Formation and structure of Ni-Ag layer by electrolytic deposition

摘要

Although nickel and silver have the same Bravais-lattice and the difference of atom radii is below 15%, the binary system Ni-Ag is generally immiscible in the solid state below 500℃ [1]. the reason for this is the positive energy of mixing (+22 kJ/mol [2]). In recent literature the formation of metastable solid solution of Ni-Ag is described [2-4]. the observed solid solution is produced by using PVD-, CVD-techniques, or mechanical alloying. i.e. techniques, which work far from the thermodynamic equilibrium. In the present work, the original aim was to deposit Ni and Ag electrolytically from the same solution as a crystal mixture with very fine grain size. Layers which consist of a ferromagnetic phase (Ni) and a diamagnetic phase (Ag) are very interesting iv view of special magnetic applications (i.e. giant magnetoresistance). The layer should consist mainly of a nickel phase and to a lesser extent part of a silver phase. Because of the large difference between the standard potentials of Ni and Ag, the authors used an electrolyte where the concentration of Ag ions is two orders of magnitude lower than the Ni ion concentration. At a strong negative polarization potential (high over-potential), this concentration of silver leads to a transport limited silver deposition and simultaneously to a charge transfer-controlled deposition of nickel. The high negative over-potential should result in a microstructure whit very fine gains. The relation of grain size and over-potential is well-known [5]. Therefore strong negative polarization potentials were chose in the following study. The electrolyte composition consisted of 0.7 mol/l NiSO_4, 0.002 mol/l AgNO_3 and 0.26 mol/l C_6H_5Na_3O_7. The pH-value of the solution was 5.5 and all depositions were carried out at room temperature (RT). The electrolyte based on the authors' own experience in the deposition of nickel layers with nanocrystalline microstructure [6] and was slightly modified. All potentials were measured against a Ag/AgCl-reference electrode.