高圧下で合成された部分充填スクッテルダイトRxCo
4Sb12 (R = La, Ce および Nd) の熱電特性
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言語: English
出版者:
公開日: 2019-06-25
キーワード (Ja):
キーワード (En):作成者: モナ, ユッタナ
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URL

https://doi.org/10.15118/00009908

Muroran IT
MURORAN INSTITUTE OF TECHNOLOGYDISSERATION THERMOELECTRIC PROPERTIES OF PARTIALLY
FILLED SKUTTERUDITES RxCo4Sb12 (R = La, Ce, and Nd)
SYNTHESIZED UNDER HIGH PRESSURE

MONA Yuttana

THERMOELECTRIC PROPERTIES OF PARTIALLY
FILLED SKUTTERUDITES RxCo4Sb12 (R = La, Ce, and Nd)
SYNTHESIZED UNDER HIGH PRESSURE

By

MONA Yuttana

A dissertation submitted in partial fulfillment of the requirements forthe degree of Doctor of Engineering

Examination Committee:

Assoc.Prof. Takeda KEIKI
Prof. Hideaki NAKANE
Prof. Chihiro SEKINE (Chairman)

Nationality:

Thai

Previous Degree:

Bachelor of Mechanical Engineering
North-Chiang Mai University, Thailand
Master of Energy Engineering
Chiang Mai University, Thailand

Muroran Institute of Technology
Division of Engineering

March 2019Acknowledgements
During the Ph.D course of my graduate career I have benefited from thehelp, support, advice and suggestions of a great many people or even supported
me in one way. Therefore, I would like to express my grateful appreciations andthanks to the followings:
Firstly, I would like to express my sincere gratitude to my advisor Prof.
Chihiro Sekine and Asst. Prof. Yukihiro Kawamura for the continuous supportof my Ph.D study, for their patience, motivation, and excellence knowledge. I am
really honored by becoming a member of his group.
Besides my advisor, I would like to thank Mr. Chul-Ho Lee, Mr.
Kunihiro Kihou and Mr. Hirotaka Nishiate from National Institute ofAdvanced Industrial Science and Technology (AIST) for their serious but
valuable suggestions especially in an experiment and analyze data.
My sincere thanks to Asst. Prof. Chatchawan Chaichana, who provided
me with an opportunity to join an intern at MurorunIT. Without they precious
support it would not be possible to study in Phd.
My sincere thanks Dr. Enda Toumey, for support everything, for his
continuous encouragement and guidance since my master degree until now. And
providing many valuable comments that improved the presentation and contentsof this thesis.
Thank you for my roommate A 405 (Ju Boom, Ju Odd, Ju Ahn) andNamphon, Ping and Moll and all students now in Physics of Strongly CorrelatedElectron Systems Lab for all happy days.
Finally, I am thankful to my family, especially my beloved Maw Paweena,
for her continuous support. Her sacrificed her comfort for my higher education.
MuroranIT, March 2019

MONA Yuttana

– Sometimes it”s very hard to move on, but once you move on, you”ll realize it
was the best decision you”ve ever made.

To my beloved

For their endless love, support and encouragement

Abstract
Improved of thermoelectric materials were synthesized using high pressureand high temperature methods (HPHT). The limits of filling guest ions in CoSb3
synthesized by HPHT were explored. The used of high-pressure techniqueexpanded the practical filling fraction limit. Explorations of rare-earth naturemischmetal (Mm) were also undertaken, using our group previous work to guide
synthesis condition. High filling with guest ions resulted in lower thermal
conductivity of approximately half the total thermal conductivity of originalcompound CoSb3 compound over a wide range in temperatures. The electricaltransportivity was enhanced, producing a high-efficiency TE material.
High pressure and high temperature synthesis techniques were thenemployed to high filling guest ions of individual mischmetal guest ions for TE
materials. The performance was created to justify the use of high pressure. The
effects of high filled guest ions in this study shows improvement of the ZT
performance for Ce0.37Co4Sb12 and Nd0.33Co4Sb12, up to 0.26, 0.48 at 700 K and
La0.28Co4Sb12 is 0.03 at 300 K respectively. The increased R concentration is not
only improved the ZT performance due to electron doping but also decreased thethermal conductivity due to a rattling behavior.
Suggestion for future work of high-pressure synthesis technique sometime
the sample is very small and fragile. Therefore, we should be finding the new
combine technique to get a large bulk sample and high density that suitable tomeasure at high temperature and must be preserving the impact of high filling
fraction. Finally, it’s should be providing a great boon with high ZT value.

iList Of Contents
Pages
Abstract ................................................................................................................ i
List Of Contents ................................................................................................. ii
List Of Figures ................................................................................................... iv
List Of Symbols ................................................................................................ vii
Introduction................................................................................... 1-1

Experimental Methods ................................................................. 2-1

Experimental Results of CexCo4Sb12 ........................................... 3-1

Experimental Results of NdxCo4Sb12 .......................................... 4-1

Experimental Results of LaxCo4Sb12 ........................................... 5-1ii

TE performance at high temperature......................................... 6-1

Conclusion ..................................................................................... 7-1
Bibliography .......................................................................................................A
Appendix A Sample preparation for SPS ......................................Appendix A
Appendix B Conference list and Publication list ..........................Appendix Biii

List Of Figures......................................................................................................... Pages
Figure 1-1. Thermodynamic circuit for the relative Seebeck coefficient ............................. 1-2
Figure 1-2. Schematic diagram of thermoelectric power generation ................................... 1-3
Figure 1-3. Thermodynamic circuit for the Peltier effect ..................................................... 1-4
Figure 1-4. Schematic of thermoelectric refrigeration ......................................................... 1-4
Figure 1-5. Schematic illustration of skutterudite crystal (a) Binary skutterudite structure; (b)Filled skutterudite structure ................................................................................................... 1-6
Figure 2-1. Kawaii-type multi-anvil apparatus UHP 1500 and guide block. ....................... 2-2
Figure 2-2. Cell assembly anvil high-pressure apparatus. .................................................... 2-2
Figure 2-3. Schematic diagram of Spark Plasma sintering (SPS)......................................... 2-4
Figure 2-4. Basic working of X-Ray diffraction ................................................................... 2-5
Figure 2-5. Schematic diagram of a typical Scanning Electron Microscopy. ....................... 2-6
Figure 2-6. Sketch map of sample connect for TTO............................................................. 2-8
Figure 2-7. Shape of copper lead. ......................................................................................... 2-9
Figure 2-8. Adhere lead to bottom end of sample................................................................. 2-9
Figure 2-9. Adhere lead to the top end of sample. .............................................................. 2-10
Figure 2-10. Illustrating sockets shoe. ................................................................................ 2-11
Figure 2-11. Sample holder with the shield. ....................................................................... 2-11
Figure 2-12. PPMS Option Manager. ................................................................................. 2-12
Figure 2-13. TTO connect diagram..................................................................................... 2-13
Figure 2-14. Model 6000, Model 6500 and Model 7100. ................................................... 2-13
Figure 2-15. Chamber control window ............................................................................... 2-14
Figure 2-16. Sample insert tool. .......................................................................................... 2-14
Figure 2-17. Charcoal and Contact Baffle Assembly. ........................................................ 2-14
Figure 2-18. Option manager .............................................................................................. 2-15
Figure 2-19. Windows after activate the Thermal Transport option................................... 2-15
iv

Figure 2-20. Change the Config File in Thermal Transport Log ........................................ 2-16
Figure 2-21. Data file. ......................................................................................................... 2-16
Figure 2-22. Sample information. ....................................................................................... 2-17
Figure 2-23. Thermal Transport Measurement. .................................................................. 2-18
Figure 2-24. Thermal Transport Waveform. ....................................................................... 2-19
Figure 2-25. Example of TTO Sequence. ........................................................................... 2-19
Figure 2-26. Log Data. ........................................................................................................ 2-20
Figure 2-27. Four-wire resistivity measurement. ................................................................ 2-21
Figure 2-28. Schematic used for the determination of Hall voltage VH. ............................. 2-22
Figure 2-29. Four-wire Van der Pauw Hall effect measurement. ....................................... 2-23
Figure 2-30. Schematic of Seebeck measurement for ZEM-3 unit..................................... 2-24
Figure 2-31. A picture of ZEM-3 unit................................................................................. 2-24
Figure 2-32. A laser flash apparatus LFA 457. ................................................................... 2-25
Figure 3-1. XRD pattern of a polycrystalline of CexCo4Sb12................................................ 3-5
Figure 3-2. Lattice constant and actual Ce filling fraction vs nominal Ce doping rate. ....... 3-7
Figure 3-3. Temperature dependences of the electrical resistivity of CexCo4Sb12................ 3-8
Figure 3-4. Temperature dependences of  of CexCo4Sb12................................................. 3-12
Figure 3-5. Temperature dependences of the lattice thermal conductivity of CexCo4Sb12. 3-13
Figure 3-6. Temperature dependences of the Seebeck coefficient of CexCo4Sb12.............. 3-14
Figure 3-7. Temperature dependences of ZT value of CexCo4Sb12. .................................... 3-15
Figure 4-1. XRD pattern of a polycrystalline of NdxCo4Sb12. .............................................. 4-5
Figure 4-2. Lattice constant and actual Nd filling fraction vs nominal Nd doping rate. ...... 4-6
Figure 4-3. Temperature dependences of the electrical resistivity of NdxCo4Sb12. .............. 4-7
Figure 4-4. Temperature dependences of the thermal conductivity of NdxCo4Sb12. .......... 4-11
Figure 4-5. Temperature dependences of the lattice thermal conductivity of NdxCo4Sb12. 4-11
Figure 4-6. Temperature dependences of the Seebeck coefficient of NdxCo4Sb12. ............ 4-13v

