RF MAGNETRON SPUTTERED YSZ THIN FILMS: FABRICATION AND
CHARACTERIZATION
BY
SHAHRUL RAZI BIN MESKON
A dissertation submitted in fulfilment of the requirement for the degree of
Master of Science (Materials Engineering)
Kulliyyah of Engineering International Islamic University
Malaysia
JUNE 2011
ii
ABSTRACT
In order to obtain high ionic conductivity of a solid electrolyte at intermediate temperatures for intermediate-temperature solid oxide fuel cells (IT-SOFC), a very thin dense film is crucial due to its reduced ohmic losses. This work describes the fabrication of yttria-stabilized zirconia (8YSZ) thin films using radio frequency (RF) magnetron sputtering method. The thin films are deposited onto carbon paint coated stainless steel sheets with varying substrate temperature (Ts) of 150, 200, 250 and 300°C. Other sputtering parameters, i.e., argon gas flow rate, RF power and deposition time are fixed. The sputtering targets used are sintered YSZ pellets. Ultra- thin YSZ films were successfully deposited with a thickness range of 300 to 600 nm as determined from the scanning electron microscopy (SEM). Phase composition analysis using X-ray diffraction (XRD) revealed cubic matrix with tetragonal and monoclinic crystalline phases of zirconia. Impedance spectroscopy (IS) is conducted at room temperature (RT) to measure the alternating-current (AC) total conductivity of the thin films. The conductivity increases with increasing substrate temperature Ts
from 150°C to 250°C but drops slightly at Ts of 300°C. The highest room temperature AC total conductivity obtained is 1.81 x 10-6 Ω-1.cm-1 and the value is comparable with the reported direct-current (DC) bulk conductivity measured at a significantly higher temperature around 283°C.
iii
ﺚﺤﺒﻟا ﺔﺻﻼﺧ
ﻲﻓ ﺔﺒﻠﺼﻟا لرﺎﻬﻜﻟا ﻦﻣ ﺔﻴﻟﺎﻋ ﺔﻴﻧﻮﻳأ ﺔﻴﻠﺻﻮﻣ ﻰﻠﻋ لﻮﺼﺤﻟا ﻞﺟأ ﻦﻣ ةراﺮﺤﻟا وذ ﺐﻠﺼﻟا ﺪﺴآﺆﻤﻟا دﻮﻗﻮﻟا ﺎﻳﻼﺨﻟ ﺔﻄﺳﻮﺘﻤﻟا ةراﺮﺤﻟا تﺎﺟرد ﺔﻄﺳﻮﺘﻤﻟا
) SOFC - IT (
ماﺪﺨﺘﺳا يروﺮﻀﻟا ﻦﻣ
،اﺪﺟ ﺔﻓﺎﺜﻜﻟا ﺔﻘﻴﻗر ﺔﺤﻳﺮﺷ
ﺔﻀﻔﺨﻨﻤﻟا ﺔﻴﻣوﻷا ﺎهﺮﺋﺎﺴﺧ ﺐﺒﺴﺑ .
ﺢﺋاﺮﺷ ﻞﻴﻜﺸﺗ ﺔﻴﻠﻤﻋ ﻒﺼﻳ ﺚﺤﺒﻟا اﺬه
) yttria-stabilized zirconia (8YSZ)
ﺔﻘﻳﺮﻃ ماﺪﺨﺘﺳﺎﺑ ﺔﻘﻴﻗﺮﻟا
( Radio Frequency (RF) )Magnetron Sputtering
.
