Synthesis of -SiAlON-AlN-Polytypoid Ceramics from Aluminum Dross

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1 Materials Transactions, Vol. 51, No. 5 (2010) pp. 844 to 848 Special Issue on Growth of Ecomaterials as a Key to Eco-Society IV #2010 The Japan Institute of Metals Synthesis of -SiAlON-AlN-Polytypoid Ceramics from Aluminum Dross Jiajing Li, Jun Wang*, Haiyan Chen, Baode Sun and Junbiao Jia The State Key Laboratory of Metal Matrix Composites, Shanghai Jiao Tong University, Shanghai , P. R. China Aluminum dross is one of the main secondary wastes during aluminum recycling procedure. It is utilized to synthesize -SiAlON-AlNpolytypoid ceramics by the reduction-nitridation method in this paper. The effects of the reaction parameters, such as the Si/Al ratio, reaction temperature and holding time are studied in detail. The results indicate that the synthesized products are mainly composed of -SiAlON and Mg- AlN-polytypoid at 1450 C to 1650 C for 6 h. As the Si/Al ratio changes from 1.0 to 2.5, the amount of -SiAlON increases while the amount of Mg-AlN-polytypoid decreases. At 1750 C, the samples consist mainly of AlN and AlN-polytypoid because SiO gas is significantly evaporated. With the increment of synthesis temperature, the growth of elongated Mg-AlN-polytypoid grains is significantly accelerated. SEM and EDS analysis results for the samples formed at 1650 C for 6 h show that the -SiAlON grains display a short platelet morphology, and Mg-AlNpolytypoid grains possess needle or lath morphology. This study suggests that it is possible to synthesize the low cost SiAlON ceramics with high toughness from aluminum dross. [doi: /matertrans.mh200913] (Received November 2, 2009; Accepted March 4, 2010; Published April 15, 2010) Keywords: aluminium dross, SiAlON, AlN polytypoid, reduction-nitridation 1. Introduction Aluminum dross is one of the main secondary wastes during aluminum recycling procedure. 1) Being a valuable industrial waste due to its chemical composition containing mainly metallic Al, Al 2 O 3, SiO 2, MgO and CaO, many methods to reuse the aluminum dross have been studied, such as recovering of metallic Al, reinforcing material in aluminum dross composites, and synthesizing spinel by solid-state reaction with MgO. 2 5) SiAlON ceramics are considered as an attractive material for engineering applications because of their excellent mechanical, chemical, and thermal properties. 6) However, the low fracture toughness of ceramics has restricted their practical use. In recent years, multiphase SiAlON ceramics have been widely investigated. It is expected that the fracture toughness of SiAlON ceramics could be improved with a whisker-reinforced microstructure. 7 10) On the other hand, the applications of SiAlON ceramics have always been limited by their high cost for the use of high-purity raw materials. Since the end of the 1970s, many research works indicate that SiAlON also can be synthesized from kaolinite, clay, bauxite, and industrial wastes such as fly ash, slag, etc ) It has been revealed that AlN-polytypoids can offer in-situ strengthening and toughening effects in multiphase SiAlON ceramics because they possess fiber-like or lath morphology ) According to the phase diagram of the Si-Al-O-N system (as Fig. 1), -SiAlON can coexist with AlNpolytypoid, so it is proposed that the metallic Al in aluminum dross should be used to synthesize AlN-polytypoid and introduced into SiAlON ceramics as a toughening phase. In this paper, the synthesis of -SiAlON-AlNpolytypoid ceramics with aluminum dross and silicon is investigated, especially the effect of high temperature (1450 C1750 C) on the composition and microstructure of SiAlON ceramics. *Corresponding author, junwang@sjtu.edu.cn Fig. 1 Phase relation ships in Si-Al-O-N system at 1700 C. 7) 2. Experimental Procedure The aluminum dross (<50 mm) used in this work is provided by the aluminum recycling company Shanghai Sigma Metals Inc. The main chemical contents are Al (31.8%, mass fraction) (the same as following), Al 2 O 3 (25.1%), SiO 2 (9.9%), CaO (3.6%), MgO (9.7%), Fe 2 O 3 (4.8%) and some other