Potentiostatic current transient measurements

Potentiostatic current transient measurements for testing individuai siurry additives... 

To design slurry compositions for metal CMP, usually it is necessary to determine if and how the individual additives would affect the faradaic activity of the selected metal surface. The potentiostatic current transient technique, usually combined with measurements of OCP transients, is useful for performing such tests. Figure 3.13 displays a set of experimental results to demonstrate this approach. Here the CMP metal is Cu,... 

For the individual types of transient measuring techniques, special names exist but their terminology lacks uniformity. The potentiostatic techniques where the time-dependent current variation is determined are often called chronoamperometric, and the galvanostatic techniques where the potential variation is determined are called chronopotentiometric. For the potentiodynamic method involving linear potential scans, the term voltammetry is used, but this term is often used for other transient methods as well. 

In Fig. 6D we show schematically the change of the measured potential E and the potential across the interphase E when a potential step is applied, corresponding to the current transient shown by curve 2 in Fig. 4D. It is clear that, whereas the potentiostat... 

Consider now the effect of uncompensated iR on the shape of the potentiostatic transients. This was shown in Fig. 6D. The point to remember is that although the potentiostat may put out an excellent step function - one with a rise time that is very short compared to the time of the transient measured - the actual potential applied to the interphase changes during the whole transient, as the current changes with time (cf. Section 10.2). This effect is not taken into account in the boundary conditions used to solve the diffusion equation, and the solution obtained is, therefore, not valid. The resulting error depends on the value of R, and it is very important to minimize this resis-tance, by proper cell design and by electronic iR compensation. 

First, Harrison et al. [3.36] studied Me UPD in the systems Ag(lll)/Pb, Cf, Ag(polycrystalline)/Tl, Cf, and Ag (polycrystalline)/Pb, acetate by cyclic voltammetry and potentiostatic pulse measurements. The authors claimed that a non-monotonous current transient represents a necessary criterion for 2D nucleation and growth involved in the 2D Meads overlayer formation. However, the experimental results presented did not give evidence for a first order phase transition. 

Bewick and Thomas [3.110-3.114, 3.270] measured electrochemically and by optical means different Me UPD systems Ag(A 0/Pb, H, ClOd", acetate and citrate, CnQikt)/ h H C104, acetate, and AgQikt)m SOd with Qikt) = (111), (100), and (110). Potentiostatic pulse measurements showed non-monotonous current transients for Ag(lll) substrates which are attributed to a first order phase transition. As an example, a current transient in the system Pig hkt)/Vf, H, SOd is shown in Fig. 3.46. In the case of Ag(lOO) and Ag(llO) substrates, higher order phase transitions were supposed. Clear evidence of a participation of 2D nucleation and growth steps in the 2D Meads phase formation process was found in the system Cu(lll)/Pb H", ClOd", acetate [3.270]. Non-monotonous current transients and a discontinuity in the q(lsE,fi) isotherm were observed (Fig. 3.13). 

Lorenz et al. [3.92-3.94, 3.98, 3.101] demonstrated that potentiostatically measured non-monotonous current transients represent a necessary, but not a sufficient criterion for 2D nucleation and growth. For example, such transients observed in the systems Ag(lll)/Pb, H, CIO4 and Ag(lll)/TT, If, S04 were quantitatively explained by Mefj g charge transfer and bulk diffusion as rate determining steps as illustrated in Fig. 3.48 [3.92, 3.93]. 

Kolb et al. [3.155, 3.224, 3.225] studied experimentally the system Au(lll)/Cu, H, so/. Along with observed discontinuous-like q E) isotherms (Fig. 3.50), non-monotonous current transients were potentiostatically measured (Fig. 3.51). These findings were explained in terms of first order phase transitions related to the peaks in the cyclic voltammograms. 

