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

Industrial ultrasonic cleaning plays a crucial role in optimizing processes and production in various industrial equipment by eliminating the need for harmful chemicals and minimizing production downtime. While previous research has focused on optimizing ultrasonic cleaning for larger and complex geometries, by controlling acoustic pressure fields and cavitation, the potential contribution of online cleaning using elastic vibrations of the equipment wall remains insufficiently explored. To bridge this gap, this study employs a combination of simulation techniques and laboratory experiments to investigate the significance of elastic guided waves and evaluate the influence of different pulsed driving waveform properties on cleaning efficacy. Specifically, the study focuses on the delivery of ultrasonic energy along the pipe wall. While confirming the effective delivery of ultrasonic energy across the length of pipes, the mechanisms underlying scale removal are still open to interpretation. Notably, specific flexural wave modes of a fluid-filled pipe, e.g. F_FFP (n,1) mode family, are identified as potential carriers of the cleaning effect. The research highlights the significance of high momentary power and total effective power in achieving efficient cleaning. The presence of liquid introduces complexities such as mode conversions and the possibility of cavitation. Further investigations are recommended to explore the individual roles of normal and tangential vibration components at the wall-scale interface. The study emphasizes the need for comprehensive analysis to optimize ultrasonic cleaning processes for larger and complex geometries, determine effective parameters for particle detachment, and enhance overall cleaning efficiency.

The aim of the thesis was to study enzymatic treatment as a way to modify paper grade pulp to be a suitable raw material for the future textile industry. Wood as a raw material is an environmentally friendly option for textile production but its sustainable exploitation is not easy. Currently, ionic liquids are assumed to enable a safe and sustainable process for the production of wood-based regenerated fibres. These processes commonly use dissolving pulp as their raw material but replacing dissolving pulp with a paper grade kraft pulp would decrease environmental impact and production expenses. In this work, molar mass distribution of softwood paper grade kraft pulp was selectively modified using enzymes. Enzymes were utilized instead of acids because of their favourable abilities to selectively modify targeted polymers and to increase fibre porosity. Enzymatic modifications of softwood kraft pulp were performed to decrease degree of polymerization of cellulose and lower the quantity of hemicellulose. Hydrolysis of cellulose was catalysed with endo-1,4-β-glucanase (endoglucanase) and hemicellulose was degraded using endo-1,4-β-mannanase and endo-1,4-β-xylanase. The treatments were carried out both at high (20%) and low (3%) pulp consistency to examine the synergistic effect of enzymatic and mechanical action arising in the high consistency treatment. Additionally, influence of different enzyme combinations on the pulp properties was studied. The modified pulp samples were characterized by determining intrinsic viscosity, molar mass distribution, yield loss, and its composition. The fibres were imaged with light microscopy. The degree of polymerization of the pulp cellulose was successfully decreased with a relatively small endoglucanase dose. The amount of hemicellulose was reduced by removing 11% of the total galactoglucomannan and 40% of the total arabinoglucuronoxylan. The high consistency treatments decreased intrinsic viscosity 1.9 times more on average than the low consistency treatments. The high consistency treatments were effective with low enzyme doses, easy to control, and reliably repeated. Therefore, enzymatic pulp treatment at high consistency seems to be a compatible way to modify paper grade kraft pulp to suitable raw material for textile production. Further studies related to pulp dissolution in ionic liquids, fibre spinning, and fibre regeneration should be concluded to confirm applicability of the modified fibres.

