Boron-modified perhydropolysilazane towards facile synthesis of amorphous SiBN ceramic with excellent thermal stability

SiBN ceramics are widely considered to be the most promising material for microwave-transparent applications in harsh environments owing to its excellent thermal stability and low dielectric constant. This work focuses on the synthesis and ceramization of single-source precursors for the preparation of SiBN ceramics as well as the investigation of the corresponding microstructural evolution at high temperatures including molecular dynamic simulations. Carbon- and chlorine-free perhydropolysilazanes were reacted with borane dimethyl sulfide complex at different molar ratios to synthesize single-source precursors, which were subsequently pyrolyzed and annealed under N2 atmosphere (without ammonolysis) to prepare SiBN ceramics at 1100, 1200, and 1300 °C with high ceramic yield in contrast to previously widely-used ammonolysis synthesis process. The obtained amorphous SiBN ceramics were shown to have remarkably improved thermal stability and oxidation resistance compared to amorphous silicon nitride. Particularly, the experimental results have been combined with molecular dynamics simulation to further study the amorphous structure of SiBN and the atomic-scale diffusion behavior of Si, B, and N at 1300 °C. Incorporation of boron into the Si—N network is found to suppress the crystallization of the formed amorphous silicon nitride and hence improves its thermal stability in N2 atmosphere.


Introduction
Nitride ceramics, such as Si 3 N 4 and BN, have been ever-increasingly applied in several key areas owing to their chemical stability, relatively high thermal conductivity, and outstanding oxidation resistance even under extremely harsh environments [1][2][3][4][5][6][7]. However, the intrinsic weakness of creep and thermal shock resistance limits their scope of application. To tackle these shortcomings, adjustments can be performed at the microscale by lowering the atomic diffusivity and reducing stress concentrations [8][9][10][11][12][13], which are achievable in materials with amorphous networks. It has been reported that in materials having three or more elements with different and specific coordination numbers, the formation of a crystalline phase can be sufficiently suppressed [1,[14][15][16][17]. However, the adaptability of this concept to nitride materials is challenging. Due to their low self-diffusion coefficients, ternary nitrides are difficult to be prepared via solid-state synthesis routes. In this context, the polymer-derived ceramic (PDC) route proves itself to be a promising method for preparing multielement ceramic nanocomposites. As polymer precursors can be tailored at the molecular level, end products with tunable chemical composition and homogeneous element distribution can be consequently achieved [18][19][20][21][22][23].
In recent years, ternary ceramics composed of Si, B, and N have attracted widespread attention due to their advanced properties as compared to single-component ceramics of Si 3 N 4 and BN. Particularly, SiBN ceramics and Si 3 N 4 /BN composites with low dielectric constant are considered to be the most promising microwavetransparent materials for use in microwave windows and radomes under harsh environments. Various studies have been reported regarding the synthesis of ternary SiBN ceramics via molecular and PDC routes [14,[24][25][26][27][28][29][30][31]. However, most of the reported polymer precursors used to prepare SiBN ceramics inevitably contain a certain amount of carbon. In order to eliminate the influence of carbon, complex and expensive pyrolysis (e.g., pyrolysis under NH 3 atmosphere) and annealing (under N 2 atmosphere) processes have to be applied to synthesize carbon-free SiBN ceramics [14,24,26,27,32]. Not only that, chlorine-containing precursors were employed, in which the chlorine is difficult to be removed during pyrolysis. Moreover, most studies have focused on the modification of precursors and the polymer-to-ceramic transformation, exclusively, and there is limited research on amorphous SiBN ceramic and its thermal stability.
In this work, carbon-and chlorine-free single-source precursor (boron-modified perhydropolysilazane) derived amorphous SiBN ceramics with varying chemical compositions were successfully prepared under N 2 (without expensive ammonolysis process). Molecular structure and the possible reaction mechanisms of the polymer precursors, as well as the relationship between microstructural evolution and the early stage crystallization of SiBN ceramics were investigated. Besides, the influence of Si/B ratio in the SiBN ceramics on both high temperature stability and resistance to crystallization was assessed. In addition to the experimental techniques, molecular dynamics (MD) simulation is employed to shed light onto the molecular structure and the atomic-level diffusion behavior of amorphous silicon nitride and SiBN, which provides a rational explanation for the polymer-toceramic transformation of SiBN ceramics.  The dried solid  single-source precursor was subsequently transferred  into Schlenk tube furnace under the protection of argon  and heated to 1100 for 2 h at a heating rate of ℃ 50 /h under a constant flow of N ℃ 2 . The as-pyrolyzed specimen was annealed at 1200 and 1300 for 2 h in ℃ N 2 atmosphere to investigate their high-temperature crystallization behavior and stability against thermal decomposition.

