초록 ▼

○ 본 연구는 프로톤전도성세라믹의 다종수송현상에 대한 실험 및 전산모사연구로 1단계에서는 벌크에서의 다종수송현상을 직접적으로 모니터할 수 있는 실험 연구방법으로 고온 van der Pauw 법과 AC 모니터링법을 개발하였고 AC법의 경우 다종수송현상을 전극과 입계의 선택적 차단효과를 바탕으로 이종의 수송자의 병렬회로에 기초한 트랜스미션라인 임피던스 모델을 적용함으로서 온도와 분위기 전환에 따른 프로톤전도체의 다종수송 양상을 DC법과 차별되는 멀티패러미터의 정보로 얻게 하였음. 입계저항은 쇼트키장벽의 성격보다는 구조변형에 의한 프로톤

○ 본 연구는 프로톤전도성세라믹의 다종수송현상에 대한 실험 및 전산모사연구로 1단계에서는 벌크에서의 다종수송현상을 직접적으로 모니터할 수 있는 실험 연구방법으로 고온 van der Pauw 법과 AC 모니터링법을 개발하였고 AC법의 경우 다종수송현상을 전극과 입계의 선택적 차단효과를 바탕으로 이종의 수송자의 병렬회로에 기초한 트랜스미션라인 임피던스 모델을 적용함으로서 온도와 분위기 전환에 따른 프로톤전도체의 다종수송 양상을 DC법과 차별되는 멀티패러미터의 정보로 얻게 하였음. 입계저항은 쇼트키장벽의 성격보다는 구조변형에 의한 프로톤전달 차단효과에 의한 전자(정공)의 전도도를 나타내는 것으로 규명되고 다양한 양상의 프로톤전도체에 적용할 수 있었음.
○ 병행된 전산모사는 BZO와 BCO의 다양한 결함의 형성에너지 및 이동도를 계산하고 도펀트 종류 및 농도의 효과 입계 효과에 대한 원자적 계산을 진행하여 매크로 분석에 의한 결과와 연계되어 다종수송현상에 대한 이해의 진일보를 이루게 함.

Abstract ▼

Ⅳ. Results
1. Multi-species transport phenomena in protonic ceramics: Experiments
A. New electrical model for protonic ceramic conductors
Impedance spectroscopy is the basic tool for the characterization of solid electrolytes. A new electrical model for protonic ceramic conductors was devel

