Papers by Author: Darko Makovec

Abstract: Conditions for the formation of (1-x)BaTiO₃–xLi_0.5Bi_0.5TiO₃ (0 ≤ x ≤0.6) solid solutions with positive temperature coefficient of resistance (PTCR) effect were studied. Solid solutions were prepared by solid state reaction technique. Samples were sintered under reducing atmosphere N₂/H₂ in the temperature range 1200–1450 °C with subsequent oxidation in air. The phase composition was investigated by X-ray powder diffraction method. It was found that samples of (1-x)BaTiO₃–xLi_0.5Bi_0.5TiO₃ (0 ≤ x ≤0.6) solid solutions at room temperature exhibit perovskite structure. Unit cell parameters of unstable at the room temperature compound Li_0.5Bi_0.5TiO₃ were determined by extrapolation of concentration dependence of the unit cell parameters in the (1-x)BaTiO₃–xLi_0.5Bi_0.5TiO₃ system. It was shown that minimum value of resistivity ρ_min rises with increase in x value. Complex impedance method shown that ceramic grains of (1-x)BaTiO₃–xLi_0.5Bi_0.5TiO₃ materials consist of three areas with different electrical properties. Boundary and outerlayer region of grains make the main contribution to the PTCR effect in lithium-containing solid solutions. It was shown that magnitude of the potential barrier's decreases with increasing x.

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: 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.