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Texture development in 3 mol% yttria-stabilized tetragonal zirconia
Materials Research Bulletin 44 (2009) 1802–1805
Contents lists available at ScienceDirect
Materials Research Bulletin journal homepage: www.elsevier.com/locate/matresbu
Short communication
Texture development in 3 mol% yttria-stabilized tetragonal zirconia Gustavo Sua´rez a,b, Yoshio Sakka a,c,*, Tohru Suzuki a, Tetsuo Uchikoshi a, Esteban F. Aglietti b a
Fine Particle Processing Group, Nano Ceramics Center, National Institute for Materials Science (NIMS), 1-2-1, Sengen, Tsukuba, Ibaraki 305-0047, Japan Technological Center of Mineral Resources and Ceramics, CETMIC, Camino Centenario y 506, Gonnet, Argentina c WPI-MANA World Premier International Research Center Initiative, Center for Materials Nanoarchitectonics, NIMS, Japan b
A R T I C L E I N F O
A B S T R A C T
Article history: Received 22 January 2009 Received in revised form 12 February 2009 Accepted 12 March 2009 Available online 25 March 2009
We have developed a method of forming textured tetragonal zirconia. A suspension containing 10 vol% solid loading of monoclinic ZrO2 mixed with 3 mol% Y2O3 was prepared, and then a bead-milling process was performed using 50 mm diameter zirconia beads resulting in a well-dispersed suspension. The mixture suspension of monoclinic zirconia and yttria nanoparticles was slip cast under a magnetic field of 12 T to produce oriented monoclinic zirconia with yttria. The reaction sintering between yttria and the oriented monoclinic zirconia produces a final 3 mol% Y2O3 doped tetragonal zirconia that remains oriented. ß 2009 Elsevier Ltd. All rights reserved.
Keywords: A. Structural materials C. X-ray diffraction D. Diffusion D. Microstructure
1. Introduction It has already been reported that many diamagnetic and paramagnetic ceramics such as Al2O3, TiO2, ZnO, HAP (hydroxyapatite), Si3N4, AlN, and SiC [1–7] can be oriented by slip casting in a strong magnetic field. Also reaction sintering of textured ceramics can lead in textured products [8–12]. The principle of the orientation process is that a crystal with anisotropic magnetic susceptibility is rotated by an angle that minimizes the system energy after placing it in a magnetic field and applying an impulse with magnetic torque T by the following Eq. (1) [13]: T ¼ DE sin 2 u ¼ DxVB2 sin 2u=2m0
(1)
where Dx(=jxa,b xcj) is the anisotropy of the magnetic susceptibility, V is the volume of each particle, m0 is the permeability in vacuum, B is the applied magnetic field and u is the angle between the easy magnetization axis of the crystal and the imposed magnetic field direction. To obtain oriented materials in a high magnetic field, various conditions are required: (1) the crystals with asymmetric structure used as starting materials have a noncubic crystal system, (2) the suspension is well dispersed with sufficiently low viscosity to rotate the particles with a low energy,
* Corresponding author at: Nano Ceramics Center, National Institute for Materials Science, 1-2-1, Sengen, Tsukuba-shi 305-0047, Japan. Tel.: +81 29 859 2461; fax: +81 29 859 2401. E-mail address: [email protected] (Y. Sakka). 0025-5408/$ – see front matter ß 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.materresbull.2009.03.003
and (3) the energy of the magnetic anisotropy of the crystals should be larger than that of their thermal motion. Yttria-stabilized tetragonal zirconia is one of the most important structural ceramics [14,15], and its properties are expected to be improved by texturing. 3 mol% yttria-stabilized zirconia has a two-phase tetragonal structure and was negligibly oriented in a magnetic field of 12 T in our previous experiments. Monoclinic zirconia is expected to exhibit magnetic anisotropy upon orientation in a magnetic field if it is well dispersed. Commercially available monoclinic zirconia powder consists of fine particles but is heavily agglomerated. In this case, dispersion by ultrasonication is not effective. Recently it has been shown that bead-milling using small beads of less than 50 mm diameter is effective for redispersing fine but heavily agglomerated powder [16–18]. In this study we investigate a two-component suspension. To perform a reaction sintering it is necessary to form a homogeneous distribution of yttria and m-ZrO2. Deagglomerated fine and welldispersed particles make possible to form a good contact between particles; thus, it is expected that tetragonal zirconia can be oriented at a low temperature. For these reasons, beads milling is necessary. It is possible to induce a reaction between commercially available yttria and monoclinic zirconia to generate the stable crystalline tetragonal form. The cation diffusion in this type of system is very low [19–26], however a very homogeneous dispersion of small particles obtained by bead-milling is necessary to obtain homogeneous tetragonal zirconia. The aim of this study is to fabricate 3 mol% yttria-stabilized tetragonal zirconia with an oriented structure by sintering oriented m-ZrO2 with homogeneously dispersed yttria.
