Bismuth telluride-based thermoelectric materials: Coatings as
protection against thermal cycling effects
Witold Brostow,a) Tea Datashvili, Haley E. Hagg Lobland, Travis Hilbig, Lisa Su, and Carolina Vinado
Laboratory of Advanced Polymers and Optimized Materials (LAPOM), Department of Materials Science and
Engineering and Department of Physics, University of North Texas, Denton, Texas 76207
John White
Laboratory of Advanced Polymers and Optimized Materials (LAPOM), Department of Materials Science and
Engineering and Department of Physics, University of North Texas, Denton, Texas 76207; and
Marlow Industries, Inc., Dallas, Texas 75238-1645
(Received 17 June 2012; accepted 4 September 2012)
Thermoelectric (TE) devices, both TE generators (TEGs) and TE coolers (TECs), have short
service lives as TE materials undergo degradation from sublimation, oxidation and reactions in
corrosive environments at high temperatures. We have investigated four high-temperature
polymers (HTPs) as candidates for TE element coatings and/or TE device fillers to minimize
or prevent this degradation. Two of these HTPs have shown good thermal stability in the
400–500 °C temperature range. The coatings were initially applied to bismuth telluride (Bi2Te3)based TE materials that are used for commercial power generation devices specified for operation
up to 250 °C. The HTPs protect the Bi2Te3 from both weight loss and weight gain up to 500 °C.
This is clearly outside the optimum TE operation range of Bi2Te3 materials, but demonstrates the
ability of the HTP coatings to protect the Bi2Te3 materials at least up to 250 °C. The properties that
HTP materials demonstrated during the examination of suitability of their use for TE element
coatings and/or TE device fillers using Bi2Te3 are expected to hold good for higher operating
temperature TE materials also.
I. INTRODUCTION
Solid-state thermoelectric (TE) devices are utilized in
both cooling and electrical power generation applications
and have several advantages over other methods, including: absence of moving parts, low maintenance, long life,
and high reliability.1–3 A device typically comprised two
ceramics that are metalized with copper,4 an array of
TE elements, and a solder that joins the device together.5–7
TE generator (TEG) applications have a wide range of
temperature requirements with sensor energy harvesting
TEGs operating around 85 °C,8 standard consumer product
TEGs up to about 250 °C,9 automotive waste heat recovery
TEGs up to about 500 °C,10 and radioisotope TEGs (RTGs)
operating up to about 1025 °C.11 Different TE materials are
required to optimize the peak TE performance at the different temperatures. For example, bismuth telluride (Bi2Te3)
alloys are satisfactory up to ;250 °C, various lead telluride
alloys and skutterudites satisfactory in the temperature range
of 500–700 °C, and silicon germanium (SiGe) and other
skutterudites up to ;1025 °C.12,13 Much effort has gone into
improving the bulk TE material performance for materials in
a)
Address all correspondence to this author.
e-mail: wbrostow@yahoo.com
DOI: 10.1557/jmr.2012.335
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each of these temperature ranges including novel engineering of nanoscale grains and interfaces.14 Our work, however,
focuses on protection of the TE material at a more macroscale, i.e., at the device assembly level.
At TEG operating temperatures, the different TE
materials need protection to prevent sublimation and
reaction with oxidative and corrosive environments
depending on the specific application. TEGs experience
preferential sublimation of the thermoelectric material.
For example, in (Bi2Te3) alloys, tellurium (Te) sublimes
before either bismuth (Bi) or lead (Pb) and deposits in
empty spaces as a film or as Te oxide needles; similarly
antimony (Sb) preferentially sublimes in the skutterudite
antimonides, germanium (Ge) in SiGe, and Te in lead
telluride (PbTe).15,16 We note that n type Bi2Te3 materials
typically contain selenium (Se).
