Browsing by Author "Hakala, Mikko"

Now showing 1 - 12 of 12
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    Atomic and electronic transport on surfaces and interfaces
    (Aalto University, 2013) Hakala, Mikko; Teknillisen fysiikan laitos; Department of Applied Physics; COMP/SIN; Perustieteiden korkeakoulu; School of Science; Nieminen, Risto, Distinguished Prof., Aalto University, Department of Applied Physics, Finland; Foster, Adam, Prof., Aalto University, Department of Applied Physics, Finland
    The properties of interfaces and surfaces play a key role in the functional design of many technologies, particularly in the development of next generation micro-electronic devices and nanocatalysts. In micro-electronics, hafnia is seen as a reliable replacement for silica in modern transistors, yet little is known about its interface with silicon and probable defects formed. Similarly, the formation of metallic nanoparticles on insulators is a promising route to new catalytically active materials, but much work is needed to understand the dynamical growth of these particles on surfaces from deposited metal atoms. In this thesis we have modeled the properties of defects in silicon-hafnia interfaces and metal adatoms on alkali halide surfaces. The calculations have been performed within the density-functional theory (DFT), supported by electron transport calculations for the interface studies. Although these results have been successful in building our understanding, we have identified the need to enhance the accuracy of the standard DFT approach without sacrificing computational speed. For this we have implemented efficient hybrid functionals into the SIESTA code and shown that it indeed improves our description of critical materials' properties.
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    Computational scheme for core-electron annihilation in solids
    (1996) Hakala, Mikko; Puska, Martti; Tietotekniikan osasto; Teknillinen korkeakoulu; Helsinki University of Technology; Hautojärvi, Pekka
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    Containerization and Cloud Migration of Legacy Web Services
    (2020-06-16) Piiroinen, Miika; Kotimäki, Jaakko; Hakala, Mikko; Perustieteiden korkeakoulu; Hirvisalo, Vesa
    A research group has multiple web services running on an outdated server hardware. Many of the services are old and not actively developed anymore and also often depend on outdated software, which is problematic from the security point of view. It is time to decommission the old hardware and therefore the services needs to be migrated onto a more modern platform. While migrating away from the old servers, we want to make the services easier for the research group to maintain and improve security where possible. Containerization technologies are an increasingly popular way to build, package and deploy software. Containers provide a convenient way to package software along with it dependencies to be easily run across different computers and operating systems. While being more lightweight than virtual machines, containers provide a layer of isolation between services running on a same host. Containerized services can be hosted on a cloud container platforms such as Kubernetes or OpenShift. In this thesis work, multiple existing web services built on top of varying techonologies are containerized. The containerized services are then deployed onto an OpenShift cloud container platform. We see how containerization can lead to better maintainability and security of olded services. Containerization provides a layer of isolation between the services improving security and makes it easier to deploy them on different plaftorms if needed. The OpenShift platform provides container orchestration and tools for automating builds and deployment, which we utilize to make sure that the services and their dependencies are always kept upt-do-date.
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    Defect complexes in silicon : electronic structures and positron annihilation
    (Helsinki University of Technology, 2001-02-09) Hakala, Mikko; Department of Engineering Physics and Mathematics; Teknillisen fysiikan ja matematiikan osasto; Laboratory of Physics; Fysiikan laboratorio
    In silicon processing technology one of the most important current objectives is to achieve a controlled impurity doping in the crystal. Point defects and defect complexes present in the crystal influence in an important way the electrical activity and the diffusion properties of the dopants. In this thesis, defect complexes in silicon are studied by using quantum-mechanical electronic-structure calculations and by modeling positron annihilation experiments. The electronic-structure calculations are based on the density-functional theory and its state-of-the-art implementations, such as a plane-wave pseudopotential computer code. For the calculation of the momentum density of annihilating electron-positron pairs a new method is presented and tested. It is based on a two-particle description of the correlated pair so that the contact density depends explicitly on the whole spatial distribution of the electron state in question. The new method is found to be superior to the state-independent methods for the momentum density and provides a basis for identifying defect complexes with different chemical surroundings from their momentum distribution fingerprint. In this work, the computational methods are used to study the positron annihilation characteristics at small vacancy clusters in silicon and the properties of typical dopant atoms, which include arsenic and boron. In highly arsenic-doped silicon an electrically inactive defect complex consisting of a vacancy decorated by three arsenic atoms is identified. In boron-doped silicon the defect structures containing one boron atom are analyzed and an estimate is given for the activation energy of boron diffusion.
