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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.

Objective: To create and validate a specific tool to evaluate the pressure ulcer risk in cancer patients. Method: The Pressure Ulcer Scale in Oncology (PUSO) was developed and subsequently validated against the Norton and Braden scales in order to efficiently and reliably evaluate the pressure ulcer risk in adult cancer patients, with various sites and stages of the illness (both curative and palliative phases). Two series of prevalence surveys (three surveys over two days, at two-month intervals) were conducted in one and then two French Cancer Centres in 2002 and 2009, respectively. The surveys investigated a total of 933 patients. Results: Multivariate logistic regression analysis identified three key items as being predictive of the development of pressure ulcers in cancer patients (mobility, incontinence and moisture/shearing), all with similar odds ratio weighting. A score was defined using this logistic model, the PUSO score (0-3) = bedridden/chair-ridden + incontinence + moisture/shearing. Conclusion: The extreme simplicity of this scale may appear disconcerting, but the PUSO is a simple, reliable and validated tool, which allows standardised evaluation of each patient's risk and, thereby, rapid introduction of appropriate measures. Conflicts of interest: None

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 high temperature microstructure of Ca-doped NiO single crystals was analysed to clear the experimental data on electrical conductivity, Seebeck's effect, diffusion and kinetic demixing. Combined transmission light microscopy and transmission electron microscopy examinations coupled with EDS analyses revealed the presence of CaO precipitates in localized (eutectic-like) zones for a wide range of analysed oxygen partial pressure. Their presence in the structure was confirmed for equilibrium (air) conditions in NiO-CaO system at about 1700 degrees C (approximately 42 mol% CaO), according to phase diagrams (Smith et al., 1969). For the wide range of oxygen partial pressure and high temperature (1000 / 1200 degrees C) the spinel phase was not observed, which is opposite to our previous results obtained for Ca-doped CoO single crystals (Kusinski et al., 2006). The chemical diffusion and electrical field kinetic demixing experiments have been combined to prove that the presence of Ca leads to an increase of the chemical diffusion coefficient of NiO. This unexpected result (D(Ca) > D(Ni)), confirmed during electrical conductivity experiment, indicates that correlation effects between Ni and Ca cations play a decisive role in diffusion processes in (Ni,Ca)O.

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.

This work concerns the ageing of solid oxide fuel cells in operative conditions due to matter transport processes, which occur in the electrolyte and oxide electrode material cationic sublattice. The surface stability and kinetic demixing processes in presence of chemical potential gradients through the materials are reviewed. Available experimental results concerning yttria-doped zirconia and iono-covalent oxides are reported. These results are discussed in relation with the microstructure and composition changes near the electrolyte-electrode material interfaces.

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.

International audience; A statistical physics approach is used to calculate the thermodynamic functions of uranium dioxide at high temperature when the presence of polarons is taken into account. This derivation explains why the electric conductivity, the thermal expansion and the thermal capacity at constant pressure follow modified Arrhenius type laws with the same apparent activation energy.

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 relationship between the microstructure and ionic conductivity of 9mol% yttria-doped zirconia is deduced from a comparative study performed on polycrystalline samples prepared either from commercial powder (sample ZC) or from freeze-dried powder (sample ZF). The grain boundary ionic conductivity of the ZF samples increases with the sintering temperature and this effect is due both to an increase in grain size and to a decrease in the number of glassy triple points. Furthermore, the grain boundary conductivity of the ZF sample is 30 times higher than that of the ZC sample sintered in the same conditions and with the same grain size. From the microstructural characterizations, it is concluded that this effect is due to the poor microstructure of the ZC sample and in particular to the presence of a glassy film on a large number of grain boundaries. On the contrary, the microstructure of the Zp samples is cleaner and more homogeneous with larger lens-shaped glassy pockets at triple points and no evidence for continuous boundary films. In spite of the differences in the ZF and ZC microstructure the activation energy for the grain boundary conductivity is the same; this is consistent with a partially-wetted grain boundary model in which conductivity occurs across unwetted grains in direct contact. A comparison with recent work on other Y2O3-compositions shows excellent agreement.

