<p>The experiment work pertaining to this thesis can be divided into two parts: a) The study of the ion beam mixing process in the platinum-iron system and b) Electrocatalysis measuremnts on the mixed platinum-iron alloys. The ion beam mixing was studied using a 120 keV Fe+ ion over a rang of temperatures from 298K to 523K. A thin film of platinum was evaporated onto an oxide free substrate of iron to form a bi-layer sample. In order to check whether the interface was clean and oxide-free, Auger electron spectrometry was used along with sputtering. The mixing was studied primarily using RBS. The TEM was also used to characterize the samples before and after mixing.</p> <p>At low temperatures (<373 K), the mixing is very small and found to take place by collisional processes. At higher temperatures (>473 K) iron moves rapidly into the platinum. The activation energy for the platinum migration into the iron was found to be ~0.5 eV. This suggests that the vacancy mechanism is operating about 423 K. The films produced by mixing at low temperatures are highly stressed and there are a considerable amount of twins formed. It was also found that the grain size increases with dose and temperature.</p> <p>The surface concentration Pt in the mixed film is high ~90%. This results in an improvement of ~25% in the overvoltage for the ion beam mixed films compared to an iron electrode. Ion beam mixed films were found to be more stable than iron electrodes simply coated with films with an evaporated platinum layer. This appears to be the result of the improved adhesion between the platinum and iron as a result of the ion beam mixing process. For unmixed samples, an oxide layer is able to form on the iron surface at the platinum./iron interface, possibly because of cracks in the platinum layer, and this results in platinum pealing off the electrode leaving just the iron electrode.</p> / Doctor of Philosophy (PhD)

<p>The galvanic nature of crevice corrosion is a generally accepted concept but the coupled electrochemical behaviour and its role in crevice corrosion has not been really studied until recently. Among many arguments regarding the mechanism of crevice corrosion, whether or not being able to reasonably interpret the shape of crevice attack is an indication of whether or not the physical processes involved in crevice are profoundly understood. Based on a critical review of the state-of-the-art of the crevice corrosion studies, the necessity of studying the role of galvanic coupling effectiveness in crevice corrosion is then proposed. The concept of a uniform anode/cathodic pair with IR-drop is developed. It is shown that the galvanic coupling effect may play a significant role in crevice corrosion. For a uniform anode/cathode pair with IR-drop, the coupled potential/current relationship is a function of the kinetics on both the anode and cathode. It is found that when the anodic environmental aggressiveness exceeds certain value, the whole process is shifted from anode control to cathodic control and, therefore, the anodic dissolution rate is significantly enhanced. What complicates the process is the inevitably existing IR-drop which increases while the anodic-to-cathodic control shift occurs. This increased IR-drop is the consequence of the increase in anodic dissolution and on the other hand impedes the further increase in anodic dissolution. Therefore, the current/potential relationship in this case is being treated in a "covariant" way. In order to define the degree of the enhancement in anodic dissolution due to the coupling effect, a dimensionless parameter, the coupling coefficient η, is proposed, which is a function of anodic solution aggressiveness as well. A real corroding crevice is considered as an array of the uniform anode/cathode pair with different solution ohmic resistance. The concept of coupling effectiveness and the procedure of obtaining coupled dissolution rate as a function of solution aggressiveness and ohmic resistance of the solution phase are applied to a real corroding crevice, attempting to explain the shape of crevice attack and its evolution. Micro-electrodes techniques for measuring solution aggressiveness inside a real corroding crevice and a proper method for calculating the coupled dissolution rate at any location inside the crevice are developed. By (i) determining the local solution aggressiveness at different locations inside a corroding crevice, (ii) obtaining the kinetic information for the interior anodes in corresponding environments, and (iii) calculating the coupled dissolution rates at these locations, we are able to obtain the distribution and evolution of the coupled dissolution rate and the coupling effectiveness along the crevice. The shape of the crevice attack and its evolution with time is then attempted by integrating the coupled dissolution rate over the total time elapsed. The results show reasonably good agreement between the calculated shape of attack and its evolution and the experimental result. An alternative criterion for crevice corrosion of materials is proposed based on the response of the material to coupling effect.</p> / Doctor of Philosophy (PhD)

