Abstract
The results of theoretical and experimental investigations of the polymer composites that belong to a class of magnetoactive elastomers with mixed magnetic content (MAEs-MC) are presented. The fundamental distinction of such composites from ordinary magnetoactive elastomers is that the magnetic filler of MAEs-MC comprises both magnetically soft (MS) particles of size 3–5 µm and magnetically hard (MH) particles whose size is an order of magnitude greater. Since MH particles of the magnetic filler are mixed into a composition in a non-magnetised state, this can ensure preparation of samples with fairly homogeneous distribution of the filler. The ‘initiation’ process of a synthesised MAE-MC is done by its magnetisation in a strong magnetic field that imparts to the sample unique magnetic and mechanical properties. In this work, it is shown that the presence of MS particles around larger MH particles, firstly, causes an augmentation of magnetic moments, which the MH particles acquire during initiation, and secondly, enhances the magnetic susceptibility and remanent magnetisation of MAEs-MC. These magnetic parameters are evaluated on the basis of the macroscopic magnetostatics from the experimental data of spatial scanning of the field over the space around MAEs-MC made in the shape of a spheroid. A set of samples with a fixed MH and varying MS volume contents that are initiated in two different fields, is used. The developed mesoscopic model of magnetic interactions between the MH and MS phases is able to explain the experimentally observed dependencies of the magnetic parameters on the concentration of the MS phase. The problem is solved numerically under the assumption that the elastic matrix of MAEs-MC is rigid, i.e. the mutual displacements of the particles are negligible. The model helps to elucidate the interaction of the magnetic phases and to establish that the MS phase plays thereby a dual role. On the one hand, the MS phase screens out the field acting inside MH particles, and on the other hand, it forms mesoscopic magnetic bridges between adjoining MH particles, which in turn enhance their field. The combined interplay of these contributions defines the resulting material properties of MAEs-MC on the macroscopic scale.
Highlights
Magnetoactive elastomeric composites of mixed magnetic content are rather different from both their ‘predecessors’: magnetoactive elastomers (MAEs) filled exclusively with micropowders of either a magnetically soft (MS) or a magnetically hard (MH), i.e. highly coercive, ferromagnetic material
From the afore-mentioned considerations an important conclusion can be drawn that applies to any MAE-MC no matter what is the actual proportion between the MH and MS phases: When an MAE-MC is subjected to an external magnetic field that does not bring the embedded MH particles to complete saturation, one should take into account that the internal field acting on these particles, exceeds the external one by an amount proportional to the concentration of the MS phase
The applied uniform magnetic field generates inside MH particles of an MAE-MC sample a field HiMniH, which imparts to them a remanent magnetisation
Summary
Magnetoactive elastomeric composites of mixed magnetic content are rather different from both their ‘predecessors’: magnetoactive elastomers (MAEs) filled exclusively with micropowders of either a magnetically soft (MS) or a magnetically hard (MH), i.e. highly coercive, ferromagnetic material. The idea of the magnetoactive elastomers of mixed magnetic content, see figure 1 – in what follows we denote them as MAEs-MC—is rather new [22,23,24,25,26] Their novel qualities stem from the fact that in such composites the magnetic interaction occurs from the coupling of each fraction of the particles with external field and within the same fraction. The active tuning under the action of externally applied field is available as for any usual MAE filled just with an MS powder This double control over mechanics, including rheology, is a unique quality acquired by such composites due to their mixed magnetic content [23, 26, 27].
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