Rheology of uncured magnetorheological elastomers

The aim of this paper is to study rheological properties of uncured magnetorheological elastomers comprising iron particles dispersed in silicon elastomer in relation to particle rearrangement by external magnetic field into oriented structures as this process is strongly affected by viscosity. Studied systems vary in used filler volume concentration (0 to 30 vol. %). From measured flow curves flow consistency index is extracted first by fitting experimental data to Herschel-Bulkley model followed by applying two concentration dependency models (Maron-Pierce and Krieger-Dougherty) to normed consistency. Results and model predictions are discussed. © Published under licence by IOP Publishing Ltd.Czech Science FoundationGrant Agency of the Czech Republic [17-24730S]; Ministry of Education, Youth and Sports of the Czech Republic -Program NPU I [LO1504

Accurate measurement of the true plane-wave shielding effectiveness of thick polymer composite materials via rectangular waveguides

This paper presents a methodology for accurately gauging the true plane wave shielding effectiveness of composite polymer materials via rectangular waveguides. Since the wave propagation of the waveguides is not in the form of plane wave patterns, it is necessary to post-process the S-parameters for the measured data of the waveguide lines to obtain such patterns and ascertain the effectiveness of true plane wave shielding. The authors propose two different methods to achieve this. The first applies simple renormalization of S-parameters, where reference impedance is changed from the value for the waveguide to that for free space, which ensures good accuracy of shielding effectiveness with a small degree of discontinuity across the range of frequencies. The other relies on rigorous extraction of the composite materials' effective permittivity and permeability ascertained from rectangular waveguides; afterward, plane wave shielding effectiveness is calculated analytically and gives very high accuracy. Both procedures assume the given samples are isotropic in character. We validated the accuracy of the methodologies by conducting tests on a set of synthetic samples of 2 mm thickness with unit permittivity and variable conductivity and on a dielectric material of known permittivity (FR4 laminate). The applicability of both methods was further proven by analyzing the isotropic composite materials, a process involving the use of iron particles embedded in a dielectric matrix. The synthetic samples and an FR4 material were tested to check the accuracy of the methods. Based on numerical studies and measurements, we concluded that materials with a shielding effectiveness of up to 25 dB could be measured at a maximum amplitude error of 1 dB to 3dB to a frequency of 18 GHz, depending on the relative permittivity of the material; hence, the first method was suitable for approximation purposes. For maximal accuracy, the second method typically demonstrated an amplitude error of below 0.5 dB to the same frequency across the entire range. © 2019 by the authors. Licensee MDPI, Basel, Switzerland.Czech Science FoundationGrant Agency of the Czech Republic [17-24730S]; Ministry of Education, Youth, and Sports of the Czech Republic-under the programme NPU I [LO1504]; project entitled Support of Sustainability and Development by the Centre for Security, Information and Advanced Technologies [LO1303

Fractional viscoelastic models of porcine skin and its gelatin-based surrogates

Viscoelasticity of porcine skin and its material substitute, modelled by variously concentrated bovine gelatin, was determined in static (creep test) and dynamic (oscillatory test) mode by the means of rotational rheometry to obtain creep compliance and complex shear modulus. Mechanical properties characterization was also supplemented with large deformation compression test in order to determine and correlate shear and compression moduli of gelatin with its concentration dependence. Obtained data was fitted with fractional viscoelastic models (Poynting-Thomson, Maxwell) in order to quantify in detail gelatin's transition from viscous-like behavior towards solid-like state with increasing gelatin concentration and hence crosslinking density. Potential of gelatin as biomaterial for skin surrogate was identified as well as a concentration region in which gelatin exhibits closest viscoelastic behavior to native porcine skin used. © 2023 The AuthorsMinisterstvo Školství, Mládeže a Tělovýchovy, MŠMT; Grantová Agentura České Republiky, GA ČR: 23-07244

Mechanical properties of bulk Sylgard 184 and its extension with silicone oil

Due to its simple curing and very good mechanical properties, Sylgard 184 belongs to the most widely and frequently used silicones in many industrial applications such as microfluidics and microengineering. On top of that its mechanical properties are further controllable through the curing temperature, which may vary from ambient temperature up to 200 °C; the lower the curing temperature the lower the mechanical properties (Johnston et al. in J Micromech Microeng 24:7, 2014. 10.1088/0960-1317/24/3/035017). However, certain specialised application may require even a softer binder than the low curing temperature allows for. In this study we show that this softening can be achieved with the addition of silicone oil into the Sylgard 184 system. To this end a series of Sylgard 184 samples with varying silicone oil concentrations were prepared and tested (tensile test, rotational rheometer) in order to determine how curing temperature and silicone oil content affect mechanical properties. Curing reaction of the polymer system was found to observe 2nd order kinetics in all cases, regardless the oil concentration used. The results suggest that within the tested concentration range the silicone oil addition can be used to soften commercial silicone Sylgard 184. © 2021, The Author(s).Ministry of Education, Youth and Sports of the Czech RepublicMinistry of Education, Youth & Sports - Czech Republic [RP/CPS/2020/006]Ministerstvo Školství, Mládeže a Tělovýchovy, MŠMT: RP/CPS/2020/00

