Synthesis and characterization of nanocrystalline aluminum nitride from a low temperature combustion precursor

Abstract

In recent years, a lot of interest has been shown in aluminum nitride nanopowder. This is because that AlN has high thermal conductivity, high thermal stability, high electrical resistivity, low dielectric constant, and a thermal expansion coefficient matched with silicon [1]. AlN is a promising material for hybrid substrates in circuits, heat sinks, and high power/high frequency electronic devices [2], and is also important for infrared and ultraviolet optoelectronic parts and surface acoustic wave devices [3, 4]. In addition, AlN can be used for melting crucibles, cutting tools, and fillers for polymer. Although AlN powders have been synthesized by many methods [5], the commercially available AlN powders have been mainly produced by carbothermal reduction process. However, a high calcining temperature (more than 1600 ◦C) and long mixing times of the alumina and carbon source is still required to obtain high-quality AlN powder [6]. The present work is concerned with the synthesis of AlN nanocrystalline powder from a new carbothermal reduction process with a low temperature combustion precursor. The method was adopted due to the possibility of obtaining a uniform mixture of nano-sized alumina and carbon from a low temperature combustion reaction of a solution of aluminum nitrate, urea, and cane sugar. Stoichiometric amounts of aluminum nitrate (Al(NO3)3·9H2O) and urea were first dissolved in double distilled water and then mixed with aqueous cane sugar solution in a beaker. The amount of urea as fuel, which can react completely with aluminum nitride, was calculated using the method of Jain and Adiga [7], while the amount of cane sugar was calculated according to the molecular ratio of aluminum and carbon (nAl/nc = 1.8–2.5). The resultant solution was heated at 200–300 ◦C in a muffle furnace to produce loose and brown-black precursor powders containing amorphous alumina and carbon. The precursor powder was placed in graphite crucible, and calcined in a graphite furnace at 1500 ◦C for 2 h with a flowing nitrogen atmosphere. After the carbon was removed at 700 ◦C for 2 h in air, the resultant nanocrystalline powder was uniaxially pressed at ∼100 MPa into cylindrical compacts 10 mm in diameter and 8 mm in height, which were then pressurelessly sintered in N2 at 1800 (5 ◦C/min) for 2 h, in a graphite furnace.