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In the present study we have analysed the thermal oxidation of Ni and dilute ( 1 − x O single crystals. In the temperature range 900°–1200 °C, Al 3 + or Cr 3 + additions decrease the oxidation rates of Ni 1 − x O single crystals in their stability range, due to the kinetic demixing of cations. At T 3 + and Cr 3 + in the outer layer and to the decrease of the mobility of Ni 2 + when the amount of Cr 3 + or Al 3 + decreases in Ni 1 − x O.

The electrical conductivity and thermoelectric power of CaO (2 and 2.6 mol%)-doped Ni 1 − x O single crystals have been measured in the temperature range 1000–1400 °C, as a function of oxygen partial pressure. These results show that Ca 2 + leads to the shift of the Ni/Ni 1 − x O phase boundary to higher P O2 , to a doping effect and to a mixed conductivity ( e ′ and h ), at P O2 - 4 atm. From electrical conductivity measurements in transient state, it was found that the chemical diffusion coefficient and the cationic vacancy diffusion coefficients are higher in the doped samples than in undoped Ni 1 − x O. Furthermore, kinetic demixing experiments in an applied electric field show that D Ca > D Ni , due to the decisive role of correlation effects in the Ca-doped samples. These results have allowed us to explain the beneficial influence of CaO coatings on the oxidation of Ni polycrystals, at T P O2, which increases with the amount of calcium, and blocking effects due to CaO precipitates near the position of the original metal surface, all the more important that the temperature is low and the oxidation time short.

International audience; Explaining and predicting the radiation resistance of structural and functional materials is a primary goal for engineering materials able to withstand severe radiation environments. Szenes has developed an empirical criterion based on the thermal behaviour of a compound at high temperature. Though the specific heat at high temperature of most materials obeys the classic Dulong–Petit law, this is not true for uranium dioxide, perhaps the most important ceramic compound in a nuclear power plant. An original analysis of the different contributions to the heat capacity of UO2 is presented showing that the large increase of UO2 heat capacity at high temperature (T > 1300 K) is microscopically connected to a high concentration of polarons that are responsible for the departure from the Dulong–Petit law. This is in particular related to the contribution of the uranium sublattice. At the microscopic scale, this thermodynamic anomaly can be related to the thermally activated charge disproportionation of U atoms that is experimentally observed by electrical conductivity measurements. This singular behaviour of the polaron concentration has a direct impact on the uranium sublattice partial molar heat capacity and an indirect effect on the energy interactions between the electronic and ionic structure of the target mediated by these polarons. This could explain, at least partially, the irradiation resistance to amorphisation of UO2.

International audience; In this paper, we describe two experimental set-ups which enable the measurement of electrical properties and intrinsic diffusion coefficients in UO 2. Electrical conductivity measurements are insured by a standard four point Kelvin-Bridge method. In parallel, the gas-solid isotopic exchange method is used to load the samples with 18O tracer atoms, the concentration profile of which are subsequently characterized using SIMS and chromatic confocal microscopy. An application of both types of measurements on a UO 2 single crystal is given. The diffusion study was carried out at 750degC, and the electrical conductivity study was performed between 1000degC and 1300degC at oxygen potentials at which the material exhibits extrinsic behaviour. We show how a careful use of both measurements in conjunction can be an indication of the operative migration mechanism.

The present study is concerned with the influence of sputter-coatings CaO on the oxidation behavior of Ni polycrystals. The experiments were performed in air, in the temperature range 800°-1200°C. Below 1200°C, CaO coatings reduce the oxidation rate, while this beneficial effect disappears at 1200 °C. The oxidized specimens were examined by SEM and X-Ray diffraction, but also by EPMA depth profiling to evaluate the scale composition. Furthermore, electrical conductivity measurements and kinetic demixing studies were carried out on Ca-doped NiO single crystals, to get a better insight regarding the transport processes involved during oxidation. These last results show that the key features allowing to explain the effect of CaOcoatings on the oxidation rate of Ni are the influence of calcium on the increase of the dissociation pressure of NiO, which delays the oxidation of nickel, the kinetic demixing of the cations, which controls the distribution of CaO precipitates in the scale responsible for blocking effects, and the increase of the diffusion coefficient of both the cations and the cationic vacancies, which play a decisive role at high temperature, when the scale growth is dominated by lattice diffusion.

