Thermoelectric properties of p-type Bi2Sr2Co2O9 glass-ceramics

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Thermoelectric properties of p-type Bi2Sr2Co2O9 glass-ceramics
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2014 Semicond. Sci. Technol. 29 124011
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Semiconductor Science and Technology
Semicond. Sci. Technol. 29 (2014) 124011 (6pp)
doi:10.1088/0268-1242/29/12/124011
Invited Article
Thermoelectric properties of p-type
Bi2Sr2Co2O9 glass-ceramics
Matthias Jost1,2, Julian Lingner1,2, Martin Letz2 and Gerhard Jakob1
1
Department of Physics, Johannes Gutenberg-University Mainz, Staudinger Weg 7, D-55128, Mainz,
Germany
2
SCHOTT AG, Material Development, Hattenbergstraße 10, D-55122, Mainz, Germany
E-mail: [email protected]
Received 30 September 2014
Accepted for publication 15 October 2014
Published 14 November 2014
Abstract
In the oxide system of Bi–Sr–Co glass melts have been prepared by adding a small amount of
glass formers. A crystallization leads to crystalline phases of Bi8Sr8Co4O25, BiSrCo2Ox and
Bi2Sr2Co2O9 (BC-222) densely embedded into a residual glass phase. This work shows the
possibility of obtaining microstructured bulk material with low thermal conductivity and
relatively high electrical conductivity via such a glass ceramic approach. Furthermore the
stability of these materials under thermal cycling for temperatures up to 700 °C is shown. A
characterization of the thermoelectric properties leads to a figure of merit (ZT) between 0.008
and 0.018.
Keywords: Bi2Sr2Co2O9, oxide thermoelectrics, thermal cycling, glass-ceramic
(Some figures may appear in colour only in the online journal)
Introduction
Although having high ZT values up to unity, well
established thermoelectric materials suffer from a limited
stability at elevated temperatures (e.g. Bi2Te3) [2], consist of
rare and expensive raw materials (e.g. SiGe) or environmentally harmful materials like lead (e.g. PbTe), for which
they will be forbidden in the EU for applications in the
automotive industry [3, 4]. In contrast, oxide thermoelectric
materials with ZT close to unity, which were first investigated
by Ichiro Terasaki in 1997 [5], are neither toxic nor harmful
to the environment, the raw materials are cost-efficient as well
as available in large quantities and they have a high chemical
and thermal stability. This makes them competitive due to
economic reasons even despite the lower performance,
reaching values of about ZT = 0.65 for polycrystalline ZnO
[6] or ZT = 0.87 for Ca3Co4O9 single crystals [7]. Recent
development on oxide thermoelectric materials can be found
in [8] and [9].
Most of today's bulk oxide thermoelectric materials are
produced via a ceramic route. The advantage of such ceramics
in comparison to glass-ceramics is the high electrical conductivity, resulting from the purity of the thermoelectric
In recent decades the demand for renewable energy has grown
steadily since more than 60% of the energy from fossil fuels is
dissipated as waste heat during the combustion process.
Based on the Seebeck effect [1] thermoelectric devices can
convert this waste heat into electrical energy. By this, the
consumption of fossil fuels as well as the emission of CO2
can be reduced. A thermoelectric module is composed of two
different materials, for example, a p-type semiconductor and
an n-type semiconductor, which are connected electrically in
series and thermally in parallel. The quality of a thermoelectric material is determined by the dimensionless figure of
merit
ZT =
S 2σT
,
κ
(1)
where S is the Seebeck coefficient (a material dependent
constant), σ the electrical conductivity, κ the thermal conductivity and T the absolute temperature.
0268-1242/14/124011+06$33.00
1
© 2014 IOP Publishing Ltd Printed in the UK
M Jost et al
Semicond. Sci. Technol. 29 (2014) 124011
phase. The disadvantages are the porosity and the associated
instability due to exposing a larger surface to chemical
influences or the high thermal conductivity. Nanostructuring,
which offers a further possibility to enhance the thermoelectric properties [10], is also very challenging, due to grain
growth during the sintering process.
These problems can be overcome by using a glass-ceramic route for the fabrication process. The glass-ceramics
represent a promising new class of materials for thermoelectrics and are also recently considered as novel optical
materials [11–14]. They are essentially pore free, have a low
thermal conductivity and both a higher chemical and
mechanical stability, due to their additional glassy phase. A
high amount of the thermoelectric phase is still important to
ensure the interconnection of these crystals, which is mandatory for providing a sufficient electrical conduction.
Nanostructuring can be expected by applying a special heat
treatment since the thermoelectric crystals are grown inside
the glass matrix. By varying the parameters of this heat
treatment the phase or crystal size can be influenced.
