球磨条件下Fe合金化改善MgH_2体系性能的机理  

Mechanism of improved properties of MgH_2 systems with ball milling and iron addition

作　　者：周惦武[1] 张健[1,2] 彭平[2] 刘金水[2] 

机构地区：[1]湖南大学汽车车身先进设计制造国家重点实验室,长沙410082 [2]湖南大学材料科学与工程学院,长沙410082

出　　处：《中国有色金属学报》2008年第12期2233-2244,共12页The Chinese Journal of Nonferrous Metals

基　　金：国家自然科学基金资助项目(50771044);国家重点基础研究发展计划资助项目(2006CB605104)

摘　　要：基于密度泛函理论的第一原理方法,通过计算清洁、空位缺陷Mg(0001)表面吸附氢分子(H2)前后以及Fe合金化镁氢化合物(MgH2)体系的能量与电子结构,对球磨条件下Fe合金化改善MgH2体系性能的原因进行初步探讨。结果表明:与清洁Mg表面相比,由于球磨改变了Mg颗粒的表面结构,使Mg表面产生较多缺陷,而缺陷的存在增强了H2的物理吸附能力,并且Mg表面向H2转移的电荷数增多,因而体系球磨后具有较好的吸氢性能;而在Fe合金化MgH2体系中,Fe固溶于MgH2中形成(MgFe)H2固溶体和合金化形成少量Mg2FeH6相,MgH2系结构稳定性均降低,对应体系解氢性能增强。分析电子结构发现:空位缺陷有助H2吸附于Mg表面,与Mg(0001)表面最上层与H2直接产生吸附作用的金属原子在费米能级(EF)附近s轨道的成键电子数密切相关;在Fe合金化MgH2体系中,与合金化元素Fe近邻的H原子形成空位的难度增加,H原子较难释放;与Mg近邻的H原子形成空位的难度减少,H原子容易释放;Fe合金化导致Mg-H之间存在较弱的成键作用,因此,MgH2体系的解氢性能得到提高。The energy and electronic structure of the adsorption of H2 on the clean, vacancy defective Mg （0001） surfaces and iron alloying magnesium hydride were calculated by using a first-principles plane-wave pseudopotential method, the mechanism of improved properties on MgH2 systems were analyzed with milling and iron addition. The results show that the vacancy defect system benefits enhancing the physical sorption between Mg surface and H2 compared with the clean Mg （0001） surface, because the quantity of the charges transferring from Mg surface to H2 adsorbed may be increased significantly, the H2 adsorption properties are improved after ball milling. The structure stability of the alloying system is reduced when a little iron dissolves into the magnesium hydride, and is further reduced when the iron additions form Mg2FeH6 compound. The analyses of electronic structures shows that the catalytic reactivities for H2 adsorption of the different surfaces are dependent on the numbers of s orbital bonding electrons around Fermi level for the uppermost layer metal atoms which interact directly with H2. It is easy to form vacancy for hydrogen atoms next to iron atom, which indicates that hydrogen atom cannot be escaped, but it is difficult to from vacancy for hydrogen atoms next to magnesium atom, which indicates that hydrogen atom can be escaped. Hence it is thought that the change of dehydrogenating properties of MgH2 with or without a little iron addition attributes to the weakened bonding between magnesium and hydrogen.

关 键 词：MgH2 Mg(0001)表面 吸附 形成热 

分 类 号：TG146.2[一般工业技术—材料科学与工程]