This thesis mainly treats the fabrication and applications of micro-optical elements in the semiconductor materials gallium arsenide (GaAs) and diamond. The recent trend in high-capacity data transfer using light as the information carrier creates new demands on the optoelectronic systems, such as small size, low cost and the integration of many components. Micro-optical components are key elements for building compact optoelectronic systems and are well suited for integration with other devices. Another area where micro-optical elements can play an important role is the use of lasers in medicine, industrial machining, metrology, etc. In most cases, the laser beam characteristic is not directly suited for the application and external optics is needed to focus, shape or split the laser beam. In the first part of this thesis, the fabrication of continuous-relief diffractive optical elements, such as diffractive lenses and blazed gratings, in GaAs is examined. The manufacturing technology uses electron-beam lithography followed by plasma etching in an inductively coupled plasma etching system. In the next step, these diffractive elements were monolithically integrated with vertical-cavity surface-emitting lasers. In the second part of this thesis a novel topic is examined, diamond micro-optics. Diamond is a unique material in many aspects, it is the hardest material mankind knows, it has an extremely wide optical transmission window, and it possesses the highest thermal conductivity of all solids. Until today, due to difficulties in machining diamond, the realization of diamond optics has been limited. By using the same technology we earlier developed for the fabrication of GaAs optics we demonstrate for the first time continuous-relief structures in diamond of optical quality. Several diamond micro-optical structures are presented; sub-wavelength gratings for reduction of unwanted Fresnel reflections, diffractive fan-out elements used to split a CO2-laser beam and refractive microlens arrays. The accuracy of the fabrication process by plasma etching was evaluated by optical and topographical measurements, in all cases the optical components were of very high quality.

No description available.

No description available.

<p>A comprehensive study of the softening behaviour of two different non-heat treatable AlMn-alloys has been carried out. These alloys were a laboratory processed and an industrially processed AA3103-alloy. The primary objective of the laboratory processed alloy has been directed towards the relationship between the amount of manganese in supersaturated solid solution and the material behaviour during deformation and annealing. The focus for the industrially processed alloy was a detailed material characterisation during hot and cold rolling as well as to follow the softening behaviour of the alloy.</p><p>The cold rolled microstructures were characterised with respect to subgrain size, misorientation across the subgrain boundaries, particle break-up and global texture. As far as it concerns the cold deformed microstructure it was found that the different homogenisation treatments, resulting in different amount of Mn in supersaturation, had little effect on the deformed microstructure.</p><p>Detailed experimental work on the softening behaviour after cold deformation and the interaction between recrystallisation and precipitation, i.e. concurrent precipitation, has been carried out. Hardness and yield stress measurements, which defines the strength of the material, revealed that the softening behaviour was significantly slowed down in case of concurrent precipitation. It was further found that the precipitation reaction in this case occurred on the subgrain boundaries. Thus the precipitates considerably retarded the recrystallisation reaction as compared to the case when no precipitation occurred.</p><p>TTT-diagrams have been constructed on the basis of hardness and conductivity measurements. From these diagrams a characteristic temperature, T_c , for the different material conditions are identified. It was found that as the annealing was carried above this temperature the microstructure consisted of a fine grained equiaxed microstructure. Below T_c the grains become elongated in the rolling direction and the average grain size became much coarser.