(موﺎﻘﻤﻟا ذﻻﻮﻔﻟا ﺢﺋﺎﻔﺻ ﻰﻠﻋ ﺔﻘﻴﻗﺮﻟا ﺢﺋاﺮﺸﻟا ﻊﺿﻮﺗ
ةﺰﻴآﺮﻟا ةراﺮﺣ تﺎﺟرد توﺎﻔﺗ ﻊﻣ ﻲﻧﻮﺑرﺎآ ءﻼﻄﺑ ﻒﻠﻐﻤﻟا أﺪﺼﻠﻟ
s) T
ﻦﻣ
(150
، 250
،
ﻰﻟا
200ﺔﻳﻮﺌﻣ ﺔﺟرد
300. ﻢﺘﻳ ﺖﻴﺒﺜﺗ ىﺮﺧﻻا ﺔﺣازﻹا تﻼﻣﺎﻌﻤﻟا
ﻞﺜﻣ
،viz
اﺮﻴﺧأو ﺔﻴﻜﻠﺳﻼﻟا تاددﺮﺘﻟا ﺔﻗﺎﻃو ،نﻮﺟرﻷا زﺎﻏ ﻖﻓﺪﺗ لﺪﻌﻣو
ﺐﺳﺮﺘﻟا ﺖﻗو .
تﺎﻳﺮآ ﻦﻋ ةرﺎﺒﻋ ﺔﻣﺪﺨﺘﺴﻤﻟا ﺔﺣاﺰﻤﻟا فاﺪهﻻا
) YSZ
ةﺎﻤﺤﻣ
(.
ﺢﺋاﺮﺷ
) YSZ
قﻮﻓ عﻮﻧ ﻦﻣ ﺔﻘﻴﻗر
(- ﻚﻤﺴﺑ و حﺎﺠﻨﺑ ﺎﻬﺒﻴﺳﺮﺗ ﻢﺗ ﺔﻘﻴﻗﺮﻟا
ﻦﻣ حواﺮﺘﻳ ﻰﻟا
300ﻲﻧوﺮﺘﻜﻟﻻا ﺮﻬﺠﻤﻟا دﺪﺣ ﺎﻤﺒﺴﺣ ﺮﺘﻴﻣﻮﻧﺎﻧ
600 )SEM
.
(ﺎﻤآ
ماﺪﺨﺘﺳا ﺔﻄﺳاﻮﺑ نﻮﻜﺘﻤﻟا رﻮﻄﻟا ﻞﻴﻠﺤﺗ ﻒﺸآ ﺔﻴﻨﻴﺴﻟا ﺔﻌﺷﻷا دﻮﻴﺣ
) XRD (
ﻦﻣ يدﺎﺣأو ﺎﻳاوﺰﻟا ﻲﻋﺎﺑﺮﻟ ﺔﻳرﻮﻠﺒﻟا ﻞﺣاﺮﻣ ﻊﻣ ﺔﺒﻌﻜﻣ ﺔﻓﻮﻔﺼﻣ مﻮﻴﻧﻮآرﺰﻟا .
ﺔﻌﻧﺎﻤﻤﻠﻟ ﻲﻔﻴﻄﻟا ﻞﻴﻠﺤﺘﻟا
) IS
ﺔﻓﺮﻐﻟا ةراﺮﺣ ﺔﺟرد ﻲﻓ ىﺮﺟ
() RT
ا ﺔﻴﻠﺻﻮﻤﻟا بﺎﺴﺤﻟ
(بوﺎﻨﺘﻤﻟا رﺎﻴﺘﻠﻟ ﺔﻴﻠﻜﻟ
) AC
ﺔﻘﻴﻗﺮﻟا ﺢﺋاﺮﺸﻠﻟ
(. تدادزإ
ةﺰﻴآﺮﻟا ةراﺮﺣ ﺔﺟرد ةدﺎﻳز ﻊﻣ ﺔﻴﻠﺻﻮﻤﻟا
) Ts
ﻦﻣ
(ﻰﻟا
150ﺔﻳﻮﺌﻣ ﺔﺟرد
250ﺖﻠﺻو ﺎﻣﺪﻨﻋ ﻼﻴﻠﻗ ﺖﻄﺒه ﺎﻬﻨﻜﻟو
) Ts
ﻰﻟا
(ﺔﻳﻮﺌﻣ ﺔﺟرد
300. ﺔﻴﻠﺻﻮﻣ ﻰﻠﻋأ
ﺖﻧﺎآ ﺔﻓﺮﻐﻟا ةراﺮﺣ ﺔﺟرد ﻲﻓ ﺎﻬﻴﻠﻋ لﻮﺼﺤﻟا ﻢﺗ ددﺮﺘﻤﻟا رﺎﻴﺘﻠﻟ
) 1.81 X 10-6
Ω-1.cm-1
ﺮﻤﺘﺴﻤﻟا رﺎﻴﺘﻠﻟ ﺔﻠﺠﺴﻤﻟا ﻚﻠﺘﻟ ﺔﺑرﺎﻘﻣ ﺔﻤﻴﻗ ﻲهو
(ﻲﻓ ﺔﻠﺠﺴﻤﻟاو
(DC)برﺎﻘﺗ ﺮﻴﺜﻜﺑ ﻰﻠﻋأ ةراﺮﺣ ﺔﺟرد ﺔﻳﻮﺌﻣ ﺔﺟرد
283.
