Background and objective: Removal of boron is one of the main challenges in the purification of metallurgical grade silicon (MG-Si) for solar cells and a simple low cost method is therefore needed. Boron removal by slag treatment is today regarded as the most promising method, but the efficiency of the refining method is relatively low. Slag refining as a method for boron removal can be improved by optimization of the slag composition by changing the components and/or the composition of the slag. Another method for improving the slag refining efficiency is to remove boron containing species from the slag by evaporation. The aim of this work is to study the refining properties of the Al2O3-MgO-SiO2 slag and the CaO-SiO2-TiO2 slag, respectively. Furthermore, the possibility of evaporating boron from a CaO-SiO2 slag when refined in a hydrogen containing atmosphere will be investigated. Methods: In all, 12 experiments were performed at 1600˚C for determination of the distribution- and mass transfer coefficient of boron in the Al2O3-MgO-SiO2 and CaO-SiO2-TiO2 slag, respectively. In the study of boron evaporation from CaO-SiO2 slag in hydrogen atmosphere 14 experiments were performed. Boron evaporation was investigated in the temperature range 1500-1600˚C, the refining time was 0-6 hours and the slag/silicon mass ratio was varied between 0.25 and 1.5. Results: The distribution- and mass transfer coefficient of boron in the Al2O3-MgO-SiO2 (29%-23%-48%) slag was found to be 1.9 and 2.3x10-6 m/s, respectively. The amount of TiO2 in the CaO-SiO2-TiO2 slag was found to rapidly decrease due to reduction of TiO2 by Si. The distribution- and mass transfer coefficients found for the system were therefore not the same as for the original system. A decrease in the boron content in the slag was observed with increasing refining time in a hydrogen atmosphere at 1600˚C. This indicated that boron evaporated from the system, but the evaporation rate was found to be low. Temperature was not found to significantly influence the evaporation rate of boron in the temperature range 1500-1600˚C. When varying the slag/silicon mass ratio, a considerable increase in the refining efficiency compared to conventional slag refining was observed for the lower slag/silicon mass ratios. The effect diminished for the higher slag/silicon mass ratios. Conclusion: The kinetic and thermodynamic properties of the Al2O3-MgO-SiO2 slag evaluated in this thesis were shown to be in the same range as comparable slags in previous studies. In the CaO-SiO2-TiO2 slag most of the TiO2 was reduced during the refining process which makes it difficult to use this compound as a part of a slag for refining of silicon. When slag refining was performed in a hydrogen atmosphere the refining efficiency compared to conventional slag refining was shown to increase.

Det har blitt estimert at 30 % av verdensuoppdagede gassreserver og 13 % av verdens uoppdagede oljereserver ligger i de polare områdene i nord. Man trenger derfor materialer som tåler lave temperaturer for å hente opp denne oljen og gassen. HSLA-stål har generelt lav omslagstemperatur, men dette kan endres under sveising. Sveisevarmen gjør at mikrostrukturen endres i den varmepåvirkede sonen (HAZ). Dette kan gjøre stålet sprøere. Den grovkornede varmepåvirkede sonen (CG HAZ) er den sprøeste sonen ved avkjøling fra austenittområdet. Målet med denne oppgaven er å undersøke forskjeller i mikrostrukturen og forskjeller i initieringsmekanismen til kløvningsfasetter i CG HAZ med to forskjellige avkjølingstider. Det blir også sett på hva som kan ha innvirkning på initieringen av fasettområdene. Prøver av et 420 MPa HSLA-stål fra Nippon Steel har blitt sveisesimulert for å oppnå mikrostrukturen til CG HAZ. Halvparten av prøvene hadde en avkjølingstid fra 800 til 500 ◦C (∆τ8/5) på 5 sekunder, og andre halvparten ∆τ8/5 på 15 sekunder. Prøvene undergikk avbrutt CTOD-testing. Det foregikk ved at prøvene ble bøyd ved -30 ◦C til det ble avgitt et akustisk emisjons-signal (AE-signal), for så å bli avlastet. Etter avlastning ble prøven utmattet videre til endelig brudd. Hensikten med avlasting og utmatting av prøvene er at man får et tydelig definert fasettområde, slik at det blir lettere å identifisere det i scanning elektronmikroskopet (SEM). Mikrostrukturen til prøven simulert med ∆τ8/5 på 5 sekunder hadde en høyere andel martensitt i bainittstrukturen enn prøven simulert med ∆τ8/5 på 15 sekunder. Det ble også funnet partikler i de to mikrostrukturene. I prøven simulert med ∆τ8/5 på 5 sekunder ble det funnet massive partikler inne i noen av bainittområdene. De ble antatt å være MA-partikler. Det ble i tillegg funnet runde partikler på noen av de tidligere austenittkorngrensene som ble antatt å være korngrenseferritt. I prøven simulert med ∆τ8/5 på 15 sekunder ble det funnet liknende massive partikler. Det ble i tillegg funnet avlange partikler inne i noen av bainittpakkene. De ble antatt å være MA-fase. De fleste prøvene simulert med ∆τ8/5 på 5 sekunder hadde fasettområder med partikler i nærheten av initieringspunktet. Disse partiklene hadde omtrent samme sammensetning som stålmatriks, men det ble i noen tilfeller funnet forhøyede verdier av silisium. Partiklene ble antatt å være MA-partikler. To av prøvene simulert med ∆τ8/5 på 15 sekunder hadde en partikkel i initieringspunktet som hadde omtrent lik sammensetning som stålmatriks, og disse ble også antatt å være MA-partikler. Arealet av alle fasettene i alle prøvene ble målt og sammenlignet med størrelsen på AE-signalet. Av dette ser man en sammenheng der størrelsen på AE-signalet øker med størrelsen på fasettene for prøvene simulert med ∆τ8/5 på 5 sekunder. For prøvene simulert med ∆τ8/5 på 15 sekunder er det flere som skiller seg ut fra denne sammenhengen. Dette forklares ved at disse fasettene oppstod under utmatting. Lengden fra sprekkspissen til initieringspunktet ble også målt og sammenlignet med CMOD-verdien. Det ser ut til å være en tilnærmet lineær sammenheng der økende CMOD-verdi gir bruddinitiering lengre bort fra sprekkspissen. Hovedforskjellen mellom de to avkjølingstidene er at fasettene i prøvene simulert med ∆τ8/5 på 5 sekunder ligger mye nærmere sprekkspissen og er mer topografiske enn prøvene simulert med ∆τ8/5 på 15 sekunder. Når det gjelder kimdanningsmekanisme ser det ut til at de fleste av fasettene i prøvene simulert med ∆τ8/5 på 5 sekunder initieres ved hjelp av spenninger fra partikler som ligger i nærheten av initieringsområdet. Det ble ikke bevist at partiklene er MA-partikler, og man kan derfor ikke si noe sikkert om MA-partiklenes innvirkning på bruddet. Det er vanskelig å si hva kimdanningsmekanismen er for prøvene simulert med ∆τ8/5 på 15 sekunder ettersom initieringspunktene er svært forskjellige.

