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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
11

Transport mechanisms of uranium and thorium in fractured rock aquifers

Van Wyk, Yazeed 24 June 2011 (has links)
The Karoo has been receiving considerable attention since the early 1970’s when uranium mining was at its peak, with numerous research studies being instigated to look at all aspects of uranium mining. It has recently been observed that there seems to be resurgence in uranium exploration in and around the town of Beaufort West. A study on the transport mechanisms of uranium and thorium in fractured-rock aquifers, initiated in the hope of understanding the actual processes controlling radionuclide mobilisation, is reported here. Hydrochemical investigations of the various boreholes were sampled for water quality in June, 2009. The hydrochemical description is typical of shallow fresh groundwater, changing composition to a more sulphate hydrochemical facies along the flow path. While the geochemistry of groundwater in the study area seems to have minimal effects on uranium concentrations, the low levels of uranium in boreholes sampled suggest the importance of hydrological and lithological variability on the measured concentrations. Nevertheless, the uranium concentration is within the recommended levels as specified in the US-EPA, WHO and SA water quality guidelines and thus poses no immediate threat to the general public. Analysis of pumping and tracer tests, reveals that the fractured-rock aquifer can be highly transmissive and that transport can take place via multiple flow paths having different hydraulic properties. Tracer diffusing into stagnant water zones within fracture asperities and the rock matrix are seen as an important retardation mechanism, that has implications for remediation should the aquifer be contaminated by radionuclides. In terms of conceptualising flow at a local scale, aperture sizes ranging from (563-828ìm) along with high flow velocities (1.90E-03m/s), points to the importance of bedding-plane fractures as conduits of groundwater flow. The groundwater flow has been influenced by dolerite dykes creating compartments isolated from each other, suggesting a highly complex aquifer system. Based on the conceptual model, it is shown that these structures can create unique, site specific flow conditions. The integration of all available data into the conceptual model provides an effective research tool that can be built upon as a basis for further research. / Dissertation (MSc)--University of Pretoria, 2010. / Geology / unrestricted
12

MFI-Type Zeolite Nanosheets Laminated Membranes for Ion Separation in Aqueous Solutions

Cao, Zishu 27 September 2020 (has links)
No description available.
13

Sorption, Transport and Gas Separation Properties of Zn-Based Metal Organic Frameworks (MOFs) and their Application in CO₂ Capture

Landaverde Alvarado, Carlos Jose 13 October 2016 (has links)
Adsorption, separation and conversion of CO₂ from industrial processes are among the priorities of the scientific community aimed at mitigating the effects of greenhouse gases on the environment. One of the main focuses is the capture of CO₂ at stationary point sources from fossil fuel emissions using porous crystalline materials. Porous crystalline materials can reduce the energy costs associated with CO₂ capture by offering high adsorption rates, low material regeneration energy penalties and favorable kinetic pathways for CO₂ separation. MOFs consist of polymeric inorganic networks with adjustable chemical functionality and well-defined pores that make them ideal for these applications. The objective of this research was to test the potential for CO₂ capture on Zn-based MOFs by studying their sorption, transport and gas separation properties as adsorbents and continuous membranes. Three Zn-based MOFs with open Zn-metal sites were initially studied. Zn4(pdc)4(DMF)2•3DMF (1) exhibited the best properties for CO₂ capture and was investigated further under realistic CO₂ capture conditions. The MOF exhibited preferential CO₂ adsorption based on a high enthalpy of adsorption and selectivity of CO₂ over N₂ and CH₄. Sorption dynamics of CO₂ indicated fast adsorption and a low activation energy for sorption. Diffusion inside the pores is the rate-limiting step for diffusion, and changes in the process temperature can enhance CO₂ separation. Desorption kinetics indicated that CO₂ has longer residence times and lower activation energies for desorption than N₂ and CH₄. This suggests that the selective adsorption of CO₂ is favored. MOF/Polymer membranes were synthesized via a solvothermal method with structural defects sealed by a polymer coating. This method facilitates the permeation measurements of materials that cannot form uniform-defect-free layers. The membrane permeation of CO₂, CH₄, N₂ and H₂ exhibited a linear relation to the inverse square root of the molecular weight of the permanent gases, indicating that diffusion occurs in the Knudsen regime. Permselectivity was well-predicted by the Knudsen model with no temperature dependence, and transport occurs inside the pores of the membrane. MOF (1) exhibits ideal properties for future applications in CO₂ capture as an adsorbent. / Ph. D.
14

