Search Results
Abstract
Thermoluminescence and optical properties of LiF:Mg,Cu,P; BaSO4:Dy; BaSO4; Eu and α-Al2O3:C were investigated as a part of a broader research project on TL mechanisms in various materials. The effects of ionizing radiation on these phosphors are determined in this work by means of thermoluminescence and optical absorption experiments with the aim of identifying new defects formed in these systems and of testing dosimetric characteristics.
Abstract
Gamma-alumina membrane was prepared from anodic (amorphous) alumina (AA) obtained in a sulphuric acid electrolyte. The transformation scheme, i.e., the crystallization to form metastable alumina polymorphs and the final transition to α-Al2O3 with heating was studied by TG-DTA and X-ray diffraction (XRD) using fixed time (FT) method. When heating at a constant rate, the crystallization occurred at 900C or higher and the final formation of α-Al2O3 occurred at 1250C or higher, which temperatures were higher than the case of using anodic (amorphous) alumina prepared from oxalic acid electrolyte. Relative content of S of the products was obtained by transmission electron microscope (TEM)-energy dispersive spectroscopy (EDS). The proposed thermal change of anodic alumina membrane prepared from sulphuric acid is as follows: 1. At temperatures lower than ca 910C: Formation of a quasi-crystalline phase or a polycrystalline phase (γ-, δ- and θ-Al2O3); 2. 910–960C: Progressive crystallization by the migration of S toward the surface within the amorphous or the quasi-crystalline phase, forming S-rich region near the surface; 3. 960C: Change of membrane morphology and the quasi-crystalline phase due to the rapid discharge of gaseous SO2; 4. 960–1240C: Crystallization of γ-Al2O3 accompanying δ-Al2O3; and 5. 1240C: Transition from γ-Al2O3 (+tr. δ-Al2O3) into the stable α-Al2O3. The amorphization which occurs by the exothermic and the subsequent endothermic reaction suggests the incorporation of SO3 groups in the quasi-crystalline structure.
Sintering of a nano-crystalline metastable alumina
Influence of the firing parameters on the phase development and microstructural evolution
Abstract
The influence of the heating rate on phase transformation and microstructural evolution during sintering of a de-agglomerated nanocrystalline, transition alumina was investigated. A low heating rate treatment allowed to decrease the α-Al2O3 crystallization temperature as well as to displace densification at lower temperatures, also implying a refinement of the fired microstructures. In addition, the set-up of sintering cycles in which the heating rate changed in the range 0.5–20°C min−1 starting from intermediate, selected temperatures resulted in a further retention of a very fine and homogeneous grain size in final materials.
Abstract
Spinel (MgAl2O4) was synthesized mechanochemically (MC) by grinding MgO with γ-Al2O3 up to 10 hours. Examination of the MC product by neutron diffraction and infrared spectroscopy showed that it has a higher degree of inversion than its thermally produced counterpart—47% as against 10% respectively. X-ray studies showed that MgAl2O4 crystallites grow equidimensionally at a much higher rate than in the case of α-Al2O3. The higher degree of inversion and higher formation rate when γ-Al2O3 is used, is attributed to similarities in oxygen framework of MgO, Al2O3 and spinel and to the higher retention of the cations coordination number. The equidimensional growth is attributed to the presence of multiple soft modes, the {111} planes.
Abstract
De-agglomeration of a nanocrystalline transition alumina powder was performed in distilled water at its natural pH under magnetic stirring for 170 h or by ball milling for 3 h. Gibbsite appeared near transition aluminas in the magnetic stirred sample. In addition, a relevant lowering of the α-Al2O3 crystallization temperature was observed in the dispersed materials with respect to the as-received powder. However, the activation energy of the above transformation, determined by the Kissinger method, was in any case about 480–500 kJ/mol and unaffected by the dispersion route. On the contrary, it was reduced of about 10% in α-alumina seeded samples, obtained by flash plunging the powders at 1,290 °C for 10 min.
Abstract
An attempt to obtain aluminium hydroxide that could give aluminium oxides of increased thermal stability was made. Aluminium hydroxide was precipitated during a hydrolysis of aluminium chloride in ammonia medium. The influence of preparative conditions, such as a dosing rate of aluminium precursor, pH, duration of the precipitate refluxing and temperature of calcination, on the properties of obtained hydroxides and oxides was investigated. The materials were studied with the following methods: thermal analysis, IR spectroscopy, low-temperature nitrogen adsorption and adsorption–desorption of benzene vapours. Precipitated boehmites had high values of S BET determined from nitrogen adsorption (220–300 m2g–1), good sorption capacity for benzene vapours, developed mesoporous structure and hydrophilic character. It has been proved that a high pH value during the precipitation of aluminium hydroxide favoured better crystallisation of boehmite structure, higher temperature of its dehydroxylation into γ-Al2O3, and delayed transformation of γ phase into α-Al2O3. Aluminium oxides derived from the hydroxides precipitated at a high pH were the most stable at high temperatures, and were characterised with the best surface properties.
