A. Castellero et al. / Journal of Alloys and Compounds 653 (2015) 54e60
55
conditions close to the equilibrium. Conversely, a quenched sample
is likely highly defective as a consequence of the non-equilibrium
synthesis conditions. Thus, a high density of defects improves
atomic mobility, favouring the loss of Zn.
for 10 h, and subsequently cooled to room temperature in 5 h.
Rapidly solidified samples of Zn4Sb3 were obtained in the form
of fragmented ribbons with a planar flow casting apparatus
(Edmund Bühler GmbH). The master alloy was induction melted in
a BN crucible and ejected by an Ar overpressure (0.2 bar) on a
copper wheel rotating at 20 m/s. Powders obtained by grinding the
master alloy (MA) or the rapidly solidified (RS) ribbons were sin-
tered by open die pressing (ODP) at 558 K for 5 min: the sample
obtained were named ODP e MA and ODP e RS, respectively. Prior
to sintering, the powders were packed under Ar atmosphere in a Fe
sheath internally coated by a BN thick layer, annealed up to 558 K in
10 min and immediately placed between the heated plates of the
compression machine. Compaction of the powders was obtained by
increasing the load up to 343 kN until a deformation of about 55%
(measured by the bar displacement) was obtained, afterwards the
load was reduced to 196 kN for the remaining time, in order to
maintain the deformation constant.
Metallographic characterization of the samples was performed
with an optical microscope (Leica DMLM).
Density of the samples was measured in distilled water with a
picnometer.
Inductively Coupled Plasma-Atomic Electronic Spectroscopy
(ICP-AES) analysis was used for elemental analysis of the samples.
Structural characterization was performed by X-ray diffraction
(XRD) analysis. Measurements were performed on powdered
samples using a PANalytical X'Pert Pro diffractometer with Bragg-
Concerning mechanical properties, Ueno et al. [9] highlighted
that a significant limit of Zn4Sb3 for application in thermoelectric
generators (TEGs) is its brittle behaviour. It was estimated that
thermal stress can exceed fracture strength, leading to fracture of
the thermoelectric leg in the module [9]. Zn4Sb3 samples prepared
from the melt by free cooling [6] or Bridgman method [10] undergo
to a solid state reaction at about 767 K [4,5] and develop micro-
cracks because of volumetric changes, due to the different thermal
expansion coefficients, during the phase transformation [6].
Therefore, the typical preparation of bulk and dense Zn4Sb3 is based
on powder metallurgy processing, where compaction of single
phase powders is usually performed by hot pressing [6,9,11e13] or
spark plasma sintering (SPS) [14,15]. In the literature, different
approaches were attempted in order to improve mechanical
properties (e.g. fracture toughness and fracture strength) of Zn4Sb3
[11e15].
A first possible approach is to optimize the mechanical prop-
erties of the single phase material by tuning the particles size of the
powders used for compaction [13]. An increase of fracture tough-
ness was observed by decreasing the average particle size. No
correlation between thermoelectric properties and particles size
was established. However, samples prepared with smaller particles
showed a higher oxygen contamination after crushing and sieving,
due to the larger surface area exposed to the atmosphere, and,
consequently, a decrease of electrical conductivity.
A step forward consists in refining the microstructure of the
crystalline Zn4Sb3 single phase by non-equilibrium techniques,
such as rapid solidification [15]. In this case, a submicrometric
multi-phase microstructure can be obtained after quenching that
develops into a submicrometric single phase microstructure after
compaction by SPS. Microstructure refining allows to reach values
of compressive fracture strength above 300 MPa, while as cast in-
gots do not exceed 150 MPa. Dimensionless figure of merit ZT is
improved with respect to as cast ingots, as a consequence of the
increase of Seebeck coefficient, and the decrease of electrical con-
ductivity and thermal conductivity. An alternative approach for
increasing mechanical properties of Zn4Sb3 consists in developing a
composite by dispersion of a second phase with different proper-
ties from the matrix [11,12]. An in situ composite can be obtained by
adding an excess of Zn with respect to the stoichiometry of the
compound [11]. It was observed that an increase of the amount of
ductile Zn phase in the composite leads, on the one hand, to an
increase of fracture toughness and, on the other hand, to a decrease
of power factor. Finally, addition of SiC whiskers to the Zn4Sb3
matrix allows to obtain ex situ composite by hot extrusion or hot
pressing [12]. Increasing volume fraction of SiC whiskers (up to 5
vol. %), fracture toughness increases, power factor remains constant
and thermal conductivity increases, leading to a decrease of ZT.
In conclusion, both thermoelectrical and mechanical properties
of Zn4Sb3 compound strongly depends on the powder production
and on the sintering process. In this work, we investigate the
synthesis of Zn4Sb3 after compaction by open die pressing [16]
using powders produced from slowly cooled ingots and from
rapid solidified ribbons. Mechanical and thermoelectric properties
are investigated and the results are discussed in terms of structural
and microstructural stability.
Brentano geometry and Cu K
a radiation. Lattice parameters and
quantitative analysis of the phases were determined by Rietveld
refinement of the XRD patterns using MAUD software [17].
A Scanning Electron Microscope (SEM) Leica Steroscan 410
equipped with an Oxford Instruments INCAx-sight probe for En-
ergy Dispersive X-ray analysis (EDX) was used to examine the
microstructure and determine phase composition.
Thermal stability of the samples was investigated by differential
scanning calorimetry (DSC) with a power compensation Perkin
Elmer DSC7 using scanning rates of 4 K/min and 8 K/min upon
heating and cooling, respectively.
Seebeck coefficient of the processed samples was measured as a
function of the temperature using MMR Technology apparatus
between 300 K and 700 K. Average heating and cooling rates were
4 K/min and 8 K/min, respectively.
Room temperature stress-strain tests were performed in
compressive mode with a MTS 2/M mechanical test machine with a
strain rate of 1.7 10ꢀ5 sꢀ1
.
Microhardness measurements were performed with a Buehler
microhardness tester using a load of 4.9 N.
3. Results and discussion
3.1. Microstructure, phase evolution and thermoelectric properties
The optical micrographs of the cross section of MA, ODP e MA
and ODP e RS samples (Fig. 1(a), (b) and (c), respectively) show a
progressive reduction of residual porosity (dark spots). Conversely,
when density is measured with the picnometer (Table 1) the values
are similar for all the samples and very close to those reported in
the literature (e.g. 6.36 g/cm3 [2]). This apparent contradiction is
due to the presence of open porosity that, on the one hand, is visible
by optical observations and, on the other hand, cannot be evaluated
by density measurements with the picnometer.
According to ICP-AES analysis (Table 1), the master alloy has a
composition very close to the nominal one (Zn57.1Sb42.9 at.%),
whereas both samples processed by ODP show slight excess of Sb
with respect to the master alloy, suggesting a loss of Zn during open
die pressing process.
2. Materials and methods
For master alloy preparation, pure elements (99.999%), sealed in
a silica vial under Ar, were annealed in a muffle furnace at 1023 K