Research on the Influence of the Defects of Materials on Thermal Runaway in Microwave Hybrid Heating for Sintering Processes  
[PERSON]  
1 Department of Engineering and Management of Technological Systems, Faculty of Mechanics, University of Craiova, 200585 Craiova, Romania; [EMAIL_ADDRESS] (I.S. ); [EMAIL_ADDRESS] (G.C.B. ); [EMAIL_ADDRESS] or [EMAIL_ADDRESS] (I.D.S.) [PERSON]  
3 Department of Applied Mechanics, Faculty of Mechanics, University of Craiova, 200  
magnetic materials with a high coercive magnetic field are used for the fabrication of permanent magnets, but these materials are costly and the classic sintering technology takes too long a time, making them expensive products. The materials for Barium Ferrite W types are widespread as natural resources, and the industry needs sustainable technologies for magnet fabrication as well. Microwave hybrid heating (MHH) has proved to be a reliable technology for materials processing in terms of sustainability and the efficiency of the process, but it is usually unstable from a sintering process point of view. [PERSON] et al. reported advanced research on microwave welding using MHH technology. They reported that MHH welding is effective in producing completely fused, homogeneous, metallurgical-bonded, and crack-free weld structures. They also concluded that achieving precise control of the complex microwave heating process remains one of the major challenges of microwave welding [1]. [PERSON] et al. reported better heat transfer when soldering metals using SAC305 solder and the MHH process [2]. Other research has been focused on the shape and dimensions of the susceptors and their influences on the performance of MHH processes. [PERSON] et al. reported better results in terms of the strength of the joints in the case of MHH soldering [2]. Also, the efficiency of MHH has been studied by [PERSON] et al. They reported that MHH is an environmentally benign and cost-effective technology that may increase manufacturing product quality [3,4]. The sintering of ceramics using microwave technology has been studied by [PERSON] et al. in order to improve the hardness of alumina. They succeeded in sintering alumina through the MHH process at 1500 \({}^{\circ}\)C [5]. The absorbance properties of ferrites in a microwave field have been studied by [PERSON] et al. They studied to what extent the losses in dielectrics influence the microwave absorbers' effectiveness. They concluded that exceptional microwave absorption properties can be achieved not by a single material but by using composite-materials-based carbon. These findings support the MHH approach as a reliable technology for materials processing [6]. Also, [PERSON] et al. reported the specific absorbance properties of W-type hexagonal ferrite. They concluded that W-type ferrite has a strong electromagnetic absorption ability at high frequencies [7]. The thermal runaway phenomenon represents the major challenges in microwave processing technologies. Ceramics particularly suffer from a large increase in temperature in a short time, which leads to loss of process stability, initiation of arc discharge and damage to the products. [PERSON] carried out a detailed multiplicity and stability analysis for microwave heating of ceramic fibers in a resonant cavity. He concluded that a stable non-uniform and asymmetric pair of temperature profiles emerges and a hot temperature front is formed at the boundary of the sample and moves along its axis, triggering a local thermal runaway at the far end while leaving the front end at a relatively low temperature [8]. [PERSON] et al. studied the hotspot evolution in materials under various gaps and shapes. They reported that, different from the gradual decrease in the enhanced electric field, the increasing gap rapidly improves heating uniformity by nearly 30 times [9]. An effort towards the reduction of thermal runaway has been also performed by [PERSON] et al.