Papers by Keyword: Magnesium Aluminate

Abstract: Magnesium aluminate (MgAl₂O₄) was synthesized via combustion reaction, using as fuel glycerine from biodiesel, for the production of cellular ceramics. A rheologically stable suspension was prepared in a ball mill and polyurethane sponges were impregnated. In order to define a firing cycle which would result in ceramic foams with high density ceramic walls (struts), synthesized and calcined powders were uniaxially pressed at 5.4 MPa and, after drying (110°C/120 min), fired at different temperatures (1450-1650°C) for 120 min. Thus, ceramic foams (fired at 1600°C/120 min) containing MgAl₂O₄ as the only formed crystalline phase with a porosity of 88 ± 0.5% and compression strength of 1.0 ± 0.3 MPa were obtained.

Abstract: Lithium fluoride as well as calcium oxide were used as sintering additives to magnesium aluminate ceramics in order to provide both transparency and better densification. This work assessed the dynamic behavior of MgAl₂O₄ by performing split Hopkinson bar dynamic tests. The total amount of additives was 1.5 wt% in which the percentage of LiF and CaO varied from 0 to 100 wt% with an increment of 25 wt%. The obtained results indicated that CaO low concentrations induced low strength values to the ceramics of different compositions. By increasing the amount of CaO, the MgAl₂O₄ dynamic strengths were higher. The strain rate was reduced as the amount of CaO increased, indicating a higher trend of energy absorption corroborated by K_Ic measured values. On the other hand, the ceramic strain increased with the increment CaO additions.

Abstract: This work aims to investigate the glycerin from the biodiesel as fuel in the combustion synthesis into two routes, one for high temperature called direct route, in order to inhibit the formation of acrolein and compare it with the conventional combustion synthesis at low temperatures, using as fuels saccharose, pure glycerin, pure glycerin gel, glycerin from biodiesel, biodiesel from glycerin dried 24 h, biodiesel from glycerin gel dried 24 h to obtain the magnesium aluminate spinel (MgAl₂O₄). The synthesis was performed from the mixture of the precursors (Al (NO₃)₃.9H₂O and Mg (NO₃)₂.6H₂O) in water with the addition of the fuels. In the direct route case, the mixture was placed in a furnace heated to 500°C followed by calcination at different temperatures for 2 h. In the conventional route cases, the mixtures were slowly heated to 800°C for 2 h. The obtained powders, product of the combustion reactions, were characterized by X-ray diffraction (XRD), laser scattering particle size analyzer and scanning electron microscopy (SEM). The results showed that the product (powder) obtained by direct reaction was the MgAl₂O₄ as the only crystalline phase formed with particle sizes of approximately 41 μm. Even so, small particles with sizes of about 30 nm were also observed according to measurements on SEM images.

Abstract: Effect of magnesium addition on rapid transformation of α-alumina prepared from route of ammonium aluminum carbonate hydroxide during thermal heating and microwave radiation heating was investigated. The phase transformation and the final particle size of the transient alumina composite powder were significantly affected by amount of magnesium added in the aluminum precursor during the microwave radiation heating in various environmental atmospheres. Rapid transformation from γ- to α-phase was found in the magnesium added transient alumina by microwave-assisted transformation and nano-sized α-alumina was obtained. When the 3 wt% magnesium added ammonium aluminum carbonate hydroxide was heated by microwave radiation under nitrogen atmosphere, the transformation temperature from γ- to α-alumina was considerably lowered to 1000°C and the average particle size of 27.6 nm was attained for the α-alumina-spinel composite powder.

Abstract: The microwave-assisted combustion synthesis as a route to obtain ultrafine α-alumina and magnesium aluminate composite powders starting from the ammonium aluminum carbonate hydroxide precursor was investigated. The synthetic temperature and the crystallite size of the α- alumina nanocomposite powder were significantly affected by the environmental atmosphere in the microwave assisted combustion. The α-alumina and spinel composite powder was obtained by the microwave-assisted combustion at temperature of 1000°C under H2/N2, but at 1150°C by the normal heat process under air. The rapid transformation from γ- to α-phase was achieved by microwave assisted nucleation at low temperature of 900°C under H2/N2 atmosphere. The least crystallite size of 26.2 nm was obtained under H2/N2 atmosphere at 1000°C for 10 min.

Abstract: Microwave-assisted combustion synthesis of α-alumina-magnesium spinel nanocomposite powders from a route using ammonium aluminum carbonate hydroxide (AACH) as a precursor was studied. γ- to α-phase transformation temperature of microwave-assisted alumina composite powders is lowered by 100°C compared to that of normal heat-treated alumina composite powders. The crystallite size of the microwave-assisted composite powders was considerably decreased from 42 nm to 29 nm compared to that of the normal heat treated composite powders. Nanosized α- alumina-magnesium aluminate composite powders were successfully synthesized by microwave-assisted combustion under nitrogen atmosphere at 1050°C.