- Email: [email protected]
The influence of different titanium sources on flaky α-Al2O3 prepared by molten salt synthesis
Available online at www.sciencedirect.com
CERAMICS INTERNATIONAL
Ceramics International 41 (2015) 12288–12294 www.elsevier.com/locate/ceramint
The influence of different titanium sources on flaky α-Al2O3 prepared by molten salt synthesis Hong Chena, Qidan Wua, Tianxi Yanga, Mengqi Lia, Hucheng Xiea, Cong Lina,b,n b
a School of Material Science and Engineering, Fuzhou University, Fuzhou 350000, People's Republic of China Key Laboratory of Eco-materials Advanced Technology, Fujian Province University, Fuzhou 350000, People's Republic of China
Received 3 January 2015; received in revised form 11 June 2015; accepted 11 June 2015 Available online 19 June 2015
Abstract Flaky α-Al2O3 with different sizes and morphologies was synthetized by molten salt synthesis using Al2(SO4)3 as raw material. The formation mechanism of flaky alumina and the influence of different titanium-containing additives (TiOSO4, TBOT, and TiO2) on the growth of α-Al2O3 crystal were discussed by SEM, XRD and EDS. Results showed that pure α-Al2O3 can be obtained by adding all the three kinds of titaniumcontaining additives, but the diameters, thickness, and the morphologies of flaky α-Al2O3 were greatly affected by the different titanium sources. The adding of TiOSO4 additive resulted in the largest diameter of alumina platelets, while adding TiO2 additive would greatly reduce the diameters. The growth velocities of crystal planes in diameter and thickness direction were influenced by the distribution differences of the titanium ions, which resulted in different morphologies of alumina platelets. Diverse reaction routes for different titanium sources in preparation process were also discussed in this paper. & 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved. Keywords: Flaky α-Al2O3; Molten salt synthesis; Additives; Crystal growth
1. Introduction Flaky α-Al2O3 powders [1] is an important special alumina powder material, which not only has excellent properties [2] of high temperature resistance, corrosion resistance, extreme hardness and high-melting temperature, but also owns special two dimension plate-like structure and excellent anisotropic mechanical properties. Therefore, it has been widely used in the fields such as ceramic reinforcements [3,4], refractory materials [5], pearlescent pigments [6], and fillers to plastic materials [7], etc. Molten salt synthesis (MSS) [8–11] is one of the most important methods to prepare pure α-Al2O3 platelets. Alumina platelets with controlled morphologies can be synthesized by using the molten salts or the compounded salts as reaction medium upon their n Corresponding author at: School of Material Science and Engineering, Fuzhou University, Fuzhou 35000, People's Republic of China. Tel: þ 86 15060664367. E-mail address: [email protected] (C. Lin).
http://dx.doi.org/10.1016/j.ceramint.2015.06.054 0272-8842/& 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
melting temperatures. Generally, there are a few factors that will greatly affect the morphologies of alumina during MSS procedure including the heating temperatures processes, the precursors properties, the molten salts categories and the additives. The inorganic additives have been proved to play a positive role in the morphology control during the crystal growth in the molten salts. Up to now, lots of efforts have been made by inducing different ions into the lattices of alumina or on the crystal surface to optimize the powders morphologies in the preparation process. Hae [12] found that adding AlF3 to aluminum nitrate is a helpful way to obtain α-Al2O3 with plate-like shapes. Hsiang [13] discovered that the addition of Fe3 þ and Ti4 þ can help to change the morphology of alumina from a vermicular structure into hexagonal platelets. Zhu [14] found that α-Al2O3 platelets with irregular shape were obtained under the existence of PO3 3 , and the usage of the titanium ions helped to achieve the α-Al2O3 platelets with discal shape. Although the effects of Ti4 þ ions on the morphology of αAl2O3 powders had been studied [9,13–15], the influences of titanium ions on the formation mechanisms of flaky alumina were
H. Chen et al. / Ceramics International 41 (2015) 12288–12294
still obscure. In this paper, TBOT, TiOSO4, and TiO2 were chosen as additives to prepare the α-Al2O3 platelets based on the molten salt synthesis method, and the influences of the additives on the sizes, morphologies, and the crystal growth mechanisms as well were carefully studied. 2. Experimental procedures Al2(SO4)3 18H2O (Fuchen Chemical reagents Factory, China, 99.5% in purity) and Na2CO3 (Fuchen Chemical reagents Factory, China, 99.5% in purity) were used as raw materials. Al2(SO4)3 18H2O was dissolved in de-ionized water to form an Al3 þ solution with 0.22 mol/L in concentration. K2SO4 and Na2SO4 were added separately into the Al2(SO4)3 solution as molten salts with the molar ratio of 2: 2: 1. Na2CO3 solution (n (Na2CO3): n(Al2(SO4)3 18H2O)=3:1) was dropped slowly into the Al2(SO4)3 18H2O solution at 60 1C followed by stirring rapidly for 15 min. The obtained gel was dried at 120 1C for 24 h and heated at 1150 1C for 4 h after ball-milling. The final powders were ultrasonic cleaned with de-ionized water for 5 times to remove the residual salts and then dried. Different amounts of titanium-containing additives including tetrabutyl titanate (TBOT), titanyl sulfate (TiOSO4), and titania (TiO2) were added into Al2(SO4)3 18H2O solution with different amounts before reacted with Na2CO3. Characterization of solid TiO2 particles additive was shown in Fig. 1. The annotation symbols of different additives amounts were listed in Table 1. The phase of final powders was examined by X-ray diffraction (XRD, UltimaIII, Rigaku, Japan). The morphologies of particles were investigated by scanning electron microscopy ( SEM, Supra55, Zeiss, Germany) to measure the diameters and thicknesses of the platelets. More than 300 particles on SEM pictures were chosen to be measured for every component before the average values and radius–thickness ratios were calculated. The energy-dispersive X-ray (EDS) was used to analyze elemental composition by scanning corresponding Al2O3 crystal planes and whole system. 3. Results and discussions The XRD patterns of alumina powders produced by the addition of three different titanium sources are shown in Fig. 2. The sample without any additives was also examined for comparison. The results showed that α-Al2O3 powders with high purity and crystallinity can be obtained without any
