## 3 Results and Discussion
In Figure 8, the samples \(P_{2}\) and \(P_{3}\) present similar curves in terms of the uniformity of the absorption process and the conversion of microwaves into heat. The sample \(P_{2}\) presents a stable evolution of the absorbed/reflected ratio. However, after 300 s, the absorption rate increases to above 500 W, which ensures a very good heating process, but, later, the absorption process decreases to 450 W, which is the average absorbed power for the samples  
Figure 8: Absorbed microwave power using matching load impedance in the MHH process. Figure 7: Top 5 values of temperatures on sphere surface. compacted at 200 MPa. On the other hand, sample \(P_{3}\) presents the best correlation between the stability of the process and the absorbed/reflected power ratio. Finally, the samples compacted at 100 MPa seemed to be the most unstable in terms of absorption/reflection. Even after 100 and 400 s, the absorption properties were consistent with \(P_{2}\) and \(P_{3}\), and the alternation between the maximum of the absorbed and reflected microwave power indicated the occurrence of the thermal runaway phenomenon. The high rate of microwave conversion into heat followed by a cooling led to the assumption that more absorbance leads to fast heating, with temperature peaks above what was expected. A series of snapshots during the MHH processing of the \(P_{1}\), \(P_{2}\) and \(P_{3}\) samples is presented in Figure 9 with recorded values presented in Table 4, showing the occurrence of the thermal runaway phenomenon. Figure 9: Snapshots during MHH process: (**a**) \(P_{1}\)—local overheating, (**b**) \(P_{1}\)—thermal runaway, (**c**) \(P_{1}\)—arc discharge, (**d**) \(P_{2}\)—local overheating, (**e**) \(P_{2}\)—thermal runaway, (**f**) \(P_{2}\)—no arc discharge, (**g**) \(P_{3}\)—local overheating, (**h**) \(P_{3}\)—no thermal runaway, (**i**) \(P_{3}\)—no arc discharge. The MHH process suffered a thermal runaway phenomenon that 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. The gradient of increasing temperature was recorded to be 24 \({}^{\circ}\)C/second, and was followed by a cooling and stabilization of the heating. The second step of the thermal runaway phenomenon was established after 400 s when an arc discharge was initiated between the sample core and the dolomite support. The temperature increased to 935 \({}^{\circ}\)C, followed by cooling to 612 \({}^{\circ}\)C. After this rapid heating/cooling cycle of the sample, the thermal runaway could not be controlled, and, after 425 s, the temperature increased up to 1010 \({}^{\circ}\)C and the MHH process was stopped. The MHH process was restarted for the samples compacted at 200 and 300 MPa. According to the parameters recorded during processing, the sample \(P_{2}\) suffered a local overheating after 260 s and the recorded temperature was 587 \({}^{\circ}\)C. After a short cooling-down period, the samples underwent thermal runaway 120 s later by increasing the temperature up to 788 \({}^{\circ}\)C followed by fast cooling. The process continued to be relatively unstable with a temperature gradient of 3 \({}^{\circ}\)C/second, but no arc discharge was reported. The most compacted sample, \(P_{3}\) = 300 MPa, was also the most stable in terms of microwave heating. The temperature increased very slowly, reaching 371 \({}^{\circ}\)C after 75 s, 20 s later than \(P_{1}\) and \(P_{2}\), due to the small dimensions of the defects and lack of local overheating. The thermal runaway phenomenon usually occurs as a result of local overheating in materials that contain polar particles when they are activated by the microwave field. In addition to the mechanical frictions between dipoles, the empty spaces inside the materials undergo high polarizations at the edges of the holes, which usually leads to local overheating and subsequently to thermal runaway.