Browsing by master's degree program "Materiaalitutkimuksen maisteriohjelma (Materials Research)"

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.

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.

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.

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.

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.

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.

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.

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.

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.