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One pot synthesis of a soluble polymer for zirconium carbide
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Chinese Chemical Letters 21 (2010) 620–623 www.elsevier.com/locate/cclet
One pot synthesis of a soluble polymer for zirconium carbide Xue Yu Tao a,b, Wen Feng Qiu a, Hao Li a, Tong Zhao a,* a
Laboratory of Advanced Polymer Materials, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China b Graduate School of Chinese Academy of Sciences, Beijing 100049, China Received 6 November 2009
Abstract A new polymer, polyzirconoxanesal (PZS), is synthesized from the reaction of zirconium oxychloride octahydrate (ZrOCl28H2O) with acetylacetone (Hacac) and salicyl alcohol (SA) by one-pot protocol. The polymer is soluble in common organic solvents, such as ethanol, methanol, acetone, tetrahydrofuran and chloroform, and exhibits rheology with viscosity of 100– 300 mpa s at 25 8C. These properties are suitable for uses in fabrication of ceramic matrix fiber composites. Pyrolysis of this polymer at 1300 8C in argon provides nanosized ZrC with spherical morphology and size of 20–100 nm. # 2010 Tong Zhao. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved. Keywords: Zirconium carbide; Preceramic polymer; Morphology
Zirconium carbide (ZrC) is an ultra-high temperature ceramic with combined properties of metals and ceramics because it has mixed ionic, covalent and metallic bonding in NaCl-type lattice structure [1]. It has ultrahigh hardness (25 GPa), high melting point (3420 8C), resistance to corrosion and wear, and structure and chemical stabilities [2–4]. These outstanding physical and chemical properties offer ZrC potential applications at high temperatures. Conventional fabrication of ZrC powders is by carbon thermal reduction. The synthesis requires a high temperature because of the relatively large enthalpy for the formation of ZrC from zirconia powders and carbon (Hf = 207.1 kJ/ mol) [5]. Moreover, a longer time is required for the reaction because of the solid reaction with coarse powders of raw materials. The carbothermal reduction usually yields powders with uncontrolled particulate shapes and sizes. This should be another factor that limits the broad applications of this material. The preceramic polymer method is an effective method for low-temperature synthesis of ceramics because of homogeneous atomic distribution of component elements [6]. The conversion of preceramic polymers to ceramics enables the use of processing techniques not attainable with more traditional powder forming methods, particularly in the areas of fiber and film formation and the development of fiber-reinforced ceramic matrix composites with complex shapes. Sacks et al. prepared ZrC and HfC powders using zirconium n-butoxide and hafnium isopropoxide, and polyhyclric alcohol as the carbon sources [7]. Preiss et al. used chelated derivatives of zirconium n-propoxide and various soluble carbon-yielding compounds to prepare ZrC fibers, films and powders [8]. Although ZrC powders and fibers were successfully synthesized using alkoxides, there are still several drawbacks such as high cost and toxicity.
* Corresponding author. E-mail address: [email protected] (T. Zhao). 1001-8417/$ – see front matter # 2010 Tong Zhao. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved. doi:10.1016/j.cclet.2010.01.002
X.Y. Tao et al. / Chinese Chemical Letters 21 (2010) 620–623
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Scheme 1. .