Many applications of this strategy are based on extensions of the concepts of impedance developed earlier in this chapter (41-43). However, the excitation waveform is usually an impulse in potential (rather than a periodic perturbation), and a transient current is measured. One records both E t) and i t) as observed functions. Then both are subjected to transformations, and comparisons are made in the frequency domain between E s) and i s). Ratios of the form i s)IE s) are transient impedances, which can be interpreted in terms of equivalent circuits in exactly the fashion we have come to understand. The advantages of this approach are (a) that the analysis of data is often simpler in the frequency domain, (b) that the multiplex advantage applies, and (c) the waveform E(f) does not have to be ideal or even precisely predictable. The last point is especially useful in high-frequency regions, where potentiostat response is far from perfect. Laplace domain analyses have been carried out for frequency components above 10 MHz. 

A good insight into the anodic oxide formation is gained from potentiostatic pulse measurements. Figure 19 shows current transients i t) of anodic oxide formation on aluminum at pH = 6.0. Various potential steps from 0 V (hess) were chosen to an oxide formation potential between 3.3 and 5.9 V [77]. This corresponds to an increase in field strength from 6.6 to 10.1 MVcm . The initial film thickness of 7.4 nm is given by a prepolarization to 3V (hess). Each experiment must be performed on a different sample with respect to the irreversible... 

Fig. 23 Potentiostatic current density transients i (t) for Ti/0.5 M H2SO4. icorr was determined by analytical measurements (AAS). = i — icon was obtained by subtraction. The rate of 02-evolution (CBR with involved ETR) depends on the formation of a band structure and becomes dominant after 100 s.

Fig.5 shows that our potentiostat system is capable of handling fast transient measurements. It should also be noted that although the current scale extends over several decades the autoranging system produces a smooth curve. 

As was mentioned in Sects. 2.8.1 and 2.8.2, application of the Laplace transform to the transient potential and current permits determination of the operational impedance. Such a method was initially introduced by Pilla [90-93] and applied to studies using mercury electrodes. Using a fast potentiostat a small potential step was applied, and both voltage and current transients were measured. Of course, because of the nonideal potentiostat response, the potential increase was not a rectangular step but occiured more slowly. Examples of the measured potential and current transients are shown in Fig. 3.6. Such data acquisition was extended to longer times and then extrapolated as the integration had to be continued to inlinity. 

Gupta, R.K., Sukiman, N.L., Cavanaugh, M.K. et al. (2012) Metastable pitting characteristics of aluminium alloys measured using current transients during potentiostatic polarisation. Electrochimica Acta, 66, 245-254. 

It has long been recognized, originally in the time domain (transient measurements) and later in the frequency domain (impedance measurements), that the nonsteady-state response of anodically dissolving metals displays in most cases an inductive-like behavior. With the galvanostatic step used in the early work, this behavior showed up as a voltage peak (overshoot), its counterpart in potentiostatic mode being a minimmn of current density. In the frequency domain an inductive loop of the Nyquist plot was first identified by Keddam et al. [43-46] in the case of iron in acidic media and interpreted by the so-called consecutive dissolution mechanism (see later). 

The complex transient r vs t, or equivalently r vs 0Na or r vs Uwr behaviour of Fig. 4.15 parallels the steady-state rvs UWr behaviour shown in Fig. 4.16, where for each point UWr has been imposed potentiostatically, until the current I has vanished and the corresponding rate value, r, has been measured. This shows that the catalyst surface readjusts fairly fast to the galvanostatically imposed transient 0Na values (Fig. 4.15). The dashed and dotted line transients on the same figure were obtained with the same gaseous composition but with initial Uwr values of 0 and -0.3 V respectively. It is noteworthy that the three transients are practically identical which shows the reversibility of the system. 

Potentiostatic Transient Technique, In the potentiostatic technique the potential of the test electrode is controlled, while the current, the dependent variable, is measured as a function of time. The potential difference between the test electrode and the reference electrode is controlled by a potentiostat (Fig. 6.21). The input function, a constant potential, and the response function, i = f(t), are shown in Figure 6.22. 

---