The possibilities for using CaNb2O6 as a photocatalyst in direct water splitting have been evaluated by investigating the electronic structure of the material. In addition the oxide was doped with nitrogen in order to modify the electronic structure and obtain visible light absorption. Experimental techniques such as electrochemical impedance spectroscopy (EIS), photocurrent, and diffuse reflectance spectroscopy (DRS) were combined with theoretical approaches to determine the bandgap, flatband potential and quasi-Fermi levels of the photocatalyst. CaNb2O6 was prepared by a sol-gel synthesis and doped with nitrogen by heat treatment of the oxide powder in an ammonia atmosphere. X-ray diffraction (XRD) confirmed phase pure orthorhombic CaNb2O6 for both pure and N-doped oxide and excluded a possible transformation of the oxide into an oxynitride. Upon illumination anodic photocurrents were observed implying that CaNb2O6 was an n-type semiconductor due to oxygen vacancies in the lattice. From the wavelength dependency of the photocurrent a direct bandgap of 3.7eV and an indirect bandgap of 3.4eV were determined for undoped CaNb2O6. Doping with nitrogen altered the optical properties of the oxide and shifted the absorption edge into the visible light region. Calculations using the density functional theory (DFT) attributed the change in absorption properties to the formation of narrow energy bands above the valence band of pure CaNb2O6. An alternative explanation could be a hybridization of N 2p and O 2p bands. Correspondingly a reduction of the bandgaps for N-doped CaNb2O6 with respect to the undoped oxide was identified. Impedance was applied to determine the flatband potential of CaNb2O6 from Mott-Schottky plots. However the obtained results seemed to be dominated by contributions from the electrode substrate. Theoretical investigations concluded that pinhole-free oxide layers creating an ohmic contact with the substrate are required in order to designate the observed impedance response to the space charge capacitance. Quasi-Fermi level measurements indicated a low photocatalytic activity of CaNb2O6 as no photocurrent could be detected. Further investigations are needed to identify the cause of the photocurrent limitations. Nevertheless probable explanations could be low conductivity in CaNb2O6, high concentrations of recombination centers or slow charge transfer kinetics. The latter was confirmed for porous oxide layers as the addition of a hole scavenger increased the measured photocurrent. Positive photocurrent transients were also observed for porous CaNb2O6 films and could be related to either the diffusion of electrons through the porous oxide layer or to a photoanodic decomposition of the photocatalyst.

Silicon undergoes oxidation at room temperature, causing a formation of an oxide layer on its surface. Gaining knowledge about the oxide layer formed during ageing, is valuable with regards to production of high quality ceramics, Si3N4, especially with for the slip casting process.In this work silicon powders were exposed to humid air at different temperatures for different periods of time. From the XPS analysis, the oxygen and hydroxide content were found to be higher for powders aged at higher humidity compared to powder aged at lower humidity. This was verified by the estimated oxide thickness, which was larger for powder aged at higher humidity. No obvious difference could be observed for short and long ageing time in low humidity atmosphere. Results from Raman spectroscopy showed a more amorphous powder after storage in humid air. A clear shift and broadening of the crystalline silicon peak were observed for powders exposed to air at low and high humidity. The powders aged in humid air were further reacted with water at pH 9.4 and temperature of 60°C. The onset time for hydrogen evolution was observed to be later for samples exposed longer times to humid air. This indicates that the reactivity of the powder decreases as the oxide layer formed during ageing develops. The amount of hydrogen evolved seemed to be dependent on the hydrogen evolution time rather than the time of exposure to humid air. This was verified by the surface area, which was about the same for all samples, showing only a slightly decrease for increasing exposure time. The oxygen content measured was linearly related to the surface area for the main section of the samples. Higher hydroxide content seemed to catalyse the aqueous oxidation reacting, resulting in an earlier onset time for hydrogen evolution and a higher hydrogen flow. This was stated by a higher surface area and higher oxygen content for these samples.Powder thermally treated, at 300 °C and 900 °C, were also investigated. XPS analysis showed a thicker oxide layer for powder treated at 900 °C. No hydrogen was formed in the aqueous oxidation of powder treated at 300 °C. This was verified by a low surface area.Regarding the slip casting of silicon powders, a defect free green body can be obtained by reducing the hydrogen formation. Based on the results found, this can be obtained by the formation of a passivation oxide layer on the particle surface, formed during ageing of the powders in humid atmosphere before processing.Higher humidity and longer ageing time seems to affect the development of the oxide layer formed on the surface more than low humidity and short ageing time. To minimise the hydrogen formation, it is crucial to be able to control the hydroxide concentration in the aqueous oxidation accurately.

