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A novel precursor route for the production of Si―B―N ceramic fibers
Materials Letters 65 (2011) 2717–2720
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t
A novel precursor route for the production of Si―B―N ceramic fibers Xi Zhao, Keqing Han ⁎, Yuqing Peng, Jia Yuan, Shutong Li, Muhuo Yu State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, College of Materials Science and Engineering, Donghua University, Shanghai 201620, PR China
a r t i c l e
i n f o
Article history: Received 28 January 2011 Accepted 9 May 2011 Available online 13 May 2011 Keywords: Ceramics Polymers Polycondensation Melt-spinning Pyrolysis SiBN
a b s t r a c t A novel polymeric precursor to SiBN ceramic fiber was synthesized by reaction of tetramethylaminosilane ((CH3NH)4Si) and trimethylaminoborane ((CH3NH)3B). It was shown that the polymer contains an Si―N―B bridge bond connecting the linear silicon and borazon ring parts. This structure imparts sufficient viscosity for the material to be processed by melt spinning, so that for the first time it was easily spun into green fibers using laboratory scale equipment. SiBN ceramic fibers with diameter 35 μm were obtained after pyrolysis at 1600 °C in an NH3 atmosphere. The thermal decomposition behavior of the polymer during pyrolysis was also investigated. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Compared with binary ceramics, multinary ceramics containing silicon, boron and nitrogen have many advantages, in particular high temperature stability. SiBNC ceramics can easily withstand temperatures up to 1800 °C in a non-oxidizing atmosphere, and up to 1500 °C under oxidizing conditions for long periods, while retaining most of their original mechanical strength [1,2]. SiB(C)N fibers are mainly used as high-temperature resistant reinforcement materials, and have potential applications in the aerospace industry. SiB(C)N fibers produced from polymeric precursors have been prepared from many types of polyborosilizanes [3,4]. Jansen et al. [5] synthesized the precursor Cl3Si–NH–BCl2 then transformed it into a borosilazane polymer by ammonolysis. The as-obtained polymer N-methylpolyborosilazane (PBS-Me) was melt-spinnable, but the degree of crosslinking was hard to control. Jansen et al. [5] partially replaced the silicon-bonded chlorine atoms with methyl groups, and synthesized Cl 2 MeSi–NH–BCl 2 (MADB) and ClMe 2 Si–NH–BCl 2 (DADB). By further polymerization, processable polymers with lower crosslink density were obtained [6]. Bernard and Weinmann et al. [7,8] reported another route to fusible precursors. Polymers based on boron-modified polysilazane, such as [B(C2H4SiCH3NCH3)3]n, showed thermoplastic properties, and green fibers could be obtained and subsequently converted to SiBCN ceramic fibers by appropriate curing and pyrolysis processes. In the present study, a novel polymeric precursor for SiBN ceramic fibers was synthesized by co-polycondensation of tetramethylamino-
⁎ Corresponding author. Tel.: + 86 21 67792904; fax: + 86 21 67792904. E-mail address: [email protected] (K. Han). 0167-577X/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2011.05.042
silane and trimethylaminoborane. These monomers were easily synthesized, and the procedures were straightforward because the by-product could be readily eliminated with an inert gas instead of filtration. By controlling the ratio of the reacting monomers, a series of polymeric precursors with varying Si/B ratio could be obtained, leading to ceramic fibers with varying performance characteristics. 2. Experimental 2.1. Materials All reactions were carried out in a purified argon atmosphere. Boron trichloride (BCl3) was obtained from Beijing Multi Technology Corporation, and silicon tetrachloride (SiCl4) from Acros Organics. Methylamine (CH3NH2) was obtained from Zhejiang Jiangshan Chemical Co. Ltd., and was freshly condensed in toluene at − 50 °C before use. Toluene obtained from Sinopharm Chemical Reagent Co. Ltd. was purified by distillation from potassium hydroxide under nitrogen. Argon and nitrogen with 99.999% purity were used during sample preparation. 2.2. Synthesis of polymeric precursor 2.2.1. Synthesis of tetramethylaminosilane In a 500 mL three-neck flask, 29.6 g (0.17 mol) of SiCl4 was dissolved in 20 mL of toluene and cooled at −78 °C. CH3NH2 (53.94 g, 1.74 mol) was slowly added dropwise to the solution with vigorous stirring, and then the temperature of the solution was increased to room temperature. The amine hydrochloride that was formed was dissolved in excess methylamine and separated from the mixture using a separating funnel. A solution of (CH3NH)4Si in toluene was thus obtained.