iv
APPROVAL PAGE
I certify that I have supervised and read this study and that in my opinion it conforms to acceptable standards of scholarly presentation and is fully adequate, in scope and quality, as a dissertation for the degree of Master of Science (Materials Engineering).
...
Raihan Othman
Supervisor
...
Agus Geter Edy Sutjipto
Co-supervisor I certify that I have read this study and that in my opinion it conforms to acceptable
standards of scholarly presentation and is fully adequate, in scope and quality, as a dissertation for the degree of Master of Science (Materials Engineering).
...
Zuraida Ahmad
Internal Examiner
...
Muhd Zu Azhan Yahya External Examiner
This dissertation was submitted to the Department of Manufacturing and Materials Engineering and is accepted as a fulfilment of the requirement for the degree of Master of Science (Materials Engineering).
...
Erry Yulian T. Adesta
Head, Dept. of Manufacturing and Materials Engineering
This dissertation was submitted to the Kulliyyah of Engineering and is accepted as a fulfilment of the requirement for the degree of Master of Science (Materials Engineering).
...
Amir Akramin Shafie Dean, Kulliyyah of Engineering
v
DECLARATION
I hereby declare that this dissertation is the result of my own investigations, except where otherwise stated. I also declare that it has not been previously or concurrently submitted as a whole for any other degrees at IIUM or other institutions.
Shahrul Razi bin Meskon
Signature ……… Date ………..
vi
OF FAIR USE OF UNPUBLISHED RESEARCH
Copyright © 2011 by Shahrul Razi Bin Meskon. All rights reserved.
RF MAGNETRON SPUTTERED YSZ THIN FILMS: FABRICATION AND CHARACTERIZATION
No part of this unpublished research may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording or otherwise without prior written permission of the copyright holder except as provided below.
1. Any material contained in or derived from this unpublished research may only be used by others in their writing with due acknowledgement.
2. IIUM or its library will have the right to make and transmit copies (print or electronic) for institutional and academic purposes.
3. The IIUM library will have the right to make, store in a retrieval system and supply copies of this unpublished research if requested by other universities and research libraries.
Affirmed by Shahrul Razi Bin Meskon.
………. ………..
Signature Date
INTERNATIONAL ISLAMIC UNIVERSITY MALAYSIA
DECLARATION OF COPYRIGHT AND
AFFIRMATION OF FAIR USE OF UNPUBLISHED RESEARCH
Copyright © 2011 by International Islamic University Malaysia. All rights reserved.
RF MAGNETRON SPUTTERED YSZ THIN FILMS: FABRICATION AND CHARACTERIZATION
I hereby affirm that The International Islamic University Malaysia (IIUM) holds all rights in the copyrights of this work henceforth any reproduction or use in any form or by means whatsoever is prohibited without the written consent of IIUM.
No part of this unpublished research may be reproduced, stored in a retrieval system, or transmitted, in any form or by means, electronic, mechanical, photocopying, recording or otherwise without prior written permission of copyright holder.