Ⅳ. Results
1. Multi-species transport phenomena in protonic ceramics: Experiments
A. New electrical model for protonic ceramic conductors
Impedance spectroscopy is the basic tool for the characterization of solid electrolytes. A new electrical model for protonic ceramic conductors was developed which is in contrast to the conventional series circuit model for the grain, grain boundary and electrodes. It is shown that a parallel network of two resistor elements, R1 and R2, where the R1 resistor rail is selectively blocked, can describe the impedance response of the protonic ceramic conductors more appropriately. As well as describing the characteristic impedance spectral features, the decisive difference between the two models is that the conventionally known grain boundary impedance of higher activation energy determines the total resistance of the sample at high temperature. Selective blocking of protonic carriers at grain boundaries leads to chemical polarization in the grains, which is also known as Hebb-Wagner polarization, by the selectivly blocking electrodes. Therefore the overall sample resistance is suggested to represent the electron (hole) conduction. This model has been previously suggested by Jamink et al. based on the Schottky barrier effects on oxide ions and electron holes. However, selective blocking effects is not expected for electron holes and protons of the same charge. Atomistic simulation work showed that the structural disorder increases the hopping barrier across the grain boundaries to support selective blocking effects. The physical origin of circuit components can be consistently described.
B. Application to various ceramic proton conductors
The Y-doped barium zirconate electrolyte has been chosen as the target electrolyte material for PCFCs by the hub institute. The new electrical modeling for ceramic proton conductors has been applied to various yttrium-doped barium zirconates (BZY) at temperatures below 500K where interaction with the gas phase is assumed to be negligible. High quality impedance spectra can be measured to test the new electrical model over a wide frequency range. Capacitance Bode plots and AC conductivity curves can provide direct information on the specific properties of various BZY materials and also allow the examination of the appropriateness of the model parameters obtained by fitting analysis. While more hydrated samples exhibit a lower R1 resistance the overall resistance R2 increases. This cannot be explained by standard defect chemistry but by the increased blocking effects. Cerates and zirconium cerates were also studied.
C. Temperature and time dependence by AC method
As reported by other research groups and also observed in the present work, relaxation occurs over several days even at temperatures as high as 600℃ upon water in/excorporation. This does not allow exact determination of the equilibrium conductivity of protonic ceramic conductors. Therefore measurements upon constant cooling from a high temperature equilibirum state are suggested for the systematic monitoring of the electrical properties. Automated scanning for about 40 seconds using fast response LCR meters allows the monitoring of AC response as a function of temperature. The AC conductivity curves indicates the presence of the R2 component, directly supporting the parallel network model. Similarly, the relaxation upon water in/excorporation can be measured by the AC method. The model analysis of AC relaxations can deconvolute the hidden large R1 values for the protonic conduction and capacitive responses which can be related to the mass incorporation in the system. The AC method with proper modeling can be used as an `electrochemical thermogravimetry.’ The quasi-equilibrium resistance values obtained by AC relaxation are significantly different from those obtained by the constant cooling method but similarly indicate the modeled parallel network of protons and electron holes. Different R parameters were used for the evaluation of preliminary kinetic parameters by the two-fold relaxation mechanism. In contrast to previous work, the diffusion model of non-instantaneous surface activity was used to describe the small time response in the logarithmic time scale and the surface reaction limited control was modeled for the long time relaxation as suggested by in-situ optical monitoring.
D. DC relaxation by High Temperature van der Pauw Method
The transmission line model suggested in the present work can describe properly the mass in/excorporation across the electrodes. The conductivity relaxation using the DC 4-probe method assumes that transport takes place through the surface between electrodes but the catalytic reaction at the electrode wiring cannot be excluded. A high temperature van der Pauw method has been developed and applied to dense BZY disks prepared by NiO-reactive sintering. Compared to the conventional DC 4-probe method on bar samples, the high temperature van der Pauw method minimizes the catalytic electrode response, enables the surface treatment for systematic reaction control, allows simple one-dimensional diffusion analysis and eliminates machining procedures by using as-prepared disk samples. Electrodes at the lateral surface through the sample thickness allow the exact estimation of the conductivity. The gas electrode polarizations can be substantially different between electrodes but the full permutation provides accurate conductivity results. Laboratory-made automation allows monitoring of the relaxation and evaluation of the kinetic parameters.
E. Liquid Phase Sintering and dopant effects
Attempts to grow single crystalline BZY samples for the multi-species transport in bulk were made via solid state crystal growth (SSCG) method using seed crystals of ZrO2, BZT or STO in a BZY matrix.
They turned out to be unsuccessful leading to the conclusion that the SSCG was not applicable to the zirconate system. To obtain dense BZY samples, Bi2O3 and MnCO3 were chosen as sintering aids. Dense BZY samples (~93%) were prepared by 3 wt% MnCO3 addition but Mn doping was found to substantially affect the electrical properties of BZY. Similarly, the electrical property of dense BZY samples prepared by NiO reactive sintering has not been well understood. The characterization method developed in the present study can evaluate the electrical properties of Mn and Ni doped samples to provide more insight into the dopant effects.
F. Multi-species Transport in single crystalline Fe-SrTiO3 model system
Unprecedentedly clear observation of the multi-species transport phenomena has been made using the Fe-SrTiO3 single crystal model system by in-situ optical spectroscopy with a spatial resolution to describe the diffusion phenomena inside 6x6 cm2 samples. The full description of the diffusion processes involving multi-species should be accomplished in the second stage of the project based on the progress during the 1st stage. The parallel network model for the grain boundary effects in polycrystalline BZY was found to be also applicable to the single crystalline Fe-STO by assuming selectively blocking electrodes. Analysis allowed the deconvolution of the R1 parameter for the proton conduction as well as the R2 parameter for electron holes and other capacitive effects indicating the thermogravimetric response in Fe-STO by water in-/excorporation.
2. Multi-species transport phenomena in protonic ceramics: Atomic-scale computer simulations
A. Bandgap energy and defect formation energy calculation
Band gap energy and defect formation energy were calculated by using PBE0, PBE, and HSE06 potentials. Though the formation energy of a particular defect is different depending on whether the lattice is Ba-poor or Ba-rich, by using the HSE06 potential, substitutional Y ions, O vacancies, interstitial Zr, and protons could easily be created at the shallow acceptor levels in p-type BZO. A band gap energy of 5.2 eV was obtained in the case of PBE0, which was close to the experimental value. Defect formation energies were calculated by using different types of exchange-correlation functions and were plotted as a function of the valence band width. Since none of the defects showed a linear relationship between their formation energies and the valance band widths, we could confirm that the method proposed by Ramprasad et al. was not appropriate for ternary materials such as BZO.
The effect of temperature and pressure were considered by using ab-initio thermodynamics techniques and the results showed a decreased formation energy for O vacancies at high temperature and low pressure.
B. Defect transport mechanism by atomistic simulation
The behavior of defects was investigated via simulation. It was confirmed that for BCO having the orthorhombic structure unlike BZO, structure distortion was required during proton transfer. Rotation of the Ce-O-Ce unit during proton transfer reduced the energy barrier for proton transfer to 0.26 eV and the energy barrier for structure distortion to 0.06 eV. Since Zr and Ba vacancies required high energy barriers of 5 eV and 3 eV respectively for transfer, they would not migrate in BZO.
C. Defect interactions by atomistic simulation
The interactions between various defects were investigated on the atomic scale. The effect of Y ion concentration on the energy barrier for proton transfer was investigated. The energy barrier for proton transfer increased up to 15 at% of Y ion and then decreased again. The energy decreased when the proton was located near another proton but a high energy barrier of 0.41 eV was required when the proton passed by the other proton. Energy barriers of about 0.4 eV were obtained regardless of Y ion concentration when the interaction between the protons was considered, which was in good agreement with the experimental values. It became energetically stable when the proton or Y ion were located near an O vacancy.
D. Grain boundary modeling and defect interactions at grain boundaries
In order to investigate the effect of the grain boundary at the atomic scale, a stable Σ3(111)/[110] tilt grain boundary in BZO was constructed where the core was terminated by Ba and O ions. The energy decreased by –.81 and 0.65 eV when the proton and O vacancy were located at the grain boundary core. These results provided an atomic mechanism of a positively charged grain boundary model for explaining the Schottky barrier in the BZO system. There were no significant differences when the proton transfers or rotates at the grain boundary, compared with those in the bulk. However, a higher energy barrier than that in the bulk was obtained when the proton transfers in a direction perpendicular to the grain boundary. The results supported the selective blocking effect of the proton at the grain boundaries which is suggested by the experimental studies, and the theory of the high energy barrier due to the structural disorder at the grain boundary core claimed by Kreuer.
E. Calculation of diffusivity and dopant effects
Diffusion coefficients for proton intra-transfer, inter-transfer, and rotation where the temperature dependences were 0.21, 0.71, and 0.17 eV, respectively were obtained by calculating the attempt frequencies. The effect of the Zn ion used as a sintering aid and the Y ion on proton migration was calculated within a BZO 2x2x2 super cell. The protons are mainly present around the Zn ions within the Zn-added BZO. The results could explain the experimentally measured low proton conductivity in Zn-added BZY. It was possible to know that the attempt frequencies were affected by the interaction between protons in pure BZO and more precise calculations are needed.

