Improved crystallisation of polysilazane-derived Si 3 N 4 /SiC nanocomposites with Fe 2 O 3 catalyst

In the present study, the use of a 3 wt% Fe 2 O 3 catalyst improved the crystallisation of polysilazane PSN2. Crystallisation peaks of higher intensity were observed for Si 3 N 4 and SiC at 1250 ◦ C by using a 3 wt% Fe 2 O 3 catalyst than at 1500 ◦ C without Fe 2 O 3 . Besides, the effects of temperature on the morphology of the samples were determined through scanning electron microscopy. Porous structures were observed at 1250 ◦ C and these structures were transformed to ﬂaky and near-spherical nanoparticles as the temperatures were increased to 1350 and 1450 ◦ C, respectively. Finally, the mechanism of Fe origination from Fe 2 O 3 reduction, the production of a supersaturated liquid alloy, and the release and diffu-sion of N 2 were proposed for Si 3 N 4 formation.


INTRODUCTION
Si 3 N 4 /SiC composite ceramics have attracted increasing attention for many engineering applications, such as turbine engines, garbage incineration boilers, and rocket nozzle, because of their outstanding mechanical properties, excellent thermal shock resistance, and corrosion resistance [1,2]. Polysilazane, a class of inorganic polymers with silicon nitrogen bonds as a repeating unit in the main chain of the molecule, could be an appropriate precursor for Si 3 N 4 /SiC synthesis [3]. In recent years, the processing of polymer-derived ceramics (PDCs) has gained a remarkable interest because of its unique advantages compared with conventional ceramic processes, such as low ceramization temperature, simple processing technology, outstanding performance at high temperature, and excellent uniformity of the composition [4]. Polysilazane polymers are precursors to amorphous SiCN when they are pyrolysed at a lower temperature (≈1100 • C). Afterwards, they continue to crystallise into Si 3 N 4 /SiC as the treatment temperature is increased. Numerous studies have investigated the crystallisation behaviour and phase evolution of polysilazane under different conditions such as annealing temperature, atmosphere, and dwell time [4][5][6][7][8][9]. Polysilazane-derived SiCN ceramics are basically amorphous and do not crystallise up to 1400 • C or even higher [4,8]. Strong et al. [7] showed that a novel NH 3controlled pyrolysis approach can be used to tailor the C:N content of SiCN powders for Si 3 N 4 /SiC-based nanocompos-ites obtained at 1650 • C. Golczewski et al. [8] reported that polysilazane could be transformed into Si 3 N 4 and SiC with free carbon by a carbothermal reaction (Equation (1)) at a temperature of more than 1484 • C. PDC processing is known to be associated with several disadvantages, including (i) Si 3 N 4 /SiC cannot be prepared efficiently at lower temperatures and (ii) at a temperature higher than 1484 • C, a low Si 3 N 4 /SiC ceramic yield is obtained, wasting energy and increasing the production costs.
The introduced transition metals and their compounds are known to effectively accelerate the formation of Si 3 N 4 and SiC at a reduced temperature (1300 • C) [10][11][12]. In several studies, catalysts were used to synthesise nano-scaled Si 3 N 4 and SiC by the PDC route. Yang et al. [13][14][15] reported the synthesis of Si 3 N 4 nanobelts, Si 3 N 4 nanowires, and SiC nanorods with the addition of FeCl 2 into the cross-linked polysilazane powder, and this mixture is subsequently pyrolysed at the elevated temperatures in N 2 atmosphere. Vakifahmetoglu et al. [16] produced Si 3 N 4 and SiC nanowires on the cell walls of polymer-derived ceramic foams through catalyst-assisted pyrolysis in the presence of FeCl 2 . Other researchers mainly focused on the synthesis of Fe-containing Si-C-N magnetic ceramic by adding various Fe compounds [17][18][19][20]. However, no study has reported the synthesis of highly crystallised Si 3 N 4 /SiC nanocomposites derived from polysilazane in the presence of a Fe-based catalyst.
In this study, we investigated a more convenient pathway of synthesising highly crystallised Si 3 N 4 /SiC nanocomposites from the polysilazane PSN2 by adding 3 wt% Fe 2 O 3 additives at a relatively low temperature (1250 • C) and under an inert (N 2 ) atmosphere. The influence of temperature on the microstructures and phase compositions of the prepared materials was studied systematically, and the growth mechanism was discussed in detail.

