Surface composition, deviations

Various pc electrode models have been tested.827 Using the independent diffuse layer electrode model74,262 the value of E n = -0.88 V (SCE) can be simulated for Cd + Pb alloys with 63% Pb if bulk and surface compositions coincide. However, large deviations of calculated and experimental C,E curves are observed at a 0. Better correspondence between experimental and calculated C,E curves was obtained with the common diffuse-layer electrode model,262 if the Pb percentage in the solid phase is taken as 20%. However, the calculated C, at a Ois noticeably lower than the experimental one. It has been concluded that Pb is the surface-active component in Cd + Pb alloys, but there are noticeable deviations from electrical double-layer models for composite electrodes.827... 

Figure 3. Variation of the surface composition of ThNi Fe5 x alloys as a function of bulk Ni/Th ratio (relative standard deviation of i 10%). Reproduced from Ref. 17. Copyright 1984, Amer. Chem. Society.

XPS has been used by several authors to identify the surface and bulk composition of ruthenium based, valve metal stabilized DSA electrodes for chlorine evolution. Augustynski et al. [45] investigated the composition of Ru02-Ti02 electrodes before and after electrochemical treatment. They found a surface composition which deviated significantly from that of the bulk. The Ru/Ti ratio corresponded to 0.15 at the surface while in the bulk 0.28 was measured. Similar results were obtained by De Battisti et al. [46] for Ru02-Ti02 electrodes with different composition ratios on Ti substrates. Fig. 10 shows the Ru/Ti ratio for different solution compositions as a function of the depth. 

The ascertaining by classical and modern methods that the surface composition of alloys can strongly deviate from the composition of the bulk. These methods have opened up the possibility of correlating catalytic phenomena with catalyst surface composition, thus removing a major drawback of older studies with alloy catalysts. 

Fain and McDavid (16) measured the surface composition of Ag-Au alloys with low-energy Auger electrons. The surface concentration proved to be linear and the work function nonlinear in the bulk concentration. This clearly shows that a deviation from linearity of the work function is in itself no proof of surface enrichment. 

The diagrams in Fig. llc-f can be measured by the force probe method, when the amplitude and phase are measured as the tip approaches and retracts the surface vertically. In the non-contact range, both the amplitude and the phase retain their constant values (Fig. llc,e). When the tip enters the intermittent contact range (Zphase reduces almost linearly on approaching the surface. The deviation of the amplitude signal from a certain set-point value As is used by a feedback loop to maintain the separation Zc between the tip and sample constant, and hereby visualise the surface structure. When the surface composition is uniform, the amplitude variation is mainly caused by the surface topography. However, if the surface is heterogeneous, the variation in the amplitude can be affected by local differences in viscoelasticity [108-110 ] and adhesion [111] of the sample (Sect. 2.2.2). 

II.2a) Reactive primary ion bombardment Yield enhancement is brought about by direct implantation of a reactive species in the sample by using an O2+ (or 0") or a Cs+ primary beam. Liebl [5] has described the build-up in the surface concentration of implanted species during sputter erosion. After sputtering to a depth of approximately Rp + 2ARp, where Rp is the projected range and p the standard deviation, the sputter rate of previously implanted primary ions will exactly balance the implantation rate. In this steady state situation, an equilibrium surface composition is achieved and the secondary ion yields stabilize. Figure 5 shows that upon commencement of bombard-... 

Recent analyses of contact angle hysteresis has treated surface heterogeneity in terms of random fluctuations see the excellent review by deGennes The approach is to treat both surface roughness and variations in surface composition as weak fluctuations, i.e. deviations from the ideally smooth surface, du/dy (Fig. 11) and from the difference in solid/liquid and solid/vapor surface energies, — yj - Both fluctuations are considered to be equivalent in perturbing the contact line and are analyzed in terms of their effect on the elastic line energy. 

The parameters listed above can be supplemented with more specialized parameters, such as viscosity, color, surface tension, and density. These secondary parameters will not generally be needed since they are dependent on the first set. For example, the density of a liquid is likely to be a function of temperamre and composition. Therefore, the discussions to do with temperature and composition deviations will incorporate any concerns to do with density. 

Both theoretical equations have been used to estimate the deviations of the surface composition from that of the solution, by plotting the mole fraction h-ddpo, at the surface, as calculated by the Cassie and Israelachvili ... 

The specimens described in the previous section (ionic liquids without MBT and with 10 M MBT) were studied. The surface composition reported in this study is a result of measurement from 9 different spots the extreme results were rejected and an average was taken from the other results. A standard deviation in a set of concentrations from different spots of one specimen was below 30 %. 

The surface tension of a liquid alkali metal mixture also shows pronounced deviation from the mean value. The surface tension of liquid cesium (74 N m x 10 ), lies much below that for sodium (197 Nm x 10 ), and since surface tension reflects the attraction between atoms in the liquid, the concentration of cesium in the surface of the liquid metal mixture should be much greater than in the bulk of the liquid. Consistent with this, the surface tension of pure sodium falls rapidly upon addition of cesium, and in the range 40-100% cesium the surface tension is near to that for pure cesium. A similar behavior has been observed for sodium-potassium mixtures. Many chemical reactions of the liquid alkali metals and their mixtures involve reaction with gases, and care must then be exercised in relating reaction rates with bulk composition when it is the surface composition that is the relevant factor. 

The double-layer capacity (Cai), which is a simple electrical capacity, is often represented by a nonlinear capacity or a constant-phase element (CPiidi) so as to adjust the difference of the electrochemical system from the ideal behavior of an electrical system. Indeed, the use of a simple capacity does not facilitate a perfect adjustment of the real and simulated spectra. The origin of this deviation from the ideal is essentially attributed to irregularities on the surface of the electrode (roughness, presence of impurities, variation in thickness or surface composition of the electrode). 

More complex mixed oxides such as Lai-jcCCiCoOs usually display important deviations in surface composition with respect to bulk composition. In the family of Lai-jcCCjcCoOs (x = 0.0 to 0.5) oxides calcined at 1173 K, it was found that... 

It was simultaneously assumed [178] that, as a result of the alloy dissolution, its surface composition, (y, 1 - y), can deviate from the bulk one. Consequently, the proportionality of the atomic fluxes of A and B going into the solution to their alloy content is expressed by... 

Melter-Created Glass Defects. Despite best efforts to generate a perfectly uniform glass melt, within any melter there are naturally occurring processes that oppose those efforts. These include refractory corrosion (dissolution), electrode corrosion, and preferential volatilization of some species from the melt surface. These produce localized and sometimes more global deviations from the desired glass composition. Localized composition deviations lead to inhomogeneities in the product, called cord and striae. 

It is not necessary to put the importance of the surface preparation and its sensitivity under the spotlight, especially for composite surfaces. For economical reasons, manual sanding is still often the way for (1) removing dust and surface decontamination (aliphatic hydrocarbon, release agent residue contaminants) and (2) chemical activation (electron peeling, microroughness generation) and everybody knows how sensitive the epoxy adhesives are to deviations in the surface composition. These operations will be entrusted to skilled workers. 

The parameter a corresponds to the original Flory-Huggins interaction parameter and V accounts for the unlike surfaces of different segments with identical volumes. These dissimilarities, which are due to the particular shapes of the different types of segments (51), need to be taken into consideration because they determine how many neighbors are affected by the insertion of a fiorther molecule to a mixture of given composition. Deviations in the entropy of mixing (at constant volume of the system) caused by this particularity are also contained in V. 

---