3.1 PCS solution preparation and spinnability analysis
3.1.1 Dissolution of PCS in various solvent formulas
The mixed solvents were prepared as designed. First, DMF and C8H10 were blended according to the predetermined ratios and stirred with a magnetic mixer at 300 rpm for 1 h. Next, PCS was added to the mixed solvent, and the solution was placed in a thermostatic water bath under ultrasonic shock for 8 h at room temperature.
The results demonstrated that when the concentration of PCS exceeded 1.4 g/ml, the solution reached saturation and solute deposition was observed. Furthermore, when DMF accounted for more than 40% of the mixed solvent, PCS could no longer be completely dissolved, regardless of the amount added to the solvent, and solute deposition was also observed. This accords with the results of Yuan .
The solubility of PCS in each prepared solution is displayed in Fig. 7. The crosses represent situations where the solute could not be completely dissolved, and the circles indicate situations where solutes were completely dissolved. Solubility decreased with an increasing amount of DMF because of its inability to dissolve PCS.
The addition of DMF may benefit the spinning process  and reduce the formation of pores of the fiber surface because of its low volatility. To account for the importance of DMF in electrospinning, four solutions with relatively high amounts of DMF were selected (as shown in Fig. 7) to serve as specimens for the electrospinning experiment.
3.1.2 Viscosity in solution formulas
Electrospun fibers are easily affected by solution characteristics (viscosity, surface tension, polarity); therefore, four of the solution formulas were selected to perform a rheology experiment and examine the rheological characteristics of electrospun fibers. The results are displayed in Fig. 8.
As evident in Fig. 8, viscosity became high with an increase of PCS content. Under the same amount of PCS, the higher the amount of DMF taken up in a solvent was, the higher the solution’s viscosity would become because the increase in DMF led to a decrease in C8H10. Because DMF is a type of polar solvent in which PCS cannot dissolve, the increase in DMF content signified less C8H10 to dissolve PCS, which in turn increased viscosity. The fourth solution exhibited the highest viscosity because it had the highest ratio of C8H10 to PCS, whereas the third solution exhibited the lowest viscosity because it had the lowest ratio of C8H10 to PCS.
As depicted in Fig. 8, the solution concentration decreased with the increasing shear rate; viscosity remained constant when the shear rate reached 100 (1/s). This indicated that the PCS solutions were a type of pseudoplastic fluid among non-Newtonian fluids. A non-Newtonian fluid features high viscosity in a quiescent condition, whereas once it begins to flow, the viscosity drops instantly until it reaches a constant level.
PCS is a type of polymer. When PCS solutions were in a quiescent condition, the solutes became aggregated; therefore, the solution exhibited greater viscosity and seemed denser. However, when the shear rate was increased progressively, the solutes within the solutions began to flow and extend instead of aggregating; this led to shear thinning. When the shear rate reached a specific level, the solutes in the solution became completely extended, and the viscosity no longer decreased with increasing shear rate.
3.1.3 Measurement of contact angles of different solution formulas
The contact angle of a solution is one of the factors that affect electrospinning. The degree of surface tension determines whether the solution is able to form a hemispherical droplet at the tip of a spinning tube, in addition to affecting the success of the electrospinning process. A corrosion-resistant plate was employed as the base plate in this experiment to simulate a corrosion-resistant spinning tube. The measurement results are displayed in Fig. 9.
The result indicated that the viscosity increased with increasing contact angle width. The contact angles of the four solutions were all >90°. In general, the contact angle of a solution should be wider than 90° to garner sufficient cohesion to form a hemispherical droplet at the tip of the spinning tube. Under the effect of an electric field, a hemispherical droplet can easily form a Taylor cone and therefore a stable electrospinning jet.
3.2 PCS solution spinnability and fiber analysis
3.2.1 Comparison of the spinnability of PCS solutions
Table 4 depicts the characteristics of electrospinning products acquired from the four solutions under various voltages. In this table, the term “droplets” represents the circumstances where droplets could not be stretched into fibers under static electricity but were directly extruded onto the collector. The term “short fibers” denotes the circumstances in which fibers were broken into tiny fragments. The term “fibers and droplets” indicates how some fibers were obtained using the collector but PCS granules were observed within these fibers. Such granules were formed after tiny droplets, which were extruded onto the collector, became dried. The term “long fibers” indicates that a layer of nonwoven fabric was collected, and no granules were observed using unaided vision.