Figure 4-7. Temperature dependences of the dimensionless figure of merit of NdxCo4Sb12.. 413
Figure 5-1. XRD pattern of a polycrystalline of NdxCo4Sb12. .............................................. 5-4
Figure 5-2. Lattice constant and actual La filling fraction vs nominal La doping rate. ........ 5-5
Figure 5-3. Temperature dependences of the electrical resistivity of LaxCo4Sb12. ............... 5-6
Figure 5-4. Temperature dependences of the thermal conductivity of LaxCo4Sb12. ........... 5-10
Figure 5-5. Temperature dependences of the lattice thermal conductivity of LaxCo4Sb12. 5-10
Figure 5-6. Temperature dependences of the Seebeck coefficient of LaxCo4Sb12. ............. 5-11
Figure 5-7. Temperature dependences of the dimensionless figure of merit of LaxCo4Sb12. . 512
Figure 6-1. Temperature dependences of the electrical resistivity at T  300 K. ................. 6-2
Figure 6-2. Temperature dependences of the thermal conductivity at T  300 K. ............... 6-4
Figure 6-3. Temperature dependences of the thermal conductivity at T  300 K. ............... 6-4
Figure 6-4. Temperature dependences of the Seebeck coefficient at T  300 K. ................. 6-5
Figure 6-5. Temperature dependences of the Seebeck coefficient at T  300 K. ................. 6-6
Figure 6-6. Temperature dependence of carrier mobility µ for Ce0.13Co4Sb12 andNd0.17Co4Sb12. ........................................................................................................................ 6-7
Figure 6-7. Temperature dependences of ZT at T  300 K. .................................................. 6-8

vi

List Of Symbols
TE

Thermoelectric material

ZT

Thermoelectric Figure of MeritS

Seebeck CoefficientElectrical ResistivityElectrical Conductivity

𝜅Total Thermal Conductivity

𝜅eElectronic Thermal Conductivity

𝜅lLattice Thermal ConductivitynCarrier concentrationµCarrier mobility

S2

Power Factor

I

Current

V

Voltage

SPS

Spark Plasma Sintering

PGEC

Phonon Glass Electron Crystal

XRD

X-ray Diffraction

SEM

Scanning Electron Microscopy

EDX

Energy Dispersive X-ray Spectroscopy

PPMS

Physical Property Measurement System

TTO

Thermal Transport Option

PPMS

Physical Properties Measurement Systemvii

Introduction
In this chapter, the basic thermoelectric (TE) concepts will be discussed. The
concepts that are related to this thesis will be introduced and summarized, withno extensive theoretical details. Terms for thermal transport and electronictransport will be discussed, and there will be a brief overview of the usability ofTE devices.

Energy Usage
The current consumption of fossil fuel has grown dramatically, whereas the
new discovery of new sources has slowed down. With the reliance on fossil fuels,
environmental pollution and global warming are growing. To generate energy
through fossil fuel combustion results in a typical efficiency of 30-40%. The
remaining 60-70% of the energy is lost to the environment.1,2) This has led toefforts to develop new sources of energy, as well as advances in energy efficiency
by recovering waste heat as energy.
Thermoelectric materials is one of the most interesting for new energy
resources.3,4) These materials convert waste heat energy directly into electricalenergy from inefficient waste heat.5–9) However, thermoelectric materials need tobe tuned to achieve optimal performance. In order to enhance efficiency andproperly understand their electronic transport and thermal transport properties, it
is important to develop TE performance.

Overview of Thermoelectrics
Thermoelectric energy was discovered in the 1820s by Thomas Johann
Seebeck. His investigations found that when two dissimilar materials were joined
together, and the junctions held at different temperatures, a voltage difference
develops which is proportional to the temperature difference,10,11) as shown in
1-1

Figure 1-1. The ratio of the voltage developed to the temperature gradient(ΔV/ΔT) is related to an intrinsic property of the materials. This is because charge
carriers (electrons or holes) at the hot side of the material have more thermalenergy than carriers at the cold side, causing a net diffusion of mobile carriers to
the cold side. Since there are more mobile carriers at the cold side than the hotside, the inhomogeneous charge distribution forms an electric field which
opposes the diffusion. If the material is in an open circuit, equilibrium will bereached when the rate at which carriers diffuse from the hot side to the cold sideis balanced by the rate at which carriers move from the cold side to the hot side,
due to the electric field. Thus, in equilibrium potential will form in response to atemperature gradient; this potential is known as the Seebeck voltage. The amountof voltage generated per unit temperature gradient is called the Seebeckcoefficient. If the material is connected to a circuit, the potential will drive acurrent which can be used to perform electrical work, which is the fundamental
for thermoelectric power generation, as shown in Figure 2-2.

A
T+ΔT

T
B

Figure 1-1. Thermodynamic circuit for the relative Seebeck coefficient1-2

Figure 1-2. Schematic diagram of thermoelectric power generation i
Thermoelectrics can also be used as solid-state refrigerators or heat pumps
by driving a current in a circuit with two dissimilar materials. This mode exploits
the Peltier effect, discovered by Jean-Charles Peltier in 1834. Basically, heat isabsorbed or rejected at the interface of two dissimilar materials, when a current
flowed around a circuit as shown in Figure 1-3. This is explained by introducing
the Peltier coefficient ∏ = ST, which is a material dependent parameter, related to
the Seebeck coefficient and described on how much thermal energy is carried per
charge carrier. Since the heat current must be continuous across the interface of
two materials. If the materials have different Peltier coefficients, heat will beeither ejected or absorbed at the interface, depending on the sign of the difference
between the Peltier coefficient and the direction of the current. If the current is
injected in one direction the junction will extract heat, which is the basis forthermoelectric refrigeration. A current in the other direction will produce heat at
the junction, and the device is acting as a heat pump. The rate at which the Peltieri

http://www.1-material.com/organic-thermoelectric-ote-materials/
1-3heat is liberated or rejected at the junction (QP) is givenby QP = SIT, where I is
the current through the junction and T is the temperature in Kelvin (K). Usually
the two materials are chosen to be thermoelectric materials with the opposite
majority carrier in Figure 1-4 (one n-type leg and one p-type leg) to maximize thedifference between the Peltier coefficient and hence the amount of heat absorbed
or generated

Figure 1-3. Thermodynamic circuit for the Peltier effectFigure 1-4. Schematic of thermoelectric refrigeration ii

ii

https://sites.google.com/site/cl663lcj1/home/thermoelectric-materials
1-4The guideline to develop applications to be accepted broadly are a focus
on developing the performance of material by improving thermopower ordecrease the thermal transport of material. The efficiency of thermoelectric
devices is determined by the dimensionless figure of merit;
ZT = S2σT/κ(1)Where Z is the figure of merit, S is the Seebeck coefficient, σ is the electricalconductivity, T is the absolute temperature, κ is the total thermal conductivity. In
the case of the total thermal conductivity can be separate with two terms is (κ =κe+κl), the electronic and the lattice contributions.12–15) In order to maximize ZTof the material should be improved power factor (S2σ) are required to be thehighest and decrees κ to the lowest. Due to their transport property is needed to
be optimized to achieve maximum ZT.

Improving Thermoelectric Performance
Improving thermoelectric performance is difficult due to the coupled natureof the relevant properties. Thermoelectric performance can be improved throughtwo directions: improving the power factor, or lowering the thermal conductivity;
however, a loss in one factor results in a losses in other factors. For example,
increasing the carrier concentration is an easy way to raise electrical conductivity.
However, doing so will generally decrease the Seebeck coefficient throughchanges in the Fermi level. In addition, if carrier concentration is increased toboost electrical conductivity, the electron contribution to thermal conductivity
will increase as well. Therefore, ZT is obtained by carefully considering thecarrier concentrations property.
Phonon Glass Electron Crystal (PGEC) 16) is a concept for improving ZT
performance, by scattering phonons heat (rattle effect). This is possible due to the

1-5differing lengths scales on lattice conductivity and electron conductivity, then
PGEC materials attempt to reduce lattice thermal conductivity.
Thermal conductivity in TE materials comes from two sources: Electrons orholes transporting heat (e) and phonons transporting of lattice (l). By electronic
term (e) is directly related to the the Wiedemann–Franz law:T = l + e(2)e = LT(3)

Where L is the Lorenz number, 2.443 x 10-8 V2/K-2 for free electrons.15,17,18)

Skutterudites-type thermoelectric
Thermoelectric Skutterudites-type one of the most promising class ofthermoelectric materials, because they exhibit high hole mobility values, 3,16)
leading to high electrical conductivities, and they present a unique approach to
reduce lattice thermal conductivity. The CoSb3-skutterudite structure includes
two empty voids per unit cell, which can be filled with guest atoms for doping,with the aim of reducing lattice thermal conductivity.14,18,19) The CoSb3skutterudite structure of unfilled Skutterudite (a) and filled (b) are shown in
Figure 1-5.

Figure 1-5. Schematic illustration of skutterudite crystal (a) Binary skutterudite
structure; (b) Filled skutterudite structure

1-6

High Pressure and High Temperature Synthesis
High-pressure synthesis techniques, more familiar in experimental, in orderto reach favorable synthesis conditions for the high filling fractions rate, whichconventional under ambient pressure method can not improve upper limit. Whichcontrast of high-pressure technique, they can increase upper limit and improve
ZT performance.
High pressure synthesis benefits to increase upper limit of filled guest atoms to
the voids of the skutterudite structure than ambient pressure. MmxCo4Sb12,
YbxCo4Sb12 and SnxCo4Sb12 have been succeeded synthesized by using highpressure and high-temperature method.18–20) Those methods show various
interesting physical properties. Furthermore, we can fill guest atoms to voids ofskutterudite structure with high filling ratio and they show the advantages of fast
process.

Research objective
The main objective of this thesis is to synthesize individual nature
mixchmetal (Mm) filled CoSb3-skutterudites TE materials for improved
performance.
1. To study individually of Mm-filled skutterudite compounds with highfilling fraction by using a high-pressure and high-temperature technique.
2. To synthesize and characterize the properties of high pressure filled
skutterudite.
3. To investigate which element is more effective to fill skutterudite and canbe used to design an artificial Mm-filled skutterudite for high ZT
performance by using HPHT method.

1-7

Experimental Methods
Chemical reagents
1) Cerium chunk, Ce, Rare Metallic co., Ltd.
2) Neodymium, chunk, Nd, Rare Metallic co., Ltd.
3) Lanthanum, chunk, Ce, Rare Metallic co., Ltd.
4) Cobalt, powder, Co, New materials and Chemical Ltd.
5) Antimony, powder, Sb, Soekawa Chemicals.
6) Ethanol, C2H6O.
7) Acetone, C3H6O.

