[en] The change of the microstructure of hydrated burnt illitic-kaolinitic clay-lime blends as a fonction of curing time and line addition were investigated using X-ray diffraction and scanning electron microscope. The relation between physical properties - bending strenght, density and water absorption - and the operating factors were formulated using response surface methodology. It was found that floculation-agglomeration, carbonation and hydrates formation where the main happening transformations. The pozzolanic reactions essentially involved metakaolin, derived from heated kaolinite. Based on the RSM results, both factors had positive effects on the strength and their interactions were synergistic. However, they manifested opposite effects and significant antaginistic interactions on density and water absorption

[en] The solvus curve of the nickel-rich domain of nickel-hafnium phase diagram is determined by the parametric method based on the cell parameter measurements by X-ray diffraction. The maximum Hf solid solubility at the eutectic temperature is determined to be 1.3 at.%. On the other hand, it was found that the effect of Hf additions on the cell parameter of the Ni(Hf) solid solution is not predictable by the elastic theory. Furthermore, the determined enthalpy of Hf in Ni(Hf), containing more than 0.3 at.%, is around 113 kJ/mol. It is however about 17 kJ/mol for the Hf-lean Ni(Hf) solid solution. (orig.)

[en] Samples of conventionally cast nickel-2 at% hafnium alloy were aged at 505, 605 and 695 C for different times and investigated by X-ray diffraction, optical metallography, transmission electron microscopy and microhardness testing. It was found that, for annealing at 605 and 695 C, the Ni₅Hf phase precipitated firstly at grain boundaries, then formed within the matrix. In contrast, only localized precipitation was observed at 505 C. Moreover, for long ageing times and high temperatures (605 and 695 C) a small amount of ZrO developed as a result of the reaction between zirconium impurity in the hafnium and the residual oxygen in the ageing chamber. (orig.)

[en] Oxidation kinetics of (Co,Ni)CrAlY powders containing respectively 6, 8, and 10 wt.% of Al was studied between 850 and 1200 C. Powder grain size has no significant influence on oxidation kinetics of these powders. All the weigth gain curves can be interpreted according to a parabolic kinetics. Oxidized powders were examined and analyzed by XRD, SEM and EDS, cross-section metallography. At test temperatures lower than 1000 C, the main components of oxide scales are a complex spinel (Co,Ni)Cr₂O₄ and Cr₂O₃. Internal oxidation of Al occurs leading to the consumption of β-NiAl and the formation of a β-free layer of γ. At temperatures above 1000 C, the major component of oxide scales is α-Al₂O₃. Some Y-rich oxides are also detected by XRD or SEM and EDS analysis. (orig.)

[en] Kinetics and equilibrium processes of the methylene blue (MB) retention from aqueous solution by a mixture of fibrous clay minerals, which was isolated from a naturally occurring clay, were investigated. For these purposes, the effects of contact time, initial adsorbate concentration, adsorbent content, pH and ionic strength were determined. The results show that the MB retention obeys a pseudo-first order equation and the process is a diffusion controlled solid-state reaction. Moreover, the isotherm data fitted the Langmuir equation and the MB binding process became more energetic with the increase of the adsorbent concentration. In addition, the augmentation of the clay content or the initial MB concentration reduced the adsorption capacity, presumably because of the clay particles microaggregation and/or the occurrence of MB deriving species. On the other hand, it is observed that the MB uptake limit is reduced in low acid pH, particularly below the PZC, as well as in ionic strengthen solutions. These facts are linked to the silanol group protonation and to the reduction of the electrostatic forces induced by the clay particles, respectively

[en] Microstructure of vitreous ceramic samples manufactured from kaolinitic-clay and feldspars raw materials from Cameroon was investigated in the range 1150-1250 deg. C by X-ray diffraction and scanning electron microscopy and by measuring some technological properties. Moreover, the simultaneous influence of feldspars content, heating temperature and soaking time on water absorption and firing shrinkage was evaluated by adopting the response surface methodology (Doehlert matrix), using the New Efficient Methodology for Research using Optimal Design (NEMROD) software. The results show that a spinel phase, mullite, glassy phase and some amount of hematite were formed. However, the spinel phase and potassic feldspar, as compared to the sodic one, disappeared at moderate firing temperature and soaking time. Apparently, mullite developed from spinel phase, which is formed from the demixion of metakaolin. On the other hand, it is found that the effects of fluxing content and firing temperature on the measured properties were almost similar and more influent than soaking time. Antagonistic and synergetic interactions existed between the considered parameters, and their importance differed for the considered properties. By using this mathematical tool, suitable operating conditions for manufacturing vitreous bodies were determined.