2 Characterizations
The synthesis and the crosslinking process of the single-source precursors were analyzed ex situ by Fourier transform infrared (FTIR) spectroscopy on a Varian IR-670 spectrometer (Agilent Technologies, USA), using attenuated total reflection (ATR) mode in the range of 550-4000 cm -1 . Background corrections and baseline corrections of the spectra have been done automatically by the device. The quantitative elemental analyses of the synthesized precursor (BPSZ2) and BPSZ2-derived ceramic (pyrolyzed at 1100 ) were ℃ performed at Mikroanalytisches Labor Pascher (Remagen, Germany). Silicon and boron contents were measured using inductively coupled plasma-atomic emission spectrometry (ICP-AES). Hydrogen content was determined by combustion in pure oxygen and the detection of water using infrared spectroscopic. Nitrogen and oxygen contents in ceramic samples were detected by thermal conductivity and IR-analysis, respectively. Nitrogen and oxygen contents in polymeric sample were detected volumetrically and by thermal conductivity detection, respectively. Carbon analysis of samples annealed at 1300 was carried out by an LECO C ℃ -200 analyzer (LECO Corporation, St. Joseph, Michigan, USA). The phase compositions of the ceramic samples were investigated by X-ray diffraction (XRD) with a STADI P powder diffractometer (STOE & Cie GmbH, Germany, Mo Kα1 radiation source). To investigate the microstructure of the as-obtained ceramics, transmission electron microscopy (TEM) experiments were performed on an atomic resolution microscope (ARM) Jeol JEM-200F (JEOL Ltd, Tokyo, Japan) under electron beam energy of 200 keV. The high-resolution TEM images were filtered using an average background subtraction filter or a Wiener filter [33]. In parallel, scanning electron microscopy (SEM) was employed with a Philips XL30 FEG high-resolution scanning electron microscope (FEI Company, Hillsboro, Oregon, USA), coupled with an energy-dispersive X-ray (EDX) spectroscope (Mahwah, New Jersey, USA). The polymerto-ceramic transformation of the different precursors was characterized by means of thermogravimetric analysis/differential thermal analysis (TGA/DTA) in flowing nitrogen atmosphere with an STA 449C Jupiter (Netzsch Gerätebau GmbH, Germany) in situ coupled with a mass spectrometer (MS, QMS 403C Aëolos, Netzsch Gerätebau GmbH, Germany). The heating rate was set at 5 ℃/min. TGA was employed as well to investigate the oxidation behavior of SiBN ceramic powders by observing their weight change from room temperature to 1350 ℃ with a heating rate of 5 ℃/min in synthetic air.

3 Molecular dynamics simulation
Molecular dynamics simulations of amorphous silicon nitride and SiBN ceramics have been conducted using the large-scale atomic/molecular massively parallel simulator (LAMMPS) [34]. Many-body Tersoff potentials [35] were employed to describe the interactions among Si, B, and N. Structural properties, e.g., coordination number and pair distribution functions, of amorphous silicon nitride and SiBN materials simulated using this Tersoff potential have been demonstrated to be reasonably reproduced compared with experimental measurements [36]. Four amorphous ceramics systems, namely Si 3 N 4 , Si 3 B 0.6 N 4.6 , Si 3 B 1.5 N 5.5 , and Si 3 B 3 N 7 , have been simulated to study the influence of the boron content on the thermal stability of the amorphous ceramics at temperature T = 1300 ℃, similar to the experiments. Initially, around 10,000 atoms were randomly placed in the simulation box at a density of ~2.5 g/cm 3 (consistent with the experimental measurement for the bulk material at ~1500 K [37]) for all systems, giving a cubic dimension of ~5 nm. Details of the simulated systems can be found in the Electronic Supplementary Material (ESM). Subsequently, the system was pre-equilibrated for 20 ps at T = 8000 K to eliminate the impact of the initial atomic arrangement and to avoid its trapping in local energy minima. A fast quench under constant volume to T = 1300 ℃ followed. The quenched system was then subjected to an additional equilibration of 100 ps. The equilibration of the simulation was monitored via tracking its total potential energy. After about 20 ps, the potential energy reaches a plateau as seen in an example of Si 3 B 3 N 7 given in the ESM. After equilibration, the simulation was continued as production run (~5000 ps) for the www.springer.com/journal/40145 extraction of data. All simulations were performed with a time step 0.2 fs under constant temperature and volume conditions using a Nose-Hoover thermostat with a coupling parameter of 0.02 ps [38,39].
