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- Bertilsson, Kent, et al.
(författare)
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The Effect of Different Transport Models in Simulation of High Frequency 4H-SiC and 6H-SiC Vertical MESFETs
- 2001
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Ingår i: Solid-State Electronics. - 0038-1101 .- 1879-2405. ; 45:5, s. 645-653
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Tidskriftsartikel (refereegranskat)abstract
- A full band Monte Carlo (MC) study of the high frequency performance of a 4H-SiC Short channel vertical MESFET is presented. The MC model used is based on data from a full potential band structure calculation using the local density approximation to the density functional theory. The MC results have been compared with simulations using state of the art drift-diffusion and hydrodynamic transport models. Transport parameters such as mobility, saturation velocity and energy relaxation time are extracted from MC simulations
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- Jönsson, Jörgen, 1969-
(författare)
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Phase transformation and surface chemistry of secondary iron minerals formed from acid mine drainage
- 2003
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The mining of sulphidic ore to extract metals such as zinc and copper produces huge quantities of waste material. The weathering and oxidation of the waste produces what is commonly known as Acid Mine Drainage (AMD), a dilute sulphuric acid rich in Fe(II) and heavy metals. This thesis serves to summarise five papers reporting how the precipitation of Fe(III) phases can attenuate the contamination of heavy metals by adsorption processes.Schwertmannite (Fe8O8(OH)6SO4) is a common Fe(III) mineral precipitating in AMD environments at pH 3-4. The stability and surface chemistry of this mineral was investigated. It was shown that the stability depended strongly on pH and temperature, an increase in either promoted transformation to goethite (α-FeOOH). Two pH dependent surface species of SO42- were detected with infrared (ATR-FTIR) spectroscopy.The adsorption of Cu(II), Pb(II) and Zn(II) to schwertmannite occurred at lower pH than to goethite, whereas Cd(II) adsorption occurred in a similar pH range on both schwertmannite and goethite. Extended x-ray absorption fine structure (EXAFS) spectroscopy suggests two surface species for Cu(II) and Cd(II) at the schwertmannite surface. Cu(II) adsorbs monodentately and Cd(II) bridging bidentately to adsorbed SO42-. Both metal ions also adsorb in a bridging bidentate mode to the surface hydroxyl groups. At pH 7.5 up to 2.7 μmol Cd(II) m-2 could be adsorbed to schwertmannite, indicating a large adsorption capacity for this mineral.The acid-base properties of two NOM samples were characterised and could be well described as diprotic acids below pH 6. The adsorption of NOM to schwertmannite and goethite was very similar and adsorption occured in a very wide pH range.High concentrations of NOM increased the adsorption of Cu(II) to goethite at low pH whereas a slight decrease was noted at low concentrations of NOM. No effect was detected in the schwertmannite system.The formation of Fe(III) phases from precipitation of AMD was shown to be very pH dependent. At pH 5.5 a mixture of minerals, including schwertmannite, formed whereas at pH 7 only lepidocrocite (γ-FeOOH) formed. The concentration of Zn(II) in AMD could by adsorption/coprecipitation be reduced to environmentally acceptable levels.
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- Persson, Kent
(författare)
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Micromechanical Modelling of Wood and Fibre Properties
- 2000
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Wood is a material with mechanical properties that vary markedly, both within a tree and among trees. Moisture changes lead to shrinkage or swelling and modify the mechanical properties. In the present study both experimental and numerical work concerning the stiffness and the hygroexpansion properties of wood and of fibres and variations in them is presented. The experimental work involves both characterizing the structure of wood at the microstructural level and the testing of clear-wood specimens. The experiments at the microstructural level provide valuable information concerning the cellular structure of wood, information needed for modelling wood on the basis of its microstructure. Deformations in the microstructure due to loading, as characterized by use of a SEM, was also studied. The longitudinal modulus of elasticity, three hygroexpansion coefficients and the density along the radius from pith to bark in the stem were determined by the testing of clear wood specimens. The longitudinal modulus of elasticity and the three shrinkage coefficients were shown to vary considerably along the radial direction of the stem. Models based on the microstructure for determining the stiffness and shrinkage properties of wood are proposed. The models investigated include the chain of modelling from the mechanical properties of the chemical constituents of the cell wall to the average mechanical properties of a growth ring. The models are based mainly on results of the experiments that were performed. Models of the microfibril in the cell wall as well as models of the cellular structure of wood were developed with the aim of determining the stiffness and shrinkage properties of wood from simply a few key parameters. Two models of the cellular structure of wood were investigated. In one of these, the structure was composed of irregular hexagonal cells, whereas in the other the cell structures were obtained from micrographs. Parametric studies performed by use of the hexagonal cell model are presented. The results of these studies showed the parameters governing the stiffness and the hygroexpansion properties of wood to be the microfibril angle of the S2-layer, density and the properties of the chemical constituents. An introductory study of the nonlinear behaviour of cell structures was also carried out. The results of numerical analyses of the deformations in cell structures that occur in compression loading in the radial and tangential directions are presented. The mechancial behaviour of chemically unaltered fibres of simplified geometrical shape was also studied in a preliminary way by means of micromechanical modelling. Three-dimensional finite element models of straight fibres of undeformed and of collapsed cross-sectional shape were involved. Both the force-displacement relationship and the moisture-induced deformations needed for characterization the behaviour of the fibre were determined. The results of simulations of the stiffness behaviour of fibres revealed two unique coupled deformation modes: coupling between extension and twist and coupling between in-plane bending and out-of-plane shear deformation. The deformation modes obtained were shown to be dependent on the value of the microfibril angle in the S2-layer.
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