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- Hedman, Daniel, 1989-
(författare)
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A Theoretical Study: The Connection between Stability of Single-Walled Carbon Nanotubes and Observed Products
- 2017
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Over the past 20 years’ researchers have tried to utilize the remarkable properties of single-walled carbon nanotubes (SWCNTs) to create new high-tech materials and devices, such as strong light-weight composites, efficient electrical wires and super-fast transistors. But the mass production of these materials and devices are still hampered by the poor uniformity of the produced SWCNTs. These are hollow cylindrical tubes of carbon where the atomic structure of the tube wall consists of just a single atomic layer of carbon atoms arranged in a hexagonal grid. For a SWCNT the orientation of the hexagonal grid making up the tube wall is what determines its properties, this orientation is known as the chirality of a SWCNT. As an example, tubes with certain chiralities will be electrically conductive while others having different chiralities will be semiconducting.Today’s large scale methods for producing SWCNTs, commonly known as growth of SWCNTs, gives products with a large spread of different chiralities. A mixture of chiralities will give products with a mixture of different properties. This is one of the major problems holding back the use of SWCNTs in future materials and devices. The ultimate goal is to achieve growth where the resulting product is uniform, meaning that all of the SWCNTs have the same chirality, a process termed chirality-specific growth. To achieve chirality-specific growth of SWCNTs requires us to obtain a better fundamental understanding about how they grow, both from an experimental and a theoretical point of view.This work focuses on theoretical studies of SWCNT properties and how they relate to the growth process, thereby giving us vital new information about how SWCNTs grow and taking us ever closer to achieving the ultimate goal of chirality-specific growth. In this thesis, an introduction to the field is given and the current state of the art experiments focusing on chirality-specific growth of SWCNTs are presented. A brief review of the current theoretical works and computer simulations related to growth of SWCNTs is also presented. The results presented in this thesis are obtained using first principle density functional theory. The first study shows a correlation between the stability of SWCNT-fragments and the observed products from experiments. Calculations confirm that in 84% of the investigated cases the chirality of experimental products matches the chirality of the most stable SWCNT-fragments (within 0.2 eV). Further theoretical calculations also reveal a previously unknown link between the stability of SWCNT-fragments and their length. The calculations show that at specific SWCNT-fragment lengths the most stable chirality changes. Thus, introducing the concept of a switching length for SWCNT stability. How these new results link to the existing understanding of SWCNT growth is discussed at the end of the thesis.
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| 2. |
- Khajavizadeh, Lida, 1978-
(författare)
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Materials Design for the Improvement of SiC Field Effect Gas Sensor performance in High Temperature Process Control applications
- 2023
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Following growing concerns about the effect these last decades of accelerating energy consumption, escalating use of natural (not the least fossil) resources, and waste generation has had on earth’s climate, human health, and the environment, industry is facing mounting pressure to increase the energy-, resource-, and cost-efficiency of both processes and products while drastically reducing pollutant emissions. In response, industry is, besides increasing their utilization of renewable energy and introducing new bio-based products, taking advantage of the rapid development in 5G technology and associated wireless interconnectivity to move towards full automation of production processes and facilities. With the ability to connect and collect information from a large number of instruments and process steps as well as to analyse huge amounts of multi-dimensional data, entire production lines can be automatically controlled and/or adapted, e.g., through the application of machine learning, for optimized efficiency and minimized greenhouse gas and pollutant emissions. In order to fully benefit from this so-called 4th industrial revolution and the Industrial Internet of Things (IIoT), however, access to cost-efficient and long-term reliable means for measuring various process parameters in multiple locations is required.The basis for this thesis is therefore the characterization of silicon carbide-based Metal Oxide Semiconductor Field Effect Transistor (MOSFET) gas sensors for their applicability in real-time monitoring of process gases/ gas mixtures and pollutant emissions in high-temperature applications, e.g., control of flue gas/ exhaust after-treatment systems, as well as investigations performed to gain a better understanding of