| dc.description.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. | |
