MOCVD growth andproperties of GaN and AlGaN layers

In this work GaN and AlGaN layers were grown by metal-organic chemical vapor deposition (MOCVD) on sapphire substrates. The research was carried out at Micro and Nanoscience Laboratory of Helsinki University of Technology. The objective of this thesis is the study of MOCVD technique for the growth of GaN and AlGaN films and optimization of growth parameters in purpose to improve crystal quality of the films. The widely used two-step and the new multistep methods have been used for GaN, AlGaN MOCVD growth on c-plane sapphire. Properties of the GaN and AlGaN layers were studied using in-situ reflectance monitoring during MOCVD growth, atomic force microscopy and x-ray diffraction. Compared to the two step method, the multistep method has produced even better qualities of the GaN and AlGaN layers and significant reduction of threading dislocation density.

Hiilinanoputkireaktorin suunnittelu

Hiilinanoputki on vasta 90-luvun alussa löydetty uusi hiilestä koostuva materiaali, jonka erinomaiset mekaaniset ja fysikaaliset ominaisuudet tarjoavat niiden käytölle useita mahdollisia sovelluskohteita. Teknologian puute ja valmistusmenetelmien korkeat kustannukset ovat kuitenkin estäneet tehokkaasti niiden käytön nykyisten materiaalien ja puolijohteiden korvaajana. Tämän työn tarkoituksena on esitellä yleisimmät menetelmät hiilinanoputkien syntetisoimiseksi sekä suunnitella laite yksiseinäisten hiilinanoputkien tuottamiseen kemiallisen höyrydeposition avulla. Lisäksi tavoitteena on luoda laitteelle modulaarinen rakenne, jolloin sen eri osien korvaaminen rajapintojen sallimissa rajoissa on helppoa. Reaktorin mekaanisen suunnittelun ja komponenttien valinnan lisäksi työssä käsitellään laitteen kaasu- ja lämpövirtauksia, prosessissa tärkeiden katalyyttipartikkelien tuotantoa sekä laitteessa tarvittavien jäähdytysjärjestelmien mitoituksia. Tuloksena syntyi helposti toteutettava suunnitelma yksiseinäisiä nanoputkia tuottavan reaktorin valmistamiseksi. Työ jatkuu laitteen rakentamisella, testaamisella sekä jatkokehittelyllä.

Reducing contact resistance in graphene devices

In this thesis, the contact resistance of graphene devices was investigated because high contact resistance is detrimental to the performance of graphene field-effect transistors (GFET). Method for increasing so-called edge-contact area was applied in device fabrication process, as few nanometers thick Ni layer was used as a catalytic etchant during the annealing process. Finally, Ni was also used as a metal for contact. GFETs were fabricated using electron beam lithography using graphene fabricated by chemical vapor deposition (CVD). Critical part of the fabrication process was to preserve the high quality of the graphene channel while etching the graphene at contact areas with Ni during the annealing. This was achieved by optimizing the combination of temperature and gas flows. The structural properties of graphene were studied using scanning electron microscopy, scanning confocal μ-Raman spectroscopy and optical microscopy. Evaluation of electric transport properties including contact resistance was carried out by transmission line method and four-probe method. The lowest contact resistance found was about at 350 Ωμm. In addition, different methods to transfer CVD graphene synthesized on copper were studied. Typical method using PMMA as a supporting layer leaves some residues after its removal, thus effecting on the performance of a graphene devices. In a metal assisted transfer method, metal is used as an interfacial layer between PMMA and graphene. This allows more effective removal of PMMA. However, Raman spectra of graphene transferred by metal assisted method showed somewhat lower quality than the PMMA assisted method

Droplet Deposition in the Last Stage of Steam Turbine

During the expansion of steam in turbine, the steam crosses the saturation line and hence subsequent turbine stages run under wet condition. The stages under wet condition run with low efficiency as compared to stages running with supersaturated steam and the life of the last stage cascade is reduced due to erosion. After the steam crosses the saturation line it does not condense immediately but instead it becomes supersaturated which is a meta-stable state and reversion of equilibrium results in the formation of large number of small droplets in the range of 0.05 - 1 μm. Although these droplets are small enough to follow the stream lines of vapor however some of the fog droplets are deposited on the blade surface. After deposition they coagulate into films and rivulets which are then drawn towards the trailing edge of the blade due to viscous drag of the steam. These large droplets in the range of radius 100 μm are accelerated by steam until they impact on the next blade row causing erosion. The two phenomenon responsible for deposition are inertial impaction and turbulent-diffusion. This work shall discuss the deposition mechanism in steam turbine in detail and numerically model and validate with practical data.