High Temperature and High Pressure Synthesis
Although skutterudites could be synthesized without high pressure, highfilling fractions targeted in this project required high pressure. All the samples
reported in this work were synthesized by using the high pressure and hightemperature apparatus (UHP-1500 , Sumitomo Heavy Industries), as shown in
Fig. 2 -1 . Kawai-type multi-anvil apparatus (6–8 system, sample pressure is
generated by successive com- pression:(1) A hydraulic ram drives six first-stage anvils.
(2) The six first-stage anvils compress eight second-stage anvils.
(3) The second-stage anvils compress an octahedral cell assembly.
Tungsten-carbide cubes with a truncated edge are used as the second-stage anvils.
The sample cell assembly is similar to that used for the cubic anvil apparatus (Fig.
2.2). The sample container, made of magnesia (MgO + 5% Cr 2O3), wastransformed into an octahedron. The starting materials were put in a crucible
made of BN. The crucible, with a graphite heater surrounded with a zirconia(ZrO2) thermal insulator, was inserted into the magnesia octahedron.13)

2-1

The heat system is achieved by current heating method. The AC current ispassed from the cubic tungsten, transfer to graphite heater. The temperature inside
the graphite tube will be increased. A hole were punched in the place which
tangency to the graphite. The temperature is detected by putting a K-type
thermocouple in the hole.

Figure 2-1. Kawaii-type multi-anvil apparatus UHP 1500 and guide block.

BN diskMgO medium

Graphite

Thermocouple

BN sleeve

SampleZrO2
ZrO2 disk

Figure 2-2. Cell assembly anvil high-pressure apparatus.

2-2

High-purity Cerium (Ce), Neodymium (Nd) or Lanthanum (La) powders weremixed and milled in stoichiometric ratio using the agate mortar and then sealedin the BN capsule. The octahedral magnesia in Fig. 2-2 was pressed to 4 GPa andheat generated. The heating rate was 37 °C/min and the target temperature 900°C. Keep the temperature at 900 °C for 2 minute and then decrease to 600 °C andkeep at 600 °C for 120 minutes. The displayed temperature depends on the
precious of the parts, especially the place of the thermocouple. Therefore, thetemperature cannot be controlled only depend on the temperature, but also need
pay attention on the current power.

Spark Plasma Sintering (SPS)
Spark Plasma Sintering (SPS) is a sintering process that make large bulk
sample and densification. By SPS pulse current flow through the graphite moldand stacked sample powder. The system consists of a SPS sintering machine witha vertical single-axis pressurization mechanism, two punch electrodes with builtin water cooler, a water-cooled vacuum chamber, a vacuum/air/argon-gas
atmosphere control mechanism, a cooling water control unit, a position
measuring unit, a special DC-pulse sintering power generator, an applied pressure
display unit and other interlock safety unites. All samples from HPHT after
remove impurity were pulverized into powders with a mortar. The powders wasthen placed into a 10 mm diameter graphite die with graphite sheet in between
the powder and graphite punches for the ease of removal after sintering and amore uniform current flow. The graphite die was then placed in between graphite
spacers with a thermocouple placed in a hole on the side of the die to measure thetemperature of the powder during the sintering. After setting up the place for thegraphite die, the chamber was closed and the furnace was purged and backfilledwith Argon three times to ensure a vacuum environment before sintering. This
study is sintering sample was pressed at up to 60 MPa and kept at 600 °C for 120
min.
2-3

Figure 2-3. Schematic diagram of Spark Plasma sintering (SPS).

X-Ray Diffraction (XRD)
X-Ray Diffraction is an analytical tool to obtain crystallographic
information of materials. It is a non-destructive, reproducible and fastcharacterization technique to identify crystalline phases and orientation, todetermine structural properties such as crystallite size, lattice parameters, grainsize, phase composition, atomic arrangement etc. The interaction of the incident
beams with the sample produces diffracted beams that satisfies the Bragg’s law
nλ= 2d sinθ

(2.1)

This equation relates the wavelength of electromagnetic radiation (λ) to thediffraction angle (θ) and the lattice spacing in a crystalline sample (d). As
illustrated in figure 2-3

2-4

Figure 2-4. Basic working of X-Ray diffraction
by scanning the sample through a range of 2θ angles, all possible diffraction
patterns of the lattice is obtained due to the random orientation of the material.
Diffracted X-ray beams are detected, processed and counted. By analyzing theposition, shape and intensity of the diffraction peaks important atomic andmolecular structure of the crystal can be obtained. Rigaku RINT-RAPID-II using
Co Kα1 radiation and silicon as a standard wavelength source of 1.5406 Å
wavelength.

SEM and EDX
Scanning Electron Microscopy (SEM) is a very convenient and dynamic
characterization tool to obtain high-resolution microstructure images of desired
materials. SEM utilizes electrons for imaging as they have much shorter
wavelengths than light, enabling better resolution and magnification. SEM can be
operated in various modes for analyzing surface morphology, elemental
composition, microstructure studies etc.

2-5Figure 2-5. Schematic diagram of a typical Scanning Electron Microscopy.
The basic working principle of SEM is a beam of incident electrons, which is
generated in an electron column above the sample chamber. Depending on theevaluation objectives the energy of the incident electrons may vary from 100eV
up to 30 keV. The electrons generated are focused into a small beam by a seriesof electromagnetic lenses in the SEM column. Scanning coils near the bottom of
the column direct and position the focused beam onto the sample surface. The
incident electrons cause electrons to be emitted from the sample due to elastic
and inelastic scattering from the sample’s surface and nearsurface material. Highenergy electrons that are ejected by an elastic collision of an incident electron arereferred to as backscattered electrons. Back scattered imaging mode provides
image contrast as a function of elemental composition, as well as, surface
topography. Emitted lower energy electrons resulting from inelastic scattering arereferred as secondary electrons. The SEM column and sample chamber are
maintained at a moderate vacuum to allow the electrons to travel freely from the2-6electron beam source to the sample and then to the detectors. Energy Dispersive
Spectroscopy (EDS) is an analytical tool used for the elemental analysis or
chemical composition of the materials. Backscattered electron images in the SEM
shows compositional contrast that results from different atomic number elements
and their distribution. EDS allows to identify what those particular elements are
and their relative proportions (Atomic % or weight %).

TE properties characterizations at low Temperature.
Thermoelectric measurements, including Seebeck coefficient (S), andthermal conductivity (κ), were performed on synthesized pellets in a Quantum
Design Physical Properties Measurement System (PPMS) coupled with thethermal transport option (TTO). The measurements were done at room
temperature.
TTO option is used to measure the thermal transport properties, thermal
conductivity κ, Seebeck coefficient S and electrical resistivity ρ. However, theepoxy bond paste which used to fix sample influenced the precious of resistivity,
therefore, we will measure resistivity using the standard four-probe method is forthe measurement with AC transport mode. The advantage of the four-probe
method is that it can prevent the problem from a contact resistance occurred in a
two-probe configuration. That is, only thermal conductivity κ and Seebeckcoefficient S value were provides the information from TTO option.
The TTO system determines the thermal conductivity κ and Seebeckcoefficient S by creating a specified temperature drop between the two ends ofsample. The sketch map of sample assembled for TTO measurement is shown in
Fig. 2-6. The sample end is adhere to the leads by epoxy bonds. Heat is applied
to one end of the sample by running current through the heater (Q+/-). In the TTO
system, the thermal conductivity the heater source applies heat to a sample andthe amount of heat passes through the sample is collected from the observed
temperature drop. Thus, thermal conductivity can be calculated directly from2-7

three parameters: an applied heater power, a resulting ΔT, and a sample
dimension and the Seebeck coefficient provides the information about the thermal
diffusion of free charge carriers, which can be an electron or a hole. According
to the majority carrier, charge carrier diffusion generates an electric field inside amaterial which creates a voltage gradient when a temperature gradient is applied.
The TTO system measures Seebeck coefficient by originating a particularized
temperature difference (ΔT) and monitoring an electrical voltage difference (ΔV).
The ratio of these two measured parameters (ΔV/ΔT) is equal to S are calculated
as follows:

𝜅 =Q∙

𝑆=

𝑑

(2. 2)

𝑇ℎ𝑜𝑡 −𝑇𝑐𝑜𝑙𝑑

𝑉+ −𝑉−(2. 3)

𝑇ℎ𝑜𝑡 −𝑇𝑐𝑜𝑙𝑑

where Thot, Tcold is the temperature measured at the thermometer shoes (copperlead); V+, V- is the voltage responded at the sample two ends during the heatpulse; Q is the heat energy and d is the height of the sample.
CURRENTHEATER

Copper lead(Thot, V+)d

SampleEpoxy
bonds
Copper lead(Thot, V+)

Figure 2-6. Sketch map of sample connect for TTO.
For TTO measurement, the sample should be modified to rectangular bulk about(2.0mm×1.0mm×0.5mm). The surface should be polished with abrasive paper
2-8

(#3000 and #8000) and cleaned. After measured the size of sample, the sectional
area and the total surface area of the other 4 surface need calculated by self. The
mounting sequence proceed as below with the help of a puck-mounting station.
(1) Prepare adhesive agent (Silver-filled H20E epoxy). Take out equivalentpart A and part B of epoxy bond (made by EPO-TEX. Co.) on the clean
paper by using toothpick. (The toothpick for taking Part A and Part B
should be separated.) Mixed the part A and Part B thoroughly using anothernew toothpick.
(2) Prepare copper lead. As shown in Fig. 2-6, there are two kinds of copper
lead. Shape a can be used for small sample while shape B can be used forbig sample. Besides, the shape b need to be cut to shape c for the convenientof assembling. Those copper lead can be used again after measurement.
(3) Adhere copper leads to the ends of sample. Adhere sample to copper leadas shown in Fig. 2-7 and heat it at 150°C for 5 min. A(4) few of epoxy bond would be enough because too much epoxy will
influenced the measurement result.
Shape aShape bShape cFigure 2-7. Shape of copper lead.

Figure 2-8. Adhere lead to bottom end of sample.

2-9After the bottom of the sample is fixed to the lead, adhere the other lead tothe top of sample. Before adhere the lead to the top end of sample, the leadshould be bended to the following shape as fig 2-8.

Figure 2-9. Adhere lead to the top end of sample.
(5) Install shoes for sample. Because the shoe is very easy soft, so excessive
force or mis-threading of the piece can easily damage the threads. The
corresponding sample lead need inserted and held in the shoe by hands anda small stainless steel metric M1 screw carefully. The metal tweezers
cannot touch the shoes at any time. The sequence of assembling shoes is:
Hot-Heat-Cold.
(6) Install sample on TTO puck. Place the puck in the TTO puck-mounting
station and clamp the cold-foot lead to the cold-foot on the puck with thehelp of the small screwdriver. Then, the other end of each shoe should also
inserted to corresponding in Fig. 2-9. In addition, all of the line should
neither touch each other nor any part of the puck.
(7) Place the shield on the puck. Remove the shield cap and screw the shieldcarefully into the base of the puck. Be care to verify that no wires, shoes,or the sample do not touch the shield. Replace the shield cap after fixed theshield well.
(8) Checking the sample contact. In order to ensure the condition of sample
contact is well or not, the electrical contact to the sample must be checked
2-10

after mounting it on the puck. Plug the puck into the test box and checkresistivity between V+/- and I+/- with an ohmmeter. The corresponding
resistivity value should be:
3-4: around 200Ω;
5-6: around 2 kΩ;
7-8: around 100Ω;
9-10: around 100Ω;
11-12: show different resistivity value according to sample property;
13-14: show different resistivity value according to sample property.
2.Heat shoe
1.Hot shoe
6.Hot shoe

5.Cold shoe
3.Cold shoe

4.Cold-foot7.Heat shoe

Figure 2-10. Illustrating sockets shoe.

