The material characterization of magnetorheological elastomers consisting of oxidized iron particles and cobalt particles

Magnetorheological (MR) elastomer is classified as a smart material whose mechanical properties, such as stiffness, natural frequency, and damping capacity are tunable and controllable under the influence of magnetic field. This tunability makes it suitable for various engineering applications that require adjustable devices. Generally, MR elastomer contains magnetic particles and a matrix. Iron particles are typically used as magnetic particles in MR elastomer because of their low residual magnetism and high magnetization. Nevertheless, iron particles are prone to oxidation in a practical environment that can weaken the effectiveness of MR elastomer in terms of its field-dependent viscoelasticity. As an alternative, MR elastomer contains cobalt particles that offer better oxidative resistance and favorable electrical properties, yet their properties under oxidation have not been explored. In this work, we conducted an oxidation process using diluted 5vol% hydrochloric acid (5 vol% HCl). Two different magnetic particles (iron particles and cobalt particles) were used and tested to identify the better filler for MR elastomer subjected to oxidation conditions. The characterizations of magnetic particles were accomplished using high-resolution field emission scanning electron microscopy and energy dispersive X-ray spectroscopy (HR-FESEM and EDS) to observe the morphology and elemental compositions, and X-ray diffraction (XRD) to determine the crystallinity of magnetic particles. In addition, Fourier-transform infrared (FTIR) analysis was conducted to identify the molecular compounds of the MR elastomers. A vibrating sample magnetometer (VSM) was also utilized to obtain magnetic properties and a rheometer to measure rheological properties. The oxidation on the layer of the magnetic particles undoubtedly affected the MR elastomers’ properties showing the degradation of magnetic properties and rheological properties (storage modulus and loss modulus) for both iron particles and cobalt particles. However, cobalt particles exhibit greater oxidation resistance. It is shown that MR elastomers with cobalt particles showed a higher storage modulus and loss modulus compared to MR elastomers with iron particles both before and after oxidation. Specifically, MR elastomers with cobalt particles and MR elastomers with iron particles showed a decrement of the MR effect by 119.36% and 139.26%, respectively.

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