[dipl] Perustieteiden korkeakoulu / SCI

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3-D Motion-Tracking for High-Intensity Focused Ultrasound Therapy
(2012) Suomi, Visa
Perustieteiden korkeakoulu | Master's thesis
Respiratory motion during high-intensity focused ultrasound (HIFU) therapy reduces the efficiency of the treatment in abdominal organs. Hence, for the purpose of motion-correction during HIFU therapy, two different ultrasound-based 3-D motion-tracking methods were presented and adapted to the existing Philips Sonalleve MR-HIFU platform. The displacement estimation accuracies of these two techniques were determined using a metal pin and an in vitro tissue sample as targets. The measurement data was collected on all movement directions using element clusters consisting of one, three and 32 transmitting transducer elements. Simulations of the acoustic fields were also performed in order to discuss the theoretical limitations of the motion-tracking with the existing HIFU system. The displacement estimation accuracies did not differ significantly for the two different techniques introduced but was rather dependent on the transmitting element cluster size and the orientation of the ultrasound beam axis. Using smaller number of elements and defining the beam axis angle accurately in the calculation algorithm yielded more stable results. The motion-tracking using the in vitro tissue sample was significantly more difficult to achieve than with the metal pin target. This was due to the incorrect time-shift values given by the cross-correlation algorithm. Hence, the failed channels had to be manually excluded from the calculations in order to yield the correct displacement estimation values. The introduced motion-tracking methods cannot be readily used as such during clinical HIFU therapy treatment, because the incorrect time-shift values had to be manually excluded from the calculation algorithm. This process could probably be automated by observing the peak count and the amplitude of the cross-correlation curves. However, this method was not verified in this thesis and hence requires further research about its feasibility.
Adsorption of maleic anhydride on platinum (111) and palladium (110) surfaces
(2012) Heikkinen, Olli Veikko
School of Science | Master's thesis
Maleic anhydride (C₄H₂O₃) is an organic, planar and cyclic molecule. At room temperature it is a solid and crystalline substance that can take part in many reactions and form various compounds. The most important of them is maleic acid, formed by dissolving maleic anhydride into water. Maleic anhydride has industrial and commercial potential and it has already been used for instance in production of pharmaceuticals, agricultural chemicals and polyester resins. In this thesis, adsorption and desorption of maleic anhydride has been studied by using two different single crystal substrates. Platinum (111) is a hexagonal surface lattice and palladium (110) a rectangular one. There are no published reports on maleic anhydride adsorption on noble metal single crystal substrates, although the phenomenon has been studied with a few oxygen and silicon surfaces. Both experimental and computational studies are available. Adsorption of maleic anhydride in the experiments reported in this thesis has been carried out in an ultrahigh vacuum chamber with a base pressure of 5 x 10^-10 millibars. Chemical vapour depostion has been used as the deposition technique. Solid maleic anhydride has been evaporated and the vapour has been introduced to the vacuum chamber via a leak valve. The incoming maleic anhydride molecules have been directed towards the single crystal surfaces which have been at a temperature of 190 kelvins. Three measurement techiques have been applied to study the sample. The relative proportions of different compounds on the sample surfaces as a function of temperature and dosage have been measured with X-ray photoelectron spectroscopy. Desorption of molecules from the surfaces as a function of temperature has been measured with temperature programmed desorption (or thermal desorption spectroscopy). The crystallographic superstructures of the adsorbed molecules has been studied with low-energy electron diffraction. By the experimental results a model has been proposed to show the nature of desorption of maleic anhydride molecules from the surfaces.
