Petr Filip

Rheological properties of magnetorheological suspensions based on core–shell structured polyaniline-coated carbonyl iron particles

The sedimentation caused by the high density of suspended particles used in magnetorheological fluids is a significant obstacle for their wider application. In the present paper, core–shell structured carbonyl iron–polyaniline particles in silicone oil were used as a magnetorheological suspension with enhanced dispersion stability. Bare carbonyl iron particles were suspended in silicone oil to create model magnetorheological suspensions of different loading. For a magnetorheological suspension of polyaniline-coated particles the results show a decrease in the base viscosity. Moreover, the polyaniline coating has a negligible influence on the MR properties under an external magnetic field B. The change in the viscoelastic properties of magnetorheological suspensions in the small-strain oscillatory shear flow as a function of the strain amplitude, the frequency and the magnetic flux density was also investigated.

Improved thermooxidation and sedimentation stability of covalently-coated carbonyl iron particles with cholesteryl groups and their influence on magnetorheology

Sedimentation of particles in magnetorheological suspensions represents a crucial problem that concerns their efficient long-term application in practice. Prepared carbonyl iron (CI) microparticles coated with a low density substance, cholesteryl chloroformate, via a two-step reaction and immersed in silicone oil, exhibit three positive aspects: (1) the CI particle modification increased the compatibility between the particles and the silicone oil resulting in improved long-term stability (reduction in sedimentation); (2) the coating provided the particles with enhanced thermal stability in the oxygen atmosphere; and (3) rheological measurements proved a promising magnetorheological performance at different particle weight fractions.

A dimorphic magnetorheological fluid with improved oxidation and chemical stability under oscillatory shear

The aim of this work was to prepare a dimorphic magnetorheological (MR) fluid for which sedimentation stability, oxidation and chemical stability are enhanced in comparison with common MR fluids, while at the same time preserving the MR effect at its fullest practical level. A dimorphic MR fluid exhibiting these properties was prepared in two steps. The first step involved the partial substitution of carbonyl iron (CI) spherical microparticles with Fe rod-like particles synthesized via a surfactant-controlled solvothermal method. This improved sedimentation stability in comparison with the application of CI particles alone. In the second step both spherical CI and Fe rod-like particles were coated with a polysiloxane layer through the hydrolysis–condensation polymerization of tetraethylorthosilicate. This ensured better oxidation and chemical stability balance with an acceptable decrease in the MR effect. This effect is still markedly better than that based on Fe3O4 particles.

Cholesteryl-coated carbonyl iron particles with improved anti-corrosion stability and their viscoelastic behaviour under magnetic field

In principle, bare particles used in magnetorheological suspensions exhibit apparent corrosion instability. To suppress substantially this adverse phenomenon, the carbonyl iron particles modified with cholesteryl group (CI-chol) were suspended in silicone oil. There was found a deterioration of magnetorheological efficiency in comparison when only bare carbonyl iron (CI) particles are used; nevertheless, from the viewpoint of applicability, this change is fully acceptable. However, an anti-corrosion stability was significantly improved. Furthermore, dynamic oscillatory measurements and other characterizations were carried out and analyzed when both CI and CI-chol particles are applied.

THE ROLE OF PARTICLES ANNEALING TEMPERATURE ON MAGNETORHEOLOGICAL EFFECT

The spinel nanocrystalline cobalt ferrite (CoFe2O4) particles were prepared via a sol–gel method followed by the annealing process. Their structural, magnetic and magnetorheological (MR) properties depending upon the annealing temperature were investigated. The X-ray diffraction analysis revealed that the higher annealing temperature, the larger grain size of CoFe2O4 particles resulting in larger magnetic domains in particles. The saturation magnetization, determined via a vibrating sample magnetometry, increased with annealing temperature and, in contrast, the coercivity decreased. The rheological behavior of CoFe2O4 particles based MR suspensions determined under the small-strain oscillatory shear flow in magnetic field showed that higher annealing temperature reflects in larger changes of rheological properties.

