Introduction

Magnetorheological (MR) materials, such as MR fluids [1, 2], have been recognized for their ability as smart and intelligent materials with easy conversion between a liquid-like and solid-like state within a very short time under a magnetic field. Although the properties of MR fluids are controlled by the shear viscosity and yield stress, the properties of MR elastomers are controlled by the modulus according to the external magnetic field [3, 4]. MR elastomers can overcome the problems of sealing and particle settling by mixing the magnetic particles in a solid matrix, such as natural rubber, silicone rubber, and polyurethane [5,6,7,8]. Because of these characteristics, MR elastomers have a range of engineering applications, including electromagnetic shielding, 3D printing, rolling friction controllers, and engine mounts in automobiles [9,10,11].

Among the various soft magnetic particles for MR elastomers, such as carbonyl iron (CI), ferrite, and nickel particles [12, 13], CI is a representative MR material owing to its soft magnetic features, appropriate size, and high magnetic permeability [14]. In addition, while different materials involving carbon nanotubes, nickel-zinc ferrite, and polyaniline [15,16,17], have been applied as additives to improve the dispersion stability of MR fluids, the MR effect is reduced in many cases. On the other hand, the introduction of a hard magnet particle as an additive can increase both the dispersion stability and MR effect for MR fluids. In particular, gamma-ferrite (γ-Fe2O3) particles were found to be advantageous for improving the MR effect by broadening the contact surface in the formation of a chain in a rod-like shape [18].

In this study, rod-shaped hard magnetic γ-Fe2O3 particles were used as an additive in a CI particle-based natural rubber (NR) composite MR elastomer to improve the MR efficiency of CI-based MR elastomers in Fig. 1. A magnetic field was applied in one direction during fabrication to produce anisotropic MR elastomers with uniformly aligned magnetic particles for improving MR and dielectric properties [19]. The morphology of the particles and the alignment of the MR elastomer were confirmed by scanning electron microscopy (SEM), and their rheological properties were measured and analyzed using a rotational rheometer.

Fig. 1
figure 1

Schematic diagram of additive system of CI particles with γ-Fe2O3 particles

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Materials and synthesis method

Materials

CI (Standard CC-grade, BASF, Germany) particles with a density of 7.8 g/cm3 and 2 wt% rod-shaped γ-Fe2O3 (HR-350, Magnox) particles with a density of 4.7 g/cm3 were used as magnetic substances. Natural rubber (NR) (CV-60) was used as solid matrix, and carbon black (CB) (N990, Cancarb, Canada) was added as both filler and hardening agent.

Preparation of CI@ γ-Fe2O3-based MR elastomer

Pure CI and 2 wt% rod-shaped γ-Fe2O3 particles were mixed evenly. Natural rubber (NR) of 100 phr, CI@ γ-Fe2O3 particles of 200 phr, and carbon black (CB) of 20 phr were mixed using a general rubber mixing machine (Banbury mixer, HYB-3L, Hyupyoung machinery Co., Korea) for 15 min. Subsequently, this composite rubber was placed into a 1-mm circle-shaped mold and heated to 160 °C for 10 min using customized heat-magnet coupled equipment under a pressure of 10 Mpa and a magnetic field intensity of 840 mT. Finally, the MR elastomer containing CI@ γ-Fe2O3 particles aligned along a uniform direction was fabricated.

Characterization

The morphology of the CI@ γ-Fe2O3 particles and mapping images of the fractured surfaces of the MR elastomer were examined by high-resolution SEM (HR-SEM) (SU8010, Hitachi, Japan) and energy dispersive X-ray spectroscopy (EDX), respectively, with a high accelerating voltage of approximately 15 kV. Note that the EDX was to examine alignment of the dispersed magnetic particles through its mapping focusing on the Fe element in the magnetic particles. The MR characteristics of the MR elastomers based on both CI and CI@ γ-Fe2O3 were confirmed using a rotational rheometer (MCR 300, Anton Paar, Austria) with a high voltage generator and special geometry (PP20/MRD/TI/S) with a sand-blasted surface to prevent a potential slip phenomenon because the MR elastomeric sample disc slips easily between the two plates.