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2. Experimental procedure The starting materials are high-purity monoclinic zirconia (TZ0Y, Tosoh Co., Tokyo, Japan, primary particle size 75.8 nm and a BET surface area of 13.6 m2/g) and Y2O3 powder (NanoTek, average primary particle size 28 nm and a BET surface area of 44 m2/g). The powders were dispersed in distilled water with ammonium polycarboxylate (A-6114, Toagosei Co., Tokyo, Japan), and aqueous suspensions containing 10 vol% solid loading of ZrO2 mixed with 3 mol% Y2O3 were prepared (pH 8.5). The suspensions were ultrasonicated using a 35 W ultrasonic stirrer (Nissei USS-1 Nihonseki Kaisha Ltd., Japan), for 15 min, and then using an ultrasonic homogenizer (Nissei 1200T) at a frequency of 19.6 kHz (Nihonseki Kaisha Ltd., Japan). To achieve as good dispersion as possible a deagglomeration treatment was performed with bead-milling equipment (Ultra Apex Mill Type UAM-015, Kotobuki Industries Co. Ltd., Kure Japan) using small ZrO2 beads with a diameter of 50 mm at a rotation speed of 4000 rpm. The particle size distribution was measured using a particle size analyzer to evaluate the dispersion of the suspension (UPA-UT151, 0.8–6500 nm NANOTRAC1 Nikkiso, Japan). Drops of dispersed and dried suspension on were observed silicon wafer at room temperature under ambient conditions by atomic force microscopy AFM (Seiko Instruments Inc., SPA-400 + SPI-3800N). After out gassing in a desicator the suspension was cast in a porous alumina mold under a magnetic field of 12 T. The compacts were sintered at various temperatures in the range of 750–1500 8C with a heating rate of 5 8C/min and a holding time of 3 h. X-ray diffraction was carried out to evaluate the phase transformation and orientation of the samples (JEOL, JDX-3500d using Cu Ka radiation at 35 kV, 300 mA). The microstructure of the sintered samples was observed by field-emission scanning electron microscopy FE-SEM (JEOL, JSM-840F). Measurements of the Lotgering factor is a semiquantitative technique for evaluating the degree of texturing. In our case we observed the (1¯ 1 1) peak of the top surface of the monoclinic zirconia and the (2 0 2) peak of the top surface of tetragonal zirconia. Eq. (2) is used to calculate the Lotgering factor of the monoclinic and tetragonal structures. f ¼ p p0 =ð1 p0 Þ
(2)
Fig. 1. Particle size distribution vs. milling time for a suspension of mixed monoclinic zirconia and yttria.
where p ¼ Ið1¯ 1 1Þ=SIðhklÞ and p0 ¼ I0 ð1¯ 1 1Þ=SI0 ðhklÞ for the monoclinic phase and p = I(2 0 2)/SI(hkl) and p = I0(2 0 2)/SI0(hkl) for the tetragonal phase. 3. Results and discussion To achieve the orientation of monoclinic zirconia, it is very important to prepare a fully dispersed suspension before the casting process. In this work we used bead-milling equipment, which was very effective for obtaining fully dispersed suspensions. The change in particle size distribution with the milling time is shown in Fig. 1. The d90 value of 270 nm for the initial suspension indicates the existence of agglomerated particles. After beadmilling for 60 min the average particle size was 90 nm, which is close to the primary particle size. The initial difference between d90 and d10 is large but becomes narrow after 60 min due to the brake up of the agglomerated particles. Fig. 2 shows AFM images of the dried suspension before and after deagglomeration, which clearly
Fig. 2. AFM images of the suspension containing Y2O3 and ZrO2 (m) before and after bead-milling.