As evident in Table 4, the fourth solution could only produce a small amount of tiny fibers at 20 kV. The reason was that the fourth solution had the highest viscosity; once the solution came into contact with the air, it would block the tip of the spinning tube and impede the spinning process, as depicted in Fig. 10.
The third solution produced droplets only. This was because the solution exhibited the lowest viscosity; therefore, the solution particles aggregated because of the surface tension, becoming unable to be spun, and was spattered onto the collector as droplets.
Only the first and second solutions demonstrated greater spinnability. The two solutions contained higher amounts of DMF, exhibited high polarity, and were easily stretched under an electric field, all of which enabled the generation of fibers. The viscosity of the two solutions was between those of the third and fourth solutions. Droplets were less likely to be observed when the first or second solution was used, which implied that the viscosity of these two solutions was more applicable. Comparing the fibers synthesized using the first and second solutions under 20 kV and 25 kV revealed that the first solution generated long fibers of similar lengths, whereas tiny droplets were observed when the second solution was used. This indicated that the viscosity and surface tension in the first solution were more favorable. By contrast, the viscosity of the second solution was lower, and the surface tension was relatively higher, which led to a sharper and unstable jets during the spinning process and therefore yielded fibers of uneven size.
3.3 Analysis of PCS fiber electron microscopy
Fig. 11 displays the fibers synthesized using the first solution at 10, 15, 20, and 25 kV and under a 1000× magnification with a scanning electron microscope (SEM).
As evident in Fig. 11, the fibers synthesized at 10 kV possessed large diameters of approximately 10 μm; furthermore, droplets with diameters of 20 μm were also observed, which indicated that fibers were of uneven size. Because of the relatively low voltage, the tensile strength was insufficiently strong, which resulted in fibers with relatively large or even diameters. The diameters of the fibers became smaller when the voltage was increased to 15 kV, but droplets were still observed among fibers. On the surface of the droplets, tiny pores emerged because of the evaporation of the solvents. After the solution was extruded and made contact with the air, the outer layer began to solidify. The vapors within the solvent would then have to break through the outer solid layer to evaporate into the air. This resulted in the formation of surface pores. Additionally, PCS fibers have to be calcinated to become SiC. The pores would further expand under calcination, which affected the mechanical property of the fibers. Accordingly, pore formation should be avoided when synthesizing such fibers.
At 20 kV, the fibers exhibited even size and droplets emerged; the fiber diameter was approximately 1 μm. At 25 kV, no notable morphological differences were observed. However, comparing the fibers generated under 20 kV and 25 kV revealed that the fibers generated under 25 kV were thinner and more compressed. This indicated that the optimal voltage was 25 kV.
Fig. 12 depicts the fibers synthesized using the second solution at 20 kV and 25 kV and viewed under a 1000× magnification with an SEM. The fibers generated at 15 kV had too many droplets; therefore, they were not observable with an SEM.
In Fig. 12, the fibers formed under 20 kV included many droplets, and round droplets and solids were detected among the long fibers, which suggested that during the process of electrospinning, the fibers were extruded onto the collector before the static electricity was able to stretch them out. However, at 25 kV, long fibers were formed with fewer droplets, but spherical extrusions were clearly seen in the image. This was because the viscosity and the surface tension of the solvents were not balanced.
According to the images displayed in Fig. 11 and 12, increasing the voltage increased the fiber length. However, when the voltage was further increased, the influence of voltage on the fiber diameter became smaller, which corresponds to the results of . The viscosity of the first solution was higher and could offset the surface tension; therefore, the resulting fibers were most even in size. Given that this set of process parameters had the best performance, it was applied to subsequent processes to produce SiC fibers.
3.4 Curing process
3.4.1 Oxygen content of cured PCS fibers
The fibers were placed in a high-temperature furnace and heated to 200°C at an incremental rate of 5 °C/minute. Fig. 13 presents a graph based on the energy dispersive spectrometer analysis on the oxygen content with the temperature maintained for 0.25, 0.5, 1.0, and 2.0 h. The oxygen content increased with the temperature maintenance time, but the increased amount became smaller with a longer time. Increasing the oxygen content reduced the strength of the resulting SiC fibers. Accordingly, the fiber oxygen content should not be overly high. Previous scholars  concluded that fibers morphology can be maintained during calcination when the fiber oxygen content is kept at approximately 8.0 at%.
