## 3 Results and Discussion
### Curing Ability Characterization  
Suitable rheological behavior is important for a satisfactory layer recoating and getting homogeneous green body microstructure. In addition, the suspension must exhibit enough curing ability because it is essential to ensure a good connection and cohesion between the photo-polymerized layers. According to Equations (1) and (2), the larger the refractive index difference between ceramic powder and resin, the lower the cure depth. Many researches contributed the difficulty of stereolithography Si\({}_{3}\)N\({}_{4}\) to its high refractive index [31; 36; 43]. However, there are also a few works considering the absorbance of Si\({}_{3}\)N\({}_{4}\) powder [35]. Therefore, we firstly characterized the cure depth of slurries with different filler type but same solid loading. As shown in Table 3, for the white-colored system (Al\({}_{2}\)O  
Figure 3: Rheological curves of slurries with different formulation. and ZrO\({}_{2}\)), the cure depth decreased significantly with the increase in refractive index of powder. At a relatively low exposure condition (Light intensity: 10%, exposure time: 1.5 s), the Al\({}_{2}\)O\({}_{3}\)-slurry's cure depth is more than twice that of ZrO\({}_{2}\)-slurry. However, when comparing Si\({}_{3}\)N\({}_{4}\)- and ZrO\({}_{2}\)-slurries, it was found that the Si\({}_{3}\)N\({}_{4}\) powder filler can make it difficult for the slurry to be curable, although the refractive index of Si\({}_{3}\)N\({}_{4}\) is slightly lower than that of ZrO\({}_{2}\) and even under an extreme high exposure condition (Light intensity: 80%, exposure time: 10 s) at the meantime. To explain the curing ability differences of Si\({}_{3}\)N\({}_{4}\)- and ZrO\({}_{2}\)-slurries, a simple RGB method was adopted to identify the colors of raw powders referring to Ref. [34]. RGB color mode combines the primary colors, red (R), green (G) and blue (B), in various degrees (ranging from 0 to 255) to create a variety of different colors. When all three of the colors are combined and displayed to their full extent, the result is a pure white (R: 255, G: 255, B: 255). Oppositely, it appears pure black (R: 0, G: 0, B: 0). As shown in Figure 5, the Si powder is very dark in color and its RGB values are all small. It can also be seen that these three kinds of Si\({}_{3}\)N\({}_{4}\) powders have their different colors, which can be due to different producing processes. Among them, DENKA-Si\({}_{3}\)N\({}_{4}\) powder has relatively lower RGB values, and its color was a little deeper than those of others accordingly. The colors of Al\({}_{2}\)O\({}_{3}\) and ZrO\({}_{2}\) powders can be viewed as nearly pure white, corresponding to their much higher RGB values. Similar results were obtained in the absorbance study of powders. The respective absorbance value at 405 nm (i.e., the wavelength of the 3D printer's light source) was listed in Figure 6 for comparison. It can be seen that in the visible light wavelength range, the deeper the powder's color, the higher the absorbance value. The absorbance values of white Al\({}_{2}\)O\({}_{3}\) and ZrO\({}_{2}\) powders were near zero, so these two materials should be transparent to the radiation in this experiment. Consequently, the low curing ability of ZrO\({}_{2}\) in comparison with Al\({}_{2}\)O\({}_{3}\) can be largely attributed to its high refractive index [44; 45]. For ceramic stereolithography, the solidification of slurries depends on how much \"out-put\" light energy will reach to the resin itself, which could trigger photo-crosslinking reaction and subsequently bind the powder together to form the parts with desired shape. Therefore, the larger the absorbance  
Figure 4: Rheological property of the 40 SN-HDDA slurry at different temperature.