# Additive Manufactured Waveguide for E-Band Using Ceramic Materials
## 3 Results and Discussion  
### Characterization of the Ceramic Filled Resin and the Printed Structures  
The rheology of the resin, as well as the ceramic filled resin, which is a crucial parameter for the printing process, was measured using a stress-controlled viscometer (Anton  
Figure 3: Photography of the different prototypes with (**right**) and without (**left**) fixtures for mounting purposes. Figure 2: Temperature profile of the debinding and sintering process. Paar(r) MCR 302) at 20 \({}^{\circ}\)C. The viscosity of the pure resin as well as the ceramic filled resin (with 40 wt% powder content) was investigated at shear rates ranging from 0.2 to 500 1/s (Figure 4). The pure resin shows a Newtonian behavior, whereas the ceramic filled resin shows a shear-thinning behavior which is desirable for the printing process. The high viscosity at low shear rates results in good stability of the suspension and prevents the ceramic powder from sedimentation. At higher shear rates, the resin flows more easily, which is advantageous for the printing process. The viscosity settles at a value of about 1.5 Pas, which corresponds to the specifications in the literature (\(\leq\)3 Pas) and thus enables good printability [31; 39; 41; 33]. The geometrical accuracy of the manufactured prototypes have a critical influence on their high-frequency performance. In particular, deviations of the waveguides' width and height [1; 42], the position of the slots [14] as well as the surface roughness, surface impedance and surface texture [8; 43] can lead to deterioration of the wave propagation. The mechanical properties were, therefore, characterized using a surface microscope (Keyence(r) VK-X1100) and a scanning electron microscope (SEM, FEI Ltd.(r) Quanta 200). In order to compare the mechanical properties with the state of the art, the ceramic waveguide was also compared to a waveguide made from epoxy resin (Figure 3) using the DLP method with the same printer. The outer geometry of this waveguide was adapted to the geometry of the sintered ceramic waveguide for sake of comparability. In Figure 5, polished cross-sections of the ceramic and the epoxy waveguides are shown. On the surface of the samples, the metal layer is clearly visible. Due to the sintering process, the ceramic sample features significant deviations and irregularities from the target geometry (Figure 1). However, the ceramic waveguide features a higher thickness of the metal layer and, therefore, a lower surface impedance. The higher roughness of the ceramic waveguide surface allows a better adhesion during the metallization process. The surface roughness of the metallic layer has a significant influence on the electrical loss mechanism within the waveguide [8; 43]. Therefore, the surface roughness on the inside of the waveguides was determined with the surface microscope (Figure 6) by evaluating the root mean square of the surface profile. Due to the large grain size of the ceramic powder (d\({}_{50}\)-particle size 2.65 um), the ceramic waveguide has a significantly higher surface roughness (\(R_{\mathrm{q,c}}=2.43\) um) compared to the epoxy resin-based substrate (\(R_{\mathrm{q,e}}=0.53\) um). As a result of the better surface quality of the epoxy resin-based waveguide, the layers of the DLP printing process appear in its close-up view (Figure 6b). Furthermore, the surface of the ceramic waveguide has significant inhomogeneities due to the different particle sizes of the ceramic powder used. In order to characterize the surface roughness in more detail, SEM images were taken from the inside of the ceramic waveguide (Figure 6c). Figure 4: Measured viscosity of the pure resin and the ceramic filled resin (with 40 wt% powder content). However, when comparing the geometry of the printed samples (Figure 7) and not just the surface quality, it is clear to see that both samples are comparable and do not deviate much from the CAD file (Figure 1).