Acetylacetone has been used to react with zirconium oxychloride to synthesize polyzirconoxanes (PZOs) as precursors for preparing zirconia (ZrO2) fibers [9–11]. Acetylacetone is realized to be an effective ligand chelated to zirconium to form stable sols against both precipitation and hydrolysis [9–11]. Ethylene glycol (EG) was reported to react with zirconium oxychloride to synthesize EG-PZO [12]. We found that salicyl alcohol also contains active hydrogen atoms and it should be able to react with zirconium oxychloride. However, the reaction of salicyl alcohol with zirconium oxychloride has not been reported in the literature. Therefore, here, we use inorganic zirconium oxychloride as zirconium source, both acetylacetone and salicyl alcohol as ligands to synthesize a new precursor for ZrC; and its characterization, properties and pyrolytic behavior were also investigated. 1. Experimental The synthesis route is according to Scheme 1. 6.44 g (0.02 mol) of zirconium oxychloride octahydrate (ZrOCl28H2O), acetylacetone (Hacac) and salicyl alcohol (SA) and 4.1 g (0.04 mol) triethylamine was added into 100 mL of methanol dropwise below 5 8C at molar ratios of Et3N/ZOC = 2.0. The reaction mixture was stirred at room temperature for 4 h and then concentrated in vacuo to get a white solid. THF 50 mL was added, the precipitate was filtrated, and the filtrate was concentrated to highly viscous solution. Reprecipitation of the solution with hexane, followed by drying in vacuo for 3 h, gave the product as a yellow powder. The dried polymers were pyrolyzed in Ar at 1300 8C for 2 h using heating rate 3 8C/min, cooled at 5 8C/min. Fourier transform infrared spectra (FTIR) were recorded between 4000 and 400 cm 1 on Bruker Tensor 27 spectrometer. 1H NMR spectra were recorded in methanol-d4 solvent with a Bruker AV400 spectrometer. X-ray diffraction (XRD) measurements were performed on the pyrolyzed samples with a powder diffractometer (Rigaku D/ M4X 2500) using a Cu/Ka radiation. The morphology of the samples was examined using scanning electron microscope (SEM, S-4800). 2. Results and discussion ZrOCl28H2O reacts with acetylacetone and salicyl alcohol in the presence of triethylamine. Triethylamine promotes the reaction by removing its hydrogen chloride salt. The product was isolated as an air-stable yellow solid, PZS. 1H NMR (400 MHz, CD3OD): d 6.70–7.25(m, 4H, phenyl ring H), 4.55(m, 2H, CH2OH in phenyl ring), 3.1(s, 1H, CH), 1.2(s, 3H, CH3CO enol form), 2.1(m, 3H, CH3CO keto form). IR (n, cm 1): 3062, 2923, 2862, 1590, 1529, 1483, 1452, 1278, 1022 and 543. The molecular weight of the as-synthesized polymer is about 700–1000. According to the results of FTIR, 1H NMR and GPC, PZS might be a linear Zr–O–Zr chain polymer, with acetylacetone and salicyl alcohol as ligands. This polymer structure has not been reported in the literature up to now. There are several advantages on the used process compared with the literature method. First, inorganic zirconium oxychloride was used as zirconium source, it is cheap and less toxic than zirconium alkoxides; second, the polymer PZS was prepared by the facile one-pot protocol and subsequent hydrolysis, condensation steps was unnecessary. Last, the synthesized polymer exhibited good solubility in common organic solvents, such as ethanol, methanol, acetone, tetrahydrofuran and chloroform. When this precursor was dissolved in ethanol solvent, the viscosity is 100–300 mpa s at 25 8C. This is useful for preparing Cf/ZrC ceramic matrix composites (CMCs) via polymer infiltration and pyrolysis (PIP) process.
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X.Y. Tao et al. / Chinese Chemical Letters 21 (2010) 620–623
Fig. 1. XRD pattern of the polymer pyrolyzed at 1300 8C for 2 h.
To study the conversion process from PZS to the ceramics, we performed a series of pyrolysis trials in tube furnace and TG experiments. The TGA curve of PZS indicated that PZS suffers a three-step mass loss: room temperature to 600 8C, 600–1100 8C, and from 1100 to 1375 8C. The weight loss below 600 8C was due to the bonded water and the decomposition of organic groups; and no further weight loss was detected between 600 and 1100 8C. After 1100 8C, the weight loss accelerates again and does not stabilize up to 1375 8C, which is believed to be the carbothermal reduction process. The curve also suggested that the polymer decomposed completely at 600 8C and ceramic yield is 68.5% at 1300 8C. Fig. 1 shows XRD pattern of ZrC after pyrolysis of PZS in Ar at 1300 8C. The XRD pattern is in agreement with the typical structure ZrC diffraction (JCPDS No. 35-0784). Sharp diffraction peaks indicate good crystallinity of ZrC nanoparticles and no characteristic peak related to any impurity was observed. This temperature of forming ZrC is much lower than the conventional solid state synthesis method (1800 8C). To the best of our knowledge, this is the lowest temperature reported in the literatures for preparing ZrC ceramics from preceramic polymers. Quantitative phase analysis reveals that the ZrC content reaches 91 wt%. The morphology and size of the as-prepared ZrC sample were investigated using SEM (Fig. 2). SEM results indicated that three-dimensional nanoscale ZrC particles were obtained although with some agglomerations. The particle sizes distribute over 20–100 nm with sphere structure and the average size is 45 nm, which is in good agreement with that calculated from the XRD pattern by Scherrer’s formula.
