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Browsing by master's degree program "Magisterprogrammet i materialforskning"

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  • Laakso, Jarno (2021)
    Halide perovskites are a promising materials class for solar energy production. The photovoltaic efficiency of halide perovskites is remarkable but their toxicity and instability have prevented commercialization. These problems could be addressed through compositional engineering in the halide perovskite materials space but the number of different materials that would need to be considered is too large for conventional experimental and computational methods. Machine learning can be used to accelerate computations to the level that is required for this task. In this thesis I present a machine learning approach for compositional exploration and apply it to the composite halide perovskite CsPb(Cl, Br)3 . I used data from density functional theory (DFT) calculations to train a machine learning model based on kernel ridge regression with the many-body tensor representation for the atomic structure. The trained model was then applied to predict the decomposition energies of CsPb(Cl, Br)3 materials from their atomic structure. The main part of my work was to derive and implement gradients for the machine learning model to facilitate efficient structure optimization. I tested the machine learning model by comparing its decomposition energy predictions to DFT calculations. The prediction accuracy was under 0.12 meV per atom and the prediction time was five orders of magnitude faster than DFT. I also used the model to optimize CsPb(Cl, Br)3 structures. Reasonable structures were obtained, but the accuracy was qualitative. Analysis on the results of the structural optimizations exposed shortcomings in the approach, providing important insight for future improvements. Overall, this project makes a successful step towards the discovery of novel perovskite materials with designer properties for future solar cell applications.
  • Lindblom, Otto (2020)
    Due to its exceptional thermal properties and irradiation resistance, tungsten is the material of choice for critical plasma-facing components in many leading thermonuclear fusion projects. Owing to the natural retention of hydrogen isotopes in materials such as tungsten, the safety of a fusion device depends heavily on the inventory of radioactive tritium in its plasma-facing components. The proposed methods of tritium removal typically include thermal treatment of massive metal structures for prolonged timescales. A novel way to either shorten the treatment times or lower the required temperatures is based performing the removal under an H-2 atmosphere, effectively exchanging the trapped tritium for non-radioactive protium. In this thesis, we employ molecular dynamics simulations to study the mechanism of hydrogen isotope exchange in vacancy, dislocation and grain boundary type defects in tungsten. By comparing the results to simulations of purely diffusion-based tritium removal methods, we establish that hydrogen isotope exchange indeed facilitates faster removal of tritium for all studied defect types at temperatures of 500 K and above. The fastest removal, when normalising based on the initial occupation of the defect, is shown to occur in vacancies and the slowest in grain boundaries. Through an atom level study of the mechanism, we are able to verify that tritium removal using isotope exchange depends on keeping the defect saturated with hydrogen. This study also works to show that molecular dynamics indeed is a valid tool for studying tritium removal and isotope exchange in general. Using small system sizes and spatially-parallelised simulation tools, we have managed to model isotope exchange for timescales extending from hundreds of nanoseconds up to several microseconds.
  • Flinck, Oliver (2022)
    In this thesis, sputtering of several low- and high-index tungsten surface crystal directions are investigated. The molecular dynamics study is conducted using the primary knock-on atom method, which allows for an equal energy deposition for all surface orientations. The energy is introduced into the system on two different depths, on the surface and on a depth of 1 nm. Additionally to the sputtering yield of each surface orientation, the underlying sputtering process is investigated. Amorphous target materials are often used to compare sputtering yields of polycrystalline materials with simulations. Therefore, an amorphous surface is also investigated to compare it's sputtering yield and process with crystalline surface orientations. When the primary knock-on atom was placed on the surface all surface orientations had a cosine shaped angular distribution with little variation in the sputtering yield for most of the surface orientations. Linear collision sequences were observed to have a large impact on the sputtering yield when the energy was introduced deeper inside the material. In these linear collision sequences the recoils are traveling along the most close packed atom rows in the material. The distance from the origin of the collision cascade to the surface in the direction of the most close packed row is therefore crucial for the sputtering yield of the surface. Surface directions with high angles between this direction and the surface normal hence show a reduction in the sputtering yield. The amorphous material had a little lower sputtering yield than the crystalline materials when the primary knock-on atoms was placed on the surface whereas the difference rose into several orders of magnitude when the energy was given at 1 nm. It is impossible for linear collision sequences to propagate long distances in the amorphous material and therefore the angular distribution in both cases is cosine shaped. The amorphous material has no long range order and was therefore unable to reproduce the linear collision sequences, which are characteristic for the crystalline materials. The difference in the sputtering yield was hence up to several orders of magnitude as a result when the energy was introduced at 1 nm depth.
