Miha Drofenik

University of Maribor, Maribor, Maribor, Slovenia

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Publications (195)273.76 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: This article reports the synthesis of novel, rare-earth coordination complexes with nicotinic acid. Three compounds with the general formula Ln2[(C5H4NCOO)6(H2O)4] (Ln = Yb, 1; Ln = Gd, 2; Ln = Nd, 3) were prepared from relatively cheap and readily available reactants. Their compositions and structure were characterized by IR spectroscopy and single-crystal X-ray diffraction. The magnetic and thermogravimetric properties were also studied. The complexes consist of centrosymetric, dimeric molecules having all six nicotinato ligands coordinated with the central atom in the bidentate mode. The coordination environment of the Ln3+ for all three compounds is 8. Here we describe the crystal structure of Yb and Gd complexes with nicotinic acid.
    Central European Journal of Chemistry 02/2014; 12(2). · 1.17 Impact Factor
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    ABSTRACT: Nickel–copper (NiCu) alloy magnetic nanoparticles with a narrow size distribution were prepared by reducing a Ni, Cu-oxide mixture in a silica matrix, obtained with a sol–gel method. In order to obtain the NiCu nanoparticles, the silica matrix was treated with an etching solution in an argon atmosphere. By selecting the nominal NiCu alloy composition of 67.5:32.5, a Curie temperature close to 65 °C was achieved. The nanoparticles were superparamagnetic with a magnetization of 8 emu/g at 293 K and exhibited a significant heating ability in an alternating magnetic field. The simplicity of the synthesis allows the preparation of NiCu alloy nanoparticles on a large scale. The method could also be used for the preparation of other mixed-alloy nanoparticles.
    Materials Letters. 01/2014; 124:39–42.
  • Igor Zajc, Miha Drofenik
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    ABSTRACT: We have identified a PTCR anomaly in undoped BaTiO3 (BT) ceramics. This anomaly was ascribed to a disconnection of the semiconducting grains, due to dimensional changes of the BT grains at the Curie point, in a composite composed of two constituent BT phases, one with a low electrical resistivity and the other with a high electrical resistivity. The composite exhibits a significant PTCR effect of three orders of magnitude at the Curie temperature. Using a similar approach, a PTCR anomaly was observed in a composite where copper was used as the conductive matrix, while BT particles were used as the second constitutive phase in the composite, acting as a circuit-breaker, and disconnecting the continuous conducting phase at the Curie temperature.
    Ceramics International. 01/2014; 40(6):8033–8036.
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    ABSTRACT: Magneto-electric (ME) ceramic composites of cobalt ferrite (CoF) and lead zirconate titanate (PZT) were prepared by mechanical mixing of the constituent powders followed by cosintering. The cosintering conditions for nano-sized CoF and submicrometer-sized PZT powders were studied in detail. It was found that the CoF powder needs to be presintered at 700°C for 2 h to minimize the differences in the sintering kinetics of the constituent powders. Despite the low cosintering temperatures (900°C–1000°C) the interdiffusion of the cations from both phases was confirmed with energy-dispersive X-ray analysis and X-ray diffraction. Efforts were made to optimize the cosintering conditions to prepare dense ceramic ME composites, which showed the converse ME effect.
    Journal of the American Ceramic Society 01/2014; 97(1). · 2.11 Impact Factor
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    ABSTRACT: Mg1+xFe2-2xTixO4 ferrite nanoparticles for Self-Regulating Magnetic-Fluid Hyperthermia were synthesized by the co-precipitation of Fe3+, Mg2+ and Ti4+ ions using NaOH. The hydroxide precursors were heated to 1000 °C in air and the product, (Mg,Ti)-ferrite, was milled in the presence of a surfactant in a high-energy mill, resulting in the magnetic fluid. The magnetic particles were characterized with transmission electron microscopy, X-ray diffractiometry and magnetic measurements. The Mg1+xFe2-2xTixO4 nanoparticles with x=0.37 exhibited a room-temperature magnetization of 8 emu/g at 1 T and a Curie temperature (Tc) of 46 °C. The heating ability of the nanoparticles in an AC magnetic field was measured using a calorimeter.
