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Vacancy effect of dopant cation on the high-temperature creep resistance in polycrystalline Al2O3
Materials Science and Engineering A319– 321 (2001) 843– 848 www.elsevier.com/locate/msea
Vacancy effect of dopant cation on the high-temperature creep resistance in polycrystalline Al2O3 Hidehiro Yoshida a,*, Yuichi Ikuhara b, Taketo Sakuma a a
Graduate School of Frontier Sciences, The Uni6ersity of Tokyo, 7 -3 -1 Hongo, Bunkyo-ku, Tokyo 113 -8656, Japan b Graduate School of Engineering, The Uni6ersity of Tokyo, 7 -3 -1 Hongo, Bunkyo-ku, Tokyo 113 -8656, Japan
Abstract High-temperature creep resistance in polycrystalline Al2O3 with 0.1 mol% oxides of YO1.5, ZrO2 or MgO has been examined by uniaxial compression creep testing at 1250 °C. The creep resistance is highly improved by the doping of Y or Zr even in the dopant level of 0.1 mol%, but is retarded by Mg doping. The dopant effect on the creep resistance cannot be explained in terms of, for example, ionic radius of the dopant cation or eutectic point in Al2O3-oxide of dopant cation system. Each dopant cation was found to segregate in grain boundaries, and is likely to influence grain boundary diffusion in Al2O3. The ionic bonding and the covalent bonding of Al–O are lowered by the introduction of V%%O or V%%%Al but the values of the net charge in Al and O are increased by the cations doping. The change in the value of Net Charge is correlated well with the high-temperature creep resistance in Al2O3 with cation doping. It is suggested that the ionicity in Al and O is an important factor to determine high-temperature creep resistance in polycrystalline Al2O3. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Grain boundaries; Al2O3; Resistance
1. Introduction It has been reported that high-temperature creep deformation or plastic flow in polycrystalline Al2O3 is sensitively affected by a small amount of dopant cation. An addition of 0.1wt% ZrO2 in Al2O3 with a grain size of 1 mm severely suppresses the creep rate by a factor of 15 at 1250 °C under the stresses of 10 – 200 MPa [1]. Cho et al. reported that 1000 ppm yttrium or 500 ppm lanthanum-doped Al2O3 has improved the creep resistance in comparison with pure Al2O3 at 1200 – 1350 °C under the stress of 50 MPa [2]. Such a dopant effect of MgO in the level of 0.1 wt.% was also observed in high-temperature flow stress of fine-grained Al2O3 in conventional tensile testing; the peak stress decreases by the doping of MgO at 1450 °C under an initial strain rate of 1 × 10 − 4 s − 1 [3]. It has been pointed out that the high-temperature creep resistance in Al2O3 with a grain size of 1 mm is markedly improved by a doping of lanthanoid oxide such as Y2O3 or Lu2O3 even in the level of 0.05 mol% [4]. In this case, * Corresponding author. E-mail address: [email protected] (H. Yoshida).
the dopant cation did not form the second phase precipitations, but segregated in the vicinity of grain boundaries to improve the creep resistance in Al2O3. The segregation must be an origin of the change in creep deformation, but the mechanism has not been clarified yet. The change in the creep resistance in lanthanoid oxide-doped Al2O3 cannot be explained in terms of ionic radius of dopant cations, but the improvement is related to the grain boundary chemical bonding state, which was influenced by the dopant segregation [5]. The purpose of this paper is to discuss the vacancy effect due to dopant addition. The first-principle molecular orbital calculations were done by solving the HartreeFock-Slater equations self consistently using DV-Xa method developed by Adachi et al.[6].
