Thermoelectric Exploration of Silver Antimony Telluride and Removal of the Second Phase Silver Telluride a Thesis Presented in P

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Thermoelectric Exploration of Silver Antimony Telluride and Removal of the Second Phase Silver Telluride

A Thesis

Presented in Partial Fulfillment of the Requirements for the Degree Masters of Science in the Graduate School of The Ohio State University

Michele D. Nielsen, B.S.

Graduate Program in Mechanical Engineering

The Ohio State University

2010

Thesis Committee:

Joseph Heremans, Advisor

Walter Lempert

Michele D. Nielsen

2010 Abstract

As demands for energy increase throughout the world, the desire to create energy efficient technologies has emerged. While the field thermoelectricity has been around for well over a century, it is becoming increasingly popular, especially for automotive applications, as new and more efficient materials are discovered. Thermoelectricity is a technology in which a temperature difference can be applied to create a potential difference for the application of waste heat recovery or a potential difference can be used to create a temperature difference for heating and cooling applications.

Materials used in thermoelectric devices are semiconductors with high Figure of Merit, zT. The dimensionless thermoelectric Figure of Merit is a function of Seebeck coefficient, S, electrical resistivity, , and thermal conductivity, . Experimental testing is used to determine the properties of these materials for optimized zT.

This thesis covers a new class of thermoelectric semiconductors based on rocksalt I-V-

VI 2 compounds, which intrinsically possess a lattice thermal conductivity at the amorphous limit. It has been shown experimentally that AgSbTe 2, when optimally doped, reaches a zT =1.2 at 410 K. 3 Unfortunately, there is a metallurgical phase transition at 417 K (144 °C). The phase transition at 417 K was identified to be a potential problem in thermal cycling, and is expected with all heavy chalcogenides of group Ib elements. This issue has to be resolved before I-V-VI 2 can be considered practical. After examination, two routes to avoid the phase transition were planned: (1) alloying AgSbTe 2 with Na, and (2) using off-stoichiometry formulations Ag 1-xSb 1+y Te 2.

Experimental results show that with increased sodium concentration, resistivity increases substantially, causing this method to be impractical for optimizing zT, however it did ii eliminate the phase transition at 417K. Using off-stoichiometric silver antimony telluride shows promising results with high S, on the order of 250-400 V/K, and maintains the low thermal conductivity on the order of 0.6-0.7 W/mK.

iii Dedication

This is dedicated to my parents, sister, and grandparents for their continual support and encouragement throughout my entire life and especially during my time in graduate school.

iv Acknowledgments

I would like to acknowledge Dr. Heremans for the guidance that he has given me during my time as a graduate student. His teaching in lab and encouragement in conferences and presentations has genuinely helped expand my knowledge and confidence in the field.

I would also like to thank Vladimir Jovovic for teaching me the fundamentals of thermoelectricity and measurement methods. Additionally, I would like to thank Christopher Jaworski for helping me to develop an in depth understanding of the physics and chemistries of the material systems I was working on and for being a part of the daily bagel break and brainstorming session.

v Vita

April 23, 1985……………………….Born – Mansfield, Ohio 2003…………………………………Monroeville High School 2008…………………………………B.S. Mechanical Engineering, The Ohio State University 2008 to present……………………...Graduate Research Associate, Department of Mechanical Engineering, The Ohio State University

Fields of Study

Major Field: Mechanical Engineering

vi Table of Contents Abstract...... ii Dedication...... iv Acknowledgments ...... v Vita ...... vi List of Figures ...... viii Chapter 1: Introduction to Thermoelectricity ...... 1 Seebeck coefficient, Peltier, & Thomson Effects and Joule Heating...... 3 Transverse and Magnetically Induced Thermoelectric Effects...... 5 Hall Effect ...... 6 Thermoelectric Material and Device Efficiency ...... 7 Device Setup & Efficiency...... 7 Material Figure of Merit...... 10 Thermal Conductivity ...... 11 Thermoelectric Material Overview ...... 12 Experimental Methods...... 16 Sample Preparation...... 16 Experimental Setup...... 16 Error Considerations ...... 18 Chapter 2: Introduction to Silver Antimony Telluride ...... 20 Chapter 3: Sodium Substitution in Silver Antimony Telluride...... 22 Background ...... 22 Conclusions ...... 31 Chapter 4: Off-Stoichiometric Silver Antimony Telluride...... 32 Intrinsic Doping...... 36 Extrinsic Doping...... 44 Conclusions ...... 49 References...... 50

vii List of Figures

Figure 1. Simplified thermoelectric circuit...... 4 Figure 2. Hall Effect ...... 6 Figure 3. Typical thermoelectric device setup ...... 8 Figure 4. Relationship between electric field and direction of carrier velocity...... 15 Figure 5. Cryostat experimental setup...... 17 Figure 6. Ternary phase diagram for Na-Sb-Te system. 5 ...... 22 Figure 7. X-Ray diffraction of Ag (1-x) Na xSbTe 2. ── ── : AgSbTe 2 , ─ ─ ─: Ag 0.5 Na 0.5 SbTe 2 , ─── : NaSbTe 2 ...... 23 Figure 8. Lattice constant calculation of 2 nd peak from X-ray diffraction demonstrates a linear relationship with increasing amounts of Sodium...... 24 Figure 9. 2 nd peak analysis of X-ray diffraction shows leftward shift of peaks with increasing Sodium concentrations...... 24 Figure 10. Differential Scanning Calorimetry (DSC) analysis was conducted to determine phase transition temperatures. (bottom to top) Red: Ag0 .97 Na 0.03 SbTe 2 , Blue: Ag 0.9 Na 0.1 SbTe 2 , Light Green: Ag 0.8 Na 0.2 SbTe 2 , Purple: Ag 0.7 Na 0.3 SbTe 2 , Pink: Ag 0.5 Na 0.5 SbTe 2 , Brown: Ag 0.25 Na 0.75 , Dark Green: NaSbTe 2...... 25 Figure 11. Resistivity as a function of Temperature. + AgSbTe 2 , ●Ag 0.97 Na 0.03 SbTe 2 , Ag 0.9 Na 0.1 SbTe 2 , Ag 0.8 Na 0.2 SbTe 2 , * Ag 0.7 Na 0.3 SbTe 2 , ˟ Ag 0.5 Na 0.5 SbTe 2 , Ag 0.25 Na 0.75 SbTe 2 , NaSbTe 2...... 26 Figure 12. Seebeck coefficient through the entire range of samples. (left), ●Ag 0.97 Na 0.03 SbTe 2 , Ag 0.9 Na 0.1 SbTe 2 , Ag 0.8 Na 0.2 SbTe 2 , * Ag 0.7 Na 0.3 SbTe 2 , Ag 0.25 Na 0.75 SbTe 2. (right) NaSbTe 2...... 27 Figure 13. Seebeck coefficient (left) and resistivity (right) as a function of sodium concentration at 300K...... 28 Figure 14. Thermal conductivity. The symbols are & AgSbTe 2 Ag 0.97 Na 0.03 SbTe 2 ● Ag 0.8 Na 0.2 SbTe 2 ▪ Ag 0.5 Na 0.5 SbTe 2...... 29 Figure 15. Hall coefficent for + AgSbTe 2, • Ag 0.97 Na 0.03 SbTe 2, Ag 0.9 Na 0.1 SbTe 2 .....30 Figure 16: (left) Ag-Sb-Te ternary phase diagram and (right) psuedobinary Ag 2Te-Sb 2Te 3 diagram from ASM International 6...... 32 Figure 17. DSC latent heat trace for various off-stoichiometric compounds ...... 34 Figure 18. XRD of AgSbTe 2 (bottom) and Ag 0.366 Sb 0.558 Te (top) ...... 35 Figure 19. DSC trace of single phased Ag 0.366 Sb 0.558 Te...... 35 Figure 20. (left) Low temperature Seebeck coefficient measurement of Ag 0.366 Sb 0.558 Te. (right) Low temperature resistivity measurement of Ag 0.366 Sb 0.558 Te...... 36

