학위논문 (박사)-- 서울대학교 대학원 : 재료공학부, 2017. 2. 주영창.Precise phase control in nanoscale synthesis and spontaneous assembly in macroscopic dimension are important but difficult for practical implementation of nanomaterials. In frequently-encountered situations where multiple components are involved in synthesis, the most of difficulties about mass production and reproducibility are related to the lack of thermodynamic and kinetic predictability. Inspired by chemical metallurgy theories, an insight to solve the limitations in nanomaterial synthesis has been suggested in this thesis.
Metal compound/carbon nanofibers are receiving great attention for the energy and environmental applications. Metal compound acts an active material which shows high electrochemical activity. Electrically conductive carbon (C) nanofiber is a supporter which maintains high-aspect ratio one-dimensional (1D) structures. In this thesis, a new functionality of C nanofiber has been pioneered as a determinant to control the spontaneous of atomic species and final structures of metal compound. It is based on oxidation based C decomposition, which produces gaseous decomposition products such as carbon monoxide (CO) and carbon dioxide (CO2). Based of the reaction between O2 and C, the degree of C decomposition can be precisely controlled by the oxygen partial pressure (pO2). In multi-atomic component system, the reaction between the elements can generate many intriguing compounds. For inducing the reaction between selected elements, Ellingham diagram and phase diagram can contribute to predict processing parameters and material composition. Based on this theoretical background, an innovative fabrication methodology for controlling the phase and structures of active materials and C nanofibers has been developed via predictive synthesis. Escaping from the control in solution process, it is based on gas-solid reactions which can realize the mass production of complex multi-atomic nanomaterials with high uniformity.
The first focus is the demonstration of selective C oxidation induced reductive metal structure control. Based on the oxidation Gibbs free energy (ΔG) of C, metals are categorized into reductive and oxidative metals. In the case of reductive metals such as Co, Ni, Cu, Pt, etc., selective redox reaction induces C oxidation and metal reduction. According to the pO2 in this condition, full-filled C nanofibers with embedded metal nanoparticles, metal/C core/shell nanofibers, hollow and porous C nanofibers are formed. The mechanism of this structure evolution according to the pO2 has been studied by the catalyzed C oxidation at the surface of metallic species.
The second focus is an in-depth study about the effect of C porosity on the structures of oxidative metallic species. The size and distribution of Sn, which shows high capacity active material in Li-ion battery anode, was optimized by managing the outward Sn diffusion during calcination. For minimizing the volumetric expansion based material degradation, the strategy for inducing Sn nanoparticles fully-embedded has been established. By utilizing the pressure equilibrium to derive reverse direction of Boudouard reaction, C porosity could be controlled by ambient conditions.
Finally, selective redox reaction scheme has been verified in multi-atomic component system to control molybdenum disulfide (MoS2) inside C nanofibers. The redox reactions of Mo-S-C-O were categorized according to the pO2. Especially, at the pO2 between ΔG of C and Mo oxidation, C is decomposed by combustion with MoS2 formation. In ternary phase diagrams, Mo-S-C-O mole fraction was determined to form MoS2 and gaseous C oxide such as CO and CO2. The calcination in this region was induced to control MoS2 structures by modulating the vertical stacking and lateral growth of MoS2 stacking unit. Through this methodology various MoS2 structures such as length, stacking number, distribution, and alignment were induced.
In this thesis, it has been revealed that the structure and phase of metal compound/C nanofibers can be precisely controlled by the predicted parameters from the same metal precursor and polymer matrix nanofibers. This customized fabrication has a potential to tuning the properties of metal compound/C nanofibers according to the various applications such as electronic, chemical, energy, and environment fields.Chapter 1. Introduction 1
1.1. Metal compound/carbon hybrid nanofibers 1
1.2. Phase and structure control issues of metal/C nanofibers 7
1.2.1. Solution process based active material loading inside C nanofibers 7
1.2.2. Thermodynamics for phase control 12
1.2.3. Kinetics for atomic diffusion induced structure control 15
1.3. Objective of the thesis 17
1.4. Organization of the thesis 23
Chapter 2. Theoretical Background 26
2.1. Nanofibers 26
2.2. Electrospinning 31
2.2.1. Effect of solution parameters 33
2.2.2. Effect of processing and ambient parameters 35
2.2.3. Coaxial-Electrospinning 36
2.3. Calcination 38
2.3.1. Selective oxidation 40
2.3.2. Boudouard reaction 42
2.4 Thermodynamic tools for predictive synthesis 46
2.4.1. Ellingham diagram 47
2.4.2. Ternary phase diagram 50
2.4.3. Reciprocal phase diagram 56
2.4.4. Relationship between phase diagram and Ellingham diagram 58
Chapter 3. Experimental Procedures 60
3.1. Sample preparation by electrospinning 60
3.1.1. Reductive metal/C nanofibers 60
3.1.2. Oxidative metal/C nanofibers 61
3.1.3. Metal compound/C nanofibers 61
3.2. Calcination 64
3.2.1. Oxygen partial pressure controlled calcination 64
3.2.2. Oxygen empty ambient condition induced calcination 64
3.3. Microstructure analysis 67
3.4. Chemical analysis 67
3.5. Electrical and electrochemical property 68
3.5.1. Electrical conductivity 68
3.5.2. All solid state Li-ion battery anode performance 68
3.6. Computation 71
3.6.1. FactsageTM calculation 71
3.6.2. Density Functional Theory (DFT) calculation 71
Chapter 4. Selective Carbon Oxidation based Reductive Metal/Carbon Nanofiber Structure Control 72
4.1. Introduction 72
4.2. Ambient effect on the thermal decomposition during calcination 73
4.3. Thermodynamics in calcination of Cu/C nanofibers 76
4.4. Microstructural changes of Cu/C nanofibers by selective oxidation 80
4.5. Mechanism of Cu/C nanofiber formation 87
4.6. Effect of Cu/C nanofiber structures on electrical properties 92
4.7. Summary 98
Chapter 5. Selective Carbon Oxidation induced Hollow Structure Formation in Metal/Carbon Nanofibers 99
5.1. Introduction 99
5.2. Thermodynamic consideration on the calcination of reductive metal/carbon nanofibers 101
5.3. Structure evolution of reductive metal/carbon nanofibers with the hollow structure formation 106
5.4. Catalytic carbon oxidation on metal surface 111
5.5. Mechanism of hollow C structure formation 120
5.6. Summary 123
Chapter 6. Phase Equilibrium based Carbon Porosity Engineering for Oxidative Metal Structure Control 125
6.1. Introduction 125
6.2. Gas-solid reaction based Sn/C structure control 128
6.3. Effect of ambient condition on porosity of Sn/C nanofibers 136
6.4. Material properties of Sn/C nanofibers 143
6.5. All solid state Li-ion battery performance 149
6.6. Summary 155
Chapter 7. Thermodynamic based Reaction Prediction of Multi-component Atomic System for Kinetic van-der Waals Solid Structure Control 156
7.1. Introduction 156
7.2. Thermodynamic calculation for redox control in Mo-S-C-O quaternary system 159
7.3. Validation of molybdenum sulfidation and carbon oxidation 169
7.4. MoS2 rational design according to the thermodynamic-driven processing parameters 175
7.5. Kinetics of MoS2 layer formation 182
7.6. Summary 188
Chapter 8. Conclusion 190
8.1. Summary of results 190
8.2. Future works and suggested research 193
References 196
Abstract (In Korean) 203Docto