Affirmed by Shahrul Razi bin Meskon
……… ………
Signature Date
vii
ACKNOWLEDGEMENTS
Alhamdulillah thanks and praise to Allah (S. W. T.) for His blessings, guidance and plentiful bounties. Prayers and salutations for Prophet Muhammad (peace and blessing may be upon him), his family and his companions (may Allah bless them). I would like to take this opportunity to express my deepest gratitude to those who have helped me in completing this dissertation.
Firstly I would like to express my high appreciation to my supervisor Dr.
Raihan Othman for his invaluable lessons and continuous solid support in making this study to become a sound research. His determination in pioneering a research in this area should be followed. I would also like to express my thankfulness to my co- supervisor Dr. Agus Geter Edy Sutjipto for giving me beneficial advice in making this research an accomplishment. Not to miss, the same credit also goes to Dr. Mohd.
Hanafi Ani for his lessons and brilliant suggestions in solving some of the problems in this study.
My special thanks are dedicated to Br. Hens Saputra for giving me ideas, advice as well as assisting me in doing this research. Not to forget the same appreciation goes to Br. Jufriadi and Br. Husni for his assistance in the sputtering work, Br. Ibrahim for the SEM, and Br. Rahimi for the XRD. I would like to express my real appreciation and thanks to my lab mates for their help, moral support and understanding, viz.: Aziz, Drajat, Akbar, Sr. Liza and Sr. Laila.
I also would like to take this opportunity to thank my beloved parents, Mr.
Meskon Dariman and Mrs. Rosmah Menon as well as my siblings for their prayers, substantial support, positive advice and good mutual understanding in making this dissertation a lifetime success.
Finally, I would like to acknowledge the Ministry of Higher Education Malaysia (MOHE) for the funding of this research through the Fundamental Research Grant Scheme (FRGS 0106-30).
SHAHRUL RAZI BIN MESKON 22 Sya’ban 1431
viii
TABLE OF CONTENTS
Abstract ………. ii
Abstract in Arabic ………. iii
Approval Page ……….. iv
Declaration Page ……….………. v
Copyright Page ………. vi
Acknowledgements ……….. vii
List of Tables ……….….. x
List of Figures ……….. xi
List of Abbreviations ………..………. xv
List of Symbols ………... xvii
CHAPTER 1: INTRODUCTION ………. 1
1.1 Overview ………... 1
1.2 Problem Statement and Its Significance ……….. 2
1.3 Research Objectives ………... 3
1.4 Research Methodology ……….... 4
1.5 Research Scope ……….... 5
1.6 Dissertation Outline ………... 6
CHAPTER 2: LITERATURE REVIEW ……… 8
2.1 Introduction ……….… 8
2.2 Fuel Cell ………... 8
2.3 Solid Oxide Fuel Cell (SOFC) ……….... 9
2.3.1 Working Principles of SOFC ……….………... 11
2.4 Microstructural Properties of Zirconia ……….... 12
2.5 Sputtering Process ………... 15
2.5.1 Plasma and Its Initiation Process ……….. 16
2.5.2 RF Magnetron Sputtering ………. 17
2.6 Impedance Spectroscopy (IS) ……….... 19
2.6.1 Advantages and Disadvantages of IS ……….….. 25
2.7 Summary ………. 28
CHAPTER 3: EXPERIMENTAL PROCEDURE ………... 31
3.1 Introduction ………... 31
3.2 Fabrication of Sputter Targets ……….... 31
3.3 Substrate Preparation ……….………... 35
3.4 RF Magnetron Sputtering Process ………... 35
3.5 Characterizations of YSZ Thin Films ………... 38
3.5.1 X-Ray Diffraction (XRD) Analysis ……….……. 38
3.5.2 Surface Morphology Analysis and Thickness Measurement ………. 39
ix
3.5.3 Impedance Spectroscopy (IS) ……….... 39
3.6 Summary ………... 41
CHAPTER 4: RESULTS AND DISCUSSION ……… 42
4.1 Introduction ………... 42
4.2 X-Ray Diffraction (XRD) Analysis ……….. 42
4.3 Surface Morphology Analysis and Thickness Measurement ……... 44
4.4 Impedance Spectroscopy Analysis and Electrical Conductivity of YSZ Thin Films ………... 49
4.5 Summary ………... 61 CHAPTER 5: CONCLUSION AND RECOMMENDATION …………... 62
5.1 Conclusion ……….... 62
5.2 Recommendation ……….. 63 BIBLIOGRAPHY ……….….. 65
x
LIST OF TABLES
Table No. Page No.
