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In-situ synthesis and growth mechanism of silicon nitride nanowires on carbon fiber fabrics
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In-situ synthesis and growth mechanism of silicon nitride nanowires on carbon fiber fabrics Kaiyuan Li, Ke Zhao, Yiguang Wang
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Cite this article as: Kaiyuan Li, Ke Zhao, Yiguang Wang, In-situ synthesis and growth mechanism of silicon nitride nanowires on carbon fiber fabrics, Ceramics International, http://dx.doi.org/10.1016/j.ceramint.2014.05.058 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
In-situ Synthesis and Growth Mechanism of Silicon Nitride Nanowires on Carbon Fiber Fabrics Kaiyuan Li, Ke Zhao, Yiguang Wang* Science and Technology on Thermostructural Composite Materials Laboratory, Northwestern Polytechnical University, Xi’an, Shaanxi 710072, P. R. China *
email: [email protected]
Abstract: Silicon nitride (Si3N4) nanowires were in-situ fabricated on carbon fiber fabrics up to 5 cm × 11 cm in area via catalyst-assisted pyrolysis of polymeric precursors. The obtained products were randomly oriented around the carbon fibers with the diameter of 30-150 nm and the length of several hundred micrometers. The effects of process parameters including the preparation temperature, flow rate of nitrogen, catalyst proportion, and volume fraction of acetone on the synthesis of Si3N4 nanowires were discussed. Accordingly, a set of optimized process parameters was determined. The microstructure of the nanowires indicated that their formation obeyed a solid-liquid-gas-solid (SLGS) growth mechanism.
Keywords: Si3N4 nanowire; Carbon fiber fabric; In-situ synthesis; Growth mechanism
* Author to whom correspondence should be addressed, Email: [email protected] (Y. Wang); Phone: +86-29-88494914, Fax: +86-29-88494620.
1. Introduction Continuous fiber-reinforced ceramic matrix composites (CFCMCs) have been considered as advanced high-temperature structural materials in aerospace and energy industry, due to their non-brittle mechanical behavior, high strength, low density, great thermal stability, and excellent abrasion resistance [1,2]. Chemical vapor infiltration (CVI) is a typical commercialized process and one of the most promising techniques to fabricate the CFCMCs. Its major advantages are the low processing temperature,
excellent
mechanical
behavior
achieved
during
the
process,
near-net-shape fabrication without extensive post-processing machining and the ability of fabricating complex structural composite [3,4]. However, time-consuming densification process is a main disadvantage of CVI technique, which makes the CFCMCs costly [5,6].. In addition, the low matrix cracking stress of many CFCMCs limits their application in high-load corrosive environments, for example the hot-section components of gas-turbine engines which work under the coupled conditions of high temperatures, complex loading, long working hours and oxidation [7]. In order to fabricate high cracking-stress CFCMCs by the CVI technique, different short fibers/whiskers were firstly introduced into continuous fiber preforms as matrix reinforcements. The short fibers/whiskers could also provide extra surfaces for accelerating matrix deposition [8-12]. However, alternative reinforcement growing and matrix deposition were required in some researches, since the lengths of these
short fibers/whiskers were limited. This led to a longer and more complex preparation process instead of accelerating it. Moreover, the mechanical properties of as-prepared CFCMCs were not obviously enhanced due to either a low volume fraction of matrix reinforcements, or an earlier clogging of composites caused by the large diameters and high number density of short fibers/whiskers. One-dimensional nanostructures appear several amazing characteristics such as high aspect ratio, large surface area to volume ratio, flexibility in surface functionalities, and superior mechanical performance compared with any other known form of the materials [13-15]. These outstanding properties make them be optimal candidates for the rapid densification and matrix enhancement of CFCMCs [16-22]. Among all the one-dimensional nanostructures, the Si3N4 nanowire is a typical and important one, which has been studied extensively in recent years owing to its low-cost synthesis and excellent comprehensive performance [23-27]. Although many methods were developed to synthesize Si3N4 nanowires on planar substrates, their growth on fiber fabrics for the many-sided improvements of advanced structural composites is rarely studied. In this work, Si3N4 nanowires were in-situ grown on large-scale carbon fiber fabrics by pyrolysis of polyureasilazane (PSN) solution with ferrocene catalyst. This method is a simple, stable and cost-efficient technique to fabricate ceramic nanowires. The effects of preparation temperature, flow rate of nitrogen, catalyst proportion and volume fraction of acetone on the generation, composition, microstructure and yield of the nanowires were investigated, respectively. The optimized process parameters were then chosen to precisely control the morphology, component and number density
of the nanowires synthesized on the fabrics, so that we can lay a solid foundation for the future study of Si3N4 nanowire-modified CFCMCs. Besides, the detailed growth mechanism of these nanowires was also discussed.
