A Multifunctional Thermal Barrier Application Composite with SiC

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Functional Nanostructured Materials (including low-D carbon)

A Multifunctional Thermal Barrier Application Composite with SiC Nanowires Enhanced Structural Health Monitoring Sensitivity and Interface Performance Liwen Yan, Changqing Hong, Jilei Liu, Bin Du, Shanbao Zhou, Guangdong Zhao, Ping Hu, and Xinghong Zhang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 27 Jul 2018 Downloaded from http://pubs.acs.org on July 27, 2018

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is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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ACS Applied Materials & Interfaces

A Multifunctional Thermal Barrier Application Composite with SiC Nanowires Enhanced Structural Health Monitoring Sensitivity and Interface Performance Liwen Yana ,b, Changqing Hong*a, Jilei Liub, Bin Dua, Shanbao Zhoua, Guangdong Zhaoa, Ping Hu*a and Xinghong Zhang*a a

National Key Laboratory of Science and Technology on Advanced Composites in Special

Environments, Center for Composite Materials and Structures, Harbin Institute of Technology, Harbin, 150080, P. R. China b

School of Physical and Mathematical Sciences, Nanyang Technological University,

Singapore, 637371, Singapore KEYWORDS: Silicon carbide nanowires, Carbon fiber, Structural health monitoring, Multifunctional composites, Interface structure, Thermal barrier, Lightweight

ABSTRACT: Carbon fiber reinforced ceramic composite shows the attractive potential for the next generation thermal protection material due to their outstanding reliability and excellent high-temperature resistance, but are facing great challenges in the combination of the engineering practicality and versatility. Herein, it is demonstrated that silicon carbide nanowires can be grown on the surface of carbon fiber to create a multifunctional thermal barrier application composite. The embedding of the silicon carbide nanowires in the interface of carbon fiber and ceramic matrix significantly increased the structural health monitoring sensitivity and interface strength of the composites. Compared to the conventional CF/ZrC

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composites, the structural health monitoring sensitivity of the composites with SiC nanowires is greatly elevated with a 14-fold improvement. Additional investigations revealed that the multifunctional SiCnws-CF/ZrC nanocomposites enjoyed a low thermal conductivity of 0.49 W/(m·K), a light weight (0.76 - 1.85 g/cm3) and a relative high compressive strength of 23.64 MPa which is favourite in applying as thermal barrier material. Furthermore, the interface design strategy could be extended as a universal method in fabricating various fiber reinforced composites for wide range of other applications.

1. INTRODUCTION Sensors have been widely used in modern daily life due to their extended application such as damage sensing, temperature sensing, pressure sensing, mass sensing, human-motion detection, etc.1-3 Stress or strain in many structures, especially the aerospace structures under external loading need to be detected as the overload might lead to breakdown or collapse of the entire structure.4 However, the relatively large size, complex designs and difficult access to some elements of their structures make the detecting challenging and time-consuming. Structure health monitoring (SHM) has attracted widespread concerns from researchers and many techniques are being applied in SHM based on the piezoelectricity, piezoresisticity, dielectrics, and so on.5-8 The piezoresistive SHM sensors provide greater advantages such as higher sensitivity, cost effectiveness, stable performance using in the engineering sensing application than piezoelectric sensors.9 One of the premises in using the piezoresistive SHM sensors is that the active material must have low enough electrical resistance. In this condition, the carbon fiber is one of the appropriate candidates owing to its outstanding electrical

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conductivity and easy processing into conductive composites. Owing to their excellent mechanical property, light weight, superior chemical stability, good thermal shock resistance, outstanding high temperature tolerance, and low thermal expansion,10-12 carbon fiber (CF) have been widely used as reinforcement materials in polymer- or ceramic-based composites in the past few decades.13,14 Carbon fiber was regarded as preferential reinforcement materials of the composites applied in the fields of aerospace, military, civil engineering, motorsports, and other competition sports equipment.15-17 However, the smooth and sluggish surface of carbon fiber severely restricted the possibility of the compound of CF with more polymer or ceramic matrix, and deteriorate the mechanical or thermal performance of the final composites due to the poor interfacial combination and low compatibility between CF and matrix.18 Therefore, the proper surface treatment of CF and appropriate interface tune in CF reinforced composites has always been pursued by the materials researchers during the half past century. Generally, a high interfacial bonding between the carbon fiber and polymer/ceramic matrix always results in an outstanding comprehensive performance, especially the excellent mechanical properties of the composites.19,20 Therefore, the interface structure between matrix and carbon fiber plays a dominant role in stress and heat transfer in the composites, which is one of the most important factors that must be considered in composite materials design. To achieve the optimized and ideal interface structure of the CF-reinforced composite, some researchers modifying the polymer or ceramic matrix,21,22 and others treated the surface of carbon fiber.23,24 Different technologies, such as chemical grafting,25 chemical vapor reaction routes,26 electrochemical method,27 were developed in modifying the surface of CF to