  • Välinen, Lauri (2023)
    Emulsion polymerization is used to make high molecular weight polymers with a fast reaction rate. In emulsion, the temperature is well controlled and the viscosity of the continuous phase remains constant since all polymer chains are inside colloidal particles. Colloid dispersions have the advantage of being used as they are without further purification, which is great for industrial purposes. Emulsion polymerization is also well-scalable to fit the standards of the industry. Adhesives serve an important role in the furniture and construction industry. Many adhesives used for such purposes are derived from non-renewable resources and are not reusable. Additionally, when such strong adhesives are being used in attaching wooden parts, they cannot be separated and once the lifetime of the product is finished, it ends in a landfill. The possibility to remove such strong adhesives from the wooden product would give the wood possibility to be used in other applications. Additionally, the possibility to reapply the adhesive would decrease the amount of adhesive needed to be produced and increase the lifetime of the glue product. In this thesis polyvinyl acetate (PVAc) adhesives are modified by introducing hydrogen bonding units to the polymer chain by copolymerization of vinyl acetate with monomers having urea and bis-urea hydrogen bonding motifs. Comonomers suitable for vinyl acetate are designed, synthesized and characterized.
  • Tavaststjerna, Miisa (2020)
    Further proof of the unique morphologies of water-soluble poly(2-isopropyl-2-oxazoline)-block-poly(DL-lactide) and poly(2-isopropyl-2-oxazoline)-block-poly(L-lactide) (PiPOx-b-PDLLA and PiPOx-b-PLLA) nanoparticles was obtained via Fluorescence Spectroscopy. Additionally, loading and release studies were carried out with hydrophobic curcumin molecules to outline the potential of the amphiphilic block copolymers in drug delivery applications. To study the morphology of the nanoparticles, absorption and emission spectra of pyrene were measured in water dispersions of the nanoparticles at several concentrations. The obtained I1/I3, I337/I333.5 and partitioning constant (Kv) values were compared to corresponding data from a control core/shell nanoparticle poly(ethylene glycol)-block-poly(DL-lactide) (PEG-b-PDLLA). Of the three different amphiphilic polymers, PEG-b-PDLLA showed the smallest and PiPOx-b-PDLLA the highest Kv value. This indicates, that PiPOx-b-PDLLA core is less hydrophobic and looser compared to the dense cores of PEG-b-PDLLA and PiPOx-b-PLLA, making it capable of encapsulating the greatest amount of pyrene. In the loading and release studies, the nanoparticles were loaded with curcumin and placed in dialysis against PBS Tween® 80 solution. Curcumin content of the samples was monitored over a week by measuring the emission spectra of curcumin. PiPOx-b-PDLLA showed greater potential as a drug delivery agent: It formed more stable nanoparticles, showed higher loading capacities, higher encapsulation efficiencies and slower release rates. Flash nanoprecipitation method (FNP) was also used to prepare the same nanoparticles with and without encapsulated curcumin. In addition to the encapsulation efficiencies, sizes of the nanoparticles were determined via dynamic light scattering (DLS). PiPOx-b-PLLA forms the smallest nanoparticles with lowest encapsulation efficiencies, thus agreeing well with the higher density of PLLA core. All three investigated amphiphilic copolymers formed stable nanoparticles in water at room temperature. On the contrary, stability of the nanoparticles was found to be poor in saline solutions at body temperature. Mixing PEG-b-PDLLA with PiPOx-b-PLA in a ratio of 20:80 w-% increased the stability of the nanoparticles in physiological conditions simultaneously uncovering the thermoresponsive character of the PiPOx-blocks. Turbidity measurements of PEG-b-PDLLA mixed with PiPOx-b-PDLLA in ratio of 20:80 w-% showed slight decrease in transmittance at the 30 °C, which corresponds to the cloud point of PiPOx-b-PDLLA in PBS solution. However, it remains unclear, whether the increased stability is due to the PEG-b-PDLLA mixing in the same micelles with PiPOx-b-PDLLA, thus hindering the aggregation of the nanoparticles upon the cloud point of the PiPOx-blocks.