This paper gives an overview of the materials transport processes which occur in oxide solid electrolytes subjected to a generalized thermodynamical potential gradient. The general equations allowing to describe the transport processes on the anionic and cationic sublattices are given. Kinetic demixing processes and microstructural changes leading to surface instabilities are analysed. Experimental results obtained with yttria-doped zirconia and gadolinium doped ceria illustrate the importance of these phenomena on the transport properties of the materials but also on their long term stability in operating conditions.

This paper concerns an analysis of the transport processes at high temperature in anionic conducting oxides subjected to a chemical potential gradient or an applied electrical field. The general equations are given. The principle of the cationic kinetic demixing under a “generalized“ thermodynamical potential gradient is reviewed. Experimental results obtained with yttria-doped zirconia are reported. An experimental procedure for the determination of the oxygen diffusion coefficient in ionic and semiconducting oxides is also described. The results obtained with yttriastabilized zirconia are compared to both self diffusion and conductivity data. This has allowed us to obtain information concerning the defect structure.

The temperature variation of UV–VIS–NIR optical spectra of UO 2 have been investigated from room temperature up to 1173 K with careful in situ oxygen partial pressure control. The deduced optical absorption edge exhibits a strong temperature dependence. Its value decreases from ∼2 eV at room temperature to ∼0.8 eV at 1173 K. Such thermal behaviour is interpreted as the consequence of the existence of a strong electron–phonon coupling (small polaron). In the temperature range 300–1173 K, the model yields a hopping radius of ∼2 A and a polaron self-energy of E p =−0.38 eV.

The chemical diffusion coefficient of UO 2+ x was determined from electrical conductivity measurements performed during transient state, for departure from stoichiometry in the range 0 x D is a decreasing function of the departure from stoichiometry, while the oxygen diffusion coefficient reported in literature is an increasing function in the same range of departure from stoichiometry. For x ′ Willis defects evidenced by electrical conductivity measurements. The enthalpy of formation of these clusters amounts to Δ H f =−1.7±0.6 eV. For x ⩾0.07, the decreasing function of D with x can be explained either by Willis defects α time ionized or more complex defect aggregates or via a dynamic exchange between mobile small defects and larger clusters or domains.

Transmission electron microscopy characterizations and XPS analyses have allowed us to show the influence of the microstructure and nanochemistry on the transport properties of Y2O3-(9 mol%)-stabilized zirconia (YSZ) and Gd2O3 (10 mol%)-doped ceria (GDC). The grain boundary electrical conductivity (σgb) and oxygen diffusion coefficient (Do) of conventional YSZ ceramics increase with the grain size, while an opposite behavior was found for GDC samples. This difference was attributed to glassy precipitates present at YSZ grain boundaries. Furthermore, it was shown that kinetic demixing processes take place during cooling, at the end of sintering. This causes important changes in the cationic species distribution at interfaces and plays an important role on the transport properties of these two materials.

The influence of alumina additions on grain boundary electrical conductivity of yttria-doped zirconia has been investigated by impedance spectroscopy. The results have been interpreted taking into account the microstructural and microchemical characterizations. Experiments have been conducted on polycrystals sintered from powders prepared through two different processing routes. Samples with the cleaner microstructure and no evidence for grain boundary glassy films show the highest conductivities, which decrease in the presence of alumina additions. On the contrary, alumina additions (≤2 mol%) lead to an increase of conductivity of samples showing grain boundary glassy films. These results have been attributed to a change in the glassy phase wettability due to the higher amount of Al found in the glassy phases in the presence of alumina additions. The same grain boundary activation energy found for the different samples suggests that only “clean” grain boundaries contribute to the transport processes.

Electrical conductivity measurements have been performed on yttrium-doped zirconia and on YSZ-alumina composites sintered from powders prepared by conventional techniques (commercial powder) or by freeze-drying. The results have been analyzed taking into account the microstructure of the interfaces which were characterized by electron microscopy. The samples sintered from freeze-dried powders show the highest conductivity values and this conductivity decreases in the presence of alumina. The microstructure of these polycrystals is clean, homogeneous with lens-shaped glassy pockets at triple-points and there is no evidence for continuous boundary films. Contrary, the samples prepared and sintered from commercial powders show a poor microstructure and the presence of a glassy film in a large number of grain boundaries. Furthermore, alumina leads to an increase in conductivity, which reaches a maximum at around 2 mol-% alumina. This result may be attributed to the influence of alumina on the viscosity and wetability of the glassy phase. However, all samples show the same grain boundary activation energy. This confirms that the transport mechanism is the same in all cases and that only clean grain boundaries contribute to the transport processes.

