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Study on the densification of composite coating particles of α-Al2O3–SiO2
Materials Chemistry and Physics 75 (2002) 265–269
Study on the densification of composite coating particles of ␣-Al2 O3–SiO2 Yue-Feng Tang a,∗ , Zhi-Da Ling b , Yi-Nong Lu b , Ai-Dong Li a , Hui-Qin Ling c , Yi-Jun Wang a , Qi-Yue Shao a a
c
National Laboratory of Solid State Microstructures, and Department of Materials Science and Engineering, Nanjing University, Nanjing 210093, PR China b School of Materials Science and Engineering, Nanjing University of Technology, Nanjing 210009, PR China National Laboratory of Solid State Microstructures, and Department of Physics, Nanjing University, Nanjing 210093, PR China
Abstract Composite coating particles consisting of alpha alumina cores (␣-Al2 O3 , average particle size 0.26 m) with outer homogeneous amorphous silica layer were prepared by the heterogeneous nucleation-and-growth processing. Densification of composite coating particles was studied at different sintering temperatures. Density of samples sintered at different temperatures was measured by the Archimedes method. Densification procedure of samples was observed and analyzed by scanning electron microscopy (SEM) and X-ray diffraction (XRD). Based on the density of samples, XRD and SEM, densification procedure of samples is divided into two stages: one is the transient viscous flow of amorphous silica layer below 1350 ◦ C, the other is mullitization of composite coating particles at 1500 ◦ C. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Densification; Transient viscous sintering; Mullitization; Composite coating particles; The heterogeneous nucleation-and-growth processing
1. Introduction Sintering of mullite powders to high densities normally requires high temperatures, special forming and sintering treatments (such as ultra-high cold isostatic pressing [1], slip casting [2,3], hot-pressing [4]) and adding sintering aids [5] because of the low bulk and grain-boundary diffusion coefficients for mullite [3]. Activation enthalpy calculations on mullite grain growth yield very high values (ca. 700 kJ mol−1 ) which is of the order of the activation energy of Si4+ lattice diffusion (702 kJ mol−1 ) [6]. In this paper, composite coating particles (␣-Al2 O3 cores with homogeneous silica layer) were prepared by the heterogeneous nucleation-and-growth processing. The densified mullite ceramics were fabricated by traditional dry-press forming processing, and densification behavior of composite coating particles of ␣-Al2 O3 –SiO2 was studied. The densification procedure is consistent with transient viscous sintering (TVS) theory submitted by Sacks et al. [3,7]. Based on our experiments, we not only confirm the densification procedure of composite coating particles by density of the samples but also observe and analyze the densification ∗ Corresponding author. Fax: +86-25-359-5535. E-mail address: [email protected] (Y.-F. Tang).
procedure of composite coating particles by scanning electron microscopy (SEM) and X-ray diffraction (XRD).
2. Experimental 2.1. Processing Composite coating particles of ␣-Al2 O3 –SiO2 with mullite stoichiometric ratio (alumina/silica = 71.8/28.2 wt.%) were prepared by the heterogeneous nucleation-and-growth processing. Alpha alumina (␣-Al2 O3 , average particle size 0.26 m) was dispersed in an ethanol solution by ball milling and ultrasonication. The pH value of the suspension was adjusted to 11.5 by adding ammonia aqueous solutions. Tetraethylorthosilicate (TEOS) was added to the core particles (␣-Al2 O3 )/ammonia solution/ethanol suspension step-by-step, while the suspension was stirred at 55 ◦ C for 6 h. The alumina/silica ratio was adjusted to the mullite stoichiometric ratio by controlling TEOS concentration. Composite coating particles were collected by centrifuge, washed with deionized water and dried in an oven at 110 ◦ C. In order to prepare high density green body, using PVA as binder, dried composite coating particles without adding any sintering aids were packed into a cylindrical
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Fig. 2. XRD patterns of raw materials and composite coating particles (a is amorphous silica and A denotes the ␣-Al2 O3 ).
