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  • Author or Editor: R. Lazǎu x
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Abstract  

The crystallization mechanism of the glass-ceramics obtained from Romanian (Şanoviţa) basalt in the presence of 3 and 5% CaF2 as nucleation agent has been investigated under non-isothermal conditions using DTA technique. The activation energies of the crystallization processes were calculated using the Kissinger-Akahira-Sunose, Ozawa-Flynn-Wall, Starink and Tang isoconversional methods. The monotonous decreases in the activation energy (E a) with the crystallized fraction (α) confirms the complex mechanism of the glass-ceramics crystallization process. It has been proved that the Johnson-Mehl-Avrami model cannot be applied for the studied glass-ceramics crystallization process.

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Abstract  

The influence of the specific surface area on the crystallization processes of two silica gels with different specific surface areas has been investigated in non-isothermal conditions using DTA technique. The activation energies of the crystallization processes were calculated using four isoconversional methods: Ozawa-Flynn-Wall, Kissinger-Akahira-Sunose, Starink and Tang. It has been established that, the decrease of the surface area from S=252.62 m2 g−1, in the case of sample GS2, to S=2.52 m2 g−1, in the case of sample GS1, has determined a slight increase of the activation energy of the crystallization process of the gels. Regardless of the isoconversional method used, the activation energy (E α) decreases monotonously with the crystallized fraction (α), which confirms the complex mechanism of gels crystallization. It has been proved that the Johnson-Mehl-Avrami model cannot be applied for the crystallization processes of the studied silica gels.

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Abstract  

The crystallization kinetics of some glass-ceramics obtained from Romanian (Şanoviţa) basalt has been studied in non-isothermal conditions using DTA technique. The activation energies of the crystallization processes were calculated using the isoconversional methods Kissinger-Akahira-Sunose and Ozawa-Flynn-Wall. The results obtained show a dependence of the activation energy (E α) on the crystallized fraction (α) that proves the complex mechanism of the glass-ceramics crystallization process. It has been proved that the Johnson-Mehl-Avrami model cannot be applied for the studied glass-ceramics crystallization process. The effect of 2% TiO2 as nucleating agent upon the crystallization kinetics and upon the microstructure of the studied glass-ceramics was analyzed.

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Abstract  

A kinetic study of the crystallization processes was performed for some decorative ceramic glazes in the PbO-SiO2-Na2O-K2O-CaO(BaO)-Al2O3-B2O3 system with addition of 10% TiO2 and ZnO. The crystallization kinetics has been studied in non-isothermal conditions using DTA technique. The apparent activation energies of the crystallization processes were calculated using the Kissinger method. The main crystalline phase, which provides the decorative effect, is rutile. This has been identified by X-ray diffraction and it is clearly visible in the optical microscopy images taken in transmitted light, as needle-like or even prismatic crystals arranged in radial-fibrous aggregates.

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Abstract  

The paper presents the kinetic study of the crystallization processes which take place in basalt glasses containing variable amount of nucleation agent (CaF2, 310%). The activation energies have been calculated using Kissinger's equation and verified with the Ozawa-Flynn-Wall equation. In this order, the DTA curves have been registered with different heating rates, between 4 and 20 degrees/min. A correlation equation between the activation energy and the amount of nucleation agent (% of CaF2) was established. By X-ray diffraction it was proved that the crystalline phase formed in the crystallization process represents a pyroxenic solid solution, Ca(Mg,Fe)SiO3.

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Abstract

Single-phase Ca3Al2O6 was prepared via polymeric precursor method. The influence of the reactants nature in the Ca3Al2O6 synthesis was investigated. For this purpose, citric acid and soluble salts of calcium (nitrate, chloride, carbonate) and aluminium (nitrate, chloride, acetate) were used as starting materials, in the presence and, respectively, in the absence of ethylene glycol. Ca3Al2O6 resulted as single-phase after annealing at 1050 °C for 1 h only starting from calcium nitrate or carbonate and aluminium nitrate or acetate as salts precursor for Ca2+ and Al3+ cations. The formation of Ca3Al2O6 is not conditioned by the ethylene glycol presence in these mixtures. Using calcium and aluminium chlorides, the phases present at 1050 °C are Ca12Al14O33 and unreacted CaO.

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Abstract

CaAl12O19 was synthesised using three different precursors: (a) a polyesteric type precursor resulted from the traditional Pechini method; (b) a polyesteric type precursor resulted from the reaction between citric acid and calcium and aluminum nitrates; and (c) a polymeric type precursor resulted from the reaction between acrylic acid and calcium an aluminum nitrates. The thermal behavior of the three precursors used in the CaAl12O19 synthesis was monitored to underline the thermal effects associated to the CaAl12O19 formation. Thermal analyses performed on precursors do not reveal clear differences regarding the thermal effects assigned to calcium aluminates formation, at temperatures over 800 °C. In contrast, thermal analysis of samples pre-fired at 200 °C, and especially at 600 °C, show clear differences between samples obtained in different ways. It is noted that in samples obtained from acrylic acid and nitrates, and citric acid and nitrates, CA6 is practically single phase after calcination at 1,200 °C. However, in the sample obtained from citric acid, ethylene glycol, and nitrates, calcined at 1,200 °C, CA6 is present along with CA2 and α-Al2O3.

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Abstract  

The synthesis of strontium-doped lanthanum chromite, La1−xSrxCrO3 (x=0.1 and 0.3), used as an interconnect material for solid oxide fuel cells (SOFC), was investigated using two unconventional synthesis methods: (1) organic precursors’ method based on the thermal conversion of complex combination resulted in the oxidation reaction of 1,2-ethanediol by La3+, Sr2+ and Cr3+ nitrates; (2) combustion synthesis based on the exothermic redox reaction of La3+, Sr2+ and Cr3+ nitrates with urea and glycine as fuels. We also used a mixture of urea and glycine as fuel. The samples were characterized by means of thermal analysis and X-ray diffraction.

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Abstract

Non-isothermal crystallization of hematite in an aventurine glaze was investigated using DTA technique. The apparent activation energy as a function of the crystallized fraction was evaluated by the Kissinger–Akahira–Sunose (KAS), Ozawa–Flynn–Wall (OFW), Starink, Tang, and Vyazovkin isoconversional methods. The apparent activation energy of the hematite crystallization in the studied aventurine glaze ranges with the crystallization fraction between 150 and 180 kJ mol−1. The Avrami exponent values, calculated by Ozawa method confirm the complex mechanism of the crystallization process.

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Abstract  

The crystallization kinetic of the basalt glass ceramic of the oxide composition, (%): SiO2 − 50.82; Al2O3 − 12.05; Fe2O3 − 9.28; CaO − 15.48; MgO − 11.08; Na2O+K2O − 1.14; TiO2 − 0.15, with addition of 10% TiO2 as nucleating agent has been studied using thermal analysis under non-isothermal conditions. In this order, the non-isothermal DTA curves were obtained at different heating rates between 4 and 20°C min−1 in the temperature range of 25–1000°C using a Derivatograph-C (MOM, Hungary). The kinetic parameters of the crystallization process were calculated on the basis of Ozawa-Flynn-Wall, Friedman, Budrugeac-Segal and non-parametric kinetic methods.

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