J. Mater. Sci. Technol., 2011, 27(11), 1011-1015. Low Temperature Synthesis of Single-crystal Alpha Alumina Platelets by Calcining Bayerite and Potassium Sulfate Xinghua Su 1) and Jiangong Li 2) 1) School of Materials Science and Engineering, Chang an University, Xi an 710061, China 2) Institute of Materials Science and Engineering, Lanzhou University, Lanzhou 730000, China [Manuscript received February 24, 2011, in revised form May 8, 2011] Single-crystal alpha alumina (α-al 2 O 3 ) platelets were synthesized by calcining a powder mixture of bayerite (α-al(oh) 3 ) and potassium sulfate (K 2 SO 4 ) at 900 C. The crystalline phase evolutions and morphologies of the samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The synthesized samples mainly consisted of single-crystal α-al 2 O 3 platelets with a diameter of 0.5 1.5 µm and a thickness of 50 150 nm. Moreover, with 3, 5, and 8 wt% (referred to the obtained alumina) α-al 2 O 3 seeds adding into the powder mixture of bayerite and potassium sulfate, the average diameter of α-al 2 O 3 platelets can be reduced to 450, 240, and 220 nm, respectively. It is found that the sequence of the phase transformation is the bayerite (α-al(oh) 3 ) boehmite (γ-alooh) γ-al 2 O 3 α-al 2 O 3. Further analysis indicated that K 2 SO 4 can promote the phase transformation from γ-al 2 O 3 to α-al 2 O 3 and the formation of single-crystal α-al 2 O 3 platelets might be attributed to the liquid phase K 3 Al(SO 4 ) 3. KEY WORDS: Alpha alumina; Platelets; Phase transformation; Calcination 1. Introduction Alpha alumina (α-al 2 O 3 ) platelet is a very useful materials, which exhibits high hardness, high modulus, high strength, chemical resistance, and good refractoriness. The combination of these excellent properties and the plate-like shape makes α-al 2 O 3 platelets useful as reinforcements in various composites because of the energy dissipation induced by the incorporation of platelets [1,2]. The α-al 2 O 3 platelets can also be used as fillers to plastics for thermal conductivity improvement because they can provide surface contact between particles [3]. Compared with reinforcements of polycrystalline α-al 2 O 3 fibers, singlecrystal α-al 2 O 3 platelets can avoid the problem of grain growth at high temperatures, which will cause the deterioration of mechanical strength [4]. Moreover, platelets can also be easily dispersed into a matrix Corresponding author. Tel.: +86 029 82337343; E-mail address: xinghuasuchd@gmail.com (X.H. Su). phase with lower cost; thus, conventional physical mixing methods can be used to produce ceramic or metal matrix composite [5]. Up till now, there have been several methods to prepare α-al 2 O 3 platelets. For example, Hill et al. [6] produced α-al 2 O 3 platelets by calcining the alumina and aluminum fluoride mixture powder. Shaklee and Messing [7] prepared α-al 2 O 3 platelets in the HF-γ- Al 2 O 3 system. Unfortunately, fluoride required by these methods is harmful to the environment. Miao and Sorrell [4] obtained α-al 2 O 3 platelets by calcining mixture of natural topaz and zirconi. Hashimoto and Yamaguchi [8] synthesized α-al 2 O 3 platelets using sodium sulfate flux. However, these methods need a calcination temperature as high as 1200 C and will consume too much energy. Recently, Park et al. [9] synthesized α-al 2 O 3 platelets using flux method in 2.45 GHz microwave field; Wu et al. [10] prepared α- Al 2 O 3 platelets by laser scanning alumina powders; Wei et al. [11] synthesized α-al 2 O 3 hexagonal platelets using electrostatic spray assisted chemical vapour de-