viii Figure 21. . Room temperature Seebeck coefficient (top), and resistivity (bottom) of samples, ● with low temperature phase transition, ● without low temperature phase transition. Silver concentration for all samples held constant at 0.366...... 38 Figure 22. Room temperature power factor (bottom) of samples, ● with low temperature phase transition, ● without low temperature phase transition. Silver concentration for all samples held constant at 0.366...... 39 Figure 23. Seebeck coefficient, resistivity, and zT for ( ) Ag 0.366 Sb 0.563 Te 0.995 ,( ˟ ) Ag 0.366 Sb 0.54 Te 1.05 ,...... 40 Figure 24. Seebeck coefficient, resistivity, and zT with no phase transition below 200 oC ( ● ) Ag 0.336 Sb 0.558 Te, ( ) Ag 0.366 Sb 0.53 Te, ( ) Ag 0.366 Sb 0.5 Te, ( )Ag 0.366 Sb 0.58 Te...... 42 Figure 25. Ternary diagram of off-stoichiometric silver antimony telluride compounds with described thermal treatment. • Single phase materials multiple phase materials .44 Figure 26. Seebeck coefficient, resistivity, and zT for extrinsically doped off- stoichiometric silver antimony telluride: ( ) Ag 0.366 Sb 0.53 Te, (+)Ag 0.366 Sb 0.52 Te + 2%Co, (*)Ag 0.366 Sb 0.525 Te + 1%Co, ( ) Ag 0.366 Sb 0.53 Te + 2%Fe, ( )Ag 0.366 Sb 0.53 Te+2%Ni ...... 45 Figure 27. Extrinsic doping of off-stoichiometric silver antimony telluride using Na 2Se and transition metals: (+)Ag 0.366 Sb 0.558 Te + 0.5%(Na 2Se+Te), ( *)Ag 0.366 Sb 0.558 Te + 0.5%(Na 2Se+Te) + 2%Co, ( □)Ag 0.366 Sb 0.558 Te + 0.5%(Na 2Se+Te) + 2%Fe, ()Ag 0.366 Sb 0.558 Te + 0.5%(Na 2Se+Te) +2%Ni ...... 47 Figure 28. VSM of extrinsically doped samples: black - Ag 0.366 Sb 0.558 Te + 0.5%(Na 2Se+Te) +2%Ni, red - Ag 0.366 Sb 0.558 Te + 0.5%(Na 2Se+Te) +2%Co, blue - Ag 0.366 Sb 0.558 Te + 0.5%(Na 2Se+Te) +2%Fe, green - Ag 0.366 Sb 0.53 Te +2%Co, orange - Ag 0.366 Sb 0.53 Te +1%Co...... 48

ix Chapter 1: Introduction to Thermoelectricity

As demands for energy increase throughout the world, the need to create energy efficient technologies has emerged. While the field of thermoelectricity has been around for over a century, it is becoming increasingly popular, especially for automotive applications, as new and better performing materials are discovered. With fuel prices fluctuating and increased concern over the environmental impact of these fuels, increased focus on improving the efficiency of automotive vehicles has been especially important.

Approximately 2/3 of the energy input from fuels is lost in the form of waste heat through engine coolant, oil, and exhaust gasses.1

Due to the inefficiencies of typical engines, waste heat recovery can be viewed as one potential way to improve overall fuel efficiency in automotive vehicles. For vehicles, design aspects must be incorporated to make up for unsteady heat dissipation due to varying speeds, varying climates, the relatively small area which they can occupy, and weight constraints. Thermoelectric devices provide direct heat to electricity conversion, and therefore can be used to supplement electrical systems in a vehicle.

1 Thermoelectric materials are capable of converting heat to electricity, and vice versa.

Thermoelectricity is the science of designing such materials. A temperature difference can be applied to create a voltage. This can be applied in waste heat recovery. A potential difference can be used to create a temperature difference for cooling applications.

The size of thermoelectric devices is a major advantage because they are light weight and very compact. Additionally, thermoelectric modules are steady state devices with no moving parts, which reduces the chance of having costly repairs needed from wear. The disadvantages of this technology include low efficiencies exhibited in thermoelectric materials. Many of the current materials in use in thermoelectric devices are rare metals and some of the materials are toxic in their element form (lead, thallium, etc.). While the device itself does not pose harm to the consumer, these materials pose a potential hazard for workers creating and recycling devices.

A simple air-liquid thermoelectric heat exchanger, where hot gas from the engine exhaust is used to create a large temperature gradient, has been studied in some depth by the automotive industry. The application of a thermoelectric device into a vehicle could be used in such a way that the thermoelectric generator is heated through the hot exhaust gasses and cooled either through a cooling system or ambient air. Thermoelectric devices can also be used for cooling applications. Recently, these devices have been used commercially for heated and cooled seats and cup holders. Thermoelectricity can also be used for cooling applications such as in electronics. Peltier cooling devices use electricity to create nearly instantaneous cooling. Additionally, these devices are light

2 weight, portable, and superior to traditional cooling methods because they do not require the use of environmentally harmful refrigerants and have no moving parts.

Thermoelectric devices are the only alternative for cooling to the vapor-compression cycle, and their efficiency does not scale with device size.

