| 1. |
- Höglund, Linda, 1974-
(författare)
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Growth and characterisation of InGaAs-based quantum dots-in-a-well infrared photodetectors
- 2008
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis presents results from the development of quantum dot (QD) based infrared photodetectors (IPs). The studies include epitaxial growth of QDs, investigations of the structural, optical and electronic properties of QD-based material as well as characterisation of the resulting components.Metal-organic vapour phase epitaxy is used for growth of self-assembled indium arsenide (InAs) QDs on gallium arsenide (GaAs) substrates. Through characterisation by atomic force microscopy, the correlation between size distribution and density of quantum dots and different growth parameters, such as temperature, InAs deposition time and V/III-ratio (ratio between group V and group III species) is achieved. The V/III-ratio is identified as the most important parameter in finding the right growth conditions for QDs. A route towards optimisation of the dot size distribution through successive variations of the growth parameters is presented.The QD layers are inserted in In0.15Ga0.85As/GaAs quantum wells (QWs), forming so-called dots-in-a-well (DWELL) structures. These structures are used to fabricate IPs, primarily for detection in the long wavelength infrared region (LWIR, 8-14 μm).The electron energy level schemes of the DWELL structures are revealed by a combination of different experimental techniques. From Fourier transform photoluminescence (FTPL) and FTPL excitation (FTPLE) measurements the energy level schemes of the DWELL structures are deduced. Additional information on the energy level schemes is obtained from tunneling capacitance measurements and the polarization dependence studies of the interband transitions. From tunneling capacitance measurements, the QD electron energy level separation is confirmed to be 40-50 meV and from the polarization dependence measurements, the heavy hole character of the upper hole states are revealed.Further characterisation of the IPs, by interband and intersubband photocurrent measurements as well as dark current measurements, is performed. By comparing the deduced energy level scheme of the DWELL structure and the results of the intersubband photocurrent measurements, the origin of the photocurrent is determined. The main intersubband transition contributing to the photocurrent is identified as the QD ground state to a QW excited state transition. Optical pumping is employed to gain information on the origin of an additional photocurrent peak observed only at temperatures below 60 K. By pumping resonantly with transitions associated with certain quantum dot energy levels, this photocurrent peak is identified as an intersubband transition emanating from the quantum dot excited state. Furthermore, the detector response is increased by a factor of 10, when using simultaneous optical pumping into the quantum dots states, due to the increasing electron population created by the pumping. In this way, the potentially achievable responsivity of the detector is predicted to be 250 mA/W.Significant variations of photocurrent and dark currents are observed, when bias and temperature are used as variable parameters. The strong bias and temperature dependence of the photocurrent is attributed to the escape route from the final state in the QW, which is limited by tunneling through the triangular barrier. Also the significant bias and temperature dependence of the dark current could be explained in terms of the strong variation of the escape probability from different energy states in the DWELL structure, as revealed by interband photocurrent measurements. These results are important for the future optimisation of the DWELL IP.Tuning of the detection wavelength within the LWIR region is achieved by means of a varying bias across the DWELL structure. By positioning the InAs quantum dot layer asymmetrically in a 8 nm wide In0.15Ga0.85As/GaAs quantum well, a step-wise shift in the detection wavelength from 8.4 to 10.3 μm could be achieved by varying the magnitude and polarity of the applied bias. These tuning properties could be essential for applications such as odulators and dual-colour infrared detection.
