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Hollow fibers of yttria-stabilized zirconia (8YSZ) prepared by calcination of electrospun composite fibers
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Materials Letters 62 (2008) 2396 – 2399 www.elsevier.com/locate/matlet
Hollow fibers of yttria-stabilized zirconia (8YSZ) prepared by calcination of electrospun composite fibers J.Y. Li a,⁎, Y. Tan a , F.M. Xu a , Y. Sun a , X.Q. Cao b , Y.F. Zhang b a
b
School of Materials Science and Engineering, Dalian University of Technology, Dalian 116024, China Key Lab of Rare Earth Chemistry and Physics, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, Jilin, China Received 11 October 2007; accepted 8 December 2007 Available online 3 January 2008
Abstract 8YSZ fibers were synthesized by calcination of PVP/zirconium oxychloride/yttrium nitrate composite fibers (PVP-Precursor) obtained by electrospinning. Scanning electron microscopy (SEM) indicated that the 8YSZ fibers are hollow and the gas released during organic binder decomposition resulted in the formation of hollow center in fibers. Proper controlling heating rate is necessary to the formation of hollow structure. Both X-ray diffraction (XRD) and Raman spectroscopy analysis indicated that the 8YSZ fibers have crystal structure of tetragonal phase. The hollow structure should make the fibers to have a higher resistance to sintering than solid fibers at elevated temperature. This result has important applications in catalytic combustion. © 2007 Elsevier B.V. All rights reserved. Keywords: Yttria-stabilized zirconia; Ceramic; Fibers; Electrospinning; Sintering
1. Introduction In the past decade, more and more attention has been drawn to the synthesis of inorganic hollow fibers due to their novel potential applications, including catalysis, supports, inorganic membrane separation and reactors, gas sensors, and drug release [1–5]. The properties of higher resistance and larger surface area/volume ratio have been demonstrated for the hollow fibers [6]. A large number of synthetic methods, such as chemical vapor deposition [7], hydrothermal reduction method [8], template synthesis method [9] and electrospinning [10], have already been demonstrated for generating hollow fibers. Nanofibers prepared by electrospinning can be used as sacrificial templates for generating tubular fibers [11]. In this method, additional coating and etching steps are often required and the quality of the resultant tubes is strongly dependent on
⁎ Corresponding author. Tel./fax: +86 411 84707583. E-mail address: [email protected] (J.Y. Li). 0167-577X/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2007.12.011
the yield and control of each step. The hollow fibers can also be prepared by electrospinning two immiscible liquids through a coaxial, two-capillary spinneret, followed by selective removal of the cores [12,13]. In our paper, the hollow fibers were synthesized by calcination of electrospinning precursor fibers. Compared with the reported electrospun methods for hollow fibers, the fabrication technique used in this work possesses the virtues including simplicity, low-cost, and absence of template. ZrO2 is an important material in industry and technology due to their properties, such as high melting point, low thermal conduction, high strength and high ionic conductivity. 8YSZ (8 wt.% Y2O3 stabilized ZrO2) is most widely studied and used as thermal barrier coating (TBC) material because it provides the best performance in high-temperature applications such as diesel engines and gas turbines [14–16], and reports about this material are numerous. It has high thermal expansion coefficient and a low thermal conductivity [15]. It had been reported in Ref. [17] that the solid fibers of 8 mol% Y2O3 stabilized ZrO2 could be obtained by electrospinning. Compared with the result of that paper, the hollow fibers of 8 wt.% Y2O3 stabilized ZrO2 could be prepared by calcinations of electrospun precursor fibers in this paper.