This paper concerns an analysis of the behavior of ionic conducting oxides brought into a chemical potential gradient, and the development of an experimental procedure for the determination of the diffusion coefficients of the most mobile species, in these materials as well as in mixed conducting compounds. The principle of the method is to place the sample between short-circuited reversible electrodes which are exposed to different chemical potentials. The ionic flux which develops then in the sample is determined from the measure of the electronic current in the external circuit. The relevant equations are given. Yttrium-doped zirconia has been used as model material. The results obtained in this work are complementary to previous data of self-diffusion and conductivity and have allowed us to obtain some information concerning the correlation factors and the structure of the defects, in this oxide.

The ageing behavior reported in this work concerns the consequences of the matter transport processes on the cationic sublattice which occur in solid electrolytes, mixed ionic conducting compounds and semiconducting oxides subjected to a chemical potential gradient, an applied electrical field or a mechanical stress gradient. The principle of the kinetic demixing under a “generalized” thermodynamic potential gradient is reviewed. Available experimental results concerning yttria-doped zirconia and iono-covalent oxides are reported. The results are discussed in relation with the microstructure and composition evolution of the surfaces and the electrode resistance.

This work concerns the characterization of α-Al 2 O 3 powders, doped with MgO (∼500 and 1650 ppm by weight), whose grain size is between 10 to 400nm. These powders have been treated in air, at 1300°C, for different times (between 2 and 80 minutes) and different heating and cooling rates. Their sintering has been followed by dilatometry. The results have been analysed in terms of the physical (TEM, SEM, BET), chemical and nano-chemical (XPS) characteristics of the powder.

The kinetics of the oxygen exchange reactions at the electrodes of a galvanic cell using yttria-doped zirconia single crystals (9.5 mole-% Y2O3) as solid electrolyte and Pt or Ag as electrode materials was studied by complex impedance spectroscopy. The electrode resistance when using silver was found to have negligible values over the temperature range 180 – 900 °C. In agreement with these results, oxygen sensors were tested successfully at temperatures as low as 200 °C. According to the performance of silver as electrode material, an electrochemical method was developed to determine the oxygen diffusion coefficient in doped zirconia. The results obtained, compared to those of conductivity and oxygen tracer diffusion measurements, have allowed us to obtain information both on the structure of the defects in yttria-doped zirconia and on the correlation factor.

We study the influence of the thermal treatment conditions on the solute cation redistributions at the near surface of alumina samples. Experiments have been done both on Mg-doped α-alumina powders and on Mg- and Ti-doped α-alumina single crystals. The results have been analyzed taking into account the influence of two competing effects: the cation redistribution kinetics and the driving force of matter transport related to the cooling rate and due to the electrochemical potential gradient which appears in the material during cooling. The influence of the alumina powder characteristics on the sintering kinetics has been followed using dilatometry. As expected, densification is favoured when the surface area increases. However, a lower shrinkage is observed for powders highly contaminated with silicon.

The electrical conductivity of monocrystalline (3.0, 9.5, 12.0 and 17.5 mol% yttria) and polycrystalline (9.9 mol% yttria) yttria-doped zirconia samples was studied using impedance spectroscopy in the temperature range of 250–1200 °C. Results show that the bulk ionic conductivity is independent of the P O 2 but varies with the amount of yttrium. The highest conductivities are obtained with monocrystalline zirconia samples doped with 9.5 mol% yttria. This set of results and recent oxygen diffusion data, suggest that different kinds of defects are present in the material, depending on the range of temperature and the amount of yttria. The behaviour of the electrical conductivity of the grain boundaries (with and without alumina) is close to that of the higher doped monocrystalline sample and suggests that associated point defects control grain boundary transport properties. A maximum of conductivity (“composite effect”) has been observed when the amount of alumina in the doped (9.9 mol%) polycrystalline zirconia is approximately 2 mol%.

Transmission electron microscopy (TEM) characterization has allowed us to show the influence of the microstructure on the grain boundary electrical conductivity (σ gb ) of Y-doped zirconia (YSZ) and Gd-doped ceria. For the Y 2 O 3 (9mol%)-doped zirconia samples, σ gb increases with the grain size and the results depend on the powder elaboration process and sintering conditions. These effects are due to wettability changes of the glassy phases at the grain boundaries. On the contrary, for the Gd 2 O 3 (10 mol%)-doped ceria samples, σ gb decreases when the grain size increases and the results are not influenced by the batch of powder likely due to grain boundaries free of detectable glassy phases.