2. Experimental procedure The 2D T-300TM woven carbon fiber fabrics (HTA-1000, TOHO, Japan) were firstly cut into the 5 cm × 11 cm substrates. A suspension composed of the commercially available PSN (Institute of Chemistry, Chinese Academy of Sciences, Beijing, China), ferrocene (Fuchen Chemical Reagent Factory, Tianjin, China) and acetone was then uniformly mixed for 3 h by a magnetic stirrer. The mass ratio of PSN to ferrocene ranged from 97:3 to 90:10, and the volume fraction of acetone varied from 33% to 56%. The substrate was fully immerged into the suspension for 3 h, followed by a curing process from room temperature (RT) to 300 °C at 3 °C/min in a horizontal tube furnace. After that, the as-prepared fabric was heated to a specified temperature (1200-1400 °C) at 5 °C/min, and then the Si3N4 nanowires were synthesized at this temperature within 3 h. The sample was naturally cooled down to room temperature at last. All these heat treatments were performed under the protection of flowing ultra-high purity nitrogen (N2). The flow rates of N2 varied from 10 sccm (standard cubic centimeters per minute) to 60 sccm. All the adopted parameters were listed in Table 1. The microstructure and composition of carbon fiber fabrics and pyrolysis products were characterized using scanning electron microscopy (SEM, JEOL 6700F,
Tokyo, Japan) and transmission electron microscope (TEM, FEI TECNAI G20, Hillsboro, OR, USA) both equipped with energy-dispersive spectrum (EDS). All X-ray diffraction (XRD) measurements were taken by a Philipps X’Pert PRO X-ray diffraction system (Cu Kα radiation, 0.15406nm) operated at 40 mA, 40 kV and 0.02 °/s.
3. Results and discussion 3.1. Characterization of Si3N4 nanowires Within the parameter space established by Table 1, a set of parameters (1400 °C preparation temperature, 60 sccm N2 flow rate, 93:7 PSN-to-ferrocene mass ratio, and 43% acetone volume fraction) is considered to be ideal for the fabrication of Si3N4 nanowires on carbon fiber fabrics. The macrostructure, microstructure and composition of obtained nanowires are shown in Fig. 1-3, respectively. The detailed optimization processes of these parameters will be discussed in the next part. Figure 1 shows a photograph containing original and as-prepared carbon fiber fabrics, indicating that the primitive fabric is covered by a white layer after the growth process. From Fig. 2b, it can be seen that a relatively loose network is built up by many randomly oriented nanowires with the length of several hundred micrometers on the uneven surface of stacked carbon fibers (Fig. 2a). Both straight and highly curved nanowires were observed. Although the diameters of these nanowires ranged from 30 nm to 150 nm, the diameter of individual nanowire was usually constant along its entire length. The observations also indicated that the surfaces and tops of all
the nanowires were smooth and clean without any attached particles. As can be seen in Fig. 2c, the nanowire network with a thickness up to 200 µm is fabricated on both sides of the carbon fiber fabric. No nanowires were found in the carbon fiber bundles (Fig. 2d). The XRD pattern of the powders ground from the nanowires (Fig. 3) indicates that the synthesized products are pure crystal α-Si3N4.
3.2. Effects of process parameters The effects of preparation temperatures on the generation of Si3N4 nanowires are revealed in Fig. 4. At 1200 °C, the PSN precursor was sufficiently pyrolyzed into many irregular Si-C-N ceramic pieces on the fabrics (Fig. 4a) [28]. No nanowires were found on the surfaces of ceramic pieces and carbon fibers below this temperature. Although the carbon fiber fabrics were still mainly covered by Si-C-N pieces at 1300 °C, the nanowires began to grow from some liquid droplets (Fig. 4b and c). When the temperature reached to 1400 °C, the ceramic pieces were fully converted to Si3N4 nanostructures with the length of a few tens of micrometers (Fig. 4d). It is believed that 1400 °C is a suitable and relatively low preparation temperature for the nanowire growth. In order to study the effect of N2 flow rate on nanowire growth, contrast experiments were conducted at a lower N2 flow rate of 10 sccm. Under such a condition, a grey layer was fabricated on the carbon fiber fabric. Figure 5a and b show that the grey layer is composed of many large-size rods with attached particles on their tops. These rods were several hundred micrometers in length and 500-1000
nanometers in diameter. EDS pattern in Fig. 5c reveals that Si, C and N can be observed in the rod. However, in the attached particle, except for Si, C and N, Fe which came from the ferrocene catalyst was also found (Fig. 5d), indicating that the Si-C-N rods grew through a typical vapor-liquid-solids (VLS) mechanism. Based on the published literatures [29-31], it is believed that, at the low partial pressure of N2, the Si-C-N ceramic could react with residual oxygen in the tube furnace at high temperatures, producing SiO, CO and N2 gases. These gases could be absorbed by the liquid Fe-Si-C-N droplets at the vapor-liquid interface to form a liquid phase supersaturated with Si, C and N. Then the solid Si-C-N rod would be continuously precipitated from the liquid-solid interface at the bottom of liquid droplet. The present study suggests that high N2 flow rate (60 sccm) should be used during the synthesis of Si3N4 nanowires. According to the previous studies [13,15], the morphologies of different nanostructures fabricated using catalysts are closely related to the amounts and sizes of liquid phases formed at high temperatures. In this work, the type of catalyst and preparation temperature were settled, so that the catalyst proportion in the PSN solution became an important variable factor for the precise control of Si3N4 nanowire microstructure. Figure 6 indicates that as the PSN-to-ferrocene mass ratio increases, the morphologies of obtained products on the fabrics change a lot. At the mass ratio of 97:3, some Si-C-N ceramic pieces were not sufficiently converted into Si3N4 wires, due to the lack of liquid alloy droplets formed via the reaction between ferrocene and Si-C-N ceramic (Fig. 6a). As shown in Fig. 6b, no residual Si-C-N can be found;
desired Si3N4 nanowires with small diameters and long lengths are fully grown around the carbon fibers, suggesting that the 93:7 PSN-to-ferrocene mass ratio is appropriate for nanowire synthesis. When the mass ratio reached to 90:10, Si3N4 nanobelts were obtained, which were 40-80 nm in thickness, 400-4000 nm in width, and a few hundred micrometers in length (Fig. 6c-e). No particles were found on the tops of nanobelts either. The formation of Si3N4 nanobelts instead of nanowires at this mass ratio is attributed to the size increase of liquid droplet [32]. During the CVI process, earlier clogging on the surface of CFCMCs leads to low density and high porosity, which are harmful for a composite, especially its mechanical property [33]. Accordingly, although higher yield of Si3N4 nanowires may offer more deposition surfaces and reinforcements for the matrix, the number density of nanowires should be controlled within a suitable range in order to avoid an over-quick densification on the surface of CFCMCs. As shown in Fig. 7a, the yield of Si3N4 nanowires decreases as the volume fraction of acetone increases. The 43% acetone volume fraction is considered to be appropriate for the synthesis of Si3N4 nanowires, since at this volume fraction a relatively loose network was formed by adequate nanowires (Fig. 2b and c). At a lower volume fraction (33%), the obtained nanowires were so dense that the earlier clogging may happen on the surface of CFCMCs (Fig. 7b and c); at a higher volume fraction (56%), the synthesized Si3N4 nanostructures were too few and short to influence the CVI process and mechanical property of CFCMCs (Fig. 7d and e). In facts, the reasonable volume fraction of acetone can be chosen in a certain range according to the requirements. The results
indicate that a designable Si3N4 nanowire network is achieved in this work, which is very helpful for the fabrication of high-performance CFCMCs in a short time.