The structural properties of amorphous Si 3 N 4 , Si 3 B 0.6 N 4.6 , Si 3 B 1.5 N 5.5 , and Si 3 B 3 N 7 have been characterized via pair distribution functions, defined as where V is the volume of the simulation box. i N and j N denote the number of atoms of elements i and j , respectively.
( ) j N r  represents the number of atoms of element j within a shell of thickness dr at the distance of r . The coordination number of element j around element i can then be calculated by where c r is the position of the first minimum in the pair distribution function between elements i and j . j  is the average number density of element j .
The atomic dynamics have been measured via the mean-square-displacement (MSD), given by where <·> denotes the ensemble average and ( ) r t is the atomic position vector at time t . The diffusion coefficient has been derived by the Einstein relation: 3 Results and discussion  (2), indicating boron loss during the   synthesis process. The loss of boron is discussed in terms of (i) the evaporation of BMS during the synthesis and drying process and (ii) borane did not react with PHPS stoichiometrically due to steric hindrance imposed by the polymer structure, which is in good agreement with the observation of Viard et al. [40]. The found amounts of carbon and oxygen are attributed to the residual solvent dibutyl ether and the water contamination during the elemental analysis. The elemental content of the ceramics will be discussed in Section 3.2.

2 Polymer-to-ceramic conversion
The polymer-to-ceramic transformation and ceramic yield of the single-source precursors BPSZX were investigated via a combination of ex situ FTIR and in situ TGA/DTA measurements coupled with evolved gas analysis (EGA). As demonstrated in the TG curves ( Fig. 3(a)), starting from room temperature to approximately 170 , the dried PHPS shows a minor ℃ weight loss of 1 wt%, while the BPSZ5, BPSZ2, and BPSZ1 samples present obvious mass loss of 4, 3, and 8 wt%, respectively. Volatiles (m/z = 27-72) identified in the MS results (partly shown in Fig. 3(c) and Figs. S1(b) and S1(c) in the ESM) indicate that the mass loss of boron-modified BPSZX samples is due to the volatilization and decomposition of low-molecular-weight oligomers, especially silane and solvent (dibutyl ether). In addition, the exothermic peak of BPSZ2 shown in Fig. 3(b) indicates proceeding dehydrocoupling reactions between Si-H (N-H) and B-H groups, with the release of H 2 that has been detected with MS shown in Fig.  3(c). This assumption is further supported by the FTIR spectra of BPSZ2 shown in Fig. 4. The characteristic absorption bands of Si-H, N-H, and B-H groups decrease considerably, and the bands of C-H vibrations caused by the solvent disappear completely.
In the temperature range of 170-800 , the dried ℃ PHPS shows a major weight loss of 22 wt%, where typical crosslinking reactions via dehydrocoupling and transamination take place, along with bond redistribution and thermal decomposition [43]. The weight loss is completed at ~800 ℃ with a ceramic yield of 77 wt%, which is in accordance with former reported data [44]. With the addition of boron, the weight loss of the precursor is significantly reduced from 23 wt% to at least 12 wt%. The second weight loss of BPSZX samples occurs between 170 and ~600 ℃. Similar to the dried PHPS, crosslinking of residual Si-H, N-H, and B-H groups takes place through dehydrocoupling reactions, which is confirmed by the continuous release of H 2 detected by MS and supported by the corresponding ex situ FTIR spectra. However, due to the large consumption of N-H groups during the synthesis of the precursors, there is no obvious detection of NH 3 in the TGA/EGA experiment, indicating that only few transamination reactions occur. Thanks to the  high crosslinking degree of BPSZX samples due to the incorporation of boron into PHPS, depolymerization and thermolysis at higher temperature are diminished. Therefore, a significantly increased ceramic yield is obtained with the boron-modified precursors.
