Hydrogen grain-boundary diffusion in nano-crystalline Mg-films - experimental and theoretical study

氢的晶界（GB）扩散与取代元素的晶界（GB）扩散有很大不同：取代元素的晶界（GB）扩散比体扩散快3个数量级左右，而间隙氢的晶界（GB）扩散比体扩散慢或与体扩散相似。1986年，m<s:1> tschele和Kirchheim在低氢浓度下证明了这一点。此外，假设GBs不转变为氢化物而保持其非晶性质，可以很容易地解释纳米晶Pd-H的不同等温线。在这个项目中，我们计划强调GB主要在整个浓度范围内保持其特性的想法，从而导致高浓度下GB的扩散应该与低浓度下测量的结果相似。这对于合金转变为具有低体积扩散率的超稳定氢化物具有特殊的意义，因为GBs可以作为氢扩散的途径。为了获得最快的扩散速率，可以对材料的微观结构和应力状态进行优化。Mg转移成稳定的Mg-二氢化物相，体积扩散系数极低。此外，由于其高重量存储密度，Mg对移动存储应用特别感兴趣。通过提出的研究，我们希望将GBs作为氢扩散途径的基础研究与纳米晶，更快动力学储氢材料的设计有希望的新方面联系起来。

Grain boundary (GB) diffusion of hydrogen differs strongly from that of substitutional elements: while GB diffusion of substitutional elements is about three orders of magnitude faster than bulk diffusion, GB diffusion of interstitial hydrogen is slower or similar to hydrogen bulk diffusion. This was proven by Mütschele and Kirchheim in 1986 for nano-crystalline Pd for low hydrogen concentrations. Moreover, different isotherms of nano-crystalline Pd-H can be easily explained under the assumption that GBs do not transform into a hydride phase but keep their amorphous-like properties. In this project we plan to stress the idea that GBs mainly keep their properties over the full concentration range leading to the result that GB diffusion at high concentrations should be similar to that measured at low concentrations. This is of special interest for alloys transforming into extra-stable hydrides with low bulk diffusivity, because GBs could act as pathways for hydrogen diffusion. Microstructure and the stress state could be optimized for fastest diffusivity. Mg transfers into a stable Mg-dihydride phase with extremely low bulk diffusivity. Mg is, further, of special interest for mobile storage applications because of its high gravimetric storage density. With the proposed studies we hope to link basic research on the idea of GBs acting as pathways for hydrogen diffusion with promising new aspects for the design of nano-crystalline, faster kinetics hydrogen storage materials.