ield was Ecrit = Vcrit/hcrit ~ 8.6 MV/m. This is ~36% lower than the dielectric strength of compact alumina, Ebreak = 13.4 MV/m . Whereas this could be partly due to the porous nature of our alumina, we do not think that the origin of this behavior is the dielectric breakdown of the oxide due to the high V reached. Instead, we assign the discontinuity in the curve to a temperature activated fast etching of the Al at the air–EL meniscus, where on fluctuations of the interface the bare Al can locally come into contact with the EL. In fact, when using a 10-fold diluted oxalic acid we could reach a Vcrit ~ 160 V before that any similar catastrophic event occurred, with a respective hcrit ~ 13 μm, which gives Ecrit ~ 12.3 MV/m, much closer to Ebreak. Furthermore, in the latter case, the final steep change of voltage was toward the zero (closed circuit with virtually no resistance), and there was no anode resection of the air–EL meniscus.
The limited improvement inh obtained in the considered time periodtendon increasingJ in bare oxalic acid resulted in a maximum (non-linear) mean growth ratevgmax = hcrit/tcrit ~ 0.83 μm/min (obtained forJ = 200 mA/cm2).
All the quantities in Fig. 2a have been plotted versus the anodization current density J. One can see that dout is approximately constant within the errors, as expected, since it should depend only on the type of EL and on its concentration. The weak increase actually observed can be due to a local rise of the EL temperature and thus of the oxide dissolution rate, probably occurring during anodization. On the contrary, Dout is clearly increasing with J. Indeed, Dout should increase with V [5, 14, 17], and our V(t) curves in Fig. 1a showed a V that increased during each anodization. As a consequence, the roughness is almost constant for the higher J values (i.e., the larger Dout), whereas it is significantly depressed for the lowest J (i.e., the smallest Dout). We attribute this effect to the spatial “low-pass” characteristic of the AFM probe tip, which can hardly penetrate the smaller pores and thus senses the respective APA as an almost continuous, smooth surface.
Anodizations in Oxalic Acid–IL Solutions
The general effect of addition of an IL in the EL, as compared to all the possible different anodization conditions for our setup, can also be seen in Fig. 3a. With respect to the dotted line curve, obtained for anodization run with external cooling and without stirring, the dashed line curve beneath it was obtained in the same EL also without stirring yet at RT. This curve presents a quite constantV level (after the initial transients). Obviously the higher EL temperature allowed for maintaining a higher ionic mobility as well, and no local ion depletion at the anode occurred, different from the cooled EL condition, (Figs. 1a,2a dotted line). A similar effect of approximately constantV was observed when the anodization was run at RT and stirring was also activated, as shown by the continuous line curve beneath the dashed one. In this case, theV level was even lower, as probably ion exchange and transport was further eased by the mechanical agitation, which was added to the thermal one. As a drawback obviously some instabilities were generated in the system, which resulted in strong fluctuations ofV. The situation of both cooled and stirred 0.03 M oxalic acid EL is not shown, as it was not experimentally accessible in our setup, but we would also expect a similar situation of approximately constantV(t), with intermediateV level lying between the high (RT, stirring) and the low (cooling, no stirring) ion mobility conditions. TheV(t) curves for anodization with cooled EL and no stirring (i.e., low ion mobility condition) but with IL contentc = 0.5% are also reported in Fig. 3a (thick black line for IL2 and thick gray line for IL1, respectively). One can see that theV level at regime was constant also in the latter cases, but significantly lower than for all the no-IL containing EL conditions. This was obviously due to the increased electrical conductivity of the EL after inject