Volume 7, Issue 10

Magnetoactive elastomers (MAEs) are a special type of smart materials consisting of an elastic matrix with embedded microsized particles that are made of ferromagnetic materials with high or low coercivity. Due to their composition, such elastomers possess unique magnetic field-dependent material properties. The present paper compiles the results of investigations on MAEs towards an approach of their potential application as vibrating sensor elements with adaptable sensitivity. Starting with the model-based and experimental studies of the free vibrational behavior displayed by cantilevers made of MAEs, it is shown that the first bending eigenfrequency of the cantilevers depends strongly on the strength of an applied uniform magnetic field. The investigations of the forced vibration response of MAE beams subjected to in-plane kinematic excitation confirm the possibility of active magnetic control of the amplitude-frequency characteristics. With change of the uniform field strength, the MAE beam reveals different steady-state responses for the same excitation, and the resonance may occur at various ranges of the excitation frequency. Nonlinear dependencies of the amplification ratio on the excitation frequency are obtained for different magnitudes of the applied field. Furthermore, it is shown that the steady-state vibrations of MAE beams can be detected based on the magnetic field distortion. The field difference, which is measured simultaneously on the sides of a vibrating MAE beam, provides a signal with the same frequency as the excitation and an amplitude proportional to the amplitude of resulting vibrations. The presented prototype of the MAE-based vibrating unit with the field-controlled “configuration” can be implemented for realization of acceleration sensor systems with adaptable sensitivity. The ongoing research on MAEs is oriented to the use of other geometrical forms along with beams, e.g. two-dimensional structures such as membranes.

Hybrid fibers consisting of biopolymers and inorganic nanoparticles are receiving increasing attention due to their unique properties. Commonly, the nanoparticles are chosen for their intrinsic properties such as magnetic, thermal, or electrical conductivity. The biopolymer component of the hybrid fiber is chosen for its mechanical properties and ability to act as a scaffold or matrix for the nanoparticles. While there are many fiber-forming synthetic polymers, there has been a recent interest in replacing these systems with biopolymers due to their sustainability, biocompatibility, nontoxicity, and biodegradability. Fibers made from biopolymers have one additional benefit over synthetic polymers as they make good scaffolds for embedding nanoparticles without the need of any additional bonding agents. In particular, naturally occurring biopolymers such as proteins exhibit a myriad of interactions with nanoparticles, including ionic, H-bonding, covalent, Van der Waals, and electrostatic interactions. The diverse range of interactions between magnetic nanoparticles and biopolymers makes resulting hybrid fibers of particular interest as magnetic-responsive materials. Magnetically responsive hybrid biopolymer fibers have many features, including enhanced thermal stabilities, strong mechanical toughness, and perhaps most interestingly multifunctionality, allowing for a wide range of applications. These applications range from biosensing, filtration, UV shielding, antimicrobial, and medical applications, to name a few. Here, we review established hybrid fibers consisting of biopolymers and nanoparticles with a primary focus on biopolymers doped with magnetic nanoparticles and their various putative applications.

The paper gives an overview of tunable elastic magnetic composites based on silicon rubber matrix highly filled with a magnetic soft and hard filler. The magnetic soft phase, which is represented by iron microparticles, allows active control of the physical properties of the composites, while the magnetically hard phase (e.g. neodymium–iron–boron alloy microparticles) is mainly responsible for passive adjustment of the composite. The control is performed by the application of an external magnetic field in situ , and passive adjustment is performed by means of pre-magnetization in order to change material remanent magnetization, i.e. the initial state. The potential and limits of active control and passive tuning of these composites in terms of their magneto-mechanical behavior are presented and discussed.

Hybrid magnetic elastomers (HMEs) belong to a novel type of magnetocontrollable elastic materials capable of demonstrating extensive variations of their parameters under the influence of magnetic fields. Like all cognate materials, HMEs are based on deformable polymer filled with a mixed or modified powder. The complex of properties possessed by the composite is a reflection of interactions occurring between the polymer matrix and the particles also participating in interactions among themselves. For example, introduction of magnetically hard components into the formula results in the origination of a number of significantly different behavioral features entirely unknown to magnetorheological composites of the classic type. Optical observation of samples based on magnetically hard filler gave the opportunity to establish that initial magnetization imparts magnetic moments to initially unmagnetized grains, as a result of which chain-like structures continue to be a feature of the material even after external field removal. In addition, applying a reverse field causes them to turn into the polymer as they rearrange into new ring-like structures. Exploration of the relationship between the rheological properties and magnetic field conducted on a rheometer using vibrational mechanical analysis showed an increase of the relative elastic modulus by more than two orders of magnitude or by 3.8 MPa, whereas the loss factor exhibited steady growth with the field up to a value of 0.7 being significantly higher than that demonstrated by elastomers with no magnetically hard particles. At the same time, measuring the electroconductivity of elastomers filled with a nickel-electroplated carbonyl iron powder made it possible to observe that such composites demonstrated an increase of variation of the resistivity of the composite influenced by magnetic field in comparison to elastomers containing untreated iron particles. The studies conducted indicate that this material exhibits both magnetorheological and magnetoresistive effect and does indeed have the potential for use in various types of devices.

