## 4 Discussion  
The problems with photopolymerization of stereolithography-enabled BT pastes and suspensions are often attributed to the large difference in refractive indices of the ceramic powder and the organic binder used in pastes and suspensions [1; 4; 10]. In this study, we demonstrated that the powder and the organic binder with a large mismatch in the refractive indices could be efficiently used for 3D printing by merely changing the spectral range of photopolymerization. In particular, the change from 355/405 nm (as in standard SLA/DLP technologies) to 465 nm improved the photopolymerization tenfold. The scattering coefficient of BT (Figure 6a) was computed for the range 400-800 nm per equation \(n^{2}\ =\ 1+4.187\lambda^{2}/\left(\lambda^{2}-0.223^{2}\right)\), following [11]. An extrapolation was made to the range of 300-400 nm to estimate an approximate value of the refractive index at 355 nm. The spectral absorption data for BT powder presented in [12] for the range of 350-800 nm were recomputed with equation \(T\ =\ 10^{-A}\) to obtain the light transmission spectra (Figure 6b). On the basis of the light transmission data, we found that BT ceramics absorbed almost 70% of light at 355 nm (a typical wavelength for laser-based commercial  
Figure 4: The results of 55 vol % BT paste polymerization test, using a 465 nm industrial laser: (**a**) the effect of powder loading, (**b**) the impact of laser marking speed variation, (**c**) the effect of hatching spacing variation. Figure 5: The 3D-printed BT greenbody samples, produced using a 465 nm industrial laser: (**a**) a 50 \(\times\) 2 \(\times\) 2 mm sample, composed of 20 layers and 3D-printed with the unmasked laser, and (**b**) a 10 \(\times\) 10 \(\times\) 5 mm rectangular sample, composed of 50 layers and 3D-printed using the masked laser. SLA 3D-printers), 14% of light at 405 nm (a typical wavelength for commercial DLP 3D-printers), and only 4% of light at 465 nm. The latter spectral range is not typical for photopolymerization-based 3D-printing. The refractive index of BT material reduced in the range of 355-465 nm from 2.81 to 2.54. Despite the gradual reduction, the BT refractive index at 465 nm was still much greater than the worst-case scenario for some SLA-enabled technical ceramics (i.e., refractive index = 2.26 for ZrO\({}_{2}\) or yttrium stabilized zirconia) that require thin 25 \(\upmu\)m layers for efficient 3D printing. An additional factor that affects the polymerization efficiency is the small size (hundreds of micrometers) of the powder particles used for paste preparation. Such small particles augment light scattering, decreasing the photopolymerization depth. However, as demonstrated in this study, highly loaded BT pastes polymerize to the thicknesses in excess of 100 \(\upmu\)m when the refractive index is still quite high and powder particles are small. Thus, the low light absorption of BT at 465 nm is considered to be a more important factor of successful 3D printing than the slight reduction in the refractive index. Thus, the powdered materials have to undergo a comprehensive optical testing prior to their usage as solid fillers in SLA-ready ceramic pastes. Attention must be paid to not only the refractive index of the powder, but also to its reflection, absorption, and transmission characteristics as functions of wavelength. The best spectral region could be selected for the efficient SLA application for selected materials, subject to the accessibility of the corresponding light source, photoinitiators, and reactive organic components (monomers/oligomers). We demonstrated that BT exhibits high light absorption in the spectral range of 350-400 nm, which led to adverse effects when conventional SLA/DLP 3D printers were used. The shift to the spectral region of low light absorption, while BT's refractive index remained high, resulted in successful 3D printing of piezoceramic greenbody samples.