Figure 2-11. Sample holder with the shield.

2-11After assembled sample on the parts, the measurement sequence need to beprepared as following.
(1) For TTO measurement, the thermal transport option need to be active if not
currently active. Do the following at the PPMS Multivu window: selectUtilities – Activate Option, click on Thermal Transport under the
Available Options heading, and then select Connection Diagram.

Figure 2-12. PPMS Option Manager.
(2) Change cable according to the Connection Diagram.
(3) Check the Power. TTO measurement use the MODEL6000, MODEL6500
and MODEL7100 (Fig.2-13). Sometimes only MODEL6000 is active, if
so, the power of MODEL6500 and MODEL7100 need to be turn on andthen restart the PC.

2-12

Figure 2-13. TTO connect diagram.

MODEL600
0
MODEL650
0
MODEL710
0Figure 2-14. Model 6000, Model 6500 and Model 7100.
(4) Ensure temperature setting at 300 K. Before open the chamber, first ensure
the temperature is 300 K. If the temperature of the chamber is low, open
the chamber will result frost in the chamber and cannot vacuum.
(5) Insert sample in the chamber. Do the following fig 2-14: click Vent Cont.
button at the Chamber windows; After the state became Flooding at the

2-13bottom of the status bar. Insert the sample puck inside the chamber using
the sample insert tool as shown in the Fig. 2-15.
(6) Assembled Charcoal (4038-010) in the bottom of the Contact Baffle (asillustrated in Fig. 2-16 and insert it inside the chamber and then close thechamber. The Charcoal will help to create a more uniform thermal
environment for the puck when high vacuum is enabled in the chamber.
(7) Click Purge/Seal button to vacuum the chamber. Wait until the status
become purged, then click the HiVac button. The status will show
AtHiVac when the chamber became high vacuum.

Figure 2-15. Chamber control window

Figure 2-16. Sample insert tool.
Charcoal
Holder(For absorb
He below
10 K)

Contact
Fingers

Contact Baffle(For uniform temperture)

BaffleAssembly

Blanking
FlangeFigure 2-17. Charcoal and Contact Baffle Assembly.

2-14

(8) Active TTO option. Choose the Thermal Transport and the click the
Activate button. Because only one measurement option may be active at atime, first click Deactivate if the another option is activate and then activate
Thermal Transport option.

Figure 2-18. Option manager(9) Ensure the Config file. After activate the Thermal Transport option, thewindows in Fig. 2-19 will be shown. Thermal transport log window signify
the Config file of the Hot, Cold and heat, respectively. The number
indicated in Thermal Transport log should be same with the Hot, Cold andHeat shoe number which used in sample puck.

Figure 2-19. Windows after activate the Thermal Transport option.

2-15

Figure 2-20. Change the Config File in Thermal Transport Log(10) Click the data file in the Thermal transport windows as shown in Fig.2-20.
Click the Browse button to make a data file. Fig.2-21 shows the input
windows of sample information. Input the sample length, cross section and
Surface Area.

Figure 2-21. Data file.

2-16

Figure 2-22. Sample information.
(11) Click the Measure in the bottom of the Thermal transport window. The
Thermal Transport Measurement windows shown in Fig. 2-22 will beshown. Change the parameter shown in the red circle and click Start button.
Wait few minutes to check the waveform. The corrected waveform isshown in Fig. 2-23.

2-17

Figure 2-23. Thermal Transport Measurement.

2-18

Figure 2-24. Thermal Transport Waveform.
(12) Write Sequence. As shown in Fig. 2-25. To get stable data at 300K, thetemperature was keep at 300 K for 1 hour and then decrease to 5 K with
0.3 K/min.

Figure 2-25. Example of TTO Sequence.

2-19

(13) Run sequence and clicking the View in the Data File panel of the Thermal
Transport Windows to open the result figure.

In addition, the Log need to be taken for any measurement. Do the following:
Utilities/Log PPMS Data/General-Browse, make a new folder and click open,
Select all the Standard Item and Diagnostic Items/Click Start button to take theLog data/Click the View Data to show the Log Data.

Figure 2-26. Log Data.
(14) After the measurement finish, the status in left will became Sequence Idle.
Ensure the chamber temperature is 300K. Then, open the chamber by click
Vent Cont. Button. Then open the Dewar and take off the sample. Closethe Dewar and click Purge/Seal button to seal the chamber.

2-20

Resistivity
The electrical resistivity at T  300 K were evaluated by the cryostat
apparatus using four-wire resistance measurement method, the sample resistivity
can be precious measured with greatly reduction of the contribution of the leads
and the joints to the resistance. Fig. 2-27 shows the attach way of wires on thesample. After attached the wire on sample one by one, the sample with four –wireis mounted on resistivity plate and wired to the four contact. The outer wire
contact applied current on sample and the inside two wire detect the voltage
between the inside wires. The resistance then can be calculated with Ohm’s law.
Fig. 2-27 shows the schematically and actual four-wire resistivity sample.

Figure 2-27. Four-wire resistivity measurement.

Hall effect
The importance of the Hall effect was needed to determine accurately carrier
density and the mobility of carriers in sample materials. The objective of the Hallmeasurement in the van der Pauw technique was to determine the sheet carrier
density by measuring the Hall voltage VH. The Hall voltage measurement
2-21

consisted of a series of voltage measurements with a constant current I, and aconstant magnetic field B applied perpendicular to the plane of the samples.
Conveniently, the samples showed in Figure 2-28, were also used for the Hallmeasurement. To measure the Hall voltage VH, a current I was forced through the
opposing pair of contacts 1 and 3 and the Hall voltage VH (= V24) was measured
across the remaining pair of contacts 2 and 4. Once the Hall voltage VH was
acquired, the sheet carrier density was able to be calculated via VH = IB/nqt or
RHIB/t, (RH = 1/nq ) from the known values of I, B, and q, t (where q was theelementary charge, 1.602 x10-19 Coulomb and t is a thickness.).

Figure 2-28. Schematic used for the determination of Hall voltage VH.
We measure Hall effect by using van der Pauw technique, that is, a combinationof a resistivity measurement and a Hall measurement. The sample shape for Hall
effect measurement is a rectangle, as shown in fig. 2-29. A Fig.2-29, a current I
is forced through the contacts I+ and I- and the Hall voltage VH (=V+/-) is measured
across the remaining contacts V+ and V-. The hall coefficient can be expressed
as:

2-22

𝑉 𝑑

𝑅𝐻 = 𝐻 × 108 (𝑐𝑚3 /𝐶)
𝐼𝐵

(2.4)

where d is the thickness of sample (cm) and B is the applied magnetic field (Oe),
I is the applied current (A), and VH is the corresponding voltage. According thevalue of Hall coefficient, the information of main carrier type can be obtained.
The corresponding carrier concentration can be calculated by using:1𝑛 = |𝑅 |𝑒

(2.5)

𝐻

Combining with the results of resistivity measurement, the carrier mobility canbe estimated by:

𝜇=

|𝑅𝐻 |
𝜌

(𝑐𝑚2 /𝑣 ∙ 𝑠)

(2.6)

Figure 2-29. Four-wire Van der Pauw Hall effect measurement.

TE Transport Evaluation at High Temperature.
Thermoelectric power is essential to evaluate the performance at hightemperature. ZEM-3 is a device for simultaneous measurement of both Seebeck

2-23

coefficient and electrical resistivity under an inert gas. The instrument permits
measurement of both 6-22 mm long prism and cylindrical samples. The sample
holder used as a unique balance contact mechanism, permitting measurement ofhigh reproducibility. V-I plots were made to judge if the lead is in intimate contact
with a set sample. Measurement was controlled by a computer, permitting
automatic measurement with each temperature difference. Temperature range
was to be raised from room temperature to 500 o C in this study.

Figure 2-30. Schematic of Seebeck measurement for ZEM-3 unit.

Figure 2-31. A picture of ZEM-3 unit.

2-24

In the ZEM-3 system, during the Seebeck measurement, The Seebeck voltage is
detected from two voltage probe leads. Together with the temperature signal fromthe same two thermal couples, Seebeck coefficient is determined. Fig.2-30 showsa detailed schematic of the Seebeck measurement configuration. The actual
equipment for the Seebeck measurement is shown in Fig. 2-31.

Thermal Conductivity Measurement at High Temperature.
A Laser Flash apparatus (LFA) were used to measure three thermal
constants: thermal conductivity, thermal diffusivity, and specific heat. Thermal
conductivity () was calculated from thermal diffusivity (dif), heat capacity (Cp),and sample density (d) based on the relationship  = dif Cp d. The Cp and dif wasmeasured under vacuum using the laser flash apparatus (LFA, 457) as shown in
Figure 2-32. Whereas the density can calculate by sample size. Thermal
diffusivity represents how fast a body can dissipate thermal energy. Or in otherwords, how fast a body can change its temperature when there is a heat flow
through it.

Figure 2-32. A laser flash apparatus LFA 457.

2-25

Experimental Results of CexCo4Sb12
Introduction
Highly mischmetal (Mm) filled skutterudite Mm0.6Co4Sb12, prepared underhigh-pressure technique, exhibits a low κl of 1.34 W/Km and shows a relatively
high ZT of 0.25 at 700 K.13) The large reduction of κl suggests that the rattlingeffect and the mass fluctuation scattering are significant in this system. 17)
Mm is an alloy of rare earth elements in various naturally occurring
proportions. A typical composition includes approximately 50% Ce and 25% La,with small amounts of Nd.13) If we can decide the ideal proportion of Mm (thebest combination of each element) instead of the natural proportion, the TE
performance could be improved for the system. In order to decide the
combination of each element, it is important to investigate the filling effect ofeach element individually. Therefore, in Chapter 3, we first studied the effect ofCerium (Ce) filled skutterudite under high pressure. The results have confirmedthat the high filling rate markedly decreased the lattice thermal conductivity.
Furthermore, the result show that Ce ions can be used to fill skutterudites with
better TE performances than CoSb3. Then, this section describes thecharacterization techniques on the samples in term of phases structures, lattice
parameter, actual compositions, and thermoelectric properties of CexCo4Sb12.

