部分充填スクッテルダイト化合物 YbxRh4Sb12の高圧合成と熱電特性
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言語: English
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公開日: 2020-06-08
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キーワード (En):作成者: シリマート, ジラッタガン
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URL

https://doi.org/10.15118/00010185

令和元年度

博士学位論文題目
High-pressure synthesis and thermoelectric properties ofpartially filled skutterudite compounds YbxRh4Sb12（部分充填スクッテルダイト化合物 YbxRh4Sb12の高圧合成と熱電特性）

提出者

室蘭工業大学大学院
工学研究科 博士後期課程
工学専攻
強相関電子物性研究室
平成28年度入学
学籍番号 16096503

Jirattagan Sirimart
提出日

令和 2 年 2 月３日

Abstract

The “phonon-glass electron-crystal” model is an ideal thermoelectric materials andthe imagining of the extra framework cation rattling with in a cage to yield very low
thermal conductivities and produce excellent thermoelectric properties. The
thermoelectric performance of a material is characterized by the figure of merit, ZT= (S2/ρκ)T, where S is Seebeck coefficient, ρ is electrical resistivity, κ is thermal
conductivity and T is temperature. To enhance performance of ZT, though, doping
heavy atom into skutterudite structure. Skutterudite structure have strong inthermoelectric properties. In skutterudite structure contain of transition metal andpnictogen atom, in structure consist of a square ring of pnictogen atoms that sharing
bonding and form with transition metal then created empty cages. Yb has been filled
in CoSb3-base skutterudite, It provide effect on low-vibration frequency according to
reduce thermal conductivities. Yb-filled skutterudite. Whereas, heavy mass andsmall radius have affected on strong interaction in structure
A new candidate thermoelectric material is YbxRh4Sb12, because RhSb3-basestructure have large void than CoSb3-base skutterudite. This work reported new
information of YbxRh4Sb12. Yb content was filled from 0 – 0.8. This compounds wereprepared under high pressure and high temperature method at 2 GPa, 750oC for 2
min with heating rate at 50oC/min and after decrease temperature to 450-550oC. The
maximum actual filling fraction of Yb was found at 0.4. This compound is in bodycentered cubic structure (Im3). From this condition, the temperature dependence ofelectrical resistivity decrease and seebeck coefficient decrease (that mean show in ntype behaviors) at the same time, thermal conductivity also decrease from 8.11
W/mK (Rh4Sb12) to 2.43 W/mK (Yb0.8Rh4Sb12) the ZT value increase to 0.042 at 300K
for Yb0.8Rh4Sb12. Maxmimum actual Yb doped Rh4Sb12 was improved thermoelectricperformance by spark plasma sintering method. The maximum ZT was found 0.1 at
420K

CONTENTS
Chapter 1 Introduction
1.1
1.2

Thermoelectric effect
Skutterudite

1
7

1.3
1.4
1.5

Thermoelectric properties of skutterudite
High pressure synthesis
Motivation

10
11
15

Chapter 2 Experimental method
2.1
2.2
2.3

Preparation of material
Synthesis
Characterization

16
16
18

2.4

Measurements electrical and thermal properties

19

Chapter 3 Results and discussion of YbxRh4Sb12 (2 GPa)
3.1
3.2
3.3
3.4
3.5

Experimental details and characterization
Electrical properties
Thermal conductivity
Dimensionless figure of merit
Specific heat

48
59
65
70
71

3.6

Magnetic properties

74

Chapter 4 Results and discussion of YbxRh4Sb12 (SPS)
4.1
4.2
4.3
4.4

Experimental details and characterization
Electrical properties
Thermal properties
Figure of merit

79
82
85
87

Chapter 5 Summary
5.1

Summary

Reference
Acknowledgement

88CHAPTER 1
INTRODUCTION

In the present energy is an important factor in human life, in all of manufacturing
which use energy, always have an energy lost, most 60% of energy lost are in formof heat. It is better if we can recovery heat lost back to electrical energy.
Thermoelectric (TE) is technology that can directly and reversibly convert heat toelectrical energy or transform electrical energy into thermal power for cooling byusing the temperature different. The advantages of TE is small size, high reliability,
no pollution and friendly with environment. Now thermoelectric technology canapply to use in many manufacturing. For example in power generation such as(automobiles, industrial processes) hand watch and radioisotope and for cooling side
they are also use in refrigerator system and CPU
Thermoelectric module consist of two semiconductor we call n type and p type. The
semiconductor is mean thermoelectric material that has electrical conductivity
greater than insulator but less than metal.

1.1 Thermoelectric effectThermoelectric effect is direct conversion of temperature difference can createdvoltage. When heat applied at one side, electron at hot end are more energetic andfaster than electron at cold end. Therefore, electron at hot end are move towards
cold end and leave positive ion at hot end. So, cold end is present negative charge
and hot end is present positive charge. If electron diffuse from hot to cold end, thenthe cold end is negative with respect to the hot side and seebeck coefficient isnegative (P-type). Holes would diffuse from the hot to cold end. The cold end ispositive which would make seebeck coefficient positive.
1

Thermoelectric efficiency can be quantified by the dimensionless figure of merit (ZT),
which is derived from eq. 1.1.1
ZT = 𝑆 2 𝑇/𝜌𝜅
when

1.1.1

S = Seebeck coefficient
ρ= electrical resistivity
ĸ = thermal conductivity (ĸ = ĸe + ĸL)ĸe = electrical thermal conductivityĸL = lattice thermal conductivity- Seebeck coefficient
Seebeck coefficient is a temperature difference between two points in a conductor
or semiconductor results in a voltage difference between these two point- Thermal conductivity
Thermal conductivity is kinetic energy of atomic motion and potential energy of
distortion of interatomic bond, which is consist of electrons and holes transporting
heat (ĸe) and phonon traveling through the lattice (ĸL).
When temperature increase, the large is the mean atomic velocity and the amplitude
of atomic vibration lead to larger thermal energy. The vibration of individual atoms
in solids are not independent from each other. The coupling of atomic vibration of
adjacent atoms results in waves of atomic displacement. Each wave is characterized
by its wavelength and frequency for a wave of a given frequency, there is the smallest
quantum of vibrational energy that call phonon. Therefore, thermal energy is theenergy is energy of all phonon (all vibration waves) present in the crystal at a given
temperature.
In thermoelectric materials, free electron stabilized with lattice vibrations in hot
end, migrate to cold end and transfer a part of their thermal energy back to latticeby phonon scattering. So, heat is transferred by phonon (lattice vibration) andelectron.

2Since free electron are responsible for both electrical and thermal conduction. The
two conductivities are related to the Wiedemann-Franz law
L=

𝜅𝑒 𝜌
𝑇

1.1.2

The decreasing of electrical resistivity not only produces an increase in theelectronic thermal conductivity but also decrease thermopower. While the power
factor can be increased by varying the concentration of charge carrier in materials.
The power factor (PF) is given as

𝑆2

𝑃𝐹 = 𝜌

1.1.3

In generally, the power factor is difficult to manner the numerical to improvementof ZT 1 . The optimization of PF can be raised performance of ZT.
In the part of lattice thermal conductivity (ĸL) is determined by structure, rigidity,
atomic masses and other characteristic of lattice.
The enhance ZT by reduce lattice thermal conductivity depend on factor as
1. Use of compound with complex crystal structure
2. Insert heavy atoms with weakly bonded to the structure.
3. The existence of inclusion or impurities
4. The formation of solid solution
5. The existence of a large number of grain boundaries.
- Electrical resistivity
The value of electrical resistivity at room temperature is indicative of whether amaterial. If the lattice is perfect, the electron would travel infinitely through it andthe material would only exhibit finite conductivity because of the thermal motion of
the lattice and impurity.
A variation of electrical resistivity depend on changes in the carrier concentrationand the mean free path of the charge carriers.

3

The rattling atom

Fig 1.1.1 rattling harmonic of atom
There is a number of metallic materials in which atom can make very extended
oscillations around their equilibrium position. Such a situation occurs in particularin materials in which certain atoms are surrounded by an oversized “cage structure”
of other atoms.
In these case the atomic vibrations are highly anharmonic and one speaks of a“rattling” motion.

In the fig 1.1.1, the effective potential for an atom is illustrated

for a conventional harmonic case (left) and a strongly rattling type case with a hardwall potential (right). Such materials are promising as high efficiency, thermoelectricmaterials because they have high electrical conductivity but low thermal
conductivity due to the rattling.
The anharmonic motion strongly influences the nuclear spin-lattice relaxation rate
and the electrical resistivity of such materials.

4

Thermoelectric are solid-state devices that consist of two or more elements of ntype doped semiconductor and p-type doped semiconductor material connected
together with conductive metal. Their electrical interconnected are mounted
between two ceramic substrates and connected electrically in series and thermallyin parallel.

