Synthesis and Applications of Size and Shape Controlled Magnetic Oxide Particles for Magnetorheological Fluids

Abstract

Magnetorheological fluids (MRFs) are non-colloidal stable suspensions of polarizable mesoscale soft magnetic particles, usually metallic Fe-particles, in a carrier liquid such as oil or water; the solidity of which can be tuned by varying the applied magnetic field strength. Magnetorheological fluids are agile candidates for impact mitigation due to their tunable “solidity”, quick and complete reversibility of physical states, durability and reusability in comparison to their mechanical counterparts. The highly desirable property of an MRF is its yield strength and hence the conventional MRFs are Fe-based. However, uncoated Fe-particles suffer from poor chemical and thermo-oxidative stabilities, poor sedimentation stability and redispersibilities necessitating the coatings / additives; which always lead to compromised performance when used in MRFs. An alternative (to Fe) magnetic filler phase is the use of magnetic oxide particles. Soft magnetic spinel ferrites and garnets (though with moderate yield strength in an MRF) with their excellent chemical, thermo-oxidative and sedimentation stabilities, ready-redispersibility, less stringent synthesis and preservation conditions, lower cost, need no stabilizers and additives make them potential contenders for use in MRFs which can provide reliable MR performance. As the microstructure and magnetic nature of particles have direct influence on the MR property, the effects of these were studied by preparing MRFs with magnetic oxide particles of different sizes and shapes. These MRFs were simple bi-phasic as magnetic oxide particles were dispersed in versatile carrier fluid (silicone oil) without any additives; where the magnetic fill fraction was decided based on off state viscosity and the wettability criteria. As the MRFs in a device can undergo different stress / strain conditions of varying amplitudes and frequencies during their service, such a response was studied in laboratory using magnetorheometer via different modes of operation which mimic the service conditions. By varying the applied magnetic field strength and applied shear conditions, the performance of MRFs was evaluated and correlated to the physical and magnetic properties of the particles. Such a study provides a basis for the choice of magnetic phase in MRFs and their required concentration in the base fluid to provide highest efficiency. The dynamic yield strengths (field dependent yield stress) of MRFs extracted from steady shear measurements showed that the yield strength was strongly dependent on the saturation magnetization as well as on the microstructure of the particles used in MRF. The yield strength scaled with the saturation magnetization, magnetic fill fraction and applied magnetic field strength due to stronger magnetic column formation. The stability of MRFs (via the absence of wall slip) was found to depend predominantly on the microstructure of magnetic particles in the fluid such that MRFs containing structured particles showed the absence of wall slip while the MRFs containing irregular shaped powder particles showed poor stability via the occurrence of wall slip. The steady shear tests highlight the importance of using particles of definite shape with superior magnetic properties at a certain magnetic fill fraction for an efficient and reliable MR performance. The MRFs subjected to different oscillatory shear conditions showed that sturdier structures form in-field (exhibited via high gain modulus or low loss factor) when the particles have certain shapes (and size distribution) which result in high surface contact and are highly magnetic. Hence, the MRF containing Fe3O4 micro-octahedrons with high magnetization and large surface area for contact with other octahedron showed the large value of gain modulus and low loss factor compared to all other MRF samples. Poly-dispersity in spheres was found to be advantageous over monodisperse spherical magnetic particles due to void-bridging effects that strengthen the magnetic structuration. The irregular shaped particles based MRFs showed lower gain (higher loss factor) due to weak structuration. Anomalously high loss factor observed for rod shaped LZFP particles based MRF at medium strains and low field strengths is attributed to the rotation hindrance and low density of particles. The polydisperse particles based MRFs showed need for higher applied field strengths to decrease the loss and irregular particles based MRFs showed noisy response. The magnetosweep results showed that shape anisotropic particles based MRFs respond faster to applied field manifested as a faster decrease in loss factor with field. With magnetorheological parameters showing high dependence on the physical and magnetic nature of particles, oscillatory shear tests can serve as a means to select and assess the suitability of these particles for magnetorheological fluid for specific applications. The time dependent magneto-mechanical behaviour such as creep-recovery in MRFs showed that the strain recovery was dependent on the microstructure and magnetic nature of the particles such that fluids containing structured particles with high saturation magnetization showed higher recovery (due to better in-field structuration) compared to the irregular shaped and lower magnetization particles based MRF counterparts. The endurance of the MRFs (sustenance of strength of the MRF) under sustained stress conditions were estimated by a novel method which showed that MRFs containing ‘structured’ particles with high saturation magnetization showed high creep strength. In case of spherical particles based MRFs, the polydispersity of particles was found to aid in better column strength due to void-filling. The high surface contact between rod-shaped particles in the fluid resulted in good creep-strength among all MRFs. Among all the particles, the octahedron shaped Fe3O4 particles with large surface contact coupled with high saturation magnetization makes the Fe3O4 micro-octahedron particles based MRF the best amongst all the MRFs studied in this work. In case of irregular shaped particles based MRFs, the creep strength lagged behind the yield strength suggesting that such MRFs are not suitable for applications which demand sustained strength over prolonged action of stresses. Thus, the present work highlights the importance of considering the physical and magnetic