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Structural characterization of CuO-doped alkali niobate piezoelectric ceramics by electron paramagnetic resonance spectroscopy [Elektronische Ressource] / vorgelegt von Ebru, Erünal

157 pages
Structural Characterization ofCuO-doped Alkali NiobatePiezoelectric Ceramics byElectron Paramagnetic ResonanceSpectroscopyINAUGURALDISSERTATIONzur Erlangung des Doktorgradesder Fakultät für Chemie, Pharmazie und Geowissenschaftender Albert-Ludwigs-Universität Freiburg im Breisgauvorgelegt vonEbru, Erünalaus Ankara, Türkei2011Vorsitzender des Promotionsausschusses : Prof. Dr. Rolf SchubertReferent : PD. Dr. Rüdiger-A. EichelKorreferent : Prof. Dr. Stefan WeberDatum der Promotion : 02.02.2011AbstractAlkali niobate ceramics are promising ‘lead-free’ alternatives for currently usedPb(Zr,Ti)O piezoelectric materials. In this thesis, the defect structure of CuO-doped3alkali niobate ceramics were investigated depending on the alkali and niobiumnon-stoichiometry, process conditions (calcination and sintering) and CuO-dopingamount by means of multi-frequency Electron Paramagnetic Resonance (EPR)Spectroscopy. Moreover, the atomic scale information provided through EPR analysiswas supported by theoretical DFT calculations and compared to a microstructural(with XRD and SEM characterization) analysis. The samples under study wereprepared through a mixed-oxide-carbonate route. The CuO-doping content was keptcomparatively low at 0.25 mol% to minimize the secondary-phase formation effects.
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Structural Characterization of
CuO-doped Alkali Niobate
Piezoelectric Ceramics by
Electron Paramagnetic Resonance
Spectroscopy
INAUGURALDISSERTATION
zur Erlangung des Doktorgrades
der Fakultät für Chemie, Pharmazie und Geowissenschaften
der Albert-Ludwigs-Universität Freiburg im Breisgau
vorgelegt von
Ebru, Erünal
aus Ankara, Türkei
2011Vorsitzender des Promotionsausschusses : Prof. Dr. Rolf Schubert
Referent : PD. Dr. Rüdiger-A. Eichel
Korreferent : Prof. Dr. Stefan Weber
Datum der Promotion : 02.02.2011Abstract
Alkali niobate ceramics are promising ‘lead-free’ alternatives for currently used
Pb(Zr,Ti)O piezoelectric materials. In this thesis, the defect structure of CuO-doped3
alkali niobate ceramics were investigated depending on the alkali and niobium
non-stoichiometry, process conditions (calcination and sintering) and CuO-doping
amount by means of multi-frequency Electron Paramagnetic Resonance (EPR)
Spectroscopy. Moreover, the atomic scale information provided through EPR analysis
was supported by theoretical DFT calculations and compared to a microstructural
(with XRD and SEM characterization) analysis. The samples under study were
prepared through a mixed-oxide-carbonate route. The CuO-doping content was kept
comparatively low at 0.25 mol% to minimize the secondary-phase formation effects. In
consideration of a complete picture, the technologically promising morphotropic-phase
boundary materials (K Na )NbO (KNN 50/50) were studied together with the end0:5 0:5 3
members of the pseudo-binary solid-solution system, KNbO (KN) and NaNbO (NN).3 3
Besides, in order to transfer the determined set of spin Hamiltonian parameters into
structural information, a semi-empirical relationship between the spin-Hamiltonian
2+parameters and the defect structure of Cu systems with low spin (S = 1/2) was
2+developed. By this way, the existence of ‘isolated’ Cu centers in a complete oxygen
2+octahedron or the formation of a dimeric defect complex (V Cu ) or a trimericO
2+
defect complex (V Cu V ) with charge-compensating oxygen vacanies (V ) canO O O
be directly assigned. The alkali or niobium non-stoichiometry has a strong influence
on the defect structure for CuO-doped NN and KNN 50/50. As the niobium amount
2+increased in the system, more Cu incorporation is observed. Furthermore, the
2+ 5+site of incorporation of Cu has been detected as the Nb site. This triggers the
formation of mutually compensating dimeric and trimeric defect complexes. However,
+ +their relative concentrations depend strongly on the alkaline (Na , K ) and niobium
5+(Nb ) non-stoichiometry. Differently from the KN and KNN 50/50, for NN samples a
2+ 2+secondary phase invoking a Cu - Cu -dimeric functional center was obtained. Apart
from this, for KNN 50/50, as the doping amount of CuO is increased above 0.25
mol%, the formation of a secondary phase (K CuNb O ) was distinguished both by4 8 23
2+ +XRD and EPR. Lastly, the nature of chemical bonding of Cu with the Na ion in
CuO-doped KNN 50/50 was analyzed by ‘Hyperfine Sublevel Correlation Experiments’
+(HYSCORE). The transferred spin density to the Na ions in KNN 50/50, showed a
considerably reduced covalent bonding to the A-site ion than in CuO doped PZT
system.