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2.3. Fibers preparation Polymer green fibers were prepared with laboratory scale meltspinning equipment. Co-polyborosilizane was heated at 160 °C in a reactor under an argon atmosphere until an appropriate viscosity was obtained. The molten polymer was then passed through a spinneret with ten 0.3 mm capillaries, and the resulting continuous fibers were collected on a rotating spool in a large glove box filled with argon. The as-spun fibers were cured in a pure ammonia atmosphere by heating from 25 to 300 °C at 1 °C/min. Fiber pyrolysis was achieved by further heating at the same rate up to 800 °C in flowing ammonia, maintained for 2 h at that temperature, then heated to 1600 °C in flowing nitrogen and maintained at that temperature for a further 2 h. 2.4. Characterization
Fig. 1. FT-IR spectrum of co-polyborosilizane.
2.2.2. Synthesis of trimethylaminoborane BCl3 (28.6 g, 0.24 mol) condensed in 20 mL of toluene was added dropwise to 60.5 g (1.95 mol) of CH3NH2 at −78 °C in a three-neck flask. The reaction mixture was then allowed to warm up to room temperature and maintained at ambient temperature for 20 h. When the reaction was complete, CH3NH3Cl was removed from the reaction mixture by dissolution in methylamine and separation using a separating funnel. A solution of (CH3NH)3B in toluene was thus obtained. 2.2.3. Synthesis of co-polyborosilizane Solutions of (CH3NH)4Si and (CH3NH)3B in toluene were mixed in a three-necked flask fitted with a reflux condenser. After the solvent was removed by distillation, the remaining mixture was heated to 150 °C and maintained at that temperature for 40 h with continuous stirring until a fusible polymer was obtained.
Fourier transform infrared (FT-IR) spectra of samples in KBr pellets were obtained with a Nicolet 8700 spectrometer. 1H and 11BNMR spectra were recorded with a Bruker Avance 400 spectrometer operated at 400.13 and 128.37 MHz, respectively. 13C and 29Si solid state NMR spectra were recorded at 100.61 and 79.49 MHz, respectively, under MAS conditions. Elemental analyses were carried out with Elementar Vario III Analyzer and Leeman Prodigy ICP-OES. Thermogravimetric analysis (TGA) was conducted with a Netzsch 209 F1 Iris instrument: samples were heated in a flowing nitrogen atmosphere at 1 °C/min from 30 to 1000 °C. Fiber morphologies were revealed by scanning electron microscopy (SEM) using a Hitachi S-3000N instrument. 3. Results and discussion 3.1. Characterization of co-polyborosilizane The FT-IR spectrum of co-polyborosilizane is shown in Fig. 1. N―H stretch absorptions appear at 3426 cm − 1 while the band at 1630 cm − 1 is assigned to deformation of N―H. Three intense absorptions at 2891, 2810 and 2947 cm − 1 are attributed to C―H
Fig. 2. NMR spectra of co-polyborosilizane.
X. Zhao et al. / Materials Letters 65 (2011) 2717–2720
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3.2. Pyrolysis of co-polyborosilizane
Fig. 3. TGA curve of co-polyborosilizane. (Heating rate of 1 °C/min, N2 atmosphere).
stretching vibrations [8,9] and the band at 1458 cm − 1 is assigned to CH3 bending. B―N (in plane ring) vibrations appear at 1395 cm − 1 while B―N (out-of-plane bending) vibrations are visible at 1199, 1100 and 709 cm − 1 [10]. Si―N―Si vibrations appear at 894 cm − 1 [11] and those of Si―N units at about 948 and 1051 cm − 1 [12]. The 13C solid-state MAS-NMR spectrum in Fig. 2 shows the expected methyl resonance at 27 ppm which is attributed to NCH3. In the 29Si solid-state MAS-NMR spectrum, a broad signal related to Si―N―B is present at − 32.8 ppm. The 1H NMR spectrum displays two resonance signals at 0.8 and 2.4 ppm which are attributed to NH and NCH3 protons, respectively. In the 11B NMR spectrum, the broad signal at 27.2 ppm may arise from BN in six-membered borazine rings. The weak signal (shoulder) at about 56 ppm can be attributed to the relevant out of borazine rings. Based on the FTIR and NMR results, during co-polycondensation the main active groups −NHCH3 in the two molecular precursors are condensed to form an Si―N(CH3)―B structure. Self-polymerization of the molecular precursor was manifested by the presence of sixmembered borazine and four-membered SiN rings.
