Title Page
Abstract
Contents
Chapter 1. Introduction 23
1.1. Dielectrics 23
1.1.1. Polarization in dielectrics 23
1.1.2. Electrical polarizability and relative permittivity in electromagnetism 25
1.1.3. Colossal dielectric constant 28
1.1.4. Control of dielectric permittivity in dielectric materials by external stimuli 30
1.2. Reversible manipulation of dielectric responses for sensing applications 31
1.3. Motivation: Protonation control of physical properties in complex oxides 32
1.4. Dielectric-based sensing applications 33
1.4.1. Multi-functional sensors with high efficiency and sensitivity 33
1.4.2. Multi-Layer Ceramic Capacitor (MLCC) 34
1.5. Our strategy: Hydrogen control of relative permittivity in dielectric oxides 35
1.5.1. Hydrogenation to oxygen-deficient oxides through water dissociation 35
1.6. Hypothesis 37
1.6.1. Protonation-driven ultrahigh dielectric constant in oxide ceramics 37
1.6.2. 6H-hexagonal BaTiO₃ : A material system for examining the protonation effect 38
References 40
Chapter 2. Experimental detail (method) 43
2.1. Material selection/synthesis: 6H-hexagonal BaTiO₃ by cation substitution 43
2.1.1. Phase stabilization of the 6H-hexagonal polymorph via a solid-state reaction method 43
2.2. Material characterization 44
2.2.1. X-ray diffraction (XRD) 44
2.2.2. Transmission electron microscopy (TEM) 45
2.2.3. X-ray photoelectron spectroscopy analyses 45
2.2.4. X-ray absorption spectroscopy measurements 45
2.2.5. Field emission scanning electron microscope (FE-SEM) and energy dispersive spectroscopy (EDS) analyses 46
2.2.6. Raman spectroscopy 46
2.2.7. Polarization-electric field hysteresis and piezoresponse force microscopy (PFM) measurements 46
2.2.8. Dielectric constant measurements 47
2.2.9. Impedance analyses 47
2.2.10. Time-of-flight secondary ion mass spectrometry measurements 47
2.3. Chemical environment treatments for protonation 48
2.3.1. Hydrogenation to oxide ceramics through specific treatments under ambient environments 48
2.4. Device tests for potential applications to multi-functional sensors 51
2.4.1. Sensing experiments in the 6H-hexagonal BaTiO₃ ceramics 51
Chapter 3. Polymorphic phase transition in BaTiO₃ by Ni substitution 53
3.1. Structural phase transition in complex oxides 53
3.2. Crystallographic structure and structural phase transition in BaTiO₃ 53
3.3. Hypothetical realization of 6H-hexagonal BaTiO₃ polymorph by cation substitution 54
3.4. Polymorphic phase transition in BaTiO₃ ceramics by Ni substitution 56
3.4.1. Structural evolution depending on Ni concentration 56
3.4.2. Ferroelectric-paraelectric-dielectric phase transition in BaTiO₃ by Ni substitution 64
3.5. Possible origin of room-temperature phase transition by cation substitution 69
3.7. Future studies in a 6H-hexagonal BaTiO₃ system 74
References 75
Chapter 4. Hydrogenation control of dielectric permittivity in 6H-hexagonal BaTiO₃ ceramics 78
4.1. Dielectric materials with ultrahigh dielectric permittivity 78
4.1.1. CaCu₃Ti₄O₁₂ 78
4.1.2. Undoped/doped BaTiO₃ 79
4.1.3. TiO₂-based materials 80
4.1.4. Double perovskite oxides 81
4.2. Engineering of physical properties in complex oxides by protonation 82
4.2.1. Manipulation of crystal structure in SrCoO₂.₅ by protonation 83
4.2.2. Hydrogen control of magnetic properties in (La, Sr)MnO₃ 84
4.2.3. Effect of hydrogenation on electrical properties in VO₂ 85
4.3. Hydrogenation-induced colossal dielectric responses in oxygen-deficient oxide ceramics 86
4.4. Experimental methods 88
4.5. Characterization of the Ni-substituted BaTiO₃ ceramics 90
4.5.1. Crystal and electronic structures 90
4.5.2. Surface morphology and chemical composition analyses 92
