A solidification processing approach to the refinement of lead-tellurium-antimony alloy microstructure is described. Liquid alloys with eutectic, hyper-eutectic and hypo-eutectic compositions (relative to lead) were cooled to the solid state in three distinct ways, i.e. by water quenching, air cooling and furnace cooling. The structures of the alloys resulting from the three different solidification paths were examined using electron microscopy and the micrographs were quantified. Classical solidification methods were used to interpret the structures in relation to the cooling histories

Azizgolshani, Hesham

Haile, Sossina M.

Ikeda, Teruyuki

Ravi, Vilupanur A.

Snyder, G. Jeffrey

English

Caltech Authors - Main

Solidification Processing of Te–Sb–Pb Alloys For Thermoelectric Applications 
 
T. Ikeda*, H. Azizgolshani**, S. M. Haile*, G. J. Snyder** and V. A Ravi** 
*Materials Science, California Institute of Technology 
**Jet Propulsion Laboratory/California Institute of Technology 
 
Abstract 
A solidification processing approach to the refinement of 
lead-tellurium-antimony alloy microstructure is described.   
Liquid alloys with eutectic, hyper-eutectic and hypo-eutectic 
compositions (relative to lead) were cooled to the solid state 
in three distinct ways, i.e. by water quenching, air cooling and 
furnace cooling.  The structures of the alloys resulting from 
the three different solidification paths were examined using 
electron microscopy and the micrographs were quantified.  
Classical solidification methods were used to interpret the 
structures in relation to the cooling histories.   
Keywords 
Tellurium-antimony-lead, solidification, dendrite 
structure, secondary dendrite arm spacing, lamella structure 
Introduction 
The development of efficient thermoelectric devices for 
both space and terrestrial applications requires the discovery 
of materials with a high thermoelectric figure of merit, zT, 
defined as S2σT/κ, where S is the Seebeck coefficient, σ the 
electrical conductivity and κ the thermal conductivity.  
Materials investigated and optimized over the past 50 years 
have been conventional, simple semiconductors.  Examples 
include bismuth telluride alloys, lead telluride alloys and 
silicon germanium alloys.  Here we examined a system of two 
immiscible thermoelectric materials: PbTe-Sb2Te3. While the 
phase diagram of this psuedo-binary system is in dispute, the 
literature agrees that a moderately deep eutectic occurs at 
approximately 40 mol% PbTe with a melting temperature of 
approximately 585°C. Such a feature suggests that under the 
right synthesis conditions, the material will exhibit a nano-
scale lamellar structure composed of the two thermoelectric 
end-member compounds. Ideally, such structures can result in 
quantum confinement effects, increasing S, and/or in 
enhanced phonon scattering, decreasing κ. 
Traditional approaches to thermoelectric fabrication are 
based on a powder metallurgical route.  One of the key 
advantages of the P/M route is the homogeneity in the final 
microstructure; however, this method offers very little 
opportunity for structural refinement unless the raw materials, 
i.e., the powders themselves are engineered to have the 
desired shape and size. Liquid-solid transformations offer the 
opportunity to significantly alter the final microstructure by 
controlling the cooling history of the material.  In addition, 
considerable advantages can be achieved by altering the 
composition of the alloy around the ternary eutectic 
composition, i.e., 40 mol% (PbTe)-60 mol% (Sb2Te3).  
The specific objectives of the present study were to refine 
the microstructure by altering solidification rates and to 
investigate the effect of alloy composition. 
Experimental procedure 
Elemental Pb, Sb and Te granules or powders (99.999 % 
purity) were sealed in quartz tubes 10 mm in diameter under 
vacuum. The compositions of the alloys were chosen to be 
21.7 mol% (PbTe)-78.3 mol% (Sb2Te3)(Te–36 at.% Sb–5 
at.% Pb), 40 mol% (PbTe)-60 mol% (Sb2Te3)(Te–31.5 at.% 
Sb–10.5 at.% Pb) and 62.5 mol% (PbTe)-37.5 mol% 
(Sb2Te3)(Te–24 at.% Sb–20 at.% Pb), all of which can be 
located on the psuedo-binary PbTe - Sb2Te3 phase diagram. 
For notational ease, the Sb2Te3 content is hereafter omitted. 
The alloys were then melted by induction heating and were 
held at temperatures above 924°C for approximately 5 
minutes and then were solidified at three different cooling 
rates, i. e. by water quenching and air cooling.  
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Figure 1: Microstructure of Te-Sb-Pb alloys solidified by air 
cooling. The compositions are 21.7 mol% (PbTe) (a), 40 
mol% (PbTe) (b) and 62.5 mol% (PbTe) (c), respectively.  
0-7803-9552-2/05/$20.00 ©2005 IEEE                                    132                      2005 International Conference on Thermoelectrics 
The alloys thus prepared were cut, mounted in epoxy 
pucks and polished finally with 0.3 µm Al2O3 paste. The 
microstructures were observed using a field emission-
scanning electron microscope (Carl Zeiss LEO 1550 VP) 
equipped with a Robinson backscattered electron (BSE) 
detector for its high compositional contrast capabilities. The 
microstructures were analyzed using a personal computer 
with an image processing program (Macscope, Mitani Corp.) 
for quantitative description. The compositions of constituent 
phases in each alloy were measured using an energy 
dispersion X-ray spectrometry (EDS, INCAEnergy energy 
dispersion X-ray microanalysis system, Oxford Instruments). 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Figure 2: Microstructure of Te-Sb-Pb alloys solidified by 
water quenching. The comositions are 21.7 mol% (PbTe) (a), 
40 mol% (PbTe) (b) and 62.5 mol% (PbTe) (c), respectively.  
Microstructural morphology and phase assemblage  
Figures 1 and 2 show the backscattered electron image of 
microstructures in the alloys solidified by air cooling and 
water quenching, respectively. Lamellar structures are 
observed in the alloy with the composition of 21.7 mol% 
(PbTe) regardless of cooling rates. On the other hand, the 
62.5 mol% (PbTe) alloy has a dendrite structure, and the 40 
mol% (PbTe) alloy, which has a composition near the eutectic 
composition, is composed of a gray matrix phase, light 
dendritic features and dark lamellar phases.  
Composition analyses using EDS in the present work 
showed that the matrix phase in all alloy compositions 
investigated are near Pb2Sb6Te11. The dark lamellar phase in 
21.7 mol% (PbTe) is Sb2Te3. The dendrites in 62.5 mol% 
(PbTe) and 40 mol% (PbTe) alloys are PbTe. The existance of 
an intermediate phase of composition Pb2Sb6Te11 has been 
proposed by Abrikosov, On the other hand, recently, 
Shelimova et al. [2] suggested, on the basis of X-ray 
diffraction experiments, that PbSb4Te7 and PbSb2Te4 phases 
can be present rather than Pb2Sb6Te11. The current results are 
consistent with the phase diagram presented by the former 
authors, Fig. 3. We attribute the lamellar microstructure of the 
eutectic and Sb rich alloys to the layered crystalline structures 
of the constituent phases, Pb2Sb6Te11 and Sb2Te3 [2, 3]. In 
contrast, PbTe has a cubic structure (rocksalt type) [4]. 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Figure 3: Equilibrium phase diagram of the psuedo-binary 
PbTe-Sb2Te3 system [1]. 
 
