Demixing In Oxides Research Articles

Prediction of high temperature demixing in oxides in an electric field or an oxygen potential gradient

During exposure to an oxygen potential gradient or an electric field at high temperatures, semi-conducting and initially homogeneous mixed ceramic oxides of the rock-salt type show demixing of the cations. The main assumption made in previous theoretical treatments of demixing is that the off-diagonal phenomenological transport coefficients can be ignored. In this paper, off-diagonal coefficients are calculated by making use of the exact sum-rule expression relating the phenomenological transport coefficients. The demixing analysis is then greatly simplified and made exact. The analysis is verified with Monte Carlo computer simulation. Results are presented for steady state composition profiles in oxygen potential gradients and electric fields. Unusual steady state demixing composition profiles are possible in the quaternary oxide (A,B,C)O. Demixing profiles in (Co,Mg)O (oxygen potential gradient) and (Co,Ni)O (electric field) are analysed.

Electrotransport and demixing in oxides

Electrotransport in oxides is an interesting phenomenon in two respects: (i) it gives insight into the microscopic mass and charge transport mechanisms in oxides, and (ii) due to the directed ionic fluxes, macroscopic phenomena such as demixing of an initially homogeneous multicomponent oxide can be studied. The electric field induces defect fluxes as a result of which ionic fluxes occur. Due to the different mobilities a concentration gradient will build up, and the initially homogeneous material becomes inhomogeneous. A formal analysis of the demixing phenomenon due to electrotransport is given for semiconducting oxide solutions and for doped oxygen ion conductors as well. Some experimental results and consequences for applications are discussed.

Demixing of (Ni, Co)O Under an Oxygen Potential Gradient Using a YSZ‐Based Galvanic Cell

Diffusion in a multicomponent system with different diffusivities of the constituent components leads to demixing, which can be a source of material deterioration. A new experimental technique has been investigated to study demixing in oxides under an oxygen potential gradient. The oxygen potential gradient across initially homogeneous polycrystalline (Ni, Co)O was established by applying an electric potential across a yttria‐stabilized‐zirconia (YSZ) tablet in the galvanic cell: Air/Pt/YSZ/Pt/(Ni, Co)O/Air. A low oxygen partial pressure was established at the left side of (Ni, Co)O, while the right side was exposed to flowing air (0.21 atm ). Steady‐state demixing profiles were observed after annealing for 10 days. Discrepancies were sometimes noticed between runs because of possible leakage problems. Growth of the sample toward the high oxygen pressure side was observed, and was used to estimate the steady‐state velocity. An attempt is made to show how demixing experiments combined with atomistic calculations can be used as a possible means to estimate the microscopic properties of materials.

Demixing, decomposition and degradation of oxides in chemical potential gradients

The thermodynamic and kinetic behaviour of binary and higher oxide solid compounds and solid solutions as open systems in oxygen potential gradients is investigated. Demixing of oxide solid solutions and decomposition of compounds occurs under (quasi-) steady-state conditions. Other kinetic problems such as the morphological stability of interfaces, point-defect relaxation phenomena and the build-up of oxygen potential gradients in reacting oxide systems are of importance in the given context.

Time Evolution of Demixing in Oxides under an Oxygen Potential Gradient

Time evolution of concentration profiles leading to steady‐state demixing in oxides under oxygen partial pressure gradient is treated based on a microscopic model by applying the path probability method of irreversible statistical mechanics. The redistribution of vacancies and cations toward the steady‐state distribution has essentially been reduced to the diffusion of vacancies and the interdiffusion of cations along the specimen. Atomistic features of the problem, such as a vast difference in the relaxation times for the redistribution of vacancies and cations, are emphasized. Time required for the system to reach the steady state is then estimated in terms of fundamental material properties.

Demixing of Oxides under a Temperature Gradient

The microscopic mechanism of a demixing of binary oxide solid solution in an oxidizing atmosphere under a temperature gradient is investigated by the path probability method of irreversibly statistical mechanics. The problem is reduced to a steady‐state metal ion transport under the coexistence of an oxygen partial pressure gradient and a temperature gradient. Calculations are performed under two different conditions: (i) demixing in a steady‐state diffusion process under the two driving forces and (ii) a redistribution of constituent metal ions in a closed system under a given temperature gradient (Soret effect). The results can be interpreted as the superposition of the contributions of two driving forces plus a cross term which tends to zero as the driving forces become small. The difference between these two cases is discussed in detail and some controversies in the past work with respect to the difference are pointed out.