J.Y. Li et al. / Materials Letters 62 (2008) 2396–2399
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2. Experiment The 8YSZ fibers were prepared as follows: Based on the conventional sol–gel process, Y2O3 (99.9%, Guangdong Chenghai Sanxing Chemicals Ltd.) and ZrOCl2•8H2O (99%, Shanghai Chemicals Ltd.) were used as staring materials. To achieve 8YSZ, the 0.8 g Y2O3 was first converted to the yttrium nitrate by dissolving in concentrated nitric acid. Excess nitric acid was removed by slow evaporation. 24.3036 g ZrOCl2•8H2O was dissolved into the 20 ml deionized water and then added to the yttrium nitrate solution, followed by the addition of ethanol containing poly(vinyl pyrrolidone) (PVP, Beijing Yili Fine Chemicals Ltd.) containing 7 ml C2H5OH and 10 g PVP. Subsequently, the mixture was magnetically stirred at room temperature for 4 h, and then ejected from the conductive stainless steel capillary with a voltage of 30 kV. The distance between the capillary and the collector was 15 cm. The collector was coated with an aluminium foil. The composite fibers of PVP-Precursor were collected on the aluminium foil to form non-woven mats. This fiber mat was dried at 80 °C for 12 h, and then calcined at 600 °C, 800 °C, 1000 °C, 1200 °C and 1400 °C and remained at the required temperature for 12 h. TG-DTA (thermal gravimetry-differential thermal analysis) was performed on a thermoanalyzer (TA SDT 2960) in air. XRD patterns were collected on a Rigaku D/Max-IIB diffractometer with Cu-Kα radiation. The morphology of the 8YSZ fibers was analyzed by the scanning electron microscope (SEM, XL 30 ESEM FEG, Micro FEI Philips). FT-Raman spectrums were recorded on a Nicolet 960 instrument using 1064 nm excitation laser. 3. Results and discussion 3.1. Thermoanalysis Fig. 1 shows the thermal behavior of the precursor fibers of PVPPrecursor in air atmosphere, measured by thermoanalyzer with a heating rate of 10 °C/min. No weight loss above 600 °C was observed, indicating that the organic material (PVP, ethanol), the nitrate, the –OCl2 group of zirconium oxychloride and other volatiles (H2O, CO x,
Fig. 1. The TG-DTA curve of the fibers of PVP-Precursor.
Fig. 2. The XRD patterns of 8YSZ fibers calcinated at various temperatures: (a) 600 °C 12 h (b) 800 °C 12 h (c) 1000 °C 12 h (d) 1200 °C 12 h.
etc.) were removed completely. The endothermic peak at 94 °C in the DTA curve could be attributed to the loss of the absorbed water and trapped solvent (ethanol) from the PVP-Precursor fibers. The two exothermic peaks at about 370 °C and 507 °C in the DTA curve correspond to the decomposition of nitrates and the degradation of PVP, which has two degradation mechanisms involving both intra- and inter-molecular transfer reactions [18]. 3.2. Crystal structure development of 8YSZ fiber The XRD patterns of the 8YSZ fibers after calcination are shown in Fig. 2 for determining the crystalline phase at various temperatures. As confirmed by XRD studies, the 8YSZ fibers calcined at 600 °C were made of tetragonal phase ZrO2 (JCPDS cards: No. 50-1089), with an average grain size around 8 nm. Furthermore, the peaks of 8YSZ crystal became sharper and narrow with increasing calcination temperature to 1200 °C, which indicated that the crystallinity was higher and the grain size was larger at high calcination temperature than those obtained at low calcination temperature. All the XRD peaks of
Fig. 3. Raman spectra of 8YSZ calcinated at various temperatures.
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J.Y. Li et al. / Materials Letters 62 (2008) 2396–2399
8YSZ crystalline calcined at different temperature form 600 °C to 1000 °C match well with the tetragonal phase zirconia. This result was also determined by Raman analysis (Fig. 3).
All the Raman spectrums of samples calcined at 600 °C, 800 °C and 1200 °C give five major bands at 148, 259, 322, 466 and 633 cm− 1, which are characteristic bands of tetragonal phase ZrO2 [19]. The peaks
Fig. 4. SEM images of 8YSZ hollow fibers calcined at different temperatures (a, b) 600 °C (c, d) 800 °C (e, f) 1000 °C (g, h) 1200 °C (i, j) 1400 °C.