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    Effect of extractives in birch kraft pulping
    (2022-10-18) Hakala, Mikko; Ruuttunen, Kyösti; Kemian tekniikan korkeakoulu; Vuorinen, Tapani
    This thesis investigated the role of extractives in birch kraft pulping. In the experimental part saw dust was produced from frozen birch wood and extracted with acetone. Some saw dust was dried prior to the extraction. Extracted saw dust samples were pulped along with non-extracted fresh and dried saw dust samples. The effect of extractive removal was assessed by analysing pulp and black liquor for pulp yield, viscosity, kappa number, carbohydrate content, lignin content and residual alkali. Acetone extracts of fresh and dried samples were also analysed by FT-IR spectrometry to study possible drying induced changes in their chemical composition. The acetone-extracted saw dust samples demonstrated higher pulping yield, higher alkali consumption and higher kappa number when compared to the non-extracted samples. Carbohydrate analysis indicated higher xylose content of pulp and lower xylose content of black liquor in the acetone-extracted samples. Proposed explanation for these chemical differences is the behaviour of dissolved xylan, which partially readsorbs from black liquor back to cell wall. Lignin analysis indicated that the lignin content was very similar in all the pulp samples. The higher kappa number level of the acetone-extracted samples may be due to higher hexenuronic acid content of the pulps. FT-IR -analysis revealed only minor drying induced changes in the extract composition. The results indicate that extractives present in pulping might inhibit xylan readsorption and thus effect pulp yield and alkali consumption.
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    First principles modelling a silicon high-k oxide interface
    (2004) Hakala, Mikko; Foster, Adam; Teknillisen fysiikan ja matematiikan osasto; Teknillinen korkeakoulu; Helsinki University of Technology; Nieminen, Risto
    Puolijohdeteollisuus pohjautuu vahvasti integroitujen piirien jatkuvaan kehitykseen. Näissä keskeisenä elementtinä on ollut piioksidi, joka on mahdollistanut nykyisten mikroprosessorien kehityksen. Lähitulevaisuudessa tämän materiaalin rajat kuitenkin tulevat vastaan, ja jotta laitteiden kehitystä voitaisiin jatkaa edelleen, täytyy piioksidi korvata niin sanotulla korkeadielektrisellä materiaalilla. Tässä työssä lupaavinta piioksidin korvaavaa kandidaattia, hafniumoksia, on mallinnettu numeerisesti. Mallintamiseen käytettiin tiheysfunktionaaliteoriaan pohjautuvaa Siesta-koodia ja DFT-GGA-menetelmää. Päätavoite työssä on tarkastella piin ja hafniumoksidin muodostamaa rajapintaa, erityisesti sen atomirakennetta sekä elektronitiloja. Tutkittu rajapinta on rakennettu kerros kerrokselta piialustan päälle. Jokainen kerros on laskennallisesti lämpökäsitelty ja sen jälkeen tuotu perustilaan viemällä lämpötila nollaan asteeseen. Tällä tavoin piipohjan päälle on rakennettu yksi kerros happea ja kolme kerrosta hafniumoksidia. Lopullinen tulos osoittaa, että rakenne ei ole vielä kolmen kerroksen jälkeen eristävä. Syynä ovat saturoitumattomat hafniumsidokset ja rakennetta tulee edelleen kasvattaa, jotta materiaalista tulee eristä.
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    Hydrogen adsorption on doped MoS2 nanostructures
    (2017-11-10) Hakala, Mikko; Kronberg, Rasmus; Laasonen, Kari; Department of Chemistry and Materials Science; Computational Chemistry
    Electrochemical devices for efficient production of hydrogen as energy carrier rely still largely on rare platinum group metal catalysts. Chemically and structurally modified metal dichalcogenide MoS2 is a promising substitute for these critical raw materials at the cathode side where the hydrogen evolution reaction takes place. For precise understanding of structure and hydrogen adsorption characteristics in chemically modified MoS2 nanostructures, we perform comprehensive density functional theory calculations on transition metal (Fe, Co, Ni, Cu) doping at the experimentally relevant MoS2 surfaces at substitutional Mo-sites. Clear benefits of doping the basal plane are found, whereas at the Mo- and S-edges complex modifications at the whole edge are observed. New insight into doping-enhanced activity is obtained and guidance is given for further experiments. We study a machine learning model to facilitate the screening of suitable structures and find a promising level of prediction accuracy with minimal structural input.
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    Hydrogen adsorption on MoS2-surfaces: a DFT study on preferential sites and the effect of sulfur and hydrogen coverage
    (2017-06-07) Kronberg, Rasmus; Hakala, Mikko; Holmberg, Nico; Laasonen, Kari; Department of Chemistry and Materials Science; Computational Chemistry
    We report a comprehensive computational study of the intricate structure–property relationships governing the hydrogen adsorption trends on MoS2 edges with varying S- and H-coverages, as well as provide insights into the role of individual adsorption sites. Additionally, the effect of single- and dual S-vacancies in the basal plane on the adsorption energetics is assessed, likewise with an emphasis on the H-coverage dependency. The employed edge/site-selective approach reveals significant variations in the adsorption free energies, ranging between ∼±1.0 eV for the different edges-types and S-saturations, including differences of even as much as ∼1.2 eV between sites on the same edge. The incrementally increasing hydrogen coverage is seen to mainly weaken the adsorption, but intriguingly for certain configurations a stabilizing effect is also observed. The strengthened binding is seen to be coupled with significant surface restructuring, most notably the splitting of terminal S2-dimers. Our work links the energetics of hydrogen adsorption on 2H-MoS2 to both static and dynamic geometrical features and quantifies the observed trends as a function of H-coverage, thus illustrating the complex structure/activity relationships of the MoS2 catalyst. The results of this systematical study aims to serve as guidance for experimentalists by suggesting feasible edge/S-coverage combinations, the synthesis of which would potentially yield the most optimally performing HER-catalysts.
