*2.4. Non-Magnetic Fillers*

Researches have been conducted to enhance the mechanical and MR characteristics of MREs by adding non-magnetic fillers as additives to MREs. Di fferent types of non-magnetic fillers include reinforcing agents [92–95], plasticizers [96–98], and crosslink agents [99,100] that are related to the mechanical characteristics of MREs, in addition to the important magnetic fillers to increase MR properties. Mechanical reinforcing agents include carbon nanotube (CNT) [92], graphene [93], graphite [94], and carbon black [95]. Li et al. [101] fabricated MRE by adding multi-walled CNTs. The addition of a small amount of CNTs can e ffectively increase the mechanical properties of MREs. In addition to increasing dynamic sti ffness and damping without an applied magnetic field, it also showed a higher field-induced increment. Chen et al. [102] fabricated MREs with di fferent carbon black content and observed the microstructure and mechanical performance of the prepared MREs. As the carbon black is added, the MR e ffect, damping ratio, and tensile strength of the MRE are all increased.

On the other hand, plasticizers are the most common additive in rubbery materials. Generally, plasticizers are added to the mixing and vulcanization processes of elastomers to increase the flexibility, distensibility, and workability of elastomers. The addition of plasticizer to the MRE helps the magnetic particles to be arranged by the magnetic field strength during the curing of the elastomer, thus increasing the absolute MR e ffect. Khairi et al. [97] examined the addition of silicone oil as a plasticizer to the SR based MRE. It resulted in an increase of particle alignment and the MR e ffect. The addition of silicone oil generally lowers the viscosity of the uncured rubber, increasing the alignment of the magnetic particles. Kimura et al. [98] analyzed the properties of MREs by adding a plasticizer such as dioctyl phthalate, dimethyl phthalate, butyl benzyl phthalate and bis(2-methyl octyl) phthalate. As the plasticizer content increases, the G' at 0 mT decreases and the G' at 500 mT increases. This is because the mobility of magnetic particles of MRE is increased by the addition of plasticizer. Fan et al. [103] varied the sulfur content to examine the e ffect of the crosslink density of the MRE on the damping behaviors. As the crosslinking density of MRE decreases, the movement of magnetic particles increases, and the damping property increases. This confirms that the rearrangemen<sup>t</sup> of magnetic particles plays an important role in improving magnetic field-induced change of loss factor, G', and relative MR performance.

While most additives for MREs are normally considered as non-magnetic, recently, addition of magnetic filler additives to the typical CI-based MRE composites has been reported to show a significant effect on the MR properties. Magnetic nano-sized fillers include both soft-magnetic and hard-magnetic nanoparticles. Lee et al. [104] prepared MRE by using micron-sized CI as a main magnetic particle in the NR matrix and adding nano-sized gamma-ferrite as an additive. The gamma-ferrite is a rod-shaped hard magnetic particle, which has been confirmed to be more uniformly aligned with the CI particles of the elastomer added gamma-ferrite than the neat CI-base elastomer due to the morphological characteristics. In addition, strain sweep tests confirmed that MREs with gamma-ferrite added had higher modulus. Kramarenko et al. [105] fabricated MRE by using SR as matrix and CI and NdFeB, a hard-magnetic filler particle, in various sizes and concentrations. MR properties and viscoelastic behavior were measured according to NdFeB size and concentration. The addition of hard magnetic NdFeB caused the non-linear viscoelastic behavior in small strains and caused a large increase in modulus.
