Fysik // Physics

Använd denna länk för att länka till samlingen:

https://odr.chalmers.se/handle/20.500.12380/13

Utforskar och använder fysik för att förstå världen och bidrar med tillämpningar, nya material och innovationer som möter dagens behov och framtidens utmaningar.
Stimulerade av vår tids behov och utmaningar är institutionens ambition att främja en kreativ miljö för forskning, lärande och samverkan. Vi tillför en konkurrensfördel genom att länka de bästa internationella forskarna i materialvetenskap, nanoteknologi och energiforskning med ledande industriella partners.

Läs mer om våra utbildningar

För forskning och forskningspublikationer, se https://research.chalmers.se/organisation/fysik/

Explores and uses physics to understand the world, and provide applications, new materials and innovations that meet today's needs, and the challenges of the future.
Stimulated by major needs and challenges in society and industry, our ambition is to foster a creative environment for academic research, learning, innovations and utilisation. We provide a competitive advantage by linking top-level international and interdisciplinary academic performance in the areas of material science, nanotechnology, life science engineering and energy research with world-leading industrial R&D&I projects.

Studying at the Department of Physics at Chalmers

For research and research output, please visit https://research.chalmers.se/en/organization/physics/

Browse

Visar 1 - 20 av 145

3D SLAM using Microsoft Kinect
(2011) Johansson, Elias; Johansson, Sebastian; Chalmers tekniska högskola / Institutionen för teknisk fysik; Chalmers University of Technology / Department of Applied Physics
In this thesis, the Microsoft Kinect sensor has been considered for the purpose of simultaneous localization and mapping. A simple system for 6D pose estimation and 3D mapping is presented. The system uses the Kinect sensor, feature detection and recognition based on the speeded up robust features algorithm, estimation of movement based on the iterative closest point and random sample consensus algorithms and mapping with RGB colored 3D point clouds. The results indicate that the Kinect, which provides both 3D depth and RGB color images, is a suitable sensor for simultaneous localization and mapping systems. The system shows robustness for frame-to-frame rotation angles of 5 or less.
A FRET based assay for the quantification of synthetic and native lipid vesicles
(2018) Thorsteinsson, Konrad; Chalmers tekniska högskola / Institutionen för fysik (Chalmers); Chalmers University of Technology / Department of Physics (Chalmers)
Lipid nanoparticles, both of artificial and biological origin, have attracted significant attention in recent years. Biological lipid nanoparticles in the form of extracellular vesicles are involved in intercellular communication and biological material transport. Synthetic liposomes have also been proposed as promising drug delivery systems. In view of this broad interest, methods capable of accurately quantifying the content of lipid nanoparticles in a sample are urgently needed. To date, quantification is most commonly achieved by counting the particles after visualization, or by quantifying the total protein content in the case of particles of biological origin. In this thesis we present an alternative method allowing for the quantification of the total lipid surface area of an unknown sample. Our approach is based on Förster Resonance Energy Transfer (FRET), where the unknown lipid nanoparticle sample is sonicated with vesicles containing a FRET-fluorophore pair, leading to membrane fusion. The change in FRET fluorescence can then be correlated to the total surface area of the unknown sample. We first calibrated the method using synthetic vesicles of known surface area. We then tested the method on synthetic vesicles containing cholesterol, herpes simplex virus type 2, and two species of outer membrane vesicles secreted from E. coli bacteria. Finally, we benchmarked our results against alternative established methods and discussed potential and limitation of each. Our results indicate that the FRET assay is suitable to quantify all the lipid nanoparticle samples tested here and serves as a viable measurement technique to quantify lipid surface areas.
A Global Optimization Scheme for Bimetallic Nanoparticles
(2011) Takahashi, Keisuke; Chalmers tekniska högskola / Institutionen för teknisk fysik; Chalmers University of Technology / Department of Applied Physics
