Formation of Porous Oxides

Dibasic and tribasic acids, such as sulfuric, oxalic, malonic, and phosphoric acids,96 cause the appearance and development of a very regular porous structure of the oxide (cf. Fig. 3). Here, the kinetics of galvanostatic anodization are characterized by an initial linear potential rise, followed, however, at relatively low anodic 

It should be noted here that the barrier-film-promoting electrolytes are also characterized by VA(t) curves similar to those of the pore-forming ones, if comparatively small current densities are used (less than 0.5mA/cm2).20 

The maximum and steady-state anode potentials depend on the pH of the electrolyte in a manner shown in Fig. 13b, which was obtained for anodization in sulfuric acid solutions with the pH adjusted to constant ionic strengths with Na2S04. This dependence can be expressed as 

The parameters of the VA(t) function are very sensitive to temperature, as seen in Fig. 13a. As the temperature rises, the position of the maximum shifts to lower anodization times, with the value of the potential maximum also decreasing. 

In the case of a potentiostatic oxidation with different applied potentials, as shown in Fig. 14,98 there is always a dip in the current density corresponding to the potential maximum in galvanostatic 

---

The Role of Metal Dissolution or Volatiiisation in the Formation of Porous Oxides... 

As we have seen, a consequence of the formation of porous oxide is that the rate-controlling step reverts to that of a phase boundary reaction and... 

Figure 6.11 shows a schematic representation of migration processes accompanying the formation of porous oxide hlms by anodization of metals, illustrated here for... 

Courtright et al. (1991) have observed the formation of porous oxide scales on HfC and PrC at high temperatures. At temperatures of 1200 to 1530 °C, Holcomb and St. Pierre (1993) have applied a counter-current diffusion model for carbon oxidation through this porous oxide. This is based on the two-step oxidation of carbon in the HfC, as shown in Fig. 7-46 and the following reactions ... 

Jorne et al. [36] investigated the reactivity of graphites in acidic solutions that are typically used for Zn/Cl2 cells. The degradation of porous graphite is attributed to oxidation to C02. The rate of C02 evolution gradually decreased with oxidation time until a steady state was reached. The decline in the C02 evolution rate is attributed to the formation of surface oxides on the active sites. 

In the case of roasting, the pretreatment process destroys the sulfide matrix by driving off sulfur from the structure. This results in the formation of iron oxide particles that are made of concentrically zoned and porous hematite and maghemite (Paktunc et al. 2006). Arsenic is volatilized as As203 and oxidised to... 

Figure 2.15. The formation of an oxide film on an aluminum film by anodic oxidation. Arrows show the path PM or PN of oxygen through the compact oxide film AB (barrier layer). BC is the already formed porous oxide film, M and Hare located in the metal film (Hoar and Mott 1959).

Hoar, T. P. and N. F. Mott. 1959. Mechanism for the formation of porous anodic oxide films on aluminum. J. Phys. Chem. Solids 9 97-99. 

Enhanced oxidation resistance was also found at elevated temperatures for Co-Cr3C2,35 Ni-Cr 15 and Ni-Si3N415 composites. In contrast Ni-SiC15 and Ni-TiC23 composites have a higher hot oxidation rate than nickel. During hot oxidation porous metal oxide scales are formed at the metal-air interface. At elevated temperature interdiffusion between the particles and the metal in composites affects the formation of these scales. The break down of TiC particles in Ni-TiC composites accelerates corrosion by favoring the formation of nickel oxide.23 In... 

The complexity of the system implies that many phenomena are not directly explainable by the basic theories of semiconductor electrochemistry. The basic theories are developed for idealized situations, but the electrode behavior of a specific system is almost always deviated from the idealized situations in many different ways. Also, the complex details of each phenomenon are associated with all the processes at the silicon/electrolyte interface from a macro scale to the atomic scale such that the rich details are lost when simplifications are made in developing theories. Additionally, most theories are developed based on the data that are from a limited domain in the multidimensional space of numerous variables. As a result, in general such theories are valid only within this domain of the variable space but are inconsistent with the data outside this domain. In fact, the specific theories developed by different research groups on the various phenomena of silicon electrodes are often inconsistent with each other. In this respect, this book had the opportunity to have the space and scope to assemble the data and to review the discrete theories in a global perspective. In a number of cases, this exercise resulted in more complete physical schemes for the mechanisms of the electrode phenomena, such as current oscillation, growth of anodic oxide, anisotropic etching, and formation of porous silicon. 

---