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Preparation of hollow Si-B-N ceramic fibers by partial curing and pyrolysis of polyborosilazane fibers
Materials Letters 78 (2012) 1–3
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Preparation of hollow Si-B-N ceramic fibers by partial curing and pyrolysis of polyborosilazane fibers Wenhua Li, Jun Wang ⁎, Zhengfang Xie, Hao Wang, Yun Tang State Key Lab. of Advanced Ceramic Fibers & Composites, College of Aerospace & Materials Engineering, National University of Defense Technology, Changsha 410073, PR China
a r t i c l e
i n f o
Article history: Received 18 December 2011 Accepted 15 February 2012 Available online 28 February 2012 Keywords: Polymers Fiber technology Ceramics Dielectric properties
a b s t r a c t Hollow silicon boron nitride (Si-B-N) ceramic fibers with composition of Si0.3BN1.4 were prepared by meltspinning, partial curing with trichlorosilane (HSiCl3), and pyrolysis of a novel polyborosilazane under NH3 up to 1000 °C. The polyborosilazane fibers with low ceramic yield were partially cured to make sure the hollow Si-B-N ceramic fibers could be fabricated after pyrolysis. The hollow Si-B-N ceramic fibers were ~ 16 μm in diameter with inner hollow pore diameter of ~ 4 μm, and showed good average tensile strength of 1.03 GPa and elastic modulus of 106 GPa. Moreover, the hollow Si-B-N ceramic fibers also exhibited excellent dielectric properties with the average dielectric constant real part and loss tangent about 3.06 and 0.0032 at 2–18 GHz, respectively, making them to be promising microwave-transparent materials. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Ceramic fibers composed of Si, B, and N exhibit high thermal stability, low thermal conductivity, low and thermally stable dielectric constant, and high mechanical strength. They are suitable candidates as reinforcement in ceramic matrix composites for high temperature microwave-transparent applications [1,2]. Recently, the Si-B-N fibers were successfully prepared in our lab and elsewhere [1,3–5]. All these Si-B-N fibers are dense without pores. However, hollow fibers (e.g. hollow silica fibers [6]) have lower dielectric constant and thermal conductivity, and could reduce the weight of the ceramic matrix composites reinforced by them. Commonly, to fabricate hollow ceramic fibers as reinforcements in ceramic matrix composites such as hollow SiC fibers [7,8], the preceramic fibers were firstly spun into hollow preceramic fibers, and then the hollow preceramic fibers were conversed into hollow ceramic fibers by pyrolysis. Up to now, there are not any reports about hollow Si-B-N ceramic fibers. In this Letter, we report a novel method to fabricate hollow Si-B-N ceramic fibers. The polyborosilazane was firstly spun into dense polyborosilazane fibers, and then the dense polyborosilazane fibers were partially cured by chemical (HSiCl3) vapor reactions and pyrolyzed directly in NH3 atmospheres into hollow Si-B-N ceramic fibers. 2. Experimental 2.1. Preparation of hollow Si-B-N ceramic fibers The polyborosilazane fibers with diameter of ~ 23 μm were prepared as before [9]. Briefly, the polyborosilazane was synthesized ⁎ Corresponding author. Tel./fax: + 86 731 84573161. E-mail address: [email protected] (J. Wang). 0167-577X/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2012.02.068
via “one-pot” approach using boron trichloride and hexamethydisilazane as the starting materials, and could be easily melt-spun into polyborosilazane fibers at 190 °C in N2 atmospheres with a lab scale melt-spinning apparatus. For most of fabrication of ceramic fibers from polymer fibers, the whole polymer fibers must be cured well before pyrolysis. However to fabricate hollow Si-B-N ceramic fibers in this work, the polyborosilazane fibers were cured at 80 °C for 0.5 h under HSiCl3 (heating rate 4 °C/min). This cure temperature (80 °C) and holding time (0.5 h) make sure that the outer parts of the polyborosilazane fibers were cured, while the inner parts were not. Then, the partial cured fibers were pyrolyzed at 1000 °C (heating rate 5 °C/min) under a flowing NH3 atmosphere (25 mL/min). The cured outer parts remained and conversed to ceramic after pyrolysis, while the uncured inner parts decomposed mostly and the hollow pores were formed. So, the off-white hollow Si-B-N ceramic fibers were fabricated. The schematic illustration of the process for fabricating Si-B-N ceramic fibers was shown in Fig. S1 of the Supporting Information. 2.2. Characterization The silicon and boron elements in the fibers were quantified by means of ICP-AES using an Arl 3580B spectrometer. Quantitative analysis of nitrogen and oxygen were carried out using a Leco TC436 O/N Determinator, and carbon was measured by a Leco CS444C/S analyzer. Fourier transform infrared (FT-IR) spectra were obtained using a Nicolet Avatar 360 apparatus in a KBr pellet. The microstructure of the fiber was observed by a scanning electron microscopy (JSM-6360LV). TG was conducted using a NETZSCH STA 449C instrument under Ar atmospheres with a heating rate of 5 °C/min. The XRD of the obtained pyrolyzed specimens was performed using a D8 ADVANCE powder X-ray diffraction, using Cu-Ka radiation.