High-intensity and -amplitude focused ultrasound has been used to induce cavitation for decades. Well known applications are medical (lithotripsy and histotripsy) and industrial ones (particle cleaning, erosion, sonochemistry). These applications often use low frequencies (0.1-5 MHz), which limits the spatial precision of the actuation, and the chaotic nature of inertial cavitation is rarely monitored or compensated for, constituting a source of uncertainty. We demonstrate the use of high-frequency (12 MHz) high-intensity (ISPTA=90 W/cm2 ) focused-ultrasound- induced cavitation to locally remove solid material (pits with a diameter of 20 µm to 200 µm) for non- contact sampling. We demonstrate breaking cohesion (aluminium) and adhesion (thin film on a substrate, i.e. marker ink on microscope glass). The eroded surfaces were analyzed with a scanning acoustic microscope (SAM). We present the assembly and the characterization of a focused ultrasound transducer and show quantification of the effect of different sonication parameters (amplitude, cycle count, burst count, defocus) on the size and shape of the resulting erosion pits. The quantitative precision of this method is achieved by systematic calibration measurements, linking the resulting erosion to acoustic parameters to ensure repeatability (sufficient probability of cavitation), and inertial cavitation monitoring of the focal echoes. We discuss the usability of this method for localized non-contact sampling.

Nonlinear acoustic and electric effects for the purposes of fluid and fluid-fluid interface manipulation find many applications in the literature. Some examples include: electrospinning, electrospraying, ultrasonic sonoreactors, acoustic drop sampling and microfluidic particle manipulation via acoustic streaming. Ultrasound-enhanced electrospinning (USES) is one application in which both an acoustic and an electric field deform the surface of an aqueous polymer solution in order to achieve electrospinning of nanofibers. In this thesis, the nonlinear physics involved in USES are reviewed and applied to a finite element method based model of the system. This work builds on my previous publication on acoustic fountain formation and subsequent electrostatic deformation of a liquid-air interface in USES by also considering the effects of acoustic streaming. Results for acoustic streaming near a liquid-air interface in a case where the acoustic field is also focused around the interface are studied with simulations and compared against experiments. The results display an intricate balance of the shape and strength of the acoustic streaming field as the liquid-air interface simultaneously deforms. This even leads to situations where the streaming field could completely change direction. Finally, simulation predictions for acoustic streaming, fountain formation and electrostatic deformation of liquid-air interface in the USES set-up in its standard configuration are given. This simulation predicts a very weak acoustic streaming field and a smaller contribution from the electric field, compared to the acoustic field, on the interface forces. This implies that in the simulated configuration, the electric field serves more as force to pull the acoustic fountain a bit more in order for the acoustic field to find its new balance and exert an even stronger force than it was able to by itself. The simulation also indicates that for the robust and reproducible operation of USES, and possibly for the resulting nanofibers, one needs to have precise control of the process parameters, acoustic field and surface level, due to the complex nature of the fountain formation.

This thesis examines the implementation of general purpose graphics processing unit (GPGPU) acceleration to a non-equilibrium Green’s function (NEGF) equation solver in the context of a computational photoelectrochemical (PEC) cell model. The goal is to find out whether GPGPU acceleration of the NEGF equation solver is a viable option. The current model does not yet have electron-photon scattering, but from the results it is possible to assess the viability of GPGPU acceleration in the case of a complete PEC cell model. The viability of GPGPU acceleration was studied by comparing the performance difference of two graphics processing unit (GPU) solutions against a multi core central processing unit (CPU) solution. The difference between the two GPU solutions was in the used floating-point precision. The GPU solutions would use LU factorization to solve the NEGF equations, and the CPU solution a banded solver (Gauss tridiagonal) provided by Scipy Python package. The performance comparison was done on multiple different GPU and CPU hardware. The electrical transport properties of the PEC cell were modeled by a self-consistent process in which the NEGF and Poisson equations were solved iteratively. The PEC cell was described as a semiconductor device connected with a metal and electrolyte contacts. The device was assumed to be a simple one dimensional tight-binding atom chain made of GaAs, where the transverse modes in the y–z plane are treated with a logarithmic function. The computational model did lack electron-photon scattering, which would be implemented in the future. From the benchmark results, it can be concluded that the GPGPU acceleration via LU factorization is not a viable option in the current code or in the complete model with electron-photon scattering and the assumed approximations. The parallel multi-core CPU code generally outperformed the GPU codes. The key weakness of the GPU code was the usage of LU factorization. Despite of this, there could be an opportunity for GPGPU acceleration if a more complex lattice structure and more exact scattering terms would be used. Also, a GPU accelerated tridiagonal solver could be a possible solution.