  • Petrow, Pauliina (2022)
    Acuros XB -annoslaskentamalli on Varianin Eclipse-annossuunnitteluohjelmistoon kehitetty annoslaskenta-algoritmi, joka on tarkoitettu ulkoisen sädehoidon fotoniannoslaskentaan. Sen toiminta perustuu lineaaristen Boltzmannin siirtoyhtälöiden ratkaisemiseen. Acuros XB -algoritmi laskee annoksen kudoksessa, minkä johdosta se pystyy tuottamaan tarkan laskennan myös heterogeenisessa väliaineessa. Algoritmin laskentatarkkuus on verrattavissa Monte Carlo -menetelmän tarkkuuteen, mutta tarvittava laskenta-aika on lyhyempi. Tässä tutkimuksessa konfiguroitiin ja testattiin Acuros XB -algoritmi HUS Syöpäkeskuksen sädehoito-osaston lineaarikiihdyttimille. Lisäksi tutkittiin kahden mittalaitteen, OCTAVIUS Detector 1000 SRS -ionisaatiokammiomatriisin ja SRS MapCHECK -puolijohdeilmaisimen, toimintaa pienillä säteilykentillä eli 2cm x 2cm ja tätä pienemmillä säteilykentillä. Acuros XB -algoritmi konfiguroitiin optimoimalla kolmea parametria: fokuspisteen kokoa, moniliuskarajoittimen dosimetrista aukkoa ja moniliuskarajoittimen läpäisykerrointa. Parametrien optimointi tehtiin vertailemalla mitattuja ja Acuros XB -algoritmilla laskettuja annosjakaumia ja säteilykentän keskiakseliannoksia. Konfigurointiprosessissa huomioitiin erilaiset kenttäkoot ja annossuunnittelutekniikat tutkimalla erikokoisia staattisia säteilykenttiä sekä IMRT (Intensity-Modulated Radiation Therapy)-, VMAT (Volumetric Modulated Arc Therapy)- ja SRS (Stereotactic Radiosurgery)-tekniikoilla toteutettuja kenttäjärjestelyjä. Mittalaitteita vertailtiin tutkimalla kolmen pienen staattisen kentän, kolmen dynaamisen kaarikentän ja kolmen VMAT-tekniikalla toteutetun kenttäjärjestelyn tuottamia annosjakaumia. Acuros XB -algoritmin konfiguroinnin tuloksena HUS Syöpäkeskuksen sädehoito-osaston lineaarikiihdyttimille määritettiin fotonienergiakohtaiset arvot fokuspisteen koolle, moniliuskarajoittimen dosimetriselle aukolle ja moniliuskarajoittimen läpäisykertoimelle, ja algoritmi otettiin kliiniseen käyttöön. Mittalaitteiden vertailun tuloksena havaittiin, että SRS MapCHECK -puolijohdeilmaisin toimi pienillä säteilykentillä paremmin kuin OCTAVIUS Detector 1000 SRS -ionisaatiokammiomatriisi. Mittalaitteiden annosjakaumien analysointiohjelmat kuitenkin käsittelevät mittaustulokset eri tavoin, joten lisätutkimusta voidaan tarvita yksiselitteisen tuloksen saamiseksi.
  • Lasonen, Valtteri (2022)
    Modern semiconductor devices require sophisticated patterning techniques that not only offer excellent resolution but also high throughput, low cost, and low number of errors. And because these devices require several patterning steps, even a slight improvement in a patterning technique can have a huge impact. New patterning technique that has a great potential to be used in many of these patterning steps is area-selective etching of polymers by catalytic decomposition. The catalytic effect can either be an intrinsic property of the underlying material, or materials can be catalytically activated/deactivated to achieve the desired pattern. This new technique is self-aligning and extremely simple, and therefore has a potential to significantly reduce the number of errors and cost, while having excellent resolution and throughput. In the literature review part of this thesis, we will have an overview of different aspects that must be considered when using polymers as thermocatalytically decomposable resists. Polymers are already widely used as resists in several patterning techniques due to an immense number of different polymers available, allowing almost endless possibilities to adjust the properties of the resist. Important polymer properties to consider include adequate gas permeability for the etching gases and the decomposition products, decomposition and degradation mechanisms, reflow, integrity during the patterning and the deposition processes, and adhesion to the substrate. Different catalysts and catalytic decomposition mechanisms of polymers as well as other carbon-containing compounds in different atmospheres are reviewed. Because area-selective etching of polymers is a new technique many challenges are still unknown. Therefore, this thesis is mainly aimed to give ideas and directions for the future research. In the experimental part, several metals and metal oxides were tested for their catalytic effect for decomposing poly(methyl methacrylate) (PMMA) in air and H2-atmosphere. Pt, Ti, and CeO2 were confirmed to have a catalytic effect in air, whereas SiO2 and Al2O3 showed no catalytic effect. In the H2-atmosphere, only Ti and Cu showed some promising catalytic effect, whereas SiO2, Al2O3, CeO2, Pt, W, Ni, and Co did not. Additionally, experiments were conducted to find out how thin CeO2 film has an adequate catalytic effect. And finally, the area-selectivity of this patterning technique was tested in the air atmosphere using CeO2 as a catalytic surface and Al2O3 as a non-catalytic surface.
