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1-2011

Thermal, Electrical, and Structural Behavior ofSilver Chalcogenide CompositesJeremy CappsClemson University, jcapps@clemson.edu

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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

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