Liquid-phase exfoliation of bismuth telluride iodide (BiTeI): structural and optical properties of single-/few-layer flakes

Bismuth telluride halides (BiTeX) are Rashba-type crystals with several potential applications ranging from spintronics and nonlinear optics to energy. Their layered structures and low cleavage energies allow their production in a two-dimensional form, opening the path to miniaturized device concepts. The possibility to exfoliate bulk BiTeX crystals in the liquid represents a useful tool to formulate a large variety of functional inks for large-scale and cost-effective device manufacturing. Nevertheless, the exfoliation of BiTeI by means of mechanical and electrochemical exfoliation proved to be challenging. In this work, we report the first ultrasonication-assisted liquid-phase exfoliation (LPE) of BiTeI crystals. By screening solvents with different surface tension and Hildebrandt parameters, we maximize the exfoliation efficiency by minimizing the Gibbs free energy of the mixture solvent/BiTeI crystal. The most effective solvents for the BiTeI exfoliation have a surface tension close to 28 mN m(-1) and a Hildebrandt parameter between 19 and 25 MPa0.5. The morphological, structural, and chemical properties of the LPE-produced single-/few-layer BiTeI flakes (average thickness of & SIM;3 nm) are evaluated through microscopic and optical characterizations, confirming their crystallinity. Second-harmonic generation measurements confirm the non-centrosymmetric structure of both bulk and exfoliated materials, revealing a large nonlinear optical response of BiTeI flakes due to the presence of strong quantum confinement effects and the absence of typical phase-matching requirements encountered in bulk nonlinear crystals. We estimated a second-order nonlinearity at 0.8 eV of |chi((2))| & SIM; 1 nm V-1, which is 10 times larger than in bulk BiTeI crystals and is of the same order of magnitude as in other semiconducting monolayers (e.g., MoS2)

Cu2Se and Cu Nanocrystals as Local Sources of Copper in Thermally Activated in Situ Cation Exchange

Among the different synthesis approaches to colloidal nanocrystals, a recently developed toolkit is represented by cation exchange reactions, where the use of template nanocrystals gives access to materials that would be hardly attainable via direct synthesis. Besides, postsynthetic treatments, such as thermally activated solid-state reactions, represent a further flourishing route to promote finely controlled cation exchange. Here, we report that, upon in situ heating in a transmission electron microscope, Cu2Se or Cu nanocrystals deposited on an amorphous solid substrate undergo partial loss of Cu atoms, which are then engaged in local cation exchange reactions with Cu “acceptor” phases represented by rod- and wire-shaped CdSe nanocrystals. This thermal treatment slowly transforms the initial CdSe nanocrystals into Cu2−xSe nanocrystals, through the complete sublimation of Cd and the partial sublimation of Se atoms. Both Cu “donor” and “acceptor” particles were not always in direct contact with each other; hence, the gradual transfer of Cu species from Cu2Se or metallic Cu to CdSe nanocrystals was mediated by the substrate and depended on the distance between the donor and acceptor nanostructures. Differently from what happens in the comparably faster cation exchange reactions performed in liquid solution, this study shows that slow cation exchange reactions can be performed at the solid state and helps to shed light on the intermediate steps involved in such reactions