Abstract
The thermal change of anodic alumina (AA), particularly the exothermic peak followed by the endothermic peak at ca 950C was studied in detail by mainly using simultaneous TG-DTA/FTIR. The gradual loss of mass up to ca 910C is attributed to dehydration. When heated at a constant rate by using TG-DTA, an exothermic peak with subsequent endothermic peak is observed at ca 950C, but the exothermic peak becomes less distinct with decreasing heating rate. It has been found that gaseous SO2 accompanying a small amount of CO2 is mainly discharged at this stage. The reaction in this stage can be considered roughly in two schemes. The first scheme can be said collectively as crystallization, in which the migration of S or C trapped inside the crystal lattice of the polycrystalline phase (γ-, δ-, and θ-Al2O3, which presumably accompanies a large amount of amorphous or disordered phase) occurs. In the second scheme, the initial polycrystalline (+amorphous) phase crystallizes into a quasi-crystallineγ-Al2O3-like metastable phase after amorphization. Conclusively,after the distinct exo- and endothermic reactions, the amorphous phase crystallizes intoγ-Al2O3, presumably accompanying small amount of δ-Al2O3. It is also found that, when maintained isothermally, the metastable phases undergo transformation into the stable α-Al2O3 at 912C.
Influence of aluminium precursor on physico-chemical properties of aluminium hydroxides and oxides
Part IV. Al(OH)(CH3COO)2
Abstract
The paper concerns aluminium hydroxides precipitated during hydrolysis of aluminium acetate in ammonia medium, as well as aluminium oxides obtained through their calcination at 550, 900 or 1200�C for 2 h. The following techniques were used for analysing of obtained materials: thermal analysis, IR spectroscopy, X-ray diffraction, low-temperature nitrogen adsorption, adsorption-desorption of benzene vapours and scanning electron microscopy. Freshly precipitated boehmite/pseudoboehmite had high value of S BET, very good sorption capacity for benzene vapours, developed mesoporous structure and hydrophilic character. After prolonged refluxing at elevated temperature its crystallinity increased which was accompanied by a decrease of specific surface determined from nitrogen adsorption, decrease of sorption capacity for benzene vapours and weakening of the hydrophilic character. Calcination of all hydroxides at the temperature up to 1200�C resulted in the formation of α-Al2O3 via transition forms of γ-, δ-and θ-Al2O3. The samples of aluminium oxides obtained after calcination at 550 and 900�C were characterised with high values of specific surface area and displayed quite high heat resistance, probably due to a specific morphology of starting hydroxides. The process of ageing at elevated temperature developed thermal stability of aluminium oxides.
Abstract
The process of oxygen chemisorption on coal in the temperature range ≈150–300 °C was studied under different experimental conditions using TG-DSC apparatus. As changing experimental conditions, oxygen flow (20 or 200 cm3 min−1), material of crucible (α-Al2O3 or Pt–Rh alloy), and initial sample mass (2–13 mg) were examined with respect to reliability and reproducibility of the parameters derived from TA curves. As parameters quantifying coal oxidation, temperatures of minimal T min and maximal T max sample mass, mass changes (mass loss W H below T min and mass increase W O above T min), heat evolution during oxygen chemisorption Q O (related to the coal mass increase), and kinetic parameters (activation energy E and frequency factor A) were evaluated. Values of T max, E, and A were found to lie in very close intervals independently on experimental conditions (95% confidence intervals were T max = 270.2 ± 0.7 °C, E = 81 ± 3 kJ mol−1, log10 A = 5.9 ± 0.3 s−1). Thus, these parameters can be used as actual characteristics of oxygen chemisorption stage of coal oxidation irrespective on conditions of TA measurements. Opposite, parameter Q O was confirmed to depend clearly on initial sample mass. The dependence is different for crucible materials used; however, it tends to the same value (≈50 kJ g−1) with increasing sample mass. Further, precision of values W H, W O, and T min determined from TG was found to be poor. This fact complicates evaluation of the effect of experimental conditions. Finally, the effect of oxygen flow on all above parameters was found to be negligible. Its influence (if any) was hidden by common experimental errors.
Influence of aluminium precursor on physico-chemical properties of aluminium hydroxides and oxides
Part III. Al2(SO4)3·18H2O
Abstract
The process of hydrolysis of aqueous aluminium sulfate was carried on in ammonia medium at 100°C and for different time intervals (0, 20, 39 or 59 h). The products thus obtained were calcined at 550, 900 or 1200°C for 2 h with the aim to obtain aluminium oxides. The materials were studied with the following methods: thermal analysis, IR spectroscopy, X-ray diffraction, low-temperature nitrogen adsorption, adsorption–desorption of benzene vapours and scanning electron microscopy.
Freshly precipitated material was an amorphous basic aluminium sulfate which after prolonged refluxing at elevated temperature in a mother liquor underwent a phase transformation into highly crystalline NH4Al13(SO4)2(OH)6 containing tridecameric unit Al13. It was accompanied by a decrease of specific surface area and the formation of a porous structure less accessible for benzene molecules. Regardless of the duration of the hydrolysis process, all products were characterised with poorly developed porous structure and hydrophilic character. Their calcination at the temperature up to 1200°C resulted in the formation of α-Al2O3 via transition forms of γ/η- and δ-Al2O3. The samples of aluminium oxides obtained after calcination at 550 and 900°C had higher values of specific surface area than starting materials due to processes of dehydroxylation and desulfurization. The process of calcination up to 900°C was reflected in developing of not only porous structure but also hydrophobic character of prepared materials. The S BET values calculated for the oxide samples obtained from aged products of hydrolysis at 1200°C were lower than for the analogous sample prepared without the ageing step. It was concluded that prolonged refluxing at elevated temperature of the products of hydrolysis of aluminium sulfate decreased thermal stability of final aluminium oxides.
