DEVELOPMENT OF NANOCRYSTALLINE Fe80Cr20 ALLOY USING

COMBINATION TECHNIQUE OF BALL MILLING AND

ULTRASONIC TREATMENT FOR FUEL CELL

INTERCONNECTOR

DAFIT FERIYANTO

A thesis submitted in fulfillment of the requirement for the award of the

Degree of Master of Mechanical Engineering

Faculty of Mechanical and Manufacturing Engineering

Universiti Tun Hussein Onn Malaysia

December 2014

v

ABSTRACT

Solid Oxide Fuel Cell (SOFC) system consists of anode, cathode, electrolyte and

interconnect. This research is focused on interconnect material. The objective of this

study is to explore the high energy ball milling (milled) combined with ultrasonic

treatment (UT) in obtaining smaller crystallite size, finer surface morphology, higher

thermal stability and more homogenous nanocrystalline Fe80Cr20 alloys. This

condition was motivated by the previous research that some of the grain growth was

observed in a high temperature. At first, this process was carried out by high energy

ball milling with milling time of 60 h and later, the samples experienced the

ultrasonic treatment with frequency of 35 kHz at various periods of 3 h, 3.5 h, 4 h,

4.5 h, and 5 h. Moreover, it was found that there are no works on these combination

treatments (milled and UT). Characterization and analysis were carried out to all

samples by using X-Ray Diffraction (XRD), Scanning Electron Microscope (SEM)

and Energy dispersive X-ray Diffraction (EDS), Thermo Gravimetric Analysis

(TGA) and Particle Size Analyzer (PSA). The results showed that the combination

treatment samples increases effectively to the solid solubility of Cr to Fe up to 62.1%

and decreased the crystallite size up to 2.71 nm at milled and UT 4.5 h sample, these

resulted and produces finer surface structure. From EDS results, the combination

treatment samples are at suitable composition of 20.05 wt% Cr and 79.95 wt% Fe as

compared to other samples. Higher thermal stability was observed on combination

treatment sample at 1100 0C up to 12.7 mg or convenient to 63 wt%, 62 wt% and 25

wt% as compared to raw material, UT samples and milled 60 h sample, respectively.

The particle size decreased up to 5.23 µm and particle size distribution of

combination treatment relatively increased up to 89.57%. It can be concluded that the

combination treatment at milled and UT 4.5 h is appropriate to achieve high solid

solubility, nano crystallite size, fine surface morphology, high thermal stability and

homogenous Fe80Cr20 alloys.

vi

ABSTRAK

Sistem sel fuel oksida pepejal (SOFC) terdiri daripada anod, katod, elektrolit dan plat

penyambung. Perkara utama yang ditekankan dalam kajian ini ialah bahan plat

penyambung yang digunakan. Objektif kajian ini adalah untuk memperoleh saiz

hablur Fe80Cr20 yang lebih kecil, permukaan morfologi yang lebih halus, kestabilan

terma yang lebih tinggi serta mempunyai nano hablur yang lebih seragam dengan

menggunakan mesin bebola pengisar yang digabungkan dengan rawatan ultrasonik.

Kaedah ini berdasarkan kajian lepas yang menunjukkan pertumbuhan butiran berlaku

pada suhu tinggi. Pada mulanya, proses dimulakan dengan mengisar bahan dengan

meggunakan bebola pengisar bertenaga tinggi dalam tempoh 60 jam. Kemudian

bahan diberi rawatan ultrasonik dengan frekuensi 35 kHz pada tempoh yang berbeza

iaitu 3, 3.5, 4, 4.5 dan 5 jam. Tambahan lagi, sebelum ini tiada kajian yang

menggunakan teknik gabungan antara pengisaran dan rawatan ultrasonik. Perincian

dan analisis bahan diperolehi menggunakan belauan sinar X (XRD), mikroskop

imbasan elektron (SEM), tenaga serakan belauan sinar X (EDS), analisis gravimetrik

terma (TGA) dan penganalisis saiz zarah (PSA). Keputusan menunjukkan kaedah

penggabungan teknik pengisar dan UT 4.5 jam meningkatkan kelarutan pepejal Cr ke

Fe sehingga 62.1 %, mengurangkan saiz hablur sehingga 2.71 nm dan menghasilkan

struktur permukaan yang lebih halus. Keputusan daripada EDS, penggabungan

rawatan bahan adalah sesuai pada komposisi 20.05 wt% Cr and 79.95 wt% Fe

berbanding dengan sampel lain. Penggabungan rawatan bahan pada suhu 1100 0C

mempunyai penambahan berat 12.7 mg yang menunjukkan ketahanan panas yang

tinggi berbanding dengan bahan mentah, UT dan pengisar 60 jam menunjukkan

pengurangan berat sebanyak 63 wt%, 62 wt% and 25 wt%. Penggabungan rawatan

mengurangkan saiz partikel sehingga 5.23 µm dan pengagihan saiz meningkat secara

relatif sehingga 89.57%. Secara kesimpulannya penggabungan rawatan pengisar and

UT 4.5 jam adalah yang paling sesuai untuk mendapatkan kelarutan pepejal yang

tinggi, saiz nano hablur, permukaan morfologi yang halus, kestabilan terma yang

tinggi serta mempunyai nano hablur yang seragam.

