The main aim of this work was to analyze an impedance model for porous semiconductor electrodes consisting of spherical particles. The model should make it possible to analyze the flatband potential for this type of electrodes. The analysis was conducted by simulating the model in MATLAB®. Cyclic voltammetry and electrochemical impedance spectroscopy was performed on titanium oxide, TiO2 P25, anodized titanium and some iridium tin oxides, Ir(1-x)SnxO2. The aim was to use the experimental data as a reference and compare the simulated data with the experimental results. This could not be done because the recorded data for the oxides were too strongly influenced by the support material. The supports tested in this work were Au, Ti and ITO. The simulations show that the capacitance of the models spherical particle is only weakly dependent on the particles surface potential. This indicates that this one-dimensional version of the model might not be sufficient to analyze the spherical particles. However, another analysis method for investigation of Mott-Schottky behavior for porous electrodes was confirmed by the result for the anodized titanium.

Ceramics with mixed ionic and electronic conductivity are being investigated for oxygen separation from air, intended for the production of syngas. Asymmetric membranes, consisting of a dense membrane co-sintered with a porous support are expected to achieve a high flux of oxygen and at the same time adequate strength. As the porous substrate is mainly intended for strength contribution, it is crucial that the flux of oxygen through the dense membrane is not limited by the flow of air through the porous substrate. An oxygen flux of 10 ml min-1 cm-2 should be achieved at operating temperatures (800-1000 °C) for the substrate to be commercially attractive. A biaxial strength of 34MPa has been obtained by similar porous substrates.Four different strategies for achieving high permeating substrates made by solid state La0.2Sr0.8Fe0.8Ta0.2O3-x have been evaluated. Pressed porous substrates were produced both with and without the use of pore formers. The most promising compositions with respect to porosity and permeability were tape casted and characterized with respect to porosity, permeability and strength.

Coin cells with lithium and graphite electrodes were assembled using different combinations of graphite material and electrolyte. Specifically, three commercially available graphite materials and five electrolyte compositions were studied. The cells were discharge-charge cycled with varying parameters in order to determine the performance of the graphite materials and electrolytes. Particularly, a temperature chamber was employed to cycle some cells at temperatures between 0 and 40°C to find the significance of the electrolyte composition and graphite material on the cell performance at these temperatures. The cycled cells were disassembled and samples from the graphite electrode soaked with electrolyte were prepared for thermal analysis, specifically differential scanning calorimetry (DSC). The thermal stability of the graphite electrodes and the influence from the graphite and electrolyte properties and the cycling parameters were analysed. In order to facilitate the interpretation of the results from discharge-charge cycling at different temperatures, DSC analysis from -80 to +50°C was performed on the pure electrolytes.Confirming previous studies, it was found that both the thermal stability and cycling performance were highly influenced by the properties of a solid electrolyte interphase (SEI), situated between the graphite surface and the electrolyte and formed during cycling. The three graphites were good substrates for stable SEI formation, exhibited by high thermal stability after being cycled at room temperature. After cycling with a temperature program, subjecting the cells to temperatures between 0 and 40°C, the thermal stability was generally reduced. This was attributed to increased SEI formation. The properties of both the electrolyte and graphite influenced the SEI and consequent thermal stability, though in different ways.The cell capacity was considerably reduced upon cycling at lower temperatures, such as 10 and 0°C. The results indicate that the electrolyte properties, particularly the viscosity and resulting conductivity, played the most important role in determining the cell performance. Low viscosity electrolyte components should be utilised, maintaining the electrolyte conductivity even at reduced temperatures. The graphite properties did not influence the cell performance at the temperatures studied. Advice is given on which electrolyte components should be avoided to build Li-ion cells performing acceptably at temperatures from 0 to 40°C.

High purity silicon material in solar cell fabrication constitutes 40% of the total cost for conventional solar cell production. One approach to reduce costs would be to use less of this expensive silicon by making thin film solar cells and use a cheaper substrate as mechanical carrier.In this work the main objective has been to manufacture silicon substrates from powder by hot-pressing. The effect of the sintering parameters has been characterized. A secondary objective was to look at the possibility to achieve larger grains by recrystallization.Samples processed by hot-pressing silicon powder of metallurgical grade with varying temperatures (1200-1375 °C), pressures (30-50 MPa) and sintering time (30-60 min) has been carried out. Halogen lamps were used for heat treatment for specific samples after hot-pressing. Microstructure and porosity were characterized using optical and electronic microscopy. EBSD was used to determine the grain size and grain orientation. The density was determined by Archimedes’ method. Resistivity was measured by a conductive probe.Densities higher than 90 % were obtained at high temperatures and pressures. The time conducted at maximum temperature during hot-pressing was not of vital importance with respect to density.The mean particle size of the powder was determined to ~20 μm, while hot-pressed samples had an average grain size of ~30 μm. The samples showed low resistivity due to high impurities of the silicon powder. High surface porosity was found for the less dense samples. Recrystallization was successfully achieved for the sample hot-pressed at 1350 °C, 30 MPa and 30 min, resulting in elimination of pores and significant grain growth from 31,83 to 56,96 μm.Characterizations of the hot-pressed samples are limited to the methods and techniques described above.