2.1 Different types of fuel cells 9
3.1 Sputtering process conditions for the YSZ thin film fabrication
37
3.2 Details of XRD experimental setup 38
3.3 The impedance spectroscopy experimental setup conditions 40
4.1 Average grain size values of the YSZ thin films’ surface
morphology 46
4.2 Average thickness values of the YSZ thin films’ cross- section
49
4.3 Average sputter deposition rate values according to sample category
49
4.4 Rs, Rt, Rb, Rgb and Re values as a function of Ts 54
4.5 AC total conductivity and AC bulk conductivity values of the thin films according to sample category
58
xi
LIST OF FIGURES
Figure No. Page No.
1.1 Research methodology in a flow chart 5
2.1 Schematic of a SOFC showing the basic operating principle. 12
2.2 High zirconia part of zirconia-yttria phase diagram.
Commercial PSZ and ZTP composition and processing temperatures are indicated by shaded regions.
14
2.3 Paschen curves for a number of gases. 18
2.4 Impedance Z plotted as a planar vector using rectangular and polar coordinates.
21
2.5 Complex plane (a) and Bode (b) plots for a series connection of a resistance and a capacitance, (inset in (a)); R = 100 Ω, C
= 20 µF (Lasia, 1999).
23
2.6 Complex plane (a) and Bode (b), (c), plots for a parallel connection of R and C (inset in (a)); R = 100 Ω, C = 20 µF (Lasia, 1999).
24
2.7 Complex plane (b) and Bode (c), (d) plots for the circuit.
(inset in (a)); Rs = 10 Ω, Rct = 100 Ω, Cdl = 20µF (Lasia, 1999).
26
2.8 Circuit equivalent for a ceramic electrolyte according to Bauerle (1969) and modeling the impedance of the grain interiors (gi), grain boundaries (gb), and electrode (e).
27
xii
Figure No. Page No.
2.9 Impedance spectra for a zirconia solid electrolyte (ZrO2: 6 mole % Y2O3 at 240°C: (a) Experimental impedance spectrum. (b) Simulated impedance spectrum, using the circuit of Figure 2.8.
28
3.1 Metal pellet die set which consists of a punch, a die body
and a base. 33
3.2 Hydraulic pellet press. 33
3.3 YSZ green pellets.
34
3.4 Box-type laboratory muffle furnace.
34
3.5 Sintering profile of the YSZ pellets. 34
3.6 RF magnetron sputtering machine (a) and its control panel (b).
36
3.7 Schematic diagram showing the arrangement of the sintered pellets on a SS foil.
36
3.8 Schematic diagram of the sample experimental setup for IS. 40
4.1 Overlay of the XRD patterns of the YSZ thin films as a function of substrate temperature.
43
4.2 XRD pattern for sample S250C showing multiphase peaks with the corresponding diffraction planes.
43
4.3 SEM images showing surface morphologies of the thin films according to sputtering batches i.e. 150°C (a), 200°C (b), 250°C (c), 300 °C (d).
45
xiii
Figure No. Page No.
4.4 SEM images of the cross-sections of samples S150C (a) and S200C (b).
47
4.4 SEM images of the cross-sections of samples S250C (c) and S300C (d) showing the thin film layer (bright strip) and carbon paint layer (darker area). The black area is a gap due to exfoliation. .