</p><p>With respect to recrystallisation texture, a very strong P-texture and in addition some ND-rotated cube texture was found in case of concurrent precipitation. This viistrong P-texture was investigated in detail, and it can be concluded that particle stimulated nucleation of recrystallisation (PSN) plays a significant role in the nucleation of these components. It was found that these texture components had a growth advantage in the early stage of annealing and that they are most probably a result of microgrowth selection, which often are related to a nucleation effect. When precipitation did not interact with recovery and recrystallisation the recrystallisation texture was either random or consisted of a weak cube texture.</p><p>The softening behaviour of the different materials has been modelled by a physically based softening model, which involves both the recovery and recrystallisation reactions. Both for the AlMn-alloys and for an additional commercially pure Al-alloy, relatively good model predictions were obtained for the softening behaviour when recrystallisation occurred prior to precipitation. However, when the softening reactions were retarded by heavy precipitation of dispersoids the model, in its original form, was not capable to predict the softening behaviour. In this case an additional retarding drag, which reduces the total number of viable recrystallisation nuclei, was added. With the addition of this drag relatively good model predictions were also obtained in case of concurrent precipitation. However, the model was not capable to predict the softening behaviour of the industrially processed AlMn-alloy particularly well.</p>

<p>Cold chamber high pressure die casting, (HPDC), is an important commercial process for the production of complex near net shape aluminium and magnesium alloy castings. The work presented in the thesis was aimed at investigating the microstructure formation in this type of casting. The solidification characteristics related to the process and the alloys control the formation of grains and defects. This again has a significant impact on the mechanical properties of the castings.</p><p>The investigations were carried out mainly using the AM60 magnesium alloy and the A356 aluminium alloy. Two different casting arrangements were used: the cold chamber HPDC and the gravity die casting methods, which allowed for different flow and solidification conditions. The microstructures in the castings were investigated using optical microscopy, image analysis, scanning electron microscopy, electron back scatter diffraction measurements and electron probe microanalysis.</p><p>In the HPDC experiments, the shot sleeve solidification conditions were investigated primarily by changing the melt superheat on pouring. This significantly affected the microstructures in the castings. The fraction of externally solidified crystals (ESCs) was consistently found to be largest near the gate in both the AM60 and the A356 die castings. This was attributed to the inherent shot sleeve solidification conditions and the flow set up by the plunger movement. When the superheat was increased, a lower fraction of ESCs was found in the castings. Furthermore, a high superheat gave ESCs with branched dendritic/elongated trunk morphology whilst a low superheat generated coarser and more globular ESCs, both in the AM60 and the A356 castings. The ESCs typically segregated towards the central region of the cross sections at further distances from the gate in the die castings.</p><p>When a thin layer of thermal insulating coating was applied on the shot sleeve wall in the production of AM60 die castings, it nearly removed all ESCs in the castings. Using an A356 alloy, (and no shot sleeve coating), with no Ti in solution gave a significantly lower fraction of ESCs, whereas AlTi5B1 grain refiner additions induced an increase in the fraction of ESCs and a significantly finer grain size in the castings. The formation of globular ESCs was enhanced when AlTi5B1 grain refiner was added to the A356 alloy.</p><p>In controlled laboratory gravity die casting experiments, typical HPDC microstructures were created by pouring semi-solid metal into a steel die: The ESCs were found to segregate/migrate to the central region during flow, until a maximum packing, (fraction of ESCs of ~35-40%), was reached. The extent of segregation is determined by the fraction of ESCs, and the die temperature affects the position of the ESCs. The segregation of ESCs was explained to occur during flow as a result of lift forces.