Kinetics of oxidation of Fe-Cr steel containing 25 wt.-percent Cr was studied as a function of temperature (1023–1173 K) for
up to 480 h in flowing air, which corresponds to SOFC cathode environment operating conditions. The oxidation process was
found to be a parabolic, suggesting that the diffusion of ionic defects in the scale is the slowest, rate determining step
and it occurs predominantly by short-circuit diffusion paths. Comparison of the determined activation energy of oxidation
of the studied steel with literature data indicates that at 1098–1173 K the chromia scale grows by the outward solid-state
diffusion of chromium interstitials, whereas at 1023–1098 K — through a significant contribution of counter-current oxygen/chromium
diffusion along Cr2O3 grain boundaries. The oxide scales were composed mainly of Cr2O3 with a continuous thin Mn1.5Cr1.5O4 spinel layer on top of the chromia scale. The oxidation test results on Fe-25Cr steel demonstrate the applicability of the
commercial type DIN 50049 stainless steel as interconnect for SOFC.
Thermal analysis complimented with evolved gas mass spectrometry has been applied to hydrotalcites containing carbonate prepared
by coprecipitation and with varying divalent/trivalent cation ratios. The resulting materials were characterised by XRD, and
TG/DTG to determine the stability of the hydrotalcites synthesised. Hydrotalcites of formula Mg4(Fe,Al)2(OH)12(CO3)·4H2O, Mg6(Fe,Al)2(OH)16(CO3)·5H2O, and Mg8(Fe,Al)2(OH)20(CO3)·8H2O were formed by intercalation with the carbonate anion as a function of the divalent/trivalent cationic ratio.
XRD showed slight variations in the d-spacing between the hydrotalcites. The thermal decomposition of carbonate hydrotalcites
consists of two decomposition steps between 300 and 400°C, attributed to the simultaneous dehydroxylation and decarbonation
of the hydrotalcite lattice. Water loss ascribed to dehydroxylation occurs in two decomposition steps, where the first step
is due to the partial dehydroxylation of the lattice, while the second step is due to the loss of water interacting with the
interlayer anions. Dehydroxylation results in the collapse of the hydrotalcite structure to that of its corresponding metal
oxides and spinels, including MgO, MgAl2O4, and MgFeAlO4.
A series of samples in the system Ni0.65Zn0.35CuxFe2−xO4 (x=0.0, 0.1, 0.2, 0.3, 0.4 and 0.5) were prepared by the usual ceramic technique. X-ray analysis showed that they were cubic spinel (single phase). Young's modulus, the dielectric loss and the change in capacitance under mechanical stress were measured for the samples. Young's modulus decreased with increasing Cu content. This is due to the fact that Cu2+ ions entered the lattice substitutionally for Fe3+ ions at the octahedral sites, creating lattice vacancies gave rise to lattice strain.
Kinetics of the oxidation of magnetite (Fe3O4) to hematite (a-Fe2O3) are studied in air using simultaneous TG/DSC. The mechanism is complex and the differences between the kinetic conclusions
and Arrhenius parameters based on either TG or DSC are discussed. As in our previous work on CaCO3 , the determination of a satisfactory baseline for the DSC results adds considerable uncertainty to those kinetic results.
Consequently the calculations based on the TG data are considered superior. Solid state reactivity varies from one source
of material to another and the results are compared for two different commercial samples of magnetite, both presumably prepared
by wet chemical methods. These materials are much more reactive than the material studied previously , which had been coarsened
and refined at high temperatures. In that earlier study, the metastable spinel, g-Fe2O3, was formed as an intermediate in the oxidation to the final stable form, a-Fe2O3. The exothermic reaction of the gamma to alpha form of the product during the oxidation process destroys the direct comparison
between the TG and DSC results, since the former only detects the change in mass of the sample and not the crystallographic
transformation. The TG results, however, represent the true oxidation process without superposition of the structural aspects.
Authors:M. Jiménez de Haro, L. Pérez Maqueda, E. Stepkowska, J. Ma Martínez, and J. Pérez-Rodríguez
Grinding and contact with water or salt solution increased the specific surface (ssa) but lowered the first dehydration effect
(escaping up to 150C) and increased the second dehydration effect (150 to 500C). The dehydroxylation was moved to lower
temperatures and was only ΔM(500-1100C)=3.70.3 % as compared to 5.5% in the parent vermiculite (V). Except ΔM(20-150C),
the mass losses measured at the remaining T ranges, were consistent in the ground samples, thus the grinding for 2 min caused
the homogenization of the crystal structure of vermiculite [ΔM(150-500C)=7.60.7%]. DTA curves after grinding and cation
exchange indicate an important exothermal peak at 795-870C, its temperature depending on exchangeable cation. It indicates
the formation of high temperature phases (enstatite, forsterite, spinel). The lowest temperature of the peak (795C) was observed
in V-gr-Li, here lithium silicate was formed. The highest peak temperature (870C) was found in V-gr-K, where almost only
forsterite developed. These exothermal peaks were very weak in unground V with various exchangeable cations.