2. Experimental procedure The materials used in this study are undoped highpurity Al2O3 and Al2O3 with 0.1 mol% of YO1.5, ZrO2 or MgO. High-purity alumina powders with 99.99% purity (TM-DAR, Taimei Chemicals, Japan), the yt-
0921-5093/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 5 0 9 3 ( 0 1 ) 0 1 0 3 8 - 3
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Table 1 Sintering conditions and grain size of undoped- and YO1.5 or ZrO2 or MgO-doped Al2O3 Sample
Sintering conditions
Grain size (mm)
Ionic radius of dopant cation (A, )a
High-purity Al2O3 YO1.5–doped Al2O3 ZrO2–doped Al2O3 MgO–doped Al2O3
1300 1400 1400 1300
0.9 1.0 1.0 0.9
– 0.900 0.720 0.720
a
°C×2 °C×2 °C×2 °C×2
h h h h
All for six-fold coordination. Ionic radius of Al3+ = 0.535 A, [7].
trium acetate (99.9%, Rare Metallic, Tokyo, Japan), colloidal zirconia particles dispersed in water (99.9%, NZS-30B, Nissan Chemical Industry, Japan) and MgO powders (99.98%, Ube Chemical, Japan) were used for starting materials. The powders were mixed, ball-milled in ethanol together with 5 mm diameter high-purity(\ 99.9%) alumina balls for 24 h, dried and shifted through a 60 mesh sieve for granulation. The green compacts were prepared by pressing the mixed powders into bars with a cemented carbide die under a pressure of 33 MPa, and then isostatically-pressed under a pressure of 100 MPa. The green compacts were sintered at a temperature in the range 1300– 1400 °C for 2 h in air to obtain an average grain size of 1 mm in all samples as shown in Table 1. High-temperature creep experiments were carried out under uniaxial compression in air at a constant load using a lever-arm testing machine with a resistance-heated furnace (HCT-1000, Toshin Industry, Tokyo, Japan). The applied stress and temperature were 50 MPa and 1250 °C, respectively. The test temperature was measured by a Pt-PtRh thermocouple attached to each specimen and kept to within 9 1 °C. The size of the specimens was 6×6 mm2 in cross-section and 8 mm in height for compression tests. Microstructures were examined with a scanning electron microscope (SEM; JSM-5200, JEOL). The grain size was measured by a linear intercept method using SEM photographs. High-resolution electron microscopy (HREM) was also performed to analyse the grain-boundary structure using a JEOL-2010 field-emission-type electron microscope. Chemical analysis was carried out by an X-ray energy dispersive spectrometer (EDS) attached to the microscope with a probe size of B1 nm.
ference of creep strain rate in the present materials is not caused by the change of grain growth rate, because the grain growth during creep deformation was negligible in the present materials at the temperature examined. Fig. 2 shows a HREM image of YO1.5 –doped Al2O3 (a) together with the data of EDS analyses taken with a probe size of 0.8 nm from a grain interior (b) and from a grain boundary (c). The HREM image is taken for the boundary at the edge-on view. No second phase or amorphous phase at the boundary was observed along the grain boundary. The EDS analysis reveals that yttrium ions are present only in the grain boundary, but not in the grain interior. Yttrium ions must segregate in grain boundaries. The microstructure in Zr4 + or Mg2 + -doped Al2O3 is essentially similar to that in Y + 3 -doped one. It is suggested that the hightemperature creep deformation is likely to take place mainly by Al3 + grain boundary sliding controlled by grain boundary diffusion [1,3–5]. The change in the creep resistance must be caused by the segregation of dopant cations at grain boundaries. Fig. 3 shows a plot of creep strain at 1250 °C under an applied stress of 50MPa in the present materials
3. Results and discussion The creep curves at 1250 °C under an applied stress of 50 MPa in the present materials are shown in Fig. 1. The creep deformation in polycrystalline Al2O3 is very much suppressed by doping of YO1.5 or ZrO2 ions [1,4,5], but is slightly accelerated by MgO doping. The doping of YO1.5 is much effective to improve the creep resistance in Al2O3 in comparison with ZrO2. The dif-
Fig. 1. The creep curves in undoped, high-purity Al2O3 and 0.1 mol% of YO1.5 or ZrO2 or MgO-doped Al2O3 under an applied stress of 50MPa at 1250 °C.
