Phase stability and thermoelectric properties of Cu10.5Zn1.5Sb4S13 tetrahedrite

Article abstract of DOI:10.1016/j.jallcom.2016.01.094

Cu_10.5Zn_1.5Sb₄S₁₃ tetrahedrite compound was prepared by mechanical milling of Cu₂S, ZnS and Sb₂S₃ powders and spark plasma sintered (SPS) to dense samples. The phase formation, chemical homogeneity, thermal stability of the compound and the thermoelectric properties of the sintered samples were evaluated. Single phase tetrahedrite with the crystallite size of 40 nm was obtained after 30 h of milling followed by annealing at 573 K for 6 h in an argon atmosphere. In-situ high-temperature X-ray diffraction studies revealed that the phase is stable up to 773 K. The Seebeck coefficient of the sintered samples of density >98% shows p-type behavior with maximum thermopower of 170 μV/K at 573 K. The electrical resistivity (ρ) decreases with temperature up to 475 K and then increases. A low thermal conductivity of 0.5 W/(m?K), in combination with moderate power factor gave a maximum ZT of ～0.038 at 573 K in Cu_10.5Zn_1.5Sb₄S₁₃ sample having a grain size of ～200 nm.

Full text of DOI:10.1016/j.jallcom.2016.01.094

Accepted Manuscript
Phase stability and thermoelectric properties of Cu
Zn Sb S tetrahedrite
0.5 1.5 4 13
1
Subramaniam Harish, Duraisamy Sivaprahasam, Manjusha Battabyal, Raghavan
Gopalan
PII:
S0925-8388(16)30094-9
10.1016/j.jallcom.2016.01.094
JALCOM 36445
DOI:
Reference:
To appear in: Journal of Alloys and Compounds
Received Date: 5 November 2015
Revised Date: 17 December 2015
Accepted Date: 12 January 2016
Please cite this article as: S. Harish, D. Sivaprahasam, M. Battabyal, R. Gopalan, Phase stability and
thermoelectric properties of Cu
Zn Sb S tetrahedrite, Journal of Alloys and Compounds (2016),
0.5 1.5 4 13
1
doi: 10.1016/j.jallcom.2016.01.094.
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ACCEPTED MANUSCRIPT
Phase stability and thermoelectric properties of Cu Zn Sb S
1
0.5
1.5
4 13
tetrahedrite
a, b
a,
a
Subramaniam Harish , Duraisamy Sivaprahasam *, Manjusha Battabyal ,
Raghavan Gopalan
a
a- Centre for Automotive Energy Materials (CAEM), International Advanced Research
Centre for Powder Metallurgy and New Materials (ARCI), IIT Madras Research Park,
Taramani, Chennai-600113, India.
b- Department of Metallurgical and Materials Engineering, Indian Institute of
Technology Madras, Chennai-600036, India
*
- Corresponding author
E-mail : sprakash@arci.res.in
Phone: +914466632812
Abstract
Cu10.5Zn Sb S tetrahedrite compound was prepared by mechanical milling of Cu S,
1.5
4
13
2
ZnS and Sb S powders and spark plasma sintered (SPS) to dense samples. The phase
2
3
formation, chemical homogeneity, thermal stability of the compound and the
thermoelectric properties of the sintered samples were evaluated. Single phase
tetrahedrite with the crystallite size of 40 nm was obtained after 30 h of milling
followed by annealing at 573 K for 6 h in an argon atmosphere. In-situ high-
temperature X-ray diffraction studies revealed that the phase is stable up to 773 K. The
Seebeck coefficient of the sintered samples of density >98% shows p-type behavior
with maximum thermopower of 170 µV/K at 573 K. The electrical resistivity (ρ)
decreases with temperature up to 475 K and then increases. A low thermal conductivity
of 0.5 W/(m⋅K), in combination with moderate power factor gave a maximum ZT of
∼
0.038 at 573 K in Cu10.5Zn Sb S sample having a grain size of ∼ 200 nm.
1.5 4 13
Keywords: Thermoelectric, tetrahedrite, solid state reactions, spark plasma sintering,
Figure of merit.

