DTA study of an Indian Kaolinite has been performed by varying packing density and rate of heating. Both these two parameters
influence the intensities of both endothermic and exothermic peaks related to the dehydration, dehydroxylation and then crystallizations
of Al-Si spinel, mullite and cristobalite phases. Significantly, the study reveals that mullitization takes place by two separate
reactions as indicated by two exotherms in the 1200-1400C range of DTA trace.
Authors:R. Zboril, M. Mashlan, V. Papaefthymiou, and G. Hadjipanayis
The mechanism of the thermal decomposition of Fe2(SO4)3 in air has been studied at different temperatures (520-700 °C) using mainly 57Fe Mössbauer spectroscopy. Iron(III) oxides with corundum (), bixbyite (), spinel () and orthorhombic () structures were identified as solid products of this conversion. A significant influence of the heating temperature on the decomposition mechanism and on the phase composition of reaction products was found.
The thermal decomposition kinetics of nickel ferrite (NiFe2O4) precursor prepared using egg white solution route in dynamical air atmosphere was studied by means of TG with different
heating rates. The activation energy (Eα) values of one reaction process were estimated using the methods of Flynn–Wall–Ozawa (FWO) and Kissinger–Akahira–Sunose (KAS),
which were found to be consistent. The dependent activation energies on extent of conversions of the decomposition reaction
indicate “multi-step” processes. XRD, SEM and FTIR showed that the synthesized NiFe2O4 precursor after calcination at 773 K has a pure spinel phase, having particle sizes of ~54 ± 29 nm.
Authors:R. Frost, A. Musumeci, J. Kloprogge, M. Weier, M. Adebajo, and W. Martens
The thermal decompositions of hydrotalcites
with hexacyanoferrate(II) and hexacyanoferrate(III) in the interlayer have
been studied using thermogravimetry combined with mass spectrometry. X-ray
diffraction shows the hydrotalcites have a d(003)
spacing of 11.1 and 10.9 which compares with a d-spacing
of 7.9 and 7.98 for the hydrotalcite with carbonate or sulphate in
the interlayer. XRD was also used to determine the products of the thermal
decomposition. For the hydrotalcite decomposition the products were MgO, Fe2O3
and a spinel MgAl2O4. Dehydration
and dehydroxylation take place in three steps each and the loss of cyanide
ions in two steps.
Authors:S. Arriola, E. Nava, G. Aguilar, and H. Armendariz
The Mössbauer parameters were determined on a series of catalyst mixtures of iron and zinc oxides with variable quantities of zinc. From these results, a change in the crystal structure of the iron oxide when introducing zinc into the samples was observed. The corundum structure of the -Fe2O3 phase was transformed into the spinel type of zinc ferrite when zinc oxide was present in any quantity. By means of these parameters a strong electronic interaction between the zinc ferrite and the zinc oxide present in excess was evident. The catalysts were analyzed using X-ray fluorescence /XRF/ and X-ray diffraction /XRD/.
A method of detecting inaccessible metallic corrosion is proposed, in which a gamma-emitting radionuclide added in aqueous solution to the contents of a pipe or vessel attaches itself selectively to corrosion products, and then may be detected from outside using a hand-held gamma detector. This was simulated in the laboratory in preliminary studies. Corrosion products studied were oxidised iron, FeS, goethite, magnetite, spinel and oxidised stainless steel.Kd values were derived for various radionuclides. Of the radionuclides studied,60Co,152Eu,214Pb,214Bi and226Ra seem potentially useful, but not22Na or137Cs.
Authors:Z. Homonnay, P. Szilágyi, E. Kuzmann, K. Varga, Z. Németh, A. Szabó, K. Radó, J. Schunk, P. Tilky, and G. Patek
57Fe-conversion electron Mössbauer spectroscopy (CEMS) — a sensitive tool to analyze the phase composition of corrosion products
on the surface of stainless steel — was applied to study real specimens from the Paks Nuclear Power Plant, Hungary. The primary
circuit side of the heat exchanger tubes was studied on selected samples cut out from the steam generators during regular
maintenance. Mostly Cr-and Ni-substituted magnetite, amorphous Fe-oxides/oxyhydroxides as well as the signal of bulk austenitic
steel of the tubes were detected. The level of Cr-and Ni-substitution in the magnetite phase could be estimated from the Mössbauer
spectra. It is suggested that Cr-Ni substitution occurs simultaneously so that the inverse spinel structure of magnetite is
preserved up to a certain limit which appears to be roughly at [Fe3+]tet[Fe2+1/4Ni2+3/4Fe3+1/4Cr3+3/4]octO4. Further decrease of the iron content of this phase results in the formation of nickel chromite of regular spinel structure,
with very low Fe content. This transformation may be responsible for the hybrid structure of the protective oxide layer, being
substantially accelerated by previously performed, factory developed and proposed AP-CITROX decontamination cycles.
Authors:N. Padmanaban, B. N. Avasthi, and J. Ghose
Thermal studies on aluminium substituted copper chromite spinel oxide, CuCr2−xAlxO4 (x=0.2 tox=0.8) by DSC show that with 20% replacement of Cr by Al, the phase transition temperature of the spinel is lowered but on further increasing the percentage of Al the phase transition temperature becomes higher than that of CuCr2O4. The enthalpy change for phase transition however decreases gradually with increasingx.
Sol–gel auto combustion process was employed to synthesize nanosized Mn–Zn ferrite at different pH values (<1, 5, 6, 7, 8
and 10). Although self propagating combustion behavior of gel was noted at pH 5 but more effective combustion was observed
at pH 6. The smoldering effect was observed in gel prepared at pH 7, 8 and 10, whereas pH < 1 showed localized burning. Thermogravimetric
(TG) and X-ray diffraction (XRD) analyses were done to investigate the effect of pH on the combustion behavior, particle size
and the formation of desired magnetic (spinel) phase. From TG curves of burnt powders, activation energy of ignition reaction
at each pH value was calculated. The results showed that fuel to oxidant ratio and the amount of gel residuals decided the
value of activation energy required to further purify the burnt powders. Calcination parameters (time and temperature in air)
of powders P1 and P6 synthesized at pH < 1 and pH 6 were also determined. B–H loop results showed that calcined powder C6
was more ferromagnetic than C1 due to fully developed spinel phase and larger particle size.
The objective of present research was to sinter nanosized Mn–Zn ferrites (MZF) at low temperature (≤1,000 °C) by avoiding
the formation of nonmagnetic phase (hematite). For this purpose, MZF powder was synthesized by sol–gel auto combustion process
at 220 °C and further calcined at 450 °C. In calcined powder, single phase (spinel) was confirmed by X-ray diffraction analysis.
Pellets were pressed, having 43% of the theoretical density and showing 47 emu gm−1 saturation magnetization (Ms). Various combinations of heating rate, dwelling time and gaseous environment were employed to meet optimum sintering conditions
at low temperature (≤1,000 °C). It was observed that sintering under air or N2 alone had failed to prevent the formation of nonmagnetic (hematite) phase. However, hematite phase can be suppressed by retaining
the green compacts at 1,000 °C for 180 min in air then further kept for 120 min in nitrogen. Under these conditions, spinel
phase (comprising of nano crystallites), 90% of theoretical density and 102 emu gm−1 of saturation magnetization has been achieved.