The nature of atomic clusters has increasingly attracted the attention of researchers over the last few decades. One of the primary reasons for the rise in interest is that the physical and chemical properties of atomic metal clusters are different from their corresponding bulk metals. To understand cluster properties, knowledge of the relevant structures are needed. However, structural information is very difficult to acquire from experiments. In this thesis, a theoretical approach is used instead. In particular, a Basin-Hopping global optimization algorithm is implemented where the atomic interactions are calculated from first principles by use of the Density Functional Theory (DFT). The Basin-Hopping code is written with the Python programming language and the Siesta and Dmol softwares are used to run the DFT calculations. Structures of bimetallic structures are the primary focus, as they are known as highly-effective hydrogenation catalysts. In addition, the monometallic cluster structures of Sn and Ru are explored. Sn clusters in a range from 2 to 20 atoms with higher stability than previously reported structures were obtained. The optimized Ru clusters match with previous reported results. The electronic, magnetic, and chemical properties of ground state clusters are discussed. The new method was, furthermore, used to study structures of a range of bimetallic systems including Ru-Sn, Ru-Pd, Pd-Au, and other similar structures.
A Neural Network Approach to Absolute State-of-Health Estimation in Electric Vehicles Battery Degradation Study Based on Fleet Data
(2018) Johansson, Herman; Chalmers tekniska högskola / Institutionen för fysik (Chalmers); Chalmers University of Technology / Department of Physics (Chalmers)
Electrification is a trend within the automotive industry. Many car manufacturers are launching electric vehicles, which are believed to be more sustainable and environmentally friendly. A major component in these cars is the battery, and its performance is crucial to the success of the electric vehicle. Therefor, the degradation of battery properties is interesting, especially the capacity decline. To understand and counter this degradation it must be measured with high precision in the cars, and be connected to car use. This project approaches this challenge by: using real fleet data, the aggregation of the data into events, and a neural network to estimate the state of the battery. The result is a proof of concept that gives an improved measure of the battery state and how different usage affects the capacity degradation. The result is, however, not validated at this point, shows unexplained properties, and should be further developed.
A question of aggregation A comparative study of objective function creation methods applied to missile defense for helicopters
(2014) djurfeldt, hanna; Chalmers tekniska högskola / Institutionen för teknisk fysik; Chalmers University of Technology / Department of Applied Physics
This report describes an investigation done about method one can use when creating a target function for optimization and the e ects of those choices. Three simple and four advanced target function creation methods are described in detail and the results from optimizing with these target functions are analyzed. Lastly suggestions into possible continuations of this investigation is made.
Adsorbate induced core level shifts of transition metal surfaces
(2013) Nilsson, Viktor; Chalmers tekniska högskola / Institutionen för teknisk fysik; Chalmers University of Technology / Department of Applied Physics
A shift in the core electron binding energy detected with X-ray photoelectron spec- troscopy (XPS) gives insight into the local chemical configuration of atoms. Such infor- mation can be used, for instance, to determine the surface structure upon adsorption. By comparing the experimental spectra with theoretical predictions, the core level shifts can be attributed to specific atomic configurations. In this thesis, core level shifts for metal surfaces upon adsorption have been computed by electronic structure calculations within density functional theory (DFT), using the software VASP. The transition metals Ni, Cu, Ru, Rh, Pd, Ag, Cd, Pt and Au have been investigated in terms of their surface structure and adsorption sites for CO, H, O and S have been tested. Core level shifts for the hcp(0001)/fcc(111) and fcc(100) facets have been deter- mined for each adsorbate. Since the involved mechanisms contributing to these energy shifts are difficult to decouple, trends among shifts as well as a model for explaining them have been sought. A clear dependence on coordination number is seen, where the binding energy is lower for a lower coordination. Trends depending on element are also found. The shift upon adsorption is towards higher binding energies for almost all elements and adsorbates. Notable exceptions are Ag and Cd. The correlation between d-band centre shift and core level shift is confirmed whereas the common model of viewing charge transfer as a dominant effect for the shifts is found insufficient.
Advanced Physico-Chemical Modeling of Lithium Plating in Lithium-Ion Batteries
(2018) Fors, Angelica; Nilsson, Simon; Chalmers tekniska högskola / Institutionen för fysik (Chalmers); Chalmers University of Technology / Department of Physics (Chalmers)