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1
.
Introduction
Thermoelectric (TE) materials required for power generation applications need
to possess the high figure of merit (ZT), readily available in large-scale, stable at the
operating temperatures [1, 2]. Most of the currently investigated TE materials use
costly rare earth elements, whose availability are limited and have toxicity issues.
Recently, environmentally friendly and earth-abundant materials are being explored for
thermoelectric applications [3, 4]. Tetrahedrite, a naturally occurring mineral is one
such material of interest as it shows promising ZT, a parameter used to evaluate the
2
efficiency of TE materials. ZT is a dimensionless parameter given by ZT=S T/ρκ
where S, ρ and T represent Seebeck coefficient, electrical resistivity and absolute
temperature T. κ is thermal conductivity which is the sum of carrier thermal
conductivity (κ ) and lattice thermal conductivity (κ ).
e
L
The tetrahedrite compound is a p-type ternary I-V-VI semiconductor of copper
antimony sulfide (Cu Sb S ) with an estimated band gap of 1.7 eV [5]. Naturally
1
2
4 13
occurring tetrahedrite has the composition of Cu12-xM (Sb, As) S which is a solid
x
4 13
solution of As rich tennantite (Cu As S ) and Sb rich tetrahedrite (Cu Sb S ).
12
4
13
12
4 13
Famatinite (Cu SbS ) and skinnerite (Cu SbS ) are some of the other compounds in
3
4
3
3
Cu-Sb-S system [6]. The pure form of the tetrahedrite phase does not occur naturally,
and normally transition metals such as Zn, Fe, Hg, Ni, and Mn substitutes the Cu site in
Cu12-xTr Sb S . Tetrahedrite has a complex crystal structure with 58 atoms arranged in
x
4 13
a high symmetry cubic unit cell (I 43m) constituted by CuS tetrahedra, CuS triangles
4
3
and SbS pyramids [7].
3

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The thermoelectric properties of synthetic tetrahedrite Cu12-xTr Sb S (Tr =
x
4 13
Mn, Ni, Fe, Co, Zn) have been studied and reported in the recent past [8-10]. Mn
substituted tetrahedrite of nominal composition Cu MnSb S showed ZT of as high as
1
1
4 13
1
.13 at 575 K [11]. Ni and Te substitution also yielded an impressive ZT of 0.7 at 665
K [10] and 0.92 at 723 K [12] respectively. Natural tetrahedrite was also found to give
power factor and ZT similar to synthetic compound [13] with modification in their
chemical compositions. The thermal conductivity of both synthetic and purified natural
mineral tetrahedrites are < 1 W/(m.K), which mainly arises due to inherent nature of
the complex crystal structure having a large number of atoms in a unit cell of the
tetrahedrite phase. Grain refinement of this phase can further enhance the ZT through
the reduction in the lattice thermal conductivity (κ ) by enhanced phonon scattering.
L
Until now, only a few studies have been reported on the synthesis of nanocrystalline
tetrahedrite and their electronic properties [14-15].
In the present investigation, we report synthesis of single phase nanocrystalline
Zn-doped tetrahedrite (Cu10.5Zn Sb S ) compound. The phase stability was studied
1.5
4 13
using in-situ high-temperature X- ray diffraction data obtained at different temperatures
298-773 K). The thermoelectric properties of the materials after processing the
(
compound by spark plasma sintering into bulk samples having relative density >98%
are reported.