1012 X.H. Su et al.: J. Mater. Sci. Technol., 2011, 27(11), 1011 1015 Fig. 1 Flow chart for synthesizing single-crystal α-al 2O 3 platelets position. Nevertheless, these techniques require complicated equipments and high temperature. Therefore, it is still important to explore a low temperature, easy-control, and green technique to produce high quality α-al 2 O 3 platelets. In this work, a simple and cost-effective method was developed to synthesize single-crystal α-al 2 O 3 platelets at a relatively low temperature. Finally, single-crystal α-al 2 O 3 platelets with average diameters varying from 220 to 1000 nm were prepared. 2. Experimental Bayerite was prepared by a chemical precipitation route. The 3 mol/l ammonia solution (NH 3 H 2 O) was added slowly into the aluminum nitrate (Al(NO 3 ) 3 ) solution under strong stirring until ph=9. To avoid agglomeration of the ultrafine aluminum hydroxide, a polyethyleneglycol (PEG) solution with equal amounts of PEG 400 and PEG 2000 was used as a dispersant. The wet gel was first washed eight times with distilled water and then washed five times with anhydrous alcohol to remove the ammonium nitrate completely. The filtered precipitated product was dried at 80 C. The dry precipitates were ground manually in an agate mortar for 10 min and then added into a K 2 SO 4 solution to form suspension. The K/Al atomic ratio can be adjusted from 3 to 12. After a vigorous stirring for 2 h, the suspension was dried in an ultrasonic bath at 80 C. The obtained dry powder mixture was ground manually in an agate mortar for 10 min and then calcined at different temperatures for 2 h (with a heating rate of 5 C/min). After a cooling to room temperature, the products were repeatedly washed with distilled water to remove the remaining salts thoroughly. The resulting powders were oven-dried at 80 C for characterizations. A flow chart of the process for synthesizing α-al 2 O 3 platelet is given in Fig. 1. Additionally, in order to investigate the effect of α-al 2 O 3 seeds on the particle size of the samples, 3, 5, and 8 wt% (referred to the amount of the obtained alumina) dispersed α-al 2 O 3 seed particles (about 50 nm in diameter) were added into the aluminum nitrate solution before the precipitation process. The subsequent processes were the same as those mentioned above. The crystal structures of the samples were investigated by X-ray diffraction (XRD) on an X-ray diffractometer (Rigaku D/max-2400, Japan) with CuKα radiation (40 kv, 60 ma) in the 2θ range of 10 80 deg. The measurement was performed at a scan rate of 5 deg./min with the step size of 0.02 deg. The morphologies of the prepared samples were observed by field-emission scanning electron microscopy (SEM, Hitachi S-4800, Tokyo, Japan) at an accelerating voltage of 15 kv. Secondary electron SEM images were obtained. The microstructures and morphologies of the samples were further examined by transmission electron microscopy (TEM, Hitachi H- 600, Tokyo, Japan) with a voltage of 80 kv. 3. Results and Discussion Figure 2 shows the XRD patterns of the powder mixture of bayerite and K 2 SO 4 (with a K/Al atomic ratio of 12) calcined at various temperatures for 2 h, then cooled to room temperature, and washed with distilled water. The XRD pattern of the sample at room temperature shows the main diffraction peaks of the bayerite phase. When the powder mixture was calcined at 300 C, the XRD pattern exhibits the major diffraction peaks of boehmite (γ- AlOOH) phase; and the additional diffraction peaks corresponding to the γ-al 2 O 3 phase. As the calcination temperature reached 400 C, the diffraction peaks of boehmite disappear completely and only diffraction peaks corresponding to γ-al 2 O 3 exist. These γ-al 2 O 3 diffraction peaks become intense after calcination at 600 C. When the calcination temperature was increased to 700 C, the XRD pattern shows strong diffraction peaks of γ-al 2 O 3. Meanwhile, the visible weak diffraction peaks corresponding to α- Al 2 O 3 phase appear. As the calcination temperature further increased to 800 C, stronger α-al 2 O 3 diffraction peaks as well as weak diffraction peaks of γ-al 2 O 3 can be observed. After calcination at 900 C, the single-phase α-al 2 O 3 is obtained. From