    Journal of Magnetism and Magnetic Materials 01/2014; · 2.00 Impact Factor
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    ABSTRACT: More than four decades ago, Brochard and de Gennes proposed that colloidal suspensions of ferromagnetic particles in nematic (directionally ordered) liquid crystals could form macroscopic ferromagnetic phases at room temperature. The experimental realization of these predicted phases has hitherto proved elusive, with such systems showing enhanced paramagnetism but no spontaneous magnetization in the absence of an external magnetic field. Here we show that nanometre-sized ferromagnetic platelets suspended in a nematic liquid crystal can order ferromagnetically on quenching from the isotropic phase. Cooling in the absence of a magnetic field produces a polydomain sample exhibiting the two opposing states of magnetization, oriented parallel to the direction of nematic ordering. Cooling in the presence of a magnetic field yields a monodomain sample; magnetization can be switched by domain wall movement on reversal of the applied magnetic field. The ferromagnetic properties of this dipolar fluid are due to the interplay of the nematic elastic interaction (which depends critically on the shape of the particles) and the magnetic dipolar interaction. This ferromagnetic phase responds to very small magnetic fields and may find use in magneto-optic devices.
    Nature 12/2013; 504(7479):237-241. · 38.60 Impact Factor
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    ABSTRACT: There is a growing interest in the synthesis of nanostructured copper sulfides due to their ability to form compounds with various stoichiometries. We report a sonochemical route for the preparation of copper sulfides with different compositions in aqueous solutions, using different, general and convenient copper sources such as copper acetate, copper hydroxide or basic copper carbonate and thiourea or thioacetamide as sulfur precursors under ambient air. Phase analysis, purity and morphology of the products were studied by X-ray powder diffraction (XRD), transmission electron microscopy (TEM), and thermogravimetric analysis (TGA). The results revealed that nanoparticles of covellite, CuS, with crystallite sizes between 7 and 18 nm can be obtained by using different precursors and complexing agents and that chalcocite, Cu2S, can also be prepared sonochemically.
    Materials Research Bulletin. 03/2013; 48(3):1184–1188.
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    ABSTRACT: A chemical and a physical method have been applied for the preparation of chromium-nickel alloy nanoparticles. These particles were designed to be used for controlled magnetic hyperthermia applications. Microemulsions with Ni2+ and Cr3+ and/or NaBH4 as precursors were prepared using the isooctane/CTAB, n-butanol/H2O system. The samples of CrxNi1-x nanoparticles with the desired composition were obtained after the reduction of their salts with NaBH4 and afterwards heat treated in a TGA in a N2 atmosphere at various temperatures. The CrxNi1-x materials were also prepared by mechanical milling. Utilizing a ball-to-powder mass ratio of 20 : 1 and selecting the proper alloy compositions we were able to obtain nanocrystalline CrxNi1-x particles. Thermal demagnetization in the vicinity of the Curie temperature of the nanoparticles was studied using a modified TGA-SDTA method. The alloy's phase composition, size and morphology were determined with XRD measurements and TEM analyses.
    Acta Chimica Slovenica 01/2013; 60(4):750-5. · 1.14 Impact Factor
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    Darja Lisjak, Petra Jenus, Miha Drofenik
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    ABSTRACT: We have studied the assembly of cobalt ferrite nanoparticles in a magnetic field. Nanoparticles of different sizes, i.e., 5-40 nm, were synthesized with coprecipitation or under hydrothermal conditions. Their saturation magnetization values ranged between 31 and 68 Am2 /kg, respectively, resulting in a strong magnetic attraction and agglomeration between the nanoparticles. In aqueous ferrofluids this was prevented by the adsorption of citric acid on the nanoparticles' surfaces. The estimated interaction energies show that the DLVO (Derjaguin, Landau, Verwey, Overbeek) theory fails to explain the stability of cobalt ferrite ferrofluids and that the solvation interaction cannot be neglected. Cobalt ferrite ferrofluids were deposited on alumina substrates and dried in a magnetic field of 0.03 or 0.5 T. The homogeneity of the deposits decreased with the increasing saturation magnetization of the nanoparticles and/or with the increasing magnetic field, until columnar structures were formed from the nanoparticles with Ms ≥ 55 Am2/kg under a magnetic field of B = 0.5 T.