Aerosol synthesis of single-walled carbon nanotube thin films using acetylene as carbon precursor
(2013) Parjanne, Joonas
School of Science | Master's thesis
Single-walled carbon nanotubes (SWCNTs) are tubular arrangements of sp² hybridized carbon atoms. Their unique structure leads to extraordinary electronic and optical properties that depend sensitively on their geometry. The extraordinary electronic, optical and mechanical properties of SWCNTs make them a potential material for conducting, transparent and flexible thin films. These films can be used in various optoelectronic devices such as touchscreens, solar cells and thermo-acoustic loudspeakers. Previous experiments and computational simulations have shown that the sheet conductivity of the thin films is proportional to the nanotube mean bundle length. Therefore, the synthesis of the films should be developed to be able to grow longer SWCNTs. One way to attempt this is to study the use of alternative, more reactive carbon precursors in the synthesis of SWCNT thin films. The aim of this work was to synthesize SWCNT thin films by an aerosol method in a ferrocene reactor using acetylene as the carbon precursor, study how the performance of the films depends on the synthesis parameters, and search for optimal synthesis conditions. Acetylene was used since it is a very reactive hydrocarbon due to its triple bond and could thus enable a higher growth rate and produce longer SWCNTs. In this work, SWCNT thin films were synthesized from acetylene in various conditions and characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), optical absorption spectroscopy (OAS), four-point probe and Raman spectroscopy. The best films produced in this work were synthesized with hydrogen and by diluting ferrocene. Their performance was as good as that of the best thin films produced earlier with a similar setup but using carbon monoxide as the precursor. However, electron microscopy showed that the bundle lengths were much higher with acetylene and there are large amounts of impurities that may lower the performance below the value expected based on the bundle length. After washing the films with toluene, their performance (0.9 komega sheet resistance at 90 % transmittance) was similar to the films produced previously using CO in a scale-up reactor (0.8 komega at 90 %). In the future, this performance improvement may be reached without toluene if the impurities can be etched by CO₂.
An algorithm for opposite side flavor tagging in the B_s -> J/vø decay
(2012) Visuri, Anne-Maria
School of Science | Master's thesis
The flavor of the oscillating B₈ meson at the time it is produced is an essential input to the measurement of the CP-violating phase phi₈ in the B₈ -> J/psi(-> µ⁺µ^-)phi(-> K⁺K^-) decay. The final state is the same for both the particle and antiparticle, and therefore flavor tagging methods are required for determining the initial flavor. Opposite side tagging can be done using the charge of leptons or the combined charge of a secondary vertex produced in the decay of the b-hadron on the opposite side. In this work, these methods are investigated using simulated data of the signal decay. The muons and electrons are selected on the basis of high transverse momentum, and the secondary vertex on the basis of high chi² vertex probability. A tagging algorithm is implemented within the CMS software framework, using the different tags in a decision-tree manner. The highest tagging power, (0:5 +- 0:3) %, is obtained by using the muon tag alone, and using the electron or secondary vertex tag does not improve the tagging power. The efficiency of the muon tag is (3:5 +- 0:2) % and dilution 0:38 +- 0:12. The result obtained here are preliminary and further optimization using real data with a reference decay is needed to make the algorithm efficient enough for tagged B₈ -> J/psi phi analyses.
Analysis of Unpublished High-Resolution Martian Meteorological Data from Viking Landers 1 and 2
(2011) Kemppinen, Osku
School of Science | Master's thesis
Application of Database in Nuclear Material Verication Measurement Program
(2010) Kuusi, Antero
School of Science | Master's thesis
The goal of the present Master's thesis is to develop a database system for use with nuclear material verification measurements. Nuclear material verification measurements have long been an important part of international nuclear safeguards. However, so far, most measurement results are used only once. The results, once analysed, are not used later - and often even not stored systematically - even though they could be useful in later verification activities. Thus, a database system which would allow storing, reviewing and using the results later for comparison would enhance the value of the obtained results. As spent nuclear fuel assemblies are placed in final repository, where they cannot be reached, the systemic storage of results becomes even more important. After this disposal, the historical measurements are the only verification information available for the nuclear material records. However, the long time lines of final disposal - the final repository is expected to function for more than hundred years - mean that any system storing information on historical measurements has to be designed to function centuries, which is much longer period than what is typically used as basis in the design of corresponding systems. The requirements that are placed on a database system that is designed to store these measurement results by the measurement systems themselves and by the long required operational time are examined in this thesis. Based on these requirements, a database is developed and implemented, and the first experiences are examined and resulting areas of improvement are identified.