Conversion of co-polyborosilizane into the desired amorphous ceramics was followed by TGA up to 1000 °C in flowing nitrogen. Fig. 3 indicates that pyrolysis occurred in two stages. In the first stage further crosslinking occurred, accompanied by elimination of methylamine and a continuous mass loss of 17% between 150 and 400 °C. Between 400 and 700 °C the sample lost a further 25% of its original mass. This mass loss can be attributed to various changes such as further condensation of methylamine, methane or its fragments from terminal methyl groups, and to hydrogen elimination [11]. The final ceramic yield was about 55%. The relatively low yield could be a consequence of the small degree of crosslinking in copolyborosilizane, which is a consequence of improved spinnability. The carbon content was reduced by pyrolysis from 27.44 wt.% to less than 0.8 wt.%, which confirms the changes during pyrolysis discussed above. The contents of nitrogen and boron both increased, but not as much as that of silicon which increased from 26.88 to 46.58 wt.%. The rather high boron content (13.87%) helps the fibers to achieve remarkable stability to high temperature and oxidation [13]. 3.3. Fiber morphology Scanning electron micrographs (SEM, Fig. 4A and B) show that the green fibers had almost smooth surfaces with some cracks. The fibers were typically 50 μm in diameter. After pyrolysis, the diameter was reduced to 35 μm (Fig. 4C and D), and the ceramic fibers were relatively uniform. However, due to the size of the polymer fibers and the defects induced by the crude spinning conditions, the ceramic fibers were found to be too weak for mechanical testing. Enhancement of the strength of the ceramic fibers is expected to be achieved by use of more sophisticated spinning apparatus. 4. Conclusions (CH3NH)4Si and (CH3NH)3B were polycondensed into highly processable polymers containing Si―N―B bridge bonds, with suitable viscosity for the polymers to be melt spun into 50 μm in diameter green fibers. After thermolysis, the diameters of the fibers were reduced from 50 to 35 μm, mainly because of elimination of
Fig. 4. SEM images of green fibers (A, B) and ceramic fibers (C, D) after thermolysis at 1600 °C.
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organic groups. Further research is in progress to optimize the conditions (in terms of Si/B ratio, degree of polycondensation and the pyrolysis process) to improve the mechanical properties of the resulting ceramic fibers. Acknowledgement This work was financially supported by the National Natural Science Foundation of China (grant no. 20704005, no. 90916025) and sponsored by Program of Shanghai Subject Chief Scientist (type A). References [1] Baldus P, Jansen M, Sporn D. Science 1999;285:699–703. [2] Baldus HP, Jansen M. Angew Chem Int Ed Engl 1997;36(4):328–43.
[3] Jansen M. Solid State Ionics 1997;101-103(Pt 2):l–7. [4] Bernard S, Weinmann M, Cornu D, Miele P, Aldinger F. J Eur Ceram Soc 2005;25(2–3): 251–6. [5] Jansen M, Jungermann H. Curr Opin Solid St M 1997;2(2):150–8. [6] Müller U, Weinmann M, Jansen M. J Mater Chem 2008;18:3671–9. [7] Weinmann M, Haug R, Bill J, Aldinger F, Schuhmacher J, Müller K. J Organomet Chem 1997;541(1–2):345–53. [8] Bernard S, Weinmann M, Gerstel P, Miele P, Aldinger F. J Mater Chem 2005;15: 289–99. [9] Weinmann M, Kroschel M, Jäschke T, Nuss J, Jansen M, Kolios G, et al. J Mater Chem 2008;18:1810–9. [10] Cornu D, Miele P, Faure R, Bonnetot B, Mongeot H, Bouix J. J Mater Chem 1999;9: 757–61. [11] Hörz M, Zern A, Berger F, Haug J, Müller K, Aldinger F, et al. J Euro Ceram Soc 2005;25(2–3):99–110. [12] Bouillon E, Pailler R, Naslain R. Chem Mater 1991;3(2):356–67. [13] Wideman T, Cortez E, Remsen EE, Zank GA, Carroll PJ, Sneddon LG. Chem Mater 1997;9(10):2230.