12289
Table 1 Correspondence to the annotation symbols of different additive concentrations. Annotation symbols
Ti-0
Ti-1
Ti-2
Ti-3
Ti-4
Ti-8
3.2 10 3 mol/L
0
1
2
3
4
8
additives at 1150 1C. No obvious phase transformation happened when the titanium additives were added. It can be concluded that the addition of titanium additives would not affect the phase formation of α-Al2O3 during the molten salt synthesis procedures. SEM micrographs of flaky α-Al2O3 with different additives in the amount of Ti-3 are shown in Fig. 3. Alumina platelets with imperfect hexagonal shape were obtained without any additives, and there were a great number of flakes with irregular shape and agglomerations in final product, as shown in Fig. 2(a). The thicknesses of the product were smaller than those with titanium additives. Flaky α-Al2O3 with hexagonal shape and less overlaps can be obtained when adding different additives. It is obvious that adding titanium-containing additives would help to obtain the flaky products with completed hexagonal shapes. The micrographs of α-Al2O3 platelets prepared by different amounts of additives are shown in Fig. 4. It revealed that the concentrations of the additives have a significant impact on the sizes and morphologies of the final products. For all of three additives, regular and hexagonal flaky α-Al2O3 can be achieved when the adding concentrations were relatively low. When the addition concentrations increased, most of the α-Al2O3 platelets were still mainly in the shape of hexagon with reducing diameter and increasing thickness. The thickness of the platelets increased along with the adding amounts until they reached to very high values, which may greatly change the morphology of particles. For example, in the case of Ti-8, block structure products with more numbers of exposing planes and smaller diameter can be observed instead of sheet-shape products. The diameter, thickness and radius–thickness ratios of alumina particles prepared by different concentrations of additives are shown in Fig. 5. For the case of TiOSO4 and TBOT additives, the diameters of the flaky α-Al2O3 increased and reached to their highest values when the additives
35
D=118.5 nm
Frequency/%
30 25 20 15 10 5 0
50
75 100 125 150 175 200 225 Diameter/nm
Fig. 1. Characterization of TiO2 particles additive. (a) SEM micrographs of TiO2 particles; (b) diameter distribution of TiO2 particles.
12290
H. Chen et al. / Ceramics International 41 (2015) 12288–12294
concentrations got to Ti-2, and then decreased with the further increasing of additive contents. However, the diameters of flaky products doped by TiO2 showed the tendency of decreasing when increasing the additive concentrations, and had the smaller values in diameter when compared with the ones of the other two additives under the same condition of titanium additive concentrations. Moreover, it is beneficial from adding TiOSO4 to achieve the largest diameter when comparing with other two additives. As to the thickness, it can be found that the thickness of the alumina platelets increased with the increase of the adding concentrations while the increasing tendency gradually reduced. The variation of radius–thickness ratio with the additive contents is shown in Fig. 5(c). Very similar curves of radius–thickness ratio can be
Fig. 2. XRD patterns of alumina adding three different additives in Ti-3.
found in the case of TiOSO4 and TBOT additive, while the ratio of products adding with TiO2 is a little bit lower. The ratios dropped to only about 2 for all three cases when the concentrations of titanium additives rose to Ti-8, which indicated that the morphology of the alumina particles changed from the platelets into the polyhedron shapes. The final morphology for crystal growth is determined by both the crystal structure and the circumstance for growth. To be specific, the hexagonal crystal structure of α-Al2O3 makes it possible to have the potential of anisotropic grain growth. On the other hand, the external circumstance, such as the molten salts and different additives sources would also have great effects on changing its growth habit to some degree. It is well known that the surface of the alumina crystal will be absorbed by some ions from the salts after nucleation in the molten salts, and then ultimately change the surface energies of the absorbed planes. In the case of adding titanium additives [14], Ti4 þ will diffuse into the lattices of alumina to substitute Al3 þ with substitute ratio of 1:1 at high temperatures, which lead to lattice deformation due to the existence of extra O2 vacancies. Table 2 shows the EDS results of flaky α-Al2O3 with additives of Ti-2 and Ti-8, and the appearance of titanium element testified that Ti4 þ diffused into alumina lattices. The increasing defect concentration will result in the increase of the diffusion rate in alumina crystal, and accelerate the growth of alumina, whatever at the diameter direction or at the thickness direction. On the other hand, the surface energies of different
Fig. 3. SEM micrographs of flaky alumina adding different additives in the amount of Ti-3. (a) Without additive; (b) TiOSO4; (c) TBOT; (d) TiO2.
