Interfacial reactions in Sn/Bi2Te3, Sn/Bi 2Se3 and Sn/Bi2(Te1-xSe x)3 couples

Article abstract of DOI:10.1016/j.jallcom.2014.05.127

Bi₂(Te_1-xSe_x)₃ is an important kind of n-type thermoelectric material. In this study, interfacial reactions at 250 °C in the Sn/Bi₂Te₃, Sn/Bi₂Se ₃ and Sn/Bi₂

Full text of DOI:10.1016/j.jallcom.2014.05.127

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Interfacial reactions in Sn/Bi2Te3, Sn/Bi2Se3 and Sn/Bi2(Te1-xSex)3 couples
Sinn-wen Chen, Chih-yu Wu, Hsin-jay Wu, Wan-ting Chiu
PII:
S0925-8388(14)01224-9
DOI:
http://dx.doi.org/10.1016/j.jallcom.2014.05.127
JALCOM 31316
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17 May 2014
19 May 2014
Please cite this article as: S-w. Chen, C-y. Wu, H-j. Wu, W-t. Chiu, Interfacial reactions in Sn/Bi2Te3, Sn/Bi2Se3
and Sn/Bi2(Te1-xSex)3 couples, (2014), doi: http://dx.doi.org/10.1016/j.jallcom.2014.05.127
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Interfacial reactions in Sn/Bi2Te3, Sn/Bi2Se3 and Sn/Bi2(Te1-xSex)3 couples
Sinn-wen Chen, Chih-yu Wu, Hsin-jay Wu, and Wan-ting Chiu
Department of Chemical Engineering, National Tsing Hua University
Abstract
Bi2(Te1-xSex)3 is an important kind of n-type thermoelectric material. In this
study, interfacial reactions at 250℃ in the Sn/Bi2Te3, Sn/Bi2Se3 and Sn/Bi2(Te1-xSex)3
couples are examined. In the Sn/Bi2Te3 couple, the reaction zone is found to be
non-planar and the reaction path is liquid/(liquid+SnTe)/Bi2Te3. But with only 2.0
at.%Se addition to the Bi2Te3 substrate, very different reaction results are observed. In
the Sn/Bi2(Te1-xSex)3 couple, the reaction layers are planar, and the reaction path is
liquid/SnTe/BiTex/ Bi2(Te1-xSex)3. In the Sn/Bi2Se3 couple, the reaction layers are also
planar, the reaction path is liquid/SnSe/BiSe/Bi2Se3, and the Bi content in the SnSe
phase varies. The growth rates in the Sn/Bi2Te3 couples are very fast, and the reaction
layer is as thick as 1174 m in just 15 minutes. Very dramatic reduction of growth
rates has been observed with Se addition to the substrates, and the reaction layer is
only 9 m in the Sn/Bi2(Te2.9Se0.1) couples after reaction for 30 minutes.
Key words: Bi2(Te1-xSex)3, Interfacial reaction, Thermoelectric
 Corresponding author: E-mail:swchen@mx.nthu.edu.tw; Tel: +886-3-5721734; Fax:+886-3-5715408
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1. Introduction
Energy shortages are an increasingly critical challenge. Thus, inspired mostly by
power generation from waste heat and cooling with more compact solid state coolers
based on the Seebeck and Peltier effects [1-5], thermoelectric devices have attracted
intensive study. Over the past 10 years, from 2003 to 2012, there were 483 American
patents and 6421 publications with the title word "thermoelectric" listed in the Web of
Science database. Among various kinds of thermoelectric materials, the p-type Bi2Te3
with Sb2Te3, and the n-type Bi2Te3 with Bi2Se3 are the earliest and most popularly
used commercial thermoelectric materials [1-15].
There are a series of p-n junction units and numerous joints in thermoelectric
devices, and the quality of these joints is crucial to the properties of devices. However,
there are few studies related to joining [6-15]. Sn-based solders are often used in
Bi2Te3-based devices. Although barrier layers exist between solders and
thermoelectric materials, it has been reported that the Sn might penetrate and react
with thermoelectric materials [6,7], so it is fundamentally important to examine their
interfacial reactions. This study investigates the interfacial reactions between Sn and
the n-type Bi2(Te1-xSex)3 and provides information not previously available in the
literature.
2. Experimental procedures
Thermoelectric material substrates, Bi2Te3, Bi2Se3, and Bi2(Te1-xSex)3 were
prepared with pure Bi shots (99.9 wt.%, Admat Midas, Norristown, PA), Te shots
(99.99 wt.%, Alfa Aesar, Ward Hill, MA), and Se shots (99.999 wt.%, Alfa Aesar,
Ward Hill, MA). Proper amounts of pure constituent elements were weighed and
encapsulated in a quartz tube at 10-2 torr.
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The sample capsule was first heated up to 500℃ for 6 hours, 700℃ for another
18 hours, and then quenched in air. The quenched sample capsules were annealed at
550℃ to homogenize the thermoelectric ingots. After one week, the capsules were
removed from the furnace. The ingots were removed from the quartz tubes and cut
into 2 mm-thick pieces of substrates.
The thermoelectric substrates were encapsulated with pure Sn shots (99.99 wt.%,
Alfa Aesar, Ward Hill, MA) at 10-2 torr, and the sample capsules were placed at 250
℃ until the Sn shots became molten. Liquid Sn surrounded the thermoelectric
substrates, and a liquid (molten Sn)/solid thermoelectric substrate [Bi2Te3, Bi2Se3, and
Bi2(Te1-xSex)3] couple was formed. After predetermined lengths of reaction time, the
