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

氢的晶界（GB）扩散与替代元素的晶界扩散有很大不同：虽然替代元素的晶界扩散比体扩散快大约三个数量级，但间隙氢的晶界扩散比氢体扩散慢或相似。 Mütschele 和 Kirchheim 于 1986 年通过纳米晶钯在低氢浓度下证明了这一点。此外，假设晶界不转变为氢化物相而是保持其非晶态性质，纳米晶 Pd-H 的不同等温线可以很容易地解释。在这个项目中，我们计划强调这样的想法，即 GB 主要在整个浓度范围内保持其特性，从而导致高浓度下的 GB 扩散应与低浓度下测量的扩散相似。这对于合金转变成具有低体积扩散率的超稳定氢化物特别重要，因为晶界可以充当氢扩散的途径。可以优化微观结构和应力状态以获得最快的扩散率。镁转变为稳定的二氢化镁相，具有极低的体积扩散率。此外，由于镁的重量存储密度高，因此对于移动存储应用特别感兴趣。通过所提出的研究，我们希望将GBs作为氢扩散途径的基础研究与纳米晶体、更快动力学储氢材料设计的有前景的新方面联系起来。

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.