impurities such as Na 2 O, K 2 O, P 2 O 5, etc. Silicon powder (98.5%, <50 mm) used as a reducing agent and Si 3 N 4 powder (96%, <40 mm) added as seed 19) are commercial products. The compositions of all the samples are designed according to the Si/Al ratio, which are listed in Table 1. The mixtures of raw powder are mixed by balling mill with silicon nitride media in absolute alcohol for 12 h. The powders are dried and pressed into cylindrical pellets of 15 mm in diameter and 23 mm in height under a pressure of 10 Mpa. The samples are sintered in a graphite resistance furnace with flowing nitrogen at 1450 C, 1550 C, 1650 C and 1750 C for 6 h, respectively. The phase constitution is identified by powder X-ray diffraction (XRD, D/MAX 2550VL/PC) analysis with

2 Synthesis of -SiAlON-AlN-Polytypoid Ceramics from Aluminum Dross 845 Table 1 Compositions of mixtures (mass fraction, %). Samples Si/Al Aluminum dross Silicon Si 3 N 4 D D D D Fig. 3 XRD patterns of sample D2 synthesized at 1450 C to 1750 C for 6 h. in polytypoids. When the initial material compositions shift towards high Si/Al ratio, it is thought that the formation of -SiAlON prior to polytypoids. Moreover, a small amount of Fe 3 Si is occurred in the final products. Fig. 2 (a) XRD patterns for the synthesized products at 1650 C for 6 h. (b) Relationship between the Si/Al ratio and the relative phase compositions of the synthesized products at 1650 C for 6 h. nickel-filtered Cu K radiation. The microstructures are characterized by scanning electron microscopy (SEM, JSM- 6460) with EDS on the sintered fractured surfaces. 3. Results and Discussion 3.1 Effects of the raw material compositions The XRD patterns of the samples synthesized at 1650 C for 6 h with various initial compositions are shown in Fig. 2(a). The intensities of the integral area of the strongest characteristic peak of each phase are presented as a function of Si/Al ratios in Fig. 2(b). The products are all composed of -SiAlON and Mg-AlN-polytypoid, which are similar with different Si/Al ratios (as Fig. 2(a)). But the percentage of -SiAlON increases with the silicon content increasing (as Fig. 2(b)). Comparing with -SiAlON and AlN-polytypoids, it is found that the Si/Al ratio in -SiAlON is higher than 3.2 Effects of the synthesis temperature and holding time The XRD patterns of sample D2 sintered at C for 6 h are shown in Fig. 3. All the samples fabricated from 1450 to 1650 C consist of -SiAlON and Mg-AlN-polytypoid. At the same time, as the synthesis temperature increasing, the amount of -SiAlON increases while the amount of Mg-AlN-polytypoid decreases. When the synthesis temperature increases to 1750 C, the synthesized products of sample D2 consist mainly of AlN and AlNpolytypoid. Previous research 20) indicates that a lot of SiO may evaporate from the sample while the synthesis temperature is higher than 1400 C, which can make the Si/Al ratio in an uncontrolled manner during the synthesis process. According to the XRD pattern of synthesized products, SiO gas would have significantly evaporated in sample D2 at 1750 C. The XRD patterns for the products synthesized at 1550 C for 3 h and 6 h in flowing nitrogen are shown in Fig. 4. From D1 to D4 (as Fig. 4(a)), the synthesized products have the Ca--SiAlON phase and the amount of Ca--SiAlON increases with the increment of the Si/Al ratio. When the holding time increases to 6 h, the synthesized products of all the samples consist mainly of -SiAlON and Mg-AlNpolytypoid (as Fig. 4(b)). Earlier research 21) indicates that to phase transformation can be explained by crystallization of the grain boundary liquid phase. Because the initial composition has many oxides, it is apparent that the liquid phase exists in sintering process. Prolonging holding time, the crystallization of the liquid phase enhances (as Fig. 5). Therefore, the transformation of Ca--SiAlON to -SiAlON is promoted and Ca--SiAlON disappears. It seems that the composition of synthesized products is dependent on the final temperature and holding time. The Si/Al ratio just affects the relative content of composition in the synthesized products.

