Michal Sedlacik

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.

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.

A facile controllable coating of carbonyl iron particles with poly(glycidyl methacrylate): a tool for adjusting MR response and stability properties

This study is focused on the controllable coating of the carbonyl iron (CI) particles widely applied in magnetorheology. These particles were grafted with poly(glycidyl methacrylate) (PGMA) with narrow polydispersity via surface-initiated atom transfer radical polymerization. Two types of core–shell particles differing in molecular weights of grafted polymer chains were synthesized. The effect of shell thickness on the thermo-oxidation stability of particles as well as the sedimentation stability of their silicone oil suspensions was evaluated. The successful coating process was confirmed by Fourier transform infrared spectroscopy and energy-dispersive spectrometry. The differences in the magnetic properties of bare and coated CI particles were clarified through vibrating sample magnetometry. Due to the controllable length of the PGMA grafts, the magnetic properties remain almost the same as those for bare CI. The magnetorheological (MR) behavior of silicone oil suspensions containing 60 wt% of bare CI particles as well as PGMA-coated analogues was investigated in the absence and in the presence of various magnetic field strengths, demonstrating the negligible impact of surface modification on final MR performance. Thus, the grafting of the particles with PGMA negligibly affected magnetic properties but considerably enhanced thermo-oxidation and sedimentation stabilities. Finally, a novel tensiometric method for sedimentation stability measurements of MR suspensions was successfully implemented.

The observation of a conductivity threshold on the electrorheological effect of p-phenylenediamine oxidized with p-benzoquinone

p-Phenylenediamine was oxidized with p-benzoquinone in the presence of 0.1–5 M methanesulfonic acid (MSA) solutions. The resulting methanesulfonate salts of 2,5-(di-p-phenylenediamine)-1,4-benzoquinone are semiconducting and the particles were further suspended in silicone oil in a weight ratio of 1 : 9 in order to create novel electrorheological fluids. Conductivity measurements using the two-point method along with dielectric spectroscopy were carried out in order to investigate their electrical and dielectric properties, including their silicone-oil suspensions. The higher the concentration of MSA was present during the synthesis, the higher the conductivity was observed. Nevertheless, a certain threshold of the ER effect has been found and a further significant increase in conductivity causes only a slight ER effect enhancement. At an electric field strength of 1.5 kV mm−1, the observed yield stresses read at the low shear rate values were 11.5 Pa, 20.3 Pa, 24.5 Pa, and 28.2 Pa for particles with conductivities 1.5 × 10−12, 8.5 × 10−11, 1.0 × 10−8, and 1.5 × 10−7 S cm−1, respectively. From dielectric spectra, it was observed that the conductivity of the particles determines the relaxation times of their silicone-oil suspensions.

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.

Plasma-enabled sensing of urea and related amides on polyaniline

The atmospheric pressure plasma jet (APPJ) was used to enhance the sensitivity of industrially important polyaniline (PANI) for detection of organic vapors from amides. The gas sensing mechanism of PANI is operating on the basis of reversible protonation or deprotonation, whereas the driving force to improve the sensitivity after plasma modifications is unknown. Herein we manage to solve this problem and investigate the sensing mechanism of atmospheric plasma treated PANI for vapor detection of amides using urea as a model. The results from various analytical techniques indicate that the plausible mechanism responsible for the improved sensitivity after plasma treatment is operating through a cyclic transition state formed between the functional groups introduced by plasma treatment and urea. This transition state improved the sensitivity of PANI towards 15 ppm of urea by a factor of 2.4 times compared to the non-treated PANI. This plasma treated PANI is promising for the improvement of the sensitivity and selectivity towards other toxic and carcinogenic amide analytes for gas sensing applications such as improving material processing and controlling food quality.

Temperature-dependent electrorheological effect and its description with respect to dielectric spectra

Electrorheological fluids consisting of large number and type of electrically polarizable particles have been presented in the literature. Nevertheless, there is a lack of temperature-dependent electrorheological effect analysis, which is their major feature from the application point of view. In this work, aniline oligomers were synthesized and carbonized in order to obtain suitable materials to be used in electrorheological fluids. The silicone oil suspensions were prepared and their temperature-dependent electrorheological performance was investigated in the temperature range between 25°C and 65°C. The electrorheological fluid based on particles with larger size exhibits higher sensitivity to the increase in temperature than the electrorheological fluid based on smaller particles. As an evaluative tool, dielectric spectra of the prepared electrorheological fluids were investigated. It has been shown that the activation energy of the relaxation process is higher for the electrorheological fluid based on larger size particles. The enhanced electrorheological effect at high temperature was ascribed to the shift of dielectric relaxation of the electrorheological fluids to higher frequencies.