  • Ikaheimonen, Patrik (2023)
    In order to streamline the diagnosis process and increase the accessibility of both magnetic resonance imaging (MRI) and magnetoencephalocraphy (MEG), the department of Neuroscience and Biomedical Engineering at Aalto University is developing a new hybrid MEG--MRI device capable of performing both of these imaging methods. Both methods are measured using an array of Superconducting QUantum Interference Device (SQUID) magnetometers, which are extremely sensitive detectors capable of detecting the small magnetic fields generated by the human brain. However, the changing magnetic fields utilized in the MRI implementation cause eddy currents in the magnetically shielded walls of the MEG--MRI room, causing artefacts in the measured signals. In order to nullify these currents, an additional magnetic field of a specifically designed pulse waveform is fed into the room in a new technique called Dynamical Coupling for Additional dimeNsions (DynaCAN). To help in the development of DynaCAN, a program designed to detect and flag SQUIDs which saturate due to various different reasons, such as the induced eddy current fields, has been created and is presented in this work. This program finds saturated and faulty signals using four different Fault Detection Filters (FDFs) and tests if they are physically consistent with the signals of their neighboring detectors using Consistency Analysis (CA). It was found that the FDFs are able to find unambiguously faulty signals repeatably, while CA was more unreliable and was very susceptible to bad data present in the data set it was analysing.
  • Ojala, Juha (2022)
    Photocatalysis is a versatile method to use solar energy for chemical processes. Photocatalytic materials absorb light to generate energetic electron-hole pairs that can be used for redox reactions in production of hydrogen and other chemicals, degradation of pollutants, and many other applications. BiVO4 is a visible light absorbing oxide semiconductor with a band gap of about 2.4 eV, and it has received a lot of attention as a standalone photocatalyst and as a photoanode material. The literature part of this thesis explores how the electronic structure of semiconductors and the different processes in photocatalysis together affect the efficiency of the method. Semiconductor materials are classified based on their chemical composition and compared by selecting most researched materials as examples. Various strategies to improve the photocatalyst material properties are also discussed. Many strategies, such as nanostructured photocatalysts, benefit from deposition of semiconductor thin films. Atomic layer deposition (ALD), as a highly conformal and controllable chemical vapor deposition method, is an excellent choice for depositing semiconductors and various interfacial layers. The literature review also includes a survey of ALD processes for Bi2O3 and V2O5 and a thorough analysis of the existing BiVO4 ALD processes. From the selection of binary ALD processes, bismuth(III) 2,3-dimethyl-2-butoxide (Bi(dmb)3), tetrakis(ethylmethylamido)-vanadium(IV) (TEMAV), and water were chosen as precursors to develop a new ALD process for BiVO4. The binary processes were combined in various metal precursor ratios both completely mixed in supercycles and as nanolaminates, and the resulting films were annealed to crystallize the BiVO4. X-ray diffraction was used to characterize the crystalline phases of the films, and it was noticed that TEMAV reacts with Bi2O3 to make metallic bismuth, but it is reoxidized by annealing. Composition of the films was investigated with energy dispersive X-ray spectrometry and time-of-flight elastic recoil detection analysis (ToF-ERDA). Some sensitivity to process conditions was observed in the deposition, as the metal stoichiometry varied in unexpected manner between some sets of experiments. ToF-ERDA depth profiles also revealed that mixing of the nanolaminate layers was incomplete with annealing temperatures below 450 °C and with laminate layers over 10 nm in thickness. Scanning electron microscopy was used to study the morphology of the films and revealed a granular, non-continuous structure. The optical properties of the films grown on soda-lime glass were investigated with UV-vis spectrophotometry. The band gaps of the films were estimated to be 2.4–2.5 eV. The nanolaminate approach to depositing the films was deemed the best, as it avoids most of the reduction of bismuth by TEMAV. However, it is still not clear why this process is so sensitive to process conditions. This should be investigated to further optimize the film stoichiometry. The morphology of the films might be improved by using different substrates, but it is not a critical aspect of the process as there are methods to passivate the exposed substrate surface. Overall, this process has potential to deposit excellent BiVO4 films that are suitable for further research pertaining their photocatalytic properties and modifications such as nanostructured or doped photoanodes.
