Browsing by study line "Experimental Materials Physics"
Now showing items 1-9 of 9
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(2024)Fibrous meat analogues (FMAs) are an emerging category of synthetic plant-based food materials intended to mimic animal meat in both taste and mouthfeel. However, replicating the texture of meat with plant matter is far from trivial. Designing a manufacturing process that gives the produced FMA the desired properties requires proper characterisation of the material and its behaviour. One source of such knowledge could be obtained through computed x-ray absorption microtomography (µCT) --- a microscopic technique capable of producing a 3D model of the internal structure of the target object without requiring its disassembly. Unfortunately, while the scanning itself can be rather straightforward, issues arise when the recorded imagery is to be processed for analysis, as the researcher's choices of the employed methods, algorithms and their parameters may become a potential source of bias, affecting the end results in unintended ways. In this work, we examine the data processing pipeline of µCT studies on FMA samples, discuss the shortcomings of some of the employed methods, and attempt to address these problems by modifying the steps as needed, the aim being to reduce the reliance of the utilised methods on the researcher's judgment. Our main focus is in particular on the segmentation step, where the values provided by the µCT scan are interpreted and simplified to a form that subsequent algorithm-based analysis methods can operate on.
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(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.
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(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.
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(2021)This thesis describes the data and data analysis concerning Compton scattering experiments to obtain the Compton profiles of metallic sodium (Na) as a function of temperature. The temperatures used in the experiment were 6 K and 300 K. The purpose of the work was to visualize the effect of temperature in the electron momentum density in a free electron gas. The effects of temperature were expected to be manifested through changes to the Fermi momentum according to the free-electron theory, but also more subtle changes could have been possible owing to possible deviations from the free electron theory. The measurements were done at the European Synchrotron Radiation Facility (Grenoble, France) beamline ID20. The data was analyzed with a help of a program written with Matlab, and it converted the measured Compton spectra from photon energy space to electron momentum space, while applying self-absorption corrections to the data, subtracting background, and normalizing the data using trapezoidal numerical integral to yield final Compton profiles. Results were obtained as valence Compton profiles and their differences between 300 K and 6 K, and compared with the prediction from free-electron gas theory. The Compton profiles followed the predictions of the free-electron gas theory well, although the theoretical profiles had a higher amplitude than the measured profiles. This is a commonly found phenomenon in Compton spectroscopy and assigned to originate from electron-electron correlations. The effect of the temperature in the Compton profiles is in good agreement with the free-electron theory.
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(2023)Lentoaika-rekyylispektrometria (TOF-ERDA) on materiaalin tutkimusmenetelmä, minkä avulla kyetään selvittämään näytteen alkuainepitoisuudet syvyyden funktiona. Menetelmässä on huomattu esiintyvän systemaattista poikkeamaa teoriasta. Poikkeaman syy on toistaiseksi epäselvä. Tutkielmassa tutkittiin poikkeaman syytä sekä suuruutta. Tutkimiseen käytettiin Helsingin yliopiston kiihdytinlaboratorion 5 MV EPG-10-II-tandemkiihdyttimeen liitettyä TOF-ERDA-laitteistoa. Laitteistolla tutkittiin ilmiön esiintymistä eri hiukkasilla, eri energioilla ja eri näytteillä. Kokeiden tuloksia verrattiin teorian ehdottamiin tuloksiin. Suurimmat poikkeamat kokeiden ja teorian välillä esiintyi pienienergisillä ja keveillä ammusioneilla, kun tutkittiin raskasalkuaineista näytettä. Kokeissa käytettyjen hiukkasten sirontaa TOF-ERDA-laitteiston lentoaikaporteista selvitettiin simuloinneilla. Simulointityökaluna työssä käytettiin SRIM-ohjelmistoa. Simulointien pohjalta hiukkasille laskettiin korjauskertoimet. Korjauskertoimen suuruuden huomattiin olevan riippuvainen hiukkasen energiasta sekä järjestysluvusta. Korjauskertoimista luotiin malli, jonka avulla kyetään arvioimaan ja korjaamaan kokeissa tapahtuvaa sirontaa lentoaikaporteista. Myös varjostumisen vaikutusta tuloksiin tutkittiin. Varjostumista tutkittiin jo olemassa olevan Andersenin mallin