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Hydrogen-induced degradation in CaCu3Ti4O12 ceramics
Journal of Alloys and Compounds 422 (2006) L9–L12
Letter
Hydrogen-induced degradation in CaCu3Ti4O12 ceramics Wan Ping Chen a,b,∗ , Wang Xiang a , Ming Sen Guo a , Wen Chao You a , Xing Zhong Zhao a , Helen Lai Wah Chan b b
a Department of Physics, Wuhan University, Wuhan 430072, PR China Department of Applied Physics and Materials Research Center, The Hong Kong Polytechnic University, Hong Kong, PR China
Received 17 November 2005; accepted 7 December 2005 Available online 24 January 2006
Abstract Hydrogen-induced degradation in CaCu3 Ti4 O12 (CCTO) ceramics was studied using an electrochemical hydrogen charging method, in which the silver electrodes of CCTO ceramic pellets were made a cathode in 0.01 M NaOH solution to deposit hydrogen through the electrolysis of water. The dielectric loss was greatly increased and the insulation resistance was greatly decreased after the treatment. X-ray diffraction analysis showed that no new phases were formed. It is proposed that atomic hydrogen is diffused into CCTO lattice and free electrons are formed through the ionization of hydrogen atoms, which lead to the observed degradation. Hydrogen-induced degradation occurs very quickly in CCTO ceramics and it is important to prevent hydrogen-induced degradation in CCTO ceramics. © 2005 Elsevier B.V. All rights reserved. Keywords: Degradation; Hydrogen; Dielectric; Ceramics
1. Introduction In recent years, the rapid development of many electronic products have benefited greatly from the miniaturization of various discrete components, especially capacitors, in them. The volume efficiency of a capacitor is directly related to its dielectric constant and there have been intensive researches on high dielectric constant materials. Generally speaking, these researches can be classified into two categories. In the first category, investigations are aimed at improving the properties of those widelyapplied high dielectric constant materials, including barium titanate and lead zirconate titanate; while in the second category, efforts are made to develop other new high dielectric constant materials, such as CaCu3 Ti4 O12 (CCTO). With a giant roomtemperature dielectric constant in the order of 104 for ceramics and ∼350 000 for single crystals, CCTO has attracted particular attention [1,2]. So far, the origin of the high dielectric constant of this material is not clear and several mechanisms have been put forwarded. Sinclair et al. [2] used the well-known internal
∗ ∗ Corresponding
author. Tel.: +86 27 6131 2396; fax: +86 27 6875 2697. E-mail addresses: [email protected], [email protected] (W.P. Chen). 0925-8388/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2005.12.013
barrier layer capacitive model to interpret the impedance spectroscopy data on CCTO, which is famous for perovskite-type titanates that are processed in such a way as to produce grains that are reduced and conducting but coupled with grain boundaries that are oxidized and insulating. When many investigations are still devoted to understanding the interesting physics that CCTO presents, more and more researches have turned to other practical works for its potential applications. CCTO films have been fabricated through pulsed laser deposition [3,4], composites with other materials have also been reported [5]. Another important work regarding its practical applications should be studies on its chemical stability. It is well known that the dielectric properties and resistance of many electronic ceramic devices and components sometimes degrade slowly under prolonged application of electrical field [6,7] and their lifetime has thus been seriously limited. Several origins have been recognized for the degradation, among which an important one is the ambient-temperature reaction of hydrogen from the electrolysis of water. This degradation process is quite common since electroceramic components and devices always operate under some voltages while moisture is ubiquitous in the environment [8,9]. Presently, we have studied the ambient-temperature reaction of hydrogen with CCTO ceramics. The influences of the reaction on the properties of CCTO
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W.P. Chen et al. / Journal of Alloys and Compounds 422 (2006) L9–L12
ceramics will be reported and the reaction mechanism will be discussed in this paper. 2. Experimental Reagent-grade powders of CaCO3 , CuO and TiO2 were mixed to yield a composition of CaCu3 Ti4 O12 . The mixture was ball-milled for 24 h and calcined at 1000 ◦ C for 12 h. Then both the ball-milling and the calcination were repeated for a second time. The calcined powder was planetary ball-milled for 1 h before pressing into pellets of 15.80 mm in diameter and 1 mm thick, which were sintered at 1100 ◦ C for 24 h and furnace-cooled to room-temperature. Two major faces of the sintered pellets were lightly polished with SiC paper and Ag electrodes were coated on them through sputtering. To study the influence of water and dc voltage, some CCTO pellets were placed in a 0.01 M NaOH solution and a 4.5 V dc voltage was imposed between the silver electrodes of the pellets and a counter Pt electrode in the solution. The silver electrodes of the ceramic pellets acted as the cathode while the counter Pt electrode acted as the anode. The imposed dc voltage induced electrolysis of water with hydrogen deposited on the silver electrodes of the pellets, and this treatment is referred to as electrochemical hydrogen charging hereafter. The dc voltage was removed after some designed periods of time and the pellets were taken out of the solution, washed with de-ionized water and dried. The frequency spectra of capacitance and dielectric loss of the pellets were measured on an Agilent 4294A impedance analyzer. Resistance of the pellets was recorded through a Keithley 2000 multimeter. Phase detection was performed using an X-ray diffractometer (Philips PW 3719) with Cu K␣ radiation. A scanning electron microscope (STEROSCAN) was used to observe the microstructure of the pellets.
