Daniel Nižňanský

Preparation of ZnFe2O4/SiO2 and CdFe2O4/SiO2 nanocomposites by sol–gel method

Abstract The purpose of this paper is to characterise the phase relations in the system ZnFe2O4/SiO2 and CdFe2O4/SiO2 by dependence on the thermal treatment. Both ferrites have the structure of normal spinels. The samples were prepared by the sol–gel method, heated at a temperature of 800–1100 °C and characterised by high-resolution transmission electron microscopy (HRTEM), X-ray diffraction, and Mossbauer spectroscopy. HRTEM revealed nanocrystals of the size of about 5–10 nm, depending on the heat treatment temperature. The spinel structure of ZnFe2O4 in the amorphous silica matrix proved to be very stable up to 1100 °C without decomposition to the zinc silicate and iron (III) oxide. On the other hand, CdFe2O4 already starts to decompose at 900 °C and complete decomposition to the α-Fe2O3 and cadmium silicate can be observed at 1100 °C. This decomposition is also accompanied with the recrystallisation of the silica matrix to quartz and cristobalite.

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.

Hydrothermally grown porous FeVO4 nanorods and their integration as active material in gas-sensing devices

Controllable fabrication of highly porous iron vanadate (FeVO4) thick film consisting of disordered nanorods suitable for gas penetration and permeation was achieved by hydrothermal synthesis of fervanite-like FeVO4·1.1H2O. The subsequent dehydration to FeVO4 was investigated by 57Fe Mossbauer spectroscopy (ΔQS), DTA, magnetic susceptibility (χ) and electron microscopy (REM/TEM). Their integration in gas sensing devices as porous layer via polymer-blended (PVDF) doctor-blading approach was successfully demonstrated followed by investigations of their electric properties and oxygen sensing capability. The probed I–U behaviour and UV-Vis measurements confirmed the semiconducting nature of triclinic FeVO4 (Eg = 2.72 eV) and exhibited an activation energy for electric conduction of 0.46 eV. The best sensitivity of 0.29 ± 0.01 (m = −3.4 ± 0.1) could be obtained at an optimal working temperature of 250 °C.

Preparation of Y3Fe5O12/SiO2 nanocomposites by sol–gel method ☆: Influence of modifiers

Abstract This paper aims to characterise the phase relations in the system Y 3 Fe 5 O 12 /SiO 2 prepared by sol–gel method. Samples were characterised by X-ray diffraction, magnetic measurements and Mossbauer spectroscopy. The final phase composition strongly depended on preparation conditions in the early stages of preparation. The samples treated in a vacuum, which were subsequently heated up to 1000°C, did not contain hematite (the most stable phase at higher temperatures) for the case of gels modified by formamide and only a small amount in the case without formamide. The samples heated in air displayed high content of hematite.

Frequency-dependent susceptibility and other magnetic properties of Celtic and Mediaeval graphitic pottery from Bohemia: an introductory study

Frequency-dependent magnetic susceptibility, its anisotropy (AMS), its temperature variation, natural remanent magnetization and time-dependent isothermal remanent magnetization as well as Mössbauer spectroscopy of a small collection of Celtic and Mediaeval graphitic pottery from Southern Bohemia were investigated. The mineral composition of the pottery is dominated by fragments of quartz, accompanied mainly by various silicates from granitoids and paragneisses, or by calcite, within the plastic component being probably illite but also graphite. No ferrimagnetic minerals were found in optical microscope, among Fe-oxides only limonite was observed, even though the bulk susceptibility of the pottery varies in the orders of 10−4 to 10−2 [SI]. This may indicate presence of ferromagnetic particles in the ultrafine (superparamagnetic, SP) state, which is confirmed by frequency-dependent susceptibility ranging from 3% to almost 16%. The low temperature susceptibility vs. temperature curves are only moderately sloped, showing the Verwey transition only in one case. The high temperature curves mostly show presence of two magnetic phases, maghemite and magnetite. Cooling curves show distinctly lower susceptibilities than the heating curves indicating instability of the assemblage of ferrimagnetic minerals, particularly in temperatures slightly under 700 °C. Mössbauer spectroscopy confirmed the results of the frequency-dependent susceptibility, showing the increase of ferrimagnetic sextets in the spectra measured at 4.2K, likely indicating maghemite as the distinct ferrimagnetic phase. The frequency-dependent AMS indicates preferred orientation of SP1,16 particles, coaxiality between SP1,16 grain AMS and whole specimen AMS indicate that all grains, ultrafine and coarser ones, were oriented by the same process, i.e. copying the pottery structure created during wheel-turning.

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.

Hyperfine interaction of 155Gd in gadolinium iron garnet

Abstract The NMR spectrum of the polycrystalline sample of gadolinium iron garnet enriched by 155Gd isotope (91.6%) was detected. The interpretation of this spectrum consisting of 12 lines gives the values of electric field gradient (EFG) tensor, isotropic part and anisotropic components of hyperfine field on 155Gd nuclei. NMR measurements were accompanied by the ab initio calculations of the EFG tensor.

Absolute up- and down-conversion luminescence efficiency in hexagonal Na(Lu/Y/Gd)F4: Yb, Er/Tm/Ho with optimized chemical composition

Up- and down-conversion luminescence efficiency of hexagonal NaYF4, NaLuF4 and NaGdF4, prepared by a novel hydrothermal method using succinic acid and codoped with Yb and Er/Tm/Ho, was investigated over a broad spectral range 350 – 1640 nm. For this synthesis method NaLuF4 matrix provided the best up-conversion efficiency, while NaYF4 and NaGdF4 give about twice and eight-times reduced efficiency, respectively. We systematically varied dopant concentrations of Yb3+ and Er3+/Tm3+/Ho3+ in order to find the optimum composition. The best performing material was found to be NaLuF4: Yb3+ 15%, Er3+ 1.5%, for which we performed detailed study of absolute luminescence power efficiency under different excitation power using an integrating sphere approach. The integrated up-converted signal reaches about 17% in the excitation power range of 70-100 W/cm2, while the down-converted emission power efficiency is about 10%. An important fraction of absorbed power is (re)emitted close to the excitation wavelength. This close-to-resonance emission represents 2–3 (anti-Stokes) and 10–30% (Stokes shifted) of absorbed power depending weakly on the excitation power. In order to prove Yb3+ luminescence-related origin of this loss channel we exploited the time-resolved luminescence kinetics using modulated laser beam (both 405 and 978 nm), which allows also determination of the absorption cross section in the range 1 – 3 × 10-21 cm2 at 978 nm matching the published values for Yb3+ ion. The determined efficiency parameters are comparable to the best values reported so far and we discuss the way to further increase the efficiency limit.

Hyperfine interactions in lutetium iron garnet

Nuclear Magnetic Resonance/Near Quadupole Resonace specra of Lu175 in lutetium iron garnet (LuIG) were measured in the range of 10–500MHz in zero external magnetic field at a temperature of 4.2K. Experimentally observed spectra had complicated structure and a theory was needed to interpret them. To this end we calculated the electronic structure of LuIG and from it the values of magnetic hyperfine fields and the components of electric-field-gradient tensor at the lutetium nuclei were determined. These parameters were used to simulate the theoretical spectra of Lu175 in LuIG. Simulated spectral lines of Lu175 at dodecahedral sublattice correspond reasonably well with the system of measured lines in the range of 10–200MHz. Several spectral lines in the range of 300–500MHz can be interpreted as the resonance of Lu175 at the octahedral sites that are nominally occupied by the ferric cations.