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1
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Temperature Dependence of a Dielectric Relaxation in Weakly Polar Ferrofluids

100%
Rajňák M., Kurimský J., Dolník B., Cimbala R., Paulovičová K., Kopčanský P., Timko M.
Acta Physica Polonica A
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2017
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vol. 131
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issue 4
943-945
EN
In this paper, we report on the temperature dependent broadband dielectric response of a ferrofluid based on transformer oil and magnetite nanoparticles covered with oleic acid molecules. For that purpose the method of dielectric spectroscopy has been chosen in the frequency range from 20 Hz up to 100 kHz. The experiments were carried out on thin film ferrofluid samples confined in a glass plate capacitor containing indium tin oxide (ITO) plate electrodes. The obtained complex permittivity spectrum shows a pronounced dielectric dispersion in the low frequency range. Taking into account the ferrofluid composition we associate this relaxation with ion impurity polarization at the nanoparticle-oil interface. The strong temperature dependence of the relaxation process has been found when conducting the experiments in the temperature range from 298 K to 358 K. The relaxation time of the revealed process exhibits a typical Arrhenius behavior. Based on the conducted experiments and analysis, we propose some reasonable practical applications of the studied ferrofluid in the field of electrical engineering.
2
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Generation of Fe₃O₄ Nanoparticle Aggregates in a Ferrofluid Driven by External Electric Field

84%
Kurimský J., Rajňák M., Cimbala R., Paulovičová K., Timko M., Kopčanský P., Kolcun M., Kosterec M., Kruželák L.
Acta Physica Polonica A
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2017
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vol. 131
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issue 4
907-909
EN
In the paper the experimental study of magnetic nanoparticle aggregation in a transformer oil based ferrofluid driven by an external electric field is reported. The studied ferrofluid was composed of the magnetite nanoparticles, oleic acid surfactant, and transformer oil. Generally, it is considered that superparamagnetic nanoparticles do not interact in the absence of external magnetic field. In the paper we present an experimental observation of the particle assembly formation in a direct current external electric field by optical microscopy. During the observation no external magnetic field was applied. A diluted low-polarity ferrofluid drop on a glass surface was exposed to the external static inhomogeneous electric field. It is assumed that induced dipole-dipole interaction and subsequent dielectrophoretic motion give rise to the electrohydrodynamic flow in the fluid after a certain time period. As a result, a visible particle chain was formed at the drop electrical equator. This demonstration is expected to contribute to the understanding of the streamer formation and electrical breakdown in transformer oil based ferrofluids.
3
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AC Magnetic Susceptibility of Ferrofluids Exposed to an External Electric Field

84%
Rajňák M., Dolník B., Kováč J., Paulovičová K., Mitróová Z., Kurimský J., Cimbala R., Kopčanský P., Timko M.
Acta Physica Polonica A
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2017
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vol. 131
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issue 4
887-889
EN
It is known that ferrofluid superparamagnetic nanoparticles response to external magnetic fields, often resulting in the formation of elongated clusters along the field. This has a notable impact on dielectric properties of ferrofluids. Here we report on indications of a contrary effect when the magnetic susceptibility of ferrofluids based on transformer oil is influenced by an external electric field. This effect is associated with structural changes in the ferrofluids induced by the external electric fields. Particularly, we focus on a steady state electric field effect, which gives rise to forces acting on the magnetite nanoparticles, leading to the formation of aggregates. In this condition we have measured the ferrofluid AC magnetic susceptibility in parallel and perpendicular configuration of magnetic and electric fields at room temperature. The measurements in both configurations yielded a noticeable decrease in the real susceptibility values with increasing electric field intensity. The result is believed to be caused by the reduction in the total magnetic moment of the ferrofluid. This can be a consequence of the superspin interactions in the aggregates, minimizing the aggregate's energy. Finally, we highlight the necessity of NMR and small angle scattering of polarized neutrons investigations in order to obtain exact information on the magnetic structure induced by the electric forces.
4
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Analysis of Thermal Field in Mineral Transformer Oil Based Magnetic Fluids

