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Structural Evolutions in Polymer-Derived Carbon-Rich Amorphous Silicon Carbide Kewei Wang, Baisheng Ma, Xuqin Li, Yiguang Wang, and Linan An J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/jp5093916 • Publication Date (Web): 09 Dec 2014 Downloaded from http://pubs.acs.org on December 17, 2014

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The Journal of Physical Chemistry

Structural Evolutions in Polymer-Derived Carbon-Rich Amorphous Silicon Carbide









‡,∗

Kewei Wang, Baisheng Ma, Xuqin Li, Yiguang Wang, ,∗ and Linan An †

Science and Technology on Thermostuctural Composite Materials Laboratory, Northwestern Polytechnical University, Xi’an, Shaanxi 710072, P. R. China ‡

Department of Materials Science and Engineering, Advanced Materials Processing and Analysis Center, University of Central Florida, Orlando, FL 32816, USA



Corresponding authors. Email: [email protected] (Y.W.), [email protected], (L.A.); Phone: +86-29-

88494914 (Y.W.), +1-407-823-1009 (L.A.).

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Abstract The detailed structural evolutions in polycarbosilane-derived carbon-rich amorphous SiC were investigated semi-quantitatively by combining experimental and analytical methods. It is revealed that the material comprised of a Si-containing matrix phase and a free carbon phase. The matrix phase is amorphous comprising of SiC4 tetrahedra, SiCxOx-4 tetrahedra and Si-C-C-Si/Si-C-H defects. With increasing pyrolysis temperature, the amorphous matrix becomes more order accompanied by a transition from SiC2O2 to SiCO3. The transition was completed at 1250oC, where the matrix phase started to crystallize by forming a small amount of -SiC. The free-carbon phase comprised of carbon nano-clusters and C-dangling bonds. Increasing pyrolysis temperature led to the transition of the free carbon from amorphous carbon to nanocrystalline graphite. The size of the carbon clusters decreased first and then increased, while the C-dangling bond content decreased continuously. The growth of carbon clusters was attributed to the Ostward ripening and described using 2-dimension grain growth model. The calculated activation energy suggested that the decrease in C-dangling bonds is directly related to the lateral growth of the carbon clusters.

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Introduction Polymer derived ceramics (PDCs) has attracted extensive attentions for years due to a variety of unique and superior properties, including excellent thermal stability, 1 great creep resistance,2,3 as well as unusual electric behavior.4-8 The materials are promising for a variety of applications such as fibers,9-11 coatings,12, 13 porous components,14, 15 and sensors.16-18 It is revealed that the outstanding and unique properties of PDC are mainly attributed to its unique structure, which is amorphous at large scale (determined by X-ray diffraction) but heterogeneous at nanometer scale. Therefore, detailed understanding of the structure of PDCs becomes important.

The understanding of the structure of PDCs has been significantly improved in the past ten years. Typically, PDCs consist of two sections: an area containing silicon and an area containing carbon only (referred as to free carbon), together forming a so-called nanodomain structure.19 The Si-area consists of various Si-based tetrahedral; and the free-carbon area is comprised of highly disordered carbon clusters. Recent results indicated that the Sitetrahedral with the same coordination tends to aggregate together to form sub-domains.20 It is well-known that the structure of PDCs evolves with increasing pyrolysis temperature, leading to increase in the degree of order and crystallization eventually. Such structural evolution is very important since it determined the stability and properties of the materials. While structural evolution was the topic of tremendous previous research, these studies were 3 ACS Paragon Plus Environment

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primarily qualitative, and quantitative (or semi-quantitative) structural evolutions are rarely discussed.21-26

In this study, we report a detailed study on the structural evolution of a carbon-rich amorphous silicon carbide (C/SiC(O)) derived from polycarbosilane. Polycarbosilane-derived silicon carbide is one of the most important PDCs, given its applications in ceramic fibers and high-temperature sensors. In this paper, the structural evolution of the material was investigated by combining composition measurement, X-ray diffractometer (XRD), X-ray photoelectron spectroscopy (XPS), Raman microscope, and Electron paramagnetic resonance (EPR). The results lead to a quantitative/semi-quantitative understanding of the structural transitions in this material.