The transport sector is responsible for about 14 % of the global greenhouse gas emissions. To make the transition to sustainable transportation it is important to find sustainable alternatives that can compete with today’s petrol and diesel vehicles. The most promising alternative is electricity stored in lithium-ion batteries (LIBs) in hybrid electric vehicles (HEVs) and electric vehicles (EVs). Two challenges that need to be solved to fully compete with petrol and diesel cars are the charging time of LIBs and the service life of EVs and HEVs, where LIBs typically are the limitation due to ageing phenomena. One ageing phenomenon that is problematic in both of these challenges is lithium metal deposition (or lithium plating). To predict lithium plating accurately complex physics-based models are needed. If accurate enough such models could be used to predict optimal charging conditions to enable for faster battery charging, at low temperatures without damaging the battery. In this project a physics-based model to predict lithium plating in LIBs and fast charging at low temperature has been developed in Simulink. The model has been validated by comparison with results from models found in literature with very good agreement. A sensitivity analysis of the parameters pointed out the most important parameters for the simulations to be parameters in the negative electrode as well as the diffusion coefficient and the ionic conductivity in the electrolyte. To make accurate simulations at low temperatures an Arrhenius dependence is introduced on relevant material parameters. Even though the results are promising the model validation is mostly done versus existing models. In order to use this model to improve charging routines for LIBs in HEVs and EVs it is necessary to collect data on lithium plating and determine material parameters experimentally. Keywords:
An efficient approach for extracting anharmonic force constants from atomistic simulations
(2018) Eriksson, Fredrik; Chalmers tekniska högskola / Institutionen för fysik (Chalmers); Chalmers University of Technology / Department of Physics (Chalmers)
Phonon theory is an important tool when analyzing solid state systems which is of high importance in modern technology. Central to the theory is the so called force constants (FCs) which determines the thermal behaviour. The super cell method is one way to extract the FCs from the force field of a displaced specimen but they are computationally expensive by requiring a lot of carefully prepared input data. By constructing an interatomic potential which uses the harmonic and higher order FCss as parameters the FCs can be extracted by a simple fitting procedure. Some small number of quasi random input configurations and the resulting force fields can contain enough information to extract all FCs with comparably low amount of computation. The underlying symmetries of the lattice must be used to reduce the number of free parameters in the model. This thesis demonstrates the implementation and application of the Force Constant Model. The model performs well and anharmonic FCs can be extracted. Given the FCs the model can be used as a potential and molecular dynamics can be performed yielding a direct method for computing thermal properties. Possible applications include analysis of thermal stabilization and phonon life time determination.
An Investigation Into the Effects of Ozone on a Nanoplasmonic Gas Sensor
(2020) Pierce, Damien; Chalmers tekniska högskola / Institutionen för fysik; Langhammer, Christoph; Andersson, Olof; Sech, Martin
Gas sensing is critical to the continued good health and well-being of humankind. The sensing of NO2 and O3 are of particular importance, due to their negative effects on the respiratory system. Nanoplasmonic sensing is a relatively unexplored method that shows significant potential in gas detection. The nanoplasmonic sensor in this study has already been successful in detecting NO2. However, its behaviour when exposed to O3 has yet to be investigated. Using an experimental approach, it was found that the optimum temperature for O3 sensing was 300°C, and 200°C for NO2 sensing. The detection of 1 ppm NO2 was possible with a constant 20 ppb O3 background, for 2 hour long exposure times. The sensor response to O3 showed rapid kinetics, with peak shifts of up to 2 nm, compared to 0.4 nm for NO2, at the same concentration of 100 ppb. This thesis presents a starting point for a further study of nanoplasmonic sensor behaviour in the presence of these two harmful gases.
An investigation of the water vapour resistance
(2013) Börjesson, Frida; Chalmers tekniska högskola / Institutionen för teknisk fysik; Chalmers University of Technology / Department of Applied Physics