  • De Meulder (2022)
    Amorphous metal oxides have proven to deform in a plastic manner at microscopic scale. In this study the plastic deformation and elastic properties of amorphous metal oxides are studied at microscopic scale using classical molecular dynamics simulations. Amorphous solids differ from crystalline solids by not having a regular lattice nor long range order. In this study the amorphous materials were created in simulations by melt-quenching. The glass transition temperature (Tg) depends on the material and cooling rate. The effect of cooling rate was studied with aluminiumoxide (Al2O3) by creating a simulation cell of 115 200 atoms and melt-quenching it with cooling rates of 1011 , 1012 and 1013 K/s. It was observed that faster cooling rates yield higher Tg. The Al2O3 was cooled to 300 K and 50 K after which the material was stretched. The stress-strain curve of the material showed that samples with higher Tg deforms in plastic manner with smaller stresses. The system stretched at 50 K had higher ultimate tensile strength than the system stretched at 300 K and thus confirming the hypothesis proposed by Frankberg about activating plastic flow with work. In order to see if the plastic phenomena can be generalized to other amorphous metal oxides the tensile simulation was performed also with a-Ga2O3 by creating a simulation cell of 105 000 atoms, melt-quenching it and then stretching. Due to the lack of parameters for Buckingham potential these parameters were fitted with GULP using the elastic properties and crystalline structure of Ga2O3. The elastic properties of Ga2O3 with the fitted potential parameters agreed very well with the literature values. The elongated a-Ga2O3 behaved in a very similar fashion compared to a-Al2O3 cooled with the same cooling rate. Further work is needed to establish the Buckingham potential parameters of a-Ga2O3 by experimen tal work. The potential can also be developed further by using the elastic constants and structures of amorphous a-Ga2O3 in the fitting process, although the potential shows already very promising results.
  • Toijala, Risto (2019)
    Ion beams have been the subject of significant industry interest since the 1950s. They have gained usage in many fields for their ability to modify material properties in a controlled manner. Most important has been the application to semiconductor devices such as diodes and transistors, where the necessary doping is commonly achieved by irradiation with appropriate ions, allowing the development of the technology that we see in everyday use. With the ongoing transition to ever smaller semiconductor devices, the precision required of the manufacturing process correspondingly increases. A strong suite of modeling tools is therefore needed to advance the understanding and application of ion beam methods. The binary collision approximation (BCA) as a simulation tool was first introduced in the 1950s. It allows the prediction of many radiation-related phenomena for single collision cascades, and has been adopted in many experimental laboratories and industries due to its efficiency. However, it fails to describe chemical and thermodynamic effects, limiting its usefulness where ballistic effects are not a sufficient description. Parallel to BCA, the molecular dynamics (MD) simulation algorithm was developed. It allows a more accurate and precise description of interatomic forces and therefore chemical effects. It is, however, orders of magnitude slower than the BCA method. In this work, a new variant of the MD algorithm is developed to combine the advantages of both the MD and the BCA methods. The activation and deactivation of atoms involved in atomic cascades is introduced as a way to save computational effort, concentrating the performed computations in the region of interest around the cascade and ignoring surrounding equilibrium regions. By combining this algorithm with a speedup scheme limiting the number of necessary relaxation simulations, a speedup of one order of magnitude is reached for covalent materials such as Si and Ge, for which the algorithm was validated. The developed algorithm is used to explain the behavior of Ge nanowires under Xe ion irradiation. The nanowires were shown experimentally to bend towards or away from the ion beam, and computational simulations might help with the understanding of the underlying physical processes. In this thesis, the high-fluence irradiation of a Ge nanowire is simulated and the resulting defect structure analyzed to study the bending, doubling as a second test of the developed algorithm.
  • Paulamäki, Henri (2019)
    Tailoring a hybrid surface or any complex material to have functional properties that meet the needs of an advanced device or drug requires knowledge and control of the atomic level structure of the material. The atomistic configuration can often be the decisive factor in whether the device works as intended, because the materials' macroscopic properties - such as electrical and thermal conductivity - stem from the atomic level. However, such systems are difficult to study experimentally and have so far been infeasible to study computationally due to costly simulations. I describe the theory and practical implementation of a 'building block'-based Bayesian Optimization Structure Search (BOSS) method to efficiently address heterogeneous interface optimization problems. This machine learning method is based on accelerating the identification of a material's energy landscape with respect to the number of quantum mechanical (QM) simulations executed. The acceleration is realized by applying likelihood-free Bayesian inference scheme to evolve a Gaussian process (GP) surrogate model of the target landscape. During this active learning, various atomic configurations are iteratively sampled by running static QM simulations. An approximation of using chemical building blocks reduces the search phase space to manageable dimensions. This way the most favored structures can be located with as little computation as possible. Thus it is feasible to do structure search with large simulation cells, while still maintaining high chemical accuracy. The BOSS method was implemented as a python code called aalto-boss between 2016-2019, where I was the main author in co-operation with Milica Todorović and Patrick Rinke. I conducted a dimensional scaling study using analytic functions, which quantified the scaling of BOSS efficiency for fundamentally different functions when dimension increases. The results revealed the target function's derivative's important role to the optimization efficiency. The outcome will help people with choosing the simulation variables so that they are efficient to optimize, as well as help them estimate roughly how many BOSS iterations are potentially needed until convergence. The predictive efficiency and accuracy of BOSS was showcased in the conformer search of the alanine dipeptide molecule. The two most stable conformers and the characteristic 2D potential energy map was found with greatly reduced effort compared to alternative methods. The value of BOSS in novel materials research was showcased in the surface adsorption study of bifenyldicarboxylic acid on CoO thin film using DFT simulations. We found two adsorption configurations which had a lower energy than previous calculations and approximately supported the experimental data on the system. The three applications showed that BOSS can significantly reduce the computational load of atomistic structure search while maintaining predictive accuracy. It allows material scientists to study novel materials more efficiently, and thus help tailor the materials' properties to better suit the needs of modern devices.