4.6. Ultrahigh dielectric responses by a specific treatment under ambient environments 94
4.7. Possible mechanism: Protonation-driven ultrahigh dielectric constant in oxide ceramics 103
4.7.1. Structural inhomogeneity induced by hydrogenation and the subsequent dielectric proximity effect 113
4.7.2. Formation of interfacial dielectric layers between electrodes and ceramics 115
4.7.3. Maxwell-Wagner effect 117
4.7.4. Electrical polarization induced by surface hydroxyl ions 123
4.7.5. Internal barrier layer capacitor at grain boundaries 126
References 128
Chapter 5. Reversible control of dielectric permittivity in oxide ceramics for sensing devices 133
5.1. Potential applications of the 6H-hexagonal BaTiO₃ ceramics for gas sensors 133
5.2. Set-up of sensing performance experiments 133
5.3. Change of dielectric permittivity under different ambient conditions 135
5.3.1. Ambient air environment 135
5.3.2. Humidity condition 137
5.3.3. Acetic acid environment 138
5.3.4. Water treatment 139
5.4. Reversible control of dielectric responses in Ni-substituted BaTiO₃ ceramics 141
5.5. Realization of oxide ceramic-based humidity sensors 143
References 146
Chapter 6. Further application of dielectric oxides to multi-functional devices 147
6.1. Hydrogen storage with high capacity 147
6.2. High-efficiency oxide catalysts for water splitting 148
References 150
Chapter 7. Summary and perspective 151
7.1. Summary 151
7.2. Perspective 152
Appendix: Detail user manual 154
A. Fabrication of Ni-substituted BaTiO₃ ceramics by solid-state reaction 154
B. X-ray diffraction measurements 158
C. Manual for impedance measurements 163
D. Temperature-dependent dielectric constant analyses 167
Publication List 171
Figure 1.1. The definition of electrical dipole moment. 24
Figure 1.2. Schematic of dielectric materials under the applied electric field. The dielectrics are polarized resulting in the formation of electrical polarization (P). 24
Figure 1.3. Real and imaginary parts of complex dielectric permittivity as a function of frequency. 27
Figure 1.4. Comparison of several dielectric compounds exhibiting the colossal dielectric permittivity in the previous studies. 28
Figure 1.5. Schematic illustration of the applications of dielectric materials in various fields. 30
Figure 1.6. Control of dielectric responses in complex oxides by external stimuli. (a) The photo- dielectric effect. (b) The magneto-dielectric coupling. (c) The mechanical stress-induced dielectric... 31
Figure 1.7. Reversible control of physical properties in complex oxides by hydrogenation. 33
Figure 1.8. A illustration of the construction of a multi-layer ceramic capacitor. 35
Figure 1.9. Introduction of hydrogen ions to oxygen-deficient oxides via water dissociation. (a) Oxygen vacancy defects as active sites for the water dissociation on the surface of complex oxides.... 36
Figure 1.10. The schematic illustrates a hypothesis of a change in dielectric responses of oxide ceramics induced by hydrogenation via water dissociation. 38
Figure 1.11. (a) A schematic 3D view of the 6H-hexagonal BaTiO₃ polymorph. (b) the hexagonal BaTiO₃ structure viewed along the [1120] direction.[이미지참조] 39
Figure 2.1. Synthesis of Ni-substituted BaTiO₃ ceramics by the conventional solid-state reaction method. 44
Figure 2.2. Experimental set-up of Ni-substituted BaTiO₃ ceramics under vacuum, nitrogen (N₂), carbon dioxide (CO₂), and high humidity environments. 49
Figure 2.3. A schematic represented the experimental set-up of the acid acetic treatment. 51
Figure 2.4. Experimental set-up of humidity sensing experiments 52
Figure 3.1. The schematic illustration of the polymorphic phase transition in perovskite BaTiO₃ with increasing temperature. 54
