Design,preparation And Property Optimization Of Al-12wt%Si Alloy Microcapsules

High-temperature phase change heat storage materials are widely used in solar thermal power generation,industrial waste heat recovery,industrial furnace heat storage and other fields.Compared with other high-temperature phase change materials,Al-Si alloy has the advantages of high latent heat,high phase change temperature,large thermal conductivity,and relatively stable chemical properties.It is an ideal material in the field on high-temperature phase change heat storage.It is the prerequisite to encapsulate Al-Si alloy for using its high latent heat because the liquid phase appears during the alloy phase transition.This paper focused on the preparation of Al-12wt%Si alloy microcapsules.The effect of microcapsule sphericity,shell thickness,shell structure and the physical parameters of shell material on the shell stress distribution were simulated by using Ansys commercial software.Based on this,two kinds of Al-12wt%Si alloy particles with different impurity contents and sphericities were selected as raw materials.Al-12wt%Si@Al₂O₃ microcapsules were fabricated by heating the Al-12wt%Si@Al(OH)₃ particles prepared by treating the two kinds of raw materials via steam oxidation method.The influence of impurity content,steam pressure,pressure holding time,sphericity and heating temperature on the morphologies and properties of Al-12wt%Si@Al₂O₃ microcapsules were studied and shell formation mechanism was investigated.Then,double-layer shell microcapsules,Al-12wt%Si@Al₂O₃@mullite with alumina as inner shell and mullite as outer shell,were fabricated by heating Al-12wt%Si@Al(OH)₃ particles which had been immersed in silica sol.The mechanism of excellent thermal cycling porfermance of the double-layer shell microcapsules was studied.Finally,the pre-microcapsules formed after being steam oxidized followed by impregnated in silica sol were introduced in mullite ceramic material.The microcapsules with Al-12wt%Si core formed in-situ in the ceramic material during heat treatment,which meaning the high-temperature phase change heat storage compsite ceramic material with latent heat,high thermal conductivity,high specific heat capacity and excellent thermal cycling performance was fabricated.The effect of addition amount of pre-microcapsules and heating temperature on the properties of high-temperature phase change heat storage compsite ceramic material were studied.The main conclusions are as follows:(1)The sphericity,shell thickness,shell structure and parameters of shell material of microcapsules affected the shell stress distribution.When the sphericity of the Al-12wt%Si alloy microcapsule was 1,the shell stress was distributed uniformly.When the shell of the Al-12wt%Si alloy microcapsule was a single shell,the stress at the interface of core and shell decreased with the increase of shell thickness.If the Al-12wt%Si alloy microcapsule shell was a double-layer shell and the total thickness remained unchanged and the inner and outer shell were both dense ceramic materials,the thickness ratio of the inner and outer shell had little impact on the stress distribution of the double-layer shell.If the material parameters of the inner and outer shell differed greatly,the stress distribution would be greatly impacted: when the inner shell and the outer shell were made of dense ceramic materials and porous materials,respectively,the stress at the interface of the two layers decreased suddenly;when the material of the inner shell was metal,the stress at the interface of the two layers increased suddenly.(2)The impurity content of Al-12wt%Si alloy,steam concentration,pressure and holding time in pressurized steam were closely related to the preparation of Al-12wt%Si@Al(OH)₃ particles.Impurity content influenced the morphologies of Al(OH)₃ crystals covering on the surface of Al-12wt%Si@Al(OH)₃ particles.In the same steam environment,the surface of raw material with high impurity content was likely to form Bayerite due to cathodic polarization,whereas the surface of raw material with low impurity content was prone to generate Boehmite due to anodic polarization.Bayerite and Boehmite tended to grow together to form crystal platforms with the increase of steam pressure.The amount of Bayerite increased and then decreased with the increase of holding time in pressurized steam,while the amount of Boehmite increased with that.(3)Al-12wt%Si@Al₂O₃ microcapsules prepared from Al-12wt%Si alloy particles with different sphericities had different morphologies and properties.The Al-12wt%Si@Al₂O₃ microcapsules prepared from Al-12wt%Si alloy particles with better sphericity had good morphologies and complete shells.The latent heat of Al-12wt%Si@Al₂O₃ microcapsules decreased with the increase of heating temperature and the increase of thermal cycling number.The shell formation mechanism was as follows: during the temperature increase,Al₂O₃ shell formed by the dehydration and decomposition of Al(OH)₃ wrapped the Al-12wt%Si alloy.Meanwhile,O2 in the air diffused through the Al₂O₃ shell that was not dense yet,and reacted with the Al in the alloy to form Al₂O₃ crystals.Until the end of the heat treatment,the Al₂O₃ crystals grew and penetrated the Al₂O₃ shell,forming a microcapsule shell embedded by Al₂O₃ crystal clusters.(4)On the basis of the above,double-layer shell microcapsules Al-12wt%Si@Al₂O₃@mullite were prepared by heating Al-12wt%Si@Al(OH)₃particles at 1100 °C which had been impregnated in silica sol.The double-layer shell microcapsules had much better morphologies,more complete shell and excellent thermal cycling performance.The latent heat of it was 367.1 J·g-1 and the latent heat could still remained 90.4% even after 3000 thermal cycles.The reason of such excellent thermal cycling performance was as follows: the alumina inner shell provided strength support for the microcapsule and the mullite outer shell provided thermal stress buffer.Moreover,the “sponge effect” of outer shell could offer volume buffer for the solid-liquid phase change of Al-12wt%Si.According to the deduced formula,it is known that,when a quarter of the product of the specific surface area of the microcapsule and the pore size is larger than the difference between the reciprocal of the liquid and solid phase change material density,the phase change material inside the microcapsule is not prone to overflow during phase change.(5)On the basis of the above experiment,the microcapsule precursor,Al-12wt%Si@Al(OH)₃ particles that had been immersed in silica sol,was introduced into mullite ceramic material to prepare high-temperature phase change heat storage composite ceramic material.The cold crushing strength,cold modulus of rupture,thermal shock resistance,thermal conductivity and latent heat of this composite ceramic material increased with the increase of microcapsule precursor content.The cold crushing strength and cold modulus of rupture increased with the heat treatment temperature,while the latent heat decreased and the thermal shock resistance and thermal conductivity firstly increased and then decreased.When the microcapsule precursor content was 45wt% and the heat treatment temperature was 1300 °C,the latent heat of this composite ceramic material after 3000 times thermal cycle was only6.63% lower than that of before thermal cycle and the heat absorption of it in heating range of 50 500 °C was 1.7 times that of the blank sample with no latent heat.