Effect of starting powder morphology on AlN prepared by combustion reaction
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The particle size and shape effects of starting raw powders on the synthesis of aluminum nitride by combustion reaction technique were investigated with four sizes of AlN powder as diluent and two shapes of Al powder as reactant. It was found that the structure of beds of starting particles significantly affected the pore channels for nitrogen gas accessibility into a mixture compact and the passages for combustion wave propagation through particles, resulting in changes of AlN product morphology and purity. Through control of the starting particle size and shape, high-purity (over 98%) AlN products several tens of microns in size were synthesized.
I. INTRODUCTION
Aluminum nitride has properties such as high thermal conductivity, high mechanical strength, and low thermal expansion. Furthermore, it has excellent thermal shock resistance and is stable up to 1000 °C under oxidizing conditions and above 2000 °C in inert atmosphere.1 Therefore, AlN is one of the most promising candidates for refractory and structural applications, in addition to its use for electronic packaging.2 Many synthesis techniques for this functional material have been researched, and it has been commercially produced either by direct nitridation of aluminum powder or by carbothermal reduction of alumina with carbon in nitrogen.3,4 These processes, however, require high temperature for a relatively long time. A low energy consuming process known as combustion reaction or selfpropagating high-temperature synthesis (SHS) is available to produce aluminum nitride powder.5–7 In spite of its merits, it is difficult to control the reaction kinetics, such as combustion propagation velocity, and reaction temperature and time, so that AlN powder with desired purity and grain size can be difficult to obtain. Many researchers have focused on studying the reaction variables in combustion synthesis,8–10 and as a result, it has been confirmed that the diluent content and porosity of a green compact of starting powders have an important role in controlling the reaction.11 Usually, AlN diluent is mixed with Al reactant to control the reaction temperature and velocity for the combustion synthesis of aluminum nitride. The reaction a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2005.0089 J. Mater. Res., Vol. 20, No. 3, Mar 2005
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between the solids and N2 gas depends on the local availability of the gas throughout pore channels within the green compact sample.12 Thus, to sustain a full reaction across the entire green compact, the rate of the gas permeation through the pore channels must be greater than that of the gas consumption. However, it is expected that the structure of packed particles could limit the nitrogen gas permeation and the combustion wave movement through the compact before and during the reaction. The main purpose of this paper is to investigate the particle size and shape effects of reactant and diluent on the morphology of AlN products along with
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