Magnetorheology of carbonyl iron particles coated with polypyrrole ribbons: The steady shear study

The aim of this study is a preparation and application of polymer coating on the surface of carbonyl iron particles. Oxidative polymerization of the pyrrole provides ribbon-like morphology. Compact coating of particles has a slightly negative impact on their magnetic properties (measured for magnetic field strength in the range from 0 to 300 mT); however, there is a significant increase in sedimentation stability. The effect of the particle concentration and temperature on the toughness of the internal structures was also investigated.

Magnetorheological behaviour and electrospinning of poly(ethylene oxide) suspensions with magnetic nanoparticles

The properties of poly(ethylene oxide) aqueous suspensions with magnetic nanoparticles synthesized under microwave-assisted radiation are studied. The magnetic nanoparticles are formed by iron (III) chloride hexahydrate (FeCl3·6H2O) dissolved in ethylene glycol (C2H4(OH)2) and subsequently in aqueous ammonia solution (approx. 25 wt% aq.). The polymer suspension exhibits substantial advantages over a suspension when ‘classical’ carrier fluids (water and silicone oil) are used. First, the presence of poly(ethylene oxide) significantly contributes to a fabrication of nanofibrous webs, the morphology of which is documented by scanning electron microscopy technique. Second, better sedimentation stability of the processed suspension during electrospinning reflects in a uniform distribution of magnetic nanoparticles along the nanofibres, thus ensuring even magnetic performance of the resulting membranes.

The role of the Gordon–Schowalter derivative term in the constitutive models—improved flexibility of the modified XPP model

Constitutive models that complete the set of equations describing the flow of polymer melts should respect objective thermodynamics and stability conditions ensuring their validity in the whole range of possible deformation flow. However, in practice, a very good description of flow situations can be achieved with the models not complying with the physical assumptions in all respects. Analogously to the term characterizing yield stress in empirical viscoplastic models, the term represented by the Gordon–Schowalter (GS) derivative in the differential constitutive models contributes to better fitting the experimental data, especially shear thinning. Efficiency of the recently presented modified eXtended Pom-Pom model (just one non-linear parameter per mode) implementing the GS derivative term (one additional non-affine motion parameter per mode) is improved (documented on LDPE, HDPE, and polyvinyl butyral (PVB) materials), and a comparison with the exponential Phan-Tien–Tanner (PTT) and PTT-XPP models (a priori containing the GS derivative term) are presented.

Core‐shell Structured Polypyrrole‐coated Magnetic Carbonyl Iron Microparticles and their Magnetorheology

The substantial sedimentation problem of magnetorheological (MR) suspensions, due to the large density mismatch between dispersed particles and carrier medium, is a significant obstacle for their broader applications. The present paper reports the MR properties and enhanced sedimentation stability of novel core‐shell structured particles with carbonyl iron as a core and polypyrrole as a shell layer dispersed in silicone oil. The coating morphology was analyzed via scanning electron microscopy and magnetic properties via vibrating sample magnetometry. The steady shear flow and small‐amplitude dynamic oscillatory shear measurements were carried out to show improved MR performance. The sedimentation test showed positive role of polymeric coating, as well.

Magnetorheological characterization and electrospinnability of ultrasound-treated polymer solutions containing magnetic nanoparticles

In order to fabricate a magnetic nanofibrous membrane by electrospinning, it is necessary to follow a suitable method for incorporating nanoparticles into a polymer solution. Ultrasound treatment represents a very effective technique for distributing magnetic nanoparticles within polymer solutions. Adverse effects caused by sonication over time on the given nanofibrous membrane (polymer degradation and appearance of defects) were evaluated by using rotational (magneto)rheometry, transmission and scanning electron microscopy, and magnetometry. A magnetorheological approach was selected to estimate the optimal duration of sonication, and findings were experimentally verified. It was concluded that the processed nanofibrous membrane showed promise as an advanced magnetoactive device.