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.

Electrical conductivity, ionic transference number and chemical diffusion values of titanium-doped α-alumina single crystals were measured at high temperature as a function of orientation, oxygen partial pressure and temperature. This set of results is consistent with a model in which aluminum vacancies (three times ionized) are the prevailing defects. A small anisotropic effect was observed on the electrical conductivity values. This effect is more pronounced on the chemical diffusion values, obtained in air. A decrease of the chemical diffusion was observed between 1350 and 1450 °C for the sample cut parallel to the c axis and between 1400 and 1530 ° C for the sample cut perpendicular to the c axis. This unexpected variation has been attributed to short circuit effects caused by the formation of TiO2 precipitates in preferred planes. The solubility limit of titanium in alumina deduced from these results is close to 0.05% in wt at 1530 °C. Also, the electrical conductivity measurements performed under CO CO 2 mixtures have allowed us to show the effect of carbon on the transport properties of alumina. This effect has been only detected when titanium forms TiO2 precipitates in alumina. XPS analysis shows the presence of carbon bound to titanium and suggests, in agreement with phase diagram calculations, that carbon reacts with TiO2 to form oxy-carbide precipitates.

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%.

Spinel coatings on steel substrates have been obtained by a thermal spraying process. The projection has been done on cold or hot substrates. A further thermal treatment has been performed on some coatings formed on cold substrates. A good adhesion has been observed only on steels rich in oxidizable elements (Z38 CDV5). These coatings have been characterized and tested in a foundry. Adhesion tests have been done by the pulling method and by indentation. The concentration profiles of the most important elements from the metal/oxide interface to the bulk have been determined by EPMA analysis. This set of results has allowed us to show the importance of the coating microstructure and of the diffusion processes on the metal/ceramic bond strength and of the coating stability in contact to Al Si melted alloys. Such coatings are then good candidates to be used in a foundry to protect the casting tools, mainly when they are performed on heated surfaces.

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.

International audience; UO2 has a specific behaviour at high temperature related to a high concentration of charged defects, polarons, affecting its thermodynamic properties. We discuss how polarons can improve the radiation resistance of UO2.

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.

In this work we have analyzed the conditions leading to a kinetic demixing of species in ceramics exposed to an electrical field. 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

This paper considers the characterization of submicron α-Al2O3 powders, doped with MgO (550 and 1650 ppm by weight), and treated at 1300°C for different times and different cooling rates. X-Ray photoelectron spectroscopy (XPS) shows a strong surface segregation of Mg. The extent of segregation is higher for 1650-ppm-doped material than for 550-ppm-doped material. The outer surface segregation of Mg depends on the thermal treatment conditions. The powders have also been characterized by TEM, electron energy loss spectroscopy (EELS), SEM, surface area measurements and chemical analysis. TEM reveals a considerable agglomeration and some degree of sintering with the higher-doped powders. The size of the particles is between 50 and 500 nm and their thickness is around 100 nm, depending on the amount of magnesium and the thermal treatment.

Electrical conductivity, ionic transference number and chemical diffusion values of titanium-doped α-alumina single crystals were measured at high temperature as a function of orientation, oxygen partial pressure and temperature. This set of results is consistent with a model in which aluminum vacancies (three times ionized) are the prevailing defects. A small anisotropic effect was observed on the electrical conductivity values. This effect is more pronounced on the chemical diffusion values, obtained in air. A decrease of the chemical diffusion was observed between 1350 and 1450 °C for the sample cut parallel to the c axis and between 1400 and 1530 ° C for the sample cut perpendicular to the c axis. This unexpected variation has been attributed to short circuit effects caused by the formation of TiO2 precipitates in preferred planes. The solubility limit of titanium in alumina deduced from these results is close to 0.05% in wt at 1530 °C. Also, the electrical conductivity measurements performed under CO CO 2 mixtures have allowed us to show the effect of carbon on the transport properties of alumina. This effect has been only detected when titanium forms TiO2 precipitates in alumina. XPS analysis shows the presence of carbon bound to titanium and suggests, in agreement with phase diagram calculations, that carbon reacts with TiO2 to form oxy-carbide precipitates.

