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Clemson UniversityTigerPrints
All Theses Theses
1-2011
Thermal, Electrical, and Structural Behavior ofSilver Chalcogenide CompositesJeremy CappsClemson University, [email protected]
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Recommended CitationCapps, Jeremy, "Thermal, Electrical, and Structural Behavior of Silver Chalcogenide Composites" (2011). All Theses. 1144.https://tigerprints.clemson.edu/all_theses/1144
THERMAL, ELECTRICAL, AND STRUCTURAL BEHAVIOR OF SILVER
CHALCOGENIDE COMPOSITES
___________________________________________
A Thesis
Presented to
The Graduate School of
Clemson University
___________________________________________
In Partial Fulfillment
of the Requirements for the Degree
Master of Science
Physics
___________________________________________
by
Jeremy Patrick Capps
August 2011
___________________________________________
Accepted by:
Dr. Fivos R. Drymiotis
Dr. Jeremy King
Dr. Eric Skaar
ii
ABSTRACT
In this thesis and the contained publications, it is demonstrated that it is not only
possible to improve the dimensionless figure of merit (ZT) of silver chalcogenide
composites, but it is also possible to move the maximum ZT value into a more favorable
temperature range for power production. It is shown that by introducing disorder, stress,
and phase competition into a system, a reduction in the material’s thermal conductivity
can be realized. The binaries Ag2Te and Ag2Se are re-examined in detail before moving
on to examine Ag2Te1-xSex composites. The maximum ZT (ZT = 0.92) of Ag2Te0.5Se0.5
is measured at T= 500 K, which is a more favorable temperature range for power
production. Lastly, the high performing thermoelectric material Cu0.2Ag2.8SbSeTe2is
presented, which produced a ZT = 1.75 and ZT = 1.5 at T= 680 K from its respective
slow-cooled and quenched samples. These results suggest that the formation of
composites is a viable way to develop a phonon-glass-electron-crystal (PGEC).
iii
DEDICATION
I would like to dedicate this thesis to my parents, Jerry and Patricia Capps, for
their continued support throughout the years.
iv
ACKNOWLEDGMENTS
I would like to thank my advisor Dr. Fivos Drymiotis for his dedication to our
group’s research and willingness to help, offer advice, and give direction when needed. I
would also like to thank the other members of the Drymiotis research group for their
contributions to my project and for sharing their knowledge on the various research
techniques needed to get me started on my own.
I would also like to thank all of my friends for their support and for keeping my
spirits high throughout my first two years of graduate school. Those in particular are Bart
Snyder, Manjeet Singh, and Michael Powers, as well as other graduate students in the
Physics Department.
Lastly, I would like to thank the National Science Foundation (NSF-DMR-
0905322) for providing the funding to make my research possible.
v
TABLE OF CONTENTS
Page
TITLE PAGE ......................................................................................................................... i
ABSTRACT.......................................................................................................................... ii
DEDICATION .....................................................................................................................iii
ACKNOWLEDGMENTS .................................................................................................. iv
LIST OF FIGURES ............................................................................................................. vi
CHAPTER
1. INTRODUCTION ............................................................................................. 1
2. RECENT RESULTS ......................................................................................... 6
Purpose of the Study ................................................................................... 6
Thermoelectric Performance Data .............................................................. 6
Discussion and Conclusions ..................................................................... 15
3. PUBLICATIONS ............................................................................................ 18
Significant enhancement of the dimensionless thermoelectric figure of
merit of the binary Ag2Te ................................................................... 19
The effect of Ag concentration on the structural, electrical and thermal
transport behavior of Pb:Te:Ag:Se mixtures and improvement of
thermoelectric performance via Cu doping ....................................... 24
Excess vibrational modes and high thermoelectric performance of the
quenched and slow-cooled two-phase alloy Cu0.2Ag2.8SbSeTe2 ...... 30
4. REFERENCES ................................................................................................ 37
vi
LIST OF FIGURES
Figure Page
1.1 Relation of ZT and Efficiency to the Carnot Efficiency ........................... 3
2.1 XRD of Ag2Te1-xSex .................................................................................... 7
2.2 EDX of Ag2Te0.5Se0.5 .................................................................................. 8
2.3 SEM of Ag2Te0.5Se0.5 (left) vs. Ag2Te0.75Se0.25 (right).............................. 9
2.4 Seebeck Coefficient of Ag2Te1-xSex ......................................................... 10
2.5 Resistivity of Ag2Te1-xSex ......................................................................... 11
2.6 Power Factor of Ag2Te1-xSex .................................................................... 12
2.7 Thermal Conductivity of Ag2Te1-xSex ...................................................... 13
2.8 Thermal Conductivity of Ag2Te0.5Se0.5 vs. Pure Binaries ...................... 14
1
CHAPTER ONE
INTRODUCTION
Research on sustainable energy sources and power production is currently of great
interest and importance to the scientific community. With the energy crisis becoming
more apparent every year, new and innovative solutions are needed to not only allow our
society to continue operating at its current consumption rate but also to be able to sustain
the enormous population growth as of late. One of the most promising research areas to
potentially offer a feasible solution to these problems is that of thermoelectric materials.
