°C. It seems the synthesized SiGe

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alloy doped with B2.0 at.% and B1.5 at.% delivers the 5.8% and 4% enhancement in the superior

dashed straight line in Fig.4c ).

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power factor performance than the commercial RTG SiGe alloy (maximum value is denoted as

The total thermal conductivity (k) contributed by both lattice and phonon of SiGe alloys doped with B is presented in the Fig.4d. In general, it is observed that the k value of synthesized SiGe alloys observed enhancing with the increase in B concentration. The k curves of SiGe alloys are

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monotonically reducing from 100°C to 800°C. At 100°C, SiGe doped with B0.5 at.% shows k value of 3.95 W/m-K and heavily doped SiGe with B2.0 at.% contribute the k value of

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4.47 W/m-K. On the other hand, with the increase in the temperature till 800°C the same SiGe alloy samples (doped with B0.5 and B2.0 at.%) demonstrates favorable reduction in k to 3.47 W/mand

3.95

W/m-K,

respectively. At

higher

temperature

(800°C),

the

reduction

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K

of k corresponding to the synthesized SiGe alloy doped with B of 0.5 at.% to 2.0 at.% are quantified to be 15.7%, 7.7%, 10.6% and 4.1%, respectively. Figure 4e exhibits the temperature dependent figure of merit (ZT) of SiGe alloy doped

with varying B concentration. The dimensionless ZT plot shows the increasing trend and reaches the higher value at 800°C. The high temperature (at 800°C) ZT is quantified and compared in the Fig.4f. The higher ZT value of 0.525 and 0.520 are recorded for SiGe alloys doped with B1.5 and B0.5 at.%, respectively. Where in, other composition SiGe doped with B2.0 and B1.0 at.% show the 18

ACCEPTED MANUSCRIPT ZT value of 0.498 and 0.462 respectively. However, the variation among the ZT values are in the negligible range. Notably, the recorded ZT values of synthesized SiGe alloys (with B doping) shows an significant enhancement as compared to RTG SiGe alloy (p-type) of ZT=0.48 [10]. The significant enhancement (in %) of ZT comparing to the RTG SiGe alloy (p-type) are 8.33% ,

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9.38% and 3.75%, respectively for the SiGe alloys with B concentration of 0.5, 1.5 and 2.0 at.%. These variation and the enhancement in the ZT value of SiGe alloys are remarkably attributed by the dependent variables such as power factor (S2σ) and thermal conductivity (k). These variables

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lies on the various crucial and tailorable factors such as chemical gradient, concentration of crystallographic defects such as lattice mismatch, dislocation due to extrinsic B doping and

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corresponding carrier concentration [32]. In addition to that, the crystallite size and the porosity are the noticeable entities that could also have the significant effect on the band gap in the SiGe

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alloys that influences transport phenomena to achieve higher ZT [33].

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Fig.4. Temperature dependent thermoelectric properties of SiGe alloy doped with B at varying concentration a) Seebeck coefficient, b) electrical conductivity, c) power factor, d) thermal conductivity, e) Figure of merit (ZT) and f) comparison of high temperature ZT at 800°C

20

ACCEPTED MANUSCRIPT Thermoelectric compatibility factor of synthesized p-type SiGe alloys is shown in Fig.5a. The S (Fig.4a) and higher ZT (Fig.4e) values are the notable factor in deciding the TE compatibility phenomena. The temperature dependent TE compatibility is observed monotonically reducing with the increase in the temperature (from 100° to 800°C). The high temperature (at 800°C) TE

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compatibility is compared in the Fig.5b. The SiGe doped with B of 1.5 and 2.0 at.% exhibits the superior values of 0.968 V-1 and 0.911 V-1, respectively. These observations provides the significant enhancement of 15.65% and 8.84% as compared to RTG SiGe (p-type) alloy

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pertaining the value of 0.837 V-1 (maximum value denoted as straight line in the Fig.5b). Where in, SiGe doped with lower doping concentration of 0.5 at.% and 1.0 at.% respectively, yields the

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lower values of 0.738 V-1 and 0.801 V-1.

