J. Plocek

Magnetism of sol-gel fabricated CoFe2O4∕SiO2 nanocomposites

Details of synthesis and characterization of sol-gel-produced CoFe2O4 nanoparticles embedded in the amorphous SiO2 matrix are presented together with results of an extended magnetization study of these materials. The particle size was found to increase from 6to15nm by varying the temperature of a subsequent annealing from 800to1100°C. All samples exhibited superparamagnetic behavior with values of the blocking temperature TB increasing with the particle size. At temperatures above TB the magnetization curves follow the expected Langevin scaling of M vs H∕T, which is consistent with the formation of the superparamagnetic state. For T<TB, the coercivity field Hc was found to be proportional to T and the frequency-dependent ac susceptibility was found to obey the Neel–Arrhenius law. Both observations are compatible with a model of noninteracting randomly oriented single-domain particles.

Preparation and Characterization of the Nanocomposites SiO2/H-Bond Hydrogensulphate (Hydrogenselenate)

The nanocomposites representing interesting H-bond compounds (Na3H(SO4)2, RbHSO4, RbHSeO4, and Rb3H(SeO4)2) in the silica matrix were prepared by the sol-gel method. Resulting samples were characterised by X-ray diffraction, IR spectroscopy and electron microscopy. Transition temperatures of the nanocomposites were determined using DSC and compared with the TC of the pure bulk active compounds. The shifts of the TC towards lower temperatures or the disappearing of the certain effects in the case nanocomposites were observed. A possible explanation of these phenomena was proposed using the analogy with the superparamagnetic compounds.

Preparation of KHSO4/SiO2 Nanocomposites and Their Physical Properties

A series of SiO2/KHSO4 nanocomposites with various SiO2/salt ratios was prepared where the active compound was added before gelation. The sol was prepared by mixing of these hydrogen salts, TEOS (tetraethyl orthosilicate) and water. After gelation and heat treatment (heating slowly to 200–220°C under vacuum), the samples were characterized by X-ray diffraction, Differential Scanning Calorimetry (DSC), IR spectroscopy, Scanning Electron Microscopy (SEM), and High Resolution Transition Electron Microscopy (HR TEM). DSC measurements showed phase transition temperature shifts that depended on the SiO2/salt ratio. The properties of the nanocomposite samples were compared with the bulk materials. The shift in the phase transitions to lower temperatures was attributed to the particle size effect.