avulla. Andersenin mallin mukaan varjostumisen vaikutus tuloksiin on hyvin vähäistä sekä ilmiötä pienentävää, ei ilmiötä selittävää. Huomioitavaa kuitenkin on mallin soveltumattomuus kyseisiin vuorovaikutuksiin, täten varjostumisen vaikutusta ilmiöön ei voida kokonaan poissulkea. Tutkielman tulokset valottavat TOF-ERDA:n systemaattisen poikkeaman syitä. Lentoaikaporttisironnan vaikutus tuloksiin on merkittävä, selittäen noin puolet ilmiöstä. Tulosten pohjalta luotu malli on voimassa vain kyseiselle TOF-ERDA-laitteistolle, sillä sironnan suuruus on riippuvainen lentoaikaporttien materiaalista, paksuuksista sekä laitteiston geometriasta. Kuitenkin mille tahansa TOF-ERDA-laitteistolle on mahdollista luoda vastaava malli, noudattamalla tämän tutkielman prosesseja. Jäljelle jäävän selittämättömän poikkeaman huomattiin olevan myös riippuvainen energiasta sekä järjestysluvuista. Tämä viittaisi jäljelle jäävän poikkeaman aiheutuvan mahdollisesta monikertasironnasta näytteessä, ja tai varjostumisesta, jota Andersenin malli ei kykene huomioimaan.
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(2023)Radiation therapy is one of the key treatments for cancer, utilizing ionizing radiation to destroy cancer cells. Proton therapy uses high-energy proton beams since protons have a favorable depth-dose curve. Clinical proton beams must meet strict quality standards in order to maximise the efficacy of the treatment while ensuring the patient safety. Real-time knowledge of the beam’s intensity profile is essential for an accurate beam delivery. While gas-filled ionization chambers have traditionally been used as the standard beam monitor, the swift development of the beam delivery techniques demands for more accurate beam monitors. Semiconductor detectors potentially offer more accurate and efficient alternative for ionization chambers. In this study, the feasibility of using a silicon pixel detector in proton beams was investigated. The detector was originally designed for tracking minimum ionizing particles at the CMS experiment at CERN. Two experiments — one with an alpha source and one in a proton beam — were carried out to characterize the detector. The response to protons with different intensities and energies was investigated more closely in the proton beam. The results show that the detector response to different proton energies agrees with theoretical expectations. The saturation of the pixels limits measuring the full energy of the protons, however measuring the full energy is not essential in beam profile measurements. The detector also has a linear response to the beam intensity, although, the counting efficiency of the detector should be improved with new readout electronics. With different readout electronics, the detector might be a viable option for the beam profile measurements in clinical proton beams.
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(2022)Positron Annihilation Spectroscopy is a powerful tool for defect characterisation, especially vacancies. Various defect properties can be studied, including defect behaviour at low and high temperatures. Despite the technique having its roots in the mid-20th century, there is little research on fundamental positron behaviour at ultralow temperatures. In this thesis, Positron Annihilation Lifetime Spectroscopy and Doppler Broadening Spectroscopy, two sub-methods of the spectroscopy technique, were used to measure positron trap-free Ge in the temperature range of 14 mK-300 K. Since a positron trap-free sample was used, the purpose was not to study defect processes. Instead, the aim of the thesis was to investigate whether any interesting positron processes could be seen at ultralow temperatures in the annihilation data. Previous research in Al has shown no change in either lifetime or Doppler broadening below 77 K. Measuring the positron lifetime in the sample located in a cryostat required designing a special detector setup, as the count rate was greatly reduced due to geometry. To tackle this, lifetime detectors consisting of BaF2 scintillators and quartz-windowed photomultiplier tubes were used. In addition, both analogue and digital signal processing techniques were tested for the lifetime setup, with the digital method proving to be preferable. Doppler Broadening was measured with a high-purity germanium detector connected to a digital gamma spectrometer. The results show a decrease in S-parameter and an increase in W-parameter with decresing temprature, with the rate of change being greatest at ultralow temperatures. This behaviour is concluded to be due to incomplete positron thermalization. The positron lifetime results are more difficult to interpret, as setup challenges resulted in results of questionable accuracy. Still, the trend suggests no change in lifetime over the whole temperature interval, which is in accordance with previous research.
