Batch Preparation and Thermal Conductivity on Working Condition of Mg/Ti-Cr-V Composite
The problem of lacking safe and efficient hydrogen storage and transportation technology was one of the key factors restrict-ing the large-scale commercial application of hydrogen energy.High-capacity hydrogen storage materials were the effective means to solve this problem.Mg-based hydrogen storage materials have become one of the most attractive hydrogen storage materials due to its high hydrogen storage capacity,abundant resources,and low price.However,due to the high enthalpy of the hydrogen absorption and desorption reaction of Mg-based hydrogen storage material and the poor thermal conductivity of the powder,it is urgent to improve the heat and mass transfer performance of the bed to meet the application needs.At present,the calculation models for the heat and mass transfer performance of Mg-based hydrogen storage materials simplified the effective thermal conductivity of Mg-based hydrogen stor-age material bed to a constant,resulting in deviation between the calculated value and the actual results under application conditions,which affects the accuracy of the model calculation.Therefore,there was an urgent need to systematically study the effective thermal conductivity of Mg-based hydrogen storage material beds with working condition parameters,such as hydrogen pressure,temperature and hydrogen content to provide key data support for the optimization of heat and mass transfer in the system.Based on the above prob-lems,the in-situ hydrogenation reaction ball milling method was used to achieve the 200 g per batch preparation of Mg-3％Ti0.16Cr0.24V0.6 hydrogen storage composites.Sievert's method was applied to determine the hydrogen absorption/desorption kinetic properties of the composites.X-ray diffraction(XRD),scanning electron microscopy(SEM)and laser particle size analysis method were used to characterize the phase composition,microscopic morphology and particle size distribution of the composite samples.The hydrogenated Mg-3％Ti0.16Cr0.24V0.6 composite sample(MH)was mixed with expanded graphite(EG)in a certain proportion,such as 90％MH and 10％EG(MH/EG).MH/EG compact with 40 mm in diameter was prepared by uniaxial die pressing method under a pres-sure of 351 MPa.The transient plane heat source method was used to measure the thermal conductivity of the hydrogen storage compact samples under different atmospheres,hydrogen pressure,temperature and hydrogen content.The results showed that with increasing of the ball milling time,the hydrogen absorption and desorption kinetics and capacity of Mg-3％Ti0.16Cr0.24V0.6 composite material were significantly improved,the particles continue to be refined,and the hydrogenation is basically completed after 40 h of ball milling.The phase composition of the final products was mainly composed of β-MgH2,γ-MgH2 and Ti0.16Cr0.24V0.6H,(y=0.3～1.2).The batch sta-bility of the product could be ensured by fixing the ball milling parameters,and the hydrogen desorption capacities at 300℃ were be-tween 6.8％～6.95％.In MH/EG compact,EG was arranged in the layers perpendicular to the pressing direction,which formed the heat conduction pathways.The thermal conductivity value could reach 6.66 W·m-1·K-1 at room temperature and 0.1 MPa hydrogen pres-sure.MH/EG compact had higher thermal conductivity in hydrogen atmosphere,and the thermal conductivity showed logarithmic in-creasing trend with increasing hydrogen pressure,and linear decreasing trend with increasing temperature.And it showed"S"type trend with hydrogen content.As the hydrogen content decreased,the thermal conductivity increased slightly firstly,and when the de-hydrogenation amount was about 3％,it significantly increased to a stable value.Then,an empirical formula was proposed for the rela-tionship between the thermal conductivity of MH/EG compact with the working conditions such as temperature,hydrogen pressure and hydrogen content and discussed the corresponding mechanism.
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