sample capsules were removed from the furnace and quenched in air.
Reacted couples were then removed from the quartz tubes, cut, mounted, and
metallographically analyzed using scanning electron microscopy (SEM, Hitachi
S-2500). Compositions of the reaction layers were determined using electron probe
microanalysis (EPMA, JEOL, JXA 8200-SX). The accelerating voltage and beam
current were at 12.0 kV and 15 nA, respectively. PbTe, Sn, Se and Bi were used as
standards for the ZAF corrections [16] from intensities of Te Lα, Sn Lα, Se Lα and Bi
Mα to concentrations. The spot size of the EPMA analysis is chosen to be smaller
than the dimension of each reaction layer and the obtained composition is an averaged
value of at least five times measurements; in general, the quantitative accuracy is
about 0.1% for compositional analysis. Elemental mapping analysis is also conducted
using the EPMA to highlight the distribution of the elements of the reaction layers.
The reaction layer thickness was determined using a microscope equipped with an
image analyzer (Image-Pro Plus, Media Cybernetics Inc.).
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3. Experimental results and discussion
Sn/Bi2Te3 couples
Figure 1(a) is the BEI (back-scattered electron image) micrograph of the
Sn/Bi2Te3 couple reacted at 250℃ for 5 minutes. Since Sn is molten at 250℃, the
couple is a liquid Sn/solid Bi2Te3 couple. An irregular reaction zone is observed. It is
also noted that the reaction zone is not a single phase region. Figures 1(b) and 1(c)
show the results after reaction times of 10 minutes and 15 minutes, respectively.
Similar microstructures are observed in these samples, and the reaction zones consist
of various phases. As revealed in Figure 1(b), the reaction zone consists of at least
two regions. The region near the Bi2Te3 substrate is brighter, and this brighter region
has homogenously distributed fine bright spots. The region near the Sn side is darker
but contains larger bright pieces of materials.
Figure 2(a) is the elemental mapping of the reaction zone in a 10-minute reacted
sample, and Figure 2(b) is its SEI (secondary electron image) micrograph of the
etched reaction zone. Compositional analysis indicates that the larger bright pieces in
the reaction zone are unreacted Bi2Te3. The reacted microstructures are similar to
those observed in Sn/Te, Sn-Bi/Te and related systems [8,10,15]. The reaction zone
could consist of SnTe and liquid at 250℃, and the bright fine spots are Bi precipitates
formed during solidification. The average EPMA-determined composition of the
reaction zone near the Sn side is Sn-1.4 at.%Bi-35.1 at.%Te. This is in agreement
with the presumption that reaction zone is a three phase mixture, Sn+Bi+SnTe. It
should be mentioned that phase identification is conducted in this study only by
compositional analysis together with the available related phase diagrams, and not
confirmed by diffraction analysis.
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Comparing the results from Sn/Te, Sn-Bi/Te and related systems [8,10,15], the
following reaction mechanism is proposed. Molten Sn erodes and reacts with the
Bi2Te3 substrate at a very fast rate, a porous SnTe phase is formed which is mixed
with molten Sn, and excess Bi dissolves in molten Sn. This reaction-formed region is
the brighter region as shown in Figure 1(b). Then Te moves toward the Sn side, and
thus Te content is higher and Bi content is lower near the Sn side. The reaction path is
liquid/SnTe+liquid/Bi2Te3. When the sample is removed from the furnace, the liquid
solidifies, and Sn and Bi phases are formed. Since molten Sn erodes the substrate
quickly and irregularly, as shown in Figure 2(c), some tips of the unreacted Bi2Te3
substrates might be fragmented, taken from the substrates, and entrapped by the very
fast-growing reaction zone. This region primarily formed during solidification is the
darker region near the Sn phase.
As shown in Figures 1(a)-1(c), the reaction phases in the Sn/Bi2Te3 couples are
similar for the reaction at 250℃ with various lengths of reaction time. As shown in
Figure 3, the reaction layer grows thicker with longer reaction time, and the reaction
rates are very fast. The reaction layer is as thick as 1174 m after only 15 minutes
reaction. Since the reaction zone is irregular, the thickness values shown are average
results of at least three different measurements in order to have reliable data.
Sn/Bi2Se3 couples
Figure 4(a) is a BEI micrograph of the Sn/Bi2Se3 couple reacted at 250℃ for 10
minutes. A reaction zone is formed that is relatively planar compared to that in the
Sn/Bi2Te3 couple, as mentioned above. Analysis of the reaction zone reveals that it
consists of four different reaction phase regions: the dark layer, the gray layer, the
bright layer, and the brightest spots. The micrographs of the couples reacted for 30
minutes, 60 minutes and 90 minutes are shown in Figures 4(b), 4(c) and 4(d),
5