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.

This paper presents time-dependent cation redistribution simulations in multi-component semiconducting oxides under a temperature gradient. Such effects are an important source of material deteriorations at high temperature. This calculation has allowed the authors to show how kinetic demixing takes place and how the phenomena is related to the transport properties of the material. Both the magnitude and the time-dependent kinetic demixing have been determined for the (Co, Mg)O system for which cation diffusion coefficient values and defect concentrations are available.

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.

This paper concerns the dynamic segregation of impurities in iono-covalent semiconducting compounds subject to large temperature changes. Simultaneously to the temperature gradient a chemical potential gradient takes place in these compounds. From the expression of the flux of mobile species (vacancies and cations in the considered materials) it has been shown that segregation (or precipitation) effects must be observed near surfaces. These effects are directly related to the mobilities of the different cations. The predicted effects are consistent with segregation layer thicknesses found experimentally in ceramic materials after cooling. This analysis confirms the fact that in ceramic materials the dynamic segregation effects near surfaces during cooling must be taken into account in the interpretation of segregation results observed at room temperature. These changes in composition near surfaces can have important technological consequences, for example in ceramic powder preparation, and consequently in powder sintering or in the aging of ceramics subject to large temperature changes at high temperature.

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.

Ca-doped (1, 1.7, 5 and 10 mol% CaO) cobalt oxide single-crystal samples, with an [001] orientation, were annealed at elevated temperatures of 1000-1200 degrees C for different times and at different oxygen partial pressures. The microstructure was examined by means of transmission light and electron microscopy. High-temperature X-ray diffractometry was used, with the aim of determining the temperature of the CoO Co(3)O(4) transition in these materials. Extensive precipitation of Ca-free Co(3)O(4) spinel crystals was observed with increasing Ca content and oxygen activity. It is suggested that the electrical conductivity changes in this material may be related to this precipitation, because it changes the electronic state of cobalt cations.

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).

An experimental method, based on measurements of the expansion of a single crystal as a function of the p(O-2) was used to determine the concentration of uranium vacancies in hyperstoichiometric Uranium dioxide. The results are consistent with the concentration of electrical carriers measured in the same conditions, they are also in qualitative agreement with a theoretical modelling proposed in the literature. However, an improved interpretation would require to take into account the clusters of oxygen interstitials and vacancies.

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

We have optimized 3 key RF devices realized in standard logic 90 nm CMOS technology and report a record performance in terms of n-MOS maximum oscillation frequency f/sub max/ (280 GHz), varactor tuning range and varactor and inductor quality factor.

The transformation of UO2 into U3O8 is of technological and academical interest because of the severe consequences on the spent nuclear fuel management. The structural mechanism responsible for the isothermal transformation of UO2 into U3O8 seems still unclear. Several phases (UO2+x, U4O9, β-U3O7, α-U3O7, U3O8 were reported but their true structures, phase boundaries between their existence domains and matter transport processes are still a matter of debate. Gathering accurate information on the behaviour of uranium oxide is a key issue for understanding the behaviour of spent nuclear fuel. The chemical diffusion coefficient ( ~ D) of UO2+x was determined by electrical conductivity experiments. Measurements were performed in transient state for departure from stoichiometry in the range 0-112)-8 atm.)and for 973D is a decreasing function of the departure from stoichiometry x. This behaviour was attributed to the presence of singly charged (2:2:2) Willis defects as suggested by equilibrium conductivity measurements. The decrease of Dchim can be explained by transport processes occurring via a dynamic exchange between isolated mobile defects and complex defects frozen in clusters or domains. At higher P(O2), near U4O9, the time to reach an equilibrium electrical conductivity value becomes increasingly long. This suggests the presence either of large defect aggregates or of complex defects arranged into domains. Furthermore, the analysis of the transport processes in non equilibrium conditions has allowed us to show that the results of ~ D are consistent with those of the oxygen diffusion coefficient within the P(O2) and temperature range of stability of the [2:2:2] clusters.

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.