I denne masteroppgaven ble det syntetisert iridiumbaserte katalysatorer på en bærerstruktur for oksygenutviklingsreaksjonen i en PEM vannelektrolysør, ved hjelp av en polyolmetode. Katalysatorpulverne ble karakterisert ved hjelp av både elektrokjemiske (syklisk voltammetri, polarisasjon og impedans), og fysiske metoder (SEM, TEM, BET, EDS, and XPS). I tillegg ble det konstruert en apparatur for måling av elektronisk ledningsevne, og denne ble benyttet for å sammenlikne den elektroniske ledningsevnen til to ulike bærermaterialer, antimondopet tinnoksid (ATO) og wolframoksid (WO3).Målet for arbeidet var å studere effekten som bruk av bærermaterialer og ulike synteseforhold har på de elektrokatalytiske egenskapene til iridiumbaserte katalysatorer på bærere.Resultatene fra dette arbeidet viser at av de to valgte bærermaterialene så har ATO de mest egnede egenskapene for bruk som bærermateriale i et elektrodesystem, i det at den elektroniske ledningsevnen var mye høyere og det spesifikke overflatearealet var større enn hos WO3. Karakterisering av de syntetiserte katalysatorene på ATO viser elektrokatalytiske egenskaper som er på samme nivå som, eller bedre enn, andre katalysatorer laget til samme formål. Forandringer gjort på synteseforholdene (pH og konsentrasjon av materialer) virket ikke å ha noen effekt av betydning på verken morfologi, partikkelstørrelse eller den katalytiske aktiviteten til katalysatorene.

By applying a texture to the front surface of solar cells, less light may be reflected and the incoming light may be given a longer path length inside the solar cell causing a better light absorbance for the solar cell. The textured layer is today made by etching with an acidic etch with a large part hydrogen fluoride. In the future a more environmental approach for creating textured solar cells may be favoured. There are many ways of creating textured surfaces, one of them are by imprinting the surface by a template, and remove the template when the process is completed.In this project the diatom species Coscinodiscus walesii and an undefined Coscinodiscus species were cleaned and processed for investigation. The work done can be divided into four tasks. The first was characterisation and investigation of the pore structure. The second were manufacturing and characterisation of gold for use as templates. The third were deposition and characterisation of thin films of silicon and silicon nitride on diatom frustules. The last part were to make structures are very similar to solar cells with diatom frustules incorporated into them.The diatom frustules were characterised by SEM and the pore structure were cut through with FIB to characterise the structure. There were structural differences between the two species. The Coscinodiscus walesii was grown in a cultivation chamber, and lacked the circular inner pore structure which the undefined Coscinodiscus species had. This was the only real difference between the two species.On the cultivated species were deposited a gold film, which a small rectangle were lifted off by a tungsten needle in FIB. The small rectangle were characterised with the SEM-column in a dual beam FIB. Pore structures down to 40 nm were replicated by the gold film lifted off the frustule. By depositing a relatively thick film of gold on a glass substrate covered with diatom frustules and dissolving the glass substrate and diatom frustules with hydrogen fluoride, a template were made with a replication of the diatom frustule pore structure.Silicon and silicon nitride were deposited on diatom frustules and characterised with FIB to investigate how those materials followed the frustule topography. A good conformity of films made of those materials was confirmed, and 4 different samples which were similar to solar cells were manufactured and characterised by light microscopy, SEM and FIB. Two samples were made to be similar to crystalline silicon solar cells. One had dried frustules on top of a silicon surface and the entire surface of the sample were coated with silicon nitride, a blue colour were observed in the entire surface and the silicon nitride had also been deposited on the diatom frustules. For the other sample similar to crystalline silicon solar cells another layer were deposited between the frustule and the silicon substrate. The double silicon layer made the sample surface yellow, the diatom frustules did not get darker due to a layer above and a layer beneath them. Only in some places where the frustules had loosened from the sample the sample were blue as the silicon nitride layer were the same as a single layer of silicon nitride. For the last two samples, thin film solar cell structures based on amorphous silicon deposited with PECVD were made. A sample there a aluminum coating were deposited on a glass substrate and diatom frustules were dried on top of the aluminum coating, afterwards a 5 µm thick film of amorphous silicon were deposited. SEM images showed that the film were distributed even across a diatom frustule and a light microscopy investigation showed that light were spread when it hit the diatom frustules, hence creating a longer mean path through the solar cell.