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| 2. |
- Törnsén, Monika, 1952-
(författare)
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Successful Principal Leadership: : Prerequisites, Processes and Outcomes
- 2009
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis' main theme is successful principal leadership in secondary schools within the Swedish education system. Successful principal leadership is examined from three perspectives: What are the processes of a successful principal? Do the leadership processes relate to successful academic and social outcomes of schools? What are the prerequisites for successful principal leadership? The Frame Factor Model and the three concepts of prerequisites, processes and outcomes constitute an overarching framework. The prerequisites are categorized as internal prerequisites (the particular characteristics of individual principals) and external prerequisites operating within the Swedish educational environment. The successful principal processes are viewed as pedagogical leadership processes, on one hand as providing prerequisites for teaching and learning and, on the other hand as leading the core processes of teaching and learning. The definitions build on the empirical data, on the Swedish national curriculum and demands for pedagogical leadership, and on international findings on what successful principals do. The outcomes of successful principal leadership are here defined as the academic and the social outcomes of schools. The research undertaken is part of the research project 'Structure, Culture, Leadership - Prerequisites for Successful Schools?' The empirical data for this thesis are gathered in twenty-six Swedish secondary schools whereof five are regarded successful schools based on both academic and social outcomes. The findings, reported on in four articles, derive from interviews and questionnaires to principals and teachers. The principals in the main identify prerequisites of importance that are within their own realm of influence, such as themselves, teachers and school district level. They consider a limited area of responsibility and support from district level specialists as providing possibilities for their success. The principals accept the national governance of schools and principals via the national curriculum. The principals in the five successful schools however take a higher degree of responsibility for setting direction towards national goals, for processes inside schools and for school outcomes than do principals in less successful schools.They as pedagogical leaders attend to a higher degree both to providing prerequisites for teaching and learning and to leading the core processes of teaching and learning than do principals in less successful schools. In schools with a successful implementation of social goals, which shows as successful social outcomes, the principals, according to teachers, overall take responsibility for their national objectives and obligations to a higher degree than principals in schools with a less successful implementation of social goals. The implementation of social goals is of importance not only from an outcome perspective but also from a process perspective. It requires collaborative interpretation which can promote principal-staff professional relations and ultimately student learning. The identified overall differences between principals' leadership processes and processes in the twenty-six schools raise questions around consequences for equivalence in education.
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| 3. |
- Björkman, Conny, 1950-
(författare)
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Internal capacities for school improvement : Principals' views in Swedish secondary schools
- 2008
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The aim of this thesis was to describe and analyse principals´ views of collaboration forms, staff development and leadership, as critical internal capacities for school improvement, in five more successful and four less successful Swedish secondary schools, and compare the qualitative similarities and/or differences in the principals´ views at the level of schools. A successful school is understood to be a school where pupils accomplish both the academic objectives and the social/civic objectives in the National Curriculum. The empirical materials used were collected through semi-structured interviews with the principals and deputy principals, and through general school observations in the nine schools. The perspective of principals´ views was used as the unit for analysis, in order to reflect the principals´ way of thinking about the internal capacities, as principals´ views were expected to be an important indication of how principals act and interact with teachers in their specific context. To create such a model for analysis meant creating views, generated from empirical text, that deepened the understanding of the meaning of collaboration forms, staff development, and leadership, as critical internal capacities for school improvement. These views were then interpreted with the help of two theoretical concepts; structure and culture. The creation of the model made it possible to analyse and describe the school observations and the principals´ views of the three critical internal capacities, in the same usage. The question of what is decided helped to describe and understand the structure in a school, which in educational sociology is understood as the division of labour. The question of how the decisions are realised helped to describe and understand the culture in a school, the distribution of work. By using the theoretical concepts of structure and culture it was possible to unfold the power relations and the modes of control in the schools, regarding the three internal capacities for school improvement. One part of the result was the constructed view types for collaboration forms, staff development and leadership. It was possible to construct three qualitatively different view types: A principal distributed and team-based/involving view type, a principal distributed and teacher-based/traditional view type, and a politically distributed and principal-based view type. The last view type only appears in relation to staff development. When connecting the principals´ views of the three internal capacities in the different schools to the different view types, the results show that the ´team-based` view type dominates in all of the more successful schools, as well as in one of the less successful schools. In two of the less successful schools the ´team-based´ view type has become a vision for the principals to strive for in relation to the experienced reality of the ´teacher-based´ view type. The remaining less successful school is dominated by the ´teacher-based´ view type. Principals´ views of external collaboration forms, the connections with the world outside the school-house, are interesting, as all schools no matter the level of success, are ´teacher-based´.
