Effect of Ni and carbon coated Ni addition on the thermoelectric [616951]

Materials Chemistry and Physics
Manuscript Draft

Manuscript Number: MATCHEMPHYS -D-16-02636

Title: Effect of Ni and carbon coated Ni addition on the thermoelectric
properties o f 25Bi2Te3+75Sb2Te3 base composites

Article Type: Full Length Article

Keywords: Bismuth telluride, Gas -atomization, Nanoparticle,
Thermoelectric performance, Nanocomposite.

Abstract: This paper report the effect of Nickel (Ni) and carbon coated
Nickel (C -Ni) on the thermoelectric and mechanical properties of
25Bi2Te3+75Sb2Te3 base composites. Ni and C -Ni powders were synthesized
by pulse wire evaporation and mixed with 25Bi2Te3+75Sb2Te3 in a planetary
ball milling. The morphology of the Ni and C -Ni powders and x/BST (x=0,
Ni and C -Ni) composites were examined using transmission electron
microscope (TEM) and scanning electron microscope (SEM). The
thermoelectric properties of the GA/x (x = 0, Ni and C -Ni) composites
shows that the addition of Ni in creases the carrier concentration while
the presence of C -Ni reduces the carrier concentration to a level
comparable to the bare sample (x = 0). Subsequently, the Seebeck
coefficient of the C -Ni sample increases by about 18% more than bare
sample (x = 0). The thermal conductivity of the Ni and C -Ni sample were
considerably lower at room temperature and T < 350 K compare to the bare
sample (x = 0). The mechanical properties of the Ni/BST and C -Ni/BST
composite samples show a three -fold improvement compared t o the bare
sample (x = 0).

Graphical abstract

Graphical Abstract

Highlights
 Ni and Carbon coated Ni nanoparticles were incorporated into 25Bi 2Te3+75Sb 2Te3
(BST) matrix.
 Seebeck coefficient increased by 18% for BST/carbon coated Ni composite s.
 BST/carbon coated Ni composite reduces the thermal conductivity ( 21%).
 The Vickers hardness of the BST/C-Ni composite samples significantly improved. Highlights (for review)

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65 1 Effect of Ni and carbon coated Ni addition on the thermoelectric properties of
25Bi 2Te3+75Sb2Te3 base composites
Sang Min Yoon , Peyala Dharmaiah , Olu Emmanuel Femi , Chul Hee Lee, and Soon -Jik Hong*
Division of Advanced Materials Engineering, Kongju National University, 275, Budae -dong, Cheonan City,
Chungcheongnam -do, 330 -717, Republic of Korea.
*Corresponding author E -mail: [anonimizat]

Abst ract
This paper report the effect of Nickel (Ni) and carbon coated Nickel (C -Ni) on the
thermoelectric and mechanical properties of 25Bi 2Te3+75Sb2Te3 base composites . Ni and C –
Ni powders were synthesized by pulse wire evaporation and mixed with 25Bi 2Te3+75Sb2Te3
in a planetary ball milling. The morphology of the Ni and C -Ni powders and x/BST ( x=0, Ni
and C -Ni) composites were examined usi ng transmission electron microscope (TEM) and
scanning electron microscope (SEM). The thermoelectric properties of the GA/x (x = 0, Ni
and C -Ni) composites shows that the addition of Ni increases the carrier concentration while
the presence of C-Ni reduces the carrier concentration to a level comparable to the bare
sample ( x = 0). Subsequently, the Seebeck coefficient of the C-Ni sample increases by about
18% more than bare sample ( x = 0). The thermal conductivity of the Ni and C -Ni sample
were considerably lower at room temperature and T < 350 K compare to the bare sample ( x =
0). The mechanical properties of the Ni /BST and C-Ni/BST composite samples show a three –
fold improvement compared to the bare sample ( x = 0).
Keywords: Bismuth telluride, Gas-atomization, Nanoparticle, T hermoelectric performance ,
Nanocomposite.

