Preparation of Ca-α SiAlON powder by reduction-nitridation in a gas mixture of C₃H₈ and NH₃

Abstract

Ca-¡ SiAlON powders were synthesized from a SiO 2 Al 2 O 3 CaCO 3 powder mixture by reduction-nitridation with C 3 H 8 NH 3 gas. Ca-¡ SiAlON with small amounts of AlN and ¢-SiAlON phases was produced at 1450°C for 120 min. Angular particles and agglomerated and bonded particles with irregular shapes were formed. In the product prepared at 1450°C without any holding time, Si 2 N 2 O was detected as an intermediate phase, in addition to the Ca-¡ SiAlON, ¢-SiAlON and AlN phases. A large number of agglomerated and bonded particles were observed.©2012 The Ceramic Society of Japan. All rights reserved.Key-words : SiAlON, Powder, Reduction-nitridation, Gas, Morphology, Formation process [Received December 13, 2011; Accepted February 2, 2012] SiAlON ceramics possess high hardness and excellent oxidation and corrosion resistance. In our previous study, Ca-¡ SiAlON powders were prepared from mixtures of SiO 2 , Al 2 O 3 , CaCO 3 and carbon powders by CRN.13)16) The powders prepared had hollow spheres composed of nanosized particles.13) Solid spherical particles that consisted of a liquid phase containing Si, Al, Ca, O were generated at the initial stage of the CRN process. As the reaction proceeded from the liquid phase to the crystalline Ca-¡ SiAlON, the solid spheres changed into hollow spheres. 14),15)High-purity and fine-grained AlN powders were obtained by gas reduction-nitridation (GRN) of fine-grained Al 2 O 3 using a gas mixture of C 3 H 8 and NH 3 as a reduction-nitridation agent.17),18) This indicated that the resultant particles retained the morphology of the starting powders because of a gas-solid reaction between Al 2 O 3 and NH 3 , and that the use of a hydrocarbon gas as reducing agent promoted more reduction-nitridation compared to the case of CRN with carbon powder.In the present work, Ca-¡ SiAlON powders were prepared through GRN of a mixture of SiO 2 , Al 2 O 3 , and CaCO 3 powders using a C 3 H 8 NH 3 gas mixture, and the crystalline phases and particle morphology of the products were investigated. The effect of hydrocarbon gas as a reducing agent on the particle morphology of the products was also discussed.As starting materials, SiO 2 (QS-102, Tokuyama Co., particle size: 0.0050.05¯m), Al 2 O 3 (AKP-50, Sumitomo Chemical Co., Ltd., 0.21¯m) and CaCO 3 (Junsei Chemical Co., Ltd., 1.0¯m) powders were used. The proportions of these powders were adjusted so as to obtain a nominal composition of Ca 13) The starting powders were mixed with a small amount of ethanol. The mixture was dried, sieved, placed in an Al 2 O 3 boat, and heated in a horizontal electric furnace under a flowing gas mixture of C 3 H 8 (20 mL/min) and NH 3 (4 L/min) at a rate of 10°C/min up to 1450°C, for holding times of 0 to 120 min. The crystalline phases that appeared in the specimens were identified by X-ray diffractometry (XRD). The content of the Ca-¡ SiAlON phase was calculated from the XRD peak intensities.15) Scanning electron microscopy (SEM) was used to characterize the particle morphology of the specimens. Figure 1 shows ...