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| 4. |
- Lindberg, Gunnar
(författare)
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Valuation and pricing of traffic safety
- 2006
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis consists of six essays covering the topic of valuation and pricing of accidents. In the first essay a theory of the external marginal cost of accidents is presented. The external cost is dependent on the value of statistical life, the proportion internal cost and the so called risk elasticity, i.e. the relationship between traffic flow and accident risk. The two following essays present CV-studies on value of statistical life. Both CV-studies are based on the community approach built on the Swedish Vision Zero and ask respondents about their willingness-to-pay for absolute safety from fatal and severe accidents in one city.The first study focuses on the problem of scope and hypothetical bias and shows that ex post calibration based on respondents’ certainty in answering the WTP question reduces the willingness-to-pay with up to 30% for a public good. Based on these certain responses a lower bound VSL value is presented of SEK 53 million for a private good and 20 million for a public good. The second essay discusses the topic of nonselfish preferences and willingness-to-pay for children’s safety as well as for relatives and friends. The essay suggests that WTP for children safety is higher than private safety and that a WTP for the safety of relatives and friends is prevailing. Also this essay shows a big difference between the WTP for a private good and for a public good. The two following essays estimate the risk elasticity for railway level crossing accidents as well as for heavy goods vehicles in Sweden. Both essays find that the risk decreases with increased traffic i.e. negative risk elasticity. The last essay presents the result from a field experiment with speed related charges and bonuses. The experiment shows that internalisation of the external accident cost through speed related economic incentives strongly will affect drivers choice of speed. It is proposed that this can be achieved on a voluntary basis.
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| 5. |
- Almgren, Ellen, 1970-
(författare)
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Att fostra demokrater : Om skolan i demokratin och demokratin i skolan
- 2006
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- A central task for the Swedish school is to foster democratic citizens. The purpose of this thesis is to evaluate some aspects of the fostering of democrats in Swedish schools, focusing on the political knowledge of young people. For more than a decade the Swedish educational policy has considered a democratic school environment to be a fundamental right for the students. Furthermore, the school democracy is assumed to have a positive effect on the students' development of democratic competence. This assumption of a causal relationship between school democracy and political knowledge, inspired by integrative theories of democracy, is tested in this study. Another important goal for the Swedish educational policy is the equivalence of education for all students. This can be interpreted in more or less ambitious terms regarding the equality of the students' education. School segregation, however, poses a threat to any form of equivalent education. The effects of school segregation on the students' political knowledge is therefore also tested in this thesis.Using multi-level analysis on a sample of over 6000 Swedish students between 14 and 15 years of age, this study shows that a deliberative open classroom-climate has a positive effect on the students' political knowledge while the direct student influence, surprisingly, has a negative effect. Both ethnic and socioeconomic segregation have negative effects on the students political knowledge. Even more troubling, though, is the fact that the different dimensions of school democracy are related to the segregational factors. The favourable open classroom-climate is more frequent in schools where the students' parents have higher education. The counter-productive direct student influence is, on the other hand, more frequent in schools with a high number of immigrants and in schools where the students' parents have lower education. It is therefore argued that even though student influence is construed as a fundamental right, the Swedish government needs to appreciate the conflict between the goals of the educational policy: Student influence and the fostering of democrats. This conflict is especially serious since student influence in fact has a negative effect not only on the fostering of democrats but also on the equality of citizenship education.
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| 6. |
- Andersson, Henric, 1963-
(författare)
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Aircraft Systems Modeling : Model Based Systems Engineering in Avionics Design and Aircraft Simulation
- 2009
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Aircraft developers like other development and manufacturing companies, are experiencing increasing complexity in their products and growing competition in the global market. One way to confront the challenges is to make the development process more efficient and to shorten time to market for new products/variants by using design and development methods based on models. Model Based Systems Engineering (MBSE) is introduced to, in a structured way, support engineers with aids and rules in order to engineer systems in a new way.In this thesis, model based strategies for aircraft and avionics development are studied. A background to avionics architectures and in particular Integrated Modular Avionics is described. The integrating discipline Systems Engineering, MBSE and applicable standards are also described. A survey on available and emerging modeling techniques and tools, such as Hosted Simulation, is presented and Modeling Domains are defined in order to analyze the engineering environment with all its vital parts to support an MBSE approach.Time and money may be saved by using modeling techniques that enable understanding of the engineering problem, state-of-the-art analysis and team communication, with preserved or increased quality and sense of control. Dynamic simulation is an activity increasingly used in aerospace, for several reasons; to prove the product concept, to validate stated requirements, and to verify the final implementation. Simulation is also used for end-user training, with specialized training simulators, but with the same underlying models. As models grow in complexity, and the set of simulation platforms is expanded, new needs for specification, model building and configuration support arise, which requires a modeling framework to be efficient.