*Manuscript
Click here to view linked References

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65 2 1. Introduction
Thermoelectric materials have attracted much interest for use in electronic devices for
solid -state energy conversion , power generation, and infrared sensing [1-3]. Materials such as
Bi2Te3 based alloys; PbTe, skutterudites , FeSi 2, SiGe, and oxide have been reported as
suitable materials for energy conversion and waste heat recovery applications . Among these,
Bi2Te3 based alloys exhibit excellent therm oelectric performance around room temperature
(300-500 K). The performance of a thermoelectric material is characterized by the
dimensionless figure of merit, ZT, which is defined as ZT = S2σT/ κ (κ = κL+ κe). Where S, σ,
κ, (κL, κe) and T are the Seebeck coefficient, electrical conductivity, thermal conductivity (the
lattice κL and carrier κe contributions), and absolute temperature, respectively [3]. The
parameters (S, σ, κ) are interrelated because they are determined fundamental ly by
transportation of electrons and phonons. Currently, research trends are concentrated on
reducing th ermal conductivity and increasing electrical conductivity for enhance d ZT values.
Many experimental and theore tical efforts have been very successful in decoupling the
electrical and thermal transport properties via nano structuring and nanocomposite approach
to reducing the thermal conductivity by scattering of phonons more efficiently without
adversely affecting the electrical conductivity [4-7]. Grain size reduction and embedded
nanoinclusions have been suggested as an efficient means of re ducing thermal conductivity
[8]. Conventionally, the reduction in the thermal conductivity is accepted as an efficient
approach to improve the ZT value via the dispersion of secondary phases or nanoparticles
because phonons can be effectively blocked by the interfaces and grain/phase boundaries
created by these secondary phases while charge carriers can percolate through the matrix [9].
Following this approach, many report s in the literature show simultaneous increase in the
Seebeck coefficient and electrical conductivity due to energy filtering effect/quan tum effect at
the interface of these small grain structures [10, 11]. Recently, a ZT value of 1.33 at 373 K

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65 3 was reported by Li. et al. in BiSbTe nanocomposite [12] as a result of interfaces created by
the dispe rsion of SiC nanop artices in the matrix of BiSbTe . A similar result was reported by
Dou et al. [13] via the incorporation of Si3N4 nanoparticles into BiSbT e matrix . The reported
ZT value of 1.38 at 383 K was attributed to a significant reduction of the thermal
conductivity by enhanced phonon scattering by the interfaces and phase boundaries of the
nanoparticles with the matrix . Improved performances have been reported in other
nanocomposites such as CoSb 3/ZrO 2, Ba 0.22Co4Sb12/TiO 2, SiGe/BN due to a reduction in κL
through enhanced phonon scattering [14-16]. More recently, L. Fu et al. [17] obtained a high
ZT of 1.07 at 723 K by the dispersion of Ni nano particles with a core-shell morphology in
Yb-filled skutterudites, which was prepared by thermal decomposition of Ni(OAc) 2. The
enhanced figure of merit was attributed to scattering of phonons by the interface s between the
shell and the core. Xiao et al. [18] report ed a lower thermal conductivity and power factor in
2.0 wt .% of P-type BiSbTe/RuO 2 composite fabricated by melting and spark plasma sintering.
The report claimed that the lower thermal conductivity could be attributed phonon scattering
at the interface of the RuO 2 and the matrix. A report by Dou et al. [19] reveals an improved
Seebeck coefficient, power factor, and thermal conductivity in (B i2Te3)0.2(Sb 2Te3)0.8 base
nanocomposite dispersed with amorphous SiO 2. The report indicated that the observed
improvement in the thermoelectric properties could be attributed to the energy -filtering effect
arising from enhanced carrier scattering by the in terface potential due to the embedded
amorphous SiO 2. Similar results have been reported in -Zn4Sb3/Cu 3SbSe 4 and -
Zn4Sb3/(Bi 2Te3)0.2(Sb 2Te3)0.8 base nanocomposite thermoelectric materials. The observed
results were ascribe d to energy -filtering effect caused by carrier scattering at the interface
barrier leading to 30% increase in power factor and 15 % reductions in thermal conductivity
[20-22].
While there has been an improvement in the thermoelectric properties of nanocomposites

