The Observation of Reaction Process between Siliceous Sand and Al in N₂ Atmosphere by Thermobalance

Abstract

The reaction process between siliceous sand and Al in N₂ atmosphere during heating up to 1500°C was examined by thermobalance and X-ray diffraction analysis with respect to N₂ flow rate, forming pressure and Al content in the starting mixture.
Following results were obtained:
(1) Nitriding of Al took place above about 500°C and its reaction rate increasd at the melting point of Al (660°C). The exothermic reaction AlN formation at 660°C induced the reaction between siliceous sand and Al to form Si and α-Al₂O₃ so rapidly.
(2) AlN⋅Al₂O₃ phase (spinel type) was observed in the heat treated compacts at the temperature range from about 660° to 1200°C. This AlN⋅Al₂O₃ formation revealed that the temperature of local parts in compacts reached at about near 2000°C by two exothermic reaction of AlN formation and alumino-thermite reaction.
(3) Si formed through alumino-thermite reaction reacted with N₂ to form β′-Si₃N₄(AlN⋅Al₂O₃) above 1150°C. The temperature of Si nitriding observed varied with forming pressure and Al content in starting mixture. At low N₂ flow rate of 0-30cc/min and at high forming pressure of 2000, 3000kg/cm², Si remained unreacted with N₂ in the compacts even if they were heated up to 1500°C, kept at the temperature for 1h.
(4) The reaction products heat-treated up to 1500°C varied with Al content in starting mixture. In the case of 40wt% Al, the compact consisted of β′-Si₃N₄, α-Al₂O₃, O′, mullite and X phases. With increasing Al content in starting mixture from 40wt% Al to 50 and 60wt% Al, silica rich phase such as O′, mullite and X phases disappeared and β′-Si₃N₄, α-Al₂O₃, 15R-AlN and 15R-AlN, β′-Si₃N₄, α-Al₂O₃ were observed, respectively.