This project has been performed at SP, Technical Research Institute of Sweden, in Boras. SP is a research institute where they are not only involved in different research areas, they are also able to provide quality assurance and certifications. Different companies turn to SP for those services and it is therefore important that those tests are reliable but also time efficient. This project will focus on one quality that the subgroup Polymer Technologies provide; the measurement of the permeability on polymeric building materials. Traditionally the permeability of polymeric building materials is measured by the well-established gravimetric method. This method tends to take a very long time, from weeks up to several months depending on the materials and in which humidities those are measured. It is therefore preferable to find alternative methods for those measurements. SP has recently invested in a new machine called Versaperm MkIV WVTR Meter, which measures the permeability using a RH-sensor based method, i.e. by measuring the relative humidity (RH) change. This RH-sensor based method is much faster than the gravimetric method. Those measurements can be done in some hours or in a couple of days, also depending on the sample and the RH. In the end one would like to find a correlation between those two methods. This thesis will include an investigation of the machine itself and Versaperm MkIV WVTR Meter turned out to have some limitations of how permeable materials that can be tested. The range of a material's water vapour resistance needs to be between 2:5 104 5:0 106 s/m to be able to give a result when measuring against a RH of 2% on the dry side. When measuring against a higher RH Versaperm will give a higher water vapour resistance than the gravimetric method which will give almost the same value in all conditions.
Analysis of empirical data on the tumbling of microrods in a shear
(2012) Johansson, Anton; Chalmers tekniska högskola / Institutionen för fundamental fysik; Chalmers University of Technology / Department of Fundamental Physics
This thesis builds upon an existing experiment [1] investigating the tumbling of microrods in a shear flow. The orientational dynamics of microrods in simple shear flows and in turbulent flows is a subject of great importance. This is because the orientational dynamics of particles strongly affects the bulk properties of the suspension [2]. There is now a large number of studies theoretically investigating the tumbling of microrods in flows [2, 3, 4, 5, 6], but there comparatively little experimental data [1, 7, 8]. The laboratory setup [1] allows for recording of large amounts of data for these rods in the form of grayscale movies at high framerates. The primary aim of this thesis is to analyse this data and to compare the results to existing theory. There are three observables in this experiment, the projected position of the rod in the channel, the projected length of the rod, and the projected orientation vector of the rod, all of these in the image plane of the recorded movies. The recorded movies are analysed using Matlab [9]. Each frame is analysed and the Canny edge-detection algorithm [10] is used to find the particle edges. The particle edges are subjected to an elliptic fit in order to approximate the position, orientation and length of the projected particle with the center, orientation, and length of the fitted ellipse. These properties are translated into the three dimensional components of the rod orientation vector n(t). Computer simulations of the equations of motion in the form given in [2, 4] were performed. The results are compared to the trajectories observed in the experiment. There is qualitative agreement between the experimental and the theoretical results.
Analysis of experimental data on -delayed proton and -particle emission
(2012) Klintefjord, Malin; Chalmers tekniska högskola / Institutionen för fundamental fysik; Chalmers University of Technology / Department of Fundamental Physics
The spin and parity of the resonance level 2645 keV in 20Na have been studied and discussed for decades among physicists, due to the astrophysical importance of the 19Ne(p,)20Na reaction. The level is situated 450 keV above the threshold for (19Ne+p). In this thesis data from an experiment investigating 20Mg -delayed proton emission, done at CERN-ISOLDE, has been calibrated and analyzed in order to obtain pure proton spectra in search for the resonance level. Simulations of energy deposition in the detectors have also been performed for the setup used and for potential future setups. It was found that the setup used was not sufficiently sensitive to put new limits on the population of the 2645 keV state. Therefore different detectors are discussed, which could allow to lower the amount of detected -background and recoils of 16O, from the decay of 20Na. New findings were -particles from -delayed -emission from 21Mg, identified in the used E E telescope.
Analytic Continuation of Electronic Green’s Functions from Imaginary to Real Time using Maximum Entropy
(2014) schött, johan; Chalmers tekniska högskola / Institutionen för teknisk fysik; Chalmers University of Technology / Department of Applied Physics
This thesis is mainly a computational work studying the analytical continuation of Green’s functions using the Maximum Entropy method. A strongly correlated electron system is described with the single-band Hubbard model and paramagnetic solutions are studied using Dynamic Mean Field Theory on a Bethe lattice. Continuous Time Quantum Monte Carlo is used as Impurity solver, for the infinite Anderson model at a finite temperature, to obtain the Matsubara single-particle Green’s function propagator. Both metallic and insulating spectral functions are obtained using the Maximum Entropy Method. General properties of the Maximum Entropy Method as an analytic continuation method from imaginary to real time are also discussed.
