Absorption kinetics and hydride formation in magnesium films : Effect of driving force revisited
Absorption kinetics and hydride formation in magnesium films : Effect of driving force revisited
复制标题
DOI：
10.1016/j.actamat.2014.11.031
复制
发表时间：
2015-02
期刊：
影响因子：
9.4
通讯作者：
H. Uchida;S. Wagner;M. Hamm;J. Kürschner;R. Kirchheim;B. Hjörvarsson;A. Pundt
中科院分区：
文献类型：
--
作者：
H. Uchida;S. Wagner;M. Hamm;J. Kürschner;R. Kirchheim;B. Hjörvarsson;A. Pundt
电化学氢渗透测量和原位气体加载的X射线衍射测量进行多晶镁薄膜。在300 K时，测定了不同驱动力下的氢扩散常数、氢化物体积含量和面内应力。对于α-Mg-H，氢扩散常数D H Mg= 7（±2）· 10-11 m 2 s-1。对于更高的浓度，发现不同的动力学机制，其表观扩散常数D H tot降低，取决于驱动力，降低到约D H tot= 10− 18 m 2 s− 1。这一最低的扩散常数比体相β-MgH 2的扩散常数大两个数量级，这种差异归因于沿沿着晶界的快速扩散。不同的动力学制度归因于空间分布的？一个非均相氢化物成核和生长模型的建议，是基于半球形的空间分布，根据作为驱动力的函数表示的核密度。该模型使我们能够定性地解释复杂的应力发展，不同的扩散制度和阻挡层厚度。由于阻挡层厚度与驱动力成反比，因此小的驱动力允许大的膜体积分数的阻挡。最大应力的情况下发生的氢化物的距离达到四倍的氢化物半径和氢化物的距离等于膜厚度。
Electrochemical hydrogen permeation measurements and in situ gas-loading X-ray diffraction measurements were performed on polycrystalline Mg films. Hydrogen diffusion constants, the hydride volume content and the in-plane stress were determined for different values of driving forces at 300 K. For α-Mg–H, a hydrogen diffusion constant of D H Mg= 7 (±2)· 10-11 m 2 s− 1 was determined. For higher concentrations, different kinetic regimes with reduced apparent diffusion constants D H tot were found, depending on the driving force, decreasing to about D H tot= 10− 18 m 2 s− 1. This lowest measured diffusion constant is two orders of magnitude larger than that of bulk β-MgH 2, and the difference is ascribed to a contribution from a fast diffusion along grain boundaries. The different kinetics regimes are attributed to the spatial distribution of hydrides. A heterogeneous hydride nucleation and growth model is suggested that is based on hemispherical hydrides spatially distributed according to the nuclei densities expressed as a function of the driving force. The model allows us to qualitatively explain the complex stress development, the different diffusion regimes and the blocking-layer thickness. As the blocking-layer thickness inversely scales with the driving force, small driving forces allow the hydriding of large film volume fractions. Maximum stress situations occur for hydride distances reaching four times the hydride radius and for hydride distances equaling the film thickness.