H. Chen et al. / Ceramics International 41 (2015) 12288–12294
12291
Fig. 4. SEM micrographs of flaky α-Al2O3 adding three additives in different amounts. (a) TiOSO4-2; (b) TiOSO4-4; (c) TiOSO4-8; (d) TBOT-2; (e) TBOT-4; (f) TBOT-8; (g) TiO2-2;(h) TiO2-4; (i) TiO2-8.
planes were also changed due to the diffusion of Ti4 þ and reduced the growth velocity differences of each plane which may lower the effect of anisotropic growth. Fig. 6 shows the EDS testing schematic diagrams on the different planes [16] in one single alumina crystal. More than 10 crystals were randomly checked and the results are summarized in Table 3, which indicates that plane (0001) contained the largest amount of titanium elements. According to the calculation of the crystal structure of alumina, the (0001) plane is the plane has the largest density of aluminum atoms compared with other planes, therefore it is statistically reasonable that a large amount of Ti atoms would be absorbed and diffused into this plane, and lead to the increase of the surface energy of
(0001) plane. As a result, the surface energies difference among different planes lowed down when the adding amount of additives increased, and the growth velocity in diameter the direction ([1010] direction) began to decrease when the adding concentrations of additives increased and the surface energies difference lowered down to some extent. It is well known that the crystal growth with very high velocities at some crystal orientations would be prohibited because of the balance of the surface energies difference among various planes [17]. As a result, some new planes in these directions would emerge. As shown in Fig. 6, the newly emerged planes will result in more exposing crystal planes in samples with Ti-8, and transform the hexagon shape of
12292
H. Chen et al. / Ceramics International 41 (2015) 12288–12294
Fig. 5. Variation of flaky α-Al2O3 diameter, thickness and radius–thickness ratio with three additives in different concentrations. (a): diameter; (b) thickness; (c): radius–thickness ratio.
Table 2 EDS results of α-Al2O3 in Ti-2 and Ti-8. No.
TiOSO4-2 TBOT-2 TiO2-2 TiOSO4-8 TBOT-8 TiO2-8
Element (at%) Ti
Al
O
1.63 1.05 0.99 3.21 2.96 1.61
55.20 56.09 55.56 52.98 51.75 55.56
43.17 42.86 43.45 43.81 45.29 42.83
particles from hexagon into polygon or even roundness shapes, that is, lower down the degree of anisotropy. However, the different titanium species would lead to different titanium element contents [18] due to their diverse reaction routes as shown in Table 2. These differences may be relate to the various additive properties during reaction. The reactions in whole process are shown in Fig. 7. TiOSO4 firstly reacted with Na2CO3 in the solution and precipitated into TiO(OH)2. The reaction between Al2(SO4)3 and Na2CO3 happened accompany with the generation of TiO (OH)2 precipitation, resulted in the co-precipitation of Al(OH)3 and TiO(OH)2, which lead to very homogeneous mixture between aluminum and titanium species. Al(OH)3 and TiO (OH)2 decomposed to their own oxides during the calcination process, and then Ti4 þ diffuse into adjacent crystals of Al2O3 when the temperature elevated. It can be assumed that the coprecipitation of Al(OH)3 and TiO(OH)2 played a positive role
in the uniform diffusion of Ti4 þ into the alumina lattice, and might benefit to the uniformity of the final microstructures. The reactions for TBOT additives are similar to TiOSO4, except that TBOT would firstly hydrolyze [19] in the water to form Ti(OH)4 precipitation. Since the precipitation of Ti(OH)4 was formed before the reaction happened between Al2(SO4)3 and Na2CO3, Al(OH)3 would like to precipitate on the surface of Ti(OH)4, which can be treated as crystal nucleus for the growth of Al(OH)3. And then Ti4 þ diffused out from the attached Ti(OH)4 into outer Al2O3 crystal during the thermal decomposition. TiO2 would not like to dissolve or hydrolyze in the water. In this case, the particle sizes of TiO2 that used in this case were much larger (as shown in Fig. 1(b)), compared with the fine nucleus size in the case of TiOSO4 and TBOT, which will greatly reduce the diffusion rate between titania to alumina [20,21], and result in the uneven distribution of titanium
H. Chen et al. / Ceramics International 41 (2015) 12288–12294
12293
Fig. 6. EDS testing schematic diagrams of flaky α-Al2O3 different planes under different additives in Ti-8; (a): TiOSO4; (b): TBOT; (c): TiO2.
Table 3 Titanium contents on different planes of alumina under different additives in Ti-8 by EDS. Ti (wt%)
Plane C
Plane A
Plane B
TiOSO4 TBOT TiO2
1.36 1.29 1.02
0.82 0.57 0.43
0.69 0.54 0.39
Fig. 7. Schematic diagram of diverse reaction routes formed by different titanium species in preparation process.