respectively. Similar results are observed in the Sn/Bi2Se3 couples reacted with
various lengths of time, and as shown in Figure 3, the reaction zone is thicker with
longer reaction time.
Figure 5 is the elemental mapping of the Sn/Bi2Se3 couple reacted at 250℃ for
90 minutes. There is a thin layer adjacent to the substrate, whose composition is
Sn-51.5 at.%Bi-47.5 at.%Se, and is the BiSe phase [17]. Sn and Se are the faster
moving species, while Bi is slowest [18]. Sn diffuses toward the substrate, Se diffuses
toward the Sn side, and the relatively immobile Bi is left behind, so a Bi-rich layer is
created.
Although there are variations, the concentrations of Se in the reaction zone are
primarily divided into two regions. The gray one adjacent to the Bi2Se3 substrate has
lower Se concentration, and is Sn-20.2 at.%Bi-33.1 at.%Se. That of the Se-rich region,
i.e. the dark region in Figure 4(d), is Sn-0.4 at.%Bi-46.5 at.%Se. Both of these layers
are the SnSe phase [19] although their Bi and Se contents are different. The
compositional analysis indicates the brightest spots are the Bi phase, although the
measured compositional values vary because the bright spots are small and their
signals might be influenced by the background phase.
Figure 6 is the reaction path in the Sn/Bi2Se3 couple superimposed on the
Sn-Se-Bi 250℃ phase equilibria isothermal section [20]. In the initial contact, part of
the Bi2Se3 substrate is dissolved into the molten Sn. The liquid phase becomes a
Sn-Se-Bi melt near the interface, and it is still pure Sn away from the interface. Sn
and Se react to form the SnSe phase at the interface, and the Bi is dissolved in the
liquid phase. As the reaction continues, Sn diffuses toward the substrate and the entire
reaction zone is primarily the SnSe phase with different compositions. Se diffuses
toward liquid side, and BiSe is formed adjacent to the Bi2Se3 phase. The reaction path
6