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| 7. |
- Grundestam, Olof, 1976-
(författare)
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Modelling and simulation of turbulence subject to system rotation
- 2006
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Simulation and modelling of turbulent flows under influence of streamline curvature and system rotation have been considered. Direct numerical simulations have been performed for fully developed rotating turbulent channel flow using a pseudo-spectral code. The rotation numbers considered are larger than unity. For the range of rotation numbers studied, an increase in rotation number has a damping effect on the turbulence. DNS-data obtained from previous simulations are used to perform a priori tests of different pressure-strain and dissipation rate models. Furthermore, the ideal behaviour of the coefficients of different model formulations is investigated. The main part of the modelling is focused on explicit algebraic Reynolds stress models (EARSMs). An EARSM based on a pressure strain rate model including terms that are tensorially nonlinear in the mean velocity gradients is proposed. The new model is tested for a number of flows including a high-lift aeronautics application. The linear extensions are demonstrated to have a significant effect on the predictions. Representation techniques for EARSMs based on incomplete sets of basis tensors are also considered. It is shown that a least-squares approach is favourable compared to the Galerkin method. The corresponding optimality aspects are considered and it is deduced that Galerkin based EARSMs are not optimal in a more strict sense. EARSMs derived with the least-squares method are, on the other hand, optimal in the sense that the error of the underlying implicit relation is minimized. It is further demonstrated that the predictions of the least-squares EARSMs are in significantly better agreement with the corresponding complete EARSMs when tested for fully developed rotating turbulent pipe flow.
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| 8. |
- Heldt, Tobias, 1972-
(författare)
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Sustainable Nature Tourism and the Nature of Tourists’ Cooperative Behavior : Recreation Conflicts, Conditional Cooperation and the Public Good Problem
- 2005
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis consists of five essays.Essay I (with David Vail) explains why neither Maine, USA's comparatively laissez faire economic and land use institutions nor Dalarna, Sweden's more heavily regulated economy seems well designed to make tourism a powerful economic development engine. The paper focuses on three clusters of institutions that have a major influence on tourism's scale, economic structure, and long term sustainability. Labor laws and labor market institutions, Land ownership and property rights and Commodity taxes. The paper employs institutional contrasts between Dalarna and Maine to frame hypotheses that will guide future studies of sustainable tourism in forest regions. Essay II (with David Vail) Snowmobiling growth in North America and Sweden creates challenges in “governing the commons.” Snowmobiling contributes to the economy of distressed rural regions and enhances residents’ quality of life; but quasi-open access to winter landscapes also breeds conflicts: among snowmobilers, with landowners, with other recreationists, and with environmentalists and ecosystem health. Case studies in Sweden and Maine are used to illustrate how innovative governance institutions, complemented by infrastructure investments, can mitigate conflicts, re-align incentives, and internalize costs. Essay III (with Kreg Lindberg and Peter Fredman) Many natural areas are visited by diverse groups of recreationists, and in some cases the presence or behavior of one group may negatively impact the experience of another group. This recreation conflict may lead to access restrictions for the "offending" group. However, the magnitude of the gains and losses from such management interventions remain unknown. The present study utilizes choice experiment analysis to provide estimates of the economic value to cross-country skiers of conflict reduction from various levels of snowmobile presence. Essay IV uses a natural experiment to study the impact of an informal sanctioning mechanism on individuals’ voluntary contribution to a public good. Cross-country skiers’ actual cash contributions in two ski resorts, one with and one without an informal sanctioning system, are used. I find the contributing share to be higher in the informal sanctioning system (79 percent) than in the non-sanctioning system (36 percent). Furthermore, a CC-function, i.e. the relationship between expected average contributions of other group members and the individual’s own contribution, is elicited and compared between the two systems. Essay V tests for conditional cooperation and social comparisons in a natural field experiment, using decisions from a sample of cross-country skiers in Sweden on the issue of voluntary cash contributions to the preparation of ski tracks. Testing by experimentally varying the beliefs about others’ behavior, I find the share of subjects giving a contribution to be significantly greater in the group receiving information about others’ behavior than in the group that does not. Regression analysis cannot reject that subjects are affected by social comparisons and express a behavior classified as conditional cooperation.