Authors:D. Bhosale, N. Choudhari, S. Sawant, V. Patil, P. Kulkarni, and V. Kelkar
Homogeneous solid solution oxalates of Fe2+, Cu2+, Mg2+ and Zn2+ metals were prepared by co-precipitation from respective metal acetate solutions with oxalic acid solution. The thermogravimetric
(TG) analysis of co-precipitated oxalate complexes with general formula MgxCu(0.50-x)Zn0.50Fe2(C2O4)3nH2O (x=0.00, 0.05, 0.10, 0.15, 0.20, 0.25, 0.30, 0.35, 0.40, 0.45, 0.50) were carried out by manual method in static air atmosphere.
The total mass loss % and stepwise mass loss % values are in good agreement with theoretically calculated mass loss % values.
The thermal decomposition of oxalate complexes occur at relatively lower temperatures (561 to 698 K). The lowering of decomposition
temperatures may be attributed to earlier initiation of Fe2+ oxalate in oxalate complexes.
At temperatures between 598–698 K the thermal decomposition of Cu-Mg-Zn-Fe solid solution oxalate complexes leads to formation
of ferrites of spinel structure. After tampering at 873 and 1273 K, homogeneous ferrites arise, which is revealed from XRD
Comparative thermogravimetric and heat-flux DSC investigations of phase formations by heating of sinteractive powders, which were prepared by thermal decomposition of a NiCO3∶ MnCO3=1∶2 mixture and thermal decomposition of oxalate mixed crystals NiMn2(C2O4)3.6H2O, show the metastability of the defect spinel from the oxalate precursor and its high reactivity.
Authors:R. Marinkovic-Neducin, E. Kiss, T. Cukic, and D. Obadovic
The purified bentonite parent clay, fraction ≤; 2 mm of montmorillonite type, has been pillared by various polyhydroxy cations,
Al, AlFe and AlCu, using conventional pillaring methods. The thermal behavior of PILCs was investigated by combination of
X-ray diffraction (XRD), thermal analysis (DTA, TG) and low temperature N2 adsorption/desorption (LTNA). Thermal stability
of Al-, AlFe- and AlCu-PILC samples was estimated after isothermal pretreatment in static air on the temperatures 300, 500,
600 and 900C. Crucial structural changes were not registered up to 600C, but the fine changes in interlayer surrounding
and porous/microporous structure being obvious at lower temperatures, depending on the nature of the second pillaring ion.
AlFe-PILC showed higher thermal stability of the texture, the AlCu-PILC having lower values and lower thermal stability concerning
both overall texture and micropore surface and volume. Poorer thermal stability of AlCu-PILC sample at higher temperatures
was confirmed, the presence of Cu in the system contributing to complete destruction of aluminum silicate structure, by 'extracting'
aluminum in stabile spinel form.
Authors:V. Š. Fajnor, H. Gerthofferová, Ľ. Kuchta, and J. Masár
The high-temperature (950–1500°) changes in synthetic montmorillonite of relatively simple chemical composition, studied by X-ray diffraction analysis, infrared spectroscopy and electron microscopy, are described. It was found that this montmorillonite belongs to the Wyoming type and the high-temperature phases involve cristobalite, mullite, anorthite and spinel. Only mullite crystallized from this sample on heating for two hours at 1500°.
Authors:K. Nomura, S. Kobayashi, K. Hashimoto, Ts. Sawada, Z. Homonnay, and A. Vértes
The CO2 absorption properties and the microstructure of (Ba,Ca)(Fe,Mg)O3- have been studied by TGA, XRD, and Mössbauer spectrometry. Paramagnetic doublets of FeIV and FeIII appeared in the Mössbauer spectra of cubic (Ba0.5Ca0.5)(Fe0.5Mg0.5)O3- heated in CO2 up to 600 °C, and a pair of sextets of tetrahedral FeIII (Hin = 43 T) and octahedral FeIII (Hin = 51 T) were produced above 800 °C, and an additional sextet characteristic of FeIII in a spinel structure (Hin = 48 T) was observed at 1000 °C. On the other hand, a pair of sextets of tetrahedral and octahedral FeIII of the orthorhombic (Ca0.95Ba0.05)(Fe0.5Mg0.5)O3- showed hardly any change after absorption of CO2. It is concluded that only a small portion of Mg entered the orthorhombic phase of (Ca0.95Ba0.05)(Fe,Mg)O3- and Mg preferred the octahedral B site of the perovskite lattice. The excess Mg formed separate CaO-MgO mixed oxide, and the primary mechanism of CO2-trapping is the formation of CaMg(CO3)2.