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2
. Experimental Details
The Cu10.5Zn Sb S tetrahedrite compound was synthesized by mixing
1.5
4 13
Copper (I) sulfide (Cu S, 99.5%, Alfa Aesar), Zinc Sulfide (ZnS, 99.99%, Aldrich) and
2
Antimony (III) Sulfide (Sb S 99.995%, Aldrich) in stoichiometric ratio (the exact
2
3,
stoichiometry was Cu10.5Zn Sb S12.75) followed by ball milling and annealing process.
1.5
4
Ball milling was carried out in a hardened steel vial with 10 mm stainless steel balls in
a planetary ball mill (Fritsch Pulverisette-5) with the ball to powder ratio of 20:1 at 150
rotations per min. (RPM). The duration of milling was varied from 1 to 30 h. The
9
9.99 % pure argon atmosphere at 1.2 bar pressure created by evacuation and
backfilling ensures that O partial pressure in the vial is very low during milling.
2
Annealing of the milled powder was carried out at 573 K for 6 h in 99.99 % argon
atmosphere after compacting the powder to 50-60 % density. The annealed compacts
were crushed into powder and spark plasma sintered to 20 mm diameter, 4 mm thick
samples for Seebeck (S) and electrical resistivity (ρ) measurements. Thermal
conductivity (κ) measurement was done on 10 mm dia., 2 mm thick samples. The spark
plasma sintering (DrSinter1050, Sumitomo Co, Japan) was done at 673 and 723 K
under 50-100 MPa pressure for the duration of 2-10 min. using two kinds of die
cavities viz. graphite and WC-Co. The heating and cooling rate was kept at 100 K/min.
for graphite die and ∼ 25 K/min. for WC/Co die. All the SPS was carried out at
chamber pressure of 10 Pa.
X-ray diffraction (XRD) analysis of the milled powders and the SPS samples
were performed using Rigaku X-ray diffractometer (Rigaku SmartLab), which has the
in-situ high-temperature measurement facility. The heating rate was maintained at 10

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K/min. The XRD patterns were analyzed using X′pert HighScore software. Differential
Scanning Calorimetry (DSC) and Thermo Gravimetric Analysis (TGA) were carried
out to evaluate the phase transition and mass loss of the tetrahedrite powders using
Netzsch STA 449 F1 apparatus. The DSC/TG measurements were carried out from
room temperature (RT) up to 800 K at the rate of 2.5 K/min. under flowing argon
cover. The surface morphology and homogeneity of the samples were examined by
SEM (FEI Inspect F50). The microstructure of the milled powders was carried out
2
using high-resolution transmission electron microscopy (HRTEM; TECNAI T20 G ).
For TEM investigation, the milled powders were prepared by hand grinding and then
dispersing in methanol followed by dropping off the obtained suspension onto a typical
carbon coated Au grid and subsequently drying it. The density of the powder and the
sintered samples was experimentally determined using a gas pycnometer (AccuPyc
1
330 Pycnometer) and Archimedes’ methods. Seebeck coefficient (S) and electrical
conductivity (σ) was measured using Seebsys - a combined Seebeck coefficient and
resistivity measurement system developed by M/s NorEcs, Norway in samples of 4 mm
×
4 mm square cross-section and 12 mm height cut from the 20 mm diameter, 4 mm
thick spark plasma sintered samples. Thermal conductivity (300-630 K) was evaluated
from the thermal diffusivity (D=κC ρ) measured by laser flash technique using a
p
3
Netszsch LFA-457 device. The density (ρ) of 4.91g/cm used in the calculation is
obtained from pycnometer measurement at 298 K. The C at different temperatures was
p
estimated by extrapolating the data reported by Curzio et al. [16] for Cu Zn Sb S up
10
2
4 13
to 400 K. The thermal conductivity of the samples was also measured directly using
transient plane source technique (Hot Disk, Sweden) using two samples of 20 mm