In this work we have analyzed the conditions leading to a kinetic demixing of species in ceramics exposed to an electrical potential gradient. Mathematical expressions have been derived from the expression of the cationic fluxes. They have allowed us to relate the driving force of diffusion to the cation concentration profiles. Time dependent cation redistribution profiles have been estimated for (Co, Mg)O and (ZrO 2 , Y 2 O 3 ) solid solutions. The computed data are compared to the experimental results.

International audience; In this paper we show how electrical conductivity and intrinsic oxygen diffusion coefficient measurements can be used in conjunction to further our understanding of oxygen related point defects in UO2. Electrical conductivity measurements have enabled an estimate to be made of the positive charge carrier concentration in two sets of samples containing different levels of doping agents. The gas-solid isotopic exchange method was then used to load the samples with 150 tracer atoms the concentration profile of which were subsequently characterised using SIMS and chromatic confocal microscopy. At the oxygen potential and temperature studied (750 degrees C), application of point-defect theory to our experimental results points to oxygen migration proceeding via an interstitial mechanism and to the fact that impurities control the concentration of negatively charged point defects responsible for atomic migration.

A formal analysis is given of the kinetic demixing effects in multicomponent semiconducting oxides within their range of stability. We have shown that the relative diffusion coefficient values of the cationic species control the shift velocity of the oxidation or reduction front in the material. Calculated kinetic demixing profiles are given for (Co, Mg)O and (Fe, Cr)O solid solutions. From the theoretical and experimental results obtained it has been possible to analyse the behavior of chromium in oxide layers formed on transition metal alloys independently of all other transport processes occuring during scale thickening.

This paper presents a general analysis of the effect of impurities on oxidation kinetics of alloys. From kinetic studies performed in the stability range of oxide solid solution it is shown that it is possible to follow a step in the complex mechanism of oxidation. This has allowed us to analyse the mass transport process in the oxidation layer independently of other transport processes occuring during the scale thickening. Such analysis has allowed us to explain the effect of impurities both on the shift velocity of the oxidation front in the material and on the kinetic demixing of the cations. It is shown that these effects are directly related to the relative diffusion coefficient values of the different cations in the oxide layer.

When an initially homogeneous oxide solid solution is brought under an oxygen potential gradient, a redistribution of cations and cationic vacancies can occur in the material until a steady state has been reached. This paper concerns a computer simulation of the gradual segregation development leading to the steady state. The numerical model has been tested with the (Co,Mg)O system for which thermodynamic and kinetic results are available. This has allowed us to follow the time evolution of concentration profiles of cations in the material and to estimate the time required to reach the steady state for which the concentration profiles remain constant.

International audience; Nanocrystalline CeO2-doped (5, 7.5, 10, and 15 mol%) Gd2O3 powders, with a particle size of about 17 nm, were synthesized through the combustion of glycine/nitrate gels. Dense nanocrystalline materials were obtained by hot uniaxial sintering. Optical microscopy, scanning electron microscopy and transmission electron microscopy examinations, as well as X-ray diffraction analyses, have allowed us to characterize these polycrystals. The grain sizes, included between ∼10 and 80 nm, depend on both the sintering temperature and the amount of dopant. A comparison of the transport properties of these nanocrystalline samples to the values obtained with coarsened grained materials of same composition shows that the ionic conductivity passes through a maximum for mean grain sizes included between 300 and 500 nm. Furthermore, an enhancement of the ionic conductivity is observed when the amount of dopant increases. This was attributed to a grain-size-dependent gadolinium segregation at the periphery of the grains confirmed by X-ray photoelectron spectroscopy characterizations.

This paper presents a formal analysis of the effect of impurities on the kinetics of oxidation or reduction of p -type semiconducting oxides within their range of stability. Due to the simultaneous diffusion and drift of the cations when the material is exposed to a chemical potential gradient a segregation phenomenon appears if the diffusion coefficients of the cations are different. In the materials considered the shift velocity of the oxidation or reduction front is related to the cationic vacancy fluxes. These fluxes were expressed as a function of the diffusion coefficient of the cations and of their demixing tendency under the influence of an oxygen potential gradient. The predicted effects on the redox kinetics are discussed and experimental examples are given.