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| 9. |
- Johansson, Anders
(författare)
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Impregnation of concrete structures : transportation and fixation of moisture in water repellent treated concrete
- 2006
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Water repellent agents, today mainly consisting of alkylalkoxysilanes, are often used on concrete to prolong the service life of the structure. This is accomplished by protecting the reinforcement bars from chlorides or by changing the moisture content inside. When the concrete is treated with a water repellent agent the properties of the surface layer turn from hydrophilic to hydrophobic and thereby water droplets are stopped from entering, still allowing water vapour to pass through. This property change can reduce chloride ingression and stop heavy rain from penetrating through the surface layer. This thesis presents results concerning how the moisture transport and fixation in the surface layer of the concrete is affected by a water repellent treatment. It also presents an investigation in which the effective penetration depth and the factors that influence it are studied. The methods used covers uni-dimensional transport of moisture using the so called cup method, measurements on moisture fixation using climate boxes with saturated salt solutions, penetration depth by cracking samples and spraying water on them, and field tests in a harsh tunnel environment. The moisture diffusion coefficient for a water repellent treated concrete is close to constant and not nearly as dependent on the relative humidity (RH) as for untreated concrete. Unlike untreated concrete, where capillary suction plays an important role for the moisture transport at high RH, the vapour transport is the dominant transport mechanism even at high RH for water repellent treated concrete. The moisture fixation is affected by a water repellent treatment and the effect is clearest at high moisture levels. There is, however, a certain amount of moisture present in a concrete treated with a water repellent agent. It can also be seen that the main reason for this is that the capillary porosity is affected by the treatment to a relatively high degree while the gel porosity to a large extent remains unaffected. The three most important factors for the penetration of any water repellent agent into concrete is time, porosity and degree of saturation. An empirical equation is derived that gives an idea on how much these factors affect the efficient penetration depth of the water repellent agent. Measures prolonging the service life of a concrete structure will lead to savings of natural resources and thus both economical and environmental savings for the community. The aim with the PhD-project is to develop explanation models to the promising results that have been obtained from the empirical research during the last decade and by doing this also create a better knowledge about when and how to apply a water repellent agent in order to benefit as much as possible from the product. The results presented in this Lisenciate thesis will be used as input in these models in the planned second phase of this project.
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| 10. |
- Johansson, Nicklas, 1973-
(författare)
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Control of Dynamically Assisted Phase-shifting Transformers
- 2008
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- In this thesis, controllers for power oscillation damping, transient stability improvement and power flow control by means of a Controlled Series Compensator (CSC) and and a Dynamic Power Flow Controller (DPFC) are proposed. These devices belong to the group of power system components referred to as Flexible AC Transmission System (FACTS) devices. The developed controllers use only quantities measured locally at the FACTS device as inputs, thereby avoiding the risk of interrupted communications associated with the use of remote signals for control. For power systems with one dominating, poorly damped inter-area power oscillation mode, it is shown that a simple generic system model can be used as a basis for damping- and power flow control design. The model for control of CSC includes two synchronous machine models representing the two grid areas participating in the oscillation and three reactance variables, representing the interconnecting transmission lines and the FACTS device. The model for control of DPFC is of the same type but it also includes the phase shift of the internal phase-shifting transformer of the DPFC. The key parameters of the generic grid models are adaptively set during the controller operation by estimation from the step responses in the FACTS line power to the changes in the line series reactance inserted by the FACTS device. The power oscillation damping controller is based on a time-discrete, non-linear approach which aims to damp the power oscillations and set the desired power flow on the FACTS line by means of two step changes in the line reactance separated in time by half an oscillation cycle. A verification of the proposed controllers was done by means of digital simulations using power system models of different complexities. The CSC and DPFC controllers were shown to significantly improve the small-signal- and transient stability in one four-machine system of a type commonly used to study inter-area oscillations. The CSC controller was also tested for 18 different contingencies in a 23-machine system, resulting in an improvement in both the system transient stability and the damping of the critical oscillation mode.
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