Previous studies have evaluated silicon dioxide (SiO2),
metals and silica aerogel17 as potential protective coatings
on the TE elements. Ba0.3In0.2Ni0.05Co3.95Sb12/SiO2 nanocomposite material with a nano-SiO2 coating showed
improved thermal stability after cycling to 450 °C.18 Thin
metallic coatings on RTG unicouples suppressed Sb sublimation at 687 °C from CeFe3.5Co0.5Sb12 and Bi0.4Sb1.6Te3
p-type segments and CoSb3 and Bi2Te2.95Se0.05 n-type
segments.15 However, none of the metals tested by El-Genk
et al., tantalum (Ta), titanium (Ti), molybdenum (Mo) and
Ó Materials Research Society 2012
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W. Brostow et al.: Bismuth telluride-based thermoelectric materials: Coatings as protection against thermal cycling effects
vanadium (V), satisfied both minimal degradation of the
unicouple’s performance combined with ease of fabrication
and long life.19
We explore high-temperature polymer (HTP) filler and/
or coating materials as an alternative to aerogel, SiO2, or
metal coatings to prevent TE material degradation from
sublimation, oxidation, and corrosion up to a temperature
range of 400–500 °C. Bi2Te3 alloys are used as the initial
TE material to test HTP candidates as there are immediate
commercial applications in the 230–250 °C temperature
range. HTPs that perform well at 400–500 °C will be
transferable to PbTe, skutterudite and other relatively hightemperature TE devices, specifically automotive waste heat
recovery.
Attempts have been made to lower sublimation rates but
we would like to eliminate sublimation entirely. There are
large temperature differences (∆T up to 250 °C) across the
device as well as relatively high temperatures of power
generation applications. Moreover, there are applications
that require sealing the device from corrosive environments.
II. EXPERIMENTAL METHODS AND MATERIALS
A. Materials
TE material was fine-grained Bi2Te3 Micro-Alloyed
Material (MAM) from Marlow Industries and samples
were diced to needed dimensions from 17-mm diameter
wafers that had been sliced perpendicularly to the ingot
growth direction, nominally the a-axis of the Bi2Te3
rhombohedral crystal structure. Aluminum isopropoxide
[Al(OC3H7)3] from Sigma-Aldrich was the precursor of
boehmite AlOOH. The classic Yoldas process20–23 was
used to prepare boehmite suspensions in methanol. Hydrochloric acid and methanol from Sigma Chemicals Co.
were analytically pure and used as received. Several HTPs
were prepared at Laboratory of Advanced Polymers &
Optimized Materials (LAPOM). HTP1 is a two-component
system, with one component based on a phenyl glycidyl
ether and the other on aniline. The system is transformed
into a final product through etherification reaction at high
temperature. HTP2 is made from a solution of polyimide
precursor polyamic acid. The solution is heated to high
temperatures to vaporize the solvent, accelerate the imidization reaction and make a nonsoluble, nonmelting insulating material with good heat resistance, chemical
resistance and insulation properties. HTP4 is a thermoset
cyanate ester resin with the glass transition temperatures
280 °C. The term “cyanate ester resin” is used to describe
both a family of monomers and oligomers with reactive
cyanate end groups (-O-C[N) on an aromatic ring and the
cured resin networks into which the monomers are formed.
This family of thermosetting monomers and oligomers
contain at least two cyanate functional groups and will
homopolymerize with the addition of heat and/or catalyst
into a thermosetting material (so called polycyanurate).
HTP5 is a thermally stable polymer based on stiff aromatic
backbones, infusible and insoluble due to its planar aromatic
structure and it is processed via the solvent route.
B. Encapsulation of Bi2Te3 materials
Encapsulation was achieved by dip coating and/or by
painting the free surface sides of Bi2Te3 substrates. The
coatings were dried and/or cured at elevated temperatures.
Before electrical conduction tests, the coating on the material
was carefully peeled off so as to test the thermoelectric material instead of the polymeric material.
C. Thermogravimetric analysis
Thermogravimetric analysis (TGA) was performed on
a Perkin Elmer TG-7 apparatus (Waltham, MA). Thermal
analysis techniques are well described by Menard,24
Lucas et al.,25 by Gedde26 and by Saiter et al.27 10 mg
of each dried sample were heated from 1 50 to 1 600 °C
at 10 °C/min.
D. Environmental scanning electron microscopy
and energy-dispersive spectroscopy
Energy-dispersive spectroscopy (EDS) of the samples
was collected using a FEI Quanta ESEM (Hillsboro, OR)
configured with EDS.
E. Profilometry
The roughness of the samples was measured with a
Veeco Dektak 150 Profilometer (Plainview, NY). The
stylus tip had a radius of 12.5 lm, the applied force was
1.0 mg and the scan rate 0.033 lm/s.
