Papers by Author: Miha Drofenik

Abstract: Stable suspensions of nanosized barium hexaferrite particles were prepared with the addition of surfactants. Hydrothermally prepared particles with sizes from 5 to 20 nm and commercial particles with sizes from 10 to 160 nm were used. The films with thicknesses from 1.5 to 9.5 μm were prepared with electrophoretic deposition in an external magnetic field. The orientation of the grains in the films was due to the magneto-crystalline anisotropy of the barium hexaferrite and was determined with X-ray diffraction analysis, scanning electron microscopy and magnetic measurements. The highest degree of orientation in the films with anisotropic magnetic behaviour was 90 %.

Abstract: Nanoparticles of barium hexaferrite were prepared by controlled hydrothermal synthesis at 280oC. During the synthesis, the composition of the precursor and the suspension concentration were controlled. The as-synthesized crystalline barium hexaferrite platelets of about 50 nm in length and 5 nm in thickness exhibited a saturation magnetization of 40 emu/g.

Abstract: The trend for integrating and miniaturizing electronic devices requires the co-firing of ferrites and Ag electrodes at 900°C. Here we report on compatibility studies using Z- and Y-type BaCo hexaferrites, which are interesting for microwave applications. Samples with compositions BaB3BCoB1.4BCuB0.6BFeB24BOB41 Band BaB2BCoB0.6BCuB1.4BFeB12BOB22 Bwere prepared with solid-state reactions at 1250°C and 1000°C, respectively. For compatibility tests, samples with additions of 0-25 wt.% of Ag were sintered at 900°C for 3h. The ceramics were characterized with X-ray powder diffraction and electron microscopy combined with energy-dispersive analysis. When co-sintered with Ag the hexaferrites decomposed to BaFeB12BOB19B and (Co,Cu)FeB2BOB4B or BaFeB2BOB4B. The incorporation of Ag in the spinel phase was also detected. The degree of decomposition increased with the amount of Ag and finally resulted in the complete disintegration of the samples with minimum 23 wt.% of Ag. The influence of the observed phase-composition changes on the electromagnetic properties and their dependence on frequency was evaluated.

Abstract: In this paper the adsorption and oxidation of CO on nanocrystalline Au/γ-Fe2O3, Au/α- Fe2O3, γ-Fe2O3 and a series of mixed ferrite with MnxZn1-xFe2O4 composition are investigated. The catalysts were prepared by co-precipitation method and characterized using X-ray diffraction (XRPD), specific surface (BET), transition electron spectroscopy (TEM) and magnetization measurements. A temperature programmed desorption (TPD) study of the water and CO interaction with nanocrystalline adsorbents is presented. The catalytic activities towards CO oxidation increased in the following order: γ-Fe2O3 < MnxZn1-xFe2O4 < Au/γ-Fe2O3 .

Abstract: Sonochemically assisted co-precipitation has been used to prepare nanosized manganesezinc- ferrite powder. A suspension of constituent hydroxides was ultrasonically irradiated for various times with high-intensity ultrasound radiation (20 kHz, 750 W) using a direct-immersion titanium horn. The average grain size and magnetization of the synthesized MnZn-ferrite nanoparticles gradually increases with the time of ultrasonic irradiation.

Abstract: Investigations of the influence of synthesis procedures and LaSr-manganite stoichiometries on the Curie point and other magnetic characteristics of the material, being the topic of this work, are attractive since they present a necessary primal step in the intended development of nano-magnetic drug carriers based on these materials. In the course of such an investigation, differences in the mechanisms of the formation of desired manganite compounds by following a classical solid-state preparation method, depending on various stoichiometries used, were noticed, with the reaction pathways thereof outlined onward.

Abstract: A water-in-oil microemulsion method was used for the preparation of silica-coated permalloy (Ni0.80Fe0.20) nanoparticles. The particles were prepared by the reduction of their salts with sodium borohydride NaBH4 in a cationic water-in-oil (w/o) microemulsion of water/cetyltrimethyl- amonium bromide (CTAB) and n-butanol/isooctane at 25oC. According to the TEM and x-ray diffraction analyses, the synthesized material was in the form of nanoparticles. The magnetization of the silica-coated nanoparticles was lower than that of the uncoated particles, reflecting the influence of the nonmagnetic silica surface layer on the magnetization.

Abstract: Nanostructured lanthanum-strontium manganites have been synthesized using two different co-precipitation approaches, one in bulk solution, and the other in reverse micelles of CTAB/1-hexanol/1-butanol/water microemulsion. In both cases, precursor cations were precipitated by alkali precipitating agents. The properties of the material synthesized by using these two methods were compared in order to reveal potential advantages of microemulsion-assisted approach. The influence of the annealing conditions on the properties of synthesized manganites was investigated by using X-ray diffraction, transmission electron microscopy, differential thermal analysis, thermogravimetric analysis and magnetic measurements.

Abstract: Magnetic nanoparticles are materials of great interest because of the remarkable fundamental properties exhibited by these materials as well as their technological potential in the area of biomedicine and other areas. The technologically useful properties of magnetic nanomaterials are not limited to their structural, chemical or mechanical behaviour, but also involve the phenomena that arise from their finite size and the surface effects that dominate the magnetic behaviour of individual nanoparticles. New techniques that have been developed recently have permitted researchers to produce larger quantities of nanomaterials and characterize them better. Here, some of the most promising procedures are reported, including techniques based on hydrothermal syntheses, sol-gel syntheses, co-precipitation syntheses, mechanochemical syntheses, sonochemical syntheses, spray pyrolysis and microemulsion-assisted syntheses.

Abstract: MnZn-ferrite nanoparticles with a narrow size distribution were prepared in situ in water-CTAB-hexanol microemulsions using a two-step process: the precipitation of the corresponding hydroxides, followed by the oxidation of the Fe2+. The influence of the temperature and the time of the precipitation step was systematically studied.