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promote the interface performance of the carbon fiber reinforced composite materials. Organic groups,27-29 graphene oxide,30,31 carbon black,32 and carbon nanotubes33 were grafted or grown on the surface of carbon fiber to improve the interfacial bonding between CF and matrix. In the carbon fiber reinforced ceramic matrix composites, carbon nanotubes34 and carbon nanofibers35 were also grown on the surface of carbon fiber to adjust the interface performance between CF and matrix and meanwhile increase the versatility of the composites. All the above mentioned reports shown that the carbon nanotubes or carbon nanofibers can bridge the carbon fiber skeleton and the ceramic matrix, which is beneficial to the improvement of the comprehensive properties of the final composites. Recently, Li et al.36 firstly reported the in-situ growth of SiC nanowires (SiCnws) in the carbon-bonded carbon fiber composites (CBCFCs) and found that the mechanical properties of CBCFCs were significantly improved due to the growth of SiC nanowires. Silicon carbide nanowires are widely researched 1D materials with high melting point, high hardness, superior Young’s modulus, outstanding oxidation and corrosion resistance, excellent high temperature strength, good thermal shock resistance and outstanding physical and chemical stability.37-40 However, the in-situ growth of SiC nanowires on the surface of carbon fiber to enhance the structural health monitoring sensitivity and improve the interface performance of the multifunctional thermal barrier composites has not been investigated, to the best of our knowledge. Herein, in order to significantly enhance the sensitivity of compressible strain sensors and improve the interface performance between carbon fiber and the matrix, we reported a facial chemical vapor reaction synthesis of silicon carbide nanowires on the surface of 3-D

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carbon fiber substrate. The typical multiscale SiCnws-CF reinforced ZrC multifunctional thermal barrier composites were fabricated from the pyrolysis of ZrC polymer precursor at 1600 °C after coating around the SiCnws-CF. The growth of SiC nanowires on the surface of carbon fiber effectively increased the structural health monitoring sensitivity of the composites and powerfully bridged the CF and ceramic matrix. Additionally, the results shown that the compressive strength of SiCnws-CF/ZrC composites were significantly increased by 35% due to the growth of SiC nanowires and the thermal conductivity of the SiCnws-CF/ZrC composites were relatively low even in 1000 °C. The strategy of employing SiC nanowires can be further applied to other multifunctional structural health monitoring sensors with different constituent materials. The results in our report also supply deeper understanding as well as other nanoscale interface engineering strategies to boost the fiber-reinforced composites. 2. EXPERIMENTAL SECTION 2.1. Materials Three dimensional carbon fiber (Toray T-300) braid were purchased from Jiangsu Tianniao High Technology Co., Ltd., China. The 3-D carbon fiber braid was constructed by the layer by layer nonwoven carbon fiber cloth in XY direction and long continuous carbon fiber in Z direction to form a fully integrated three dimensional structure. Evenly distributed short continuous carbon fibers were also assembled in Z direction to reinforce the connection between the layers of the nonwoven carbon fiber cloth. The 3-D carbon fiber braid was prepared by a modified needle punching process. Zirconium carbide precursor were supplied by Institute of Process Engineering, Chinese Academy of Sciences, China and synthesized on

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the basis of their previous report41 from ZrCl4, acetyl acetone, methanol, and 1,4-butanediol. Co(NO3)2·6H2O (as catalyst in the synthesis of SiCnws), HNO3, and xylene (solvent of ZrC polymer precursor) were purchased from Aladdin Biochemical Technology Co., Ltd., Shanghai, China. All the chemicals were analytical-grade and used as received. 2.2. Synthesis of SiC nanowires on the 3-D carbon fiber substrate Before growing SiC nanowires, the cubic block (25×25×25 mm) of 3-D carbon fiber braid were pretreated with acetone at 70 °C to clean the surface residue. Then the cleaning carbon fiber braid were surface activation by immersing in the 65% nitric acid solution for 3 h at room temperature. In a typical experiment, cobalt-containing catalyst were loaded on the surface of CF braid by keeping the braid in 0.06 M Co(NO3)2·6H2O aqueous solution for 5 h. The 3-D carbon fiber braid loaded with catalyst was dried in a vacuum at 60 °C overnight. The dry 3-D carbon fiber braid were placed in the reaction chamber of chemical vapor infiltration (CVI) system, which were then heated to 700 °C in hydrogen atmosphere with a flow rate of 20 sccm. The system were kept at 700 °C in hydrogen for 30 min for the reduction of the cobalt ions. Then the chamber was heated to 1000 °C and kept for 1 h with injecting the mixture of 20% H2/CH3SiCl3 to assemble the SiC nanowires on the surface of CF. The targeted 3-D SiCnws-CF architecture were obtained by cooling down the materials to room temperature under the protection of pure Ar gas. 2.3. Fabrication of SiCnws-CF/ZrC multifunctional nanocomposite 100 g of zirconium carbide polymer precursor powder was dissolved in xylene by continuous ultrasonic assistant stirring for about 8 h to form a uniform solution with mass fraction of 65%. The 3-D SiCnws-CF architectures were immersed in the ZrC polymer

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precursor solution and kept for 30 min in vacuum to fill up the holes and gaps in the porous architectures. The 3-D SiCnws-CF architectures crammed with ZrC polymer precursor were collected and solvent removed at 120 °C in vacuum. The dry SiCnws-CF/ZrC-precursor cubic blocks were transferred to the high-temperature pyrolysis furnace which equipped with vacuum pump. Subsequently, with a heating rate of 10 °C/min, the furnace was heated to 1000 °C and kept for 1 h in vacuum, then further heated to 1600 °C with a heating rate of 5 °C/min and maintained for another 1 h. Finally, the SiCnws-CF/ZrC product were obtained after the furnace was cooled down to room temperature in vacuum. The polymer infiltration and pyrolysis procedure was conducted six cycles for every SiCnws-CF sample to make sure all the SiCnws-CF architectures were fully covered with ZrC coating. CF/ZrC nanocomposites were prepared with the same PIP process to act as reference samples. 2.4. Characterization The X-ray diffraction patterns of the synthesized SiCnws-CF and SiCnws-CF/ZrC products were conducted by an X’Pert PRO (PANalytical, Holland) X-ray diffractometer with Cu Kα radiation. The microstructure and morphology of as-prepared SiC nanowires and SiCnws-CF/ZrC products were performed on scanning electron microscopy (SEM, FEI Helios Nanolab 600i) and transmission electron microscopy (TEM, FEI Tecnai F30) with an acceleration voltage of 20 kV and 300 kV respectively. The SiCnws-CF products was characterized with a Fourier transform infrared spectrometer (FT-IR, PerkinElmer 2000). X-ray photoelectron spectra was examined on a Perkin-Elmer Escalab 250 spectrometer with a Kratos Analytical spectrometer. Porosity of SiCnws-CF/ZrC nanocomposites were analyzed by mercury porosimetry (Micromeritics' AutoPore IV 9500). The compressive strength of the