H. Yoshida et al. / Materials Science and Engineering A319–321 (2001) 843–848
Fig. 2. HREM image of as-sintered 0.1 mol% YO1.5 –doped Al2O3 (a), and the EDS spectra taken with a probe size of 1 nm from: (b) the grain interior; and (c) the grain boundary.
against ionic radius of dopant cations. The dopant effect cannot be explained solely in terms of the ionic radius of the dopant cation; for example, Zr4 + and Mg2 + ions have similar ionic radii, which is much larger than that in Al as shown in Table 1, but the role of the dopant cations is different from each other. Fig. 3(b) shows the eutectic point in Al2O3-oxide of dopant cation system. No correlation is seen between the creep strain rate and the eutectic point. In order to find the
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origin of the change in the creep resistance due to the cation doping, change in chemical bonding state in the cation-doped Al2O3 was examined by a first-principle molecular orbital calculation. The model clusters of undoped-Al2O3 and Al2O3 with dopant cation used are shown in Fig. 4(a–f). Fig. 4(a) shows the model clusters for undoped Al2O3, [Al5O21]27 − , which represents a part of the corundum structure. One can estimate the interaction between an Al ion at the center of the cluster (marked as AlC in Fig. 4(a)) and surrounding O or Al ions by using the model cluster. Fig. 4(b–c) show model clusters of [Al5O20]25 − and [Al4O21]30 − , which have O2 − ion vacancy (V%%O) and Al3 + ion vacancy (V%%%Al), respectively. In order to estimate the interaction between AlC and vacancy, the vacancy is introduced at the nearest O or Al site as shown in Fig. 4(b–c). Fig. 4(d) shows a model cluster of [Al2Y3O21]27 − , in which Y3 + ions are substituted at Al sites. Fig. 4(e–f) shows the [Al3Mg2O20]27 − and [AlZr3O21]27 − clusters, which include the dopant cations and accompanying O2 − or Al3 + vacancies for electronic neutrality. From the first-principle molecular orbital calculations, the ionic net charges (NC) of each atom and the bond overlap population (BOP) between atoms can be obtained as the parameters of chemical bonding state. The NC and the BOP are regarded as the effective ionic charge and the covalency between atoms, respectively [6,8– 10]. Fig. 5 shows the BOP of (a) Al–O ions and (b) dopant cation-O obtained from the cluster models. The horizontal broken lines denote the value of Al–O in pure-Al2O3 cluster. The BOP of Al–O in V%%O or V%%%Al-introduced Al2O3 and cation-doped Al2O3 is smaller than that in pure-Al2O3. This result indicates that the covalency between Al and O ions decreases by the presence of vacancy or dopant cations. The BOP of dopant cation–O is also smaller than that of Al – O in pure Al2O3. This fact indicates that the covalency between dopant cation and O ions is lower than that of Al –O. The BOP seems not to be related to the change in the high-temperature creep resistance in Al2O3. Fig. 6 shows the NC of: (a) Al and O ions; and (b) dopant cations. The horizontal lines in the figure denote the value in pure-Al2O3. The absolute values of NC of Al and O ions become smaller in the V%%%Al or V%%O-introduced clusters. The decrease of NC is clearer in Al3 + vacancy-introduced cluster. This result suggests that the chemical bonding strength is reduced by the presence of vacancies and that the reduction is larger in V%%%Al –doped cluster. On the other hand, the value of NC in Y-doped cluster is larger than that in Al2O3 cluster, and NC of Al and O in [Al3Mg2O20]27 − or [AlZr3O21]27 − is larger than that in [Al5O20]25 − or [Al4O21]30 − , respectively. One can conclude that the dopant cations have an effect to increase the ionicity between Al and O ions in Al2O3. The values of NC of
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Al, O and dopant cations in the Y or Zr-doped cluster becomes larger, but the NC value in the Mg-doped cluster becomes smaller than that in pure-Al2O3. This result indicates that NC is likely to correlate to the creep resistance in cation-doped Al2O3. Fig. 7 shows the creep strain rate against: (a) BOP of Al –O; and (b) the product of NC in Al and O ions. The effect of dopant cations on the creep resistance cannot be explained solely from the order of the BOP in cation-doped Al2O3; the BOP of Mg or Zr-doped cluster is nearly the same, but the dopant effect on the creep strain rate is very different. However, the absolute
value of the production of NC shows a good correlation with the creep resistance in cation-doped Al2O3. The product of NC of Al and O must correspond to the Coulomb’s attraction force between cation and anion. The present result must reflect the fact that the chemical bonding strength in Al2O3 is mainly associated with ionic bonding, i.e. a Coulomb’s electrical force [11]. In our previous report, the change in chemical bonding state in Al2O3 grain boundaries with segregation of Lu ions was detected by HREM-electron energy loss spectroscopy (EELS) analysis [12]. The change in the energy-loss near-edge structure (ELNES), which con-
Fig. 3. A plot of creep strain rate at 1250 °C under the applied stress of 50 MPa in the present materials against: (a) ionic radius of dopant cations; and (b) eutectic point of Al2O3 –oxide of dopant cation system.
