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

Röntgenabsorptiospektroskopia (X-ray absorption spectroscopy, XAS) kuvaa röntgensäteilyn absorboitumistodennäköisyyttä tutkittavaan materiaaliin röntgenfotonien energian funktiona. XAS-spektroskopiaa hyödynnetään monilla tieteenaloilla kuten fysiikassa, materiaalitutkimuksessa, kemiassa ja biologiassa. Menetelmällä saadaan yksityiskohtaista tietoa valitun alkuaineen ympäristöstä ja materiaalin rakenteesta atomitasolla. Kun röntgenfotonin energia vastaa tutkittavan atomin sidosenergiaa, absorptio kasvaa jyrkästi. Tämän nousun eli absorptioreunan paikkaa ei ole yksiselitteisesti määritelty, vaan kirjallisuudessa esiintyy yhä toisistaan poikkeavia menetelmiä absorptioreunan paikan määritykseen XAS-spektristä. Tutkielmassa hyödynnetään XANES-spektroskopiaa (X-ray Absorption Near Edge Structure), joka kuvaa absorptioreunan läheistä aluetta XAS-spektrissä, ja keskitytään koboltin K-kuoren XANES-spektroskopiaan. Työssä tutkitaan absorptioreunan paikan eri määritysmenetelmiä kobolttioksidien CoO, Co2O3, Co3O4 ja LiCoO2 XANES-spektreistä. Käytetyt reunan määritysmenetelmät ovat 1) absorptioreunan puolivälikorkeutta vastaavan energian etsiminen, 2) keskiarvon laskeminen niistä energioista, joilla reunan korkeus saavuttaa 20% ja 80% maksimiabsorptiosta, 3) reunan ensimmäisen käännepisteen etsiminen ja 4) reunan jyrkimmän käännepisteen etsiminen. Tutkielman kirjallisuuskatsauksessa käsitellään röntgenabsorptiospektroskopiaa keskittyen XANES-spektroskopiaan. Kirjallisuuskatsauksessa esitellään eri menetelmiä absorptioreunan määritykseen. Kokeellisessa osiossa mitataan kobolttioksidien spektrit ja verrataan eri menetelmillä määritettyjä absorptioreunojen paikkoja tunnettuihin hapetustiloihin ja pyritään etsimään lineaarista riippuvuutta reunan paikan ja hapetustilan välille. Pienin epätarkkuus, noin 10%, suoran sovituksessa saatiin hapetustilojen ja jyrkimmän käännepisteen menetelmällä määritettyjen reunan paikkojen välille. Tällä menetelmällä sovitussuoran kulmakerroin oli myös suurin, joten voidaan todeta tämän menetelmän herkkyyden olevan suurin. Toisaalta kulmakerroin poikkeaa myös selvästi muilla menetelmillä määritetyistä kulmakertoimista.

