Electron diffraction studies of electro-deposits

Abstract

The work described in this thesis has been directed towards the design and fabrication of an electron diffraction camera, and its use to study the dependence of the structure of electrodeposits upon electrochemical conditions such as current density and bath composition. Attention has been mainly focused on electrodeposits of brass obtained from alkaline glycerate–zincate baths; but, in addition, a brief study has been made of manganese electrodeposits from the manganous sulphate–ammonium sulphate bath. Chapter 1 includes a general survey of studies on the structure and growth of electrodeposits. This survey brings out the important role that the electron diffraction technique has played in these studies. Chapter 2 outlines the broad aim and scope of the work and highlights the absence of any systematic attempt to correlate the structure of any electrodeposits with electrochemical variables. Chapter 3 deals with the design of a 50 kV electron diffraction camera and describes the unit which has been fabricated as part of the present work. The design and constructional features of the cold?cathode electron gun, the magnetic focusing and deflecting lens, the high?vacuum system and the camera column are discussed in detail. In addition, the initial testing of the electron diffraction camera is described. Chapter 4 contains a report of a brief study of manganese electrodeposits, undertaken partly to test out the diffraction camera and partly to elucidate the structure of thin manganese electrodeposits, the structure of which has been the subject of controversy. Reflection electron diffraction has shown that thin deposits (< 20 microns thickness) exist in the gamma form as a mixture of the face?centred tetragonal and the hitherto unobserved face?centred cubic modifications. It has also been observed that preferred orientations were developed in the thicker deposits, and that the texture axis changed with the addition of sulphur dioxide to the bath. The systematic study of the structure of brass electrodeposits from glycerate–zincate baths is reported in Chapter 5. The following are some of the more interesting aspects of the work:

Liquiddensitometry has been used for the first time to facilitate the interpretation of electron diffraction patterns.

In contrast to the many phases observed with thermally prepared brass and with brass electrodeposits from the cyanide bath, it has been found that brass from the glycerate–zincate bath is electrodeposited either with a predominantly face?centred cubic copper?rich alpha?structure or with a hexagonal close? packed zinc?rich epsilon?structure.

The variations of structure with current density and copper content of the bath have been conveniently summarised in the form of a “phase diagram”.

The discovery of the zinc?rich epsilon phase has been used to explain the well?known observation that deposits from the glycerate–zincate bath de?zincify faster than those from the cyanide bath, even though both the brass deposits have the same copper content.

Texture development has been observed with brass electrodeposits, as also changes of texture axis with current density. These changes in texture axis have been attributed to changes in current efficiency resulting from changes in current density.

With small changes in current density or copper content in the bath, it has been observed that there are marked changes of zinc content in the deposits. These changes are associated with the change from a predominantly alpha to an epsilon structure. This change, however, is not reflected in the current–potential curves unlike the jump in zinc content in nickel–zinc alloys.

The final chapter summarises the main conclusions emerging from this work and makes some suggestions regarding future work.