3 846 J. Li, J. Wang, H. Chen, B. Sun and J. Jia Fig. 4 Fig. 5 Fig. 6 XRD patterns of the products synthesized at 1550 C for 3 h (a) and 6 h (b). SEM micrographs of sample D2 synthesized at 1550 C for different holding time: (a) 3 h; (b) 6 h. SEM micrographs of the products synthesized at 1650 C for 6 h with various initial compositions: (a) D1; (b) D2; (c) D3; (d) D Fracture surface analysis The fracture surface morphology of the samples synthesized at 1650 C for 6 h with various initial composition are observed (as Fig. 6). XRD analysis of these samples shows that they all consist of -SiAlON and Mg-AlN-polytypoid. The major phase in D1 is Mg-AlN-polytypoid while in D2 is

4 Synthesis of -SiAlON-AlN-Polytypoid Ceramics from Aluminum Dross -SiAlON. Many polytypoid grains in D1 appear with more needle-like morphology, sample D2 show the lath morphology of the polytypoid phases. In addition, the analysis results of the pores in sample D2 clearly show the lath polytypoid phases (as Fig. 7). The EDS spectra also shows the lath grains are Mg-AlN-polytypoid, the -SiAlON grains exist as short platelet and the spherical particles are Fe3 Si. The needle-like morphology of polytypoid phases in D1 is attributed to the abundance of polytypoid in the microstructure. The large number of polytypoid phases would restrict the space for polytypoid grains growing, so the polytypoid phases display the needle-like morphology in D1. However, it is believed that the growth of the polytypoid grains prior to the SiAlON grain growth in sample D2. So the aspect ratios of polytypoid phases in sample D2 is larger than that in sample D The fracture surface morphology of sample D3 is similar to sample D4. They are both composed of -SiAlON as the dominant phase. SEM shows that some evidence of grain debonding and pullout, a few of lath shaped imprints of the polytypoid phases is present. Fracture surface of sample D3 and D4 produces a jagged shape on the -SiAlON phase regions where the intergranular fractures take place. Comparing the fracture surface morphology of these samples, it is indicated that the toughening mechanisms may be induced by the lath morphology of polytypoid grains. The fracture surface morphology of sample D2 synthesized at different temperatures (as Fig. 8) reveals that the synthesis temperature could significantly affect the morphology of grains. At 1450 C, the morphology occurs mainly as petal aggregates. The clear lath morphology of AlNpolytypoid grains begin to appear at 1550 C. As the synthesis temperature increases to 1650 C, the grain coarsening of AlN-polytypoid is obviously accelerated. At 1750 C, the synthesized products consist mainly of AlN and AlNpolytypoid, and many elongated lath shaped imprints of AlN-polytypoid phases is present. 3.4 Discussion of the reaction mechanism A series of reactions take place during the synthesis process from room temperature to the reaction temperature. In the range of 600 C 1100 C, the following reactions happen:22) 2Al þ N2! 2AlN ð1þ 3SiO2 þ 4Al! 2Al2 O3 þ 3Si ð2þ Fig. 7 6 h. SEM observation of a pore in sample D2 synthesized at 1650 C for Fig. 8 Above 1100 C, the reaction of Si and N2 occurs, yielding Si3 N4. At the same time, Si also reacts with both SiO2 and N2 to yield Si2 N2 O. The following is the main reactions in this stage:23) SEM micrographs of sample D2 synthesized at different temperature: (a) 1450 C; (b) 1550 C; (c) 1650 C; (d) 1750 C.

