The Structure and Growth of Electrodeposits, and the Modifications Caused by Brightening Agents

Abstract

Recent electron-diffraction evidence of the structure and conditions of growth of condensed deposits and of electrodeposits leads to the conclusion that during electrodeposition under the conditions usually used, the surface quickly reaches a temperature often as high as 600°C. or even more. A detailed study of electrodeposits of silver from a cyanide bath on electropolished (110), (100) and (111) faces of a silver single crystal has shown that between the initial stage of parallel overgrowth and the random polycrystalline (and later one-degree orientated growth) there is, above a minimum critical c.d., an intermediate twinning of the single-crystal overgrowth, which accounts for the development of the random crystals. This critical c.d., for a given bath, is highest on the crystal faces having the highest number of atoms (or potential troughs) per unit area, and the more this c.d. is exceeded the more rapidly is twinning developed and in turn gives place to random crystals. This c.d. increases with increasing bath temperature. The effect of varying the bath composition was also explored. For comparison, the effects caused by presence of a small proportion of addition agents (CS₂, thiosemicarbazide, thiourea, urea and KCNS) is also briefly described. Widely different effects on the crystal habit were caused by the various agents investigated. It is concluded that brightening results when the crystals, whether of microscopic size or not, form surfaces which are large and smooth relative to the light wavelength.