</p><p>The formation of banded defects has also been studied: the position of the bands was affected by the die temperature and the fraction of ESCs. Based on the nature of the bands and their occurrence, a new theory on the formation of defect bands was proposed: During flow the solid distribution from the die wall consists of three regions: 1) a solid fraction gradient at the wall; 2) a low solid fraction region which carries (3) a network of ESCs. A critical fraction solid exists where the deformation rate exceeds the interdendritic flow rate. When the induced stress exceeds the network strength, deformation can occur by slip, followed by liquid flow. The liquid flow is caused by solidification shrinkage, hydrostatic pressure on the interior ESC network, and gaps forming which draw in liquid.</p>

<p>The present work reports on the effect of Mn-, Zr- and Sc-additions upon hot deformation properties, recrystallization properties and mechanical properties for different temper conditions of Al-Mg alloys.</p><p>It can be stated that the addition of Mn, Zr and Sc improves the recrystallization properties and the mechanical properties of Al-Mg alloys. It should be emphasised that the precipitation of the metastable cubic Al₃Zr and the stable cubic Al₃(Sc,Zr) is favourable in an aluminium-magnesium matrix due to a close similarity of the lattice structures. The Al₃(Sc,Zr)-phase is similar to the equilibrium Al₃Sc-phase and has a high thermal stability and thus the coherency with the aluminium matrix is retained to very high temperatures. The present work has demonstrated the beneficial features of the Al₃(Sc,Zr)- phase upon recrystallization and strength. This also results in an increase in the deformation resistance and a reduction in the hot ductility. In particluar, manganese reduces hot ductility.</p><p>After casting most of the Zr and Sc remained in solid solution. The Mn was partly present in large primary constituent particles and partly in solid solution. Segregations of all three elements were detected. Decomposition of solid solutions of these elements resulted in the formation of dispersoids of the type Al₃Mn (orthorombic), Al₃Zr (cubic) and Al₃(Sc,Zr) (cubic).</p><p>It was found that the flow stress increased in the presence of the dispersoids. As compared to the alloy without dispersoids, the presence of Al₆Mn and Al₃Zr or Al₃(Sc,Zr) increased the flow stress by 20-100% depending on the temperature and strain rate. The effect of the particles decreases as the Zener- Hollomon parameter increases. Extrusion experiments also confirm these results. In addition, manganese reduces the hot ductility considerably.</p><p>Furthermore, the present work has demonstrated that the recrystallization properties of Al-Mg alloys may be affected considerably by introducing Mn, Zr and Sc. The recrystallization behaviour after hot deformation may be effectively determined by the Zener drag exhibited by the dispersoids on grain boundaries. Al₆Mn showed to be least effective while Al₃(Sc,Zr) is extremely effective in retarding recrystallization.</p><p>After cold deformation, however, the recrystallization behaviour is different due to a higher amount of stored energy. In the alloy without dispersoids, recrystallization occurred by classical nucleation at microstructural heterogeneities, while particle stimulated nucleation operates in the other alloys. Recrystallization of cold rolled material resulted in an extremely finegrained microstructure. Once recrystallized, extensive grain growth occurs in alloys containing Al₆Mn and/or Al₃Zr. Contrary, alloys containing Al₆Mn and Al₃(Sc,Zr) are very stable and the fine-grained structure seems to be very stable up to 550°C. This clearly proves that Al₃(Sc,Zr) are thermally stable and efficiently pin grain boundaries up to very high temperatures.</p><p>In the last part of this thesis the mechanical properties of the investigated alloys were mechanically tested in several temper conditions. It was found that the presence of Al₆Mn and Al₃(Sc,Zr) caused an increase in the flow stress of 36 MPa in the O-temper condition, probably due to the Orowan mechanism. The effect of Al₆Mn and Al₃Zr alone or in combination was less pronounced.</p><p>Furthermore, the retained deformation microstructure after extrusion was associated with the Zener drag forces exhibited by the dispersoids and resulted in considerable strengthening. For instance, the combination of Al₆Mn and Al₃(Sc,Zr) increased the strength by approximately 100 MPa compared to the dispersoid free alloy. Again the effect of Al₆Mn and Al₃Zr is less pronounced due to the lower capacity in retarding recrystallization.</p><p>The capability of the dispersoids to retard recrystallization should be an opportunity to increase the strength of the heat-affected zone after fusion welding. This is an important aspect since strain hardened conditions are used commercially. However, it has been demonstrated that a complete utilisation of the strength increase in the base material is not achieved as long as the weld metal is the weakest part in the weldment. However, a yield strength of 160 MPa was achieved for the material containing both Al₆Mn and Al₃(Sc,Zr), while somewhat lower values were obtained for the alloys with Al₆Mn and/or Al₃Zr.</p>