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SiCnws-CF/ZrC products were evaluated by the Electronic Universal Testing Machine (Instron 5569, USA) with displacement rate of 0.2 mm/min. The final discussed compressive strength value was averaged from five test results. To characterize the structural health monitoring sensitivity of the SiCnws-CF/ZrC nanocomposites, the Electronic Universal Testing Machine (Instron 5848, USA) was employed to apply compression tests with the displacement rate of 0.6 mm/s. The compression samples were sectioned and polished to 10(length)×10(width)×12(height) mm. An out-of-plane through width electrode configuration was achieved by adhering common copper foil on both sides of the SiCnws-CF/ZrC cube sample. Two copper foils were placed apart and connected to Keithley 4200-SCS digital multimeter to record the electrical resistance. The testing was conducted by a displacement-controlled method that induced a specified strain to each sample. The sample was compressed to the specified strain level with the speed of 0.6 mm/s and held for 15 seconds, and then returned to zero displacement and held for another 15 seconds before next compression. The program was repeated more than 10 times at every strain levels. The in-plane and out-of-plane thermal conductivity of SiCnws-CF/ZrC nanocomposites at different temperature were calculated according to the following equation: κ = αρCp

(1)

where α represents the thermal diffusivity, ρ is the density and Cp is the heat capacity of the sample. Thermal diffusivity (α) and heat capacity (Cp) of the samples were measured through thermal conductivity measurement equipment (LFA 467 HT HyperFlash, NETZSCH GmbH, Germany) using the laser flash method. The specimens (10 × 10 × 3 mm) were polished and

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carbon sprayed before conducting. The thermal diffusivity and heat capacity were detected five times and the average values were used to calculate the thermal conductivity at every temperature point. The α and Cp were measured at 25 °C, 200 °C, 400 °C, 600 °C, 800 °C, and 1000 °C representatively, and the thermal conductivity of the samples were calculated in these temperature points. 3. RESULTS AND DISCUSSION As illustrated in Figure 1, the fabrication of SiCnws-CF/ZrC porous low thermal conductive nanocomposites is based on catalyst assisted self-assembly of SiC nanowires on the surface of 3-D carbon fiber in CVI system, which is followed by an in-situ coating of zirconium carbide ceramic through the polymer infiltration and pyrolysis method. Co(NO3)3·6H2O, act as catalyst precursor, was coated on the surface of CF, which would be reduced to zero valence state and form a liquid alloy phase in the reaction to adsorb Si and C atoms

according

to

the

vapor-liquid-solid

(VLS)

mechanism.42

CH3SiCl3

(Methyltrichlorosilane, MTS) was selected as Si and C source precursor owing to the 1:1 atomic ratio of Si and C and the low decomposition temperature. Hydrogen was used as reduction and carrier gas. At high temperature, the CH3SiCl3 molecules were decomposed into Si and C atoms when they contact with the vapor-liquid surface. Subsequently, the Si and C atoms were dissolved into the liquid Co catalyst. The supersaturation of Si and C atoms in liquid Co catalyst could be easily reached due to the rapid absorption, and then the growth of SiC crystal was started.42-44 On the basis of the above synthesis solution, SiC nanowires were radially grown on the surface of carbon fiber. Figure S1 shows a large perspective SEM observing of the 3-D

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SiCnws-CF architecture, which present that every single CF in braid was covered with crowded wire-like SiC nanostructures. Figure 2a-d show the representative morphologies of the commercial carbon fiber and the SiC nanowires assembled on the surface of carbon fiber. The original carbon fiber is surface-smooth and has a uniform diameter about 7 ± 0.2 µm (Figure 2a). The as-synthesis SiC samples are wire-like nanostructures and homogeneous in their size, showing a diameter about 80-100 nm and the length more than tens of micrometers, with a large aspect ratio up to 100 (Figure 2b). The EDS elemental mapping results (Figure 2c and d) identify the homogeneous distribution of C and Si element in SiC nanowires.

Figure 1. Schematic diagram illustrating of synthesis procedure of SiCnws-CF architecture and SiCnws-CF/ZrC nanocomposites. To clearly known the crystallographic structure, chemical bonding state and the elements distribution of SiC nanowires, the samples were further studied by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), and X-ray photoelectron spectroscopy (XPS). As shown in Figure 2e, the diffraction peaks in the XRD pattern match well with the standard pattern of β-SiC (JCPDS Card. No. 29-1129), which is of cubic structure with lattice constant α = 0.4358 nm. The low intensity SF peak ahead of the (111) peak indicates that stacking 10


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faults were existed in the SiC crystals according to the previous literature45,46 and the subsequent TEM characterization results of SiC nanowires. Typical FT-IR transmittance spectrum of the as-synthesized β-SiC nanowires was shown in Figure 2f. Two strong peaks at 810.04 cm-1 and 921.58 cm-1 were attributed to the stretching vibration of the Si-C bonds.47 The slight shift and broadening of the FT-IR peaks, comparing with that of bulk β-SiC, could be ascribed to the structure defects and size confinement effects.47,48