Polyurethane-carbon nanotubes composite dual band antenna for wearable applications

The design of a unipole and a dual band F-shaped antenna was conducted to find the best parameters of prepared antenna. Antenna radiator part is fully made of polymer and nonmetal base composite. Thermoplastic polyurethane (PU) was chosen as a matrix and multi-wall carbon nanotubes (MWCNT) as an electrical conductive filler, which creates conductive network. The use of the composite for the antenna has the advantage in simple preparation through dip coating technique. Minor disadvantage is the usage of solvent for composite preparation. Composite structure was used for radiator part of antenna. The antenna operates in 2.45 and 5.18 GHz frequency bands. DC conductivity of our PU/MWCNT composite is about 160 S/m. With this material, a unipole and a dual band F antenna were realized on 2 mm thick polypropylene substrate. Both antenna designs were also simulated using finite integration technique in the frequency domain (FI-FD). Measurements and full wave simulations of S11 of the antenna showed good agreement between measurements and simulations. Except for S11, the gain and radiation pattern of the antennas were measured and simulated. Maximum gain of the designed unipole antenna is around −10.0 and −5.5 dBi for 2.45 and 5.18 GHz frequency bands, respectively. The manufactured antennas are intended for application in wearable electronics, which can be used to monitor various activities such as walking, sleeping, heart rate or food consumption. © 2020 by the authors. Licensee MDPI, Basel, Switzerland.Ministry of Education, Youth and Sports of the Czech Republic-DKRVO [RP/CPS/2020/006]; national budget of the Czech Republic [CZ.1.05/2.1.00/19.0409

The enhanced MR performance of dimorphic MR suspensions containing either magnetic rods or their non-magnetic analogs

A co-precipitation method was used to prepare non-magnetic rod-like ferrous oxalate dihydrate (Fe2CO4 • 2H2O) particles that were further turned into iron oxide (Fe3O4) magnetic rod-like particles. A simple precursor-assisted thermal decomposition technique enabled the preservation of the morphology and size of the precursor ferrous oxalate dihydrate particles, thus allowing their magnetic analogs to be obtained. Both types of rod-like particles were used as an additive together with spherical carbonyl iron (CI) particles in novel dimorphic magnetorheological (MR) suspensions. Controlled shear rate mode experiments were performed using a rotational rheometer with a source of an external magnetic field in order to investigate their MR behavior. Moreover, the properties of the novel prepared dimorphic MR (DMR) suspensions were compared with conventional MR suspensions based on spherical CI microparticles. It was found that the DMR suspensions exhibit enhanced MR performance as well as enhanced sedimentation stability in comparison with the MR suspension based on pure CI. The dimorphic suspensions containing magnetic rod-like additives further exhibited significant MR hardening at low shear rates. The properties of CI-based suspensions can be thus optimized by using various additive substances. © 2017 IOP Publishing Ltd.MOE, Ministry of EducationMinistry of Education [LO1504]; Operational Program Research and Development for Innovations-European Regional Development Fund (ERDF); national budget of the Czech Republic, within the framework of the project CPS - strengthening research capacity [CZ.1.05/2.1.00/19.0409

A highly flexible supercapacitor based on MnO2/RGO nanosheets and bacterial cellulose-filled gel electrolyte

The flexible supercapacitors (SCs) of the conventional sandwich-type structure have poor flexibility due to the large thickness of the final entire device. Herein, we have fabricated a highly flexible asymmetric SC using manganese dioxide (MnO2) and reduced graphene oxide (RGO) nanosheet-piled hydrogel films and a novel bacterial cellulose (BC)-filled polyacrylic acid sodium salt-Na2SO4 (BC/PAAS-Na2SO4) neutral gel electrolyte. Apart from being environmentally friendly, this BC/PAAS-Na2SO4 gel electrolyte has high viscosity and a sticky property, which enables it to combine two electrodes together. Meanwhile, the intertangling of the filled BC in the gel electrolyte hinders the decrease of the viscosity with temperature, and forms a separator to prevent the two electrodes from short-circuiting. Using these materials, the total thickness of the fabricated device does not exceed 120 μm. This SC device demonstrates high flexibility, where bending and even rolling have no obvious effect on the electrochemical performance. In addition, owing to the asymmetric configuration, the cell voltage of this flexible SC has been extended to 1.8 V, and the energy density can reach up to 11.7 Wh kg-1 at the power density of 441 W kg-1. This SC also exhibits a good cycling stability, with a capacitance retention of 85.5% over 5000 cycles. © 2017 by the authors.Ministry of Education, Youth, and Sports of the Czech Republic [LTACH17015]; NPU Program I [LO1504]; Operational Program Research and Development for Innovations - European Regional Development Fund (ERDF); national budget of the Czech Republic within the framework of the CPSstrengthening research capacity [CZ.1.05/2.1.00/19.0409]; Tomas Bata University in Zlin, Czech Republic [IGA/CPS/2015/008, IGA/CPS/2016/003