<p>This thesis work consists of two parts, each with a different motivation. Part II is the main part and was partly conducted in industry, at ScanWafer ASA’s plant no.2 in Glomfjord.</p><p>The large growth in the Photo Voltaic industry necessitates a dedicated feedstock for this industry, a socalled Solar Grade (SoG) feedstock, since the currently used feedstock rejects from the electronic industry can not cover the demand. Part I of this work was motivated by this urge for a SoG- feedstock. It was a cooperation with the Sintef Materials and Chemistry group, where the aim was to study the kinetics of the removal reactions for dissolved carbon and boron in a silicon melt by oxidative gas treatment. The main focus was on carbon, since boron may be removed by other means. A plasma arc was employed in combination with inductive heating. The project was, however, closed after only two experiments. The main observations from these two experiments were a significant boron removal, and the formation of a silica layer on the melt surface when the oxygen content in the gas was increased from 2 to 4 vol%. This silica layer inhibited further reactions.</p><p>Multi-crystalline (mc) silicon produced by directional solidification constitutes a large part of the solar cell market today. Other techniques are emerging/developing and to keep its position in the market it is important to stay competitive. Therefore increasing the knowledge on the material produced is necessary. Gaining knowledge also on phenomenas occurring during the crystallisation process can give a better process control.</p><p>Part II of this work was motivated by the industry reporting high inclusion contents in certain areas of the material. The aim of the work was to increase the knowledge of inclusion formation in this system. The experimental work was divided into three different parts;</p><p>1) Inclusion study</p><p>2) Extraction of melt samples during crystallisation, these were to be analysed for carbon- and nitrogen. Giving thus information of the contents in the liquid phase during soldification.</p><p>3) Fourier Transform Infrared Spectroscopy (FTIR)-measurements of the substitutional carbon contents in wafers taken from similar height positions as the melt samples. Giving thus information of the dissolved carbon content in the solid phase.</p><p>The inclusion study showed that the large inclusions found in this material are β-SiC and β-Si3N4. They appear in particularly high quantities in the top-cuts. The nitrides grow into larger networks, while the carbide particles tend to grow on the nitrides. The latter seem to act as nucleating centers for carbide precipitation. The main part of inclusions in the topcuts lie in the size range from 100- 1000 µm in diameter when measured by the Coulter laser diffraction method.</p><p>A method for sampling of the melt during crystallisation under reduced pressure was developed, giving thus the possibility of indicating the bulk concentration in the melt of carbon and nitrogen. The initial carbon concentration was measured to ~30 and 40 ppm mass when recycled material was employed in the charge and ~ 20 ppm mass when no recycled material was added. Since the melt temperature at this initial stage is ~1500 °C these carbon levels are below the solubility limit. The carbon profiles increase with increasing fraction solidified. For two profiles there is a tendency of decreasing contents at high fraction solidified.</p><p>For nitrogen the initial contents were 10, 12 and 44 ppm mass. The nitrogen contents tend to decrease with increasing fraction solidified. The surface temperature also decreases with increasing fraction solidified. Indicating that the melt is saturated with nitrogen already at the initial stage. The proposed mechanism of formation is by dissolution of coating particles, giving a saturated melt, where β-Si3N4 precipitates when cooling. Supporting this mechanism are the findings of smaller nitride particles at low fraction solidified, that the precipitated phase are β-particles, and the decreasing nitrogen contents with increasing fraction solidified.