Fig. 4. Atomic structure of cluster models: (a) [Al5O21]27 − ; (b) [Al5O20]25 − ; (c) [Al4O21]30 − ; (d) [Al2Y3O21]27 − ; (e) [Al3Mg2O20]27 − ; and (f) [AlZr3O21]27 − .
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tains fine structure of unoccupied density of states in the conduction band (DOS) [13], can be explained by the DOS obtained from the molecular orbital calculations using Lu-doped Al2O3 cluster [12]. This result suggests that the chemical bonding state in Al2O3 is influenced by the presence of dopant cations rather than by the grain boundary atomic structure. The model clusters used in this study were very simple ones, but seem to be useful to explain the change in the chemical bonding state in Al2O3 grain boundaries due to the segregation of dopant cations. The configuration of dopant ions at grain boundaries in Al2O3 would be different from those of Fig. 4. But, we can assumed the simple clusters for the grain boundary models with the segregation in order to estimate the change in the atomic bonding state on introducing vacancy or dopant cations into Al2O3.
4. Conclusions
Fig. 5. Bond overlap population of: (a) Al –O; and (b) dopant cation – O obtained from molecular orbital calculations.
The creep resistance of polycrystalline Al2O3 with a grain size of 1 mm is highly improved by the doping of YO1.5 or ZrO2 even in the level of 0.1 mol%, but is reduced slightly by MgO doping at temperature of 1250 °C under an applied stress of 50 MPa. The high-temperature creep deformation is likely to take place mainly by grain boundary sliding accommodated by grain boundary diffusion, and the change in the creep resistance is likely to be caused by the change in the grain boundary diffusivity due to the segregation of cations at grain boundaries. The ionic bonding and the covalent bonding of Al–O are lowered by the introduction of V%%O or V%%%Al but the values of NC in Al and O are increased by the cations doping. The change in NC value is correlated well with the high-temperature creep resistance in Al2O3 with cation doping. It is suggested that the ionicity in Al and O is an important factor to determine high-temperature creep resistance in polycrystalline Al2O3.
Acknowledgements
Fig. 6. Net charges for: (a) Al and O; and (b) dopant cation obtained from model clusters.
The authors wish to express their gratitude to the Ministry of Education, Science and Culture, Japan, for the financial support by a Grant-in-Aid for Developmental Scientific Research (2)–1045 0254 for Fundamental Scientific Research. We also wish to express our thanks to Research Fellowships of the Japan Society for the Promotion of Science for Young Scientists for their financial aid. A part of this study was supported by Kurata Science and Technology Foundation.
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Fig. 7. A plot of creep strain rate at 1250 °C under the applied stress of 50 MPa in the present materials against: (a) bond overlap population of Al– O; and (b) the product of Al and O net charges obtained by molecular orbital calculations.