CHARACTERIZATION, MODELING AND DESIGN OF ULTRA-THIN VAPOR CHAMBER HEAT SPREADERS UNDER STEADY-STATE AND TRANSIENT CONDITIONS

Gaurav Patankar (5930123) 10 June 2019 (has links)
This dissertation is focused on studying transport behavior in vapor chambers at ultra-thin form factors so that their use as heat spreaders can be extended to applications with extreme space constraints. Both the steady-state and transient thermal transport behaviors of vapor chambers are studied. The steady-state section presents an experimental characterization technique, methodologies for the design of the vapor chamber wick structure, and a working fluid selection procedure. The transient section develops a low-cost, 3D, transient semi-analytical transport model, which is used to explore the transient thermal behavior of thin vapor chambers: 1) The key mechanisms governing the transient behavior are identified and experimentally validated; 2) the transient performance of a vapor chamber relative to a copper heat spreader of the same external dimensions is explored and key performance thresholds are identified; and 3) practices are developed for the design of vapor chambers under transient conditions. These analyses have been tailored to ultra-thin vapor chamber geometries, focusing on the application of heat spreading in mobile electronic devices. Compared to the conventional scenarios of use for vapor chambers, this application is uniquely characterized by compact spaces, low and transient heat input, and heat rejection via natural convection.
15

Contribuição ao estudo da durabilidade e do transporte de fluidos em concretos contendo adições minerais / Contribution to the study of durability and fluid´s transport in concretes containing mineral additions

