Surface segregation of Sn and Sb in the low index planes of Cu
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Date
2005-05
Authors
Asante, Joseph Kwaku Ofori
Journal Title
Journal ISSN
Volume Title
Publisher
University of the Free State
Abstract
In this study, the segregation parameters for Sn and Sb in Cu were determined for the first
time using novel experimental procedures. Sn was first evaporated onto the three low
index planes of Cu(111), Cu(110) and Cu(100) and subsequently annealed at 920°C for
44 days to form three binary alloys of the same Sn concentration. Experimental
quantitative work was done on each of the crystals by monitoring the surface segregation
of Sn. Auger electron spectroscopy (AES) was used to monitor the changes in
concentration build up on the surface by heating the sample linearly with time (positive
linear temperature ramp, PLTR) from 450 to 900 K and immediately cooling it linearly
with time (negative linear temperature ramp, NLTR) from 900 to 650 K at constant rates.
The usage of NLTR, adopted for the first time in segregation measurements, extended the
equilibrium segregation region enabling a unique set of segregation parameters to be
obtained.
The experimental quantified data points were fitted using the modified Darken model.
Two supportive models - the Fick integral and the Bragg- Williams equations - were used
to extract the starting segregation parameters for the modified Darken model that
describes surface segregation completely. The Fick integral was used to fit part of the
kinetic section of the profile, yielding the pre-exponenrial factor and the activation
energy. The Bragg- Williams equations were then used to fit the equilibrium profiles
yielding the segregation and interaction energies. For the first time, a quantified value for
interaction energy between Sn and Cu atoms through segregation measurements was
determined (ΩCuSn = 3.8 kJ/mol). The different Sn segregation behaviours in the three Cu
orientations were explained by the different vacancy formation energies (that make up the
activation energies) for the different orientations. The profile of Sn in Cu(110) lay at
lowest temperature which implies that Sn activation energy was lowest in Cu(110).
Sb was evaporated onto the binary CuSn alloys and annealed for a further 44 days
resulting in Cu(111)SnSb and Cu(100)SnSb ternary alloys. Sn and Sb segregation
measurements were done via AES. The modified Darken model was used to simulate Sn
and Sb segregation profiles, yielding all the segregation parameters. Guttman equations
were also used to simulate the equilibrium segregation region that was extended by the
NLTR runs to yield the segregation and interaction energies. These segregation values
obtained from the modified Darken model for ternary systems completely characterize the
segregation behaviours of Sn and Sb in Cu. For the ternary systems, it was found that Sn
was the first to segregate to the surface due to its higher diffusion coefficient, which
comes about mainly from a smaller activation energy (ESn(100)= 175 kJ/mol and ESb(100)
186 kJ/mol). A repulsive interaction was found between Sn and Sb (ΩSnSb = - 5.3
kJ/mol) and as a result of the higher segregation energy of Sb, Sn was displaced from the
surface by Sb. This sequential segregation was found in Cu(100) (∆GSb(100)= 84 kJ/mol;
∆GSn(100)= 65 kJ/mol) and in Cu(111) (∆GSb(111) = 86 kJ/mol; ∆GSn(l1l) = 68 kJ/mol). It
was also found that the profile of Sn in the ternary systems lay at lower temperatures due
the higher pre-exponential factor (DoSn(binary) = 9.2 x 10-4 m2/mol and DoSn(ternary) = 3.4 X
10³ m2/mol) if compared to the binary systems.
This study successfully and completely describes the segregation behaviour of Sn and Sb
in the low index planes of Cu.
Description
Keywords
Surface chemistry, Segregation (Metallurgy), Surfaces (Physics), Thesis (Ph.D. (Physics))--University of the Free State, 2005