Figure 2. Characterization of SiCnws-CF architecture. (a) SEM image of carbon fiber. (b) SEM image of SiCnws-CF (insert is the high-magnified SEM image of the SiC nanowires). (c and d) EDS elemental mapping of C and Si of the SiCnws-CF architecture in (b). (e) XRD pattern of SiCnws-CF. (f) FT-IR transmittance spectrum of SiCnws-CF. (g) XPS spectra of the untreated CF and SiCnws-CF architecture. (h) High-resolution XPS spectra of C 1s for SiCnws-CF architecture. X-ray photoelectron spectroscopy (XPS) (Figure 2g and h) was used to investigate the chemical nature of the untreated CF and SiCnws-CF architecture. As shown in Figure 2g, two characteristic peaks around 285 eV and 532 eV could be assigned to C 1s and O 1s, which indicated that the surface of untreated carbon fiber mainly composed of C and O element. 11



However, the Si 2s and Si 2p peaks at binding energy of 150.5 eV and 100.1 eV were obviously presented in the XPS curve of SiCnws-CF architecture, which further confirmed the presence of SiC nanowires on the surface of CF. Figure 2h presents the high-resolution XPS scan spectrum of C 1s for SiCnws-CF. The C 1s spectrum reveals the C-Si peak at 283.4 eV due to the growth of SiC nanowires besides the prominent C-C peak at 284.8 eV. Moreover, the fitting peak with binding energy of 285.6 eV assigned to C-O bonds is possible due to the adsorbed CO2. The transmission electron microscopy (TEM) image and selected area electron diffraction (SAED) pattern in Figure 3 further reveal the structural information of the single SiC nanowire. Figure 3a shows the TEM image of the one-dimensional SiC nanowire with the diameter of ≤ 100 nm. The high resolution transition electron microscopy (HRTEM) image of SiC nanowire in Figure 3b presents that the lattice fringes are perpendicular to the axis and the d spacing are measured to be 0.25 nm (the insert of Figure 3b), corresponding to the interplanar spacing of (111) planes of β-SiC. The results implied that the SiC nanowires grown on the surface of CF along the [111] direction. Generally, the nanowires always grow in the crystal direction which minimize the total free energy. And in most cases, the total free energy is determined by the surface free energy of the interface between the SiC and the metallic catalyst. For the transition metal catalyst, many reports show that the SiC-catalyst always form a single surface at the (111) plane with the lowest energy and thus, the SiC nanowires grow along the [111] direction in most fabrication conditions.42,49 Besides, many stacking faults could be found in the HRTEM image, which results in the low intensity SF peak in XRD pattern in Figure 2e. Figure 3c is the corresponding selected area electron

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diffraction pattern of the SiC nanowire. The clear diffraction spots in the SAED pattern could be indexed to β-SiC crystal which is agree with the XRD analysis result.

Figure 3. (a) TEM image of the SiC nanowires. (b) HRTEM image of the SiC nanowire. (c) SAED pattern of the SiC nanowire in (b). ZrC ceramic thermal barrier coating was prepared by a polymer infiltration and pyrolysis method on the surface of three dimensional porous SiCnws-CF architecture. The ZrC preceramic polymer were synthesized from zircomiun tetrachloride (ZrCl4), acetylacetone (Hacac, CH3COCH2COCH3), methanol (CH3OH), and 1,4-butanediol (C4H8(OH)2) as the reaction details illustrated in Figure 4. There are four steps in the fabrication of ZrC ceramic on the surface of SiCnws-CF architecture. In the first step, ZrCl4 acted as the Zr source and the four chlorine atoms were replaced by two CH3O- and two acac- group and chelated to Zr(acac)2(CH3O)2. Here, acac- represents the -CH(COCH3)2 group resulted from Hacac taking off a hydrogen atom. Then the zirconium-containing metal chelates Zr(acac)2(CH3O)2 reacted with C4H8(OH)2 molecule and synthesized to Zr(acac)2(OC4H8OH)2. In the third step, the intermediate

Zr(acac)2(OC4H8OH)2

molecule

connected

one

by

one

through

a

polycondensation reaction and produced ZrC precursor polymer ([Zr(acac)2O2C4H8]n). The porous SiCnws-CF architecture were infiltrated in the xylene solution of ZrC precursor

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polymer to load the ZrC precursor on the surface of SiCnws-CF architecture. At last, the SiCnws-CF loaded with ZrC precursor were curing and then pyrolysis treated at 1600 °C to generate the final SiCnws-CF/ZrC products.

Figure 4. Schematic illustration of reaction principle in preparation of ZrC precursor and pyrolysis of ZrC precursor. The pyrolysis behavior and reaction yield of the ZrC precursor polymer were studied by synchronous thermal gravimetric (TG) and differential scanning calorimetry (DSC) analyzer and reported in our previous work35. The pyrolytic products of the ZrC precursor under different pyrolysis temperature were investigated by XRD and the analysis was also shown in our previous report35. Here, the pyrolysis temperature was 1600 °C and the final products, SiCnws-CF/ZrC, were characterized by XRD and the patterns were shown in Figure 5d. Based on the XRD patterns, five strong peaks with the 2θ at 33.0° (111), 38.3° (200), 55.3° (220), 66.0° (311), and 69.3° (222) are well matched with the standard value of cubic-ZrC (JCPDS,