1. Experimental Section The C/SiC(O) studied here was prepared by thermal decomposition of polycarbosilane (PCS) purchased from Xiamen University, which was synthesized using the procedure described in Ref. 8. The as-received liquid-phased PCS was first cross-linked by heattreatment at 400°C for 1hr in a quartz tube furnace under a steady flow of ultrahigh purity argon. The cross-linked product was then ground to powder of 1 µm in a planetary ball miller for 60 min. The powder was compressed into disk samples of 16 mm in diameter and 1 mm in thickness under uniaxial pressure of 20MPa followed by cold isostatic pressing at 200MPa 4 ACS Paragon Plus Environment

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for 5 min. The compacts were pyrolyzed at temperatures between 1000 to 1400°C for 3 hrs at a heating rate of 1°C/min.

The composition of the ceramics was measured using a combination of IR carbon-sulfur analyzer (Horiba Ltd., EMIA-320V, Kyoto, Japan) and oxygen-nitrogen analyzer (Horiba Ltd., EMGA-620W, Kyoto, Japan). The silicon content of the samples was estimated by

M Si = 1 − M C − M O

where MSi, MC, MO are silicon mass fraction, carbon mass fraction and oxygen mass fraction, respectively. The X-ray diffraction patterns of the materials were collected on Rigaku D/max2400 Diffractometer (Tokyo, Japan) using Cu Kα radiation. The data were digitally recorded in a continuous scan mode in the angle (2θ) range of 10-75° at a scanning rate of 0.12o/s.

The X-ray photoelectron spectroscopy (XPS) was carried out on an AXIS ULTRA (Kratos, Manchester, U.K.) apparatus, using monochromatic Al Kα radiation (hv = 1486.6 eV) at 150 W. The spectra were recorded at room temperature under high vacuum (10-9 Torr). The instrument analysis chamber is directly attached to a UHV preparation chamber via a gate valve, avoiding air exposure during sample transfer. In order to obtain the internal structure (instead of surface) of bulk materials, the testing surface of the sample was first polished to remove the top layer of 5µm, and then etched by Ar+ ions to remove additional 1µm from the surface before testing. In order to further ensure that the signal is not from the surface, the 5 ACS Paragon Plus Environment

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surface was etched to remove additional 1µm and tested again. It was found that the results from the two tests were very similar, indicating the structural information is indeed from bulk. The bind energies were calibrated using O1s peak as a reference.27-29 The XPS spectra were curve-fitted using the XPS Peak 4.1 program with a Shirley background subtraction.

The Raman spectra were collected using Renishaw in-Via Raman microscope (Renishaw, London, U.K.) with the 514 nm line of Ar laser as the excitation source and a sensitive Peltier-cooled couple charged device detector. The laser beam of 10 µm in diameter was focused on the sample surface. The laser power on the sample was kept below 2.5 mW to avoid possible material damage. 20 Raman spectra were collected for each sample at different positions to minimize errors associated with material heterogeneity.

The G-band (406 GHz) EPR spectra were recorded on a home-built spectrometer. The microwave of 406.4000(4) GHz (g ≈ 2.00 around 14.52 T) was generated by a phase locked Virginia Diodes source and propagated along a 10 mm cylindrical waveguide to the sample compartment and further towards the bolometer detector. A superconducting magnet of 17 T was employed. All measurements were carried out at room temperature. The experimental peak-to-peak distances and relative signal intensities were obtained from the spectra with the aid of the Easyspin software package.

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2. Results Table 1 gives the chemical composition of the materials obtained at different temperatures. Table 1. Composition of the polymer-derived amorphous SiC. Pyrolysis C (wt%)

O (wt%)

Si (wt%)

Composition

1000

37.584

2.200

60.216

SiC1.46O0.06

1100

37.168

2.830

60.002

SiC1.45O0.08

1200

37.477

2.381

60.142

SiC1.45O0.07

1250

37.209

2.220

60.271

SiC1.43O0.06

1300

37.854

2.031

60.115

SiC1.47O0.06

1400

37.454

2.887

59.659

SiC1.46O0.08

Temperature (oC)

It can be seen that all samples contain silicon and carbon as the main compositions with a small amount of oxygen, which did not exist in the original precursor and likely came from the processing contamination. The carbon content is much higher than that required to form stoichiometric silicon carbide, indicating the existence of free carbon in the materials. The composition of the samples remains constant within experimental error regardless pyrolysis temperature, suggesting the material is stable against thermal decomposition within the experimental temperature range; which is consistent with the TGA results that showed no detectable weight change in the temperature range. Figure 1 shows the X-ray diffraction

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Figure1. XRD patterns of the C/SiC(O) pyrolyzed at different temperatures as labeled. patterns for the materials pyrolyzed at different temperatures. It is seen that the samples pyrolyzed at lower temperatures (1250oC) show weak diffraction peak(s), indicating that they contain a small amount of crystalline phase. The peak(s) can be indexed to be is β−SiC. The increase in the intensity of the peak(s) with increasing pyrolysis temperature suggests that the content of the crystalline phase increases with the temperature.