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t
A novel precursor route for the production of Si―B―N ceramic fibers Xi Zhao, Keqing Han ⁎, Yuqing Peng, Jia Yuan, Shutong Li, Muhuo Yu State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, College of Materials Science and Engineering, Donghua University, Shanghai 201620, PR China
a r t i c l e
i n f o
Article history: Received 28 January 2011 Accepted 9 May 2011 Available online 13 May 2011 Keywords: Ceramics Polymers Polycondensation Melt-spinning Pyrolysis SiBN
a b s t r a c t A novel polymeric precursor to SiBN ceramic fiber was synthesized by reaction of tetramethylaminosilane ((CH3NH)4Si) and trimethylaminoborane ((CH3NH)3B). It was shown that the polymer contains an Si―N―B bridge bond connecting the linear silicon and borazon ring parts. This structure imparts sufficient viscosity for the material to be processed by melt spinning, so that for the first time it was easily spun into green fibers using laboratory scale equipment. SiBN ceramic fibers with diameter 35 μm were obtained after pyrolysis at 1600 °C in an NH3 atmosphere. The thermal decomposition behavior of the polymer during pyrolysis was also investigated. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Compared with binary ceramics, multinary ceramics containing silicon, boron and nitrogen have many advantages, in particular high temperature stability. SiBNC ceramics can easily withstand temperatures up to 1800 °C in a non-oxidizing atmosphere, and up to 1500 °C under oxidizing conditions for long periods, while retaining most of their original mechanical strength [1,2]. SiB(C)N fibers are mainly used as high-temperature resistant reinforcement materials, and have potential applications in the aerospace industry. SiB(C)N fibers produced from polymeric precursors have been prepared from many types of polyborosilizanes [3,4]. Jansen et al. [5] synthesized the precursor Cl3Si–NH–BCl2 then transformed it into a borosilazane polymer by ammonolysis. The as-obtained polymer N-methylpolyborosilazane (PBS-Me) was melt-spinnable, but the degree of crosslinking was hard to control. Jansen et al. [5] partially replaced the silicon-bonded chlorine atoms with methyl groups, and synthesized Cl 2 MeSi–NH–BCl 2 (MADB) and ClMe 2 Si–NH–BCl 2 (DADB). By further polymerization, processable polymers with lower crosslink density were obtained [6]. Bernard and Weinmann et al. [7,8] reported another route to fusible precursors. Polymers based on boron-modified polysilazane, such as [B(C2H4SiCH3NCH3)3]n, showed thermoplastic properties, and green fibers could be obtained and subsequently converted to SiBCN ceramic fibers by appropriate curing and pyrolysis processes. In the present study, a novel polymeric precursor for SiBN ceramic fibers was synthesized by co-polycondensation of tetramethylamino-
⁎ Corresponding author. Tel.: + 86 21 67792904; fax: + 86 21 67792904. E-mail address: [email protected] (K. Han). 0167-577X/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2011.05.042
silane and trimethylaminoborane. These monomers were easily synthesized, and the procedures were straightforward because the by-product could be readily eliminated with an inert gas instead of filtration. By controlling the ratio of the reacting monomers, a series of polymeric precursors with varying Si/B ratio could be obtained, leading to ceramic fibers with varying performance characteristics. 2. Experimental 2.1. Materials All reactions were carried out in a purified argon atmosphere. Boron trichloride (BCl3) was obtained from Beijing Multi Technology Corporation, and silicon tetrachloride (SiCl4) from Acros Organics. Methylamine (CH3NH2) was obtained from Zhejiang Jiangshan Chemical Co. Ltd., and was freshly condensed in toluene at − 50 °C before use. Toluene obtained from Sinopharm Chemical Reagent Co. Ltd. was purified by distillation from potassium hydroxide under nitrogen. Argon and nitrogen with 99.999% purity were used during sample preparation. 2.2. Synthesis of polymeric precursor 2.2.1. Synthesis of tetramethylaminosilane In a 500 mL three-neck flask, 29.6 g (0.17 mol) of SiCl4 was dissolved in 20 mL of toluene and cooled at −78 °C. CH3NH2 (53.94 g, 1.74 mol) was slowly added dropwise to the solution with vigorous stirring, and then the temperature of the solution was increased to room temperature. The amine hydrochloride that was formed was dissolved in excess methylamine and separated from the mixture using a separating funnel. A solution of (CH3NH)4Si in toluene was thus obtained.