12294
H. Chen et al. / Ceramics International 41 (2015) 12288–12294
diffusion in alumina lattices. For these reasons, it was extremely difficult in this case to achieve uniform microstructures in the final products, especially when the addition titania was pretty high. 4. Conclusions Pure α-Al2O3 with well-developed hexagonal shape and good dispersion were synthesized by adding three kinds of titaniumcontaining additives. The diameter of the flaky products increased and then reduced with the increasing concentrations of TiOSO4 and TBOT additives, while the thickness showed out the tendency of increasing for all three titanium-containing additives. The different contents of titanium ions on different planes affected the crystal growth velocities at both diameter and thickness directions, and resulted in various morphologies of alumina. It can be attibuted to the diverse reaction routes for different titanium species in the preparation process which created different decomposition products, and then affected the diffusion rate of titanium. It can be concluded that uniform distribution of titanium sources in the processor would be beneficial to achieve homogeneous microstructures in the final products than using TiOSO4 and TBOT. Acknowledgment The project is supported by the National Natural Science Foundation of China (No. 51102046) and the Natural Science Foundation of Fujian Province (No. 2013J05067). The authors would also like to thank for the financial supports from Fuzhou University (No. 046022328). References [1] T. M. S. K. Nitta, J. Sugahara, Flaky aluminum oxide and pearlescent pigment, and production thereof, EP0763573A2, 1997. [2] S. Hashimoto, A. Yamaguchi, Synthesis of α-Al2O3 platelets using sodium sulfate flux, J. Mater. Res. 14 (1999) 4667–4672. [3] Matthew M. Seabaugh, Ingrid H. Kerscht, G.L. Messing, Texture development by templated grain growth in liquid-phase-sintered αalumina, J. Am. Ceram. Soc. 80 (1997) 1181–1187. [4] L. Zhang, J. Vleugels, O. Van Der Biest, Fabrication of textured alumina by orienting template particles during electrophoretic deposition, J. Eur. Ceram. Soc. 30 (2010) 1195–1202.
[5] A. Cemail, The role of fine alumina and mullite particles on the thermomechanical behaviour of alumina–mullite refractory materials, Mater. Lett. 57 (2002) 708–714. [6] P. Karlsson, A.E.C. Palmqvist, K. Holmberg, Surface modification for aluminium pigment inhibition, Adv. Colloid Interface Sci. 128–130 (2006) 121–134. [7] M. Egashira, Y. Utsunomiya, N. Yoshimoto, M. Morita, Effects of the surface treatment of the Al2O3 filler on the lithium electrode/solid polymer electrolyte interface properties, Electrochim. Acta 52 (2006) 1082–1086. [8] X.-H. Jin, L. Gao, Size control of alpha-Al2O3 platelets synthesized in molten Na2SO4 flux, J. Am. Ceram. Soc. 87 (2004) 533–540. [9] H.-I. Hsiang, T.-H. Chen, C.-C. Chuang, Synthesis of alpha-alumina hexagonal platelets using a mixture of boehmite and potassium sulfate, J. Am. Ceram. Soc. 90 (2007) 4070–4072. [10] X. Su, J. Li, Low temperature synthesis of single-crystal alpha alumina platelets by calcining bayerite and potassium sulfate, J. Mater. Sci. Technol. 27 (2011) 1011–1015. [11] L.-H. Zhu, R.-R. Tu, Q.-W. Huang, Molten salt synthesis of α-Al2O3 platelets using NaAlO2 as raw material, Ceram. Int. 38 (2012) 901–908. [12] H.J. Kim, T.G. Kim, J.J. Kim, S.S. Park, S.S. Hong, G.D. Lee, Influences of precursor and additive on the morphology of nanocrystalline αalumina, J. Phys. Chem. Solids 69 (2008) 1521–1524. [13] H.-I. Hsiang, C.-C. Chuang, T.-H. Chen, F.-S. Yen, Ti4 þ addition effect on alpha-Al2O3 flakes synthesis using a mixture of boehmite and potassium sulfate, Ceram. Int. 36 (2010) 1467–1472. [14] L.-H. Zhu, Q.-W. Huang, Morphology control of α-Al2O3 platelets by molten salt synthesis, Ceram. Int. 37 (2011) 249–255. [15] H.-B. Tan, Effect of ferric oxide on plate-like α-Al2O3 particles morphology development, Mater. Manuf. Process. 28 (2013) 154–156. [16] J.H. Adair, S.-B. Cho, N.S. Bell, A.J. Perrotta, Recent developments in morphological control of α-Al2O3 particles synthesized in 1,4-butanediol solution, J. Dispers. Sci. Technol. 22 (2001) 143–165. [17] W.-J. Li, E.-W. Shi, Z.-W. Yin, Theoretical model of anionic coordination polyhedron growth uints and the growth habits of crystal, Sci. China Ser. E 6 (2001) 487–495. [18] J. Aguado-Serrano, M.L. Rojas-Cervantes, Titania aerogels: influence of synthesis parameters on textural, crystalline, and surface acid properties, Microporous Mesoporous Mater. 88 (2006) 205–213. [19] O.A. Fouad, R.A. Geioushy, S.M. El-Sheikh, M.H. Khedr, I.A. Ibrahim, Metalorganic chemical vapor deposition of Ti–O–C–N thin films using TBOT as a promising precursor, J. Alloy. Compd. 509 (2011) 6090–6095. [20] U.O. Akkaya Arıer, F.Z. Tepehan, Influence of Al2O3:TiO2 ratio on the structural and optical properties of Al2O3–TiO2 nano-composite films produced by sol gel method, Compos. Part B: Eng. 58 (2014) 147–151. [21] R. Dong, S. Liu, Z. Li, Z. Chen, H. Zhang, TiO2 microspheres with variable morphology, size and density synthesized by a facile emulsionmediated hydrothermal process, Mater. Lett. 123 (2014) 135–137.
