Microstructure and Thermoelectric Properties of
n-type Chalcogenide Nanopowders Sintered by
ECAE
S. Ceresara#1, C. Fanciulli#2, D. Vasilevskiy*3
1-CNR-IENI, Corso Promessi Sposi 29, 23900 Lecco, Italy
1ceresara
2c.fanciulli
@tin.it
@ieni.cnr.it
2-École Polytechnique de Montréal - Département de Génie Mécanique, H3C 3A7 Montréal, Canada
3dimitri.vasilevskiy@polymtl.ca
Abstract— Nanograined powders of Bi1.9Sb0.1Te2.85Se0.15 alloy
have been consolidated into bulk ingots by warm ECAE process.
Sintering has been achieved after only two ECAE passes at
300 °C, with a total holding time at this temperature of 19
minutes. The morphological characterization of ECAE processed
samples, performed by HRTEM, has shown grain sizes varying
from 20 to 50 nm. Thermoelectric properties, measured at 300 K
show that the thermal conductivity is considerably reduced by
the nanostructure, but the value of the Seebeck coefficient is
relatively low, due to a non optimized value of the electron
concentration. The latter result has been attributed to oxygen
contamination.
Keywords— thermoelectricity, nanopowders, warm ECAE, low
temperature sintering, Bismuth Telluride alloys
I. INTRODUCTION
Chalchogenides belonging to the (BixSb1-x)2(TeySe1-y)3
family are known as the best TE materials for the temperature
range from 0 to 250 °C and are used in most present
applications of thermoelectricity. Their dimensionless figure
of merit, ZT, has remained around 1 for several decades, until
a breakthrough of ZT=2.3 at 300K has been achieved in a
Bi2Te3/Sb2Te3 superlattice [1]. The concept of “nanostructured
thermoelectricity” has been soon extended to bulk materials:
as a consequence, a considerable enhancement of ZT, in a
wide temperature range, has been achieved in p-type
BixSb2-xTe3, by reducing the crystalline grain size to the
nanometer scale [2, 3]. In both papers [2] and [3],
nanoparticles were fabricated by ball milling, either starting
from alloy ingots [2], or from elemental chunks [3]. In both
cases, consolidation of the powders into dense bulk was
achieved by hot pressing.
From a general point of view, it appears that other
consolidation processes, alternative to hot pressing, may help
preserving the nanocrystalline grain size. Among these, spark
plasma sintering (SPS) is often recommended [4] because of
its relatively short sintering time. Another potentially suitable
process is represented by equal channel angular extrusion
(ECAE), since it allows powder sintering at considerably
lower temperature than hot pressing [5].
In this paper we have followed the latter approach to
consolidate n-type chalcogenide nanopowders.
II. EXPERIMENTAL
Nanopowders of Bi1.9Sb0.1Te2.85Se0.15 alloy were prepared
from shots of 99.995 % purity Bi, Sb, Te, Se elements by
mechanical alloying (M.A.) Attrition milling was performed
in argon atmosphere for 10 hours. The average size of the
nanograins was estimated as 20-25 nm, both by X-ray peak
diffraction broadening and by direct observation at HRTEM
[6].
By operating in an Ar filled dry box, the nanoparticles were
pressed into pellets 10 mm in diameter and 8 mm in height, by
applying a pressure of 500 MPa. A number of pellets were
inserted into a Cu can, 25 mm in external diameter and
120 mm in length, with the interposition of a Nb diffusion
barrier, 1 mm in thickness. Three composite billets were
vacuum sealed and ECAE processed at 300°C, with 2 (sample
A), 4 (sample B), and 6 (sample C) passes, respectively. The
holding time in temperature was 19, 37, and 50 minutes, for
samples A, B, and C, respectively.
The principles of warm ECAE have been described in a
previous paper [7]. For the present experiments we have used
an upgraded equipment which allows processing of billets
25 mm in diameter and 120 mm in length. The true plastic
strain after each pass is ε = 0.98.
The Harman method has been used to measure the figure of
merit, Z, at 300 K, both along the longitudinal (i.e. parallel to
the extrusion axis) and the transverse directions. Typical
samples, 6 x 6 x 7 mm3, were cut from the centre of the ECAE
processed billets. The values of the electric resistivity, ρ, and
of the Seebeck coefficient, α, were measured on the same
samples; the thermal conductivity, λ, was calculated from the
relationship λ=α2/Zρ.
Hall measurements, under 0.5 T magnetic field, were
performed by the Van der Pauw method on thin (0.6 mm)
specimens sliced normal to the extrusion direction. In such a
way, the carrier concentration, n, was determined and the
mobility, μ=1/neρ, (e = charge of the electron) was calculated.
III. RESULTS AND DISCUSSION
A. Microstructure characterization
The XRD pattern of the nanopowders, reported in Fig.1,
confirms the formation of the Bi1.9Sb0.1Te2.85Se0.15 phase by
mechanically alloying of the elemental Bi, Sb, Te, Se shots.
However, DSC analysis, reported in Fig.2, shows that the
powder is not single-phase: as a matter of fact, a small
endothermic peak is observed at 410 °C. By considering that
the Te-rich eutectic temperature of the Bi-Te binary alloy falls
at 413 °C [8], it appears that a small fraction of Te-rich
eutectic is also present in the powders, although unrevealed by
XRD. It is believed that the eutectic has not affected the
sintering of nanopowders, since ECAE has been performed at
a temperature far below 410 °C
The morphological characterization of ECAE processed
samples has been performed by collecting several HRTEM
images. Examples are given in Figs. 3, 4, and 5, for samples A,
B, and C, respectively.