Thermal Conductivity of Die Cast AlSi10Mg(Fe) Aluminium

Tiivistelmä: Painevalettuja alumiinikomponentteja käytetään usein sähköelektroniikkateollisuuden laitteissa. Prosessorien ja tehovahvistimien kehittyessä niiden tuottama lämpökuorma kohdistuu yhä pienemmälle alalle. Jos lämpöä ei voida tehokkaasti johtaa ja levittää tukirakenteisiin, voivat elektroniikkakomponentit rikkoutua. Alumiini on hyvän lämmönjohtavuutensa ja keveytensä vuoksi yleinen materiaali elektroniikan jäähdytysrivastoissa. Kohtalaisen jäykkyytensä ansiosta se tarjoaa lisäksi tukevan kiinnitysalustan elektroniikalle. Painevalumenetelmällä kotelointi ja lämpöäjohtavat alumiiniset jäähdytysrakenteet saadaan yhdistettyä kustannustehokkaasti samaan komponenttiin. Hyvin johtavilla materiaaleilla voidaan välttää kalliit ja vikaantumisalttiit aktiiviset jäähdytyskomponentit. Työn tavoitteena oli selvittää, mitkä kemialliset ja mikrorakenteelliset ominaisuudet vaikuttavat painevaletun alumiinin lämmönjohtavuuteen. Johtavuuden parantamiskeinoja tutkittiin. Diplomityön tuloksia tullaan hyödyntämään asiakasräätälöinnissä ja laadunvalvonnassa. Työ jakaantui kahteen osaan: Kirjallisuustutkimuksessa selvitettiin alumiinin mikrorakenteen ja koostumuksen vaikutusta lämmönjohtavuuteen. Kokeellisessa osassa tutkittiin sulan kemiallisen modifioinnin sekä valukappeleiden lämpökäsittelyn vaikutuksia lämmönjohtavuuteen. Koesuunnittelussa käytettiin Taguchi-menetelmää. Lämmönjohtavuutta mitattiin Hot Disk menetelmällä. Tutkimusaineistona toimi AlSi10Mg(Fe)-painevaluseos, jonka valu ja lämpökäsittelyt tehtiin Alteams Oy:n Laihian painevalimolla. Mikroskooppitarkastelut tehtiin Tampereen teknillisen yliopiston Materiaalitekniikan laitoksella. Tuloksista selviää, että lämpökäsittelylämpötilaa ja -aikaa nostamalla saadaan johtavuus nousemaan arvosta 120 W/m·K arvoon 164 W/m·K. Suuret rautapitoiset faasit sitovat johtavuutta haittaavia seosaineita, pienentäen niiden pitoisuutta alumiinimatriisissa. Lämpökäsittelyssä matriisista erkautuu muita seosaineita, jolloin johtavuus paranee. Strontiumseostuksella voidaan muokata johtavuutta muun muassa paikalliseen jähmettymisnopeuteen ja rautapitoisen faasin sekä piifaasin muodostumiskinetiikkaan vaikuttamalla.Abstract: As the circuit packing density increases, thermal concentrations of high power, small footprint electronic components get more intense. Failure of the component will follow, unless heat is transferred away. Aluminium, thanks to its good thermal conductivity, is used in heat dissipating components, such as heat sinks. Die casting allows a seamless integration of such components into aluminium chassis in a very cost efficient way. Improving the conductivity of materials reduces the need of using costly active cooling components, which are also more prone to fail during operation. The scope of the study was to find effects of heat treatment, chemical composition and microstructure on thermal conductivity of die cast aluminium. The thesis consists of a literature study and a practical study. Thermal conductivity values were measured using the Hot Disk method and optical and electron microscope examination were performed. Design of experiment was done using the Taguchi method in order to find the optimal heat treatment scheme and the optimal chemical composition. Samples of hypoeutectic AlSi10Mg(Fe) alloy were die cast and heat treated in Alteams Oy in Laihia. Microscopy studies were performed at Tampere University of Technology. The results will be used for product customization and quality control in a die cast alu-minium foundry. Increasing the heat treatment times and temperatures improved ther-mal conductivity from 120 W/m·K up to 164 W/m·K. Formation of large iron contain-ing intermetallics were found to improve on conductivity by arresting chromium and manganese. Heat treatment further lowered solute content in the aluminium matrix, hence improving the total conductivity. High levels of strontium inhibited the formation of said intermetallics and transformed the solidification kinetics and the morphology of silicon phases

