Elucidation of the Formation Mechanism of b-SiAlON from a Zeolite Feng Jin Li, Toru Wakihara, Junichi Tatami, w Katsutoshi Komeya, and Takeshi Meguro Graduate School of Environment and Information Sciences, Yokohama National University, Hodogaya-ku, Yokohama 240-8501, Japan Kenneth J.D. Mackenzie School of Chemical and Physical Sciences, Victoria University of Wellington, Wellington, New Zealand b-SiAlON was synthesized from Y-type zeolite and its forma- tion mechanism was determined using magic-angle spinning nuclear magnetic resonance (NMR) spectroscopy and powder X-ray diffraction (XRD). Y-type zeolite and carbon powders were mixed for 36 h, fired at 10501 to 14501C for 0–2 h in flowing gas (0.5 L/min N 2 ), then heat treated at 7001C for 2 h in air to remove residual carbon. The resulting product was, ac- cording to XRD analysis, mono-phase b-SiAlON; but minor amounts of unreacted SiO 2 and SiC (possibly amorphous) were detected in the NMR spectra. It was found that b-SiAlONs with higher and lower Z values are formed by different reaction pathways. I. Introduction T HE SiAlONs are silicon aluminium oxynitrides of varying compositions and crystal structures. 1,2 The SiAlON ceram- ics, of which a-SiAlON and b-SiAlON are representative phases, are useful for engineering applications because of their excellent mechanical and chemical properties. 3 The structure of a-SiA- lON is based on the a-Si 3 N 4 structure in which Si 41 and N 3 are partially replaced by Al 31 and O 2 , respectively. Charge balance is achieved by interstitial solution of other ions such as Li 1 , Ca 21 ,Y 31 , Mg 21 , or rare-earth ions. The formula of a-SiAlON is generally written M x Si 12mn Al m1n O n N 16n , where M is a metal cation and x 5 m/n (n is the electric charge). Similarly, b-SiAlON is based on the b-Si 3 N 4 structure where Si 41 and N 3 are partially replaced by Al 31 and O 2 . The formula is written Si 6z Al z O z N 8z (0ozr4.2). SiAlON ceramics, which exhibit high strength and hardness and are resistant to oxidation and corrosion, are typically pro- duced by high temperature liquid phase sintering of either a mixture of Si 3 N 4 –AlN–M x O y powders (where M is a metal such as Li, Mg, Ca, or a rare earth element), or of Si 3 N 4 –AlN– Al 2 O 3 . 4,5 The corrosion resistance and high-temperature strength of these materials is known to decrease with the for- mation of a glassy phase in the sintered body. 6 As it is difficult to fabricate SiAlON ceramics by liquid phase sintering without producing a glassy phase in the grain boundary, the preparation of SiAlON powders 7 and their sintering by other methods such as spark plasma sintering (SPS), has received considerable at- tention. 8 The availability of high-quality SiAlON powders is es- sential to achieve structural reliability of the corresponding ceramics. a- and b-SiAlON powders are often synthesized by carbothermal reduction-nitridation (CRN) of silicate-based ox- ides. Several reports describe the synthesis of a-SiAlON by heat- ing carbon-containing powder mixtures of SiO 2 –Al 2 O 3 – CaCO 3 , 9 SiO 2 –Al 2 O 3 –metal (Ca or Y), 10,11 clay-metal compounds 12 and talc (Mg 3 Si 4 O 10 (OH) 2 )-halloysite clay miner- als. 13,14 Other reports have also shown that b-SiAlON powder can be synthesized from kaolinite (Al 2 Si 2 O 5 (OH) 4 ), 15 SiO 2 – Al 2 O 3  2H 2 O powders, 16 SiO 2 –Al 2 O 3 powders, 17 an alkoxide- derived SiO 2 –Al 2 O 3 co-precipitate 18 and SiO 2 –Al 2 O 3 gels, 19 all heated in the presence of carbon under flowing N 2 gas. In this study, we focus our attention on the use of zeolites as the pri- mary source for preparing b-SiAlON. Zeolites are typical host aluminosilicate minerals with rigid frameworks of (Si, Al)O 4 tetrahedra that form cavities containing cations such as H 1 , NH 4 1 , Na 1 , and Ca 21 for charge balance. Recently, we have developed a new synthesis method for SiAlON by CRN of zeolite with different Si/Al ratios. 20 X-ray diffraction (XRD) continues to be the primary tech- nique for characterizing these reaction sequences, but 29 Si and 27 Al solid-state nuclear magnetic resonance (NMR) with magic- angle spinning (MAS NMR) has provided useful complemen- tary information about related thermal reactions in clay miner- als. 21 The 29 Si and 27 Al NMR spectra have also been reported for a number of phases encountered in the reduction nitridation reaction sequence, 22–26 and a brief study has indicated the potential of MAS NMR in monitoring the progress of SiAlON formation in halloysite 27 and kaolinite, 28 and for shedding light on the intermediates formed during the formation of b-SiAlON from organosilicon polymers. 29 Therefore, in this study, b-SiAlON was synthesized from Y-type zeolite and its forma- tion mechanism was determined using 27 Si and 27 Al NMR spec- troscopy and powder XRD. II. Experimental Procedure High purity of Y-type zeolite (see Fig. 1, FAU,330HUA, Tosoh Co. Ltd., Tokyo, Japan) and carbon black (650B, Mitsubishi Chemical Co. Ltd., Tokyo, Japan) were used as the starting 60 50 40 30 20 10 Intensity (a.u) 2 /degree Y-type zeolite Fig. 1. X-ray diffraction pattern of Y-type zeolite (sample 1). R. Kerans—contributing editor w Author to whom correspondence should be addressed. e-mail: tatami@ynu.ac.jp Manuscript No. 22279. Received September 22, 2006; approved December 22, 2006. J ournal J. Am. Ceram. Soc., 90 [5] 1541–1544 (2007) DOI: 10.1111/j.1551-2916.2007.01576.x r 2007 The American Ceramic Society 1541