Polydimethylsiloxane based magnetoactive elastomers demonstrate above the melting transition range (e.g. at room temperature) an induced uniaxial magnetic anisotropy, which grows with increasing ...magnetic field. By freezing a material down to 150 K, displaced iron microparticles are immobilized, so that the magnetic anisotropy can be measured. Magnetic anisotropy “constant” is a consequence of particle displacements and a characteristic of the energy of internal deformations in the polymer matrix. The maximum anisotropy constant of the filling is at least one order of magnitude larger than the shear modulus of the pure elastomer (matrix). In a magnetic field, the gain in the rigidity of the composite material is attributed to the magnetomechanical coupling, which is in turn a source of anisotropy. The concept of effective magnetic field felt by the magnetization allows one to explain the magnetization curve at room temperature from low-temperature measurements. The results can be useful for developing vibration absorbers and isolators.
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•Magnetic anisotropy induced by a magnetic field is experimentally observed.•To investigate magnetic anisotropy, a sample was frozen in a magnetic field.•Magnetic anisotropy “constant” depends on the magnitude of the magnetic field.•Magnetic anisotropy is a characteristic of internal deformations of the polymer matrix.•Effect of the effective magnetic anisotropy field on the magnetization is found.
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•The effect of blocking of particles displacement in MAE was observed from ZFC-FC.•M-H loops change the shape at solidification temperature of MAE.
The magnetization of a ...magnetoactive elastomer (MAE) with microparticles of soft magnetic carbonyl iron embedded in a highly elastic matrix has been studied. It is shown that at high temperatures its magnetization curve has the form of a specific hysteresis loop. This hysteresis is attributed to the influence of displacement of magnetized particles in the elastically soft elastomer matrix under the effect of magnetic forces, leading to the change of magnetic interaction between the particles. In this case, there is a maximum in the field dependence of the magnetic susceptibility, the occurrence of which has been associated with the competition between re-arrangement of particles, when they are displaced in a magnetic field, and saturation of particles’ magnetization. When the MAE is cooled below approximately 225 K, both the magnetic hysteresis and the maximum in the field dependence of the magnetic susceptibility disappear. When the MAE material is cooled below the solidification temperature of the elastomer matrix, the displacements of the magnetic particles during magnetization are blocked by the rigid matrix. The magnetization reversal of the MAE is reversible. This means that the shape of subsequent magnetization loops remains constant and the sample returns into the initial non-magnetized state after the magnetic field is turned off.
Magnetic field and temperature dependencies of the critical current density, J/sub c/(H/spl par/c, T) were measured by SQUID-magnetometry, ac magnetic susceptibility, and dc transport current ...techniques in the single-crystalline epitaxially-grown by off-axis dc magnetron sputtering YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// (YBCO) films with J/sub c/(H/spl par/c, 77 K) /spl ges/ 2 /spl middot/ 10/sup 6/ A/cm/sup 2/. The mechanism of vortex depinning from growth-induced linear defects, i.e., out-of-plane edge dislocations in low-angle tilt domain boundaries, is shown to describe quantitatively measured J/sub c/(H/spl par/c, T). The developed model takes into account a statistical distribution of the dislocation domain boundaries ordered in a network as well as the interdislocation spacing within boundaries. Actual structural features of YBCO film known from HREM data turn out to be extracted from J/sub c/(H/spl par/c, T)-curves by a fitting procedure within the proposed model.
Out-of-plane edge dislocations (mean 2D density is 1011 lines/cm2) in low angle tilt domain boundaries of single-crystalline YBCO films are shown to play a crucial role in Jc(H∥c,T) behavior and Jc ...angular dependence. Dislocation cores and their vicinity, where Tc is locally suppressed, are shown to provide an extremely strong pinning force resulting in Jc(77K)≈2×106 A/cm6. Peculiarities of Jc(H∥c) are found in the low-field range. A simple model of pinning by dislocation domain boundaries is developed. The model consistently describes the Jc(H∥c,T)/Jc(0,T)-dependencies, accounting for the fraction of vortices pinned by dislocations. Angular dependencies of Jc in rotating magnetic field were measured by four-probe transport technique. A satisfactory agreement with theoretically predicted behavior has been found.
It is shown that in external magnetic fields, a uniaxial magnetic anisotropy comes into being in a magnetoactive elastomer (MAE). The magnitude of the induced uniaxial anisotropy grows with the ...increasing external magnetic field. The filler particles are immobilized in the matrix if the MAE sample is cooled below 220 K, where the anisotropy can be read out. The cooling of the sample is considered as an alternative methodological approach to the experimental investigation of the magnetized state of MAEs. The appearance of magnetic anisotropy in MAE is associated with restructuring of the filler during magnetization, which leads to an additional effective field felt by the magnetization. It is found that the magnitude of the effective magnetic anisotropy constant of the MAE is approximately two times larger than its effective shear modulus in the absence of magnetic field. It is proposed that the experimentally observed large (about 40) ratio of the magnetic anisotropy constant of the filler to the shear modulus of the matrix deserves attention for the explanation of magnetic and magnetoelastic properties of MAEs. It may lead to additional rigidity of the elastic subsystem increasing the shear modulus of the composite material through the magnetomechanical coupling.
Magnetic properties of a magnetoactive elastomer (MAE) filled with {\mu}m-sized soft-magnetic iron particles have been experimentally studied in the temperature range between 150 K and 310 K. By ...changing the temperature, the elastic modulus of the elastomer matrix was modified and it was possible to obtain magnetization curves for an invariable arrangement of particles in the sample as well as in the case when the particles were able to change their position within the MAE under the influence of magnetic forces. At low (less than 220 K) temperatures, when the matrix becomes rigid, the magnetization of the MAE does not show a hysteresis behavior and it is characterized by a negative value of the Rayleigh constant. At room temperature, when the polymer matrix is compliant, a magnetic hysteresis exists and exhibits local maxima of the field dependence of the differential magnetic susceptibility. The appearance of these maxima is explained by the elastic resistance of the matrix to the displacement of particles under the action of magnetic forces.
Robust Wireless Network Jamming Problems Commander, Clayton W.; Pardalos, Panos M.; Ryabchenko, Valeriy ...
Lecture notes in control and information sciences,
2009, Letnik:
381
Book Chapter, Conference Proceeding
We extend the results presented in a previous paper which focused on deterministic formulations of the wireless network jamming problem. This problem seeks to determine the optimal number and ...placement locations for a set of wireless jamming devices to sufficiently suppress a communication network according to some specified criterion. We now introduce robust variants of those formulations which account for the fact that the exact topology of the network to be jammed may not be known entirely. Particularly, we consider instances in which several topologies are considered likely, and develop robust scenarios for placing jamming devices which are able to suppress the network regardless of which candidate topology is realized. We derive several formulations and include percentile constraints to account for a variety of scenarios. Case studies are presented and the results are analyzed. We conclude with directions of future research.
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