Search

Abstract: Single-phase LaAlO3 was prepared by a simple combustion synthesis procedure, which relies on using urea and β-alanine fuel mixture, instead of a single fuel. The aqueous solution of La(NO3)3, Al(NO3)3, urea and β-alanine underwent a strong exothermic reaction at 281°C, thus enabling the formation of single-phase LaAlO3 directly from the combustion reaction—without the need of supplementary annealing. The combustion synthesized LaAlO3 had an average crystallite size of 46nm and a BET surface area of 3.0m2/g. The addition of NaCl to the reaction mixture decreased the combustion temperature, promoting an increase of the BET surface area (8.5m2/g). At the same time the LaAlO3 crystallite size decreased to 36nm. After sintering at 1500°C for 5h, LaAlO3 pellets developed a fine microstructure, reaching 94% of the theoretical density. [Copyright &y& Elsevier]

Abstract: In order to achieve BaAl2O4 formation via combustion synthesis, two types of recipes were designed: single fuel recipes (urea, glycine, β-alanine or hexamethylenetetramine) and innovative fuel mixture recipes (urea+glycine, urea+β-alanine and urea+hexamethylenetetramine respectively). No combustion reactions were noticed in the case of single fuel recipes based on urea or hexamethylenetetramine. The only crystalline phase present in the case of the powders obtained in such a way was unreacted Ba(NO3)2. Glycine and β-alanine generated smoldering reactions, leading to the formation of black powders, which consist of Ba(NO3)2, BaCO3 and traces of BaAl2O4 (in the case of β-alanine). Fuel mixture recipes proved to be better than single fuel recipes, yielding BaAl2O4 as main crystalline phase. The specific surface area of the resulted powders ranges between 2.0 and 3.9m2/g. The urea and glycine fuel mixture was the most efficient yielding BaAl2O4 with an average crystallite size of 60nm. [Copyright &y& Elsevier]

The influence of urea, β-alanine and glycine on combustion synthesized Y2O3 powders was studied. Standard enthalpies of reactions suggested that the heat released from combustion reactions increases from urea (-1778 kJ/mol) to glycine (-1934 kJ/mol) and then to β-alanine (-1995 kJ/mol). Since the heat released during combustion reactions is generated at different rates, we suggested the use of a new concept - heat generation velocity - in order to explain the different characteristics of Y2O3 powders. Urea had the smallest heat generation velocity (0.4 kJ/s), therefore the resulted powder was amorphous. In the case of β-alanine, the higher heat generation velocity (0.7 kJ/s) led to the formation of nanocrystalline (5 nm) Y2O3 having a surface area of 36 m²/g. Glycine triggered the combustion reaction with the highest heat generation velocity (3.9 kJ/s). Therefore the resulted Y2O3 had the largest crystallite size (21 nm) and the smallest surface area (19 m²/g). [ABSTRACT FROM AUTHOR]

Zinc aluminate powders with tuned surface area were prepared by solution combustion synthesis, using different oxidizer-fuel ratios. As the amount of urea increased, standard heat of reaction and standard Gibbs free energy decreased, whilst adiabatic temperature increased. The larger amount of energy released during combustion facilitated grain growth and sintering, causing the decrease of specific surface area from 156 to 27 m 2 g −1 . The particle size estimated from TEM increased from 8 to 40 nm. The adsorption capacity of zinc aluminate powders with respect to methyl orange was highly dependent on the specific surface area. For an adsorbent dose of 3 g L −1 the removal efficiency of methyl orange was much larger (86 versus 18%) in the case of the sample with larger surface area. The adsorption kinetics followed a pseudo-second-order model and the equilibrium data were correlated with the Freundlich isotherm. [ABSTRACT FROM AUTHOR]