Uptake of heavy metals by plants from airborne deposition and polluted soils

Selostus: Kasvien raskasmetallien otto ilmasta ja saastuneesta maasta

Seasonal dry deposition and canopy leaching of base cations in a subtropical evergreen mixed forest, China

Abstract

Water vapor tightness of packages: methods and optimizing

Työn tarkoituksena oli testata jo tutkimuskeskuksella käytössä ollutta ja tutkimuskeskukselle tässä työssä kehitettyä pakkauksen vesihöyrytiiveyteen liittyvää mittausmenetelmää. Saatuja tuloksia verrattiin keskenään sekä materiaalista mitattuihin arvoihin. Elintarvikepakkauksia tutkittiin myös kosteussensoreiden, säilyvyyskokeen sekä kuljetussimuloinnin avulla. Optimoinnilla tutkittiin pakkauksen muodon vaikutusta vesihöyrytiiveyteen. Pakkauksen vesihöyrynläpäisyn mittaamiseen kehitetty menetelmä toimi hyvin ja sen toistettavuus oli hyvä. Verrattaessa sitä jo olemassa olleeseen menetelmään tulokseksi saatiin, että uusi menetelmä oli nopeampi ja vaati vähemmän työaikaa, mutta molemmat menetelmät antoivat hyviä arvoja rinnakkaisille näytteille. Kosteussensoreilla voitiin tutkia tyhjän pakkauksen sisällä olevan kosteuden muutoksia säilytyksen aikana. Säilyvyystesti tehtiin muroilla ja parhaan vesihöyrysuojan antoivat pakkaukset joissa oli alumiinilaminaatti- tai metalloitu OPP kerros. Kuljetustestauksen ensimmäisessä testissä pakkauksiin pakattiin muroja ja toisessa testissä nuudeleita. Kuljetussimuloinnilla ei ollutvaikutusta pakkausten sisäpintojen eheyteen eikä siten pakkausten vesihöyrytiiveyteen. Optimoinnilla vertailtiin eri muotoisten pakkausten tilavuus/pinta-ala suhdetta ja vesihöyrytiiveyden riippuvuutta pinta-alasta. Optimaalisimmaksi pakkaukseksi saatiin pallo, jonka pinta-ala oli pienin ja materiaalin sallima vesihöyrynläpäisy suurin ja vesihöyrybarrierin määrä pienin.

Study of a vapor-compression air-conditioning system for jetliners

Most modern passenger aeroplanes use air cycle cooling. A high-speed air cycle is a reliable and light option, but not very efficient. This thesis presents research work done to design a novel vapour cooling cycle for aeroplanes. Due to advancements in high-speed permanent magnet motors, the vapour cycle is seen as a competitive option for the air cycle in aeroplanes. The aerospace industry places tighter demands on the weight, reliability and environmental effects of the machinery than those met by conventional chillers, and thus modifications to conventional design are needed. The thesis is divided into four parts: the initial screening of the working fluid, 1-D design and performance values of the compressor, 1-D off-design value predictions of the compressor and the 3-D design of the compressor. The R245fa was selected as the working fluid based the study. The off-design range of the compressor was predicted to be wide and suitable for the application. The air-conditioning system developed is considerably smaller than previous designs using centrifugal compressors.

Superconducting properties and their enhancement in ReBa2Cu3O7-delta (RE = Y and Gd) films prepared by pulsed laser deposition

Due to font problem on the tilte field the titlte of the thesis is corrected here. The title of the thesis is: Superconducting properties and their enhancement in ReBa₂Cu₃O₇-delta (RE = Y and Gd) films prepared by pulsed laser deposition

PET and MR imaging in Parkinson’s disease patients with cognitive impairment. A study of dopaminergic dysfunction, amyloid deposition, cortical hypometabolism and brain atrophy