Almeida, Marina Augusta Malagoni de 21 September 2016 (has links)
Submitted by Erika Demachki (erikademachki@gmail.com) on 2017-05-04T19:01:17Z No. of bitstreams: 2 Dissertação - Marina Augusta Malagoni de Almeida - 2016.pdf: 12520522 bytes, checksum: 853486dc2fbdd4513ff1c2cf33d112b0 (MD5) license_rdf: 0 bytes, checksum: d41d8cd98f00b204e9800998ecf8427e (MD5) / Approved for entry into archive by Luciana Ferreira (lucgeral@gmail.com) on 2017-05-05T12:57:03Z (GMT) No. of bitstreams: 2 Dissertação - Marina Augusta Malagoni de Almeida - 2016.pdf: 12520522 bytes, checksum: 853486dc2fbdd4513ff1c2cf33d112b0 (MD5) license_rdf: 0 bytes, checksum: d41d8cd98f00b204e9800998ecf8427e (MD5) / Made available in DSpace on 2017-05-05T12:57:04Z (GMT). No. of bitstreams: 2 Dissertação - Marina Augusta Malagoni de Almeida - 2016.pdf: 12520522 bytes, checksum: 853486dc2fbdd4513ff1c2cf33d112b0 (MD5) license_rdf: 0 bytes, checksum: d41d8cd98f00b204e9800998ecf8427e (MD5) Previous issue date: 2016-09-21 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - CAPES / Brazil's current water crisis, generated by the low level of reservoirs, has threatened the national energy production. Faced with this problem, the wind towers present themselves as a viable solution. It is known that these towers are constructed in regions of high aggressiveness and therefore, it is necessary to develop high performance concretes that are able to withstand environmental action to which they are submitted, so that these structures reach the lifespan previously defined in project, ensuring the viability of the towers. In this sense, the present study evaluates the mass transport mechanisms (permeability and absorption) that takes place inside the concrete containing mineral pozzolanic admixtures of high performance, in order to evaluate the influence of the presence of these additions in these properties. Three different study mixes were prepared containing, respectively, 9% of silica fume in composition with 1% of nanossílica, 5% of a metakaolin with high fineness in composition with 5% of a metakaolin slightly less fine, both with high pozzolanic activity, a mix containing only 10% of the metakaolin with higher fineness, and a reference proportioning. The analysis presented in this work involve the scanning electron microscope (SEM) to characterize the internal structure of the material, the trace reconstruction test to analyze the aggregate/binder ratio in the regions of the cover and the interior of concrete, and the analyzes related to the mass transport: water absorption by immersion and capillarity and the air permeability obtained by the Figg’s method, in addition to the mechanical characterization. The concrete with addition of pozzolanic admixtures showed significant improvements in compressive strength, in the modulus of elasticity, in the air permeability and in the refinement of pores. Furthermore, the evaluation of the microstructure by scanning electron microscope showed that the use of mineral admixtures may cause an improvement in the adhesion and in the densification of the transition zone between aggregate and cement paste. The trace reconstitution test showed that the concrete cover has lower ratio aggregate/binder, which corroborates the results of the capillary absorption test in which it was found that the cover of concrete presents a mean capillarity absorption lower than the interior region. Thus, it was concluded that the incorporation of the different mineral admixtures studied in this work significantly alter the properties of concrete, with positive emphasis on the properties related to the durability, in which the benefits of using mineral admixtures is more pronounced and, in addition, it was also concluded that the concrete cover presents itself more favorable to the durability than the interior region. / A atual crise hídrica brasileira, gerada pelo baixo nível dos reservatórios, tem ameaçado a produção energética nacional. Frente a esse problema, as torres eólicas se apresentam como uma solução viável. Sabe-se que essas torres são construídas em regiões de alta agressividade e, portanto, faz-se necessário o desenvolvimento de concretos de alto desempenho que sejam capazes de resistir às ações ambientais às quais são submetidos de maneira que essas estruturas atinjam a vida útil de projeto previamente definida, garantindo a viabilidade das torres. Nesse sentido, o presente trabalho avalia as propriedades mecânicas e os mecanismos de transporte de massa (permeabilidade e absorção) que ocorrem no interior dos concretos que contêm adições minerais pozolânicas de alto desempenho, com o intuito de avaliar a influência da presença dessas adições nessas propriedades. Foram elaborados três diferentes proporcionamentos de estudo contendo respectivamente, 9% de sílica ativa em composição com 1% de nanossílica, 5% de um metacaulim de elevada finura em composição com 5% de um metacaulim um pouco menos fino, ambos de elevada pozolanicidade e um proporcionamento contendo exclusivamente 10% do metacaulim de maior finura, além de um proporcionamento de referência. São apresentadas análises em microscópio eletrônico de varredura (MEV) visando à caracterização da estrutura interna do material, reconstituição de traço com vistas à análise do teor de agregado/ligante nas regiões do cobrimento e do interior do concreto, análises relacionadas ao transporte de massa: absorção de água por imersão e por capilaridade e permeabilidade ao ar pelo método de Figg, além da caracterização mecânica. Os concretos com incorporação de adição pozolânica apresentaram melhorias significativas na resistência à compressão, no módulo de elasticidade, na permeabilidade ao ar e no refinamento dos poros. Além disso, a avaliação da microestrutura por meio de microscopia eletrônica de varredura mostrou que o uso das adições minerais pode ter causado densificação e melhoria na aderência da zona de transição entre os agregados e a pasta de cimento. O ensaio de reconstituição de traço mostrou que a região do cobrimento apresenta menor relação agregado/ligante, o que corrobora o resultado do ensaio de absorção capilar, em que se constatou que a região do cobrimento apresentou absorção capilar média inferior à região do interior. Assim, se concluiu que a incorporação das diferentes adições minerais estudadas no presente trabalho alterou significativamente as propriedades do concreto, com destaque positivo para as propriedades associadas à durabilidade, nas quais os benefícios do uso da adição mineral são mais pronunciados e, além disso, também se concluiu que a região do cobrimento se apresenta mais favorável à durabilidade que a região interna.
16