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Fig.5. a) Thermoelectric compatibility factor at varying temperature and b) high temperature thermoelectric compatibility factor at 800°C

3.5 Mechanical properties of bulk SiGe alloy Figure 6 shows the micro hardness and fracture toughness properties of bulk SiGe alloy. It is observed that SiGe doped with B shows that higher hardness than the undoped bulk SiGe alloy. The hardness is observed increasing with the increase in B doping concentration from B0.5 at.% (9.4±0.10 GPa) to B1.5 at.% (9.9 ±0.180 GPa) concentration and decreases for SiGe doped 21

ACCEPTED MANUSCRIPT with B2.0 at.%. Where as, the lower hardness value is recorded for bulk undoped SiGe as 8.59 ±0.18 GPa. The significant improvement in hardness with the incorporation of B in SiGe is primarly due to the two factors namely physical nature and crystallographic changes. In first case, B possess higher hardness than the SiGe, hence incorporating B in SiGe acts as a

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dislocation barrier in the SiGe solid solution thereby offers the resistance. In addition to that, B occupies the interstitial position in SiGe lattice. Hence, it causes the higher tensile strain fields that results in strain hardening phenomena. The both factors contributed towards the higher

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hardness values of SiGe. At the same, it is strongly expected that when B is in the range of 1.5 at.% to 2.0 at.% certainly offers negligible dislocation in SiGe, which is due to the ductility

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nature of B. Therefore, it could reduce the brittle failure of SiGe and slightly reduces hardness from 9.93±0.123 (SiGeB1.5) to 9.702±0.13 (SiGeB2.0). Similarly, the bulk SiGe shows the fracture toughness value of 2.17±0.1 MPa√m and the values are gradually reduces to 1.88 MPa√m corresponds to SiGe doped with B1.0 at.%. Then, it increases to 2.36±0.173 and corresponding to SiGe heavily doped with B1.5 at.% and B2.0 at.%,

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2.09±0.12 MPa√m

respectively. The decreasing trend of fracture toughness values indicates the brittle fracture behaviour. SiGe doped with B1.5 at.% and B2.0 at.% significantly reduces the brilltle nature by

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offering significant ductility. This phenomena provides negiligle deformation thereby offers

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higher the resistance to the SiGe alloys doped with B1.5 and B2.0 at.% concentration. Therefore, it offers the increasing of fracture toughness values. In general, the hardness shows increasing and the fracture toughness decreasing for the SiGe alloys doped with B of 0.5 at.% and 1.0 at.%. After that, the hardness and fracture toughness phenomena seems going in the parellel in trend for the SiGe alloys doped with B1.5 at.% and B2.0 at.%. Although the variation is observed in the neglible range, it is expected that SiGe doped with B2.0 provides the significant chemical gradient and the corresponding crystallographic evolution are notably reduces the hardness values. The compariosn of fracture toughness property of synthesized SiGe alloys (with varying B

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ACCEPTED MANUSCRIPT concentration) with the state of art thermoelectric materials (Fig. 6b) [34-37]. The superior

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enhancement of 36% is witnessed for undoped SiGe alloy and 48% for SiGeB1.5 at.%.

Fig.6. Mechanical properties of SiGe alloys a) micro hardness cum fracture toughness and b) comparison of fracture toughness with state-of-art TE material systems

4. Conclusion

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The SiGe alloy doped with B (0.5-2.0 at.%) synthesized through mechanical alloying and consolidated using spark plasma sintering. The detailed characterization of powder and bulk

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SiGe alloy samples were characterized from XRD and electron microscopy studies. The thermoelectric and mechanical properties of the SiGe alloys doped with B were evaluated.

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Following are the significant observations were obtained and it is summarized in brief as follows. •

Short duration milling (10h) elemental powders such as Si and Ge with B (0.5 – 2.0 at.%) using high energy ball milling results in higher fracturing due to the brittle nature of elemental powders resulting in nanostructuring of SiGe alloy powder. Furthermore, while milling the mechano-chemical reaction involving solid state diffusion between Si and Ge followed by intermittent diffusion of doping of B in SiGe alloy.

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ACCEPTED MANUSCRIPT •

During SPS, the densification kinetics reveals that mechanically alloyed SiGe powder doped with B of varying concentration (0.5 – 2.0 at.%) provides the higher stability till 900°C and beyond that till 1150°C as the temperature and pressure ramp up the densification. The near theoretical densification (100%) is obtained for undoped SiGe and

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with the increase of doping element B from 0.5 to 2.0 at.% lowering the density of bulk SiGe to 98%.