The use of offshore wind turbines as a source of renewable energy is promising. However, many challenges have to be solved before they will be cost effective. The operation time, without any maintenance required, is desired to be as long as possible, because downtime and maintenance costs are high. Thus it is important that the rotating parts in the turbine have a long operational time. The harsh marine environment, combined with the desire for elongated operating times and little or no maintenance, give rise to many tribological challenges. It is important that the chosen materials and lubricants can withstand the marine environment, and the high loads. The effect of marine environment on lubricating properties of lubricants is not well understood. The objective of this thesis has been to investigate this effect. Two lubricants that are commonly used as gear lubricants in onshore wind turbines were tested in this work; polyalphaolefin and polyalkylene glycol. They were both contaminated with different amounts of artificial seawater in order to investigate the effect on the lubricating properties. The lubricants were tested in a rotating ball-on-disc tribometer, with self-mated stainless steel, and self-mated silicon carbide. Stainless steel was selected as it is commonly used in gear bearings, whereas silicon carbide was chosen due to its promising excellent properties. The results obtained from this work show that PAO has a very low saturation limit for water, and an emulsion will be formed even at low contamination levels. This made the lubricant unstable, and the measured COF were unstable. It was found that the amount of two-body abrasive wear increased as a function of seawater content. The PAG lubricant managed to dissolve much larger quantities of seawater that PAO. But even though the system was one-phased, the results for COF were unstable. Wear induced pitting was found for both clean and contaminated lubricant. It is believed that it is caused by the additive package of the lubricant. For dry tribological testing COF was found to be mush less for self-mated silicon carbide than for self-mated stainless steel. This is as expected, since silicon carbide has shown outstanding tribological properties in previous work.For self-mated silicon carbide testing only abrasive wear could be found for both dry contact, and lubricated. The results from PAO contaminated with seawater, showed a clear increase in COF as a function of seawater content. Further, the COF all stabilized after the running-in period. PAG showed no such trend with increasing amounts of seawater, but the standard deviation of the measurements increased.

Denne masteroppgaven har tatt for seg undersøkelser av stål designet for lavtemperaurappliksjoner ved bruk av elektronmikroskop i kombinasjon med diffraksjon av tilbakespredte elektroner (Electron Backscatter Diffraction - EBSD). Målet med oppgaven var å bruke EBSD-teknikken i kombinasjon med in situ deformasjon og en spesiallaget kaldfinger for å kunne studere materialene ved lave temperaturer. To ulike stål ble undersøkt; et smidd finkornet strukturelt stål (F70) og et varmvalset supermartensittisk rustfritt stål (Supermartensitic Stainless Steel - SMSS).F70-stålet ble sveisesimulert for å oppnå en mikrostruktur tilsvarende interkritisk grovkornet HAZ (Heat Affected Zone). Dette ble gjort ved en tosykel sveisesimulering. Prøvestaver av materialet ble varmet opp med en ønsket makstempertur på Tp1= 1350°C og Tp2= 780°C. For å undersøke effekten av ulike avkjølingshastigheter ble halvparten av prøvestavene avkjølt med Dt8/5 = 5 sekund, og den andre halvparten med Dt8/5 = 15 sekund. Ved sveising av stål kan det dannes lokale sprø soner som martensitt/austenitt-faser (MA-faser). Bilder tatt i lysmikroskop viser at strukturen til sveisesimulert F70-stål består av martensitt og noe bainitt. MA-faser er lokalisert langs korngrensene.EBSD-teknikken krever at materialet på forhånd gjennomgår en tilfredsstillende prøvepreparering for å gi gode resultater. Det var tidligere etablert en tilfredsstillende prepareringsteknikk for SMSS. I denne oppgaven ble to ulike prepareringsmetoder testet på sveisesimulerte prøver av F70-stål, som er et forholdsvis nytt materiale. Metodene som ble testet var ionesputtering og elektropolering. Preparerte prøver ble undersøkt med EBSD for å analysere kvaliteten på prepareringen. Det viste seg at MA-fasene reagerte annerledes på prepareringen enn resten av strukturen, og