## 3 Results and Discussion
### Curing Ability Characterization
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Powder**} & **Refractive** & **Solid** & **Light** & **Exposure** & **Cure** \\  & **Index** & **Loading/vol.\%** & **Intensity/\%** & **Time/s** & **Depth/\(\upmu\)m** \\ \hline Al\({}_{2}\)O\({}_{3}\) & 1.77 & 42 & 10 & 1.5 & \(121\pm 4\) \\ ZrO\({}_{2}\) & 2.20 & 42 & 10 & 1.5 & \(58\pm 2\) \\ Si\({}_{3}\)N\({}_{4}\) (Eno) & 2.10 & 42 & 80 & 10 & \(38\pm 1\) \\ \hline \hline \end{tabular}
\end{table} 
Table 3: The influence of powder type with different refractive index on slurry curing ability. of powder, the less light energy will act on the photosensitive resin, and the worse the curing ability will exhibit [34]. As Figure 6 shows, in addition to the high refractive index, the absorbance of Si\({}_{3}\)N\({}_{4}\) powder also should not be ignored. As a result, the stereolithography of Si\({}_{3}\)N\({}_{4}\) is more difficult than that of ZrO\({}_{2}\) and seldom reported. Unless otherwise specified, the Eno-Si\({}_{3}\)N\({}_{4}\) powder was selected as the filler in slurries because it owns low absorbance. Figure 7 shows the cure depth results and corresponding optical pictures of different slurries. For the ceramic printer Adamaflex 130, about 0.58 mW/cm\({}^{2}\) corresponds to a percentage of the light intensity. As predicted, the cure depth gradually increases as exposure energy dose increases (Figure 7a,d). According to Equation (1), the cure depth has a linear correlation with the mean particle size. This was studied and reported in many papers by numerical model [46] and conducting experiments [34]. However, the larger grain can impose negative impact on the mechanical performance of structural ceramics in agreement with the Hall-Petch relationship [47]. Due to the grain growth during the densification process, researchers tend to use the as-received powder with a size of nano- to submicrometer to produce high-performance ceramic materials. As for the fabrication of Si\({}_{3}\)N\({}_{4}\) ceramics, a forming method named sintered reaction-bonded silicon nitride (SRBSN) is well-known, which uses the relative cheaper Si raw powders instead of Si\({}_{3}\)N\({}_{4}\) powder [48]. Herein, we replaced a portion of Si\({}_{3}\)N\({}_{4}\) powder with the coarse 5 \(\upmu\)m Si powder to see whether it can help to improve the curing ability of slurries. Figure 7a-c shows the results of the slurries with a mixture of silicon and Eno-Si\({}_{3}\)N\({}_{4}\) powder. It can be seen that a higher solid loading can result in a lower cure depth. However, even under an extreme high exposure condition (light intensity: 70%, exposure time: 40 s, energy density: 1.61 J/cm\({}^{2}\)) the cure  
Figure 5: Colors and RGB values of raw powders. Figure 6: The absorbance of raw powders. depth of slurries containing silicon powder was still extreme low (less than 7 \(\upmu\)m) when compared with those slurries without containing silicon powder. It can be attributed to the higher absorbance of silicon powder (0.86 at 405 nm) than that of Si\({}_{3}\)N\({}_{4}\) powder (0.18 at 405 nm). As shown in Figure 7d, for the slurries without silicon powder, using NPG2 PODA as crosslinking agent can obtain a higher cure depth than that of using HDDA at the same condition. In the meantime, however, it can be seen in Figure 7e,\(f\) that a more serious \"over-cure\" phenomenon in the lateral direction can be observed with the naked eye by adding NPG2 PODA. This result, appearing as the probable cause of the higher molecular weight which NPG2 PODA possesses, deserves further investigation and will be discussed in our future work. Based on the as-obtained Si\({}_{3}\)N\({}_{4}\) slurry (35 SN-HDDA), stereolithography-based additive manufacturing was conducted. The printing parameters were set as 30% for the light intensity, 10 s for the exposure time and 20 \(\upmu\)m for the layer thickness in this work. Figure 8 presents the stereolithography of both complex-shaped parts and cuboid-like samples for testing.