Mixed ionic electronic conducting ceramic oxides are being investigated for use as membranes for oxygen separation from air and as electrodes in solid oxide fuel cells [1, 2]. Oxygen surface exchange rate is an important parameter controlling the oxy- gen permeation rate of these membranes [3]. In this work surface structuring by wet chemical etching is investigated as a possible route to improve the surface exchange rate of La0.2Sr0.8Fe0.8Ta0.2O3−δ (LSFT). Several different etchant solutions have been in- vestigated (HCl, HNO3, H2SO4, H2C2O4, NaHCO3, Na2CO3 and KOH) and evaluated with respect to etch rate, the resulting surface morphology, selectivity and special effects caused by the etchant or the nature of the sample being etched. The resulting surface structure after etching proved to be very dependent on microstructure, showing both el- evated grain boundary and mid grain etching. The mid grain etching was unexpected, and additional experiments were done to investigate the cause of this behaviour. The effect of surface structuring using wet chemical etching was investigated using electri- cal conductivity relaxation at 800◦C and 900◦C. This showed that an increase of the specific surface area 3.6 times gave an increase of the chemical surface exchange coef- ficient about 3.16 times at 800◦C. The influence of temperature and partial pressure of oxygen on the effect of surface structuring is discussed based on the obtained results. LSF-based membranes (La1−xSrxFe1−yMyO3−δ where M = transition metal cation) show limited stability towards CO2 containing atmospheres, forming surface layers of SrCO3 [4, 5]. It is suggested that a subsequent removal of this layer by i.e. chemical etching could lead to a fine surface structuring. In this work, exposure of LSFT-samples to CO2- rich atmospheres was done at 900◦C and 1000◦C without the formation of such a layer. Reasons for this are discussed and alternate experimental conditions are suggested.