48
4.5 Impedance spectra for YSZ thin films with varying Ts for the room temperature IS measurement.
50
4.6 Nyquist plot of the impedance spectrum for sample S150C (a). The experimental curve is the solid lines. The fitted curves (dashed lines) form the underlying semicircle components thus estimating Rs value. The inset depicts the equivalent circuit model for S150C, S200C, 250C and S300C spectra. The Rb, Rgb, Re and Rt are shown in the plots.
52
4.6 Nyquist plots of the impedance spectra for samples S200C (b) and S250C (c).
53
4.6 Nyquist plots of the impedance spectrum for sample S300C (d).
54
4.7 Variation of Rb and Rgb as a function of substrate temperature Ts.
56
4.8 Variation of electrode resistance Re and average grain sizes as a function of substrate temperature Ts.
57
4.9 Variation of total resistance Rt and total conductivity σt as a function of substrate temperature Ts.
58
4.10 Variation of the thin film average grain size as a function of Ts.
59
xiv
Figure No. Page No.
4.11 Bulk conductivity of YSZ pellets at various measurement temperatures by Abelard and Baumard in comparison with total conductivity of this work.
60
4.12 SEM image of the surface morphology of sample S300C after being heated at a temperature of 400°C.
60
xv
LIST OF ABBREVIATIONS
8YSZ 8-mol % yttria-doped/ - stabilized zirconia
m monoclinic
AC alternating-current MCFC molten carbonate fuel cell
AFC alkaline fuel cell MgO magnesia
Ar argon OCV open circuit voltage
a.u. arbitrary unit Pa Pascal
c cubic PAFC phosphoric acid fuel cell
CaO calcia PEFC polymer electrolyte fuel
CeO2 ceria PLD pulsed cell laser deposition
cps count per second PSZ partially stabilized
i i
CVD chemical vapor deposition PVD physical vapor deposition DAFC direct alcohol fuel cell RF radio frequency
DC direct-current RT room temperature e electrode S150C thin film sample with
150°C of Ts e-beam electron beam S200C thin film sample with
200°C of Ts
ed. /eds. edition/editions; editor, edited by
S250C thin film sample with 250°C of Ts
e.g. (exempligratia); for example
S300C thin film sample with 300°C of Ts
EIS electrochemical impedance
spectroscopy SAED selected area electron diffraction
et al. (et alia): and others sccm standard cubic centimeter per minute
EVD electrochemical vapor
deposition
SEI secondary electron imaging
FFT fast Fourier transform SEM scanning electron microscopy FRA frequency response analyzer SS stainless steel
gb grain boundaries S. W. T. Subhanahu Wa Ta'ala gi grain interiors t tetragonal i.e. id est; that is (to say) TZP tetragonal zirconia
HRTEM high resolution transmission electron
XRD x-ray diffraction
Hz hertz Y2O3 yttria IBAD ion-beam-assisted
deposition
YSZ yttria-stabilized zirconia IS impedance spectroscopy ZrO2 zirconia
xvi
IT-SOFC intermediate temperature solid oxide fuel cell
xvii
LIST OF SYMBOLS
θ diffraction angle
θ phase difference between the voltage and the current
A neutral gas atom
A electrolyte-electrode contact area
A+ positively charged ion
C capacitance
Cdl double-layer capacitance
d distance between electrodes
E electric field
e- electron
g gravitational acceleration
I current
i0 initial current
l thickness
L inductance
OхO lattice oxygen
P sputter gas pressure
Q constant phase element
R0 electrolyte resistance
R1 grain interior resistance
R2 grain boundary resistance
R3 electrode resistance
xviii
Rb bulk resistance
Rct charge transfer resistance
Re electrode interface resistance
Rgb grain boundary resistance
Rs ohmic resistance
Rt total resistance
σb AC bulk conductivity
σt total electrical conductivity Ts substrate temperature
f frequency
V voltage
VB breakdown voltage for the ionization of the sputter gas Vpeak AC voltage amplitude
vacancy in the oxygen site with double positive charge
ω angular frequency
Y admittance
Y’Zr Y in the Zr site with the apparent negative charge Z’ or Re(Z) real part of the complex impedance
-Z” or Im(Z) imaginary part of the complex impedance
Z(ω) complex impedance
1
CHAPTER 1 INTRODUCTION
1.1 OVERVIEW
The most important aspects in the study of oxygen ion conductors are the abilities to enhance their ionic conductivity and reaction kinetics. Both features are essential for the development of electrochemical devices including fuel cells, gas sensors and ionic membranes. These devices have the potential to deliver high economic and ecological benefits (non-polluting); however to achieve satisfactory performance, it is necessary to optimize the ionic conductivity of the solid electrolytes (Kosacki, 2005).