3.3. Growth mechanism of Si3N4 nanowires As mentioned previously, there were no particles on the tops of Si3N4 nanowires and nonobelts, and the diameter of individual nanowire was usually constant. Although some Si3N4 nanowires with cone-shaped tips were observed (Fig. 8a), the EDS pattern (Fig. 8b) indicates that the cone-shaped tips only contain Si and N as well. The formation of cone-shaped tips may be caused by the cone-shaped liquid droplets formed between the adjacent two carbon fibers. At the beginning of the nanowire synthesis, a gradually shrinking growth of Si3N4 firstly happened along the length direction in the cone-shaped liquid droplets, followed by a continuously homogeneous growth. Thus, the Si3N4 nanowires with cone-shaped tips were finally obtained. Compared with the Si-C-N rods with Fe-containing spherical caps (Fig. 5b and d), which were synthesized via VLS mechanism, the microstructures of Si3N4 nanowires were different. In addition, the Si-containing vapor phase was absent during the whole nanowire growth under the protection of N2 gas. So that, the VLS mechanism, which is widely used for the explanation of nanostructure growth, is not applicable in the current study of Si3N4 nanowires. In order to propose a reasonable growth mechanism, the roots of Si3N4 nanowires were observed by TEM. From Fig. 8c, it can be seen that a nanowire directly grew from a broken particle at its root. The EDS pattern (Fig. 8d) reveals that
the particle contains Fe, Si, C, and N. Based on the observations and analyses mentioned above, a detailed SLGS growth model of Si3N4 nanowires synthesized on carbon fiber fabrics was established (Fig. 9). The mixed PSN solution containing ferrocene catalysts was firstly solidified at ~200-300 °C and then decomposed to Si-C-N ceramic at ~1000 °C [34]. The Si-C-N ceramic was separated into many irregular pieces during the pyrolysis process, since the substance loss and volume shrinkage happened at high temperature [35]. At ~1300 °C, the Si-C-N pieces started to react with the Fe-containing catalysts to form liquid Fe-Si-C-N alloy droplets. The adjacent small liquid droplets tended to aggregate together, so that larger liquid droplets with various sizes were generated. That is the main cause of variable Si3N4 nanowire diameters. A few short nanowires were obtained in some areas at this temperature, because of the local heat or N2 concentration. Under the optimized conditions (1400 °C preparation temperature, 60 sccm N2 flow rate, 93:7 PSN-to-ferrocene mass ratio, and 43% acetone volume fraction), Si3N4 is the most stable phase. The chemical reactions between the liquid Fe-Si-C-N alloy and N2 happened at the liquid-gas interface to form Si3N4, and meanwhile free carbon was fabricated [24]. At the same time, Si, C and N in the liquid alloy droplets were continuously provided by the Si-C-N ceramic via the solid-liquid interface. When the Si3N4 phase was supersaturated in the liquid alloy, it nucleated and crystallized in a row from the liquid. Due to the size-confining effect of liquid phase on the growth of crystalline phase, the Si3N4 can only grow along length direction into a one-dimensional nanostructure. Although many nanowires kept
growing straightly since the beginning, some highly curved ones were also fabricated owing to the fluctuations of surrounding environment. After 3 h growth, the Si-C-N ceramic pieces were exhausted. As a result, the whole process ended with sufficiently in-situ growth of Si3N4 nanowires on carbon fiber fabrics.
4. Conclusions In summary, α-Si3N4 nanowires were in-situ synthesized via catalyst-assisted pyrolysis of a polymer precursor on large-scale carbon fiber fabrics. The effects of preparation temperature, flow rate of nitrogen, mass ratio of PSN to ferrocene and volume fraction of acetone on the generation, composition, microstructure and yield of the nanowires were investigated, respectively. Using the optimized parameters (1400 °C preparation temperature, 60 sccm N2 flow rate, 93:7 PSN-to-ferrocene mass ratio, and 43% acetone volume fraction), a relatively loose network was formed by randomly oriented Si3N4 nanowires which were 30-150 nm in diameter and hundred micrometers in length. The analyses indicated that the Si3N4 nanowire network was designable. A detailed SLGS growth model was established for a better understanding of Si3N4 nanowire synthesis on the fabrics. Acknowledgments This work was financially supported by the National Nature Science Found of China (51172181, 51372202, 51272210 and 50902112), Research Fund of State Key Laboratory of Solidification Processing (82-TZ-2013), Program for New Century Excellent Talents in University, and “111” Project (B08040).
