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Title Page

Abstract

Contents

Explanation of terms and abbreviations 13

1. Introduction 15

References 16

2. Theory and Literature Reviews 17

2.1. Brief History of Ferroelectric Ceramics 17

References 21

2.2. Fundamentals of ferroelectrics 24

2.2.1. Dielectrics and polarization mechanism 24

2.2.2. Ferroelectric phase transition 42

References 45

2.3. Relaxor ferroelectrics 46

2.3.1. Phenomenological signatures of relaxors 46

2.3.2. Models describing relaxation mechanisms 49

2.3.3. Phase transition from ferroelectric to relaxor state 55

2.3.4. Lead-free relaxor ferroelectrics 57

References 59

3. Concept and Aims 63

4. Results and Discussions 65

4.1. Dielectric spectra of a relaxor PLZT 65

4.1.1. Introduction 65

4.1.2. Experimental procedure 66

4.1.3. Experimental results 67

4.1.4. Summary 74

References 75

4.2. Dielectric spectra of a relaxor PST 77

4.2.1. Introduction 77

4.2.2. Experimental procedure 77

4.2.3. Experimental results 79

4.2.4. Summary 83

References 83

4.3. The role of A-site deficiency of BNT-Based relaxor ferroelectrics. 84

4.3.1. Introduction 84

4.3.2. Experimental procedure 85

4.3.3. Experimental results 86

4.3.4. Summary 93

References 93

4.4. in-situ monitoring of a polarization reversal from a ferroelectric-to-relaxor state 96

4.4.1. Introduction 96

4.4.2. Experimental procedure 96

4.4.2. Experimental results 98

4.4.4. Summary 105

References 106

5. Summary 108

Research Achievements 110

List of Tables

Table 1. B-site ordered/disordered complex perovskite. 53

Table 2. Summary of α and β values at different temperatures from the fitting of Havriliak-... 70

Table 3. Fullprof simulation parameter for R3m and R3c 101

Table 4. The Ed and EF-R of PLZT and BNT-6BT[이미지참조] 104

List of Figures

Fig. 1. a) Rochelle salt and b) potassium dihydrogen phosphate (KDP). 17

Fig. 2. (a) ABO₃ perovskite structure, (b) phase transition of BaTiO₃ 17

Fig. 3. Comparison of temperature dependent dielectric permittivity between typical a)... 18

Fig. 4. Hubble space telescope and its relaxor application (PMN multilayers tilt and... 19

Fig. 5. Compliances with restriction of harmful material usage 19

Fig. 6. Applications of piezoceramics: a) and b) small size cooler, c) speaker, d) linear actuator,... 20

Fig. 7. Schematics of polarized materials in a unit volume. 24

Fig. 8. Schematics of (a) vacuum permittivity and (b) a capacitor 25

Fig. 9. Schematics of the individual polarization p and the local electric field Eloc.[이미지참조] 26

Fig. 10. A gaussian sphere surface surrounding a point charge. 27

Fig. 11. Polarization process of electrons, ions, permanent dipoles, space charge and domain... 29

Fig. 12. Frequency dependence of the dielectric permittivity. 31

Fig. 13. (a) An ideal capacitor with sinusoidal voltage application. Phasor diagrams for (b) an... 32

Fig. 14. (a) Capacitative Ic and 'loss' components Iloss of total current I and (b) the equvalent...[이미지참조] 34

Fig. 15. The relaxation behavior of polarization after electric-field (a) on and (b) off. 35

Fig. 16. The Debye relaxation. 38

Fig. 17. History of the complex permittivity equation from Debye model to Havriliak-Negami equation. 39

Fig. 18. Comparison between a linear spring and the ionic and electric oscillation. 39

Fig. 19. a) the phase shifting between the input electric field and output displacement, and b)... 41

Fig. 20. A phase transition of BaTiO₃ single crystal with lattice parameter, spontaneus... 42

Fig. 21. Burns temperature and Curie-weiss law. 46

Fig. 22. Vogel-Fulture relationship with poled and unpoled state 47

Fig. 23. Schematics of electrically induced long-range order 48

Fig. 24. Aging effect of PLZT aged at RT for a serveral month 48

Fig. 25. Conceptual draws of a) normal ferroelectrics b) the diffuse phase transition with local... 49

Fig. 26. Schematic paragraph of superparaelectric model in a rhombohedral symmetry (a) free... 50

Fig. 27. a) Comparison between Arrhenius fit and Vogel-Fulture fit for dielectric... 51

Fig. 28. Comparision of PST relaxor (a) B-site ordered perovskite (b) B-site disordered... 52

Fig. 29. conceptual draws of random-field induced by different charge cations of a) ABO₃ and... 53

Fig. 30. Oxygen octahedral tilting in ABO₃ structure. 54

Fig. 31. Phase transition near TF-R.[이미지참조] 55

Fig. 32. Phase transition of PLZT and schematic draw of NR to ER transtion. 56

Fig. 33. Strain measured with an initial unpoled state of BNT-BT and BNT-6BT-2KNN system... 57

Fig. 34. Compared strain of incipient piezoelectricity with commercial PZT (PIC151) and... 58

Fig. 35. Comparison of Tf and TF-R of PLZT (left) and BNT-6BT (right).[이미지참조] 59

Fig. 36. Puzzle of general consensus of results and causes for lead-based relaxors. 63

Fig. 37. Puzzle of results and causes for lead-based relaxors. 64

Fig. 38. Hot pressed PLZT made by (Boston Applied Technologies, MA, US) 66

Fig. 39. Examples of multiple fitting of Havrilriak-Negami equation using WINFIT program. 66