    IEEE Transactions on Magnetics 11/2012; 48(11):3303-3306. · 1.42 Impact Factor
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    ABSTRACT: A new, simple, low-temperature method for the synthesis of superparamagnetic, photocatalytic, nanocomposite particles for applications in the decomposition of pollutants in water is presented. The method is based on the coating of clusters of superparamagnetic maghemite (γ-Fe2O3) nanoparticles with a photocatalytic anatase layer using the hydrolysis of aqueous TiOSO4. The clusters of an appropriate size between 100 and 200 nm form by the simultaneous agglomeration of the aminopropyl-triethoxy-silane-grafted maghemite nanoparticles with a size of approximately 15 nm in a suspension of diluted TiOSO4. During a sudden increase of pH with the addition of NaOH the titania is heterogeneously nucleated at the cluster surfaces. If the hydrolysis was conducted at an elevated temperature of 90 °C, the titania layer was nanocrystalline anatase. The composition of the nanocomposite particles, i.e., the thickness of the anatase layer, can be controlled simply by changing the starting TiOSO4/Fe2O3 ratio for low titania contents, and by multiple coatings to get high titania contents. The photocatalytic activity of the nanocomposites was evaluated in the photocatalytic decomposition of formic acid. The activity seems to increase with an increase in the thickness and the crystallinity of the anatase coating, whereas it decreased after the calcination of the as-synthesized nanocomposite. The coating of the maghemite nanoparticles with a thin layer of insulating silica also slightly improves the photocatalytic activity.
    Materials Chemistry and Physics. 09/2012; 136(1):230–240.
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    ABSTRACT: Crystal structure of ultrafine Ba-hexaferrite (BaFe12O19) nanoparticles was studied using X-ray diffractometry (XRD), high-resolution transmission electron microscopy (HRTEM), energy-dispersive X-ray spectroscopy (EDXS), X-ray absorption fine structure (XAFS), and Mossbauer spectroscopy (MS), ¨ to be compared to the structure of larger nanoparticles and the bulk. The nanoparticles were synthesized with hydrothermal treatment of an appropriate suspension of Ba and Fe hydroxides in the presence of a large excess of OH � . The ultrafine nanoparticles were formed in a discoid shape, �10 nm wide and only �3 nm thick, comparable to the size of the hexagonal unit cell in the c-direction. The HRTEM image analysis confirmed the hexaferrite structure, whereas EDXS showed the composition matching the BaFe12O19 formula. XAFS and MS analyses showed considerable disorder of the structure, most probably responsible for the low magnetization.
    Journal of Solid State Chemistry 07/2012; 196:63-71. · 2.04 Impact Factor
  • Simona Ovtar, Darja Lisjak, Miha Drofenik
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    ABSTRACT: Oriented deposits of hard-magnetic barium ferrite were prepared using electrophoretic deposition (EPD). A stable suspension of highly anisotropic particles with diameters of a few 100 nm and thicknesses of 10 nm was prepared in 1-butanol. The electrophoretic velocities for the two orientations of particles with respect to the electrode were calculated. The particles migrated to the electrode with the basal plane perpendicular to the electrode. The oriented deposits were formed under the influence of the hydrodynamic force near the electrode. The sintering was accompanied by an anisotropic and abnormal grain growth and the oriented, larger grains affected most significantly the overall orientation of the films. The grain size was investigated by SEM image analysis and the orientation of the films was calculated from XRD patterns and determined from magnetic measurements. The films with the best orientations were prepared with the lowest applied voltage, where the electrophoretic velocity was the slowest. The films that were prepared with step EPD and sintered at 1150 °C had a remanence-to-saturation magnetization of 0.9. The density of the film and the magnetization values were further increased by iso-static pressing of the deposits before sintering.
    Colloids and Surfaces A Physicochemical and Engineering Aspects 06/2012; 403:135-147. · 2.11 Impact Factor
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    ABSTRACT: CM-dextran-covered maghemite particles for applications in magnetic hyperthermia treatments were synthesized and their physical, magnetic and morphological properties were examined. Magnetic fluids were prepared and their heating properties in an alternating magnetic field were studied. The results reveal that the particle size and the thickness of the carboxy-methyl-dextran (CM-dextran) coatings have a decisive influence on the heating properties: specific absorption rate (SAR). The majority of the magnetic dissipation comes from the Neel relaxation, while the Brown contribution is small. A thermal steady state at the selected temperature (42 °C) can be achieved using synthesized maghemite particles with proper particle morphology and by controlling the magnetic field intensity or the frequency.