  • Kontinen, Joona (2020)
    Tutkielman kirjallisuusosuudessa on käyty läpi erilaisia kaupallisia biopolymeerejä, niiden synteesiä, käyttöä ja biohajoamista. Tutkielman pääpaino on erilaisten materiaalien biohajoamisessa ja näiden materiaalien kaupallisessa käytössä. Biohajoamisen evaluointiin tarkoitettuja standardeja, tutkimusmenetelmiä ja hyväksyntäkriteerejä on esitelty laajasti. Tutkimusosuudessa on valmistettu PLA:n ja PBAT:n seoksesta puukomposiitti ja materiaalin termomekaaniset ominaisuudet on karakterisoitu. Tavoitteena oli luoda biohajoava materiaali, jonka ominaisuudet ovat sellaisia, että sen kaupallinen hyödyntäminen kertakäyttömuovin korvikkeena on järkevää. Materiaalin mekaaniset ominaisuudet karakterisoitiin lopputuotteen kestävyyden, ja sulaominaisuudet kaupallisen tuotannon mahdollistamisen takia. Termomekaanisia analyysejä tehtiin materiaalin säilyvyyden ja lämpöominaisuuksien karakterisoimiseksi. Työssä on tutkittu myös puhtaan PLA/puukomposiitin biohajoamista meriympäristössä. Tutkimuksen tuloksena saatiin luotua riittävällä nopeudella biohajoava puukomposiitti, jonka mekaaniset ominaisuudet ovat riittäviä korvaamaan erilaisia kertakäyttöisiä muovituotteita ja joka on prosesoitavissa nykyisillä ekstruusiolaitteistoilla.
  • Väisänen, Vesa (2023)
    The diffusion of a single hydrogen atom through solid tungsten was studied using two separate computational methods: Molecular dynamics simulations and nudged elastic band calculations. Molecular dynamics was used to track the atom’s path in the tungsten lattice in several successive simulations to obtain its mean squared displacement. Results were obtained for temperatures between 400 K and 700 K. These were used to derive a temperature relation for the diffusion coefficient and an estimate for the diffusion energy barrier, which was 0.1985 eV. The nudged elastic band method was used to directly obtain an estimate for the diffusion energy barrier. The same tungsten lattice system as in the molecular dynamics simulations had the hydrogen atom relaxed into an interstitial position and then moved over the energy barrier into an adjacent site. If the tungsten atoms were allowed to relax during the motion, a comparable value of 0.2022 eV was obtained. Further computations were made with a fixed-length tungsten bond next to the hydrogen starting position, yielding energy barriers from 0.1 eV to 1.2 eV depending on the bond length, direction of the diffusion jump and tungsten relaxation. In conclusion, the two different computational methods do give results for the energy barrier that are comparable in magnitude, but further measurement of the tungsten bonds near the hydrogen atom during the molecular dynamics simulation may be needed for a more matching comparison from the nudged elastic band results.
  • Koitermaa, Roni (2022)
    The complex physical mechanisms involved in the formation of vacuum arcs have been of interest for many decades. Vacuum arcs are relevant in many engineering disciplines, but the physics behind them is not yet fully understood. In recent years, there have been many experimental and computational studies focused on understanding aspects of vacuum arcs. This thesis focuses on further development of a simulation model to describe the physical processes starting from electron emission and leading to the formation of an ionized plasma. The FEMOCS code is extended to include plasma simulation based on previous work on ArcPIC. Emission of electrons and heating of the cathode is simulated using the finite element method, while plasma simulation is performed using the particle-in-cell method. We add evaporation of neutral atoms from the cathode, as well as ionization processes for multiple species of ions. Monte Carlo collisions for elastic, Coulomb, impact ionization, charge exchange and recombination collisions between particles are added. Direct field ionization of neutrals is included to account for ionization at high electric fields. A dynamic weighting scheme is described for adjusting superparticle weights during the simulation. Ion bombardment effects such as bombardment heating and sputtering are added to account for additional supply of neutrals resulting from energetic ions accelerated by the electric field. Finally, we add a circuit model for coupling to an external circuit. A static nanotip is simulated with different parameters to study local field thresholds leading to thermal runaway. We find that our simulations agree with experimental results. The most significant interactions contributing to initial formation of vacuum arcs are identified. We find most neutrals are created via evaporation rather than sputtering. The most important collision for plasma formation is impact ionization of neutrals into Cu+ ions, while higher-order ions are found to play a lesser role. Direct field ionization of neutrals is also found to be significant at high fields on the order of 10 GV/m.