vii

TABLE OF CONTENT

TITLE i

DECLARATION ii

DEDICATION iii

ACKNOWLEDGMENT iv

ABSTRACT v

TABLE OF CONTENT vii

LIST OF TABLES xi

LIST OF FIGURES xiii

LIST OF SYMBOLS AND ABBREVIATION xvi

CHAPTER 1 INTRODUCTION

1.1 Research background 1

1.2 Problem statement 3

1.3 Hypothesis 4

1.4 Research objectives 5

1.5 Scope of study 5

CHAPTER 2 LITERATURE REVIEW

2.1 Introduction of SOFC 7

2.1.1 SOFC Material 8

2.1.2 Crystal structure for solid oxide fuel cell (SOFC) 9

2.1.3 Operating Principle of SOFC 10

2.1.4 SOFC component 12

2.1.4.1 Anode 12

2.1.4.2 Cathode 13

2.1.4.3 Electrolyte 15

viii

2.1.4.4 Interconnect 17

2.2 Oxidation of metallic alloy 18

2.3 Metallic interconnect 20

2.3.1 Chromium based alloys 22

2.3.2 Iron based alloys 24

2.3.3 Iron-Chromium Alloys 25

2.4 Mechanical alloying 27

2.4.1 Control agent in ball milling process 29

2.4.2 Temperature of ball milling 30

2.4.3 Temperature increase during ball milling process 31

2.4.4 Ball milling mechanism 32

2.4.5 Ductile-ductile Fe and Cr sample 34

2.4.6 Solid solubility 35

2.5 Nanocrystalline 37

2.6 Effect of crystallite size 39

2.7 Determining crystallite size 40

2.7.1 Measuring interplanar spacing and lattice parameter 41

2.7.2 Measuring solid solubility (Xss) 43

2.7.3 Measuring Bcorrected 43

2.7.4 Measuring crystallite size and strain of the material 45

2.8 Particle size and distribution 47

2.9 Ultrasonic treatment 48

2.9.1 Maximizing the cavitations 51

2.9.2 Mechanism of ultrasonic treatment 51

2.9.3 Broader application area for ultrasonic treatment 52

2.10 Summary of literature review 53

CHAPTER 3 METHODOLOGY

3.1 Sample designation 55

3.2 Flow chart 56

3.3 Starting material 58

ix

3.4 Powder blending 58

3.5 Ultrasonic treatment 62

3.6 Combination technique 64

3.7 Characterization and analysis 65

3.7.1 X-Ray diffraction analysis 65

3.7.2 Morphology characterization 66

3.7.3 Thermal analysis 67

3.7.4 Determining particle size and distribution 67

CHAPTER 4 RESULT AND DISCUSSION

4.1 XRD pattern analysis 69

4.1.1 XRD pattern of raw material (Fe80Cr20) 70

4.1.2 XRD pattern of the ultrasonic treatment (UT)

samples and raw material

71

4.1.3 XRD peaks of the milled 60 h and combination

technique (milled and UT) Samples

73

4.1.4 Comparison XRD pattern of treated and untreated

samples

75

4.2 Determining crystallite size using Williamson-Hall method 76

4.2.1 Crystallite size and strain of the raw material and

the ultrasonic (UT) samples

77

4.2.2 Crystallite size and strain of the milled 60 h and

combination technique (milled and UT) samples

79

4.2.3 Comparison of crystallite size and strain of treated

and untreated samples

82

4.3 The relation between lattice parameter and the solid

solubility (Xss)

85

4.4 Surface morphology of the treated and untreated samples 87

4.4.1 Surface Morphology of Cr and Fe powder as raw

materials

88

4.4.2 Surface morphology of ultrasonic treatment (UT)

x

samples 89

4.4.3 Surface morphology of milled 60 h and

combination technique(milled and UT) samples

90

4.5 Identification of the chemical composition 93

4.6 Determining the thermal stability 95

4.6.1 Thermal stability of the UT samples 96

4.6.2 Thermal stability of the milled 60 h samples and

combination technique samples

97

4.6.3 Comparison thermal stability of treated and

untreated samples

100

4.7 Particle size and distribution of the treated and untreated

samples

102

4.8 Summaries 104

CHAPTER 5 CONCULSIONS AND RECOMMENDATIONS

5.1 Conclusions 106

5.2 Recommendations 107

REFERENCE 109

APPENDIX A XRD analysis 122

APPENDIX B SEM analysis 135

xi

LIST TABLES

Table No. Item Page

Table 2.1 Type of material SOFC components within several operation

temperatures

8

Table 2.2 Atomic radius and crystal structures for 16 metals 10

Table 2.3 Advantages and disadvantages of several type anode materials 12

Table 2.4 Advantages and disadvantages of several type cathode materials 14

Table 2.5 Advantages and disadvantages of several type electrolyte

materials

16

Table 2.6 Advantages and disadvantages of several types interconnect

materials

17

Table 2.7 Varies metallic candidate materials for interconnect application 21

Table 2.8 Material properties of chromium powder 23

Table 2.9 Material properties of iron powder 24

Table 2.10 Temperature rise during ball milling process 32

Table 3.1 Samples designation 56

Table 3.2 Detail information of raw material 58

Table 3.3 Various diameter and amount of ball mill 59

Table 3.4 Technical data of ultrasonic bath machine 63

Table 4.1 XRD data of the Fe80Cr20 alloys powders 70

Table 4.2 Result of XRD analysis of the treated and untreated samples 76

Table 4.3 Crystallite size and strain of the sample with ultrasonic treatment 78

xii

Table 4.4 Crystallite size and strain of the Fe80Cr20, milled 60 h and

combination technique

80

Table 4.5 Comparison crystallite sizes of the milled 60 h sample and

combination technique samples

81

Table 4.6 Crystallite size and strain of the treated and untreated samples 83

Table 4.7 Comparison crystallite size of the treated and untreated samples 84

Table 4.8 The solid solubility (xss) of Cr into Fe 86

Table 4.9 TGA data of the treated and untreated samples 100

Table 4.10 Comparison mass gain of the treated and untreated samples 101

Table 4.11 Particle size and distribution of the treated and untreated samples 102

Table 4.12 Comparison particle size of the treated and untreated samples 104

Table 4.13 The correlation of the all results 104

xiii

LIST OF FIGURES

Figure No. Item Page

Figure 2.1 Configuration for planar design SOFC 8

Figure 2.2 Unit cell of crystal structure 9

Figure 2.3 Operating Concept of a SOFC 11

Figure 2.4 Anode with Ni-YSZ material 13

Figure 2.5 Cathode with LaMnO3 material 14

Figure 2.6 Temperature dependence of conductivity for undoped and Sr-

dopend LaMnO3

15

Figure 2.7 TGA oxidation curve 19

Figure 2.8 Phase diagram of Fe-Cr system 26

Figure 2.9 Oxidation in air of a Fe 26 wt% Cr 1 wt% Mo alloy 27

Figure 2.10 Grain size vs milling time for CoZr 31

Figure 2.11 Collision of the powder when mixture during mechanical

alloying

33

Figure 2.12 Particle size distribution at various ball milling times under

steady-state conditions

33

Figure 2.13 Refinement of particle and grain sizes with milling time 34

Figure 2.14 Deformation characteristic of starting powder in ball milling

process

35

Figure 2.15 Schematic diagram of solid solubility vs milling 35

Figure 2.16 Solid solubility of Fe in Cu 36

Figure 2.17 Classification of nanocrystalline materials 37

Figure 2.18 Volume fractions of the crystallite, grain boundary, triple line

and quadruple node as a function of crystallite size

38

Figure 2.19 Grain size and hardness of a ball milled Fe20Cr alloy versus

annealing time at 6000C

40

Figure 2.20 X-Ray Diffraction 42

xiv

Figure 2.21 FWHM of the standard reference material for LaB6 on an in-

house Cu K diffractometer

44

Figure 2.22 Principle of the laser diffraction 47

Figure 2.23 Schematic of particle size analyzer 48

Figure 2.24 Sound frequency ranges 49

Figure 2.25 Microscopy analysis of FeCrY2O3 alloy by SEM treated by

ultrasonic with frequency 22 kHz and holding time of 2.5 h

50

Figure 2.26 Eutectic silicon morphology of a AlSi9Cu3: (a) non-US treated

and (b) US treated at 400 W and 615 0C melt temperature

53

Figure 3.1 Flow chart of the research works 57

Figure 3.2 Various size of ball mill media 59

Figure 3.3 The grinding jar 60

Figure 3.4 Glove box 60

Figure 3.5 RETCH PM 400 planetary ball mill 61

Figure 3.6 Schematic depicting of the ball motion inside ball milling

machine

61

Figure 3.7 Schematic diagram of ultrasonic treatment 62

Figure 3.8 Fe and Cr powder before XRD process 65

Figure 3.9 Fe and Cr powder for SEM preparation 66

Figure 3.10 CILAS 1180 particle size Analyzer (PSA) 68

Figure 3.11 Schematic diagram of CILAS 1180 particle size analyzer work 68

Figure 4.1 Planes crystal structure of Fe80Cr20 69

Figure 4.2 XRD peaks of the Fe80Cr20 alloys powders 70

Figure 4.3 XRD peaks of the raw material and UT treatment samples 72

Figure 4.4 XRD peaks of the raw material, milled 60 h sample and

combination technique (milled and UT) samples

74

Figure 4.5 XRD pattern of the treated and untreated samples 75

Figure 4.6 Crystallite size vs strain of the raw material and UT treatment

samples

79

Figure 4.7 Crystallite size vs strain of the milled 60 h and combination

xv

technique samples 81

Figure 4.8 Effect of strain on the diffraction peaks and crystallite size 82

Figure 4.9 Crystallite size and lattice parameter of treated and untreated

samples

83

Figure 4.10 Crystallite size and strain of treated and untreated samples 84

Figure 4.11 Lattice parameter vs solid solubility of the treated and untreated

samples

86

Figure 4.12 SEM morphology images of the raw material 88

Figure 4.13 Surface morphology of a); Ultrasonic treatment 3 h; b):

