Authors:C. U. Aniz, P. P. Robinson, K. K. Abdul Rashid, and T. D. Radhakrishnan Nair
Ferrite spinels are important catalysts for various chemical reactions like the water gas shift reaction, carbon monoxide (CO) oxidation, alkylation reactions etc. [ 1 – 3 ] due to their structural and redox
Authors:Marcela Stoia, Paul Barvinschi, Lucian Barbu Tudoran, Mirela Barbu, and Mircea Stefanescu
Development of spinel ferrite nanoparticles has been intensively pursued because of their technological and fundamental scientific importance. Ferrites have received great attention as a result of their magnetic
Authors:A. P. Surzhikov, A. M. Pritulov, E. N. Lysenko, A. N. Sokolovskii, V. A. Vlasov, and E. A. Vasendina
–zinc ferrite phases with x Zn = 0, 0.2, 0.4, 0.6, and 0.8 was included in the program of full-profile analysis. The given set reflected quasi-continuous distribution of Li 0.5(1– x ) Zn x Fe 2.5–0.5 x O 4 type spinel phases that could be formed in different
Key subjects related to the present status of mantle convection theory are reviewed in this paper. Rheology of the polycrystalline mantle material is known from laboratory experiments. Diffusion and dislocation creeps must predominate in the long-term deformation of the mantle at high temperatures; their effective viscosities can be estimated from measured creep parameters. Inhomogeneities in the chemical and phase composition of the mantle can influence the convective pattern considerably. Perhaps the most significant heterogeneities in this respect are those produced by the phase changes of the transition zone. The endothermic spinel-perovskite transition at 660 km depth can create an efficient obstacle to vertical flow. Available seismic tomographic images show clear signs of such an obstacle in some subduction zones, at other places however the downwellings seem to be unimpeded. The exact degree of flow layering caused by the 660 km discontinuity is not known, but some sort of flow stratification is strongly suggested by the isotopic and trace element geochemistry which shows that different chemical reservoirs must exist in the mantle on the geological time scale. Equations and the main governing forces are analyzed, and the basic structures of the convective flow are demonstrated in examples of numerical solutions. In particular, recent modelling results are discussed with regard to the plume forms allowed by a semi-permeable internal phase boundary. It is shown that three different kinds of plumes can reach the surface and produce hotspots: those arising from the internal phase boundary, those coming from the basal boundary layer of the mantle, and a recently described new plume type which breaks through the phase transition starting from a diffuse volume below the transition zone.
Authors:A. A. Said, E. A. Hassan, K. M. Abd El-Salaam, and M. M. Mohamed
Thermal decomposition of pure basic zinc carbonate (BZC) and doped or mixed with iron ions were studied using TG, DTA and kinetics of isothermal decomposition. The TG and DTA investigations revealed that, the presence of iron ions retards the decomposition processes of (BZC). Also, the retardation effect increases on increasing of iron concentration up to 50 at.%. The curves of isothermal decomposition show the usual sigmoidal character. The decomposition velocity contsant (K) values are plotted vs. 1/T according to Arrhenius equation gave a plot of good straight lines with activation energies of 43.7, 48.2, 53.2 and 57.1 kJ mol−1 for pure (BZC) and incorporated with 1, 10, 30 and 50 at.% Fe2+ respectively. The products of the thermal decomposition of pure BZC and mixed with iron ions are characterized using X-ray diffraction, IR spectroscopy, surface area determination and the surface porosity. These investigations showed that iron ions are effectively incorporated into zinc oxide lattice in the range of 30–50 at.%, which gave a ZnFe2O4 spinel.
Authors:Lucjan Chmielarz, Małgorzata Rutkowska, Piotr Kuśtrowski, Marek Drozdek, Zofia Piwowarska, Barbara Dudek, Roman Dziembaj, and Marek Michalik
high surface mixed metal oxides, with a high dispersion of introduced transition metals oxide species [ 2 – 6 ]. An increase in the calcination temperature results in the aggregation of these species and formation of the spinel phases [ 2 – 6
Starting from oxalate mixed crystals NixMn3−x(C2O4)3·6H2O thermal decomposition at reduced oxygen partial pressure (po2= 2%) leads to the formation of NiMn2O4 (x = 1) at metastable conditions. Ni1.5Mn1.5O4 (x = 1.5) existing in the metastable state only has been also prepared. The spinel compounds both are of the highly inversed type. Following a sol-gel preparation route Mg2TiO4 has been also found to be formed in the metastable state. Annealing results in decomposition of the compounds providing NiMnO3 and 1/2α-Mn2O3 or NiMnO3 only or MgTiO3 and MgO, respectively. The reaction rates observed are lower for NiMn2O4 and Ni1.5Mn1.5O4 than for Mg2TiO4 decomposition. The reverse reaction of NiMn2O4 formation above 730°C shows an endothermic enthalpy of +61 kJ·mol−1. For Mg2TiO4 formation above 1050°C an endothermic enthalpy of +19.3 kJ·mol−1 is found. The results are discussed in terms of structural features of the oxides.
The morphology of kaolinite thermally processed to 1600° in a thermoanalyzer was studied and related to DTA/TG/DTG data. The results show that DTA/TG/DTG data refined with scanning electron microscopic information offers insight into some of the controversies concerning the kaolinite-mullite transformation. In particular, the study supports the hypothesis that a spinel phase forms in the 950–1000° region. Further, the study shows that during dehyroxylation in the 450–700° region, water escapes by a process opposite to that generally supposed.
Authors:A. P. Surzhikov, T. S. Frangulyan, S. A. Ghyngazov, and E. N. Lysenko
Lithium pentaferrite (LPF) (LiFe 5 O 8 or Li 0.5 Fe 2.5 O 4 ) is the simplest basic material for a variety of chemical compositions of lithium ferrospinels. LPF has the structure of inverted spinel. Its crystal