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diameter and 4 mm thickness. The specific heat (C ) of the sample was also measured
p
by the above technique.
3
3
. Results and Discussions
.1 XRD investigation:
Figure 1a shows the XRD patterns of the powders milled for various time.
From the XRD peaks, it can be seen that, after 10 h milling, the Cu Sb S tetrahedrite
1
2
4 13
phase (diffraction peaks corresponding to d (222) of ICDD: 042-0560) starts appearing.
The powder is converted into nearly single phase tetrahedrite after 30 h milling with
traces of unreacted Sb and ZnS phases.
The formation of the tetrahedrite phase during ball milling of Cu S, ZnS and
2
Sb S can be understood by the following chemical reactions.
2
3
(1)
5.25Cu S+2Sb S +1.5ZnS
1.75Cu S +4Sb+0.75S+1.5ZnS
6 6
2
2
3
(2)
1.75Cu S +4Sb+1.5ZnS+0.75S
Cu10.5Zn Sb S
1.5 4 12.75
6
6
During the initial stage of milling, the Sb S powder decomposes first into Sb and S
2
3
phases. Since the Sb S is a highly unstable compound, it readily decomposes into Sb
2
3
and S even while mixing in agate mortar and pestle. The other constituent viz. Cu2S
phase that is in hexagonal chalcocite form reacts with S that has phase separated from
Sb S and forms Cu S (hexagonal covellite) phase. With the increase of milling time,
2
3
6 6
both Sb and Cu S phases undergo refinement and at 10 h milling both the phases
6
6
reacts to nucleate the Cu Sb S tetrahedrite phase. The ZnS (wurtzite structure) was
1
2
4 13

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stable at this stage as there is no evidence for the presence of diffraction peaks
corresponding to the elemental Zn. However with prolonged milling, it decomposes
and alloys with tetrahedrite phase. This phenomenon can be clearly observed in 30 h
milled powder, which predominantly contains the tetrahedrite phase with traces of Sb
and ZnS phases. Annealing of the above 30 h milled powder has resulted in the
formation of the tetrahedrite phase that is devoid of the residual phases as shown in Fig.
1
b.
3
.2
Microstructural investigation
Figure 2 shows the SEM image of the Cu10.5Zn Sb S tetrahedrite synthesized
1.5
4 13
by 30 h milling followed by annealing. From the microstructure, it can be seen that
most of the particles have the size in the range 300-500 nm. The elemental mapping
carried out by energy dispersive X-ray spectroscopy (EDS) showed a slight
inhomogeneity in the distribution of the constituent elements particularly Sb and S
(Supplementary data S1). It was found that the overall composition of sulfur in the
tetrahedrite was ∼ 3 % lesser than that of the initial stoichiometric composition of
Cu10.5Zn Sb S . However, such a lesser sulfur content does not result in the formation
1.5
4 13
of any other phases other than tetrahedrite phase (Fig. 1b) in the powder. Figure 3
shows the TEM micrograph of the tetrahedrite nanocrystalline particles and their
corresponding selected area electron diffraction (SAED) pattern. It can be seen from
the TEM microstructure that the 300-500 nm size particles observed in SEM are
primarily the agglomerates of nanocrystalline particles. The average size of these
nanocrystalline particles estimated from TEM was 26 nm.

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3
.3
In-situ high-temperature XRD analysis
Figure 4 shows the in-situ high-temperature XRD pattern of the nanocrystalline
tetrahedrite phase at different temperatures. The measurements were carried out in 30 h
milled and annealed sample up to 773 K under 0.02 mbar vacuum atmosphere. At all
the temperatures investigated (373-773 K with 100 K increment), the XRD patterns
showed mainly peaks corresponding to the tetrahedrite phase (Cu Sb S ). The
1
2
4 13
intensities of the diffraction peaks corresponding to the tetrahedrite phase are relatively
same as shown in the inset of Fig. 4 suggesting that no major atomic rearrangement
could have occurred in the compound. However, the DSC/TG (Supplementary data
S2) carried out from RT to 800 K in the powder disagrees with the XRD observations.
The DSC of Cu10.5Zn Sb S shows a broad endothermic peak at 518 K and a weak
1.5
4 13
endothermic peak between 690 K and 710 K, both of which are associated with notable
weight loss. The weight loss can be attributed to sulfur volatilization as observed
during annealing of the milled powder at 573 K (3 wt. % sulfur loss). The absence of
any XRD peaks corresponding to any decomposition product notwithstanding the
sulfur volatilization can be attributed to the complex structure of tetrahedrite, which can
easily accommodate vacancies leading to Cu+Zn rich tetrahedrite (Cu+Zn)12+xSb4+yS13
with 0.65≤x≤1.9.
Un-substituted polycrystalline tetrahedrite is usually unstable beyond 791 K,
and it undergoes decomposition into various phases such as CuSbS and Cu SbS [17].
2
3
3
Substitution of tetrahedrally coordinated Cu in the structure with Ni and Fe [17, 18]
increases the phase stability significantly up to 840 K. However the Sb substitution