Fig. 1. TEM micrograph of the composite coating particles.
die with a diameter of 13 mm and formed by double-face uniaxial pressing at 900 MPa. The compact green samples (63% of theoretical density) were heated at the heating rates of 5 ◦ C min−1 up to 1500 ◦ C and held at the end temperature for 2 h at a controlled electric furnace. The density measurement proved that the mullite ceramics were highly densified (99.7% of theoretical density). The value of 3.162 g cm−3 as the average theoretical density of the values of between 3.157 and 3.168 g cm−3 was obtained by measuring the density of sintered samples. This value is in good agreement with the theoretical density (3.171 g cm−3 , ASTM XRD card 15–776).
Fig. 3. XRD patterns of samples sintered at different temperatures (A is ␣-Al2 O3 , C is cristobalite and M is mullite).
Fig. 4. Plots of bulk density and open porosity versus sintering temperature for 2 h.
Y.-F. Tang et al. / Materials Chemistry and Physics 75 (2002) 265–269
Fig. 5. SEM micrographs of samples sintered at different temperatures for 2 h: (a) 1200; (b) 1300; (c) 1350; (d) 1450; (e) 1500 ◦ C.
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2.2. Characterization The alumina/silica ratio of composite coating particles is obtained using chemical analysis. Density of samples sintered at different temperatures is measured by the Archimedes method. Phase analysis is performed on raw materials, composite coating particles and samples sintered at different temperatures by XRD (Model Dmax/rb, Rigaku, Japan) using nickel-filtered Cu K␣ radiation. Transmission electron microscopy (TEM, Model STEM H-800, Hitachi, Japan) is used for microstructure observations on composite coating particles. Samples were sputtered coated with a thin gold layer prior to the TEM observations. Scanning electron microscopy (SEM, Model STEM H-800, Hitachi, Japan) is used for microstructure observations on samples sintered at different temperatures. 3. Results and discussion Fig. 1 shows a TEM micrograph of composite coating particles. There is a homogeneous layer on ␣-Al2 O3 cores, and the thickness of this layer is about 20 nm. The alumina/silica ratio of composite coating particles is controlled by the concentration of TEOS. When the concentration of TEOS is 0.358 mol l−1 , the alumina/silica ratio of composite coating particles is 71.3/28.7 wt.% by chemical analysis. Comparing with the XRD pattern of the ␣-Al2 O3 raw materials (Fig. 2(a)), XRD pattern of composite coating particles (Fig. 2(b)) shows that there is an expanding peak in the range of 2θ ≈ 22◦ which is characteristic of amorphous silica. On the basis of Fig. 1, the silica coating on ␣-Al2 O3 cores is postulated to be amorphous silica layer. Fig. 3 shows the XRD patterns of samples sintered at different temperatures. There are large amounts of corundum and some cristobalite in the samples sintered at 1350 ◦ C for 2 h, and no mullite phase is observed. With increasing sintering temperature, mullite phase appeared in the samples in increasing amounts. After the samples were sintered at 1500 ◦ C for 2 h, there is only mullite phase present indicating that the samples are mullitized completely at 1500 ◦ C. Fig. 4 shows plots of bulk density and open porosity versus sintering temperature for 71.3/28.7 wt.% samples. Substantial densification occurs in the range of 1200–1350 ◦ C, as indicated by the large increase in bulk density and corresponding decrease in open porosity. Microstructure observation (see Fig. 5(c)) shows that the sample sintered at 1350 ◦ C is almost 100% dense. The decrease in bulk density in the range of 1350–1450 ◦ C reflects volume expansion on the formation of cristobalite in the samples. Above 1450 ◦ C, the increase in bulk density of samples is due to the reaction between alumina and silica to form mullite. (This is confirmed by the XRD results in Fig. 3.) The bulk density value (∼3.162 g cm−3 ) for the sample sintered