det var derfor vanskelig å få disse fram på EBSD-skann. De to prepareringsmetodene ga også forskjellige utfall for andel austenitt (g-fase) i stålet. Da det i hovedsak er MA-fasene som er interessante ved in situ undersøkelser ble det besluttet å ikke gjennomføre disse på F70-stålet, da en tilfredsstillende preparering må etableres først. Ved bruk av EBSD-teknikken kan orienteringsdataene som lagres behandles i programvaren TSL OIM Analysis 5.32, som tilbyr ulike renseoperasjoner. Grain CI Standarization gir alle punktene i et korn, innenfor en gitt vinkeltoleranse, samme CI-verdi som den maksimale CI-verdien funnet blant punktene i dette kornet. Det er også mulig å legge på et CI-filter som fjerner alle punkter med en CI ≤ 0,05, og erstatter disse med svarte eller grå piksler. Effekten av disse to operasjonene ble undersøkt på et EBSD-skann av SMSS tatt ved romtemperatur, uten noe form for deformasjon. Renseoperasjonene førte til at 8,1 % av alle punkter ble fjernet. 3,8 % av disse var av g-fasen, som utgjør kun 17,3 % av skannet før renseoperasjonene ble benyttet. Punkter som fjernes ligger i hovedsak i forbindelse med korngrenser eller øyer av restaustenitt. For SMSS som inneholder rundt 20 vol.% restaustenitt i opprinnelig (dvs. interkristisk glødet) tilstand, ble det gjennomført undersøkelser av deformasjonsindusert fasetransformasjon ved bruk av in situ strekktesting med påfølgende EBSD karakterisering. Undersøkelsene ble utført ved romtemperatur og -80°C. Et spesiallaget strekkbord ble montert på SEM (Scanning Electron Microscope) stagen med prøven fastmontert, og en kaldfinger ble brukt for å kjøle ned prøven. EBSD karakteriseringen ble foretatt uten deformasjon og ved 1, 2, 3 og 4 % forlengelse. En sammenligning av resultatene ved romtemperatur og -80°C ble deretter utført. Det ble konkludert med at øyer av restaustenitt transformeres til martensitt som følge av plastisk deformasjon i større grad ved -80°C, som følge av høyere termodynamiske drivkrefter for reaksjonen. Ved romtemperatur var det mulig å verifisere at martensitten som ble dannet oppfyller Kurdjomov-Sachs kriteriet som beviser at den er korrekt indisert. Dette var vanskelig å verifisere for lavtemperatursforsøkene, da skannene har en mye lavere kvalitet.

AISI 316/316L has traditionally been used in offshore topside environments when operating under low temperatures. It has been observed that the AISI 316/316L piping suffer from severe attacks of pitting and crevice corrosion on external surfaces. It is therefore important to emphasise the comments within the statements that give the limitations on the use of the material. While temperature is the main parameter in the standards, there seem to be others, such as location, presence of tags and coating, which affect the initiation and propagation of pitting and crevice corrosion. This thesis discusses the various operating parameters that affect the susceptibility to pitting and crevice corrosion of AISI 316/316L piping and proposes procedures for identifying piping that are most prone to attack, and calculations of the probability of failure of such piping.In the first part of the thesis, a literature survey of the parameters that affect the pitting and crevice corrosion of AISI 316/316L in an offshore environment is presented. In the second part, a procedure for the identification of piping that is prone to pitting and crevice corrosion is presented. This procedure has been developed taking into consideration the parameters that are assumed to be important and suitable for use in RBI analysis. In the first step, the parameters that affect both pitting and crevice corrosion, that is, the chloride content due to the location of the pipe, coating and temperature, were considered to be the most important ones. In the second step, the possibility of crevice corrosion was assessed. In the third part, a procedure for assessing the possibility of pitting corrosion in the offshore topside environment based on the pitting potential was developed. The temperature of the external pipe surface was used to find the solubility of NaCl in water. The chloride concentration was used to determine the in order to find the possibility of pitting in the topside offshore environment. The last step was to develop a procedure for calculating the probability of failure of a pipe as a function of time. In this step, the functional life of the coating and the rate of pitting corrosion have been considered.