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W. Li et al. / Materials Letters 78 (2012) 1–3
Fig. 1. TG curves of the polyborosilazane fibers (a) and cured fibers (b) in Ar. Fig. 3. FT-IR spectra of the obtained hollow fibers.
Single filament tensile properties were determined using an YG-type tensile strength tester (Jiangsu Taicang Textile Instruments Co., China) with a gauge length of 25 mm. The dielectric properties of Si-B-N fiber at 2–18 GHz were measured by short-circuited wave guide technique. 3. Results and discussion To fabricate hollow Si-B-N ceramic fibers from polyborosilazane fibers by partial curing and pyrolysis, the polyborosilazane fibers should have low ceramic yield to make sure the inner parts of the polyborosilazane fibers could decompose mostly into hollow pores, and still could be cured to make sure the outer parts of the fibers remained after pyrolysis. The hypothesized structure of the polyborosilazane and the curing mechanism of the polyborosilazane fibers were shown in Fig. S2 of Supporting Information. As shown in Fig. 1, the polyborosilazane fibers used for the fabrication of the hollow fibers shows weight loss of 65 wt.% up to 1000 °C in Ar (Fig. 1a). The large weight loss of the polyborosilazane fibers make sure the pore of the hollow fiber could be formed. While the polyborosilazane fibers could be cured by HSiCl3 vapor. The entirely cured fibers by HSiCl3 have a high ceramic yield up to 73 wt.% (Fig. 1b), which ensures that the outer parts of the hollow fibers could remain after pyrolysis.
The polyborosilazane fibers have low ceramic yield and could be cured by chemical vapor reaction, which is just the prerequisite condition for the fabrication of the hollow fibers. The chemical vapor cure temperature (80 °C) and curing time (0.5 h) are also the key points for forming the hollow fibers. Fig. 2 shows the surfaces and crosssections of the hollow fibers obtained under optimized curing temperature and time. Typically, the hollow fibers are ~ 16 μm in diameter (Fig. 2a) with inner hollow pore diameter of ~ 4 μm (Fig. 2b). The porosity of the hollow fiber is 20%. It means that, when using them as reinforcements, the weight of fibers will be lightened 20% than that of the dense fibers with the same diameter and density. As shown in Fig. 2a, some fibers are split, which may be caused by the drastic volatilization of the pyrolysis gas, and could be improved by optimizing the heating rates in the pyrolysis process. However, the split hollow fibers (Fig. 2a) indicate that the pores of the hollow fibers are along the fiber longitudinal axis. Besides, no matter the hollow fibers are split, the outer parts of the hollow fibers have smooth surfaces and exhibit dense textures. (Fig. 2b,c). The obtained hollow fibers were amorphous, which was identified by X-ray diffraction (see Fig.S3 in Supporting Information). Although the phase compositions of the as-obtained hollow fibers could not be identified by X-ray diffraction, boron nitride and silicon
Fig. 2. SEM images of the obtained hollow fibers. a) surface microstructure. b) cross-section of hollow fiber. c) cross-section of split hollow fiber. The arrows in a) indicate the split hollow fibers.