J.Y. Li et al. / Materials Letters 62 (2008) 2396–2399
of monoclinic phase and cubic phase are not obviously observed, indicating that the 8YSZ fibers calcinated at 600 °C are made of single tetragonal phase. 3.3. Microstructure of the fiber The SEM images of the 8YSZ fibers calcined at 600 °C, 800 °C, 1000 °C and 1200 °C with heating rate of 4 °C/min are shown in Fig. 4. After calcination at 600 °C, the surface of the fiber became rough due to the removal of organic components and the crystallization of the 8YSZ phase, and then a polycrystalline hollow fiber was obtained. Due to the burning-off of PVP, the fibers show a decrease in the diameter. The fibers with a diameter of about 2–4 μm are obtained after calcination at 600 °C for 12 h and the hollow structure with a thin wall whose thickness is about 300 nm can be observed in the cross-section of the fiber (Fig. 4a, b), indicating the hollow fibers have been formed at this temperature. The forming process of hollow structure is similar to that of the spray drying of ceramic powders [20]. Carbon monoxide and other gases are released during organic binder decomposition and the ballooning of the fibers also occurs. With the increase of calcination temperature, the crystallinity was higher and the grain size was larger. After calcination at 1400 °C for 12 h, the 8YSZ grains as shown on the fiber surface are averagely smaller than 400 nm, and these grains are closely connected to each other with clear grain boundaries (Fig. 4i, j).
4. Conclusion In summary, the direct electrospinning was used in this work to prepare the PVP-Precursor composite fibers. After calcination, the composite fibers were converted into the 8YSZ fibers with hollow structure. The outer diameter and wall thickness of hollow fibers were about 2–3 µm and 300–400 nm, respectively. The formation mechanism of hollow structure was proposed and the fibers were made of polycrystalline tetragonal phase ZrO2.
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Acknowledgement This work was financially supported by the project of A1320061002. References [1] Z.Y. Liu, D.D. Sun, P. Guo, J.O. Leckie, Nano Lett. 7 (2007) 1081–1085. [2] X.Y. Tan, K. Li, Ing. Eng. Chem. Res. 45 (2006) 142–149. [3] R.H. Baughman, A.A. Zakhidov, W.A. De Heer, Science, 297 (2002) 787–793. [4] L. Hueso, N. Mathur, Nature 427 (2004) 301–304. [5] C.R. Martin, P. Kohli, Nature Rev. Drug Discov. 2 (2003) 29–37. [6] X.Y. Tan, Y. Liu, K. Li, Ind Eng Chem Res 44 (2005) 61–66. [7] H.J. Xin, A.T. Wooley, Nano Lett. 4 (2004) 1482–1484. [8] Y.D. Li, W. Wang, Z.X. Deng, Y.Y. Wu, X.M. Sun, D.P. Yu, et al., J. Am. Chem. Soc. 123 (2001) 9904–9905. [9] C.J. Brumlik, C.R. Martin, J. Am. Chem. Soc. 113 (1991) 3174–3175. [10] Z.C. Sun, E. Zussman, A.L. Yarin, J.H. Wendorff, A. Greiner, Adv. Mater. 15 (2003) 1929–1932. [11] M. Bognitzki, H.Q. Hou, M. Ishaque, T. Frese, M. Hellwig, et al., Adv. Mater. 12 (2000) 637–640. [12] D. Li, Y.N. Xia, Nano Lett. 4 (2004) 933–938. [13] J.H. Yu, S.V. Fridrikh, G.C. Rutledge, Adv. Mater. 16 (2004) 1562–1566. [14] P. Bengtsson, T. Ericsson, J. Wigren, J. Therm. Spray Technol. 8 (1999) 512–522. [15] C.R.C. Lima, R. da Exaltacao Trevisan, J. Therm. Spray Technol. 8 (1999) 323–327. [16] M. Tamura, M. Takahashi, J. Ishii, K. Suzuki, M. Sato, K. Shimomura, J. Therm. Spray Technol. 8 (1999) 68–72. [17] A.M. Azad, Mater. Lett. 60 (2006) 67–72. [18] S.J. Azhari, M.A. Diab, Polym. Degrad. Stab. 60 (1998) 253–256. [19] M.J. Li, Z.C. Feng, G. Xiong, P.L. Ying, Q. Xin, C. Li, J. Phys. Chem., B 105 (2001) 8107–8111. [20] X.Q. Cao, R. Vassen, S. Schwartz, W. Jungen, F. Tietz, D. Stoever, J. Eur. Ceram. Soc. 20 (2000) 2433–2439.
