Characteristics of Mg2Ni0.75Co0.25 alloy after surface treatment

Mg2Ni0.75Co0.25 alloy has been successfully synthesized by the Ball milling diffusion method. A part of the product was treated by different concentrations of a NH4F solution. The results showed that the alloy treated by a 0.06 M NH4F solution had the best performance. During treatment, NH3. H2O, one of the byproducts could be used to indicate the end of etching process. A study of the surface state of the alloy showed that beneath the honeycomb MgF2 layer, a metallic Ni-rich layer was formed. This layer was believed to have a high activity, the concentration of Ni approaching 70 at.%, which was much higher than 25 at.% of that in the bulk. The hydrogen desorption capacity reached the highest value after only 2 adsorption/desorption cycles. The capacity wets 372 ml(S.T.P.)H-2/g.alloy (250 degrees C) (H/M=1.18). The desorption properties of treated and untreated alloys showed that the desorption plateau could not be changed even after surface treatment. The specific surface was 4.1 m(2)/g after surface treatment. All of the above results revealed quite different properties from that of the untreated alloy. It is possible that the existence of the honeycomb MgF2 layer and the metallic Ni-rich layer was the main reason of the improvement of the hydriding/dehydriding properties. (C) 2000 Elsevier Science S.A. All rights reserved.