  • Peterzéns, Kasper (2023)
    Power ultrasound increases production efficiency in the industry, and therefore reduces emissions. This advantage arises from the ability of ultrasound to mitigate fouling. Ultrasound solution requires clamping the transducers onto the external wall of the production equipment, typically made of steel. A challenge then arises, since mechanical loading by the wall hampers the natural resonating of the ultrasonic transducer and therefore reduces power transmission. To overcome this limitation, airgap contact coupling (ACC) is proposed. ACC features an airgap to reduce the mechanical loading and two protruding elements for mechanical contacting and sound delivery. Finite-element method (FEM) simulations are employed to evaluate the physical mechanisms behind ACC. For the comparison, direct traditional contact coupling (TCC) is evaluated. To assess the acoustic power delivery by ACC and TCC, calorimetric measurements were used. A water-filled stainless-steel pipe with a 2 mm thick wall and 136 mm outer diameter was sonicated. To prevent heat transmission to ambient air, it was covered by isolating foam. ACC and TCC were sonicated at their coupled resonance frequencies, respectively at 19.2 kHz and 28.1 kHz. A power delivery ratio was determined by the calorimetric power against the sonication power. ACC resulted in a power delivery ratio of 27.4±6.3 % whereas that for TCC was 6.1±0.6 %. ACC was thereby shown to transmit 6 dB more acoustic power than TCC. In conclusion, a novel contact coupling method is proposed for industrial metal-walled equipment. The proposed new approach enhances the utility of power ultrasound for online cleaning and prevention.
  • Terletskaia, Mariia (2023)
    In recent decades, more and more attention has been paid to solar energy because of the need to ensure “green” and sustainable future. Solar cells have been treated as one of the most promising technologies for solar energy utilization. Since conversion of sunlight into electricity mainly passes through the light absorbing material (absorber), its optoelectronic properties largely determine the cell performance. Among the existing absorbers, inorganic lead-free perovskites, like CsSnI3, are of great interest due to high potential efficiency, increased stability and the absence of toxic components. However, currently used fabrication techniques limit quality of the materials and their application in large-scale production. Atomic layer deposition (ALD) is a thin film fabrication technique which is now widely used in electronics and optoelectronics. Based on the principle of sequential saturated surface reactions, it is able to provide almost atomic level control over the thickness and composition of the film. Moreover, the principle ensures the formation of uniform films on large surfaces. Since precise composition control and scalability are of great importance for efficiency of perovskite solar cells, ALD acts as an excellent tool for production of this type of absorbers. The literature review of this thesis examines perovskites as absorber material for commercially efficient solar cells. The aim is to give the reader an overview of solar cell performance, currently available absorber materials and motivation for perovskites to become promising cost-efficient solution. Additionally, the most common fabrication techniques for perovskite structures are introduced together with limitations to emphasize the expediency of further experiments. The experimental part combines development of SnI2 thin film deposition in ALD reactor with a subsequent use of the technique in conversion to perovskite for future solar cell application. Unfortunately, the applicability of SnI2 ALD with proposed chemical process became doubted due to multitude issues that arose during the investigation. However, successful results on SnI2 pulsed chemical vapor deposition (pCVD) in the same ALD reactor supported feasibility of the chemical process. Application of the optimized pCVD technique for the conversion of CsI thin films, prepared by ALD, made it possible to obtain phase-pure CsSnI3 perovskite. In addition, conversion part demonstrates that use of SnI2 pCVD allows the formation of uniform and conformal perovskite thin films with promising band gap of 1.7 eV.