Ferrofluid for radiofrequency capacitive hyperthermia: In-vitro study

The current work deals with preparation and characterization of electrically lossy ferrofluid which can be used as a mediator for radio frequency (RF) - capacitive hyperthermia method. To this end, ferrofluid that can absorb the energy of alternating electrical field at the frequencies commonly employed in RF-capacitive hyperthermia (13.56 and 27.12 MHz) has been prepared by co-precipitation method. This ferrofluid comprises of electrically conductive component with core-shell structure, i.e. magnetite nanoparticles (NPs) coated by dextran, organized in chain-like structure. The effect of RF - capacitive hyperthermia in the presence of mediator was studied on the series of tests performed on HaCaT and HepG2 cell lines using MMT test. The RF-electrical field (13.56 MHz) with controllable power output was applied using the EHY-110 SA (Oncotherm group) to increase the temperature of samples from 37°C up to target temperature of 44°C. The results of in-vitro test clearly indicate that the usage of capacitive heating of obtained ferrofluid substantially contribute to cytotoxic effect of hyperthermia treatment.Ministry of Education, Youth and Sports of the Czech Republic Program NPU I [LO1504

One-dimensional nanostructures of polypyrrole for shielding of electromagnetic interference in the microwave region

Polypyrrole one-dimensional nanostructures (nanotubes, nanobelts and nanofibers) were prepared using three various dyes (Methyl Orange, Methylene Blue and Eriochrome Black T). Their high electrical conductivity (from 17.1 to 60.9 S cm−1), good thermal stability (in the range from 25 to 150◦ C) and resistivity against ageing (half-time of electrical conductivity around 80 days and better) were used in preparation of lightweight and flexible composites with silicone for electromagnetic interference shielding in the C-band region (5.85–8.2 GHz). The nanostructures’ morphology and chemical structure were characterized by scanning electron microscopy, Brunauer–Emmett–Teller specific surface measurement and attenuated total reflection Fourier-transform infrared spectroscopy. DC electrical conductivity was measured using the Van der Pauw method. Complex permittivity and AC electrical conductivity of respective silicone composites were calculated from the measured scattering parameters. The relationships between structure, electrical properties and shielding efficiency were studied. It was found that 2 mm-thick silicone composites of polypyrrole nanotubes and nanobelts shield almost 80% of incident radiation in the C-band at very low loading of conductive filler in the silicone (5% w/w). Resulting lightweight and flexible polypyrrole composites exhibit promising properties for shielding of electromagnetic interference in sensitive biological and electronic systems. © 2020 by the authors. Licensee MDPI, Basel, Switzerland.Ministry of Education, Youth and Sports of the Czech Republicproject DKRVO [RP/CPS/2020/006]; [A2_FCHI_2020_030

Size dependent heating efficiency of iron oxide single domain nanoparticles

The iron oxide nanoparticles have been synthesized by coprecipitation and solvothermal reduction methods. The particles obtained differ in size, the mean size of particles coprecipitated is of 13 nm and the particles, prepared by solvothermal reduction method have a size of 20 nm. Both kinds of nanoparticles demonstrate narrow particles sizes distribution. The particles which are prepared by coprecipitation method have narrow particles sizes distribution with mean size diameter of 13 nm and the particles, prepared by solvothermal reduction method have a size of 20 nm. The X-ray diffraction data analysis revealed that highly crystalline and single-phase magnetite nanoparticles are formed by solvothermal reduction technique, whereas coprecipitation leads to the formation of multi-phase composition of a magnetite (72%) and maghemite (28%). According to the size of nanoparticles obtained, they are in superparamagnetic state for iron oxides. The saturation magnetization of solvothermal prepared particles is higher than those for coprecipitated due to their higher crystallinity and phase purity. Nevertheless, the glycerol dispersion of particles coprecipitated shows higher SLP values than the dispersion of the particles, synthesized by solvothermal reduction method. The heating efficiency of nanoparticles based dispersions is explained by the particles size effect and properties of carrier medium. © 2015 The Authors