KEYWORDS:Lead-Free Piezoelectrics,CuO doping, KNN,KN,NN,Solid State EPR,
Defect Chemistry, Oxygen Vacancies
iiiTo my family:
my mother, Nurten Dodanlı Erünal,
my father, Nurdoˇgan Erünal,
my life companion, Başar ÇaˇglarAcknowledgements
This thesis represents the last step of a hard, challenging, didactic, and maturing
adventure of mine in Germany. I would like to thank everyone who has been a
significant part of this special journey.
I wish to express my deepest appreciations for the never ending help, patience and
encouragement provided by my thesis advisor, Assoc. Prof. Dr. Rudiger-A. Eichel. I
would not be able to start and finish this thesis without his support. His optimistic
motivation, creative and analytical ways to solve problems gave me the strength to
keep this research going on and taught me a lot in my life. I will always admire his
patience and positive attitude in every kind of hard situation. I am also thankful to my
co-supervisor, Professor Stefan Weber, who gave me the opportunity to use the EPR
laboratories and spectrometers and as well for his guidance and helpful discussions.
Additionally, I am grateful to Prof. Gerd Kothe whose knowledge and experience
assisted me about EPR. Besides, the valuable contributions of Prof. Donald Smyth
and Dr. Josef Granwehr are greatly acknowledged.
I want to voice my appreciation of our project collaborators whose contributions,
ethusiasm, harmony and support always kept me motivated in this project and created
this incredible work: Prof. Dr. Michael J. Hoffman, Dr. Hans Kungl, Jerome Acker
(fromKarlsruheInstituteofTechnology)whoprovidedtheKNNsamples,andProf. Dr.
Christian Elsasser and Sabine Körbel who provided the theoretical calculations from
Fraunhofer Institute. Also for the financial support, many thanks to DFG EI498/1-1
and 1-2. Again, I am grateful to Sabine Körbel for her incredible friendship in Freiburg,
always being there both in my hard times and happy moments.
Moerover, I am grateful to Dr. Peter Jakes for helping me in every kind of scientific
problem, motivating me to write my thesis and especially his endless support in
EPR laboratory, without his contributions my work wouldn’t be what it is today.
Additionally, I would like to thank all co-workers, especially Thomas Berthold who
provided technical support in EPR laboratory, Marco Beaumont for his assitance with
LaTeX, Dr. Emre Erdem, Michael M. D Drahus, Dr. Mikail Lukaschek, and Dr. Erik
Schleicher for their helpful discussions. In addition, I am thankful to Ilkin Kokal for
his contributions. Lastly, among my co-workers, I want to mention my appreciation to
Müge Aksoyoˇglu whose joyful and sincere friendship brought a new dimension both in
my work and my life.
