Miha Drofenik

University of Maribor, Maribor, Maribor, Slovenia

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Publications (212)422.67 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: Abstract A series of nickel-copper alloy magnetic nanoparticles with a range of Curie points from 51°C to 63°C were prepared by the reduction of intimately mixed nickel and copper oxides in a silica matrix using the sol-gel method. The silica matrix was subsequently removed with an etching solution, assisted by sonication. The alloy nanoparticles were characterized using X-ray diffraction (XRD) analysis, thermogravimetric analysis (TGA/SDTA), thermomagnetic analysis (TMA), transmission electron microscopy (TEM), magnetic measurements (SQUID, vibrating-sample magnetometer) and specific absorption rate measurements (SAR). The synthesized nanoparticles show a size in the range 15-20 nm, exhibited superparamagnetic behavior with a blocking temperature (TB) of approximately 135 K and a room-temperature magnetization of 3-9 emu/g, depending on the composition. The nanoparticles showed a relatively high effective anisotropy constant (Keff) and a significant heating ability in an alternating magnetic field. The synthesis method is straightforward and allows the preparation of homogeneous Ni-Cu alloy nanoparticles with a relatively narrow particle size distribution.
    Journal of Alloys and Compounds 07/2015; 648:53-58. DOI:10.1016/j.jallcom.2015.06.067 · 3.00 Impact Factor
  • Gregor Ferk · Miha Drofenik · Darko Makovec · Irena Ban ·
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    ABSTRACT: A magnetic fluid (MF) consisting of water-soluble maghemite nanoparticles, stabilized by a polymerized bilayer of fatty acid, was prepared. The simplicity of the synthesis without intermediate purification phases allows the preparation of MF. The structure and composition of the nanoparticles were analyzed by transmission electron microscopy (TEM), Fourier transform infrared (FTIR) spectroscopy, thermal gravimetric analysis (TGA) and magnetic measurements. The hydrodynamic diameter of the coated nanoparticles in the magnetic liquid and their stability against dilution was monitored by dynamic light scattering (DLS).
    Materials Letters 05/2015; 157. DOI:10.1016/j.matlet.2015.05.077 · 2.49 Impact Factor
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    ABSTRACT: The coordination compounds of pyridine-2,6-dicarboxylic acid and two lanthanide(III) ions, Ho3+ and Dy3+, were hydrolytically synthesized in aqueous solutions at a slightly basic pH, and then characterized by thermogravimetric analysis, IR spectroscopy, magnetic measurements as well as X-ray powder and single-crystal diffraction analysis. The elemental analyses were performed to check the purity of the compounds. The formula for these compounds is identified as Na3[Ln(Pydc)3]⋅14H2O (Ln = Ho, 1; Ln = Dy, 2) in agreement with the X-ray structural analysis and all the other experimental data. The absence of the 1709 cm−1 band corresponding to ν(СO) in the IR spectra of the compounds evidences the deprotonating of the carboxyl group. The very strong inductive effect of the metal ion that is readily coordinated by the carboxylate group of the zwitterionic ligand is responsible for the formation of the product. The single-crystal X-ray structural analysis revealed that compounds 1 and 2 are isostructural. Their structure can be described as interchanging layers of complex anions [Ln(Pydc)3]3 (Ln = Ho and Dy for 1 and 2, respectively) and layers of hydrated sodium cations. In complex anions the holmium and dysprosium atoms are coordinated by three crystallographically independent pyridinedicarboxylate ligands in tridentate-chelate mode, via one O atom of both carboxylate groups and the ring N atom. The coordination number is nine and the coordination polyhedron is a tricapped trigonal prism with O atoms at the corners.
    Journal of Molecular Structure 01/2015; 1079:54–60. DOI:10.1016/j.molstruc.2014.09.029 · 1.60 Impact Factor
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    ABSTRACT: The synthesis and characterization of two new cobalt(II) coordination compounds with nicotinamide (nia) and isonicotinamide (isn) are reported. The products were characterized magnetically, structurally by single-crystal X-ray diffraction analysis and spectrally by FT-IR spectroscopy. Using the reaction of cobalt(II) acetate tetrahydrate and nicotinamide in methanol we obtained light-red crystals of the mononuclear complex [Co(nia)(2)(H2O)(4)](CH3COO)(2)center dot 2H(2)O (1). The synthesis in a system cobalt(II) acetate dihydrathe, isonicotinamide and dimethylformamide-methanol mixture gave a new dinuclear coordination compound with the formula [Co-2(CH3COO)(4)(isn)(4)]center dot 2C(3)H(7)NO (2). In both compounds a trans arrangement of pyridinecarboxamide ligands was found. Intermolecular hydrogen bonds in the crystal structures of both complexes are discussed. The magnetic properties were studied between 2 K and 300 K giving the result mu(eff) = 4.6 BM for 1 and mu(eff) = 4.7 BM for 2 in the paramagnetic region.
    Journal of Molecular Structure 11/2014; 1076:713-718. DOI:10.1016/j.molstruc.2014.08.031 · 1.60 Impact Factor
  • Sašo Gyergyek · Miha Drofenik · Darko Makovec ·
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    ABSTRACT: Magnetic CoFe2O4 nanoparticles were synthesized by co-precipitation at 80 °C. This co-precipitation was achieved by the rapid addition of a strong base to an aqueous solution of cations. The investigation of the samples that were quenched at different times after the addition of the base, using transmission electron microscopy (TEM) coupled with energy-dispersive X-ray spectroscopy (EDXS) and X-ray powder diffractometry, revealed the formation of a Co-deficient amorphous phase and Co(OH)2, which rapidly reacted to form small CoFe2O4 nanoparticles. The nanoparticles grew with the time of aging at elevated temperature. The colloidal suspensions of the nanoparticles were prepared in both an aqueous medium and in a non-polar organic medium, with the adsorption of citric acid and ricinoleic acid on the nanoparticles, respectively. The measurements of the room-temperature magnetization revealed the ferrimagnetic state of the CoFe2O4 nanoparticles, while their suspensions displayed superparamagnetic behaviour.