5 848 J. Li, J. Wang, H. Chen, B. Sun and J. Jia 3Si þ 2N 2! Si 3 N 4 ð3þ 3Si þ 2N 2 þ SiO 2! 2Si 2 N 2 O ð4þ When the temperature is higher than 1350 C, SiO 2 reacts with Si to yield SiO. Subsequently, some SiO reacts with N 2 to yield Si 2 N 2 O. Then Si 2 N 2 O converts to O-SiAlON via the reaction with Al 2 O 3. In this stage, Ca--SiAlON, -SiAlON and AlN-polytypoid are formed by the reaction of CaO, Si 3 N 4, AlN and Al 2 O 3. With the increase of temperature, O-SiAlON as intermediate phases converts further to - SiAlON. Mg 2þ can be incorporated into AlN-polytypoid to form Mg-AlN-polytypoid phase. 24) The main reactions are present as follows: Si þ SiO 2! 2SiO 4SiO þ 2N 2! 2Si 2 N 2 O þ O 2 Si 2 N 2 O þ Al 2 O 3! O-SiAlON CaO þ Si 3 N 4 þ AlN þ Al 2 O 3! Ca--SiAlON Si 3 N 4 þ AlN þ Al 2 O 3! -SiAlON Si 3 N 4 þ AlN þ Al 2 O 3! AlN-polytypoid O-SiAlON þ Al 2 O 3! -SiAlON ð5þ ð6þ ð7þ ð8þ ð9þ ð10þ ð11þ Above 1450 C, when the holding time increases, the transformation of Ca--SiAlON to -SiAlON is promoted and Ca--SiAlON disappears. 21) 4. Conclusions The -SiAlON-AlN-polytypoid ceramics can be synthesized by a reduction and nitridation method with aluminum dross. The results indicate that the whisker-reinforced microstructure could be obtained and adjusted in SiAlON ceramic system by controlling the synthesis parameters. When the ratio of Si/Al is 1.5, the optimal composition and microstructure are formed at 1650 C for 6 h. The further investigation on the densification and properties of SiAlON ceramic is necessary to estimate the toughening effect of AlN-polytypoid. Acknowledgements This work is financially supported by Research Project (No. 07dz12028 and No. 08xd14231) from the Shanghai Municipal Science and Technology Commission. REFERENCES 1) M. C. Shinzato and R. Hypolito: Waste Manage. 25 (2005) ) H. N. Yoshimura, A. P. Abreu, A. L. Molisani, A. C. de Camargo, J. C. S. Portela and N. E. Narita: Ceram. Int. 34 (2008) ) T. Akiyama, Y. Hirai and N. Ishikawa: Mater. Trans. 42 (2001) ) V. M. Kevorkijan: Compos. Sci. Technol. 59 (1999) ) T. Hashishin, Y. Kodera, T. Yamamoto, M. Ohyanagy and Z. A. Munir: J. Am. Ceram. Soc. 87 (2004) ) K. H. Jack: J. Mater. Sci. 11 (1976) ) G. Z. Cao and R. Metselaar: Chem. Mater. 3 (1991) ) V. A. Izhevskiy, L. A. Genova, J. C. Bressiani and F. Aldinger: J. Eur. Ceram. Soc. 20 (2000) ) A. Rosenflanz: Curr. Opin. Solid State Mat. Sci. 4 (1999) ) I. W. Chen and A. Rosenflanz: Nature 389 (1997) ) G. Lee and I. B. Cutler: Am. Ceram. Soc. Bull. 58 (1979) ) J. Y. Qiu, J. Tatami, C. Zhang, K. Komeya, T. Meguro and Y. B. Cheng: J. Eur. Ceram. Soc. 22 (2002) ) W. W. Chen, P. L. Wang, D. Y. Chen, B. L. Zhang, J. X. Jiang, Y. B. Cheng and D. S. Yan: J. Mater. Chem. 12 (2002) ) Q. Qiu, V. Hlavacek and S. Prochazka: Ind. Eng. Chem. Res. 44 (2005) ) Y. Miyamoto: Curr. Opin. Solid State Mat. Sci. 7 (2003) ) W. W. Chen, Y. W. Li, W. Y. Sun and D. S. Yan: J. Eur. Ceram. Soc. 20 (2000) ) H. Wang, W. Y. Sun, H. R. Zhuang, J. W. Feng and T. S. Yen: Mater. Lett. 17 (1993) ) P. L. Wang, W. Y. Sun and D. S. Yan: Mater. Sci. Eng. A 272 (1999) ) C. Zhang, K. Komeya, J. Tatami, T. Meguro and Y. B. Cheng: J. Eur. Ceram. Soc. 20 (2000) ) J. Zheng and B. Forslund: J. Eur. Ceram. Soc. 19 (1999) ) W. Y. Sun, P. L. Wang and D. S. Yan: Mater. Lett. 26 (1996) ) A. D. Mazzoni and E. F. Aglietti: Mater. Chem. Phys. 48 (1997) ) S. P. Li, Y. Q. Guo and X. C. Zhong: Non-Metallic Mines 31 (2008) ) P. L. Wang, C. Zhang, Y. X. Jia and W. Y. Sun: J. Inorg. Mater. 14 (1999)