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

氢的晶界扩散与替代元素的晶界扩散有很大的不同：替代元素的晶界扩散比体扩散快三个数量级，而间隙氢的晶界扩散慢于或类似于氢的体扩散。Mütschele和Kirchheim在1986年为低氢浓度的纳米晶钯证明了这一点。此外，在假设GBS不会转变为氢化物相而保持其类非晶态性质的情况下，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，我们计划强调这样一个观点，即Gb主要在整个浓度范围内保持其性质，从而导致在高浓度下Gb的扩散应该与在低浓度下测量的结果相似。这对于转变成体积扩散系数低的超稳定氢化物的合金特别有意义，因为GB可以作为氢扩散的途径。微观结构和应力状态可以优化，以获得最快的扩散率。镁转变为稳定的镁-二氢化物相，具有极低的体扩散系数。此外，由于其高的重量存储密度，MG对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作为氢扩散途径这一概念的基础研究与设计纳米晶、更快动力学的储氢材料的新方面联系起来。

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.