84%
Kosterec M., Kurimský J., Cimbala R., Čonka Z., Kruželák L., Rajňák M., Timko M., Kopčanský P., Vargová B.
Acta Physica Polonica A
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2017
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vol. 131
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issue 4
937-939
EN
Growing interest in the use of magnetic fluids in power systems especially in transformers as insulation and a coolant is nowadays registered. Magnetisable nanofluids, which are used in cooling systems as an alternative to mineral transformer oil, are characterized by lower concentration of magnetic nanoparticles. The magnetic fluid has better heat transfer and dielectric properties such as breakdown than mineral transformer oil and it can be used to improve heat flow, thereby increasing the ability of the active parts to resist failures such as electromagnetic pulses. External magnetic field may be used for forced circulation of magnetic fluid. Magnetic force inside the magnetic fluid can be adequately controlled by adjusting the incident magnetic field. This paper presents thermal distribution, fluid flow and cooling ability of mineral transformer oil and magnetic fluid based on mineral transformer oil. The concentration of Fe₃O₄ magnetic nanoparticles is 0.15% volume of mineral transformer oil. The thermal field is generated by a steel conductor. Thermal distributions in mineral transformer oil and magnetic fluid are investigated and differences for both cases are discussed in the paper.
5
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Influence of Electric Field on AC Magnetic Susceptibility of a Mineral Oil Based Ferrofluid

84%
Rajňák M., Dolník B., Tobiáš T., Paulovičová K., Kurimský J., Cimbala R., Kopčanský P., Timko M.
Acta Physica Polonica A
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2018
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vol. 133
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issue 3
567-579
EN
In a ferrofluid, the magnetic susceptibility at low magnetic fields is sensitive to the size and shape of magnetic nanoparticles and their concentration in a liquid carrier. In this paper we report on the AC magnetic susceptibility of a ferrofluid based on a mineral oil and iron oxide nanoparticles coated with oleic acid. As this type of ferrofluid is of increasing interest for electrical engineering applications, we investigate the effect of an external electric field and an electric current flowing through the ferrofluid on the AC magnetic susceptibility at ambient conditions. It is known that the action of the electric field on the oil based ferrofluids can result in the particle assembling. Then, the induced structural changes may affect the magnetic susceptibility of the bulk ferrofluid. In our experimental study we observed that the ferrofluid magnetic susceptibility decreases with increasing electric field. However, a heating effect with increasing electric field was observed too. Hence, it is concluded that besides the structural changes, the Joule heating has an obvious impact on the magnetic susceptibility of the ferrofluid in the external electric field.
6
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The Response of a Magnetic Fluid to Radio Frequency Electromagnetic Field

68%
Dolník B., Rajňák M., Cimbala R., Kolcunová I., Kurimský J., Balogh J., Džmura J., Petráš J., Kopčanský P., Timko M., Briančin J., Fabián M.
Acta Physica Polonica A
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2017
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vol. 131
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issue 4
946-948
EN
Electromagnetic pollution generated by the electrical devices has been regarded as a new form of pollution, harmful to the society as air and water pollution. The operation of electronic devices in a polluted electromagnetic environment has caused electromagnetic interference to become important concerns. Devices that are vulnerable to interference must often be shielded to protect them from the effects of electromagnetic interference. In this work we describe an interaction of a magnetic fluid based on transformer oil with alternating magnetic field. The magnetic fluid was composed of a transformer oil and dispersed magnetite nanoparticles coated with oleic acid. Among the wide range of topics covered, we pay attention to an important field related to the absorption of electromagnetic field by magnetic fluid as a suitable candidate for applications where it is necessary to electrically isolate, remove excess of heat, and to shield electromagnetic fields. We present a method for the determination of shielding effectiveness of the magnetic fluid under high-frequency excitation conditions from 750 MHz to 3 GHz by means of magnetic near field measurements and analysis. Herein, we report the effect of magnetic volume fraction in the magnetic fluid and the effect of the sample thickness on the shielding effectiveness. We have found that the magnetic fluid has a frequency dependent "windows", characterized that either absorb the magnetic field, or facilitate penetration of the magnetic field through the barrier.
7
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Ultrasound Frequency Analysis of a Magnetic Fluid in Low-Intensity External Magnetic Field

