RESUMO
In the present study two sets of nanocomposites consisting of an epoxy resin and BaFe12O19 or SrFe12O19 nanoparticles were successfully developed and characterized morphologically and structurally via scanning electron microscopy and X-ray diffraction spectra. The dielectric response of the nanocomposites was investigated by means of broadband dielectric spectroscopy and their magnetic properties were derived from magnetization tests. Experimental data imply that the incorporation of the ceramic nanoparticles enhances significantly the dielectric properties of the examined systems and their ability to store electrical energy. Dielectric spectra of all systems revealed the presence of three distinct relaxation mechanisms, which are attributed both to the polymer matrix and the nanoinclusions: Interfacial polarization, glass to rubber transition of the polymer matrix and the re-orientation of small polar side groups of the polymer chain. The magnetic measurements confirmed the ferromagnetic nature of the nanocomposites. The induced magnetic properties increase with the inclusion of hexaferrite nanoparticles. The nanocomposites with SrFe12O19 nanoparticles exhibit higher values of coercive field, magnetization, magnetic saturation and remanence magnetization. A magnetic transition was detected in the ZFC/FC curves in the case of the BaFe12O19/epoxy nanocomposites.
RESUMO
Self-organized porous TiO(2) nanotubes (NTs) were prepared on conductive glass by galvanostatic anodizing of sputtered titanium in an NH(4)F /glycerol electrolyte. DC magnetron sputtering at an elevated substrate temperature (500 degrees C) was used to deposit 650 nm thick titanium films. After anodizing, NTs, 830 nm long, with an average external diameter of 92 nm, were grown; this gave a high conversion rate of oxide from titanium (1.9), with a 220 nm thick layer of titanium, which was not oxidized, located at the base of the tubes. The NTs revealed a mainly amorphous structure, which transformed mostly to anatase upon thermal treatment in air at 450 degrees C. The tubes were sensitized by the N719 complex and the resultant photoelectrodes were incorporated into liquid dye solar cells (DSCs) and further tested under back-side illumination. High values of V(oc) (714 mV) were obtained under 1 sun (AM 1.5), assigned to low dark current magnitude and large recombination resistance and electron lifetime. In addition, typical values of fill factors (of the order of 0.62) were attained, in agreement with the estimated ohmic resistance of the cells in combination with low electron transfer resistance at the platinum/electrolyte interface. The overall moderate power conversion efficiency (of the order of 0.3%) was mainly due to the low short-circuit photocurrents (J(sc) = 0.68 mA cm(-2)), which was confirmed further by the corresponding IPCE values (5.2% at 510 nm). The magnitude of J(sc) was attributed to absorbed light losses due to back-side illumination of the cells, the low dye loading (due to the limited thickness of anodic titania) and the high charge transfer resistance at the TiO(2)/conductive substrate due to the presence of barrier layer(s) underneath the tubes. These preliminary results encourage the DSC community to explore further the galvanostatic anodizing of titanium in order to produce highly efficient porous TiO(2) NTs directly on conductive glass. Current work is focusing on achieving complete anodizing of the metal substrate and full transparency for the photoelectrode in order to increase and optimize the resultant cell efficiencies.