Ultrasonic treatment 3.5 h; c): Ultrasonic treatment 4 h; d):

Ultrasonic treatment 4.5 h; e): Ultrasonic treatment 5 h

89

Figure 4.14 Surface morphology of a); milled 60 h; b) milled and UT 3 h;

c): milled and UT 3.5 h; d): milled and UT 4 h; e): milled and

UT 4.5 h; f): milled and UT 5 h

91

Figure 4.15 EDS result of Fe80Cr20 alloy powder 93

Figure 4.16 EDS result of UT 4.5 h 94

Figure 4.17 EDS result of Milled 60 h 94

Figure 4.18 EDS result of milled and UT 4.5 h 94

Figure 4.19 Mass gain result of the raw material and UT samples 96

Figure 4.20 Mass change derivative of the raw material and UT treatment

samples

97

Figure 4.21 Mass gains of the milled 60 h sample and combination

technique samples

98

Figure 4.22 Mass change derivative of the raw material and UT treatment

samples

99

Figure 4.23 Mass gain vs crystallite size of the treated and untreated

samples

101

Figure 4.24 Particle size vs distribution size of the treated and untreated

samples

103

xvi

LIST OF SYMBOLSAND ABBREVIATION

SOFC - Solid Oxide Fuel Cell

SOx - Sulfur Oxide

NOx - Nitrogen Oxide

CO2 - Carbon Dioxide

CO - Carbon Monoxide

CH4 - Methane

rad - radian

H2 - Hydogen

µm - Micrometer (10-6

meter)

nm - Nanometer (10-9

meter)

h - Hours

YSZ - Yttrium Stabilized Zirconia

LaMnO3 - Lanthanum Manganite

Ni - Nickel

H2O - Air

Fe - Iron

FeO - Iron Oxide

Cr - Chromium

Fe-Cr - Iron Chromium

Cr2O3 - Chromium Oxide

BCC - Body Centered Cubic

FCC - Face Centered Cubic

HCP - Hexagonal Close-Packed

Å - Angstrom

D - The diameter crystallite size (nm).

k - The shape factor (k=0.9).

- Wavelength of the X-Ray ( =0.154056 nm for Cu-K ).

xvii

- Bragg diffraction angle.

B - The broadening of the diffraction peaks measured at half of its

maximum intensity (in radians).

Bcorrected - The instrumental corrected Full Width at Half Maximum

(FWHM) in radians

Bobs - The Full Width at Half Maximum (FWHM) from diffraction

peak of the specimens

Binstrument - Standard peaks of the standard reference materials

- The strain present in the material

d - Interplanar spacing (Å)

a - Lattice parameter

Xss - Solid Solubility (%)

WD - Work Distance (mm)

kV - Kilo Volt

mm - Millimeter

kHz - Kilo Hertz

SPS800 - Spark Plasma Sintering at 800 0C

UTHM - University Tun Hussein Onn Malaysia

TGA - Thermo Gravimetric Analysis

EDS - Energy Dispersive X-Ray Diffraction

PSA - Particle Size Analyzer

FeCrAl - Iron Chromium Aluminum

Fe80Cr20 - Composition of alloy powder with 80 wt% Fe and 20 wt% Cr

UT - Ultrasonic Treatment

Milled - Ball milling technique

Milled and UT - Combination technique ( Ball milling 60 h and ultrasonic

treatment)

MA - Mechanical Alloying

XRD - X-Ray Diffraction

SEM - Scanning Electron Microscopy

PCA - Process Control Agent

xviii

CTE - Coefficient of Thermal Expansion

LaCrO3 - Lanthanum Chromites

LPG - Liquefied Petroleum gas

INCO - International Nickel Company

HVOF - High Velocity Oxy-Fuel

YSZ - YttriaStabilizedZirconia

SCZ - Scandia Stabilized Zirconia

LSGM - Lanthanum Strontium Gallium Magnesium

CGO - Cerium Gadolinium Oxide

LSM - Lanthanum Strontium Manganite

LSCF - La0.6Sr0.4Co0.2Fe0.8O3

Ni-YSZ - Nickel-Yttria Stabilized Zirconia

Ni-SDC - Nickel-Samaria Doped Ceria

LCR - Lanthanum Chromites/LaCrO3

RSP - Rapid Solidification Processing

CCD - Charged Coupled Device

DSP - Digital Signal Processor

APU - Auxiliary Power Units

S/cm - Siemens per centimetre

CHAPTER 1

INTRODUCTION

This chapter consists of the research background, problem statement, hypothesis,

research objectives and scope of study.

1.1 Research Background

Solid Oxide Fuel Cells (SOFC) research develop most interest currentlyas its

potential to create an efficient system, high energy-density power generation device

which typically operates at high temperatures relatively (≅1000 0C). Nowadays,

research in SOFC focuses on the interconnect material because it must be stable

chemically in both of anode and cathode at temperature approximately 10000C

(Zhang, 2006 and Zhong et al., 2004), as well as grain growth on that temperature

was observed by Saryanto et al., (2009); Pudji et al.,(2009); Darwin et al.,(2010a)

and Deni (2011). The interconnect is required to connect thermally the different cells

and also providesthe physical barriers to keep the oxidant and fuel separated

(Yaodong et al., 2007). Metallic interconnect has interest material to promote as

interconnect because their high thermal conductivities (Deni, 2011 and Khaerudini et

al., 2012). In the current research, there is high interest in nanocrystalline iron and

chromium based alloys. The iron-chromium system has long been used by many

engineering alloys as the basis in high-strength and corrosion-resistant applications

such as for fuel cell interconnect. Traditionally, iron-based alloy with additions of

chromium has been used in high temperature (up to 10000C) applications (Basu,

2007), where the stabilization and oxidation behavior at high temperature are

fundamental in actual applications. Chromium has a phase stabilizer when added into

the iron. It is because the chromium has the same BCC crystal structure with iron and

2

made primarily to promote the formation of a dense, adherent layer of Cr2O3 on the

surface of the alloy (Craig, 1995).