One of the first themoelectric glass-ceramic systems was
investigated by [15]. The n-type SrTiO3 was fabricated using
a glass-ceramic route. Although the material had a very high
absolute Seebeck-coefficient of 480 μV K−1, a thermal conductivity lower than 1.6 W m−1 K−1 and showed thermal
cycling up to 650 °C, the electrical conductivity was very
low, because the percolation threshold was not reached.
This work represents the first attempt of synthesizing a ptype thermoelectric glass-ceramic. Based on [16], a glass melt
was prepared with the aim of obtaining a Ca3Co4O9 glassceramic. Due to the impurity of the material, the second melt
was varied in an attempt to reduce the amount of secondary
phases and to get a Bi2Sr2Co2O9 glass-ceramic, which is also
known as a good misfit-structured thermoelectric material
[17]. Due to the complete crystallization during the melting
processes, the growth of specific phases could not be affected.
Nevertheless, thermoelectric characterizations were carried out.
forming range of the composition. The mixture was melted in
a platinum–iridium crucible at 1500 °C for 30 min. After that
it was casted on a copper plate, quenched with a second
copper plate, and cooled down very slowly again.
Scanning electron microscopy (SEM) images were taken
to identify the structure of the material. For that a Zeiss LEO
1530 Gemini was used. The x-ray diffraction (XRD) patterns
were measured with a PANalytical X’Pert Pro MPD x-ray
Diffractometer. The specific heat cp(T) of the materials was
determined by Difference-Scan Calorimetry (DSC) with a
NETZSCH DSC 404 Pegasus.
For the measurements of the temperature dependent
electrical conductivities and Seebeck coefficients rods with a
geometry of 10 mm × 2 mm × 2 mm were prepared. For the
measurements of the thermal conductivities disks with a
diameter of 10 mm and a thickness of 1 mm were prepared.
The measurements for the BSCO-1 were done at the
EMPA in Switzerland. For the Seebeck coefficient and the
electrical conductivity an Ozawa Science RZ2001i unit was
used. The measurements were carried out in an O2 atmosphere in a temperature range between 50 °C and 700 °C.
According to [18], the measurement error originates from the
inaccuracy of the thermocouples, the geometry of the samples
and the averaging of five data points, which were received at
each measurement. The relative error of the Seebeck coefficient and electrical conductivity can therefore be assumed to
be 5%.
The measurements of the thermal diffusivity α were
carried out using a NETSCH LFA 457 MicroFlash®. Together with the density of the material and its specific heat cp,
the thermal conductivity κ can be calculated via
κ = α ⋅ c p ⋅ ρ.
(2)
Due to the fact that each of these measurements is errorprone, the relative error of the thermal conductivity amounts
to about 7.5%.
The measurements for the BSCO-2 sample were carried
out at the Johannes Gutenberg-University in Mainz. The
Seebeck coefficient and the electrical conductivity were
measured using a Linseis LSR-3. In contrast to the measurements of the BSCO-1 sample, these measurements were
carried out in air. The error is assumed to be 5–10%. Again,
the thermal diffusivity measurements were done using a
NETSCH LFA 457 MicroFlash®.
Experimental procedures
The samples were prepared by a conventional melting process. For the first melt (BSCO-1), powders of CoO, CaCO3,
Bi2O3 and SrCO3 were mixed in a molar ratio of 2:1:1:1, as in
[16]. Unlike in [16], a platinum–iridium crucible instead of an
alumina crucible was used and a well defined amount of
Al2O3 was added to the composition, to simulate the
decomposing of the alumina crucible. The mixture was melted at 1500 °C for 30 min, before it was casted on a copper
plate and quenched with another copper plate. To reduce the
amount of stress inside the material, it was cooled down very
slowly after the quenching.
For the second melt (BSCO-2), powders of CoO, Bi2O3
and SrCO3 were mixed in a molar ratio of 2:1:2 and some
additional B2O3 was added to the mixture. These changes
were done due to the phases grown in BSCO-1. Since B2O3 is
known to be a good glass former, it should increase the glass
Results and discussion
The first melt (BSCO-1) showed a very low viscosity and the
surface appeared to be homogeneous and clear of crystals.
Therefore we assume that the components melted without any
residuals. Nevertheless, it completely crystallized upon casting. Because of this, the crystallization was only barely
controlled, resulting in a number of secondary crystalline
phases in the material.
The XRD pattern of the BSCO-1 (figure 1) shows that
the main phase is Bi8Sr8Co4O25 (JCPDS 85-1501), which is a
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M Jost et al
Semicond. Sci. Technol. 29 (2014) 124011
Figure 1. XRD pattern of BSCO-1, measured at 25 °C.
Figure 3. SEM image of BSCO-1 containing (1, 3) CaSrBi2O5, CoO
and Ca3Al2O6, (2, 6) Ca3Al2O6, CaSrBi2O5 and CoO, and (4, 5)
Bi8Sr8Co4O25, Bi2Sr2Co4O5+δ and a small amount of the Ca3Al2O6.