Fig 1.1.2 Thermoelectric device module2
Thermoelectric efficiency (η) is defined by combining the Carnot efficiency and thefigure of merit (ZT), as following eq 1.1.4.
(√1+𝑍𝑇𝑎𝑣𝑔 −1)𝑇ℎ𝑜𝑡 √1+𝑍𝑇𝑎𝑣𝑔 + 𝑐𝑜𝑙𝑑
𝑇ℎ𝑜𝑡

∆𝑇

η=𝑇

1.1.4

Where Thot = temperature of the hot end
Tcold = temperature of the cold end∆T = temperature difference between hot end and cold end.
To enhance thermoelectric efficiency, both type of thermoelectric materials should
be high in ZT values and large temperature difference across the thermoelectricmaterials. Now a day, thermoelectric materials performance was improve to ZT>1,
especially p-type materials as show in fig 1.1.3.

5

Fig 1.1.3 Temperature dependence of ZT of difference thermoelectric materials3

6

1.2 Skutterudite
Skutterudite structure have strong in thermoelectric properties because of thisstructure is one of the class in Zintl compounds. The Zintl compounds is the productbetween alkali metal, alkaline earth, post-transition metal or metalloid, thoseelement made up of electropositive cations which donate their electrons toelectronegative anions, which in turn use them to form bonds in order to satisfy
valence. In skutterudite structure contain of transition metal and pnictogen atom,
in structure consist of a square ring of pnictogen atoms that sharing bonding andform with transition metal then created empty cages. So that, In Fig 1.2.1 represent
six pnictogen such pnictogen ring in a large square divided into eight smaller square.
It suggest that there are two small square, one at the upper right front and anotherat the lower left back, which do not contain a ring of pnictogen atom. In the otherwords, skutterudite contain two structural voids often called “cage”. This instead of
MX3, this structure in the cubic space group Im3.

Fig 1.2.1 skutterudite crystal structure

Binary skutterudite compounds AB3 can form with various elements. That waspredicted as fig 1.2.2. Transition element is 8c position and pnictogen atom is in 24h.
the octahedras create large voids in structure, that are able to accommodate
electropositive element such as rare earth, alkali metal etc. Filler element are justloosely bound in the cages giving rise to low energy optical modes.
The skutterudite form by substantial overlap and minor electronegativity differences
between cation and anion to produce a highly covalent structure with exceptional7

carrier mobility that relatively high intrinsic electrical conductivity. The concept of
introducing guest atoms into these voids without structure distortion because
unfilled skutterudite can maintain the cage structure without guest ions. A marked
rattling effect is expected owning to large vibration of small guest ions located inside
the cage. When M is transition metal and X is pnictogen This formula (MX3) isequivalently ⊡2M8X24 = 2⊡M4[X4]3 when ⊡ symbol represent

the void and [X4]represent four-member pnictogen rings. If take only one-half of the unit cell
including a single void and designate the skutterudite structure as ⊡M4X12
Rattling effects is not only decreased in lattice thermal conductivity but also
increases in electrical conductivity because fillers generally are electro positiveelements even though having a negligible effect on the band structure.

Fig 1.2.2 Formation of binary skutterudite3
Filled skutterudite
R4+[M4X12]4Where R = electronegative ion (the balance of charge)[M4X12]4- = a tetravalent negative charge[M4X12] = appears as a neutral.

8

The filled skutterudite consist of 2 groups
1. Compounds are formed by a simple filling of the 2a site of the framework built by
8c and 24h atom
2. Electronically promoted formation of filled skutterudites.
Alkaline earths, rare earths can be considered the main skutterudite because thefiller must have a size consistent with the host void and the difference ofelectronegativity (χ), χx-χfiller > 0.8.
The filler weak bonds with x, their delocatization is responsible for the decrease ofthermal conductivity.
Electronegativity of Yb, Rh and Sb
Yb = 1.1
Rh = 2.28
Sb = 2.05
The filled skutterudite consider that the lattice parameter and the size of voids
consistently increase passing from skutterudite. The lattice parameter increase with
the increase of the mass of metal within a given family of skutterudites.
The role of filler was associated with additional and independent resonant scattering
introduced by the weak interaction between the filler and the host structure.
When filled guest atom into structure, a lattice parameter of structure are increase.
These is the effect of the increasing of atomic mass in structure.
The limit of the filling fractions depends on
Radius of the void in the structureRadius of the ionicThe valence state of the guest ion

Moreover, partially filled skutterudites are very sensitive to difference ofelectronegativity between the antimony and the guest atom.

9

1.3 Thermoelectric properties of skutterudite.
As we know, thermoelectric performance can calculated from figure of merit or ZT
equation. To enhance thermoelectric performance, the figure of merit should be
develop. It have many way to increase figure of merit such as nanostructuretechniques or the doping of transition metal or pnictogen-atom. The other method isfilled atom into empty cages leading to the formula AyM4X12 y≤1 Because of large size
between filler atom and empty cage. The filler atom act as independent oscillation
then reduce in thermal conductivity. Moreover, Filled skuterudite voids has effect onelectrical conductivity, although smaller than the decreasing of total thermal
conductivity.
Many studies have investigated reduce lattice thermal conductivity of rare earth
filled skutterudite4. The filling rare-earth element is not only reduced lattice thermal
conductivity but also enhance carrier concentration in compounds, especially, Ybfilled skutterudite. Whereas, heavy mass and small radius have affected on strong
interaction in structure5. From previously, YbxCo4Sb12 had been studied6, havesucceeded in synthesis heavier rare earths filled skutterudite compounds such as
YbxCo4Sb12 by using high pressure and high temperature (HPHT) synthesis
technique. This compound shows high filling ratio, nearly theoretical investigation8,9.
Accordingly, higher ZT values may be accomplished if the lattice thermal
conductivity can be further reduce towards minimum thermal conductivity byincreasing Yb filling fraction while maintaining the high power factors observed in
Yb-filled compounds.
Therefore, the control of Yb filling fraction is an important issue in optimizing thethermoelectric properties of filling skutterudite compounds. The limit of Yb filling
fraction is known to be only 0.2 in YbxCo4Sb12. To achieve high filling fraction, it isnecessary to compensate for the charge of Yb ions by substitution on either the Cosite or the Sb site10
Moreover, it shows various interesting physical properties, indicate that highpressure can introduce heavier rare earths into voids skutterudite. High pressure
has many advantages, such as decrease impurity phase in preparation method or
prevent the disorder11,12

10

1.4 High Pressure Synthesis
The cubic-anvil type high pressure apparatus was used to synthesis thermoelectricmateraisl of skutterudite structure. In this work, DIA-type (UHP-500) was used forsynthesis, as show in fig 1.4.1. High pressure technique has ability to restrain ofdisorder phase. Moreover, High pressure method is not complicated and faster thanconvention method. Accordingly, HPHT method allow heavier rare earth atom easily
to entrance into skutterudite structure. A cubic anvil high pressure apparatus wasused in this work.
UHP-500 (DIA-type)
A cubic anvil high pressure apparatus type apparatus also known as the equipment
can deform materials under high pressure and high temperature. This apparatus
consists of six anvils, two of the anvils are fixed opposite to each other on the upperand lower guide blocks, which are driven by uniaxial hydraulic ram. The innersurfaces of the guide blocks from a tetragonal pyramid. The anvil size is 12 mm
suitable for a 16 mm pyrophyllite cube. The upper and lower anvil bases are fixed tothe upper and lower guide blocks respectively, and the four anvil bases on the side
are set slide ably on the 45 ° inclined surface machined into the upper and lower
guide blocks. When the upper and lower guide blocks thus configured are advanced
relative to each other in the vertical axial direction, the six anvils can uniformly
compress six surfaces of the sample to form a hydrostatic pressure field. Regarding
the hydraulic pressure, the cylinder first adopts a push-up method in which the guideblock attached to the lower part of the frame is pushed up and pressurized. The
maximum working pressure is 70MPa. As a system, pressurization, holding pressure
and pressure reduction control are set in advance as a pattern for the ultrahigh11

pressure generating hydraulic cylinder, and pressure control of the cylinder isperformed based on the pattern. With regard to the sample section, the sample
chamber is formed in a cubic pressure medium as shown in Figure 1.4.2, and this isplaced in line with the anvil surface. An alternating current source is used togenerate high temperature, and two opposing anvils are used as electrodes to makethe graphite heater in the sample part flow current and raise the temperature. The
other two opposing anvils were electrodes for thermocouples for temperature
measurement.
Although this size changes depending on the size of the anvil with respect to thegenerated pressure, the UHP-500 used in this research uses a 12 mm anvil, and asa pressure medium for high temperature and high pressure synthesis, 16 mm squarepyrophyllite (habitite: Al2O3 · 4SiO 2 · H2O) was used.

Fig 1.4.1 a cubic anvil apparatuses of UHP-500

12A cubic anvil high pressure apparatus can be work three step. Firstly, after the
sample was being placed in the apparatus at the center of the anvil. The pressure
was applied to anvil by the control of hydrostatic compression. Secondly, after thepressure reached a set up pressure, the temperature would increase to the set up
temperature. The pressure and temperature were held constant for a period timeand detect by using thermocouple and sent to controller (computer). Finally, thesample was quenched to the room temperature and then the sample wascharacterized composition and thermoelectric properties.