Performance Modeling, Design and Analysis of Transport Mechanisms in Integrated Heterogeneous Wireless Networks

Rutagemwa, Humphrey January 2007 (has links)
Recently, wireless access to Internet applications and services has attracted a lot of attention. However, there is no single wireless network that can meet all mobile users’ requirements. Con-sequently, integrated heterogeneous wireless networks are introduced to meet diverse wireless Internet applications and services requirements. On the other hand, integrated heterogeneous wireless networks pose new challenges to the design and development of reliable transport mechanisms. Wireless Application Protocol version 2 (WAP 2.0) is one of the promising trans-port mechanisms. It uses wireless profiled TCP (WP-TCP), which is fully compatible with TCP, as one of the reliable transport protocols to cope with the wireless link impairments. For WAP 2.0 to continue providing reliable and efficient transport services in the future, one of the key is-sues is to thoroughly study, understand, and improve its performance in integrated heterogeneous wireless networks. In this thesis, we develop analytical frameworks and propose a solution to respectively study and improve the performance of WP-TCP in integrated heterogeneous wireless networks. Spe-cifically, we consider WP-TCP short- and long-lived flows over integrated wireless local area network (WLAN) and wireless wide area network (WWAN), where WLAN can be static or mo-bile. In order to facilitate the analysis of WP-TCP performance in integrated WLAN and WWAN, we first construct a novel WLAN link model, which captures the impact of both uncor-related and correlated transmission errors, and derive mathematical expressions that describe packet loss probability and packet loss burst length over WWAN-WLAN link. Then, we develop analytical frameworks for studying the performance of WP-TCP short- and long-lived flows. Differently from those reported in the literature, our analytical framework for WP-TCP short-lived flows takes into account both correlated and uncorrelated packet losses. Furthermore, our analytical framework for long-lived flow can be used to study the short-term (during vertical handover) and long-term performances of WP-TCP and it captures the effects of vertical handover, such as excessive packet losses and sudden change in network characteristics, which are commonly experienced in integrated static WLAN and WWAN. By using the devel-oped analytical frameworks, we extensively analyze the performance of WP-TCP flows and in-vestigate the optimal protocol design parameters over a wide range of network conditions. Finally, based on our analytical studies, we propose a receiver-centric loosely coupled cross-layer design along with two proactive schemes, which significantly improve the vertical hand-over performance. The proposed solution is easy to implement and deploy, compatible with tra-ditional TCP, and robust in the absence of cross-layer information. Extensive simulations have been conducted to confirm the effectiveness and practicability of our schemes.
17

Performance Modeling, Design and Analysis of Transport Mechanisms in Integrated Heterogeneous Wireless Networks

Rutagemwa, Humphrey January 2007 (has links)
Recently, wireless access to Internet applications and services has attracted a lot of attention. However, there is no single wireless network that can meet all mobile users’ requirements. Con-sequently, integrated heterogeneous wireless networks are introduced to meet diverse wireless Internet applications and services requirements. On the other hand, integrated heterogeneous wireless networks pose new challenges to the design and development of reliable transport mechanisms. Wireless Application Protocol version 2 (WAP 2.0) is one of the promising trans-port mechanisms. It uses wireless profiled TCP (WP-TCP), which is fully compatible with TCP, as one of the reliable transport protocols to cope with the wireless link impairments. For WAP 2.0 to continue providing reliable and efficient transport services in the future, one of the key is-sues is to thoroughly study, understand, and improve its performance in integrated heterogeneous wireless networks. In this thesis, we develop analytical frameworks and propose a solution to respectively study and improve the performance of WP-TCP in integrated heterogeneous wireless networks. Spe-cifically, we consider WP-TCP short- and long-lived flows over integrated wireless local area network (WLAN) and wireless wide area network (WWAN), where WLAN can be static or mo-bile. In order to facilitate the analysis of WP-TCP performance in integrated WLAN and WWAN, we first construct a novel WLAN link model, which captures the impact of both uncor-related and correlated transmission errors, and derive mathematical expressions that describe packet loss probability and packet loss burst length over WWAN-WLAN link. Then, we develop analytical frameworks for studying the performance of WP-TCP short- and long-lived flows. Differently from those reported in the literature, our analytical framework for WP-TCP short-lived flows takes into account both correlated and uncorrelated packet losses. Furthermore, our analytical framework for long-lived flow can be used to study the short-term (during vertical handover) and long-term performances of WP-TCP and it captures the effects of vertical handover, such as excessive packet losses and sudden change in network characteristics, which are commonly experienced in integrated static WLAN and WWAN. By using the devel-oped analytical frameworks, we extensively analyze the performance of WP-TCP flows and in-vestigate the optimal protocol design parameters over a wide range of network conditions. Finally, based on our analytical studies, we propose a receiver-centric loosely coupled cross-layer design along with two proactive schemes, which significantly improve the vertical hand-over performance. The proposed solution is easy to implement and deploy, compatible with tra-ditional TCP, and robust in the absence of cross-layer information. Extensive simulations have been conducted to confirm the effectiveness and practicability of our schemes.
18