CERAMICS INTERNATIONAL
Ceramics International 41 (2015) 12288–12294 www.elsevier.com/locate/ceramint
The influence of different titanium sources on flaky α-Al2O3 prepared by molten salt synthesis Hong Chena, Qidan Wua, Tianxi Yanga, Mengqi Lia, Hucheng Xiea, Cong Lina,b,n b
a School of Material Science and Engineering, Fuzhou University, Fuzhou 350000, People's Republic of China Key Laboratory of Eco-materials Advanced Technology, Fujian Province University, Fuzhou 350000, People's Republic of China
Received 3 January 2015; received in revised form 11 June 2015; accepted 11 June 2015 Available online 19 June 2015
Abstract Flaky α-Al2O3 with different sizes and morphologies was synthetized by molten salt synthesis using Al2(SO4)3 as raw material. The formation mechanism of flaky alumina and the influence of different titanium-containing additives (TiOSO4, TBOT, and TiO2) on the growth of α-Al2O3 crystal were discussed by SEM, XRD and EDS. Results showed that pure α-Al2O3 can be obtained by adding all the three kinds of titaniumcontaining additives, but the diameters, thickness, and the morphologies of flaky α-Al2O3 were greatly affected by the different titanium sources. The adding of TiOSO4 additive resulted in the largest diameter of alumina platelets, while adding TiO2 additive would greatly reduce the diameters. The growth velocities of crystal planes in diameter and thickness direction were influenced by the distribution differences of the titanium ions, which resulted in different morphologies of alumina platelets. Diverse reaction routes for different titanium sources in preparation process were also discussed in this paper. & 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved. Keywords: Flaky α-Al2O3; Molten salt synthesis; Additives; Crystal growth
1. Introduction Flaky α-Al2O3 powders [1] is an important special alumina powder material, which not only has excellent properties [2] of high temperature resistance, corrosion resistance, extreme hardness and high-melting temperature, but also owns special two dimension plate-like structure and excellent anisotropic mechanical properties. Therefore, it has been widely used in the fields such as ceramic reinforcements [3,4], refractory materials [5], pearlescent pigments [6], and fillers to plastic materials [7], etc. Molten salt synthesis (MSS) [8–11] is one of the most important methods to prepare pure α-Al2O3 platelets. Alumina platelets with controlled morphologies can be synthesized by using the molten salts or the compounded salts as reaction medium upon their n Corresponding author at: School of Material Science and Engineering, Fuzhou University, Fuzhou 35000, People's Republic of China. Tel: þ 86 15060664367. E-mail address: [email protected] (C. Lin).
http://dx.doi.org/10.1016/j.ceramint.2015.06.054 0272-8842/& 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
melting temperatures. Generally, there are a few factors that will greatly affect the morphologies of alumina during MSS procedure including the heating temperatures processes, the precursors properties, the molten salts categories and the additives. The inorganic additives have been proved to play a positive role in the morphology control during the crystal growth in the molten salts. Up to now, lots of efforts have been made by inducing different ions into the lattices of alumina or on the crystal surface to optimize the powders morphologies in the preparation process. Hae [12] found that adding AlF3 to aluminum nitrate is a helpful way to obtain α-Al2O3 with plate-like shapes. Hsiang [13] discovered that the addition of Fe3 þ and Ti4 þ can help to change the morphology of alumina from a vermicular structure into hexagonal platelets. Zhu [14] found that α-Al2O3 platelets with irregular shape were obtained under the existence of PO3 3 , and the usage of the titanium ions helped to achieve the α-Al2O3 platelets with discal shape. Although the effects of Ti4 þ ions on the morphology of αAl2O3 powders had been studied [9,13–15], the influences of titanium ions on the formation mechanisms of flaky alumina were
H. Chen et al. / Ceramics International 41 (2015) 12288–12294
still obscure. In this paper, TBOT, TiOSO4, and TiO2 were chosen as additives to prepare the α-Al2O3 platelets based on the molten salt synthesis method, and the influences of the additives on the sizes, morphologies, and the crystal growth mechanisms as well were carefully studied. 2. Experimental procedures Al2(SO4)3 18H2O (Fuchen Chemical reagents Factory, China, 99.5% in purity) and Na2CO3 (Fuchen Chemical reagents Factory, China, 99.5% in purity) were used as raw materials. Al2(SO4)3 18H2O was dissolved in de-ionized water to form an Al3 þ solution with 0.22 mol/L in concentration. K2SO4 and Na2SO4 were added separately into the Al2(SO4)3 solution as molten salts with the molar ratio of 2: 2: 1. Na2CO3 solution (n (Na2CO3): n(Al2(SO4)3 18H2O)=3:1) was dropped slowly into the Al2(SO4)3 18H2O solution at 60 1C followed by stirring rapidly for 15 min. The obtained gel was dried at 120 1C for 24 h and heated at 1150 1C for 4 h after ball-milling. The final powders were ultrasonic cleaned with de-ionized water for 5 times to remove the residual salts and then dried. Different amounts of titanium-containing additives including tetrabutyl titanate (TBOT), titanyl sulfate (TiOSO4), and titania (TiO2) were added into Al2(SO4)3 18H2O solution with different amounts before reacted with Na2CO3. Characterization of solid TiO2 particles additive was shown in Fig. 1. The annotation symbols of different additives amounts were listed in Table 1. The phase of final powders was examined by X-ray diffraction (XRD, UltimaIII, Rigaku, Japan). The morphologies of particles were investigated by scanning electron microscopy ( SEM, Supra55, Zeiss, Germany) to measure the diameters and thicknesses of the platelets. More than 300 particles on SEM pictures were chosen to be measured for every component before the average values and radius–thickness ratios were calculated. The energy-dispersive X-ray (EDS) was used to analyze elemental composition by scanning corresponding Al2O3 crystal planes and whole system. 3. Results and discussions The XRD patterns of alumina powders produced by the addition of three different titanium sources are shown in Fig. 2. The sample without any additives was also examined for comparison. The results showed that α-Al2O3 powders with high purity and crystallinity can be obtained without any