Eye plaque radiotherapy is a treatment method of ocular tumors: A sealed radiation source is temporarily placed on the surface of the eye in day surgery. Compared to externally delivered conventional radiation treatments, more precisely targeted brachytherapy allows a higher dose in the target tissue while keeping the dose to healthy tissue relatively low. In Finland, all eye plaque treatments are centralised in Helsinki and brachytherapy of the eye is performed annually on approximately 70 patients. Patient specific anatomy takes into account determination of specific location and shape of the tumor in respect of radio-biologically critical structures of the eye. Until now, this has not been systematically modeled in dose calculation of eye plaque brachytherapy at HUS. The new version of Plaque Simulator, a 3D treatment simulation and modeling package for I-125, Pd-103, Ir-192 and Ru-106 plaque therapy of ocular tumors, enables importation and digitisation of patient imaging data (fundus imaging, CT and MRI) which consequently allows for systematically accurate estimation of dose distribution not only in the tumor but also in surrounding healthy tissues. The aim of this Master’s thesis is to prepare the new version of Plaque Simulator simulation and modeling package for clinical use in patient dose calculation at HUS. A comparison is done between the dose calculation method of the old and the new version of Plaque Simulator, and the dose calculation parameters as well as the plaque modeling parameters are reviewed. The function of the image-based dose calculation method is also tested with an anonymised patient treated for a tumor of a more peculiar shape. The absorbed dose to water on the central axis of the radiation source is measured experimentally for two individual I-125-seed along with Ru-106-CCB-, I-125-CCB-, and two I-125-COB-plaques. Experimental results are compared with the results obtained from Plaque Simulator. Individual I-125-seed is used to calibrate the detector at a distance of 10 mm, yielding to a calibration factor of 0.808. The use of the gold parameter in the dose calculation is justified, and a dosimetry modifier of Plaque Simulator is found to be 1.226 for I-125-plaques. Ru-106-plaque measurements are not calibrated, making them only relative. However, an excellent correspondence is observed between Ru-106-plaque dose calculations in Plaque Simulator and the manufacturer’s certificate. The measurements are normalized to the manufacturer’s certificate with a normalisation factor of 1.117.

Monoenergetic neutron reference fields are used in neutron metrology for the calibration of different neutron detectors, including dose rate meters. The International Standardization Organization ISO has composed guidelines and requirements for the production of narrow energy spread neutron fields using a particle accelerator. The objective of this Thesis was to investigate a target material that could be used to produce a monoenergetic neutron field by irradiating it with protons. A broader energy distribution was deemed satisfactory in regard to the initial phase of the station’s development, as significant modifications to the beamline would be necessary to acquire more precise beam current values and to achieve proton energies closer to the reaction threshold energy. The target material was chosen to be lithium fluoride (LiF) based on a literature review and Monte Carlo simulations. The simulations were executed with the proton energy of 2.5 MeV, which is close to the threshold energy of the 7Li(p, n)7Be reaction, and with the fixed energy 10 MeV of the IBA cyclotron used to conduct the experiment. The simulations were executed with the MCNP6 code, and the results were compared to those obtained from equivalent Geant4 simulations. The simulations suggested two wide peaks around 3 MeV and 0.6 MeV at the proton energy of 10 MeV. The irradiation experiment included two phases, one of which entailed the use of a shadow cone to estimate the number of scattered neutrons in the neutron yield. The maximum neutron fluence of (2.62 ± 0.78)∙109 s-1 was measured at the pop-up probe current of (8.3 ± 0.8) µA. Gamma spectrometry was utilized after the experiment to further evaluate the number of 7Li(p,n)7Be reactions taken place in the target by calculating the number of 7Be nuclei in the LiF plate. Altogether, lithium fluoride exhibits promising characteristics as a target material for accelerator-based monoenergetic neutron production, although its application demands further considerations regarding for instance, the decrement of the proton energy and the aiming and measurement of the proton beam. These results contribute to the future development of a neutron irradiation station at the University of Helsinki.