3. Results and discussion The average diameter of the pellets shrank to 12.86 mm after sintering. Pellet densities calculated using the Archimedes method were all over 95% of the theoretical X-ray density. A representative SEM micrograph taken for the surface of an assintered ceramic pellet is shown in Fig. 1, which also indicates a dense microstructure. It would be difficult for water or moisture to permeate into the pellets. As a matter of fact, for reference, we had immersed some pellets in 0.01 M NaOH solution for long periods of time with no electricity applied. After cleaning and drying, no detectable change was observed in the properties
Fig. 2. Frequency spectra of capacitance and dielectric loss obtained for a CCTO ceramic pellet as-sintered and after different periods of electrochemical hydrogen charging, respectively.
of the samples. Together with the microstructural observation, it can be concluded that water showed little influence on the pellets through permeation. For those samples treated with the application of the dc voltage, on the contrary, their properties were found seriously degraded after the treatment. Fig. 2 shows the frequency spectra of capacitance and dielectric loss obtained for a representative sample as-sintered and after different periods of electrochemical hydrogen charging. Though the capacitance exhibited some noticeable variation with the treatment, the most remarkable change occurred in the dielectric loss. After 22 h of electrochemical hydrogen charging, the dielectric loss was increased from 0.3 to 1.7 at 102 Hz, and remained almost unchanged at frequencies above 104 Hz. At the same time, the resistance of the sample measured between its two silver electrodes was found greatly decreased, as shown in Fig. 3. The resistance was decreased by almost one order of magnitude after only 1 h of hydrogen charging. So the decrease was very quick at the initial stage and then it
Fig. 1. SEM micrograph taken on the surface of an as-sintered CCTO ceramic pellet.
W.P. Chen et al. / Journal of Alloys and Compounds 422 (2006) L9–L12
Fig. 3. Resistance measured between two silver electrodes of a CCTO ceramic pellet vs. electrochemical hydrogen charging time.
was slowed down with increasing time of hydrogen charging. It is well known that low dielectric loss and high insulation resistance are two primary requirements for dielectric applications. Obviously electrochemical hydrogen charging is very harmful for the application of CCTO ceramics as high dielectric constant materials. Since no noticeable change in properties was observed in those samples immersed in the NaOH solution with no voltage applied, the degradation of CCTO-based ceramics induced by electrochemical hydrogen charging was directly related to the deposition of hydrogen on the silver electrodes of the samples, which can be expressed as: H2 O + e− → OH− + Hads ,
(1)
Hads + Hads → H2 ,
(2)
where Hads represents an adsorbed hydrogen atom. As we proposed in some previous researches [8,10], hydrogen atoms deposited on silver electrodes may diffuse into ceramics and react with them at room-temperature, though most of them combine with one another and form hydrogen molecules. The properties of the ceramics are usually greatly changed by the reaction of hydrogen, the same as the properties of the CCTO ceramics in this study. The reactions between oxides and hydrogen through electrochemical hydrogen charging can be classified into two categories: either the lattices of the oxides are decomposed after the reaction [8], or no gross structural change occurs in the lattices of the oxides with hydrogen incorporated in them [10]. Fig. 4 shows the X-ray diffraction patterns taken on the surface of a CCTO pellet as-sintered and after 42 h of hydrogen charging, respectively. It is clear that the long time treatment led to no noticeable changes in the X-ray diffraction pattern. So we believe that the reaction between hydrogen and CCTO belongs to the second category, namely hydrogen atoms diffuse into CCTO lattice and exist at interstitial sites: Hads → Hi • + e ,
(3)
L11
Fig. 4. X-ray diffraction patterns taken on the surface of a CCTO ceramic pellet as-sintered and after 42 h of electrochemical hydrogen charging, respectively.