Charge transport in two-dimensional materials and their electronic applications

Arora, Himani 01 March 2021 (has links)
Semiconducting two-dimensional (2D) materials have gained considerable attention in recent years owing to their potential in future electronics. On the one hand, the conventional 2D semiconductors, such as transition metal dichalcogenides (TMDCs (MoS2, WS2, etc.) are being exhaustively studied, on the other hand, search for novel 2D materials is at a rapid pace. In this thesis, we explore 2D materials beyond graphene and TMDCs in terms of their intrinsic electronic properties and underlying charge transport mechanisms. We introduce 2D semiconducting materials of indium selenide (InSe) and gallium selenide (GaSe), and a novel π-d conjugated Fe3(THT)2(NH4)3 metal-organic framework (MOF) as potential candidates for their use as active elements in (opto)electronic applications. Owing to the air-sensitivity of InSe and GaSe, their integration into active devices has been severely constrained. Here, we report a hexagonal boron nitride (hBN) based encapsulation, where 2D layers of InSe and GaSe are sandwiched between two layers of hBN; top hBN passivating the 2D layer from the environment and bottom hBN acting as a spacer and suppressing charge transfer to the 2D layer from the SiO2 substrate. To fabricate the devices from fully encapsulated InSe and GaSe layers, we employ the technique of lithography-free via-contacts, which are metal contacts embedded within hBN flakes. Based on our results, we find that full hBN encapsulation preserves InSe in its pristine form and suppresses its degradation with time. Consequently, the electronic properties of encapsulated InSe devices are significantly improved, leading to a mobility of 30–120 cm2 V−1 s−1 as compared to a mere ∼1 cm2 V−1 s−1 obtained for unencapsulated devices. In addition, encapsulated InSe devices are stable for a prolonged period of time, overcoming their limitation to be air-sensitive. On employing full hBN encapsulation to GaSe, a high photoresponsivity of 84.2 A W−1 at 405 nm is obtained. The full hBN encapsulation technique passivates the air-sensitive layers from various degrading factors and preserves their unaltered properties. In the future, this technique can be applied to other 2D materials that have been restricted so far in their fundamental study and applications due to their environmental sensitivity. MOFs are another emerging class of semiconducting 2D materials investigated in this thesis. They are hybrid materials that consist of metal ions connected with organic ligands via coordination bonds. In recent years, advances in synthetic approaches have led to the development of electrically conductive MOFs as a new generation of electronic materials. However, to date, poor mobilities and hopping-type charge transport dominant in these materials have prevented them from being considered for electronic applications. In this work, we investigate a newly developed π-d conjugated Fe3(THT)2(NH4)3 (THT: 2,3,6,7,10,11-hexathioltriphenylene) MOF. The MOF films are characterized with a direct bandgap lying in the infrared (IR) region. By employing Hall-effect measurements, we demonstrate band-like transport and a record-high mobility of 230 cm2 V−1 s−1 in Fe3(THT)2(NH4)3 MOF films. The temperature-dependent conductivity confirms a thermally activated charge carrier population in the samples induced by the small bandgap of the