    Acta Chimica Slovenica 06/2012; 59(2):366-74. · 1.14 Impact Factor
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    ABSTRACT: We have investigated an organic resin solution designed for EM wave absorption based on a magnetic filler, composed of phases within the Mn0.66Zn0.27Fe2.07O4 system, embedded in an absorber composite with concentration ratios of 50:50, 75:25 and 90:10 by weight. The formation of the manganese zinc ferrite particles, as the principal magnetic phases, was achieved via the conventional ceramic method. The electromagnetic parameters of the composites were measured with a vector network analyser at 100 MHz to 10 GHz. The subject of the paper was a study of the electromagnetic absorber properties and the rheological properties of the resin composite based on ferrite particles with respect to using the materials in architectural coatings.
    Ceramics International. 05/2012; 38(4):2693–2699.
  • Sašo Gyergyek, Miha Drofenik, Darko Makovec
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    ABSTRACT: Oleic-acid-coated CoFe2O4 nanoparticles were synthesized by co-precipitation and hydrothermal synthesis. The coprecipitation of the nanoparticles was achieved by the rapid addition of a strong base to an aqueous solution of cations in the presence of the oleic acid surfactant, or without this additive. The nanoparticles were also synthesized by a hydrothermal treatment of suspensions of the precipitates, coprecipitated at room temperature in the presence of the oleic acid, or without it. The influence of the synthesis conditions, such as the valence state of the iron cation in the starting aqueous solution, the temperature of the treatment and the presence of oleic acid, on the particles size was systematically studied. X-ray powder diffractometry (XRD) and transmission electron microscopy (TEM) coupled with energy-dispersive X-ray spectroscopy (EDS) revealed that, although spinel forms at room temperature, a substantial amount of Co was incorporated within the secondary, feroxyhyte-like phase when the iron cation was in the 2+ state. In contrast, when iron was in the 3+ state, the spinel forms at elevated temperatures of approximately 60 °C. The presence of the oleic acid further increased the formation temperature for the stoichiometric spinel. Moreover, the oleic acid impeded the particles’ growth and enabled the preparation of colloidal suspensions of the nanoparticles in non-polar organic solvents. The nanoparticles’ size was successfully controlled by the temperature of the synthesis in the region where superparamagnetism dominates to the region where mono-domain ferrimagnetism dominates the magnetic properties.
    Materials Chemistry and Physics. 03/2012; 133(1):515–522.
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    ABSTRACT: The magnetic properties of layered hydroxylammonium fluorocobaltate (NH(3)OH)(2)CoF(4) were investigated by measuring its dc magnetic susceptibility in zero-field-cooled (ZFC) and field-cooled (FC) regimes, its frequency dependent ac susceptibility, its isothermal magnetization curves after ZFC and FC regimes, and its heat capacity. Effects of pressure and magnetic field on magnetic phase transitions were studied by susceptibility and heat capacity measurements, respectively. The system undergoes a magnetic phase transition from a paramagnetic state to a canted antiferromagnetic state exhibiting a weak ferromagnetic behavior at T(C) = 46.5 K and an antiferromagnetic transition at T(N) = 2.9 K. The most spectacular manifestation of the complex magnetic behavior in this system is a shift of the isothermal magnetization hysteresis loop in a temperature range below 20 K after the FC regime-an exchange bias phenomenon. We investigated the exchange bias as a function of the magnetic field during cooling and as a function of temperature. The observed exchange bias was attributed to the large exchange anisotropy which exists due to the quasi-2D structure of the layered (NH(3)OH)(2)CoF(4) material.
    Journal of Physics Condensed Matter 02/2012; 24(5):056002. · 2.22 Impact Factor
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    ABSTRACT: Cadmium telluride nanoparticles with sizes between 8 and 13 nm have been synthesized via a sonochemical route using cadmium sulfate hydrate (CdSO4·8/3H2O), cadmium chloride (CdCl2) and elemental Te as precursors and aqueous solutions of NaOH and EDTA as solvents. The qualitative characterization and estimation of nanoparticle size were carried out by using X-ray powder diffraction (XRD). The morphology of nanoparticles was analyzed by transmission electron microscopy (TEM). Changes of physical and chemical properties of the prepared CdTe nanoparticles at increased temperature were studied by thermal analysis (TGA, SDTA). A probable mechanism for the sonochemical formation of CdTe is proposed.
