Porous Fe3O4 submicron particles for use in magnetorheological fluids

Macroscopic Room-Temperature Magnetoelectricity in Piezoelectric (Core)-Ferrimagnetic (Shell) Nanocomposites

The magnetoelectric (ME) effect, which involves the interaction of magnetic and electric fields within a material, has a significant potential for various applications. Our study addresses the limitations of conventional magnetostriction-based ME materials by demonstrating an alternative approach that achieves substantial ME effects in core-shell-type nanocomposites at room temperature. By synthesizing ferrimagnetic Fe3O4 nanoparticles onto piezoelectric poly(vinylidene fluoride) (PVDF) particles, we identified a distinct ME mechanism. In magnetorheological (MR) fluids, the magnetic-field-induced aggregation of Fe3O4 nanoparticles, combined with the piezoelectricity of PVDF, leads to a pronounced ME effect, significantly enhancing the performance and stability of MR fluids. This research highlights a crucial observation of distinct ME effects, which could suggest potential pathways for advancements in practical applications including microfluidics, vibration dampers, tactile technologies, and biomedical and bioengineering fields.

Significant Improvement in Magnetorheological Performance by Controlling Micron Interspaces with High Permeability Submicron Particles

The shear yield strength, sedimentation stability and zero-field viscosity of magnetorheological fluids (MRFs) are crucial for practical vibration damping applications, yet achieving a balanced combination of these performances remains challenging. Developing MRFs with excellent comprehensive performance is key to advancing smart vibration damping technologies further. Theoretically, incorporating a multiscale particle system and leveraging synergistic effects between their can somewhat enhance MRFs' performance. However, this approach often faces issues such as insignificant increases in shear yield strength and excessive rise in zero-field viscosity. In response, this study employs a DC arc plasma method to synthesize a high magnetic permeability, low coercivity submicron FeNi particles, and further develops a novel CIPs-FeNi bidisperse MRFs. The introduction of submicron FeNi particles not only significantly enhances the shear2019 yield strength of MRFs under low magnetic fields but also promotes improvements in sedimentation stability and redispersibility without excessively increasing viscosity. Comprehensive performance analysis is conducted to explore the optimal content ratio, and detailed mechanisms for the enhancement of performance are elucidated through analysis of parameters such as chain-like structure, magnetic flux density and friction coefficient. Most importantly, the superior comprehensive performance combined with straightforward fabrication methods significantly enhances the engineering applicability of the CIPs-FeNi bidisperse MRFs.

Non-Settling Super-Strong Magnetorheological Fluids

A magnetorheological (MR) fluid is generally called a suspension in which magnetic particles are dispersed in a non-magnetic medium. When an external magnetic field is applied, a pseudo-phase transition occurs within a short time to generate yield stress, and when the magnetic field is released, it returns to the suspended state. Due to these unique characteristics, it is classified as a smart material to be widely applied in various industries. High performance MR fluids require high yield stress and stability for long-term use. However, it is very difficult to improve performance and stability simultaneously due to the limited amount of magnetic particles in the suspension and particle sedimentation caused by the density mismatch between the suspending particles and the liquid phase. In this study, an MR slurry is developed that is completely different from the MR suspension, starting from the opposite concept. An innovative non-settling (i.e., permanently stable) magnetorheological slurry is successfully created that exhibits unprecedented ultra-high yield stress. This result is expected to be a turning point for applying MR fluids to more diverse industries. In addition, a simple fitting equation expressing the yield stress as a function of the particle volume fraction is proposed.

Stable Magnetorheological Fluids Containing Bidisperse Fillers with Compact/Mesoporous Silica Coatings

A drawback of magnetorheological fluids is low kinetic stability, which severely limits their practical utilization. This paper describes the suppression of sedimentation through a combination of bidispersal and coating techniques. A magnetic, sub-micro additive was fabricated and sequentially coated with organosilanes. The first layer was represented by compact silica, while the outer layer consisted of mesoporous silica, obtained with the oil–water biphase stratification method. The success of the modification technique was evidenced with transmission electron microscopy, scanning electron microscopy/energy-dispersive X-ray spectroscopy and Fourier-transform infrared spectroscopy. The coating exceptionally increased the specific surface area, from 47 m2/g (neat particles) up to 312 m2/g, which when combined with lower density, resulted in remarkable improvement in the sedimentation profile. At this expense, the compact/mesoporous silica slightly diminished the magnetization of the particles, while the magnetorheological performance remained at an acceptable level, as evaluated with a modified version of the Cross model. Sedimentation curves were, for the first time in magnetorheology, modelled via a novel five-parameter equation (S-model) that showed a robust fitting capability. The sub-micro additive prevented the primary carbonyl iron particles from aggregation, which was projected into the improved sedimentation behavior (up to a six-fold reduction in the sedimentation rate). Detailed focus was also given to analyze the implications of the sub-micro additives and their surface texture on the overall behavior of the bidisperse magnetorheological fluids.

Physical Mechanisms of Magnetic Field Effects on the Dielectric Function of Hybrid Magnetorheological Suspensions

In this paper, we study the electrical properties of new hybrid magnetorheological suspensions (hMRSs) and propose a theoretical model to explain the dependence of the electric capacitance on the iron volumetric fraction, ΦFe, of the dopants and on the external magnetic field. The hMRSs, with dimensions of 30 mm×30 mm×2 mm, were manufactured based on impregnating cotton fabric, during heating, with three solutions of iron microparticles in silicone oil. Flat capacitors based on these hMRSs were then produced. The time variation of the electric capacitance of the capacitors was measured in the presence and absence of a magnetic field, B, in a time interval of 300 s, with Δt=1 s steps. It was shown that for specific values of ΦFe and B, the coupling coefficient between the cotton fibers and the magnetic dipoles had values corresponding to very stable electrical capacitance. Using magnetic dipole approximation, the mechanisms underlying the observed phenomena can be described if the hMRSs are considered continuous media.