I ball milled Ti-V-Cr BCC alloys. I measured reduced kinetics after ball milling. The literature reported the reduced kinetics due to surface contamination while ball milling and observed peaks of ball material on XRD patterns. In my case i did not observed these peaks. The BCC crystal structure remains the same through for various milling time. From EDS only I got small Iron contamination. But it is also showing arbitrary numbers and not relevant with milling time. If it is from milling it could be expected the contamination increasing with milling time. But these numbers are up and down arbitrarily. The arbitrary numbers could be explained by iron source from Spacula used to scratch sticky powder from milling vail. But what could be more reasons behind reduced kinetics?
My recommendation is to check the particle size after ball-milling. Some metallic materials are ductile and do not tend to decrease size even after a very long ball-milling. In contrast, they tend to agglomerate forming bigger particles and hence reducing the surface area, which is directly related to the kinetics. An example is Nb and Nb2O5 in MgH2. The former cannot give better performance than the latter, because Nb2O5 which is brittle can give smaller size after ball-milling. You can refer to this paper: T Ma et al, J Phys Chem C 2013, 117, 10302. Also for that reason, Fe content will not increase linearly if you increase the ball-milling time, because once the particles agglomerate you have reduced surface area.
Three comments:
The first one was delivered a couple of hours ago... (see here above).
The second one refers to the answer of Tao Ma. effectively Nb is a ductile metal and Nb2O5 is a brittle ceramic, so the dispersion of particles is by far more effective in the second case. Anyway, during ball milling the oxide is progressively reduced and transformed to Nb metal particles, those particle are quite finer and extensively dispersed onto the Mg/MgH2 particles. The question of agglomeration to bigger and bigger Mg based particles is not so detrimental for "bulk" hydrogen diffusion (reference to the surface area that is far to be regular). The question is how easy one transforms a HCP to a rutile structure and vice versa. What is the main part driving force for such a transformation? Catalysis? Literature is plenty of catalytic effects, but no CATALYTIC mechanism was effectively demonstrated (to my knowledge).
The last comment is when you ball mill (long time enough), a mechano-synthesis process can take place, depending of the elements (that is chemistry of metal systems). So I suppose that you balls and jar are made of strenthened steel (as often). More probably this is a Fe-rich Fe-Co alloy. When you ball mill a BCC Ti-V-Cr material, you can incorporate 'alloy) Fe and Co in the BCC matrix, without any production of extra impurities since the d-elements are incorporated in the matrix of powdered materials.
It results a net decrease of both the sorption kinetics and the max hydrogen uptake. As a proof, please measure the magnetic properties of your long time ball milled BCC materials. Once, some years ago, we did that with.. Jacques.. and from my magnetic characterisations, It came that the BCC should contain several 10% Iron(Co), and was effectively improper for hydrogen storage...
Since BCC TiVCr is a resilient alloy, it is as was rather abrasive...
Good luck
Daniel
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