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G. Sua´rez et al. / Materials Research Bulletin 44 (2009) 1802–1805
Fig. 3. XRD patterns of (1) slip cast of randomly oriented monoclinic zirconia without magnetic field, (2) slip-cast monoclinic zirconia in magnetic field (top view), and (3) slip-cast monoclinic zirconia in magnetic field (side view).
show that bead-milling is an effective method of breaking agglomerated particles into primary particles. Fig. 3 shows the X-ray diffraction pattern of a green compact of the bead-milled monoclinic zirconia prepared by slip casting in a magnetic field in comparison with that reference slip cast without a magnetic field. The reference sample of the mixture suspension of monoclinic zirconia and yttria nanoparticles without magnetic field is randomly oriented; showing the top and side view identically the same. The diffraction patterns taken from the surfaces perpendicular (top view) and parallel (side view) to the magnetic field clearly show the orientation of monoclinic zirconia. Comparing the XRD patterns in Fig. 3, the (1¯ 1 1) peak is strongest in the reference sample and also in the reflection pattern of the side view. The reflection of the top view shows that the (1 1 1) peak is strongest. Another important fact to note is that the (2 0 0) peak has a similar intensity to the (0 0 2) peak in the reference sample but after casting the intensity of the (2 0 0) peak became twice that of the (0 0 2) peak in the top view. Observing the behavior of the (2 0 0) peak which is strong from the reflection of the top view, we can assume that the longer a-axis is aligned parallel to the magnetic field. From the analysis of the crystalline structure, we calculate that the angles between the (1¯ 1 1) and (2 0 0) planes and
Fig. 4. Change in the intensities of the peaks of monoclinic to tetragonal zirconia due to reaction sintering. No orientation is observed.
the magnetic field direction are 49.778 and 80.88, respectively. The Lotgering factor of the mixture of monoclinic zirconia and yttria is F = 0.15. Fig. 4 shows the change in the intensities of the (1 1 1) peak of tetragonal ZrO2 and the (1¯ 1 1) and the (1 1 1) peaks of randomly oriented monoclinic ZrO2 due to sintering. After sintering at 1300 8C, the monoclinic phase completely disappears. The patterns for top and side view in these samples show no change in peaks intensities. Fig. 5 shows a comparison of the XRD peaks of standard Tosoh zirconia 3YTZ with that of the tetragonal zirconia with same composition prepared in this study. We observe that our tetragonal zirconia remains oriented after the reaction sintering between monoclinic ZrO2 and Y2O3 at 1500 8C for 3 h. The peaks of 3YTZ and the relative intensities of the peaks from the different surfaces reveal the orientation of our tetragonal zirconia. The main peaks are (2 0 2) for the top view of the oriented zirconia and (1 1 1) for the side view. Furthermore the relative intensities of the pairs of peaks (0 0 2) vs. (2 0 0) and (1 1 3) vs. (3 1 1), for the oriented tetragonal structure exhibit a complete inversion from the top view to the side view which is evidence that the c-axis is aligned parallel to the magnetic field. In the case of tetragonal zirconia, the angles between the c-axis and the (0 0 2) and (2 0 0) planes are 908 and 08 respectively, and our results show that these planes have the highest and lowest intensities respectively, from the top view. From the side view, the trend is reversed. This observation suggests that the c-axis is aligned with the magnetic field. Moreover, the angles between the magnetic field direction and the (1 1 3) and (3 1 1) planes are 72.088 and 24.978 respectively; they are almost parallel or perpendicular to the c-axis. The results show that the (1 1 3) peak is strong and the (3 1 1) peak is weak from the top view and that this relation is reversed from the side view. This observation provides further evidence that the c-axis is aligned with the magnetic field. The Lotgering factors after sintering at 1300 and 1500 8C for 3 h are F = 0.25 and F = 0.31, respectively. The improvement from 0.15 before sintering for oriented monoclinic zirconia may be due to the fact that the orientation factor increases with the grain growth [3].
