| Homogenous-structured monotectic alloys have many potential applications in industry. They are, however, seriously restricted for having not been able to achieve homogenous structures because of phase separation during solidification. Based on the unique property of phase separation, a kind of core-shell-typed materials, which are potential to use for automobile industry, electronic packaging, conductive filler, phase change energy storage, etc., have been highly concerned in recent years. The premise of the applications is to fabricate core-shell-typed structures with mono-sized dimension and identical morphology. An effective method is urgently needed since the current methods, including atomization, drop tube and jet breakup methods etc., are unable to make the above goals come true. High frequency metals/alloys particles have been produced successfully by Pulsed Orifice Ejection Method (POEM) and it is considered to be an excellent candidate. In the present thesis, researchs on the morphology control, formation mechanism and preliminary phase transformation behavior of Bi-Ga monotectic alloy particles were performed.Firstly, Bi45Ga55alloy core-shell-structured particles with mono-sized dimension and morphology and high melting point Bi-rich phase shell were produced for the first time by the POEM. Formations of mono-sized particles and core-shell structures were analyzed from the point of technique and solidification path, respectively. Bi-rich phase shell results from the surface segregation of Bi. Temperature field of a droplet during falling was simulated by the FLUENT software and the velocities of Marangoni motion and Stokes motion of second phase globules were calculated. The calculated result indicates that Marangoni motion plays an important role in the whole solidification processing, which explains the formation of the concentric core-shell structures. The formation of Bi-Ga alloy core-shell structures is illustratly concluded. Effects of superheat, ultrasound on the morphology and the property of thermal storage were investigated. The diameter relationship between the core and the particle satisfies a linear function of Dcore=79Dparticle-51.3, with a regression coefficient of R2=0.976.Secondly, core-shell structures of six compositions in the hypermonotectic gap were investigated. Morphologies evaluate from concentric to crescent and to concentric successively with the increase of Ga content and all the core-shell structures consist of Ga-rich phase core regardless of the minor volume fraction phase. The formation of core-shell structures with different minor fraction phase was summarized schematically. Effects of ultrasound processing, particle dimension, alloy composition and superheat on the morphologies were discussed and the relationship between composition, superheat and particle dimension were investigated. It shows particle dimension increases and then decreases with Ga content at fixed superheat and decreases with superheat at fixed composition. Wetting Model and Ball Model were set up and both of their predictions are in accord with the experimental observation.Thirdly, the possibility of the core-shell structures as Microencapsulated Phase Change Materials (MPCMs) for heat energy storage was conducted using DSC thermal cycling test. The results indicate that Bi45Ga55alloy core-shell particles keep terrific stability during thermal cycling with temperature in and below the immiscible gap. All the Bi-Ga alloy core-shell structural particles with various compositions keep well stabled with thermal cycling temperature of-20-100～20℃and the thermal densities increase with the Ga content and the thermal desity of Bi15Ga85is the highest among them; while particles with thermal cycling temperature of-20-300～-20℃exhibit unstable state. Bi45Ga55alloy partiles with thermal cycling temperature of-20-100～-20℃exhibit excellent stability after numberous cycling times and the thermal density during phase change ranges from10.75to11.25J/g, showing stable thermal behavior and its thermal density increases with surpreme heating temperature. |
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