Figure 2. XRD pattern of BSCO-2, measured at 25 °C.
phase with good thermoelectric properties. In addition to this
phase three other phases crystallized, including CoO (JCPDS
48-1719), which is known to be a insulator with a band gap of
about 2.5 ± 0.3 eV [19]. The presence of CoO will thus
decrease the thermoelectric properties of the material. The
remaining phases are Bi2Sr2Co4O5+δ (JCPDS 49-0691) and
CaSrBi2O5 (JCPDS 49-1539). The first result shows that in
this experiment it was not possible to receive a clear glass by
melting down the batch, based on ‘Example 1’ in the patent of
R Funahashi.
The idea behind the second melt (BSCO-2) was to stabilize it by adding a network forming material (B2O3) to the
batch, with the intention to obtain a glass with more resilience
against devitrification. It was further necessary to suppress the
secondary phases by excluding the components of Al2O3 and
CaCO3. This time the targeted crystal phase was supposed to
be Bi2Sr2Co2O9, which is known to be a good thermoelectric
material [17]. To fulfill these efforts CaCO3 was replaced by
SrCO3 in the batch. The growth of the BC-222 phase should
therefore be supported.
This melt had a very low viscosity as well and also
crystallized completely upon casting, which made a controlled crystallization in a second step impossible. The XRD
pattern in figure 2 shows the existence of at least four different crystalline phases in the material. The main phase is
Bi2Sr2Co4O5+δ (JCPDS 49-0691). The secondary phases are
SrBi4O7 (JCPDS 46-0752), CoO (JCPDS 48-1719) and
Sr3B2O6 (JCPDS 31-1343). The SEM images of the two
samples are given in figures 3 and 4. These images show
again the existence of different crystalline phases inside the
materials.
The different phases are represented in a variety of gray
shades in the SEM images. Both materials do not show any
pores. Additionally emission spectra were measured via
Figure 4 SEM image of BSCO-2 containing (1) and (2) CoO, (3) and
(4) Bi2Sr2Co2O9 and (5) and (6) SrBi4O7 and CoO.
Figure 5. EDX measurement of BSCO-2(4). The carbon detected in
the EDX spectrum results from the coating of the sample.
energy-dispersive x-ray spectroscopy (EDX) at six different
points of each sample. In the BSCO-1, the region around 4
and 5 contains Bi8Sr8Co4O25, Bi2Sr2Co4O5+δ and a small
amount of Ca3Al2O6, which was not detected by the XRD.
Regions 1 and 3 contain CaSrBi2O5, CoO and Ca3Al2O6 as
well. Area 2 and 6 contain Ca3Al2O6, CaSrBi2O5 and CoO.
Hence the bright regions in figure 3 contain most of the
desired thermoelectric phase, whereas big parts of the material
consist of secondary phases, suppressing the good thermoelectric properties of the main phase.
All of the examined points in the BSCO-1 sample contain
a certain amount of alumina, which is a hint that it is present
as an amorphous phase.
The BSCO-2 contains secondary phases as well. However, the different areas of the crystals are much larger
compared to BSCO-1. Regions 1 and 2 consist of pure CoO,
the areas of 3 and 4 of pure Bi2Sr2Co2O9 (BC-222). The
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M Jost et al
Semicond. Sci. Technol. 29 (2014) 124011
Figure 6. XRD pattern of BSCO-2 with the identification of the
Bi2Sr2Co2Oy phase of [21].
Figure 7. Temperature dependent electrical conductivities of BSCO1 and BSCO-2.
regions in 5 and 6 consist of SrBi4O7 and CoO, while
Sr3B2O6 as determined by the XRD, was not found at these
points.
The EDX spectrum of point number 4 of the BSCO-2 is
shown in figure 5. It matches the one presented by Wang et al
[20]. The small additional peak of carbon on figure 5 results
from the coating of the SEM sample.
These measurements show the presence of the BC-222
phase inside the sample, although it was not possible to
identify it by XRD evaluations. This can be explained by the
fact, that there is no JCPDS file existing for the BC-222.
Therefore we assume the peaks of the Bi2Sr2Co4O5+δ phase in
the XRD to be related to the BC-222 phase. Comparing the
measured XRD pattern in figure 2 with the one in [21], it is
possible to match the BSC-222 hkl reflexes, shown in
figure 6. The line shows the XRD measurement results, while
the numbers represent the identification of the Bi2Sr2Co2Oy
phase according to [21]. In figure 7, the temperature dependent electrical conductivities of the samples are presented.
