157 research outputs found

    Physically motivated modelling of magnetoactive elastomers

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    Magnetoactive elastomers (MAEs) are polymer composites containing magnetically soft or hard particles incorporated into an elastomer matrix during the crosslinking procedure. In the presence of a magnetic field, the induced magnetic interactions and the corresponding particle rearrangements significantly alter the mechanical properties in dependence on the initial particle distribution and sample shape. In addition, applying magnetic fields also changes the macroscopic shape of an MAE. This thesis investigates the magneto-mechanical coupled behaviour of MAEs by means of analytical and numerical methods. The effects of particle distribution and sample shape have been studied with the help of a physically motivated model of MAEs that considers dipole-dipole interactions between magnetizable particles. The presence of a magnetic field leads to a mechanical anisotropy in MAEs with isotropic particle distribution, and the induced anisotropy is directed along the orientation of the field. Thus, MAEs exhibit direction-dependent mechanical properties with distinct elastic moduli along and perpendicular to the field direction when the MAE sample is subjected to uniaxial deformation. A good agreement is reported between the physically motivated approach and conventional transversely isotropic material models. Furthermore, we investigate the important interplay between the particle distribution and the sample shape of MAEs, where a simple analytical expression is derived based on geometrical arguments to describe the particle distribution inside MAEs. We show that the enhancement of elastic moduli arises not only from the induced dipole-dipole interactions but also considerably from the change in the particle microstructure. Moreover, the magneto-mechanical behaviour of isotropic MAEs under shear deformations is studied. Three principal geometries of shear deformation are investigated with respect to the orientation of the applied magnetic field. We show that the Cauchy stress tensor of MAEs is not always symmetric due to the generation of a magnetic torque acting on an anisometric MAE sample under shear loadings. The theoretical study of magneto-mechanical behaviour of MAEs confirms that the effect of sample shape is quite significant and cannot be neglected. On the other hand, the initial particle distribution and presumed rearrangements due to the magnetic field additionally influence the material response of MAEs. Finally, the physically motivated model of MAEs could be transformed into an invariants-based model enabling its implementation in commercial finite element software. Therefore, we have uncovered a new pathway to model MAEs based on dipole-dipole interactions, leading to a constitutive relation analogous to the macro-scale continuum approach and revealing a synergy between both modelling strategies

    Sensitivity analysis and optimal design of conventional and magnnetorheological fluid brakes

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    Mechanical and electrical brakes have dominated the braking industry for many years and will most likely continue to do so for the foreseeable future due to their low cost and adequate operating performance, wide range of applications, vehicle engineering, civil engineering, and biomedical engineering. Simple mechanical drum brake and magnetorheological (MR) fluid brake have presented in the current work. The main objective of this work is to increase braking torque, and to develop a new optimal design of MR fluid brake with better design and design control of the MR fluid design. To do so, four important steps have been accomplished. In the first step, a mathematical modeling of the conventional frictional brake and MR fluid brake has been developed to study and specify all design parameters. In the second step, a nondimensional, closedform analysis and a Taylor series expansion have used to examine the effects of perturbing dimensionless design parameters on the overall brakes performance. In the third step, two optimal designs for MR fluid brakes have been developed by taking advantage of sensitivity analysis and the design of experiments method also known as the Taguchi method. In the fourth step, controlling a MR fluid brake is performed by using two parallel PI controls for controlling the magnetic current and MR fluid thickness simultaneously. It was concluded that sensitivity analysis is a good method for identifying the parameters that have the greatest impact on brake performance and can be used as one method for the designer to obtain an optimal design. Four nondimensional design parameters were successfully used to describe the conventional frictional brake and seven nondimensional design parameters for MR fluid brake. Only two parameters for the conventional brake and five parameters for the MR fluid brake affect the performance and the others can be neglected. Two new designs for the MR fluid brake are presented and shown to be very simple in design, low in cost by removing a lot of additional auxiliaries for the frictional brake, and easy for control. By simultaneously controlling the MR fluid thickness and the electric current, a large range of brake torque is achieved without increasing the radial envelop for the brake, and saturation conditions in one controller are compensated for by the other controller. High angular velocities of the brake are primarily controlled by increasing the MR fluid thickness, while low angular velocities are primarily controlled by increasing the electric current. Good transient responses for regulating a constant speed (high, moderate, and low), and good stability while seeking to track a sinusoidal input have been achieved. In summary, the proposed control system for the MR fluid brake has demonstrated good controllability for the MR fluid brake.Includes bibliographical reference

