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- Chengxi, Liu, 1987-
(författare)
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Understanding the Impacts of Weather and Climate Change on Travel Behaviour
- 2016
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Human behaviour produces massive greenhouse gas emissions, which trigger climate change and more unpredictable weather conditions. The fluctuation of daily weather corresponds to variations of everyday travel behaviour. This influence, although is less noticeable, can have a strong impact on the transport system. Specifically, the climate in Sweden is becoming warmer in the recent 10 years. However, it is largely unknown to what extent the change of travel behaviour would respond to the changing weather. Understanding these issues would help analysts and policy makers incorporate local weather and climate within our policy design and infrastructure management.The thesis contains eight papers exploring the weather and climate impacts on individual travel behaviour, each addressing a subset of this topic. Paper I explores the weather impact on individual’s mode choice decisions. In paper II and III, individual’s daily activity time, number of trips/trip chains, travel time and mode shares are jointly modelled. The results highlight the importance of modelling activity-travel variables for different trip purposes respectively. Paper IV develops a namely nested multivariate Tobit model to model activity time allocation trade-offs. In paper V, the roles of weather on trip chaining complexity is explored. A thermal index is introduced to better approximate the effects of the thermal environment. In paper VI, the role of subjective weather perception is investigated. Results confirm that individuals with different socio-demographics would have different subjective weather perception even given similar weather conditions. Paper VII derives the marginal effects of weather variables on transport CO2 emissions. The findings show more CO2 emissions due to the warmer climate in the future. Paper VIII summaries the existing findings in relations between weather variability and travel behaviour, and critically assesses the methodological issues in previous studies.
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| 2. |
- Cortés González, Miguel Ángel, 1992-
(författare)
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Development of new Fluorination Methods Directed to Fluorine-18 Labelling
- 2019
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis deals with the development of new fluorination reactions and their application to fluorine-18 labelling. Fluorine-18 labelled compounds are employed as tracers in Positron Emission Tomography (PET), which is a powerful non-invasive imaging method in medical diagnostics.The first part of this thesis focuses on the development of a late-stage halogen exchange-based fluorination method for the synthesis of trifluoromethylated molecules. The first project in this area relies on the application of a copper(I)-based fluorinating reagent to furnish trifluoroacetates, trifluorotoluenes and trifluoroacetamides. The second project involves the translation of this methodology into the fluorine-18 labelling of tertiary and secondary trifluoroacetamides. The targeted substrates were labelled in high radiochemical yield and high molar activity using [18F]Bu4NF as fluorine source in the presence of an organic activator.In the second part, the development of electrophilic fluorination reactions using a hypervalent iodine-based reagent is discussed. The first project in this area addresses the development of an electrophilic fluorine-18 fluorination reagent: [18F]fluoro-benziodoxole. The utility of this reagent was demonstrated in the labelling of [18F]fluoro-benzoxazepines. In the second project, the same [18F]fluoro-benziodoxole reagent was used in the rhodium-mediated synthesis of α-[18F]fluoroethers. High molar activities were obtained in these electrophilic labelling processes. In the third project, the fluorine-19 analog fluoro-benziodoxole was used in the palladium-catalyzed iodofluorination of allyl benzenes, styrenes and cycloalkenes. Both iodine and fluorine atoms in the product arise from the same reagent.
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| 3. |
- Liu, Yu, 1987-
(författare)
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Modelling Evolution : From non-life, to life, to a variety of life
- 2018
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Life is able to replicate itself, e.g., a microorganism is able to divide into two identical ones, and a single plant is able to forest a whole island. But life is the only example of self-replication (note that a computer virus seems able to replicate itself, but it needs the assistance of a processor such as a CPU, and thus not a truly self-replicating entity). So before the appearance of life, nothing can self-replicate. How does life, a truly self-replicating entity, evolve from substances which is not able to self-replicate? Why can it ever happen? Is there a general underlying mechanism that governs how self-replicating entities can develop de novo on Earth, or even other plants?As long as the first life appears, it has the potential to cover the whole plant. But one single life form cannot do the job. Life has branched into a huge number of biological classes and species. Different species interact with each other, and with their environment, which, as a whole, is defined as an ecosystem. Distinct ecosystems are found at different scales and different places, e.g., microbes cross-feed and compete for resources within natural communities; and different types of cells interact by exchanging metabolite within an organism body. But, why sometimes we consider an ecosystem as an individual (such as the human body which is, in fact, an ecosystem inhabited by a huge number of microorganisms without which we cannot survive) while sometimes not? What really distinguishes an individual-level life from a system-level life? Are there general properties only a system-level life has, emerged from the interactions among its compositional individual-level life?This thesis is to investigate these two questions by mathematical models. For the evolution from non-life to life, namely the origin of life, we build an artificial chemistry model to investigate why an independent self-replicating entity can develop spontaneously from some chemical reaction system in which no reaction is self-replicating. For the evolution from life to a variety of life, we build an artificial ecosystem model to investigate general properties of ecosystems.
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| 4. |
- Liu, Yanfeng, 1992-
(författare)
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Studying Morphology Formation and Charge Separation in Organic Solar Cells
- 2021
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- We are currently living in the era of automation and artificial intelligence, which requires more energy than ever before. Meanwhile, the reduction of carbon footprint is needed for keeping the environment sustainable. Exploring green energy is crucial. Solar power is one of the green energy sources. The apparatus that converts solar energy to electricity is a solar cell. Organic solar cells (OSCs), employing organic materials absorbing solar radiation and converting to electricity have got extensive attention in the last decades due to their unique advantages like lightweight, semi-transparency, and potential industrialization. In most cases, an OSC composes of two different organic semiconductors as electron donor and acceptor to form a photoactive layer with a bulk heterojunction (BHJ) structure, and sandwiched between the electron and hole transport layers and then two electrodes. The morphology of the BHJ plays a crucial role in the device's performance, and it is a result of a complicated interplay between donor, acceptor, and solvent during the film drying from a solution. Thus, in-situ monitoring the film drying during solvent evaporation could deepen understanding of the mechanism of the morphology formation. A versatile multiple spectroscopic setup is assembled for this purpose, which can record laser scattering, steady-state photoluminescence (PL), time-resolved photoluminescence (TRPL), and white-light absorption during film formation. By comparing the drying dynamics of three different blend systems with their corresponding pristine films, we find that the blend film formation and its final morphology are more dominated by the component with a higher molecular weight. Different PL and TRPL quenching profiles between fullerene- and non-fullerene-based systems provide hints about different donor-acceptor interactions. Moreover, with the help of TRPL, the relative change of quantum yield during film formation can be calculated. Besides, this setup is also proved suitable for studying mechanisms behind device optimization processes, like the usage of solvent additives. One of the unique features of OSCs based on non-fullerene acceptors is the highly efficient hole transfer from the acceptor to the donor, sometimes even under zero or negative energetic offsets. However, in these cases the mechanism of hole transfer has not been fully understood. By studying hole transfer at the donor:acceptor interface in different material systems and device configurations, we highlight the role of electric field on the charge separation of OSCs when energetic offsets are not enough. To achieve better device performance, engineering the photoelectric properties of interfacial layers is equally essential. A good interfacial layer can facilitate carrier extraction and reduce carrier recombination. We demonstrate that adding MXenes into the PEDOT:PSS can increase the conductivity of this composite hole transport layer, without sacrificing its optical transparency and work function.
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