W. Li et al. / Materials Letters 78 (2012) 1–3
3
Undoubtedly, the low dielectric constant of hollow Si-B-N ceramic fibers is more promising than the dense Si-B-N ceramic fibers for application in microwave-transparent system. 4. Conclusions
Fig. 4. Dielectric properties of hollow Si-B-N fibers.
nitride were clearly identified by the corresponding characteristic absorption bands in the FT-IR spectrum (Fig. 3). The main peaks in the hollow fibers are 1380 and 801 cm − 1, which are characteristic of B-N [10], and 986 cm − 1 which is characteristic of amorphous Si-N [11]. Further the composition of the obtained hollow fibers was determined by elemental analysis (EA). The EA results show that the chemical compositions of the hollow fibers are Si (21.25 wt.%), B (27.54 wt.%), N (50.26 wt.%), O (0.62 wt.%) and C (0.12 wt.%). Carbon and oxygen contents were found to be b1 wt.% and can be omitted, therefore the hollow fibers display a chemical formula of Si0.3BN1.4. Moreover, the hollow Si-B-N fibers exhibit good mechanical strength with the average tensile strength of 1.03 GPa and elastic modulus of 106 GPa. They were lower than those of the dense Si-BN fibers in our previous work. Remarkably, values as high as 1.68 GPa can be found in the distribution of the tensile strength which reflects the high potential of these hollow Si-B-N fibers for reinforcing ceramic matrix composites. Considering the dependence of fiber strength with the numbers of flaws and the processing conditions, significant progress in fiber strengths are particularly expected by improving the stretchability, i.e. decreasing the green fiber diameter, during the spinning process. Furthermore, the hollow Si-B-N fibers also exhibit excellent dielectric properties as shown in Fig. 4. The average dielectric constant real part (εr) and loss tangent (tan δ) of the hollow Si-B-N ceramic fibers are 3.06 and 0.0032, respectively, which is lower than those of the dense Si-B-N fiber (4.36 and 0.0042) in our previous work [1,5].
In summary, attractive hollow Si-B-N ceramic fibers with excellent dielectric properties were prepared by partial curing method under HSiCl3 vapor of polyborosilazane fibers and subsequent pyrolysis. The average tensile strength and elastic modulus are about 1.03 GPa and 106 GPa, respectively. It is possible to improve the hollow Si-BN ceramic fiber properties by optimizing experimental parameters such as decreasing the green fiber diameter and controlling the heating rates in the pyrolysis process. Meanwhile, the hollow Si-B-N fibers exhibit low average εr of 3.06 and tanδ of 0.0032 at 2–18 GHz, which is lower than the dense Si-B-N fibers in our previous work. The desirable properties enable the hollow Si-B-N fibers to be promising candidates for microwave transparent ceramic composites. Acknowledgements This work was financially supported by the Exploring Project for W&E (No. 7130902), National Natural Science Foundation of China (Nos. 50702075 and 51172280) and Fund of Key Laboratory of Advanced Ceramic Fibers & Composites (9140C820103110C8201). Appendix A. Supplementary data Supplementary data to this article can be found online at doi:10. 1016/j.matlet.2012.02.068. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]
Tang Y, Wang J, Li XD, Xie ZF, Wang H, Li WH, et al. Chem Eur J 2010;16:6458–62. Li D, Zang CR, Li B, Cao F, Wang SQ, Li JS. Mater Lett 2012;68:222–4. Seyferth D, Plenio H. J Am Ceram Soc 1990;73:2131–3. Zhao X, Han KQ, Peng YQ, Yuan J, Li ST, Yu MH. Mater Lett 2011;65:2717–20. Tang Y, Wang J, Li XD, Li WH, Wang XZ, Wang H. Acta Chim Sinica 2009;67: 2750–4. Zou XR, Zhang CR, Wang SQ, Cao F, Li B, Song YX. J Aeronaut Mater 2010;30: 38–42. Sugimoto M, Idesaki A, Tanaka S, Okamura K. Key Eng Mater 2003;247:133–7. Kita K, Narisawa M, Mabuchi H, Itoh M, Sugimoto M, Yoshikawa M. J Am Ceram Soc 2009;92:1192–7. Li WH, Wang J, Xie ZF, Wang H, Tang Y. Acta Chim Sinica 2011;69:1936–40. Lei YP, Wang YD, Song YC, Li YH, Deng C, Wang H, et al. Mater Lett 2011;65:157–9. Janakiraman N, Weinmann M, Schuhmacher J, Müller K, Bill J, Aldinger F. J Am Ceram Soc 2002;85:1807–14.