    Materials Letters. 01/2012; 67(1):56–59.
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    ABSTRACT: The synthesis of magnetic Cu$_{1-{\rm x}}$Ni$_{\rm x}$ nanoparticles was carried out in cationic water-in-oil (w/o) microemulsions of water/cetyl-trimethyl-ammonium bromide (CTAB), n-butanol/isooctane by the reduction of nickel and copper chlorides with hydrazine and NaOH. The synthesized Cu$_{1-{\rm x}}$Ni$_{\rm x}$ particles were heat treated to maintain their proper homogeneity and Curie temperature. Some alloy particles with the composition Cu$_{27.5}$Ni$_{72.5}$ were coated with silica prior to the thermal homogenization in order to retain the pristine morphology. The magnetic particles were characterized using XRD, transmission electron microscopy (TEM) and magnetic measurements. The thermal demagnetization in the vicinity of the Curie temperature of the nanoparticles was studied using a modified TGA/SDTA method.
    IEEE Transactions on Magnetics 01/2012; 48(4):1344-1347. · 1.42 Impact Factor
  • D. Primc, M. Drofenik, D. Makovec
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    ABSTRACT: Ultrafine strontium hexaferrite (SrFe12O19) nanoparticles have been synthesized by the hydrothermal treatment of an appropriate suspension of Sr and Fe hydroxides in the presence of a large excess of OH– at temperatures between 130 and 170 °C. To avoid the parallel formation of any undesired hematite (α-Fe2O3) during the synthesis, a large excess of Sr in the starting composition (Fe/Sr = 3) had to be used. When the treatment was performed below 170 °C, ultrafine nanoparticles were formed. The TEM images show they have a disc-like shape, approximately 12 nm wide, but only around 4 nm thick. The EDS analysis showed their composition corresponded to SrFe12O19. When the temperature of the hydrothermal treatment exceeded 170 °C, larger hexagonal platelet crystals appeared as a consequence of Ostwald ripening. The evolution of the size and morphology of the nanoparticles with the temperature of the hydrothermal treatment was also monitored by XRD and measurement of the magnetic properties.
    European Journal of Inorganic Chemistry 07/2011; 2011(25):3802-3809. · 3.12 Impact Factor
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    ABSTRACT: In recent years, a pronounced interest in materials that are perspective in applications for everyday life has start to grow. Our interest has been focused on different conversions of energies in such advanced materials. We have studied a giant conversion of thermal to mechanical energy, i.e., thermomechanical effect in nematic liquid crystal elastomers, a giant conversion of electrical to thermal energy, i.e., electrocaloric effect in PLZT thin films, and an electric control of magnetization, i.e.,magnetoelectric effect, in soft multiferroics such as mixtures of SCE9 ferroelectric liquid crystal and magnetic nanoparticles.
    3rd Jožef Stefan International Postgraduate School Students Conference, JSI,; 05/2011

Publication Stats

670 Citations
273.76 Total Impact Points

Institutions

  • 1998–2014
    • University of Maribor
      • • Faculty of Chemistry and Chemical Engineering
      • • Department of Chemical Technology
      Maribor, Maribor, Slovenia
  • 1981–2014
    • Jožef Stefan Institute
      • • Department of Synthesis of Materials
      • • Department of Advanced Materials
      Lubliano, Ljubljana, Slovenia
  • 2011
    • Institute of Mathematics, Physics and Mechanics
      Lubliano, Ljubljana, Slovenia
    • University of Nova Gorica
      Gorica, Nova Gorica, Slovenia
  • 1973–2006
    • University of Ljubljana
      • Faculty of Chemistry and Chemical Technology
      Lubliano, Ljubljana, Slovenia
  • 2005
    • Kolektor Group
      Idria, Idrija, Slovenia
  • 2004
    • Martin Luther University of Halle-Wittenberg
      Halle-on-the-Saale, Saxony-Anhalt, Germany
  • 1993
    • Institute of Non-ferrous and Rare Metals - IMNR
      Pantelimon, Ilfov County, Romania