Preparation And Thermal Properties Of Al-Si Alloy Microcapsules

Al-Si alloy is an ideal high-temperature phase change heat storage material.It is important to encapsulate Al-Si alloys into microcapsules.In this paper,Al-Si alloy powders with five different Si contents were selected to prepare Al-Si alloy microcapsules through steam treatment bonded baking method and the thermal properties of microcapsules were studied.Firstly,the parameters of water vapor was numerical simulated.The influence of boundary conditions and reactor structure on water vapor flow,pressure,temperature,water vapor concentration and distribution were investigated to determine the suitable experimental conditions of water vapor for Al-Si alloy powder.On this basis,particles were obtained by treating Al-Si alloy powders in the water vapor.The Al-Si alloy microcapsules were prepared by baking the above particles.The microstructure of the microcapsules were observed by scanning electron microscope（SEM）,their thermal properties were also measured.The effect of water vapor parameters,baking temperature and Si content on the microstructure and properties of the microcapsules were studied.The formation process of the microcapsules was discussed.Based on the experimental results,combined with the thermal-structure coupling simulation of the microcapsules,the cracking mechanism of the microcapsule shell was discussed.The results of this paper are shown as follow:（1）From the numerical simulation of water vapor generation,coupling with flow and heat transfer,it is found that the initial water quantity,heating temperature affected the distributions of temperature,pressure,velocity and volume fraction for water vapor.When the heating temperature is 573 K and the height of water addition was 66 mm,the high-water vapor concentration with uniform distribution,as well as uniform distribution of temperature and pressure was obtained in the autoclave.The reliability of numerical simulation results was verified by water vapor corrosion of Al-Si alloy powder with 12wt%silicon content（mass fraction,the same below）combined with literature data.（2）Water vapor pressure,treatment time and silicon content were main factors that affected the morphology,distribution and size of products from Al-Si alloy powder hydration.Flaky boehmite that grow along the radial direction of the particle appeared on the surface of five kinds of aluminum-silicon alloy powders with different silicon contents when the alloy powders were kept in water vapor with pressure of 0.10 MPa for 1.0 h.And at this condition,the particle size of boehmite was relatively large.The Al-Si alloy powder with 10 wt%silicon content was greatly affected by water vapor pressure.The size of boehmite on the Al-Si alloy powder with 12 wt%silicon content was smaller under different water vapor conditions in the paper.Al-Si alloy powders with 30 wt%and 35wt%silicon content were easier to react with water vapor and flake boehmite could be produced under studied water vapor conditions.（3）The formation temperature of Al-Si@Al₂O₃microcapsules for Al-Si alloy powders with 10 wt%and 12 wt%silicon content was 1100°C,while the formation temperature of Al-Si@Al₂O₃-Al OOH microcapsules for Al-Si alloy powders with 20 wt%,30 wt%and 35 wt%silicon content was 800°C.The phase change latent heat densities of Al-Si alloy microcapsules decrease with the increase of silicon content.After 50 thermal cycles,the complete structure and high retention rate of the phase change latent heat densities for aluminum-silicon alloy microcapsules were obtained.（4）The formation process of microcapsules for Al-Si alloy powders with 10 wt%and12 wt%silicon content included boehmite decomposition and microcracks self-healing through oxidation of molten aluminum.Microcapsules’formation process of Al-Si alloy powders with 20 wt%,30 wt%and 35 wt%silicon contents was mainly the encapsulation of different variants of alumina decomposed by boehmite.The uneven stress distribution in the shell layer was caused by the aggregation of Si in the alloy and the segregation of Si after baking.The stress concentration was appeared at the aluminum-silicon-shell interface.Therefore,the shells were easy to rupture,leading to the poor thermal cycle stability of microcapsules.