Beyond 600 ℃, the thermal behavior of the BPSZX samples with different Si/B molar ratio differs in the TGA measurements. The BPSZ2 sample exhibits the best thermal stability with a negligible mass loss and has a final ceramic yield of 92 wt% at 1300 ℃, indicating that the polymer-to-ceramic transformation has been already accomplished at ~600 ℃. The ceramization process is supported by the ex situ FTIR spectra (Fig. 4), where the absorption bands of Si-H and N-H almost completely vanish beyond 500 . On ℃ this account, further investigations focus more on the BPSZ2 sample. With less boron content, the BPSZ5 sample goes through a sudden mass loss (3 wt%) between 800 and 930 ℃, which is attributed to the thermal decomposition of the preceramic polymer, as confirmed by the endothermic peak shown in the DTA curve ( Fig. S1(a) in the ESM). Nevertheless, the ceramic yield of BPSZ5 is still higher than that of dried and pristine PHPS, and ends up as 88 wt% at 1300 . With more boron incorporation, the BPSZ1 ℃ sample gains 3 wt% of mass between 600 and 1300 , ℃ and has a final ceramic yield of ~91 wt%. The analyzed mass gain is ascribed to the reaction between residual boron and N 2 [45,46]. Additionally, the dried PHPS shows a mass gain of 6 wt% at ~1250 ℃ due to the crystallization process in N 2 , which will be discussed in Section 3.3.
The chemical composition of the obtained BPSZ2derived ceramic pyrolyzed at 1100 shown in Table 1 ℃ confirms the formation of SiBN ceramic. During the polymer-to-ceramic conversion, another fraction of boron is lost due to the further volatilization of borane species, resulting in a final Si/B molar ratio of 10.51. In addition, the elemental analysis of BPSZ-derived ceramics (Table 2) confirms the removal of carbon after annealing at 1300 ℃. In summary, compared with the former reported ceramic yield (~55 wt% [27], 72 wt% [32]) of SiBN ceramics obtained via other synthesis methods, higher ceramic yield was achieved with the current method (88-92 wt%).

3 Crystallization and microstructural evolution of the as-prepared ceramics
The structural evolution and crystallization behavior of annealed PHPS and BPSZX samples have been investigated after heat treatment at various temperatures (1100, 1200, and 1300 ) by XRD, TEM, and EDX ℃ spectroscopy. The XRD patterns of the PHPS-and BPSZX-derived ceramics obtained upon annealing at different temperatures are shown in Fig. 5. At 1100 , ℃ all the as-prepared ceramics are predominantly X-ray amorphous. At 1200 ℃, the boron-free PHPS-derived specimen exhibits a low-intensity reflection of α-Si 3 N 4 , while all the BPSZX-derived ceramics remain X-ray amorphous. As the annealing temperature further increases to 1300 ℃, crystallization of α-Si 3 N 4 is induced in the annealed PHPS sample. According to the weight gain (6 wt%) presented in the TG curve and the exothermal peak shown in the DTA curve ( Fig. 3(b)), the crystallization temperature of PHPS-derived ceramic is suggested to be in the range of 1200-1300 ℃ [47]. In the meantime, as the boron content increases in the BPSZX samples, the emergence of α-Si 3 N 4 is hindered. The crystallization of BPSZ5-and BPSZ2-derived ceramics gradually takes place at temperature up to 1300 ℃, while the XRD pattern of the BPSZ1-derived ceramic is still X-ray amorphous. These results clearly demonstrate that the incorporation of boron into Si-N network suppresses the crystallization of Si 3 N 4 , which is attributed to the higher crystallization temperature of the formed ternary Si-B-N system, and which thereby restricts the crystallization of both Si 3 N 4 and BN [15,48].  The microstructure of the as-prepared PHPS and BPSZ2-derived ceramics annealed at 1300 were ℃ further investigated by TEM along with the selected area electron diffraction (SAED) pattern, SEM, and EDX spectroscopy. As shown in Fig. 6(a), the high-resolution TEM (HRTEM) image of the annealed PHPS-derived ceramic exhibits a typical nanocrystalline structure with a lattice spacing of 0.21 nm, which corresponds to the d-spacing of the (202) α-Si 3 N 4 lattice plane (JPCDS Card No. 09-0250). Furthermore, the SAED pattern (inset in Fig. 6(a)) presents diffraction spots of α-Si 3 N 4 crystals, which agrees with the XRD results (Fig. 5). As revealed by Fig. 6(b), both the TEM image and the SAED pattern affirm that the as-prepared BPSZ2-derived ceramic remains amorphous, which is consistent with the results obtained from the XRD as well. The SEM micrograph ( Fig. 7(a)) of BPSZ2-derived ceramic annealed at 1300 shows a dense structure. As can be ℃