The chemical bond presence within the C/SiC(O) was analyzed using XPS. Figure 2 shows

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Figure 2. Typical XPS spectra of C1s (a) and Si2p (b) measured from the C/SiC(O) pyrolyzed at 1000oC. typical XPS spectra of C1s and Si2p obtained from the C/SiC(O) pyrolyzed at 1000oC. Curvefitting of these spectra using Gaussian/Lorentzian sum function reveals that the C1s spectrum (Figure 2a) can be split into three peaks centered at 283.6, 284.4 and 285 eV, corresponding to C-Si,30-32 C=C,33-35 and C-C/H bonds,36-39 respectively; and the Si2p spectrum (Figure 2b) can be split into two peaks centered at 100.2 and 101.7 eV, corresponding to Si-C30,31,40 and Si-O bonds,41 respectively. This suggests that there are three kinds of bonds for C and two kinds of bonds for Si; and no C-O and Si-Si bonds exist in the material within the detect limitation. The concentration of these bonds was calculated from the integration areas

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underneath their corresponding peak, and was listed in tables 2 and 3. It is seen (table 1) that Table 2. Chemical bond presence in the SiC, calculated based on C1s. Pyrolysis 1000°C

1100°C

1200°C

1300°C

1400°C

C-Si

66.19%

66.19%

66.19%

66.19%

66.19%

C=C

03.14%

12.47%

28.17%

32.51%

33.13%

C-C/H

30.67%

21.13%

05.64%

01.30%

00.68%

o

Temperature ( C)

Table 3. Chemical bond presence in the SiC, calculated based on Si2p. Pyrolysis 1000°C

1100°C

1200°C

1300°C

1400°C

Si-C

90.03%

90.04%

90.99%

92.05%

92.33%

O-Si-C

09.97%

09.96%

09.01%

07.95%

07.67%

Temperature (oC)

the concentration of the C-Si bond is independent of pyrolysis temperature; while that of the C=C bond increases and C-C/H bond decreases with increasing temperature, suggesting a sp3-sp2 transition with increasing pyrolysis temperature. Table 2 reveals that the concentration of the O-Si-C bond (corresponds to SiCxO4-x tetrahedral, where x can be 0, 1, 2 and 3) and the Si-C bond (corresponds to SiC4 tetrahedral) remains constant up to 1100oC and then decreases and increases with increasing pyrolysis temperature, respectively.

The structural change of the C/SiC(O) was further analyzed using Raman spectroscopy. A typical Raman spectrum is shown in Figure 3, which exhibits two major bands corresponding

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Figure 3. A typical Raman spectrum measured from the C/SiC(O) pyrolyzed at 1000oC. to D band (centered at 1350cm-1) and G band (centered at 1600cm-1) of the free-carbon phase. Three minor peaks were observed for some samples at 1200 cm-1 for I peak, attributed to sp3 C–C bond, D’’ at 1500 cm-1 due to amount of amorphous carbon in sample, and D’ peak at 1620 cm-1 for disordered graphitic lattice.42,43 In order to obtain more information, the spectrum was curve-fitted using Lorentzian function for D-peak and Breit–Wigner–Fano function (BWF) for G-peaks.38 It is found that the position of G band persistently increased from 1566cm-1 to 1597cm-1, meanwhile its FWHM (full width at half maximum) decreased from 54.60cm-1 to 40.96cm-1 as the pyrolysis temperature increases from 1000oC to 1400oC. The changes imply that free-carbon phase transfer from amorphous carbon to nanocrystalline graphite with increasing pyrolysis temperature,44,45 which is consistent with the XPS results aforementioned.

The Raman spectra were also used to calculate the lateral size of the carbon clusters

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according to the following equation:44,45

ID = C ' (λ ) L2a IG

(1)

where ID and IG are the respective intensities of D and G bands in the Raman spectra; C'(λ) a constant depending on the laser wavelength and to be 0.0055 Å−2 for the wavelength of 514 nm;32, 44, 35 La the lateral size of the free-carbon clusters. The results are listed in table 4. The Table 4. The lateral size of the carbon clusters and the defect concentration in the C/SiC(O). Pyrolysis Temperature

1000°C

1100°C

1200°C

1250°C

1300°C

1400°C

1.89

1.89

1.79

1.58

1.61

1.67

3447

1838

/

710

680

628

(oC) Lateral Size (nm) Defect Content (a.u.) size decreases with pyrolysis temperature up to 1250°C; and then increases with further increasing the temperature.

