Vladimir Pavlinek

The effect of dielectric properties on the electrorheology of suspensions of silica particles coated with polyaniline

The flow behaviour of silicone-oil suspensions of five types of silica particles coated with a polyaniline base in a DC electric field has been linked to their dielectric properties. The relaxation frequencies corresponding to the position of the dielectric-loss maxima in the frequency spectra identify the interfacial polarization of suspension particles as a controlling factor for a strong electrorheological effect. The yield stresses of suspensions under the influence of electric field and critical shear rates, at which the chains of polarized particles were broken by shear forces, is correlated with the difference between the limit values of dielectric constants above and below the relaxation frequency. The analysis of particle dipole coefficient β showed that particle polarizability is the main factor affecting rigidity of the electrorheological structure. In contrast with this, particle shape and size, controlling the field-off suspension viscosity, become unimportant after the electric field has been applied. The plots of the relative viscosity of studied suspensions vs. Mason number characterizing the relation between shear and polarization forces have been discussed. While the results obtained at different shear rates and field strengths were reduced to a single dependence, for various particle suspensions these dependences differed.

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.

Electrorheological properties of suspensions of hollow globular titanium oxide/polypyrrole particles

Hollow globular clusters of titanium oxide (TiO2) nanoparticles were synthesized by a simple hydrothermal method. The prepared particles were consequently coated by in situ polymerization of conductive polymer polypyrrole (PPy) to obtain novel core–shell structured particles as a dispersed phase in electrorheological (ER) suspensions. The X-ray diffraction analysis and scanning electron microscopy provided information on particle composition and morphology. It appeared that PPy coating improved the compatibility of dispersed particles with silicone oil which results in higher sedimentation stability compared to that of mere TiO2 particles-based ER suspension. The ER properties were investigated under both steady and oscillatory shears. It was found that TiO2/PPy particles-based suspension showed higher ER activity than that of mere TiO2 hollow globular clusters. These observations were elucidated well in view of their dielectric spectra analysis; a larger dielectric loss enhancement and faster interfacial polarization were responsible for a higher ER activity of core–shell structured TiO2/PPy-based suspensions. Investigation of changes in ER properties of prepared suspensions as a function of particles concentration, viscosity of silicone oil used as a suspension medium, and electric field strength applied was also performed.

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.

Influence of particle concentration on the electrorheological efficiency of polyaniline suspensions

The dependence of the efficiency of positive electrorheological (ER) phenomenon, expressed as a ratio of the low-shear electroviscosity to the field-off viscosity, on the particle concentration of polyaniline base suspensions in silicone oil has been studied. A simple analysis reveals that the dependence of the efficiency on the volume fraction of suspended particles may have a maximum at which the optimum ER performance of material is expected. Experimental results confirmed this expectation. It is demonstrated that with higher temperature the maximum is shifted to higher particle concentration.

MnO2 nanoflake/polyaniline nanorod hybrid nanostructures on graphene paper for high-performance flexible supercapacitor electrodes

A facile two-step strategy is adopted to construct a free-standing composite paper of MnO2 nanoflake/polyaniline (PANI) nanorod hybrid nanostructures on reduced graphene oxide (RGO) for flexible supercapacitor electrode application. MnO2 nanoflakes are first grown on RGO paper via an electrodeposition method, followed by assembly of PANI nanorods between MnO2 nanoflakes by in situ polymerization using camphorsulfonic acid as a dopant. The morphology and structure of the composite paper are characterized and the electrochemical properties are systematically investigated. The interconnected PANI nanorods deposited on the interlaced MnO2 nanoflakes have a length of ∼100 nm and a diameter of ∼30 nm, creating plenty of open porous structures which are beneficial for ion penetration into the electrode. The RGO/MnO2/PANI composite paper shows a large specific capacitance of 636.5 F g−1 at 1.0 A g−1 in 1.0 M Na2SO4 electrolyte and excellent cycling stability (85% capacitance retention after 104 cycles). The optimized composite structure with more electroactive sites, fast ion and electron transfer, and strong structural integrity endows the ternary composite paper electrode with outstanding electrochemical performance.

Morphology-controllable synthesis of MnO2 hollow nanospheres and their supercapacitive performance

Uniform MnO2 hollow nanospheres with hierarchical (urchin-like and flower-like) and non-hierarchical structures have been synthesized via a dual-template assisted hydrothermal process. The morphology control of the MnO2 hollow spheres can be easily achieved by altering the mass ratio of Pluronic F-127 to silica spheres. Material characterizations reveal that urchin-like hollow spheres possess the highest BET surface area of 233.4 m2 g−1 among the diverse morphologies. A possible formation mechanism for the MnO2 hollow spheres with different morphologies is proposed. The supercapacitive performance of the MnO2 spheres was investigated by cyclic voltammetry and galvanostatic charge–discharge techniques. The urchin-like hollow spheres exhibit the highest specific capacitance of 266.6 F g−1 within the potential range of 0–1.0 V. The relationship between the specific capacitance and the morphology of the MnO2 hollow spheres is also discussed. The good capacitive behavior and cycling stability of the hierarchical MnO2 hollow spheres highlights the importance of the morphological design and control of materials in practical supercapacitor applications.

The effect of dispersed particle size and shape on the electrorheological behaviour of suspensions

Abstract Electrorheological (ER) and dielectric properties of suspensions of four inorganic powders (talc, magnesium hydroxide, aluminium hydroxide, and titanium dioxide) of different particle size and shape and polyaniline particles in silicone oil has been compared. It has been shown that the efficiency of ER effect expressed as a ratio of electroviscosity Δη (an increase in suspension viscosity on the application of electric field) to the field-off viscosity, η0, in addition to dielectric phenomena of dispersed and continuous phase may be strongly influenced by the hydrodynamic properties of individual particles. The non-spherical or irregular particle shape can considerably increase rigidity of the structure formed in the electric field and enhance the ER performance. The main factor controlling the ER effect, however, is particle polarisability. Thus, the electroviscosity of suspensions of highly polarisable polyaniline particles proved to be much higher than that of suspension of the most ER-efficient inorganic powder.

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.

The Electrorheological Behavior of Suspensions Based on Molten-Salt Synthesized Lithium Titanate Nanoparticles and Their Core–Shell Titanate/Urea Analogues

This paper concerns the preparation of novel electrorheological (ER) materials using microwave-assisted synthesis as well as utilizing a suitable shell-providing system with enhanced ER performance. Lithium titanate nanoparticles were successfully synthesized, and their composition was confirmed via X-ray diffraction. Rheological properties were investigated in the absence as well as in the presence of an external electric field. Dielectric properties clarified the response of the particles to the application of an electric field. The urea-coated lithium titanate nanoparticle-based suspension exhibits higher ER performance in comparison to suspensions based on bare particles.