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65 4 based thermoelectric material evidence from the preceding discussions. As far as we know ,
there are very scanty reports on BiSbTe base nanocomposites dispersed with metallic
inclusions. Therefore, in this study, we report the thermoelectric properties of Ni and Carbon –
coated Ni nano pa rticles dispersed individually in a gas-atomized p -type Bi 2Te3 powder
followed by spark plasma sintering process. The effects of Ni and carbon -coated Ni on the
thermoelectric and mechanical properties were systematically investigated .
2. Experimental procedures
Elemental Bi (99.99 %, Alfa Aesar), Sb (99.9 %, Alfa Aesar), and Te (99.65% Alfa Aesar)
granules were weighed according to required stoichiometric composition of
25Bi 2Te3+75Sb 2Te3 alloy and placed in a high-density graphite crucible in an induction
furnace. The detail processing conditions of the gas -atomiza tion technique have been
discussed elsewhere [23-25]. The nano particles of Ni and carbon coated Ni were produced
using the method described by Yilmaz et al. [26]. 2 wt.% of the nanometer -sized Ni /carbon
coate d Ni and the as-atomized BiSbTe powders were mixed in a planetary mill for 20 min
using a ball to powder weight ratio (BPR) of 15:1 at a speed of 800 rpm . The size
distributions of the as -mixed powders were estimated by laser diffraction technique using
Mastersizer 2000 particle size analyzer. The morphology and chemical composition of the as-
mixed powders were characterized using scanning electron microscopy (SEM) equipped with
energy dispersive X -ray spectroscopy (EDS). The crystal structures of the as-mixed powders
were characterized by X-ray diffraction method (XRD) using high-energy monochromatic Cu
Kα radiation (15.418 nm). Subsequently , the uniformly mixed powders were consolidated by
spark plasma sintering at 673 K for 10 min under a n axial pressure of 50 MPa in a diameter
of 20 mm graphite mold in a vacuum. The sample density (d) and micro Vickers hardness
were measured by using Archimedes principle and Vickers hardness tester respectively. The
sintered bulk samples were cut into rectangular pieces (3 × 3 × 12 mm3) used to measure the

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65 5 temperature dependence of thermoelectric ( TE) properties such Seebeck coefficient, electrical
conductivity, and power factor by a TE power measurement system . The thermal diffusivity
(D) and specific heat capacity (Cp) of the samples were measured by laser flash method. The
thermal conductivity was calculated by κ = dC pD. The carrier concentration (nc) and carrier
mobility (μc) were measured by Hall measurement system. The sample without the addition
of Ni and carbon coated Ni will be referred to as GA while the sample with the additi on of Ni
will be referred to GA + Ni and the sample with the carbon coated Ni will be called GA + C –
Ni.
3. Results and discussion
Figure 1 shows the SEM images of the Ni and C -Ni particles synthesized using Pulsed
wire evaporation method. The Ni nano particles appear spherical in shape with smooth
surface s, and uniform size , while the C-Ni nanoparticles are agglomerated . The average
particle diameters of Ni and C -Ni nanoparticles were 80 nm and 50 nm respectively.
Figure 2a and 2b shows the BET plots of Ni and C -Ni nano -particles. The surface area, pore
diameter and mean particle size are obtained from the Brunauer -Emmett -Teller (BET) plot ,
which are summarized in Table. I . The average particle size from the SEM results and BET
results of Table 1 are approximately same. The mean particle size decreases with carbon
coating s on Ni particles leading to an increase in the surface area and pore diameter . The
morphology of the Ni and carbon coated Ni samples were examined using TEM analysis and
shown in Fig. 3 a, and 3b. The thickness of the outside layer shell structure in the carbon
coated Ni samples increased as shown in Fig 3b even though the average particle size are
smaller than those of Ni nano particles . This is consistent with previously reported results [ 15].
These core -shell nano structures dispersed in the p-type Bi 2Te3 matrix are expected to
influen ce the thermoelectric response of the Bi 0.5Sb1.5Te3 system .
Fig. 4 shows the morphology of GA + Ni and GA + C-Ni composite powders after mill ing.