where Hi • represents a hydrogen ion in interstitial site. The formation of interstitial hydrogen in oxides via hydrogen charging process has been proven through Fourier-transform infrared (FTIR) absorption analysis of polished rutile single crystals [10]. A free electron is generated through the ionization of hydrogen atom, which accounts for the great increases in the dielectric loss and in the leakage current of hydrogen charged-CCTO. In practice, perovskite-type titanates are most widely used in high dielectric constant ceramic capacitors and hydrogeninduced degradation had also been observed for them. However, as we can see in Fig. 3, hydrogen-induced resistance degradation is very quick for CCTO in the early stage of electrochemical hydrogen charging, which we had not observed for barium titanate ceramics [11]. It indicates that hydrogen-induced degradation will occur more easily in CCTO than in barium titanate. On the other hand, due to the rapid development of surface mounting technology (SMT), now more and more ceramic capacitors are being manufactured as chip capacitors. Electroplating is widely applied in forming three-layer-electrodes for chip components and no polymer coating can be adopted for them. It is well known that hydrogen is always deposited during electroplating [12] while polymer coating is helpful for preventing the electrolysis of water on electronic components in service. Hydrogen-induced degradation will become a more serious problem if CCTO is fabricated as chip ceramic capacitors. It is an important challenge to minimize hydrogen-induced degradation in CCTO for capacitors application and further investigations are highly desired. 4. Conclusions The dielectric loss and the leakage current of CCTO ceramics are greatly increased when hydrogen is deposited on them through the electrolysis of water. No new phases are formed after the treatment and it is proposed that hydrogen atoms are incorporated into CCTO lattice and exist in interstitial sites. The degradation can be well explained by the formation of
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W.P. Chen et al. / Journal of Alloys and Compounds 422 (2006) L9–L12
free electrons through the ionization of interstitial hydrogen. Hydrogen-induced degradation occurs quite quickly in CCTO ceramics and efforts should be made to minimize hydrogeninduced degradation in CCTO ceramics. Acknowledgement This work has been supported by the Centre for Smart Materials of the Hong Kong Polytechnic University (Project: 1-A302). References [1] C.C. Homes, T. Vogt, S.M. Shapiro, S. Wakimoto, A. Ramireze, Science 293 (2001) 673. [2] T.B. Adams, D.C. Sinclair, A.R. West, Adv. Mater. 14 (2002) 1321.
[3] Y. Lin, Y.B. Chen, T. Garret, S.W. Liu, C.L. Chen, R.P. Bontchev, A. Jacobson, J.C. Jiang, E.I. Meletis, J. Horwitz, H.D. Wu, Appl. Phys. Lett. 81 (2002) 631. [4] L. Fang, M. Shen, D. Yao, Appl. Phys. A 80 (2005) 1763. [5] A.F.L. Almeida, P.B.A. Fechine, J.C. Goes, M.A. Valente, M.A.R. Miranda, A.S.B. Sombra, Mater. Sci. Eng. B 111 (2004) 113. [6] R. Waser, T. Baiatu, K.H. Hardtl, J. Am. Ceram. Soc. 73 (1990) 1645. [7] A. Lagrange, Mater. Sci. Eng. A 109 (1989) 113. [8] W.P. Chen, J.Q. Qi, Y. Wang, X.P. Jiang, H.L.W. Chan, J. Appl. Phys. 95 (2004) 5920. [9] J.D. Baniecki, J.S. Cross, M. Tsukada, J. Watanabe, Appl. Phys. Lett. 81 (2002) 3837. [10] W.P. Chen, Y. Wang, J.Y. Dai, S.G. Lu, X.X. Wang, P.F. Lee, H.L.W. Chan, C.L. Choy, Appl. Phys. Lett. 84 (2004) 103. [11] W.P. Chen, X.P. Jiang, Y. Wang, Z. Peng, H.L.W. Chan, J. Am. Ceram. Soc. 86 (2003) 735. [12] C.J. Raub, Plat. Surf. Finish. 80 (1993) 30.