13

6mm

Boron nitrite (BN) cap

2.5mm
8mm

SUS ring
MO plate
4mm
3mm
1mm

BN crucible

8.4mm

5mm
4mm
1mm

10.4mmGraphite heater crucible

15mm
16mm

Pyrophyllite

Thermocouple

16mm

Fig 1.4.2 Assembly of parts for UHP-500

141.5 Motivation

To enhancement thermoelectric performance, the lattice thermal conductivity
should be reduce. Skutterudite structure is one of candidates because this compound
has large voids inside structure. Guest ion could be filled inside the void. The guest
atom act as rattler interact with phonon modes in structure. From previously report,
YbxCo4Sb12 has been fill in void site with nearly theoretical by synthesis under highpressure and high temperature technique. Beside, lattice thermal conductivity can
reduce by insert Yb-atom in Co4Sb12 about 30%. Nevertheless however, Rh4Sb12
system have larger void radius from 7%. Because of Rh4Sb12 is hole dominatedsemiconductor with strongly covalent Rh-Sb bonds. The strong covalency create highcharge carrier mobility and high lattice thermal conductivity. Moreover, Yb is smalland heavy. It is believe that if can insert Yb-atom in void cage with highest
percentage. It can reduce lattice thermal conductivity then enhance with highthermoelectric performance.

15

CHAPTER 2
Experimental method
2.1 Preparation of materials
Before synthesis by using a cubic anvil high-pressure apparatus. We need toprepared materials as following,

Pyrophyllite cube (16x16x16 mm)

Pyrophyllite lid (Diameter, Ø 2.5 mm)

Graphite container (inner Ø = 4 mm, outer Ø = 5 mm, length = 10.4)

Graphite lid (Ø = 4 mm, thickness = 1 mm)

Boron nitrate (BN) container (inner Ø = 3 mm, outer Ø = 4 mm, length = 8.4)

Boron nitrate (BN) lid (Ø = 4 mm, thickness = 1 mm)



Molybdemun (Mo) plateThermocouple

2.2 Synthesis
In this study, a cubic anvil high-pressure apparatus (Sumitomo Heavy Industries
UHP-500) was used for synthesis. The cubic-anvil apparatus is shown in fig 2.2.1
The powder of starting materials, Ytterbium (Yb- 99.9%), rhodium (Rh-99.9%) andantimony (Sb- 99.9999%), were mixed by stoichiometric ratio. Those compounds
were mashed and mixed in agate mortar and then loaded to boron nitrite (BN)container and graphite to become a pellet. The pellet was insert to pyrophyllite cube.
After that, the cube was cover with molybdenum plate and stainless-ring (SUS-ring)for conduct current pass thought SUS-ring and molybdenum plat into sample. An
assembly of sample for high pressure apparatus UHP-500 shows in fig 2.2.2. Afterassemble, the cubic was place in apparatus. The upper stage was rise up andpressure to setting pressure. The compress pressure was control by hydrostatic
compression. After press sample to setting pressure, AC current was entering tosystem. The current is pass through anvil to initial sample. The temperature inside
the sample was detect by using K-type thermocouple.

In this work, the synthesis

condition of YbxRh4Sb12 is below,
Synthesis condition.
Pressure: 2 GPa
Temperature: 750oC
16450-550oC (Keeping)
The heating rate: 50oC/min, 25 W/min
Time:

2 min (over shoot)120 min (Keeping)Fig 2.2.1 A cubic anvil high-pressure apparatus (Sumitomo Heavy Industries UHP500), Muroran Institute of technology.

Starting material(Powder)

Boron Nitrate

Graphite

Pyrophyllite Cubic

Thermocouple

Fig 2.2.2 A assemble of cubic-anvil apparatus (UHP-500)

17

2.3 Characterization
After finish the synthesis, the sample was cut by diamond cutter and clean byusing sand paper number #1500, #5000 and #8000, respectively. Then sample wasanalyzed phase purity and element composition by using X-ray diffraction
instrument (XRD) and Scanning electron microscope (SEM)

X-ray diffraction (XRD) instrument

In this work, Phase composition and actual element distribution in all sample were
investigated by using Rigaku RINT-RAPID-II with with Co Kα radiation and Si
powder as a standard. The XRD instrument is shown in Fig 2.3.1

Fig 2.3.1 X-ray diffraction (Rigaku RINT-RAPID-II) instrument, Muroran instituteof technology, Hokkaido, Japan

Scanning electron microscope (SEM)

Scanning electron microscope (SEM) is one of electron microscope that using electron
beam to interact sample surface and create an images. The scanning electron
microscope (SEM) with energy dispersive X-ray Spectroscopy (EDS),JEOL JSM-6510
Series, was used for measurement chemical electron composition, as show in fig 2.3.2.
The EDS using for detects x-rays emitting of element composition to analyse actual
element composition. In this work, we use back scattered electron image (BEIW)
with magnification 3000 for measure actual composition and select 8-9 point for
reduce error.
18

Fig 2.3.2 a scanning electron microscope and EDS (JEOL JSM-6510 Series
Scanning Electron Microscope), Muroran institute of technology, Hokkaido, Japan

2.4 Measurements electrical and thermal properties.
The high quality sample, which is lowest impurity, was chosen for measurement
electrical and thermal properties. Those sample were shape to rectangle with wide x
length x height about 1.0 x 2.0 x 0.5 mm, as show in fig 2.4.1. The sample should besmall in wide and height size but large in length in order to reduce resistivity ofmaterials, as following equation 2.4.1.
𝜌𝑙

R= 𝐴
where,(2.4.1)

R= Electrical resistance of a uniform specimen of the material (Ω)
𝜌 =Resistivity of the material (Ω. 𝑚 𝑜𝑟 Ω. 𝑐𝑚)

A = Cross-sectional area of the specimen (𝑚2 𝑜𝑟 𝑐𝑚2)
L = Length of the piece of material (m or cm)

19

0.7~1.3mm

0.3~0.7mm

More than 2.0mm

Fig 2.4.1 Sample standard size for measurement thermoelectric properties.


Cryostat and vacuum system (Thermal block company, Saitama Japan).

This machine was used for measurement electrical resistivity with standards fourprobe method. We use the gold wires as a probe, and contact with silver paste. An
amount of silver paste is taken out with a toothpick on a glass plate and diluted with
α-butyl (the ethylene glycol mono-n-butyl ether acetate) to attach on gold wires andsample same as fig 3.4.2. A current is pass through two outer probe while a voltage
is measured through two inner probe. The gold wires and silver paste couldn’t touchon each other for prevent short circuit of system. A cryostat and vacuum system(Thermal block company, Saitama Japan) is shown in fig 2.4.3
V

I

V

I

Fig 2.4.2 four – probe measurement for electrical resistivity.

20

Fig 2.4.3. A cryostat and vacuum system (Thermal block company, Saitama Japan),
Muroran Institute of technology, Hokkaido, Japan

Physical property measurement system (Quantum Design: PPMS)
The quantum design PPMS can measure various physical properties such as
thermal conductivity and Seebeck coefficient as well as electric resistivity andspecific heat can be performed. Moreover, this machine can operate over 24 hours
per day and 7 days per week. In this experiment, we use PPMS to execute
thermal transport option and heat capacity option. A Quantum design PPMS isshown in fig 2.4.4

21

Fig 2.4.4 Physical property measurement system (PPMS; Quantum Design),
Muroran Institute of technology, Hokkaido, Japan- Thermal transport option (TTO)
Thermal transport option (TTO) can measure thermal conductivity (ĸ), Seebeckcoefficient (S) for determined thermoelectric figure of merit (ZT). For TTO
measurement, copper lead was use for probe. Prepare two copper lead in length about
1.5 cm and bend as in fig 3.4.5, then mount copper lead and sample by using
epoxy(Silver-filled epoxy A part and B part) on the heat plate. After that connect
sample with wires shoe in the position as following fig 2.4.6 (The wires shoe is very
easy to damage, so we should touch it softly.)
In order to measure thermal conductivity, heat is apply from heater shoe to create atemperature differential between two thermometer shoe and detect on each side ofsample.
Thermal conductivity was measured by PPMS Model 7100. The thermal was passthough the sample and then detect by each side of sample. In the parts of measure
Seebeck coefficient, while create temperature differential, the voltage drop is also
created and detect seebect coefficient.
22

Heat shoe

Hot shoe

Copper leadepoxy
Cold shoe

Cold footFig 2.4.5 Sample connection for TTO

2

1

3

z

Sample5674Fig 2.4.6 Illustration sample of TTO puck. (1. Cold shoe, 2. Heat shoe, 3. Hot
shoe, 4. Cold foot, 5. Cold shoe, 6. Hot shoe and 7. Heat shoe)23

- Hall effect measurement
Hall effect measurement is determine the carrier concentration and carrier mobility
under magnetic field. In this experiment, the magnetic was set at 5T. The assemble
sample with gold wires method as show in fig 2.4.7. The measurement method cansee as following.