Fig. 5. X-ray diffraction patterns of standard Tosoh zirconia (3YTZ) prepared without magnetic field and the oriented tetragonal phase prepared by reaction sintering.
G. Sua´rez et al. / Materials Research Bulletin 44 (2009) 1802–1805
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Fig. 6. SEM of oriented 3YTZ microstructure. Top and side view.
The average grain size is 130 nm after sintering at 1300 8C and 600 nm after sintering at 1500 8C. The presented Lotgering factor is low, even if it was calculated from the peak (1¯ 1 1) for monoclinic zirconia and from the peak (2 0 2) for the formed monoclinic zirconia. This low value of Lotgering factor is due to the way of calculation and the intrinsic mechanism of transformation from monoclinic to tetragonal. Lotgering factor calculation is based on the XRD pattern, so it is based on planes position. However the orientation in high magnetic field affects the axis. In monoclinic zirconia, the fact of orient the long a-axis give to the planes certain freedom and due to the angle between the a-axis with the planes produce different diffractions that leads to a low Lotgering factor calculation even if the a-axis is well aligned. The calculation of the tetragonal orientation degree is improved due to the grain growth but is still low and it can be due to the mechanism of planes translation from the oriented monoclinic to the oriented tetragonal. Microstructure observation was performed on the samples sintered at 1500 8C for 3 h with large grains. SEM images of the samples are shown in Fig. 6. The grains had an average particle size of 600 nm from both the top and side views with a spherical shape and no apparent elongation of the grains. 4. Conclusion An effective deagglomeration method and the application of a magnetic field are used in this study to achieve textured tetragonal ZrO2. Bead-milling is very effective for obtaining a good colloidal suspension with fully dispersed primary particles of the two different components. Slip casting the prepared suspension containing monoclinic zirconia with yttria in high magnetic field produced a textured structure with the a-axis of monoclinic zirconia parallel to the magnetic field. Upon sintering, oriented tetragonal zirconia was formed with the c-axis parallel to the magnetic field. This result indicates that the a-axis in monoclinic zirconia is transformed to the c-axis in the tetragonal phase.
Acknowledgments The authors express their gratitude to Dr. Naoto Shirahata for his help in the operation of the AFM equipment and in providing us with high quality images of the specimens. This study was supported in partly by the Grant-in Aid for Scientific Research of the JSPS and World Premier International Research Center Initiative (WPI) on Materials Nanoarchitronics (MANA), MEXT, Japan. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26]