Figure 3.2. Schematic diagram of tetragonal-hexagonal phase transition in BaTiO₃ at room temperature induced by Ni substitution. 55
Figure 3.3. (a) X-ray diffraction (XRD) patterns of as-sintered Ni-substituted BaTiO₃ ceramics with various Ni concentrations (x). (b) The evolution of the XRD pattern around the 2θ angle of 45˚ as... 57
Figure 3.4. Raman spectra of Ni-substitution BaTiO₃ at different Ni concentrations (x=0.00, 0.03, and 0.125). 59
Figure 3.5. High-revolution transmission electron microscopy (HRTEM) images of (a) pure BaTiO₃, and (b) Ni-substituted BaTiO₃ at x=0.125. The corresponding selected area electron... 61
Figure 3.6. The electron diffraction (ED) pattern simulations and [experiments] of (a) [(c)] pure BaTiO₃, and (b) [(d)] Ni-substituted BaTiO₃ at x=0.125. Tables 1 and 2 show the space groups... 63
Figure 3.7. The SEM images of Ni-substituted BaTiO₃ at different Ni concentrations. 63
Figure 3.8. (a) P-E hysteresis loops of the Ni-substituted BaTiO₃ ceramics under external electric fields (i.e., triangular pulse fields with the amplitude of 50 kV/cm and the frequency of 10 Hz). (b)... 65
Figure 3.9. (a) Out of plane PFM amplitude images of pure BaTiO₃ and Ni-substituted BaTiO₃ (x=0.175). In the PFM amplitude images, bright regions have larger amplitude values than dark... 67
Figure 3.10. The dielectric permittivity as a function of temperature in the Ni-substituted BaTiO₃ ceramics at different frequencies (i.e., 1, 10, and 100 kHz). The solid arrow indicates the Curie... 68
Figure 3.11. (a) The Ni 2p₃/₂-edge XPS spectra in the Ni-substituted BaTiO₃ ceramics at different Ni concentration (x). At x<0.03, the raw XPS data (the black solid line) was fitted by the spectrum...[이미지참조] 70
Figure 3.12. (a) The Ni concentration (x) dependence of oxygen-vacancy XPS peak area in Ni- substituted BaTiO₃ ceramics. (b) The variation of Ti 2p₃/₂ peak position depending on the Ni level...[이미지참조] 72
Figure 4.1. (a) The crystal structure of CaCu₃Ti₄O₁₂. The blue, red, and yellow balls represent the Cu, O, and Ca atoms, respectively. (b) The dielectric constant as a function of temperature in CaCu₃Ti₄O₁₂. 79
Figure 4.2. Colossal dielectric constant in (a) pure and (b) La-doped BaTiO₃ ceramics. 80
Figure 4.3. (a) The temperature-dependent dielectric constant and dielectric loss in (In, Nb) codoped-TiO₂. (b) dielectric permittivity and loss as a function of frequency in (In, Nb) codoped-... 81
Figure 4.4. Ultrahigh dielectric permittivity in double perovskite oxides. The temperature- dependent dielectric constant and dielectric loss in double perovskite (a) Ba₂CoNbO₆ and (b)... 82
Figure 4.5. Reversible control of tri-state phase transformation among brownmillerite SrCoO₂.₅, perovskite SrCoO₃ and protonated (HSrCoO₂.₅) phases. The transition from brownmillerite... 84
Figure 4.6. Reversible changes in magnetic properties La₀.₆₇Sr₀.₃ MnO₃ (LSMO) films by hydrogenation (i.e. annealing in hydrogen) and dehydrogenation (i.e. annealing in argon). (a) The... 85
Figure 4.7. Hydrogenation-induced resistivity modulation of VO₂ thin films. The reversible resistivity changes through hydrogen/dehydrogenation. 86
Figure 4.8. The schematic illustration of a change in dielectric responses of oxide ceramics induced by hydrogenation through water dissociation. (a) For the as-sintered state, oxygen vacancy defects... 88
Figure 4.9. (a) XRD pattern of the as-sintered Ni-substituted BaTiO₃ ceramics. (b) The normalized O K-edge x-ray absorption spectra of the Ni-substituted BaTiO₃ ceramics. (c) Schematic diagram... 91
Figure 4.10. The fast Fourier transform (FFT) analyses of Ni-substituted BaTiO₃ ceramics. (a) The simulated electron diffraction pattern of the 6H-hexagonal BaTiO₃ polymorph along [112 0]...[이미지참조] 92