Fig. 2. Scanning electron microscopy image of the synthesized ZrC powders at 1300 8C for 2 h.
X.Y. Tao et al. / Chinese Chemical Letters 21 (2010) 620–623
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3. Conclusions A new polymer, polyzirconoxanesal (PZS), has been synthesized and characterized by FTIR and 1H NMR. The polymer exhibits good solubility in common organic solvents. Crystalline ZrC was obtained by pyrolysis of this polymer at 1300 8C under Ar atmosphere. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12]
M.S. Song, B. Huang, M.X. Zhang, et al. J. Cryst. Growth 310 (2008) 4290. Y.S. Won, V.G. Varanasi, O. Kryliouk, et al. J. Cryst. Growth 307 (2007) 302. X.M. Cui, Y.S. Nam, J.Y. Lee, et al. Mater. Lett. 62 (2008) 1961. Q.F. Tong, J.L. Shi, Y.Z. Song, et al. Carbon 42 (2004) 2495. A. Maitre, P. Lefort, Solid State Ionics 104 (1997) 109. Y.J. Yan, Z.R. Huang, S.M. Dong, et al. J. Am. Ceram. Soc. 89 (2006) 3585. M.D. Sacks, C.-A. Wang, Z.H. Yang, et al. J. Mater. Sci. 39 (2004) 6057. H. Preiss, E. Schierhorn, K.W. Brzezinka, J. Mater. Sci. 33 (1998) 4697. Y. Abe, T. Kudo, H. Tonioka, et al. J. Mater. Sci. 33 (1998) 1863. H.Y. Liu, X.Q. Hou, X.Q. Wang, et al. J. Am. Ceram. Soc. 87 (2004) 2237. M. Pan, J.R. Liu, M.K. Lu, et al. Thermochim. Acta 376 (2001) 77. G. Takahiro, Y. Hiroshi, H. Takaaki, et al. Appl. Organomet. Chem. 14 (2000) 119.
Chinese Chemical Letters 21 (2010) 620–623 www.elsevier.com/locate/cclet
One pot synthesis of a soluble polymer for zirconium carbide Xue Yu Tao a,b, Wen Feng Qiu a, Hao Li a, Tong Zhao a,* a
Laboratory of Advanced Polymer Materials, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China b Graduate School of Chinese Academy of Sciences, Beijing 100049, China Received 6 November 2009
Abstract A new polymer, polyzirconoxanesal (PZS), is synthesized from the reaction of zirconium oxychloride octahydrate (ZrOCl28H2O) with acetylacetone (Hacac) and salicyl alcohol (SA) by one-pot protocol. The polymer is soluble in common organic solvents, such as ethanol, methanol, acetone, tetrahydrofuran and chloroform, and exhibits rheology with viscosity of 100– 300 mpa s at 25 8C. These properties are suitable for uses in fabrication of ceramic matrix fiber composites. Pyrolysis of this polymer at 1300 8C in argon provides nanosized ZrC with spherical morphology and size of 20–100 nm. # 2010 Tong Zhao. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved. Keywords: Zirconium carbide; Preceramic polymer; Morphology
Zirconium carbide (ZrC) is an ultra-high temperature ceramic with combined properties of metals and ceramics because it has mixed ionic, covalent and metallic bonding in NaCl-type lattice structure [1]. It has ultrahigh hardness (25 GPa), high melting point (3420 8C), resistance to corrosion and wear, and structure and chemical stabilities [2–4]. These outstanding physical and chemical properties offer ZrC potential applications at high temperatures. Conventional fabrication of ZrC powders is by carbon thermal reduction. The synthesis requires a high temperature because of the relatively large enthalpy for the formation of ZrC from zirconia powders and carbon (Hf = 207.1 kJ/ mol) [5]. Moreover, a longer time is required for the reaction because of the solid reaction with coarse powders of raw materials. The carbothermal reduction usually yields powders with uncontrolled particulate shapes and sizes. This should be another factor that limits the broad applications of this material. The preceramic polymer method is an effective method for low-temperature synthesis of ceramics because of homogeneous atomic distribution of component elements [6]. The conversion of preceramic polymers to ceramics enables the use of processing techniques not attainable with more traditional powder forming methods, particularly in the areas of fiber and film formation and the development of fiber-reinforced ceramic matrix composites with complex shapes. Sacks et al. prepared ZrC and HfC powders using zirconium n-butoxide and hafnium isopropoxide, and polyhyclric alcohol as the carbon sources [7]. Preiss et al. used chelated derivatives of zirconium n-propoxide and various soluble carbon-yielding compounds to prepare ZrC fibers, films and powders [8]. Although ZrC powders and fibers were successfully synthesized using alkoxides, there are still several drawbacks such as high cost and toxicity.