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65 6 The composite powders show irregular shape with a size distribution ranging from 0.3 -5 μm
as presented in Fig. 5. In addition , the GA + C -Ni composite powder exhibits slightly wide r
size distribution, which is mainly due to agglomeration of particles. Composition analysis
conduct ed using EDS mapping are shown in Fig. 6 (a, b) shows the Ni nano particles are
homogeneously distributed in BiSbTe matrix. Carbon is not detected as the limitation of EDS
for detection of element with Z (atomic number) < 11 is well known.
Figure 7 (a) shows the relative density of the as -sintered composites . A relative density of
99.99% was obtained for all the three samples after compaction.
Figure 7(b) shows the XRD patterns of the spark plasma sintered ( SPSed ) sample s with and
without Ni and Carbon coated Ni particles dispers ed in BiSbTe matrix . All diffraction peaks
of GA + Ni and GA + C -Ni composites were indexed to the Bi 2Te3 phase according to JCPDS
card No#491713 with rhombohedral crystal structure having lattice parameters of a =4.284 Å
and c=30.523 Å. Diffraction peaks corresponding to Ni could not be identified in the matrix .
This indicates the matrix phase was not distorted , but the existence Ni was confirmed by EDS
results . In addition, the XRD peak broaden ing was observed for GA + Ni and GA + C-Ni
composites samples indicating a substantial decreased in grain size.
The temperature dependence of the Seebeck coefficient is presented in Fig . 8(a) for GA/x
(x=0, Ni and C -Ni) composite samples. The positive values of S indicate that the majority of
the transports are caused by holes. The Seebeck coefficient for the GA + Ni and GA + C -Ni
composite samples increase s linearly with increasing temperature . In addition, the Seebeck
coefficient was greatly enhanced with the addition of Ni and carbon coated Ni nano particles.
In particular, the Seebeck coefficient of the GA + C -Ni sample at 400 K reaches 191 μV/K,
which is 18 % higher than the BST-GA sample. The impact of Ni and C -Ni on the Seebeck
coefficient can be understood from Fig. 8(c). The carrier concentration increased in the GA +
Ni and decreased remarkably in GA + C-Ni composite samples. Simultaneously , the carrier

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65 7 mobility decreases as well with the addition of Ni and decreases further in the GA + C-Ni
sample due to the scattering of carriers by the core-shell nano -particles in the matrix. The
reduction of the carrier concentration in GA + Ni and GA + C-Ni resulted in the high Seebeck
coefficient observed with the addition of Ni and C -Ni. This is because the Seebeck coefficient
is inversely proportional to the carrier concentration according to equation (1) bel ow

Where k B is Boltzmann’s constant, T is temperature, h is Planck constant m* is effective mass,
n is carrier concentration, e is the electronic charge and  is the scattering parameter. From
equation 1, it is clear that an increase in carrier concentration result in low Seebeck
coefficients and a decrease in carrier concentrations leads to a high Seebeck. Alternatively,
the dispersed Ni and the BST matrix could be considered to form a metal/semiconductor
interface with charge transfer between the metallic Ni and the BST semiconducting matrix.
Faleev et al. [27] argue d that this charge transfer results in band bending away from the
interfaces leading to the development of a characteristic electrostatic potential (V B) the
present of which causes an energy -dependent scat tering of electrons whereby high energy
holes are less affect by such p otential while the low energy hole s are scattered more. The
report suggested that such energy filtering is needed to improve the Seebeck coefficient,
because the Seebeck coefficient is dependent on the energy derivative of the relaxation time
at the Fermi level. We believe that this energy filtering effect is also
responsible for the high Seebeck coefficient observed in GA + C -Ni compared to the GA
sample as shown in Fig 8(a).
The temperature dependence of the electrical conductivities of the GA/x (x=0, Ni and C -Ni)
composite s samples are shown in Fig. 8(b). For all the samples, the electrical conductivity
shows a decreasing trend with increasing temperature, which indicates the characteristics of