Assessment of Thorium Fuel Utilization in an Equilibrium Cycle BWR
(2011) Vanhanen, Risto
School of Science | Master's thesis
The task of this study is to assess whether it is possible and feasible to utilize thorium as nuclear fuel in current BWRs from technical point of view. Lattice calculation code CASMO-4E is used as a part of the analysis chain. The code is based on nuclear reactor theory and should be able to model both uranium and thorium fuels. The capability is assessed by comparing few-group constants generated by CASMO-4E and a Monte Carlo based lattice calculation code Serpent. ENDF/B-VI based libraries are used. The results show that the di_erences in few-group constants between the codes remain at similar level when similar uranium and thorium fueled assemblies are used. Basic properties of thorium fuels are studied along with nuclear fuel assembly design. CASMO-4E is used to design _ve di_erent thorium assemblies. Three of them are high thorium content assemblies and two low thorium content assemblies. CASMO-4E is also used to generate few-group cross section libraries for full core simulations. The parameters from design phase are also compared to a reference uranium assembly. Considering the basic nuclear properties the results are expected. A single assembly type BWR core running an equilibrium cycle is modeled using SIMULATE-3. Each design is compared to the reference uranium design. Considered quantities are thermal limits, shutdown margin, discharge burnups, reactivity feedbacks and resource utilization. Thorium designs improved on some quantities but no thorium design could improve all considered quantities. It is shown that it is possible to use thorium fuels from the neutronics point of view in current BWRs. There are some advantages over uranium fuels. Thorium fuels could excel when it is possible to reach higher burnups.
Automated Testing of Solid Oxide Fuel Cell Units' Control System
(2011) Neuvonen, Lauri Johannes
School of Science | Master's thesis
This thesis presents a testing protocol and a testing platform for the purpose of automatically testing the control unit of a solid oxide fuel cell system. The work was commissioned by Wärtsilä - a Finnish company involved in maritime solutions and distributed power generation. The protocol was constructed to ensure all necessary things are taken into account during testing. The platform provides a set of tools for realizing the tests. The different parts of the platform were taken into use and configured to work together. Three case examples of test runs were erformed to test the functionalities of the platform.
Biomimetic fibrous patterns and composite materials with nanofibrillated cellulose
(2012) Mertaniemi, Henrikki
School of Science | Master's thesis
Learning from material and structural design principles found in biological materials, it may be possible to manufacture strong and lightweight structures difficult to obtain via traditional fabrication. Biological models for such new materials include nacre and a glass sponge skeleton. The organized layered structure in nacre results in an at least hundredfold increase in toughness compared to its main constituent, the brittle mineral aragonite. The nacre structure effectively spreads stresses and damage to a wider area around a crack propagating in the material. Thus, breaking of nacre requires a large amount of energy. In the glass sponge skeleton, a biological composite material is combined with optimized structural design. The grid structure of the glass sponge appears to provide optimum mechanical performance with minimal material use. Another promising new material from a biological source is nanofibrillated cellulose. The strength of the cellulose nanofibers, together with the abundance of wood-based cellulose, makes cellulose nanofibers a promising component in novel materials. In this work, two approaches for fabricating strong and lightweight materials were explored, finding inspiration for design from biological systems and utilizing the strong natural material, nanofibrillated cellulose. First, a concept was developed to fabricate glass-sponge-mimicking structures with macroscopic fiber grids of nanofibrillated cellulose. Employing a 3D printer and principles of additive manufacturing, programmable fabrication of the grid structures was demonstrated. The macroscopic fibers prepared from nanofibrillated cellulose were found to be strong, having a tensile strength of 300 MPa. Compared to previous art, more than two times larger ultimate elongation was achieved, making the fibers remarkably tougher. Furthermore, chemically cross-linking the nanocellulose within the macroscopic fibers improved the material performance in wet environment, potentially enabling biomedical applications. 3D-printed ultra-lightweight grid structures were found to be considerably stronger in tension than in compression. Still, some of the potential strength of individual fibers was lost during the fabrication of grid structures due to defects. Further optimizing the fabrication process of the fibrous patterns, very strong and lightweight structures may be obtained. Second, a nacre-mimicking clay/polymer composite material was employed in combination with cellulose nanofibers in preparation of aerogels, i.e., gels where water is replaced with air. It was observed that in the clay/polymer/nanocellulose aerogels, clay platelets probably arrange to fill the pores between cellulose nanofibers in the sheet-like microstructure of the aerogel, making the aerogel less porous. The resulting aerogels were soft and had so far very limited mechanical strength.