The point defects (dangling bonds) within the materials were analyzed using EPR. Figure 4(a) presents a typical EPR spectrum obtained at the frequency of 406.4Hz. The spectrum

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Figure 4. (a)An EPR spectrum obtained from the C/SiC(O) pyrolyzed at 1000oC; (b) Linewidth of EPR data pyrolyzed at different temperatures. contains only one symmetric line with a g factor of 2.0032±0.0002, suggesting that it is from carbon-related dangling bond (unpaired electrons).46,47 The absence of hyperfine satellites suggests that there is no silicon dangling bond within the C/SiC(O).48 The peak-to-peak line width of the EPR signal vs temperatures is displayed in Figure 4(b). The line width decreases with increasing temperature in the low temperature region is probably caused by the unresolved proton hyperfine couplings resulting from the loss of hydrogen.49 The relative concentrations of the unpaired electrons of carbon were calculated from the EPR spectra as the product of the square of peak-to-peak width and peak-to-peak height. The results are 13 ACS Paragon Plus Environment

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listed in table 4, which shows that the defect concentration decreases with increasing pyrolysis temperature.

3. Discussion The amorphous structure and structural evolution of the SiC have been characterized by composition measurement, XRD, Raman and EPR. As we discussed above, the results show that the structure of the material undergoes several significant changes as increasing pyrolysis temperature. In this section, we will combine these information together to obtain a better understanding of the amorphous structure and structural evolutions in the material.

Structural evolution of the Si-containing matrix: By combining data from composition measurements (table 1) and chemical bond presence measurements (tables 2 and 3), a rather clear picture about the structure of the Si-containing matrix can be depicted. Since Si has four valence electrons, one mole of SiC contains four moles of Si-based bonds (Si-M, M is O or C). Since O is only bonded to Si and there is no Si-Si bond, it can be calculated from the apparent formula (~SiC1.46O0.07) that within one mole SiC there should be 0.14 mole of Si-O bonds (oxygen has two valence electrons and can form two Si-O bonds); and 3.86 moles of Si-C bonds, which requires 0.965 mole of C (C also has four valence electrons). According to the XPS results listed in table 2, 69.19% of C is bonded to Si, which corresponds to 1.01 moles of C. Therefore, within the Si-containing area C-to-Si ratio is slightly higher than 1:1, 14 ACS Paragon Plus Environment

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suggesting that there are a small amount of Si-C-C-Si and/or Si-C-H bonds within the area. With increasing pyrolysis temperature, the composition and percentage of C bonded to Si remain almost constant. This suggests that the composition of the Si-containing area is independent of pyrolysis temperature. Previous study on the optical absorption indicated the structure became more ordering with increasing the temperature.50

The percentage of O-Si-C bond within Si-containing matrix is determined by the x value in SiCxO4-x tetrahedral. 0.14 mole of Si-O bonds should give 0.035, 0.07, 0.105 and 0.14 mole SiCxO4-x tetrahedra for x = 0, 1, 2 and 3, respectively. It is seen from table 3 that the materials pyrolyzed at lower temperatures should contain SiC2O2 as major Si-O-C tetrahedral; while those prepared at higher temperatures should contain SiCO3 as major Si-O-C tetrahedral. This suggests that increasing pyrolysis temperature caused transition from SiC2O2 to SiCO3, likely by following reaction: 3SiC2O2 →2SiCO3+SiC4. Such a change likely led to an aggregation of the O-containing tetrahedral since the SiCO3 has three O coordinators and must form a 3-dimensional network structure.

In summary, the Si-containing matrix of the C/SiC(O) is comprised of SiC4 tetrahedral as the major building block, as well as certain amounts of SiC2O2/SiCO3 tetrahedral and Si-C-CSi/Si-C-H. With increasing pyrolysis temperature, the SiC2O2 tetrahedral transformed to SiCO3 tetrahedral. Such a transition likely caused an aggregation of the tetrahedral. The data in table 3 suggests that the transition likely started at 1100oC and finished at 1300oC, which 15 ACS Paragon Plus Environment

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concurred with the onset of crystallization of the C/SiC(O).

