Stoichiometry profiles

If we switch from a situation with uniform chemical potential (A/i0 = 0) to a situation in which on one side a different but constant PQ is established, a transient occurs during which the homogeneous stoichiometry profile changes to an approximately linear profile (see chemical polarization, see Appendix 3). As long as the electrode reactions are fast, the emf measured at such a sample is always determined by the invariant boundary values of the oxygen potential (ju0,ju0 + Aju0) but, owing to the internal virtually neutral short-circuit, lower than the Nernst-value. The result is, instead of Eq. (20),56 57 now... 

Stoichiometry Profiles in SrTiC>3 after High DC Field Stress... 

Fig.l2. (a) Plane-averaged stoichiometry profile of the (100) surfaces of CusAu for TjTf. =0.8 and 0.9. (b) Short-range order for each plane parallel to the surfaces for TjTf. =0.8, 0.9, 1.1, and 1.2. Layer 0 corresponds to the Cu-terminated surface, and layer 27 to the CuAu-terminated surface (a simulation cell, 5x5x 14 in units of the lattice constant of CusAu, with two free surfaces (100) was used). According to Ref.36. 

If the partial pressure on both sides is not maintained constant, the differences in P02 level out (we switch off the gas flows in Fig. 7.2). We designate this as chemical depolarization. Its transient behaviom permits calculation of chemical diflhision coefficients or effective rate constemts of the surface reaction. Similarly and k can be obtained from the transient of the chemical polarization (i.e. one-sided steplike change in the partial pressure of o g gen starting from the homogeneous initial situation). Figure 7.11 shows the stoichiometry profiles for a diffusion-controlled chemical polarization. These profiles are obtainable via Yi dc/dx. and c(x,t) by solution of the second Fick s law with the initial condition c(x,0) = Ci and the boundary conditions c(0,t) = C2 and c(L,t) = ci = c(x,0) (see e.g. [431]). 

Fig. 7.11 Stoichiometry profiles during dififusion-controlled chemical polarization of a homogeneous initial state (see text). The stationary profile is Unear dc/dt = 0 = 9 c/0x ) corresponding to Pick s law.

Compared with XPS and AES sputter depth profiling After achieving sputter equilibrium, and until a layer with different sputtering behavior is reached [3.59], the SN flux represents stoichiometry and not altered layer concentrations evolving because of preferential sputtering effects. 

The ECALE deposition of ternary II-VI compound semiconductors such as CdxZni xS, CdxZni xSe, and CdSjcSei c, on Ag(lll), has been reported [51-53]. The compounds were prepared by sequential deposition of the corresponding binaries in submonolayer amounts for instance, alternate deposition of CdS and ZnS was carried out to form Cd cZni cS. The stoichiometry of the ternaries was seen to depend on the deposition sequence in a well-defined and reproducible way, with the limit that only certain discrete x values were attainable, depending on the adopted sequence profile. Photoelectrochemical measurements were consistent with a linear variation of the band gap vs. the composition parameter x of the mixed compounds. 

The quantity and quality of experimental information determined by the new techniques call for the use of comprehensive data treatment and evaluation methods. In earlier literature, quite often kinetic studies were simplified by using pseudo-first-order conditions, the steady-state approach or initial rate methods. In some cases, these simplifications were fully justified but sometimes the approximations led to distorted results. Autoxidation reactions are particularly vulnerable to this problem because of strong kinetic coupling between the individual steps and feed-back reactions. It was demonstrated in many cases, that these reactions are very sensitive to the conditions applied and their kinetic profiles and stoichiometries may be significantly altered by changing the pH, the absolute concentrations and concentration ratios of the reactants, and also by the presence of trace amounts of impurities which may act either as catalysts and/or inhibitors. 

Also depicted on the graph in Figure 8.5 is the number of moles of magnesium sulphate produced. It should be apparent that the two concentration profiles (for reactant and product) are symmetrical, with one being the mirror image of the other. This symmetry is a by-product of the reaction stoichiometry, with 1 mol of sulphuric acid forming 1 mol of magnesium sulphate product. 

Secondary ion mass spectrometry (SIMS) was used to characterise the coatings for their Ti, Ru and O stoichiometry on the surface and as a function of depth into the coating. A PHI 6650 Quadrupole mass spectrometer, with Cs+ as the ion source was used in these studies. The conversion of the measured secondary ion counts to concentration was performed using relative sensitivity factors, which were first determined with a standard sample containing known amounts of RuC>2 and TiC>2. All of the SIMS profiles were repeated several times, to determine the measurement precision, which was typically +10%. 

Figure 22. Local current density profiles along the channel direction for different humidification levels at Ueii = 0.65 V. Anode and cathode stoichiometries are 1.4 at 1.0...

It has been shown recently [25] that concentrations of NOj, tend to reduce with increase in the amplitude of discrete-frequency oscillations. The mechanisms remain uncertain, but may be associated with the imposition of a near-sine wave on a skewed Gaussian distribution with consequent reduction in the residence time at the adiabatic flame temperature. Profiles of NO, concentrations in the exit plane of the burner are shown in Fig. 19.6 as a function of the amplitude of oscillations with active control used to regulate the amplitude of pressure oscillations. At an overall equivalence ratio of 0.7, the reduction in the antinodal RMS pressure fluctuation by 12 dB, from around 4 kPa to 1 kPa by the oscillation of fuel in the pilot stream, led to an increase of around 5% in the spatial mean value of NO, compared with a difference of the order of 20% with control by the oscillation of the pressure field in the experiments of [25]. The smaller net increase in NO, emissions in the present flow may be attributed to an increase in NOj due to the reduction in pressure fluctuations that is partly offset by a decrease in NOj, due to the oscillation of fuel on either side of stoichiometry at the centre of the duct. 

For systems in which is not zero, the shape of the etched profile can be controlled to a certain extent by adjusting the stoichiometry of the discharge. This is illustrated in Fig. 3-7 using an idealized Si sample. As hydrogen is added to a CF. discharge the etch rate of Si will decrease as we have seen earlier in Fig. 3.2. However, the etch rate will stop on surfaces not subjected to ion bombardment (point A in Fig. 3.7) before etching stops on surfaces which are exposed to energetic ion bombardment. This means that the lateral etch rate has been eliminated and features with vertical sidewalls can be etched if an etch gas mixture of CF. — 10% is used in this example. 

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