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Table caption: Table 1. Variable process parameters for study of Si3N4 nanowires growth on carbon fiber fabrics.
Figure captions: Figure 1. Photograph of carbon fiber fabrics before (top) and after (bottom) synthesis of Si3N4 nanowires. Figure 2. Microstructure of Si3N4 nanowires grown on carbon fiber fabrics. (a) SEM image of an original carbon fiber fabric. (b), (c) and (d) are SEM images of carbon fiber fabrics covered by Si3N4 nanowires. (b) Surface. (c) Cross-section. (d) Partially enlarged morphology of (c). Figure 3. XRD pattern of Si3N4 nanowires. Figure 4. SEM images of various products obtained on carbon fiber fabrics at different preparation temperatures. (a) At 1200 ºC. (b) and (c) At 1300 ºC. (d) At 1400 ºC. (60 sccm N2 flow rate, 90:10 PSN-to-ferrocene mass ratio, and 50% acetone volume fraction) Figure 5. Surface morphology of a carbon fiber fabric covered by Si-C-N nanorods (a) and partially enlarged morphology of a single Si-C-N nanorod (b) at a lower N2 flow rate of 10 sccm. (c) and (d) are EDS patterns of the middle and tip of the nanorod in (b), respectively. (1400 °C preparation temperature, 90:10 PSN-to-ferrocene mass ratio, and 50% acetone volume fraction)
Figure 6. SEM and TEM images of various products obtained on carbon fiber fabrics at different PSN-to-ferrocene mass ratios. (a) At 97:3. (b) At 93:7. (c), (d) and (e) At 90:10. (1400 °C preparation temperature, 60 sccm N2 flow rate, and 43% acetone volume fraction) Figure 7. Effect of volume fraction of acetone on the yield of Si3N4 nanowires grown on carbon fiber fabrics. (a) Weight gain rate of carbon fiber fabrics as a function of volume fraction of acetone. (b), (c), (d) and (e) are the morphologies of Si3N4 nanowires obtained at different acetone volume fractions. (b) and (c) At 33%. (d) and (e) At 56%. (1400 °C preparation temperature, 60 sccm N2 flow rate, and 93:7 PSN-to-ferrocene mass ratio) Figure 8. (a) SEM image of the tip of a single Si3N4 nanowire and its EDS pattern (b). (c) TEM image of the root of a single Si3N4 and its EDS pattern (d). Figure 9. Schematic diagram of the SLGS growth model of Si3N4 nanowires synthesized on carbon fiber fabrics.
Table 1. Variable process parameters for study of Si3N4 nanowires growth on carbon fiber fabrics.
Figure 1. Photograph of carbon fiber fabrics before (top) and after (bottom) synthesis of Si3N4 nanowires.
Figure 2. Microstructure of Si3N4 nanowires grown on carbon fiber fabrics. (a) SEM image of an original carbon fiber fabric. (b), (c) and (d) are SEM images of carbon fiber fabrics covered by Si3N4 nanowires. (b) Surface. (c) Cross-section. (d) Partially enlarged morphology of (c).
Figure 3. XRD pattern of Si3N4 nanowires.
Figure 4. SEM images of various products obtained on carbon fiber fabrics at different preparation temperatures. (a) At 1200 ºC. (b) and (c) At 1300 ºC. (d) At 1400 ºC. (60 sccm N2 flow rate, 90:10 PSN-to-ferrocene mass ratio, and 50% acetone volume fraction)
Figure 5. Surface morphology of a carbon fiber fabric covered by Si-C-N nanorods (a) and partially enlarged morphology of a single Si-C-N nanorod (b) at a lower N2 flow rate of 10 sccm. (c) and (d) are EDS patterns of the middle and tip of the nanorod in (b), respectively. (1400 °C preparation temperature, 90:10 PSN-to-ferrocene mass ratio, and 50% acetone volume fraction)
Figure 6. SEM and TEM images of various products obtained on carbon fiber fabrics at different PSN-to-ferrocene mass ratios. (a) At 97:3. (b) At 93:7. (c), (d) and (e) At 90:10. (1400 °C preparation temperature, 60 sccm N2 flow rate, and 43% acetone volume fraction)
Figure 7. Effect of volume fraction of acetone on the yield of Si3N4 nanowires grown on carbon fiber fabrics. (a) Weight gain rate of carbon fiber fabrics as a function of volume fraction of acetone. (b), (c), (d) and (e) are the morphologies of Si3N4 nanowires obtained at different acetone volume fractions. (b) and (c) At 33%. (d) and (e) At 56%. (1400 °C preparation temperature, 60 sccm N2 flow rate, and 93:7 PSN-to-ferrocene mass ratio)
Figure 8. (a) SEM image of the tip of a single Si3N4 nanowire and its EDS pattern (b). (c) TEM image of the root of a single Si3N4 and its EDS pattern (d).
Figure 9. Schematic diagram of the SLGS growth model of Si3N4 nanowires synthesized on carbon fiber fabrics.
