Mg/Ti-Cr-V复合储氢材料批量制备与服役条件下热导率研究

Batch Preparation and Thermal Conductivity on Working Condition of Mg/Ti-Cr-V Composite

掌握储氢材料床在服役条件下的热导率性能是准确计算预测固态储氢装置吸/放氢动态响应特性的基础。采用原位氢化反应球磨法实现了Mg-3%Ti_(0.16)Cr_(0.24)V_(0.6)(质量分数)复合储氢材料200 g级的批量制备,采用Sievert's体积法测定了复合材料的吸/放氢动力学性能,并采用X射线衍射(XRD)、扫描电镜(SEM)和激光粒度法对复合材料物相组成、显微形貌和颗粒粒度分布进行表征。将氢化态的Mg-3%Ti_(0.16)Cr_(0.24)V_(0.6)复合储氢材料(MH)与一定比例的膨胀石墨(EG)混合,采用单轴模压制备Φ40 mm的MH/10%EG储氢模块,利用瞬态平面热源法测定储氢模块在不同气氛、氢压、温度和氢含量下的热导率。结果表明随着球磨时间的增加,Mg-3%Ti_(0.16)Cr_(0.24)V_(0.6)复合材料的吸/放氢动力学和容量明显提高,颗粒不断细化,并且在球磨40 h后基本氢化完全,最终产物的物相组成主要为β-Mg H_(2),γ-Mg H_(2)和Ti_(0.16)Cr_(0.24)V_(0.6)Hy(y=0.3~1.2),通过固化球磨工艺可以保证产物的批次稳定性,其在300℃对0.1 MPa的放氢量在6.8%~6.95%之间。在压实后的储氢模块中EG沿垂直于压制方向呈层状排列并形成导热通路,在室温和0.1 MPa氢压下热导率可达6.66 W·m^(-1)·K^(-1)。Mg-3%Ti_(0.16)Cr_(0.24)V_(0.6)/10%EG(MH/EG)储氢模块在氢气气氛中具有较高的热导率,且热导率随压力的提高呈对数增大的趋势,随温度的升高呈线性减小趋势。MH/EG储氢模块在放氢时,热导率呈先轻微增加,当放氢量在3%左右时后显著增大至基本稳定的值。给出了MH/EG储氢模块热导率与温度、氢压和氢含量等服役条件关系的经验公式,并探讨了相应的机制。

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%Ti_(0.16)Cr_(0.24)V_(0.6) 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%Ti_(0.16)Cr_(0.24)V_(0.6) 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%Ti_(0.16)Cr_(0.24)V_(0.6) 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 ofβ-MgH_(2),γ-MgH_(2) and Ti_(0.16)Cr_(0.24)V_(0.6)Hy(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.