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No.35-0784), which displays that the ZrC converted at 1600 °C from the precursor are cubic crystal. The strong and sharp peaks indicated that the as-prepared ZrC on the surface of SiCnws-CF were well-crystallized. Moreover, the remaining low intensity peaks in the pattern can be indexed as β-SiC resulted from the SiC nanowires in the nanocomposites. No other characteristic peaks associated with any impurity could be detected. In order to make sure every SiCnws-CF fully covered and protected with the thermal barrier ZrC coating, the porous 3-D SiCnws-CF architectures were conducted 6 cycles of precursor infiltration pyrolysis treatment. Figure 5a-c show the surface morphology of the 3-D SiCnws-CF/ZrC nanocomposites with the PIP cycles of 2, 4 and 6 respectively. Owing to the radial growth of the SiC nanowires on the surface of CF, all the fibers were covered with ZrC ceramic coating. After 2 cycles of PIP treatment, the loading of ZrC ceramic coating was relatively low and a few of SiC nanowires were exposed (Figure 5a). With the increase of the PIP circle times, the loading of ZrC ceramic coating and the apparent density were gradually increased. The SiCnws-CF was completely coated with ZrC ceramic with the PIP cycles of 4, but the ZrC coating was still scab-like loaded on the surface of SiCnws-CF (Figure 5b). However, after 6 cycles of PIP treatment, the completed ZrC ceramic coating sheath was formed on the surface of SiCnws-CF architecture and the diameter of the single SiCnws-CF/ZrC was about 40 µm (Figure 5c). The interface between SiC nanowires and ZrC ceramic matrix plays an important role in enhancing the mechanical property and maintaining the low thermal conductivity of the SiCnws-CF/ZrC composites. It is believed that the interface between SiC nanowires and ZrC ceramic is a physical adsorption interface without chemical reactions. Good wettability interfaces exist between the SiC nanowires and ZrC matrix due to

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their similar intrinsic ceramic properties. As we can see from Figure S3c, the SiC nanowires on the surface of CF are penetrated into the ZrC and resulted in an enhanced interlocking interface connection between SiCnws-CF and ZrC ceramic matrix.

Figure

5.

Morphological

and

structural

characterization

of

the

SiCnws-CF/ZrC

nanocomposites. SEM image of SiCnws-CF/ZrC with the precursor infiltration pyrolysis cycles of (a) 2, (b) 4, and (c) 6. The inserts are the HRSEM images with the corresponding PIP cycles. (d) XRD pattern of the SiCnws-CF/ZrC nanocomposites. To effectively reveal the function of the SiC nanowires in improving the interface performance of SiCnws-CF/ZrC nanocomposites, untreated 3-D CF braid was also conducted with the same PIP process, and the SEM images of CF/ZrC composites prepared with different PIP cycles were shown in Figure S2. For the untreated-CF/ZrC composites fabricated with 2 PIP cycles (Figure S2a and b), the loading of ZrC ceramic on the smooth surface of CF was obviously less than that on the SiCnws-CF. The ZrC ceramics were

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agglomerated and caked in this condition, and lots of CF were exposed without the protection of ZrC coating. In this case, many untreated CF were surface corrosion or even damaged in the pyrolysis process as shown in the insert of Figure S2b. The surface damage of carbon fiber always results in a serious decline in the comprehensive performance of CF reinforced ceramic composites, especially the mechanical property of the composites. However, the growth of SiC nanowires on the surface of CF here is facile and effective in protection of CF and enhancing the interface connection between CF and ZrC ceramic coating. Figure S3 shows the representative SEM images of CF/ZrC and SiCnws-CF/ZrC composites to reveal the different interface connection between CF and ZrC with- and without-SiC nanowires. Comparing with the partially embedded CF in Figure S3a, the CF with SiC nanowires performs an obvious improved interface connection with the ZrC ceramic matrix. As shown in Figure S3c, no interface defects or interface unsticking are found between SiC nanowires and ZrC ceramic matrix. Moreover, owing to the penetration of SiC nanowires into the ZrC matrix, an interlocking interface structure is constructed and the interface bonding strength between SiCnws-CF framework and ZrC ceramic matrix is improved. The growth of SiC nanowires on the surface of CF not only improved the interface connection between CF and ZrC matrix but also enhanced the structural health monitoring (SHM) capability of the composites effectively. The inherent piezoresistive property of the SiC nanowires could be utilized to enhance the change in electrical resistance of the SiCnws-CF/ZrC composites under the external compressive strain. The static loading instead of dynamic loading could be employed because the application of piezoresistive effect as opposed to piezoelectric effect. Previous literatures reported the piezoelectric materials had

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been applied in structural health monitoring50,51, but the dynamic loading cycles were required to generate signals. So they cannot be applied in the real-world common static loading situation. As thus, the piezoresistive SHM sensors provide greater advantages using in the engineering sensing application than piezoelectric sensors. One of the premises in using the piezoresistive SHM sensors is that the effective material must have low enough electrical resistance. In this condition, the carbon fiber is one of the appropriate candidates owing to its outstanding electrical conductivity and easy processing into conductive composites. Typical electrical resistance response with the compressive strain change cycles of CF/ZrC and SiCnws-CF/ZrC composite are shown in Figure 6a and b respectively. The cyclic compression loading was applied to calculate an average relative resistance dependence on the different strain values as shown in Figure 6c. From Figure 6a, the relative electrical resistance change of the CF/ZrC composites shows a slight increase to 0.0093 with the repeated input strain. However, for the SiCnws-CF/ZrC composites, the relative electrical resistance change can reach to 0.1353 in the same strain circle condition. The relative resistance change curves of two composites with- and without-SiCnws show good cyclic durability under 5% compressive strain. The SiCnws-CF/ZrC composites exhibit a much higher piezoresistive sensitivity than the CF/ZrC composites. The enhancement in piezoresistive sensitivity is mainly due to the inherent piezoresistive property of the SiC nanowires. Because of the semiconducting nature, the single crystalline β-SiC is proved to have a strong piezoresistive effect with the absolute gauge factor of 20 ~ 30.52 And for the β-SiC nanowires, the gauge factor was reported to be 4.5 to 46.2 with different fabrication conditions.53 Moreover, the change of the surface states of SiC nanowires may also contribute to the

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increase of piezoresistance effect of the composites.53 In our experiment, the application of external stress can change the surface states of the SiC nanowires, resulting in a change of built-in potential near the SiC nanowires surface, which could further enhance the piezoresistance effect of the SiCnws-CF/ZrC composites.