X-ray diffraction (XRD) of CexCo4Sb12
Crystalline degree and phase purity of the products were analyzed by X-ray
diffractometer (XRD) with Cu Kαl radiation (λ = 0.154060 nm) operating at 40
kV- 40 mA, in the 2 range of 10° - 160°. The products were specified by a
Rigaku RINT-RAPID-II Computer Software (search match program) of the
JCPDS database.
3-1

Skutterudites of the form CexCo4Sb12 were filled with varying fractions (x =0.2 - 1.0) using high pressure (4 GPa), heating to 900°C for two minutes, and thendecreasing the temperature to 600°C for 120 minutes, the products werecomposed of CexCo4Sb12 with some impurities. For x = 0.2, both Sb and CoSb2
impurities were left and right in the main peak respectively. When thecomposition x was varying increase to 1.0, the impurity of Sb in the left in themain peak and the impurity of CoSb2 in the right are increases, likely because thefilling fraction limit (FFL) of Ce is excessed.
The sharp peaks indicated shift to lower 2 of CexCo4Sb12 compare to mothercompound CoSb3. The strongest intensity peak is at 2 = 36.203 degrees whereas
CoSb3 the strongest intensity peak is at 2 = 36.497 degrees and the main peakdiffracts is (013) planes of the samples. Moreover, by using high pressuretechnique the main peaks diffract are split to two peaks by lower 2 are indicatedfilled skutterudite phase and higher 2 are indicated unfilled skutterudite at costate were shown in x = 0.6 and 0.8.

*

Co4Sb12Intensity (a.u)

Sb
CoSb2

 **
30

40

50

60

70

80

2 (deg)3-2

*

Ce0.2Co4Sb12

Intensity (a.u)

Sb
CoSb2

 **
30

40

50

60

70

80

2 (deg)*

Ce0.4Co4Sb12

Intensity (a.u)

Sb
CoSb2

 **
30

40

50

60

70

80

2 (deg)3-3*Intensity (a.u)

Ce0.6Co4Sb12
Sb
CoSb2

 **
30

40

50

60

70

80

2 (deg)*Intensity (a.u)

Ce0.8Co4Sb12

*
30

Sb
CoSb2

*
40

50

60

70

80

2 (deg)3-4*Intensity (a.u)

Ce1.0Co4Sb12
Sb
CoSb2

 **
30

40

50

60

70

80

2 (deg)

013

*

530
600

431

332
422

420

 ** 

330

Intensity (a.u)

123

CexCo4Sb12
Sb
CoSb2

X = 1.0
X = 0.8
X = 0.6
X = 0.4
X = 0.2
X=0

30

40

50

60

70

80

2 (deg)

Figure 3-1. XRD pattern of a polycrystalline of CexCo4Sb12.
3-5

Lattice constant and actual composition of CexCo4Sb12
The lattice constant (a) for the samples was calculated using the least-squares
fitting method as shown in Figure 3-2 (black line) indicates the effect of increased
filling fraction on CexCo4Sb12 compounds. As the Ce filling fraction increased,
the lattice constant increases. The lattice constant increased sharply up to 9.120
angstroms with increasing nominal filling of x and then remains unchanged. Thisis the highest value reported for any synthesis method of CexCo4Sb12
demonstrating that the Ce was filling the Sb voids. 21,22) The point analysis of
SEM and EDS were used to determine the actual compound composition.
Comparing the atomic ratio, it was found that the average composition of thepoint analysis (red line) the actual filling fraction is lower than the nominal Ce
doping rate. The actual filled fraction was found to be 0.37, the highest value
reported for any CexCo4Sb12 this might be due to the use of the high pressure,high temperature method.

3-6

9.14

CexCo4Sb12High Pressure

Lattice constant (Å)

9.12
9.1
9.08
9.06

Ambient Pressure [11]

9.04
9.02
0

0.2

0.4

0.6

0.8

1

Nominal doping ratio, x
Figure 3-2. Lattice constant and actual Ce filling fraction vs nominal Ce dopingrate.

The electrical resistivity of CexCo4Sb12
The temperature dependence of the electrical properties of the CexCo4Sb12
samples is shown in Figure 3-3. it was found that expected that addition of each
atom of cerium to CoSb3 unit cell would contribute additional electrons to the
conduction band and lead to n-type metallic behavior. The electrical resistivity() exhibited a small semimetal-like behavior; small increased with temperaturein the whole temperature range and reached a maximum at 300 K. This seems toindicate incomplete of the filling ions, contributing fewer conduction electrons
than expected.

3-7

-1

10

CexCo4Sb12

X=0
X=0.0005
X=0.05
X=0.18
X=0.24
X=0.37-2 (−m)

10

-3

10

-4

10

-5

10

-6

10

0

50

100

150

200

250

300

T (K)

Figure 3-3. Temperature dependences of the electrical resistivity of
CexCo4Sb12.

The thermal transport property of CexCo4Sb12
The temperature dependence of the thermal conductivity ( ) of CexCo4Sb12 isshown in Figure 3-4. The  values of sample were quite high like metals, and alittle bit decreased with temperature increasing this might be due to the boundary
scattering. CoSb3 presented the higher  values than CexCo4Sb12, because of therattling behaviors are scattering phonon heat. The lattice thermal conductivity lwas obtained by subtracting the electronic thermal conductivity l from the total
measured thermal conductivity . The value of e was calculated using e = LT,
where  is the electrical conductivity ( = 1/ ) and L is the Lorentz number (L= 2.45×10-8 WΩK-2).15) The lowest value at room temperature values of the l for
CexCo4Sb12 were 3.12 Wm-1K-1 for Ce0.37Co4Sb12. The significant decrease the3-8lattice thermal conductivity due to the mass fluctuation effect or the rattling effector boundary scattering so that the total thermal conductivity is much lower thanthat of the samples without Ce guest ions 17) as shown in Fig. 3-4 and 3-5.
10

Co4Sb12

 [W/Km]

8

6

4


.

2

. e
. l

0
0

50

100

150

200

250

300

T (K)

3-9

10

Ce0.0005Co4Sb12

 [W/Km]

8

6

4


.

2

.e
. l

0
0

50

100

150

200

250

300

T (K)

10

Ce0.05Co4Sb12

 [W/Km]

8

6

.
. e
. l

4

2

0
0

50

100

150
T (K)

200

250

300

3-10

10

Ce0.18Co4Sb12

 [W/Km]

8

6

4
.
. e
. l

2

0
0

50

100

150
T (K)

200

250

300

10

Ce0.24Co4Sb12

 [W/Km]

8

6

4

.
. e
. l

2

0
0

50

100

150

200

250

300

T (K)

3-11

10

Ce0.37Co4Sb12
8

 [W/Km]

.
. e
. l

6

4

2

0
0

50

100

150

200

250

300

T (K)

CexCo4Sb12

8
7

 [W/Km]

6
5
4
3
X=0
X=0.0005
X=0.05
X=0.18
X=0.24
X=0.37

2
1
0
0

50

100

150

200

250

300

T (K)

Figure 3-4. Temperature dependences of  of CexCo4Sb12.
3-12

12
10

l [W/Km]

CexCo4Sb12

X=0
X=0.0005
X=0.05
X=0.18
X=0.24
X=0.37

8
6
4
2
0
0

50

100

150

200

250

300

T (K)

Figure 3-5. Temperature dependences of the lattice thermal conductivity of
CexCo4Sb12.

The Seebeck coefficient and ZT value of CexCo4Sb12
The negative seebeck coefficients (S) (Fig. 3-6) indicate that electrons are
the main carriers. Sample of CoSb3 indicates an intrinsic semiconductor withpositive value, whereas Ce-doped samples decrease gradually as Ce-doping
increased. The absolute values of S with Ce-doping reach a maximum value of0.37 with S = -53 µV/K at 300 K. This behavior corresponds with the change ofthe carrier concentration from hole to electrons. The conductor changes from ptype to n-type conductor by receiving donated electrons from Ce-doping,consistent with the electrical resistivity decrease with increasing x. This behavior
due to the increase in the electron concentration. The result of large decrease 

3-13

combine with S and  which leads to the highest observed ZT value among all thesamples studied were shown in fig.3-7.
80
X=0
X=0.0005
X=0.05
X=0.18
X=0.24
X=0.37Seebeck Coef. (V/K)

60
40

CexCo4Sb12

20
0
-20
-40
-60
0

50

100

150

200

250

300

T (K)

Figure 3-6. Temperature dependences of the Seebeck coefficient of
CexCo4Sb12.

3-14

0.006
X=0
X=0.0005
X=0.05
X=0.18
X=0.24
X=0.37

0.005

ZT

0.004
0.003
0.002
0.001
0
0

50

100

150
T (K)

200

250

300

Figure 3-7. Temperature dependences of ZT value of CexCo4Sb12.

Summary
In summary, a high-pressure synthesis technique has been demonstrated to
produce TE materials with a high filling rate, the high filling fraction has
produced an enhancement of thermoelectric properties. The experimental process
was used high pressure at 4 GPa to synthesize a bulk sample. XRD diffraction
results indicate that only small amount impurity of Sb and CoSb2 were detected.
Samples of nominal composition CexCo4Sb12 ( x = 0 – 1.0) started material madethrough the high-pressure and high temperature synthesis. The increased Ceconcentration in this study not only enhance the electric transport due to a better
electron doping effect but also decrease the thermal conductivity due to a stronger
rattling effect. The highest actual composition of Ce filled CoSb3-based
skutterudite of 0.37, and was found to be optimal for achieving a peak ZT valueof about 0.006 at 300 K.
3-15

Experimental Results of NdxCo4Sb12
Introduction
In chapter 4, we have studied the effect of a a few percent of neodymium(Nd) in filling Mm skutterudites under high pressure. The results have confirmed
the high filling rate is markedly decreased the lattice thermal conductivity thanCe filled skutterudite with studied in chapter 3. This is may be due to the
distribution of fillers ions. Therefore, this chapter will discuss the detailed
experimental result data of the NdxCo4Sb12 samples in term of phases structures,
lattice parameter, actual compositions, and thermoelectric properties of
NdxCo4Sb12.