Figure 4.11. Surface morphology and elemental mapping analyses of the as-sintered Ni-substituted BaTiO₃ ceramics. (a) The field emission-scanning electron microscopy (FE-SEM) surface image... 94
Figure 4.12. The EDS measurements of the as-sintered Ni-substituted BaTiO₃ ceramics in the grain and grain boundary regions. (a) The field emission-scanning electron microscopy (FE-SEM)... 94
Figure 4.13. The change of dielectric responses in Ni-substituted BaTiO₃ ceramics under the ambient air environment. (a) A schematic diagram of the experimental sequence. Red, green, and... 97
Figure 4.14. The reversible change of the dielectric responses in BaTiO₃ ceramics through thermal annealing and air exposure. (a) A schematic figure of the experimental sequence. The treated Ni-... 99
Figure 4.15. The evolution of dielectric responses in Ni-substituted BaTiO₃ ceramics under various ambient environments. (a) Change of frequency-dependent dielectric constant in Ni-substituted... 101
Figure 4.16. Impedance analyses of Ni-substituted BaTiO₃ ceramics at two dielectric states (i.e., off- and on-states). (a) Experimental set-up of impedance analysis. b) Schematic diagram of a... 105
Figure 4.17. The impedance Cole-Cole plots in Ni-substituted BaTiO₃ ceramics at various temperatures for the off-state. 107
Figure 4.18. The impedance Cole-Cole plots in Ni-substituted BaTiO₃ ceramics at various temperatures for the on-state. 108
Figure 4.19. (a, b) XPS spectra at O 1s edge of the Ni-substituted BaTiO₃ ceramics at as-sintered state (a) and after 4 weeks in ambient air (b). The red, blue, and black solid lines in (a) and (b)... 110
Figure 4.20. X-ray diffraction (XRD) measurements for two different dielectric states. The XRD patterns of Ni-substituted BaTiO₃ ceramics for (a) the off- and (c) on-states. The corresponding... 113
Figure 4.21. A possible mechanism of ultrahigh dielectric permittivity in Ni-substituted BaTiO₃ ceramics induced by hydrogenation. 115
Figure 4.22. (a) The formation of interracial dielectric layers between the ceramic and electrodes. (b) The corresponding electrical equivalent of Ni-substituted BaTiO₃ ceramic capacitor with the... 117
Figure 4.23. (a) The capacitor system with two dielectric layers. (b) The corresponding electrical equivalent for two layer dielectrics in series. 118
Figure 4.24. The frequency-dependent dielectric response for the capacitor system with two dielectric layers. Solid lines indicate the overall dielectric response including the contribution of... 122
Figure 4.25. The possible mechanism of the ultrahigh dielectric permittivity in Ni-substituted BaTiO₃ ceramics by hydrogenation according to the Maxwell-Wagner effect. The interfacial... 123
Figure 4.26. The electrical dipole under a uniform electric field. The electrostatic force on the charges tends to rotate the dipole along the direction of electric field (E). 124
Figure 4.27. The alignment of polar hydroxyl ions under an applied electric field would induce electrical polarization resulting in high dielectric responses in Ni-substituted BaTiO₃. 125
Figure 4.28. The barrier layer capacitor (IBLC) model in polycrystalline ceramics. 127
Figure 5.1. Evolution of dielectric responses in Ni-substituted BaTiO₃ ceramics under the ambient air environment. (a) A schematic of the experimental sequence. (b) The frequency-dependent... 137
Figure 5.2. Change of (a) dielectric permittivity and (b) corresponding dielectric loss in Ni- substituted BaTiO₃ ceramics under the ambient air environment. 138
Figure 5.3. Change of dielectric responses in Ni-substituted BaTiO₃ ceramics under an acetic acid environment. (a and d) The dielectric constants as a function of frequency in (a) Ni-substituted and... 139