In-situ synthesis and growth mechanism of silicon nitride nanowires on carbon fiber fabrics Kaiyuan Li, Ke Zhao, Yiguang Wang
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PII: DOI: Reference:
S0272-8842(14)00781-0 http://dx.doi.org/10.1016/j.ceramint.2014.05.058 CERI8594
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Received date: Revised date: Accepted date:
27 February 2014 11 May 2014 12 May 2014
Cite this article as: Kaiyuan Li, Ke Zhao, Yiguang Wang, In-situ synthesis and growth mechanism of silicon nitride nanowires on carbon fiber fabrics, Ceramics International, http://dx.doi.org/10.1016/j.ceramint.2014.05.058 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
In-situ Synthesis and Growth Mechanism of Silicon Nitride Nanowires on Carbon Fiber Fabrics Kaiyuan Li, Ke Zhao, Yiguang Wang* Science and Technology on Thermostructural Composite Materials Laboratory, Northwestern Polytechnical University, Xi’an, Shaanxi 710072, P. R. China *
email: [email protected]
Abstract: Silicon nitride (Si3N4) nanowires were in-situ fabricated on carbon fiber fabrics up to 5 cm × 11 cm in area via catalyst-assisted pyrolysis of polymeric precursors. The obtained products were randomly oriented around the carbon fibers with the diameter of 30-150 nm and the length of several hundred micrometers. The effects of process parameters including the preparation temperature, flow rate of nitrogen, catalyst proportion, and volume fraction of acetone on the synthesis of Si3N4 nanowires were discussed. Accordingly, a set of optimized process parameters was determined. The microstructure of the nanowires indicated that their formation obeyed a solid-liquid-gas-solid (SLGS) growth mechanism.
Keywords: Si3N4 nanowire; Carbon fiber fabric; In-situ synthesis; Growth mechanism
* Author to whom correspondence should be addressed, Email: [email protected] (Y. Wang); Phone: +86-29-88494914, Fax: +86-29-88494620.
1. Introduction Continuous fiber-reinforced ceramic matrix composites (CFCMCs) have been considered as advanced high-temperature structural materials in aerospace and energy industry, due to their non-brittle mechanical behavior, high strength, low density, great thermal stability, and excellent abrasion resistance [1,2]. Chemical vapor infiltration (CVI) is a typical commercialized process and one of the most promising techniques to fabricate the CFCMCs. Its major advantages are the low processing temperature,
excellent
mechanical
behavior
achieved
during
the
process,
near-net-shape fabrication without extensive post-processing machining and the ability of fabricating complex structural composite [3,4]. However, time-consuming densification process is a main disadvantage of CVI technique, which makes the CFCMCs costly [5,6].. In addition, the low matrix cracking stress of many CFCMCs limits their application in high-load corrosive environments, for example the hot-section components of gas-turbine engines which work under the coupled conditions of high temperatures, complex loading, long working hours and oxidation [7]. In order to fabricate high cracking-stress CFCMCs by the CVI technique, different short fibers/whiskers were firstly introduced into continuous fiber preforms as matrix reinforcements. The short fibers/whiskers could also provide extra surfaces for accelerating matrix deposition [8-12]. However, alternative reinforcement growing and matrix deposition were required in some researches, since the lengths of these
short fibers/whiskers were limited. This led to a longer and more complex preparation process instead of accelerating it. Moreover, the mechanical properties of as-prepared CFCMCs were not obviously enhanced due to either a low volume fraction of matrix reinforcements, or an earlier clogging of composites caused by the large diameters and high number density of short fibers/whiskers. One-dimensional nanostructures appear several amazing characteristics such as high aspect ratio, large surface area to volume ratio, flexibility in surface functionalities, and superior mechanical performance compared with any other known form of the materials [13-15]. These outstanding properties make them be optimal candidates for the rapid densification and matrix enhancement of CFCMCs [16-22]. Among all the one-dimensional nanostructures, the Si3N4 nanowire is a typical and important one, which has been studied extensively in recent years owing to its low-cost synthesis and excellent comprehensive performance [23-27]. Although many methods were developed to synthesize Si3N4 nanowires on planar substrates, their growth on fiber fabrics for the many-sided improvements of advanced structural composites is rarely studied. In this work, Si3N4 nanowires were in-situ grown on large-scale carbon fiber fabrics by pyrolysis of polyureasilazane (PSN) solution with ferrocene catalyst. This method is a simple, stable and cost-efficient technique to fabricate ceramic nanowires. The effects of preparation temperature, flow rate of nitrogen, catalyst proportion and volume fraction of acetone on the generation, composition, microstructure and yield of the nanowires were investigated, respectively. The optimized process parameters were then chosen to precisely control the morphology, component and number density
of the nanowires synthesized on the fabrics, so that we can lay a solid foundation for the future study of Si3N4 nanowire-modified CFCMCs. Besides, the detailed growth mechanism of these nanowires was also discussed.