  • Halkoaho, Johannes (2022)
    MRS or magnetic resonance spectroscopy is an imagining technique which can be used to gain information about the metabolite concentration within a certain volume of interest. This can be used for example in brain imagining. The brain consists of three main types of tissue: cerebrospinal fluid, white and gray matter. It is important to know the different volume fractions of these tissues as the resolution in MRS is significantly lower than that of magnetic resonance imagining (MRI). The tissues all have different metabolite profiles and in order to get meaningful data the volume fractions need to be taken into account. This information can be gained from the segmentation of an image formed by using MRI. In this work a software tool was created to find these volume fractions with the input of a .rda file that is created by the scanner and Nifti file. The Nifti file is the image formed by using MRI and the .rda file is the manufacturers raw data format for spectroscopy data which has the relevant information about the volumes of interest. The software tool was created using Python and JavaScript programming languages and different functions of FSL. FSL is a comprehensive library of analysis tools used in brain imaging data processing. The steps for the software tool are: determining the coordinates of the volume of interest in FSL voxel coordinates, creating a mask in the correct orientation and location, removing non-brain tissue from the image using FSL’s tool tailored for that purpose (BET), segmenting the image using FSL’s segmenting tool (FAST), registering the mask on the segmented images and calculating the volume fractions. The software tool was tested on imaging data that was obtained at Meilahti Kolmiosairaala for the purpose of the testing. The testing data set included five different spectroscopy volumes from different parts of the brain and a T1 weighted image. The software tool was given the relevant information about the volume of interest in the form of a .rda file and the T1 weighted image in the form of a Nifti file. The software tool then determined the different volume fractions from all of the five volumes of interest. There is variation on the volume fraction of different brain areas within different brains and it is not possible to have an absolute reference value. The results of the test corresponded to the possible volume fractions that can be expected from the volumes in question.
  • Hunnakko, Joel (2023)
    An optical data bus is a promising solution to provide a fast data transmission from room temperature to quantum devices at low temperatures, which would minimize the heat load into the cryogenic system compared to the conventional electrical cabling. Previously, a similar measurement setup was used to drive a Josephson junction array (JJA) with an optical pulse pattern generated with a mode-locked laser (MLL). A photodiode (PD) was used to convert optical signals to photocurrent signals to drive the JJA at low temperature. There was a long and non-ideal transmission line between the PD and the JJA at their operation temperature of 4 K. The experiments and simulations revealed that the non-idealities in the transmission line caused non-desired reflections. In this work the PD is integrated on a same chip as the JJA. The new on-chip integration provides shorter transmission line with less interfaces. The new compact transmission line promises less electrical signal reflections between the PD and the JJA. A custom-made MLL emitted an original optical single pulse to the optical pulse pair generator. The MLL was operated at a well-defined pulse frequency of 2.3 GHz to produce the single pulses in the desired frequency. An optical time delay circuit (OTD) was applied to the generated pulses in the MLL to time divide the pulses on the desired time delays. The generated optical pulse pair pattern was transmitted from room temperature to the PD in 4 K via an optical polarization maintaining fiber. The fiber was integrated on the top of the PD in 4 K, which was used to drive the JJA sample. The PD was biased with a reverse voltage and the JJA sample with a current. The amplification of optical twin pulses was varied during the measurements. We measured the DC voltage of the JJA sample and the DC photocurrent of the PD simultaneously. The measurement was repeated with several different manually defined optical time delayed twin pulses. The work included optimization of the optical setup. The optimization involved setting the reflective diffraction grating to the optimized position, which was used to filter away undesired wavelengths. The optical pulse pair method used in this work can be used to investigate the maximum speed of the data signals.
  • Mäkelä, Ville (2022)
    With their ability to convert chemical energy to electrical energy through electrochemical reactions, rechargeable batteries are widely used to store energy in various applications such as electronic mobile equipment, aerospace aviation, road transportation, power grid, and national defense industry Numerous battery types are available commercially. Lithium ion-based batteries stand out due to several key advantages such as high operating voltage, high specific energy, and long cycle life. They also have a market dominance in a wide range of electric vehicles. However, like all battery technologies, lithium ion-based ones suffer from the effects of aging-induced degradation which can lead to reduced capacity, lifetime, and in some cases even safety hazards. One method of preventing/slowing down these aging reactions is to modify the standard battery materials by using dopants and additives. They are specific impurities purposely introduced into the battery during the manufacturing process. In this master’s thesis, the effect of additives (Mg/Al) on the aging of Li-ion cells was examined by using X-ray absorption spectroscopy, more specifically x-ray absorption near edge structure (XANES). For the experiment, 7 different cells, all containing lithium cobalt oxide as the major component (with 4 having a stoichiometric ration of Li/Co, and 3 being Li-rich), with 5 of them containing Mg/Al as dopants, and 2 containing no dopants were examined using XANES as a function of aging in terms of charge/discharge cycles. The dopants were introduced at different stages of the material preparation, either at the lithiation step or at the synthesis of the precursor. This thesis focuses on the XANES experiment and the data analysis, with extensive literature review on the topic of using additives and dopants. The cells were prepared by the Aalto University. The results showed that of the cells with dopant materials, the cells doped during lithiation stage aged slightly better after cycling than the undoped ones, whereas the cells doped during precursor stage aged worse than the undoped cells. This would suggest that doping might be more effective when done during the lithiation stage.