Parametric effects of low-temperature combustion synthesis of alumina
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The influences of parameters on the low-temperature combustion synthesis of alumina particles from reactant mixture of aluminum nitrate and combustion fuel were studied using the particle size of as-synthesized alumina particles as a performance index. First, when urea was used as combustion fuel, it produced a higher combustion temperature and a larger particle size than the case when carbohydrazide was the fuel. Next, the combustion in air yielded a flame propagating through the reactant mixture, in contrast to a flame simultaneously ruptured from the entire reactant when the combustion was conducted in nitrogen. The particle size of the product obtained in nitrogen was 40% smaller than that obtained in air. Increasing the heating temperature could increase the alumina particle size due to the sintering effect, while combustion failed if the heating temperature was too small. The addition of diluent, excess fuel, and gas-releasing agents reduced the particle size. The increase of stirring speed also reduced the particle size. Next, if the reactant density (the amount of reactant mixture in the reacting container) was below a certain threshold value, the combustion failed to ignite. Increasing the reactant density was found to reduce the particle size due to the simultaneous reduction of combustion time and temperature. Finally, a liquid–gas reaction model was proposed and solved to study the threshold of combustion parameters.
I. INTRODUCTION
The low-temperature combustion synthesis, initiated by Patil and coworkers, has emerged as an attractive method to produce advanced materials with fine structures. It has been used to produce various types of materials such as titania and titanate,1–3 manganite and ferroelectric,4–6 alumina and aluminate,7,8 as well as composite and solid solution.9–12 Basically, this process utilizes the energy generated from the redox exothermic reaction between oxidizers (metal nitrates) and fuel (urea, for example), which is triggered at a relatively low heating temperature (350–500 °C). The combustion temperature measured from the ruptured flame is around 1000 °C or lower, which is much lower than the temperature of self-propagating high-temperature synthesis (SHS) and reactive sintering. Thus, this method is classified as low-temperature combustion synthesis. Moreover, because the reactant mixture is aqueous, this method is also referred to as aqueous or liquid-phase combustion synthesis. There are several advantages
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2005.0052 424
http://journals.cambridge.org
J. Mater. Res., Vol. 20, No. 2, Feb 2005 Downloaded: 17 Mar 2015
associated with the low-temperature combustion synthesis. First, numerous advanced materials have been synthesized. Next, the obtained products are often nanosized and can be easily dispersed. Finally, it is a fast process that finishes in less than 5 min. Therefore, these advantages make the low-temperature combustion synthesis an attractive and efficient process to prod
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