Letter
Hydrogen-induced degradation in CaCu3Ti4O12 ceramics Wan Ping Chen a,b,∗ , Wang Xiang a , Ming Sen Guo a , Wen Chao You a , Xing Zhong Zhao a , Helen Lai Wah Chan b b
a Department of Physics, Wuhan University, Wuhan 430072, PR China Department of Applied Physics and Materials Research Center, The Hong Kong Polytechnic University, Hong Kong, PR China
Received 17 November 2005; accepted 7 December 2005 Available online 24 January 2006
Abstract Hydrogen-induced degradation in CaCu3 Ti4 O12 (CCTO) ceramics was studied using an electrochemical hydrogen charging method, in which the silver electrodes of CCTO ceramic pellets were made a cathode in 0.01 M NaOH solution to deposit hydrogen through the electrolysis of water. The dielectric loss was greatly increased and the insulation resistance was greatly decreased after the treatment. X-ray diffraction analysis showed that no new phases were formed. It is proposed that atomic hydrogen is diffused into CCTO lattice and free electrons are formed through the ionization of hydrogen atoms, which lead to the observed degradation. Hydrogen-induced degradation occurs very quickly in CCTO ceramics and it is important to prevent hydrogen-induced degradation in CCTO ceramics. © 2005 Elsevier B.V. All rights reserved. Keywords: Degradation; Hydrogen; Dielectric; Ceramics
1. Introduction In recent years, the rapid development of many electronic products have benefited greatly from the miniaturization of various discrete components, especially capacitors, in them. The volume efficiency of a capacitor is directly related to its dielectric constant and there have been intensive researches on high dielectric constant materials. Generally speaking, these researches can be classified into two categories. In the first category, investigations are aimed at improving the properties of those widelyapplied high dielectric constant materials, including barium titanate and lead zirconate titanate; while in the second category, efforts are made to develop other new high dielectric constant materials, such as CaCu3 Ti4 O12 (CCTO). With a giant roomtemperature dielectric constant in the order of 104 for ceramics and ∼350 000 for single crystals, CCTO has attracted particular attention [1,2]. So far, the origin of the high dielectric constant of this material is not clear and several mechanisms have been put forwarded. Sinclair et al. [2] used the well-known internal
∗ ∗ Corresponding
author. Tel.: +86 27 6131 2396; fax: +86 27 6875 2697. E-mail addresses: [email protected], [email protected] (W.P. Chen). 0925-8388/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2005.12.013
barrier layer capacitive model to interpret the impedance spectroscopy data on CCTO, which is famous for perovskite-type titanates that are processed in such a way as to produce grains that are reduced and conducting but coupled with grain boundaries that are oxidized and insulating. When many investigations are still devoted to understanding the interesting physics that CCTO presents, more and more researches have turned to other practical works for its potential applications. CCTO films have been fabricated through pulsed laser deposition [3,4], composites with other materials have also been reported [5]. Another important work regarding its practical applications should be studies on its chemical stability. It is well known that the dielectric properties and resistance of many electronic ceramic devices and components sometimes degrade slowly under prolonged application of electrical field [6,7] and their lifetime has thus been seriously limited. Several origins have been recognized for the degradation, among which an important one is the ambient-temperature reaction of hydrogen from the electrolysis of water. This degradation process is quite common since electroceramic components and devices always operate under some voltages while moisture is ubiquitous in the environment [8,9]. Presently, we have studied the ambient-temperature reaction of hydrogen with CCTO ceramics. The influences of the reaction on the properties of CCTO
L10
W.P. Chen et al. / Journal of Alloys and Compounds 422 (2006) L9–L12
ceramics will be reported and the reaction mechanism will be discussed in this paper. 2. Experimental Reagent-grade powders of CaCO3 , CuO and TiO2 were mixed to yield a composition of CaCu3 Ti4 O12 . The mixture was ball-milled for 24 h and calcined at 1000 ◦ C for 12 h. Then both the ball-milling and the calcination were repeated for a second time. The calcined powder was planetary ball-milled for 1 h before pressing into pellets of 15.80 mm in diameter and 1 mm thick, which were sintered at 1100 ◦ C for 24 h and furnace-cooled to room-temperature. Two major faces of the sintered pellets were lightly polished with SiC paper and Ag electrodes were coated on them through sputtering. To study the influence of water and dc voltage, some CCTO pellets were placed in a 0.01 M NaOH solution and a 4.5 V dc voltage was imposed between the silver electrodes of the pellets and a counter Pt electrode in the solution. The silver electrodes of the ceramic pellets acted as the cathode while the counter Pt electrode acted as the anode. The imposed dc voltage induced electrolysis of water with hydrogen deposited on the silver electrodes of the pellets, and this treatment is referred to as electrochemical hydrogen charging hereafter. The dc voltage was removed after some designed periods of time and the pellets were taken out of the solution, washed with de-ionized water and dried. The frequency spectra of capacitance and dielectric loss of the pellets were measured on an Agilent 4294A impedance analyzer. Resistance of the pellets was recorded through a Keithley 2000 multimeter. Phase detection was performed using an X-ray diffractometer (Philips PW 3719) with Cu K␣ radiation. A scanning electron microscope (STEROSCAN) was used to observe the microstructure of the pellets.