Parkinson’s disease (PD) is the second most common neurodegenerative disorder. It is characterized by a severe loss of substantia nigra dopaminergic neurons leading to dopamine depletion in the striatum. PD affects movement, producing motor symptoms such as rigidity, tremor and bradykinesia. Non-motor symptoms include autonomic dysfunction, neurobehavioral problems and cognitive impairment, which may lead to dementia. The pathophysiological basis of cognitive impairment and dementia in PD is unclear. The aim of this thesis was to study the pathophysiological basis of cognitive impairment and dementia in PD. We evaluated the relation between frontostriatal dopaminergic dysfunction and the cognitive symptoms in PD patients with [18F]Fdopa PET. We also combined [C]PIB and [¹⁸F]FDG PET and magnetic resonance imaging in PD patients with and without dementia. In addition, we analysed subregional striatal [¹⁸F]Fdopa PET data to find out whether a simple ratio approach would reliably separate PD patients from healthy controls. The impaired dopaminergic function of the frontostriatal regions was related to the impairment in cognitive functions, such as memory and cognitive processing in PD patients. PD patients with dementia showed an impaired glucose metabolism but not amyloid deposition in the cortical brain regions, and the hypometabolism was associated with the degree of cognitive impairment. PD patients had atrophy, both in the prefrontal cortex and in the hippocampus, and the hippocampal atrophy was related to impaired memory. A single 15-min scan 75 min after a tracer injection seemed to be sufficient for separating patients with PD from healthy controls in a clinical research environment. In conclusion, the occurrence of cognitive impairment and dementia in PD seems to be multifactorial and relates to changes, such as reduced dopaminergic activity, hypometabolism, brain atrophy and rarely to amyloid accumulation.

Polymer surface modification by atomic layer deposition

Current industrial atomic layer deposition (ALD) processes are almost wholly confined to glass or silicon substrates. For many industrial applications, deposition on polymer substrates will be necessary. Current deposition processes are also typically carried out at temperatures which are too high for polymers. If deposition temperatures in ALD can be reduced to the level applicable for polymers, it will open new interesting areas and applications for polymeric materials. The properties of polymers can be improved for example by coatings with functional and protective properties. Although the ALD has shown its capability to operate at low temperatures suitable for polymer substrates, there are other issues related to process efficiency and characteristics of different polymers where new knowledge will assist in developing industrially conceivable ALD processes. Lower deposition temperature in ALD generally means longer process times to facilitate the self limiting film growth mode characteristic to ALD. To improve process efficiency more reactive precursors are introduced into the process. For example in ALD oxide processes these can be more reactive oxidizers, such as ozone and oxygen radicals, to substitute the more conventionally used water. Although replacing water in the low temperature ALD with ozone or plasma generated oxygen radicals will enable the process times to be shortened, they may have unwanted effects both on the film growth and structure, and in some cases can form detrimental process conditions for the polymer substrate. Plasma assistance is a very promising approach to improve the process efficiency. The actual design and placement of the plasma source will have an effect on film growth characteristics and film structure that may retard the process efficiency development. Due to the fact that the lifetime of the radicals is limited, it requires the placement of the plasma source near to the film growth region. Conversely this subjects the substrate to exposure byother plasma species and electromagnetic radiation which sets requirements for plasma conditions optimization. In this thesis ALD has been used to modify, activate and functionalize the polymer surfaces for further improvement of polymer performance subject to application. The issues in ALD on polymers, both in thermal and plasma-assisted ALD will be further discussed.