Stability and compatibility of the proton conducting electrolyte material La6WO12 with the potential cathode materials LaCoO3 and La2NiO4 were investigated by means of solid-solid diffusion couples. Reactivity studies were carried out at 1450 °C for various times. Reaction products were analysed by SEM and EDS. The study suggests a high reactivity between the electrolyte and the electrodes, which is detrimental for the fuel cell system.In the case of LaCoO3 as a cathode material a secondary phase of LaCo1-xWxO3 were formed at the interface and as precipitates in La6WO12. Theoretical models for both diffusion or interface controlled reactions failed to fit the experimental data. This failure is probably related to the reaction going towards equilibrium after a certain time, or poor connectivity between the materials. The solid- state reaction between La6WO12 and La2NiO4 showed formation of a composite layer of La2O3 and La6WO12 at the interface. The reaction kinetics of this product layer was diffusion controlled. In addition to the interface reaction, precipitates of La2O3 and La6WO12 were formed in the La2NiO4 phase.

Primary silicon crystals grown from an Al-Si melt has been investigated by solidifying directionally and under electromagnetic field. The goal of this thesis was to increase the size of the primary Si crystals and to agglomerate the crystals to one part of the melt. If achieved, this could simplify the following acid leaching process that is necessary to collect the crystals from the melt. Seven experiments were conducted in a resistance furnace with directional solidification to investigate the agglomeration and size of the Si crystals. A mono-crystalline Si seed was added to the Al-Si melt in three of these experiments as an attempt to increase the crystal growth. The impact of stirring in the melt before solidification was investigated. Al-Si melt was directionally solidified without the aid of seed in three experiments. The silicon content in the alloy and pulling rate during solidification was varied in these experiments to find the appropriate Si crystal growth conditions. The growth of silicon crystals from a mono-crystalline Si seed without aluminium was performed to investigate the impact aluminium had on the seeded growth. One experiment was conducted in an induction furnace to investigate the influence electromagnetic force has on the agglomeration of primary Si crystals. Agglomeration of Si primary crystals was found not to be successful for either directional solidification or electromagnetic force method, as the crystals were not gathered to one part of the melt. The size of the primary Si crystals was not as large as expected and addition of mono-crystalline Si seed did not improve the crystal size. A single Si crystal was successfully grown from a mono-crystalline Si seed when there was no aluminium in the melt.

Renewable energy production featuring silicon photovoltaic solar cells are of considerable interest to reduce pollution and related environmental changes. Improvements in efficiency along with reductions in cost are key elements to large scale implementation of this technology. Some suggested and attempted methods of improvements that deserves mentioning are: modifying the energy of incident light to better suit the existing band gap along with reduction in losses by applying surface texturing and nitride coatings. Looking to nature for inspiration reveals diatoms and their frustules having pore structures displaying excellent light harvesting abilities. Thus an implementation of such structural features with regards to solar cell improvements would be highly desirable. This thesis was aimed at performing modifications of diatom frustules surfaces by deposition of oxides known to possess properties of up and down-conversion of light as well as attempting to convert diatom frustules to silicon nitride replicas. Coating frustules with oxides possessing properties of up and down-conversion of light combines light harvesting properties of frustules with spectral modifications of incident light. This offers possibilities of solar cell improvements upon implementation. Nitriding of diatom frustules preserves structural features of the frustules and offers increased mechanical, chemical, thermal and anti-reflecting properties for possible solar cell use. Diatom frustules of the species Coscinodiscus wailesii and Coscinodiscus sp. were utilized in all experimental work during this thesis. An initial temperature exposure experiment was performed at temperatures ranging from 400oC-1200oC with increments of 200oC, to gage thermal response of the frustules. In another set of experiments diatom frustules were subjected to one, two and four dip coatings in one of two different precursor solutions. One solution consisted of erbium and yttrium chloride dissolved in a mixture of ethylene glycol and acetonitrile while the other solution consisted of manganese and zinc chloride dissolved in ethylene glycol and acetonitrile. Post dip coating, frustules were annealed at 800oC in normal atmosphere, decomposing chloride precursors to corresponding oxides. Nitriding of frustules was attempted by simultaneous metallothermic reduction in a purpose built reactor vessel where necessary nitrogen was supplied in the form of ammonia at 650oC and 800oC. Ammonia was generated by thermal decomposition of ammonium chloride mixed with calcium oxide. Post experiments, frustules were characterized by use of scanning electron microscope (SEM) featuring energy dispersive x-ray spectrometer (EDS), fluorescence microscopy, photoluminescence spectroscopy and Raman spectroscopy. Frustules in the temperature exposure experiment were found to display small changes to the pores at 600oC. At 800oC the changes were more severe and the changes were found to increase with increasing temperature until complete destruction of the pores along with visible external changes took place at 1200oC. For the coating experiments, no photoluminescent properties were found to exist for frustules coated once or twice. Frustules coated four times were found to display photoluminescent behavior for both types of coatings. Frustules subjected to four coatings with Zn/Mn solution were found to display more temperature related changes in the pores as compared to frustules coated four times with Y/Er solution. Frustules coated with Zn/Mn solution, were found to be contaminated by elevated levels of tin, possibly influencing both thermal and photoluminescent properties. Frustules subjected to simultaneous metallothermic reduction and nitriding at 650oC were found to suffer low conversion and thus only superficial nitride formation. Frustules nitrided at 800oC were found to display higher conversion to silicon nitride. The formed nitride was determined to be β-silicon nitride by Raman spectroscopy. The frustules were also found to contain elevated levels of reduced silicon, for the frustules nitrided at 800oC, this silicon was found to be a mixture of amorphous and nanocrystalline. The nanocrystalline silicon was found to have a crystalline size of ~2.2 nm.

