Synthesis of AlN nanowhiskers
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P.G. Zhang Department of Mechanical Engineering, Louisiana State University, Baton Rouge, Louisiana 70803
L.C. Zhang Department of Chemical Materials and Biomolecular Engineering, University of Connecticut, Storrs, Connecticut 06269
S.M. Guoa) Department of Mechanical Engineering, Louisiana State University, Baton Rouge, Louisiana 70803 (Received 4 April 2010; accepted 17 May 2010)
The synthesis of hexagonal aluminum nitride (AlN) nanoparticles and nanowhiskers by using direct nitridation of aluminum powders is presented in this work. After mixing aluminum powders with carbon and silica (derived from rice bran ashes) at a cryogenic temperature, the homogeneous mixture was heat treated/examined in a differential scanning calorimeter/tube furnace under N2 atmosphere. Under a maximum temperature of 1450 C in N2 atmosphere, the as-milled aluminum-carbon-silica powder mixtures transformed completely into AlN nanoparticles and nanowhiskers with a trace of graphite. The formation mechanisms for AlN nanoparticles and nanowhiskers are discussed. I. INTRODUCTION
Aluminum nitride (AlN) has a hexagonal crystal system with a wurtzite structure. Because of its structural characteristics, AlN has unique physical and chemical properties, such as high theoretical thermal conductivity (320 W m1 K1) and perfect dielectric and sound wave transmitting properties.1 AlN powders and whiskers may also be used both as mechanical reinforcement materials in structural composites and as promising materials in functional composites such as thermal interface materials (polymer-based and silicate-based pastes and solders). Recently, low-dimensional AlN nanostructures, such as nanowire, nanobelt, and nanotube, were found to possess promising capabilities for electronics and photonics devices.2 There are two major AlN synthesis methods: the direct nitridation of aluminum or aluminum salts by nitrogen or ammonia3–5 and the carbothermal reduction and nitridation of alumina.6,7 The carbothermal process generally operates around 1700–1900 C. This high operating temperature represents a major obstacle to the large-scale application of this method. In contrast, direct nitridation of metallic aluminum can occur at around 1500 C. However, due to the low melting temperature of aluminum (660 C), which is less than the temperature required for a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2010.0240 J. Mater. Res., Vol. 25, No. 9, Sep 2010
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nitridation, aggregation of aluminum particles occurs during the nitridation process,4 thus it is difficult to generate AlN nanopowders using the direct nitridation method. Furthermore, a coherent nitride skin is usually formed on the surface of Al particles and prevents further nitridation processes. An early work on the formation of AlN whiskers was conducted by Taylor et al.8 on high-temperature treatment of aluminum powders. Aluminum nitride nanowhiskers can also be synthesized on a Si substrate by an oxideassis
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