목차 Contents

• 제출문 ... 1
• 보고서 요약서 ... 2
• 요 약 문 ... 3
• SUMMARY ... 8
• CONTENTS ... 14
• 목 차 ... 15
• 제 1 장 연구개발과제의 개요 ... 16
• 제 1절 연구목표 ... 16
• 제 2 절 연구개발대상의 필요성 ... 17
• 제 2 장 국내외 기술개발 현황 ... 18
• 제 1 절 입계임피던스 ... 18
• 제 2 절 다종수송현상 ... 19
• 제 3 절 전산모사 ... 24
• 제 3 장 연구개발수행 내용 및 결과 ... 25
• 제 1 절 프로톤전도성 세라믹의 다종수송현상: 실험 ... 25
• 1. 새로운 등가회로 모델의 개발 ... 25
• 2. 다양한 Y 첨가 BaZrO3 소재에서의 모델 적용 ... 29
• 3. AC 법에 의한 온도 및 시간 의존성 측정 ... 32
• 5. 액상소결 및 소결조제에 따른 전도특성 ... 39
• 6. 단결정 Fe-SrTiO3 에서의 다종수송현상 ... 44
• 제 2 절 프로톤전도성 세라믹의 다종수송현상: 전산모사 ... 47
• 1. 결함의 생성 ... 47
• 2, 결함의 거동 ... 50
• 3. 결함간의 상호작용 ... 52
• 4. 입계의 영향 ... 55
• 5. 결함의 이동도 계산 ... 57
• 6. 매크로 스케일 모델과의 비교 ... 61
• 제 4 장 목표달성도 및 관련분야에의 기여도 ... 62
• 제 1 절 목표달성도 ... 62
• 제 2 절 관련분야에의 기여도 ... 63
• 제 5 장 연구개발결과의 활용계획 ... 65
• 제 1 절 연구개발결과의 활용방안 ... 65
• 제 2 절 기대성과 ... 65
• 제 6 장 연구개발과정에서 수집한 해외과학기술정보 ... 66
• 제 1 절 학계 및 학회 동향 ... 66
• 제 2 절 연구분야 관련 해외 연구자 및 연구 정보 ... 66
• 제 7 장 연구시설 및 장비현황 ... 68
• 제 8 장 참고문헌 ... 70
• 끝페이지 ... 72