Calculation of positron annihilation characteristics in InGaN alloys
(2013) Norrman, Vesa
School of Science | Master's thesis
Positron annihilation spectroscopy (PAS) is a way to get information about electronic and atomic structures of materials. By measuring lifetimes and Doppler broadening spectra of annihilating positrons it is possible to detect vacancies and characterize their chemical environments. By varying the concentration of In and Ga the band gap of InGaN alloys can be tuned to cover the whole visible spectrum of light. Hence this materials family is interesting for optoelectronic devices, such as light emitting diodes (LEDs). The quality of devices is affected by the existence of defects. In this work the electronic and atomic structure of InGaN, vacancies in InGaN, and polar InN/GaN superlattices were calculated with the local density approximation (LDA) of the density functional theyry (DFT). The wave function of the positron and PAS characteristics such as lifetime and Doppler broadening spectra were calculated for those systems. I investigated how the indium mole fraction, chemical environment of vacancies, and thickness of the InN layer affect positron annihilation characteristics. The InN/GaN superlattice was modeled by stacking 1 - 4 wurtzite unit cells of InN in the direction of the hexagonal c-axis. Because of periodic boundary conditions the layer is surrounded by GaN on both sides. There is no inversion symmetry in c-direction of InN and GaN which causes polarization in the superlattice. In calculations the positron is attracted by the InN/GaN interface. Electronic structures of superlattices were also studied with hybrid functionals which are computationally more demanding but give more accurate results.
CFD modelling of heat transfer to supercritical water in vertical pipe
(2011) Rintala, Lauri
School of Science | Master's thesis
The Supercritical Water Reactor (SCWR) is one of the fourth generation nuclear power plants. One of the major challenges in its design is heat transfer which has enhanced and deteriorated modes in addition to the normal mode. There is still too little knowledge of the different modes and their occurence and for example a decent correlation for system codes does not exist. Experiments with supercritical water are expensive and demanding and hence they are more often substituted with computational modelling. The computational model has to be validated against real experiments before using it for further research. This validation is also the main point in this thesis. The computational modelling of fluid dynamics, the governing equations and turbulence models, are shortly reviewed. The computational tool, Open- FOAM, and the modifications done to render it possible for modelling supercritical water, are presented. The experiment considered here is a vertical pipe with heated section and supercritical water flowing upwards. The results obtained with too coarse mesh are false but converge to correct results when the amount of cells in the mesh is increased. The form of the mesh affects the results slightly but with the selected mesh the results do not deviate from cylindrical symmetry. The results without gravitation are surprisingly similar to results with upward flow whereas the downward flow results are different. Three different turbulence models are used: laminar, k-epsilon- and SST. The SST model outperforms the other two and the laminar one is the worst as expected. The heat transfer deterioration, the most significant aspect of heat transfer in supercritical region, is in practice totally missed by the simulations. With further research and refining the turbulence model or even moving over to direct numerical simulation, the validation is certainly possible.