Both conductivities increase with temperature, which is a
typical behavior for semiconductors, whereas the slope of the
BSCO-2 is less steep than the one of BSCO-1. The BSCO-1
reaches a value up to 16 S cm−1 at 700 °C, the BSCO-2 only
reaches about 5 S cm−1. The different slopes might have their
origin in the different doping levels of the Co defect band.
The electrical conductivities of these samples are about
one (BSCO-1)/two (BSCO-2) orders of magnitude smaller
than the electrical conductivities known from common oxidic
thermoelectric ceramics (e.g. Ca3Co4O9 ceramics [22]).
In figure 8 the temperature dependent Seebeck coefficient
is shown. For the BSCO-1 it is nearly constant over the
temperature range and reaches a value of about 86 μV K−1 at
700 °C. However the Seebeck coefficient of the BSCO-2 is
increasing from about 100 μV K−1 at 100 °C up to
233 μV K−1 at 700 °C. Hence it was possible to increase the
Seebeck coefficient significantly from one melt to the other.
The resulting power factors of the two samples are shown
in figure 9. The power factor of BSCO-2 has been improved
compared to BSCO-1, despite its lower electrical
conductivity.
The thermal conductivities (figures 10 and 11) of the two
samples do not differ very much. Only in the temperature
Figure 8. Temperature dependent Seebeck coefficients of BSCO-1
and BSCO-2.
Figure 9. Temperature dependent power factors of BSCO-1 and
BSCO-2.
range between 100 °C–250 °C and at 700 °C, significant differences can be identified.
The BSCO-1 shows an almost constant value for the
thermal conductivity over the temperature range up to 650 °C.
The value at 700 °C, however, has a much lower value than it
would be expected from the general trend. This arises from
the cp measurement, because the specific heat shows a dip
around 700 °C. The thermal diffusivity decreases continuously with increasing temperature. From this, we obtain a
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M Jost et al
Semicond. Sci. Technol. 29 (2014) 124011
2 decreased compared to BSCO-1, while the Seebeck coefficient increased, as well as the power factor of BSCO-2. The
thermal conductivity increased only slightly, resulting in an
overall higher ZT value.
The corresponding error bars are shown in each figure.
They were calculated with the help of the error assumptions,
shown in the 'Experimental' part of this work. This results in
relative errors for ZT of about 16%.
The ZT values of these glass-ceramics are 1–2 orders of
magnitude lower than the values of common bulk oxidic
ceramics. To improve these further, the melt has to be stabilized in a way that a clear glass can be obtained after
casting. Only with a true glass a controlled crystallization can
be realized via a special heat treatment.
Figure 10. Temperature dependent thermal conductivity of BSCO-1.
Conclusion
Two glass melts have been produced, one which was aimed at
obtaining a Ca3Co4O9 single-crystal like it was claimed in
[16], the other one with the goal to obtain a Bi2Sr2Co2O9
glass-ceramic. Both of the melts completely crystallized upon
casting therefore showing a variety of crystalline phases, due
to the uncontrolled crystallization. The main phases of the
samples were Bi8Sr8Co4O25/Bi2Sr2Co4O5+δ for the BSCO-1
and Bi2Sr2Co2O9 for the BSCO-2. The second material could
be significantly improved compared to the first material,
resulting in a ZT of 0.018 at 700 °C. To increase this value
even further, the electrical conductivity and the Seebeck
coefficient have to be improved by increasing the amount of
crystalline phases with good thermoelectric properties. A
single-phase glass-ceramic is a goal for the further
development.
In addition it should be mentioned, that the microstructured material BSCO-2 offers high-temperature stability
and that at least one thermal cycle is possible. The present
work represents a good starting point for further glass-ceramic
development of oxide thermoelectric materials.
Figure 11. Temperature dependent thermal conductivity of BSCO-2.
Acknowledgments
Figure 12. ZT of BSCO-1 and BSCO-2.
The authors would like to thank Dr Sascha Populoh and
Professor Dr Anke Weidenkaff from the EMPA in Dübendorf
as well as Dr Benjamin Balke and Dr Michael Schwall from
the Johannes Gutenberg-University of Mainz for the help and
support during the thermoelectric measurements.
value of 1.28 W m−1 K−1 for the thermal conductivity
at 700 °C.
The values for the BSCO-2 sample on the other hand,
show a decreasing slope with increasing temperature, for
which a value of about 1.44 W m−1 K−1 at 700 °C is reached.
The corresponding ZT values are shown in figure 12. For
the BSCO-1 a value of 0.008 is reached at 700 °C. At the
same temperature BSCO-2 reaches a value of 0.018, which is
more than twice as high as the value of BSCO-1, although it
was only measured in air instead of an O2 atmosphere. Thus it
was possible to improve the material by varying the composition of the melt. The results at 700 °C are the most interesting results, as oxidic thermoelectric materials work best at
high temperatures. Here, the electrical conductivity of BSCO-
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