Developing Fe80Cr20 based alloy using mechanical alloying via high energy

ball milling was successfully carried out by Saryanto et al., (2009). Mechanical

alloying (MA) are in general micro and nano structured often have remarkable

properties. The product depends on many parameters such as the milling conditions,

amount material which are being processed, type of the ball mill used and milling

time (Suryanarayana, 2001 and Suryanarayana, 2004). Mechanical alloying

technique is able to synthesize single-phase FeT (T = Cr, Cu and Ni) binary alloys

with average crystalline size below 50 nm and exhibiting a variety of magnetic

responses (Martinez-Blanco et al., 2011). Therefore, it is very challenging to develop

nanocrystalline FeCr alloy, because the Cr metal encourage the formation of

protective oxide (scale) (Quadakkers et al., 2003). Nanoscale particle research has

become a very important field in materials science recently. A nanocrystalline

material was characterized by a microstructural length scale in 1 to 100 nm range

(Koch, 2003). Nano powders or nano materials with physical, chemical, and the

mechanical properties can be utilized as the main building of innovative solutions for

the problems in energy, environment, health and communication (Ozlem et al.,

2012). Nanoscale particles usually have different physical properties compared with

large particles. It had been found that nanoparticles exhibit variety of previously

unavailable properties including magnetic, optical, and other physical properties as

well as the surface reactivity depending on particle size (Poole et al., 2003).

Therefore, heat-resistant nanoscale particle alloys is produced in the industrial scale

and widely used in different fields of science and technology.

In the area of ultrasonic processing in liquid media, the development of the

power generators family with extensive radiators has strongly contributed to the

implementation at semi-industrial and industrial stage of several commercial

applications. Some of the applications are in food industry, environment and process

for manufacturing (Juan et al., 2010). The use of high-intensity ultrasonic enhances

the reactivity of metals as stoichiometric reagents becamean important synthetic

technique for many heterogeneous organic and organometallic reactions, especially

for the involving reactive metals. In particular for small matter from several

nanometers on couple of microns, ultrasonic is very effective in breaking

agglomerates, aggregates and even primaries (Thomas et al., 2006; Pudji et al., 2009;

3

Krisztian et al., 2010 and Darwin et al., 2010b). The mechanism of the rate

enhancements in reactions of metals had been done by monitoring the effect of

ultrasonic vibration on the kinetics of chemical reactivity of solids, examining the

effects of vibration on the surface morphology and size distributions of powders

(Suslick, 1994; Kenneth et al., 1999; Pudji et al., 2009 and Darwin et al., 2010b).

Ultrasonic treatment also effectively refines the surface morphology on FeCrAl

substrate (Yanuandri, 2011 and Ade, 2012).However, the exploration and

improvement novel processes enhance the reducing of finer crystallite size and

homogenous distribution crystalline size of iron-chromium is still very challenging.

Many researchers field investigate the nano material using ball milling and

ultrasonic treatment in mixing raw material (Suryanarayana, 2001and Quadakkers et

al., 2003; Suryanarayana, 2004; Saryanto et al., 2009; Pudji et al., 2009; Darwin et

al., 2010a; Ade, 2012 and Ozlem et al., 2012). However, unsatisfactory performance

still resulted when applied at high temperature operation such as grain growth, rough

surface morphology, high oxidation, and the composition not appropriate with

working composition. No studies have yet been done on using combination

technique between ball milling and ultrasonic treatment. This study focus on the

development of iron-chromium alloy powder promoted using combination technique

in order to achieve smaller crystallite size, finer the surface morphology, more

homogenous and better thermal stability of Fe80Cr20 alloys powder.

1.2 Problem statement

The challenge in this research is how to produce the nano range crystallite size, finer

surface morphology, high thermal stability and homogenize powders size. Ball

milling process is significant to reduce the crystallite size into nano range size.

Nevertheless, processing techniques involves high-temperature conditions at longer

processing times cause the growth rate of crystallite size inevitable (Suryanarayana,

2001; Suryanarayana, 2003; Quadakkers et al.,2003 and Suryanarayana, 2004). In

previous research, grain growth from 5.82 nm to 38.51 nm occurred (Hendi, 2011and

Deni, 2011). Meanwhile, Ultrasonic process with fixed frequency develop and

4

achieve homogenous and fine grain structure alloys powder (Suslick, 1988; Kenneth

et al.,1999; Mikko et al., 2004; Thomas Hielscher, 2005; Darwin et al., 2010b; Ade,

2012 and Pudji et al., 2009). According toYanuandri, (2011) and Ade, (2012), the

frequency was conducted in 18.52 kHz and holding time of 10, 20, and 30 minutes

which were obtained the rough surface and less homogenous. Nanoscale material

have many advantages whencompared with large particle. It was proved by Hendi

(2011) and Deni (2011) that the cracks in commercial ferritic steel observed after

consolidated process at temperature of 900 0C. It means that large particle have low

thermodynamic stability when consolidated at temperature of 900 0C and above.

Therefore, the combination technique of ball milling and untrasonic treatment is

needed to obtain smaller grain size, finer surface morphology and more homogenous

crystallite sizes in nano range size.

1.3 Hypothesis

The hypothesis consider in this research are:

1. Ball milling will be effective to reduce the powder size into nanometer range

size.

2. Ultrasonic treatment will be improved the homogeneity and finer surface of

morphology.

3. Combination technique of ball milling and ultrasonic will be developed new

Fe80Cr20 alloy powder which have smaller crystallite size, more homogenous,

finer surface morphology and higher thermal stability.

5

1.4 Research objectives

Researchobjectives in this study are:

i. To develop nanostructured Fe80Cr20 alloy by using ultrasonic treatment, ball

milling and combination technique (milled combined with ultrasonic

treatment).

ii. To investigate the influence of different ultrasonic treatment time on particle

size and particle distribution.

iii. To investigate the influence of combination technique (milled combined with

ultrasonic) on powder size, homogeneity, fine surface morphology and

thermal stability.

1.5 Scope of study

The scopes of this research are:

1. Raw material is arranged by 80 wt% Fe, and 20 wt% Cr.

2. Developing the nanocrystalline Fe80Cr20 alloy is conducted by using ball

milling with milling time of 60 h. According to Hendi (2011), that the milling

time of 60 h is the most effective to develop nanocrystalline Fe80Cr20 alloy as

compared to higher milling time of 80 h.

3. Only nitrogen gas is used in high energy ball milling process

4. Ultrasonic process is introduced at appropriate frequency of 35 kHz and

holding times of 3 h, 3.5 h, 4 h, 4.5 h and 5 h.

5. The atmospheric pressure while ultrasonic treatment, characterization and

analysis process is assumed 1 atm.

6. The microstructure is observed by using Scanning Electron Microscopy

(SEM) and the composition of the individual particle is analyzed by using

Energy Dispersive X-Ray Spectroscopy (EDS).

7. Phase analyses is conducted by using X-Ray Diffraction (XRD).

6

8. Thermal analysis is conducted by using Thermo Gravimetric analysis (TGA)

up temperature of 1100 0C which are carried out to the all samples (treated

and untreated samples).

9. Determination of particle size and its distribution of particle size is done by

using Particle Size Analyzer (PSA).

CHAPTER 2

LITERATURE REVIEW

This literature related to this research is discussed in this chapter. It consists of the

theory about Solid Oxide Fuel Cell (SOFC), interconnect, developing nanocystalline

material, homogenizing, and refining the surface morphology.

2.1 Introduction of SOFC

According to Zhang (2006), Solid Oxide Fuel Cell (SOFC) is a promising technology

for electricity generation. It converts the chemical energy of the fuel gas to electrical

energy directly. Therefore, the electrical efficiencies can be achieved through this

system. The system can reach efficiency approximately 80% (efficiency of internal

combustion engine is no more 40%) when couple it with heat recovering system for

cogeneration (heating applications and electric power). Besides, SOFC can replace

several fuels and widely implemented to power supplies from small-scale

distribution power supplies to large-scale thermal power generation (Blein, 2005).