analyzed MOFs. Following these results, we demonstrate the feasibility of using this high-mobility semiconducting MOF as an active material in thin-film optoelectronic devices. The MOF photodetectors fabricated in this work are capable of detecting wavelengths from UV to NIR (400–1575 nm). The narrow IR bandgap of the active layer constrains the performance of the photodetector at room temperature by band-to-band thermal excitation of the charge carriers. At 77 K, the device performance is significantly improved; two orders of magnitude higher voltage responsivity, lower noise equivalent power, and higher specific detectivity of 7 × 10^8 cm Hz1/2 W−1 are achieved at 785 nm excitation, which is a direct consequence of suppressing the thermal generation of charge carriers across the bandgap. These figures of merit are retained over the analyzed spectral region (400–1575 nm) and are comparable to those obtained with the first demonstrations of graphene and black phosphorus based photodetectors, thus, revealing a promising application of MOFs in optoelectronics. / Zweidimensionale (2D) Halbleiter haben dank ihres Potenzials für elektronische Anwendungen in den letzten Jahren große Aufmerksamkeit erregt. Dabei werden einerseits konventionelle 2D-Materialien, wie die Übergangsmetall-Chalkogenide (TMDCs) (MoS2, WS2, usw.) intensiv erforscht. Andererseits schreitet auch die Suche nach neuen 2D-Materialien rasch voran. Diese Dissertation stellt Forschungsergebnisse zu elektrischen Eigenschaften und den zugrundeliegenden Ladungstransportmechanismen von 2D-Materialien jenseits von Graphen und TMDCs vor. Untersucht wurden die 2D-Halbleiter Indiumselenid (InSe) und Galliumselenid (GaSe), sowie eine neuartige π-d konjugierte Metallorganische Gerüstverbindung (Metal-Organic Framework, MOF) Fe3(THT)2(NH4)3. Diese Materialien sind vielversprechende Kandidaten für elektronische und optoelektronische Anwendungen. InSe und GaSe sind besonders luftempfindliche Materialien. Aus diesem Grund ist ihre Verwendung für aktive Bauteile trotz ihrer hervorragenden elektrischen Eigenschaften bis heute sehr begrenzt. In dieser Arbeit wird ein effektives Verkapselungsverfahren vorstellt, bei dem InSe- oder GaSe-2D-Schichten zwischen zwei Schichten aus hexagonalem Bornitrid (hBN) eingebettet werden. Die untere Schicht hBN isoliert das Material vom Substrat Siliziumdioxid (SiO2), während die obere Schicht das 2D-Material luftdicht isoliert. Um Bauteile aus komplett eingekapseltem InSe oder GaSe herzustellen, wurden lithographiefreie, sogenannte via-Kontakte hergestellt. Dies sind Metallkontakte, die bereits vor der Verkapselung in die hBN-Schichten integriert werden. Die hBN-Verkapselung erhält InSe in seiner ursprünglichen Form. Die hier vorgestellten Ergebnisse zeigen, dass sich die elektronischen Eigenschaften von InSe durch Verkapselung signifikant verbessern, was zu elektrischen Mobilitäten von 30–120 cm2 V−1 s−1 gegenüber nur rund ∼1 cm2 V−1 s−1 in unverkapselten Bauteilen führt. Darüber hinaus bleiben die Eigenschaften der verkapselten InSe-Bauteile über einen langen Zeitraum erhalten und degradieren nicht mehr bei Kontakt mit Luft. Die Verkapselung von GaSe ermöglicht den Einsatz in Fotodetektoren, bei einer Wellenlänge von 405 nm wird eine Fotoempfindlichkeit von 84.2 A W−1 gemessen; auch hier bewahrt die Verkapselung die empfindlichen Schichten vor schädlichen Einflüssen und konserviert so ihre unveränderten Eigenschaften. In