Seebeck coefficient, Peltier, & Thomson Effects and Joule Heating

The Seebeck effect was discovered by Thomas Seebeck in the early 1800s. It is defined as the potential difference created as a result of carriers (i.e. electrons, holes) diffusing to the cold side of a material while a temperature gradient is applied. The Seebeck coefficient, S, can be expressed as:

∆V S = (1.1) ∆T

The Seebeck coefficient is a state function and therefore does not depend on shape or size of the material being tested; however, it is temperature dependent. The temperature dependence of the Seebeck coefficient stems from it being a reflection of carrier entropy, which is a function of temperature. From the Nernst principle, it goes to zero at absolute zero temperature. Metals typically have a Seebeck coefficient on the order of a few

V/K, whereas more electrically insulating materials show higher Seebeck coefficient, typically on the order of a few hundred V/K.

The Peltier Effect can be viewed as the opposite of Seebeck effect, where heat flow, q from a material is caused by passing a current, I. The Peltier coefficient, П, is related to these two quantities as follows: 3 qP = −Π * I (1.2)

With current flow into the material (from a to b in Figure 1) and a positive Peltier coefficient, heat is absorbed at junction T C.

Material b

TC TH

Material a

Figure 1. Simplified thermoelectric circuit

Similarly, if current is flowing out of the material (from b to a in Figure 3), with positive

Peltier coefficient, heat will be released at that junction. Reversing the sign of П reverses the direction of heat flow for this example. Further, the Peltier effect is reversible.

The Peltier coefficient is related to the Seebeck coefficient by the Onsager relation

Π = T * S (1.3)

Combining these equations we can see that a higher Seebeck coefficient implies more heat evolved from the material.

As described before, Seebeck coefficient is a measure of entropy. In a material with a temperature gradient imposed, the amount of heat carrying ability of a charge carrier

4 changes as it moves throughout the material. As a result, when there is a current, some heat will be absorbed or emitted as the charge carriers move along the temperature gradient. The Thomson effect describes this heat from the following equation:

qT =τ ⋅ I ⋅(−∆T ) (1.4)

Where q T is the heat absorbed or released, is the Thomson coefficient, I is the current through the material, and T is the temperature gradient. Thomson coefficient is related to Seebeck coefficient in the following way:

dS τ = T (1.5) dT

When a material is at uniform temperature, the entropy remains the same, therefore heat is neither emitted nor absorbed, resulting in q T=0.

Joule heating is an irreversible process in which heat is generated within a material as a result of electrical current being passed through. Joule heat occurs because charge carriers are set into motion and collide with each other and the lattice, and thus produce small amounts of heat. As a result, the quantity of Joule heat produced is a function of material resistance, R.

2 qJ = R * I (1.6)

Transverse and Magnetically Induced Thermoelectric Effects

Transverse effects are observed in semiconductors when a magnetic field is applied the z- direction, with either an imposed electrical current or thermal gradient in the x-direction.

5 A resistance (voltage) is observed in the y-direction and gives way to Hall and Nernst-

Ettingshausen coefficients.

Hall Effect

The Hall coefficient provides a direct relationship between an imposed current, I x, magnetic field, B z, and transverse voltage measurement, V y. This occurs due to the

Lorentz force, as a charged particle travels through a magnetic field it is deflected in the mutually orthogonal direction. Because of this, the sign of the Hall coefficient is dependent on the sign of the majority carrier.

R I B V = H x z (1.7) y th

Where th is the thickness of the material in the z-direction. The following figure illustrates this effect with negative charge carriers or electrons.

∆V I

B th

Figure 2. Hall effect

The Lorentz force is defined as the force on one particle that causes it to deflect orthogonally to both the original direction of particle velocity and the magnetic field.

6 v v L = q(v × B) (1.8)

The carrier charge, q, determines the direction of deflection, and therefore the sign of the transverse voltage. Moreover, in single carrier systems and assuming a Hall prefactor of unity, we can relate carrier density to Hall coefficient through

1 n = (1.9) RH q where n is the electron (or hole) carrier density and q is the electron charge. This makes

Hall coefficient a useful measurement to determine carrier density of a material. It is important to note that when a system has both types of carriers, this equation is not valid to determine the carrier density.

Thermoelectric Material and Device Efficiency

Device Setup & Efficiency

Thermoelectric devices are set up with two branches, both a p-type and n-type material, electrically in series. The devices are designed to have a common hot side, existing at temperature T H, and a common cold side, existing at temperature Tc. An example device setup is shown in Figure 3. Both p-type and n-type are needed as one connects the device load to isothermal leads.

7 TH

p-type n-type

+ + - - + - + + - -

- + Vo

Figure 3. Typical thermoelectric device setup

For simplicity, the thermoelectric circuit in Figure 1 is analyzed to derive the device efficiency. A typical setup within a thermoelectric device is shown in Figure 3.

The connecting material is considered to be of low enough electrical resistivity that we can ignore contact losses. The efficiency of a device, , can be calculated by considering ratio of power output from device, P, to heat input to device, q, in the following way:

P η = (1.10) q

When a load resistance, R L, is connected in series, the power output term is represented by

2 P = I RL (1.11)

As discussed previously, three heating effects must be taken into account to determine the efficiency of the device: Joule & Peltier effects, as well as thermal conduction.

q = qP + qJ + qκ (1.12)

8 The heat released as a result of thermal conduction is that of classical thermodynamics and can be expressed by

qκ = κ∇T (1.13) where ∇T is the temperature gradient in the material and is the thermal conductivity of the material.

With the cross sectional area of material a taken to be unity and the assumption that the length, L, of both the p-type and n-type material are equal. With these assumptions, the heat term for Joule equation can be expressed as follows:

 I 2  ρ    b q J = −  ρ a +  L (1.14)  2  A 

Combining these equations, the expanded term for device efficiency becomes:

2 I RL η = (1.15)  ∆T   I 2  ρ    b ()κ a + κ b A   − Π * I − −  ρ a + L  L   2  A 

The circuit, which is subject to Ohm’s Law, V=IR, can be reanalyzed to include material properties including Seebeck coefficients as well as series resistances in the following way:

S V I = = ∆T (1.16) R + R + R  ρ  L a b R +  ρ + b L L  a A 

With the adjustable parameters, A, L, and R L, we can determine the maximum efficiency

9 ∆T ( 1+ Tz −1) T η = h (1.17) max T 1+ Tz + C TH

Where T is the average of T H and T C, and zT is the material thermoelectric figure of merit.

Material Figure of Merit

The measurement of efficiency of a thermoelectric material is defined as the Figure of

Merit, zT. Maximizing the material figure of merit is the ultimate goal in material development research. Materials have peak zT at different temperature ranges depending on doping concentration, melting point, band gap, etc., so thermoelectric devices can be fitted for different applications. The dimensionless thermoelectric figure of merit, zT, is a function of Seebeck coefficient, S, resistivity, , and thermal conductivity, , in the following way:

S 2σ zT = T (1.18) κ

Electrical conductivity, =1/ , where resistivity is determine from resistance along the path of the current. High S and with low is the ideal situation, however it must be noted that each of these properties are related to one another. In general, we can expect a larger Seebeck coefficient with lower carrier concentrations, however this also results in a lower electrical conductivity. Thermal conductivity has contributions from the lattice and electronic portions. The electronic portion gives way to the relationship between

10 thermal and electrical conductivity. The power factor, PF= S 2, is the numerator in equation (zT).