8

2.1.1 SOFC Material

The structure of SOFC is well stacked which consists of anode, electrolyte, cathode,

and interconnect materials as in Figure 2.1 (Quadakkers, 2003). Besides, the material

properties for SOFC component in various temperatures are tabulated in Table 2.1.

Figure 2.1 Configuration for planar design SOFC (Singhal, 2000 and Quadakkers,

2003)

There are many types of materials for SOFC components which are applied

by researcher to develop the efficient SOFC system. The most commonly material

which are used in the SOFC components and it is depend on the temperature

application as listed in the Table 2.1:

Table 2.1 Type of material SOFC components within several operation temperatures

(Rajendra, 2007)

SOFC operating

temperature

(0C)

Material

Cell

design

Electrolyte

thickness

(µm)

Electrolyte

Cathode

Anode

Interconnect

950-1000 YSZ LSM Ni-YSZ LCR Tubular 40

Planar 150-300

650-800 YSZ, SCZ LSM Ni-YSZ Ferritic steel Planar 5-20

400-650

LSGM,

CGO

LSCF

Ni-SDC

Ferritic steel Planar 500 or less

Stainless steel Planar 5 (LSGM)

Stainless steel Planar 10-30 (CGO)

Cell

Repeat

Unit

Current flow

Fuel

Interconnect

Electrolyte

Anode

Cathode

Air

interconnect

9

2.1.2 Crystal structure for solid oxide fuel cell (SOFC)

A crystalline material is one of atoms which are situated in a repeating or periodic

array over large atomic distances. Long-range crystal is observed after consolidation

the atoms. It will position itself in a repetitive three-dimensional pattern (Figure 2.2).

All metals, many ceramic materials, and certain polymers form crystalline structures

under normal solidification conditions (Smith, 2004).

Some of the properties of crystalline solids depend on the crystal structure of

the material, the manner in which atoms, ions, or molecules are arranged spatially.

The crystal structure has various structures from simple structure to complex

structure. A unit cell is chosen to represent the symmetry of the crystal structure;

where all of atom positions in the crystal may be generated by translations of the unit

cell integral distances along each its edges. Thus, the unit cell is the basic structural

unit or building block of the crystal structure and defines the crystal structure by

virtue of its geometry and the atom inside positions (Smith, 2004).

Figure 2.2 Unit cell of crystal structure (Smith, 2004)

Study of structural, electronic and magnetic behaviour of CuxFeCr1−xO2 has

been investigated by Osman (2013), they found that the partial crystal structure

deformation from delafossite CrFeO2 structure to corundum-type FeCrO3 structure is

contain CrO2 and Cr2O3 blocks. The crystal geometry is change when the Cr

substitution is increased. Therefore, the FeCr based alloys is used in this research in

order to minimize the change of crystal geometry.

10

Three simple crystal structures relatively are found for the most common

metals which are listed in Table 2.2: Face Centered Cubic (FCC), Body-Centered

Cubic (BCC) and Hexagonal Close-Packed (HCP).

Table 2.2 Atomic radius and crystal structures for 16 metals (Smith, 2004)

Metal Crystal

structurea

Atomic radiusb

(nm)

Metal Crystal

structure

Atomic

radius(nm)

Aluminium FCC 0.1431 Molybdenum BCC 0.1363

Cadmium HCP 0.1490 Nickel FCC 0.1246

Chromium BCC 0.1249 Platinum FCC 0.1387

Cobalt HCP 0.1253 Silver FCC 0.1445

Copper FCC 0.1278 Tantalum BCC 0.1430

Gold FCC 0.1442 Titanium (α) HCP 0.1445

Iron (α) BCC 0.1241 Tungsten BCC 0.1371

Lead FCC 0.1750 Zinc HCP 0.1332

aFCC (Face-Centred Cubic); HCP (Hexagonal Close-Packed); BCC (Body-Centred

Cubic).

bA nanometer (nm) equals 10

-9 m; to convert from nanometer to Angstrom units (Å),

multiply the nanometer value by 10.

2.1.3 Operating Principle of SOFC

The cell is constructed with two porous electrodes with an electrolyte in the middle.

Initially, the catalytically reaction can be described as follows, oxygen flows along

cathode in order to contact among cathode and electrolyte interface for importing the

ion oxygen to electrolyte and pass it towards anode. That reaction produces the

water, carbon dioxide, heat, and the electrons (Gorte et al., 2003). However, the

electron flow will occur when the electrical connection between the cathode and the

anode emerged, where a continuous supply of oxygen ions (O2-

) generated from

cathode to anode in order to maintain the overall electrical charge balance.

Ultimately, this process produced the residual product pure water (H2O) and heat.

The operating of SOFC technology is shown in Figure 2.3.

11

Figure 2.3 Operating Concept of a SOFC (Greneir et al., 1995; EG&G, 2002;

Benjamin, 2004; Michael et al., 2006 and Hendi, 2011)

In real application, fuel cells are connected in a series of cells, in order to

obtain higher outlet voltage. An interconnect plate is installed to provide the

electronic contact between the anode of one cell and the cathode of the next cell

(Navadol et al., 2009).

Hydrogen, carbon monoxide (CO) and hydrocarbons such as methane (CH4)

can be used as fuels in SOFC system. The fuel stream reacts with oxide ions (O2-

)

from the electrolyte to produce the water or CO2 and to deposit electrons into the

anode. The electrons pass outside fuel cell through the load and back to the cathode

where oxygen from air receives the electrons and its converted into the oxide ions

which is injected to the electrolyte (EG&G, 2002). At the temperature of 10000C, the

CO and CH4 direct oxidation will occur but it is less favoured due to the shift of the

water gas from CO to H2 and reforming from CH4 to H2 (EG&G, 2002).

The electrochemical reactions in SOFC at anode:

H2+ O2-

H2O + 2e- (2.1)

CO + O2-

CO2+ 2e-

(2.2)

CH4+ 4O2-

2H2 O + CO2 + 8e- (2.3)

At cathode:

O2 + 4e- 2O

2 (2.4)

The overall cell reactions are:

H2 + ½ O2 H2O (2.5)

e-

e-

Permeable

Cathode

impermeable

Electrolyte

Permeable

Anode

Oxidant

Fuel

H2 + CO H2O + CO2

2H2 + 2O2-

4e- + 2H2O

O2-

O2-

O2-

O2 + 4e- 2O

2-

H2 + CO

Air Depleted O2

heat

12

CO + ½O2 CO2 (2.6)

CH4+ 2O2 2H2O + CO2 (2.7)

2.1.4 SOFC Components

There are several components in SOFC system such as anode, cathode, electrolyte,

and interconnect. All the components are described as follows:

2.1.4.1 Anode

Anode materials are of several types which have advantages and disadvantages as

listed in Table 2.3. The most commonly used as the anode material is Ni-YSZ which

will be described in more detail.