The rapidly increasing global energy demand has led to the necessity of finding sustainable alternatives for energy production. Fusion power is seen as a promising candidate for efficient and environmentally friendly energy production. One of the main challenges in the development of fusion power plants is finding suitable materials for the plasma-facing components in the fusion reactor. The plasma-facing components must endure extreme environments with high heat fluxes and exposure to highly energetic ions and neutral particles. So far the most promising materials for the plasma-facing components are tungsten (W) and tungsten-based alloys. A promising class of materials for the plasma-facing components is high-entropy alloys. Many high-entropy alloys have been shown to exhibit high resistance to radiation and other wanted properties for many industrial and high-energy applications. In materials research, both experimental and computational methods can be used to study the materials’ properties and characteristics. Computational methods can be either quantum mechanical calculations, that produce accurate results while being computationally extremely heavy, or more efficient atomistic simulations such as classical molecular dynamics simulations. In molecular dynamics simulations, interatomic potentials are used to describe the interactions between particles and are often analytical functions that can be fitted to the properties of the material. Instead of fixed functional forms, interatomic potentials based on machine learning methods have also been developed. One such framework is the Gaussian approximation potential, which uses Gaussian process regression to estimate the energies of the simulation system. In this thesis, the current state of fusion reactor development and the research of high-entropy alloys is presented and an overview of the interatomic potentials is given. Gaussian approximation potentials for WMoTa concentrated alloys are developed using different number of sparse training points. A detailed description of the training database is given and the potentials are validated. The developed potentials are shown to give physically reasonable results in terms of certain bulk and surface properties and could be used in atomistic simulations.

Tutkielman kirjallisuusosassa tarkastellaan johtavien metalli-, oksidi- ja nitridikalvojen kasvattamista epitaksiaalisesti strontiumtitanaatille. Epitaksiaalisia kalvoja on kasvatettu fysikaalisilla kasvatusmenetelmillä, kuten laserpulssikasvatuksella, elektronisuihkuhöyrystyksellä ja sputteroimalla, sekä kemiallisilla kasvatusmenetelmillä, kuten atomikerroskasvatuksella, sooli-geeli-menetelmällä sekä metalliorgaanisella kemiallisella kaasufaasikasvatuksella. Useiden tekijöiden, kuten substraattien lämpötilan ja esikäsittelyn todettiin vaikuttavan kalvojen orientaatioon. Kokeellisessa osassa iridium- ja platinaohutkalvoja kasvatettiin (100)-orientoiduille strontiumtitanaattisubstraateille atomikerroskasvatuksella. Iridiumkalvojen lähtöaineina käytettiin iridiumasetyyliasetonaattia sekä happea tai otsonia ja vetyä. Platinakalvojen lähtöaineina käytettiin platina-asetyyliasetonaattia, otsonia ja vetyä tai metyylisyklopentadienyylitrimetyyliplatinaa ja happea. Kalvojen rakennetta ja tekstuuria tutkittiin θ-2θ- ja in plane -röntgendiffraktiolla. Osaa iridiumkalvojen poikkileikkauksista tutkittiin myös läpäisyelektronimikroskopialla. Iridiumkalvojen todettiin olevan vahvasti (100)-orientoituneita, mutta monikiteisiä. Platinan (h00)-piikkejä ei kyetty erottamaan substraatin (h00)-piikeistä, mutta vahvojen (111)-piikkien perusteella kalvot eivät olleet epitaksiaalisia. Kalvojen kuumentaminen lisäsi (111)-orientaatiota molemmissa metalleissa.

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.