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    Hydrogen adsorption trends on Al-doped Ni 2 P surfaces for optimal catalyst design
    (2018) Hakala, Mikko; Laasonen, Kari; Department of Chemistry and Materials Science
    Nanoparticles of nickel phosphide are promising materials to replace the currently used rare Pt-group metals at cathode-side electrodes in devices for electrochemical hydrogen production. Chemical modification by doping can be used to fine-tune the electrocatalytic activity, but this path requires theoretical, atomic-level support which has not been widely available for Ni-P. We present a density functional theory analysis of Al-doped Ni2P surfaces to identify structural motifs that could contribute to the improved behavior of the catalyst. Based on the formation energies of substitutionally Al-doped Ni sublattices, we find doping to take place preferably at the topmost layers. The Ni-Ni bridge and the P-top sites are the optimal ones in terms of hydrogen bonding energies. The Ni-Ni bridge site is not present on pristine surfaces but is a consequence of Al doping and provides a candidate to explain the experimentally observed high activities in doped Ni-P nanoparticles. Similar structural motifs can be recommended to be engineered for other Ni-P structures for improved electrocatalytic activity.
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    Hydrogen adsorption trends on various metal-doped Ni2P surfaces for optimal catalyst design
    (2019-01-01) Partanen, Lauri; Hakala, Mikko; Laasonen, Kari; Department of Chemistry and Materials Science
    In this study, we looked at the hydrogen evolution reaction on Mg-, Mo-, Fe-, Co-, V-, and Cu-doped Ni3P2 and Ni3P2 + P terminated Ni2P surfaces. The DFT calculated hydrogen adsorption free energy was employed as a predictor of the materials' catalytic HER activity. Our results indicate that doping can substantially improve the catalytic activity of the Ni3P2 terminated surface. In contrast, the Ni3P2 + P terminated one seems to be catalytically active irrespective of the type of doping, including in the absence of doping. Based on our doping energy and adsorption free energy calculations, the most promising dopants are iron and cobalt, whereas copper is less likely to function well as a doping element.
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    The influence of the production parameters on the quality of powder coatings
    (2007) Uusinoka, Mikko; Hakala, Mikko; Kemian tekniikan osasto; Teknillinen korkeakoulu; Helsinki University of Technology; Koskinen, Ari
    Työn tarkoituksena oli tutkia kolmen valmistusparametrin, esisekoitusajan, ekstruuderin lämpötilan ja ekstruuderin kierrosnopeuden, vaikutusta jauhemaalien laatuun. Tutkimuksen kohteena oli punainen polyesterijauhe ja sininen epoksipolyesterijauhe eli hybridi. Työn kirjallisuusosassa tarkasteltiin jauhemaalien raaka-aineita: erilaisia hartseja, täyteaineita ja apuaineita sekä kovettimia ja kovetusreaktioita. Kirjallisuusosassa käytiin lävitse myös jauhemaalien valmistusprosessi, ruiskutus kitka- ja koronamenetelmillä sekä kiillon säätö. Tutkimusosan alussa tarkasteltiin työn vaiheita: esisekoitus säiliösekoittimella, ekstruusio, saadun massan murskaus ja jauhatus, seulonta sekä ruiskutus teräs- ja alumiinilevyille. Kummastakin jauheesta ajettiin 27 ajoa muuttamalla kolmea tutkittavaa parametria. Jokaisesta ajosta ruiskutettiin sekä alumiinilevy, josta mitattiin kiilto ja värierot, että rautalevy, jolle tehtiin iskunkestävyystesti. Kiillossa ja iskunkestävyydessä ei kummallakaan hartsityypillä tapahtunut muutoksia valmistusparametreja muutettaessa. Pieniä värieroja syntyi sekä hybridissä että polyesterissä. Polyesterissä keltainen pigmentti dispergoitui selvästi paremmin kuin punainen. Tästä johtui, että vaikka valmistusolosuhteet rankkenivat, ei punaisuus lisääntynyt. Hybridissä selvää tulosta ei saatu. Sinisyydessä ei tapahtunut johdonmukaisia muutoksia. Kokeiden perusteella voidaan todeta, että valmistusparametreilla ei ole suurta vaikutusta kiiltoon ja iskunkestävyyteen. Värieroihin vaikutus määräytyy käytettyjen pigmenttien dispergoitumisen asteesta. Kaksipigmenttinen jauhemaali voi käyttäytyä yllättävällä tavalla, jos toinen pigmentti dispergoituu helpommin.
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    Machine vision for visual inspection of electronic equipment
    (2017-10-23) Halme, Taneli; Hakala, Mikko; Sähkötekniikan korkeakoulu; Kyrki, Ville