## 4 Conclusions  
The compaction force influences the size of defects inside materials. According to CT scanning, the defects had larger dimensions at low compaction forces. The dimensions of the defects decreased with the increase in pressing force from 515 \(\upmu\)m for 100 MPa to 110 \(\upmu\)m for 300 MPa. The dimensions of the defects influenced the behavior of the temperature distribution during the microwave heating process. Based on the simulation  
Figure 12: Cracks and melted areas of samples: (**a**) top view of \(P_{1}\), (**b**) side view of \(P_{1}\), (**c**) top view of \(P_{2}\), (**d**) side view of \(P_{2}\), (**e**) top view of \(P_{3}\), (**f**) side view of \(P_{3}\). model, the hottest spot, around 1211 \({}^{\circ}\)C, was recorded near the largest defect, and the lowest temperature, around 1160 \({}^{\circ}\)C, was obtained near the smallest defect. The local overheating, at the edge of the defects, can lead to the thermal runaway phenomenon. According to the simulation model, the level of overheating was dependent on the dimensions of the defects, with the simulation providing high temperatures of around 1512 \({}^{\circ}\)C with a peak of 2000 \({}^{\circ}\)C in the case of 100 MPa and 1377 \({}^{\circ}\)C for 100 MPa compaction forces. The main conclusion, after simulation of the dependency between compaction force and size of internal defect, was that the size of the defect leads to local overheating followed by thermal runaway and arc discharge. The experimental processing of samples using MHH has shown that the process tends to be unstable and the heating time is long. Moreover, in all three cases, the temperature recorded on the top surface of the samples was below sintering temperature: 515 \({}^{\circ}\)C for the sample compacted at 100 MPa, 475 \({}^{\circ}\)C at 200 MPa and 476 \({}^{\circ}\)C for the sample compacted using 300 MPa. The main conclusion regarding the heating process was the need of matching the load impedance to the microwave generator. The thermal runaway phenomenon caused damage to the samples processed in the microwave field, and the level of destruction was proportional to the compaction forces used for the crude samples. The phenomenon was more pronounced for the samples compacted at 100 MPa. After 110 s, the local overheating occurred, raising the temperature up to 758 \({}^{\circ}\)C in about 15 s. All three sets of samples suffered cracks due to the high temperature developed around their defects, even though the presence of the air should normally lead to a decrease in temperature. This effect is explained by the high polarization at the defects' edges, which leads to small areas with high temperatures. However, the samples did not reach the sintering temperature, the highest temperature recorded being 1010 \({}^{\circ}\)C for the sample compacted using 100 MPa. For the sample compacted at 300 MPa, small cracks and exfoliation of 20% of the sample surface were observed, but this sample presented the best behavior during the MHH process. Finally, based on previous research related to microwave hybrid heating of ceramics and the current research, three major conclusions can be extracted:  
* Barium Ferrite W-type powders should be compacted at higher pressure forces in order to reduce the size of the defects in the materials. * The MHH process can be suitable for microwave sintering if matching load impedance tuning is used. However, the level of microwave injected power should be decreased to below 300 W, taking into consideration the aggressivity of the thermal runaway phenomenon. In addition, the heating time should be increased in order to facilitate the uniform distribution of the temperature in samples by dissipating the heat from the local overheating near the defects. * Local overheating leads to major cracks and arc discharge in the case of samples compacted at low pressure forces. It is recommended to cool the surface of the samples using compressed air in order to avoid the thermal runaway phenomenon and arc discharge. Conceptualization, S.V.S.