Table 2.3 Advantages and disadvantages of several type anode materials (Gorte et

al., 2003; Maarten, 2005 and Zhang, 2006)

No. Types of anode

material

Advantages Disadvantages

1 Yttria

Stabilized

Zirconia (YSZ)

Offering the required oxygen

vacancies for ionic conductivity

Ionic conductivity comes down

significantly over time when working at

high operating temperatures

2 Nickel-Yttria

Stabilized

Zirconia (Ni-

YSZ)

Higher thermal expansion than

YSZ, an excellent catalyst for both

steam reforming and hydrocarbon

cracking

Poor redox stability, low tolerance for

sulphur and carbon deposition and the

tendency of nickel agglomeration after

prolonged operation

3 Strontium

Titanate

(SrTiO3)

Low-cost raw materials and simple

processing steps

The cubic to tetragonal structural phase

transformation may induce strain or

micro cracks in a film grown on a

SrTiO3 substrate

Anode has the smallest losses polarity if it is compared with other

components, and as the layer that often provide mechanical support. The oxidation

13

reaction between the oxygen and the hydrogen ions produces heat and water as well

as electricity. One of the advantages of anode as the catalyst for steam reformer

hydrocarbon gas becomes hydrogen fuel. It provide another operational benefit to the

fuel cell stack due to the reforming cools stack reaction internally (EG&G, 2002).

Therefore, its material should be catalytic, conductive, as well as porous to hydrogen

fuel. It may reduce the atmosphere and the thermal expansion must be compatible

with the other cell. Ni-YSZ anode is currently the material of choice for anode

material as shown in Figure 2.4 (Singhal, 2000).

Figure 2.4 Anode with Ni-YSZ material (Singhal, 2000)

Ni has a higher thermal expansion than YSZ (13.3 x 10-6

/0C for Ni and 10 x

10-6

/0C for YSZ). Therefore, the Ni-YSZ composite was created. This is due to the

YSZ that provide structural support for the separate Ni particles, preventing them

from sintering together while matching thermal expansion. The adhesion of the

anode to the electrolyte also increased (Singhal, 2003 and Zhong et al., 2004).

2.1.4.2 Cathode

Several advantages and disadvantages of the cathode materials were observed by

many engineers with different cathode materials are listed in Table 2.4. Generally,

LSM is used as the cathode material.

Ni-

YSZ

i> 0

e-

e-

14

Table 2.4 Advantages and disadvantages of several type cathode materials (Chunwen

et al., 2010 and Hyo et al., 2009)

No. Types of cathode

material

Advantages Disadvantages

1 Lanthanum

Strontium

Manganite (LSM)

High electrical conductivity at higher

temperatures, and its high thermal

expansion coefficient

Increasing costs and longer

start-up times

2 Lanthanum

Strontium Gallium

Magnesium

(LSGM)

High conducting properties at

intermediate operating temperatures

while providing stability

High thermal expansion

The most commonly used cathode material is lanthanum manganite

(LaMnO3) as shown in Figure 2.5. Typically, it is doped with rare earth elements (eg.

Sr, Ce, Pr) to enhance its conductivity.

Figure 2.5 Cathode with LaMnO3 material (Singhal, 2003)

The function of the crystal as electron is to conduct the process with no ionic

conductivity. The electrons flow back through the open circuit of cathode cells to

reduce molecular oxygen and flow it back to the electrolyte. In addition, to make it

more effective in flowing the oxygen, it can be achieved through YSZ electrolytes

material. It has adequate functionality at intermediate fuel cell temperatures (about

7000C) and used with alternative electrolyte compositions (Zhang, 2006 and

Rajendra, 2007). Using electrolytes material can reduces operating costs and expands

the materials selection, creating an opportunity for additional cost savings (Gorte et

al., 2003 and Zhong et al., 2004).

It had been found that by increasing the Sr2+

or Ca2+

content it is possible to

increase the electronic conductivity due to change in the Mn- ratio (Chakraborty et

al., 1995). It conductivity at 10000C for La0.8Sr0.2MnO3 varies in range of 100 to 200

S/cm which shows maximum value for a Sr-content close to 55 mol%. However,

LaMnO3

e-

e-

+

i>0

15

their ionic conductivity are negligibly small, ~10-7

S/cm at 8000C in air (Figure 2.6).

Therefore, this total conductivity (electronic and ionic) of LSM is not sufficient to

completely neglect the in-plane resistance in the cathode. Besides, it is possible to

minimize the corresponding losses by providing a larger geometry relatively and

location of the current collector during cell operation (Sasaki et al., 1999).

Figure 2.6 Temperature dependence of conductivity for undoped and Sr-dopend

LaMnO3 (Kuo et al., 1990)

2.1.4.3 Electrolyte

Table 2.5 shows the advantages and disadvantages of the electrolyte materials which

are investigated by many researchers. However, YSZ material is the mostly used as

the electrolyte material.

ln𝜎𝑇

(S

/cm

)

103T (K

-1)

LaMnO3

La0.95Sr0.05MnO3

La0.9Sr0.1MnO3

La0.8Sr0.2MnO3

16

Table 2.5 Advantages and disadvantages of several type electrolyte materials

(Wendy Lim, 2009; Crina et al., 2009 and Jennifer et al., 2007)

No. Types of electrolyte

material

Advantages Disadvantages

1 Yttria Stabilized

Zirconia (YSZ)

Sufficiently high oxygen ion

conductivity, suitable for tape

casting, pellet pressing and

other fabrication methods

Increasing costs and longer start-up

times

2 Scandia Stabilized

Zirconia (SCZ)

High oxygen conductivity,

chemical stability and good

electrochemical performance

There is a phase transition at 650

0C from cubic to rhombohedra

phase, lead to an abrupt change in

volume and coefficient of thermal

expansion (CTE) and thus cracking

of the electrolyte material

3 Gadolinia-Doped

Ceria (GDC)

High ionic conductivity at

intermediate operating

temperatures, low cost, reduced

ohmic losses,

Short circuiting, decreased power

performance by electronic leakage

and Lower operating

temperatures (<700 °C)

4 Lanthanum

Strontium Gallium

Magnesium

(LSGM)

High stability and ionic

conductivity

High cost

5 Samaria-doped

Ceria (SDC)

High ionic conductivity in the

intermediate temperature range

Disadvantages:

High electrolyte resistance, hence

degrade the performance

6 Cerium Gadolinium

Oxide (CGO)

higher ionic conductivity lower operating

temperatures (<700 °C)

The electrolyte conducts ionic charges to the electrodes and completes the

cell electric circuit. It also provides a physical barrier to prevent the fuel and oxidant

gas from mixing directly. Therefore, the electrolyte must possess a high ionic

conductivity without electrical conductivity. To prevent gas from permeating one

side of the electrolyte layer to another, it requires impermeable material as an

electrolyte and it also should be as thin as possible to minimize resistive losses in the

cell (Gorte et al., 2003; Zhang, 2006 and Zhong et al., 2004).

17

2.1.4.4 Interconnect

Several types of interconnect material was investigated by previous research. The

advantages and disadvantages are as listed in Table 2.6. Ceramic materials are high

cost and difficult to machine. Therefore, the FeCr as ferritic steel is currently very

attractive metallic material for interconnect and is described in more detail in this

chapter.