Hydrogen transport membranes have gained interest from industry as an alternative to pressure swing adsorption or cryogenic distillation, and are aimed to reduce cost, equipment size, energy consumption and waste generation. Doped strontium cerate stabilized with zirconium exhibit high proton conductivity and chemical stability and is therefore considered to be a promising material for hydrogen transport membranes.The purpose of the work was to investigate if phase pure strontium cerates, stabilized with zirconium and doped with thulium, ytterbium or yttrium, could be synthesized by the Pechini method, and if the membranes exhibited sufficient density. Determination of optimal calcination and sintering parameters was also important parts in the study.SrCe0.75Zr0.20M0.05O3-d, (M=Tm,Y,Yb) were synthesized via the Pechini method, followed by calcination, pressing of green bodies and sintering of dense membranes. Phase purity of powders and sintered membranes was examined by X-ray diffraction. Surface investigation and microstructure was investigated in a scanning electron microscope. Sintering behavior and thermal expansion coefficients was determined by dilatometry. Phase pure and dense orthorhombic perovskite structured SrCe0.75Zr0.20M0.05O3-d;, (M=Tm,Y,Yb) membranes, were obtained by powder calcination at 1000 C, followed by milling and conventional sintering at 1500-1600 C. SrCe0.75Zr0.20Tm0.05O3-d demonstrated the highest density of 98,6%, when sintered at 1500 C. Sintered membranes had a average grain size in the range from 3,2 – 4,9 um. Ball milling is concluded to be of vital importance to obtain sufficient density in the membranes.The membrane characterization is limited to the methods and techniques described above. Hydrogen flux across the membrane, total conductivity, stability in reducing atmosphere and thorough investigation of thermal properties are recommended for further work.

The main object was to investigate the effect of donors, thermal donors and defects on the lifetime on n-type Czochralsky (Cz) silicon. Cz is a technique for making monocrystals by dipping a monocrystalline seed into the melt, and pulling the ingot up as the melt solidifies. Samples were prepared by sawing, grinding and polishing. Copper decoration and preferential etching was done to reveal the defects. Resistivity measurements were performed to investigate the donor distribution. The lifetime and oxygen concentration was measured.The lifetime may be influenced by donors and traps. Traps may be metallic or non-metallic. Dopants and metal contaminants are usually Scheil distributed, which typically involves a relatively stable distribution in most part of the ingot, and a rapid increase in concentration close to the bottom. Metal contaminants may be introduced from the feedstock, and dopants (either n or p dopants) are added to the melt.Oxygen is dissolved from the crucible. The oxygen concentration typically decreases with increasing length, and is controlled by the rotation speed. This was confirmed by the interstitial oxygen measurements. The oxygen concentration showed no change after heat treatment, but the presence of oxygen in itself may contribute to produce lifetime reducing agents, such as precipitates and thermal donors. Thermal donors were found in the first 18cm of the ingot. Both the resistivity and lifetime measurements was used to calculate estimated lifetimes and donor concentrations, respectively. This estimated values were compared with the measured ones, revealing that the lifetime in the ingot was SRH dominated. Thermal donors were found to have less influence on the lifetime than the trapping. Oxygen precipitations may explain the low lifetime found in the center of the ingot up to about 30cm from the crown. Such precipitations are often gathered in defect bands in the sample. CDI of a Cu decorated sample revealed a pattern of different defect densities, which also showed up at the surface of the sample after preferential etching. An H- or L- band was believed to be situated here, which both consist of small oxygen precipitates. Defects arize as a funktion of the pulling parameters, and a low growth rate was connected to the low lifetime here. Indications of the presence of non-metallic traps were found throughout the ingot. However, oxygen precipitates could not explain the non-metallic traps that was found on the outside of the ingot, and the cause of these is therefore unknown. It is however likely that these traps may be connected to the pullrate/temperature gradient-ratio. Most likely metallic traps were also present. It is uncertain whether the non-metallic or metallic traps were the most important to the lifetime of this ingot, as the relative distribution of the two cannot be decided with certainty from the results in this thesis.Contrasts in CDI and steep curves using PCD measurements vs length plots were believed to be due to the lifetime degrading agent that changes the fastest. Metallic traps and the phosphorous dopant are known to be Scheil distributed, hence they appear transparent for the lifetime measurements in areas where the distribution is stable. Thermal donors and non-metallic traps are thus more likely to create contrast in the top parts of an ingot. However, it is not necessarily the phenomenon that causes the contrast that is the major lifetime degradation contributor.

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.

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