The objectives of this work include the analysis of electrical and magnetic properties of magneto-elastic hybrid materials with the intention of developing new techniques for sensor and actuator applications. This includes the investigation of dielectric properties at both low and high frequencies. The behaviour of capacitors whose dielectrics comprise magnetic hybrid materials is well known. Such interfacial magnetocapacitance can be varied according to magnetic content, magnetic flux density and the relative permittivity of the polymer matrix together with other dielectric content. The basic function of trapping electrical charges in polymers (electrets) is also established technology. However, the combination of magnetoactive polymers and electrets has led to the first electromagnetic device capable of adhering to almost any material, whether magnetically susceptible or not. During the course of this research, in addition to dielectrics, electrically conductive polymers based on (PDMS) matrices were developed in order to vary the electrical properties of the material in a targeted manner. In order to ensure repeatable results, this demanded new fabrication techniques hitherto unavailable. The 3D printing of silicones is far from being a mature technology and much pioneering work was necessary before extending the usual 3 d.o.f. to include orientation about and diffusion of particles in these three axes, thus leading to the concept of 6D printing. In 6D printing, the application of a magnetic field can be used during the curing process to control the particulate distribution and thus the spatial filler particle density as desired. Most of the devices (sensors and actuators) produced by such methods contain levels of carbonyl iron powder (CIP) embedded magnetic filler of up to 70 wt%. Contrary to this, a hitherto neglected research area, namely magnetoactive polymers (MAPs) having significantly lower magnetic particle concentrations (1 to 3 wt% CIP) were also investigated. With filler concentrations lower than 3 wt%, structures are formed which are completely absent at higher filler levels. CIP concentrations in the range of 1wt% demonstrate the formation of toroidal structures. Further development of coherent rings with a compact order results as filler concentrations increase towards 2 wt%. Above 3 wt% the structure eventually disintegrates to the usual random order found in traditional MAP with higher CIP content. Structured samples containing 1%–3 wt% CIP were investigated with the aid of X-ray tomography where solitary ring structures can be observed and eventually the formation of capillary doubles. Over wavelengths ranging from 1 to 25 µm, spectroscopic analysis of thin film MAP samples containing 2 wt% CIP revealed measurable magnetic-field-dependent changes in IR absorption at a wavenumber 2350 ( λ  = 4.255 µm). This was found to be due to the diamagnetic susceptibility of atmospheric carbon dioxide (CO 2 ). Consequently, the first potential application for sparse matrix MAPs was found.

In this contribution, a magnetoactive elastomer (MAE) of mixed content, i.e., a polymer matrix filled with a mixture of magnetically soft and magnetically hard spherical particles, is considered. The object we focus on is an elementary unit of this composite, for which we take a set consisting of a permanent spherical micromagnet surrounded by an elastomer layer filled with magnetically soft microparticles. We present a comparative treatment of this unit from two essentially different viewpoints. The first one is a coarse-grained molecular dynamics simulation model, which presents the composite as a bead-spring assembly and is able to deliver information of all the microstructural changes of the assembly. The second approach is entirely based on the continuum magnetomechanical description of the system, whose direct yield is the macroscopic field-induced response of the MAE to external field, as this model ignores all the microstructural details of the magnetization process. We find that, differing in certain details, both frameworks are coherent in predicting that a unit comprising magnetically soft and hard particles may display a nontrivial reentrant (prolate/oblate/prolate) axial deformation under variation of the applied field strength. The flexibility of the proposed combination of the two complementary frameworks enables us to look deeper into the manifestation of the magnetic response: with respect to the magnetically soft particles, we compare the linear regime of magnetization to that with saturation, which we describe by the Fröhlich–Kennelly approximation; with respect to the polymer matrix, we analyze the dependence of the reentrant deformation on its rigidity.