## 3 Results and Discussion  
### Rheological Characterization  
Figure 3 shows the flow curves of the slurry where viscosity varies as a function of applied shear rate. Although the pure monomers used in this study were Newtonian fluids and owned low viscosity (\(\sim\)0.01 Pa-s), the presence of filler can significantly change the rheological behavior. As shown in Figure 3, all the four slurries exhibited obvious viscosity decrease with increase in the shear rate,  
\begin{table}
\begin{tabular}{c c} \hline
**Designation** & **Composition** \\ \hline
35 SN-HDDA & 35 vol.\%Si\({}_{3}\)N\({}_{4}\) + Y\({}_{2}\)O\({}_{3}\) + Al\({}_{2}\)O\({}_{3}\) + HDDA \\
40 SN-HDDA & 40 vol.\%Si\({}_{3}\)N\({}_{4}\) + Y\({}_{2}\)O\({}_{3}\) + Al\({}_{2}\)O\({}_{3}\) + HDDA \\
35 SN-NPG2 PODA & 35 vol.\%Si\({}_{3}\)N\({}_{4}\) + Y\({}_{2}\)O\({}_{3}\) + Al\({}_{2}\)O\({}_{3}\) + NPG2 PODA \\ (10S + 25 SN)-HDDA & 10 vol.\%Si + 25 vol.\%Si\({}_{3}\)N\({}_{4}\) + Y\({}_{2}\)O\({}_{3}\) + Al\({}_{2}\)O\({}_{3}\) + HDDA \\ (10S + 30 SN)-HDDA & 10 vol.\%Si + 30 vol.\%Si\({}_{3}\)N\({}_{4}\) + Y\({}_{2}\)O\({}_{3}\) + Al\({}_{2}\)O\({}_{3}\) + HDDA \\ \hline \end{tabular}
\end{table}
Table 2: Designations and corresponding compositions of slurries used in this experiment.  
Figure 1: The illustration of the printing process (Step 1: slurry layer is transported, Step 2: platform descends slowly, Step 3: photocure, Step 4: platform rises).  
Figure 2: The illustration of the cure depth measurement principle.  
showing typical shear-thinning characteristic. This kind of non-Newtonian fluid behavior is desirable for the present CSL study, which combines both stereolithography and tape-casting process [40]. It can be seen that the viscosity of (10S + 25 SN)-HDDA slurry was much lower than that of 35 SN-HDDA slurry. That is, after a portion (10 vol.%) of fine Si\({}_{3}\)N\({}_{4}\) powder in slurry was substituted by the coarse Si powder, the slurry viscosity decreased markedly under the same total solid loading (35 vol.%) and monomer choice (HDDA) conditions. It was because the introduction of Si powder modified the particle size distribution (PSD) from mono-modal (0.5 \(\upmu\)m Si\({}_{3}\)N\({}_{4}\) only) to bimodal distribution (0.5 \(\upmu\)m Si\({}_{3}\)N\({}_{4}\) + 5 \(\upmu\)m Si mixture powder), which can decrease the relative viscosity at the same total solid loading according to \"Farris effect\" [41; 42]. As expected, the viscosity increases as the solid loading increases. The viscosity of 40 vol.% slurry (40 SN-HDDA or (10S + 30 SN)-HDDA) was observed to be higher than that of 35 vol.% slurry (35 SN-HDDA or (10S + 25 SN)-HDDA) in all shear rate regions. Typically, higher molecular weight diacrylates are more viscous, which may be attributed to a larger intermolecular force. It was reported that a lower starting viscosity usually results in a lower final viscosity for the suspension [31]. Accordingly, a lower viscosity can be obtained by using HDDA rather than NPG2 PODA as binder when other conditions are identical.  
Given the seasonal temperature differences in one year, we also used the rheometer MCR 301 to evaluate the rheological behavior of 40 SN-HDDA slurry at the temperature range from 5 to 35 \({}^{\circ}\)C with an interval of 10 \({}^{\circ}\)C. As expected, a rise of temperature results in a fall in suspensions viscosities (Figure 4). For instance, at 35 \({}^{\circ}\)C, the viscosity was reduced by a factor of 14 and drops down to 1.6 Pa-s (at the shear rate of 100 s\({}^{-1}\)). Furthermore, the rheological behavior of slurry also showed a strong dependence on temperature. As the shear rate increasing, the viscosity of slurry decreased and reached the plateau faster at lower temperature. In our study, the ambient temperature was kept at 26-28 \({}^{\circ}\)C which we thought is favorable for a satisfactory layer recoating.