2. Experimental procedure The 2D T-300TM woven carbon fiber fabrics (HTA-1000, TOHO, Japan) were firstly cut into the 5 cm × 11 cm substrates. A suspension composed of the commercially available PSN (Institute of Chemistry, Chinese Academy of Sciences, Beijing, China), ferrocene (Fuchen Chemical Reagent Factory, Tianjin, China) and acetone was then uniformly mixed for 3 h by a magnetic stirrer. The mass ratio of PSN to ferrocene ranged from 97:3 to 90:10, and the volume fraction of acetone varied from 33% to 56%. The substrate was fully immerged into the suspension for 3 h, followed by a curing process from room temperature (RT) to 300 °C at 3 °C/min in a horizontal tube furnace. After that, the as-prepared fabric was heated to a specified temperature (1200-1400 °C) at 5 °C/min, and then the Si3N4 nanowires were synthesized at this temperature within 3 h. The sample was naturally cooled down to room temperature at last. All these heat treatments were performed under the protection of flowing ultra-high purity nitrogen (N2). The flow rates of N2 varied from 10 sccm (standard cubic centimeters per minute) to 60 sccm. All the adopted parameters were listed in Table 1. The microstructure and composition of carbon fiber fabrics and pyrolysis products were characterized using scanning electron microscopy (SEM, JEOL 6700F,
Tokyo, Japan) and transmission electron microscope (TEM, FEI TECNAI G20, Hillsboro, OR, USA) both equipped with energy-dispersive spectrum (EDS). All X-ray diffraction (XRD) measurements were taken by a Philipps X’Pert PRO X-ray diffraction system (Cu Kα radiation, 0.15406nm) operated at 40 mA, 40 kV and 0.02 °/s.
3. Results and discussion 3.1. Characterization of Si3N4 nanowires Within the parameter space established by Table 1, a set of parameters (1400 °C preparation temperature, 60 sccm N2 flow rate, 93:7 PSN-to-ferrocene mass ratio, and 43% acetone volume fraction) is considered to be ideal for the fabrication of Si3N4 nanowires on carbon fiber fabrics. The macrostructure, microstructure and composition of obtained nanowires are shown in Fig. 1-3, respectively. The detailed optimization processes of these parameters will be discussed in the next part. Figure 1 shows a photograph containing original and as-prepared carbon fiber fabrics, indicating that the primitive fabric is covered by a white layer after the growth process. From Fig. 2b, it can be seen that a relatively loose network is built up by many randomly oriented nanowires with the length of several hundred micrometers on the uneven surface of stacked carbon fibers (Fig. 2a). Both straight and highly curved nanowires were observed. Although the diameters of these nanowires ranged from 30 nm to 150 nm, the diameter of individual nanowire was usually constant along its entire length. The observations also indicated that the surfaces and tops of all
the nanowires were smooth and clean without any attached particles. As can be seen in Fig. 2c, the nanowire network with a thickness up to 200 µm is fabricated on both sides of the carbon fiber fabric. No nanowires were found in the carbon fiber bundles (Fig. 2d). The XRD pattern of the powders ground from the nanowires (Fig. 3) indicates that the synthesized products are pure crystal α-Si3N4.
3.2. Effects of process parameters The effects of preparation temperatures on the generation of Si3N4 nanowires are revealed in Fig. 4. At 1200 °C, the PSN precursor was sufficiently pyrolyzed into many irregular Si-C-N ceramic pieces on the fabrics (Fig. 4a) [28]. No nanowires were found on the surfaces of ceramic pieces and carbon fibers below this temperature. Although the carbon fiber fabrics were still mainly covered by Si-C-N pieces at 1300 °C, the nanowires began to grow from some liquid droplets (Fig. 4b and c). When the temperature reached to 1400 °C, the ceramic pieces were fully converted to Si3N4 nanostructures with the length of a few tens of micrometers (Fig. 4d). It is believed that 1400 °C is a suitable and relatively low preparation temperature for the nanowire growth. In order to study the effect of N2 flow rate on nanowire growth, contrast experiments were conducted at a lower N2 flow rate of 10 sccm. Under such a condition, a grey layer was fabricated on the carbon fiber fabric. Figure 5a and b show that the grey layer is composed of many large-size rods with attached particles on their tops. These rods were several hundred micrometers in length and 500-1000
nanometers in diameter. EDS pattern in Fig. 5c reveals that Si, C and N can be observed in the rod. However, in the attached particle, except for Si, C and N, Fe which came from the ferrocene catalyst was also found (Fig. 5d), indicating that the Si-C-N rods grew through a typical vapor-liquid-solids (VLS) mechanism. Based on the published literatures [29-31], it is believed that, at the low partial pressure of N2, the Si-C-N ceramic could react with residual oxygen in the tube furnace at high temperatures, producing SiO, CO and N2 gases. These gases could be absorbed by the liquid Fe-Si-C-N droplets at the vapor-liquid interface to form a liquid phase supersaturated with Si, C and N. Then the solid Si-C-N rod would be continuously precipitated from the liquid-solid interface at the bottom of liquid droplet. The present study suggests that high N2 flow rate (60 sccm) should be used during the synthesis of Si3N4 nanowires. According to the previous studies [13,15], the morphologies of different nanostructures fabricated using catalysts are closely related to the amounts and sizes of liquid phases formed at high temperatures. In this work, the type of catalyst and preparation temperature were settled, so that the catalyst proportion in the PSN solution became an important variable factor for the precise control of Si3N4 nanowire microstructure. Figure 6 indicates that as the PSN-to-ferrocene mass ratio increases, the morphologies of obtained products on the fabrics change a lot. At the mass ratio of 97:3, some Si-C-N ceramic pieces were not sufficiently converted into Si3N4 wires, due to the lack of liquid alloy droplets formed via the reaction between ferrocene and Si-C-N ceramic (Fig. 6a). As shown in Fig. 6b, no residual Si-C-N can be found;
desired Si3N4 nanowires with small diameters and long lengths are fully grown around the carbon fibers, suggesting that the 93:7 PSN-to-ferrocene mass ratio is appropriate for nanowire synthesis. When the mass ratio reached to 90:10, Si3N4 nanobelts were obtained, which were 40-80 nm in thickness, 400-4000 nm in width, and a few hundred micrometers in length (Fig. 6c-e). No particles were found on the tops of nanobelts either. The formation of Si3N4 nanobelts instead of nanowires at this mass ratio is attributed to the size increase of liquid droplet [32]. During the CVI process, earlier clogging on the surface of CFCMCs leads to low density and high porosity, which are harmful for a composite, especially its mechanical property [33]. Accordingly, although higher yield of Si3N4 nanowires may offer more deposition surfaces and reinforcements for the matrix, the number density of nanowires should be controlled within a suitable range in order to avoid an over-quick densification on the surface of CFCMCs. As shown in Fig. 7a, the yield of Si3N4 nanowires decreases as the volume fraction of acetone increases. The 43% acetone volume fraction is considered to be appropriate for the synthesis of Si3N4 nanowires, since at this volume fraction a relatively loose network was formed by adequate nanowires (Fig. 2b and c). At a lower volume fraction (33%), the obtained nanowires were so dense that the earlier clogging may happen on the surface of CFCMCs (Fig. 7b and c); at a higher volume fraction (56%), the synthesized Si3N4 nanostructures were too few and short to influence the CVI process and mechanical property of CFCMCs (Fig. 7d and e). In facts, the reasonable volume fraction of acetone can be chosen in a certain range according to the requirements. The results
indicate that a designable Si3N4 nanowire network is achieved in this work, which is very helpful for the fabrication of high-performance CFCMCs in a short time.