A Computational Study of Graphene and Phosphorene for DNA Base Detection

Research into methods of deoxyribonucleic acid (DNA) sequencing has been improving with the goal of providing fast and cheap sequencing of longer strands as accurately as possible. One possible way of improving the process is utilizing a solid-state device. In fact, it has been shown in previous studies that two-dimensional (2D) materials such as graphene, molybdenum disulfide, and hexagonal boron nitride can be used to detect individual DNA bases [2, 3]. Graphene's success for nanopore DNA sequencing has shown that it is possible to explore other potential single- and few-atom thick layers of 2D elemental materials beyond graphene, and also that these materials can exhibit fascinating and technologically useful properties for DNA base detection that are superior to those of graphene. One material of interest is monolayer black phosphorus, or phosphorene. Phosphorene shares many of the remarkable properties of graphene including high carrier mobility [30] and tunable optical properties [32]. The advantage of phosphorene is its direct band gap [31] that is also dependent on thickness, making phosphorene ideal for electronic and optoelectronic applications [33]. It is therefore extremely important to perform exploratory studies to determine phosphorene's ability to detect individual DNA bases as this material is currently being sought after by many experimental groups as a promising material for designing nano-bioelectronic devices for high-speed DNA sequencing. In this thesis, using density functional theory calculations, we find that single-layer phosphorene is an extraordinary material for DNA sequencing using two advanced detection modalities (i.e., nanopore and nanoribbon). We observe that binding energies of DNA bases using nanopores and nanoribbons of phosphorene are smaller compared to graphene devices. This shows that minimal sticking of DNA bases to phosphorene's surface is expected for phosphorene devices. Furthermore, both nanopore and nanoribbon devices from phosphorene show a characteristic change in the density of states for each base. The band gap of phosphorene is significantly changed compared to other nanomaterials (e.g., MoS2, graphene, silicene, h-BN, and silicon nanowire) due to physisorption of bases on the nanoribbon surface. We also observe that the nanoribbon device performs better than the nanopore devices. Our findings confirm our hypothesis that phosphorene is a promising material for DNA base detection using advanced detection principles such as transverse tunneling current measurements. These results will provide valuable insights for other researchers in this field. Ideas for future research include examining this system using a periodic calculation; simulating a real device and calculating the current spectrum; and expanding the research to include other single-layer materials such as transition-metal dichalcogenides and Van der-Waals heterostructures

Efeito do teor de ferro na microestrutura e nas propriedades mecânicas de ligas Al-Si-Cu-Mg variando a taxa de resfriamento