Figure 5.4. Change of dielectric responses in Ni-substituted BaTiO₃ ceramics induced by water treatment. (a) The sequence of the water-treatment experiment. We measured the dielectric... 141
Figure 5.5. The control of dielectric permittivity in Ni-substituted BaTiO₃ experimentally. (a) The schematic of the experimental sequence. (b) The dielectric constant and corresponding dielectric... 142
Figure 5.6. Sensing-performance experiments. (a) Schematic diagram of a humidity sensing experiment. (b) Resistive responses of Ni-substituted BaTiO₃ ceramics to the change in relative... 144
Figure 6.1. (a) A possible mechanism of the introduction and storage of hydrogen ions into oxygen- deficient oxide ceramics via hydrogenation. (b) Experimental evidence of the presence of... 148
Figure 6.2. The potential application of Ni-substituted BaTiO₃ ceramics as an oxide catalyst for the water splitting. 149
Figure A.1. The starting materials of BaCO₃(99.9%), TiO₂(99.9%), and NiO (99.9%) from High Purity Chemicals. 154
Figure A.2. (a) The mixed powders after ball-milling for 24 hours. (a) After drying at 100℃ for 24 hours, the powders were ground. 155
Figure A.3. (a) The calcination condition of Ni-substituted BaTiO₃ powders. We calcinated the powder at 1100℃ for 8 hours with a heating/cooling rate of 5℃/min. (b) The Ni-substituted... 156
Figure A.4. (a) The sintering condition of Ni-substituted BaTiO₃ ceramic pellets. The ceramics were calcinated at 1330℃ for 6 hours. Before that, we annealed the ceramics pellets at 500℃ for... 157
Figure B.1. Bruker D8 Advance equipment. 158
Figure B.2. The setting of the soller slit and Ni filter for the XRD measurements of polycrystalline ceramics. 159
Figure B.3. A soller slit and Cu filter were installed in front of the detector. 159
Figure B.4. Setting the measurement mode. 160
Figure B.5. Initialization process. 160
Figure B.6. X-ray generator and detector settings. 161
Figure B.7. Full beam alignment. 161
Figure B.8. Mounting the ceramics sample on the holder. 162
Figure B.9. The 2Theta-Omega scan after alignment. 162
Figure C.1. Experimental set-up of the impedance measurement. The impedance analyses were carried out by a HIOKI 3522-50 analyzer. 163
Figure C.2. The silver paste (SJ-41-557, Sung Lee Tech. Co.) was used to make the electrodes of ceramics samples. (b) the Ni-substituted BaTiO₃ ceramics without electrodes and with Ag electrodes 164
Figure C.3. The Ni-substituted BaTiO₃ ceramics with electrodes were contacted to the analyzer by silver wires. 165
Figure C.4. Open the program with the path of: Computer/New Volume (F:)/HIOKI_2012/Hioki-Sk935_IS_D. 165
Figure C.5. The measurement setting. (1) Set the frequency range from 1 (startF) to 100000 Hz (stopF). (2) We put the sample name in Material Name and the folder to save data in Path_File... 166
Figure C.6. The raw data of impedance measurement. The columns of A, B, C, D, E, and F correspond to the temperature, frequency, impedance, phase, capacitance, and dielectric loss of... 167
Figure D.1. Experimental set-up of the temperature-dependent dielectric constant. A HP4192A analyzer was used to measure dielectric permittivity as a function of temperature. 168
Figure D.2. The Ni-substituted BaTiO₃ ceramics with electrodes were contacted to the analyzer by silver wires. The samples were put into the tube furnace. 169
Figure D.3. Open the program with the path of: Computer/New Volume (F:)/HP4192A/HP4192A/4192A_71_2017. 169
Figure D.4. The measurement setting. (1) Set the temperature range. (2) Set the frequency range. We put the sample name in (3) Material Name and the folder to save data in (4) Path_File Name... 170