at 1500 ◦ C indicates that the relative density is >99.5%, and open porosity is nearly zero above 1500 ◦ C. The high relative density is also confirmed by SEM observation (see Fig. 5(e)). Fig. 5(a)–(e) shows SEM micrographs of samples sintered at different temperatures. These samples were polished and etched chemically for 11 min using a 2 wt.% hydrofluoric acid (HF) dilute solution. Fig. 5(a) shows that a sample sintered at 1200 ◦ C is composed of pores and particles (0.2–0.3 m). Based on Fig. 5(a), there is hardly any transient viscous flow of amorphous silica layer at this temperature. This can also be confirmed from Fig. 4. The density (2.1 g cm−3 ) of samples sintered at this temperature is similar to the density of green body (1.98 g cm−3 ). Fig. 5(b) shows that a sample sintered at 1300 ◦ C is composed of pores and many conglutinate blocks. The formation of conglutinate blocks is due to transient viscous flow of amorphous silica layer at this temperature. But pores in Fig. 5(b) show that transient viscous flow does not occur completely. It can also be confirmed from density of the samples sintered at this temperature (2.8 g cm−3 ). Fig. 5(c) shows that a sample sintered at 1350 ◦ C is composed of only conglutinate blocks, because transient viscous flow of amorphous silica layer occurs completely at this temperature. There are no pores in the sample. Density of the sample sintered at the temperature is 3.1 g cm−3 . It is close to the theoretical density of mullite. Fig. 5(d) shows that a sample sintered at 1450 ◦ C is composed of pores, conglutinate blocks and fine structures. Pores are due to volume expansion of formation of cristobalite. Formation of fine structures is due to a reaction between alumina and silica to form fine structures of mullite. Density of the sample sintered at this temperature is 3.03 g cm−3 . The decrease of density is due to volume expansion of the reaction between alumina and silica and the formation of cristobalite. Fig. 5(e) shows that a sample sintered at 1500 ◦ C is composed of fine structures and there are no pores. Based on Figs. 3 and 4, the sample is mullitized completely and density of the sample is 3.162 g cm−3 . The relative density is more than 99%.
4. Conclusions (1) Composite coating particles (␣-Al2 O3 cores with homogeneous silica layer) of mullite stoichiometric ratio have been prepared using ␣-Al2 O3 powders and TEOS as the starting materials by the heterogeneous nucleation-and-growth processing. (2) Mullitization of the samples is studied by XRD and pure mullite phase is obtained at 1500 ◦ C. (3) TVS is observed during sintering as confirmed by density measurement and SEM observation. (4) Densification of samples is divided into two stages: one is the transient viscous flow of amorphous silica layer
Y.-F. Tang et al. / Materials Chemistry and Physics 75 (2002) 265–269
below 1350 ◦ C, the other is mullitization of composite coating particles at 1500 ◦ C. Acknowledgements This work is supported by Natural Science Foundation of Jiangsu Province, People’s Republic of China. References [1] Y.I. Cho, H. Kamiya, Y. Suzuki, M. Horio, Processing of mullite ceramic from alkoxide-derived silica and colloidal alumina with ultra-high cold isostatic pressing, J. Eur. Ceram. Soc. 18 (1998) 261.
269
[2] E. Tkalcec, R. Nass, T. Krajewski, R. Rein, H. Schmidt, Microstructure and mechanical properties of slip cast sol–gel derived mullite ceramics, J. Eur. Ceram. Soc. 18 (1998) 1089. [3] M.D. Sacks, Z. Bozkurt, G.W. Scheiffele, Fabrication of mullite and mullite-matrix composite by transient viscous sintering of composite powders, J. Am. Ceram. Soc. 74 (1991) 2428. [4] M. Mizuno, Microstructure, microchemistry, and flexural strength of mullite ceramics, J. Am. Ceram. Soc. 74 (1991) 3017. [5] L. Montanaro, J.M. Tulliani, C. Perrot, A. Negro, Sintering of industrial mullites, J. Eur. Ceram. Soc. 17 (1997) 1715. [6] A. Skoog, R. Morre, Refractory of the past for the future: mullite and its use as bonding phase, Am. Ceram. Soc. Bull. 67 (1988) 1180. [7] M.D. Sacks, G.W. Scheiffele, N. Bozkurt, R. Raghunathan, Fabrication of ceramics and composites by viscous and transient viscous sintering of composite particles, Ceram. Trans. 22 (Ceram. Powder Sci. 4) (1991) 437.