Thermal Conductivity

Thermal conductivity is an indication of how easy it is for heat to propagate through a material. Methods of heat transfer include phonons, photons, and charge carriers. These quantized heat transporters are scattered while moving heat along the thermal gradient.

Scattering processes create a finite thermal conductivity. As a result, increasing scattering mechanisms can be viewed as a way to improve zT. We can simply calculate minimal thermal conductivity for a solid by the formula

1 κ = C vl (1.19) L 3 v where C v is the specific heat at constant volume, v is the velocity of the particular heat transfer mechanism (phonons, photons, charge carriers, etc.) and l is the mean free path of that particular mechanism. A minimal lattice thermal conductivity can be calculated by setting l equal to one interatomic distance. As discussed previously, thermal conductivity can be viewed as having different contributing factors, typically from a lattice portion, L, as well as an electronic portion, e.

κ = κ L +κe (1.20)

The lattice portion of thermal conductivity is limited, in general, by phonon-phonon interactions, anharmonic bonds, as well as imperfections in the crystal structure. These imperfections include mechanisms such as alloy, impurity, dislocation, and boundary

11 scattering and can be incorporated into the structure deliberately. Introducing phonon scattering mechanism typically also decreases electrical conductivity thus reducing the power factor. The electronic portion of thermal conductivity and electrical conductivity are related through the Weidemann-Franz law.

κ e = LσT (1.21)

A portion of the Lorentz number, L, is used to determine the electronic portion of thermal

h e conductivity. The electronic contribution of holes,κ e , and electrons, κ e , are additive

and are combined with ambipolar contributions, κ a when dealing with to two carrier systems.

h e κ e = κ a + κe + κe (1.22)

Ambipolar conduction can be observed at sufficiently high temperatures where thermally excited carriers, in a material with a temperature gradient imposed, create electron-hole pairs in the hot end. As the carriers diffuse towards the cold side, the electron-hole pairs recombine and release energy in the form of heat. This can be a large contributing factor to the electronic thermal conductivity.

Thermoelectric Material Overview

Materials that can optimize all three of the properties to create a high figure of merit often fall into the classification of a semiconductor. Semiconductors are commonly classified as materials with electrical conductivity between 10 -10 (Ohm*cm) -1 and 10 4 (Ohm*cm) -1 which have an energy band gap. Semiconductors fall between metals and insulators in conductivity with metals being the highest conductivity. Certain materials are termed

12 semimetals, as they do not have as high electrical conductivity as metals due to their lower carrier densities. Semimetals also fall in the same region of electrical conductivity as semiconductors, however are not included in the semiconductor classification due to overlapping energy bands.

Electrons surrounding atoms existing independently in space exist at specific energy levels. As atoms are brought close to each other, interatomic forces come into play and the energy level that existed when the atom was independent of the force begins to stretch into a band-like structure. Electrons which were once restricted to one specific energy level now may exist at several different closely related energy levels. Accordingly, the bands move up and down in energy depending on the interatomic distance, and one can imagine how this directly creates metals, semiconductors and insulators. Metals have a band that is not completely full, or another band that overlaps in energy and creates available states for the carriers. Thus, even at 0K, any applied electric field will create conduction. In the opposite way, insulators have bands that are not adjacent; in fact, they are typically separated by several electron volts. Therefore, the carriers have no where to go, and remain trapped. In the middle are semiconductors, where the energy band gap

(the difference in a solid between the energy of the highest occupied energy band and lowest unoccupied band) is small, and with thermal energy, some conductivity is realized. Moreover, through appropriate doping, electrical transport can be realized when the impurity’s atomic energy level lies near (or in) one of the bands.

13 In undoped semiconductors, conductivity is exponentially increased by a rise in temperature as electrons are thermally excited into the conduction band. Conduction as a result of electrons being excited to the conduction band, and therefore leaving behind an equal number of holes in the valence band is called intrinsic conduction. In a highly doped semiconductor, the carrier concentration (and therefore the number of electrons already in the conduction band) is high, comparatively; therefore the change in conductivity is considerably less.

Some discussion has been made into the sign of different properties depending on whether a material is p-type or n-type, such as whether Seebeck coefficient is positive or negative. In an n-type semiconductor, the valence band is full and there are electrons in the conduction band. The electrons are the means of conduction and flow in the opposite direction of an imposed electric field. For a p-type material, a positive charge is said to flow. This is actually created by a hole left behind in place of an electron, or a space where a valence electron was. As an electric field is imposed on a material, electrons will move in the opposite direction of the field, constantly filling the hole and leaving a different hole open, causing a positive charge to propagate in the direction of the electric field. Some materials can experience both holes and electrons as carriers, but can have a dominant carrier that indicates whether the material is overall n-type or p-type.

The flow of charge carriers per unit area, or current density is given by

J x = nev dx (1.22)

14 Where n is the number of carriers, e is the carrier charge, and v d is the drift velocity.

Current density is established, so the sign of the charge carrier determines the direction of velocity. The following figure illustrates this relationship.

j I electrons

x-direction Figure 4. Relationship between electric field and direction of carrier velocity

The drift velocity is directly related to the mobility, , of the carrier and the electric field,

vd = µE (1.23)

The mobility is a function of the particle charge, the effective mass, m*, as well as the relaxation time, m.

e µ = τ (1.24) m* m

Relaxation time is the amount of time needed for the momentum of a carrier, mv d, to return to equilibrium after the electric field is removed. Effective mass is simply the observed mass of an electron while incorporating all outside influences including forces directly from the crystal structure.

Electrical conductivity, , incorporates the number and mobility of charge carriers.

15 σ = ne µ (1.25)

The relationship between current density and electric field is then revealed to be direct with electrical conductivity acting as a coefficient to electric field.

J = σE (1.26)

In the case where a material is a two carrier system, both holes and electrons must be considered in the calculation of electrical conductivity. The electronic contribution for the both sets of carriers is additive:

σ = e (pµ p + nµn ) (1.27)

Experimental Methods

Sample Preparation

Appropriate amounts of the starting elements (>5N purity) are loaded into fused quartz ampoules which are then sealed under high vacuum. Samples are heated to above the liquidus then slow cooled to the single phase region and annealed for an appropriate amount of time ranging from several hours to a week. The phase diagrams for each of the material systems were used to develop an understanding of the particular system and the temperatures for which to anneal. A main focus of the studies was to create a single phase sample, so the samples were annealed in the single phase regions indicated by their respective phase diagrams.