</p><p>The carbon profile for the solid phase goes through a maximum value appearing at a fraction solidified from 0.4 to 0.7. The profiles flatten out after the peak and attains a value of ~ 8 ppma. This drop in carbon content is associated with a precipitation of silicon carbide. It is suggested that the precipitation of silicon carbide occurs after a build-up of carbon in the solute boundary layer.</p><p>FTIR-measurements for substitutional carbon and interstitial oxygen were initiated at the institute as a part of the work. A round robin test was conducted, with the Energy Research Centre of the Netherlands (ECN) and the University of Milano-Bicocci (UniMiB) as the participants. The measurements were controlled against Secondary Ion Mass Spectrometer analyses. For oxygen the results showed a good correspondence between the FTIR-measurements and the SIMS. For carbon the SIMS-measurements were significantly lower than the FTIR-measurements. This is probably due to the low resistivity of the samples (~1 Ω cm), giving free carrier absorption and an overestimation of the carbon content.</p>

<p>Ceramic membranes made from mixed oxygen-ionic and electronic conducting perovskite oxides can selectively separate oxygen from air at elevated temperatures. These membranes have several potential applications that require a continuous supply of oxygen. For example, they may be an alternative for cryogenic production of oxygen or alternative electrode materials in solid oxide fuel cells. Of particular significance is the partial oxidation of methane to syngas (CO + H₂). By combining air separation and partial oxidation of natural gas into a single step, the need for expensive oxygen production by cryogenic means may be eliminated. Combined with existing processes for gas-to-liquid production such as Fisher-Tropsch and methanol synthesis, the MIEC membrane technology represents a very attractive route for conversion of natural gas to liquid fuels.</p><p>The research in this field was initially concerned with the search for materials with the optimum oxygen flux. Today, the long term stability of the membranes is probably the main issue. The membranes have to be stable under operating conditions, which include mechanical stability and chemically compatibility with other materials like sealing and support materials. However, the current understanding of the long term chemical and mechanical reliability is poor and this is one of the major challenges for solid state ionic research. The aim of this work has been to investigate the mechanical properties and the chemical stability of La_0.5Sr_0.5Fe_1-xCo_xO_3-δ (x = 0, 0.5, 1) materials when they are exposed to thermal and chemical gradients.</p><p>The chemically induced stresses due to reduction of the valence state of the transition metals are of particular importance with respect to the mechanical stability. In paper I, the oxygen non-stoichiometry, investigated by thermogravimetrical analysis, and thermal end chemical expansion, studied by dilatometry and high temperature X-ray diffraction, of La_0.5Sr_0.5Fe_1-xCo_xO3-δ materials are reported. The oxygen deficiency was observed to increase with decreasing partial pressure of oxygen and increasing temperature corresponding to expectations and previous reports. At ambient temperature the thermal expansion coefficient of the materials were in the range 15- 18·10^-6 K^-1. Above a certain temperature thermal reduction of the material take place, and the thermal expansion coefficient due to chemical expansion raise to 16-36·10^-6 K^-1. The chemical expansion ε_c, defined as the linear expansion due to a change in partial pressure of oxygen at constant temperature, reached a maximum in the range 0.036-0.039 for the materials studied at 800ºC. The change in ionic radii of the transition metals is the main contribution to the chemical expansion. The crystal structure of the perovskite materials were shown to be slightly rhombohedral at ambient temperatures and a transition to cubic phase were observed above 300ºC.