* Corresponding author. E-mail address: [email protected] (T. Zhao). 1001-8417/$ – see front matter # 2010 Tong Zhao. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved. doi:10.1016/j.cclet.2010.01.002
X.Y. Tao et al. / Chinese Chemical Letters 21 (2010) 620–623
621
Scheme 1. .
Acetylacetone has been used to react with zirconium oxychloride to synthesize polyzirconoxanes (PZOs) as precursors for preparing zirconia (ZrO2) fibers [9–11]. Acetylacetone is realized to be an effective ligand chelated to zirconium to form stable sols against both precipitation and hydrolysis [9–11]. Ethylene glycol (EG) was reported to react with zirconium oxychloride to synthesize EG-PZO [12]. We found that salicyl alcohol also contains active hydrogen atoms and it should be able to react with zirconium oxychloride. However, the reaction of salicyl alcohol with zirconium oxychloride has not been reported in the literature. Therefore, here, we use inorganic zirconium oxychloride as zirconium source, both acetylacetone and salicyl alcohol as ligands to synthesize a new precursor for ZrC; and its characterization, properties and pyrolytic behavior were also investigated. 1. Experimental The synthesis route is according to Scheme 1. 6.44 g (0.02 mol) of zirconium oxychloride octahydrate (ZrOCl28H2O), acetylacetone (Hacac) and salicyl alcohol (SA) and 4.1 g (0.04 mol) triethylamine was added into 100 mL of methanol dropwise below 5 8C at molar ratios of Et3N/ZOC = 2.0. The reaction mixture was stirred at room temperature for 4 h and then concentrated in vacuo to get a white solid. THF 50 mL was added, the precipitate was filtrated, and the filtrate was concentrated to highly viscous solution. Reprecipitation of the solution with hexane, followed by drying in vacuo for 3 h, gave the product as a yellow powder. The dried polymers were pyrolyzed in Ar at 1300 8C for 2 h using heating rate 3 8C/min, cooled at 5 8C/min. Fourier transform infrared spectra (FTIR) were recorded between 4000 and 400 cm 1 on Bruker Tensor 27 spectrometer. 1H NMR spectra were recorded in methanol-d4 solvent with a Bruker AV400 spectrometer. X-ray diffraction (XRD) measurements were performed on the pyrolyzed samples with a powder diffractometer (Rigaku D/ M4X 2500) using a Cu/Ka radiation. The morphology of the samples was examined using scanning electron microscope (SEM, S-4800). 2. Results and discussion ZrOCl28H2O reacts with acetylacetone and salicyl alcohol in the presence of triethylamine. Triethylamine promotes the reaction by removing its hydrogen chloride salt. The product was isolated as an air-stable yellow solid, PZS. 1H NMR (400 MHz, CD3OD): d 6.70–7.25(m, 4H, phenyl ring H), 4.55(m, 2H, CH2OH in phenyl ring), 3.1(s, 1H, CH), 1.2(s, 3H, CH3CO enol form), 2.1(m, 3H, CH3CO keto form). IR (n, cm 1): 3062, 2923, 2862, 1590, 1529, 1483, 1452, 1278, 1022 and 543. The molecular weight of the as-synthesized polymer is about 700–1000. According to the results of FTIR, 1H NMR and GPC, PZS might be a linear Zr–O–Zr chain polymer, with acetylacetone and salicyl alcohol as ligands. This polymer structure has not been reported in the literature up to now. There are several advantages on the used process compared with the literature method. First, inorganic zirconium oxychloride was used as zirconium source, it is cheap and less toxic than zirconium alkoxides; second, the polymer PZS was prepared by the facile one-pot protocol and subsequent hydrolysis, condensation steps was unnecessary. Last, the synthesized polymer exhibited good solubility in common organic solvents, such as ethanol, methanol, acetone, tetrahydrofuran and chloroform. When this precursor was dissolved in ethanol solvent, the viscosity is 100–300 mpa s at 25 8C. This is useful for preparing Cf/ZrC ceramic matrix composites (CMCs) via polymer infiltration and pyrolysis (PIP) process.