X-ray diffraction (XRD) of NdxCo4Sb12
The XRD patterns of NdxCo4Sb12 ( x = 0 - 1.0) samples are shown below,
respectively. The diffraction peaks of all samples can be indexed as the
NdxCo4Sb12 phase which has a skutterudite structure with the Im3̅ space group.
Crystalline degree and phase purity of the products were analyzed by X-ray
diffractometer (XRD) with Cu Kαl radiation (λ = 0.154060 nm) operating at 40
kV- 40 mA, in the 2 range of 10° - 160°. The products were specified by a
Rigaku RINT-RAPID-II Computer Software (search match program) of the
JCPDS database.
These studies show a small impurity of Sb and CoSb 2 amount of impurityof Sb and CoSb2, by Nd increase the impurity of Sb is relatively low but CoSb 2
is reverted, and the x-ray study of Nd filled skutterudite gives a shift of lower 2
compared to the unfilled samples x-ray study as shown in fig 4-1. When the filling
fraction of Nd is lower than 0.6 (nominal composition), the reflection lines splits
into two peaks in lower intensity, likely due to a quantity of Nd incomprehensive
4-1

all area of the samples and some area is unfilled producing a defective point in
the samples. The smaller angle with increased filling fraction is very similar tosome previous reports of filled skutterudite.23–25)

*

Co4Sb12Intensity (a.u)

Sb
CoSb2

 **
30

40

50

60

70

80

2 (deg)4-2*

Nd0.4Co4Sb12

Intensity (a.u)

Sb
CoSb2

 **
30

40

50

60

70

80

2 (deg)

Nd0.6Co4Sb12

Intensity (a.u)

*

Sb
CoSb2

 **
30

40

50

60

70

80

2 (deg)4-3*

Nd0.8Co4Sb12

Intensity (a.u)

Sb
CoSb2

 **
30

40

50

60

70

80

2 (deg)*Intensity (a.u)

Nd1.0Co4Sb12
Sb
CoSb2

 **
30

40

50

60

70

80

2 (deg)

4-4

013



Nd Co Sb
4

12

2

600

530

431

422
332



420



* *

330

Intensity (a.u)

123

*

x

Sb
CoSb

X = 0.35
X = 0.33
X = 0.27
X = 0.08
X=0

30

40

50

60

70

80

2 (deg)

Figure 4-1. XRD pattern of a polycrystalline of NdxCo4Sb12.

Lattice constant and actual composition of NdxCo4Sb12
The lattice parameter with a small filled Nd has rapidly increased to 9.10 Åand saturated although Nd doping rate was increased (shown in fig 4-2). The
actual compositions were evaluated by SEM and EDX. The SEM examination of
NdxCo4Sb12 samples indicates a dense structure with closely packed grains. The
EDX has been used to examine the actual composition by point analysis, bycomparing the atomic ratio. The actual filling fraction of Nd in Nd xCo4Sb12 islower than the nominal Nd doping ratio as shown in fig 4-2. The maximum Ndvalue obtained is 0.35, higher than Nd 0.1Co4Sb12 and Nd0.13Co4Sb12 werereported.26,27) The calculated lattice parameter from the XRD measurement isshown in fig 4-2 (red line).

4-5

0.4

9.14
NdxCo4Sb12

0.35

9.12

Lattice constant (Å)

9.1

0.25
0.2

9.08

0.15
9.06
0.1
9.04

Actual Nd filling fraction

0.3

0.05
0

9.02
0

0.2

0.4

0.6

0.8

1

Nominal Nd doping ratio, X

Figure 4-2. Lattice constant and actual Nd filling fraction vs nominal Nd
doping rate.

The electrical resistivity () of NdxCo4Sb12
Temperature dependence of the electrical resistivity were shown in fig 4-3.
The electrical conductivity () or electrical resistivity () of materials relates tocharge carrier concentration (n), electric charge (e), and electron mobility (µ) by = 1/ = neµ by higher level of charge carrier concentration significantly
increases electrical conductivity of material. In general, the charge carrier can bedivided into two types; a hole and an electron which are dominant in p- and ntype semiconductors, respectively. In this study, the  of unfilled skutterudite
decreased with the increasing temperature and shows p type semiconductor
behavior in low temperature with large . In contrast, samples doped with Nd
4-6

displayed semimetal behavior. The  decreases with elevating Nd filled in thevoids and x = 0.33 and 0.35 exhibits almost the same value of . This can be
attributed to increased electron concentration by donating electron from Nd filled
skutterudite. This might be samples having high electron concentration changes
from p-type conductivity to n-type conductivity, according to some previous
work on rare earth elemental filled skutterudites.9,28)-2

10

NdxCo4Sb12
X=0
X=0.008
X=0.27
X=0.33
X=0.35-3 (−m)

10

-4

10

-5

10

0

50

100

150

200

250

300

T (K)

Figure 4-3. Temperature dependences of the electrical resistivity of
NdxCo4Sb12.

The thermal transport property of NdxCo4Sb12
The ZT value can be further maximized by minimizing thermal
conductivity. Filled skutterudite introduce the phonon scattering. Total thermal
conductivity () comes from two sources: electronic (e) and lattice (l) part. The

e is directly related to the electrical conductivity through the Wiedemann–Franzlaw which mention above. Fig. 4-4 shows the temperature dependence of lattice
thermal conductivity l in the temperature range from 5 K to 300 K for Nd filled
4-7skutterudites and compared to the unfilled CoSb 3. The peak value for unfilled
CoSb3 in our study reaches 8 W/mK at 150 K and is suppressed a little to 6.49
W/mK at room temperature. This is due to the scattering of grain-boundary inpolycrystalline of CoSb3 as shown in fig 4-4. Filled Nd ions in the voids of CoSb3
are reducing l substantially. For the x = 0.09 and x = 0.27 skutterudite samples
exhibits the same l value even though the amount is different. This may berelated to the Co-phase of Nd-filled and unfilled phase which is observed by the
XRD reflections line (see Fig. 4-1). The l of the samples with x value 0.33
exhibits the lowest value of l of 2.49 W/mK. This is lower than Nd-filled CoSb3
skutterudite at ambient pressure of 3.7 W/mK Nd 0.25Co4Sb12 and 3.4 W/mK of
Nd0.1Co4Sb12.26,27) The markedly reduction of l due to the rattle behaviors andthe random distribution of Nd filled skutterudite demonstrating the more effective
scattering of phonons by markedly decreased in l as shown in fig 4-5.

10

Co4Sb12

 [W/Km]

8

6

4


.

2

. e
. l

0
0

50

100

150

200

250

300

T (K)

4-8

4
3.5

Nd0.008Co4Sb12

 [W/Km]

3
2.5
2
1.5

.

1

. e
. l

0.5
0
0

50

100

150

200

250

300

250

300

T (K)

4
3.5

Nd0.27Co4Sb12

 [W/Km]

3
2.5
2

.

1.5

. e
. l

1
0.5
0
0

50

100

150

200

T (K)

4-9

4

Nd0.33Co4Sb12

3.5

 [W/Km]

3
2.5
2

.

1.5

. e
. l

1
0.5
0
0

50

100

150

200

250

300

T (K)

4

Nd0.35Co4Sb12

3.5

 [W/Km]

3
2.5
2

.

1.5

. e
. l

1
0.5
0
0

50

100

150

200

250

300

T (K)

4-10

10

NdxCo4Sb12
8

 [W/Km]

X=0
X=0.0086

X=0.27
X=0.33
X=0.35

4

2

0
0

50

100

150

200

250

300

T (K)

Figure 4-4. Temperature dependences of the thermal conductivity of
NdxCo4Sb12.
10

NdxCo4Sb12
8

l [W/Km]

X=0
X=0.0086

X=0.27
X=0.33
X=0.35

4

2

0
0

50

100

150
T (K)

200

250

300

Figure 4-5. Temperature dependences of the lattice thermal conductivity of
NdxCo4Sb12.

4-11

The seebeck coefficient and ZT value of NdxCo4Sb12
Regarding the temperature dependence of the Seebeck coefficient (S), as
shown in fig 4-6, all samples of Nd filled skutterudites acquires negative Seebeckcoefficients. It indicates the dominant carriers for the material’s n-typeconductivity are electrons, meaning that the negative change of S is related to thecharge carrier concentration. Whereas the S of unfilled skutterudite is contrasted,
it increases with the increasing temperature. Samples with different Nd fillingrate shows a similar temperature dependence of S from 5 to 300 K, with theabsolute value of S increasing to the maximum of -62.6 μVK-1 at 300K. The highdoping of Nd clearly affects the S properties, however Nd filling up to 0.35 the S
is conversely lower than those of the x = 0.33. It is noteworthy, if high filling of
x=0.35 has more efficient, their S should be larger than the sample with x = 0.33,
which the ratio is contrast in our experiment observed. One possible reason is thatthe sample with x = 0.35, over-electron concentration may contribute to theelectrical conduction and then results of S is decrease. Moreover, x = 0.08 and0.27 show the same value of S although the filling rate differs. This is due to theco-state of filled and unfilled of Nd, probably due to a quantity of Nd
incomprehensive all area of the sample and some area is unfilled produced adefective point in the samples, observed in fig 4-1. The ZT value is estimatedfrom the measured thermoelectric properties. In the experimental samples, theactual filling of Nd slightly increases with increasing Nd doping, which can be
attributed to increases in the electron concentration. The ZT values increase withincreased temperature. In this experiment, an optimal ZT figure of 0.011 at 300K
was reached at x = 0.33. With filling fractions higher than 0.33, the ZT
performance decreased due to a lower Seebeck coefficient and low carrier
mobility. Therefore, tuning the carrier concentration and finding the effective
mass of guest ions to reduce thermal conductivity are important for theenhancement of the ZT value.

4-12

60
NdxCo4Sb12

Seebeck Coef. (V/K)

40
20
0
-20
X=0
X=0.008
X=0.27
X=0.33
X=0.35

-40
-60
-80
0

50

100

150

200

250

300

T (K)

Figure 4-6. Temperature dependences of the Seebeck coefficient of
NdxCo4Sb12.
0.012

NdxCo4Sb12

0.01

X=0
X=0.08
X=0.27
X=0.33
X=0.35

ZT

0.008
0.006
0.004
0.002
0
0

50

100

150

200

250

300

T (K)

Figure 4-7. Temperature dependences of the dimensionless figure of merit of
NdxCo4Sb12.
4-13

SummaryIn this study, we have successfully synthesized the n-type filled skutterudite
using HPHT technique. The actual Nd filling rate indicates an advantage of
HPHT technique which can substantially increase the filling fraction into thevoids of CoSb3 compared with conventional synthesis techniques. The highfilling fraction of Nd-filled skutterudite greatly suppresses the lattice thermal
conductivity and approaches the min. The lattice thermal conductivity of
Nd0.33Co4Sb12 has decreased to 2.49 W/mK at 300K. However, we can observedthat the low of l of single filled compared to double filled or multi-filled are
higher effective for example

In0.13Lu0.05Co4Sb12 of 2.0 W/mK

and

Ba0.08Yb0.14Eu0.10Co4Sb12.12 of 1.1 W/mK.29,30) It might possible to introduce
lower l by synthesizing double-filled or multi-filled preparing under highpressure. The optimal of Nd filled skutterudite shows significantly enhanced ZT
value due to the increment of Nd filling rate, as high as 0.011 at room temperature.
HPHT technique is one of the most attractive approaches to fabricating n-type
filled skutterudites of TE material, with the advantages of speed, increased filling
fraction and markedly decreased lattice thermal conductivity.

