| The rapid development of modern technology is demanding materials with higher qualities. AlN ceramics will be widely used in many high-tech areas because of its remarkable properties, such as high thermal conductivity, high electrical resistivity, low dielectric constant and dielectric loss, no toxicity, as well as low thermal expansion coefficient close to that of silicon. While commercial degree of AN is notl high today. The main factors that delay the further development of AlN are its high production cost and the difficulties to produce complex shape AlN parts. These problems, including improving reaction conditions, decreasing sintering temperature and fabricating complex shape ceramic parts, are discussed in this paper in order to improve the application and development of AlN ceramics.A homogenous mixture precursor of Al2O3 + C was prepared by the chemical precipitation method and AlN powders were fabricated by carbon thermal reduction nitridation of the precipitated precursor. The nitridation reaction of the precursor shows that the reaction temperature was lowered about 100# and the reaction rate increased greatly, compared with alumina and carbon black as starting materials. The factors that could affect the nitridation conditions and the properties of AlN powders, including PH value, surface active agent, solution concentration, N2 flow rate, carbon source, ratio of carbon to aluminum, were investigated. An optimal process was obtained to synthesize fine and pure AlN powders. First, a homogenous and good dispersion precursor was prepared by the precipitation process from 0.4M A1(NO3)3 solution and 156m2/g carbon black at PH=9, with the molar ratio of carbon to aluminum(C/Al) 3:1 and adding a certain amount of stearic acid and ethylene glycol as surface active agents. Then the precursor was heated at 1550# for 4 h in common pressure with 5L/min N2 flow rate. Finally AlN powders with nitrogen content 33.20%, oxygen content 0.98%, specific surface area 4.26m2/g were produced after decarbonization of the nitridation products in air for 4h at 650#.AlN powders were synthesized by the carbothermal reduction method from aluminum nitrate and glucose. Effects of urea on the morphology and composition of the precursors and their nitridation products were studied. The results show that urea could affect the morphology and composition of the precursors as well as the properties of AlN powders. In the solution added with urea, a low-temperature combustion process took place between urea and aluminum nitrate accompanied with lots of heat and gases liberated, and a precursor with high specific surface that contained nanometer particles was formed. While for the solution without urea, no combustion reaction took place and a badly agglomerated precursor was formed. Urea could also affect the morphology of AlN powders by affecting the morphology of the precursors. AlN powders obtained from the precursor prepared without urea agglomerated badly, while the powders synthesized from the precursors prepared withurea were soft aggregates of fine particles, which could be easily dispersed.The ratio of urea to aluminum nitrate (U/A) had great influence on the phase transition behaviors and the reaction temperature and rate of the precursors during the nitridation process, as well as the properties of AlN powders. For the precursors with U/A smaller than the smallest theatrical ratio(about 1.0), namely in fuel-lean region, y-AlaOa, a-AOa and A1ON phases were detected during the reaction process. Because of the appearance of a-AOs, which has a very low reactivity, a higher reaction temperature and a longer reaction period were needed for a complete conversion. The reaction rate increased and the reaction temperature decreased with U/A increasing in fuel-lean region. But for the precursors with U/A equal to or larger than 1, namely in fuel-rich region or stoichiometry region, only y-ACh was detected during the nitridation process and AIN was synthesized directly form y-AlaOs without the phase transition from y-AOa to a-AOa. T...
|
PDF Full Text Request