3.3. Growth mechanism of Si3N4 nanowires As mentioned previously, there were no particles on the tops of Si3N4 nanowires and nonobelts, and the diameter of individual nanowire was usually constant. Although some Si3N4 nanowires with cone-shaped tips were observed (Fig. 8a), the EDS pattern (Fig. 8b) indicates that the cone-shaped tips only contain Si and N as well. The formation of cone-shaped tips may be caused by the cone-shaped liquid droplets formed between the adjacent two carbon fibers. At the beginning of the nanowire synthesis, a gradually shrinking growth of Si3N4 firstly happened along the length direction in the cone-shaped liquid droplets, followed by a continuously homogeneous growth. Thus, the Si3N4 nanowires with cone-shaped tips were finally obtained. Compared with the Si-C-N rods with Fe-containing spherical caps (Fig. 5b and d), which were synthesized via VLS mechanism, the microstructures of Si3N4 nanowires were different. In addition, the Si-containing vapor phase was absent during the whole nanowire growth under the protection of N2 gas. So that, the VLS mechanism, which is widely used for the explanation of nanostructure growth, is not applicable in the current study of Si3N4 nanowires. In order to propose a reasonable growth mechanism, the roots of Si3N4 nanowires were observed by TEM. From Fig. 8c, it can be seen that a nanowire directly grew from a broken particle at its root. The EDS pattern (Fig. 8d) reveals that
the particle contains Fe, Si, C, and N. Based on the observations and analyses mentioned above, a detailed SLGS growth model of Si3N4 nanowires synthesized on carbon fiber fabrics was established (Fig. 9). The mixed PSN solution containing ferrocene catalysts was firstly solidified at ~200-300 °C and then decomposed to Si-C-N ceramic at ~1000 °C [34]. The Si-C-N ceramic was separated into many irregular pieces during the pyrolysis process, since the substance loss and volume shrinkage happened at high temperature [35]. At ~1300 °C, the Si-C-N pieces started to react with the Fe-containing catalysts to form liquid Fe-Si-C-N alloy droplets. The adjacent small liquid droplets tended to aggregate together, so that larger liquid droplets with various sizes were generated. That is the main cause of variable Si3N4 nanowire diameters. A few short nanowires were obtained in some areas at this temperature, because of the local heat or N2 concentration. Under the optimized conditions (1400 °C preparation temperature, 60 sccm N2 flow rate, 93:7 PSN-to-ferrocene mass ratio, and 43% acetone volume fraction), Si3N4 is the most stable phase. The chemical reactions between the liquid Fe-Si-C-N alloy and N2 happened at the liquid-gas interface to form Si3N4, and meanwhile free carbon was fabricated [24]. At the same time, Si, C and N in the liquid alloy droplets were continuously provided by the Si-C-N ceramic via the solid-liquid interface. When the Si3N4 phase was supersaturated in the liquid alloy, it nucleated and crystallized in a row from the liquid. Due to the size-confining effect of liquid phase on the growth of crystalline phase, the Si3N4 can only grow along length direction into a one-dimensional nanostructure. Although many nanowires kept
growing straightly since the beginning, some highly curved ones were also fabricated owing to the fluctuations of surrounding environment. After 3 h growth, the Si-C-N ceramic pieces were exhausted. As a result, the whole process ended with sufficiently in-situ growth of Si3N4 nanowires on carbon fiber fabrics.
4. Conclusions In summary, α-Si3N4 nanowires were in-situ synthesized via catalyst-assisted pyrolysis of a polymer precursor on large-scale carbon fiber fabrics. The effects of preparation temperature, flow rate of nitrogen, mass ratio of PSN to ferrocene and volume fraction of acetone on the generation, composition, microstructure and yield of the nanowires were investigated, respectively. Using the optimized parameters (1400 °C preparation temperature, 60 sccm N2 flow rate, 93:7 PSN-to-ferrocene mass ratio, and 43% acetone volume fraction), a relatively loose network was formed by randomly oriented Si3N4 nanowires which were 30-150 nm in diameter and hundred micrometers in length. The analyses indicated that the Si3N4 nanowire network was designable. A detailed SLGS growth model was established for a better understanding of Si3N4 nanowire synthesis on the fabrics. Acknowledgments This work was financially supported by the National Nature Science Found of China (51172181, 51372202, 51272210 and 50902112), Research Fund of State Key Laboratory of Solidification Processing (82-TZ-2013), Program for New Century Excellent Talents in University, and “111” Project (B08040).