der Zukunft kann diese Technik auch für andere 2D-Materialien eingesetzt werden, insbesondere für solche, deren Erforschung und Anwendung durch die große Empfindlichkeit bis heute eingeschränkt ist. Darüber hinaus untersucht diese Dissertation mit Metallorganischen Gerüstverbindungen (MOFs) eine zweite Klasse halbleitender 2D-Materialien. MOFs sind hybride Materialien aus Metallionen, die mit organischen Molekülen als Verbindungselementen eine meist kristalline Struktur bilden. In den letzten Jahren haben Fortschritte in der synthetischen Herstellung zur Entwicklung von elektronisch leitfähigen MOFs geführt. Die niedrige Mobilität und der sogenannte hopping-Ladungstransport der gängigsten MOFs haben jedoch verhindert, dass diese für Anwendungen betrachtet wurden. In dieser Arbeit wird eine kürzlich neu entwickelte, π-d-konjugierte Fe3(THT)2(NH4)3 (THT: 2,3,6,7,10,11-hexathioltriphenylene) MOF vorgestellt. Der MOF Film hat eine direkte Bandlücke im Infrarot(IR)-Bereich liegend. Mithilfe von Hall-Effekt-Messungen wurde gezeigt, dass der Transport in den Fe3(THT)2(NH4)3 MOF Filmen mit dem Drude-Modell konsistent ist. Darüber hinaus wird eine bis jetzt nicht übertroffene Mobilität von 230 cm2 V−1 s−1 gemessen. Die Temperaturabhängigkeit der Leitfähigkeit bestätigt, dass die kleine Bandlücke zu thermisch aktivierten Ladungstragerdichten in den Proben führt. Auf Grundlage dieser Ergebnisse wird die Machbarkeit von hochmobilen halbleitenden Fe3(THT)2(NH4)3 MOFs als aktives Material in dünnen optoelektronischen Bauteilen gezeigt. Die hier vorgestellten MOF Fotodetektoren reagieren auf Wellenlängen im UV bis Nahinfrarotspektrum (400–1575 nm). Die schmale Bandlücke schränkt die Leistung des Fotodetektors bei Raumtemperatur durch thermische Band-zu-Band-Anregung der Ladungsträger ein. Bei einer Temperatur von 77 K verbessert sich die Leistung des Detektors signifikant: Bei 785 nm wird eine um zwei Größenordnungen erhöhte Spannungsempfindlichkeit, eine niedrigere äquivalente Rauschleistung sowie eine höhere spezifische Empfindlichkeit von 7 × 10^8 cm Hz1/2 W−1 erhalten. Dies ist eine direkte Konsequenz der Unterdrückung thermischer Anregung von Ladungsträgern über die Bandlücke. Diese Leistungszahlen sind über das analysierte Spektrum (400–1575 nm) gültig und vergleichbar mit den ersten Fotodetektoren auf Grundlage von Graphen und Schwarzem Phosphor. Die Ergebnisse zeigen deutlich das Potenzial von MOFs für optoelektronische Anwendungen.
19

Elektrické vlastnosti nanostrukturovaných povrchů TaxOy pro kapacitní aplikace / Electrical properties of nanostructured TaxOy for capacitive applications

Nováková, Tereza January 2017 (has links)
Cílem diplomové práce bylo nastudovat a popsat mechanismy transportu náboje v tantalovém kondenzátoru. Práce obsahuje stručný teoretický úvod do problematiky kondenzátoru jako součástky a dále se zabývá jednotlivými mechanismy přenosu náboje jako je ohmická, Poole-Frenkel proudová, Schottkyho, tunelovací a emisní složka a také proudem prostorového náboje. V experimentální části byly měřeny ampér-voltové I/V, ampér-časové I/t a impedanční charakteristiky, jejichž data byla následně zpracována pomocí např. Mott-Schottkyho analýzy a byly vyhodnoceny elektrické parametry jako je aktivační energie, koncentrace dopantů, akumulační kapacita nebo potenciálová bariéra. Výsledky, vypočtené veličiny a naměřené hodnoty jsou diskutovány.

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