18000
16000
Bi1.9Sb0.1Te2.85Se0.15
14000
Powders
Counts
12000
a = 4.37 A
c = 30.36 A
10000
Fig.3 HRTEM image of sample A
8000
6000
4000
2000
0
10
20
30
40
50
60
70
80
90
100 110 120
2 deg 
Fig. 1 XRD pattern of nanopowders
Fig.4 HRTEM image of sample B
Fig.2 DSC curve of M.A. nanopowders
From the nominal alloy composition and from the lattice
parameters obtained from XRD plot refinement and reported
in Fig.1, a theoretical density of 7.84 g/cm3 has been
calculated. Although such a figure is likely to be slightly
incorrect, due to the presence of the eutectic, it may be taken
as a reference value for the comparison of the density of the
samples in different structural states.
The density of the pellets resulted 83% of the full density
value, simply by dividing its mass to its geometrical volume.
The density of ECAE processed samples were determined by
Archimedes’ method and resulted about 96%. The differences
in density with the number of ECAE passes were within the
reproducibility of the measurements (±0.5%).
Fig.5 HRTEM image of sample C
In such figures, the nanograins are represented as a selfcontained area consisting of one or more sets of fringes. It
appears that in all cases their dimension varies from 20 to
50 nm.
XRD measurements performed on the three different
samples were also used for a deeper structural analysis. From
the diffraction patterns, the Hall-Williamson plot has been
obtained and reported in Figs. 6, 7, and 8, for sample A, B,
and C, respectively.
materials when there is a distribution of grain size, since a
small number of large particles gives a relevant contribution
to the XRD measurements [9].
B. Thermoelectric properties
The thermoelectric properties of the ECAE processed
samples are shown in Tab I. For the purpose of comparison,
also the results obtained on the same material batch by direct
extrusion [6] at 440 °C, in Ar atmosphere, are reported.
TABLE I
THERMOELECTRIC PROPERTIES AT 300 K OF THE ECAE PROCESSED SAMPLES
Fig. 6 Hall-Williamson plot of sample A
Sample
α
ρ
λ
Z (K-1)
n (m-3)
μ
A
(μV/K)
(μΏm)
(W/Km)
x 103
x 10-25
(cm2/Vs)
8.0
55
1.2
331
Longit.
-164
11.2
1.45
1.68
Transv.
-158
14.2
1.02
1.47
Longit.
-147
13.5
1.16
1.37
Transv.
-146
15.7
1.08
1.27
Longit.
-167
13.7
1.15
1.76
Transv.
-159
16.1
1.01
1.55
6.6
59
-178
8.2
1.41
2.74
1.9
401
B
C
Extruded
at 440°C
Fig. 7 Hall-Williamson plot of sample B
Fig. 8 Hall-Williamson plot of sample C
In these plots, the slope of the line obtained by the linear fit
of the data is related to the strain of the nanograins. It
increases with the number of ECAE passes, from 0.08% in
sample A, to 0.21% in sample B, reaching 0.40% in sample C.
The intercept of the same line with the Y-axis is related to the
grain size, D. From this analysis, D shows tendency to
increase with the number of ECAE passes from 64, to 84, up
to 160 nm for samples A. B, and C, respectively. Using the
Scherrer’s formula to calculate D from the same diffraction
patterns, a value of 55 nm is obtained for all the samples. Both
analysis of X-ray line profiles indicate a grain size
considerably larger than the values directly measured by
HRTEM. This result is often observed in nanostructured
The most striking result emerging from the analysis of
Tab. I is the anomaly of the n values of the ECAE processed
samples. To explain this “fuzzy” behaviour of carrier
concentration with the number of ECAE passes, we assume
that the vacuum sealing system of the composite billet has
failed to fully protect the powder from oxygen contamination.
Under this hypothesis, when the contamination is feeble
(samples A and C), oxygen dissolves in the alloy, introducing
donor levels [10] and increasing the n value. When the oxygen
contamination is more important (sample B), the formation of
Bi oxides is likely to occur at 300 °C. In such a situation, the
cation sub-lattice is under-stoichiometric and Bi vacancies are
created. Ionization of the latter, according to the reaction
vBix → vBi'''+3h• [11], causes a drop in the n carrier
concentration.
The non optimized electron concentration accounts for the
low values of α and, consequently, of Z.
In previous experiments on ECAE processed chalcogenides
[7], the maximum value of Z was observed along the plane of
intersection of the entry and exit channel (at 45° to the
extrusion direction in our equipment). This was explained by
the specific texture assumed by the material, with the basal
(00l) planes of the hexagonal crystal cell preferentially
oriented parallel to such a shear deformation plane. This rule
has been confirmed also in present case. As a matter of fact,
measurements of sample B along a plane at 45° to the
extrusion direction have shown a value of Z = 1.76 x 10 -3 K-1,
namely 28% higher than in the longitudinal direction.
The positive note emerging from the analysis of tab. I is
represented by the low values of λ, which well compare with
those obtained in p-type nanostructured (BixSb1-x)2Te3
alloys [2, 3].
IV. CONCLUSIONS
Results of the present investigation confirm the validity of
the warm ECAE process in the consolidation of the
nanograined powders. Sintering by ECAE has been achieved
after only two passes at 300 °C, with a total holding time at
this temperature of 19 minutes. Further investigation is needed
to improve the thermoelectric properties of ECAE processed
nanopowders. In particular, oxygen contamination has to be
avoided, for instance by operating in an Ar atmosphere; in
addition, the starting nanograined powder has to be a
stoichiometric single phase alloy.
ACKNOWLEDGMENT
The authors are thankful to Mr. E. Bassani for technical
assistance in ECAE experiments.
[2]
[3]
[4]
[5]
[6]
[7]
[8]
[9]
[10]
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