Transmission electron microscopy, XPS analysis, electrical conductivity and diffusion measurements were used to characterize the transport properties and grain boundary segregation phenomena in 9 mol% yttria-stabilized zirconia (YSZ). The highest grain boundary electrical conductivity (σgb) and oxygen diffusion coefficient (DO) values are shown by samples with a cleaner microstructure, sintered at 1600 °C and rapidly cooled at the end of sintering. XPS measurements show that an yttrium and silicon kinetic demixing process takes place during cooling. The amount of silicon rejected in the grain boundaries decreases when the cooling rate at the end of sintering increases, in agreement with the highest grain boundary conductivity values of the quenched samples. A formal treatment has allowed us to show that these results can be explained by the two competing effects occurring during cooling: the cation redistribution kinetics and the cooling rate.

This work was directed at a comprehensive study of the role of the nanostructure and nanochemistry on the transport properties of yttria-stabilized zirconia. Alumina additions lead to a decrease Of sigma(gb) when the samples have clean grain boundaries, while sigma(gb) goes through a maximum in samples having glassy grain boundaries. The differences were attributed to the strong interaction between Al2O3 and SiO2 impurities leading to a glassy phase depletion at the grain-boundaries, due to a change in wettability. Moreover, XPS analyses show that Si and Y segregate near these interfaces according to a kinetic demixing process, explaining why a faster cooling rate after sintering has a beneficial effect on sigma(gb).

X-ray and neutron powder diffraction studies of UO2 were performed under controlled oxygen partial pressure between room temperature and 1673 K. More than 40 neutron diffraction patterns were recorded. The thermal expansion coefficient of UO2 and the temperature dependence of Debye-Waller factors for oxygen and uranium atoms were determined. The dependence of Debye-Waller factors as a function of temperature is linear and the thermal expansion coefficient follows the classical Debye regime within the temperature range 300-1000 K. Above 1200 K, a departure from this quasi-harmonic behavior is clearly observed. Both an abnormal increase of the thermal expansion and of the oxygen sublattice disorder are evidenced. The departure of the lattice parameter from a linear thermal variation is found to be thermally activated with an effective activation energy close to 1 eV, very similar to the activation energy already found for the electrical conductivity. This new result suggests that polarons may affect the mean lattice parameter. A new thermodynamic model is then proposed to explain the heat capacity thermal variation by only three contributions: harmonic phonons, thermal expansion and polarons

This work is directed towards a comprehensive study on the role of the microstructure and local chemistry of grain boundaries on the ionic conductivity of yttria (9 mol%)-stabilized zirconia and YSZ-alumina composites. It has been performed on samples prepared from two batches of YSZ powders containing ≈1.0 or 1.6 wt% SiO2. Electrical conductivity measurements show that the grain boundary conductivity (σgb) increases with the sintering temperature and the cooling rate at the end of sintering or when the amount of Si in the ceramic decreases. Alumina additions lead to a decrease in σgb of the samples containing 1.0 wt% SiO2, while σgb passes through a maximum in the highly silicon contaminated materials. These results coupled with TEM X-ray microanalysis, which have shown important gradients of the concentration ratio Al/Si in the grains, near the second phase, and in the glassy precipitates, suggest a competitive effect between the insulating alumina particles and the strong interaction of Al2O3 for SiO2, removing it from grain boundary localities. On the other hand, XPS analyses show that Si and Y segregate near the interfaces. Analysis of these results suggests a kinetic demixing process and allow us to explain the beneficial effect of a faster cooling rate at the end of sintering by the lower amount of Si rejected in grain-boundary localities.

This paper presents a formal analysis of the transport processes in yttria-doped zirconia under a temperature gradient. Due to the simultaneous diffusion and drift of the species when the material is exposed to a thermodynamical potential gradient, a kinetic demixing process appears on the cationic sublattice if the diffusion coefficients of the cations are different. Experimental results obtained with yttria-doped zirconia are discussed on the basis of this analysis. They confirm that kinetic demixing processes during cooling must be taken into account in the interpretation of the grain boundary conductivity of doped zirconia.

In this study electrical conductivity measurements have allowed us to show the effect of carbon on the transport properties of alumina and cobaltous oxide samples. The effect of the CO/CO 2 mixture in equilibrium with the samples has been detected only when the material is doped and when the solute forms a second phase in the material. XPS analysis have shown the presence of carbon bound to titanium in titanium-doped alumina. This set of results suggests that carbon reacts with the precipitate phase.

