ABSTRACT
We investigated the magnetorheological (MR) properties of the carbon nanotube (CNT)-Co0.4Fe0.4Ni0.2 composite suspension to find a high-performance MR fluid with excellent stability. The composites were fabricated by chemical reduction of Co0.4Fe0.4Ni0.2 on the surface of amine-functionalized CNTs. A synergistic effect between the high aspect ratio of the CNTs and the strong magnetic polarization of the Co0.4Fe0.4Ni0.2 led to stronger MR performance of the nanocomposite particle suspension. The MR fluid exhibits an unexpected high yield stress value that is 13 times greater than that of a CNT-Fe3O4 suspension at a magnetic field strength of 343 kA/m. Nonmagnetic CNTs form a three-dimensional networklike structure, imparting surprisingly large additional yield stress to the CNT-Co0.4Fe0.4Ni0.2 nanocomposite MR suspension. The low density of the CNTs resulted in much better long-term stability for the CNT-Co0.4Fe0.4Ni0.2 nanocomposite suspension than the MR fluid containing only Co0.4Fe0.4Ni0.2.
ABSTRACT
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.
ABSTRACT
The morphological development and thermal properties of different polyamides with long-chain branches without forming a network structure were characterized by differential scanning calorimetry, polarized optical microscopy, and nonisothermal crystallization kinetics. The crystallization characteristics were analyzed using the nonisothermal kinetic equation proposed by Seo. Polarized optical microscopy and the Avrami exponent show the effect of the structural changes on the molecular ordering during the crystallization and early morphological development. The Avrami exponent, n, determined from the analysis of the nonisothermal crystallization kinetics, indicates a reduced heterogeneous nucleation for the modified polyamides. Structural changes (branching) of the polyamides impede crystallization, as indicated by the shift of the crystallization peaks to lower temperatures.
ABSTRACT
Magnetorheological (MR) fluids are a type of smart material with rheological properties that may be controlled through mesostructural transformations. MR fluids form solid-like fibril structures along the magnetic field direction upon application of a magnetic field due to magnetopolarization of soft-magnetic particles when suspended in an inert medium. A reverse structural transition occurs upon removal of the applied field. The structural changes are very fast on the order of milliseconds. The rheological properties of MR fluids vary with the application of a magnetic field, resulting in non-Newtonian viscoplastic flow behaviors. Recent applications have increased the demand for MR materials with better performance and good long-term stability. A variety of industrial MR materials have been developed and tested in numerous experimental and theoretical studies. Because modeling and analysis are essential to optimize material design, a new macroscale structural model has been developed to distinguish between static yield stress and dynamic yield stress and describe the flow behavior over a wide range of shear rates. Herein, this recent progress in the search for advanced MR fluid materials with good stability is described, along with new approaches to MR flow behavior analysis. Several ways to improve the stability and efficiency of the MR fluids are also summarized.
ABSTRACT
The magnetorheological (MR) performance of suspensions based on core-shell-structured foamed polystyrene (PSF)/Fe3O4 particles was investigated by using a vibrating sample magnetometer and a rotational rheometer. Core-shell-structured polystyrene (PS)/Fe3O4 was synthesized by using the Pickering-emulsion polymerization method in which Fe3O4 nanoparticles were added as a solid surfactant. Foaming the PS core in PS/Fe3O4 particles was carried out by using a supercritical carbon dioxide (scCO2) fluid. The density was measured by a pycnometer. The densities of PS/Fe3O4 and PSF/Fe3O4 particles were significantly lowered from that of the pure Fe3O4 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 Fe3O4 on the PS core surface. The morphology was observed by scanning electron microscopy and transmission electron microscopy. Most Fe3O4 particles stayed on the surface of PS/Fe3O4 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/Fe3O4 and PSF/Fe3O4 showed remarkably improved stability against sedimentation, much better than the bare Fe3O4 particle suspension because of the reduced density mismatch between the nanoparticles and the carrier medium as well as the surface topology change.