4-14

Experimental Results of LaxCo4Sb12
Introduction
In chapter 5, we have studied the effect of lanthanum (La) 25% of Mm filled
skutterudite under high pressure. The results have confirmed the high filling rateof La markedly decreased the lattice thermal conductivity to a greater than thatof Ce and Nd filled skutterudite in chapter 3 and 4. Furthermore, the result of La
filler, can be used to replace Mm as fillers and greatly reduction in l. This chapter
describes the characterization techniques on the samples in term of phases
structures, lattice parameter, actual compositions, and thermoelectric properties of
LaxCo4Sb12.

X-ray diffraction (XRD) of LaxCo4Sb12
Figure 5-1 shows x-ray diffraction patterns of filled skutterudite materials,
which are mixture of each metal and antimony powder in the atomic ratio of
LaxCo4Sb12 (x = 0 - 1), at room temperature and 4 GPa. While a small amount of
impurity phase Sb and CoSb2 were detected, most of the observed diffractionlines were indexable using the skutterudite structure. However, with La content
lower than 0.8 the main peaks is split to two peaks and with increased La content
the main of split peaks were changes to lower  (red arrows) and finally, the split
peaks disappear with x = 1.0 were shown below.

5-1

*

Co4Sb12Intensity (a.u)

Sb
CoSb2

 **
30

40

50

60

70

80

2 (deg)*

La0.4Co4Sb12

Intensity (a.u)

Sb
CoSb2

 **
30

40

50

60

70

80

2 (deg)5-2*Intensity (a.u)

La0.6Co4Sb12
Sb
CoSb2


**
30

40

50

60

70

80

2 (deg)

La0.8Co4Sb12

Intensity (a.u)

*

Sb
CoSb2


* **
30

40

50

60

70

80

2 (deg)5-3*La1.0Co4Sb12

Intensity (a.u)

Sb
CoSb2

* * *
30

40

50

60

70

80

2 (deg)013

LaxCo4Sb12

530
600

422
431
332

420

 ** 

330

Intensity (a.u)

123

*

Sb
CoSb2

X=1.0
X=0.8
X=0.6
X=0.4
X=0

30

40

50

60

70

80

2 (deg)
Figure 5-1. XRD pattern of a polycrystalline of NdxCo4Sb12.

5-4

Lattice constant and actual composition of LaxCo4Sb12
The lattice constant of La filled skutterudite is almost the same with Ce andNd filled skutterudite in chapters 3 and 4. The lattice constants were determined in
the same manner in this study and the saturate point was found to be about 9.12 Å
while small filling of x = 0.4 also steeply increased to 9.12 Å, whereas for CoSb3
the lattice constant data was 9.038 Å (see fig. 5-2, black line). The actual
compositions evaluated by SEM and EDX were similar to those in the previous

chapter. The SEM examination of La filled skutterudite samples are very fragile.
The EDX has been used to examine the actual composition by point analysis, bycomparing the atomic ratio. The actual filling fraction were shown in fig 5-2. The
maximum La filling fraction of 0.28 see fig 5-2 (red line).

0.3
9.14

LaxCo4Sb12
0.25

Lattice constant (Å)

0.2
9.1
0.15
9.08
0.1
9.06

Actual La filling fraction

9.12

0.05

9.04

0

9.02
0

0.2

0.4

0.6

0.8

1

Nominal Nd doping ratio, X

Figure 5-2. Lattice constant and actual La filling fraction vs nominal La doping
rate.

5-5

The electrical resistivity () of LaxCo4Sb12
Temperature dependence of the electrical resistivity were shown in fig 5-3.
The electrical resistivity reduced with increasing La-doping content andtemperature, indicating a typical semiconductor behavior for all samples. After
doping with La to a content maximum of 0.28, the electrical resistivity reducedwith increasing La-doping. This might be due to the change of majority
concentration and carriers generated with increasing temperature, which contrast
with the mother compound of CoSb3. La0.28Co4Sb12 sample showed the lowest
electrical resistivity of 17.58 μΩm at 300 K, which is lower than CoSb3 sample
of 353.39 μΩm at same temperature. This is mainly due to the electrons suppliedby La-doping and also due to increasing the carrier concentrations. Thus, Lafilled skutterudite improved the electrical resistivity of the Co4Sb12 sample.
-210

LaxCo4Sb12

X=0
X=0.04
X=0.13
X=0.19X=0.28-3 (−m)

10

-4

10

-5

10

-6

10

0

50

100

150

200

250

300

T (K)

Figure 5-3. Temperature dependences of the electrical resistivity of LaxCo4Sb12.

5-6

The thermal transport property of LaxCo4Sb12
Figure 5-4 shown the temperature dependence of thermal conductivity ()of LaxCo4Sb12 bulk samples. The total thermal conductivity is the sum ofelectronic (e) and lattice (l) thermal conductivities. It was found that the thermal
conductivity decreased with increasing La-doped content over the temperature.
For the doped samples, La0.28Co4Sb12 sample show the lowest value of thermal
conductivity of 1.72 W/mK at 300 K, which is lower than CoSb 3 sample of 6.51
W/mK at same temperature. However, which subtraction of the electronic termthe l is lower than that of 1.31 W/mK see fig 5-5.

10

Co4Sb12

 [W/Km]

8

6

4


.

2

. e
. l

0
0

50

100

150

200

250

300

T (K)

5-7

5

 [W/Km]4

La0.004Co4Sb12

3

2

.

1

. e
. l

0
0

50

100

150

200

250

300

T (K)

5

La0.13Co4Sb12

 [W/Km]

4

3

2

.

1

. e
. l

0
0

50

100

150

200

250

300

T (K)

5-8

5

La0.19Co4Sb12

.

4 [W/Km]

. e
. l

3

2

1

0
0

50

100

150

200

250

300

T (K)

5

La0.28Co4Sb12

.

 [W/Km]

4

. e
. l

3

2

1

0
0

50

100

150

200

250

300

T (K)

5-9

12
10

 [W/K-m]

LaxCo4Sb12

X=0
X=0.04
X=0.13
X=0.19X=0.28

8
6
4
2
0
0

50

100

150
T (K)

200

250

300

Figure 5-4. Temperature dependences of the thermal conductivity of
LaxCo4Sb12.

12
10

l [W/Km]

LaxCo4Sb12

X=0
X=0.04
X=0.13
X=0.19X=0.28

8
6
4
2
0
0

50

100

150

200

250

300

T (K)

Figure 5-5. Temperature dependences of the lattice thermal conductivity of
LaxCo4Sb12.

5-10

The seebeck coefficient and ZT value of LaxCo4Sb12
Regarding the temperature dependence of the Seebeck coefficient (S), as
shows in fig 5-6, all samples of Nd filled skutterudites acquires negative Seebeckcoefficients. The absolute Seebeck coefficient values improved with increasing
La-doped content and temperature. The highest Seebeck coefficient value
reached -55.21 μV/K at 300 K for La0.28Co4Sb12 sample, which is changes topositive of undoped Co4Sb12 sample of 56.59 μV/K at same temperature. This ismainly due to the electrons supplied by La-doping. The dimensionless figure of
merit, ZT for LaxCo4Sb12 bulk samples as a function of temperature are displayedin Figure 5-7. It can be seen that the ZT values improved via increasing La-doped
content and temperature. The maximum ZT value of 0.03 has been attained forLa0.28Co4Sb12 sample at 300 K.
60
LaxCo4Sb12

Seebeck Coef. (V/K)

40
20
0
-20
X=0
X=0.04
X=0.13
X=0.19X=0.28

-40
-60
0

50

100

150

200

250

300

T (K)

Figure 5-6. Temperature dependences of the Seebeck coefficient of LaxCo4Sb12.

5-11

0.035
LaxCo4Sb12
0.03

X=0
X=0.04
X=0.13
X=0.19X=0.28

ZT

0.025
0.02
0.015
0.01
0.005
0
0

50

100

150

200

250

300

T (K)

Figure 5-7. Temperature dependences of the dimensionless figure of merit of
LaxCo4Sb12.

Summary
In this chapter, we have successfully synthesized the n-type filled
skutterudite using HPHT technique. The actual La filling rate indicates anadvantage of HPHT technique which can substantially increase the filling
fraction. The use of HPHT can increase the lattice constant to 9.12, which ishigher than any previous report so far. The high filling fraction of La-doping
greatly suppresses the lattice thermal conductivity. The lattice thermal
conductivity of La0.28Co4Sb12 has decreased to 1.31 W/mK at 300K. The
optimal of La filled skutterudite shows significantly enhanced ZT value of 0.03
at room temperature.

5-12

TE performance at high temperature
Introduction
In chapter 6, we have continued studied TE properties from chapter 3, 4 and5 evaluated at high temperature. However, the samples prepared under highpressure are very small and were sometimes fragile and easy to break become
small pieces or powder, which are not suitable to evaluate the TE properties athigh temperature. Therefore, we combined HPHT synthesis method using a
multi-anvil press and spark plasma sintering (SPS) method. First, we removed
impurity phases of samples and grind the samples, prepared under high pressure,
into powder. Then, the powder was sintered by SPS at 60 MPa and 600 °C for
120 min. to obtain large and dense bulk samples suitable to evaluate the properties
with high temperature region. The samples were shaped to discoid shape with 10
mm in diameter and 2 mm in height. The discoid shape samples were used for thethermal conductivity measurements with a laser flash method using an
NETZSCH LFA457 system. For other measurements, the discoid samples were
cut into rectangle with 2 mm in width, 2 mm in height, and 8 mm in length. The
electrical resistivity and Seebeck coefficient were measured using ULVAC ZEM3 system. Hall effect measurements were performed using the van der Pauw
method under a magnetic field of 7 and -7T, were measurement by a thermal
relaxation method (Quantum Design PPMS). However, after selected the samples
which have highest ZT performance to sintering and measure at high temperature
it was found that the actual compositions of samples after sintering by SPS
technique are decreased to 0.13 and 0.17 for Ce- and Nd- filled skutterudite andthe lattice constant decrease to 9.039 and 9.041 Angstrom respectively.