</p><p>This non-linear thermal expansion behavior is a major challenge for the applications of the mixed conductor materials. La_0.5Sr_0.5Fe_1-xCo_xO_3-δ membranes in an oxygen partial pressure gradient will have different oxygen deficiency on either side of the membrane. The increasing oxygen deficiency is accompanied by a volume expansion as shown in paper I, and this will lead to chemically induced stresses. These stresses and the failure that might follow can be prevented by creep of the materials. Creep is also important due to dimensional stability. In paper II, the steady-state creep performance under compression of La_0.5Sr_0.5Fe_1-xCo_xO_3-δ (x = 0.5, 1) as a function of temperature, atmosphere, load and two different grain sizes is reported. The stress exponent found for the materials was close to unity and an unusual low inverse grain size exponent close to one was found for one of the materials. The activation energy of the two materials was not equal and the influence of secondary phases on the creep was discussed. The obtained creep behavior and microstructural investigation after measurements point to a diffusion related mechanism for the creep. Higher creep rates are found under reducing conditions and this suggest that creep relaxation of mechanical or chemical induced stresses may enhance the mechanical stability of oxygen permeable membranes.</p><p>In Paper III, the mechanical properties of La_0.5Sr_0.5Fe_1-xCo_xO_3-δ (x = 0.5, 0.75, 1) were investigated by several methods. Fracture strength was measured by four-point bending, fracture toughness was measured by SENB and SEVNB methods and finally Young’s modulus were investigated by four-point bending and resonant ultrasound spectroscopy. Four-point bending showed a non-linear ferroelastic behavior at ambient temperature due to rhombohedral crystal structure. Above the ferroelastic to paraelastic transition temperature the materials showed elastic behavior, however, at temperatures from about 800ºC a non-elastic respond was observed due to creep. The measured fracture strength and fracture toughness were observed to increase with increasing temperature, which was attributed to frozen-in stress gradients in the materials during cooling due to different oxygen stoichiometry. These stress gradients caused the low fracture strength and fracture toughness at ambient temperature. At higher temperatures, the stresses are assumed to relax resulting in a higher strength and fracture toughness. At high temperature, the non-linear respond made systematic errors in the calculated strength and fracture toughness. The Young’s modulus was measured from four-point bending and by resonant ultrasound spectroscopy for two of the materials. These data obtained by these two different methods were not in good agreement, which demonstrate the difficulty to obtain reliable data for the Young’s modulus of such materials by four-point bending. The presented findings have demonstrated the importance of understanding ferroelasticity and chemically induced stresses in order to comprehend the mechanical properties of such mixed valence state perovskite materials.</p><p>A high oxygen flux is required in order to realize the oxygen permeable membrane technology. At the same the chemical stability of the materials in a pO₂ gradient must be good for a sufficient long period of time. The oxygen flux performance and the long term stability of La_0.5Sr_0.5Fe_1-xCo_xO_3-δ (x = 0, 0.5, 1) are the topics of Paper IV and V.</p><p>Oxygen fluxes through the membranes are found as a function of oxygengradient and temperature in a oxygen permeation cell using air and inert gas on each side. The oxygen flux was observed to increase with decreasing pO₂on the secondary side until the surface exchange became rate limiting and the fluxes reach a constant value. By further increase of the pO₂ gradient, the flux seemed to decrease and this was attributed to the pO₂ dependence of the surface exchange coefficient. The apparent activation energy of the oxygen permeation was in good accordance with previous investigation of similar materials.</p><p>After about 5 week of exposure in an oxygen gradient at about 1150°C, the membranes were carefully examined by electron microscopy for evidence for kinetic demixing and decomposition. Dependent of the overall composition of the membrane, different secondary phases were formed at the primary surface of the membrane. For the cobalt containing materials, isolated grains or clusters of grains of cobalt oxide were formed. In case of the La_0.5Sr_0.5FeO_3-δ membrane, a dense and about 20 µm thick layer of the secondary phase SrFe₁₂O₁₉was formed at the primary side. The overall (La+Sr)/(Fe+Co) ratio was also seen to influence on the phase formed at the primary side. Kinetic demixing was also demonstrated in all the membranes although the metal concentration profiles were not drastically changed from the initial concentrations. The formation of secondary phases was reflected in the (La+Sr)/(Fe+Co) ratio across the membrane. The largest deviation from the nominal stoichiometry was seen close to the surfaces indicating steeper chemical gradients close to the surfaces. These phenomena may strongly limit the long term stability of thinner membranes e. g. films on a porous substrate.</p>