Figure 6. The electrical resistance response of (a) CF/ZrC composite and (b) SiCnws-CF/ZrC composite to the repeated input strain. (c) Plots of average gauge factor of CF/ZrC and SiCnws-CF/ZrC composites with the applied strain from 0 to 5%. (d) The average gauge factor for the CF/ZrC and SiCnws-CF/ZrC composites under 5% compressive strain. Even though the curves of electrical resistance responding to external compressive strain present the strong difference in piezoresistive sensitivity of the two composites, a more quantified method was applied to effectively reveal the enhancement of the sensitivity of SiCnws-CF/ZrC composites. The typical method to characterize the sensor sensitivity is to calculate the gauge factor (GF), which is defined as the following equation:

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GF =

| ∆R | R0

Page 20 of 37

(2)

ε

where |△R| represents the relative resistance change, R0 represents the initial resistance and ε is the compressive strain. According to the equation, the gauge factors for the CF/ZrC and SiCnws-CF/ZrC composites with compressive strain from 0 to 5% were calculated and plotted in Figure 6c. As shown in Figure 6c, for the SiCnws-CF/ZrC composites, the gauge factors increased significantly with the compressive strain increased from 0 to 5%. But the gauge factors for CF/ZrC composites experienced a slight increase respond to the increase of compressive strain. The results indicated that the growth of SiC nanowires on the surface of CF greatly enhance the electrical resistance change sensitivity of the composites. The gauge factor for the CF/ZrC and SiCnws-CF/ZrC composites under 5% compressive strain was plotted in Figure 6d to better compare the sensitivity of the two composites. As shown in Figure 6d, the maximum gauge factor for the SiCnws-CF/ZrC composites was 2.57 while the value was just 0.18 for the CF/ZrC composites. For comparison, CNTs reinforced fiber sensors were reported in the previous literature and reached a gauge factor of 1.6,54 which is lower than 2.57. Comparing with the composites with no SiC nanowires, the calculated maximum structural health monitoring sensitivity of the SiCnws-CF/ZrC was improved by 14 times. Finally, the results show that growth of SiC nanowires on the surface of CF significantly enhanced the structural health monitoring sensitivity of the SiCnws-CF/ZrC nanocomposites. Comparing with the untreated CF, the growth of SiC nanowires on the surface of CF significantly improved the surface roughness and increased the attachment points for ZrC

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precursor polymer to load on the CF. As thus, the loading of ZrC precursor was increased and the pyrolytic ZrC ceramic coating was bridged with CF framework by the interlocked SiC nanowires through a mechanical connection mechanism. To explore the influence of the growth of SiC nanowires on the porosity and the weight of the composites, the porosity and density of untreated-CF/ZrC and SiCnws-CF/ZrC nanocomposites were detected. The porosity of the untreated-CF/ZrC and SiCnws-CF/ZrC nanocomposites depended on PIP cycles were presented in Figure 7a, and the density test results of them were shown in Table S1. From Figure 7a, the porosity of both untreated-CF/ZrC and SiCnws-CF/ZrC nanocomposites decreased with the increase of PIP cycles. Porosity of untreated-CF/ZrC was decreased from 82.44% to 74.60% with the PIP treatment increasing from 2 cycles to 6 cycles, while the porosity of SiCnws-CF/ZrC nanocomposites decreased from 78.88% to 68.13%. The porosity of SiCnws-CF/ZrC nanocomposites was little less than that of untreated-CF/ZrC composites under the same PIP circle due to the increased loading of ZrC ceramic coating on the SiCnws-CF comparing with the loading on the untreated CF. With the same reason, the density of SiCnws-CF/ZrC nanocomposites was little higher than that of untreated-CF/ZrC composites under the same PIP circle. Rising the PIP cycles resulted in the improved loading of ZrC ceramic coating on both untreated CF and SiCnws-CF, which finally caused the density increase. With the PIP cycles increased from 1 to 6, the density of untreated-CF/ZrC composites increased from 0.61 g/cm3 to 1.75 g/cm3, while the density of SiCnws-CF/ZrC nanocomposites increased from 0.76 g/cm3 to 1.85 g/cm3. Summary of the above analysis of porosity and density of our samples, the prepared SiCnws-CF/ZrC nanocomposites are lightweight with high porosity, which is favorite for the thermal barrier materials.