EXPERIMENTAL
In this study, a commercially available polysilazane (PSN2, Institute of Chemistry, Chinese Academy of Science) was used as the precursor. Iron oxide powder (Fe 2 O 3 , analytical-grade purity, Tianjin Yuanli Chem. Co. Ltd., Tianjin, China) was used as the catalyst. In the first instance, 3 wt% Fe 2 O 3 was mixed with liquid PSN-2 by ultrasonication for 1 h and then by electromagnetic stirring for 1 h more. Afterwards, the as-obtained mixtures were cross-linked at 130 • C for 2 h in a vacuum. The crosslinked solid was put into an alumina crucible and subsequently pyrolysed at different temperatures (i.e. 1100, 1250, 1350, and 1450 • C) for 4 h in an oven with purging in N 2 atmosphere. The experiments were also performed on the sample without Fe 2 O 3 annealed at 1100-1500 • C for 4 h. The phase composition was characterised through X-ray diffractometry (XRD, Bruker D8 Advanced, Germany) by using a Cu Kα radiation source ( CuKα = 0.15406 nm) at voltagecurrent settings of 40 kV and 40 mA in the 20 • -65 • (2θ) regime. The micro/nanostructures were characterised through scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy (SEM and EDS, S4800, Hitachi, Tokyo, Japan).