properties of magnetic particles while selecting them for application specific MRFs where high endurance is sought. The stress relaxation behaviour of MRFs showed an overall high strength (via relaxation moduli) for MRFs containing particles with definite shape and high magnetization values (increased structure strength). However, the rod shaped particles based MRF did not witness increased strain limit with increased field strength, probably due to the mass flow in fluid due to higher inter-particle interaction than the interaction with applied field. The observation of increase in critical strain with increase in field for MRFs containing irregular shaped particles is only due to the higher number of particles resulting in overall increase in viscosity with field. Among all the MRFs, octahedron Fe3O4 particles with superior magnetic properties and large surface contact between facets showed highest critical strain for flow, which is in corroboration with other magnetorheological studies discussed so far. The creep-recovery and stress relaxation behaviours of MRFs are rarely studied, yet very important when selecting an MRF for an application which seeks high retention of MR strength over prolonged action of stress or strains. A comparison of particle shapes used in the MRFs suggests that although both octahedron shaped and rod shaped particles make high surface contact during structuration, the former is better due to lack of rotation hindrance, thus useful for preparing quickly responding MRFs. The inadequacies in th e conventio nal FOMs are address ed by a new FOM which is based o n a wholistic approach formulated consideri ng all relev ant physical and magnetic paramete rs of the particles. Also, the individ ual terms of this FOM help in selecting a particular MRF for a specific application. The FOM is as follows: λ – sedimentation constant (time taken by the MRF to sediment to about 1/eth of its total volume) With the MRFs containing octahedron shape d Fe3O4 pa rticles showing the highest FOM followed by s pheres (mod erate value ) which are succeeded by irregular powder samples based MRFs, the FO M observed in all MRF cases follow the same trend as observed by results from different magnetorheologi cal studies. Hence, the highest F4 (or FAB) observed for Fe3O4 octahedron particles based MR F in comparison to a ll other MR Fs (including Fe-based) is justified by the o Mbserved large yield strength, creep-resistance, low density and ready-redispersibilities, validating the FOM. The entire thesis is organized as follows. Chapter 1 details the motivation for the present research work, introduction to the material of interest (Magnetorheological fluid) with overview of different areas of potential applications, important properties of MRF, the current status of MRF, the challenges / issues needed to be addressed followed by choice of alternate materials for addressal of these drawbacks faced by conventional (Fe based) MRFs. Chapter 2 explains the synthesis of magnetic-oxide particles of different sizes and shapes by following different synthesis techniques. This is followed by the structural, microstructural and magnetic properties characterizations carried out by employing different, standard characterization techniques. The procedure for preparation of MRFs from the synthesized magnetic oxide particles is discussed. The basis of carrier fluid selection and magnetic particle concentration in MRF is explained. Chapter 3 gives a background to magnetorheology, in terms of the instrumentation (magnetorheometer), the relation between the magnetorheological parameters and the instrumental parameters (conversion factors), the different operating modes and the relevance of characterization modes in terms of practical applications, the procedure of different characterizations and the standard response behavior of MRFs to the characterizations. Chapter 4 is comprehensive characterization of all the MRFs subjected to steady shear conditions at various applied fields. The detailed analyses in terms of MR response are given with respect to the structure, microstructure, magnetic nature, and magnetic fill fraction of the magnetic particle in the fluid. Chapter 5 is extensive study of all the MRFs subjected to dynamical shear conditions at various applied fields. The magnetorheological responses of MRFs under different dynamical conditions (amplitude sweep, frequency sweep and magnetosweep) are analyzed in regard to role of microstructure, magnetic nature and magnetic fill fraction of the magnetic particle in the fluid. Chapter 6 explains the creep-recovery response of MRFs for the best magnetic fill fraction, decided from the steady and dynamical shear responses for all concentrations of MRFs. The recovered strain is analyzed with respect to a range of applied field strength and stress values. The creep strength determined from this study is correlated to the microstructure and magnetic nature of particles constituting the MRFs. Chapter 7 elaborates the stress relaxation behaviour of MRFs for the best magnetic fill fraction, decided from the steady and dynamical shear responses for all concentrations of MRFs. The stress relaxation (plateau values) moduli for the MRFs extracted at various applied field strength and strain values are analyzed to estimate the critical stress for flow in MRFs. This relationship between the critical stress that an MRF can withstand and the microstructure and magnetic nature of the particles in the fluid are investigated. Chapter 8 is about the study of sedimentation stability (and the redispersibility) of magnetic oxide particles based MRFs and the comparison of these properties with Fe- based MRFs. The role of mass-density and microstructure of particles in the fluid on sedimentation rate is briefly explained. Chapter 9 compares the important outcome of all the magnetorheological characterizations for all the studied MRFs in terms of extent and speed of response, the sedimentation stability and eases of redispersibility, and relates the observations to the physical and magnetic properties of the magnetic particles. The method of developing a new figure of merit based on a wholistic approach for assessing the efficiency and reliability of MRF is discussed which overcomes the shortcomings of conventional figures of merit. Chapter 10 summarizes the important findings of research work and highlights the validity of the new figure of merit in assessing ‘reliability and performance’ of MRFs.