OXIDE PROPERTIES AND OXIDATION

11.6 OXIDE PROPERTIES AND OXIDATION

Metal oxides are commonly in the class of electrical conductors called semicon- ductors; that is, their conductivity lies between insulators and metallic conductors. Conductivity increases with a slight shift from stoichiometric proportions of metal and oxygen and with an increase of temperature. There are two types of semiconducting oxides, namely, p - and n - types ( p = positive carrier, n = negative carrier). In the p - type, shift of stoichiometric proportions takes the form of a certain number of missing metal ions in the oxide lattice called cation vacancies, represented by 䊐 . At the same time, to maintain electrical neutrality, an equiva- lent number of positive holes, ⊕ , form — that is, sites where electrons are missing.

A cupric ion, Cu 2+ , in a Cu 2 O lattice is an example of a positive hole. Oxides of p - type are Cu 2 O, NiO, FeO, CoO, Bi 2 O 3 , and Cr 2 O 3 . A model for the Cu 2 O lattice is shown in Fig. 11.5 . During the oxidation of copper, cation vacancies and positive holes are formed at the outer O 2 – oxide surface. These migrate to the metal surface, a process that is equivalent to the reverse migration of Cu + and electrons.

For n - type oxides, excess metal ions exist in interstitial positions of the oxide lattice, and it is these, together with electrons, that migrate during oxidation

OXIDE PROPERTIES AND OXIDATION

Figure 11.5. Lattice defects.

(Fig. 11.5 ) to the outer oxide surface. Examples of n - type oxides are ZnO, CdO, TiO 2 , and Al 2 O 3 . Wagner showed that the law of mass action can be applied to the concentration of interstitial ions and electrons, and also to cation vacancies and positive holes. Hence, for Cu 2 O, the equilibrium relations are

1 O 2 + 2 ← − ⎯ ⎯ → Cu O 2 Cu □ +⊕ 2 (11.2)

and

C 2 Cu □ − ⋅ C 2 = const p ⋅ 12 ⊕ / O 2 (11.3) For ZnO, the corresponding equilibrium is

ZnO ← ⎯ ⎯ → Zn 2 +

1 int + 2 e − + O 2 (11.4)

and const

These relations lead to predictions regarding the effect of impurities in the oxide lattice on oxidation rate; for example, if a few singly charged Li + ions are substi- tuted for doubly charged Ni 2+ ions in the NiO lattice, the concentration of positive holes must increase in order to maintain electrical neutrality. Hence, to maintain equilibrium for the reaction These relations lead to predictions regarding the effect of impurities in the oxide lattice on oxidation rate; for example, if a few singly charged Li + ions are substi- tuted for doubly charged Ni 2+ ions in the NiO lattice, the concentration of positive holes must increase in order to maintain electrical neutrality. Hence, to maintain equilibrium for the reaction

C C ⊕ Ni 2 □ 2 − ⋅ = const p ⋅ 12 O / 2

the concentration of cation vacancies must decrease. This is accompanied by a decrease in oxidation rate [22] because the rate is controlled by migration of cation vacancies. On the other hand, if small amounts of Cr 3+ are added to NiO,

the concentration of positive holes decreases, the concentration of cation vacan- cies correspondingly increases, and the oxidation rate increases. Data of Wagner and Zimens [23] showing the effect of alloyed chromium on oxidation of nickel are given in Table 11.2 .

At 10% Cr, the rate decreases again, possibly because a scale composed of Cr 2 O 3 forms instead of NiO, which alters the rate of ion migration apart from factors described previously. In a sulfur atmosphere, for reasons paralleling those pertaining in the situa- tion with O 2 , up to 2% Cr alloyed with nickel accelerates reaction at 600 – 900 ° C [24] . Outward diffusion of Ni 2+ occurs through cation vacancies in Ni 1 − x S, where x connotes a number between 0 and 1, and incorporation of Cr 3+ increases cation

vacancy concentration. In Cr – Ni alloys containing > 40% Cr, outward diffusion of Cr 3+ occurs in a scale composed of Cr 2 S 3 . Incorporation of Ni 2+ ions into the Cr 2 S 3 scale decreases cation vacancy concentration, thereby decreasing the reac- tion rate to a value below that for pure chromium. At intermediate chromium compositions, the scale is heterogeneous, consisting of both nickel and chromium sulfi des, with the rate of sulfi dation being less than that of pure chromium for Cr – Ni alloys containing > 20% Cr.

From Eq. (11.3) , it is apparent that higher partial pressures of oxygen for p - type semiconductors must be accompanied by higher concentrations of vacancies and holes at the O 2 – oxide interface. Hence, copper oxidizes at higher rates the higher the oxygen pressure, in accord with prediction [25] .

T A B L E 11.2. Oxidation of Nickel Alloyed with Chromium,

1000 ° C , 1 atm O 2

Weight Percent Cr Parabolic Rate Constant, k (g 2 cm −4 s −1 )

GALVANIC EFFEC TS AND ELEC TROLYSIS OF OXIDES

T A B L E 11.3. Oxidation of Zinc, 390 ° C , 1 atm O 2

Parabolic Rate Constant, k (g 2 cm −4 h −1 )

Zn pure 0.8 × 10 −9 Zn + 0.1% Al

× 10 1 − 11 Zn + 0.4% Li

If small amounts of Li + are added to ZnO, which is an n - type semiconductor, the electron concentration decreases in order to preserve neutrality, and the concentration of interstitial zinc ions increases in accord with the law of mass action [Eq. (11.5) ]. This increase in concentration facilitates the diffusion of

ZN 2+ int ; hence, Li + increases the oxidation rate of zinc, contrary to its effect in NiO. By similar reasoning, Al 3+ decreases the rate. Data showing these effects are presented in Table 11.3 [26] . The oxidation rate of zinc is almost independent of oxygen pressure because the concentration of interstitial zinc ions is already low

at the O 2 – oxide interface, and any further decrease brought about by increasing oxygen pressure has little effect on their concentration gradient referred to the metal surface where ZN 2+ int concentration is highest.

Therefore, traces of impurities, which play a major role in semiconductor properties, also appreciably affect rates of oxidation of metals protected by semi- conductor fi lms. On the other hand, alloying elements present in large percent- ages (e.g., > 10% Cr – Ni) affect the oxidation rate by a gross alteration of the actual composition and structure of the fi lm in addition to any effects on semi- conducting properties.