I will always remember my true friends from Darmstadt, Freiburg, Ankara and
Eindhoven for their companionship and support. Particularly, I am thankful to all my
friends who delighted my life in Germany and gave me strength to go on. Especially
Dr. Murat Cetinkaya, Dr. Daniel Gallichian, Kamil Kiraz, Agnes Sitompul-my beloved
‘Kanka’, all my CACTUS friends, Miriam Ronsdorf, my singing teacher Vera Joppig
and my crazy flatmates Michael Berger and Cristoph Öxle and all others whose names
couldn’t be mentioned here but always be in my memories. Also, I owe thanks to Prof.
Deniz Üner for her lifelong consultancy.
I am cordially thankful to my parents – Nurten Dodanlı Erünal and Nurdoˇ gan Erünal–
ivfor their endless love, guidance, and encouranging me to achieve my goals. There are
not enough words to show my gratitude to my mother who devoted her life to me and
my father who prays day and night for me. Lastly, I want to thank a very special
person, Başar Çaˇ glar, whose love, patience and faith never seperated us despite long
distances and hard times: Thanks for never stop holding my hand, always being my
true and only love. I am very lucky to have such a great love and a valuable person
like him in my life.
vContents
I Theory 1
1 Introduction 3
2 Ferroelectric Materials 5
2.1 Ferroelectrics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
2.1.1 Origin of Ferroelectricity on an Atomic Scale . . . . . . . . . . . 7
2.1.2 Ferroelectric Domains . . . . . . . . . . . . . . . . . . . . . . . 9
2.2 Piezoelectrics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
3 Defect Chemistry 13
3.1 Defect Notation: Kröger and Vink Notation . . . . . . . . . . . . . . . 14
3.2 Thermodynamics of Defects . . . . . . . . . . . . . . . . . . . . . . . . 15
3.3 Intrinsic and Extrinsic Point Defects . . . . . . . . . . . . . . . . . . . 16
3.4 Influence of Dopant Ions on Defect Structure . . . . . . . . . . . . . . . 17
3.5 Defect Chemistry in KNN . . . . . . . . . . . . . . . . . . . . . . . . . 18
4 EPR Theory 21
4.1 Understanding the EPR Spectrum . . . . . . . . . . . . . . . . . . . . . 22
4.1.1 Electron Zeeman Interaction . . . . . . . . . . . . . . . . . . . . 23
4.1.2 Hyperfine Interaction . . . . . . . . . . . . . . . . . . . . . . . . 26
4.2 EPR Spectra of Powder Samples . . . . . . . . . . . . . . . . . . . . . 28
4.2.1 Homogeneous and Inhomogeneous Line-Broadening . . . . . . . 28
4.2.2 g- and A-strain Effects . . . . . . . . . . . . . . . . . . . . . . . 29
4.3 The Full Spin Hamiltonian . . . . . . . . . . . . . . . . . . . . . . . . . 30
4.4 Methods in EPR . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
4.4.1 Continuous Wave Measurements . . . . . . . . . . . . . . . . . . 32
4.4.1.1 Multifrequency EPR . . . . . . . . . . . . . . . . . . . 32
4.4.2 Pulsed Measurements . . . . . . . . . . . . . . . . . . . . . . . . 33
4.4.2.1 Hahn Echo Sequence . . . . . . . . . . . . . . . . . . . 33
4.4.2.2 Hyperfine Sublevel Correlation Experiments
(HYSCORE) . . . . . . . . . . . . . . . . . . . . . . . 34
5 Experimental 37
5.1 Samples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
5.2 Synthesizing Conditions . . . . . . . . . . . . . . . . . . . . . . . . . . 37
vi5.2.1 Characterization Methods . . . . . . . . . . . . . . . . . . . . . 39
5.3 EPR Measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
5.3.1 Continuous Wave Measurements . . . . . . . . . . . . . . . . . . 40
