Research On Thermal Storage Characteristics Of Mg-Al-Zn Alloys As PCM

Metal based thermal storage of phase transition material with high energy storage density, good thermal conductivity, stable performance, easily controlling process and long service life, are widely used in middle or high temperature heat storage. Compared with the relative mature Al based phase transition heat storage materials, Mg based alloys can be used as a kind of potential phase transition material for heat storageIn this thesis, the microstructure and properties of Mg-Al-Zn alloy phase transition thermal energy storage materials were researched by the use of density testing, metallographic analysis, X-ray diffraction analysis (XRD), DSC thermal analysis, electron probe microanalysis (EPMA) and so on. By means of the thermal cycling test and corrosion test, the thermal cycling stability and the compatibility with container materials of Mg-25Al-15Zn-14Cu alloy were also studied. The main conclusions are as follows:(1) With increasing Al content and decreasing Zn content, the density of Mg-Al-Zn alloy decreased. After adding elements of Cu and Si, the density of alloy increased.(2) The microstructure of Mg-Al-Zn alloy is mainly composed of primary phase a(Mg) solid solution, β-Mg17Al12and Mg32(Al,Zn)49. When adding elements of Cu and Si, there are new phase formed in alloys, such as CuZn, MgAl2Cu and Mg2Si. The compound forms of as cast structure in Mg-25Al-15Zn-14Cu alloy are mainly skeleton, bulk, fishbone, lamellar, mesh and so on.(3)The phase transformation temperature of Mg-25Al-15Zn, Mg-25Al-15Zn-8Cu,Mg-25Al-15Zn-14Cu and Mg-25Al-15Zn-5Si alloys are412.1℃、405.9℃、407.5℃and434.2℃respectively, and the latent heat value are175.4J/g、209.3J/g、204.7J/g and218.4J/g respectively. The phase transition temperature and latent heat are changed by the reason of forming new phase and its morphological changes. The addition of Cu and Si contributes to the formation of some new phases and increasing the latent heat of phase transformation, and improving the phase transformation temperature of the alloy in a certain extent.(4) The phases of Mg-25Al-15Zn-14Cu alloy have none of change before and after thermal cycling, but the microscopic structure changes obviously. The fine grain becomes thick after thermal cycles, the white block and bone like compounds have a trend to solution in the matrix. After1000thermal cycles, the phase transition temperature has increased1.87%, the latent heat reduced by7.35%, the density of alloy increased by9.53%. Mg based phase transition material for heat storage shows good stability in the long-term thermal cycling.(5) The compatibility tests show that, the etching layer of carbon steel is irregular loose belt, while the etching of stainless steel is regular, smooth and compacted. The corrosion form of molten magnesium alloy on the corrosion material is that the magnesium atom in high temperature diffuses to etching sample from outside to inner and forms slight intermediate alloy phase. There are the alloy element segregation around the grain boundary in the stainless steel, which will hinder the magnesium atom diffusion in matrix. Because the inhibition is stronger than carbon atom, resulting in slow develop of etching lamellar, so the corrosion rate of stainless steel is lower than carbon steel.