Structural evolution of the free carbon: As shown in table 4, the lateral size of the carbon clusters decreased with increasing pyrolysis temperature, and then increased with further increasing the temperature. The similar changes have also been observed by previous studies on the crystallization of amorphous carbon.44,51 The results were explained by Ferrari model: during transition from amorphous carbon to nanocrystalline graphite, sp3-to-sp2 transition and rearrangement of distorted aromatic rings into six-membered rings occurred between 1000 to 1100ºC, leading to a shrinkage of lateral size; further increasing temperature resulted in the in-plane growth of nano-polycrystalline graphite. This model cannot explain the current results to complete satisfaction since (i) the temperature for the transition of the decrease-toincrease in the lateral size is much higher in the current study than for amorphous carbon; and (ii) the total decrease in the lateral size from 1000 to 1250oC corresponds to ~ 45% volume decrease, which is too high to result from structural rearrangement solely. Here, we propose that the change in the lateral size of the carbon clusters is a combining effect of the structural rearrangement and interactions between the Si-area and free-carbon area. In the lower temperature region, the decrease in the lateral size of the carbon clusters is due to the combining effect of structural rearrangements similar as those for amorphous carbon and dissolution of the clusters into Si-area. Considering that the composition of the Si-containing area is independent of pyrolysis temperature, the dissolution likely underwent in following 16 ACS Paragon Plus Environment

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way: Si-C-C-Si + nC = Si-C-[C]n-C-Si and/or Si-C-H + nC = Si-C-[C]n. In other words, the dissolved carbon formed new carbon clusters. The driving force for such reactions is likely the loss of the residual H within both Si- and C-areas and interfaces between the two areas, made the structural unstable.52,53 It is interesting to note that such dissolution process primarily occurred before ~ 1250oC, which is the temperature range where the major loss of the residual H occurred.

In the higher temperature region, the transition of amorphous carbon into nanocrystalline graphite should finish.44,51 The increase in the lateral size of the carbon clusters is primarily due to the in-plane growth of the graphite. Such growth likely occurred via an Ostward ripening process where the smaller clusters disappeared and larger clusters grew.54 The inplane growth of the graphite clusters can be treated as a 2 dimension (2D) grain growth process. Accordingly, the growth rate of the graphite nanocrystals can be described by following equation:55

L2a ∝ exp(−

G* ) RT

(2)

where G* is the activation energy of the process. Figure 5 plots the lateral size of the carbon

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Figure 5. A plot of the lateral size of the carbon cluster as a function of pyrolysis temperature according to Eq. (2). clusters as a function of pyrolysis temperatures according to Eq. (2). It is seen that the data obtained at the higher temperature region follows Eq. (2) very well, suggesting that the inplane growth of the graphite nanocrystals indeed follows 2D grain growth process. The activation energy for the growth of the nanocrystalline graphite calculated from the slop of the curve is 15.8KJmol-1.

Considering the electric behavior research in Ref. 56, the above discussion about carbon structure evolution could be relate to the increasing conductivity of amorphous SiC with raising temperature. In the lower temperature region, rearrangement of carbon phase from sp3 to sp2 and forming of new carbon clusters would lead to (i) increase the electrical conductivity of carbon phase; (ii) provide more electric transport paths in SiC ceramic which can be explained by the “electric field concentration” effect.7,57 The model proposed that the

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electric field between the closely packed nano-scaled free carbon clusters was enhanced as compared to the applied external field, thus the electric current increased, leading to an apparent high conductivity (when the conductivity is still calculated by the applied electric field). In the higher temperature region, the 2D grain growth of nanocrystalline graphite further enhanced the conductivity of the carbon phase, which would increase the conductivity of the SiC.

As shown in table 4, carbon defect concentration in the material deceases with increasing pyrolysis temperature. In order to further analysis such a change, the carbon defect concentration is plotted as a function of pyrolysis temperature according to Arrhenius relation (Figure 6):

nd ∝ exp(−

E ) RT

(3)

where nd is the carbon defect concentration and E is apparent activation energy. It is seen that

Figure 6. A plot of defect density as a function of pyrolysis temperature.

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the curve shows two regions transited at temperature between 1200-1300oC. In the low temperature region, the calculated activation energy is 101.4KJmol-1, which is much higher than that (17.4KJmol-1) for the higher temperature region. As we discussed above, in the lower temperature region the carbon phase went through a set of structural changes, including carbon cluster dissolution, loss of H and transition from amorphous carbon to nanocrystalline graphite. The carbon defect decrease should relate to the combination of these changes. While in the higher temperature region, the major structural change is the in-plane growth of the nano-sized graphite clusters, thus the decrease in the carbon defect should dominantly relate to this process. It is interesting that the activation energy for the in plane growth of the clusters is very similar to that for the carbon defect decrease, suggesting the two closely related to each other.