Table 2.6 Advantages and disadvantages of several types interconnect material

(Minh, 1993; Sammes, 1997; Jeffrey, 2005; Junwei Wuand Xingbo Liu, 2010; Costa

et al., 2006; Redjdal et al.,2013 and Sebayang et al., 2013)

No. Types of

interconnect

material

Advantages Disadvantages

1 LSC (La0.9 Sr0.1

CrO3)

High Cr fraction and high Sr

fraction

Less conducting, expensive and

difficult to fabricate as their

brittleness

2 LaCrO3 (LCR) Stable in higher sintering

temperature

Expensive and difficult to fabricate

as their brittleness

3 Ca-doped yttrium

chromites

Better thermal expansion

compatibility, especially in

reducing atmospheres

Excess A-site element tend to react

with zirconium electrolyte lead to

produce undesirable secondary

phases

4 Iron Chromium

(FeCr)

High-strength and corrosion-

resistant, high magnetic responses

Some grain growth at high

temperature operation

The cell interconnection is exposed both to cathode and anode, so it must be

stable chemically in both environments at temperature of 10000C. However, since the

operating temperature of this material is of high range, the material should have fire

resistance properties, oxidant gases, and proper coefficient of electronic conductivity

(Zhang, 2006; Gorte et al., 2003 and Zhong et al., 2004). One of the approach to

promote the formation of more conductive oxide phases could be accomplished by

depositing thin films of certain metals onto the interconnect surface before oxidation.

Upon heating, the metal coating oxidizes, reacts with Cr from the base alloy, and

18

subsequently forms another oxide layer (Cr2O3) (Jaing et al., 2002 and Tad et al.,

2005). Developing interconnect by many researchers with right material had shown

that its can improve the corrosion resistance at high temperature, protection in

aggressive environments, high temperature coating, high temperature alloys and

intermetallic, metal dusting and carburisation, role of water vapour and steam in high

temperature corrosion, lifetime prediction spalling and advanced characterization

techniques of degradation (Bastidas, 2006).

These three criteria are most important for interconnect material such as high

electrical conductivity, good stability in both condition reducing, oxidizing and

matching for coefficient of thermal expansion (CTE) with other SOFC components

(Benjamin, 2004). Ceramic interconnect such as doped lanthanum chromites (La1-

x(Sr,Ca)xCrO3) have been widely used as interconnect material (Songlin et al., 2008).

These materials are expensive and difficult to fabricate as they are very brittle. The

reduction of SOFC operating temperature is possible to consider oxidation resistance

metallic alloys. Metallic material is easier gained and interest replacement for the

LaCrO3 based ceramic (Geng et al., 2006). Therefore, there are many researches that

develop Fe-Cr metallic material as interconnect SOFC application. FeCr powder

alloy for interconnect SOFC application has the challenged to improve the electrical

conductivity (Khaerudini et al.,2012), high corrosion resistance and the decrease in

the growth rate which is provided by Cr2O3 scale (Deni, 2011). The Cr2O3 scale is

suitable for resistance application such as to decrease the migration of chromium to

SOFC cathodes and to defend cell electrochemical performance (Hendi, 2011).

2.2 Oxidation of metallic alloy

Metal alloy has superior properties as compared to LaCrO3 (traditionally material

used for interconnect material) because its greater potential to increase the efficiency

of SOFC (Benjamin, 2004). The significant advantage of metallic interconnect, it can

decrease resistance of the cell that would be directly convert to increased output and

the electrical conductivity. This is because of the reduction of temperature gradient

within the structure that could give effect the stack efficiency thermal stress

19

(Benjamin, 2004 and Minh, 1993). However, the metallic alloys have several

negatives aspect for SOFC interconnect which are potential to adversely impact

during operation. It is due to thermal expansion mismatch between the other metallic

SOFC materials. Metallic alloys have higher CTE as compared to ceramic material.

This is lead to significant level of stress developed when temperature changed. Metal

is oxidized when exposed to the range of atmospheres and temperature during SOFC

operation. Its efficiency was reduced due to higher surface resistance and this lead to

excessive oxidation (Benjamin, 2004 and Minh, 1993). Loss efficiency of the stack is

due to oxidation of interconnect. It is caused by increasing of electrical resistance of

the oxidation layer. One way to improve the oxidation resistance is to complete the

addition of alloy and coating method (Kofstad, 1996 and Benjamin, 2004). However,

this study focuses on alloying addition process via new method. The oxidation curve

of the metallic materials is shown in Figure 2.7, when oxidation time increased, the

weight change increased as well and higher temperature oxidation is produced by the

higher mass gain.

Figure 2.7 TGA oxidation curve (Fredriksson et al., 1991)

Time (hours)

Wei

gh

t ch

ang

e2.µ

g2/c

m4

0

100

200

300

400

500

600

700

800

900

0 5 10 15 25 20 30

LEGEND

= 500 0C

= 560 0C

20

2.3 Metallic interconnect

Metallic materials are easier and cheaper to fabricate, they are less brittle and easier

for machining. They have high thermal conductivities than ceramic materials (Deni,

2011 and Khaerudini et al., 2012). The most important ability of metallic alloy is

slow formation, thermal grown, protective oxide scale. If compared with ceramic

interconnect, the metallic interconnect can reduce the fabrication cost, facilitate the

production of more complex fuel cell designs and improve the efficiency of the fuel

cell (Benjamin, 2004). Metallic material such as iron, nickel and cobalt based alloys

are usually used in high temperature environment. Developing alloys metal with

chromium, aluminium and silicon encourage the formation of protective oxide

(scales) of chromia, alumina and silica, respectively. However, the protective oxide

(scales) of alumina has low electrical conductivity resulted in performance loss

(voltage loss across the insulating alumina scale) (Quadakkers et al., 2003). Iron (Fe)

as metallic interconnect has many advantages such as good work ability, low cost

and thermal expansion coefficient similar with other cell component. Chromium is

formed on the surface during vaporization operation and it decreases the cathode

activity. Nearly of all metallic materials as interconnect candidates are chromium

forming alloy since the high electrical conductivity is provided by oxide Cr2O3 scale

compared to Al2O3 and SiO2. However, the chromium based alloys have some

weaknesses such as formation of volatile Cr(VI) which occur under operating

environment of SOFC. The migration of volatile Cr(VI) may occur and it destroy the

cathode. The degradation of stack performance is over long term exposure of high

temperature (Jong-Hee Kim et al., 2004). There are several studies which are related

to the FeCr alloy as metallic interconnect for high temperature SOFC and the

modification for improving the conductivity of oxide scale is required (Jong-Hee

Kim et al., 2004). Fe-16Cr alloys (SUS430) have been studied on oxidation kinetics

under the condition of the air and fuel side environments in SOFC application

(Hideto et al., 2004). The growth rate scale in the air and fuel side is almost same

when it reaches the temperature of 1073 K (Hideto et al., 2004). In the air side, the

growth rate of iron oxide is higher than in the steam side. It might be caused by the

hydrogen permeates from the effect of oxidation behaviour on the steam side.

Interconnect of SOFC is exposed simultaneously both of air and full atmospheres.

21

Ni-based alloy Haynes 242 with low coefficient of thermal expansion (CTE)

was used as metallic interconnect by Geng et al., (2006). They found that the

parabolic law and weight gain of the alloy increased with the increase in oxidation

temperature. In the outer layer of NiO and an inner layer of Cr2O3 with (Mn,Cr)3O4

oxide is formed. In the top NiO layer did not uniformly cover the surface. They also

study the Ebrite and Haynes 230 alloy (nickel-chromium-tungsten-molybdenum

alloy) material for interconnect application. Commercial Ni-Cr based alloys divide to

three i.e. Haynes 230, Hastelloy S (nickel-chromium-iron alloy) and Haynes 242

alloy (nickel-molybdenum-chromium alloy) used to evaluate the oxidation

behaviour, scale conductivity and thermal expansion (Zhenguo Yang et al., 2006).