the corresponding sensing mechanisms and to improve sensor performance in terms of e.g., long-term reliability.Besides demonstrating the general applicability of SiC MOSFET gas sensors in ammonia (NH3) slip detection and control of Selective Catalytic Reduction and similar flue gas/ exhaust emissions abatement systems to minimize the release of nitrogen oxides into the atmosphere from the energy production and transport sectors, sensors for the selective monitoring of nitrogen oxides with negligible interference from (sensitivity/ cross-sensitivity to) ammonia have been developed. Through variation of the gold/ iridium composition of the MOSFET gate contact, the interaction of which with gaseous substances in the ambient determines the sensor signal, sensors selective to ammonia and nitrogen oxides (NOx; NO and NO2), respectively, could be developed. Furthermore, as both sensor-types exhibit decent sensitivity to the respective substance over a common temperature interval, the realization of simultaneous NH3 and NOx monitoring with one and the same sensor probe would be facilitated. In relating the sensor response under different conditions to the composition and micro- (/nano-) structure of the gate contact material another piece in the puzzle towards an understanding of the ammonia sensing mechanism of field effect sensor devices could also be obtained.Through the investigation of co-deposited platinum group metals/ metal oxides in different ratios it was also shown possible to improve the long-term stability of carbon monoxide (CO) sensors intended for emissions monitoring and combustion control in industrial processes. Both the materials composition and structure were thereby evaluated and their effect on other important sensor parameters, e.g., sensitivity and temperature dependence, analysed. To further improve sensitivity and selectivity of CO as well as NH3 sensors for combustion control and ammonia slip monitoring applications, respectively, the sensor characteristics were also studied under different ambient conditions, e.g., variations in oxygen concentration, and related to the microstructure of the deposited gate materials. From the simultaneous tuning of film structure and operating temperatures, the selectivity could be enhanced for both the CO and NH3 sensors.In summary, the obtained research results from this thesis work have contributed to a further understanding of the sensing mechanisms of silicon carbide-based MOSFET sensors for CO, NOx, and NH3 monitoring and through the tuning of materials properties and sensor operation parameters generated improvements in sensor reliability, sensitivity, and selectivity of importance to the further realization of sensors for process control and emissions reduction applications.
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| 3. |
- Pham, Ngoc Tung, 1977-
(författare)
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Three-dimensional structured carbon foam : synthesis and applications
- 2016
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Recently, due to the unique properties and structures such as large geometric surface area, electrical conductivity and light weight, 3D structured carbon materials have been attracting extensive attention from scientists. Moreover, the materials, which can provide well-defined pathways for reactants to easily access active sites, are extremely useful for energy conversion as well as environmental and catalysis applications. To date, many precursors have been used for fabrication of 3D structured carbon materials including pitch, carbon nanotubes, graphene, and polymer foams.This thesis, as shown in the thesis title, focus on two main aspects: the study of the characteristics of melamine based carbon foam synthesized at different conditions and their applications. In paper I, it was revealed that through a simple, one-step pyrolysis process, flexible carbon foam synthesized from melamine foam (BasotectÒ, BASF) was obtained. Additionally, through a pyrolysis-activation process, activated carbon foam which possesses hydrophilic nature and high surface area was successfully synthesized. The characteristics of carbon foam such as the hydrophobic/hydrophilic nature, electrical conductivity, mechanical properties and surface chemistry were studied. It was shown that carbon foam could be successfully used as an absorbent in environmental applications e.g. removing of spill oil from water (paper I) or as support for heterogeneous catalysts, which in turn was used not only in gas phase reactions (paper I and IV) but also in an aqueous phase reaction (paper II). Importantly, when combined with a SpinChem® rotating bed reactor (SRBR) (paper II), the monolithic carbon foam/SRBR system brought more advantages than using the foam alone. Additionally, the work in paper III showed the potential of carbon foam in an energy conversion application as anode electrode substrate in alkaline water electrolysis. In summary, the versatility of the carbon foam has been proven through abovementioned lab scale studies and due to the simple, scalable and cost effective pyrolysis and activation processes used for the production, it has potential to be used in large-scale applications.
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