Tietokonetomografialla (TT) on ollut jo usean vuosikymmenen ajan tärkeä asema lääketieteellisenä kuvantamismenetelmänä, jolla pystytään tuottamaan tarkkoja leikekuvia kehon rakenteista. TT on historiansa aikana muuttunut ja kehittynyt menetelmänä teknologisten harppausten ansiosta. Kuvauksista on tullut nopeampia ja entistä tarkempia. Kaksoisenergiatietokonetomografia (KETT) on yksi TT-kuvantamisen kehitysaskel, jonka teoreettiset perusteet ovat olleet jo pitkään tiedossa, mutta joka on vasta viime vuosikymmenenä saanut enemmän sovelluskohteita. KETT lisää TT-kuvauksen materiaalierottelukykyä, mikä mahdollistaa tarkemman diagnostiikan ja vähentää jatkotutkimusten tarvetta. Laajempi KETT:n kliininen käyttö vaatii kuitenkin toimintatapojen muutosta ja pysyvän laadunvalvontaprotokollan muodostamista. Tämän maisterintutkielman tarkoituksena oli auttaa laadunvalvonnan kehittämistä HUS:n Diagnostiikkakeskuksessa kartoittamalla KETT-kuvausten toimintaa, kuvausparametrien vaikutusta tuloksiin ja laitekohtaisia eroja. Työhön kuuluvat mittaukset suunniteltiin ja toteutettiin yhdessä HUS:n Diagnostiikkakeskuksen Meilahden Tornisairaalan radiologian yksikön kanssa. Kuvauksissa käytettiin Siemens Healthineersin valmistamia SOMATOM Definition Flash - ja SOMATOM Force -laitteita sekä KETT-kuvantamiseen soveltuvaa testikappaletta. Mittauksissa tutkittiin testikappaleen pystysuuntaisen asettelun vaikutusta kuvauksista saataviin tuloksiin, materiaalien elektronitiheyden ja efektiivisen järjestysluvun määritystarkkuutta, varjoaineena käytetyn jodin pitoisuuden määritystarkkuutta ja kuvauksen annostason merkitystä. Työssä käytetyllä menetelmällä määritettiin testikappaleen materiaalien koostumuksellisia eroja. Tutkimuksen avulla voitiin havaita jodipitoisuuden määrittämisen olevan haasteellisempaa hyvin pienillä alueilla, mutta halkaisijaltaan 5 mm:n kokoisilla alueilla määritystarkkuus oli jo kohtalainen. Potilaan oikeanlaisen keskityksen ja kuvanlaadun havaittiin olevan erityisen tärkeää pehmytkudoksia kuvattaessa. Laitteiden välisten erojen todettiin olevan merkittävämpää eri laitemallien välillä kuin samanmallisten laitteiden välillä. Laadunvalvonnassa tulee ottaa huomioon laitteiden kalibraatio, jotta laitteiden välisiä eroja pystytään vähentämään. Jatkossa mittauksia tulee toteuttaa lisää mittausten välisen toistettavuuden varmistamiseksi.

The spin-orbit-coupled insulator Sr 3 NiIrO 6 is a strongly correlated transition metal compound, where an interplay of geometric frustration and spin anisotropy gives rise to novel magnetic phases. Resonant inelastic x-ray scattering (RIXS) is a powerful probe of the low-lying quasi-particle excitations that underpin these emergent properties. In this work, we partition the active space into approximately non-interacting parts in order to introduce a tight-binding single-particle model Hamiltonian describing the distorted IrO6 octahedra in Sr3NiIrO6. We then use this model to calculate its RIXS spectrum at the Ir L3-edge in the sub-electronvolt range. The results of this calculation are compared with experiments performed at the European Synchrotron Radiation Facility, and with a multiplet crystal field model calculation. We find that this one electron model largely agrees with the full-multiplet model and describes the d-d excitations observed in experiment. The addition of an exchange field term explains the low-lying temperature-dependent magnetic feature, disambiguating the sign of the crystal-field term, and suggesting that the feature is well localized at low temperatures, and is best described as an orbitally- entangled local spin-flip excitation. However, the correspondence at room temperature diminishes, suggesting that dispersive description is necessary to model this regime. The drastic reduction in active space entailed by this model facilitates the creation of extended non-collinear Heisenberg-like models, which can be calculated at a lower computational cost than full multiplet extended models.

Several nuclear power plants in the European Union are approaching the ends of their originally planned lifetimes. Extensions to the lifetimes are made to secure the supply of nuclear power in the coming decades. To ensure the safe long-term operation of a nuclear power plant, the neutron-induced embrittlement of the reactor pressure vessel (RPV) must be assessed periodically. The embrittlement of RPV steel alloys is determined by measuring the ductile-to-brittle transition temperature (DBTT) and upper-shelf energy (USE) of the material. Traditionally, a destructive Charpy impact test is used to determine the DBTT and USE. This thesis contributes to the NOMAD project. The goal of the NOMAD project is to develop a tool that uses nondestructively measured parameters to estimate the DBTT and USE of RPV steel alloys. The NOMAD Database combines data measured using six nondestructive methods with destructively measured DBTT and USE data. Several non-irradiated and irradiated samples made out of four different steel alloys have been measured. As nondestructively measured parameters do not directly describe material embrittlement, their relationship with the DBTT and USE needs to be determined. A machine learning regression algorithm can be used to build a model that describes the relationship. In this thesis, six models are built using six different algorithms, and their use is studied in predicting the DBTT and USE based on the nondestructively measured parameters in the NOMAD Database. The models estimate the embrittlement with sufficient accuracy. All models predict the DBTT and USE based on unseen input data with mean absolute errors of approximately 20 °C and 10 J, respectively. Two of the models can be used to evaluate the importance of the nondestructively measured parameters. In the future, machine learning algorithms could be used to build a tool that uses nondestructively measured parameters to estimate the neutron-induced embrittlement of RPVs on site.