at 250℃ is thus liquid/SnSe/BiSe/Bi2Se3. The liquid phase solidifies when the
sample is removed from the furnace, and Sn and Bi phases are formed during
solidification, as can be found near the Sn/reaction zone interface.
It should be noted that there are numerous undetermined compounds in the
Bi-Se binary system [17], and BiSe has been reported as metastable compound in the
literature [17,21]. Since most of the Bi-Se compounds have not actually been
determined and identification of these phases is also not the primary research interest
of this study, only Bi2Se3 and BiSe are considered in Figure 6.
Sn/Bi2(Te1-xSex)3 couples
Figure 7(a) is a BEI micrograph of the Sn/Bi2(Te0.9Se0.1)3 couple reacted at 250
℃ for 10 minutes, and two reaction layers can be observed. Although only 6.0
at.%Se is introduced into the substrate, the morphology of the reaction zone has been
changed significantly and is very different from those in the Sn/Bi2Te3 couples shown
in Figures 1(a)-1(c). The composition of the dark layer is Sn-4.0 at.%Bi-43.8
at.%Te-2.2 at.%Se and it is the SnTe phase. The composition of the bright phase is
Sn-53.4 at.%Bi-44.5 at.%Te-1.3 at.%Se, and it is the Bi4Te3 phase. The SnTe phase is
a common reaction product in most of the Sn/Te (alloys) couples [8,10,12,14,15].
Similar to compounds in the Bi-Se binary system [17], there are numerous binary
compounds in the Bi-Te binary system [22], and the Bi4Te3 phase has been reported
as a metastable compound [22,23].
Micrographs of the couples reacted for 30, 60, 90 and 120 minutes are shown in
Figures 7(b)-7(e). The reaction layers grow thicker with longer reaction time, as
shown in Figure 3, and the reaction results are similar to those reacted for 10 minutes.
The darker layer is the SnTe phase. However, a closer analysis of the brighter region
of the reaction zone as shown in Figures 7(c)-7(e) reveals there are different layers in
7

the brighter region. In addition to the Bi4Te3 layer, a light gray layer with composition
of Bi-49.5 at.%Te-4.1 at.%Se is found and is identified as the BiTe phase. It has been
reported that the numerous binary Bi-Te compounds are an infinitely adaptive series
formed according to the (Bi2)m(Bi2Te3)n formula [24]. The reaction layers'
identification as Bi4Te3 and BiTe phases are based on their compositional analysis
results only. Since there are a series of undetermined compounds, these reaction
phases could have evolved according to the reaction time as well, and the BiTe
compound could be referred to as BiTex. The reaction path is
liquid/SnTe/BiTex/Bi2(Te0.9Se0.1)3.
Figures 8(a)-(c) are BEI micrographs of the Sn/Bi2(Te2.9Se0.1) couples reacted at
250℃. Similar to those in the Sn/Bi2(Te0.9Se0.1)3, the reaction layers are planar and
the reaction products are SnTe and BiTex phase, and the reaction path is liquid/SnTe/
BiTex/Bi2(Te2.9Se0.1). The question of why a small amount of 2.0 at.%Se added to the
Bi2Te3 substrate would completely alter the reaction results is interesting and needs to
be examined in the future.
Reaction layer thicknesses for the Sn/Te [8], Sn/Bi2Te3, Sn/Bi2Se3, Sn/Sb2Te3
[15], Sn/(Bi0.25Sb0.75)2Te3 [15] and Sn/Bi2(Te1-xSex)3 couples reacted at 250℃ are
shown in Figure 3. Since the reaction rates are very different, the Y-axis on the
right-hand size is for the Sn/Te, Sn/Bi2Te3, Sn/Sb2Te3 and Sn/(Bi0.25Sb0.75)2Te3, while
that on the left-hand side is for the Sn/Bi2Se3 and Sn/Bi2(Te1-xSex)3. It can be noted
that, though the reaction rates are very fast for most of these couples, when a small
amount of Se is added, the reaction rates are reduced dramatically. The reason for the
change of reaction rates is likely related to the interfacial reaction phases and
morphology and is definitely worthy of further investigations. Furthermore, the
compounds formed at the interfaces and their different morphologies would have
8