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The compressive strength of the untreated-CF/ZrC and SiCnws-CF/ZrC nanocomposites were studied and the results were shown in Figure 7b. The weaving process caused the directionality of the 3-D CF braid (as schematic shown in Figure S4), which further resulted in the direction dependent of mechanical and thermal physical properties of the untreated-CF/ZrC and SiCnws-CF/ZrC nanocomposites. The compressive strength in z direction was higher than that in xy direction for both untreated-CF/ZrC and SiCnws-CF/ZrC composites fabricated with the same PIP circle, which indicated that the strength of CF/ZrC and SiCnws-CF/ZrC composites is mainly depended on the strength of 3-D CF and SiCnws-CF architecture respectively. The compressive strength of both untreated-CF/ZrC and SiCnws-CF/ZrC nanocomposites, no matter in xy or z direction, was increased gradually with the increase of PIP cycles. Most importantly, the compressive strength of SiCnws-CF/ZrC nanocomposites was obviously higher than that of untreated-CF/ZrC composites with the same PIP circle. With 6 PIP cycles, the compressive strength of SiCnws-CF/ZrC is 23.64 MPa in z direction and 13.02 MPa in xy direction, which are higher than those of CF/ZrC composites (17.5 MPa in z direction and 10.64 MPa in xy direction) respectively. The failure of the CF reinforced ceramic matrix composites under compression load could be divided into three stages: ceramic matrix destruction, interface failure and fiber skeleton failure.55, 56 From our previous discussion, on the one hand, growth of SiC nanowires on the surface of CF increased the ZrC loading effectively, and thus further improved the compressive strength of ZrC ceramic matrix in the SiCnws-CF/ZrC nanocomposites. On the other hand, the growth of SiC nanowires greatly enhanced the interface strength between CF and ZrC ceramic coating, which could further increased the critical load for interface failure. Overall, the compressive

22


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strength of SiCnws-CF/ZrC nanocomposites was significantly increased due to the growth of SiCnws on the surface of CF comparing with CF/ZrC composites. The representative compressive stress-strain curves of the CF/ZrC and SiCnws-CF/ZrC composites with different PIP cycles are shown in Figure S5. It reveals that the growth of SiC nanowires on the surface of CF can also improve the resistance to deformation of the SiCnws-CF/ZrC composites, and lead to an increase of the failure stress comparing with the CF/ZrC composites under the same PIP cycle. The specific heat capacity of the CF/ZrC and SiCnws-CF/ZrC composites at different temperature is shown in Figure S6. With the temperature increasing from 25 °C to 1000 °C, the specific heat capacity of the two composites is gradually increased. The specific heat capacity of SiCnws-CF/ZrC composite is lower than that of CF/ZrC composite at every temperature point. Figure 7c and d presents the thermal conductivity of CF/ZrC and SiCnws-CF/ZrC nanocomposites along z and xy direction with the temperature from 25 °C (room temperature, RT) to 1000 °C. The thermal conductivity of both CF/ZrC and SiCnws-CF/ZrC nanocomposites no matter along z direction or xy direction exhibit a slow increasing trend with the temperature increasing from RT to 1000 °C due to the increase in intrinsic thermal conductivity of ZrC ceramic57 and CF58 with increasing temperature. The interesting found is that the thermal conductivity of SiCnws-CF/ZrC nanocomposites along the z direction (Figure 7c) was almost the same with that of CF/ZrC composites at every test temperature point, while the thermal conductivity of SiCnws-CF/ZrC nanocomposites along the xy direction (Figure 7d) was higher than that of CF/ZrC composites. As for the CF/ZrC composites, the overall thermal conductivity is determined by CF architecture, ZrC ceramic

23



matrix and the porosity in the composites due to their high porosity. However, for SiCnws-CF/ZrC nanocomposites, apart from the CF architecture, ZrC ceramic matrix and the porosity in the composites, the interface of SiC nanowires and ZrC ceramic also play a dominant role in determining the overall thermal conductivity due to the high interfacial area in the SiCnws-CF/ZrC nanocomposites.

Figure 7. Properties of the SiCnws-CF/ZrC nanocomposites. (a) Porosity of untreated-CF/ZrC and SiCnws-CF/ZrC nanocomposites with the PIP cycles of 2, 4, and 6. (b) Compressive strength of CF/ZrC and SiCnws-CF/ZrC in different direction with the PIP cycles changed from 2 to 6. Temperature-dependent thermal conductivity of CF/ZrC and SiCnws-CF/ZrC nanocomposites along (c) z direction and (d) xy direction. When the heat flux conducted along the z direction in the SiCnws-CF/ZrC

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nanocomposites, the SiC nanowires almost play no effect on the conduction due to their radial growth and relatively low volume fraction. As thus, the thermal conductivity along z direction of both CF/ZrC and SiCnws-CF/ZrC composites were mainly determined by the ZrC ceramic matrix with intrinsic low thermal conductivity, CF framework, and dominantly the porosity in the composites. However, the SiC nanowires were superimposed and overlapped when the heat flux conducting along the xy direction in the SiCnws-CF/ZrC nanocomposites. So, the effect of SiC nanowires in determining the thermal conductivity of SiCnws-CF/ZrC nanocomposites along the xy direction could not be ignored. The influence of SiC nanowires on the thermal conductivity along the different direction was schematic illustrated in Figure S7. Comparing with that of CF/ZrC composites, the thermal conductivity of SiCnws-CF/ZrC nanocomposites along the xy direction showing an increase at every temperature point. Overall, the high porosity of the composites and low thermal conductivity of the ceramic matrix dominantly resulted in the low thermal conductivity of the CF/ZrC and SiCnws-CF/ZrC nanocomposites. The thermal conductivity of SiCnws-CF/ZrC nanocomposites is 0.49 W/(m·K) at room temperature and 5.70 W/(m·K) at 1000 °C along the z direction respectively. 4. CONCLUSIONS In conclusion, SiCnws-CF multiscale architectures reinforced ZrC multifunctional thermal barrier composites were synthesized by a simple two-step method. The uniform growth of SiC nanowires significantly enhanced the structural health monitoring sensitivity and strongly improved the interface combination between carbon fiber and ZrC coating. The final SiCnws-CF/ZrC products after six cycles of polymer infiltration and pyrolysis process still maintained lightweight with a relatively high porosity of 68.13%. Compared to the

