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Browsing Physics by Author "Asante, Joseph Kwaku Ofori"

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    The determination of ternary segregation parameters using a linear heating method
    (University of the Free State, 2000-11) Asante, Joseph Kwaku Ofori; Roos, W. D.; Du Plessis, J.
    In this study the segregation behaviour of the ternary system Cu(lll ), Sb, Sn is investigated experimentally, as well as with the modified Darken segregation model. The model, which describes the kinetics as well as the equilibrium of segregation, had been used successfully in various studies of binary systems. A computer program based on this model was developed for ternary systems. A Cu(lll) single crystal was doped with low concentrations of 0,180 at% Sb and 0.133 at% Sn using evaporation and diffusion process.' The experimental results were gathered with the Auger electron spectroscopy technique. This technique was combined with a linear temperature ramp that makes it possible to obtain the segregation parameters in a single run. The traditional method requires various runs at different temperatures. The overlapping of Sb and Sn Auger peaks in the energy regions of interest necessitated the development of a method to successfully extract the true contributions of the elements from the measured spectra. It is clearly shown that the combination of Auger peaks is not linear and that the true contributions of Sb and Sn can be calculated if the peaks overlap in two energy regions and the standard spectra are available. The segregation profiles resulted from the Auger data show clearly the sequential segregation of the two elements (Sn and Sb). From the equilibrium conditions, it is also concluded that an interaction energy between Sb and Sn is present. By simulating the experimental results, using the theoretical Darken model, values for the segregation parameters can be obtained. The initial values for the fits are found mathematically (highenergy regions) and manually (low energy regions). The calculated profiles fit the experimental results very well. The present study confirms that Sn segregate first to the surface with Do = 1.58x10-5 m²s-¹ and E = 170 kJ/mol. Sb with a lower dimsion coefficient (Do = 1.93x10-8 m²s-¹ and E = 150 kJ/mol) segregates at higher temperatures. A further increase in temperature results in the stronger segregate Sb, (with a higher segregation energy ∆G = -74.6 kJ/mol) to displace the Sn (∆G = -59.0 kJ/mol) from the surface. From the simulations, it is clear that the maximum surface coverage for Sn is determined mainly by the attractive interaction (ΩSnCu = -8.25 kJ/mol) between Sn and Cu. The desegregation rate of Sn in this system is determined by the segregation rate of Sb. The segregation profile of Sb is similar to that in a binary system (Cu,Sb) with the desegregation rate of Sb much slower than the segregation rate. The study also shows definite attractive interaction between Sb and Cu (ΩSbCu= -17.05 kJ/mol) This trend was not observed in the studies of binary systems. There is, however, repulsive interaction between the segregates (ΩSnSb = 3.62 kJ/mol). The repeatability of the segregation parameters at different heating rates shows that this experimental method can be used successfully.
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    ItemOpen Access
    Surface segregation of Sn and Sb in the low index planes of Cu
    (University of the Free State, 2005-05) Asante, Joseph Kwaku Ofori; Roos, W. D.; Terblans, J. J.
    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.
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    ItemOpen Access
    Surface segregation of Sn and Sb in the low index planes of Cu
    (University of the Free State, 2005-05) Asante, Joseph Kwaku Ofori; Roos, W. D.; Terblans, J. J.
    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 oC 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-exponential 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 (Ω.CuSn = 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 (Ω.SnSb = - 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) (ΔGSb(100) = 84 kJ/mol; ΔGSn(100) = 65 kJ/mol) and in Cu(111) (ΔGSb(111) = 86 kJ/mol; ΔGSn(111) = 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 × 10-4 m2/mol and DoSn(ternary) = 3.4 × 10-3 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.