Y. Sakka, T.S. Suzuki, J. Ceram. Soc. Jpn. 113 (2005) 26. T.S. Suzuki, Y. Sakka, Sci. Technol. Adv. Mater. 7 (2006) 356. X. Zhu, T.S. Suzuki, T. Uchikoshi, Y. Sakka, J. Ceram. Soc. Jpn. 114 (2006) 979. T.S. Suzuki, T. Uchikoshi, Y. Sakka, Mater. Trans. 48 (2007) 2883. T.S. Suzuki, Y. Sakka, Scripta Mater. 52 (2005) 583. X. Zhu, Y. Sakka, Sci. Technol. Adv. Mater. 9 (2008) 033001. K. Inoue, K. Sassa, Y. Yokogawa, Y. Sakka, M. Okida, S. Asai, Mater. Trans. 44 (2003) 1133. Y. Sakka, A. Honda, T.S. Suzuki, Y. Moriyoshi, Solid State Ionics 172 (2004) 341. K. Tabata, A. Makiya, S. Tanaka, K. Uematsu, Y. Doshida, J. Ceram. Soc. Jpn. 115 (2007) 237. X.W. Zhu, T.S. Suzuki, T. Uchikoshi, Y. Sakka, J. Ceram. Soc. Jpn. 115 (2007) 701. X.W. Zhu, T.S. Suzuki, T. Uchikoshi, Y. Sakka, J. Eur. Ceram. Soc. 28 (2008) 929. X.W. Zhu, T.S. Suzuki, T. Uchikoshi, Y. Sakka, J. Am. Ceram. Soc. 91 (2008) 620. C.Y. Wu, S.Q. Li, K. Sassa, Y. Sakka, T.S. Suzuki, S. Asai, ISIJ Int. 45 (2005) 997. D.L. Porter, A.H. Heuer, J. Am. Ceram. Soc. 60 (1977) 183. T. Masaki, J. Am. Ceram. Soc. 69 (1986) 638. J. Choi, B.-D. Hahn, D.-S. Park, W.-H. Yoon, J.-H. Lee, J.-H. Jang, K.-H. Ko, C. Park, J. Am. Ceram. Soc. 90 (2007) 388. J.-Y. Qiu, Y. Hotta, K. Watari, K. Mitsuishi, M. Yamazaki, J. Eur. Ceram. Soc. 26 (2006) 385. Y. Sakka, T.S. Suzuki, T. Uchikoshi, J. Eur. Ceram. Soc. 28 (2008) 935. K. Matsui, H. Yoshida, Y. Ikuhara, Acta Mater. 56 (2008) 1315. M. Inkyo, T. Tahara, T. Iwaki, F. Ishandar, C.J. Hogan, K. Okuyama, J. Colloid Interface Sci. 304 (2006) 535. B. Basu, J. Vleugels, O. Van Der Biest, Key Eng. Mater. 206 (2002) 1185. T. Akao, T. Motohashi, M. Karppinen, H. Yamauchi, M. Hayakawa, Mater. Sci. Eng., A 438 (2006) 387. P. Mondal, A. Klein, W. Jaegermann, H. Hahn, Solid State Ionics 118 (1999) 331. N. Gupta, P. Mallik, B. Basu, J. Alloys Compd. 379 (2004) 228. Y. Sakka, Y. Oishi, K. Ando, J. Mater. Sci. 17 (1982) 3101. Y. Sakka, Y. Oishi, K. Ando, J. Am. Ceram. Soc. 72 (1989) 2121.
Contents lists available at ScienceDirect
Materials Research Bulletin journal homepage: www.elsevier.com/locate/matresbu
Short communication
Texture development in 3 mol% yttria-stabilized tetragonal zirconia Gustavo Sua´rez a,b, Yoshio Sakka a,c,*, Tohru Suzuki a, Tetsuo Uchikoshi a, Esteban F. Aglietti b a
Fine Particle Processing Group, Nano Ceramics Center, National Institute for Materials Science (NIMS), 1-2-1, Sengen, Tsukuba, Ibaraki 305-0047, Japan Technological Center of Mineral Resources and Ceramics, CETMIC, Camino Centenario y 506, Gonnet, Argentina c WPI-MANA World Premier International Research Center Initiative, Center for Materials Nanoarchitectonics, NIMS, Japan b
A R T I C L E I N F O
A B S T R A C T
Article history: Received 22 January 2009 Received in revised form 12 February 2009 Accepted 12 March 2009 Available online 25 March 2009
We have developed a method of forming textured tetragonal zirconia. A suspension containing 10 vol% solid loading of monoclinic ZrO2 mixed with 3 mol% Y2O3 was prepared, and then a bead-milling process was performed using 50 mm diameter zirconia beads resulting in a well-dispersed suspension. The mixture suspension of monoclinic zirconia and yttria nanoparticles was slip cast under a magnetic field of 12 T to produce oriented monoclinic zirconia with yttria. The reaction sintering between yttria and the oriented monoclinic zirconia produces a final 3 mol% Y2O3 doped tetragonal zirconia that remains oriented. ß 2009 Elsevier Ltd. All rights reserved.