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Table caption: Table 1. Variable process parameters for study of Si3N4 nanowires growth on carbon fiber fabrics.
Figure captions: Figure 1. Photograph of carbon fiber fabrics before (top) and after (bottom) synthesis of Si3N4 nanowires. Figure 2. Microstructure of Si3N4 nanowires grown on carbon fiber fabrics. (a) SEM image of an original carbon fiber fabric. (b), (c) and (d) are SEM images of carbon fiber fabrics covered by Si3N4 nanowires. (b) Surface. (c) Cross-section. (d) Partially enlarged morphology of (c). Figure 3. XRD pattern of Si3N4 nanowires. Figure 4. SEM images of various products obtained on carbon fiber fabrics at different preparation temperatures. (a) At 1200 ºC. (b) and (c) At 1300 ºC. (d) At 1400 ºC. (60 sccm N2 flow rate, 90:10 PSN-to-ferrocene mass ratio, and 50% acetone volume fraction) Figure 5. Surface morphology of a carbon fiber fabric covered by Si-C-N nanorods (a) and partially enlarged morphology of a single Si-C-N nanorod (b) at a lower N2 flow rate of 10 sccm. (c) and (d) are EDS patterns of the middle and tip of the nanorod in (b), respectively. (1400 °C preparation temperature, 90:10 PSN-to-ferrocene mass ratio, and 50% acetone volume fraction)
Figure 6. SEM and TEM images of various products obtained on carbon fiber fabrics at different PSN-to-ferrocene mass ratios. (a) At 97:3. (b) At 93:7. (c), (d) and (e) At 90:10. (1400 °C preparation temperature, 60 sccm N2 flow rate, and 43% acetone volume fraction) Figure 7. Effect of volume fraction of acetone on the yield of Si3N4 nanowires grown on carbon fiber fabrics. (a) Weight gain rate of carbon fiber fabrics as a function of volume fraction of acetone. (b), (c), (d) and (e) are the morphologies of Si3N4 nanowires obtained at different acetone volume fractions. (b) and (c) At 33%. (d) and (e) At 56%. (1400 °C preparation temperature, 60 sccm N2 flow rate, and 93:7 PSN-to-ferrocene mass ratio) Figure 8. (a) SEM image of the tip of a single Si3N4 nanowire and its EDS pattern (b). (c) TEM image of the root of a single Si3N4 and its EDS pattern (d). Figure 9. Schematic diagram of the SLGS growth model of Si3N4 nanowires synthesized on carbon fiber fabrics.
Table 1. Variable process parameters for study of Si3N4 nanowires growth on carbon fiber fabrics.
Figure 1. Photograph of carbon fiber fabrics before (top) and after (bottom) synthesis of Si3N4 nanowires.
Figure 2. Microstructure of Si3N4 nanowires grown on carbon fiber fabrics. (a) SEM image of an original carbon fiber fabric. (b), (c) and (d) are SEM images of carbon fiber fabrics covered by Si3N4 nanowires. (b) Surface. (c) Cross-section. (d) Partially enlarged morphology of (c).
Figure 3. XRD pattern of Si3N4 nanowires.
Figure 4. SEM images of various products obtained on carbon fiber fabrics at different preparation temperatures. (a) At 1200 ºC. (b) and (c) At 1300 ºC. (d) At 1400 ºC. (60 sccm N2 flow rate, 90:10 PSN-to-ferrocene mass ratio, and 50% acetone volume fraction)
Figure 5. Surface morphology of a carbon fiber fabric covered by Si-C-N nanorods (a) and partially enlarged morphology of a single Si-C-N nanorod (b) at a lower N2 flow rate of 10 sccm. (c) and (d) are EDS patterns of the middle and tip of the nanorod in (b), respectively. (1400 °C preparation temperature, 90:10 PSN-to-ferrocene mass ratio, and 50% acetone volume fraction)
Figure 6. SEM and TEM images of various products obtained on carbon fiber fabrics at different PSN-to-ferrocene mass ratios. (a) At 97:3. (b) At 93:7. (c), (d) and (e) At 90:10. (1400 °C preparation temperature, 60 sccm N2 flow rate, and 43% acetone volume fraction)
Figure 7. Effect of volume fraction of acetone on the yield of Si3N4 nanowires grown on carbon fiber fabrics. (a) Weight gain rate of carbon fiber fabrics as a function of volume fraction of acetone. (b), (c), (d) and (e) are the morphologies of Si3N4 nanowires obtained at different acetone volume fractions. (b) and (c) At 33%. (d) and (e) At 56%. (1400 °C preparation temperature, 60 sccm N2 flow rate, and 93:7 PSN-to-ferrocene mass ratio)
Figure 8. (a) SEM image of the tip of a single Si3N4 nanowire and its EDS pattern (b). (c) TEM image of the root of a single Si3N4 and its EDS pattern (d).
Figure 9. Schematic diagram of the SLGS growth model of Si3N4 nanowires synthesized on carbon fiber fabrics.