Study on the densification of composite coating particles of ␣-Al2 O3–SiO2 Yue-Feng Tang a,∗ , Zhi-Da Ling b , Yi-Nong Lu b , Ai-Dong Li a , Hui-Qin Ling c , Yi-Jun Wang a , Qi-Yue Shao a a
c
National Laboratory of Solid State Microstructures, and Department of Materials Science and Engineering, Nanjing University, Nanjing 210093, PR China b School of Materials Science and Engineering, Nanjing University of Technology, Nanjing 210009, PR China National Laboratory of Solid State Microstructures, and Department of Physics, Nanjing University, Nanjing 210093, PR China
Abstract Composite coating particles consisting of alpha alumina cores (␣-Al2 O3 , average particle size 0.26 m) with outer homogeneous amorphous silica layer were prepared by the heterogeneous nucleation-and-growth processing. Densification of composite coating particles was studied at different sintering temperatures. Density of samples sintered at different temperatures was measured by the Archimedes method. Densification procedure of samples was observed and analyzed by scanning electron microscopy (SEM) and X-ray diffraction (XRD). Based on the density of samples, XRD and SEM, densification procedure of samples is divided into two stages: one is the transient viscous flow of amorphous silica layer below 1350 ◦ C, the other is mullitization of composite coating particles at 1500 ◦ C. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Densification; Transient viscous sintering; Mullitization; Composite coating particles; The heterogeneous nucleation-and-growth processing
1. Introduction Sintering of mullite powders to high densities normally requires high temperatures, special forming and sintering treatments (such as ultra-high cold isostatic pressing [1], slip casting [2,3], hot-pressing [4]) and adding sintering aids [5] because of the low bulk and grain-boundary diffusion coefficients for mullite [3]. Activation enthalpy calculations on mullite grain growth yield very high values (ca. 700 kJ mol−1 ) which is of the order of the activation energy of Si4+ lattice diffusion (702 kJ mol−1 ) [6]. In this paper, composite coating particles (␣-Al2 O3 cores with homogeneous silica layer) were prepared by the heterogeneous nucleation-and-growth processing. The densified mullite ceramics were fabricated by traditional dry-press forming processing, and densification behavior of composite coating particles of ␣-Al2 O3 –SiO2 was studied. The densification procedure is consistent with transient viscous sintering (TVS) theory submitted by Sacks et al. [3,7]. Based on our experiments, we not only confirm the densification procedure of composite coating particles by density of the samples but also observe and analyze the densification ∗ Corresponding author. Fax: +86-25-359-5535. E-mail address: [email protected] (Y.-F. Tang).
procedure of composite coating particles by scanning electron microscopy (SEM) and X-ray diffraction (XRD).
2. Experimental 2.1. Processing Composite coating particles of ␣-Al2 O3 –SiO2 with mullite stoichiometric ratio (alumina/silica = 71.8/28.2 wt.%) were prepared by the heterogeneous nucleation-and-growth processing. Alpha alumina (␣-Al2 O3 , average particle size 0.26 m) was dispersed in an ethanol solution by ball milling and ultrasonication. The pH value of the suspension was adjusted to 11.5 by adding ammonia aqueous solutions. Tetraethylorthosilicate (TEOS) was added to the core particles (␣-Al2 O3 )/ammonia solution/ethanol suspension step-by-step, while the suspension was stirred at 55 ◦ C for 6 h. The alumina/silica ratio was adjusted to the mullite stoichiometric ratio by controlling TEOS concentration. Composite coating particles were collected by centrifuge, washed with deionized water and dried in an oven at 110 ◦ C. In order to prepare high density green body, using PVA as binder, dried composite coating particles without adding any sintering aids were packed into a cylindrical
0254-0584/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 4 - 0 5 8 4 ( 0 2 ) 0 0 0 7 4 - 3
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Y.-F. Tang et al. / Materials Chemistry and Physics 75 (2002) 265–269
Fig. 2. XRD patterns of raw materials and composite coating particles (a is amorphous silica and A denotes the ␣-Al2 O3 ).