High cost and a short lifetime are the two main reasons why the PEM fuel cell is yet to be commercialized. The bipolar plate in a PEM fuel cell is alone responsible for about 45% of the cost and 85% of the total weight of a single cell. stainless steel has been suggested as material for the bipolar plate because of its good mechanical properties, easy manufacturing and relatively low price. A problem with stainless steel is the Chromium oxide film formed on the surface which causes a high contact resistance. In order to prevent this oxide formation, the stainless steel can be coated. Gold has been suggested as coating, but it is too expensive to be considered a viable alternative. The objective of this thesis was to investigate stainless steel as bipolar plate material for PEM fuel cells. In cooperation with SINTEF polarization tests were done on stainless steel bipolar plates with and without two different coatings; gold and Coating A. The tests were performed in H2SO4 electrolytes with different molarities and additives. Before and after each polarization test Interfacial Contact Resistance (ICR) measurements where done to see how the oxide layer on the stainless steel surface changed during polarization. Gold coated stainless steel was chosen as standard for both the polarization tests and the ICR measurements because of its corrosion resistance.The results obtained from both polarization tests and corresponding ICR measurements showed that the reproducibility was not as good as one had hoped, but this can be explained by low absolute values of the current densities. Gold coated steel proved to be a good standard for the ICR measurements, but due to pitting corrosion the corresponding polarization results were not as promising. The pH in an operating fuel cell was found to be approximately 3.5, and the tests done at different molarities showed that at a lower pH the oxide layer seemed to be thinner and the stainless steel surface thus became more exposed to corrosion. Additions of fluoride and chloride in the amounts expected in an operating fuel cell did not seem to cause any changes for neither the polarization results nor the contact resistance measurements. Stainless steel plates with Coating A showed very small changes in contact resistance after being put trough the polarization tests, but at low potentials the current densities in the polarization test were very high, indicating that components in the coating either catalyzed hydrogen evolution or were reduced themselves. Out of all the ICR measurements, gold coated stainless steel was the only plate satisfying US department Of Energy’s (DOE) resistance requirement for bipolar plates of less than 10 mΩ cm2. The stainless steel plates with Coating A were close to DOE’s requirements for both corrosion current and contact resistance. Non-coated stainless steel was ruled out as bipolar plate material due to high contact resistance measurements.

A study of the effect of various amounts of manganese in solid solution on the recovery and recrystallization behavior of an AlMn-alloy, for different annealing conditions after different degrees of deformation (cold rolling) has been carried out. The alloy studied was a 3xxx-type model alloy with 0.5 wt% Fe, 0.15 wt% Si and 1.0 wt% Mn. The studies were a contribution to the MOREAL-project, where the main objective is to quantify and characterize the softening behavior of recyclable aluminium alloys, with small amounts of manganese, iron and silicon, as a basis to validate and further develop the ALSOFT-model. 5 variants of the AlMn-alloy, with respect to strain and homogenization, were prepared and further investigated.Based on electrical conductivity measurements the different amounts of Mn in solid solution, after the different homogenization (hom.) treatments, were estimated to be approximately 0.85 wt% (as cast), 0.55 wt% (hom. nr. 2) and 0.49 wt% (hom. nr. 3). The reductions of Mn in solid solution by hom. nr. 2 and hom. nr. 3 resulted in less concurrent precipitation and consequently a larger effective driving force for recovery and recrystallization. The high amount of Mn in solid solution for the as cast variants resulted in much concurrent precipitation. Concurrent precipitation was found to delay and suppress the recovery and recrystallization processes. At high annealing temperatures, with no (or little) concurrent precipitation occurring, the recrystallised grain structures were found to be homogeneous, fine grained and consisting of equiaxed grains. In these cases the recrystallised textures were found to be approximately random after a strain of 0.7, while they showed cube textures of weak and medium strength after a strain of 3.0. At low annealing temperatures, with concurrent precipitation occurring, the recrystallised grain structures were found to be inhomogeneous and coarse grained, with grains elongated in the rolling direction. In these cases the recrystallised textures were found to consist of P-textures and ND-rotated cube textures of medium and high strength.A high degree of deformation was found to increase the rate and degree of the recovery and recrystallization processes during the isothermal annealing procedures, and to result in fine recrystallised grain structures, consistent with the increased effective driving force for recovery and recrystallization. A high degree of deformation was also found to increase the rate and magnitude of precipitation during the isothermal annealing procedures.Non-isothermal annealing experiments were carried out to investigate if they could result in significantly different grain structures and textures compared to the isothermal annealing experiments. No significantly different results were achieved, only results directly comparable to the results from the isothermal annealing experiments.

