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.
Anisotropic magnetorheological elastomers (MREs) with four different mass percentages of
iron powder were prepared in an external magnetic field. The inner structure of the samples
was characterized by using computed tomography. It has been shown that this kind of
non-destructive analysis of MRE samples can be efficiently used for a detailed structural
investigation. It was found that even small changes in the mass content of the magnetic
filler led to the formation of completely different morphologies, which were reproducible for
all samples. There were the familiar column formations in patterns with a mass content of ∼ 5% iron powder. Increasing
the mass fraction to ∼ 14%
resulted in the formation of tubular structures. Samples with ∼ 23 and ∼ 33 wt%
had a densely packed structure, where the particle formations broke up: meanders without
particles penetrate the samples over the entire height like canyons.
The phase behavior of polymer solutions and composites is a complex issue and is of both technological and fundamental interest. For a better understanding of the microstructure formation in magnetorheological (MR) elastomers, x-ray micro-computed tomography (XμCT) investigations were carried out. Magnetorheological elastomers with 5% mass content of iron powder were prepared under different magnetic field strengths between 1 and 220 kA m−1. Through quantitative analysis, valuable information was obtained regarding the number, size and frequency distribution of column structures in MR elastomers, as well as the magnetic field required to force structure formation.
One of the central appealing properties of magnetic gels and elastomers is that their elastic moduli can reversibly be adjusted from outside by applying magnetic fields. The impact of the internal magnetic particle distribution on this effect has been outlined and analyzed theoretically. In most cases, however, affine sample deformations are studied and often regular particle arrangements are considered. Here we challenge these two major simplifications by a systematic approach using a minimal dipole-spring model. Starting from different regular lattices, we take into account increasingly randomized structures, until we finally investigate an irregular texture taken from a real experimental sample. On the one hand, we find that the elastic tunability qualitatively depends on the structural properties, here in two spatial dimensions. On the other hand, we demonstrate that the assumption of affine deformations leads to increasingly erroneous results the more realistic the particle distribution becomes. Understanding the consequences of the assumptions made in the modeling process is important on our way to support an improved design of these fascinating materials.
Magnetorheological elastomers are a type of smart hybrid material where elastic properties of a soft elastomer matrix are combined with magnetic properties of magnetic micro particles. This combination leads to a complex interplay of magnetic and elastic phenomena, of which the magnetorheological effect is the best described. In this paper, magnetically hard NdFeB-particles were used to obtain remanent magnetic properties. X-ray microtomography has been utilised to analyse the particle movement induced by magnetic fields. A particle tracking was performed; thus, it was possible to characterise the movement of individual particles. Beyond that, a comprehensive analysis of the orientation of all particles was performed at different states of magnetisation and global particle arrangements. For the first time, this method was successfully applied to a magnetorheological material with a technically relevant amount of magnetic NdFeB-particles. A significant impact of the magnetic field on the rotation and translation of the particles was shown.
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