Keywords: A. Structural materials C. X-ray diffraction D. Diffusion D. Microstructure
1. Introduction It has already been reported that many diamagnetic and paramagnetic ceramics such as Al2O3, TiO2, ZnO, HAP (hydroxyapatite), Si3N4, AlN, and SiC [1–7] can be oriented by slip casting in a strong magnetic field. Also reaction sintering of textured ceramics can lead in textured products [8–12]. The principle of the orientation process is that a crystal with anisotropic magnetic susceptibility is rotated by an angle that minimizes the system energy after placing it in a magnetic field and applying an impulse with magnetic torque T by the following Eq. (1) [13]: T ¼ DE sin 2 u ¼ DxVB2 sin 2u=2m0
(1)
where Dx(=jxa,b xcj) is the anisotropy of the magnetic susceptibility, V is the volume of each particle, m0 is the permeability in vacuum, B is the applied magnetic field and u is the angle between the easy magnetization axis of the crystal and the imposed magnetic field direction. To obtain oriented materials in a high magnetic field, various conditions are required: (1) the crystals with asymmetric structure used as starting materials have a noncubic crystal system, (2) the suspension is well dispersed with sufficiently low viscosity to rotate the particles with a low energy,
* Corresponding author at: Nano Ceramics Center, National Institute for Materials Science, 1-2-1, Sengen, Tsukuba-shi 305-0047, Japan. Tel.: +81 29 859 2461; fax: +81 29 859 2401. E-mail address: [email protected] (Y. Sakka). 0025-5408/$ – see front matter ß 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.materresbull.2009.03.003
and (3) the energy of the magnetic anisotropy of the crystals should be larger than that of their thermal motion. Yttria-stabilized tetragonal zirconia is one of the most important structural ceramics [14,15], and its properties are expected to be improved by texturing. 3 mol% yttria-stabilized zirconia has a two-phase tetragonal structure and was negligibly oriented in a magnetic field of 12 T in our previous experiments. Monoclinic zirconia is expected to exhibit magnetic anisotropy upon orientation in a magnetic field if it is well dispersed. Commercially available monoclinic zirconia powder consists of fine particles but is heavily agglomerated. In this case, dispersion by ultrasonication is not effective. Recently it has been shown that bead-milling using small beads of less than 50 mm diameter is effective for redispersing fine but heavily agglomerated powder [16–18]. In this study we investigate a two-component suspension. To perform a reaction sintering it is necessary to form a homogeneous distribution of yttria and m-ZrO2. Deagglomerated fine and welldispersed particles make possible to form a good contact between particles; thus, it is expected that tetragonal zirconia can be oriented at a low temperature. For these reasons, beads milling is necessary. It is possible to induce a reaction between commercially available yttria and monoclinic zirconia to generate the stable crystalline tetragonal form. The cation diffusion in this type of system is very low [19–26], however a very homogeneous dispersion of small particles obtained by bead-milling is necessary to obtain homogeneous tetragonal zirconia. The aim of this study is to fabricate 3 mol% yttria-stabilized tetragonal zirconia with an oriented structure by sintering oriented m-ZrO2 with homogeneously dispersed yttria.
G. Sua´rez et al. / Materials Research Bulletin 44 (2009) 1802–1805
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2. Experimental procedure The starting materials are high-purity monoclinic zirconia (TZ0Y, Tosoh Co., Tokyo, Japan, primary particle size 75.8 nm and a BET surface area of 13.6 m2/g) and Y2O3 powder (NanoTek, average primary particle size 28 nm and a BET surface area of 44 m2/g). The powders were dispersed in distilled water with ammonium polycarboxylate (A-6114, Toagosei Co., Tokyo, Japan), and aqueous suspensions containing 10 vol% solid loading of ZrO2 mixed with 3 mol% Y2O3 were prepared (pH 8.5). The suspensions were ultrasonicated using a 35 W ultrasonic stirrer (Nissei USS-1 Nihonseki Kaisha Ltd., Japan), for 15 min, and then using an ultrasonic homogenizer (Nissei 1200T) at a frequency of 19.6 kHz (Nihonseki Kaisha Ltd., Japan). To achieve as good dispersion as possible a deagglomeration treatment was performed with bead-milling equipment (Ultra Apex Mill Type UAM-015, Kotobuki Industries Co. Ltd., Kure Japan) using small ZrO2 beads with a diameter of 50 mm at a rotation speed of 4000 rpm. The particle size distribution was measured using a particle size analyzer to evaluate the dispersion of the suspension (UPA-UT151, 0.8–6500 nm NANOTRAC1 Nikkiso, Japan). Drops of dispersed and dried suspension on were observed silicon wafer at room temperature under ambient conditions by atomic force microscopy AFM (Seiko Instruments Inc., SPA-400 + SPI-3800N). After out gassing in a desicator the suspension was cast in a porous alumina mold under a magnetic field of 12 T. The compacts were sintered at various temperatures in the range of 750–1500 8C with a heating rate of 5 8C/min and a holding time of 3 h. X-ray diffraction was carried out to evaluate the phase transformation and orientation of the samples (JEOL, JDX-3500d using Cu Ka radiation at 35 kV, 300 mA). The microstructure of the sintered samples was observed by field-emission scanning electron microscopy FE-SEM (JEOL, JSM-840F). Measurements of the Lotgering factor is a semiquantitative technique for evaluating the degree of texturing. In our case we observed the (1¯ 1 1) peak of the top surface of the monoclinic zirconia and the (2 0 2) peak of the top surface of tetragonal zirconia. Eq. (2) is used to calculate the Lotgering factor of the monoclinic and tetragonal structures. f ¼ p p0 =ð1 p0 Þ
(2)
Fig. 1. Particle size distribution vs. milling time for a suspension of mixed monoclinic zirconia and yttria.
