**1. Introduction**

Due to functional, structure, or property needs, joining of dissimilar materials is increasingly used to achieve components with improved or tailor-engineered properties. For example, ceramic-metal joints have found wide application in electrical engineering and electronics for their combination of ceramic insulating properties and metallic conductivity. Moreover, ceramic-metal seals are used extensively in applications such as engine ignitors, vacuum tubes, high-voltage feedthroughs, magnetic recorder heads, synthetic colorless sapphire-metal windows, and ceramic sputtering targets [1]. Most ceramics have very high melting temperatures, so the applicability of conventional fusion welding methods to the bonding of ceramic-metal is not feasible in cases of extremely high power requirements. During the fusion welding process, the high temperature can cause severe property degradation or fracture in surrounding heat-affected areas. Furthermore, the great thermal expansion coefficient mismatch between ceramics and metals can cause serious residual stresses at the interface of the ceramic and

metal [2]. In lightweight construction, the bonding of difficult-to-wet materials such as aluminum alloys, magnesium alloys, or titanium alloys, which easily forms an oxide passivation layer by natural oxidation, entails numerous difficulties. Brazing and soldering with fillers as interlayers are considered to be more feasible ways to bond dissimilar materials such as ceramics and difficult-to-wet metals. However, because ceramics provide mostly covalent or ionic bonding, have a very stable electron configuration, and are chemically inert, most brazing or soldering fillers cannot be wetted on their surfaces [3]. Thus, the filler metal is the most important key factor in determining wettability. Direct bonding has been developed to simplify wetting of difficult-to-wet materials by using active fillers containing active elements that improve the wettability of the filler on the difficult-to-wet material surface and eliminate the need for pre-metallization during the joining process [4].