5.3.2 Pulsed Measurements . . . . . . . . . . . . . . . . . . . . . . . . 40
5.3.3 Numerical Spectrum Simulations . . . . . . . . . . . . . . . . . 41
II Results and Discussions 43
6 Defect Structure and Formation of Defect Complexes in
2+Cu -Modified Metal Oxides: A Semi-Empirical Spin-Hamiltonian
Parameter Analysis 47
7 CuO-doped Potassium Niobate 55
7.1 Introduction to KNbO . . . . . . . . . . . . . . . . . . . . . . . . . . . 553
7.2 Sample Characterization . . . . . . . . . . . . . . . . . . . . . . . . . . 56
2+7.3 EPR Analysis of the Cu Center in KNbO Ceramics . . . . . . . . . 583
7.3.1 Impact of Potassium and Niobium Non-stoichiometry . . . . . . 58
7.3.2 Low Temperature EPR Measurements . . . . . . . . . . . . . . 60
7.3.3 Multifrequency EPR Measurements (X- and Q-Band) and
Numerical Spectrum Simulations . . . . . . . . . . . . . . . . . 61
7.4 Comparison of EPR Results with Theoretical Calculations . . . . . . . 64
7.5 Defect Chemistry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65
7.6 The Effects of Defect Complexes on Ferroelectric and Piezoelectric
Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67
8 CuO-doped Sodium Niobate 69
8.1 Introduction to NaNbO . . . . . . . . . . . . . . . . . . . . . . . . . . 693
8.2 Sample Characterization . . . . . . . . . . . . . . . . . . . . . . . . . . 70
2+8.3 EPR Analysis of the Cu Center in NaNbO Ceramics . . . . . . . . . 723
8.3.1 Impact of Sodium and Niobium Non-Stoichiometry . . . . . . . 72
8.3.2 Low Temperature EPR Measurements . . . . . . . . . . . . . . 74
8.3.3 Impact of Synthesizing Conditions . . . . . . . . . . . . . . . . . 78
8.3.3.1 Calcination and Sintering Behaviour . . . . . . . . . . 78
8.3.3.2 Effect of Sintering Temperature . . . . . . . . . . . . . 80
8.4 Numerical Simulations . . . . . . . . . . . . . . . . . . . . . . . . . . . 81
8.4.1 A-excess Sample . . . . . . . . . . . . . . . . . . . . . . . . . . 81
8.4.2 B-excess . . . . . . . . . . . . . . . . . . . . . . . . . . . 85
8.5 Defect Chemistry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88
vii9 CuO-doped Sodium Potassium Niobates 91
9.1 Introduction to (K,Na)NbO . . . . . . . . . . . . . . . . . . . . . . . . 923
9.2 Sample Characterization . . . . . . . . . . . . . . . . . . . . . . . . . . 95
2+9.3 EPR Analysis of the Cu Center in (K,Na)NbO Ceramics . . . . . . 973
9.3.1 Impact of Potassium-Sodium and Niobium Non-Stoichiometry . 97
9.3.2 Low Temperature EPR Measurements . . . . . . . . . . . . . . 99
9.3.3 Impact of Synthesizing Conditions . . . . . . . . . . . . . . . . . 101
9.3.3.1 Calcination-Sintering Behaviour and
Secondary Phase (K CuNb O ) Formation . . . . . . 1014 8 23
9.3.3.2 Effect of Sintering Temperature . . . . . . . . . . . . . 102
9.3.3.3 CuO-Doping Amount . . . . . . . . . . . . . . . . . . 104
9.3.4 HYSCORE Measurements . . . . . . . . . . . . . . . . . . . . . 106
9.4 Numerical Simulations . . . . . . . . . . . . . . . . . . . . . . . . . . . 106
9.5 Interaction of Defect Complexes with Domain Walls . . . . . . . . . . . 108
9.6 Comparison of EPR Results with Theoretical Calculations . . . . . . . 111
9.7 Defect Chemistry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112
9.8 Comparison of CuO-doping on KNN and PZT . . . . . . . . . . . . . . 113
10 Conclusion 125
viii

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