4 Molecular dynamics simulations
According to previous theoretical calculation studies [16,36,49], boron incorporation has a significant influence on the crosslinking and pyrolysis processes of Si-N networks. Short-range atomic arrangements are characterized by total and partial pair distribution functions (PDF) of amorphous Si 3 N 4 and SiBN ceramics with different boron content. As shown in Figs. 8(a) and 8(b), all PDFs converge to a constant value of 1 for radii larger than 0.5 nm, indicating long-range amorphous disorder in the ceramics. As shown in Fig. 8(b), the first peak of SiBN ceramics at a distance of 0.149 nm is not only closer than that of any other pair, but also much higher. It corresponds to B-N distances and indicates the strong bonding between these elements. This characteristic distance is in good agreement with the range of B-N bond lengths obtained from experiments and density functional theory calculations (0.144-0.156 nm) [50,51]. Moreover, the height of this peak increases with boron content, suggesting that more boron atoms surrounding any given nitrogen atom. Sharp peaks at a distance of ~0.17 nm are seen in the total PDFs for all investigated ceramics, which are due to Si-N pairs in the partial PDFs. The Si-N bond length is found here to be roughly 0.17 nm, which is similar to the value estimated from experiments (0.173-0.175 nm) [52][53][54]. In addition, peak for Si-B pairs (~0.28 nm) has a much lower height than that for Si-N or B-N observed in the partial PDFs of SiBN ceramics, suggesting that the formation of Si-B bonds is disfavored. From the PDFs, the coordination numbers have been calculated. In Fig. 8(c), the coordination numbers of central Si and N atoms are shown as a function of the B/Si atomic ratio. The coordination number of Si surrounded by B (Si-B) increases as the B/Si atomic ratio increases, which implies that the Si-B bonds are more likely to form when more boron atoms are added. In the meantime, the coordination number of Si (N) (Si-N) slightly decreases. On the other hand, the coordination number of N (Si) (N-Si) decreases rapidly as B/Si atomic ratio increases, while the coordination number of N (B) (N-B) increases, indicating that the boron incorporation influences more on the N atoms than Si atoms.
The atomic mobility was also investigated in amorphous silicon nitride and various SiBN ceramics. The obtained mean-square displacements (MSDs) of Si, N, and B atoms are shown in Figs. 9(a)-9(c), respectively. The MSDs of Si are similar in all investigated ceramics, indicating a minor influence of boron incorporation on the Si diffusivity. In contrast, the MSDs of N in all SiBN ceramics are lower than that in Si 3 N 4, and they decrease further with increasing boron content. This suggests that the incorporation of boron into the Si-N network restricts the atomic motion of both boron and nitrogen in the Si-B-N systems because of the formation of B-N bonds. These B-N bonds are not only copious (cf. Fig. 8), but also strong, and the system stability is hence improved.
From the limiting slopes of MSD curves, the atomic self-diffusion coefficients have been evaluated using the Einstein relation (Eq. (4)) [55]. For each element, its slope has been obtained from a linear part of the respective MSD within the time period of 900-1100 ps. As shown in Fig. 9(d), the self-diffusion coefficient of silicon is basically unaffected by the boron concentration within the margin of error. In contrast, the atomic diffusion coefficients of nitrogen and boron decrease significantly with increasing boron content, consistent with the formation of B-N bonds as seen in the peaks of B-N pairs in the pair distribution functions (Fig. 8). This is due to the strong mutual attraction of B and N, which in the Tersoff potential is much stronger than Si-B and B-B interactions. Furthermore, the presence of B-N bonds prevents N atoms from diffusing in the Si-B-N network and plays the part of rigid linkages. This impedes the formation of crystallites even at high temperatures. Similar behaviors are also found in the relaxation properties such as Van-Hove correlation functions and intermediate scattering functions in the ceramics with different boron contents. Detailed discussion can be found in the ESM. In conclusion, the addition of boron enables the formation of rigid B-N bonds and consequently reduces the atomic diffusivity of nitrogen atom in the network, and therefore, the thermal stability of amorphous Si-B-N is considerably enhanced relative to amorphous Si-N.