I+

V+

V-

I-

Fig 2.4.7 Sample assembly for Hall effect measurement(1) PPMS can measure various physical properties function. It can be chose thefunction by control in option manager mode, as show in fig 2.4.8. Various operations
can be performed on the Multivu screen shown in fig 2.4.9. If Option Manager is not
displayed, it can be select the Utilities-Activate Option menu command in PPMS
MultiVu, select on option that we need to be activated in Available Option panel, andthen select Connection Diagram* Only one measurement option can be active at a time. If another option is activated.
Select Deactivate before select option that we need to measurement.

Fig. 2.4.8

PPMS Option manager(2) Change cable according to the Connection Diagram(3) Before insert sample. It is necessary to set the inside of the chamber to 1 atm. To
24

control the pressure in the chamber, select the Vent Cont. at the Chamber state panel
as shown in Fig 2.4.9. Waiting until state become flooding, then insert the sample
pack into the chamber using the sample insertion tool shown in Figure 2.4.10 (a).
(4) Insert the radiation shielding cap tool (as shown in Fig 2.4.10 (b)) into thechamber(5) Select Purge / Seal at the Chamber state panel, wait until the state become
purged. (It cannot use High-Vac)

Fig 2.4.9 Chamber Control

Fig 2.4.10 (a)Fig 2.4.10 (b)Sample insert tool.

Radiation shielding cap tool

4.1 Active Resistivity Option (Hall effect).
(1) Select Resistivity in the Available Options panel, as shown in Figure 2.4.8. And
then select Activate. As soon as when resistivity option is activated, the resistivity
control, the screen in Figure 2.4.9, appear in to PPMS MultiVu interface.
25

(2) Create a data file. Press Browse button in the resistivity control center in Figure
2.4.11. Select the drive and directory. (Do not use special characters in file names.)Fig 2.4.11 Resistivity Option(3) Enter sample description to the File and Sample Properties dialog box open
(Figure 2.4.12). After creating the data file, press Samples in Figure 2.4.13 The
Sample description represent the user bridge channel that installed sample, thefollowing information:(a) The name of each sample(b) The area in mm2 of each sample. Use “1” when set Units information toOhm.
(c) The Length in mm of each sample. Use “1” when set Units information
to Ohm.

Fig 2.4.12

File and Sample Properties dialog box

26

(4) Select Bridge Setup in Figure 2.4.13 to make various settings. Basically, we canset from 5000,3000,2000,1000,… in current limit (μA) for check resistivity in AC andDC mode in Drive mode, set power limit to1000 μV and Voltage limit to 95 mV in
Figure 2.4.13. When setting, Resistance is calculated to the right, so confirm that it
is not an abnormal numerical value (Resistance of all value should be constant).

Fig 2.4.13 Bridge Channels(5) Write Sequence. And then Run sequence

Figure 2.4.14 Sequence data
(6) Log. Open Utilities-Log PPMS Data. Since Log Data in Figure 3.4.15 opens, selectStop and then select Browse on the General tab, and create a new folder named Log
27

in the folder. Within the created new folder, set the File name as the date of themeasurement date and make a file. Select Start.
(7) Select View Data and Press Select All on all tabs, including the Standard Itemtab, and take Log of all items.

Fig 2.4.15 Log Data
(8) Display the state of measurement a graph. When select View in Figure 2.4.11,
the graph on the left of Figure 2.4.16 is displayed. Right click on the graph - Data
Selection will display the screen on the right. Select the item you want to display on
this screen. In Figure 2.4.16, x-axis select temperature.

28

Fig 2.4.16 measurement graph and Data Selection29

(9) When it is finished, press View Data to display a graph of temperature change.
All necessary screens with reference to Figure 2.4.17.

Fig 2.4.17 Measurement screen- Heat capacity
Heat capacity (HC) measurement, the sample should smaller than 3.0mm×3.0mm.
The one side of surface should be parallel with puck. Before measurement of HC thesize and the mass sample should be measured. The mounting sequence proceed as
below.
(1) Install vacuum pump (Fig 2.4.18) and sample-mounting station (Fig 2.4.19).
Turn on vacuum pump and connect puck (adjust to the center of platform holder)
and lock puck interlock arm *be careful do not break platform holder!

Fig 2.4.18 Vacuum Pump for Sample-Mounting station
30

Fig 2.4.19 Sample-Mounting station (top-view)(2) Clean platform holder by using cotton bud and an apply N grease to platform
holder by using toothpick. (Should use N grease as least as possible)(3) Place the shield on the puck (Fig 2.4.20)

Fig. 2.4.20 Sample holder with the shield.
After assembled sample on the parts, the measurement sequence needs to beprepared as following.
(4) Change cable according to the Connection Diagram(5) Connect cable, labeled “Heat Cap” (Fig 2.4.21) to P1-Option3, port on model
6500 and connect P2-system Bridge to P2-system Bridge on port model6000.

Fig 2.4.21 Heat Cap cable assembly(6) Check the Power. TTO measurement use the MODEL6000, MODEL6500.
(Fig 2.4.22)
31

MODEL60
00
MODEL6500
MODEL7100

Fig 2.4.22. MODEL6000, MODEL6500, and MODEL7100.
(7) Ensure Temperature at 300 K. Before open the chamber, first ensure thetemperature is 300 K and field is 0 Oe.
(8) Insert sample into the chamber. Do the following: click Vent Cont. button at
the Chamber windows (Fig 2.4.23); After the state became Flooding at thebottom of the status bar. Insert the sample puck inside the chamber using
the sample insert tool as shown in the Fig. 3.4.24(9) Assembled Charcoal (4038-010) in the bottom of the Contact Baffle (asillustrated in Fig. 2.4.25) and insert it inside the chamber and then close thechamber. The Charcoal will prevent helium from adsorbing on the sample
platform when temperature below 10K.
(10) Click Purge/Seal button to vacuum the chamber. Wait until the status
become purge, then click the HiVac button. The status will show At HiVac
when the chamber became high vacuum.

32

Fig.2.4.23 Chamber control window

Fig.2.4.24 Sample insert tool.

Charcoal

Contact

Contact Baffle

BaffleBlanking

HolderFingers(For uniform temperture)

Assembly

Flange(Forabsorb He
below 10

Fig. 2.4.25 Charcoal and Contact Baffle Assembly.
(11) Do the following at the PPMS Multivu window: select Utilities – Activate
Option, click on Heat capacity under the Available Options heading, and then33select Activate. (Fig 2.4.26)

Fig. 2.4.26 Option manager
1. Measurement addenda(12) Do the following: Click Prepare Addenda Measurement (Fig 2.4.27) andcheck puck serial number and confirm puck (Fig2.4.28) and click Next button.
Create New file and click Finish (Fig 2.4.29)

34

Fig 2.4.27. Windows after activate for Heat capacity

35

Fig 2.4.28. Puck Calibration

Fig 2.4.29 Data file(13) Write Sequence. As shown in Fig 2.4.30

36

Fig 2.4.30. Example for addenda Sequence.
(14) Run sequence and click the Graph in file Manu bar and select save namefile for view graph (Fig2.4.31). In data selection, select log scale in x and yaxis. In y axis should show addenda HC (μJ/K)

Fig 2.4.31 Measurement graph and Data Selection37

(15) 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 takethe Log data/Click the View Data to show the Log Data. (Fig 2.4.32)

Fig. 2.4.32 Log Data.
(16) Display the state of measurement graph. When select View in Fig 2.4.32
and measurement status viewer (Fig 2.4.33)

38

Fig 2.4.33 Log data view and Measurement status viewer
2. Measurement sample(17) Do the following (7). Adhere sample and platform holder (Fig 2.4.34) andinsert sample and do the following (7) to (10). Click Prepare sample
measurement (Fig 2.4.27 – Fig 2.4.29). Input sample mass information andselect sample heat capacity unit in μJ/K (Fig2.4.35)

39

Fig 2.4.34. Connection between sample and sample platform for Heat Capacity

Fig 2.4.35 Input sample information.
(18) Write and Run Sequence ( following (12)-(15) )40

(19) 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. Close the
Dewar and click Purge/Seal button to seal the chamber.