Fig. 1. TEM micrograph of the composite coating particles.
die with a diameter of 13 mm and formed by double-face uniaxial pressing at 900 MPa. The compact green samples (63% of theoretical density) were heated at the heating rates of 5 ◦ C min−1 up to 1500 ◦ C and held at the end temperature for 2 h at a controlled electric furnace. The density measurement proved that the mullite ceramics were highly densified (99.7% of theoretical density). The value of 3.162 g cm−3 as the average theoretical density of the values of between 3.157 and 3.168 g cm−3 was obtained by measuring the density of sintered samples. This value is in good agreement with the theoretical density (3.171 g cm−3 , ASTM XRD card 15–776).
Fig. 3. XRD patterns of samples sintered at different temperatures (A is ␣-Al2 O3 , C is cristobalite and M is mullite).
Fig. 4. Plots of bulk density and open porosity versus sintering temperature for 2 h.
Y.-F. Tang et al. / Materials Chemistry and Physics 75 (2002) 265–269
Fig. 5. SEM micrographs of samples sintered at different temperatures for 2 h: (a) 1200; (b) 1300; (c) 1350; (d) 1450; (e) 1500 ◦ C.
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2.2. Characterization The alumina/silica ratio of composite coating particles is obtained using chemical analysis. Density of samples sintered at different temperatures is measured by the Archimedes method. Phase analysis is performed on raw materials, composite coating particles and samples sintered at different temperatures by XRD (Model Dmax/rb, Rigaku, Japan) using nickel-filtered Cu K␣ radiation. Transmission electron microscopy (TEM, Model STEM H-800, Hitachi, Japan) is used for microstructure observations on composite coating particles. Samples were sputtered coated with a thin gold layer prior to the TEM observations. Scanning electron microscopy (SEM, Model STEM H-800, Hitachi, Japan) is used for microstructure observations on samples sintered at different temperatures. 3. Results and discussion Fig. 1 shows a TEM micrograph of composite coating particles. There is a homogeneous layer on ␣-Al2 O3 cores, and the thickness of this layer is about 20 nm. The alumina/silica ratio of composite coating particles is controlled by the concentration of TEOS. When the concentration of TEOS is 0.358 mol l−1 , the alumina/silica ratio of composite coating particles is 71.3/28.7 wt.% by chemical analysis. Comparing with the XRD pattern of the ␣-Al2 O3 raw materials (Fig. 2(a)), XRD pattern of composite coating particles (Fig. 2(b)) shows that there is an expanding peak in the range of 2θ ≈ 22◦ which is characteristic of amorphous silica. On the basis of Fig. 1, the silica coating on ␣-Al2 O3 cores is postulated to be amorphous silica layer. Fig. 3 shows the XRD patterns of samples sintered at different temperatures. There are large amounts of corundum and some cristobalite in the samples sintered at 1350 ◦ C for 2 h, and no mullite phase is observed. With increasing sintering temperature, mullite phase appeared in the samples in increasing amounts. After the samples were sintered at 1500 ◦ C for 2 h, there is only mullite phase present indicating that the samples are mullitized completely at 1500 ◦ C. Fig. 4 shows plots of bulk density and open porosity versus sintering temperature for 71.3/28.7 wt.% samples. Substantial densification occurs in the range of 1200–1350 ◦ C, as indicated by the large increase in bulk density and corresponding decrease in open porosity. Microstructure observation (see Fig. 5(c)) shows that the sample sintered at 1350 ◦ C is almost 100% dense. The decrease in bulk density in the range of 1350–1450 ◦ C reflects volume expansion on the formation of cristobalite in the samples. Above 1450 ◦ C, the increase in bulk density of samples is due to the reaction between alumina and silica to form mullite. (This is confirmed by the XRD results in Fig. 3.) The bulk density value (∼3.162 g cm−3 ) for the sample sintered