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Preparation of hollow Si-B-N ceramic fibers by partial curing and pyrolysis of polyborosilazane fibers Wenhua Li, Jun Wang ⁎, Zhengfang Xie, Hao Wang, Yun Tang State Key Lab. of Advanced Ceramic Fibers & Composites, College of Aerospace & Materials Engineering, National University of Defense Technology, Changsha 410073, PR China
a r t i c l e
i n f o
Article history: Received 18 December 2011 Accepted 15 February 2012 Available online 28 February 2012 Keywords: Polymers Fiber technology Ceramics Dielectric properties
a b s t r a c t Hollow silicon boron nitride (Si-B-N) ceramic fibers with composition of Si0.3BN1.4 were prepared by meltspinning, partial curing with trichlorosilane (HSiCl3), and pyrolysis of a novel polyborosilazane under NH3 up to 1000 °C. The polyborosilazane fibers with low ceramic yield were partially cured to make sure the hollow Si-B-N ceramic fibers could be fabricated after pyrolysis. The hollow Si-B-N ceramic fibers were ~ 16 μm in diameter with inner hollow pore diameter of ~ 4 μm, and showed good average tensile strength of 1.03 GPa and elastic modulus of 106 GPa. Moreover, the hollow Si-B-N ceramic fibers also exhibited excellent dielectric properties with the average dielectric constant real part and loss tangent about 3.06 and 0.0032 at 2–18 GHz, respectively, making them to be promising microwave-transparent materials. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Ceramic fibers composed of Si, B, and N exhibit high thermal stability, low thermal conductivity, low and thermally stable dielectric constant, and high mechanical strength. They are suitable candidates as reinforcement in ceramic matrix composites for high temperature microwave-transparent applications [1,2]. Recently, the Si-B-N fibers were successfully prepared in our lab and elsewhere [1,3–5]. All these Si-B-N fibers are dense without pores. However, hollow fibers (e.g. hollow silica fibers [6]) have lower dielectric constant and thermal conductivity, and could reduce the weight of the ceramic matrix composites reinforced by them. Commonly, to fabricate hollow ceramic fibers as reinforcements in ceramic matrix composites such as hollow SiC fibers [7,8], the preceramic fibers were firstly spun into hollow preceramic fibers, and then the hollow preceramic fibers were conversed into hollow ceramic fibers by pyrolysis. Up to now, there are not any reports about hollow Si-B-N ceramic fibers. In this Letter, we report a novel method to fabricate hollow Si-B-N ceramic fibers. The polyborosilazane was firstly spun into dense polyborosilazane fibers, and then the dense polyborosilazane fibers were partially cured by chemical (HSiCl3) vapor reactions and pyrolyzed directly in NH3 atmospheres into hollow Si-B-N ceramic fibers. 2. Experimental 2.1. Preparation of hollow Si-B-N ceramic fibers The polyborosilazane fibers with diameter of ~ 23 μm were prepared as before [9]. Briefly, the polyborosilazane was synthesized ⁎ Corresponding author. Tel./fax: + 86 731 84573161. E-mail address: [email protected] (J. Wang). 0167-577X/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2012.02.068
via “one-pot” approach using boron trichloride and hexamethydisilazane as the starting materials, and could be easily melt-spun into polyborosilazane fibers at 190 °C in N2 atmospheres with a lab scale melt-spinning apparatus. For most of fabrication of ceramic fibers from polymer fibers, the whole polymer fibers must be cured well before pyrolysis. However to fabricate hollow Si-B-N ceramic fibers in this work, the polyborosilazane fibers were cured at 80 °C for 0.5 h under HSiCl3 (heating rate 4 °C/min). This cure temperature (80 °C) and holding time (0.5 h) make sure that the outer parts of the polyborosilazane fibers were cured, while the inner parts were not. Then, the partial cured fibers were pyrolyzed at 1000 °C (heating rate 5 °C/min) under a flowing NH3 atmosphere (25 mL/min). The cured outer parts remained and conversed to ceramic after pyrolysis, while the uncured inner parts decomposed mostly and the hollow pores were formed. So, the off-white hollow Si-B-N ceramic fibers were fabricated. The schematic illustration of the process for fabricating Si-B-N ceramic fibers was shown in Fig. S1 of the Supporting Information. 2.2. Characterization The silicon and boron elements in the fibers were quantified by means of ICP-AES using an Arl 3580B spectrometer. Quantitative analysis of nitrogen and oxygen were carried out using a Leco TC436 O/N Determinator, and carbon was measured by a Leco CS444C/S analyzer. Fourier transform infrared (FT-IR) spectra were obtained using a Nicolet Avatar 360 apparatus in a KBr pellet. The microstructure of the fiber was observed by a scanning electron microscopy (JSM-6360LV). TG was conducted using a NETZSCH STA 449C instrument under Ar atmospheres with a heating rate of 5 °C/min. The XRD of the obtained pyrolyzed specimens was performed using a D8 ADVANCE powder X-ray diffraction, using Cu-Ka radiation.