Aluminum is the metal of modern life. Heat-treatable aluminum alloys currently occupy a prominent place in the industrial sector and with even greater prospects for use in the future. The A356 alloy is extended to automotive, aerospace and other industrial areas due to its excellent properties, including high strength, low density and excellent castability. Aiming at production cost benefits, less energy consumption and elimination of environmental damage associated with the mining and refining sectors, the recycling of aluminum alloys has been widely explored. In recycled foundry alloys, iron is the main impurity found and, due to its low solubility in aluminum, its presence leads to the formation of second phases, such as β - Al5FeSi intermetallic compounds which are presented in the form of thin and long platelets (needles ) which have a fragile character, degrading the strength and ductility of the alloy, as they act as stress concentrators. The size of the β - Al5FeSi phase needles increases the higher the iron content in the alloy and the lower the cooling rate. Thus, optimal control of iron concentration in foundry alloys is essential. In order to evaluate the effect of iron content on the microstructure and mechanical properties of the A356 alloy, the alloy was cast, containing 1% and 3% iron in its composition and then heat treated. The microstructure after remelting and solidification presents intermetallic phases with acicular morphology of the AlSiMgFe and AlFeSi composition. On the other hand, the microstructure after heat treatment presents intermetallic phases in needle shapes with β -Al5FeSi composition. The additions of iron resulted in a decrease in the cooling rate. It was found that as the iron content increases, there is a reduction in the distribution of the volume fraction of precipitates and an increase in grain size. It is also verified that with different cooling rates, the volume fraction changes, concluding that the higher the cooling rate, the greater the volume fraction. After performing the microhardness test for post-remelting, it was possible to notice an increase in the values of alloys with iron contents. The microhardness values in relation to the cooling rates used in quenching were unexpected. The recycling simulation of this work and consequent iron contamination in the A356 alloy generated changes in the solidification sequence, enabling the appearance of primary phases formed of iron before the crystallization of aluminum grains.O alumínio é verdadeiramente o metal da vida moderna. Ligas de alumínio tratáveis termicamente envolvem atualmente um lugar de destaque no setor industrial e com perspectivas ainda maiores de utilização no futuro. A liga A356 é amplamente utilizada nas áreas automotiva, aeroespacial e outras áreas industriais devido às suas excelentes propriedades, incluindo alta resistência, baixa densidade e excelente capacidade de fundição. Visando benefícios em custo de produção, menos consumo de energia e eliminação de danos ambientais associados aos setores de mineração e refino, a reciclagem de ligas de alumínio têm sido muito explorada. Nas ligas de fundição recicladas, o ferro é a principal impureza encontrada e, devido à sua baixa solubilidade no alumínio, sua presença leva a formação de segundas fases, como compostos intermetálicos β - Al5FeSi que se apresentam-se na forma de plaquetas finas e longas (agulhas) que possuem caráter frágil, degradando a resistência e ductilidade da liga, pois atuam como concentradores de tensões. O tamanho das agulhas da fase β - Al5FeSi aumentam quanto maior o teor de ferro na liga e menor a taxa de resfriamento. Assim, o controle ideal da concentração de ferro nas ligas de fundição é essencial. Com o objetivo de avaliar o efeito do teor de ferro na microestrutura e nas propriedades mecânicas da liga A356 foi realizado a fundição da liga contendo em sua composição 1% e 3% de ferro e depois foram tratadas termicamente. A microestrutura após refusão e solidificação apresentou fases intermetálicas com morfologia acicular da composição AlSiMgFe e AlFeSi. Já a microestrutura após tratamento térmico apresentou fases intermetálicas em formas de agulhas de composição β - Al5FeSi. As adições de ferro resultaram na diminuição da taxa de resfriamento. Foi constatado que a medida que o teor de ferro aumenta, verifica-se uma redução na distribuição da fração volumétrica dos precipitados e um aumento no tamanho de grão. Constata-se também que com diferentes taxas de resfriamento, a fração volumétrica se altera, concluindo que quanto maior a taxa de resfriamento, maior a fração volumétrica. A partir da realização do ensaio de microdureza para as amostras pós refusão, foi possível notar um aumento nos valores das ligas com teores de ferro. Os valores de microdureza em relação às taxas de resfriamento utilizadas na têmpera, foram inesperados. A simulação de reciclagem deste trabalho e consequente contaminação do ferro na liga A356 gerou mudanças na sequência de solidificação, possibilitando o surgimento de fases primárias formadas de ferro antes da cristalização dos grãos de alumínio

Contemporary music festivals as micronational spaces : articulations of national identity in Serbia’s Exit and Guča trumpet festivals in the post-Milošević era

