Composition Optimization Research Of Al-based Alloy Phase Change Thermal Storage Materials

There still exist some problems in the present researches of Al-based alloy phase change thermal storage materials such as poor high-temperature oxidation resistance and abated thermal storage capacities after repeated melting-solidification thermal cycles,and this has restricted its further applications and development.In order to alleviate these problems and provide more experimental basis and theoretical basis for composition optimization research of Al-based alloy phase change thermal storage materials,the variation law of the latent heat of Al-Si alloys(controversial conclusion were found in the current literatures)was studied in this paper and the Al-Si alloy with optimal performance in this experiment was sort out to be the matrix alloy.And the further alloying were achieved by adding some Cu element or mixed rare earth elements to the matrix alloy.According to the result of the DTA test,the thermal cycling test,the isothermal oxidation test,the cyclic oxidation test,the microstructure observation test,the XRD test and the EDS test,the effect rule and effect mechanism of various elements on the heat storage properties of the experimental alloys were investigated as well as the addition amount of these alloy elements were optimized.The variation law of latent heat of fusion per unit mass of Al-Si alloys(casted with a graphite mould)can be expressed as Y1 =13.14x + 372.05(4 ≤ x ≤ 12.6)for hypoeutectic/eutectic alloy and Y2 =-4.72x + 599.01(12.6<x<<20)for hypereutectic alloy in this paper.The verification test showed that the prediction accuracy of these two expression were very high.The latent heat of fusion of eutectic Al-Si alloy was higher than that of hypoeutectic/hypereutectic Al-Si alloys and the Al-12Si alloy has the highest thermal storage capacity in all Al-Si alloys which have been tested in this paper.The possible reasons which caused opposite conclusions about the variation law of latent heat of Al-Si alloys in the references were listed as follows:the random data would be obtained when the times of measurement was few;the measured object was the latent heat of solidification and the latent heat of fusion(the variation law of them were different),respectively;diverse melting history such as different superheat were existed in previous researches.For the Al-12Si-XCu(0<<X<<15)alloys in this paper,the isothermal/cyclic oxidation mass gain decreased with the increase of mass fraction of Cu element during the oxidation experiment periods.The oxide films have an good block protection during the isothermal oxidation periods.But except for Al-12Si-10Cu alloy,the oxide films of these alloys were short of effective protection since the intermittent failure of the film during the cyclic oxidation periods.Before thermal cycles,the latent heat of fusion per unit mass decreased and the phase-transformation temperature range enlarged(the temperature range of the major transformation peaks reduced)as well as the mass density linearly increased with the increase of mass fraction of Cu element.While the latent heat of fusion per unit volume did not show an regular change law due to the double effect of the latent heat of fusion per unit mass and the mass density.After 100 times of thermal cycle,the latent heat of fusion per unit mass of Al-12Si alloy and Al-12Si-10Cu alloy were decreased while that of Al-12Si-10Cu alloy and Al-12Si-15Cu alloy were increased;The phase-transformation temperature range basically showed a trend of reduction and the mass density slightly decreased(the linear law was not changed)as well as the latent heat of fusion per unit volume showed an increasing trend with the increase of mass fraction of Cu element;The thermal cycling stability of Al-12Si-XCu alloy in this paper were all good and the Al-12Si-10Cu alloy showed the best thermal cycling stability through a comprehensive comparison.Changes in the organization structure of these alloys and gradual composition variation caused by long-term high temperature thermal cycles were the major reasons which lead to different changes in the latent heat of fusion and the phase transition temperature after thermal cycles.For the Al-12Si-XRE(0<<X<<0.5)alloys in this paper,the latent heat of fusion per unit mass gradually decreased with the increase of mass fraction of RE elements because of the total increase of entropy decreased as well as the melting point slightly declined(△H=△S·Te).But the DTA peaks were almost constant.The high temperature oxidation resistance(It did not show an monotonic change trend with the increase of mass fraction of RE elements)of Al-12Si alloy could be improved by adding an appropriate amount of RE elements.This was due to the reasons listed as follows:the selective preferential oxidationof RE elements;the grain refinement caused by RE elements could increase the number of rapid diffusion channel;RE elements could improve the ductility and toughness of the oxide film;RE elements could restrain the generation of substrate/film interface defect.When the RE element content was 0.1 wt%the matrix alloy showed the best high temperature oxidation resistance,and it had the least influence on the latent heat of fusion per unit mass as well as the temperature range of melting of the matrix alloy.