6-1

The electrical resistivity () at T  300 K
Figure 6-1 shows the temperature dependence of the electrical resistivity 
for Ce0.13Co4Sb12 and Nd0.17Co4Sb12 prepared by using HPHT synthesis and SPS
technique. The data above 300 K for CoSb3 are reference from Park et al.,.31)
Based compound of CoSb3 shows semiconducting behavior, Ce- and Nd-doped
samples exhibit semimetallic behavior. Furthermore, the values of  for
Ce0.13Co4Sb12 and Nd0.17Co4Sb12 are much lower than that for CoSb3. Thissuggests that they are heavily doped semiconductor, which can be attributed toincrease of the electron concentration by doping of guest ions with high filling
ratio. Moreover, high fillers Nd-doping of 0.17 exhibit greater the electricalconductivity than Ce fillers of 0.13.

[31]

Figure 6-1. Temperature dependences of the electrical resistivity at T  300 K.

6-2

The thermal conductivity at T  300 K
Figure. 6-2 shows the temperature dependence of the total thermal
conductivity  and the lattice thermal conductivity l in fig. 6-3, in thetemperature range from 5 to 760 K for Ce0.13Co4Sb12 and Nd0.17Co4Sb12. The data
above 300 K for CoSb3 are reference from Park et al.31) By filling Ce or Nd, bothκ and κl decrease. The minimum values of  l are 2.57 Wm-1K-1 at 670 K and 2.11
Wm-1K-1 at 680 K for Ce0.13Co4Sb12 and Nd0.17Co4Sb12, respectively. A reductionin  l suggests that the rattling of Ce or Nd atoms. Furthermore, the random
distribution of partly filled Ce or Nd ions in vacant sites causes more effective
scattering of phonons. The difference in reduction of  l between Ce and Nd ions,having almost the same content as filling guest ions, could be due to thedifference of mass and size of guest ions. The smaller and heavier rattler tends to
induce more off-center behavior than the larger and lighter ions. Therefore, the lof Nd filled skutterudite is lower than that of Ce filled skutterudite because Nd
has a smaller ionic radius and larger mass than that of Ce.

6-3

10
(a)
.
CoSb
3 (this work)8 (W/K-m).
CoSb
3 [31]

6

4
BCoSb
Mm Co Sb
0.17

B

2

4

12

C

CNd
Ce0.13Co4Sb12

D

DCe
Nd

(n) [13]

Co4Sb12

0.17

0
0

100

200

300

400

500

600

700

800

T (K)

Figure 6-2. Temperature dependences of the thermal conductivity at T  300 K.
10
(b)
.
CoSb
(this
work)
CoSb
3
Co
.
CoSb
3 [31]Mm
CeNd

l (W/K-m)

8

6

4

B
Mm
0.17Co4Sb12 (n) [13]C 0.13Co4Sb12
CeD
Nd0.17Co4Sb12

2

0
0

100

200

300

400

500

600

700

800

T (K)

Figure 6-3. Temperature dependences of the thermal conductivity at T  300 K.

6-4

The Seebeck coefficient at T  300 K
The temperature dependence of Seebeck coefficients S for Ce0.13Co4Sb12 andNd0.17Co4Sb12 is shown in fig. 6-4. The data above 300 K for CoSb3 are also
reference from Park et al.31) While the S of CoSb3 is positive, those forCe0.13Co4Sb12 and for Nd0.17Co4Sb12 are negative, indicating transition from ptype to n-type conductor. The absolute values of S reach a maximum of 185 μV/K
at 480 K for Ce0.13Co4Sb12 and 166 μV/K at 690 K for Nd0.17Co4Sb12. However,after 500 K with increasing temperature the trends of both samples are stable ora little bit increase, this due to the samples are become to intrinsic behaviors.

200.CoSb3 (this work). CoSb3 [31]
Mm
.
.

150

S (µV/K)

100
50

..

0

..
.
Mm
0.6Co4Sb12 (n) [13].
. 0.13Co4Sb12
Ce
.
.
Nd
0.17Co4Sb12

-50
-100
-150
-200
0

100

200

300

400

500

600

700

800

T (K)

Figure 6-4. Temperature dependences of the Seebeck coefficient at T  300 K.

The carrier concentration (n)Figure 6-5 and 6-6 show the temperature dependence of the carrier
concentration n and the carrier mobility µ for Ce0.13Co4Sb12 and Nd0.17Co4Sb12.

6-5

The n and µ were calculated from Hall coefficient RH, namely, n = 1/(eRH) and µ= RH/, where e is the elementary charge. The n for both Ce0.13Co4Sb12 andNd0.17Co4Sb12 increases with increasing temperature because of the increased
activation of charge carriers across the band gap while the n of CoSb3 is nearly
constant in the temperature range of 150–300 K. By filling Ce or Nd, the valueof n inceases to 2.6 × 1020 cm-3 and 7.6 × 1020 cm-3 for Ce0.13Co4Sb12 andNd0.17Co4Sb12, respectively, from 0.036 × 1020 cm-3 (CoSb3) at 300 K. The  forboth Ce0.13Co4Sb12 and Nd0.17Co4Sb12 decreases with increasing temperature
while the  of CoSb3 increases (Fig. 6-6). The μ at room temperature decreases
from 220.99 cm2V-1 s-1 for CoSb3 to 7.86 and 8.66 cm2V-1 s-1 for Ce0.13Co4Sb12
and Nd0.17Co4Sb12. The  of both compounds exhibits a T-3/2 behavior above 150
K. The results suggest that the carrier scattering mechanism is mainly acoustic
phonon scattering.

Figure 6-5. Temperature dependences of the Seebeck coefficient at T  300 K.

6-6

Figure 6-6. Temperature dependence of carrier mobility µ for Ce0.13Co4Sb12 andNd0.17Co4Sb12.

The dimensionless figure of merit (ZT) at T  300 K
Figure 6-7 shows the temperature dependence of dimensionless figure of
merit (ZT). The maximum ZT values of 0.26 and 0.48 were achieved at 700 K for
Ce0.13Co4Sb12 and Nd0.17Co4Sb12, respectively. The enhancement of ZT is mainly
due to the high filling of guest ions and the change in carrier concentration. The
use of HPHT synthesis not only make it possible to fill a wide variety of elements
into Sb vacancies, but can also increase the upper limit of elements. High filling
ratio of Ce or Nd ions in voids of CoSb 3 results in an improvement of thethermoelectric performance. Unfortunately, the use of SPS technique causes a
deviation from the expected compositions, namely, reduction in filling ratio ofguest ions. We have to develop a new technique to make large bulk samples withkeeping high filling rate of samples prepared by HPHT synthesis.

6-7

0.6
.Nd0.17Co4Sb12

ZT

3

0.5

.Ce0.13Co4Sb12

0.4

.
Mm
0.6Co4Sb12 (n) [13].CoSb3 (this work).CoSb3 [31]

0.3
0.2
0.1
0
0

100

200

300

400

500

600

700

800

T (K)

Figure 6-7. Temperature dependences of ZT at T  300 K.

6-8

Conclusion
In this study, we have successfully synthesized n-type filled
skutterudite using HPHT technique. The actual Ce and Nd filling rate
indicates an advantage of HPHT technique which can substantially
increase the filling fraction into the voids of CoSb 3 compared withconventional synthesis techniques. The high filling fraction rate results ina significant suppression of l. The lowest l of this study is 2.11 W/mK
at 680 K for Nd0.17Co4Sb12. The maximum ZT values for Ce0.13Co4Sb12
and Nd0.17Co4Sb12 are 0.26 and 0.48 at 700 K, respectively. This worksuggests that Nd filling is much effective for the improvement TE
performance of CoSb3-based compounds. Furthermore, the l of double
filled or multi-filled skutterudite compounds is generally lower than thatof single filled compounds, which indicates the possibility of inducing
lower l by synthesizing double-filled or multi-filled skutterudites
prepared by HPHT synthesis. Therefore, if we prepare Nd rich Mm,
namely, “artificial Mm” instead of the nature Mm, the TE performance
could be improved. In this way, the HPHT technique is one of the most
attractive approaches to fabricating n-type filled skutterudite compounds,
with the advantages of increasing filling fraction and markedly reductionof lattice thermal conductivity.

Suggestions
1) Spark Plasma Sintering (SPS) system is not suitable for
combination of HPHT because the low pressure of SPS sintering cannot
preserve the advantage of high filling rate. Therefore, finding the new
method combine with HPHT technique is an importance.
7-12) Artificial compound of Mm synthesized under high pressuretechnique, it would be possible to achieve the higher ZT performance than(Mm) nature mischmetal.
3) If it is possible, we should synthesis a large bulk sample byusing HPHT which suitable to measure with high temperature. It might
be known the actual performance with high filling rate.
4) Studying the carrier concentration and carrier mobility isimportance for tuning the majority concentration. This can lead to
optimized TE material synthesized by HPHT technique.

7-2

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AAppendix A
Sample preparation for SPS

Appendix A

Remove impurity

10 mm

2 mm

A sample size is required for LFA 457

𝑚

= 𝑣

𝑚 = 1.2 𝑔

Appendix A

Theoretical density for rare earth filled skutterudite approximately is 7.7 g/cm3,
therefor at least 1.2 g after remove impurity.

Appendix A

Appendix B
Conference list and Publication list

Appendix BConference list
➢ 2016 Rare Earth workshop, The Westin Rusutsu, Hokkaido - JAPAN.
➢ 2016 Joint Convention, The Hokkaido chapters of the institutes ofelectrical and information - JAPAN.
➢ 2017 Joint seminar on Environmental Science and Disaster Mitigation
Research (JSED 2017) - JAPAN.
➢ 2017 The 28th Rare Earth Research conference, (28th RERC) -Ames,
IOWA STATE, USA.
➢ 2017 Japan-Thailand Joint Seminar and International Symposium (MIER 2017) - THAILAND.
➢ 2018 The 37th International Conference on Thermoelectric(ICT 2018) - CAEN, FRANCE.
➢ 2018 Rare Earth workshop, Lake Toyako, Hokkaido - JAPANPublic list
➢ Thermoelectric properties of partially filled skutterudites
RxCo4Sb12 (R = Ce and Nd) synthesized under high pressures

Appendix B