References [1] H. Yoshida, K. Okada, Y. Ikuhara, T. Sakuma, Phil. Mag. Lett. 76 (1997) 9 –14. [2] J. Cho, M.P. Harmer, H.M. Chan, J.M. Rickman, A.M. Thompson, J. Am. Ceram. Soc. 80 (1997) 1013 –1017. [3] Y. Takigawa, Y. Ikuhara, T. Sakuma, J. Mater. Sci. 34 (1999) 1991 – 1997. [4] H. Yoshida, Y. Ikuhara, T. Sakuma, J. Mater. Res. 13 (1998) 2597 – 2601. [5] H. Yoshida, Y. Ikuhara, T. Sakuma, Phil. Mag. Lett. 79 (1999) 249 – 256. [6] H. Adachi, M. Tsukada, C. Satoko, J. Phys. Soc. Jpn. 45
(1978) 875 – 883. [7] R.D. Shannon, Acta Cryst. A32 (1976) 751 – 767. [8] H. Adachi, Ceramics 27 (1992) 495 – 501. [9] I. Tanaka, J. Kawai, H. Adachi, Solid State Commun. 93 (1995) 533 – 536. [10] Y. Ikuhara, Y. Sugawara, I. Tanaka, P. Pirouz, Interface Sci. 5 (1997) 5 – 16. [11] W.D. Kingery, H.K. Bowen, D.R. Uhlman, Introduction to Ceramics, Wiley, New York, 1976, p. 185. [12] H. Yoshida, Y. Ikuhara, T. Sakuma, J. Inorganic Mater. 1 (1999) 229 – 234. [13] D.B. Williams, C.B. Carter, Transmission Electron Microscopy. Plenum Press, New York, 1996.
Vacancy effect of dopant cation on the high-temperature creep resistance in polycrystalline Al2O3 Hidehiro Yoshida a,*, Yuichi Ikuhara b, Taketo Sakuma a a
Graduate School of Frontier Sciences, The Uni6ersity of Tokyo, 7 -3 -1 Hongo, Bunkyo-ku, Tokyo 113 -8656, Japan b Graduate School of Engineering, The Uni6ersity of Tokyo, 7 -3 -1 Hongo, Bunkyo-ku, Tokyo 113 -8656, Japan
Abstract High-temperature creep resistance in polycrystalline Al2O3 with 0.1 mol% oxides of YO1.5, ZrO2 or MgO has been examined by uniaxial compression creep testing at 1250 °C. The creep resistance is highly improved by the doping of Y or Zr even in the dopant level of 0.1 mol%, but is retarded by Mg doping. The dopant effect on the creep resistance cannot be explained in terms of, for example, ionic radius of the dopant cation or eutectic point in Al2O3-oxide of dopant cation system. Each dopant cation was found to segregate in grain boundaries, and is likely to influence grain boundary diffusion in Al2O3. The ionic bonding and the covalent bonding of Al–O are lowered by the introduction of V%%O or V%%%Al but the values of the net charge in Al and O are increased by the cations doping. The change in the value of Net Charge is correlated well with the high-temperature creep resistance in Al2O3 with cation doping. It is suggested that the ionicity in Al and O is an important factor to determine high-temperature creep resistance in polycrystalline Al2O3. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Grain boundaries; Al2O3; Resistance
1. Introduction It has been reported that high-temperature creep deformation or plastic flow in polycrystalline Al2O3 is sensitively affected by a small amount of dopant cation. An addition of 0.1wt% ZrO2 in Al2O3 with a grain size of 1 mm severely suppresses the creep rate by a factor of 15 at 1250 °C under the stresses of 10 – 200 MPa [1]. Cho et al. reported that 1000 ppm yttrium or 500 ppm lanthanum-doped Al2O3 has improved the creep resistance in comparison with pure Al2O3 at 1200 – 1350 °C under the stress of 50 MPa [2]. Such a dopant effect of MgO in the level of 0.1 wt.% was also observed in high-temperature flow stress of fine-grained Al2O3 in conventional tensile testing; the peak stress decreases by the doping of MgO at 1450 °C under an initial strain rate of 1 × 10 − 4 s − 1 [3]. It has been pointed out that the high-temperature creep resistance in Al2O3 with a grain size of 1 mm is markedly improved by a doping of lanthanoid oxide such as Y2O3 or Lu2O3 even in the level of 0.05 mol% [4]. In this case, * Corresponding author. E-mail address: [email protected] (H. Yoshida).
the dopant cation did not form the second phase precipitations, but segregated in the vicinity of grain boundaries to improve the creep resistance in Al2O3. The segregation must be an origin of the change in creep deformation, but the mechanism has not been clarified yet. The change in the creep resistance in lanthanoid oxide-doped Al2O3 cannot be explained in terms of ionic radius of dopant cations, but the improvement is related to the grain boundary chemical bonding state, which was influenced by the dopant segregation [5]. The purpose of this paper is to discuss the vacancy effect due to dopant addition. The first-principle molecular orbital calculations were done by solving the HartreeFock-Slater equations self consistently using DV-Xa method developed by Adachi et al.[6].