X.H. Su et al.: J. Mater. Sci. Technol., 2011, 27(11), 1011 1015 1013 Bayerite Boehmite -alumina -alumina 900 o C Intensity / a.u. 800 o C 700 o C 600 o C 400 o C 300 o C R. T. 10 20 30 40 50 60 70 80 90 2 / deg. Fig. 2 XRD patterns of the powder mixture of bayerite and K 2SO 4 calcined at various temperatures for 2 h, then cooled to room temperature, and washed with distilled water -alumina -alumina Intensity / a.u. K/Al=12 K/Al=6 K/Al=3 10 20 30 40 50 60 70 80 90 2 / deg. Fig. 3 XRD patterns of the powder mixture of bayerite and K 2SO 4 with different K/Al atomic ratios calcined at 900 C for 2 h, then cooled to room temperature, and washed with distilled water the analysis above, it can be concluded that, during the heating process of the powder mixture of bayerite and K 2 SO 4, the sequence of the phase transformation is the bayerite (α-al(oh) 3 ) boehmite (γ-alooh) γ-al 2 O 3 α-al 2 O 3, and the formation of α- Al 2 O 3 starts at 700 C. Significantly, the single-phase α-al 2 O 3 can be obtained at 900 C, which is much lower than those via conventional method [4,6,8]. Previously, it has been reported that, when being heated in air, the bayerite would lose the physically adsorbed and crystalline water at about 120 and 250 C, respectively, and then transforms into boehmite (γ-alooh), which could transform to the final stable α-al 2 O 3 phase via a sequence of facecentered cubic transition aluminas (γ-a1 2 O 3, δ- A1 2 O 3 or η-a1 2 O 3, and θ-a1 2 O 3 ). Meanwhile, a high calcination temperature above 1100 C is necessary for the complete transformation to α-a1 2 O [12,13] 3. In this work, by heating the powder mixture of bayerite and K 2 SO 4, the sequence of the phase transformation is the bayerite (α-al(oh) 3 ) boehmite (γ-alooh) γ-al 2 O 3 α-al 2 O 3, skipping the transitional θ-a1 2 O 3 phase. Moreover, the transformation tem- Fig. 4 SEM micrograph of the α-al 2O 3 particles obtained by calcining powder mixture of bayerite and K 2SO 4 at 900 C for 2 h (a), and TEM micrograph of a single α-al 2O 3 platelet (b) (the inset in (b) shows the corresponding SAED pattern of the single α-al 2O 3 platelet) perature to α-al 2 O 3 is greatly reduced. These results indicate that K 2 SO 4 can promote the phase transformation from γ-al 2 O 3 to α-al 2 O 3. In order to investigate the influence of fraction of K 2 SO 4 in the powder mixture on the phase transformation, the powder mixtures of bayerite and K 2 SO 4 with different K/Al atomic ratios were calcined. Figure 3 shows the XRD patterns of the powder mixture of bayerite and K 2 SO 4 with different K/Al atomic ratios calcined at 900 C for 2 h, then cooled to room temperature, washed with distilled water, and dried subsequently. For the powder mixture with the K/Al atomic ratios of 6 and 12, γ-a1 2 O 3 transformed completely into α-al 2 O 3 after calcination at 900 C. However, as the K/Al atomic ratio decreased to 3, it is found that there is a little amount of γ-a1 2 O 3 remaining in the sample. This indicates that γ-a1 2 O 3 could not completely transform into α-al 2 O 3 at 900 C when insufficient K 2 SO 4 is mixed with bayerite. Figure 4(a) shows the SEM micrograph of the α-