F. Electric resistivity
A four-point Keithley SourceMeter (Cleveland, OH)
was used to measure electrical resistivity. The conductivity of Bi2Te3 is anisotropic. The electrical conductivity
perpendicular to the ingot growth direction (nominally the
a-axis direction) is approximately four times as large as
that perpendicular to the ingot growth direction (nominally
the c-axis direction).1 Given the sample orientation relative to crystal growth direction, the probes contact a planar
surface that is parallel to the ingot growth direction
(nominally c-axis direction). However, this may be of
academic interest only as the change in resistivity is the
parameter of interest and all samples were measured in the
same orientation for both pre- and post measurements.
III. EFFECTS OF HEATING ON COMPOSITION
AND MORPHOLOGY
As TE devices are subjected to thermal cycling, we
used TGA to locate temperatures of various thermal
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W. Brostow et al.: Bismuth telluride-based thermoelectric materials: Coatings as protection against thermal cycling effects
decomposition processes and to determine the amounts of
volatiles. Results for a Bi2Te3 sample are shown in Fig. 1.
Figure 1 shows a slight mass loss (note the left scale) up
to 400 °C. There is a steeper slope (wt%/°C) in the mass
loss up to ;160 °C and this is likely due to desorption and
evaporation of absorbed residues such as moisture. The
mass loss slope is more gradual from ;160 to ;400 °C
and this may be due to sublimation of tellurium. However,
starting at approximately 400 °C the diagram shows
relatively sharp weight gain. Apparently a chemical reaction with something from the environment takes place.
If tellurium is escaping, then oxidation of bismuth is
a likely candidate for that reaction. This hypothesis is
tested below by other means. Given the heating rate of
10 °C/min and the sharply upward trend of the curve, the
oxidation reaction has not been completed during the run.
To evaluate the sublimation and oxidation hypotheses
formulated above, we have applied EDS to n-type Bi2Te3
samples before [Fig. 2(a)] and after [Fig. 2(b)] the TGA
examination.
We note 0.4 to 1.3 wt% oxygen at different sample
areas of an unheated Bi2Te3 specimen prior to TGA. This
implies different levels of initial oxidation during the
production or storage period possibly caused by a nonuniform distribution of defects in Bi2Te3 crystals. Areas
with large defects might provide high oxidation levels.
The oxygen peak in Fig. 2(b) shows oxidation occurred
during the TGA run. The ratio of the Te peak to the Bi peak
after the run is smaller and the small 1.4 keV Se peak in
2(a) is absent in 2(b). We recall that n-type Bi2Te3 alloys
typically contain Se. These data suggest a loss of Te and Se
during heating of the Bi2Te3; a plausible explanation is
sublimation.
We have also studied morphology changes caused by
heating using scanning electron microscopy (SEM); see
Fig. 3. Samples were heated in a TGA apparatus up to
550 °C.
We see in Fig. 3 hollow microstructures caused by
heating—a result of sublimation. The empty domains and
degradation of the crystalline structure are more visible on
the fractured surfaces, a darker color possibly a sign of
oxidation. Thus, also environmental scanning electron
microscopy (ESEM) images support our hypotheses on
oxidation and sublimation of Bi2Te3 components during
heating.
IV. ELECTRICAL RESISTIVITY
We now consider effects of heating on the electrical
resistivity. Both n-type and p-type Bi2Te3 samples were
left at either 250 or 500 °C for an hour. Then electrical
resistivity was determined at 25 °C for each sample; see
the results in Fig. 4. The current versus voltage diagrams
are also included in that figure.
As expected, the resistivity values decrease with higher
heating temperature in all cases. Compared with initial
specimens, the resistivity values of both types of Bi2Te3
samples are slightly higher after 1 h of heating at 250 °C
temperature and much higher after heating to 500 °C.
These results are also in agreement with our model of
sublimation and oxidation. We have seen above manifestations of these processes in TGA, EDS and ESEM.
Clearly metal sublimation and/or oxidation at high temperatures increase electric resistivity.
V. ENCAPSULATED MATERIALS
FIG. 1. TGA thermogram and its derivative for Bi2Te3.
The key question is how to extend the service life of
TE devices. Our approach is based on using a coating to
FIG. 2. EDS results: n-type Bi2Te3 (a) and n-type Bi2Te3 after a TGA run (b).
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W. Brostow et al.: Bismuth telluride-based thermoelectric materials: Coatings as protection against thermal cycling effects
FIG. 3. ESEM micrographs of Bi2Te3 surfaces (S) and fractured surfaces (FS) before (A–S, A–FS) and after TGA testing (B–S, B–FS).