2. Experimental procedure The materials used in this study are undoped highpurity Al2O3 and Al2O3 with 0.1 mol% of YO1.5, ZrO2 or MgO. High-purity alumina powders with 99.99% purity (TM-DAR, Taimei Chemicals, Japan), the yt-
0921-5093/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 5 0 9 3 ( 0 1 ) 0 1 0 3 8 - 3
H. Yoshida et al. / Materials Science and Engineering A319–321 (2001) 843–848
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Table 1 Sintering conditions and grain size of undoped- and YO1.5 or ZrO2 or MgO-doped Al2O3 Sample
Sintering conditions
Grain size (mm)
Ionic radius of dopant cation (A, )a
High-purity Al2O3 YO1.5–doped Al2O3 ZrO2–doped Al2O3 MgO–doped Al2O3
1300 1400 1400 1300
0.9 1.0 1.0 0.9
– 0.900 0.720 0.720
a
°C×2 °C×2 °C×2 °C×2
h h h h
All for six-fold coordination. Ionic radius of Al3+ = 0.535 A, [7].
trium acetate (99.9%, Rare Metallic, Tokyo, Japan), colloidal zirconia particles dispersed in water (99.9%, NZS-30B, Nissan Chemical Industry, Japan) and MgO powders (99.98%, Ube Chemical, Japan) were used for starting materials. The powders were mixed, ball-milled in ethanol together with 5 mm diameter high-purity(\ 99.9%) alumina balls for 24 h, dried and shifted through a 60 mesh sieve for granulation. The green compacts were prepared by pressing the mixed powders into bars with a cemented carbide die under a pressure of 33 MPa, and then isostatically-pressed under a pressure of 100 MPa. The green compacts were sintered at a temperature in the range 1300– 1400 °C for 2 h in air to obtain an average grain size of 1 mm in all samples as shown in Table 1. High-temperature creep experiments were carried out under uniaxial compression in air at a constant load using a lever-arm testing machine with a resistance-heated furnace (HCT-1000, Toshin Industry, Tokyo, Japan). The applied stress and temperature were 50 MPa and 1250 °C, respectively. The test temperature was measured by a Pt-PtRh thermocouple attached to each specimen and kept to within 9 1 °C. The size of the specimens was 6×6 mm2 in cross-section and 8 mm in height for compression tests. Microstructures were examined with a scanning electron microscope (SEM; JSM-5200, JEOL). The grain size was measured by a linear intercept method using SEM photographs. High-resolution electron microscopy (HREM) was also performed to analyse the grain-boundary structure using a JEOL-2010 field-emission-type electron microscope. Chemical analysis was carried out by an X-ray energy dispersive spectrometer (EDS) attached to the microscope with a probe size of B1 nm.
ference of creep strain rate in the present materials is not caused by the change of grain growth rate, because the grain growth during creep deformation was negligible in the present materials at the temperature examined. Fig. 2 shows a HREM image of YO1.5 –doped Al2O3 (a) together with the data of EDS analyses taken with a probe size of 0.8 nm from a grain interior (b) and from a grain boundary (c). The HREM image is taken for the boundary at the edge-on view. No second phase or amorphous phase at the boundary was observed along the grain boundary. The EDS analysis reveals that yttrium ions are present only in the grain boundary, but not in the grain interior. Yttrium ions must segregate in grain boundaries. The microstructure in Zr4 + or Mg2 + -doped Al2O3 is essentially similar to that in Y + 3 -doped one. It is suggested that the hightemperature creep deformation is likely to take place mainly by Al3 + grain boundary sliding controlled by grain boundary diffusion [1,3–5]. The change in the creep resistance must be caused by the segregation of dopant cations at grain boundaries. Fig. 3 shows a plot of creep strain at 1250 °C under an applied stress of 50MPa in the present materials
3. Results and discussion The creep curves at 1250 °C under an applied stress of 50 MPa in the present materials are shown in Fig. 1. The creep deformation in polycrystalline Al2O3 is very much suppressed by doping of YO1.5 or ZrO2 ions [1,4,5], but is slightly accelerated by MgO doping. The doping of YO1.5 is much effective to improve the creep resistance in Al2O3 in comparison with ZrO2. The dif-
Fig. 1. The creep curves in undoped, high-purity Al2O3 and 0.1 mol% of YO1.5 or ZrO2 or MgO-doped Al2O3 under an applied stress of 50MPa at 1250 °C.