The chemical diffusion coefficient of Cu 2 O has been obtained for an oxygen partial pressure near 5 10 −4 atm as a function of the temperature in the range 700–900°C D = 1 62 10 −4 exp(−5140 ± 600 cal mol −1 )/ RT cm 2 s −1 This was easily achieved according to the electrochemical method used for the preparation of gaseous mixtures whose Po 2 ; is lower than 10 −5 atm The slight difference observed with the previously published results by Maluenda, and obtained for Po 2 values which increase with T between 10 −4 and 0.21 atm, may be due to an oxygen partial pressure effect already observed in the case of CoO. An ambipolar treatment of the chemical diffusion, in the case of p -type semiconductor M a O b , oxides, has allowed us to express the chemical diffusion coefficient as a function of the concentration of the prevailing defects and of their diffusion coefficient In the case where the prevailing defects are cationic vacancies α times ionized we have shown that the expression D = (1 + α) D v α can be generalized to the A 2 O compounds This set of results has allowed us, according to the copper self diffusion data obtained recently by Peterson et al , to estimate the apparent enthalpy of formation of the catiomc vacancies Δ H f 23 ± 0 8 kcal mol −1 .

The thermodynamic equilibrium Sn(l) + O2(g) = SnO2(s) has been studied between 773 and 1380 K by e.m.f. measurements on cells involving a solid electrolyte, of the type: Pt∥Ni(s), NiO(s)∥ZrO 2 + Y 2 O 3 ∥Sn(1), SnO 2 (s)∥Pt

The electrical conductivity of cuprous oxide has been measured in the temperature range 600–1100°C and for oxygen partial pressures between 10 −7 atm and the oxidation limit of Cu 2− δ O. Oxygen partial pressures lower than 10 −5 atm hav e been obtained either by the partial vacuum technique, the opening of an adjustable leak being tied to the response of an electrochemical gauge placed near the sample, or under a total pressure of 1 atm, the oxygen partial pressure in argon being lowered by passing through an electrochemical pump. The electrical conductivity values, obtained for oxygen partial pressures lower than 10 −5 atm, are in agreement with those measured at higher oxygen partial pressures, using a classical method for obtaining P O 2 . The results obtained may be explained assuming the existence of interactions among defects leading to the formation of complex associates of the type (V' Cu V x Cu ). At lower P O 2 , the values of the slope 1/ n = ( ∂ ln σ / ∂ ln P O 2 ) T , lower than 1/8, may be explained by impurity effects or by the prevailing influence of interstitial copper defects.

The effect of aliovalent impurities on the chemical diffusion coefficient of an oxide AO having a p -type semiconducting behavior has been determined. This treatment shows that for an oxide doped by a monovalent impurity the chemical diffusion coefficient in the extrinsic range is equal to the diffusion coefficient of the prevailing defects. In the case of doping by higher-valent ions, it predicts an increase of the chemical diffusion coefficient. The above predictions are in agreement with the experimental results obtained from electrical conductivity measurements on single crystals ofcobaltous oxide doped either by chromium or aluminium or lithium. In the case of pure or chromium doped nickel oxide, this analysis is in qualitative agreement with the experimental results but lack of data does not allow any calculation.

Electrical conductivity measurements were performed on aluminium doped cobaltous oxide monocrystalline samples. From these results it was possible to show the effect of aluminium on the transport properties and on the defect structure of CoO. A statistical analysis allowed us to estimate the evolution of the defect population as a function of the oxygen partial pressure. The equilibrium constants of formation of singly and doubly ionized cobalt vacancies were calculated from the results obtained in the low oxygen partial pressure range. Good agreement with values calculated previously for pure and chromium doped samples was observed. Furthermore, it was possible to estimate from these experimental results the solubility limit of aluminium in CoO at 1000°C (xAl ≅ 2.52 × 10−3 mol. fraction). The chemical diffusion coefficients were evaluated by analysing the kinetics of re-equilibration of the samples after a sudden change in the oxygen partial pressure at constant temperature. The values obtained are consistent with a formal treatment of the effect of aliovalent impurities on the transport properties of CoO. Further, a reaction of carbon with a precipitate phase in CoO was indicated from measurements performed under CO/CO2. These observations are in agreement with those done on titanium and silicon doped alumina.