FIG. 4. Electrical resistivity of n-type and p-type Bi2Te3 before and after heating at 250 and 500 °C.
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W. Brostow et al.: Bismuth telluride-based thermoelectric materials: Coatings as protection against thermal cycling effects
FIG. 5. Bi2Te3 1 AlOOH sample: (a) TGA and (b) ESEM.
insulate free spaces inside the TE devices and also to
encapsulate both p-type and n-type TE materials. We thus
expect to alleviate several problems listed above: moisture will not have easy access to interior of the device;
structural integrity will be enhanced; corrosive agents
will not have easy access; temperature cycling will not
cause fast degradation, in particular sublimation will be
eliminated; low thermal conductivity of the added materials will ensure concentration of heat flow through the
TE device. The overall desired results will be TE systems
operating at high temperatures with improved performance and longer lifetime.
An inorganic ceramic coating could accomplish the
above objectives–as mentioned previously. Bi2Te3 specimens were coated with boehmite via dip coating and dried
at 250 °C for 12 h to provide smoother surfaces.
Figure 5 shows that Bi2Te3 coated with boehmite displays better thermal stability all the way up to 600 °C than
the bare Bi2Te3. Below 400 °C the coating prevents
sublimation. However, above 350 °C we see a slight weight
gain. ESEM analysis shows formation of cracks and it is
likely that oxygen penetrates inside through the cracks and
chemically reacts with Bi2Te3 components. The main
reason for cracking of the ceramic coating seems to be the
coating thickness exceeding 1 lm. We have to apply fairly
thick coatings because of relatively rough surface of the
unencapsulated Bi2Te3 substrate. The minimum roughness
was 0.78 lm, the maximum 1.16 lm. The n-type
material has higher roughness than the p-type.
Given the results obtained with a ceramic coating, we
moved to polymeric coatings. Expected advantages of
HTPs for encapsulation of TE materials are flexibility,
easiness of processing and ability to tailor properties
through chemical modification or fillers. However, pitfalls are also possible. We are dealing here with the
coating very different from the substrate. As discussed by
Kopczynska and Ehrenstein28 and more in detail in a book
by Desai and Kapral,29 properties of multiphase composites are strongly controlled by interfaces. Even interfacial
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FIG. 6. TGA thermograms of “naked” and encapsulated Bi2Te3.
curvatures are important.29 Diffusion of moisture, gases or
liquids across ceramic surfaces can be neglected for
most purposes while such diffusion is fairly typical
for polymers30–32 and usually causes swelling.33–35
As before, we first used TGA for evaluation of four different HTP coatings; see Fig. 6.
Note the small scale in Fig. 5 starting at 95%. As
demonstrated above, Bi2Te3 loses weight in temperatures
up to 400 °C and then begins to gain weight through oxidation around 400 °C. The encapsulated samples show good
thermal stability and lack of oxidation.
Subsequently, Bi2Te3 encapsulated with HTP2 was
heated twice to 500 °C and each time left for 5 min at
500 °C. The respective TGA results are shown in Fig. 7;
again note the scale, beginning here at 99.8%. In the
second heating cycle there is hardly any change of weight;
the volatiles were eliminated in the first cycle. Two cycles
up to 500 °C have not caused thermal degradation,
sublimation or oxidation of the TE material.
We find that encapsulating Bi2Te3 TE material with
HTPs provided proof of concept for the capability to
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W. Brostow et al.: Bismuth telluride-based thermoelectric materials: Coatings as protection against thermal cycling effects
FIG. 7. TGA thermograms: “naked” and encapsulated Bi2Te3 samples
after two heating cycles.
extend the service life of TE devices at high temperatures
up to 550 °C. This provides a solution for Bi2Te3 TEGs
with their relatively low operating temperature of 250 °C
and it opens possibilities for applications with the higher
temperature TE materials such as PbTe and skutterudite
antimonides. We have used HTPs and their blends, but
there are also other options including polymer irradiation36,37 or using fillers.38–44 The last option should enable
adjustment of thermal expansivity.
ACKNOWLEDGMENTS
Partial financial support from the II–VI Foundation,
Bridgeville, PA, is gratefully acknowledged. Support to
one of us (L.S.) by the Texas Academy of Mathematics
and Science (TAMS), Denton, is acknowledged also.
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