5 Oxidation behavior
The BPSZX-derived ceramics were annealed at 1300 ℃ and oxidized in air at a temperature up to 1350 ℃. The oxidation resistance of the corresponding ceramics was studied by the TG curves shown in Fig. 10, in which the triangle markers represent the amorphous silicon nitride data extracted from the work of Ma et al. [56]. Starting from 30 to 1000 , the amorphous silicon ℃ nitride exhibits a significant weight gain of 25.7 wt%. ) perfor ℃ med within a temperature range from ambient temperature to 1350 ℃ in air. TG data points of amorphous silicon nitride are extracted from the work of Ma et al. [56].
It is well documented in the literature that large numbers of unsaturated Si-N and Si dangling bonds in amorphous silicon nitride, such as Si dangling bonds and N vacancies, lead to severe vacancy oxidation (nitrogen vacancies occupied by oxygen atoms) and replacement oxidation (nitrogen atoms being replaced by oxygen atoms) [56,57]. In contrast, amorphous BPSZ5-derived ceramic displays negligible weight gain of 0.8 wt%, and the BPSZ2-, and BPSZ1-derived ceramics present no weight change at the temperature of 1000 . Moreover, by oxidation at 1350 , the ℃ ℃ BPSZ5-, BPSZ2-, and BPSZ1-derived amorphous ceramics yield mass gains of 5.1, 2.1, and 2.4 wt%, respectively, indicating improved oxidation resistance if compared with amorphous silicon nitride. As aforementioned, the presence of B atoms in amorphous Si-N networks not only occupies part of the unsaturated spaces, but also promotes the formation of B-N rigid linkages, resulting in superior oxidation resistance compared to amorphous silicon nitride. The proposed oxidation mechanisms of SiBN ceramics are illustrated in Eqs. (5)-(10) [58,59], which are suggested to be responsible for the weight gains between 1000 and 1350 . Due to ℃ the formation of B 2 O 3 in the silica, boron silicate glass is formed, which limits the O 2 inward diffusion and further B 2 O 3 volatilization. Furthermore, as reported by former research studies [60,61], with the addition of boron, a dual surface layer of B-N-O/SiO 2 is formed during oxidation to protect the ceramic from further oxidation, and hence increases the oxidation resistance.

Conclusions
In the present work, ternary SiBN ceramics with different Si/B molar ratios have been successfully prepared via the PDC route starting from carbon-and chlorine-free single-source precursors, synthesized by modifying perhydropolysilazane (PHPS) with borane dimethyl sulfide complex (BMS). It has been proved that both Si-H and N-H bonds of the PHPS react with B-H bonds of BMS through dehydrocoupling reaction during the synthesis of the BPSZX precursors. The precursor-to-ceramic transformation was investigated systematically with respect to crosslinking and crystallization. The TGA and FTIR results reveal that incorporation of boron considerably improves the crosslinking degree of the preceramic polymer, and consequently, the ceramic yield increases from 77 wt% (PHPS) to 88-92 wt% (BPSZX) upon pyrolysis under N 2 atmosphere. Besides, without employing the costly ammonia, our reported method proves itself to be a facile and economic method for the synthesis of SiBN ceramic with high ceramic yield as opposed to former reported ammonolysis synthesis process. Furthermore, the resultant BPSZX-derived ceramics are shown to possess remarkable resistance against crystallization in comparison to PHPS-derived boron-free Si 3 N 4 ceramics. This characteristic is ascribed to the decreased selfdiffusion coefficient of nitrogen in the ternary ceramic, due to the formation of rigid B-N bonds in the Si-B-N network. In conclusion, the boron modification of Si-N via the PDC route presents a significant effect on its high-temperature resistance behavior with respect to crystallization and oxidation, which is of great significance for the design and manufacture of ternary Si-based and/or boron-containing ceramics. The influence of boron on the phase evolution, mechanical and functional properties of SiBN ceramics, such as Young's modulus, hardness, high-temperature self-healing capability, thermal and electrical conductivities, as well as the related applications have not been clarified unambiguously so far, and will be investigated in future studies.