Magnetic property measurement system (MPMS)

Attach the sample
The weight of sample should be measured before measurement. In the preparedsample section, we use a clear plastic drinking straw to attach a sample because thestraw has minimal magnetic susceptibility. The step of attach the sample asfollowing
1. Cut off a clear plastic drinking straw to 12 cm
2. Place the straw segment inside the drinking straw, and insert the sample into theside of straw segment as show in Fig3.4.36. Then move the straw segment andsample until the middle of the length of the drinking segment. The sample should
parallel to straw

Fig 2.4.36. Correctly positioned sample
3. Punch one side of drinking straw on top and bottom
4. Wrap a tape around the brass-coloured end of the drinking straw.
5. Measure the length between sample and the sample rod, and adjust to 122.7±0.5
cm as show in fig 2.4.37 (* the sample should in the middle of straw segment anddrinking straw)

Fig 2.4.37. Sample assemble41

Insert the sample
Turn the airlock lever to Closed position (Fig 3.4.38), the airlock level is closes

for protect the lower portion from the air flowing into the top of the chamber. SelectInstrument on the menu bar, and select Chamber and Vent Sample space. Verify the
O-rings are properly seated and lubricated
1. Insert the sample and sample rod, and push the two slide seal clamps on the socket
block assembly.
2. Select Purge sample space around 3 time (Waiting for green “Ready” LED for
insert sample)
3. Turn the airlock lever to Opened position, and slowly to insert the sample rod andslowly until the black slide clamp on the rod engages the actuator shoe. If the rod
does not move easily, apply a small amount of Grease (Fig 2.4.39) to the rod. Run
your fingers along the length of the rod even coating of grease. The knurled nut andtwo clip screws on top of the actuator shoe should be tight.

Fig 2.4.38 Sample Transport and Electronic Control Assembly42Fig 2.4.39. GreaseMeasurment

1. Select Actual field in status bar. For example, Set field to 10000 Oe, Approach tooscillate and Mode Hi-Res enabled. (at 10000 Oe with Oscillating with Hi-res enable,
if higher than 50000 Oe with on over shot with Hi-res enable)

Fig 2.4.40 Status bar
2. Select Center and DC. The DC Centering dialog box show in fig 2.4.41. SelectInitialize Transport for calibrate the sample transport.
3. Select Parameters. Set scan length 4 cm and Data points 24 (fig 2.4.42)

43

Fig 2.4.41. DC Centering dialog box

Fig 2.4.42 Parameter dialog box
4. Select Full DC Scan for scan full centering measurement, which cover length of
the sample transport`s travel path.
5. Select Center, scan the location of sample only a partial centering measurement.
Check the center of position box, the different position should around 1.5 ±0.5 cm (fig
2.4.43). If different position select Adjust Position in DC Centering dialog box, andSelect Adjust manually (fig. 2.4.44) (If still different select adjust manually again).
The sample location should show 2.0 cm. Select Adjust Automatically.

Fig 2.4.43 Center position

44

Fig 2.4.44. Adjust sample position dialog box
6. Before run sequence, check He at status bar.
7. Write sequence, Save as sequence. Select Change sample name in control center,
Select Change Sequence base data file name (fig 2.4.45).

Fig 2.4.45. Control center
8. Select Utilities and log data. Select Stop and then select Browse on the generaltab and create files name. Select Start and View Data and Press Select All on alltabs.
9. Display the state of measurement a graph as show in fig 2.4.46.

45

Fig 2.4.46. Measurement screen.


Remove sample

1. Ensure Temperature at 300 K. Before open the chamber, first ensure thetemperature is 300 K and field is 0 Oe. (Sequence indle)
2. loose two clip screws, Pull sample rod, and turn the airlock lever to closed position
3. Set lower chamber to 1 atm. by select Instrument, Chamber and Vent Sample
space.
4. Remove sample rod and drinking strew.
5. Install the airlock puck (blue color).
6. Set vacuum the bottom chamber by select Instrument, Chamber and Purge
Sample space.
3.5 High temperature range measurement.
Before measurement at high temperate range, sample was prepared by using
spark plasma sintering (SPS), at the national institute of advanced industrial
science and technology (AIST), Tsukuba, Ibaraki, Japan
High actual Yb filling fraction of YbxRh4Sb12 was synthesized using a highpressure apparatus (UHP-500, Sumitomo Heavy Industries Ltd., Japan) about 2 g.
In spark plasma sintering technique, the sample after synthesized was clean
and remove BN by using sand paper and pin then ground into powder with agate
mortar. The powder was loaded to crucible and pressed into pellet, heat to 500oC
46

under a pressure 60 MPa by Spark Plasma Sintering (SPS).


Laser flash method (LFA) (NETZSCH LFA457)
The pellet was shape to dimension 10±0.1 mm and thickness 2±0.1 mm (fig 2.5.1)

for thermal conductivity measurement, using a laser flash method (NETZSCH
LFA457).

10 mm
2 mm
Fig 2.5.1 size of sample for measuring LFA
In Laser flash analysis (LFA) is used to measure thermal diffusivity andcalculate . An energy pulse heats one side on the bottom side and detect temperature
signal versus time on the top side.
ZEM-1 (SINKU-RIKO ULVAC ZEM-1)
Electrical transport properties (electrical resistivity and Seebeck coefficient) was

measured by ZEM-1 (SINKU-RIKO ULVAC ZEM-1) with sample shape inrectangular with dimension of width (W), height (H) and length 2.0±0.1 mm x 2.0±0.1
mm x 8.0±0.1 mm, respectively. The thermal transport properties and electricaltransport properties were measured from 300 K to 723K

2 mm

2 mm

8 mm

Fig 2.5.2 size of sample for measuring ZEM-1

47

CHAPTER 3
Results and discussion of YbxRh4Sb12 (2GPa)
3.1 Experimental details and Characterization
High-quality polycrystalline sample of skutterudite compounds, YbxRh4Sb12(0≤x≤0.9) were synthesized under high temperature and pressure. At the beginning
purified element Ytterbium (Yb), Rhodium (Rh) and Antimony (Sb) were mixed bystoichiometric. Then those element were melted and pressed by a cubic anvil highpressure apparatus (UHP-500). The element was melted at 750-800oC for 2 minutethen decrease to 450-550oC in order to comply with pressure-temperature phase
diagram of base RhSb3 (see fig 3.1.1)
The synthesis condition under high pressure is very sensitive in the detection
temperature. Thus, the synthesis condition had been controlled by power withtemperature. Moreover, the thermocouple position should be good contact withgraphite cylinder for reduce error of temperature measuring position. The
preparation process, grinding element, has to be avoided, because of possibly
oxidation in this process or the oxidation of elementary constituents. Therefore, It isdifficult to make good quality sample (purity sample) in every time synthesize. For
this reason, a contain of some impurity can determine thermoelectric properties. In
fact, the impurity content act as minor carrier, the main influence will be dominatedby doping element.

48

Fig 3.1.1 Pressure-temperature phase diagram of RhSb3 preparation under highpressure technique12

49

50

51

52

Figure3.1.2 X-ray diffraction pattern of YbxRh4Sb12 (0.2≤x≤0.9)
9.27
calculated lattice constantLattice constant ( Å)

9.26

9.25

9.24

9.23

9.22
0

0.2

0.4

0.6

0.8

Nominal Yb composition,xFigure 3.1.3. The relationship between nominal Yb compotion and calculated lattice
constant.
53

Figure 3.1.2 shows the powder X-ray diffraction pattern of polycrystalline
YbxRh4Sb12(0.2≤x≤0.9)

skutterudite

sampleswithdifferent

Ytterbium

concentrations. The YbxRh4Sb12 skutterudite compounds exhibit in a body-centercubic crystal structure belongs to Im-3̅ space group (No. 204). The x-ray diffraction
results shows almost a pure skutterudites phase. A negligible peak of secondary
phase was observed in X-ray diffraction pattern (see figure 3.1.2). A low insensitivityof secondary phase is corresponding with RhSb2 and Sb. The RhSb2 is a binary
compound of antimony and rhodium, exhibit in monoclinic crystal structure in spacegroup P21/c (No. 14). In secondary phase of Sb was observed in R-3m (No. 166) with
Trigonal crystal structure. The lattice parameter was calculation from x-ray
diffraction pattern by a least-square fitting method. The result of calculation lattice
parameter of Yb-filled skutterudites corresponding with nominal composition, asshow in figure 3.1.3. From the least-square fitting method the estimated lattice
parameter of unfilled Rh4Sb12 is 9.232 Å. the lattice parameter expanded with Yb
doping filling fraction up to 9.2612 Å at Yb0.6Rh4Sb12 composition. The lattice
parameter of rare-earth filled Rh4Sb12 prepared from ambient pressure,
La0.1Rh4Sb1223 and Yb0.1Rh4Sb1217 are 9.2213 Å and 9.2289 Å, respectively. Thoselattice parameter are smaller than mother compounds, Rh4Sb12 (9.232 Å). Thisdifferent results from YbxRh4Sb12 synthesis under high pressure technique.