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does not show such behavior and the phase decomposition starts at 623 K in Cu Sb
4-
12
Te S compound [19]. The observation of stable Cu Zn Sb S tetrahedrite phase
x
x
13
10.5
1.5
4 13
with crystallite size 44 nm up to 773 K in the present investigation could originate from
2
+
+
the substitution effect of Zn ion at Cu site. The free energy of formation (∆G°)
estimated by Seal et.al. [20] shows that Cu Zn Sb S has more negative value than
1
0
2
4 13
Cu Fe Sb S . Hence, Zn doping is likely to have either similar or higher stabilizing
10
2
4 13
effect than Fe on the tetrahedrite phase.
The in-situ high-temperature XRD peaks of the tetrahedrite at different
temperature showed a shift in the diffraction peak toward lower diffraction angle (2θ)
(inset in Fig 4). Table 1 gives the lattice constant and the crystallite size of the
nanocrystalline particles at room temperature and after heating to different temperatures
estimated from in situ high-temperature XRD data. The lattice parameter (a) increases
with temperature linearly [a(Å) =10.2627+0.00021T] similar to the one observed in Ni
and Mn substituted tetrahedrite [17, 21]. The average crystallite size obtained from first
three most intense XRD peaks (222), (440), and (400) using Scherrer’s method is
relatively high (40 nm) as compared to the crystalline size observed from TEM (26
nm). The crystallite size does not vary much with temperature up to 773 K. The
crystallite size tends to grow with prolonged holding at this temperature as T/Tmelting point
is more than >0.8 at 773 K assuming the melting point of Zn-substituted tetrahedrite is
similar to other transition metal substituted compounds such as Fe [18]. It must be
noted that the dwell time at each temperature during the high-temperature XRD
measurement was around 10 min. only and hence the grain growth must have been
kinetically hindered.

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3
.4
Spark plasma sintering (SPS) and thermoelectric properties of tetrahedrite.
The spark plasma sintering of nanocrystalline tetrahedrite powders carried out
at 50 MPa pressure and 723 K in a graphite die has resulted in incomplete (93.4 %) and
non-uniform densification of the samples. However, SPS using WC-Co die with 100
MPa pressure has yielded uniform, dense samples. The density of the nanocrystalline
tetrahedrite powder consolidated under different SPS processing conditions is given in
Table 2. Sintering at 723 K for 10 min. in WC-Co die resulted in samples free from
major porosity with 98.8 % densification. The cubic tetrahedrite phase present in the
nanocrystalline powder was retained in the sintered samples. However, a noticeable
amount of other phases such as Cu S and Cu SbS was also formed in the sample as
2
-x
3
4
evident from small XRD peaks of these phases (Fig. 5a). Decreasing the sintering time
to 2 min. reduces the unwanted phases (Fig. 5b), however this also resulted in 4%
decrease in density. In Cu-Sb-S ternary system, the Cu Sb S phase exist over narrow
1
2
4 13
compositional field [18]. The minor deviation in the composition particularly due to S
volitalization can still retain the cubic I 43m symmetry in the phase. The formation of
Cu S in the sintered samples indicate the nanocrystalline tetrahedrite powder
2
-x
undergoes significant sulfur loss during SPS. The TGA analysis carried out in the
powder up to 800 K confirm this aspect (Supplementary data S2). The powder samples
showed weight loss to an extent of 15 % when heated to 750 K. Sintering below this
temperature (673 K ) had resulted in samples with a lesser fraction of the decomposed
phases with up to 1 % decrease in density. Figure 6 shows the secondary electron SEM
images of a fractured surface of the 723 K SPS samples. The density and grain size is