Materials Letters 62 (2008) 2396 – 2399 www.elsevier.com/locate/matlet
Hollow fibers of yttria-stabilized zirconia (8YSZ) prepared by calcination of electrospun composite fibers J.Y. Li a,⁎, Y. Tan a , F.M. Xu a , Y. Sun a , X.Q. Cao b , Y.F. Zhang b a
b
School of Materials Science and Engineering, Dalian University of Technology, Dalian 116024, China Key Lab of Rare Earth Chemistry and Physics, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, Jilin, China Received 11 October 2007; accepted 8 December 2007 Available online 3 January 2008
Abstract 8YSZ fibers were synthesized by calcination of PVP/zirconium oxychloride/yttrium nitrate composite fibers (PVP-Precursor) obtained by electrospinning. Scanning electron microscopy (SEM) indicated that the 8YSZ fibers are hollow and the gas released during organic binder decomposition resulted in the formation of hollow center in fibers. Proper controlling heating rate is necessary to the formation of hollow structure. Both X-ray diffraction (XRD) and Raman spectroscopy analysis indicated that the 8YSZ fibers have crystal structure of tetragonal phase. The hollow structure should make the fibers to have a higher resistance to sintering than solid fibers at elevated temperature. This result has important applications in catalytic combustion. © 2007 Elsevier B.V. All rights reserved. Keywords: Yttria-stabilized zirconia; Ceramic; Fibers; Electrospinning; Sintering
1. Introduction In the past decade, more and more attention has been drawn to the synthesis of inorganic hollow fibers due to their novel potential applications, including catalysis, supports, inorganic membrane separation and reactors, gas sensors, and drug release [1–5]. The properties of higher resistance and larger surface area/volume ratio have been demonstrated for the hollow fibers [6]. A large number of synthetic methods, such as chemical vapor deposition [7], hydrothermal reduction method [8], template synthesis method [9] and electrospinning [10], have already been demonstrated for generating hollow fibers. Nanofibers prepared by electrospinning can be used as sacrificial templates for generating tubular fibers [11]. In this method, additional coating and etching steps are often required and the quality of the resultant tubes is strongly dependent on
⁎ Corresponding author. Tel./fax: +86 411 84707583. E-mail address: [email protected] (J.Y. Li). 0167-577X/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2007.12.011
the yield and control of each step. The hollow fibers can also be prepared by electrospinning two immiscible liquids through a coaxial, two-capillary spinneret, followed by selective removal of the cores [12,13]. In our paper, the hollow fibers were synthesized by calcination of electrospinning precursor fibers. Compared with the reported electrospun methods for hollow fibers, the fabrication technique used in this work possesses the virtues including simplicity, low-cost, and absence of template. ZrO2 is an important material in industry and technology due to their properties, such as high melting point, low thermal conduction, high strength and high ionic conductivity. 8YSZ (8 wt.% Y2O3 stabilized ZrO2) is most widely studied and used as thermal barrier coating (TBC) material because it provides the best performance in high-temperature applications such as diesel engines and gas turbines [14–16], and reports about this material are numerous. It has high thermal expansion coefficient and a low thermal conductivity [15]. It had been reported in Ref. [17] that the solid fibers of 8 mol% Y2O3 stabilized ZrO2 could be obtained by electrospinning. Compared with the result of that paper, the hollow fibers of 8 wt.% Y2O3 stabilized ZrO2 could be prepared by calcinations of electrospun precursor fibers in this paper.