Angular Light Scattering of Gold Nanoparticles on Graphene Oxide
(2016) Hüseyin, Tepe; Chalmers tekniska högskola / Institutionen för fysik (Chalmers); Chalmers University of Technology / Department of Physics (Chalmers)
Lately, the study of plasmon excitations have been a rapidly growing subject. Some applications are light trapping, sensing and catalysis. The scattering characteristics of particles depends on the size of the particle and the environment. The aim of this study is to perform angular resolved measurements on gold nanoparticles imbedded in graphene oxide akes. The angular scattering intensities and peaks were measured. The e ect of changing environment on the scattering spectrum was also studied. Peak shifts were observed for changing dielectric environment. Changing the dielectric environment, from a citric bu er to graphene oxide on fused silica, led to a peak shift from 541 nm to 573 nm. The angular intensity measurements revealed possibly a di raction pattern, which changed for di erent wavelenghts. Angular scattering measurements showed the weakness of the sample preparation method. The sample was therefore modi ed, and angular scattering measurements were conducted, which showed di erent scattering peaks at di erent observation angles.
Aqueous Electrolytes for Next Generation Batteries
(2018) Hamrin, Alice; Scott, Ellen; Chalmers tekniska högskola / Institutionen för fysik (Chalmers); Chalmers University of Technology / Department of Physics (Chalmers)
In today’s society the demand for green energy is increasing, which in turn increases the demand for batteries to store the energy with. Facing a large expansion in battery production, the demands on batteries being safe, environmentally friendly and economical with natural resources are raised. This thesis aims to investigate the possibility of using water-based electrolytes in batteries, to mitigate safety problems usually associated with solvents for electrolytes. The thesis digs deeper into the area of highly concentrated aqueous electrolytes and investigates which properties in the salts are responsible for the expanded voltage window seen in previous research on the subject. The study comprises a range of combinations in anions and cations chosen by cost, abundance and previous results. The anions chosen were triflate (Tf), thiocyanate (SCN−) and bis(trifluoromethane)sulfonimide (TFSI) and the cations were lithium (Li+), sodium (Na+) and magnesium (Mg2+). Three main properties were in focus; the ionic conductivity, the solvation structure and the electrochemical stability window (ESW). The properties were examined experimentally with Raman spectroscopy, dielectric broadband spectroscopy and linear sweep voltammetry, and computationally with semi-empirical and density functional theory (DFT) calculations. The study found LiTf to have the largest ESW at 3.25 V. The majority of the investigated systems show potential for solidelectrolyte interphase (SEI) formation for highly concentrated aqueous electrolytes, and an expanded ESW, possibly as a result of this SEI formation.
Boiling in liquid hydrogen under gravity compensated with a magnetic field
(2012) Garcia, Stephane; Chalmers tekniska högskola / Institutionen för teknisk fysik; Chalmers University of Technology / Department of Applied Physics
Nowadays, the behaviour of fluids in weightlessness is a great interest for a lot of scientists all around the world. A particular aim of this study is to predict how fluids behave in space. The use of cryogenic propellant - liquid hydrogen (LH2) and liquid oxygen (LO2) - is increasing, especially in space launcher. The main questions the designers of these launchers have to solve is how heat transfer are taking place in absence of gravity and how does the gravity affect the boiling crisis. Experiments have been performed at the Low Temperature Laboratory (SBT) of the CEA/GRENOBLE in 2009 to answer this question for liquid oxygen. The experiments that are carried out in this master thesis are based on the same principle. However, the studied fluid is LH2. It consists on measuring the thermal transfers in nucleate and film boiling regime for different levels of gravity, until 0g. The compensation of gravity is done by using a huge magnetic field, leading to an important gradient of this field, used to levitate the hydrogen. Such a facility is available at the CNRS of Grenoble in the National Laboratory of the Intense Magnetic Fields (LNCMI). This resistive coil of 20MW delivers a magnetic field of 15 Teslas. For various thermal heat flux, the temperature difference between the wall and the fluid has been measured to plot the boiling curve : Nukiyama curve. These experiments have been carried out for several level of gravity and thermodynamics conditions (pressure and temperatures have been studied independently). Results were very satisfying and the main goal of this study has been achieved: these results are able to answer the main questions of designers for the new Vinci engine, used in the next generation of spacecraft Ariane.