ABSTRACT

The aluminium smelter at Hydro Sunndal has experienced problems with an unexpected increased loss of AlF3 from the electrolyte when using a certain quality of alumina. One hypothesis considered was that the fluoride loss was a direct result of unusually high HF formation when the quality was added to the cryolitic melt. This work has set out to study the HF formation potential of three different industrial alumina qualities. Alumina A was an alumina quality that worked well in the smelter. Alumina B was the alumina quality where the aforementioned drop in AlF3 was observed and alumina C, made by the same producer, controversially did not to exhibit the same loss at smelter. The present work has been divided into two parts: a characterisation section, where the aluminas have been characterised by techniques such as LOI, TGA, DSC and XRD, and an experimental section, where the industrial aluminas were added to a cryolitic melt. The HF formation and the corresponding H2O concentration were measured in-situ using a tunable diode laser during the alumina additions to the cryolitic melt. Through this work it has been found that both primary and secondary alumina A form less HF than alumina B and C. The LOI, TGA and XRD characterisation has shown that alumina B and C contain a substantial amount of gibbsite, whilst the gibbsite level in alumina A is negligible. A clear correlation between the quantity of moisture found through LOI characterisation and HF formation has been found. It has also been shown that all types of moisture found through LOI testing contribute to HF formation; both physisorbed and chemisorbed types, as well as structural hydroxyl. From the present work it cannot be explained why alumina B caused a drop in AlF3 in the electrolyte, whilst alumina C did not. One explanation postulated is that alumina C has better scrubber efficiency than alumina B. Examining this hypothesis has been outside the scope of this master work. If variations in scrubber efficiency for the two aluminas fail to explain the drop in AlF3, the solution may be found in parameters outwit the alumina quality e.g. weather conditions at the time of production or storage conditions at the calciner.

Si-nanowires are being introduced as an attempt to decrease the high recombination rate present in silicon based thin-film solar cells by employing radial pn-junctions instead of conventional planar pn-junctions. Previous publications have also shown an additional increase in the amount of absorbed light when covering a silicon-substrate in silicon nanowires which may result in a further increase in the total efficiency of a thin-film solar cell. Successful growth of Si-nanowires has earlier been performed by Chemical Vapour Deposition (CVD), employing gold (Au) as catalytic material. Au is a very stable catalytic material for nanowire growth but Au-residues are unwanted in solar cell applications, and the current experiment has therefore investigated aluminium (Al) as an alternative catalyst material. However, stable Al-catalysed growth has been proven to be difficult and is assumed to be mainly due to rapid oxidation of Al to Al2O3. Most of the nanowires were short, tapered and consisted of worm-like structures. Several unsuccessful in-situ NH3-based cleaning (CVD) processes were attempted. Tin (Sn) was also attempted as a protective coating for the Al-film in order to protect Al from exposure to air during sample transport, without any luck. As solar cells require both p-doped and n-doped sections in order to form pn-junctions, initial investigations were performed on the effect from the addition of dopant gases (B2H6 and PH3) on nanowire morphology. The addition of B2H6 to the gas flow seemed to have much larger effects than PH3 on the nanowire morphology compared to intrinsic nanowires. Both gases resulted in a continuous reduction in the average nanowire length with increasing dopant⁄SiH4 ratios, ultimately leading to a complete inhibition of nanowire growth. The highest usable dopant⁄SiH4 ratios before complete growth-inhibition were found to ~10^-3 for B2H6 and ~10^-1 for PH3. An undesirable tapering effect was also found when adding B2H6 to the gas-flow, resulting in radial growth of amorphous silicon on the nanowire walls already at the lowest dopant ratio (~10^-5). This may complicate the use of B2H6 as a dopant gas for p-type nanowires. Ignoring the fact that the addition of PH3 to the gas-flow reduces the nanowire growth rate PH3 may be assumed to be a good alternative for n-type doping of nanowires as no further effects on the nanowire morphology is observed. The actual implementation of dopant atoms into the nanowire structure may be determined by measuring the electrical resistivity in the nanowire, and a possible four-contact structure has been designed and partly optimized for this purpose. The contact structure has been designed in three layers where two of them are produced by photolithography while the smallest layer by electron-beam-lithography. Note that the structure has not been finalized because of time limitations. Some optimization of the four nanowire contacts remains as some final lift-off problems appeared, and is assumed to be related to either an incomplete development of the smallest features or an observed resist-bubbling because of high Titanium (Ti) deposition temperature. However, a robust three-point alignment procedure has been investigated and found useful for producing accurate contacts to single nanowires and leads to the conclusion of a promising structure.