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65 8 degenerate semiconducting behavior. The magnitude of the electrical conductivity also
decreases with the addition of Ni and C -Ni nano -particles into BST matrix compared to the
bare GA -BST. This observation is can be explained from the contribution of the carrier
concentration (n) and the carrier mobility ( μ) to the electrical conductivity ( σ) from the
expression σ= ne μ. From Fig. 8(c), the GA + C-Ni sample has the lowest carri er
concentration and carrier mobility while the GA + Ni shows high carrier concentration and
reduced carrier mobility compared to GA sample . The co ntribution of these materials
properties resulted in the observed values of the electri cal conductivity shown in Fig. 8(b).
The contribution of the carrier mobility in the case of GA + Ni and GA + C-Ni outweighs the
contribution of the carrier concentration while the reserve is the case in the GA sample.
The power factor (S2σ) of GA/x (x=0, N i and C -Ni) composite samples calculated from the
values of the electrical conductivity and the Seebeck coefficient are presented in Fig. 8(d).
The power factor decreases with the addition of Ni and C-Ni into BST matrix. This implies
the contribution of the electrical conductivity to the power factor outweighs that of the
Seebeck coefficients.
A well -known phenomenon is that charged carriers are created in BiSbTe type compound by
the creation of charge defects of the kind shown in the equation below [7, 28, 29].
Bi2Te3= 2Bi Te' + VTex + 2VBix + (3/2)Te 2(g) + 2h.
We believe that the addition of Ni being a metal lead to charge accumulation at the metal
semiconductor interface resulting in an increase of the carrier concentration . Likewise,
reports in the literature also suggest ed that the doping of transition element such as Ag, Sn,
and Cu in BiSbTe -type compound lead to the acceptor effect [30, 31]. The carbon coated Ni
did not show the same contribution to the carrier concentratio n probably due to the
amorphous nature of carbon. Similar results was observed by Dou et al. [19] with the

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65 9 dispersion of SiO 2 of 2.2 volume fraction in (Bi 2Te3)0.2(Sb 2Te3)0.8 base nanocomposites. They
concluded that reduction in carrier concentration at higher volume fraction of SiO 2 could be
attributed to the amorphous nature of SiO 2 the presence of which reduces the overall carriers.
Figure 9(a) shows the total thermal conductivity of GA/x (x=0, Ni and C -Ni) composite
samples as a function of temperature. At T < 325 K, the total thermal conductivity of the bare
samples (GA) is higher compared to the GA + Ni and GA + C-Ni samples. The GA + C-Ni
sample shows a 21 % reduction in the total thermal co nductivity in this regime. This suggests
that the dispersion of core -shell structure of Ni particles significantly reduce s the thermal
conductivity , especially at low temperature probably due to boundary scattering of carriers .
At T = 350 K, the therma l conductivity of the GA sample show s a higher value compared to
the GA + C-Ni sample. Beyond 350 K, the thermal conductivity of the GA samples falls
rapidly in comparison to the composites. The total thermal conductivity is the sum of the
electron and phonon contributions according to the expression in equation (2).

Where κe is the electronic contribution to the total thermal conductivity and κl is the lattice
contribution to the total thermal conductivity. The electronic contribution is given as

Where L is the Lorentz number,  is the electrical cond uctivity and T is temperature. The
Lorentz number was estimated using the temperature dependent Seebeck coefficient shown in
Fig. 8a following the approach suggested by Kim et al. [32] according to the equation (4)
below

Where L is the Lorentz number and S is the Seebeck coefficient.
The estimated Lorentz number was used to calculate the electronic contribution κe to the total