Experimental Setup

The samples are then cut using a diamond edge wire saw. Samples are cut into discs of approximately 1-2mm thickness for thermal conductivity measurements and

16 parallelpipeds with dimensions of approximately 1mm x 1mm x 5mm for cryostat measurements. Small wires are spot welded to the sample and silver epoxy is added for mechanical stability. Two copper-constantin thermocouples are attached, along the length of the sample, in order to measure the temperature at two distinct points. Voltage measurements are taken from the two copper wires to determine Seebeck coefficient. A resistance heater is placed on the top of the sample and it is attached to a thermally and electrically insulated base. Current wires are attached on both the top and the bottom of the sample, then a brass plate is attached to assure uniformity on each end. Resistivity is measured between the two copper thermocouple wires while a current is applied on the sample. Hall and Nernst measurements are taken in two different locations using the copper wire from the thermocouple and a wire placed on the opposite side of the parallelpiped while a magnetic field is imposed in a transverse direction. Each wire is attached to the base to allow the measurements to be taken from a region of uniform temperature.

Q in I

Qout

Figure 5. Cryostat experimental setup

Measurements are then taken in stepped intervals from 77K up to 630K using a conventional liquid nitrogen cryostat. Low temperature Seebeck coefficient, resistivity, and thermal conductivity measurements are taken in the range of 2K to 200K using the

Quantum Design PPMS.

Thermal conductivity is measured using Anter laser thermal conductivity measurement system, using flash diffusivity, specific heat, and density measurements. A round disk, with measurements of approximately 9mm in diameter and 1mm thickness, is coated in graphite and subjected to a flash. Thermal diffusivity specific heat is measured and density is found using a comparative method with the known sample being PbTe.

Thermal conductivity is then found with the following formula:

κ = ρ ⋅α ⋅C p (1.28)

Error Considerations

Wires that are attached to the sample for cryostat measurements are 0.001” in diameter.

Small wires reduce error in measurements by allowing a small contact area and reducing thermal losses.

Measurement error from Hall measurements exist from misalignment of wires on the sample. A method of correction involves reversing the direction of the magnetic field and taking an average of the measurements. Error estimates from Seebeck and resistivity are approximately 3% and are due to simple length measurement errors between contacts.

18 Thermal conductivity error is estimated to be a maximum of 10% stemming from physical length measurements and radiation effects

19 Chapter 2: Introduction to Silver Antimony Telluride

Silver antimony telluride is included in a class of thermoelectric semiconductors, based on rocksalt I-V-VI 2 compounds, which intrinsically possess a lattice thermal conductivity at the amorphous limit. Recalling that the lattice thermal conductivity is given by

1 κ = C vl (2.1) L 3 v

The specific heat, Cv, can be derived using the law of Dulong & Petit, which states

J g c = 3R = 24 94. . The molecular weight of AgSbTe 2 is 484 83. . The v mole ⋅ K mole

J specific heat at constant volume is found to be .0 2058 , which is in close agreement g ⋅ K to the reported constant pressure specific heat measurement.4 For solids, constant volume and constant pressure specific heats are nearly the same due to the coefficient of thermal expansion being sufficiently small. With the interatomic distance, a = .606 nm ,we can solve Equation 2.1 to conclude the theoretical limit to thermal conductivity of AgSbTe 2

W to be 73.0 . The phonon mean free path is limited to the interatomic distance due to m ⋅ K the anharmonicity of the bonds in this compound.

20 When formed in the stoichiometric ratio, AgSbTe 2 tends to form several different compounds including variations of antimony telluride, silver telluride, and an off- stoiciometric form of silver antimony telluride. Phase transitions for the compounds that are formed begin around 144C. These phase transitions cause instability within the material. As a result, in order to create a material that is useful in a wider temperature range, it is important to try to form a single phase material.

Silver antimony telluride is a p-type material, but is a two carrier system. The system has been shown at room temperature to have a high concentration of holes, with a low mobility, , and a low concentration of electrons with high mobility. 3 With an increase in temperature, thermally excited electrons cause a reduction in Seebeck coefficient with a corresponding reduction in resistivity. The increase in carriers at higher temperatures also adds to the electronic portion of thermal conductivity with an increase in ambipolar conductivity.

Silver antimony telluride has been studied in some depth and has been determined to be a promising narrow gap semiconductor. Previously, the optimum doping level was calculated, and samples of stoichiometric AgSbTe 2 were doped experimentally to the

3 optimal level reaching a zT =1.2 at 410 K when doped with Na 2Se. Unfortunately, a metallurgical phase transition exists in this system at 417 K (144 C) due to silver telluride. This thesis explores two routes to avoid the phase transition: (1) alloying

AgSbTe 2 with Na, and (2) using off-stoichiometry formulations Ag 1-xSb 1+y Te 2.

21 Chapter 3: Sodium Substitution in Silver Antimony Telluride

Background

The motivation behind studying Ag (1-x) Na xSbTe 2 as a material system stemmed from the work done on AgSbTe 2. Silver antimony telluride showed promising thermal and electrical properties with a zT of up to 1.2 when doped, however, due to a second phase of Ag 2Te, the material experiences a phase transition at 144 °Celsius. It was anticipated that replacing silver with sodium would remove the second phase. Due to the highly ionic nature of Sodium, larger energy gaps were also expected which would ultimately decrease mobility.

6 Figure 6. Ternary phase diagram for Na-Sb-Te system.

22 The ternary phase diagram for Na-Sb-Te reveals that the compound NaSbTe 2 forms easily from the stoichiometric amounts with a liquidus region above the range of 600 °C.

For this thesis, samples were prepared of Na xAg (1-x) SbTe 2 with x=0.03, 0.1, 0.2, 0.3, 0.5,

0.75, and 1.

400

300

200 Intensity

100

0 20 30 40 50 60 70 Angle (2θ) Figure 7. X-Ray diffraction of Ag (1-x) Na xSbTe 2. ── ── : AgSbTe 2 , ─ ─ ─: Ag 0.5 Na 0.5 SbTe 2 , ─── : NaSbTe 2

Each of the samples underwent X-ray analysis. Peaks shown with increasing sodium concentration demonstrated a linear shift with x and each of the samples exhibited a cubic structure. At the highest concentrations of sodium, a second peak appeared and is shown along with the shifted peaks in Figure 7. A thorough investigation of possible phases in the reference literature did not reveal this composition.

23 4.52 ) k a

e 4.48 P

n 4.44 2 (

t n

a 4.4 t s n o 4.36 C

e c i t t 4.32 a L 4.28 0 40 80 Composition (%Na) Figure 8. Lattice constant calculation of 2 nd peak from X-ray diffraction demonstrates a linear relationship with increasing amounts of Sodium.

