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Electrochemical Impedance Spectroscopy of Oxide Layers on the Titanium Surface

“…Figure 4 shows dependence of the preexponential factor Y 0 of the CPE for the thermal oxide. Element Q 1 is practically potential-independent in a broad range of polarizing biases, which corroborates the conclusion that the R 1 Q 1 circuit is responsible for the porous portion of the thermal oxide, exactly as we assumed previously in [13]. At the same time, the clearly pronounced parabolic dependence Y 0 (ϕ), which is responsible for the linearization of the dependence in the (E) coordinates in accordance with the Mott-Schottky equation (1) for constant-phase element Q 2 , which defines the SCR state, confirms cor-Y 0 2 -rectness of the selection of the equivalent circuit and the physical meaning of elements that make it up.…”

“…The model (equivalent circuit), which we have proposed for the electrode/electrolyte interface, made it possible to calculate the potential dependence of every element of the equivalent circuit, thus revealing the time constants that are responsible for the morphology (Q 1 ) and the capacitance (Q 2 ) of the spacecharge region. Time constant R 2 Q 2 provides for a maximum potential dispersion in the region of frequencies amounting to a few hertz, where, according to the data we already reported in [13], a poreless layer manifests itself, whereas the factors that are responsible for the porous layer (and the morphological factor) are the high-frequency inflection in the phase shift and the time constant R 1 Q 1 . It should be noted that the calculated values of power index n possess rather large values (n = 0.8-0.9) for both constant-phase elements, thus confirming a capacitive character of quantities Q 1 and Q 2 .…”

“…with the changes that are caused by the dwelling of the sample in the electrolyte, rather than with its polarization). According to the data we reported in [13], the chemical composition of the coating that formed in the hypophosphite-aluminate electrolyte is heterogeneous, and while its pores are smaller, its surface is more developed than that of other object under investigation (Fig. 6).…”

“…1-3) increases nearly tenfold with increasing positive polarization potential, which most likely is due to an increase in the width of the spacecharge region in a semiconductor of the n -type conduction, with which titanium dioxide is known to belong, the titanium dioxide being, according to the data reported in [7,8], one of the major components of oxide layers under study. Besides, in the systems under investigation, the method of polarization curves failed to reveal in an explicit form any contribution made by faradaic reactions [13], which allow one to interpret this particular experimental result in a different manner. Due to a lower concentration of charge carriers, the resistance of the space-charge region is larger than that of bulk layers of the semiconductor.…”

“…The model that was proposed for the electrode/electrolyte interface is additionally confirmed by comparative evaluations of values of resistance R 1 for all structures under study. At the steadystate potential, for example, at an initial period of the sample's exposure in the electrolyte, R 1 = 8 ohm cm 2 for the MAO coating formed in the phosphate electrolyte; R 1 = 31 ohm cm 2 [13] for the MAO coating formed in the hypophosphite-aluminate electrolyte; and R 1 = 2.7 × 10 3 ohm cm 2 for the thermal oxide, which possesses a homogeneous layer of low porosity.…”