H. Yoshida et al. / Materials Science and Engineering A319–321 (2001) 843–848
Fig. 2. HREM image of as-sintered 0.1 mol% YO1.5 –doped Al2O3 (a), and the EDS spectra taken with a probe size of 1 nm from: (b) the grain interior; and (c) the grain boundary.
against ionic radius of dopant cations. The dopant effect cannot be explained solely in terms of the ionic radius of the dopant cation; for example, Zr4 + and Mg2 + ions have similar ionic radii, which is much larger than that in Al as shown in Table 1, but the role of the dopant cations is different from each other. Fig. 3(b) shows the eutectic point in Al2O3-oxide of dopant cation system. No correlation is seen between the creep strain rate and the eutectic point. In order to find the
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origin of the change in the creep resistance due to the cation doping, change in chemical bonding state in the cation-doped Al2O3 was examined by a first-principle molecular orbital calculation. The model clusters of undoped-Al2O3 and Al2O3 with dopant cation used are shown in Fig. 4(a–f). Fig. 4(a) shows the model clusters for undoped Al2O3, [Al5O21]27 − , which represents a part of the corundum structure. One can estimate the interaction between an Al ion at the center of the cluster (marked as AlC in Fig. 4(a)) and surrounding O or Al ions by using the model cluster. Fig. 4(b–c) show model clusters of [Al5O20]25 − and [Al4O21]30 − , which have O2 − ion vacancy (V%%O) and Al3 + ion vacancy (V%%%Al), respectively. In order to estimate the interaction between AlC and vacancy, the vacancy is introduced at the nearest O or Al site as shown in Fig. 4(b–c). Fig. 4(d) shows a model cluster of [Al2Y3O21]27 − , in which Y3 + ions are substituted at Al sites. Fig. 4(e–f) shows the [Al3Mg2O20]27 − and [AlZr3O21]27 − clusters, which include the dopant cations and accompanying O2 − or Al3 + vacancies for electronic neutrality. From the first-principle molecular orbital calculations, the ionic net charges (NC) of each atom and the bond overlap population (BOP) between atoms can be obtained as the parameters of chemical bonding state. The NC and the BOP are regarded as the effective ionic charge and the covalency between atoms, respectively [6,8– 10]. Fig. 5 shows the BOP of (a) Al–O ions and (b) dopant cation-O obtained from the cluster models. The horizontal broken lines denote the value of Al–O in pure-Al2O3 cluster. The BOP of Al–O in V%%O or V%%%Al-introduced Al2O3 and cation-doped Al2O3 is smaller than that in pure-Al2O3. This result indicates that the covalency between Al and O ions decreases by the presence of vacancy or dopant cations. The BOP of dopant cation–O is also smaller than that of Al – O in pure Al2O3. This fact indicates that the covalency between dopant cation and O ions is lower than that of Al –O. The BOP seems not to be related to the change in the high-temperature creep resistance in Al2O3. Fig. 6 shows the NC of: (a) Al and O ions; and (b) dopant cations. The horizontal lines in the figure denote the value in pure-Al2O3. The absolute values of NC of Al and O ions become smaller in the V%%%Al or V%%O-introduced clusters. The decrease of NC is clearer in Al3 + vacancy-introduced cluster. This result suggests that the chemical bonding strength is reduced by the presence of vacancies and that the reduction is larger in V%%%Al –doped cluster. On the other hand, the value of NC in Y-doped cluster is larger than that in Al2O3 cluster, and NC of Al and O in [Al3Mg2O20]27 − or [AlZr3O21]27 − is larger than that in [Al5O20]25 − or [Al4O21]30 − , respectively. One can conclude that the dopant cations have an effect to increase the ionicity between Al and O ions in Al2O3. The values of NC of
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Al, O and dopant cations in the Y or Zr-doped cluster becomes larger, but the NC value in the Mg-doped cluster becomes smaller than that in pure-Al2O3. This result indicates that NC is likely to correlate to the creep resistance in cation-doped Al2O3. Fig. 7 shows the creep strain rate against: (a) BOP of Al –O; and (b) the product of NC in Al and O ions. The effect of dopant cations on the creep resistance cannot be explained solely from the order of the BOP in cation-doped Al2O3; the BOP of Mg or Zr-doped cluster is nearly the same, but the dopant effect on the creep strain rate is very different. However, the absolute