Analysis of the 2 nd , 3 rd , and 4 th peaks of the x-ray data revealed the linear relation between each of the material lattice constants and x. As shown in Figure 7, each of the samples exhibits this linear relation as described by Vegard’s law. Also noted in this figure is the 2 nd peak that occurs in the higher sodium concentration samples. Figure 9 shows a magnified view of Figure 7.

300 y t

i 200 s n e t n I

100

0 40 41 42 Angle (2θ) Figure 9. 2 nd peak analysis of X-ray diffraction shows leftward shift of peaks with increasing Sodium concentrations.

The peak on the left in Figure 9 is NaSbTe 2, the middle peak is Ag 0.5 Na 0.5 SbTe 2 , and the peak on the right is AgSbTe 2. The sodium antimony telluride, mixed in its stoichiometric amount demonstrated a second peak that corresponds to a second phase in that sample.

Figure 10. Differential Scanning Calorimetry (DSC) analysis was conducted to determine phase transition temperatures. (bottom to top) Red: Ag0 .97 Na 0.03 SbTe 2 , Blue: Ag 0.9 Na 0.1 SbTe 2 , Light Green: Ag 0.8 Na 0.2 SbTe 2 , Purple: Ag 0.7 Na 0.3 SbTe 2 , Pink: Ag 0.5 Na 0.5 SbTe 2 , Brown: Ag 0.25 Na 0.75 , Dark Green: NaSbTe 2

A differential scanning calorimeter was used to determine what phase transitions occurred in each composition and is shown in Figure 10. Samples with low concentrations of sodium and high concentrations of silver still exhibit a noticeable phase transition around 144C due to the presence of Ag 2Te. With increasing amounts of sodium, the low temperature phase transition fades and a very noticeable high-

25 temperature phase transition appears at 400K. The transport properties of these materials shown in Figure 11 and Figure 12 are affected by the phased transitions presented here.

Figure 11. Resistivity as a function of Temperature. + AgSbTe 2 , ●Ag 0.97 Na 0.03 SbTe 2 ,

Ag 0.9 Na 0.1 SbTe 2 , Ag 0.8 Na 0.2 SbTe 2 , * Ag 0.7 Na 0.3 SbTe 2 , ˟ Ag 0.5 Na 0.5 SbTe 2 , Ag 0.25 Na 0.75 SbTe 2 , NaSbTe 2

In a plot of resistivity as a function of temperature, increasing the amount of sodium in the material system generally trends towards higher resistivity. The substantial increase in resistivity is attributed to the highly ionic nature of sodium and the suspected increase in band gap. The samples with the lowest concentrations of sodium show a linear trend of increasing with temperature, then a light increase in at the phase transition. The

Seebeck coefficient of all samples with x ˂ 0.75 remains positive; that of NaSbTe 2 is negative.

Figure 12. Seebeck coefficient through the entire range of samples. (left), ●Ag 0.97 Na 0.03 SbTe 2 ,

Ag 0.9 Na 0.1 SbTe 2 , Ag 0.8 Na 0.2 SbTe 2 , * Ag 0.7 Na 0.3 SbTe 2 , Ag 0.25 Na 0.75 SbTe 2. (right) NaSbTe 2

In a plot of Seebeck coefficient as a function of temperature, increasing the amounts of sodium increases the Seebeck coefficient up to 20-30% Na. Additional amounts of sodium cause a decrease as shown in Figure 12. Lower concentrations of sodium exhibit a linear trend until the low temperature phase transition, while the samples with higher concentrations of sodium exhibit a turnover as temperature increases. This is characteristic of a two-carrier system where the dominant carrier becomes activated at high temperatures. The NaSbTe 2 sample showed a substantially negative Seebeck coefficient: data from room temperature and above is shown on the right in Figure 12 and indicates that the sodium antimony telluride compound that was formed has a large

Seebeck coefficient of roughly -900 V/K. It is important to note that the material formed was not single phase NaSbTe 2, so the second phase will also affect transport properties.

Figure 13. Seebeck coefficient (left) and resistivity (right) as a function of sodium concentration at 300K.

A summary of the concentration-dependence of the room-temperature electronic transport properties is given in Figure 13. Increasing concentrations of sodium show the electrons emerging as the carrier dominating the Seebeck coefficient of the system, but this is accompanied by an increase in resistivity by about five orders of magnitude. This is consistent with an increase in band gap of NaSbTe 2 over AgSbTe 2.

The specific heat, Cv, of NaSbTe 2 can be derived using the law of Dulong & Petit with

g molecular weight of NaSbTe 2 being 399 94. , the specific heat at constant volume is mole

J found to be .0 249 . With the interatomic distance, a = .0 6317 nm we can solve mole ⋅ K

Equation 2.1 to conclude the theoretical limit to thermal conductivity of NaSbTe 2 to be

28 W .0 661 . This value is lower in comparison to the calculated value for AgSbTe 2 due m ⋅ K to NaSbTe 2 having a much lower density. The larger interatomic spacing contributes to a lower specific heat per unit volume, and therefore a lower theoretical thermal conductivity.

Figure 14. Thermal conductivity. The symbols are &&& AgSbTe 2 Ag 0.97 Na 0.03 SbTe 2 ● Ag 0.8 Na 0.2 SbTe 2 ▪ Ag 0.5 Na 0.5 SbTe 2

The measured thermal conductivity of the system is reported in Figure 14. The thermal conductivity of the 20% Na sample was similar to that of pure silver antimony telluride.

The 3% Na sample had a higher thermal conductivity which is due to an increase in the

29 electronic contribution, and thus in line with a doped sample. As expected, alloy scattering has little effect on the thermal conductivity of this system, confirming the initial conjecture that the thermal conductivity is pinned to the amorphous limit by intrinsic phonon-phonon scattering processes only.

Hall coefficient was also measured on samples with low sodium concentration. As noted, this material system has two carriers, so Hall coefficient cannot be a measure of carrier density, but it does provide insight to material properties.

Figure 15. Hall coefficent for + AgSbTe 2, • Ag 0.97 Na 0.03 SbTe 2, Ag 0.9 Na 0.1 SbTe 2

30 Figure 15 shows that the samples with lower concentration of sodium have a negative hall coefficient despite the largely positive Seebeck coefficient. This is because the silver antimony telluride system has a high concentration of low-mobility holes and a smaller concentration of high mobility electrons.50 4

Conclusions

In summary, the Ag 1-xNa xSbTe 2 alloy system forms a solid solution that remains in the rocksalt crystal structure and follows Vegard's law over its entire composition range. The alloys avoid the phase transition associated with Ag 2Te for x ≥ 30 at. %. The thermal conductivity remains pinned at the amorphous limit, as was expected for all rocksalt I-V-

VI 2 compounds. Unfortunately, neither the electrical conductivity nor the Seebeck coefficient of the Ag 1-xNa xSbTe 2 alloy system with x ≥ 30 at% is favorable for the thermoelectric figure of merit. Further studies of NaSbTe 2 are warranted, and must be aimed at preparing single-phased material and then doping it.