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(2024)One of the main goals of materials research is to find the link between the properties of materials and their fundamental structures. The distinct properties of thin films, categorized as materials from a few single layers of atoms to some hundreds of nanometers, have enjoyed an unparalleled demand in modern device manufacturing, and thus the investigation of factors which determine thin film structure and morphology is a vital area of research. In the case of thin films, their final structures can often be connected back to the initial film formation stages, such as in the crystallographic growth competition during island growth and coalescence. In this thesis, thin film growth stages are studied from the perspective of how they are affected by impurities. From the initial diffusion of adatoms on a bare substrate; to the formation of islands, their growth and coalescence; to the mobility of grain boundaries and bulk diffusion leading to the formation of a fully continuous layer; impurities influence each of these thin film growth processes in a multifaceted way, either acting as growth inhibitors, promoters or potentially neutral agents. To this end, Ag and Cu thin films were synthesized by magnetron sputtering onto SiO_2/Si substrates, with thicknesses ranging from 3 nm to 30 nm using varied deposition conditions, with the addition of a 3 nm amorphous carbon layer to limit further restructuring and oxidation. Impurities were let into the deposition atmosphere via a controlled opening of a leak valve, corresponding to a step-wise increase of base pressure from 10^(-8) Torr to 10^(-6) Torr and finally 10^(-5) Torr. The full range of thin films was deposited with each base pressure (except for 10^(-5) Torr for Cu) using two deposition rates, around 0.1 Ås^(-1) and 2 Ås^(-1). Each film was characterized ex situ with ellipsometry, 4PP, XRD and AFM to map the morphological and microstructural evolution during film growth. It is found that impurities tend to inhibit island coalescence and initial grain growth, resulting in a reduction of continuous film formation thickness and average grain size, leading to the formation of flatter films with, in most cases, less surface roughness. In later stages, it is found that impurities may allow for more grain growth by their incorporation into the growing facets. In terms of crystal structure, it is shown that impurities have a more pronounced effect on (111) oriented grains, inhibiting their growth, thus altering the preferred growth orientations of Ag and Cu by allowing (200) grains to grow larger. Grain radii and equivalent ellipse distributions showed the different responses of Ag and Cu to impurities. Ag films showed more prominent effects when a lower deposition rate was used, highlighting the impact of impurities on diffusive processes, while Cu films exhibited more effects with the use of higher deposition rates, indicating that the role of impurities, in this case, was more significant after the formation of a continuous layer.
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(2023)X-ray emission spectroscopy (XES) is an elemental characteristic x-ray technique that is used in studying the electronic properties of varied materials. It is based on an x-ray emission process that occurs due to an absorption process. First an x-ray photon is shot at the sample and electron in the inner core absorbs this x-ray photon. The electron gets kicked out of its place, creating an empty electronic state called an electron hole. This leaves the atom to an excited state that then de-excites by emitting an elemental characteristic x-ray photon. One of the main aspects of x-ray emission spectroscopy is to study sensitivity of XES towards different emission lines in different chemical environments and then compare the results. X-ray emission spectroscopy is a relatively easy technique to perform because it does not require a long-range crystal order and experiments can be done in an atmospheric pressure and in room temperature. A series of measurements were performed with non-resonant XES to study chemical properties of different uranium oxides. The experiment was done at the SOLEIL Synchrotron Facility in France. Main goal of the experiment was to study the sensitivity of XES to different uranium L3-emission lines by measuring different uranium oxides with different uranium oxidation states and compare the shape of each emission spectrum. The results showed that there are no visible differences between different samples when using non-resonant XES. However, the experiment also showed that when the resonant part is included in the measurements, then there are visible differences between different samples. This indicates the resonant part is required to collect to see differences in the spectrum when comparing the speciation of different samples.
Now showing items 1-9 of 9