2 research outputs found
μΉ¨κ° μμ μ±μ΄ ν₯μλ κ³ μ±λ₯ μκΈ°μ λ³μ 체μ λν μ°κ΅¬
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Όλ¬Έ (λ°μ¬) -- μμΈλνκ΅ λνμ : 곡과λν μ¬λ£κ³΅νλΆ, 2020. 8. μμ©μ.Magnetorheological (MR) fluids are typically consist of magnetic particles (Carbonyl Iron, Fe2O3, Fe3O4 and so on) in a magnetically insulating fluid (water, silicon oil and so on). When a magnetic field induces attractive interactions between the magnetic particles, these particles form a solid-like network of fibril shapes within a few milliseconds oriented along the direction of the magnetic field. Reverse transition occurs as soon as the magnetic field is switched off. These features lead to remarkable changes in the rheological properties of the fluid which shows wide potential applications such as dampers, brakes, shock observers, drug delivery, and robotics, etc and could be controlled by adjusting the strength of the magnetic field depending on applications. Despite substantial advanced in commercialization, MR fluids have long-term stability issues that significantly limit their usefulness and also need to be predicted the precise flow behavior.
In this thesis, we propose the constitutive equation to predict the flow behavior of MR fluid and investigate a number of MR fluid composed of soft-magnetic composite particles to overcome the sedimentation drawback. Firstly, as modeling and analysis are essential to optimize material design, describe the flow behavior over a wide range of shear rate and distinguish between static yield stress and dynamic yield stress, the precise knowledge of the relationships between the suspension rheological properties and such variables as the deformation rate, the applied magnetic field strength, and the composition are required. So we re-analyze the constitutive equation proposed before to describe the MR fluids flow and propose new constitutive equation. The proposed Seo-Seo model predicted the flow behavior precisely compared to pre-exist constitutive model and also yielded a quantitatively and qualitatively precise description of MR fluid rheological behavior based on relatively few experimental measurements.
To overcome sedimentation drawback, the core/shell structured Foamed polystyrene/Fe3O4 Particles were synthesized by applying a dual-step processing comprising pickering emulsion polymerization, subsequently by the foaming of polystyrene core using the supercritical carbon dioxide fluid foaming process. Through these processes, the density of composite was dropped significantly and the long-term stability was improved. As polystyrene located core part and magnetic particle contact directly, the magnetorheological properties of the Foamed polystyrene/Fe3O4 were considerable compared to pure Fe3O4. Even though the core/shell structured Foamed polystyrene/Fe3O4 showed considerable level, the magnetorheological properties got worsen because polystyrene is magnetically non-active. So, we synthesized hollow shape Fe3O4 particles without any magnetically non-active template. As a result, compared to the core/shell structured Foamed polystyrene/Fe3O4, the density of hollow shape Fe3O4 particles rise slightly and the magnetorheological properties reached outstanding level, and the long-term stability maintained.
Also, the conformation of solid-like network of fibril shapes changes were investigated by using micro/nano size Fe3O4 particles to verify the reinforcement effect. As the particle size increases, the magnetorheological properties improve due to a rise of the magnetic saturation level. However, depending on the ratio of the nano size Fe3O4 particles, an overturning of the magnetorheological properties and the magnetic saturation was observed. This phenomenon is because of the cavity among the micro size Fe3O4 particles. The micro size Fe3O4 particles develops a relatively coarse solid-like network of fibril shapes. The chain conformation of a bidisperse MR fluid shows quite different from that of the micron size Fe3O4 particles-based fluids. The nano size Fe3O4 particles appear to fill in the cavity among the micro size Fe3O4 particles. As a result, this distinct conformation reinforced the magnetorheological properties.