31 Chapter 4: Off-Stoichiometric Silver Antimony Telluride

Due to the existence of parasitic phases in AgSbTe 2, study of off-stoichiometric was conducted to determine if the parasitic phases could be eliminated. We prepared samples in the equilibrium formation region of the silver-antimony-tellurium phase diagram shown in Figure 16.

Figure 16: (left) Ag-Sb-Te ternary phase diagram and (right) psuedobinary Ag 2Te-Sb 2Te 3 diagram from ASM International 7

Appropriate amounts of the starting elements (>5N purity) were loaded into fused quartz ampoules which were then sealed under high vacuum. Samples were heated to above the liquidous then slow cooled to the theoretical single phase region and annealed for a week.

32 A major difficulty here is that many phase diagrams for the Ag-Sb-Te system contradict each other. This pseudo-binary diagram 8 indicates a small region where off- stoichiometric material can be annealed to create a single phase material. Difficulties in preparing this material result from a phase separation where compounds such as silver telluride and antimony telluride form, thus we anneal the material in this single phase region indicated in the phase diagram.

Because the parasitic phases are often too dilute to show up in x-rays, the samples were subjected to a latent heat trace in a DSC to determine whether or not the material was single phase. Several compounds showed distinct phase transitions at 417K and in the regions of 423K, 605K, and 635K, as shown in Figure 17, indicating that some amount of second phase material was present. Many of the samples also showed a very subtle phase transition in the region of 430K and 450K.

33 2

Ag0.366Sb0.558Te Ag Sb Te 0 0.366 0.563 0.995 Ag0.366Sb0.55Te )

u Ag0.366Sb0.558Te1.05 a

( Ag0.366Sb0.558Te0.95

w Ag0.366Sb0.53Te o l Ag0.366Sb0.569Te F -2

t Ag0.366Sb0.53Te0.95 a

e Ag0.366Sb0.53Te1.05 H Ag0.366Sb0.5Te1

Ag0.366Sb0.54Te1.03 -4

0 200 400 Temperature (C)

Figure 17. DSC latent heat trace for various off-stoichiometric compounds

The term “single phase” in this thesis will refer to compositions found to not have a phase transition below 200 oC (473K).

The first step was to confirm the existence of a material composed of single phase

Ag xSb yTe, where x=0.366 y=0.558 as reported in the phase diagram. We prepared the material and XRD shown in Figure 18 reveals a cubic lattice, for a sample of this composition, with a slight shift in angle of the peaks suggesting a smaller lattice constant than material with nominal composition AgSbTe 2. DSC shows a clean latent heat trace through the Ag 2Te transition in Figure 19; other phases are not seen either up to 500 ̊C.

Figure 18. XRD of AgSbTe 2 (bottom) and Ag 0.366 Sb 0.558 Te (top)

0 )

s -0.2 t i n U

r -0.4 A (

w o l -0.6 F

t a e

H -0.8

-1 0 100 200 300 400 500 Temperature (C)

Figure 19. DSC trace of single phased Ag 0.366 Sb 0.558 Te

We test S, , and of this sample from 2-300K, and see a Seebeck coefficient higher than

350 V/K which is among the highest observed in silver antimony telluride systems. In the low temperature resistivity measurement, carrier freeze out starts to occur around

100K which corresponds to an energy gap of 8meV, which coincides closely with the reported gap of AgSbTe 2. The decreasing slope of the Seebeck coefficient with increasing temperature indicates that thermally excited electrons become more prevalent 35 at higher temperatures. Although silver antimony telluride is a strong p-type material, this stable composition corresponds to a sample which is a two carrier system because of its small energy gap. While this is stable metallurgically, it is not a good thermoelectric material.

400 36

300 32

28 cm)

V/K)

200 Ω µ (

(m S 24 ρ 100 20

0 16 0 100 200 300 0 100 200 300 T (K) T (K)

Figure 20. (left) Low temperature Seebeck coefficient measurement of Ag 0.366 Sb 0.558 Te. (right) Low temperature resistivity measurement of Ag 0.366 Sb 0.558 Te

Intrinsic Doping

In order to try to add more holes in such stable samples, selected compounds in the range of Ag 0.366 Sb 0.48-0.569 Te 0.95-1.05 were prepared and subjected to a latent heat trace. Several of the samples were found to have a subtle indication of a low temperature phase transition from Ag 2Te and possibly other compositions of silver telluride. Samples that

36 exhibited no low temperature phase transition were in the range of Ag 0.366 Sb 0.48-0.569 Te. S and were measured on the samples at room temperature and we studied their trends based on antimony and tellurium content. The results indicate that generally with decreasing antimony and increasing tellurium, Seebeck coefficient and resistivity decrease. Figure 21 shows a plot of S and as a function of antimony and tellurium content with the silver content held constant.

Figure 21. . Room temperature Seebeck coefficient (top), and resistivity (bottom) of samples, ● with low temperature phase transition, ● without low temperature phase transition. Silver concentration for all samples held constant at 0.366.

The power factor plot generated from this data indicates a general region of optimal thermoelectric properties, shown in Figure 22. The sample which exhibited the highest power factor had a composition of Ag 0.366 Sb 0.53Te. From this figure, we were able to determine the most promising samples, and are able to study their transport properties as a function of temperature.

Figure 22. Room temperature power factor (bottom) of samples, ● with low temperature phase transition, ● without low temperature phase transition. Silver concentration for all samples held constant at 0.366.

Figure 23. Seebeck coefficient, resistivity, and zT for ( ) Ag 0.366 Sb 0.563 Te 0.995 ,( ˟ ) Ag 0.366 Sb 0.54 Te 1.05 ,

( ) Ag 0.366 Sb 0.48 Te, ( AAA) AgSbTe 2

40 The data shown in Figure 23 corresponds to compounds that exhibited a low temperature phase transition below 200 oC. Varying compositions of the off-stoichiometric compound show similar thermal and electrical properties, but show disturbances in the transport at the phase transition temperature. The reported data is cut off at the phase transition. zT values up to 0.77 are achieved.

For the samples that have not yet had thermal conductivity measured show a zT determined by using the Wiedemenn-Franz law on the measured electrical conductivity plus a conservative assumption of lattice thermal conductivity of 0.7 W/mK (most samples have L~0.6 W/mK).