1014 X.H. Su et al.: J. Mater. Sci. Technol., 2011, 27(11), 1011 1015 1000 Average diameter / nm 800 600 400 200 0 2 4 6 8 Seed content / wt% Fig. 6 Variation of the average diameter of α-al 2O 3 platelets as a function of α-al 2O 3 seed content ter of K 3 Al(SO 4 ) 3 formed during the calcination of the powder mixture of bayerite and K 2 SO 4. The K 3 Al(SO 4 ) 3 could be formed at 612 C by the following reaction: γ Al 2 O 3 + 6K 2 SO 4 2K 3 Al (SO 4 ) 3 + 3K 2 O (1) Fig. 5 Morphology of the α-al 2O 3 particles obtained by calcining 3 wt% (a), and 5 wt% (b) α-al 2O 3 seeded powder mixture of bayerite and K 2SO 4 at 900 C for 2 h Al 2 O 3 particles obtained by calcining the powder mixture of bayerite and K 2 SO 4 (with the K/Al atomic ratio of 12) at 900 C for 2 h. The α-al 2 O 3 particles exhibit a plate-like morphology. The diameter and the thickness of the platelets are in the range of about 0.5 1.5 µm and 50 150 nm, respectively. Figure 4(b) displays the TEM micrograph of a single α-al 2 O 3 platelet with a hexagonal shape. The corresponding selected area electron diffraction (SAED) pattern reveals its single crystalline nature. According to the six-fold symmetric diffraction spots of the SAED pattern, the top/bottom faces of the platelet are indexed as the {0001} planes of α-al 2 O 3. As has been pointed out, Al 2 O 3 grains can grow into platelets with the presence of a liquid phase [14 17]. The surface energy of the Al 2 O 3 grains changes sensitively with even a small amount of liquid phase added into alumina ceramics. Kaysser et al. [16] demonstrated that the growth rate of the {0001} planes of an α-al 2 O 3 crystal is reduced because the surface energy of {0001} planes is lowered when only a small amount of liquid phase is present. In our previous research [18], we have found that there is a new mat- Since K 3 Al(SO 4 ) 3 has a melting point of about 654 C [19], it should melt at 654 C or even higher and be of liquid phase in the powder mixture during calcination. Therefore, it might be suggested that the formation of the single-crystal α-al 2 O 3 platelets in this work is related to the liquid phase K 3 Al(SO 4 ) 3. Depending on the difference in various applications, α-al 2 O 3 platelet powder of various particle sizes is needed. In order to control the particle size of α-al 2 O 3 platelets, α-al 2 O 3 seed particles were added into the powder mixture of bayerite and K 2 SO 4. Figure 5(a) shows the SEM micrograph of α-al 2 O 3 particles obtained by calcining powder mixture of bayerite and K 2 SO 4 with an addition of 3 wt% α-al 2 O 3 seeds at 900 C for 2 h. The α-al 2 O 3 particles exhibit a plate-like morphology. The diameter of the platelets is in the range of 300 600 nm and the thickness of platelets is in the range of 30 60 nm. Figure 5(b) exhibits the SEM micrograph of α-al 2 O 3 particles obtained by calcining powder mixture of bayerite and K 2 SO 4 with an addition of 5 wt% α-al 2 O 3 seeds at 900 C for 2 h. It is found that the diameter of the platelets is in the range of 100 300 nm. The variation of the average diameter of α-al 2 O 3 platelets as a function of α-al 2 O 3 seed content is shown in Fig. 6. The addition of α-al 2 O 3 seeds to the powder mixture of bayerite and K 2 SO 4 has brought about a significant decrease in the diameter of α-al 2 O 3 platelets. The average diameter of α-al 2 O 3 platelets decreased from 1000 to 450, 240, and 220 nm, when 3%, 5%, and 8 wt% seed particles were added, respectively. This may result from an increase in the nucleation rate of α-al 2 O 3 caused by the addition of seed particles. The formation of α-al 2 O 3 platelet should be a nucleation-growth process. During this process, the nucleation rate of α-al 2 O 3 plays