    Acta Chimica Slovenica 09/2014; 61(3):488-496. · 0.69 Impact Factor
  • 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 07/2014; 40(6):8033–8036. DOI:10.1016/j.ceramint.2014.01.001 · 2.61 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 06/2014; 124:39–42. DOI:10.1016/j.matlet.2014.03.030 · 2.49 Impact Factor
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    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). DOI:10.2478/s11532-013-0366-5 · 1.33 Impact Factor
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    Petra Jenus · Darja Lisjak · Danjela Kuscer · Darko Makovec · Miha Drofenik ·
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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). DOI:10.1111/jace.12600 · 2.61 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; 350:124-128. DOI:10.1016/j.jmmm.2013.09.037 · 1.97 Impact Factor
  • Irena Ban · Janja Stergar · Miha Drofenik · Gregor Ferk · Darko Makovec ·
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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 12/2013; 60(4):750-5. · 0.69 Impact Factor
  • Alenka Mertelj · Darja Lisjak · Miha Drofenik · Martin Čopič ·
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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(7479-7479):237-241. DOI:10.1038/nature12863 · 41.46 Impact Factor
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    ABSTRACT: Magnetic particles with a controlled Curie temperature were prepared by reducing a Ni,Cu-hydrazine complex that was synthesized in a compartmentalized state of reverse micelles. The planned Curie temperature of 43 °C was achieved by a thermally activated homogenization of as-prepared alloy particles embedded in a NaCl salt environment. The particles were superparamagnetic with a blocking temperature of 16.5 K and a room-temperature magnetization of 2.5 emu/g. The particles exhibited a therapeutic Curie temperature that is suitable for self-regulating magnetic hyperthermia.
    Journal of Alloys and Compounds 11/2013; 576:220–226. DOI:10.1016/j.jallcom.2013.04.130 · 3.00 Impact Factor
  • Matjaž Kristl · Nuša Hojnik · Sašo Gyergyek · Miha Drofenik ·
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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. DOI:10.1016/j.materresbull.2012.12.020 · 2.29 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. DOI:10.1109/TMAG.2012.2200659 · 1.39 Impact Factor
  • Darko Makovec · Marjan Sajko · Aljaž Selišnik · Miha Drofenik ·
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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. DOI:10.1016/j.matchemphys.2012.06.058 · 2.26 Impact Factor
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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. DOI:10.1016/j.jssc.2012.07.043 · 2.13 Impact Factor
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    Petra Jenuš · D Lisjak · D Kuščer · D Makovec · M Drofenik ·
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    ABSTRACT: In multiferroic composites the ferro(piezo)electric and magnetostrictive phases can be electromagnetically coupled via stress mediation (1). In our work we focused on the cosintering of cobalt ferrite (CF) - barium titanate (BT) and cobalt ferrite (CF) - lead zirconate titanate (PZT) composites. The CF nanoparticles were prepared by hydrothermal synthesis; the PbZr0,53Ti0,47O3 (PZT) powder was synthesized by solid-state synthesis; and the BT nanopowders were commercially obtained. The powder mixtures were pressed into disc-shaped samples, after which they were sintered at different temperatures. The CF-BT and CF-PZT diffusion couples were also investigated. The samples were analyzed with a scanning electron microscope (SEM) and with an electron diffraction spectrometer (EDS). The SEM studies of the samples sintered at 950°C for 4 h revealed the low relative density of the composites and cracks at the interface of the constituent phases. The elemental mapping showed the distribution of titanium over both constituent phases, while at the same time partial diffusion of the iron and cobalt into the PZT and BT was also detected. The mechanical coupling of two constituent phases is only possible in a composite with a high sintered density and without any interdiffusion or a chemical reaction between the phases. Therefore, we focused on the cosintering of the CF with BT or PZT. A study of the size effect of the constituent powders on the cosintering of the composites will be presented.
    Electroceramics XIII, June 24th - 27th 2012, Enschede, The Netherlands; 06/2012
  • 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. DOI:10.1016/j.colsurfa.2012.04.004 · 2.75 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. · 0.69 Impact Factor

Publication Stats

3k Citations
422.67 Total Impact Points


  • 2001-2015
    • University of Maribor
      • Faculty of Chemistry and Chemical Engineering
      Maribor, Maribor, Slovenia
  • 1973-2015
    • Jožef Stefan Institute
      • • Department of Synthesis of Materials
      • • Department of Advanced Materials
      Lubliano, Ljubljana, Slovenia
  • 2012
    • EN-FIST Centre of Excellence
      Lubliano, Ljubljana, Slovenia
  • 2011
    • Institute of Mathematics, Physics and Mechanics
      Lubliano, Ljubljana, Slovenia
  • 2005
    • Kolektor Group
      Idria, Idrija, Slovenia
  • 2004
    • Martin Luther University of Halle-Wittenberg
      Halle-on-the-Saale, Saxony-Anhalt, Germany
  • 1973-1997
    • University of Ljubljana
      • Faculty of Chemistry and Chemical Technology
      Lubliano, Ljubljana, Slovenia