TiZir Titanium & Iron produces high titania slag and high purity pig iron from ilmenite in Tyssedal. The ilmenite is pelletized before smelting. Bentonite is added to the ilmenite concentrate as a binder to give the pellets strength and durability. Bentonite consists mainly of silica and alumina, which are considered as impurities in the high titania slag production. The use of organic binder has therefore been suggested as a substitute for bentonite.This work has focused on developing knowledge on the pelletization process and investigating various organic binders.Two methods of agglomeration, pelletization and briquetting, have been used in this work. Three batches of pellets have been made using a laboratory scale pelletizing drum. Two organic binders, Peridur 300 and Peridur 330, have been tested and compared to pellets made with bentonite and without binder. Seven batches of briquettes have been made using a cylindrical mold and a piston. Three organic binders, Peridur 300, calcium lignosulfonate and a nano cellulose fibre have been tested and compared to briquettes made with bentonite and without binder. The characterization included drop number test (pellets only), compression strength and thermal treatment.Briquettes were employed because using pellets yielded large deviations in the results. These deviations were believed to stem from the varying geometry of the pellets and were substantially mitigated by the use of cylindrical briquettes. It was found that Peridur 300 is a potential alternative to bentonite. The findings from thermal treatment suggest that above 500 degrees celcius sintering takes over as the dominating binding mechanism. For green strength, increasing binder viscosity has a positive effect.

Electro-oxidation of ethanol on smooth platinum surfaces was studied in thetemperature range 21C to 140C for 0.2 M ethanol in 0.5 M sulphuric acid.This was done by use of cyclic voltammetry and electrochemical impedancespectroscopy. In addition cyclic voltammetry with different ethanol concentrationsfrom 0.1 M to 1 M, in 0.5 M sulphuric acid was done at room temperature.Cyclic voltammetry with different ethanol concentrations showed a shift to morepositive potentials for the first oxidation peak in positive going scan as the ethanolconcentration increased. A shift to more positive potentials was also observed forthe oxidation peak in the negative scan as the concentration increased from 0.1M to 1 M. This indicates that the optimum surface condition is reached at higherpotentials for higher ethanol concentrations. This can be because ethanol andadsorbed ethanol derivatives take up more active sites at the surface, thus leavingless active sites available for adsorbed water derivatives which is necessary for theoxidation of ethanol to acetic acid and CO2.Cyclic voltammetry was done for increasing temperatures from 21C up to 140Cfor 0.2 M ethanol in 0.5 M sulphuric acid. These results showed an increasein oxidation current for all oxidation peaks as the temperature increased. Adecrease in peak potential for the first oxidation peak was observed for increasingtemperatures. This indicates that the optimum surface condition for ethanoloxidation is reached at lower potentials at higher temperatures. There was alsoseen an decrease in the apparent onset potential of the first oxidation peak as thetemperature increases. These effects can come from increased thermal activity forwater adsorption at higher temperatures. The peak potential for the oxidationpeak in negative going scan increased with increasing temperatures. This cancome from an easier reduction of platinum oxide at higher temperatures.Dynamic electrochemical impedance spectroscopy measurements was done atdifferent temperatures from 21C up to 140C for 0.2 M ethanol in 0.5 Msulphuric acid solution. The results from the measurements at 60C was fittedto electrochemical equivalent circuits. This gave indications of one kineticallysignificant surface adsorbed species in most potential regions with a notableoxidation current. This in combination with literature suggesting that acetic acidand acetaldehyde is the major products from ethanol electro-oxidation suggestthat the adsorbed intermediate is something other than CO(ads). Results fromthis work together with existing literature on ethanol oxidation was used to givea suggested simplified reaction mechanism for ethanol electro-oxidation.