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65 10 thermal conductivity using equation 3 whil e the lattice contribution was estimated by
rearranging equation (2) ( ).
Figure 9 (b, c) illustrates the lattice and electronic contribution to the total thermal
conductivity. The electronic contribution in the GA sample to the total thermal conductivity
outweighs that of the GA + Ni and GA + C-Ni sample. This can be attributed to the high
electrical conductivity observed in the GA sample compared to the other t wo composites as
shown in Fig. 8b. The lattice contribution to total thermal conductivity, however, follows the
same trend as the tota l thermal conductivity of Fig. 9a. The low thermal conductivity
observed in the composites especially the GA + C-Ni sample compared to the GA sample at
room temperature till 350 K can be attribute d to lower electronic contribution to the total
thermal conductivity as a result of the dispersed car bon coated Ni creating interfaces and
boundaries that served as carrier scattering centers.
Figure 1 0(a) reveals t he thermoelectric figure of merit ZT estimated from the experimentally
obtained values of the Seebeck coefficient, the electrical conductivity, and the total thermal
conductivity. The ZT values of the GA samples reach’s around 1.2 at 375 K while the ZT of
the composites is around 0.69 at 400 K for the GA + Ni sample and 0.7 at 375 K for the C -Ni
sample respectively.
In Fig. 10(b) , we present the mechanical properties of the three samples measure d via the
micro hardness tester. The figure shows that the hardn ess of the GA sample is about 45 Hv
while that of the GA + Ni sample is about 150 Hv and around 165 Hv for the GA + C-Ni
sample. This amount to a three -fold increase in the mechanical properties of the composites
compared to the GA sample. The increase in hardness in the GA + Ni and GA + C-Ni
sample can be attribute d to the small grain size obtaine d in these samples as shown in F ig. 1a,
b. The dispersion of these fine grains in the matrix of BiSbTe resulted in the creation of
interfaces, wh ich we believed inhibit the propagation of cracks and dislocation movement

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65 11 through the matrix leading the high hardness obtained in Fig. 10(b) . In comparison to
reported value of Vickers hardness in BiSbTe type -compound , prepared through such routes
as hot extrusion and SPS, gas atomization and SPS, the values obtained in this report are two
to three -fold superior to those other reported value of Vickers hardness [23, 25].
4. Conclusions
In this study, GA/x (x=0, Ni and C -Ni) composites were successfully fabricated using a
combination of gas -atomization, high energy ball milling and spark plasma sintering and
subsequently thermoelectric properties were investigated over a temperature range of 300 –
400 K. The results show that the addition of core -shell str ucture of Ni and carbon coated Ni
resulted in an 18% increase in the Seebeck coefficient compared to the bare sample. The
morphology of the Ni and carbon coated Ni samples were responsible for low electronic
contribution to the total thermal conductivity leading to a 21% reduction in the thermal
conductivity. However, w e believe that the dispersion of spherical Ni and C -Ni nanoparticles
into matrix showed a detrimental effect on the figure of merit. The addition of Ni and carbon
coated Ni improved the mecha nical properties of the BiSbTe matrix by three -fold compare d
to the bare sample.
Acknowledgements
This research was supported by Basic Science Research Program through the National
Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and
Technology (NRF -2015R1D1A1A09060920). Part of this work was supported by the Korean
Institute of Energ y Technology Evaluation and Planning (KETEP) grant funded by the Korea
government Ministry of Knowledge Economy (No. 2011501010020).

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Table 1 Summary of the Brunauer -Emmett -Teller (BET) analysis shown in Fig 2. The
reduction in the particle size of the Carbon coated Ni powder results in an increase of the
surface area as well as the pore diameter compared to the Ni powder.
Sample Constant
(C) Adsorption
volume,V m
(cm3/g) Mean pore
diameter
(nm) Total pore
volume
(cm3/g) BET surface
area
SBET(m2/g) Mean
particle
size (nm)
Ni 157.20 1.8474 2.2588 0.00454 8.0408 83.767
Carbon
coated Ni 35.990 2.7567 2.3078 0.00693 11.999 56.136
Table

Fig. 1
Figure

Fig. 2

Fig. 3

Fig. 4

Fig. 5

Fig. 6(a)

Fig. 6(b)

Fig. 7

Fig. 8

Fig. 9

Fig. 10

Figure caption s
Figure 1 Scanning electron micrograph of the Pulse wire evaporation as -synthesized powder
(a) Ni (b) Carbon coated Ni. The Ni nanoparticles appear spherical in shape with smooth
surfaces, and uniform size, while the C -Ni nanoparticles are agglomerated. The average
particle diameters of Ni and C -Ni nanoparticles are 80 nm and 50 nm respectively.