Ultrasonic transducers convert electric energy into mechanical energy at ultrasonic frequencies. High-power ultrasound is widely used in the industry and in laboratories e.g. in cleaning, sonochemistry and welding solutions. To be effective in these cases, a piezoelectric transducer must deliver maximal power to the medium. Most of these systems rely on having the power delivery maximized during long driving sequences where stable performance is critical. Power ultrasonic transducers are typically narrowband, featuring high Q-value, that are finely tuned to a specific resonance frequency. The resonance frequency can vary during driving due to temperature, mechanical loading and nonlinear effects. When the transducers resonance frequency changes, drastic changes in its impedance (resonance to anti-resonance) can lead quickly to damage or failure of the driving electronics or the transducers themselves. In this work we developed a multi-channel high-power ultrasonic system with a software-based resonance frequency tracking and driving frequency control. The implementation features a feedback loop to maximize power delivery during long driving sequences in an ultrasonic cleaning vessel. The achieved total real power increased from 6.5 kW to almost 10 kW in peak with our feedback loop. The feedback loop also protected the electronics and transducers from breaking due to heating and varying impedance.

Transcranial magnetic stimulation (TMS) is a non-invasive method for stimulating cortical neurons in the brain. Combining TMS with functional magnetic resonance imaging (fMRI) shows the effects of TMS through the changes in brain metabolism. This information is necessary for developing new applications of TMS and for improving the efficacy and safety of the existing treatments. The biggest setback in current TMS–fMRI technology arises from the mechanical forces formed on the transducer as the interplay of the magnetic fields from the MRI and the changing current in the coil, leading to breakage of the transducers and additional safety risks. The objective of this Thesis was to assess and compare the mechanical stresses on multi-locus TMS (mTMS) transducers inside a high-field fMRI bore using finite element modeling, and to build and test transducer options based on the simulation results. For a transducer design for rats, six different coil former materials and three mTMS coil combinations were simulated with two commonly used current waveforms. In addition, the effect of the transducer orientation relative to the magnetic field was modeled with a transducer designed for humans. Our results show that the current pulses ran through the coils produce shock waves on the coil formers, leading to regions of maximum stresses that depend on the time instant. The intensity and location of the maximum stresses depends on the current waveform and coil combination used. Based on the results, 30% glass-fiber filled polyamide was found to be the most durable material. This Thesis provides novel insights for more durable TMS coil designs.

Vacuum breakdown is a limiting factor in the design of powerful and cost-efficient particle accelerators. Modern models have suggested that the rate of breakdowns is driven by dislocation dynamics in the electrode materials suffering from breakdowns. In order to understand why specifically the copper-2wt%beryllium alloy outperforms other electrode materials in vacuum breakdown rate and maximum electric fields in breakdown experiments at CERN, a new machine-learning interatomic potential (ML-IAP) for the CuBe alloy was developed. Density functional theory (DFT) was used in calculating a dataset of atomic forces, energies, and virials for a set of CuBe structures. This dataset was performed a fit on with Gaussian process regression, producing an IAP with close-to-DFT accuracy in its intended use cases. With the developed IAP, the interactions between single interstitial beryllium atoms and edge dislocations in a face-centered cubic (FCC) copper matrix were studied with molecular dynamics (MD). It was found that beryllium atoms bind to the edge dislocations, inhibiting their mobility under shear stress. Furthermore, beryllium atoms were found to increase the intrinsic stacking fault energy of FCC copper, possibly leading to an increase in dislocation mobility. These two findings suggest that beryllium atoms could increase copper's resistance to vacuum breakdown mainly via trapping dislocations. Future studies could look at how precipitates of beryllium, or other alloys of copper, play a role in dislocation dynamics.