In this study, I address issues of national identity articulations in post-Milošević Serbia (i.e. from 2000 onwards) using two major Serbian music festivals as case studies – the Exit and Guča trumpet festivals. The Exit and Guča festivals are particularly instructive for this line of inquiry because of fundamental aesthetic and ideological differences they are said to embody; namely, the two festivals are often narrated domestically as representing Two Serbias, thereby evoking the recurring West-East hermeneutic and its familiar taxonomy of binaries. The study first documents in detail the various ways in which Exit and Guča both reflect and construct the perceived schisms in Serbia’s national identity imaginary. Second, the study challenges the polarized representations of Exit and Guča by pointing to internal contradictions inherent in each festival. To achieve both objectives, I develop a new approach to festival research on national identity – one which arises from the idea of contemporary music festivals as micronational spaces. Informed by the larger framework of critical cultural theory and using rich research material from a wide variety of sources (including ethnographic evidence), the study ultimately illuminates the discursive practices underpinning the social production of Exit and Guča as particular types of micronational spaces, specifically, as a counter- and as an organic space respectively. Of special analytical interest is also the perception of each festival, both native and foreign, within the symbolic geographies of Serbia and the world beyond. The study concludes with a discussion on the transformation of Exit and Guča into (national) brandscapes and the effects this conceptual change has produced on the local perception of each festival as well as of the interrelated fields of popular music and national identity more generally. The final argument of the study is that the branding talk in two Serbian festivals ultimately recasts the earlier Balkanist discourse on Serbia’s indeterminate position between the West and the East, but in a way which provides little hope for alternative visions of the nation’s future. One solution to this problem is arguably the key concept of the study – specifically, the ideas of the festival microcitizenship and coming community – which may be used as an alternative perspective for exploring the political function of music festivals as much in Serbia as elsewhere.Tutkimuksessa tarkastelen kansallisen identiteetin rakentumista Miloševićin jälkeisessä Serbiassa (vuodesta 2000 alkaen) käyttäen tapaustutkimuksena kahta merkittävää serbialaista musiikkifestivaalia, Exitiä sekä Guča torvisoittokuntafestivaalia. Exit- ja Guča-festivaali toimivat havainnollistavina esimerkkeinä, sillä niiden välillä katsotaan yleisesti olevan huomattavia esteettisideologisia eroja; kansallisissa narratiiveissa festivaalien kuvaillaan edustavan kahta Serbiaa, mikä toisintaa länsi-itä-hermeneutiikkaa ja sen vastakohtaisuusajatteluun perustuvaa arvomaailmaa. Tutkimus kuvaa ensinnäkin yksityiskohtaisesti niitä keinoja, joiden avulla Exit ja Guča heijastavat ja rakentavat Serbian kansallisen itseymmärryksen osaksi miellettyjä kiistoja. Toisekseen tutkimus haastaa Exitin ja Gučan vastakkaisuutta korostavia representaatioita keskittämällä huomion kummankin festivaalin sisäisiin ristiriitoihin. Tätä varten kehitän tutkimuksessa uuden lähestymistavan kansallisen identiteetin tarkasteluun festivaalitutkimuksen kautta, joka nojaa ajatukseen nykyajan musiikkifestivaaleista mikrokansallisina tiloina. Tutkimuksen pohjana toimivat kriittisen kulttuurintutkimuksen teoriat sekä monipuolinen tutkimusaineisto (mukaan lukien etnografinen materiaali), joiden avulla tutkimus valottaa Exit- ja Guča-festivaaleja ympäröiviä diskurssin tuottamisen tapoja ja miten ne sosiaalisesti rakentavat kuvaa kyseisistä festivaaleista mikrokansallisina tiloina, ensin mainittua vastakulttuurisena ja jälkimmäistä orgaanisena tilana. Analyyttisen huomion kohteena ovat niin ikään kotimaiset ja ulkomaiset mielikuvat ja käsitykset kyseisistä festivaaleista osina Serbian symbolista maantiedettä sekä muuta maailmaa. Tutkimuksen perusteella Exit- ja Guča-festivaalien voidaan todeta muodostuneen (kansallisiksi) brändikokonaisuuksiksi (brandscape) ja että tämä käsitteellinen muutos on vaikuttanut paikallisiin mielikuviin molemmista festivaaleista sekä yleisemmin populaarimusiikin ja kansallisidentiteetin kytköksiin. Tutkimuksen johtopäätös on, että kahden festivaalin brändäyspuhe toistaa aiempaa balkanistista diskurssia Serbian häilyvästä positiosta lännen ja idän välillä, mutta tekee sen tavalla, joka ei tarjoa vaihtoehtoisia näkemyksiä maan tulevaisuudesta. Yhtenä ratkaisuna tutkimus esittää, että siinä kehitettyjä keskeisiä teoreettisia käsitteitä, festivaalien mikrokansalaisuutta ja yhteisöksi tulemista, on mahdollista käyttää vaihtoehtoisina lähestymistapoina musiikkifestivaalien poliittisten funktioiden tutkimisessa sekä Serbiassa että muualla maailmassa

Production of graphene and two dimensional crystals based functional electrodes for lithium ion batteries

Produzione di elettrodi in grafene e cristalli bidimensionali per batterie a ioni di liti

Otimização de tratamentos térmicos da Liga de Alumínio AA2024 utilizando Redes Neurais Artificiais.