Figure 24. Seebeck coefficient, resistivity, and zT with no phase transition below 200 oC ( ● ) Ag 0.336 Sb 0.558 Te, ( ) Ag 0.366 Sb 0.53 Te, ( ) Ag 0.366 Sb 0.5 Te, ( )Ag 0.366 Sb 0.58Te

42 Measurements for materials that did not show a phase transition at low temperatures are shown in Figure 6. Samples with decreased amounts of antimony showed a decrease in

Seebeck coefficient and resistivity. We conclude that varying the antimony content allowed for intrinsic doping. Although Ag 0.366 Sb 0.558 Te showed a lower zT throughout the entire temperature range, this material has an advantage in that it does not show a phase transition through 773K in DSC whereas the single phase samples of different compositions showed phase transitions around 610K.

We conclude from the Seebeck coefficient plot, at higher temperatures that thermally excited electrons start to become the dominant carrier and cause a reduction in Seebeck coefficient above 450-500K. Resistivity also starts to decrease in the same temperature region. The figure of merit for the single phase materials ranges from 0.3 to 0.7 at high temperatures depending on composition. Thermal conductivity was measured on

Ag 0.366 Sb 0.558 Te and used to determine zT. The thermal conductivity used to determine zT for the remaining samples is the electrical portion of thermal conductivity as measured in the cryostat plus 0.7 W/mK for the lattice thermal conductivity. From the thermal conductivity plots shown thusfar, we have shown that 0.7 is a high estimate for thermal conductivity. The remaining thermal conductivity measurements are currently in progress.

From the data reviewed thus far, a new experimental ternary diagram of the samples has been provided in Figure 25. With the thermal treatment described in the preparation of these samples, there is a distinct region of samples that do not exhibit a phase transition

43 o below 200 C. The dotted line indicates where the traditional Ag 2Te-Sb 2Te 3 psuedobinary intersects this group of materials.

Ag 22 30

27 25 at % Sb at % Ag

32 20

37 15 Sb 48 53 58 63 Te at % Te

Figure 25. Ternary diagram of off-stoichiometric silver antimony telluride compounds with described thermal treatment. • Single phase materials multiple phase materials

Extrinsic Doping

Attempts to extrinsically dope this material were also made. Initial attempts at doping included substituting transition metals in for antimony. The optimal composition, as determined by power factor, was used to determine the doping effects of cobalt, nickel, and iron.

Figure 26. Seebeck coefficient, resistivity, and zT for extrinsically doped off-stoichiometric silver antimony telluride: ( ) Ag 0.366 Sb 0.53 Te, (+)Ag 0.366 Sb 0.52 Te + 2%Co, ( *)Ag 0.366 Sb 0.525 Te + 1%Co, ( ) Ag 0.366 Sb 0.53 Te + 2%Fe, ( )Ag 0.366 Sb 0.53 Te+2%Ni

Seebeck coefficient and resistivity for all of the Fe, Co and Ni-doped samples are shown in Figure 26. Both S and ρ were lower than those of the pure samples of identical nominal composition; however no overall change in power factor was noted. The reduction in Seebeck coefficient and resistivity could be attributed to intrinsic doping as a result of the decreased concentration of antimony. We tentatively conclude that these

45 transition metals were not actually incorporated into the samples, but that their presence in the melt may simply have altered the level and/or nature of the defects in the parent semiconductor.

We further evaluated Na 2Se as an extrinsic dopant, because it showed promising results

3 when used as a p-type dopant in the stoichiometric AgSbTe 2 system. After creating an off-stoichiometric compound with sodium selenide and equal parts tellurium, we then again included transition metals as dopants to determine their effects. The results of this study are shown in Figure 27. The addition of Na 2Se created different effects with each of the transition metals used. The samples showed decrease in Seebeck coefficient and resistivity and an overall increase in zT. The amount of sodium selenide and tellurium used was kept constant at 0.5%: more study is needed to determine the optimal amount.

Figure 27. Extrinsic doping of off-stoichiometric silver antimony telluride using Na 2Se and transition

metals: (+)Ag 0.366 Sb 0.558 Te + 0.5%(Na 2Se+Te), ( *)Ag 0.366 Sb 0.558 Te + 0.5%(Na 2Se+Te) + 2%Co, (□)Ag 0.366 Sb 0.558 Te + 0.5%(Na 2Se+Te) + 2%Fe, ( )Ag 0.366 Sb 0.558 Te + 0.5%(Na 2Se+Te) +2%Ni

The addition of sodium selenide created different effects with each of the transition metals used. The samples showed decrease in Seebeck coefficient and resistivity and an overall increase in zT. The amount of sodium selenide and tellurium used was kept constant at 0.5%: more study is needed to determine the optimal amount. From

47 comparing Figure 26 and Figure 27, we tentatively conclude that the addition of sodium selenide and tellurium allows the transition metals to be either incorporated into the crystal structure, or made electrically active, therefore allowing variations in transport properties. This hypothesis is supported by the concept that the role of the native defects

(Sb antisite defects, Ag vacancies/interstitials) is crucial in determining the activity of the

3d-transition metals.

We tried to ascertain the actual concentration of paramagnetic 3d-metals in the samples by means of a magnetic susceptibility measurements in the Quantum Design PPMS instrument equipped with a VSM option. The results indicated that paramagnetic iron was present in the Na 2Se and Fe-doped samples, but no paramagnetic signal was observed in the Ni or Co-doped ones.

Figure 28. VSM of extrinsically doped samples: black - Ag 0.366 Sb 0.558 Te + 0.5%(Na 2Se+Te) +2%Ni, red - Ag 0.366 Sb 0.558 Te + 0.5%(Na 2Se+Te) +2%Co, blue - Ag 0.366 Sb 0.558 Te + 0.5%(Na 2Se+Te) +2%Fe, green - Ag 0.366 Sb 0.53 Te +2%Co, orange - Ag 0.366 Sb 0.53 Te +1%Co

48 We can conclude that either Co and Ni were not incorporated into the structure or that they were incorporated in their non-magnetic forms of Co 3+ and Ni 3+ .

Conclusions

A region of single phase material was determined to intersect the typical Ag 2Te-Sb 2Te 3 psuedo-binary. We conclude that it is possible to create a stable material with an approximate formula Ag 0.366 Sb 0.558±0.02 Te and that Na 2Se appears to be a favorable dopant. Different compositions within this range vary the transport properties and results in intrinsic doping. Extrinsic doping using transition metals Fe, Co, and Ni alone do not substantially and independently vary the transport properties of the system. The addition of Na 2Se in conjunction with the transition metals proved to vary Seebeck coefficient and resistivity substantial between the compounds. From this, we can conclude that extrinsic dopants, Fe, Co, and Ni must be combined with small amounts of Na 2Se in order to manipulate transport properties effectively.

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