54

RhSb2

SEM image #1: Yb0.2Rh4Sb12

Sb
RhSb2

SEM image #2: Yb0.3Rh4Sb12

RhSb2

SEM image #3: Yb0.4Rh4Sb12
55

RhSb2

SEM image #4: Yb0.5Rh4Sb12

RhSb2

SEM image #5: Yb0.6Rh4Sb12

SEM image #7: Yb0.7Rh4Sb12
Figure 3.1.4 BEW-SEM images of polycrystalline YbxRh4Sb12 (0.2≤x≤0.7)
A scanning electron microscope (SEM) is produces image of the sample surface anddetect the element composition by using energy-dispersive X-ray (EDX). In the56

backscattered electron composition, BEC-SEM images the skutterudite material of
YbxRh4Sb12 (0.2≤x≤0.9) was shown in figure 3.1.4. The surface area of YbxRh4Sb12
sample is almost homogeneous with grey colour area. On the other hand, some point
areas are inhomogeneous, which are found the secondary phase of RhSb2and Sb in
the black point and white point, respectively. The actual filling fraction ofhomogeneous compound was observed by using EDX as show in table 3.1.1
Table 3.1.1. Nominal composition, actual filling fraction and lattice parameter of
YbxRh4Sb12 skutterudite compounds.
Nominal composition

Actual filling fraction(YbxRh4Sb12)

Lattice parameter(Å)

Yb0.2Rh4Sb12

Yb0.1Rh3.9Sb12

9.237

Yb0.3Rh4Sb12

Yb0.28Rh3.64Sb12

9.237

Yb0.4Rh4Sb12

Yb0.39Rh3.75Sb12

9.243

Yb0.5Rh4Sb12

Yb0.404Rh4Sb12

9.252

Yb0.6Rh4Sb12

Yb0.44Rh3.92Sb12

9.261

Yb0.7Rh4Sb12

Yb0.434Rh4.02Sb12

9.252

Yb0.8Rh4Sb12

Yb0.39Rh3.95Sb12

9.247

Yb0.9Rh4Sb12

Yb0.206Rh3.9Sb12

9.238

57

0.5
Yb filling fraction

Actual Yb content

0.4

0.3

0.2

0.1

0

0

0.2

0.4

0.6

0.8

Nominal Yb composition,xFigure 3.1.5 Dependence of Yb actual content versus nominal Yb composition (x),YbxRh4Sb12.
The actual filling fraction is lower than nominal composition, as show in figure 3.1.5.
The actual Yb filling fraction was filled to be 0.4 or 40% of filling guest atom invacancy site of mother compounds (Rh4Sb12), which is highest than any reported.
From the previously study, high pressure synthesis of Yb filled Co4Sb12 was found Yb
actual filling fraction at 0.298, which is highest than ambient pressure synthesis.
Moreover, High pressure synthesis method of YbxCo4Sb12 can be insert Yb in Co4Sb128
structure nearly theoretical Yb filling fraction in Co4Sb1215

58

3.2 Electrical properties
This section descript the result of the Seebeck coefficient, Electrical resistivity andthermal conductivity as a function of temperature. Hall measurement, heat capacityand magnetic properties are presented in the end of this section.

50

0

S (V/K)

-50

-100
x=0
x=0.2
x=0.3
x=0.4
x=0.5
x=0.6
x=0.7
x=0.8

-150

-200
10

100
T (K)

Figure 3.2.1 Temperature dependent Seebeck coefficient of YbxRh4Sb12
Figure 3.2.1 Seebeck coefficient (S) of YbxRh4Sb12 for 0≤x≤0.8. Unfilled
skutterudite Rh4Sb12 shows positive (p-type semiconductor) overall temperature
range. Whereas all doped sample are negative (n-type), the seebeck coefficientincrease with increasing temperature. Temperature dependent of Yb0.2Rh4Sb12,
Yb0.3Rh4Sb12 and Yb0.4Rh4Sb12 are decrease fast and then increase nearly room
temperature, where the intrinsic conduction behavior is observed. The observed signand temperature dependence of YbxRh4Sb12 change from positive to negative byinserting Yb-content indicating Yb donated electron at elevated temperature. The
Seebeck coefficient values up to -150μV/K at 220 K for Yb0.4Rh4Sb12.

59

0.01

0.0008

x=0
x=0.2
x=0.3
x=0.4
x=0.5
x=0.6
x=0.7
x=0.8

ρ(Ωm)

0.008

0.00060.00040.006
0.0002

0.004

0

0

100

200

300

0.002

0

0

100

200

300

T (K)
Fig 3.2.2 Temperature dependence of the electrical resistivity for YbxRh4Sb12 for0≤x≤0.8.
Fig 3.2.2 shows electrical resistivity as a function of temperature unfilled Rh4Sb12 ismetallic behavior as electrical resistivity increase with temperature increase. On theother hand, the decrease of electrical resistivity with increase of Yb content. The Yb
filled skutterudite become semiconducting behavior, suggest that this system remainrich of rhodium composition. It seems that contribute of Yb to charge carrier
concentration is small compared with the degeneration of Yb-filled, owing to large
void-radius Rh4Sb12 system. Those trend are similar in InxRh4Sb127 and LaxRh4Sb128.
Despite of the reducing of electrical resistivity of filled Yb is inconsistent with actual
Yb but carrier concentration higher than initial sample. At room temperature,electrical resistivity of Yb doping is decrease from 149 mΩ.cm to 1.984 mΩ.cm at
Rh4Sb12 and Yb0.5Rh4Sb12, respectively. Room temperature of carrier concentration of
nominal Yb composition and carrier mobility shows in Fig 3.2.3 and Fig 3.2.4,
respectively.

60

Fig 3.2.3 Room temperature of carrier concentration of nominal Yb composition

Fig 3.2.4 Room temperature of carrier mobility of nominal Yb composition
61

From the result of fig 3.2.3 and fig 3.2.4, carrier concentration increase with nominal
Yb content increase. On the other hand, mobility in system decrease while increasingof number of Yb. It seem that when increase number of doping atom, the number ofscatter also increase. Fig 3.2.5 show temperature dependence of carrier mobility of
YbxRh4Sb12. While increasing temperature, the mobility of YbxRh4Sb12 (0.2≤x≤0.5)
increases and the mobility of Yb0.6Rh4Sb12 reduce with temperature decrease. At
temperature higher than absolute zero, the donate electron from Yb atom arescattering in structure which tends to decrease carrier mobility. In the part of, themobility of Yb0.7Rh4Sb12 and Yb0.8Rh4Sb12 are decrease at some range of temperatureand then gradually increase, suggesting intrinsic carrier are dominate. These effectcan be result of holes minority carrier by temperature excitation. From the result ofhall effect measurement obtained that at high filling fraction ratio of Yb0.44Rh3.92Sb12(nominal Yb = 0.6) exhibit high carrier concentration than Yb0.29Co4Sb11.71(n=4.96x1023 cm-3, n=6.02x1020 cm-3, respectively.), which correspond to Rh4Sb12
system can insert guest atom to void structure higher than Co4Sb12 system.

62

20
x=0.2
x=0.3
x=0.4
x=0.5

 cm2/V.s

15

(a)

10

5

0

-5

1

10

100

1000

T[K]
1
x=0.6

2

cm /V.s

(b)

0.1
1

10

100

1000

T[K]

63

0.6
x=0.7

(c)

x=0.8

 cm2/V.s

0.4

0.2

0

1

10

100

1000

T[K]
Fig 3.2.5 Temperature dependence of carrier mobility of YbxRh4Sb12

64

3.3 Thermal conductivity
10


L

 (W/mK)

8

Rh4Sb12

6

4

2

e
0
0

50

100

150

200

250

300

T (K)
10



(W/mK)8

L
Yb0.2Rh4Sb12

6

4

2

e
0
0

50

100

150

200

250

300

T (K)

65

10

Yb0.3Rh4Sb12

(W/mK)

8

6


4

L

2

e
0
0

50

100

150

200

250

300

T (K)
10

Yb0.4Rh4Sb12

 (W/mK)

8

6

4


L

2

e
0
0

50

100

150

200

250

300

T (K)

66

10

Yb0.5Rh4Sb12

(W/mK)

8

6


4

L

2

e
0
0

50

100

150

200

250

300

T (K)
10

Yb0.6Rh4Sb12

(W/mK)

8

6

4


2

L
e

0
0

50

100

150

200

250

300

T (K)

67

10

8

 (W/mK)

Yb0.7Rh4Sb12
6

4


L

2

e
0
0

50

100

150

200

250

300

T (K)
10

Yb0.8Rh4Sb12

 (W/mK)

8

6

4


2

L
e

0
0

50

100

150

200

250

300

T (K)
Fig 3.3.1 Temperature dependence of total thermal conductivity (ĸ), lattice thermal
conductivity (ĸL) and electrical thermal conductivity (ĸe) of YbxRh4Sb12
68

Temperature dependence of lattice thermal conductivity (ĸL) and electrical thermal
conductivity (ĸe) are presented in fig 3.3.1. The lattice thermal conductivity (ĸL) was
calculated fromĸ = ĸL+ ĸe
All results of YbxRh4Sb12 sample, total thermal conductivity (ĸ), lattice thermal
conductivity (ĸL) are decrease with Yb content increase. From this results assumed
that the effect of rattling. At room temperature of Rh4Sb12 to Yb0.8Rh4Sb12, lattice
thermal conductivity (ĸL) decrease from 8.11 W/mK to 2.43 W/mK for lattice thermal
conductivity in Rh4Sb12 to Yb0.8Rh4Sb12.
For whole temperature range, the electrical thermal conductivity (ĸe) increase withtemperature increasing, as the effect of electron contribution in void side iscorresponding to Wiedemann-Franz law
ĸe=LσTWhen

L = 2x10-8 V2.K-2

σ = electrical conductivity
T = absolute temperature69

3.4 Dimensionless figure of merit (ZT)
0.05
x=0
x=0.2
x=0.3

0.04

x=0.4
x=0.5
x=0.6

x=0.7
x=0.8

ZT

0.03

0.02

0.01

0
0

50

100

150

200

250

300

T (K)
Fig 3.4.1 Temperature dependent dimensionless figure of merit (ZT) of YbxRh4Sb12
Temperature dependent dimensionless figure of merit (ZT) of YbxRh4Sb12 are present
in fig 3.4.1. The figure of merit of YbxRh4Sb12 increase with temperature increase.
From these results, it seem that high Yb doping exhibit high figure of merit (ZT) at
overall temperature range. We can observe highest figure of merit at Yb0.8Rh4Sb12
with room temperature, value exceeding 0.04. It is evident that we can improve thedimensionless figure of merit (ZT) by control filling fraction ratio of Yb content.