Coarse-grained modeling of fullerene-polymer blends using the MARTINI force field
(2011) Uusitalo, Jaakko
School of Science | Master's thesis
Computational studies of disordered graphene and graphene nanoribbons
(2012) Ketolainen, Tomi
School of Science | Master's thesis
Graphene is one of the most studied carbon nanomaterials today. Carbon atoms in grapheme form a hexagonal lattice that consists of only one atom layer. This structure leads to interesting electronic properties and makes it possible to examine physical phenomena not usually seen in other materials. A special feature of graphene is the linear band structure near the corners (Dirac points) of the first Brillouin zone. Moreover, graphene has no band gap and the density of states at the Fermi level vanishes. Graphene also possesses very good mechanical and optical properties, which is important from the point of view of applications. Real graphene sheets may have some disorder that results, e.g., from the fabrication method. Small point defects, such as vacancies and impurities, have been observed to induce non-zero localized states in graphene, which causes a sharp peak in the density of states near the Fermi level. In addition, the vacancies and impurities influence the electronic and transport properties of graphene nanoribbons. In this work, the local density of states is calculated in the vicinity of point defects in grapheme and zigzag graphene nanoribbons. The computations are based on a tight-binding model and the number of atoms in the systems is 4000-16800. In particular, the coupling of a single vacancy to the edge state of a zigzag nanoribbon is studied. Furthermore, the local density of states is computed in the middle of an impurity cluster placed on a graphene sheet and on a zigzag nanoribbon. Vacancies and impurities induce similar localized states near the Fermi level and the states enhance in the cluster configuration. The local density of states at the Fermi level has a peak that can split if the defect lies close to the ribbon edge. Armchair- and zigzag-shaped impurity rows on a zigzag nanoribbon are also considered. Only the zigzagshaped impurity row is found to induce a peak at the Fermi level.
Computational study of quantum dot qubits using Lagrange mesh method and exact diagonalization
(2013) Ritala, Juha
Perustieteiden korkeakoulu | Master's thesis
Quantum computation using quantum circuit model is based on quantum bits and gates, which are quantum analogues to the bits and logical gates in classical computing. The computations are carried out by performing single- and two-qubit quantum gate operations on the input qubits (quantum bits). In this thesis, a set of computational methods to study these gate operations in the case of semiconductor quantum dot based spin qubits is presented. This set consists of three parts. Lagrange mesh method is used to calculate the single-electron states in a quantum dot system. These states are then used in an exact diagonalization calculation to obtain the many-electron ground state, which is then evolved using exact diagonalization based dynamics. The presented set of methods is used to simulate single-qubit gates, and it is found to be successful for this purpose. The Lagrange mesh method is extremely versatile as it can handle an arbitrary quantum dot confinement potential without the need to calculate any integrals. This feature is achieved by approximating the potential matrix element integrals using a Gauss quadrature. The high accuracy of the Lagrange mesh method despite the seemingly crude approximation is investigated, and a reasonable cause for it in the case of low degree polynomial potentials is found. A hypothesis that the Gauss quadrature approximation is extremely accurate for an arbitrary polynomial potential is made. The convergence of the states calculated with the Lagrange mesh method is tested and compared to an alternative method based on localized Gaussian basis functions.
Computational study on nanoscale ice flows
(2011) Heinonen, Vili
School of Science | Master's thesis
Crystal ice with its multitude of different phases and exotic surface physics is complicated beyond most of the homogeneous materials. The complex nature of ice is rooted in the hydrogen bonding that is keeping the water molecules of ice in an ordered crystal structure. The fact that ice can be melted by increasing pressure gives rise to critical behaviour when ice is pushed around some obstacle as the ice undergoes a phase transition and starts to flow. Atomistic molecular dynamics (MD) simulations are a good tool to study complex systems such as flows of ice. The complex dynamics on the boundary of ice can be reconstructed as the positions and the orientations of all the molecules are known at any time. Recently developed all-atom models for water have increased the efficiency of the computation allowing simulations of large systems. This thesis consists of two parts. In the first part a flow of ice around several different types of spheres is studied using all-atom MD simulations. The focus is in a depinning transition caused by a phase transition in ice when a large enough driving force is applied. The second part proposes a continuum flow model for ice based on the idea of pressure melting of ice. The depinning transition caused by breaking layers of ice was seen to be completely different from recent studies of flows around nanowires. The flows around spheres with same radii with the wires of earlier simulations were dominated by thermal effects as the high curvature of the spheres caused the ice to flow around the spheres with great ease. The dynamics with spheres of larger sizeswere affected by large-scale fractures in the ice and single layers of ice ceased to have any real impact on the dynamics. The continuum flow model was dominated by effects of Newtonian flows and was seen unfit for ice flows.