4. Conclusions In this paper, the amorphous structure and structural evolutions of polycarbosilanederived amorphous SiC ceramics were studied in details. The results revealed that the material contains two areas: Si-containing area and free-carbon area. The Si-containing area consists of SiC4 tetrahedra, SiCxOx-4 tetrahedra, and Si-C-C-Si/Si-C-H; and the free-carbon area consists of nano-sized carbon clusters and C-dangling bonds. The materials obtained at temperatures below 1250oC are amorphous; while those obtained at temperatures higher than

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1250oC contains a small amount of crystalline SiC. With increasing pyrolysis temperature, both Si-containing area and free-carbon area underwent several structural changes. In the Siarea, major structural changes include the transition from SiC2O2 to SiCO3 via reaction: 3SiC2O2 →2SiCO3+SiC4, which mainly occurred between 1100-1300oC and likely caused oxygen-containing tetrahedra aggregated together; and the rest of Si-area became more ordered with their chemical coordination remaining unchanged.

In the free-carbon area, the structural changes include transform from amorphous carbon to nanocrystalline graphite; the change in the size of the carbon nano-clusters; and the decrease in C-dangling bonds. Before 1250oC, the size of the carbon clusters decreased with pyrolysis temperature, likely due to the combining effects of sp3-to-sp2 structural transition and inter-reaction between the Si-containing matrix and the free carbon. After 1250oC, the size of the carbon clusters increased with pyrolysis temperature, ascribed to the lateral growth of the graphite by following 2D grain growth model. The increasing conductivity of amorphous SiC with raising temperature is related to the increasing order degree of carbon phase. We also demonstrated that the decrease in C-dangling bonds in high-temperature range is directly related to the lateral growth of the graphite.

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Acknowledgements This work was financially supported by the Chinese Natural Science Foundation (Grant#51372202), State Key Laboratory of Solidification Processing (Grant #82-TZ-2013), and the “111” project (B08040). References 1.

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List of Figure Captions: Figure1. XRD patterns of the C/SiC(O) pyrolyzed at different temperatures as labeled. Figure 2. Typical XPS spectra of C1s (a) and Si2p (b) measured from the C/SiC(O) pyrolyzed at 1000oC. Figure 3. A typical Raman spectrum measured from the C/SiC(O) pyrolyzed at 1000oC. Figure 4. (a)An EPR spectrum obtained from the C/SiC(O) pyrolyzed at 1000oC; (b) Linewidth of EPR data pyrolyzed at different temperatures. Figure 5. A plot of the lateral size of the carbon cluster as a function of pyrolysis temperature according to Eq. (2). Figure 6. A plot of defect density as a function of pyrolysis temperature.

List of Table Caption: Table 1. Composition of the polymer-derived amorphous SiC. Table 2. Chemical bond presence in the SiC, calculated based on C1s. Table 3. Chemical bond presence in the SiC, calculated based on Si2p. Table 4. The lateral size of the carbon clusters and the defect concentration in the C/SiC(O).

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Figure1. XRD patterns of the C/SiC(O) pyrolyzed at different temperatures as labeled. 43x31mm (600 x 600 DPI)

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Figure 2. Typical XPS spectra of C1s (a) and Si2p (b) measured from the C/SiC(O) pyrolyzed at 1000oC. 74x91mm (600 x 600 DPI)

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Figure 3. A typical Raman spectrum measured from the C/SiC(O) pyrolyzed at 1000oC. 46x35mm (600 x 600 DPI)

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Figure 4. (a)An EPR spectrum obtained from the C/SiC(O) pyrolyzed at 1000oC; (b) Linewidth of EPR data pyrolyzed at different temperatures. 77x100mm (600 x 600 DPI)

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Figure 5. A plot of the lateral size of the carbon cluster as a function of pyrolysis temperature according to Eq. (2). 44x33mm (600 x 600 DPI)

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Figure 6. A plot of defect density as a function of pyrolysis temperature. 44x32mm (600 x 600 DPI)

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Structural evolutions in polymer-derived carbon-rich amorphous silicon carbide.

The detailed structural evolutions in polycarbosilane-derived carbon-rich amorphous SiC were investigated semiquantitatively by combining experimental...
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