They found that the Haynes 230, Hastelloy S formed thin scale mainly comprised of

Cr2O3 and (Mn,Cr,Ni)3O4 spinel, have excellent oxidation resistance and high

electrical conductivity while the Haynes 242 shows limited oxidation resistance for

interconnect application. Three alloys above possess a Coefficient of Thermal

Expansion (CTE) that is higher than Crofer 22 Auxiliary Power Units (APU) as

candidate ferritic stainless steel.

This study focuses on the improving FeCr properties such as high thermal

stability, reducing the crystallite size, more homogenous particle size and finer

surface morphology of the material by using the combination technique. The

summaries of the description of the metallic materials for interconnect are listed in

Table 2.7.

Table 2.7 Varies metallic candidate materials for interconnect application

No. Author Metallic material for

interconnect

Analyzed to

1 Deni, 2011; Hendi,

2011

Ade, 2012

Fe80Cr20

FeCrAl

Investigate the crystallite size,

oxidation kinetics, CTE, electrical

properties, morphology and

composition analysis.

The distribution of particle size,

morphology, and oxidation kinetics.

2 Quadakkers et al., 2003 MnCrAl Investigate the excellent oxidation

protection of the alumina scale

3 Jong-Hee Kim et al.,

2004

iron, nickel, cobalt,

chromium, aluminium and

silicon

Investigate the electrical conductivity.

22

4 Hideto et al., 2004

Murugesan et al.,

1999

Rajeev et al., 2013

Fe-16Cr alloys (SUS430)

Fel-xCrx

Fe20Cr Alloy

Investigate the oxidation kinetics

Investigate the magnetism of

nanocrystalline alloys

Investigate the corrosion rate or

corrosion resistance

5 Genget al., 2006 Ni-based alloy Haynes 242

and Ebrite and Haynes 230

Investigate the parabolic law and

weight gain.

6 Zhenguo Yang et al.,

2006

Haynes 230, Hastelloy S

and Haynes 242

Investigate the oxidation behaviour,

scale conductivity and thermal

expansion

2.3.1 Chromium based alloys

Chromium based alloys have a crystal structure of Body Centred Cubic (BCC) and

are not considered as super alloys. Evaporation of chromium species occurs from the

protective Cr2O3 layer and the poisoning effects of these species occur at the

electrolyte interface (Bastidas, 2006).

Chromium-based alloys were initially developed as a replacement of ceramic

interconnects for electrolyte-supported planar SOFC. They are favoured because they

have high oxidation resistance and fairly good corrosion resistance provided by the

formation of Cr2O3 scale in the presence of oxidant. The binary metal oxide Cr2O3

has large electronic conductivity (Zhong et al., 2003 and Yang et al., 2004). Thicker

corrosion scales grow in the carbon-containing atmosphere (methane, propane,

Liquefied Petroleum Gas (LPG) and coal gas) due to the formation of carbides

(Bastidas, 2006). According to Zhu Wei-zhong and Yan Mi (2004) the Cr based

alloys have main weakness as metal interconnect i.e. developing Cr(VI) gas species

which is easy to volatilize at the fuel cell operating temperature. The further

oxidations of the Cr2O3 scale usually occur at the higher oxygen partial pressure in

the end of the cathode prior to the electrochemical reduction of the oxygen. The Cr

based alloys of this research have several properties that influence the result of the

research. Material properties of Cr powder can be seen in Table 2.8 (John Emsley,

2011).

23

Table 2.8 Material properties of chromium powder (John Emsley, 2011)

Material Properties

Atomic number 24

Atomic mass 51,996

Melting point 1 900 °C / 2 173 K

Boiling point 2 672 °C / 2 945 K

Atomic volume 1.2 · 10-29

[m3]

Vapour pressure at 1 800 °C 267 [Pa]

at 2 200 °C 7161 [Pa]

Density at 20 °C (293 K) 7.15 [g/cm3]

Crystal structure Body-Centered Cubic

Lattice constant 2.8847 · 10-10

[m]

Hardness at 20 °C (293 K) 180 - 250 [HV10]

Modulus of elasticity at 20 °C (293 K) 294 [GPa]

Poisson number 0.21

Coefficient of linear thermal expansion at 20 °C (293 K) 6.2 · 10-6

[m/(m·K)]

Thermal conductivity at 20 °C (293 K) 93.7 [W/(m·K)]

Specific heat at 20 °C (293 K) 0.45 [J/(g·K)]

Electrical conductivity at 20 °C (293 K) 7.9 · 106 [1/Ω·m)]

Specific electrical resistance at 20 °C (293 K) 0.127 [(Ω·mm2)/m]

Sound speed at 20 °C (293 K) Longitudinal wave 6 850 [m/s]

Transverse wave 3 980 [m/s]

Electron work function 4.5 [eV]

High oxidation resistance of the alloy and relatively low Coefficient of

Thermal Expansion (CTE) of 9 to 10 x10-6

K-1

are shown by chromium based alloys

with dispersion of oxide particle. Oxide dispersion is used to improve mechanical

properties of Cr based alloys and reduce the detrimental effect of ambient

temperature from ductile to brittle transition (Greneir et al., 1995). Complete

reduction of chromium has occurred when added to iron oxide by addition of 25 wt%

Cr or less. It due to the chromium based alloy cannot be produced successfully using

honeycomb extrusion process (Nadler, 2003).

24

2.3.2 Iron based alloys

Iron-based alloys have higher ductility than the chromium-based alloys, it is cheap

and easy to machine. Currently, there are two (2) types of iron based alloys

(Fe/Cr/Mn and Fe/Cr/W) which are explored by many researchers due to of their

relatively low Coefficient Thermal Expansion (CTE) (Kim et al., 1999). Interconnect

metal contains at least 17 wt% Cr which serves as forming corrosion resistance

which provided by Cr2O3 scale compared to manganese and tungsten addition. The

Fe-Cr alloy interconnect has been proposed by some researchers due to its good

oxidation resistance (Kim et al., 1999). Material properties of iron powder can be

seen in Table 2.9.

Table 2.9 Material properties of iron powder (John Emsley, 2011)

Material properties

Atomic number 26

Atomic mass 55.85 g.mol -1

Electro negativity 1.8

Density 7860 (kg/m3)

Melting point 1537 (oC)

Specific heat 460 (J/kgK)

Thermal conductivity 74 (W/mK)

Coefficient of thermal expansion 11.5 (µm/m-oC)

Electrical resistivity 9.5 x 10-8

( -m)

Boiling point 2861 °C

Vanderwaals radius 0.126 nm

Ionic radius 0.076 nm (+2) ; 0.064 nm (+3)

Isotopes 8

Electronic shell [ Ar ] 3d6 4s

2

Energy of first ionization 761 kJ.mol -1

Energy of second ionization 1556.5 kJ.mol -1

Energy of third ionization 2951 kJ.mol -1

Standard potential - 0.44 V (Fe2+

/ Fe ) ; 0.77 V ( Fe3+

/ Fe2+

)

Formation of a protective single phase chromium layer requires a chromium

content of 17-20% approximately; it depends on temperature, surface treatment,

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