Fig. 40. frequency dependent (a) real part and (b) imaginary part of dielectric permittivity 67

Fig. 41. (a) Cole-Cole plots of selected temperature. (b) Deconvoluted cole-cole plot at 60℃. (c)... 68

Fig. 42. Real and imaginary permittivity of the (a) slow, (b) intermediate and (c) fast relaxation... 71

Fig. 43. Summary of involvement of each relaxation process at 1 ㎑. 72

Fig. 44. (a) Time dependent ε' and ε" with the exponential decay fitting at various temperature... 73

Fig. 45. Experimental procedure of PST sample fabrication. 77

Fig. 46. Closed crucible filled with sacrificial powder (a) Closed crucible like 'Russian-... 78

Fig. 47. Differences of major properties of B-site ordered PST (PST-O) and disordered PST... 79

Fig. 48. Electric-field-induced properties of PST-O. 80

Fig. 49. Electric field induced properties of PST-D. 80

Fig. 50. The temperature-dependent dielectric permittivity response with polarization hysteresis... 81

Fig. 51. Colo-Cole plot of a) PST-D and b) PST-D 81

Fig. 52. the real part and imaginary part of (a) PST-O and (b) PST-D 82

Fig. 53. Experimental procedure 85

Fig. 54. X-ray diffraction patterns of BNT-xBT with the Na deficiencies (the subscript "pc"... 86

Fig. 55. Comparison of electric-field-induced properties of BNT-xBT according to the BT... 88

Fig. 56. Concept of stress field induced by depect dipoles 89

Fig. 57. Temperature dependent dielectric permittivity and losses of poled samples of a) BNT, b)... 90

Fig. 58. TEM analysis of BNT-40BT-0A and BNT-40BT-4A ceramics, the apparent splitting in... 91

Fig. 59. TEM analysis of Na-deficient BNT-40BT-4A ceramics. TEM analysis of grains occupied... 92

Fig. 60. Concept of polarization swtiching of ferroelectrics and relaxors 96

Fig. 61. Schematics draw of in-situ nuetron diffraction. 97

Fig. 62. Schematic draw of in-situ TEM sample praperation. 97

Fig. 63. Polarization hysteresis and switching current compared with electrocaloric effect of (a)... 98

Fig. 64. Change in structure with poling from unpoled state at 23℃ and maximum field of 2kV/mm. 99

Fig. 65. (111) lattice strain, 1/2(311) and (210) peak intensity, and polarization hysteresis of PLZT...[이미지참조] 100

Fig. 66. Comparison of the neutron diffraction pattern simulation between R3m and R3c. 101

Fig. 67. In-situ TEM direct observation on the electric field-induced relaxor-to-ferroelectric and... 102

Fig. 68. Determination of Ed, ER-F, and EZP.[이미지참조] 104

초록보기

Since the first discovery of relaxor ferroelectrics (relaxors), a lot of dedication have been fulfilled to extend the knowledge and understanding for relaxor features. Even though the true origins of their high performances are not fully understood, relaxors have been widely used due to their versatile ferroelectric/piezoelectric properties and temperature stability. To figure out the true origin of relaxor features still remains extremely hard since various results indicate that the causes of relaxation processes are appearing in atomic level scale with an application of electric field.

Various consensual models, describing correlations between relaxor features and causes, have been made thanks to a lot of dedications. The existence of polar nanoregions (PNRs) is considered as a key role to carry out relaxor features. It leads to frequency dispersive dielectric maxima, which is the most important features introducing relaxor ferroelectrics. Therefore, a proper understanding of PNRs is a key to unveiling all the existing questions in relaxor ferroelectrics.

Presently, lead-free relaxors become essential since we are under the pressure to find replacements for lead-containing relaxors in response to environmental regulations. However, extensive researches indicate that commonly accepted concepts from lead-based relaxors are not always applicable to lead-free relaxors; for example, the double dielectric permittivity maxima and a deviation of temperature between depolarization (Td) and a transition from a ferroelectric-to-relaxor transition (TF-R).

This work contains dedications to explain major relaxor enigmas as followings,

1) The relaxation processes mainly affect to the frequency dispersive dielectric response were investigated using canonical relaxor Pb0.92La0.08(Zr0.65Ti0.35)0.98O3 (PLZT) and Pb(Sc1/2Ta1/2)O3 (PST). It was able to deconvolute the plots, revealing three distinct processes, namely, slow, intermediate, and fast relaxtor, in terms of their characteristic relaxation time.

2) Lead-free (1-x)Bi1/2Na1/2TiO3-xBaTiO3 (BNT-xBT) system with artificially made deficiencies were investigated to define the correlation between the degree of random-field and relaxor features. The random-field is considered as the origin of creation of PNRs caused from random charges and/or random cation vacancies, which is well an accepted model for both lead-based and lead-free relaxors.

3) The conceptually suggested two-step process for a transition from a ferroelectric-to-relaxor state is observed in PLZT by electric-field induced in-situ monitoring methods such as an electrocaloric effect measurement, neutron diffraction and transmission electron microscopy.

4) The correlation between temperature-dependent and electric-field-induced transition from a ferroelectric-to-relaxor state, related to a deviation between the Td and the TF-R, is defined using ferroelectric PIC151, lead-based relaxor PLZT and lead-free BNT-BT.

The major contribution of this dissertation is to expand our insight into what truly happens in relaxors. New systematic approaches will be discussed in detail in the body of this dissertation.

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