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found to be uniform throughout the cross section, and the sample is also found to be
free from residual pores. The grain size is in the range of 150-250 nm, which is
significantly higher than that of the crystallite size (40 nm) of the powder.
The thermoelectric properties measurements were carried out on
Cu10.5Zn Sb S samples (density >98 %) in the temperature range 273-623 K. Figure
1.5
4 13
7
(a-c) shows the temperature dependence of the Seebeck coefficients (S), resistivity
(ρ) and the thermal conductivities (κ) of the samples. The Seebeck coefficient is
positive over the entire temperature range of the measurement suggesting that the
majority carriers in this compound are holes (p-type). The Seebeck coefficient
monotonously increases with temperature from 100 µV/K (at 310 K) to 170 µV/K (at
6
23 K). The temperature dependent electrical resistivity (ρ) measurement shows that ρ
decreases sharply with increase in T up to 475 K, which is typical of a non-degenerate
semiconductor behavior due to thermal excitation of the charge carriers. Above 475 K,
the ρ increases continuously till 625 K, which can be attributed to decrease in the
carrier mobility due to carrier-phonon scattering [12]. The value of ρ at 300 K is
-2
0
.095×10 Ω.m, which is one order higher than the previously reported results on Zn-
substituted tetrahedrite [8]. It could be due to the presence of cubic secondary phase
Cu SbS (XRD data from Fig. 5) which is a wide bandgap semiconductor [15]. The
3
4
EDS analysis carried out in the sintered sample shows up to 3-6 at.% lower sulfur than
the stoichiometric composition. The relatively higher proportion of Zn due to the S
loss also could make the compound more insulating.

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Figure 7c depicts the T vs. κ variation for the spark plasma sintered
nanocrystalline Cu10.5Zn Sb S sample. The κ reported here is from laser flash
1.5
4 13
technique and the thermal conductivity measured for the same sample by hot disk
method also showed similar (∼ 8 % higher) values. It can be seen from the figure that
the thermal conductivity (∼ 0.5 W/m.K) varies marginally with temperature and is
comparable with the values reported for polycrystalline tetrahedrite compound of
similar composition [8]. In Cu Sb S tetrahedrite, it was observed that Zn and other
12
4 13
transition metal substitution at Cu site have the tendency to reduce the thermal
conductivity significantly (0.2 W/m.K in Cu Zn Sb S ) due to the increased phonon
1
0
2
4 13
scattering by randomly distributed substitutional atoms [11]. A nanocrystalline
structure is expected to decrease the thermal conductivity further by scattering of high-
frequency phonons at grain boundaries. However, for this mechanism to play a
significant role, the crystallite size in the sintered samples have to be within few tens of
nanometers [22]. In the present study, the crystallite size has grown from 40 nm to 150-
2
50 nm during sintering. As observed by Yan et al. [23] in 200 nm half Heusler
samples, it is possible that crystallite size could contribute to decreasing the lattice
thermal conductivity (κ ). However, to get a substantial benefit in κ , crystallite size
L
L
has to be reduced further in this compound, as already κ is close to its lower limit. The
highest ZT obtained in the present study is around ∼ 0.038, which is one order lower
than ZT ∼ 0.4 reported for the polycrystalline compound of Cu10.5Zn Sb S [8] but
1.5
4 13
comparable to Cu Zn Sb S that shows ZT<0.1 at similar temperature range [11]. The
1
0
2
4 13
low ZT is mainly due to high ρ value in our sample that appears to be dependent upon
the extent of Zn substitution in the tetrahedrite. The density functional theory (DFT)
band struture calculations carried out by Lu et al. [8] predict the Cu12-xZn Sb S
x
4 13