In oxygen ion conductors, current flow occurs by the movement of oxide ions through the crystal lattice. This movement is a result of thermally-activated hopping of the oxygen ions, moving from a crystal lattice site to another crystal lattice site, with a superimposed drift in the direction of the electric field. The ionic conductivity is consequently strongly temperature dependent. In fact, at high temperatures it can approach values close to 1 S.cm-1 that is comparable to the levels of ionic conductivity found in liquid electrolytes. This is clearly a remarkable property of these solids.
(Skinner and Kilner, 2003).
There are several requirements for these solid electrolytes to operate in various applications, among which the crystal must contain unoccupied sites equivalent to those occupied by the lattice oxygen ions. The energy involved in the process of migration from one site to the unoccupied equivalent site must be small, certainly less than about 1 eV. In order to overcome this small energy barrier the materials have to
2
possess a special and open crystal structures with partially-occupied oxygen sites for the oxygen ions to migrate in the electric field (Skinner and Kilner, 2003).
Most oxygen ion conductors are mixed conductors and only very few are capable of being classed as pure ionic conductors. In order to obtain a material that is a pure oxygen ion conductor (a solid electrolyte), the level of any electronic contribution to the total electrical conductivity must be negligible (Skinner and Kilner, 2003).
However, in most technological applications these materials are used under extreme conditions where the cathode and the anode are exposed to oxidizing and reducing atmospheres respectively at a temperature of 800°C and above. Under such extreme conditions, many oxides will reduce and the reduction process will liberate electrons and give rise to electronic conductivity. Very few materials meet the stringent requirements needed to function satisfactorily as an electrolyte in a device such as a solid oxide fuel cell (SOFC) (Skinner and Kilner, 2003). Yttria – stabilized zirconia is the most widely used solid electrolyte material in SOFCs due to its thermal and chemical stability.
Both ionic and mixed conductors have their applications. Ionic conductors can be used as the electrolyte in devices such as the SOFC and electrolytic oxygen separators. However, the mixed conductors are particularly useful and find applications as electrodes for both devices (Skinner and Kilner, 2003).
1.2 PROBLEM STATEMENT AND ITS SIGNIFICANCE
A key requirement for the intermediate temperature SOFC (IT-SOFC) is the reduction of ohmic losses. These occur across mixed ionic–electronic conducting electrodes as well as ionic conducting electrolyte, but the main contribution is from
3
the electrolyte, hence reduction of its thickness is most effective (Wang and Kim, 2008). An alternative solution to enhance the ionic conductivity involves increasing the mobility of the ion species. A number of recent studies have shown that this can be promoted when the material microstructure is in the nanometre range. This is attributed to grain boundary and interfacial effects, which can exhibit orders of magnitude greater diffusivity than that of the lattice (Philibert, 1991; Siegel, 1992;
Kosacki and Anderson, 2001). Approaches to improve device performance by lowering the internal resistance allowing operation at lower temperatures aim at processing these materials in the form of thin films, i.e. <1 μm thickness, as well as microstructural enhancement of the transport properties (Heiroth, Lippert, Wokaun and Döbeli, 2008).
1.3 RESEARCH OBJECTIVES
The objectives of the research are as follows:
i) To fabricate 8YSZ thin films (< 1 μm thickness) by using RF magnetron sputtering deposition technique.
ii) To characterize the fabricated thin films in terms of their crystallographic, microstructural and electrical properties (electrical conductivity).
iii) To analyze the factors which contribute to such high or low levels of electrical conductivity by correlating them with the characterization properties of the thin films.