J.Y. Li et al. / Materials Letters 62 (2008) 2396–2399
2397
2. Experiment The 8YSZ fibers were prepared as follows: Based on the conventional sol–gel process, Y2O3 (99.9%, Guangdong Chenghai Sanxing Chemicals Ltd.) and ZrOCl2•8H2O (99%, Shanghai Chemicals Ltd.) were used as staring materials. To achieve 8YSZ, the 0.8 g Y2O3 was first converted to the yttrium nitrate by dissolving in concentrated nitric acid. Excess nitric acid was removed by slow evaporation. 24.3036 g ZrOCl2•8H2O was dissolved into the 20 ml deionized water and then added to the yttrium nitrate solution, followed by the addition of ethanol containing poly(vinyl pyrrolidone) (PVP, Beijing Yili Fine Chemicals Ltd.) containing 7 ml C2H5OH and 10 g PVP. Subsequently, the mixture was magnetically stirred at room temperature for 4 h, and then ejected from the conductive stainless steel capillary with a voltage of 30 kV. The distance between the capillary and the collector was 15 cm. The collector was coated with an aluminium foil. The composite fibers of PVP-Precursor were collected on the aluminium foil to form non-woven mats. This fiber mat was dried at 80 °C for 12 h, and then calcined at 600 °C, 800 °C, 1000 °C, 1200 °C and 1400 °C and remained at the required temperature for 12 h. TG-DTA (thermal gravimetry-differential thermal analysis) was performed on a thermoanalyzer (TA SDT 2960) in air. XRD patterns were collected on a Rigaku D/Max-IIB diffractometer with Cu-Kα radiation. The morphology of the 8YSZ fibers was analyzed by the scanning electron microscope (SEM, XL 30 ESEM FEG, Micro FEI Philips). FT-Raman spectrums were recorded on a Nicolet 960 instrument using 1064 nm excitation laser. 3. Results and discussion 3.1. Thermoanalysis Fig. 1 shows the thermal behavior of the precursor fibers of PVPPrecursor in air atmosphere, measured by thermoanalyzer with a heating rate of 10 °C/min. No weight loss above 600 °C was observed, indicating that the organic material (PVP, ethanol), the nitrate, the –OCl2 group of zirconium oxychloride and other volatiles (H2O, CO x,
Fig. 1. The TG-DTA curve of the fibers of PVP-Precursor.
Fig. 2. The XRD patterns of 8YSZ fibers calcinated at various temperatures: (a) 600 °C 12 h (b) 800 °C 12 h (c) 1000 °C 12 h (d) 1200 °C 12 h.
etc.) were removed completely. The endothermic peak at 94 °C in the DTA curve could be attributed to the loss of the absorbed water and trapped solvent (ethanol) from the PVP-Precursor fibers. The two exothermic peaks at about 370 °C and 507 °C in the DTA curve correspond to the decomposition of nitrates and the degradation of PVP, which has two degradation mechanisms involving both intra- and inter-molecular transfer reactions [18]. 3.2. Crystal structure development of 8YSZ fiber The XRD patterns of the 8YSZ fibers after calcination are shown in Fig. 2 for determining the crystalline phase at various temperatures. As confirmed by XRD studies, the 8YSZ fibers calcined at 600 °C were made of tetragonal phase ZrO2 (JCPDS cards: No. 50-1089), with an average grain size around 8 nm. Furthermore, the peaks of 8YSZ crystal became sharper and narrow with increasing calcination temperature to 1200 °C, which indicated that the crystallinity was higher and the grain size was larger at high calcination temperature than those obtained at low calcination temperature. All the XRD peaks of
Fig. 3. Raman spectra of 8YSZ calcinated at various temperatures.
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J.Y. Li et al. / Materials Letters 62 (2008) 2396–2399
8YSZ crystalline calcined at different temperature form 600 °C to 1000 °C match well with the tetragonal phase zirconia. This result was also determined by Raman analysis (Fig. 3).
All the Raman spectrums of samples calcined at 600 °C, 800 °C and 1200 °C give five major bands at 148, 259, 322, 466 and 633 cm− 1, which are characteristic bands of tetragonal phase ZrO2 [19]. The peaks
Fig. 4. SEM images of 8YSZ hollow fibers calcined at different temperatures (a, b) 600 °C (c, d) 800 °C (e, f) 1000 °C (g, h) 1200 °C (i, j) 1400 °C.