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W. Li et al. / Materials Letters 78 (2012) 1–3
Fig. 1. TG curves of the polyborosilazane fibers (a) and cured fibers (b) in Ar. Fig. 3. FT-IR spectra of the obtained hollow fibers.
Single filament tensile properties were determined using an YG-type tensile strength tester (Jiangsu Taicang Textile Instruments Co., China) with a gauge length of 25 mm. The dielectric properties of Si-B-N fiber at 2–18 GHz were measured by short-circuited wave guide technique. 3. Results and discussion To fabricate hollow Si-B-N ceramic fibers from polyborosilazane fibers by partial curing and pyrolysis, the polyborosilazane fibers should have low ceramic yield to make sure the inner parts of the polyborosilazane fibers could decompose mostly into hollow pores, and still could be cured to make sure the outer parts of the fibers remained after pyrolysis. The hypothesized structure of the polyborosilazane and the curing mechanism of the polyborosilazane fibers were shown in Fig. S2 of Supporting Information. As shown in Fig. 1, the polyborosilazane fibers used for the fabrication of the hollow fibers shows weight loss of 65 wt.% up to 1000 °C in Ar (Fig. 1a). The large weight loss of the polyborosilazane fibers make sure the pore of the hollow fiber could be formed. While the polyborosilazane fibers could be cured by HSiCl3 vapor. The entirely cured fibers by HSiCl3 have a high ceramic yield up to 73 wt.% (Fig. 1b), which ensures that the outer parts of the hollow fibers could remain after pyrolysis.
The polyborosilazane fibers have low ceramic yield and could be cured by chemical vapor reaction, which is just the prerequisite condition for the fabrication of the hollow fibers. The chemical vapor cure temperature (80 °C) and curing time (0.5 h) are also the key points for forming the hollow fibers. Fig. 2 shows the surfaces and crosssections of the hollow fibers obtained under optimized curing temperature and time. Typically, the hollow fibers are ~ 16 μm in diameter (Fig. 2a) with inner hollow pore diameter of ~ 4 μm (Fig. 2b). The porosity of the hollow fiber is 20%. It means that, when using them as reinforcements, the weight of fibers will be lightened 20% than that of the dense fibers with the same diameter and density. As shown in Fig. 2a, some fibers are split, which may be caused by the drastic volatilization of the pyrolysis gas, and could be improved by optimizing the heating rates in the pyrolysis process. However, the split hollow fibers (Fig. 2a) indicate that the pores of the hollow fibers are along the fiber longitudinal axis. Besides, no matter the hollow fibers are split, the outer parts of the hollow fibers have smooth surfaces and exhibit dense textures. (Fig. 2b,c). The obtained hollow fibers were amorphous, which was identified by X-ray diffraction (see Fig.S3 in Supporting Information). Although the phase compositions of the as-obtained hollow fibers could not be identified by X-ray diffraction, boron nitride and silicon
Fig. 2. SEM images of the obtained hollow fibers. a) surface microstructure. b) cross-section of hollow fiber. c) cross-section of split hollow fiber. The arrows in a) indicate the split hollow fibers.