12289
Table 1 Correspondence to the annotation symbols of different additive concentrations. Annotation symbols
Ti-0
Ti-1
Ti-2
Ti-3
Ti-4
Ti-8
3.2 10 3 mol/L
0
1
2
3
4
8
additives at 1150 1C. No obvious phase transformation happened when the titanium additives were added. It can be concluded that the addition of titanium additives would not affect the phase formation of α-Al2O3 during the molten salt synthesis procedures. SEM micrographs of flaky α-Al2O3 with different additives in the amount of Ti-3 are shown in Fig. 3. Alumina platelets with imperfect hexagonal shape were obtained without any additives, and there were a great number of flakes with irregular shape and agglomerations in final product, as shown in Fig. 2(a). The thicknesses of the product were smaller than those with titanium additives. Flaky α-Al2O3 with hexagonal shape and less overlaps can be obtained when adding different additives. It is obvious that adding titanium-containing additives would help to obtain the flaky products with completed hexagonal shapes. The micrographs of α-Al2O3 platelets prepared by different amounts of additives are shown in Fig. 4. It revealed that the concentrations of the additives have a significant impact on the sizes and morphologies of the final products. For all of three additives, regular and hexagonal flaky α-Al2O3 can be achieved when the adding concentrations were relatively low. When the addition concentrations increased, most of the α-Al2O3 platelets were still mainly in the shape of hexagon with reducing diameter and increasing thickness. The thickness of the platelets increased along with the adding amounts until they reached to very high values, which may greatly change the morphology of particles. For example, in the case of Ti-8, block structure products with more numbers of exposing planes and smaller diameter can be observed instead of sheet-shape products. The diameter, thickness and radius–thickness ratios of alumina particles prepared by different concentrations of additives are shown in Fig. 5. For the case of TiOSO4 and TBOT additives, the diameters of the flaky α-Al2O3 increased and reached to their highest values when the additives
35
D=118.5 nm
Frequency/%
30 25 20 15 10 5 0
50
75 100 125 150 175 200 225 Diameter/nm
Fig. 1. Characterization of TiO2 particles additive. (a) SEM micrographs of TiO2 particles; (b) diameter distribution of TiO2 particles.
12290
H. Chen et al. / Ceramics International 41 (2015) 12288–12294
concentrations got to Ti-2, and then decreased with the further increasing of additive contents. However, the diameters of flaky products doped by TiO2 showed the tendency of decreasing when increasing the additive concentrations, and had the smaller values in diameter when compared with the ones of the other two additives under the same condition of titanium additive concentrations. Moreover, it is beneficial from adding TiOSO4 to achieve the largest diameter when comparing with other two additives. As to the thickness, it can be found that the thickness of the alumina platelets increased with the increase of the adding concentrations while the increasing tendency gradually reduced. The variation of radius–thickness ratio with the additive contents is shown in Fig. 5(c). Very similar curves of radius–thickness ratio can be
Fig. 2. XRD patterns of alumina adding three different additives in Ti-3.
found in the case of TiOSO4 and TBOT additive, while the ratio of products adding with TiO2 is a little bit lower. The ratios dropped to only about 2 for all three cases when the concentrations of titanium additives rose to Ti-8, which indicated that the morphology of the alumina particles changed from the platelets into the polyhedron shapes. The final morphology for crystal growth is determined by both the crystal structure and the circumstance for growth. To be specific, the hexagonal crystal structure of α-Al2O3 makes it possible to have the potential of anisotropic grain growth. On the other hand, the external circumstance, such as the molten salts and different additives sources would also have great effects on changing its growth habit to some degree. It is well known that the surface of the alumina crystal will be absorbed by some ions from the salts after nucleation in the molten salts, and then ultimately change the surface energies of the absorbed planes. In the case of adding titanium additives [14], Ti4 þ will diffuse into the lattices of alumina to substitute Al3 þ with substitute ratio of 1:1 at high temperatures, which lead to lattice deformation due to the existence of extra O2 vacancies. Table 2 shows the EDS results of flaky α-Al2O3 with additives of Ti-2 and Ti-8, and the appearance of titanium element testified that Ti4 þ diffused into alumina lattices. The increasing defect concentration will result in the increase of the diffusion rate in alumina crystal, and accelerate the growth of alumina, whatever at the diameter direction or at the thickness direction. On the other hand, the surface energies of different
Fig. 3. SEM micrographs of flaky alumina adding different additives in the amount of Ti-3. (a) Without additive; (b) TiOSO4; (c) TBOT; (d) TiO2.