Results and discussion

Figure 2 represents SEM images of the pristine CI (a) and CI@ γ- Fe2O3 (b) particles, in which the surface of pure CI appears smooth with an almost spherical shape and a size of 1–4 μm. On the other hand, the CI@γ-Fe2O3 particles contained rod-shaped γ-Fe2O3 particles on not only the surface of the CI particles, but also in the empty space among the CI microspheres because of the magnetic interaction of the hard magnetic γ-Fe2O3 particles. Therefore, when the CI particles form a chain under an applied magnetic field, higher MR performance can be expected because γ-Fe2O3 particles act as a support among CI particles [18].

Fig. 2
figure 2

SEM images of pure CI (a) and CI@ γ-Fe2O3 particles

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To examine aligned magnetic particles of the MR elastomers, a MR elastomer sample was placed in liquid nitrogen, frozen, and then broken to produce a smooth fracture surface. Figure.3a, b presents images of the fractured surface profiles of both the CI and γ-Fe2O3 particle-based MR elastomers confirmed by HR-SEM. Furthermore, the Fe components in the samples were more distinctively mapped by EDX to exhibit the alignment of magnetic particles in each anisotropically prepared MR elastomer (as shown in Fig. 3c, d). In the case of a CI-based MR elastomer, the CI particles were aligned like fibrillar chains [20], but they were discontinuous and irregular. In contrast, the MR elastomer based on CI@ γ-Fe2O3 particles showed an almost parallel chain structure that was regular and continuous because the added γ-Fe2O3 particles were located in the empty spaces among the CI particles to support and stabilize the chain. These results suggest that the CI@ γ-Fe2O3-based MR elastomer will have a higher modulus and efficiency than the CI-based MR elastomer. Note that while various additives including carbon nanotube increase dynamic stiffness and loss factor of CI-based MR elastomers, its addition was found to reduce the MR effect [21].

Fig. 3
figure 3

Fractography and mapping images of a, c pure CI- and b, d CI@ γ-Fe2O3-based anisotropic MR elastomer

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Figure 4 shows storage modulus of the samples as a function of shear strain from the strain amplitude sweep test, demonstrating the viscoelastic characteristics for both CI-based (open) and CI@ γ-Fe2O3-based (closed) MR elastomers. This test was conducted at a fixed frequency of 1 Hz, varying the strain from 0.01 to 1%. Overall, the storage modulus of the MR elastomer based on CI@ γ-Fe2O3 was higher than those of the CI-based MR elastomer under an applied external magnetic field. Moreover, the storage modulus tended to decrease with increasing strain and increase with increasing magnetic field strength [22]. The linear viscoelastic (LVE) region, where the storage modulus remained constant before a sharp decrease, appeared at a low strain region because the parallel chain structures were maintained in this region under the influence of an external magnetic field. In addition, the storage modulus was also independent in this LVE region.

Fig. 4
figure 4

Storage moduli of MR elastomers based on pure CI (open) and CI@ γ-Fe2O3 (closed) as a function of strain under a variety of magnetic field strengths

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Figure 5 demonstrates the relationship between storage modulus and strain under a variety of strains at a constant angular frequency of approximately 6.28 rad/s. The storage moduli of the two MR elastomer samples were proportional to the magnetic field strength, and the slope tended to increase with increasing magnetic field. On the other hand, the slope of the CI@γ-Fe2O3-based MR elastomer increased sharply with increasing magnetic field intensity compared to the slope of the CI-based MR elastomer [23]. This suggests that γ-Fe2O3 particles have a higher storage modulus because of the formation of a stable chain. Note that in the case of silicon rubber matrix, poly(trimethylsilyloxyethyl methacrylate)-coated CI particles with improved wettability and dispersibility showed enhanced MR efficiency [24], and the issue on affinity between CI particles and elastomer matrix could be also important [25].

Fig. 5
figure 5

The tendency of storage modulus of anisotropic MR elastomer based on CI (square) and CI@ γ-Fe2O3 (circle) under different magnetic field strengths

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Conclusion

A CI@ γ-Fe2O3-based MR elastomer was fabricated by adding γ-Fe2O3 nanoparticles as an additive and the behaviors were compared with those of a pure CI-based MR elastomer. Mapping showed that the CI@ γ-Fe2O3-based MR elastomer was aligned more uniformly than the CI-based MR elastomer. Moreover, the strain sweep test confirmed that the CI@ γ-Fe2O3-based MR elastomer had a higher modulus. The modulus increased significantly with increasing magnetic field strength.