Figure 4 (a) shows the fraction of dendrite in the 62.5 
mol% (PbTe) alloy, which was determined by image analyses. 
The fraction of dendrite is found to be around 26 % regardless 
of cooling rate and to agree with the fraction of PbTe phase 
expected from the equilibrium phase diagram proposed by 
Abrikosov [1].  
For the 21.7 mol% (PbTe) alloy, solidification was also 
performed by furnace cooling. The fraction of lamella in this 
alloy slightly decreases with increasing cooling rate as shown 
in Fig. 4 (b). In the figure, the fraction of Sb2Te3 phase 
expected from the equilibrium phase diagram is indicated by 
broken lines. Since lead is slightly soluble in the Sb2Te3 phase 
and the solubility varies with temperature, the fraction of the 
Sb2Te3 phase in solidified alloys can be expected to vary with 
cooling rate; the upper bound corresponds to the maximum 
solubility of lead and the lower limit to zero solubility, 
respectively. The experimental fraction is found to lie within 
this range. Therefore, the fraction of lamella experimentally 
observed is also consistent with the equilibrium phase 
diagram proposed by Abrikosov.  
Secondary dendrite arm spacing 
For the 62.5 and 40 mol% (PbTe) alloys, the secondary 
dendrite arm spacing (SDAS) was determined by image 
analysis. There is no significant difference between the SDAS 
near the perimeter and at the center of the ingot samples (data 
not shown). This is attributed to the fact that the thermal 
conductivity of the quartz container (~1.5 mm thick) [5] is 
roughly comparable to that of the Te-Sb-Pb alloys [6, 7].  
Thus, the resulting cooling rate of the alloy adjacent to the 
container wall (usually the highest in permanent mold 
castings) is reduced. Therefore, the temperature gradient 
within the alloys is expected to be small. This can lead to 
similar cooling rates near the perimeter and at the center.  
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Figure 4: Fraction of dendrite in the 62.5 mol% (PbTe)- alloy 
(a) and lamella in the 21.7 mol% (PbTe) alloy (b). The broken 
lines show the fraction expected from the equilibrium phase 
diagram [1]. The upper line is calculated from the solibility 
limit of Pb in Sb2Te3 and lower line to zero solubility in 
Sb2Te3.  
 
