Study On Thermal Storage Properties Of Mg-Ni-La/Ce Alloy

Due to the low price and high reversible hydrogen storage,the research of Mg-based alloy materials for heat storage and release has attracted extensive attention.In this paper,firstly,the thermal storage materials were classified,and the performance indexes of Mg-based thermal storage materials were defined.At the same time,the research progress of Mg based thermal storage materials and thermal storage devices were summarized.On this basis,Mg-Ni-Ce and Mg-Ni-La alloys were selected as the research carriers,XRD and SEM were used to study the microstructure,PCT and DSC were used to study the enthalpy change,heat storage and activation energy of hydrogen evolution.The effects Mg and Ni ratios on the thermal storage properties of Mg-based alloys were investigated.The Mg_77+xNi_20-xCe₃?x=0,5,10,15?alloy was prepared by suspension induction smelting.The SEM-EDS showed.the difference between the Mg-Ni-Ce alloy microstructure before and after hydrogen absorption Before hydrogen absorption,the alloy is composed of four main phases of Mg,Mg₂Ni,CeMg₁₂ and Ce₂Mg₁₇,and after hydrogen absorption it changed as MgH₂,Mg₂NiH₄ and CeH_2.29 phases.The CeH_2.29 can significantly improve the hydrogen absorption and desorption rate of Mg-Ni-CE alloy.The hydrogen absorption of Mg₈₇Ni₁₀Ce₃alloy is 5.55wt.%at 683K for two hours,which is 1.6 times that of Mg₇₇Ni₂₀Ce₃ alloy.Similarly,at 683K,the Mg₈₇Ni₁₀Ce₃ alloy releases hydrogen for 5.45wt.%.For two hours,the alloy can completely absorb and release hydrogen.The heat release per unit mass of Mg₈₇Ni₁₀Ce₃ alloy is 2033kJ and the heat release density is 2.03MJ/kg;the heat storage per unit mass of Mg₈₇Ni₁₀Ce₃ alloy is 1997kJ and the heat storage density is 2.0MJ/kg.At a heating rate of 5K/min,the enthalpy change value of Mg₈₇Ni₁₀Ce₃ alloy reached of 1471J/g.The Mg_77+xNi_20-xLa₃?x=0,5,10,15?alloy was also prepared by induction smelting.The XRD and SEM inspection results showed that Mg,Mg₂Ni,LaMg₁₂,La₂Mg₁₇ reacted as MgH₂,Mg₂NiH₄,and LaH_3.05 after hydrogen absorption.The Mg₂Ni phase becomes the eutectic layered structure of Mg-Mg₂Ni.The hydrogen absorption and desorption process was a reversible reaction of Mg/MgH₂ and Mg₂Ni/Mg₂NiH₄.The increase of Mg content significantly improved the hydrogen absorption rate of the alloy at low temperature,which is due to the fact that the hydrogen absorption capacity of Mg is much larger than that of Mg₂Ni phase.The enthalpy change value of Mg₈₇Ni₁₀La₃ at a hydrogen release rate of5K/min was 1529J/g.The hydrogen absorption capacity of Mg₈₂Ni₁₅La₃ alloy at 683k for 2hours is 4.92wt%,that is,the heat release capacity of unit mass of Mg₈₂Ni₁₅La₃ alloy is1802kJ,and the heat release density is 1.8MJ/kg;the hydrogen release capacity of Mg₈₂Ni₁₅La₃ alloy at 683k for 2 hours is 4.69wt%,and the heat storage capacity of unit mass of Mg₈₂Ni₁₅La₃ alloy is 1718kJ,and the heat storage density is 1.7MJ/kg.The comparison results of the microstructures Ce and La-based alloys showed that different rare earth elements Ce and La had the similar effect on the micro-morphology and phase composition of the alloy.Both Ce and La-based alloys reacted as the metal hydrides with high thermal stability and difficult to dissociate after completely absorbing and releasing hydrogen.During the absorption and release of hydrogen,metal hydride nanocrystals are evenly distributed on the surface and body of the alloy,introducing a large number of active sites,the number of grain boundaries and the interface area for the alloy.This greatly increases the diffusion rate of hydrogen atoms in the alloy.the combined Mg-Mg₂Ni eutectic layered structure provides an effective channel for the diffusion of hydrogen atoms in the alloy,The metal hydride could effectively promote the hydrogen absorption and heat release kinetic properties of the alloy.The heat storage capacity of Mg₈₇Ni₁₀Ce₃ alloy is 279kJ higher than that of Mg₈₂Ni₁₅La₃ alloy,and the heat storage density is 0.3MJ/kg higher.