H. Chen et al. / Ceramics International 41 (2015) 12288–12294
12291
Fig. 4. SEM micrographs of flaky α-Al2O3 adding three additives in different amounts. (a) TiOSO4-2; (b) TiOSO4-4; (c) TiOSO4-8; (d) TBOT-2; (e) TBOT-4; (f) TBOT-8; (g) TiO2-2;(h) TiO2-4; (i) TiO2-8.
planes were also changed due to the diffusion of Ti4 þ and reduced the growth velocity differences of each plane which may lower the effect of anisotropic growth. Fig. 6 shows the EDS testing schematic diagrams on the different planes [16] in one single alumina crystal. More than 10 crystals were randomly checked and the results are summarized in Table 3, which indicates that plane (0001) contained the largest amount of titanium elements. According to the calculation of the crystal structure of alumina, the (0001) plane is the plane has the largest density of aluminum atoms compared with other planes, therefore it is statistically reasonable that a large amount of Ti atoms would be absorbed and diffused into this plane, and lead to the increase of the surface energy of
(0001) plane. As a result, the surface energies difference among different planes lowed down when the adding amount of additives increased, and the growth velocity in diameter the direction ([1010] direction) began to decrease when the adding concentrations of additives increased and the surface energies difference lowered down to some extent. It is well known that the crystal growth with very high velocities at some crystal orientations would be prohibited because of the balance of the surface energies difference among various planes [17]. As a result, some new planes in these directions would emerge. As shown in Fig. 6, the newly emerged planes will result in more exposing crystal planes in samples with Ti-8, and transform the hexagon shape of
12292
H. Chen et al. / Ceramics International 41 (2015) 12288–12294
Fig. 5. Variation of flaky α-Al2O3 diameter, thickness and radius–thickness ratio with three additives in different concentrations. (a): diameter; (b) thickness; (c): radius–thickness ratio.
Table 2 EDS results of α-Al2O3 in Ti-2 and Ti-8. No.
TiOSO4-2 TBOT-2 TiO2-2 TiOSO4-8 TBOT-8 TiO2-8
Element (at%) Ti
Al
O
1.63 1.05 0.99 3.21 2.96 1.61
55.20 56.09 55.56 52.98 51.75 55.56
43.17 42.86 43.45 43.81 45.29 42.83
particles from hexagon into polygon or even roundness shapes, that is, lower down the degree of anisotropy. However, the different titanium species would lead to different titanium element contents [18] due to their diverse reaction routes as shown in Table 2. These differences may be relate to the various additive properties during reaction. The reactions in whole process are shown in Fig. 7. TiOSO4 firstly reacted with Na2CO3 in the solution and precipitated into TiO(OH)2. The reaction between Al2(SO4)3 and Na2CO3 happened accompany with the generation of TiO (OH)2 precipitation, resulted in the co-precipitation of Al(OH)3 and TiO(OH)2, which lead to very homogeneous mixture between aluminum and titanium species. Al(OH)3 and TiO (OH)2 decomposed to their own oxides during the calcination process, and then Ti4 þ diffuse into adjacent crystals of Al2O3 when the temperature elevated. It can be assumed that the coprecipitation of Al(OH)3 and TiO(OH)2 played a positive role
in the uniform diffusion of Ti4 þ into the alumina lattice, and might benefit to the uniformity of the final microstructures. The reactions for TBOT additives are similar to TiOSO4, except that TBOT would firstly hydrolyze [19] in the water to form Ti(OH)4 precipitation. Since the precipitation of Ti(OH)4 was formed before the reaction happened between Al2(SO4)3 and Na2CO3, Al(OH)3 would like to precipitate on the surface of Ti(OH)4, which can be treated as crystal nucleus for the growth of Al(OH)3. And then Ti4 þ diffused out from the attached Ti(OH)4 into outer Al2O3 crystal during the thermal decomposition. TiO2 would not like to dissolve or hydrolyze in the water. In this case, the particle sizes of TiO2 that used in this case were much larger (as shown in Fig. 1(b)), compared with the fine nucleus size in the case of TiOSO4 and TBOT, which will greatly reduce the diffusion rate between titania to alumina [20,21], and result in the uneven distribution of titanium
H. Chen et al. / Ceramics International 41 (2015) 12288–12294
12293
Fig. 6. EDS testing schematic diagrams of flaky α-Al2O3 different planes under different additives in Ti-8; (a): TiOSO4; (b): TBOT; (c): TiO2.
Table 3 Titanium contents on different planes of alumina under different additives in Ti-8 by EDS. Ti (wt%)
Plane C
Plane A
Plane B
TiOSO4 TBOT TiO2
1.36 1.29 1.02
0.82 0.57 0.43
0.69 0.54 0.39
Fig. 7. Schematic diagram of diverse reaction routes formed by different titanium species in preparation process.
