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Synthesis and Properties of Magnetorheological (MR) Fluids

Genc, Seval (2003) Synthesis and Properties of Magnetorheological (MR) Fluids. Doctoral Dissertation, University of Pittsburgh. (Unpublished)

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Magnetorheological (MR) fluids are dispersions of fine (~ 0.05-10 micron) magnetically soft, multi-domain particles. The apparent yield strength of these fluids can be changed significantly within milliseconds by the application of an external magnetic field. MR fluid devices are being used and developed for shock absorbers, clutches, brakes, and seismic dampers. The major goals of this research were to advance the science of MR fluids. More specifically, the goals were: (a) influence of interparticle forces on stability and redispersibility of MR fluids and (b) factors affecting the "on" and "off" state rheological properties of MR fluids. In first part, the influence of the remnant magnetization of soft magnetic particulates on the redispersibility of MR fluids was investigated. The ratio of magnetic dipole interaction energies to thermal energy (Vmag/kBT) for 33 vol% iron based MR fluids (average particle size ~6micron), manganese zinc ferrite (average particle size ~2.3micron) and nickel zinc ferrite (average particle size ~2.1micron) were calculated as -161000, -6400, and -3900. Our calculations showed that even small levels of remnant magnetization levels in the magnetic particles introduce significant dipole-dipole interparticle interactions. These can lead MR fluids to show a tendency to undergo agglomeration. The second part of this study was concerned with the magnetic properties of the dispersed phase and "on-state" rheological behavior. The effects of dispersed phase saturation magnetization and applied magnetic fields (H) on the "on-state" apparent yield stress of MR fluids was investigated. Rheological measurements were conducted on MR fluids based on two different grades of carbonyl iron powder. Grade A (average size 7-9micron, saturation magnetization ~ 2.03T) and Grade B (average size ~2micron, saturation magnetization ~ 1.89T). The yield stresses of 33 and 40 vol% Grade A were 100 ± 3 and 124 ± 3 kPa, respectively at 0.8 ± 0.1T. The yield stress values of MR fluids were based on finer particles (Grade B) were consistently smaller. For example, the yield stresses for 33 and 40 vol% Grade B based MR fluid were 80 ± 8 and 102 ± 2 kPa respectively at 0.8 ± 0.1T. These experimental results were in good agreement with the analytical models developed by Ginder and co-workers. The decrease in the apparent yield strength of MR fluids based on smaller particles was attributed to the smaller saturation magnetization of these particles. The third part of this research was directed to better elucidate the "off-state" rheological behavior of MR fluids. MR fluids composed of 40 vol% Grade B and 100 cSt silicone oil exhibited shear thinning behavior with a viscosity of 199±52 and 1.9±0.3 Pa-s at a shear rate of 0.1 and 100 1/s respectively at 25 C. One of the major findings of this study was establishment the existence a measurement of a critical yield strain. The creep-recovery measurements revealed multiple yielding in the MR fluid where the first yield strain occurred between 0.01 and 0.02 strain values at stress levels between 10-120 Pa and the second yielding occurred at larger strains ranging from 0.08-20. This research shows that rheologically MR fluids exhibit a time and shear dependent behavior. Further research is needed to fully understand the rheological behavior of these complex materials. The effects of resting and shearing periods on the yielding were also investigated. Finally, in an effort to address issues concerning durability of MR fluids, the effect of exposing MR fluids to higher temperatures was investigated. MR fluids were exposed to high temperatures (175 C) for 24 hours. The "on" state apparent yield stress did not show any decrease, however the "off" state apparent viscosity showed an increase at shear rates > 15 1/s. The viscosity at a shear rate of 501/s was 3.9 Pa-s and the yield strain increased ~10 times the first yield strain observed in the MR fluids without heat treatment.


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Item Type: University of Pittsburgh ETD
Status: Unpublished
CreatorsEmailPitt UsernameORCID
ETD Committee:
TitleMemberEmail AddressPitt UsernameORCID
Committee ChairPhule, Pradeep
Committee MemberPlazek, Donal Jplazek@pitt.eduPLAZEK
Committee MemberNettleship, Ian
Committee MemberGinder, John M
Committee MemberEror, Nicholas
Committee MemberClark, William W
Date: 7 January 2003
Date Type: Completion
Defense Date: 18 July 2002
Approval Date: 7 January 2003
Submission Date: 6 August 2002
Access Restriction: No restriction; Release the ETD for access worldwide immediately.
Institution: University of Pittsburgh
Schools and Programs: Swanson School of Engineering > Materials Science and Engineering
Degree: PhD - Doctor of Philosophy
Thesis Type: Doctoral Dissertation
Refereed: Yes
Uncontrolled Keywords: coercivity; magnetic interaction energy; remnant magnetization; rheometry; shear thinning; soft magnetic materials; thixotropy; van der waals energy; yield strain; yield stress; redispersibility; rheology
Other ID:, etd-08062002-162348
Date Deposited: 10 Nov 2011 19:57
Last Modified: 15 Nov 2016 13:48


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