应力诱导下亚稳镁基储氢材料释氢热力学特性及其调控研究

The formation of metastable phases induced by stress provides a new idea to solve the dehydrogenation thermodynamics obstacle of magnesium based hydrogen storage materials. The keys of manipulating the dehydrogenation thermodynamics of magnesium based hydride are to master the rule of stress-induced metastable structures and to establish a method of controllable stress modulation. In the present project, the studies on two-dimentional nanoscale MgH2 based multilayers with abundant interface will be performed starting from the correlation among stress, structure and performance. Using first-principles calculations in combination with the method of modulating interfacical stress of multilayers through magnetostriction, the effects of interfacical stress on the metastable structures and dehydrogenation thermodynamics of MgH2 will be investigated from theoretical and experimental aspects. The influence rules of interfacical stresses with different attributes (including uniaxial or biaxial or triaxial, different magnitude, tensile or compressive, isotropic or anisotropic) on the macro- and micro-structures as well as dehydrogenation enthalpy will be ascertained. The intrinsic mechanism of stresses will be revealed. The corresponding correlation among stress status, metastable structures and dehydrogenation thermodynamics of MgH2 will be established. Based on the correlation as an optimization criterion for the interfacial stress, the metastable MgH2 in multilayers will be further designed and prepared to realize the stress manipulation of dehydrogenation thermodynamics. These results can provide the theoretical and technological basis for structural designing and thermodynamic manipulation of magnesium and other metal hydrogen storage materials.

利用应力诱生亚稳相为解决镁基储氢材料的释氢热力学障碍提供了新思路。掌握镁基氢化物亚稳结构的应力诱导规律、并建立可控的应力调制方法，是实现其释氢热力学有效应力调控的关键。本项目拟从应力-结构-性能三者之间关联性的角度出发，以界面丰富的二维MgH2基纳米多层薄膜为研究对象，采用第一性原理计算与磁致伸缩调制多层膜界面应力的研究手段，从理论与实验两方面逐层深入研究MgH2亚稳结构及其释氢热力学的界面应力诱导效应。探明不同属性(单轴/双轴/三轴、不同大小、拉/压、各向同/异性)界面应力对MgH2微、宏结构及其释氢反应焓的影响规律，揭示应力作用的本质机理；建立应力状态、MgH2亚稳结构及释氢热力学三者之间的对应关系，并以此作为界面应力的优化准则，指导多层膜中MgH2亚稳结构的设计与制备，从而实现对其释氢热力学的有效应力调控。研究成果可为镁及其它金属储氢材料的结构设计与热力学性能调控提供理论与技术基础。

金属Mg因具有储氢密度高(7.6wt%)、质轻、价廉、资源丰富等优点，被认为是极具发展前景的车载储氢材料之一。然而，其氢化物MgH2热力学性质过于稳定导致其释氢温度偏高，进而限制了实际应用。从晶体结构角度入手，诱使MgH2晶格变形，使其呈现为弱稳定性的亚稳结构，进而削弱Mg-H键强，无疑是突破镁基储氢材料释氢热力学障碍的有效途径。本项目以MgH2纳米薄膜和纳米颗粒为研究对象，采用第一性原理计算与实验研究相结合的方法，系统研究了MgH2亚稳结构及其释氢热力学的应力诱导效应。首先，基于表面能计算，确定了MgH2薄膜的晶面择优取向；进而系统计算了不同大小的单/双/三轴拉/压应力作用下MgH2薄膜的微观结构及其释氢反应焓，发现随着应变维数的增加以及应变程度的增大，MgH2薄膜总能量不断上升，释氢反应焓不断降低，且拉应变致使体系释氢热力学的改善效果尤为显著；在部分单轴和双轴应变条件下，MgH2经历由四方结构向正交结构的转变，从而有效削弱了MgH2的相结构稳定性，改善其释氢热力学性能。此外，进一步研究了不同应力对MgH2释氢动力学的影响，发现MgH2薄膜释氢动力学也随着应变维数的增多以及应变程度的增大而逐渐增强，其中，压应变更有助于改善MgH2的释氢动力学。在理论计算基础上，采用磁控溅射法和高能球磨法分别制备了MgH2纳米薄膜和纳米颗粒，基于结构与性能表征，发现MgH2纳米薄膜或纳米颗粒的近邻接触相及其固溶原子，会诱使MgH2产生长程或局域晶格应变，从而使其结构产生变形，呈现为亚稳结构，进而导致其释氢温度降低，实验与计算结果保持一致。进一步研究了应力对MgH2释氢热力学影响的电子结构起源，发现在应力作用下，MgH2成键峰和反键峰彼此互相接近，导致费米能级附近的能隙变窄，从而明确了应力诱导MgH2释氢热力学改善的本质原因。项目研究结果为改善镁基储氢材料释氢性能提供了新思路与理论依据。项目执行过程中以第一作者发表学术论文16篇，其中SCI收录12篇，EI收录2篇；授权发明专利2项；出版学术专著1部；培养硕士研究生3名。