The NV⁻ center is an optically active defect consisting of a nitrogen atom next to a vacancy with a trapped electron. The defect is ideally suited for many quantum applications and thanks to its long coherence time at elevated temperatures, it can be used as quantum bits for quantum information processing even at room temperature. The defect is found in diamond, which is an exemplary host to a wide range of optically active defects due to its unique properties. As nitrogen is the most common impurity in diamond, NV centers occur naturally but at concentrations that are often deemed insufficient for applications. Thus, understanding the formation process of NV centers and how to efficiently produce them with high spatial resolution is of great interest. In this thesis, the formation of NV centers through irradiation has been studied both in the nuclear and electronic stopping power regimes with molecular dynamics. The analysis of the atomic configurations and displacement resulting from the irradiation revealed two different formation mechanisms, in which either a carbon vacancy is created next to a nitrogen atom or a nitrogen atom becomes mobile to be trapped by a vacancy. While the probability of NV formation from irradiation alone was shown to be low in the nuclear regime, two-temperature molecular dynamics simulations of swift heavy ions in the electronic regime showed the direct formation of NV-centers along the ion’s path. By producing NV centers along an almost one dimensional chain, swift heavy ions offer high spatial resolution in addition to high conversion rates from nitrogen to NV centers due to their high energies.

Energy is an essential input for any non-spontaneous mechanism. In biological organisms, the process of producing energy currency, adenosine triphosphate, is called cellular respiration. It is made of three smaller steps, out of which the last one is oxidative phosphorylation that is responsible for the largest production of adenosine triphosphate molecules in the whole process. Oxidative phosphorylation is performed by the electron transport chain made of five protein complexes, named respiratory complex I-V. Complex I is the first and largest protein complex in the electron transport chain, and it is the least understood. Its primary function is to transfer electrons from nicotinamide adenine dinucleotide to ubiquinone, which is coupled to the pumping of four protons across the mitochondrial inner membrane. Although the overall reaction of complex I is understood, the intricate detail of the mechanism is still largely unknown. There is significance in the details because there are numerous point mutations, which have been strongly correlated with neurogenerative diseases, such as Leigh’s syndrome, and aging. Therefore, a more thorough understanding of its mechanism can give insight into potential target drug development. Complex I is made of 14 highly conserved subunits that can be found in most species that use the electron transport chain. They create an L-shape, where seven subunits are embedded in the inner membrane, the membrane domain, and the others are floating in the mitochondrial matrix, the peripheral arm. In mitochondrial complex I, however, there are in addition around 30 accessory subunits. It has been previously thought that the main mechanism is conducted by the 14 subunits that are found in all species. However, in the past couple of years, it has been shown that accessory subunits can play an important role in the mechanism of mitochondrial complex I. The work presented in this thesis uses a multiscale computational approach to study the effect of three mutations, F89A, Y43A and L42A, from an accessory subunit LYRM6 on the function of complex I. Previous experiments demonstrated that the mutations decreased the overall activity of complex I by 76-86 %. The LYRM6 subunit is located at the pivot of the membrane and periplasmic domains. The results of this study show that the point mutations have a long-range effect on the conformations of three loops from three conserved subunits in this region. The shift in the loop dynamics causes a drop in water occupancy. The observed water pathway is tested for the capability of proton transfer. The findings are demonstrated with the help of molecular dynamics and quantum mechanics/molecular mechanics simulations.

This thesis examines the optical response of tuneable chiral plasmonic nanostructures in linear cross-polarization. Plasmonic gold-silver nanostructures composed of silver-coated gold nanorods, and dynamic DNA origami are investigated because of their optical properties of interest in the visible light wavelength region, and because of their controllable rotational asymmetry, which results in tuneable chirality in dimer structures. These plasmonic nanostructures present optical properties such as circular dichroism and optical rotatory dispersion. In this thesis we establish the relationship between perceived color, spectrometry, circular dichroism and optical rotatory dispersion of the samples, depending on the chiral geometry of the nanostructures within. The motivation is to predict perceived color from the chiral geometry of the nanostructures, which will enable visual detection for biosensing applications. Circular dichroism and optical rotatory dispersion give us detailed knowledge about the polarization state of a sample, but visible light detection and spectrometer measurements are more accessible and portable methods for characterizing the polarization state of a sample. We achieve color modulation from green to blue with the switching of chiral geometry, under cross-polarized white light. This has potential for biosensing applications, based on the perceived color change depending on the chiral geometry of the sample. The DNA origami structures react to the presence of an analyte by changing their chiral geometry. Possible applications in biosensing of analytes can be made more practical if the orientation of the DNA origami template can be determined from the perceived color or the transmission spectra, rather than from the less accessible circular dichroism or optical rotatory dispersion measurements.