XRD and TEM studies confirms the SiGe phase formation with the nano/ultrafine grain

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•

feature observed in the sintered bulk SiGe. The Vegard’s plot shows the increase in the

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lattice parameter (Å) with the increase in B doping concentration. The undoped SiGe possess the 5.438 Å and SiGeB2.0 at.% shows 5.463 Å. This observation clearly confessed the lattice mismatch and defect concentration. The SEM-EDX elemental mapping and quantification confirm the chemical homogeneity of bulk SiGe heavily

•

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doped with B2.0 at.% and near theoretical stoichiometry.

In SiGe, doping of B and substantial increase in its concentration significantly attributes in the temperature dependent transport properties. Seebeck coefficient (S) plot of SiGe

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alloy doped with B (0.5 – 2.0 at.%) showcase the increasing trend with the increase in the temperature. However, higher the B concentration reduces the S values. The SiGe doped

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with B0.5 and B2.0 at.% exhibits the higher and lower S values of 294µV/K and 229µV/K respectively, at higher temperature of 800°C.

•

In the case of electrical conductivity (σ), SiGe doped with B reduces from 100°C to

800°C. At 800°C, the higher electrical conductivity of 38 x 103 S/m is recorded for SiGe with B1.5 at.% and the lowest electrical conductivity value of 21 x 103 S/m for SiGe doped with B0.5 at.%. The reducing trend of σ plot is attributed due to higher order lattice

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ACCEPTED MANUSCRIPT distortion and dislocation concentration leading towards the lattice mismatch that scatters the charge carriers induced in SiGe by an extrinsic B doping. •

The SiGe with B2.0 at.% exhibits the superior power factor (S2σ) of 20 µW/cm-K2 with

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the optimum S and σ values of 229 µV/K and 37 x 103 s/m respectively. On the other hand, the lower S2σ value of 17.5µW/cm-K2 is recorded for SiGe doped with B1.0 at.% which has the S and σ values of 243 µV/K and 30 x 103 s/m respectively.

The total thermal conductivity (K) attributed by both phonons and lattice is observed

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monotonically decreasing with the increase in both temperature. The SiGe doped with

•

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B0.5 at.% and B2.0 at.% shows 3.47 W/m-K and 3.95 W/m-K respectively, at 800°C. The synthesized SiGe alloys exhibits the enhanced Figure of merit (ZT) than the RTG SiGe alloys. The superior enhancement in ZT are observed for the SiGe alloys doped with B0.5 at.% and B1.5 at.% with 0.520 and 0.525. These values shows the enhancement

•

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of 8.33% and 9.38% as compared to RTG SiGe alloy. The thermoelectric compatibility value was derived from the obtained S and ZT values.

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The SiGe doped with B1.5 at.% and B2.0 at.% reveals the values of 0.968 V-1 and 0.911 V-1 with the enhancement of 15.6% and 8.84%, respectively as compared to RTG SiGe alloy

•

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(p-type) that has 0.837 V-1.

Beyond thermoelectric properties, the increase in B doping concentration in SiGe significantly increases the hardness and fracture toughness. The SiGe alloy hardness is increasing with the increase in B doping concentration from B0.5 at.% (9.4±0.10 GPa) to B1.5 at.% (9.9 ±0.180 GPa) concentration and decreases for SiGe doped with B2.0 at.%. The synthesized SiGe (undoped) shows the fracture toughness of 2.17 ± 0.10 MPa√m. This value witnessed the enhancement of 36% compared to the earlier reported SiGe with 25

ACCEPTED MANUSCRIPT the fracture toughness of 1.6 ± 0.06 MPa√m. On the other hand, the ever reported bench mark fracture toughness is observed for the SiGe alloy doped with B1.5 at.% with 2.36 ± 0.17 MPa√m. Comparing with other start of art thermoelectric systems the mechanical

Acknowledgement

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properties of synthesized alloys exhibits superior among that ever reported so far.

The authors would like to express their sincere thanks to the Indian Space Research (ISRO)

for

the

financial

support

ISRO/RES/3/652/2013-2014).

RESPOND

scheme

(Ref.:

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Highlights

•

Synthesis of SiGe alloys doped with Boron with superior density (>97%) by MA and

•

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SPS. Potential enhancement in the ZT of 0.525 (9.38% higher than the p-type RTG SiGe alloy). •

Higher thermoelectric compatibility factor of 0.968 V-1 (16 % higher than RTG SiGe alloy). Bench marked micro hardness of 9.93±0.12 GPa.

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Giant enhancement in fracture toughness of 2.36 ± 0.173 MPa√m (48% higher than

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undoped SiGe).

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•