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2.3. Fibers preparation Polymer green fibers were prepared with laboratory scale meltspinning equipment. Co-polyborosilizane was heated at 160 °C in a reactor under an argon atmosphere until an appropriate viscosity was obtained. The molten polymer was then passed through a spinneret with ten 0.3 mm capillaries, and the resulting continuous fibers were collected on a rotating spool in a large glove box filled with argon. The as-spun fibers were cured in a pure ammonia atmosphere by heating from 25 to 300 °C at 1 °C/min. Fiber pyrolysis was achieved by further heating at the same rate up to 800 °C in flowing ammonia, maintained for 2 h at that temperature, then heated to 1600 °C in flowing nitrogen and maintained at that temperature for a further 2 h. 2.4. Characterization
Fig. 1. FT-IR spectrum of co-polyborosilizane.
2.2.2. Synthesis of trimethylaminoborane BCl3 (28.6 g, 0.24 mol) condensed in 20 mL of toluene was added dropwise to 60.5 g (1.95 mol) of CH3NH2 at −78 °C in a three-neck flask. The reaction mixture was then allowed to warm up to room temperature and maintained at ambient temperature for 20 h. When the reaction was complete, CH3NH3Cl was removed from the reaction mixture by dissolution in methylamine and separation using a separating funnel. A solution of (CH3NH)3B in toluene was thus obtained. 2.2.3. Synthesis of co-polyborosilizane Solutions of (CH3NH)4Si and (CH3NH)3B in toluene were mixed in a three-necked flask fitted with a reflux condenser. After the solvent was removed by distillation, the remaining mixture was heated to 150 °C and maintained at that temperature for 40 h with continuous stirring until a fusible polymer was obtained.
Fourier transform infrared (FT-IR) spectra of samples in KBr pellets were obtained with a Nicolet 8700 spectrometer. 1H and 11BNMR spectra were recorded with a Bruker Avance 400 spectrometer operated at 400.13 and 128.37 MHz, respectively. 13C and 29Si solid state NMR spectra were recorded at 100.61 and 79.49 MHz, respectively, under MAS conditions. Elemental analyses were carried out with Elementar Vario III Analyzer and Leeman Prodigy ICP-OES. Thermogravimetric analysis (TGA) was conducted with a Netzsch 209 F1 Iris instrument: samples were heated in a flowing nitrogen atmosphere at 1 °C/min from 30 to 1000 °C. Fiber morphologies were revealed by scanning electron microscopy (SEM) using a Hitachi S-3000N instrument. 3. Results and discussion 3.1. Characterization of co-polyborosilizane The FT-IR spectrum of co-polyborosilizane is shown in Fig. 1. N―H stretch absorptions appear at 3426 cm − 1 while the band at 1630 cm − 1 is assigned to deformation of N―H. Three intense absorptions at 2891, 2810 and 2947 cm − 1 are attributed to C―H
Fig. 2. NMR spectra of co-polyborosilizane.
X. Zhao et al. / Materials Letters 65 (2011) 2717–2720
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3.2. Pyrolysis of co-polyborosilizane
Fig. 3. TGA curve of co-polyborosilizane. (Heating rate of 1 °C/min, N2 atmosphere).
stretching vibrations [8,9] and the band at 1458 cm − 1 is assigned to CH3 bending. B―N (in plane ring) vibrations appear at 1395 cm − 1 while B―N (out-of-plane bending) vibrations are visible at 1199, 1100 and 709 cm − 1 [10]. Si―N―Si vibrations appear at 894 cm − 1 [11] and those of Si―N units at about 948 and 1051 cm − 1 [12]. The 13C solid-state MAS-NMR spectrum in Fig. 2 shows the expected methyl resonance at 27 ppm which is attributed to NCH3. In the 29Si solid-state MAS-NMR spectrum, a broad signal related to Si―N―B is present at − 32.8 ppm. The 1H NMR spectrum displays two resonance signals at 0.8 and 2.4 ppm which are attributed to NH and NCH3 protons, respectively. In the 11B NMR spectrum, the broad signal at 27.2 ppm may arise from BN in six-membered borazine rings. The weak signal (shoulder) at about 56 ppm can be attributed to the relevant out of borazine rings. Based on the FTIR and NMR results, during co-polycondensation the main active groups −NHCH3 in the two molecular precursors are condensed to form an Si―N(CH3)―B structure. Self-polymerization of the molecular precursor was manifested by the presence of sixmembered borazine and four-membered SiN rings.