Atomikerroskasvatuksen soveltaminen kemiantekniikassa

Tässä kirjallisuustyössä tutkittiin atomikerroskasvatuksen (ALD) soveltamista kemiantekniikassa. Työn alussa kerrottiin atomikerroskasvatuksesta, sen toimintaperiaatteista ja prosessitekniikasta. Tämän jälkeen tutkittiin viittä eri kemiantekniikan sovellusta, jotka olivat polymeerien pinnoittaminen, heterogeenisten katalyyttien syntetisointi, membraanien modifiointi, korroosionesto ja kaasunilmaisimet. ALD on ohutkalvotekniikka, jolla voidaan valmistaa nanometrin tai jopa Ångströmin (1 Å = 0.1 nm) tarkkuudella epäorgaanisia materiaalikerroksia, jotka yleensä ovat metallioksideja, kuten alumiinioksidi. ALD perustuu kaasu-kiintoainereaktioihin, joissa kaasumaiset kemialliset prekursorit reagoivat vuorotellen kasvualustan kanssa. Tyypilliset prekursorit ovat metalliligandi ja vesi, joka on yleisin hapen lähde ALD-reaktioissa. ALD−reaktiot suoritetaan yleensä matalassa paineessa (100−200 Pa) ja korkeassa lämpötilassa (200–400 °C) suljetussa reaktorikammiossa. ALD-prosesseissa voidaan hyödyntää myös plasmaa alentamaan reaktiolämpötiloja. Plasman avulla prekursoreista luodaan hyvin reaktiivisia radikaaleja, jotka voivat reagoida jopa huoneenlämmössä. Lämpöherkkiä polymeerejä voidaan pinnoittaa ohutkalvoilla, joilla voidaan lisätä esimerkiksi pakkausmateriaalien suojaa happea ja vesihöyryä vastaan. ALD:llä voidaan syntetisoida tarkasti nanomittakaavan heterogeenisiä katalyyttejä, joilla on korkea dispersio tukimateriaalin pinnalla. ALD:n avulla voidaan säästää katalyyttimateriaalia menettämättä katalyytin aktiivisuutta, mikä on tärkeää monien katalyyttisovellusten taloudellisuuden kannalta, esimerkiksi polttokennot. ALD soveltuu hyvin membraanien modifiointiin, koska kaasumaiset prekursorit leviävät tasaisesti membraanin huokosiin. Membraanien pinnoittamisella pyritään vaikuttamaan, selektiivisyyteen, hydrofiilisyyteen, liuotinkestävyyteen, huokoskokoon ja sen jakaumaan. Lisäksi membraaneja voidaan pinnoittaa katalyyttisillä ohutkalvoilla, mikä on tärkeää nanoreaktoreiden kehityksen kannalta. ALD:llä voidaan pinnoittaa esimerkiksi terästä, ja vähentää täten teräksen korroosiota. Puolijohtavia metallioksideja voidaan käyttää kaasunilmaisimina, joiden valmistuksessa ALD:n tarkkuudesta on suurta hyötyä.

Application of cerium oxide thin films grown by atomic layer deposition for soot removal

The objective of the thesis is to study cerium oxide thin films grown by the atomic layer deposition (ALD) for soot removal. Cerium oxide is one of the most important heterogeneous catalysts and can be used in particulate filters and sensors in a diesel exhaust pipe. Its redox/oxidation properties are a key factor in soot oxidation. Thus, the cerium oxide coating can help to keep particulate filters and sensors clean permanently. The literature part of the thesis focuses on the soot removal, introducing the origin and structure of soot, reviewing emissions standards for diesel particulate matter, and presenting methods and catalysts for soot removal. In the experimental part the optimal ALD conditions for cerium oxide were found, the structural properties of cerium oxide thin films were analyzed, and the catalytic activity of the cerium oxide for soot oxidation was investigated. Studying ALD growth conditions of cerium oxide films and determining their critical thickness range are important to maximize the catalytic performance operating at comparatively low temperature. It was found that the cerium oxide film deposited at 300 °C with 2000 ALD cycles had the highest catalytic activity. Although the activity was still moderate and did not decrease the soot oxidation temperature enough for a real-life application. The cerium oxide thin film deposited at 300 °C has a different crystal structure, surface morphology and elemental composition with a higher Ce3+ concentration compared to the films deposited at lower temperatures. The different properties of the cerium oxide thin film deposited at 300 °C increase the catalytic activity most likely due to higher surface area and addition of the oxygen vacancies.

Een eenfalligh Kennedoom af nödtorftelighe articklar vthdraghne aff then Helga schrifft / Laurentius Olai Gestricius. (Coll. 762, Aholansaari Foundation's deposition)

Quire structure: 9xIV⁷² + (IV–1)⁷⁹. Text in one column, on (mostly) 26–33 lines, not ruled. Catchwords frequently used at the foot of each page to indicate the next word when a sentence continues on the next page. Not foliated. Gothic Cursive in one saec. XVI² hand. The first initial of the book has been touched with what looks like red pencil. This may be a later addition, as also the green tint to the outer edges of the block. Titles and rubrics distinguished, if at all, through layout only. A note fol. [47]v (see also the main text on fol. [31]v) indicates that the text was copied in 1580. The Anti-Papist poem mentions the Jesuit Antonio Possevino, active in Sweden in 1577–1580 (Kiiskinen ed. 2010, 25, 27). The texts appear to be copies of printed works, with the possible exception of the Anti-Papist poem. The publication of Eric Falck’s Een Tröstbook is known, but apparently no printed copies survive; Laurentius Olai Gestricius’ catechism is otherwise unknown. He was a teacher in Gävle from 1557 or 1558, then curate of Västerås from 1561 and of Stockholm in 1562, where he died in 1565 (Kiiskinen ed. 2010, 15, 349; Collijn ed. 1927–1938, vol. 2, 324).