Nickel vacancies

In this experiment the non-stoichiometric nickel oxide is expressed as NiOj + y. Since y < 10 the concentration of nickel vacancies [Vn ] (this denotes the total concentration nickel vacancies having different electronic states, such as V i, V i, and VnD can be described as... 

In the case of NiO which is p-type metal-deficient material we have a structure with Ni2+, O2- and Ni3+ in nickel vacancy sites. Addition of lithium ion impurity to NiO makes it a p-type conductor and oxide growth rate is reduced compared with the Ni-02 system alone. On the other hand addition of Cr to Ni results in greater ease of oxidation than nickel alone due to higher cation diffusivity. 

The fraction of Schottky defects in nickel oxide, NiO, at 1000 °C is 1.25 x 10 1 The cubic unit cell contains four nickel atoms and has a cell edge of 0.417 nm. Calculate the number of nickel vacancies present. 

As discussed earlier, NiO is ap-type cation-deficit semiconductor and, therefore, the cations will migrate with electrons from the scale-metal interface to the scale-gas interface during oxidation. Correspondingly, there will be a flow of defects, cation vacancies and electron holes, in the opposite direction. Consequently, the driving force for the reaction will be reflected by the concentration gradient of cation vacancies across the scale. The nickel vacancies are formed according to Equation (4.2),... 

The net result is that nickel vacancies are consumed and electron holes are produced to maintain equilibrium in Equation (Cl3). Thus the cation conductivity is reduced and electronic conductivity is increased. A corresponding reduction in oxidation rate would be expected. 

The electrical properties of NiO, doped with monovalent ions (Li+) and trivalent ions (Ga +, Cr +) have been studied quite extensively. The study of Cr-doped NiO is of particular interest for understanding the oxidation behavior of Ni—Cr alloys. When trivalent Cr ions are introduced into a NiO crystal, the electrical compensation could occur by formation of doubly ionized nickel vacancies, according to... 

This electric field will oppose the migration of the nickel vacancies in the bulk, and we observe a slowing down of this process. We can thus suppose that a stationary condition self-regulated by the resulting electric field will be established. 

The diffusion of Ni in NiO, taking another example, depends on the oxygen pressure because this affects the nickel vacancy concentration, which in this case determines the diffusion coefficient ... 

The four nickel materials in Eqs. 13.1 and 13.2 are distinct phases and have been characterized bothelectrochemically and by the XRD patterns exhibited in the chemically precipitated phases. A point defect, nonstoichiometric, structural model correctly describes the structure and interactions of nickel electrode active materials (22). In this model, the intercalated water is incorporated as interlameUar protons, with the oxygen effectively adding to the Ni02 layers. The model introduces nickel vacancies that explain the empirical Ni/O ratios (less than 1/2). The Raman spectra gives no indication of water in the lattice as the excess protons and alkali cations... 

Nickel oxide Assume that doubly charged nickel vacancies and electron holes are the dominating defects in Nii.yO under oxidising conditions. At 1245°C and po2 = 1 atm we know the following for the compound ... 

The other group of oxides is that with a metal deficit or excess oxygen. NiO can be quoted as an example which shows a metal deficit due to nickel vacancies being present in the cation sublattice. Such nickel vacancies are introduced when oxygen is taken into the nickel oxide lattice at the outer part of the scale as a result of the oxidation process... 

For this type of oxide, the controlling step in increasing scale thickness is the ability of the nickel ions to migrate outward toward the surface. If a small quantity of a higher valence ion (Cr +, for example) is added to the nickel, as in Fig. 15.11, charge balance will require a greater number of nickel vacancies and hence an easier. 

We treat the ease of the selective formation of nickel oxide starting from the nickel-platinum alloys [LAL 73] (nickel and platinum are miscible in all proportions). We choose mole fractions of nickel vacancies in the alloy as variables of compositiom If we assume non-ionized point defects of nickel oxide (cationic vacancies and associated electron holes), the various oxidation steps are as follows ... 

This equation indicates that the concentration of the nickel vacancies is strongly dependent on the partial pressure of the nonmetal species in the corrosive environment. 

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