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compound become electically insulating at x=2.0. The sulfur losses by volatilization
during processing of this high surface area nanocrystalline powder, which increses the
overall Zn content in comparision to the starting stochiometry of x=1.5 might be one
major contributing factor for the high ρ value of the samples.
4
. Conclusions
In summary, we have synthesized nanocrystalline tetrahedrite powder of
nominal composition Cu10.5Zn Sb S successfully by a simple ball milling and
1.5
4 13
annealing of Cu S, Sb S and ZnS powders. The high temperature in situ x-ray
2
2 3
diffraction of the nanocrystalline powders showed stable tetrahedrite phase up to 773 K
with marginal growth in the crystallite size. Samples densified to >98 % by spark
plasma sintering at 723 K for 10 minutes, showed p-type behavior with highest
Seebeck coefficient of 170 µV/K and thermal conductivity of 0.5 W/(m.K). A
maximum figure of merit ZT close to 0.038 at 573 K was obtained in the above sample.

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Figures and Tables
Tables
Table 1 Crystallite size and lattice parameter (a) of the nanocrystalline tetrahedrite
powder estimated from in-situ high temperature XRD at different temperatures.
Table 2 Density of nanocrystalline tetrahedrite powder consolidated under different
SPS conditions.
Figures
Figure 1 (a) XRD pattern showing the evolution of various phases during mechanical
milling of Cu S, ZnS and Sb S powders with the milling time, (b) XRD pattern of the
2
2 3
3
0 h milled and 6 h annealed powder.
Figure 2 SEM images of (Cu10.5Zn Sb S ) powder synthesized by 30 h milling and 6
1.5
4 13
h annealing in argon atmosphere.
Figure 3 TEM micrograph of a Cu10.5Zn Sb S tetrahedrite powder and inset showing
1.5
4 13
selected area electron diffraction (SAED) pattern corresponding to this area.
Figure 4 In-situ high-temperature x-ray diffraction patterns of the
Cu10.5Zn Sb S tetrahedrite powder from 373 K to 773 K.
1.5
4 13
Figure 5 XRD pattern of Cu10.5Zn Sb S powder spark plasma sintered at 723 K for
1.5
4 13
(a) 10 min. and (b) 2 min.

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Figure 6 SEM micrograph of spark plasma sintered Cu10.5Zn Sb S samples (a)
1.5
4 13
secondary electron image showing crystallite size of 150-250 nm and (b) backscattered
electron image showing the nanocrystalline secondary phases (arrows marked).
Figure 7 Temperature dependence of (a) Seebeck coefficients and electrical resistivity
and (b) power factor and (c) thermal conductivity of Cu10.5Zn Sb S .
1.5
4 13

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ACKNOWLEDGEMENT
The authors are thankful to Prof. G. Sundararajan, Director, ARCI for his keen interest
and support in carrying out this work. The DSC/TG thermal analysis support by Dr.
N.Rajalakshmi, ARCI, Chennai, INDIA and thermal diffusivity measurement by Mr.
Anilkumar Rai, IGCAR, Kalpakkam, INDIA are gratefully acknowledged.

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Table 1 Crystallite size and lattice parameter (a) of the nanocrystalline tetrahedrite powder
estimated from in-situ high temperature XRD at different temperatures.
Temperature
K)
Crystallite size
(nm)
a
(
(
Å)
3
4
5
6
7
73
73
73
73
73
39.7
43.3
40.6
41.0
44.4
10.323
10.333
10.347
10.361
10.385

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Table 2 Relative densities of nanocrystalline tetrahedrite powders consolidated under
different SPS processing conditions.
Sintering
Pressure (MPa)
Time (min.)
Rel.Density
Temperature (K)
6
7
7
7
73
23
23
23
100
50
10
10
2
97.8
93.4
94.8
98.8
100
100
10

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