The magnetorheological (MR) performance of suspensions based on core-shell-structured foamed polystyrene (PSF)/FeO particles was investigated by using a vibrating sample magnetometer and a rotational rheometer. Core-shell-structured polystyrene (PS)/FeO was synthesized by using the Pickering-emulsion polymerization method in which FeO nanoparticles were added as a solid surfactant. Foaming the PS core in PS/FeO particles was carried out by using a supercritical carbon dioxide (scCO) fluid. The density was measured by a pycnometer. The densities of PS/FeO and PSF/FeO particles were significantly lowered from that of the pure FeO particle after Pickering-emulsion polymerization and foaming treatment. All tested suspensions displayed similar MR behaviors but different yield strengths. The important parameter that determined the MR performance was not the particle density but rather the surface density of FeO on the PS core surface. The morphology was observed by scanning electron microscopy and transmission electron microscopy. Most FeO particles stayed on the surface of PS/FeO particles, making the surface topology bumpy and rough, which decreased the particle sedimentation velocity. Finally, Turbiscan apparatus was used to examine the sedimentation properties of different particle suspensions. The suspensions of PS/FeO and PSF/FeO showed remarkably improved stability against sedimentation, much better than the bare FeO particle suspension because of the reduced density mismatch between the nanoparticles and the carrier medium as well as the surface topology change.
The effect of shape anisotropy of a magnetic particle on the performance and stability of the magnetorheological (MR) suspension was investigated using a rotational rheometer and a vibrating sample magnetometer. A flaky Sendust (FS) suspension demonstrated surprisingly high MR performance at low magnetic field strength because of the shape anisotropy effect which induced a rapid ascent of the particles' magnetic moment and, thus, suspension's high yield stress. Despite its lower iron content (85%) than carbonyl iron (almost pure iron), the Sendust suspension exhibited a yield stress value greater than that of the carbonyl iron suspension at low magnetic field strength. Because of the low demagnetization factor in a nonisometric Sendust particle, the alloy particles were easily magnetized along the easy axis direction, which led to an unexpectedly high yield stress of the suspension at a low magnetic field strength. At a high magnetic field strength, all of the tested suspensions of flaky Sendust, bulk Sendust (BS), and carbonyl iron (CI) displayed similar MR behaviors but different yield stress values. The highest yield stress value at high magnetic field strength was that of the carbonyl iron suspension, followed by those of the FS suspension and the BS suspension; however, the difference between the yield stress values of the two Sendust suspensions was not substantial. A master curve for different MR fluids was obtained by plotting dimensionless viscosity as a function of the shear rate and the square of the magnetic field strength, which indicates that the particle suspensions exhibited similar responses to the external magnetic stimuli. Though the density of FS is larger than the bulk Sendust, the FS suspension demonstrates remarkably improved stability compared with the CI suspension or the BS suspension because of the large drag coefficient of the flaky Sendust particles.
The
magnetorheological (MR) performance of suspensions based on
magnetic (flaky Sendust (FS))–magnetic (Co0.4Fe0.4Ni0.2) nanocomposite particles was investigated
by using a vibrating sample magnetometer and a rotational rheometer.
Flaky Sendust@Co0.4Fe0.4Ni0.2 nanocomposite
particles were fabricated through wet chemical synthesis of Co0.4Fe0.4Ni0.2 on the surface of FS. The
density of the resultant FS@Co0.4Fe0.4Ni0.2 was less than that of FS due to the pore/void formation
in the composite particles. Because of the high saturation magnetization
of Co0.4Fe0.4Ni0.2 (165 emu/g), FS@Co0.4Fe0.4Ni0.2 (145 emu/g) exhibited greater
magnetization than bare FS (130 emu/g), which resulted in the good
performance of FS@Co0.4Fe0.4Ni0.2-based MR fluids: the suspension exhibited a remarkably high yield
stress, almost one order greater than that of MR fluids based on hierarchically
structured (HS) Fe3O4 particles. In addition,
the high drag coefficient of FS@Co0.4Fe0.4Ni0.2 in the liquid medium, in conjunction with its lower density,
resulted in a substantially improved long-term stability, better than
that of Co0.4Fe0.4Ni0.2- or FS-based
suspensions. Although the density of the FS@Co0.4Fe0.4Ni0.2 nanoparticles is higher than that of HS-Fe3O4 particles, their stability is much better than
the stability of HS-Fe3O4 particle’s
suspension. Manufactured magnetic–magnetic nanocomposite particles
provide a feasible MR suspension of high MR performance and long-term
stability.
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