where p ¼ Ið1¯ 1 1Þ=SIðhklÞ and p0 ¼ I0 ð1¯ 1 1Þ=SI0 ðhklÞ for the monoclinic phase and p = I(2 0 2)/SI(hkl) and p = I0(2 0 2)/SI0(hkl) for the tetragonal phase. 3. Results and discussion To achieve the orientation of monoclinic zirconia, it is very important to prepare a fully dispersed suspension before the casting process. In this work we used bead-milling equipment, which was very effective for obtaining fully dispersed suspensions. The change in particle size distribution with the milling time is shown in Fig. 1. The d90 value of 270 nm for the initial suspension indicates the existence of agglomerated particles. After beadmilling for 60 min the average particle size was 90 nm, which is close to the primary particle size. The initial difference between d90 and d10 is large but becomes narrow after 60 min due to the brake up of the agglomerated particles. Fig. 2 shows AFM images of the dried suspension before and after deagglomeration, which clearly
Fig. 2. AFM images of the suspension containing Y2O3 and ZrO2 (m) before and after bead-milling.
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Fig. 3. XRD patterns of (1) slip cast of randomly oriented monoclinic zirconia without magnetic field, (2) slip-cast monoclinic zirconia in magnetic field (top view), and (3) slip-cast monoclinic zirconia in magnetic field (side view).
show that bead-milling is an effective method of breaking agglomerated particles into primary particles. Fig. 3 shows the X-ray diffraction pattern of a green compact of the bead-milled monoclinic zirconia prepared by slip casting in a magnetic field in comparison with that reference slip cast without a magnetic field. The reference sample of the mixture suspension of monoclinic zirconia and yttria nanoparticles without magnetic field is randomly oriented; showing the top and side view identically the same. The diffraction patterns taken from the surfaces perpendicular (top view) and parallel (side view) to the magnetic field clearly show the orientation of monoclinic zirconia. Comparing the XRD patterns in Fig. 3, the (1¯ 1 1) peak is strongest in the reference sample and also in the reflection pattern of the side view. The reflection of the top view shows that the (1 1 1) peak is strongest. Another important fact to note is that the (2 0 0) peak has a similar intensity to the (0 0 2) peak in the reference sample but after casting the intensity of the (2 0 0) peak became twice that of the (0 0 2) peak in the top view. Observing the behavior of the (2 0 0) peak which is strong from the reflection of the top view, we can assume that the longer a-axis is aligned parallel to the magnetic field. From the analysis of the crystalline structure, we calculate that the angles between the (1¯ 1 1) and (2 0 0) planes and
Fig. 4. Change in the intensities of the peaks of monoclinic to tetragonal zirconia due to reaction sintering. No orientation is observed.