Building Software for an Ordered Study of Single Long Polymer Chains Confined in Nano-channels
(2011) Sandlund, Eric; Chalmers tekniska högskola / Institutionen för teknisk fysik; Chalmers University of Technology / Department of Applied Physics
Since the discovery of bio-molecules and their signifi-cance they have been the object of much study. In order to better understand the structure and dynamics of bio-molecules devices and methods to capture and manipulate features of the molecule has been developed, such as confining long polymers in nano-channels and epifluorescence microscopy. In this report focus has been given to the study of the data properties of data collected from viewing confined long polymers homoge-neously stained along the length of the molecule in order to reduce noise and trace significant features along the molecule in the data. An overview of the polymer physics involved will also be presented along with an overview of the techniques used to capture the data. From the the study of the experimentally obtained data software tools for viewing large single molecules confined in nano-channels is developed. The software is used to view the data in an ordered way by reducing noise, using redun-dancy, and presenting subsets of the data in an interface. The software is also able to detect and trace simple features along the molecule.
Carbon as a particle-transfer material in hole-mask colloidal lithography
(2012) Syrenova, Svetlana; Chalmers tekniska högskola / Institutionen för teknisk fysik; Chalmers University of Technology / Department of Applied Physics
In this thesis a method for fabrication and optimization of the gold nanodisc dimer structure was developed. In particular, a new nanofabrication strategy using carbon nanocones as transfer material for placing a particle of interest (palladium (Pd) in this work) in the plas-monic “hot-spot” of the dimer gap was realized by means of hole-mask colloidal lithography (HCL). Series of samples with various gap sizes, gold disc sizes and Pd particle sizes/positions in the hot-spot were successfully fabrica-ted and characterized by scanning electron microscopy and spectrophotometry. The possibility to plasmonically probe a nanoparticle located in the gap of the dimers was investigated in a mass flow reactor system by monitoring the hydride formation in Pd. The long term vision motiva-ting this work is to facilitate the understanding and development of plasmonic sensing at a single nanoparticle level by providing a novel versatile nanofabrication tool based on a bottom-up self-assembly strategy.
CFD simulation of a safety relief valve for improvement of a one-dimensional valve model in RELAP5
(2012) Budziszewski, Anna; Thoren, Louise; Chalmers tekniska högskola / Institutionen för teknisk fysik; Chalmers University of Technology / Department of Applied Physics
In the Swedish nuclear power plants a structural verificatio of the pipe systems is a necessity to ensure that the pipes are strong enough to withstand the forces which can result from a sudden event. One example of a component which generates forces in the systems while operating is the safety relief valve. Safety relief valves are used in order to prevent overpressure in a process system by releasing a volume of fluid from the process when a predetermined maximum pressure is reached. In order to analyze the forces from water and steam in the pipe systems the software RELAP5, which performs calculations in one dimension, is commonly used within nuclear engineering. The valve model which is currently used when simulating a safety relief valve in RELAP5 is the motor valve model. However, the usage of this model with present settings results in forces higher than in reality in the pipe systems. The purpose of this project was to investigate how a safety relief valve can be modeled with CFD and to find interesting parameter relations to be implemented in RELAP5 in order to obtain more realistic results of generated forces in the pipe systems. The aim was to modify the currently used motor valve model and to develop a servo valve model which is a more exible model to use in RELAP5. The purpose of this project was also to investigate if a CFD simulation in 2D of the valve gives similar results as a 3D simulation. The investigated valve in this project was a proportional valve. It starts to open at a set pressure of 31 bar(g) and is completely opened at 10 % overpressure, i.e. 34.1 bar(g), where the maximum lift of 8.5 mm is reached. The movement of the spindle is determined by the different forces acting on it. In this project the hydraulic forces, the spring force and the gravity force were considered. The