Figure 2 Brunauer -Emme tt-Teller (BET) plot s of (a) Ni (b) C -Ni nano -particles. The surface
area, pore diameter and mean particle size are obt ained from the plot as shown. The mean
particle size decreases with carbon coatings on Ni particles leading to an increase in the
surface area and pore diameter.

Figure 3 Bright field TEM analysis of the Pulse wire evaporation as -synthesized powder (a)
Ni (b) Carbon coated Ni. The thickness of the outside layer shell structure in the carbon
coated Ni samples increased as shown in Fig 3b. The morphology of the carbon coated Ni
reveals a core -shell structure.

Figure 4 Scanning electron micrograph of the c omposite powders after milling (a) Ni addition
(b) Carbon coated Ni addition. The composite powders show irregular shape with a size
distribution ranging from 0.3 -5 μm.

Figure 5 Particle size distributions of Ni and Carbon coated Ni distributed in GA. Th e
composite size distribution ranges from 0.3 -5 μm. The carbon coated Ni/BST powder exhibits
slightly wider size distribution, which is mainly due to agglomeration of particles

Figure 6 (a) Elemental EDS mapping showing uniform distribution Ni, Bi, Sb and Te in the Figure Captions

composite dispersed with Ni.
(b) Elemental EDS mapping showing uniform distribution Ni, Bi, Sb, and Te in the composite
dispersed with Ni. Carbon is not detected as the limitation of EDS for detection of element
with Z (atomic number) < 11 i s well known.

Figure 7 (a) Relative densities of GA, GA + Ni, and GA + C-Ni samples. Relative density of
99.99 % was obt ained in all the three samples. (b) X-ray diffraction patterns of the SPSed
sample with and without addition of Ni and carbon coated N i. All diffraction peaks were
indexed to the Bi 2Te3 phase according to JCPDS card No.JCPDS#491713 with rhombohedral
crystal structure. Diffraction peaks corresponding to Ni and Carbon were not observed in the
pattern. XRD peak broadening was observed for N i and C -Ni composites samples indicating
a substantial decreased in grain size .

Figure 8 Temperature dependence transport properties (a) Seebeck coefficient (b) Electrical
conductivity (c) carrier concentration/mobility (d) power factor; The Seebeck coeff icient
reveals a p-type behavior for the entire three samples . An 18 % increase in Seebeck was
observed for the GA + C-Ni sample compared to the GA sample. The electrical conductivity
decreases with increasing temperature indicating a metallic behavior. The carrier
concentration in the GA + Ni sample is higher compared to the GA and GA + C-Ni sample
due to the acceptor effect of Ni

Figure 9 Temperature dependence thermal transport (a) Total thermal conductivity (b) lattice
contribution to thermal conductivi ty (c) Electronic contribution to thermal conductivity. At T
< 325 K (Fig (a)), the total thermal conductivity of the bare samples (GA) is higher compared
to the GA + Ni and GA + C-Ni samples. The GA + C-Ni sample shows a 21 % reduction in

the total thermal conductivity in this regime. The electronic contribution outweighs the lattice
contribution in the GA sample.

Figure 1 0 (a) Temperature dependent thermoelectric fig ure of merit (ZT). The ZT value of the
GA samples reaches around 1.2 at 375 K whil e the ZT of the composites is around 0.69 at
400 K for the GA + Ni sample and 0.7 at 375 K for the GA + C-Ni sample respectively.
(b) Mechanical properties of the bare (GA) sample and the composites dispersed with Ni and
C-Ni. The hardness of the composite s samples shows a three -fold increase compared to the
GA sample.