Thermoelectric materials may be used to convert thermal heat energy into
electrical energy. This ability of thermoelectric materials makes them a very promising
candidate to aid in the world-wide energy crisis. It should immediately be noticed that
there are many sources of heat where wasted energy could be harnessed and utilized by
converting it into a usable source of energy. Obvious examples of this are the exhaust
gases in internal combustion engines and natural hot springs. By utilizing just a small
portion of wasted energy in automobiles, we could save millions of dollars per year by
cutting the amount of oil products used.
The figure of merit (ZT) is the dimensionless quantity used to rate the
performance and efficiency of a thermoelectric material and is given below as:
(1)
2
The Seebeck coefficient is given by α, the electrical conductivity is given by σ,
and the thermal conductivity is given by κ. The thermal conductivity is divided into an
electrical contribution, denoted by κe, and a lattice contribution (sometimes referred to as
the phonon contribution), denoted by κl. In order to improve the ZT of a material, one
can try to increase the electrical conductivity or reduce the thermal conductivity. We
chose to manipulate the thermal conductivity. However, these variables are not entirely
independent of one another. For example, the electrical conductivity can be improved by
adding charge carriers to the system, but this usually also raises the value of the electrical
thermal conductivity.
Here it should be noted that the more promising materials in thermoelectrics are
those that have high levels of complexity. Complex single-phase materials are able to
scatter long wavelength phonons, which leads to low thermal conductivity values. In
addition to single-phase materials, two-phase compound materials are also very
promising. However, only a small percentage of these compounds are capable of
demonstrating good thermoelectric performance because they often also have high
electrical resistivity values despite having low thermal conductivity values. Most state of
the art thermoelectric materials today operate with a ZT ~1. The current goal in
thermoelectrics research is to produce physically stable materials that possess ZT values
greater than 2. It is also a requirement that the materials should not have the problem of
reproducibility. The returns in efficiency diminish once ZT values become greater than 3
such that the added efficiency does not outweigh the research costs. This relation can be
seen in the following:
3
Figure 1.1: Relation of ZT and Efficiency to the Carnot Efficiency (Yang and Caillat)
To produce a high thermoelectrically performing material, it is useful to have a
good starting platform to build on. This means starting with materials that already
possess both low thermal conductivity values and favorable electrical resistivity values.
Because of these requirements, our research group was drawn to the chalcogenides, and
more specifically, the silver-chalcogenides Ag2Te and Ag2Se. A chalcogenide is a
compound formed with a chalcogen ion (Group 16 in Periodic Table, usually O, S, Se, or
Te) and at least one electropositive element. An electropositive element is one that is
4
willing to donate electrons for bonding. The silver-chalcogenides have intrinsically low
thermal conductivity values usually less than 1 W/m-K, and resistivity values on the
order of 1 mΩ-cm.
Before working with Ag2Te1-xSex composites, the pure binaries Ag2Te and Ag2Se
were re-examined. It was also important to be able to reproduce the thermoelectric
performance from the previous work on these materials. The thermoelectric performance
of pure Ag2Se was reproduced and in agreement with the previous literature. Two factors
that strongly correlate with high thermoelectric performance of these systems are
synthesis procedure and elemental purities. Using a different synthesis method and
higher elemental purities, it was shown that Ag2Te had much greater thermoelectric
potential than previously reported.
The research project concludes with the investigation of Ag2Te1-xSex composites.
The motivation for this work is built on the idea that the introduction of stress, disorder,
and phase competition into a system can effectively lower a materials’ thermal
conductivity. This phase competition comes about from the different lattice structures of
Ag2Te and Ag2Se below their phase transitions at approximately 407 K. Ag2Te is
monoclinic below the transition, whereas Ag2Se is orthorhombic below the transition. It
was clearly demonstrated that competing phases can lead to a reduction in thermal
conductivity, seeing as most of the Ag2Te1-xSex composites had lower thermal
conductivity values than both of the pure binaries. In addition to this, the composite
Ag2Te0.5Se0.5 maintained a lower thermal conductivity with respect to the pure binaries
5
even after the phase transition temperature. It was verified by EDX and XRD that these
composites are in fact two-phase systems.