A study of the influence of artificial ageing temperature on the precipitation behaviour in three distinct direct chill casted and extruded Al-Mg-Si alloys has been carried out. The total amount of alloying elements is approximately the same in these alloys, but the difference is the ratio between these elements. The master thesis is a continuation of the work reported in Characterization of precipitates at maximum hardness in Al-Mg-Si alloys which was written in connection to the course TMT4500 Materials Technology, Specialization Project. The primary objective of this work was to identify the value of maximum hardness, and at what time it is obtained for these alloys after they have been artificially aged, one set at 200 °C and another at 250 °C, i.e. six cases in total. The aim in the master thesis has been to investigate which type and relative amount of needle shaped precipitates that are present, and what size, number density and volume fraction that is responsible for these conditions by using transmission electron microscopy (TEM). In addition, all three alloys have been shown to obtain a local hardness maximum at overaged conditions in the 200 °C case. These conditions have also been analysed by comparing with an overaged condition for KK5 for an ageing time shorter than the one were the local maximum appears. Therefore, ten different conditions have been investigated in total.One sample has been selected from each case based on the hardness curves in the project work, and prepared for analysis in two different TEM instruments. A number of general trends between alloy composition, heat treatment, and the resulting microstructure and hardness that already have been reported, have been verified during the master thesis. The maximum hardness conditions after artificial ageing at 200 °C have been found to be quite different for KK5, KK6 and KK7 in terms of type and relative amount of precipitates, and precipitate size and number density. Artificial ageing at 250 °C has been observed to give very similar values of maximum hardness for the three alloys primarily due to the size and number density of the precipitates. The local maximum hardness peaks observed at overaged conditions after artificial ageing at 200 °C have been seen to be purely due to precipitate size and number density, but the reason for their appearance has not been clarified. The observed differences between the three alloys in each condition and the differences between the conditions themselves for each alloy have been seen to be reflected in both type and relative amount of precipitates, and precipitate size and number density.

A thermal barrier coating (TBC) is used as thermal protection of metallic components exposed to hot gas streams in e.g. gas turbine engines. Due to a high thermal expansion coefficient, low thermal conductivity, chemical- and thermal stability, yttria stabilized zirconia (YSZ) is the most widely used material for TBCs today. In the work presented in this master thesis an aqueous nitrate precursor solution was prepared and deposited on stainless steel substrates by spray pyrolysis to produce 8YSZ coatings (8 mol% of Y2O3 in ZrO2). The precursor solution concentration and deposition parameters, including set-point temperature and volume sprayed, were optimized to produce continuous and crack-free green coatings.The deposited green coatings were characterized by scanning electron microscopy, thermogravimetry and Fourier transform infrared spectroscopy to study the influence of substrate temperature on the microstructure of the green coatings. A substantial change in microstructure was observed for the green coatings in a certain temperature range indicating that a minimum deposition temperature was necessary to obtain crack-free green coatings.Heat treatment was necessary to decompose the nitrate species in the deposited film. During heat treatment, vertical cracks were introduced into the coatings due to the nitrate decomposition. The cracking behavior of the coatings was studied for different drying times and conditions, and it was found that the crack propagation can be controlled to obtain the preferred size and geometry of the cracks. Due to built-up stresses in the coating, which can exceed the fracture toughness of the material, it was found that there was a maximum film thickness achievable before spallation of the coating for a given precursor solution. Therefore, the possibility of spraying multi-layered coatings was investigated. The introduction of a second layer showed that it was possible to double the thickness of the coating.

There are currently no standard test methods for testing the cathodic disbonding properties of organic coatings at temperatures above 100 C. There are several subsea oil and gas reservoirs with high temperatures, some as high as 200 C. The main goal of this work was the development of a new apparatus and testing procedure for high temperature cathodic disbonding, hereby referred to as HTCD (High Temperature Cathodic Disbonding).A test method for the cathodic disbonding of organic coatings from submerged steel subjected to high temperatures was studied. The test method requires the use of a specialized HTCD apparatus. In this test method, sample plates were mounted between two channels, one containing a hot (150 C) oil flow and the other containing a pressurized, cold, salt (3.5 % NaCl) water flow. Accelerated conditions made it possible to test the cathodic disbonding properties of several coatings in four weeks. Four weeks is a typical duration for coating prequalification tests. Several commercial coating products of different generic types provided by different manufacturers were tested. The results indicate that adequate coating products for high temperature underwater exposure are available.The required cathodic protection current for the samples tested in the HTCD apparatus was continuously monitored. No correlation between the required cathodic protection current and the extent of cathodic disbonding was observed.A long term test with more field like conditions and a duration of 400 days was performed. Low levels of disbonding for most of the tested products in the 400 day test made comparison to the accelerated tests difficult.An attempt was made to determine the oxygen diffusion coefficient of five coating products. The attempt was unsuccessful. The same method had previously been used to study coatings with a thickness of up to 300 micro m, the coatings studied in this work were between 600 micro-metres and 1200 micro-metres. It remains uncertain whether the chosen method can be used for coatings this thick.Results from electrochemical impedance spectroscopy, performed in a pressurized vessel, showed a large reduction in the ionic resistance of a submerged organic coating upon heating from 30 C to 150 C. This showed that elevated temperatures throughout the coating can reduce the ionic resistance to a level where even an intact coating is incapable of protecting the substrate. Studies of coating samples at ambient temperatures after exposure to higher temperatures showed that exposure to heat causes a lasting reduction in impedance. High impedances correlated with good performance in the HTCD tests.Investigation with a scanning electron microscope (SEM) provided images where the extent of the cathodic disbonding was clearly visible. Electron-dispersive X-ray spectroscopy (EDS) enabled the identification of oxides discovered at the holiday and beneath the disbonded coating. Zinc and calcium oxides were identified at and near the holiday; iron oxide was identified beneath the disbonded coating.