Fabrication And Thermal Cycling Stability Of Monodispersed Bi-Ga Microencapsulated Phase Change Materials

Phase change materials (PCMs) have drawn more and more attention by all countries, because they are featuring high energy storage density, high energy conversion efficiency and almost constant temperature during heat absorption and release. However, active molten metal PCMs are prone to cause corrosion of the container at high temperature. How we can reliably package the PCMs must be considered. Microencapsulated phase change materials (MEPCMs) are an important way to solve this problem. In this thesis, a novel durable encapsulation of metal-based PCMs, POEM, is proposed because there are a series of problems arose from MEPCMs prepared by the conventional metal electric plating. This method is called pulsated orifice ejection (POEM). In this method, monotectic Bi-Ga droplets are generated and then form spherical core-shell particles during solidification because of phase separation in Bi-Ga droplets. The influence of fabrication parameters, vibration frequency of piezoelectric ceramic, melting holding time and the cooling atmosphere are discussed. These MEPCMs are experienced 0-600 times heat absorption and release thermal cycling test in order to examime the thermal properties and structural stability.First,5 Bi-Ga monotectic alloys with component of Bi50Ga50, Bi45Ga55, Bi30Ga70, Bi25Ga75 and Bi20Ga80 (at%) are chosen according to Bi-Ga monotectic binary phase diagram. Bi-Ga particles with uniform size and small dispersion are prepared by using POEM. All of these particles are in perfect core-shell structures, high melting point Bi-rich phase shell and low melting point Ga-rich phase core. The parameter of vibration frequency will influence the particles size distribution and sphericity. Vibration frequency of 10 Hz can meet all the components of the preparation work, but the vibration frequency of 100 Hz unfavorable for some components. For low Ga content, high Bi content alloy, extending melting holding time can ensure sufficient alloying and obtain particles with better morphology. As to high Ga content, low Bi content component, in order to ensure the particles solidify completely, high cooling capacity atmosphere should be used. For critical components, Bi30Ga70, both of the two parameters should be considered.Secondly, TEM and EDS are used to characterize the monodispersed MEPCMs fabricated. It is found that nearly all particles are in perfect core-shell structure. Take the component of liquid miscibility gap (Bi30Ga70, in Bi-Ga binary phase diagram) as the critical point, on its left side, Bi-Ga alloys are of low Ga content. Perfect core-shell structure can be formed when the holding time 90 min and Ar atmosphere are used. While on its right side, Bi-Ga alloys are of high Ga content. Holding time 40 min and He atmosphere can ensure to perfect cor-shell structured particles be obtained. As to the critical component, the best parameters are holding time 60 min and gas mixture Ar and He (Ar:He=3:2).Finally, the three representative component, Bi45Ga55, Bi30Ga70 and Bi25Ga75 were selected for 0-600 times thermal cycling test. SEM was employed to observe the core-shell structure change after different times thermal cycling. The results indicated that the core-shell structure retained even though the morphology of the core changed a little bit. The measured melting temperatures and the latent heats of fusion were acquired from Differential Scanning Calorimetry (DSC) technique. It is demonstrated that the effect of thermal cycling on the onset of melting temperature and latent heat of fusion is small. Good structure stability and thermol-physical properties stability indicate that all the particles of different alloys fabricated by POEM are able to satisfy the requirement of MEPCMs.