G.V. Stepanov

Motion of ferroparticles inside the polymeric matrix in magnetoactive elastomers

Ferroelastic composites are smart materials with unique properties including large magnetodeformational effects, strong field enhancement of the elastic modulus and magnetic shape memory. On the basis of mechanical tests, direct microscopy observations and magnetic measurements we conclude that all these effects are caused by reversible motion of the magnetic particles inside the polymeric matrix in response to an applied field. The basic points of a model accounting for particle structuring in a magnetoactive elastomer under an external field are presented.

Effect of a homogeneous magnetic field on the mechanical behavior of soft magnetic elastomers under compression

The mechanical properties of new magnetic composite materials were studied. The above materials represent rubbery silicon matrices filled with magnetic microparticles of metallic iron or magnetite. In homogeneous magnetic fields with an intensity of up to 0.4 T, the shear modulus of the composites was abnormally high (up to 10 000%). The variation of elastic properties of new materials on the type and volume content of the magnetic filler was investigated. In the presence of a sufficiently strong magnetic field, the above composites were shown to behave as elastoplastic materials with strengthening.

Magnetic and viscoelastic response of elastomers with hard magnetic filler

Magnetic elastomers (MEs) based on a silicone matrix and magnetically hard NdFeB particles have been synthesized and their magnetic and viscoelastic properties have been studied depending on the size and concentration of magnetic particles and the magnetizing field. It has been shown that magnetic particles can rotate in soft polymer matrix under applied magnetic field, this fact leading to some features in both magnetic and viscoelastic properties. In the maximum magnetic field used magnetization of MEs with smaller particles is larger while the coercivity is smaller due to higher mobility of the particles within the polymer matrix. Viscoelastic behavior is characterized by long relaxation times due to restructuring of the magnetic filler under the influence of an applied mechanical force and magnetic interactions. The storage and loss moduli of magnetically hard elastomers grow significantly with magnetizing field. The magnetic response of the magnetized samples depends on the mutual orientation of the external magnetic field and the internal sample magnetization. Due to the particle rotation within the polymer matrix, the loss factor increases abruptly when the magnetic field is turned on in the opposite direction to the sample magnetization, further decreasing with time. Moduli versus field dependences have minimum at non-zero field and are characterized by a high asymmetry with respect to the field direction.

Strong magnetodielectric effects in magnetorheological elastomers

The effect of a uniform magnetic field on the permittivity of magnetorheological elastomers (MREs) is studied. MREs were synthesized on the basis of silicone rubber and magnetic fillers of various chemical nature (Fe, NdFeB and Fe3O4) and particle sizes. The value of permittivity was obtained from the measurements of the capacity of a plane capacitor with MRE samples. A strong increase of the permittivity (magnetodielectric effect) was observed when the applied field was perpendicular to the capacitor plates. The value of the magnetodielectric effect was found to be strongly dependent on the type of magnetic filler as well as on the size and concentration of magnetic particles within MRE composites. The highest magnetic response reaching 150% was observed for the MRE based on a magnetically hard NdFeB filler. A simple model explaining physical reasons for the magnetodielectric effect in a MRE is proposed. The developed MRE with a strong magnetodielectric effect is very promising for a wide range of applications, in particular, as magnetic field sensors and actuators.

Tuning the tensile modulus of magnetorheological elastomers with magnetically hard powder

It has been experimentally determined the tensile modulus of magnetorheological elastomers based on magnetically hard particles. Samples of the elastomer consisting of a soft elastic matrix and micron-sized particles of FeNdB powder have been magnetized in uniform magnetic fields of varying strength in order to provide different remanence magnetizations. The tensile modulus of these samples was measured small and large strain regimes (up to 6.6%) through mechanical elongation with a table top machine. The relative change in the tensile modulus after the sample was magnetized can reach 360%, depending on the remanence magnetization and the strain.

Shape instability of a magnetic elastomer membrane

Shape instability occurring in a thin plate (membrane) made of a soft magnetoelastic material under a uniform magnetic field is predicted and analysed. The instability onset is shown to be similar to the second-order transition; the dependence of the threshold field on the magnetic and geometric parameters of the membrane is derived analytically; the membrane shapes (domes) are evaluated with the aid of numerical simulation. The theory proposed is in general agreement with experiments performed on a siloxane rubber/iron carbonyl composite.

Magnetoactive elastomer based on magnetically hard filler: Synthesis and study of viscoelastic and damping properties

This work is focused on the magnetic response of magnetoactive elastomer (MAE) based on silicone polymer matrices filled with magnetic particles of magnetically hard NdFeB-alloy. Viscoelastic properties of MAE were studied by the method of oscillation shear. After magnetization in an external magnetic field of 2 T MAE samples demonstrate more than two-time increase in the storage and loss moduli and 25% increase in the loss factor. Performed study of the damping properties of the materials has shown that the oscillation time of the pendulum hammering the magnetized sample is in two times shorter than in case of the non-magnetized sample. Viscoelastic and damping properties of MAE are defined by magnetic interactions between the magnetized particles of the magnetic filler.

Magnetorheological effect of magneto-active elastomers containing large particles

The magnetorheological effect of elastomer composites containing a mixture of large (50-80 μm) and small (3-5 μm) particles has been experimentally examined. The data shows that elasticity in the range of small deformations (1%) for a magnetic field strength of 290 mT increased by two order of magnitude. This effect can be explained with the presence of the large particles in the structure of the composite assisting the aggregation effect. Due to the strong increase of the interparticle interaction compared with the restoring elastic forces, the presence of the large particles leads to the observed steep increase of Youngs modulus.

Low Frequency Rheology of Magnetically Controlled Elastomers with Isotropic Structure

The method of torsion oscillations is used to measure the dynamic modulus of elasticity of magnetically controlled elastomers that comprise silicone rubber and carbonyl iron in the low-frequency (up to 100 Hz) range. The samples are synthesized in the absence of a magnetic field; therefore, they have an isotropic structure. In the measurements, a constant magnetic field (up to 24 kA/m) is superimposed along the axis of forced torsion oscillations of the sample. A simple model of the rheological behavior of magnetically controlled elastomers is proposed; the problem of torsion oscillations of a cylindrical sample is solved. From the comparison with the experiment for the materials under study, we determine the coefficients of the theoretical model and the corrections to them, which are made because of variations in the rheology of magnetically controlled elastomers under the influence of a magnetic field. The derived relations make it possible to exclude artifacts and to adequately describe dependences of the storage and loss moduli on the frequency of mechanical loading and the strength of the applied magnetic field.

Deformation of a Circular Ferroelastic Membrane in a Uniform Magnetic Field

The loss of the stability of a thin flat soft ferroelastic plate (membrane) fixed along its periphery in a magnetic field normal to its surface is considered using a continuum model of nonlinear magnetoelasticity. The relation between the instability threshold field and the magnetic and geometric parameters of the membrane is found, and the supercritical shape of the membrane (dome) is calculated. The calculation and experimental results agree well without fitting parameters.