X.H. Su et al.: J. Mater. Sci. Technol., 2011, 27(11), 1011 1015 1015 a dominant role in controlling the particle size of α- Al 2 O 3 platelet. The added α-al 2 O 3 seed particles will provide a large number of sites for the heterogeneous nucleation of α-al 2 O 3, and hence lead to an increase in the nucleation frequency of α-al 2 O 3 in the system. Obviously, an increase in nucleation frequency results in an increased nucleation rate of α-al 2 O [20,21] 3, leading to smaller particle size of α-al 2 O 3 platelets. Generally speaking, in intragranular structure materials, the grain size of matrix is about 0.5 5 µm, so these α- Al 2 O 3 platelets with smaller particle size are suitable for the intragranular reinforcement of nanocomposites. 4. Conclusion Single-crystal α-al 2 O 3 platelets with a diameter of 0.5 1.5 µm and a thickness of 50-150 nm were prepared by calcining the powder mixture of bayerite and K 2 SO 4 at 900 C for 2 h. Moreover, with the 3%, 5%, and 8 wt% (referred to the obtained alumina) α- Al 2 O 3 seeds adding into the powder mixture, the average diameter of α-al 2 O 3 platelets was reduced from 1000 to 450, 240, and 220 nm, respectively. It was found that K 2 SO 4 can promote the phase transformation from γ-a1 2 O 3 to α-al 2 O 3. The γ-a1 2 O 3 did not completely transform into α-al 2 O 3 at 900 C when insufficient K 2 SO 4 was mixed with bayerite. The formation of the α-al 2 O 3 platelets may be related to the liquid phase K 3 Al(SO 4 ) 3. This work presents a simple and cost-effective method to synthesize single-crystal α-al 2 O 3 platelets at a relatively low temperature. Acknowledgement This work was supported by the National Natural Science Foundation of China (Grant Nos. 51102022 and 50872046), the China Postdoctoral Science Foundation (Grant No. 2011M501413), the Special Fund for Basic Scientific Research of Central Colleges, Changan University (Grant No. CHD2012ZD015) and the Special Fund for Basic Research support programs of Chang an University. REFERENCES [1 ] M. Kotoul, J. Pokluda, P. Šandera, I. Dlouhý, Z. Chlup and A.R. Boccaccini: Acta Mater., 2008, 56, 2908. [2 ] S.H. Lim, K.Y. Zeng and C.B. He: Mater. Sci. Eng. A, 2010, 527, 5670. [3 ] R.F. Hill and P.H. Supancic: J. Am. Ceram. Soc., 2002, 85, 851. [4 ] X. Miao and C.C. Sorrell: J. Mater. Sci. Lett., 1998, 17, 2087. [5 ] I. Wadsworth and R. Stevens: J. Mater. Sci., 1991, 26, 6800. [6 ] R.F. Hill, R. Danzer and R.T. Paine: J. Am. Ceram. Soc., 2001, 84, 514. [7 ] C.A. Shaklee and G.L. Messing: J. Am. Ceram. Soc., 1994, 77, 2977. [8 ] S. Hashimoto and A. Yamaguchi: J. Mater. Res., 1999, 14, 4667. [9 ] H.C. Park, S.W. Kim, S.G. Lee, J.K. Kim, S.S. Hong, G.D. Lee and S.S. Park: Mater. Sci. Eng. A, 2003, 363, 330. [10] Y. Wu, K.L. Choy and L.L. Hench: J. Am. Ceram. Soc., 2004, 87, 1606. [11] M. Wei, D. Zhi and K.L. Choy: Nanotechnology, 2006, 17, 181. [12] W.H. Gitzen: Alumina as a Ceramic Material, Columbus, OH, 1970. [13] X. Du, X. Su, Y. Wang and J. Li: Mater. Res. Bull., 2009, 44, 660. [14] H. Song and R.L. Coble: J. Am. Ceram. Soc., 1990, 73, 2077. [15] H. Song and R.L. Coble: J. Am. Ceram. Soc., 1990, 73, 2086. [16] W.A. Kaysser, M. Sprissler, C.A. Handwerker and J.E. Blendell: J. Am. Ceram. Soc., 1987, 70, 339. [17] M.M. Seabaugh, G.L. Messing and M.D. Vaudin: J. Am. Ceram. Soc., 2000, 83, 3109. [18] X. Su, S. Li and J. Li: J. Am. Ceram. Soc., 2010, 93, 1904. [19] S.C. Srivastava, K.M. Godiwalla and M.K. Banerjee: J. Mater. Sci., 1997, 32, 835. [20] N.S. Bell, S.B. Cho and J.H. Adair: J. Am. Ceram. Soc., 1998, 81, 1411. [21] X. Jin and L. Gao: J. Am. Ceram. Soc., 2004, 87, 533.