It is clearly apparent from Figs 1 and 2 that the water 
quenched samples have finer microstructure than the air 
cooled samples. Figures 5 and 6 show the histograms of the 
SDAS obtained by the image analyses for the 40 and 62.5 
mol% (PbTe) alloys, respectively. The SDAS in the water 
quenched samples are significantly smaller than that in the air 
cooled ones; the average SADS in the water quenched 40 
mol% (PbTe) sample is 10.9 µm whereas that in the air 
cooled sample is 21.1 µm, Fig. 5, with a similar trend evident 
for the 62.5 mol% (PbTe) alloy, Fig. 6. This result reflects the 
higher cooling rate of water quenching than air cooling.  
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Figure 5: Distribution of secondary dendrite arm spacings 
observed in 62.5 mol% (PbTe) alloys solidified by air 
cooling (a) and water quenching (b). 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Figure 6: Distribution of secondary dendrite arm spacings 
observed in 40 mol% (PbTe) alloys solidified by air cooling 
(a) and water quenching (b). 
 
Inter-lamellar spacing 
For the 21.7 mol% (PbTe) alloy, inter-lamellar spacing 
was determined by image analyses on the microstructures. 
Figure 7 shows the distribution of lamella spacing for air 
cooling and water quenching. The average separation between 
lamellae is smaller for the water quenched samples in 
comparison to the air cooled ones. Thus, it is found that the 
inter-lamellar spacing can also be controlled by changing 
solidification rates. 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Figure 7: Distribution of lamella thickness observed in 21.7 
mol% (PbTe) alloys solidified by air cooling (a) and water 
quenching (b). 
 
Conclusions 
The microstructure transitions from lamellar to dendritic 
as the fraction of PbTe increases from 21.7 mol% to 62.5 %. 
It is to be noted that the Pb2Sb6Te11 phase appears to be 
present in all microstructures.  
Water quenched samples have finer microstructures 
compared to air cooled ones. This includes the decrease in the 
interlamellar spacing for the Sb2Te3-rich alloys (21.7 mol% 
(PbTe)-78.3 mol% (Sb2Te3)) and decrease in SDAS for the 
PbTe-rich alloys (40 mol% (PbTe)-60 mol% (Sb2Te3) and 
62.5 mol% (PbTe)-37.5 mol% (Sb2Te3)) when the materials 
are water quenched. Thus, the characteristic microstructural 
lengthscales were found to be tunable by changing the 
cooling rate during solidification. More rapid cooling can be 
expected to produce nanoscale structural features, which is 
desirable for enhancing the thermoelectric figure of merit. 
Acknowledgments 
TI is supported by NSF through Caltech’s Center for the 
Science and Engineering of Materials (CSEM). Portions of 
this work were carried out at the Jet Propulsion 
Laboratory/California Institute of Technology; HA is 
supported by the JPLUS program at JPL/Caltech. We 
gratefully acknowledge the assistance of Dr. Chi Ma of 
Caltech for assistance with electron microscope image 
acquisition. 
References 
1 Abrikosov, N. Kh. et al., “The system Pb-Sb2Te3,” 
Inorganic matter., Vol. 1 (1965), pp. 1944-1946. 
2 Shelimova et al., “Synthesis and Structure of Layered 
Compounds in the PbTe-Bi2Te3 and PbTe-Sb2Te3 
Systems,” Inorganic matter., Vol. 40 (2004), pp. 1440-
1447. 
3 Belotskii, D. P. et al., “Crystal structure and electrical 
properties of alloys the system In2Se3-Sb2Te3,” Inorganic 
matter., Vol. 8 (1972), pp. 1677-1681.  
4 Noda, Y. et al., “Temperature dependence of atomic 
thermal parameters of lead chalcogenides, PbS, PbSe and 
PbTe,” Acta crystallographica, Vol. 43C (1987), pp. 1443-
1445.  
5 Thermophisical Properties of High Temperature Solid 
Materials, Vol. 4: Oxide and their solutions and mixtures, 
Part 2, ed. Touloukian, Y. S., The Macmillan Company 
(New York, 1967), pp. 1657-1658. 
6 Lost’ák, P. et al., “Preparation and some physical 
properties of tetradymite-type Sb2Te3 single crystals 
doped with CdS,” J. cryst. growth, Vol. 222 (2001), pp. 
565-573. 
7 Yoneda, S. et al., “Crystal grain size dependence of 
thermoelectric properties for sintered PbTe by spark 
plasma sintering technique,” 16th International Conference 
on Thermoelectrics, Dresden, Germany, Aug. 1997, pp. 
247-250. 
  
 
 
 
 
 
 


Solidification processing of Te-Sb-Pb alloys for thermoelectric applications

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Solidification processing of Te-Sb-Pb alloys for thermoelectric applications

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