3.4.2 FTIR analysis before and after the curing process
Fig. 14 displays the FTIR spectra before and after the PCS curing process. The main apparent differences are the absorption peaks at 1080 cm-1 and 3700 cm-1, which correspond to the presence of Si-OH and Si-O-Si bonds, respectively. These bonds form an SiOx coating with a high melting point to maintain the fiber morphology during calcination.
During the curing process, after PCS was heated to 200°C, the Si-H and Si-C bonds were oxidized to form Si-OH and Si-O-Si bonds. Other side chains, including Si-CH3, Si-H, and C-H, reacted with one another and released H2 and CH4. Thus, the corresponding absorption peaks were weakened. In addition,  reported that the C-H stretching vibration at 2900 cm-1 was induced by the trace amount of DMF in the fibers.
3.4.3 SEM analysis after the curing process
Fig. 15 depicts the SEM images of the fibers after the curing process. The fibers were even in size, with an approximate diameter of 1 μm. The effect of curing on the fibers nonapparent because the temperature was only sustained for 0.5 h and because the oxygen content was low.
3.5 Fiber calcination
3.5.1 Thermal analysis of the calcination process
Fig. 16 displays the thermal analysis of the calcination process. The rate of temperature increase was 10°C/minute, and the experiment was conducted under a nitrogen flux. The thermogravimetric weight loss was divided into two phases. First, from 400°C to 800°C, the rapid weight loss (up to 40wt%) occurred due to the pyrolysis of the side chains and chemical bonds within the polymer. Second, from 800°C to 1100 °C, the weight loss converged (<2wt%) because of the emission of residual C, H, and related compounds. Three peaks were discovered when employing differential scanning calorimetry. The absorption peak at 200°C indicated the melting of PCS in the fibers. From 400°C to 800°C, the absorption peak corresponded to the pyrolysis of the fibers. This indicated that PCS was converted from organic compounds to ceramics in this stage. The absorption peak from 1100°C to 1200°C was speculated to correspond to the energy released by the SiC crystallization process.
3.5.2 X-ray diffraction analysis of the PCS fibers after calcination
The calcination temperature and its sustained time would affect the final SiC crystallization; hence, a small amount of the specimen was first applied to conduct the calcination experiment. An X-ray diffraction (XRD) spectrum analysis was employed to examine whether the crystallization was in a β-crystalline phase. The calcination occurred at an incremental rate of 10°C/min. Fig. 17 presents the XRD spectrum of PCS fibers after calcination. In the spectrum, no significant diffraction peaks were observed with temperature sustained for 1 h at 1100°C, which indicated that the specimen was an amorphous solid. After extending the temperature maintenance time to 3 h at the same temperature, weak peaks began to occur, and they all corresponded to the β-SiC diffraction peaks. This indicated that a β-crystalline phase could not be formed at 1100°C. Even if the temperature retention time was extended to 3 h, crystallization could not occur among the molecules. The specimen calcinated at 1200°C with temperature sustained for 2 h displayed notable diffraction peaks when the angle 2θ was 36°, 42°, 60°, and 72°, and the XRD curve corresponds to that recorded in the JCPDS file No.29-1129. This verified that the resulting product was β-SiC.
3.5.3 SEM analysis of SiC fibers
The fibers were inserted into a tubular furnace after the curing process and were heated to 1200°C at an increasing rate of 10°C /min in a nitrogen flux for 2 h, which pyrolyzed PCS into SiC. The diameter of the fibers was 0.8 μm, as presented in Fig. 18. The fibers were long, even, complete, and without pores.
3.5.4 Energy-dispersive X-ray spectroscopy analysis of SiC fibers
SiC fibers were subjected to energy-dispersive X-ray spectroscopy (EDS) analysis, as shown in Fig. 19. The percent composition of the SiC fibers was 57.3% C, 33.5% Si, and 9.2% O. Oxygen was introduced during the curing process to form a protective coating. The excessive amount of carbon was derived from the polymer side chain, a common byproduct in the Si-H compound pyrolysis process.
3.5.5 Transmission electron microscope analysis of SiC fibers
The selected area diffraction and the crystal lattice of SiC fibers are displayed in Fig. 20 and 21, respectively. In Fig. 20, comparing the SiC selected area diffraction results to those in  revealed that the crystal phase was a simple cubic structure and the Miller index was 111, which accords with the relevant literature. The result of the selected area diffraction was verified to be β-SiC fibers because the β crystal phase was in a simple cubic structure. Previous scholars  indicated that the crystal lattice spacing was approximately 0.26 nm. The result in Fig. 21 matches the result of that study.