70

3.5 Specific heat
500
x=0.2
x=0.3
x=0.4
x=0.6
x=0.8

Cp (J/mol.K)

400

300

200

100

0
0

50

100

150

200

250

300

T (K)

Cp (J/mol.K)

6

4

2

0
1

2

3

4

5

T (K)
Fig 3.5.1 Specific heat C(T) as a function of temperature (T) for YbxRh4Sb12.
71

4

x=0.2
x=0.3
x=0.4
x=0.6
x=0.8

Cp/T (J/mol.K2)

3

2

1

0
0

50

100

150

200

250

300

T [K ]
2

2

Fig 3.5.2 Specific heat C/T as a function of T2 for YbxRh4Sb12.
Specific heat measurement can observe how energy heat can absorbed into electron
or vibrations of the positive charges in lattice at low temperature. A summary ofelectronic and phonon contribution can fitted by Debye T3 law
C(T)/T = γ+βT2
Where γ = electronic specific heat coefficient.
βT3term = lattice term (Debye constant)
From fig 3.5.1 show specific heat C(T) as a function of temperature (T). The overall
sample are steep increase at low temperature and lone tail at high temperature. At
low temperature show magnetic contribution with a Schottky anomaly at 2.3 K at
all sample.
At low temperature of specific heat can be calculated γ function from fig 3.5.2. And
Debye temperature (θD) can be calculated from
12𝜋 4 𝑅𝑛 1/3
𝜃𝐷 = ()
5𝛽Where

R = the gas constantn = the number of atoms/f.u. (17)72

From the fitted and calculation above, we can obtainedγ and θD are shown in table
3.5.1. From the specific heat measurement results, when the foreign atoms areinserted into the Rh4Sb12 structure, the heat capacity should increase as a result of
the increased occupancy of the unit cell. However, the occupancies of the guest atoms
are smaller, this structure system cannot calculated Einstein vibration at low
temperature.
Table 3.5.1 Electronic specific heat coefficient γandθD of YbxRh4Sb12.
Nominal Yb (x)Actual Yb

γ (mJ/mol.K2)

θD (K)

0.2

0.1

65.362

257.8

0.3

0.26

19.663

247.7

0.4

0.39

36.176

220.1

0.6

0.44

70.100

215.4

0.8

0.39

52.215

216.9

73

3.6 Magnetic properties
0.04

1000

Yb0.3Rh4Sb12
800

600
0.02
400

1/x (mol/emu)(x)emu/mol

0.03

0.01
200

0
0

50

100

150

200

250

0
300

T (K)
0.06

800

Yb0.4Rh4Sb12

700

0.05

0.04
500

0.03

400

300

1/x (Mol/emu)(x)emu/mol

600

0.02
200
0.01
100

0

0
0

50

100

150

200

250

300

T (K)

74

0.08

500

Yb0.6Rh4Sb12
400

300
0.04
200

1/x (Mol/emu.)(x) emu/mol

0.06

0.02
100

0
0

50

100

150

200

250

0
300

T (K)
0.1

300

Yb0.8Rh4Sb12
250

200
0.06
150
0.04
100

0.02

1/x (Mol/emu)(x) emu/mol

0.08

50

0

0
0

50

100

150

200

250

300

T (K)
Fig 3.6.1 Temperature dependence of magnetic susceptibility χ and the inverse
magnetic susceptibility 1/χ of YbxRh4Sb12

75

10

Yb0.3Rh4Sb12
8

M (B/Yb)

2K
6

10 K
4

2

0
0

2

4

6

8

6

8

B (T)
1.5

M (B/Yb)

Yb0.4Rh4Sb12

1

2K

10 K
0.5

0
0

2

4

B(T)

76

1.2

Yb0.6Rh4Sb12

M (B/Yb)

2K
0.8

10 K

0.4

0
0

2

4

6

8

6

8

B(T)
2.5

Yb0.8Rh4Sb12
2

M ( B/Yb)

2K
1.5

10 K
1

0.5

0
0

2

4

B (T)
Fig 3.6.2 Magnetization curve of YbxRh4Sb12 at 2 K and 10 K

77

Table 3.6.1 Curie constant C, effective magnetic moment μeff and Weisstemperature θp of YbxRh4Sb12
Nominal Yb (x)Curie constant,Cμeff

θp (K)

0.3

0.35

1.68

23.37

0.4

0.45

1.90

16.03

0.6

0.79

2.52

37.90

0.8

1.21

3.10

53.94

Fig 3.6.1 shows temperature dependence of magnetic susceptibility χ at 1 Tesla andthe inverse magnetic susceptibility 1/χ of YbxRh4Sb12. The temperature dependenceofχ follows the Curie-Weiss. From the fitting linear slope of 1/χversus temperature.
When nominal Yb content increase, the effective magnetic moment is also increase,as show in table 3.6.1. It seem that the effective magnetic momentμeff are lower thanYb3+ ion (μeff = 4.54μB), suggesting Yb exhibits an intermediate valence. From themagnetization curve, no anomaly sign at 2 K and 10 K, as show in fig 3.6.2.
Magnetization cure at 2 K exhibit paramagnetic behaviour.

78

CHAPTER 4
Results and discussion of YbxRh4Sb12 (SPS)
4.1 Experimental details and Characterization
The high filling fraction ratio of YbxRh4Sb12 (x=0.7) were prepared by
combination of a high-pressure apparatus (UHP-500, Sumitomo Heavy Industries
Ltd., Japan) and spark plasma sintering.

high-pressure method, the purity ofstarting material (Yb, 99.9%, Rh 99.9% and Sb, 99.9999%) powder were mixed welland load into boron nitrate (BN) container and assemble to pyrophyllite.[3],[4] The
sample was prepared under pressure 2 GPa, overshoot temperature up to 750oC for
2 min, kept temperature 450-550oC for 120 min (heating rate at 50oC/min). The
purity of specimen was investigated by X-ray diffraction with Si powder standard(Rigaku RINT-RAPID-II). In spark plasma sintering technique, the sample after
synthesized was clean and remove BN by using sand paper and pin then ground into
powder with agate mortar. The powder was loaded to crucible and pressed into pellet,
heat to 500oC under a pressure 60 MPa by Spark Plasma Sintering (SPS). The pellet
was shape to dimension 10±0.1 mm and thickness 2±0.1 mm for thermal conductivity
measurement, using a laser flash method (NETZSCH LFA457). Electrical transport
properties (electrical resistivity and Seebeck coefficient) was measured by ZEM-1(SINKU-RIKO ULVAC ZEM-1) with sample shape in rectangular with dimension ofwidth (W), height (H) and length 2.0±0.1 mm, 2.0±0.1 mm, 8.0±0.1 mm, respectively.
The thermal transport properties and electrical transport properties were measured
from room temperature to 723K
Figure 4.1.1 shows the powder X-ray diffraction pattern of polycrystalline
YbxRh4Sb12 (x=0.7). This pattern shows skutterudite structure in a body-center cubic
crystal structure belongs to Im-3̅ space group (No. 204). RhSb2, RhSb and Sb were
found in this sample as a secondary phase. From the least square fitting method,lattice parameter shows 9.225 Å

79Intensity (arb.unit)

732

730

622
631

ox

433
600

+

420
323
422
431

312

310

o: RhSb2
x: RhSb
+: Sb

D2

ox

D1

o

C2

o

C1

o

B2
B1

o x

A2
+

A1

+
20

40

60

80

100

120

2 theta (deg)Fig 4.1.1 X-ray diffraction pattern from SPS method.
A scanning electron microscope (SEM) and using energy-dispersive X-ray (EDX), asshow result in fig 4.1.2 and table 4.1.1. The backscattering image of SPS sample isalmost homogeneous with grey color area. In black area is the roughness or cracking
in sample. Before sintering method by SPS, this stating sample have actual Yb filling
fraction about 40%. After sintering method, the actual filling ratio is found to be
Yb0.25Rh4Sb12.51. The lower filling ratio might be the effect of lower pressure in SPS
method.