O alumínio é verdadeiramente o metal da vida moderna. As ligas de alumínio tratáveis termicamente ocupam atualmente um lugar de destaque no setor industrial e com perspectivas ainda maiores de utilização no futuro. O interesse na melhoria de suas propriedades se torna cada vez mais um fator atual e preponderante, pois essas ligas apresentam uma combinação extremamente interessante de propriedades mecânicas, térmicas, físico-químicas, elétricas e tecnológicas, aliadas ao seu alto grau de reciclagem. A contribuição deste trabalho é a otimização dos tratamentos térmicos da liga AA 2024 e para tal propósito foram utilizadas três técnicas de avaliação: simulação computacional (utilizando as Redes Neurais Artificiais), ensaio de condutividade elétrica (utilização do método de corrente parasita ou de Foucault) e a comparação dos tratamentos de envelhecimentos interrompidos com os contínuos (em termos de propriedades mecânicas e gastos de energia). O banco de dados da RNA foi composto pelos valores de dureza e microdureza obtidos experimentalmente com a realização de vários tratamentos de solubilização e envelhecimento, tanto contínuos como interrompidos. A variável escolhida como de entrada da rede foi a microdureza resultante e as de saída as temperaturas e tempos de solubilização e envelhecimento. As RNAs se mostraram eficientes, possibilitando então o emprego da engenharia reversa: a partir da propriedade desejada prever as possíveis condições/parâmetros que devem ser utilizados no processamento da liga e ainda com economia de tempo de tratamento e gastos de energia. O envelhecimento da liga foi monitorado pelos ensaios de dureza e de condutividade elétrica e os resultados demonstraram uma boa correlação entre ambas, validando então o ensaio de condutividade para o controle do envelhecimento, com a vantagem de ser um ensaio rápido, não destrutivo e de baixo custo. Uma análise comparativa de gastos de energia também foi feita com a realização de todos os tratamentos térmicos. Também foi possível fazer a previsão do limite de escoamento a partir dos resultados de microdureza Vickers para as diferentes condições de processamento da liga AA 2024, com resultados muito satisfatórios

A SHARED INFORMATION SYSTEM FOR TOURISM ENTERPRISES IN DEVELOPING ECONOMIES

In a developing economy's business environment, introduction of new technology creates a large potential for more effective and streamlined production of tourism services. This study shows guidelines of design of an inter-organizational information system for small tourism enterprises. The entrepreneurial goal is to support strategic alliances in order to obtain better market-fit and sustainable competitive advance. This requires that the enterprises are capable of evaluating their existing processes, identify and outline improvements, and implement them. More than that, enterprises have to execute profound strategic changes in their business processes. For this change, study shows that adaption of supportive information system can be a key factor to satisfy these demands. Firstly, using systematic literature review the study identifies global trends of e-tourism. Secondly the trends are compared with the reality of small tourism enterprises in Nicaraguan Caribbean coast. With interview and brain storming sessions with hotel managers and local tourism specialists, desired state of e-tourism enhanced business processes is defined. Performance gaps and solutions are identified and outlined in order to reach new customer segments and better customer satisfaction with use of inter-organizational information system. With help of shared information system, enterprises search for sustainable economic growth and more stable business environment for their activities. The scientific domain of research is Information systems science. The method used for data collection and interpretation is systematic literature review and human and institutional capacity development -method. As result, the research identifies critical business processes when implementing e-Tourism services into tourism enterprises in developing economies. Strategic solutions for sustainable improvements in business processes supported by use of shared information system are outlined. As a practical result, the study lists required steps in order to reach desired changes in tourism enterprises with e-tourism initiative. Specification of requirements for information system is made. The implementation process and construction of information system is left out from this research and it requires later its own case study.fi=Opinnäytetyö kokotekstinä PDF-muodossa.|en=Thesis fulltext in PDF format.|sv=Lärdomsprov tillgängligt som fulltext i PDF-format

Kylmän sodan pelkoja ja fantasioita. Muukalaisten invaasio 1950-luvun yhdysvaltalaisessa tieteiselokuvassa

Siirretty Doriast