RESULTS AND DISCUSSION
First, we investigated the crystallisation of polysilazane PSN2derived samples without Fe 2 O 3 . Figure 1 presents the XRD patterns of the PSN2-derived samples at various temperatures without an additive after a thermal treatment of 4 h. The PSN2 pyrolysed at 1100 • C was completely amorphous. The sample obtained at 1300 • C gave two large diffuse peaks at approximately 2θ = 36 • and 2θ = 61 • , revealing that its amorphous network was starting to become ordered. As the temperature of thermal treatment increased from 1300 to 1500 • C, the decrease in the full-width at half maximum and the increase in the intensity of the two broad peaks suggested a slightly increase in the crystallisation degree. However, the PSN2-derived Si 3 N 4 /SiC was poorly crystallised without the catalyst Fe 2 O 3 even at 1500 • C.   Figure 2(a), the sample pyrolysed at 1100 • C consists of only two phases: C (PDF#89-8487) and C-Fe-Si (PDF#47-1293). When the temperature increased to 1250 • C, more diffraction peaks were detected; these peaks were attributable to α-Si 3 N 4 (PDF #41-0360), SiC (PDF #49-1430), C, and C-Fe-Si. With a further increase in temperature, the intensity of FW(S) is the width among two or more diffraction peaks with half height; 2θ is the Brag angle (rad); d is the average crystal size (Å); ε is the microstrain; and CuKα = 0.15406 nm.
The effect of the Fe 2 O 3 catalyst on Si 3 N 4 crystallisation was determined. As displayed in Figure 2(b), 1 wt% Fe 2 O 3 induced the formation of Si 3 N 4 at 1250 • C. However, the crystallisation was low. With an increase in the Fe 2 O 3 dosage to 3 wt%, a high degree of crystallisation was observed for Si 3 N 4 , and it remained unchanged with a further increase in the Fe 2 O 3 dosage to 5 wt%. Therefore, 3 wt% Fe 2 O 3 was chosen in the present study.
SEM was performed on samples pyrolysed at 1100, 1250, 1350, and 1450 • C with 3 wt% Fe 2 O 3 and on the sample pyrolysed at 1100 • C without Fe 2 O 3 , used as a reference for comparison. Figure 3(a) shows that the sample obtained without the catalyst was amorphous SiCN, and this is consistent with XRD analysis results. Compared with Figure 3(a,b) shows that lots of particles are distributed in the SiCN matrix of the first sample (1100 • C) obtained with the catalyst, which is related to cooled particles of C-Fe-Si alloy. When the temperature was increased to 1250 • C, many pores appeared (Figure 4(a)). The formation of pores might be related to the reaction between SiCN and Fe, which lead to the formation of the molten C-Fe-Si alloy and release of the nitrogen in SiCN as N 2 . Although this reaction also occurred at 1100 • C (as the phase of C-Fe-Si was identified), no apparent pores were observed through SEM (Figure 3(b)), because the reaction rate and the formation and the release of N 2 were probably slower at 1100 • C. Therefore, the pores formed were smaller and were below the detection limit of SEM. When the temperature was increased to 1250 • C, the reaction occurred more strongly and larger pores were formed (Figure 4(a)). When the temperature of the thermal  Figure 3(b) entirely transformed into large amounts of Si 3 N 4 or SiC nano-scaled particles with flaky and near-spherical shapes (Figure 4(b-d)) of diameters 20-100 nm. Meanwhile, as displayed in Figure 3(c), Fe 2 O 3 particles of sizes 50-200 nm could be obtained. Upon ceasing heating, the particle size increased significantly to 250 nm, as shown in Figure 3(b), hinting that Fe 2 O 3 particles facilitate the formation of Si 3 N 4 /SiC nanocomposites.
Moreover, the elementary compositions of the prepared nanocomposites were analysed, as shown in Figure 5. The Based on the aforementioned results of XRD and SEM investigations, the synthesis mechanism of Si 3 N 4 /SiC nanocomposites can be proposed as shown in Figure 6. Our studies revealed that the growth mechanism of Si 3 N 4 /SiC nanocomposites involves dissolution, reaction, and precipitation. As shown in Figure 6(a), Fe 2 O 3 is first reduced by pyrolysis products CH 4 and H 2 into Fe, and PSN2 is decomposed thermally into SiCN at approximately 1000 • C. As shown in Figure 6(b), the amorphous SiCN reacts with Fe to form a liquid C-Fe-Si alloy at a high temperature and N 2 gas is released, which leads to the formation of pores. The further reaction between SiCN and C-Fe-Si leads to the formation of a supersaturated liquid alloy, which is rich in Si and C and then precipitates. Under the N 2 atmosphere, the silicon nitride has the most Meanwhile, a small amount of SiC is precipitated.
The EDS results (Figure 7) showed the difference in the distribution of the N element at 1250 and 1450 • C. The difference between the two temperatures could be explained with the introduction of the nitrogen element from the N 2 atmosphere, instead of being generated by the SiCN matrix. Moreover, temperature is an essential factor affecting the formation of Si 3 N 4 /SiC nanomaterials, as the energy at 1100 • C is not sufficient for the reaction between N 2 and Si in the liquid C-Fe-Si alloy. However, on the other hand, the reaction occurred and Si 3 N 4 and SiC started to precipitate and grow subsequently with the increased temperature ( Figure 2(b-d)).

CONCLUSION
In this work, crystallised Si 3 N 4 /SiC nanocomposites were successfully synthesised from the polysilazane PSN2. The 3 wt% Fe 2 O 3 catalyst lowered down the crystallisation temperature from 1500 to 1250 • C. As the pyrolysis temperature was elevated from 1250 to 1350 • C and 1450 • C, the porous structure transformed into flaky and near-spherical nanoparticles on Si 3 N 4 /SiC nanocomposites. Furthermore, the production of Fe from Fe 2 O 3 reduction by CH 4 and H 2 caused the transformation of amorphous SiCN to a liquid C-Fe-Si alloy. Afterwards, the reaction of SiCN with C-Fe-Si led to the formation of a Siand C-rich supersaturated liquid alloy, which then precipitated. Besides, the N 2 that diffused through the pores facilitated the formation of Si 3 N4 through its reaction with Si in the liquid C-Fe-Si alloy.