68%
Kurimský J., Rajňák M., Cimbala R., Dolník B., Tóthová J., Paulovičová K., Timko M., Kopčanský P., Kolcunová I., Petráš J., Džmura J., Balogh J.
Acta Physica Polonica A
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2017
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vol. 131
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issue 4
910-912
EN
This work deals with an interaction of a magnetic fluid of a dielectric nature with a magnetic field by means of ultrasound waves measurements and analysis. Ultrasound analysis is known as a non-destructive inspection tool often used in technical diagnostics, moreover, it has numerous applications in medicine and biology, too. We report the low-frequency ultrasound analysis of a dielectric magnetic fluid in a low-intensity external static magnetic field. The studied magnetic fluid was composed of a transformer oil and dispersed magnetite nanoparticles coated with oleic acid. Experiments were carried out by using an ultrasonic testing cell. The cell was exposed to a magnetic field of 50 mT in both parallel and perpendicular direction to the waves propagation. A through-transmission mode measurement was applied, where two fixed narrow-band transducers with completely shielded crystal for maximum RFI/EMI immunity (Physical Acoustic R15I-AST, the resonant frequency 150 kHz) served as a transmitter and a receiver. In this way we carried out the measurement of the frequency-dependent ultrasonic response to a rectangle calibrating signal of 5 μs pulse width. Digitized signals were recorded for further analysis. We present the frequency domain analysis of the low-frequency ultrasound in magnetic fluid. The frequency spectrum in magnetic fluid colloidal system was calculated by the Fourier transformation method. Results show that there is a frequency shift in the amplitude-frequency spectrum caused by the step-up magnetic field. The higher the magnetic field, the higher the frequency of the peaks. The effect of particle aggregation in magnetic field on the ultrasound wave propagation is discussed in the paper.
8
Content available remote

Variation of Magnetic Fluid Deformation Related to Nanoparticle Concentration in Steady Electric Field

68%
Kosterec M., Kurimský J., Cimbala R., Špes M., Farkaš R., Dolník B., Gamcová M., Rajňák M., Timko M., Kopčanský P., Paulovičová K., Vargová B.
Acta Physica Polonica A
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2018
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vol. 133
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issue 3
570-573
EN
Today, it is important to know the behaviour of magnetic fluids applied in the power electrical machines, e.g. in power transformers, when exposed to an electric field. Besides their promising applications in high voltage engineering, they are of increasing interest from designed assembly and pattern formation point of view. The structure of such magnetic fluids is easily controllable by external magnetic fields. However, less attention has been paid to structural phenomena in magnetic fluids induced by electric fields. The core of this paper is dedicated to the experimental observation of a magnetic fluid droplet deformation in a steady electric field. The mutual relation between the deformation parameter and magnetic nanoparticles concentration is analysed. Spatio-temporal analysis of the droplet shape is presented in the paper. The phenomena of the droplet deformation were recorded by a camera. The detailed experimental procedure is presented. The method of deformation parameter calculation based on the linear pixel as the smallest-size unit in digital image is written. Finally, the relation between the deformation parameter and the nanoparticle volume concentration, as well as the time and magnitude of the DC field application are thoroughly evaluated. The results show that the deformation parameter decreases with increasing concentration at constant applied steady electric field but increases with increasing applied steady electric field.
9
Content available remote

The Shielding Effectiveness of a Magnetic Fluid in Radio Frequency Range

68%
Dolník B., Rajňák M., Cimbala R., Kolcunová I., Kurimský J., Džmura J., Petráš J., Zbojovský J., Kosterec M., Kopčanský P., Timko M.
Acta Physica Polonica A
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2018
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vol. 133
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issue 3
585-587
EN
This article builds on the previous work and describes the interaction of transformer oil-based magnetic fluid (MF) with the radio frequency (RF) magnetic near-field. Three prepared samples of the MF used as a barrier to magnetic near-field, consist of transformer oil and dispersed magnetite nanoparticles coated with oleic acid. We pay attention to the important area related to the electromagnetic field shielding by the MF. Such sample of the MF may be a good candidate for applications where it is necessary to simultaneously electrically isolate, remove the excess of heat and to shield electromagnetic interference (EMI). We present a method for the determination of shielding effectiveness (SE) of the MF under RF excitation conditions ranging from 500 MHz to 3 GHz. We report the effect of magnetic volume fraction in the MF and the effect of the sample thickness on the SE.
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