The economical feasibility of commercial, single-junction solar cells is limited by high costs and limited efficiencies. New solar cell concepts and materials are sought to decrease the production costs and increase the efficiency. Intermediate band solar cells (IBSCs) show a promising concept for increased efficiency up to 46.77% as they employ three band gaps that can be matched to the solar spectrum to minimize fundamental losses. Doping of copper gallium disulphide (CuGaS2) with transition metals like Fe and Ni to high concentrations can theoretically form an intermediate band (IB), which for Fe doping gives nearly optimal band gaps for IBSC applications. Copper gallium disulphide is synthesized in an environmentally friendly, inexpensive and simple hydrothermal synthesis which may contribute to decreased costs of solar cell production.The hydrothermal synthesis is developed to produce copper gallium disulphide from copper(I) chloride (CuCl), gallium(III) chloride (GaCl3) and excess thiourea (Tu) (SC(NH2)2) in deionized water. The influence of varying synthesis parameters on product purity, yield and morphology has been investigated through X-ray diffraction (XRD) and scanning electron microscopy (SEM). Energy dispersive X-ray spectroscopy (EDS) is used to investigate doping of copper gallium disulphide and identify particle morphologies formed by different phases. Formation of copper gallium disulphide proceeds through slow decomposition of Tu, driven by an equilibrium shift due to hydrogen disulphide (H2S) evolution and precipitation of sulphides like the intermediate phase digenite (Cu2-δS) into which Ga3+ ions are incorporated. An additional impurity of copper(II) sulphide (CuS) is commonly formed, and gallium(III) hydroxyoxide (GaO(OH)) forms at pH > 0.5. Products of high purity and yield are obtained at 250 °C with concentrations above 0.060 M copper(I) chloride and gallium(III) chloride with the complexing agent 1-pentanethiol, and 0.319 M without 1-pentanethiol. Introducing nickel(II) chloride (NiCl2) or iron(III) chloride (FeCl3) in the hydrothermal synthesis forms vaesite (NiS2) or iron pyrite (FeS2) impurities, respectively, and copper gallium disulphide is not doped to a desirable concentration for IB formation.Color variations in the products reveal off-stoichiometries which contribute to a wide range of particle and crystallite morphologies within each product. Yellow, stoichiometric copper gallium disulphide particles have been deposited on a Si(100) substrate and growth of a red, Ga-rich film was achieved with 1-pentanethiol. These products were subject to photoluminescence spectroscopy (PL) along with off-stoichiometric powders of doped and undoped products, but no luminescence was obtained, possibly due to high defect densities and non-radiative recombination. Dispersions of powders were also subject to absorption spectroscopy which indicate extensive scattering due to wide ranges of particle sizes. The morphology of powder products shows particularly large variations within and between the products. Nanoplate and pyramidal crystallites are produced through nucleation and growth to form network structures and polycrystalline spheres, rods and rose-like particles, which along with the crystallites have varying irregularities and sizes.

The aim of this thesis was to create an understanding between the electrochemical performance of a porous Ni-CGO anode used in SOFC, and its microstructural parameters when subjected to redox-cycling. The anode samples were produced consisting of two different layers, and subjected to different number of redox-cycling. Their electrical resistance was measured using electrochemical impedance spectroscopy (EIS), and their parameters were obtained by analyzing cross-section images from the samples obtained by the focused ion beam (FIB) or from simulated 3D-models. A large increase of Ohmic resistance was observed after 3-4 redox-cycles, while there was only a slight increase of the polarization resistance. The high increase of the Ohmic resistance is believed to be due to the observed Ni-coarsening and the net increase of the anode volume due to oxidation of Ni to NiO and subsequently reduced back to Ni. Both the Ni-coarsening and the net volume expansion decrease the Ni connectivity which increases the Ohmic resistance. More electrons are also forced to go through the lower-conducting CGO phase, which increases the Ohmic resistance, though preventing a higher increase since it is connecting the isolated Ni grains together. The low increase of polarization resistance is believed to be due to the large pore-CGO interfacial area, which compensates for the decrease in triple-phase boundaries (TPB). The resolution of synthetic microstructures is not enough to capture constrictivity and tortuosity effects realistically. The dimensions of most of the bottle necks are below 100 nm and hence the size classes must be in the range of 10 nm, which is similar as the resolution of FIB-tomography.