profound effects upon the properties of the thermoelectric joints and the devices,
detail analyses and understanding are fundamentally important and need to be
examined in the future.
4. Conclusions
Interfacial reactions at 250℃ in the Sn/Bi2Te3, Sn/Bi2Se3 and Sn/Bi2(Te1-xSex)3
couples are examined. The reaction paths are liquid/(liquid+SnTe)/Bi2Te3 in the
Sn/Bi2Te3 couple, liquid/SnSe/BiSe/Bi2Se3 in the Sn/Bi2Se3 couple, and liquid/SnTe/
BiTex/Bi2(Te1-xSex)3 in the Sn/Bi2(Te1-xSex)3 couple. With only 2.0 at.%Se addition to
the Bi2Te3 substrate, very different morphologies and different reaction rates are
observed. The reason for this dramatic change needs to be further examined in the
future.
5. Acknowledgment
This study is financially sponsored by the National Science Council of Taiwan
(NSC99-2221-E-007-093-MY3).
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Figure captions
Figure 1: BEI micrograph of the Sn/Bi2Te3 couple reacted at 250℃ for (a) 5 minutes,
(b) 10 minutes and (c) 15 minutes.
Figure 2: (a) Elemental mapping of the reaction zone in the Sn/Bi2Te3 couple reacted
at 250℃ for 10 minutes. (b) SEI micrograph of the deep-etched reaction zone in
the Sn/Bi2Te3 couple reacted at 250℃ for 10 minutes using (CH3OH) 93(HCl) 2
(HNO3)5 etchant. (c) Schematic illustration of the mechanism of the entrapped
pieces of unreacted Bi2Te3.
Figure 3: The reaction layer thickness in the Sn/Te [8], Sn/Bi2Te3, Sn/Bi2Se3,
Sn/Sb2Te3 [15], Sn/(Bi0.25Sb0.75)2Te3 [15] and Sn/Bi2(Te1-xSex)3 couples reacted
at 250℃. The Y-axis on the right is for Sn/Te, Sn/Bi2Te3, Sn/Sb2Te3 and
Sn/(Bi0.25Sb0.75)2Te3 and that on the left is for the Sn/Bi2Se3 and
Sn/Bi2(Te1-xSex)3.
Figure 4: BEI micrograph of the Sn/Bi2Se3 couple reacted at 250℃ for (a) 10
minutes, (b) 30 minutes (c) 60 minutes and (d) 90 minutes.
Figure 5: Elemental mapping of the reaction zone in the Sn/Bi2Se3 couple reacted at
250℃ for 90 minutes.
Figure 6: The reaction path in the Sn/Bi2Se3 couple superimposed on the Sn-Se-Bi
250℃ phase equilibria isothermal section.
Figure 7: BEI micrograph of the Sn/Bi2(Te0.9Se0.1)3 couple reacted at 250℃ for (a) 10
minutes, (b) 30 minutes, (c) 60 minutes, (d) 90 minutes and (e) 120 minutes.
Figure 8: BEI micrograph of the Sn/Bi2(Te2.9Se0.1) couple reacted at 250℃ for (a) 30
minutes, (b) 60 minutes, and (c) 90 minutes.
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Figure 1(a) TIF

Figure 1(b) TIF

Figure 1(c) TIF

Figure 2(a) TIF

Figure 2(b) TIF

Figure 2(c) TIF

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Figure 4(a) TIF

Figure 4(b) TIF

Figure 4(c) TIF

Figure 4(d) TIF

Figure 5 TIF

Figure 6 TIF

Figure 7(a) TIF

Figure 7(b) TIF

Figure 7(c) TIF