    Fabrication of strong magnetic micron-sized supraparticles with anisotropic magnetic properties for magnetorheology

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    Dr Tavacoli is acknowledged for useful discussions. This work was supported by MICINN PID2019-104883GB-I00 project (Spain), Junta de Andalucฤฑยดa P18-FR-2465 project and European Regional Development Fund (ERDF). J. R. M. acknowledges FPU14/01576 fellowship. E. C.-G. acknowledges financial support by CONACYT (Ref. #232347).We propose three different techniques to synthesize anisotropic magnetic supraparticles for their incorporation in the formulation of magnetorheological fluids with novel potential applications. The techniques include microtransfer molding, electrodeposition and microfluidic flow-focusing devices. Although the yield of these methods is not large, with their use, it is possible to synthesize supraparticles with anisotropy in both their magnetic content and shape. The magnetorheological characteristics (yield stress) of the resulting field-induced structures were computed using finite element method simulations and demonstrated to be strongly dependent on the microstructural anisotropy of the supraparticles. In anisotropic particles, the simulated yield stress is always larger than that of the isotropic ones consisting of magnetically homogeneous spherical particles.MICINN PID2019-104883GB-I00 project (Spain)Junta de Andalucรญa P18-FR-2465 projectEuropean Regional Development Fund (ERDF)FPU14/01576CONACYT (Ref. #232347

    Steady shear flow of magnetic fiber suspensions: theory and comparison with experiments

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    International audienceThis paper is focused on the rheology of magnetic fiber suspensions in the presence of a magnetic field applied perpendicular to the flow. At low Mason numbers, Mn<0.1, the experimental flow curves show a steep initial section corresponding to the inclination and stretching of the gap-spanning aggregates formed upon magnetic field application. At higher Mason numbers, aggregates no longer stick to the walls and the flow curves reach a Bingham regime, with the dynamic yield stress growing with the magnetic field intensity. This yield stress appears to be about three times higher for the fiber suspensions than for the suspensions of spherical particles. Such difference, measured at relatively low magnetic field intensities, H0<30 kA/m, is explained in terms of the enhanced magnetic susceptibility of the aggregates composed of fibers compared to the aggregates composed of spherical particles. For weak magnetic fields, the forces of solid friction between fibers are expected to play a minor role on the stress level of the suspension. In order to confirm these findings, we propose a new theoretical model, taking into account hydrodynamic interactions. The flow curve and the yield stress predictions are in a good agreement with the experimental results for semi-diluted suspensions

    Effect of polydispersity in concentrated magnetorheological fluids

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    Magnetorheological fluids (MRF) are smart materials of increasing interest due to their great versatility in mechanical and mechatronic systems. As main rheological features, MRFs must present low viscosity in the absence of a magnetic field (0.1 - 1.0 Pa.s) and high yield stress (50 - 100 kPa) when magnetized, in order to optimize the magnetorheological effect. Such properties, in turn, are directly influenced by the composition, volume fraction, size, and size distribution (polydispersity) of the particles, the latter being an important piece in the improvement of these main properties. In this context, the present work aims to analyze, through experiments and simulations, the influence of polydispersity on the maximum packing fraction, on the yield stress under field (on-state), and on the plastic viscosity in the absence of field (off-state) of concentrated MRF (phi = 48.5 vol.%). Three blends of carbonyl iron powder in polyalphaolefin oil were prepared. These blends have the same mode, but different polydispersity indexes, ranging from 0.46 to 1.44. Separate simulations show that the random close packing fraction increases from about 68% to 80% as the polydispersity index increases over this range. The on-state yield stress, in turn, is raised from 30 +/- 0.5 kPa to 42 +/- 2 kPa (B ~ 0.57 T) and the off-state plastic viscosity, is reduced from 4.8 Pa.s to 0.5 Pa.s. Widening the size distributions, as is well known in the literature, increases packing efficiency and reduces the viscosity of concentrated dispersions, but beyond that, it proved to be a viable way to increase the magnetorheological effect of concentrated MRF. The Brouwers model, which considers the void fraction in suspensions of particles with lognormal distribution, was proposed as a possible hypothesis to explain the increase in yield stress under magnetic field