the magnetic field direction are 49.778 and 80.88, respectively. The Lotgering factor of the mixture of monoclinic zirconia and yttria is F = 0.15. Fig. 4 shows the change in the intensities of the (1 1 1) peak of tetragonal ZrO2 and the (1¯ 1 1) and the (1 1 1) peaks of randomly oriented monoclinic ZrO2 due to sintering. After sintering at 1300 8C, the monoclinic phase completely disappears. The patterns for top and side view in these samples show no change in peaks intensities. Fig. 5 shows a comparison of the XRD peaks of standard Tosoh zirconia 3YTZ with that of the tetragonal zirconia with same composition prepared in this study. We observe that our tetragonal zirconia remains oriented after the reaction sintering between monoclinic ZrO2 and Y2O3 at 1500 8C for 3 h. The peaks of 3YTZ and the relative intensities of the peaks from the different surfaces reveal the orientation of our tetragonal zirconia. The main peaks are (2 0 2) for the top view of the oriented zirconia and (1 1 1) for the side view. Furthermore the relative intensities of the pairs of peaks (0 0 2) vs. (2 0 0) and (1 1 3) vs. (3 1 1), for the oriented tetragonal structure exhibit a complete inversion from the top view to the side view which is evidence that the c-axis is aligned parallel to the magnetic field. In the case of tetragonal zirconia, the angles between the c-axis and the (0 0 2) and (2 0 0) planes are 908 and 08 respectively, and our results show that these planes have the highest and lowest intensities respectively, from the top view. From the side view, the trend is reversed. This observation suggests that the c-axis is aligned with the magnetic field. Moreover, the angles between the magnetic field direction and the (1 1 3) and (3 1 1) planes are 72.088 and 24.978 respectively; they are almost parallel or perpendicular to the c-axis. The results show that the (1 1 3) peak is strong and the (3 1 1) peak is weak from the top view and that this relation is reversed from the side view. This observation provides further evidence that the c-axis is aligned with the magnetic field. The Lotgering factors after sintering at 1300 and 1500 8C for 3 h are F = 0.25 and F = 0.31, respectively. The improvement from 0.15 before sintering for oriented monoclinic zirconia may be due to the fact that the orientation factor increases with the grain growth [3].
Fig. 5. X-ray diffraction patterns of standard Tosoh zirconia (3YTZ) prepared without magnetic field and the oriented tetragonal phase prepared by reaction sintering.
G. Sua´rez et al. / Materials Research Bulletin 44 (2009) 1802–1805
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Fig. 6. SEM of oriented 3YTZ microstructure. Top and side view.
The average grain size is 130 nm after sintering at 1300 8C and 600 nm after sintering at 1500 8C. The presented Lotgering factor is low, even if it was calculated from the peak (1¯ 1 1) for monoclinic zirconia and from the peak (2 0 2) for the formed monoclinic zirconia. This low value of Lotgering factor is due to the way of calculation and the intrinsic mechanism of transformation from monoclinic to tetragonal. Lotgering factor calculation is based on the XRD pattern, so it is based on planes position. However the orientation in high magnetic field affects the axis. In monoclinic zirconia, the fact of orient the long a-axis give to the planes certain freedom and due to the angle between the a-axis with the planes produce different diffractions that leads to a low Lotgering factor calculation even if the a-axis is well aligned. The calculation of the tetragonal orientation degree is improved due to the grain growth but is still low and it can be due to the mechanism of planes translation from the oriented monoclinic to the oriented tetragonal. Microstructure observation was performed on the samples sintered at 1500 8C for 3 h with large grains. SEM images of the samples are shown in Fig. 6. The grains had an average particle size of 600 nm from both the top and side views with a spherical shape and no apparent elongation of the grains. 4. Conclusion An effective deagglomeration method and the application of a magnetic field are used in this study to achieve textured tetragonal ZrO2. Bead-milling is very effective for obtaining a good colloidal suspension with fully dispersed primary particles of the two different components. Slip casting the prepared suspension containing monoclinic zirconia with yttria in high magnetic field produced a textured structure with the a-axis of monoclinic zirconia parallel to the magnetic field. Upon sintering, oriented tetragonal zirconia was formed with the c-axis parallel to the magnetic field. This result indicates that the a-axis in monoclinic zirconia is transformed to the c-axis in the tetragonal phase.
Acknowledgments The authors express their gratitude to Dr. Naoto Shirahata for his help in the operation of the AFM equipment and in providing us with high quality images of the specimens. This study was supported in partly by the Grant-in Aid for Scientific Research of the JSPS and World Premier International Research Center Initiative (WPI) on Materials Nanoarchitronics (MANA), MEXT, Japan. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26]
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