3. Results and discussion The average diameter of the pellets shrank to 12.86 mm after sintering. Pellet densities calculated using the Archimedes method were all over 95% of the theoretical X-ray density. A representative SEM micrograph taken for the surface of an assintered ceramic pellet is shown in Fig. 1, which also indicates a dense microstructure. It would be difficult for water or moisture to permeate into the pellets. As a matter of fact, for reference, we had immersed some pellets in 0.01 M NaOH solution for long periods of time with no electricity applied. After cleaning and drying, no detectable change was observed in the properties
Fig. 2. Frequency spectra of capacitance and dielectric loss obtained for a CCTO ceramic pellet as-sintered and after different periods of electrochemical hydrogen charging, respectively.
of the samples. Together with the microstructural observation, it can be concluded that water showed little influence on the pellets through permeation. For those samples treated with the application of the dc voltage, on the contrary, their properties were found seriously degraded after the treatment. Fig. 2 shows the frequency spectra of capacitance and dielectric loss obtained for a representative sample as-sintered and after different periods of electrochemical hydrogen charging. Though the capacitance exhibited some noticeable variation with the treatment, the most remarkable change occurred in the dielectric loss. After 22 h of electrochemical hydrogen charging, the dielectric loss was increased from 0.3 to 1.7 at 102 Hz, and remained almost unchanged at frequencies above 104 Hz. At the same time, the resistance of the sample measured between its two silver electrodes was found greatly decreased, as shown in Fig. 3. The resistance was decreased by almost one order of magnitude after only 1 h of hydrogen charging. So the decrease was very quick at the initial stage and then it
Fig. 1. SEM micrograph taken on the surface of an as-sintered CCTO ceramic pellet.
W.P. Chen et al. / Journal of Alloys and Compounds 422 (2006) L9–L12
Fig. 3. Resistance measured between two silver electrodes of a CCTO ceramic pellet vs. electrochemical hydrogen charging time.
was slowed down with increasing time of hydrogen charging. It is well known that low dielectric loss and high insulation resistance are two primary requirements for dielectric applications. Obviously electrochemical hydrogen charging is very harmful for the application of CCTO ceramics as high dielectric constant materials. Since no noticeable change in properties was observed in those samples immersed in the NaOH solution with no voltage applied, the degradation of CCTO-based ceramics induced by electrochemical hydrogen charging was directly related to the deposition of hydrogen on the silver electrodes of the samples, which can be expressed as: H2 O + e− → OH− + Hads ,
(1)
Hads + Hads → H2 ,
(2)
where Hads represents an adsorbed hydrogen atom. As we proposed in some previous researches [8,10], hydrogen atoms deposited on silver electrodes may diffuse into ceramics and react with them at room-temperature, though most of them combine with one another and form hydrogen molecules. The properties of the ceramics are usually greatly changed by the reaction of hydrogen, the same as the properties of the CCTO ceramics in this study. The reactions between oxides and hydrogen through electrochemical hydrogen charging can be classified into two categories: either the lattices of the oxides are decomposed after the reaction [8], or no gross structural change occurs in the lattices of the oxides with hydrogen incorporated in them [10]. Fig. 4 shows the X-ray diffraction patterns taken on the surface of a CCTO pellet as-sintered and after 42 h of hydrogen charging, respectively. It is clear that the long time treatment led to no noticeable changes in the X-ray diffraction pattern. So we believe that the reaction between hydrogen and CCTO belongs to the second category, namely hydrogen atoms diffuse into CCTO lattice and exist at interstitial sites: Hads → Hi • + e ,
(3)
L11
Fig. 4. X-ray diffraction patterns taken on the surface of a CCTO ceramic pellet as-sintered and after 42 h of electrochemical hydrogen charging, respectively.