at 1500 ◦ C indicates that the relative density is >99.5%, and open porosity is nearly zero above 1500 ◦ C. The high relative density is also confirmed by SEM observation (see Fig. 5(e)). Fig. 5(a)–(e) shows SEM micrographs of samples sintered at different temperatures. These samples were polished and etched chemically for 11 min using a 2 wt.% hydrofluoric acid (HF) dilute solution. Fig. 5(a) shows that a sample sintered at 1200 ◦ C is composed of pores and particles (0.2–0.3 m). Based on Fig. 5(a), there is hardly any transient viscous flow of amorphous silica layer at this temperature. This can also be confirmed from Fig. 4. The density (2.1 g cm−3 ) of samples sintered at this temperature is similar to the density of green body (1.98 g cm−3 ). Fig. 5(b) shows that a sample sintered at 1300 ◦ C is composed of pores and many conglutinate blocks. The formation of conglutinate blocks is due to transient viscous flow of amorphous silica layer at this temperature. But pores in Fig. 5(b) show that transient viscous flow does not occur completely. It can also be confirmed from density of the samples sintered at this temperature (2.8 g cm−3 ). Fig. 5(c) shows that a sample sintered at 1350 ◦ C is composed of only conglutinate blocks, because transient viscous flow of amorphous silica layer occurs completely at this temperature. There are no pores in the sample. Density of the sample sintered at the temperature is 3.1 g cm−3 . It is close to the theoretical density of mullite. Fig. 5(d) shows that a sample sintered at 1450 ◦ C is composed of pores, conglutinate blocks and fine structures. Pores are due to volume expansion of formation of cristobalite. Formation of fine structures is due to a reaction between alumina and silica to form fine structures of mullite. Density of the sample sintered at this temperature is 3.03 g cm−3 . The decrease of density is due to volume expansion of the reaction between alumina and silica and the formation of cristobalite. Fig. 5(e) shows that a sample sintered at 1500 ◦ C is composed of fine structures and there are no pores. Based on Figs. 3 and 4, the sample is mullitized completely and density of the sample is 3.162 g cm−3 . The relative density is more than 99%.
4. Conclusions (1) Composite coating particles (␣-Al2 O3 cores with homogeneous silica layer) of mullite stoichiometric ratio have been prepared using ␣-Al2 O3 powders and TEOS as the starting materials by the heterogeneous nucleation-and-growth processing. (2) Mullitization of the samples is studied by XRD and pure mullite phase is obtained at 1500 ◦ C. (3) TVS is observed during sintering as confirmed by density measurement and SEM observation. (4) Densification of samples is divided into two stages: one is the transient viscous flow of amorphous silica layer
Y.-F. Tang et al. / Materials Chemistry and Physics 75 (2002) 265–269
below 1350 ◦ C, the other is mullitization of composite coating particles at 1500 ◦ C. Acknowledgements This work is supported by Natural Science Foundation of Jiangsu Province, People’s Republic of China. References [1] Y.I. Cho, H. Kamiya, Y. Suzuki, M. Horio, Processing of mullite ceramic from alkoxide-derived silica and colloidal alumina with ultra-high cold isostatic pressing, J. Eur. Ceram. Soc. 18 (1998) 261.
269
[2] E. Tkalcec, R. Nass, T. Krajewski, R. Rein, H. Schmidt, Microstructure and mechanical properties of slip cast sol–gel derived mullite ceramics, J. Eur. Ceram. Soc. 18 (1998) 1089. [3] M.D. Sacks, Z. Bozkurt, G.W. Scheiffele, Fabrication of mullite and mullite-matrix composite by transient viscous sintering of composite powders, J. Am. Ceram. Soc. 74 (1991) 2428. [4] M. Mizuno, Microstructure, microchemistry, and flexural strength of mullite ceramics, J. Am. Ceram. Soc. 74 (1991) 3017. [5] L. Montanaro, J.M. Tulliani, C. Perrot, A. Negro, Sintering of industrial mullites, J. Eur. Ceram. Soc. 17 (1997) 1715. [6] A. Skoog, R. Morre, Refractory of the past for the future: mullite and its use as bonding phase, Am. Ceram. Soc. Bull. 67 (1988) 1180. [7] M.D. Sacks, G.W. Scheiffele, N. Bozkurt, R. Raghunathan, Fabrication of ceramics and composites by viscous and transient viscous sintering of composite particles, Ceram. Trans. 22 (Ceram. Powder Sci. 4) (1991) 437.