Conductive atomic force microscopy studies and computational modelling of single-walled carbon nanotube thin films
(2012) Laiho, Patrik
School of Science | Master's thesis
Transparent, conductive thin films are a key component of devices such as thin film displays and photovoltaic cells. Thin films of single-walled carbon nanotubes (SWCNTs) are a promising candidate material for replacing or supplementing currently-used metal oxides such as indium tin oxide (ITO). While SWCNT thin films are and have been a topic of active research, the microscopic basis of electrical conduction in SWCNT thin films is understood relatively poorly and a better understanding of it could help enhance film properties. In this work, electrical conduction in SWCNT thin films prepared by an aerosol synthesis method was studied by conducting atomic force microscopy (C-AFM) of the junctions between SWCNTs and their bundles, which are expected to be the dominant source of electrical resistance in the films. The C-AFM data has been used, together with an experimental estimate of the relationship between film density and optical transmittance, in a substitute circuit model to simulate the bulk electrical and optical properties of thin films with different morphologies. Results from the computational model were found to be within the experimental errors of properties measured from thin films, and they can be used in judging which controllable parameters in SWCNT thin film preparation should be changed to obtain the largest enhancement in thin film performance.
Considerations of factors in the front end of the nuclear fuel cycle
(2011) Rantala, Tuomas
School of Science | Master's thesis
This thesis examines the whole front end of the nuclear fuel cycle. The first focus of the thesis is ill exploring literature. The relevance and inevitability of nuclear energy is introduced. All the procurement steps are discussed at first separately examining history, techniques, physics and markets, and secondly together trying to figure the balance between supply and demand in the history and in the future scenarios by underlining eight factors that have an effect on the balance. The eight factors are divided into two categories; four quantitative primary factors and four factors affected by political and technical issues. The second focus of the thesis is to estimate the development of nuclear fuel costs based on calculated and simulated data, and data from the literature, and to create scenarios for the future. The price of nuclear fuel cumulates during the front end steps. The front end step costs are discussed separately and combined. The bases for future scenarios are extrapolations of historical discharge burn-up evolution in LWR reactors, presented by OECD/NEA, and estimated connection between average discharge burn-up and average enrichment assay. The effect of variations in the front end step prices is compared to the effect of other factors, which have effects on the fuel consumption. It is found that a 10 % increase in uranium concentrate and enrichment prices has an effect of circa 4 % on the total fuel procurement price, respectively, but an effect of natural uranium conversion price increase is only circa 0.3 %. The scenario predicts that the fuel cost proportion in electricity production with nuclear power in 2025 is between 6.15 $/MWh_e and 8.72 $/MWh_e (circa 7.05 $/MWh_e in 2010).
Designing and implementing a temperature solver routine for Serpent
(2012) Valtavirta, Ville
School of Science | Master's thesis
The goal of this thesis was the design and implementation of a temperature solver routine for the Monte Carlo neutronics code Serpent 2. The routine is used in conjunction with the on-the-fly Doppler processor already found in Serpent 2 to provide high resolution temperature discretization with minimal memory usage. This leads, however, to a significant increase in the computation time needed for simulations. The current routine addresses the radial heat transfer in fresh 2D pin-geometries. The consideration of 3D-problems and addressing the effect of the burnup on the problem are, however, deemed to be realistic directions of future development for the routine. Likewise, the temperature distribution could also be solved for non-pin-like geometries as long as certain simplifications such as symmetry considerations can be applied. A thorough literature review was made for choosing default internal material property correlations for the routine and an integral comparison was performed against the fuel code femaxi-6. The correspondence between the results was found to be good. Two assembly level calculations verified the capability of the code to handle problems while monitoring various fuel pins of different compositions. The simulations also gave quantitative results on the differences between using the temperature feedback system and a single homogeneous volume-averaged temperature for the fuel part of the bundle combined with Doppler pre-processing. At the time of the completion of this thesis, the routine seems very promising, although it does still require a significant amount of development and validation in order to unleash its full capabilities.