Finally, the shape effect of the magnetic particle on magnetorheological properties and sedimentation stability was investigated by using two types of sendust which are bulk and flake type. The flake type sendust has a small demagnetization factor because its domain orients one direction. This feature lead to extraordinary behavior which is a rapid transition to solid-like network at low magnetic field. Also, its high aspect ratio leads to a large drag coefficient which improve the long-term stability.μκΈ°μ λ³μ 체λ λ¬Ό λλ λΉμκ³(μ€λ¦¬μ½ μ€μΌ λ±)μ μ 체μ μν κ°λ₯ν λ―ΈμΈμ
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ν‘λΉλ‘ μΈν νλ ₯κ³μλ μ₯κΈ° μμ μ±μ ν₯μμμΌ°λ€.Chapter 1. Introduction and Background . 0
1.1. Magnetorheological (MR) Fluids 0
1.2. Applications of MR fluids . 2
1.3. Rheology 2
1.3.1. Flow behavior . 3
1.3.1.1. Definition of terms 3
1.3.1.1.1. Shear stress 5
1.3.1.1.2. Shear rate 5
1.3.1.1.3. Shear viscosity . 5
1.3.1.2. Flow and viscosity curve 7
1.3.1.2.1. Ideal viscous flow. 7
1.3.1.2.2. Shear-thinning flow and Shear-thickening . 9
1.3.1.2.3. Yield stress 9
1.3.2. Viscoelastic behavior 11
1.3.2.1. Storage modulus and Loss modulus . 11
Reference 12
Chapter 2. Constitutive Equation . 14
2.1. Introduction . 14
2.2. Rheological Models for the Yield Stress . 18
2.2.1. Static Yield Stress versus Dynamic Yield Stress . 18
2.2.2. Yield Stress Dependency on the Magnetic Field Strength 22
2.2.3. Mechanism of Structure Evolution . 24
2.3. Conclusion . 26
Reference . 27
Chapter 3. High-Performance Magnetorheological Suspensions of Pickering Emulsion Polymerized Polystyrene/Fe3O4 Particles with Enhanced Stability 31
3.1. Introduction 31
3.2. Experimental Section 33
3.2.1. Synthesis of Polystyrene/Fe3O4 particles . 33
3.2.2. Synthesis of Foamed Polystyrene/Fe3O4 particles 34
3.2.3. Characterization 37
3.3. Results and Discussion 41
3.3.1 Morphology . 41
3.3.2. Magnetorheological Behaviors . 42
3.3.3. Yield Stress of the MR Fluids 47
3.3.4. Structure Evolution Mechanism and the Suspension Stability . 54
3.4. Conclusion . 59
References . 61
Chapter 4. Template Free Hollow Shaped Fe3O4 Micro-Particles for Magnetorheological Fluid . 65
4.1 Introduction . 65
4.2. Experiment Section . 67
4.2.1. Synthesis of Fe3O4 particles (Pure Fe3O4) . 67
4.2.2. Synthesis of PS/Fe3O4 particles (Picker) . 68
4.2.3. Synthesis of PS/Fe3O4@Fe3O4 particles (C-picker) 68
4.2.4. Synthesis of templet free hollow shaped Fe3O4 (H-Picker) . 69
4.2.5. Characterization 69
4.3. Results and Discussion . 70
4.3.1. Particle Morphologies and Magnetic Hysteresis Curve 70
4.3.2. Magnetorheological Behaviors . 76
4.3.3. Yield Stress of the MR Fluids . 80
4.3.4. Mechanism of Structure Evolution and Suspension Stability . 84
4.4. Conclusion 89
Reference 90
Chapter 5. Bidisperse MR Fluids Using Nano/micro Size Fe3O4 particles . 95
5.1. Introduction 95
5.2. Experiment Section 99
5.2.1. Material. 99
5.2.2. Characterization Methods . 99
5.3. Results and Discussion . 99
5.4. Conclusion 106
References . 107
Chapter 6. Shape effect of magnetic particle on magnetorheological (MR) properties and sedimentation stability 108
6.1. Introduction . 108
6.2. Experiment Section . 109
6.2.1. Material . 109
6.2.2. Characterization Methods 109
6.3. Results and Discussion 110
6.3.1. Particle Morphologies and Magnetic Hysteresis Curve . 110
6.3.2. Magnetorheological Behaviors 116
6.3.3. Yield Stress of the MR Fluids . 120
6.3.4. Mechanism of Structure Evolution and Suspension Stability . 124
6.4. Conclusion . 129
References 130
Chapter 7. Conclusions 135
κ΅λ¬Έμ΄λ‘ 139
List of Publication 141
Appendix . 142
Appendix A. Improvement of Mechanical Properties by Introducing Curable Functional Monomers in Stereolithography 3D PrintingDocto