W. Li et al. / Materials Letters 78 (2012) 1–3
3
Undoubtedly, the low dielectric constant of hollow Si-B-N ceramic fibers is more promising than the dense Si-B-N ceramic fibers for application in microwave-transparent system. 4. Conclusions
Fig. 4. Dielectric properties of hollow Si-B-N fibers.
nitride were clearly identified by the corresponding characteristic absorption bands in the FT-IR spectrum (Fig. 3). The main peaks in the hollow fibers are 1380 and 801 cm − 1, which are characteristic of B-N [10], and 986 cm − 1 which is characteristic of amorphous Si-N [11]. Further the composition of the obtained hollow fibers was determined by elemental analysis (EA). The EA results show that the chemical compositions of the hollow fibers are Si (21.25 wt.%), B (27.54 wt.%), N (50.26 wt.%), O (0.62 wt.%) and C (0.12 wt.%). Carbon and oxygen contents were found to be b1 wt.% and can be omitted, therefore the hollow fibers display a chemical formula of Si0.3BN1.4. Moreover, the hollow Si-B-N fibers exhibit good mechanical strength with the average tensile strength of 1.03 GPa and elastic modulus of 106 GPa. They were lower than those of the dense Si-BN fibers in our previous work. Remarkably, values as high as 1.68 GPa can be found in the distribution of the tensile strength which reflects the high potential of these hollow Si-B-N fibers for reinforcing ceramic matrix composites. Considering the dependence of fiber strength with the numbers of flaws and the processing conditions, significant progress in fiber strengths are particularly expected by improving the stretchability, i.e. decreasing the green fiber diameter, during the spinning process. Furthermore, the hollow Si-B-N fibers also exhibit excellent dielectric properties as shown in Fig. 4. The average dielectric constant real part (εr) and loss tangent (tan δ) of the hollow Si-B-N ceramic fibers are 3.06 and 0.0032, respectively, which is lower than those of the dense Si-B-N fiber (4.36 and 0.0042) in our previous work [1,5].
In summary, attractive hollow Si-B-N ceramic fibers with excellent dielectric properties were prepared by partial curing method under HSiCl3 vapor of polyborosilazane fibers and subsequent pyrolysis. The average tensile strength and elastic modulus are about 1.03 GPa and 106 GPa, respectively. It is possible to improve the hollow Si-BN ceramic fiber properties by optimizing experimental parameters such as decreasing the green fiber diameter and controlling the heating rates in the pyrolysis process. Meanwhile, the hollow Si-B-N fibers exhibit low average εr of 3.06 and tanδ of 0.0032 at 2–18 GHz, which is lower than the dense Si-B-N fibers in our previous work. The desirable properties enable the hollow Si-B-N fibers to be promising candidates for microwave transparent ceramic composites. Acknowledgements This work was financially supported by the Exploring Project for W&E (No. 7130902), National Natural Science Foundation of China (Nos. 50702075 and 51172280) and Fund of Key Laboratory of Advanced Ceramic Fibers & Composites (9140C820103110C8201). Appendix A. Supplementary data Supplementary data to this article can be found online at doi:10. 1016/j.matlet.2012.02.068. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]
Tang Y, Wang J, Li XD, Xie ZF, Wang H, Li WH, et al. Chem Eur J 2010;16:6458–62. Li D, Zang CR, Li B, Cao F, Wang SQ, Li JS. Mater Lett 2012;68:222–4. Seyferth D, Plenio H. J Am Ceram Soc 1990;73:2131–3. Zhao X, Han KQ, Peng YQ, Yuan J, Li ST, Yu MH. Mater Lett 2011;65:2717–20. Tang Y, Wang J, Li XD, Li WH, Wang XZ, Wang H. Acta Chim Sinica 2009;67: 2750–4. Zou XR, Zhang CR, Wang SQ, Cao F, Li B, Song YX. J Aeronaut Mater 2010;30: 38–42. Sugimoto M, Idesaki A, Tanaka S, Okamura K. Key Eng Mater 2003;247:133–7. Kita K, Narisawa M, Mabuchi H, Itoh M, Sugimoto M, Yoshikawa M. J Am Ceram Soc 2009;92:1192–7. Li WH, Wang J, Xie ZF, Wang H, Tang Y. Acta Chim Sinica 2011;69:1936–40. Lei YP, Wang YD, Song YC, Li YH, Deng C, Wang H, et al. Mater Lett 2011;65:157–9. Janakiraman N, Weinmann M, Schuhmacher J, Müller K, Bill J, Aldinger F. J Am Ceram Soc 2002;85:1807–14.