Development of laser-induced breakdown spectroscopy for analyzing deposited layers in ITER
(2013) Karhunen, Juuso
School of Science | Master's thesis
Formation of different mixed deposited layers can degrade the performance level of plasma-facing components and enhance fuel retention in future fusion reactors. Thus, first walls of the reactors should be actively monitored in between plasma discharges to determine the locations, thicknesses, and material compositions of the mixed layers. Laser-Induced Breakdown Spectroscopy (LIBS), where the spectrum emitted in a laser-induced ablation process is studied, is a potential method for such in-situ studies. In this thesis, the feasibility of LIBS on analyzing ITER-relevant material mixtures was tested. The results were compared with data obtained by the standard ion-beam methods Rutherford backscattering spectroscopy (RBS), nuclear reaction analysis (NRA), and secondary ion mass spectrometry (SIMS). For the experiments, various 500-2500-nm thick coatings with different Be/W ratios were produced by the TVA method at MEdC, Romania. To investigate fuel retention in these coatings, part of the samples were doped with deuterium during their deposition, and some samples were implanted with 200-eV deuterons at IPP Garching. By studying the evolution of intensities of selected spectral lines, the depth profiles of Be and W were found to match with the SIMS profiles. The elemental composition of the mixed layers was determined using the Calibration-Free LIBS method (CF-LIBS), and the results agreed well with those obtained by RBS. Only the implanted coatings contained enough deuterium for reliable analysis. Due to limitations in the laser fluence, the retention studies in this work were done just by using SIMS and NRA. The results indicated that the implantation depth of D is independent of the thickness of the coating but depends on its composition: in pure Be coatings, the D signal vanished after 300 nm, whereas D was present throughout the Be-W layers. The ablation rate of the mixed layers was found to be slightly higher than those of pure Be coatings, indicating a degraded structure of the W-rich alloys compared to bulk Be. For the D-implanted samples, the ablation rates were further increased due to lattice defects caused by D bombardment. The results show that LIBS is a feasible method for both qualitative and quantitative in-situ studies of mixed deposited layers in fusion devices. However, in future studies a larger laser fluence is required for detection of deuterium.
Development of thin film and crystalline silicon solar cells with advanced light management based on plasmonic and dielectric nanostructures
(2012) Salpakari, Jyri
School of Science | Master's thesis
In this thesis, light scattering plasmonic nanostructures were studied as light management techniques in silicon solar cells with an experiment and a literature review, and solar cell designs was developed for future research on plasmonic and dielectric nanostructures. The broad literature review on the state of the art of plasmonic scatterers revealed that such nanostructures on the front side of Si solar cells have been shown to be superior to flat cells with an optimized anti-reflection coating, but the use of texturing has not been outperformed. Plasmonic nanostructures at the back, combined with Mie scatterers at the front side, have been shown to outperform the Asahi U texture for ultra-thin film a-Si solar cells. The effect of computationally optimized arrays of Ag and Al nanoparticles on the front surface of thin-film a-Si:H solar cells on the photovoltaic performance of the device was studied experimentally. The nanostructures enhanced EQE at long wavelengths, and decreased at short. The Ag structure enhanced JSC by 4.6 % compared to the best measured flat cell structure with an anti-reflection coating, and the Al structure by 3.4 %. To the best of the author's knowledge, an experimental demonstration of performance enhancement with an Al nanostructure has not been presented thus far in the literature. Crystalline Si solar cell designs were developed for experiments with metallic and dielectric photonic nanostructures. The designs are based on the IBC Si-HJ architecture and processed on mirror-polished FZ mono-Si wafers, with surface passivation implemented with SiNx for the cells with metallic nanoparticles and a-Si:H for the cells with dielectric nanoparticles.

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