where Hi • represents a hydrogen ion in interstitial site. The formation of interstitial hydrogen in oxides via hydrogen charging process has been proven through Fourier-transform infrared (FTIR) absorption analysis of polished rutile single crystals [10]. A free electron is generated through the ionization of hydrogen atom, which accounts for the great increases in the dielectric loss and in the leakage current of hydrogen charged-CCTO. In practice, perovskite-type titanates are most widely used in high dielectric constant ceramic capacitors and hydrogeninduced degradation had also been observed for them. However, as we can see in Fig. 3, hydrogen-induced resistance degradation is very quick for CCTO in the early stage of electrochemical hydrogen charging, which we had not observed for barium titanate ceramics [11]. It indicates that hydrogen-induced degradation will occur more easily in CCTO than in barium titanate. On the other hand, due to the rapid development of surface mounting technology (SMT), now more and more ceramic capacitors are being manufactured as chip capacitors. Electroplating is widely applied in forming three-layer-electrodes for chip components and no polymer coating can be adopted for them. It is well known that hydrogen is always deposited during electroplating [12] while polymer coating is helpful for preventing the electrolysis of water on electronic components in service. Hydrogen-induced degradation will become a more serious problem if CCTO is fabricated as chip ceramic capacitors. It is an important challenge to minimize hydrogen-induced degradation in CCTO for capacitors application and further investigations are highly desired. 4. Conclusions The dielectric loss and the leakage current of CCTO ceramics are greatly increased when hydrogen is deposited on them through the electrolysis of water. No new phases are formed after the treatment and it is proposed that hydrogen atoms are incorporated into CCTO lattice and exist in interstitial sites. The degradation can be well explained by the formation of
L12
W.P. Chen et al. / Journal of Alloys and Compounds 422 (2006) L9–L12
free electrons through the ionization of interstitial hydrogen. Hydrogen-induced degradation occurs quite quickly in CCTO ceramics and efforts should be made to minimize hydrogeninduced degradation in CCTO ceramics. Acknowledgement This work has been supported by the Centre for Smart Materials of the Hong Kong Polytechnic University (Project: 1-A302). References [1] C.C. Homes, T. Vogt, S.M. Shapiro, S. Wakimoto, A. Ramireze, Science 293 (2001) 673. [2] T.B. Adams, D.C. Sinclair, A.R. West, Adv. Mater. 14 (2002) 1321.
[3] Y. Lin, Y.B. Chen, T. Garret, S.W. Liu, C.L. Chen, R.P. Bontchev, A. Jacobson, J.C. Jiang, E.I. Meletis, J. Horwitz, H.D. Wu, Appl. Phys. Lett. 81 (2002) 631. [4] L. Fang, M. Shen, D. Yao, Appl. Phys. A 80 (2005) 1763. [5] A.F.L. Almeida, P.B.A. Fechine, J.C. Goes, M.A. Valente, M.A.R. Miranda, A.S.B. Sombra, Mater. Sci. Eng. B 111 (2004) 113. [6] R. Waser, T. Baiatu, K.H. Hardtl, J. Am. Ceram. Soc. 73 (1990) 1645. [7] A. Lagrange, Mater. Sci. Eng. A 109 (1989) 113. [8] W.P. Chen, J.Q. Qi, Y. Wang, X.P. Jiang, H.L.W. Chan, J. Appl. Phys. 95 (2004) 5920. [9] J.D. Baniecki, J.S. Cross, M. Tsukada, J. Watanabe, Appl. Phys. Lett. 81 (2002) 3837. [10] W.P. Chen, Y. Wang, J.Y. Dai, S.G. Lu, X.X. Wang, P.F. Lee, H.L.W. Chan, C.L. Choy, Appl. Phys. Lett. 84 (2004) 103. [11] W.P. Chen, X.P. Jiang, Y. Wang, Z. Peng, H.L.W. Chan, J. Am. Ceram. Soc. 86 (2003) 735. [12] C.J. Raub, Plat. Surf. Finish. 80 (1993) 30.