622
X.Y. Tao et al. / Chinese Chemical Letters 21 (2010) 620–623
Fig. 1. XRD pattern of the polymer pyrolyzed at 1300 8C for 2 h.
To study the conversion process from PZS to the ceramics, we performed a series of pyrolysis trials in tube furnace and TG experiments. The TGA curve of PZS indicated that PZS suffers a three-step mass loss: room temperature to 600 8C, 600–1100 8C, and from 1100 to 1375 8C. The weight loss below 600 8C was due to the bonded water and the decomposition of organic groups; and no further weight loss was detected between 600 and 1100 8C. After 1100 8C, the weight loss accelerates again and does not stabilize up to 1375 8C, which is believed to be the carbothermal reduction process. The curve also suggested that the polymer decomposed completely at 600 8C and ceramic yield is 68.5% at 1300 8C. Fig. 1 shows XRD pattern of ZrC after pyrolysis of PZS in Ar at 1300 8C. The XRD pattern is in agreement with the typical structure ZrC diffraction (JCPDS No. 35-0784). Sharp diffraction peaks indicate good crystallinity of ZrC nanoparticles and no characteristic peak related to any impurity was observed. This temperature of forming ZrC is much lower than the conventional solid state synthesis method (1800 8C). To the best of our knowledge, this is the lowest temperature reported in the literatures for preparing ZrC ceramics from preceramic polymers. Quantitative phase analysis reveals that the ZrC content reaches 91 wt%. The morphology and size of the as-prepared ZrC sample were investigated using SEM (Fig. 2). SEM results indicated that three-dimensional nanoscale ZrC particles were obtained although with some agglomerations. The particle sizes distribute over 20–100 nm with sphere structure and the average size is 45 nm, which is in good agreement with that calculated from the XRD pattern by Scherrer’s formula.
Fig. 2. Scanning electron microscopy image of the synthesized ZrC powders at 1300 8C for 2 h.
X.Y. Tao et al. / Chinese Chemical Letters 21 (2010) 620–623
623
3. Conclusions A new polymer, polyzirconoxanesal (PZS), has been synthesized and characterized by FTIR and 1H NMR. The polymer exhibits good solubility in common organic solvents. Crystalline ZrC was obtained by pyrolysis of this polymer at 1300 8C under Ar atmosphere. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12]
M.S. Song, B. Huang, M.X. Zhang, et al. J. Cryst. Growth 310 (2008) 4290. Y.S. Won, V.G. Varanasi, O. Kryliouk, et al. J. Cryst. Growth 307 (2007) 302. X.M. Cui, Y.S. Nam, J.Y. Lee, et al. Mater. Lett. 62 (2008) 1961. Q.F. Tong, J.L. Shi, Y.Z. Song, et al. Carbon 42 (2004) 2495. A. Maitre, P. Lefort, Solid State Ionics 104 (1997) 109. Y.J. Yan, Z.R. Huang, S.M. Dong, et al. J. Am. Ceram. Soc. 89 (2006) 3585. M.D. Sacks, C.-A. Wang, Z.H. Yang, et al. J. Mater. Sci. 39 (2004) 6057. H. Preiss, E. Schierhorn, K.W. Brzezinka, J. Mater. Sci. 33 (1998) 4697. Y. Abe, T. Kudo, H. Tonioka, et al. J. Mater. Sci. 33 (1998) 1863. H.Y. Liu, X.Q. Hou, X.Q. Wang, et al. J. Am. Ceram. Soc. 87 (2004) 2237. M. Pan, J.R. Liu, M.K. Lu, et al. Thermochim. Acta 376 (2001) 77. G. Takahiro, Y. Hiroshi, H. Takaaki, et al. Appl. Organomet. Chem. 14 (2000) 119.