Conversion of co-polyborosilizane into the desired amorphous ceramics was followed by TGA up to 1000 °C in flowing nitrogen. Fig. 3 indicates that pyrolysis occurred in two stages. In the first stage further crosslinking occurred, accompanied by elimination of methylamine and a continuous mass loss of 17% between 150 and 400 °C. Between 400 and 700 °C the sample lost a further 25% of its original mass. This mass loss can be attributed to various changes such as further condensation of methylamine, methane or its fragments from terminal methyl groups, and to hydrogen elimination [11]. The final ceramic yield was about 55%. The relatively low yield could be a consequence of the small degree of crosslinking in copolyborosilizane, which is a consequence of improved spinnability. The carbon content was reduced by pyrolysis from 27.44 wt.% to less than 0.8 wt.%, which confirms the changes during pyrolysis discussed above. The contents of nitrogen and boron both increased, but not as much as that of silicon which increased from 26.88 to 46.58 wt.%. The rather high boron content (13.87%) helps the fibers to achieve remarkable stability to high temperature and oxidation [13]. 3.3. Fiber morphology Scanning electron micrographs (SEM, Fig. 4A and B) show that the green fibers had almost smooth surfaces with some cracks. The fibers were typically 50 μm in diameter. After pyrolysis, the diameter was reduced to 35 μm (Fig. 4C and D), and the ceramic fibers were relatively uniform. However, due to the size of the polymer fibers and the defects induced by the crude spinning conditions, the ceramic fibers were found to be too weak for mechanical testing. Enhancement of the strength of the ceramic fibers is expected to be achieved by use of more sophisticated spinning apparatus. 4. Conclusions (CH3NH)4Si and (CH3NH)3B were polycondensed into highly processable polymers containing Si―N―B bridge bonds, with suitable viscosity for the polymers to be melt spun into 50 μm in diameter green fibers. After thermolysis, the diameters of the fibers were reduced from 50 to 35 μm, mainly because of elimination of
Fig. 4. SEM images of green fibers (A, B) and ceramic fibers (C, D) after thermolysis at 1600 °C.
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organic groups. Further research is in progress to optimize the conditions (in terms of Si/B ratio, degree of polycondensation and the pyrolysis process) to improve the mechanical properties of the resulting ceramic fibers. Acknowledgement This work was financially supported by the National Natural Science Foundation of China (grant no. 20704005, no. 90916025) and sponsored by Program of Shanghai Subject Chief Scientist (type A). References [1] Baldus P, Jansen M, Sporn D. Science 1999;285:699–703. [2] Baldus HP, Jansen M. Angew Chem Int Ed Engl 1997;36(4):328–43.
[3] Jansen M. Solid State Ionics 1997;101-103(Pt 2):l–7. [4] Bernard S, Weinmann M, Cornu D, Miele P, Aldinger F. J Eur Ceram Soc 2005;25(2–3): 251–6. [5] Jansen M, Jungermann H. Curr Opin Solid St M 1997;2(2):150–8. [6] Müller U, Weinmann M, Jansen M. J Mater Chem 2008;18:3671–9. [7] Weinmann M, Haug R, Bill J, Aldinger F, Schuhmacher J, Müller K. J Organomet Chem 1997;541(1–2):345–53. [8] Bernard S, Weinmann M, Gerstel P, Miele P, Aldinger F. J Mater Chem 2005;15: 289–99. [9] Weinmann M, Kroschel M, Jäschke T, Nuss J, Jansen M, Kolios G, et al. J Mater Chem 2008;18:1810–9. [10] Cornu D, Miele P, Faure R, Bonnetot B, Mongeot H, Bouix J. J Mater Chem 1999;9: 757–61. [11] Hörz M, Zern A, Berger F, Haug J, Müller K, Aldinger F, et al. J Euro Ceram Soc 2005;25(2–3):99–110. [12] Bouillon E, Pailler R, Naslain R. Chem Mater 1991;3(2):356–67. [13] Wideman T, Cortez E, Remsen EE, Zank GA, Carroll PJ, Sneddon LG. Chem Mater 1997;9(10):2230.