Formation of porous $\alpha$ tantalum in films

Ta–Cd alloys in the form of coatings with up to 66.2 at% Cd are fabricated for the first time by ion-plasma sputtering and the subsequent codeposition of ultradispersed Ta and Cd particles. At a concentration higher than 44.0 at% Cd, tetragonal $\beta$-Ta transforms into bcc $\alpha$-Ta. Beginning from 74.4 at%, cadmium forms its hexagonal lattice and tantalum in the coatings is represented by an amorphous phase. During vacuum heat treatment at 700–750$^\circ$C, cadmium evaporates from the binary tantalum–cadmium system containing more than 74.4 at% Cd, and a porous tantalum coating with a well-developed surface forms. Tantalum in the coating is represented by various flakelike crystallites, and tantalum in the near-surface layer mainly has a globular shape. A comparison of the specific surface areas of a tantalum film and the porous tantalum coating, which is performed by the BET method, demonstrates that the specific surface area increases by 277.5 m$^2$/g Ta. Our results can be applied to other binary systems having similar properties.