6
CHAPTER TWO
RECENT RESULTS
Purpose of the Study
The purpose of this study is to demonstrate that introducing phase competition is
an effective method to lower the thermal conductivity of a system, particularly in the case
of Ag2Te1-xSex. As discussed earlier, reducing thermal conductivity is one way to
improve the dimensionless figure of merit ZT. There are other methods to try and
improve the ZT of a system, such as manipulating the electrical resistivity, but the focus
of our research is to investigate the effects of phase competition on thermal conductivity.
Thermoelectric Performance Data
XRD, SEM, and EDX
7
Figure 2.1: XRD of Ag2Te1-xSex
8
Figure 2.2: EDX of Ag2Te0.5Se0.5
9
Figure 2.3: SEM of Ag2Te0.5Se0.5 (left) vs. Ag2Te0.75Se0.25 (right)
10
Seebeck Coefficient
Figure 2.4: Seebeck Coefficient of Ag2Te1-xSex
11
Resistivity
Figure 2.5: Resistivity of Ag2Te1-xSex
12
Power Factor
Figure 2.6: Power Factor of Ag2Te1-xSex
13
Thermal Conductivity
Figure 2.7: Thermal Conductivity of Ag2Te1-xSex
14
Figure 2.8: Thermal Conductivity of Ag2Te0.5Se0.5 vs. Pure Binaries
15
Discussion and Conclusions
To start on this section, I would first like to restate the goal of this project, which
was to see how phase completion can influence the thermoelectric performance of a
material. It was hypothesized that having relatively high levels of stress and disorder
introduced into a system via phase competition will lower the thermal conductivity of a
material. That being said, it is important to understand what the XRD and EDX data
implies.
From the XRD data shown in the previous section, it should be noted that the
bottom portion (x=0) of the graph shows the x-ray profile, or fingerprint, of pure Ag2Te,
and the top data shows the profile of pure Ag2Se. What we see is that as the Se
concentration is increased from x=0 to x=0.1, there is no evidence (conclusive) to suggest
that the Ag2Se phase is present. Also note the opposite is true when a small amount of Te
is introduced to pure Ag2Se. The region of interest is when 0.25 < x < 0.75. In this
region of the XRD graph, we see evidence of both the Ag2Te and Ag2Se phase being
present in the material. Therefore, it is expected that the greatest level of disorder will
occur at x=0.5. The XRD suggests that a two-phase composite was formed, which was
the goal. The EDX data provide further evidence that we are working with a two-phase
system in the region between 0.25 < x < 0.75. In this region we clearly see traces of
Ag2Te and Ag2Se. Not only are these traces observed, but it appears that the two phases
are uniformly distributed throughout the sample.
16
Now that it has been confirmed that we are indeed working with two-phase
composites, it is interesting to see how the thermoelectric quantities behave before and
after the phase transition. It should be noted here that the pure binaries exhibit very
different behavior around the transition temperature and also in the higher temperature
regions. It seems to be the case for the Seebeck, resistivity and thermal conductivity that
the dominant element in the composite has an influence of the composites’ thermoelectric
behavior at the transition and in the higher temperature regions. Composites that contain
more Te behave more like the pure Ag2Te sample, and those that contain more Se behave
more like the pure Ag2Se sample. There is some ambiguity with the Ag2Te0.5Se0.5 sample
behavior in these measurements, which is to be expected because the higher level of
phase competition present in the sample.
When looking at the thermal conductivity data, it seems that the goal of lowering
thermal conductivity below the phase transition temperature has been achieved. All of
the mixed composites have lower thermal conductivity values than the pure binaries
below the transition temperature. Above the transition temperature, most of the
composites have thermal conductivity values that fall between the two pure binaries, but
it should be noted that the Ag2Te0.5Se0.5 composite’s values fall below the pure binary
values even after the phase transition. It is believed that this is because at this
concentration the maximum level of disorder and phase competition has been introduced
into the system. From the above data, the ZT of Ag2Te0.5Se0.5 was calculated to be
approximated ZT = 0.92 at a temperature of T = 500 K. While this ZT value is no higher
than the ZT of pure Ag2Se, the temperature at which the maximum occurs has been
17
shifted from T = 300 K to a more favorable temperature for power production (ideally T
should be close to ~700 K).
It would appear from the data that the goal of the research was reached. It has
been demonstrated that phase competition can have a positive effect on the thermoelectric
performance of a system. It has also been shown that when the level of disorder is high
enough, it is not only possible to improve ZT but to also move the temperature of
maximum performance to a more suitable temperature range for power production. In
order to see how much more improvement is possible, the system should be doped with
other elements to either introduce more charge carriers into the system or to introduce
more disorder into the system in hope to scatter more of the long wave-length phonons.
18
CHAPTER THREE
PUBLICATIONS
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
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