80

Fig 4.1.2 backscattering image
Table 4.1.1 Actual filling faction of YbxRh4Sb12 composition analysis by SEM-EDS
Rh

Sb

Yb

gray 1

4

12.23

0.27

gray 2

4

12.01

0.29

gray 3

4

12.83

0.32

gray 4

4

12.30

0.18

gray 5

4

11.45

0.19

gray 8

4

12.66

0.21

gray 9

4

13.39

0.40

gray 10

4

12.65

0.07

gray 11

4

13.14

0.22

gray 13

4

12.47

0.32

Average

4

12.51

0.25

81

4.2 Electrical properties

Fig 4.2.1 shows the temperature dependence of electrical resistivity ρ and Fig
4.2.2 shows temperature dependence of seebeck coefficient. Electrical resistivity
exhibit semiconductor behavior, electrical resistivity decrease with temperature
increase through temperature range. Seebeck coefficient exhibit negative and increase
roughly linearly with temperature. When temperature range enters, both carrier ofelectron and hole are excited from valence band to conduction band, It can be
explained in equation (4.2.1),(4.2.2) as following
𝜎 = 𝜎𝑒 + 𝜎ℎ = 𝑒(𝑛𝑒 𝜇𝑒 + 𝑛ℎ 𝜇ℎ )𝑆=𝑆𝑒 𝜎𝑒 +𝑆ℎ 𝜎ℎ𝜎𝑒 +𝜎ℎ

4.2.1
4.2.2

Where both symbol 𝑛, 𝜇 are present carrier concentration and carrier mobility,
respectively. At low temperature, carrier mobility of SPS sample decrease andcarrier concentration increase, suggesting below 300K filling atom (Yb-atom) is
donated electron in this structure. That results according to seebeck coefficient at
low temperature range. The carrier mobility and carrier concentration were
measurement from 2-300 K, as shows in fig 4.4.3, fig 4.4.4, respectively. The fact that,Yb element is heavy element and small in ionic radii. While high Yb-filled wasinserted in large void site of Sb in Rh4Sb12 system as a result of lattice expansion andsmall energy band gap (Eg = 0.11 eV.). When temperature increase, the carrier
contribution (electron and hole) are easier to excited indication of Seebeck coefficient
become positive and electrical resistivity become small at high temperature.
At higher temperature indicates the onset of intrinsic semiconductor behavior. The
results of seebeck coefficient measurements which implies that the bipolar
conduction is at play at temperatures above 300 K. this can be a consequence of thefine grain structure of SPS sintering samples which enhances the charge carrier
scattering.
the energy band gap can be fitting from electrical conductivity or electricalresistivity and calculated from Arrhenius law,−𝐸𝑎

σ = Aexp( 𝑘𝑇 )4.2.3

where, Ea = the activation energyk = Boltzmann`s constant (1.3806 x 10-22 J/K)

82

3.5 10-5

(m)

2.5 10-5

1.5 10-5

5 10-6

0

200

400

600

800

T (K)
Fig 4.2.1 Temperature dependence of electrical resistivity
0

S (V/K)

-40

-80

-120

0

200

400

600

800

T (K)
Fig 4.2.2 Temperature dependence of seebeck coefficient.

83(cm2V-1s-1)

10

1

0.1
1

10

100

1000

T[K]
Fig 4.2.3 Temperature dependence of carrier mobilityn (cm-3)

10 24

10 23
0

50

100

150

200

250

300

T[K]
Fig 4.2.4 Temperature dependence of carrier concentration

844.3 Thermal property6 [W/k-m]

5

4

L

3

2

1

e
0
0

50

100

150

200

250

300

T [K]
Fig 4.3.1 Temperature dependence of thermal conductivity (ĸ), lattice thermal
conductivity (ĸL) and electrical thermal conductivity (ĸe) below 300 K
76 [W/k-m]

5

L

4

3

2

e

1

0
300

400

500

600

700

T(K)

Fig 4.3.2 Temperature dependence of thermal conductivity (ĸ), lattice thermal
conductivity (ĸL) and electrical thermal conductivity (ĸe) higher 300 K
85

Fig 4.3.1 show temperature dependence of thermal conductivity (ĸ), lattice
thermal conductivity (ĸL) and electrical thermal conductivity (ĸe) below 300 K and Fig

4.3.2 show temperature dependence of thermal conductivity (ĸ), lattice thermal
conductivity (ĸL) and electrical thermal conductivity (ĸe) higher than 300 K. At below

300 K, lattice thermal conductivity gradually decrease from 150 K.
At high temperature, heat flow is carried by carrier, lattice and bipolar
diffusion. The total thermal conductivity increases due to the intrinsic excitation as
observed in the electrical properties (bipolar diffusion). In bipolar thermal diffusion(𝜅𝑏 ) is an appreciable by both charge carriers in electrical conductivity and Seebeckcoefficient, can estimated by𝜎 𝜎𝜅𝑏 = 𝜎 𝑒+𝜎ℎ (𝑆𝑒 − 𝑆ℎ )2 . 𝑇𝑒Where symbol of 𝑒 𝑎𝑛𝑑 ℎ

ℎ

4.3.1

are present electron and hole, respectively and T

is absolute temperature. The total thermal conductivity increases over the entire
temperature range, which mean that bipolar diffusion had occurred. This result
suggests that, both carrier in intrinsic and extrinsic were activated and vibrated
with lattice vibration inside void site in Sb atom as a result of reduction lattice
thermal conductivity by heat transfer dependent temperature. Bipolar thermal
diffusion has been reported in alkaline earth and rare earth doped small band-gap
skutterudite structures.

864.4 Figure of merit
0.12

ZT

0.08

0.04

0

0

200

400

600

800

T [K]

Figure 4.4.1 temperature dependence of figure of merit

Figure 4.4.1 shows the temperature dependence of dimensionless figure of
merit ZT. The maximum ZT value of YbxRh4Sb12 (x=0.7) reaches 0.1 at 420K. The ZT
exhibit low value might be the effect of transport carrier of electron and hole. It is
expected that double fill, triple fill or substitution should be insert in Yb-doped
Rh4Sb12 structure for improve higher thermoelectric performance.

87CHAPTER 5
SUMMARY
5.1 Summary
Skutterudite structure of YbxRh4Sb12 were synthesis under high pressure and hightemperature method. Because of skutterudite structure have high performance ofthermoelectric properties. Filled - into CoSb3 based skutterudite have been best forcandidate especially heavy rare earth element. From previous report, YbxCo4Sb12 are
successfully to filling with maximum of Yb nearly theoretical by using high pressure
method. So, Yb-atom was tried to filling in Rh4Sb12 system because this structure
have large void site than Co4Sb12. Moreover, Yb-atom have small and heavy atoms
that should be more effective in reduce lattice thermal conductivity and improve
performance of thermoelectric properties.
In this experiment, the maximum Yb-filled in Rh4Sb12 system was found to be 0.4 or
the filing fraction ratio up to 40%. As a matter of fact, Thermal transport properties
have effect on efficiency of thermoelectric properties, is clearly seen in Yb-content(x=0.8), When consider only electronic transport properties, the PF value reach up
to 3.6 µW/cmK2 at 300 K. While thermal conductivity reducing with temperature
increase, the ZT value increase to 0.042 at 300K for YbxRh4Sb12 (x=0.8). Thisobviously seen that the reduction of thermal conductivity because phonon interact
between carrier and lattice structure in compounds attributed to the rattling motionof the heavy rare earth element in its Sb atom. At maximum filling fraction ratio of
YbxRh4Sb12 (x=0.6) exhibit show small ZT because large electrical resistivity, indicatethat small energy band gap in this compound. It is expected that further onsubstitution effect and combination method would result in even higher performancen-type skutterudite material.
To enhancement efficiency of thermoelectric of YbxRh4Sb12. The maximum of actual
filling fraction was sintering by using spark plasma sintering (SPS). Yb-atom
insertion into void-site of Rh4Sb12 had effect on Seebeck coefficient and thermal
conductivity, especially in lattice thermal conductivity at elevated temperature. This
result exhibit low ZT due to high thermal conductivity that results from excitation
in intrinsic.

88

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ACKNOWLENDGEMENTFirstly, I would like to express my sincere gratitude to my advisor Prof.
Chihiro SEKINE for the continuous support of my Ph.D study and related
research, for his patience, motivation, and immense knowledge. His
guidance helped me in all the time of research and writing of this thesis. I
could not have imagined having a better advisor and mentor for my Ph.D
study. Besides my advisor, I would like to thank Assis. Prof. Yukihiro
KAWAMURA and technical staff Junichi HAYASHI who kindly supportingin experiment detail and analysis.
My thanks also goes to Dr. Chul-Ho LEE, Dr. Kunihiro KIHOU, Hirotaka NISHIATE,
who provided me an opportunity to join their team as intern, and who gave
access to the laboratory and research facilities. Without they precious support
it would not be possible to conduct this research.

A special thanks to all student in Sekine lab member, who help me inexperiment, living life support, all happy day.