25



Page 26 of 37

conventional CF/ZrC composites, the structural health monitoring sensitivity of the SiCnws-CF/ZrC is greatly elevated with a 14-fold improvement, and the compressive strength of SiCnws-CF/ZrC nanocomposites is significantly increased by 35% due to the growth of SiC nanowires. Moreover, the introduction of SiC nanowires on the surface of CF almost does no effect on the low thermal conductivity of the porous SiCnws-CF/ZrC nanocomposites. The high SHM sensitivity, high compressive strength (23.64 MPa), lightweight and low thermal conductivity

(0.49

W/(m·K))

of

SiCnws-CF/ZrC

shows

that

the

SiCnws-CF/ZrC

nanocomposites have great potential in using as thermal protection materials. Importantly, this study demonstrates a radically new strategy to combine the structural health monitoring with the thermal barrier application and improve the functionality of the conventional thermal protection material, which further enables their practical applications.

ASSOCIATED CONTENT Supporting Information. SEM images of 3-D SiCnws-CF architecture. (a) A large perspective SEM image of SiCnws-CF. (b) SEM image of cross-section for single SiCnws-CF. Morphology of CF/ZrC composites with different PIP cycles. (a) Low resolution SEM image of CF/ZrC composites with 2 PIP cycles. (b) High resolution SEM image of CF/ZrC composites with 2 PIP cycles, insert is the single fiber from CF/ZrC composites with 2 PIP cycles and the scale bar is 5 µm. (c) Low resolution SEM image of CF/ZrC composites with 6 PIP cycles. (d) High resolution SEM image of CF/ZrC composites with 6 PIP cycles. Comparison of the connection between CF and ZrC matrix with and without SiC nanowires. (a) Representative SEM image of CF/ZrC composites with 4 PIP cycles. (b) Representative SEM image of SiCnws-CF/ZrC composites 26


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with 4 PIP cycles. (c) High resolution SEM image of SiCnws-CF/ZrC to show the penetration of SiC nanowires into the ZrC matrix. Density of CF/ZrC and SiCnws-CF/ZrC nanocomposites fabricated with different PIP cycles. Schematic illustration of the directionality of three dimensional carbon fiber architecture. Typical compressive stress vs. compressive strain curves of CF/ZrC and SiCnws-CF/ZrC composites with the PIP cycles of a) 2, b) 4, and c) 6. Specific heat capacity of CF/ZrC and SiCnws-CF/ZrC composites at different temperature. Schematic illustration of the different effect of SiC nanowires on the heat flux conduction along z and xy direction. (PDF)

AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] (C. Hong) *E-mail: [email protected] (P. Hu) *E-mail: [email protected] (X. Zhang) Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS This research was financially supported by the National Natural Science Foundation of China (No. 51772061 and 51602076), the National Fund for Distinguished Young Scholars (No. 51525201), the Foundation for Innovative Research Groups of the National Natural Science Foundation of China (No. 11421091), the China Postdoctoral Science Foundation Funded Project (No. 2016M601426) and the Major State Basic Search Program

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(No. 2014CB46505).

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Figure Captions Figure 1. Schematic diagram illustrating of synthesis procedure of SiCnws-CF architecture and SiCnws-CF/ZrC nanocomposites. Figure 2. Characterization of SiCnws-CF architecture. (a) SEM image of carbon fiber. (b) SEM image of SiCnws-CF (insert is the high-magnified SEM image of the SiC nanowires). (c and d) EDS elemental mapping of C and Si of the SiCnws-CF architecture in (b). (e) XRD pattern of SiCnws-CF. (f) FT-IR transmittance spectrum of SiCnws-CF. (g) XPS spectra of the untreated CF and SiCnws-CF architecture. (h) High-resolution XPS spectra of C 1s for SiCnws-CF architecture. Figure 3. (a) TEM image of the SiC nanowires. (b) HRTEM image of the SiC nanowire. (c) SAED pattern of the SiC nanowire in (b). Figure 4. Schematic illustration of reaction principle in preparation of ZrC precursor and pyrolysis of ZrC precursor. Figure

5.

Morphological

and

structural

characterization

of

the

SiCnws-CF/ZrC

nanocomposites. SEM image of SiCnws-CF/ZrC with the precursor infiltration pyrolysis cycles of (a) 2, (b) 4, and (c) 6. The inserts are the HRSEM images with the corresponding PIP

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cycles. (d) XRD pattern of the SiCnws-CF/ZrC nanocomposites. Figure 6. The electrical resistance response of (a) CF/ZrC composite and (b) SiCnws-CF/ZrC composite to the repeated input strain. (c) Plots of average gauge factor of CF/ZrC and SiCnws-CF/ZrC composites with the applied strain from 0 to 5%. (d) The average gauge factor for the CF/ZrC and SiCnws-CF/ZrC composites under 5% compressive strain. Figure 7. Properties of the SiCnws-CF/ZrC nanocomposites. (a) Porosity of untreated-CF/ZrC and SiCnws-CF/ZrC nanocomposites with the PIP cycles of 2, 4, and 6. (b) Compressive strength of CF/ZrC and SiCnws-CF/ZrC in different direction with the PIP cycles changed from 2 to 6. Temperature-dependent thermal conductivity of CF/ZrC and SiCnws-CF/ZrC nanocomposites along (c) z direction and (d) xy direction.

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Table of Contents 266x156mm (300 x 300 DPI)


A Multifunctional Thermal Barrier Application Composite with SiC

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