PHASE STABIITY OF ELECTRODEPOSITED ALLOYS OF TIN AND IB METALS

摘要

Phase structure and stability of electrochemically deposited (ECD) alloys is a topic of great theoretical interest. The phenomenological factors affecting structure of ECD alloys are well known and they from the basis for well sound qualitative explanations. The structural characteristics are determined by electrokinetic supersaturation effect and the related peculiarities of crystal growth in electrocrystallization: near two-dimensional growth front, ineffective surface diffusion and incorporation of atoms at randomly distributed kink sites at the surface. In this work the phase structure of ECD binary alloys of noble metals with Sn is reported and differences with respect to metallurgical alloys and their equilibrium phase diagrams are discussed. Starting from the observation that formation of ordered phases in ECD alloys appears forbidden, it is shown that ECD phase formation can be related to a thermodynamic phase stability interpretation, even though simplistic, when long-range order structures are not considered. Au-Sn deposits were prepared from solutions of thiourea, CS(NH_2)_2 1M and sulphuric acid, H_2SO_4 1M, and containing [Au~+] 0.01÷0.03M and [Sn~(2+)] up to 0.1M, under both direct current (DC) and pulse current (PC) conditions, with cd in the range 0.5÷6 A/dm~2 and 40 deg C on SiTaAu or SiCrNiAu substrates. Ag-Sn alloys electrodeposition was carried out from the following base solution: CS(NH_2)_2 2M, H_2SO_4 0.75M, [Ag~+] 0.1M and [Sn~(2+)] in the range from 0.02 to 0.05 M. Ag-Sn coatings were obtained at cd in the range 4 to 10 mA/cm~2, from quiescent solutions at room temperature on polished brass sheets, coated with a thin silver displacement film. Cu-Sn alloys were electrodeposited from acid solutions, containing either cupric (sulphate) or cuprous (oxide) compounds, on brass or steel substrates. Cu(II) acid solutions were basically simple sulphuric acid and sulphate solutions, with addition of additives such as reducing agents and/or tin ions ligands. Copper as Cu(I) was stabilized in acidic solution (H_2SO_4 1 M) by thiourea addition in concentration from 0.2 to 1.5 M. The source of stannous ions was, for both solutions, SnSO_4 added to a concentration of about 0.1 M. The structure of ECD alloy deposits was studied by X-ray diffraction. Microhardness measurements were used as additional data source for the assessment of phase stability limits.