value of the production of NC shows a good correlation with the creep resistance in cation-doped Al2O3. The product of NC of Al and O must correspond to the Coulomb’s attraction force between cation and anion. The present result must reflect the fact that the chemical bonding strength in Al2O3 is mainly associated with ionic bonding, i.e. a Coulomb’s electrical force [11]. In our previous report, the change in chemical bonding state in Al2O3 grain boundaries with segregation of Lu ions was detected by HREM-electron energy loss spectroscopy (EELS) analysis [12]. The change in the energy-loss near-edge structure (ELNES), which con-
Fig. 3. A plot of creep strain rate at 1250 °C under the applied stress of 50 MPa in the present materials against: (a) ionic radius of dopant cations; and (b) eutectic point of Al2O3 –oxide of dopant cation system.
Fig. 4. Atomic structure of cluster models: (a) [Al5O21]27 − ; (b) [Al5O20]25 − ; (c) [Al4O21]30 − ; (d) [Al2Y3O21]27 − ; (e) [Al3Mg2O20]27 − ; and (f) [AlZr3O21]27 − .
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tains fine structure of unoccupied density of states in the conduction band (DOS) [13], can be explained by the DOS obtained from the molecular orbital calculations using Lu-doped Al2O3 cluster [12]. This result suggests that the chemical bonding state in Al2O3 is influenced by the presence of dopant cations rather than by the grain boundary atomic structure. The model clusters used in this study were very simple ones, but seem to be useful to explain the change in the chemical bonding state in Al2O3 grain boundaries due to the segregation of dopant cations. The configuration of dopant ions at grain boundaries in Al2O3 would be different from those of Fig. 4. But, we can assumed the simple clusters for the grain boundary models with the segregation in order to estimate the change in the atomic bonding state on introducing vacancy or dopant cations into Al2O3.
4. Conclusions
Fig. 5. Bond overlap population of: (a) Al –O; and (b) dopant cation – O obtained from molecular orbital calculations.
The creep resistance of polycrystalline Al2O3 with a grain size of 1 mm is highly improved by the doping of YO1.5 or ZrO2 even in the level of 0.1 mol%, but is reduced slightly by MgO doping at temperature of 1250 °C under an applied stress of 50 MPa. The high-temperature creep deformation is likely to take place mainly by grain boundary sliding accommodated by grain boundary diffusion, and the change in the creep resistance is likely to be caused by the change in the grain boundary diffusivity due to the segregation of cations at grain boundaries. The ionic bonding and the covalent bonding of Al–O are lowered by the introduction of V%%O or V%%%Al but the values of NC in Al and O are increased by the cations doping. The change in NC value is correlated well with the high-temperature creep resistance in Al2O3 with cation doping. It is suggested that the ionicity in Al and O is an important factor to determine high-temperature creep resistance in polycrystalline Al2O3.
Acknowledgements
Fig. 6. Net charges for: (a) Al and O; and (b) dopant cation obtained from model clusters.
The authors wish to express their gratitude to the Ministry of Education, Science and Culture, Japan, for the financial support by a Grant-in-Aid for Developmental Scientific Research (2)–1045 0254 for Fundamental Scientific Research. We also wish to express our thanks to Research Fellowships of the Japan Society for the Promotion of Science for Young Scientists for their financial aid. A part of this study was supported by Kurata Science and Technology Foundation.
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Fig. 7. A plot of creep strain rate at 1250 °C under the applied stress of 50 MPa in the present materials against: (a) bond overlap population of Al– O; and (b) the product of Al and O net charges obtained by molecular orbital calculations.
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