12294
H. Chen et al. / Ceramics International 41 (2015) 12288–12294
diffusion in alumina lattices. For these reasons, it was extremely difficult in this case to achieve uniform microstructures in the final products, especially when the addition titania was pretty high. 4. Conclusions Pure α-Al2O3 with well-developed hexagonal shape and good dispersion were synthesized by adding three kinds of titaniumcontaining additives. The diameter of the flaky products increased and then reduced with the increasing concentrations of TiOSO4 and TBOT additives, while the thickness showed out the tendency of increasing for all three titanium-containing additives. The different contents of titanium ions on different planes affected the crystal growth velocities at both diameter and thickness directions, and resulted in various morphologies of alumina. It can be attibuted to the diverse reaction routes for different titanium species in the preparation process which created different decomposition products, and then affected the diffusion rate of titanium. It can be concluded that uniform distribution of titanium sources in the processor would be beneficial to achieve homogeneous microstructures in the final products than using TiOSO4 and TBOT. Acknowledgment The project is supported by the National Natural Science Foundation of China (No. 51102046) and the Natural Science Foundation of Fujian Province (No. 2013J05067). The authors would also like to thank for the financial supports from Fuzhou University (No. 046022328). References [1] T. M. S. K. Nitta, J. Sugahara, Flaky aluminum oxide and pearlescent pigment, and production thereof, EP0763573A2, 1997. [2] S. Hashimoto, A. Yamaguchi, Synthesis of α-Al2O3 platelets using sodium sulfate flux, J. Mater. Res. 14 (1999) 4667–4672. [3] Matthew M. Seabaugh, Ingrid H. Kerscht, G.L. Messing, Texture development by templated grain growth in liquid-phase-sintered αalumina, J. Am. Ceram. Soc. 80 (1997) 1181–1187. [4] L. Zhang, J. Vleugels, O. Van Der Biest, Fabrication of textured alumina by orienting template particles during electrophoretic deposition, J. Eur. Ceram. Soc. 30 (2010) 1195–1202.
[5] A. Cemail, The role of fine alumina and mullite particles on the thermomechanical behaviour of alumina–mullite refractory materials, Mater. Lett. 57 (2002) 708–714. [6] P. Karlsson, A.E.C. Palmqvist, K. Holmberg, Surface modification for aluminium pigment inhibition, Adv. Colloid Interface Sci. 128–130 (2006) 121–134. [7] M. Egashira, Y. Utsunomiya, N. Yoshimoto, M. Morita, Effects of the surface treatment of the Al2O3 filler on the lithium electrode/solid polymer electrolyte interface properties, Electrochim. Acta 52 (2006) 1082–1086. [8] X.-H. Jin, L. Gao, Size control of alpha-Al2O3 platelets synthesized in molten Na2SO4 flux, J. Am. Ceram. Soc. 87 (2004) 533–540. [9] H.-I. Hsiang, T.-H. Chen, C.-C. Chuang, Synthesis of alpha-alumina hexagonal platelets using a mixture of boehmite and potassium sulfate, J. Am. Ceram. Soc. 90 (2007) 4070–4072. [10] X. Su, J. Li, Low temperature synthesis of single-crystal alpha alumina platelets by calcining bayerite and potassium sulfate, J. Mater. Sci. Technol. 27 (2011) 1011–1015. [11] L.-H. Zhu, R.-R. Tu, Q.-W. Huang, Molten salt synthesis of α-Al2O3 platelets using NaAlO2 as raw material, Ceram. Int. 38 (2012) 901–908. [12] H.J. Kim, T.G. Kim, J.J. Kim, S.S. Park, S.S. Hong, G.D. Lee, Influences of precursor and additive on the morphology of nanocrystalline αalumina, J. Phys. Chem. Solids 69 (2008) 1521–1524. [13] H.-I. Hsiang, C.-C. Chuang, T.-H. Chen, F.-S. Yen, Ti4 þ addition effect on alpha-Al2O3 flakes synthesis using a mixture of boehmite and potassium sulfate, Ceram. Int. 36 (2010) 1467–1472. [14] L.-H. Zhu, Q.-W. Huang, Morphology control of α-Al2O3 platelets by molten salt synthesis, Ceram. Int. 37 (2011) 249–255. [15] H.-B. Tan, Effect of ferric oxide on plate-like α-Al2O3 particles morphology development, Mater. Manuf. Process. 28 (2013) 154–156. [16] J.H. Adair, S.-B. Cho, N.S. Bell, A.J. Perrotta, Recent developments in morphological control of α-Al2O3 particles synthesized in 1,4-butanediol solution, J. Dispers. Sci. Technol. 22 (2001) 143–165. [17] W.-J. Li, E.-W. Shi, Z.-W. Yin, Theoretical model of anionic coordination polyhedron growth uints and the growth habits of crystal, Sci. China Ser. E 6 (2001) 487–495. [18] J. Aguado-Serrano, M.L. Rojas-Cervantes, Titania aerogels: influence of synthesis parameters on textural, crystalline, and surface acid properties, Microporous Mesoporous Mater. 88 (2006) 205–213. [19] O.A. Fouad, R.A. Geioushy, S.M. El-Sheikh, M.H. Khedr, I.A. Ibrahim, Metalorganic chemical vapor deposition of Ti–O–C–N thin films using TBOT as a promising precursor, J. Alloy. Compd. 509 (2011) 6090–6095. [20] U.O. Akkaya Arıer, F.Z. Tepehan, Influence of Al2O3:TiO2 ratio on the structural and optical properties of Al2O3–TiO2 nano-composite films produced by sol gel method, Compos. Part B: Eng. 58 (2014) 147–151. [21] R. Dong, S. Liu, Z. Li, Z. Chen, H. Zhang, TiO2 microspheres with variable morphology, size and density synthesized by a facile emulsionmediated hydrothermal process, Mater. Lett. 123 (2014) 135–137.