    ์นจ๊ฐ• ์•ˆ์ •์„ฑ์ด ํ–ฅ์ƒ๋œ ๊ณ ์„ฑ๋Šฅ ์ž๊ธฐ์œ ๋ณ€์œ ์ฒด์— ๋Œ€ํ•œ ์—ฐ๊ตฌ

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    ํ•™์œ„๋…ผ๋ฌธ (๋ฐ•์‚ฌ) -- ์„œ์šธ๋Œ€ํ•™๊ต ๋Œ€ํ•™์› : ๊ณต๊ณผ๋Œ€ํ•™ ์žฌ๋ฃŒ๊ณตํ•™๋ถ€, 2020. 8. ์„œ์šฉ์„.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.์ž๊ธฐ์œ ๋ณ€์œ ์ฒด๋Š” ๋ฌผ ๋˜๋Š” ๋น„์ˆ˜๊ณ„(์‹ค๋ฆฌ์ฝ˜ ์˜ค์ผ ๋“ฑ)์˜ ์œ ์ฒด์— ์žํ™” ๊ฐ€๋Šฅํ•œ ๋ฏธ์„ธ์ž…์ž(์ฒ  ๋งˆ์ดํฌ๋กœ ์ž…์ž)๋ฅผ ๋ถ„์‚ฐ์‹œํ‚จ ํ˜„ํƒ์•ก์œผ๋กœ์„œ, ์™ธ๋ถ€๋กœ๋ถ€ํ„ฐ ์ œ๊ณต๋˜๋Š” ๊ฐ•ํ•œ ์ž๊ธฐ์žฅ์— ๋”ฐ๋ผ ์งง์€ ์‹œ๊ฐ„์•ˆ์— ํƒ„์„ฑ, ์†Œ์„ฑ, ์ ๋„ ๊ฐ™์€ ์ž๊ธฐ์œ ๋ณ€ํšจ๊ณผ๋ฅผ ๋‚˜ํƒ€๋‚ด๋Š” ์œ ์ฒด๋ฅผ ๋งํ•œ๋‹ค. ์ž๊ธฐ์œ ๋ณ€์œ ์ฒด๋Š” ์™ธ๋ถ€ ์ž๊ธฐ์žฅ์— ์˜ํ•ด ์œ ๋ณ€ํšจ๊ณผ๋ฅผ ์กฐ์ ˆํ•  ์ˆ˜ ์žˆ๊ธฐ ๋•Œ๋ฌธ์— ๋‹ค์–‘ํ•œ ์‘์šฉ๋ถ„์•ผ๋กœ์˜ ์ ์šฉ ๊ฐ€๋Šฅ์„ฑ์— ๋Œ€ํ•œ ๊ด€์‹ฌ์ด ์ฆ๊ฐ€ํ•˜๊ณ  ์žˆ๋‹ค. ๊ทธ๋Ÿฌ๋‚˜ ์ž์„ฑ์ž…์ž์™€ ํ˜„ํƒ ์œ ์ฒด์™€์˜ ๋ฐ€๋„ ์ฐจ์— ์˜ํ•ด ๋ฐœ์ƒํ•˜๋Š” ์นจ์ „ํ˜„์ƒ์œผ๋กœ ์ธํ•ด ์ž๊ธฐ์œ ๋ณ€์œ ์ฒด์˜ ์‹ค์ œ์ ์ธ ์‘์šฉ์ด ์ œํ•œ๋˜๊ณ  ์žˆ๋‹ค. ๋ณธ ์—ฐ๊ตฌ์—์„œ๋Š” ์ž๊ธฐ์œ ๋ณ€์œ ์ฒด์˜ ๊ฑฐ๋™์„ ์˜ˆ์ธกํ•˜๋Š” ๊ตฌ์„ฑ๋ฐฉ์ •์‹์„ ์ œ์•ˆํ•˜๊ณ , ์นจ์ „ ๋ฌธ์ œ๋ฅผ ๊ทน๋ณตํ•˜๊ธฐ ์œ„ํ•ด ์—ฐ์ž์„ฑ ๋ณตํ•ฉ์ฒด๋กœ ๊ตฌ์„ฑ๋œ ์ž๊ธฐ์œ ๋ณ€์œ ์ฒด๋ฅผ ์กฐ์‚ฌํ•œ๋‹ค. ์žฌ๋ฃŒ ์„ค๊ณ„๋ฅผ ์ตœ์ ํ™”ํ•˜๊ธฐ ์œ„ํ•ด ํ•„์ˆ˜์ ์œผ๋กœ ๊ด‘๋ฒ”์œ„ํ•œ ์ „๋‹จ ์†๋„์— ๊ฑธ์นœ ํ๋ฆ„ ๋™์ž‘์„ ์„ค๋ช…ํ•˜๊ณ  ์ •์  ํ•ญ๋ณต ์‘๋ ฅ๊ณผ ๋™์  ํ•ญ๋ณต ์‘๋ ฅ์„ ๊ตฌ๋ถ„ํ•˜์—ฌ์•ผ ํ•œ๋‹ค. ๋˜ํ•œ, ํ˜„ํƒ์•ก์˜ ์œ ์ „ํ•™์  ํŠน์„ฑ๊ณผ ๋ณ€ํ˜•๋ฅ , ์ ์šฉ๋œ ์ž๊ธฐ์žฅ ๊ฐ•๋„ ๋ฐ ๊ตฌ์„ฑ๊ณผ ๊ฐ™์€ ๋ณ€์ˆ˜ ์‚ฌ์ด์˜ ๊ด€๊ณ„์— ๋Œ€ํ•œ ์ •ํ™•ํ•œ ์ง€์‹์ด ํ•„์š”ํ•˜๋‹ค. ๋”ฐ๋ผ์„œ, ์ž๊ธฐ์œ ๋ณ€์œ ์ฒด์˜ ํ๋ฆ„์„ ์„ค๋ช…ํ•˜๊ธฐ ์œ„ํ•œ ๊ธฐ์กด์˜ ์ œ์•ˆ๋œ ๊ตฌ์„ฑ๋ฐฉ์ •์‹์„ ๋ถ„์„ํ•˜๊ณ  ์ƒˆ๋กœ์šด ๊ตฌ์„ฑ๋ฐฉ์ •์‹์„ ์ œ์•ˆํ•œ๋‹ค. ์ƒˆ๋กญ๊ฒŒ ์ œ์•ˆํ•œ ๊ตฌ์„ฑ ๋ฐฉ์ •์‹์ธ ์„œ-์„œ ๋ชจ๋ธ์€ ๊ธฐ์กด์— ์กด์žฌํ•˜๋Š” ๊ตฌ์„ฑ๋ฐฉ์ •์‹๊ณผ ๋น„๊ตํ•˜์—ฌ ์œ ์ฒด์˜ ํ๋ฆ„์„ ์ •ํ™•ํ•˜๊ฒŒ ์˜ˆ์ธกํ•˜์˜€๊ณ , ๋น„๊ต์  ์ ์€ ์‹คํ—˜ ๊ฐ’์„ ๋ฐ”ํƒ•์œผ๋กœ ์ž๊ธฐ์œ ๋ณ€์œ ์ฒด์˜ ํ๋ฆ„์— ๋Œ€ํ•œ ์ •๋Ÿ‰์ , ์งˆ์ ์œผ๋กœ ์ •๋ฐ€ํ•œ ์„ค๋ช…์„ ๋„์ถœํ•˜์˜€๋‹ค. ์นจ์ „ ๋ฌธ์ œ๋ฅผ ๊ทน๋ณตํ•˜๊ธฐ ์œ„ํ•ด ํ”ผ์ปค๋ง ์—๋ฉ€์ „ ์ค‘ํ•ฉ์„ ๋ฐ ์ดˆ์ž„๊ณ„ ์ด์‚ฐํ™”ํƒ„์†Œ๋ฅผ ์ด์šฉํ•œ ๋ฐœํฌ๊ณต์ •์˜ ์ด์ค‘ ๊ณต์ • ์ฒ˜๋ฆฌ๋ฅผ ํ†ตํ•ด ์ฝ”์–ด-์‰˜ ๊ตฌ์กฐ์˜ ๋ฐœํฌ ์Šคํƒ€์ด๋ Œ ๊ณ ๋ถ„์ž-์ฒ  ๋ณตํ•ฉ์ฒด๋ฅผ ํ•ฉ์„ฑํ•˜์˜€๋‹ค. ์ด์ค‘ ๊ณต์ • ์ฒ˜๋ฆฌ๋ฅผ ํ†ตํ•ด ๋ณตํ•ฉ์ฒด์˜ ๋ฐ€๋„๊ฐ€ ํ˜„์ €ํžˆ ๋–จ์–ด์ง€๊ณ  ์žฅ๊ธฐ ์•ˆ์ •์„ฑ์ด ํ–ฅ์ƒ๋˜์—ˆ๋‹ค. ๋˜ํ•œ, ์Šคํƒ€์ด๋ Œ์ด ์ฝ”์–ด ๋ถ€๋ถ„์— ์œ„์น˜ํ•˜์—ฌ, ์ฒ  ์ž…์ž๊ฐ€ ์ง์ ‘์ ์ธ ์ ‘์ด‰์„ ํ†ตํ•ด ๋†’์€ ์ž๋ ฅ ํŠน์„ฑ์„ ์–ป์—ˆ๋‹ค. ์ฝ”์–ด-์‰˜ ๊ตฌ์กฐ์˜ ๋ฐœํฌ ์Šคํƒ€์ด๋ Œ ๊ณ ๋ถ„์ž-์ฒ  ๋ณตํ•ฉ์ฒด์˜ ์ž๋ ฅ ํŠน์„ฑ์ด ์ƒ๋‹นํ•œ ์ˆ˜์ค€์„ ๋ณด์˜€์Œ์—๋„ ๋ถˆ๊ตฌํ•˜๊ณ , ์Šคํƒ€์ด๋ Œ์ด ์ž๋ ฅ์ ์œผ๋กœ ๋น„ํ™œ์„ฑํ™” ๋ฌผ์งˆ์ด๋ฏ€๋กœ ์ˆœ์ˆ˜ํ•œ ์ฒ ์— ๋น„ํ•ด ์ž๋ ฅ ํŠน์„ฑ์€ ์•ฝํ™”๋˜์—ˆ๋‹ค. ๋”ฐ๋ผ์„œ ์ž๋ ฅ์ ์œผ๋กœ ๋น„ํ™œ์„ฑํ™” ๋ฌผ์งˆ์ธ ์Šคํƒ€์ด๋ Œ์„ ์ œ๊ฑฐํ•˜์—ฌ ์ง€์ง€๋Œ€๊ฐ€ ์—†๋Š” ์ค‘๊ณตํ˜•์ƒ์˜ ์ฒ  ์ž…์ž๋ฅผ ํ•ฉ์„ฑํ•˜์˜€๋‹ค. ๊ทธ ๊ฒฐ๊ณผ, ์ฝ”์–ด-์‰˜ ๊ตฌ์กฐ์˜ ๋ฐœํฌ ์Šคํƒ€์ด๋ Œ ๊ณ ๋ถ„์ž-์ฒ  ๋ณตํ•ฉ์ฒด์— ๋น„ํ•ด ์ค‘๊ณตํ˜•์ƒ์˜ ์ฒ  ์ž…์ž๋Š” ๋ฐ€๋„๊ฐ€ ์•ฝ๊ฐ„ ์ƒ์Šนํ•˜์˜€์œผ๋‚˜ ๋†’์€ ์ž๋ ฅํŠน์„ฑ์„ ๋ณด์˜€๊ณ  ์žฅ๊ธฐ ์•ˆ์ •์„ฑ์ด ์œ ์ง€๋˜์—ˆ๋‹ค. ์ถ”๊ฐ€์ ์œผ๋กœ ๋งˆ์ดํฌ๋กœ/๋‚˜๋…ธ ํฌ๊ธฐ์˜ ์ฒ  ์ž…์ž๋ฅผ ์‚ฌ์šฉํ•˜์—ฌ ํ”ผ๋ธŒ๋ฆด ๊ตฌ์กฐ์˜ ๋ณด๊ฐ•ํšจ๊ณผ๋ฅผ ๊ฒ€์ฆํ•˜์˜€๋‹ค. ์ž…์ž ํฌ๊ธฐ๊ฐ€ ์ฆ๊ฐ€ํ•จ์— ๋”ฐ๋ผ ์ž๊ธฐ ํฌํ™” ์ˆ˜์ค€์˜ ์ƒ์Šน์œผ๋กœ ์ž๋ ฅํŠน์„ฑ์ด ๊ฐœ์„ ๋˜์—ˆ๋‹ค. ๊ทธ๋Ÿฌ๋‚˜, ๋‚˜๋…ธ ํฌ๊ธฐ์˜ ์ฒ  ์ž…์ž์˜ ๋น„์œจ์— ๋”ฐ๋ผ ์ž๋ ฅํŠน์„ฑ๊ณผ ์ž๊ธฐ ํฌํ™” ํ˜„์ƒ์˜ ์—ญ์ „ํ˜„์ƒ์ด ๊ด€์ฐฐ๋˜์—ˆ๋‹ค. ์ด ํ˜„์ƒ์€ ๋งˆ์ดํฌ๋กœ ํฌ๊ธฐ์˜ ์ฒ  ์ž…์ž์˜ ํ”ผ๋ธŒ๋ฆด ๊ตฌ์กฐ๋ฅผ ํ˜•์„ฑ์‹œ์— ์ฒ  ์ž…์ž ์‚ฌ์ด์˜ ๊ณต๋™๋•Œ๋ฌธ์ด๋‹ค. ๋งˆ์ดํฌ๋กœ ํฌ๊ธฐ์˜ ์ฒ  ์ž…์ž๋Š” ๋น„๊ต์  ๊ฑฐ์นœ ํ”ผ๋ธŒ๋ฆด ๊ตฌ์กฐ๋ฅผ ํ˜•์„ฑํ•œ๋‹ค. ํ˜ผ์„ฑ ์ž๊ธฐ์œ ๋ณ€์ฒด๋Š” ๋งˆ์ดํฌ๋กœ ํฌ๊ธฐ์˜ ์ฒ  ์ž…์ž์™€๋Š” ๋‹ค๋ฅธ ํ”ผ๋ธŒ๋ฆด ๊ตฌ์กฐ๋ฅผ ํ˜•์„ฑํ•œ๋‹ค. ๋‚˜๋…ธ ํฌ๊ธฐ์˜ ์ฒ  ์ž…์ž๋“ค์ด ๋งˆ์ดํฌ๋กœ ํฌ๊ธฐ์˜ ์ฒ  ์ž…์ž ์‚ฌ์ด์˜ ๊ณต๋™์„ ์ฑ„์›€์œผ๋กœ ์ธํ•ด์„œ ์ž๋ ฅํŠน์„ฑ์ด ํ–ฅ์ƒ๋˜์—ˆ๋‹ค. ๋งˆ์ง€๋ง‰์œผ๋กœ, ๋ฒŒํฌํ˜•๊ณผ ๋ฐ•๋ฆฌํ˜•์˜ ์„ผ๋”์ŠคํŠธ๋ฅผ ์ด์šฉํ•˜์—ฌ ์ž๊ธฐ์ž…์ž์˜ ๋ชจ์–‘์ด ์œ ๋ณ€์  ํŠน์„ฑ์— ๋ผ์น˜๋Š” ์˜ํ–ฅ์„ ์กฐ์‚ฌํ•˜์˜€๋‹ค. ๋ฐ•๋ฆฌํ˜• ์„ผ๋”์ŠคํŠธ์˜ ์ž๊ตฌ๋Š” ํ•œ ๋ฐฉํ–ฅ์œผ๋กœ ์ •๋ ฌ๋˜์–ด ์žˆ์–ด ์ž‘์€ ๊ฐ์ž์œจ์„ ๊ฐ–๊ณ , ์ด ํŠน์ง•์€ ์ €์ž๊ธฐ์žฅ์—์„œ ํ”ผ๋ธŒ๋ฆด ๊ตฌ์กฐ๋กœ์˜ ๋น ๋ฅธ ์ „ํ™˜์„ ๊ฐ€๋Šฅํ•˜๊ฒŒ ํ•œ๋‹ค. ๋˜ํ•œ, ๋ฐ•๋ฆฌํ˜• ์„ผ๋”์ŠคํŠธ์˜ ๋†’์€ ์ข…ํšก๋น„๋กœ ์ธํ•œ ํ•ญ๋ ฅ๊ณ„์ˆ˜๋Š” ์žฅ๊ธฐ ์•ˆ์ •์„ฑ์„ ํ–ฅ์ƒ์‹œ์ผฐ๋‹ค.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 ๊ตญ๋ฌธ์ดˆ๋ก 139 List of Publication 141 Appendix . 142 Appendix A. Improvement of Mechanical Properties by Introducing Curable Functional Monomers in Stereolithography 3D PrintingDocto

    Studies of Electroconductive Magnetorheological Elastomers

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    Electroconductive magnetorheological elastomers (MREs) have attracted a wide scientific attention in recent years due to their potential applications as electric current elements, in seismic protection, in production of rehabilitation devices, and sensors or transducers of magnetic fields/mechanical tensions. A particular interest concerns their behavior under the influence of external magnetic and electric fields, since various physical properties (e.g., rheological, elastic, electrical) can be continuously and/or reversibly modified. In this chapter, we describe fabrication methods and structural properties from small-angle neutron scattering (SANS) of various isotropic and anisotropic MRE and hybrid MRE. We present and discuss the physical mechanisms leading to the main features of interest for various medical and technical applications, such as electrical (complex dielectric permittivity, electrical conductivity) and rheological (viscosity) properties
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