4
1.4 RESEARCH METHODOLOGY
The principles of the research methods are as follows:
i) RF magnetron sputtering technique is used to fabricate YSZ thin films from the bulk material. The thin film deposition is conducted at a low pressure, using plasma and employing heating of the substrates at varying temperatures.
ii) Characterizations of the formed YSZ thin films are conducted to study their crystallographic, microstructural and electrical properties.
Comparisons of the properties were made for each batch of the thin films with the theoretical data and reported data of other researchers.
iii) The response of the thin films to small AC signal of current and voltage is studied using impedance spectroscopy (IS) in order to determine the electrical conductivity of the thin films. The aim is to achieve high electrical conductivity and the resulted value is evaluated by comparison with the available data obtained by other researchers. The key variable i.e.
substrate temperature Ts is set to several values with an expectation that a sound trend of electrical conductivity, resistances, average grain sizes and film thickness can be obtained.
The research methodology can be simplified in a flow chart as shown in Figure 1.1:
5
Figure 1.1: Research methodology in a flow chart
1.5RESEARCH SCOPE
Many efforts have been carried out to enable solid oxide fuel cell to operate at lower temperatures (i.e. from 600 – 800°C) than the normal operating temperatures (i.e.
from 600 – 1000°C) to avoid longer start-up time and electrolyte-electrode mechanical/chemical suitability problems.
Yttria-stabilized zirconia (YSZ) is an interesting material because of its high chemical stability in both oxidizing and reducing atmospheres, high level of ionic conductivity (Heuer and Hobbs, (Eds.), 1981; Subbarao, 1981; Baumard and Abelard, 1984; Minh, 1993) and low thermal conductivity. Also, YSZ is the most popular electrolyte material for solid oxide fuel cells (SOFCs) because it conducts only
RF magnetron sputtering process
Sample characterizations
XRD Surface morphology
analysis
Impedance spectroscopy Preparation of sputtering
substrates
Fabrication of sputter targets
Result analysis
RF magnetron sputtered YSZ thin films
6
oxygen ions over a wide range of oxygen partial pressures (Laukaitis, Galdikas, Čerepaitė-Trušinskienė, Dudonis and Milčius, 2006).
YSZ thin films are fabricated from a wide range of deposition techniques such as electrochemical vapor deposition (EVD) (Inaba et al., 1997), electrostatic spray deposition (Wang and Kim, 2008), plasma spraying (Zhang, Li, Liao, Planche, Li and Coddet, 2008), pulsed laser deposition (Heiroth, et. al, 2008), electron beam (e-beam) deposition (Laukaitis et al., 2006) and electrophoretic deposition(Ishihara, Sato and Takita, 2005). Physical vapor deposition (PVD) could be one of the best techniques for getting good quality YSZ thin films for SOFC electrolyte. It is easier to control thin film properties using PVD technology, in comparison with the other techniques (Laukaitis, Dudonis and Milcius, 2005).
In the present work ultra thin films of YSZ with various crystalline phases of controlled chemical composition are deposited by RF magnetron sputtering technique.
Their microstructural, crystallographic, and electrical properties are analysed as a function of deposition substrate temperature Ts. XRD and SEM are used to deduce the microstructural and crystallographic properties. Whereas impedance spectroscopy is utilized to determine the AC total and bulk conductivities of the YSZ thin films.
1.6 DISSERTATION OUTLINE
Chapter 1 of the dissertation gives a brief introduction of the research. The literature review is presented in Chapter 2. This chapter reviews the main aspects of YSZ material application in SOFC, the microstructural properties of zirconia, sputtering deposition process and impedance spectroscopy (IS) characterization technique.
Chapter 3 presents the experimental section of the research which details the fabrication of sputter targets, substrate preparation, RF magnetron sputtering process