CFD simulations were performed in ANSYS FLUENT v.13. Dynamic layering was used in order to change the mesh during the opening process of the valve. The 2D and 3D geometries were created and meshed in ANSA v.13.2.1. Axisymmetry was used as a boundary condition in the 2D model, and in the 3D model mirror symmetry was used. The used turbulence model was SST k !. A sensitivity analysis was performed in order to investigate if and to which extent different mesh densities, turbulence models and time step sizes in fluence the results of the CFD simulations. A verification of the 3D geometry and force calculations was performed, with the conclusion that they seem to be consistent with reality. The transient 2D and 3D simulations were conducted with both an instant and a gradual increase of inlet pressure. Differences could be observed between the 2D and 3D simulations but similarities were also evident. The simulations performed with a gradual increase of inlet pressure were verified with experimental data. Interesting relations were found such as that the total hydraulic force acting on the spindle is a function of different pressures in the valve and the mass flow through the valve. In the currently used motor valve model in RELAP5 an opening time of 1 ms, an instant increase of inlet pressure and the abrupt area change model are used. This model was modified by using an opening time of 41 ms which was a result from the 3D CFD simulation. This modification resulted in lower forces generated in the pipe right after the valve. The generated forces also reached more realistic magnitudes than the forces generated from the currently used model. A servo valve model was developed in RELAP5 by specifying all necessary relations, needed for the valve to function, in control variables. One relation from the CFD simulations, describing the total hydraulic force acting on the spindle, was implemented successfully. The usage of the abrupt area change model in combination with short pipes resulted in a stable system and realistic forces. The trends in the opening process were fairly consistent with reality when the inlet pressure was gradually increased. Both the motor and the servo valve model were also modified by using the smooth area change model including the implementation of a Cv table. This modification did not decrease the magnitude of the forces and instabilities were observed in the system. The opening process of the valve, simulated both with CFD and in RELAP5, is faster than the opening process observed in experimental data. This concludes that the models are conservative, which is a requirement within the nuclear industry.
Characterisation of high-frequency noise in graphene FETs at different ambient temperatures
(2019) Li, Junjie; Chalmers tekniska högskola / Institutionen för fysik; Stake, Jan; Yang, Xinxin
Graphene is a promising channel material for high-frequency field-effect transistors, owing to its intrinsically high carrier velocity and purely two-dimensional structure. At high frequencies, the noise originated in device itself, especially the thermal noise, becomes very crucial for the performance of transistors. The thermal noise can be influenced by ambient temperature or self-heating due to high drain bias. This motivates the study of the effect of temperature on the noise performance of graphene field-effect transistors (GFETs). In this thesis, the results on high-frequency noise characterisation and modelling of the GFETs at different temperature and bias conditions are presented. The basic idea and main procedures of high-frequency noise modelling are based on Pospieszalski’s noise model. Two different high-frequency noise characterisation techniques, i.e., the Y-factor and cold-source methods, and two calculation methods of highfrequency noise parameters, i.e., the source-pull and 50 impedance termination (F50) methods, have been analysed and discussed. The high-frequency noise of the GFETs at an ambient temperature range from -60 C to 25 C is presented. The minimum noise figure (Fmin) of the GFETs decreases with the drain bias and saturates above approximately -1 V due to the carrier mobility saturation in the channel. The noise performance shows a rather strong dependence on both temperature and gate bias mainly due to the change of carrier density and the contact resistance. The minimum noise figure (Fmin) is 1.2 dB at 6.5 GHz at room temperature, which is comparable with that of the best metal-semiconductor field effect transistors. And it decreases down to 0.3 dB at 8 GHz for an ambient temperature of -60 C. An empirical noise model for the GFETs considering both temperature and gate voltage has been proposed and verified by the experimental results. In conclusion, a way to characterise the temperature dependence of noise performance of the GFETs is discussed, which allows for further development of low-noise GFETs for high-frequency applications.

Browse

Browsar Fysik // Physics efter Program "Applied physics (MPAPP), MSc"

Sökresultat per sida

Sortera efter