J.Y. Li et al. / Materials Letters 62 (2008) 2396–2399
of monoclinic phase and cubic phase are not obviously observed, indicating that the 8YSZ fibers calcinated at 600 °C are made of single tetragonal phase. 3.3. Microstructure of the fiber The SEM images of the 8YSZ fibers calcined at 600 °C, 800 °C, 1000 °C and 1200 °C with heating rate of 4 °C/min are shown in Fig. 4. After calcination at 600 °C, the surface of the fiber became rough due to the removal of organic components and the crystallization of the 8YSZ phase, and then a polycrystalline hollow fiber was obtained. Due to the burning-off of PVP, the fibers show a decrease in the diameter. The fibers with a diameter of about 2–4 μm are obtained after calcination at 600 °C for 12 h and the hollow structure with a thin wall whose thickness is about 300 nm can be observed in the cross-section of the fiber (Fig. 4a, b), indicating the hollow fibers have been formed at this temperature. The forming process of hollow structure is similar to that of the spray drying of ceramic powders [20]. Carbon monoxide and other gases are released during organic binder decomposition and the ballooning of the fibers also occurs. With the increase of calcination temperature, the crystallinity was higher and the grain size was larger. After calcination at 1400 °C for 12 h, the 8YSZ grains as shown on the fiber surface are averagely smaller than 400 nm, and these grains are closely connected to each other with clear grain boundaries (Fig. 4i, j).
4. Conclusion In summary, the direct electrospinning was used in this work to prepare the PVP-Precursor composite fibers. After calcination, the composite fibers were converted into the 8YSZ fibers with hollow structure. The outer diameter and wall thickness of hollow fibers were about 2–3 µm and 300–400 nm, respectively. The formation mechanism of hollow structure was proposed and the fibers were made of polycrystalline tetragonal phase ZrO2.
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Acknowledgement This work was financially supported by the project of A1320061002. References [1] Z.Y. Liu, D.D. Sun, P. Guo, J.O. Leckie, Nano Lett. 7 (2007) 1081–1085. [2] X.Y. Tan, K. Li, Ing. Eng. Chem. Res. 45 (2006) 142–149. [3] R.H. Baughman, A.A. Zakhidov, W.A. De Heer, Science, 297 (2002) 787–793. [4] L. Hueso, N. Mathur, Nature 427 (2004) 301–304. [5] C.R. Martin, P. Kohli, Nature Rev. Drug Discov. 2 (2003) 29–37. [6] X.Y. Tan, Y. Liu, K. Li, Ind Eng Chem Res 44 (2005) 61–66. [7] H.J. Xin, A.T. Wooley, Nano Lett. 4 (2004) 1482–1484. [8] Y.D. Li, W. Wang, Z.X. Deng, Y.Y. Wu, X.M. Sun, D.P. Yu, et al., J. Am. Chem. Soc. 123 (2001) 9904–9905. [9] C.J. Brumlik, C.R. Martin, J. Am. Chem. Soc. 113 (1991) 3174–3175. [10] Z.C. Sun, E. Zussman, A.L. Yarin, J.H. Wendorff, A. Greiner, Adv. Mater. 15 (2003) 1929–1932. [11] M. Bognitzki, H.Q. Hou, M. Ishaque, T. Frese, M. Hellwig, et al., Adv. Mater. 12 (2000) 637–640. [12] D. Li, Y.N. Xia, Nano Lett. 4 (2004) 933–938. [13] J.H. Yu, S.V. Fridrikh, G.C. Rutledge, Adv. Mater. 16 (2004) 1562–1566. [14] P. Bengtsson, T. Ericsson, J. Wigren, J. Therm. Spray Technol. 8 (1999) 512–522. [15] C.R.C. Lima, R. da Exaltacao Trevisan, J. Therm. Spray Technol. 8 (1999) 323–327. [16] M. Tamura, M. Takahashi, J. Ishii, K. Suzuki, M. Sato, K. Shimomura, J. Therm. Spray Technol. 8 (1999) 68–72. [17] A.M. Azad, Mater. Lett. 60 (2006) 67–72. [18] S.J. Azhari, M.A. Diab, Polym. Degrad. Stab. 60 (1998) 253–256. [19] M.J. Li, Z.C. Feng, G. Xiong, P.L. Ying, Q. Xin, C. Li, J. Phys. Chem., B 105 (2001) 8107–8111. [20] X.Q. Cao, R. Vassen, S. Schwartz, W. Jungen, F. Tietz, D. Stoever, J. Eur. Ceram. Soc. 20 (2000) 2433–2439.