Nonlinear Analysis of Composite Beams under Random Excitations

The vibration responses of three unsymmetrically laminated beams, that are excited with a Gaussian random forcing function, are studied in this thesis. The beams are analyzed nonlinearly and compared to linear results, indicating some important corrections. The solution procedure begins with the derivation of the general equation of motion using Galerkin\u27s method. Then, two approaches are taken in the solution. First, the equation of motion is attacked directly employing a real time Runge-Kutta numerical analysis. Second, the method of equivalent linearization is used. The thesis finds the results from the two approaches to be in a close agreement, although some discrepancies at high loads could be found. However, the most important achievement of the thesis is undoubtedly that the same response equation can be used for any type of laminate layup, and any of the three types of beams. One of many applications that the solution can be used for is the assessment of fatigue tolerances

A Biomimetic, Energy-Harvesting, Obstacle-Avoiding, Path-Planning Algorithm for UAVs

This dissertation presents two new approaches to energy harvesting for Unmanned Aerial Vehicles (UAV). One method is based on the Potential Flow Method (PFM); the other method seeds a wind-field map based on updraft peak analysis and then applies a variant of the Bellman-Ford algorithm to find the minimum-cost path. Both methods are enhanced by taking into account the performance characteristics of the aircraft using advanced performance theory. The combined approach yields five possible trajectories from which the one with the minimum energy cost is selected. The dissertation concludes by using the developed theory and modeling tools to simulate the flight paths of two small Unmanned Aerial Vehicles (sUAV) in the 500 kg and 250 kg class. The results show that, in mountainous regions, substantial energy can be recovered, depending on topography and wind characteristics. For the examples presented, as much as 50% of the energy was recovered for a complex, multi-heading, multi-altitude, 170 km mission in an average wind speed of 9 m/s. The algorithms constitute a Generic Intelligent Control Algorithm (GICA) for autonomous unmanned aerial vehicles that enables an extraction of atmospheric energy while completing a mission trajectory. At the same time, the algorithm automatically adjusts the flight path in order to avoid obstacles, in a fashion not unlike what one would expect from living organisms, such as birds and insects. This multi-disciplinary approach renders the approach biomimetic, i.e. it constitutes a synthetic system that “mimics the formation and function of biological mechanisms and processes.

Epitaxial growth and characterization of (001) [NiFe/M]20 (M = Cu, CuPt and Pt) superlattices

We present optimization of [(15 \uc5) Ni80Fe20/(5 \uc5) M]20 single crystal multilayers on (001) MgO substrates, with M being Cu, Cu50Pt50 and Pt. These superlattices were characterized by high resolution X-ray reflectivity (XRR) and diffraction (XRD) as well as polar mapping of important crystal planes. It is shown that cube on cube epitaxial relationship can be obtained when depositing at substrate temperature of 100 \ub0C regardless of the lattice mismatch (5% and 14% for Cu and Pt, respectively). At lower substrate temperatures poly-crystalline multilayers were obtained while at higher substrate temperatures {111} planes appear at ∼10\ub0 off normal to the film plane. It is also shown that as the epitaxial strain increases, the easy magnetization axis rotates towards the direction that previously was assumed to be harder, i.e. from [110] to [100], and eventually further increase in the strain makes the magnetic hysteresis loops isotropic in the film plane. Higher epitaxial strain is also accompanied with increased coercivity values. Thus, the effect of epitaxial strain on the magnetocrystalline anisotropy is much larger than what was observed previously in similar, but polycrystalline samples with uniaxial anisotropy (Kateb et al. 2021)

Tailoring microstructure and stress through energetic ion bombardment: A molecular dynamic simulation

Pre-print (óritrýnt handrit)We studied high power impulse magnetron sputtering (HiPIMS) and different substrate bias for the epitaxial growth of Cu film on Cu (111) substrate by molecular dynamics simulation. We assumed a fully ionized deposition flux to represent the high ionization fraction in the HiPIMS process. Three different substrate biases, roughly low, moderate and high, were applied to the kinetic energy of the deposition flux with a flat energy distribution in each range. In low energy regime, the results were compared to the case of completely neutral flux, in analogy with thermal evaporation. In the low energy range, HiPIMS presents a slightly smoother surface and more interface mixing compared to that of thermal evaporation. However, in the moderate energy HiPIMS an atomically smooth surface was obtained with a slight increase in the interface mixing compared to low energy HiPIMS. In the high energy regime, HiPIMS presents severe interface mixing with a smooth surface with a limited growth due to resputtering from the surface. The results also indicate that in the film obtained by moderate energy HiPIMS fewer crystal defects appear. This behavior can be attributed to the repetition frequency of collision events demonstrated recently by Kateb et al. (2019). In particular high energy HiPIMS suffers from high repetition of collision events which does not allow recrystalization of the film. In the low energy HiPIMS, collision events are not enough to overcome island growth. At moderate energy, collision events repeat in a manner that provides enough time for recrystalization that results in a smooth surface, fewer defects and limited intermixing.This work was partially supported by the University of Iceland Research Funds for Doctoral students, the Icelandic Research Fund Grant Nos. 196141, 130029 and 120002023

The importance of HiPIMS ionization flux fraction on the film microstructure and surface roughness: A molecular dynamic simulation

Pre-print (óritrýnt handrit)We demonstrate the effect of ionization flux fraction on the epitaxial growth of Cu film on Cu (111) substrate at room temperature. We compare thermal evaporation, dc magnetron sputtering (dcMS) and high power impulse magnetron sputtering (HiPIMS) with fully neutral, 50 % ionized and 100 % ionized flux, respectively. It is shown that higher ionization flux fraction of the deposition flux leads to smoother surfaces by two ma-jor mechanisms i.e. decreasing clustering in the vapor phase and bi-collision of high energy ions at the film surface. The bi-collision event consists of local amorphization which fills the gaps between islands followed by crystallization due to secondary collisions. We found bi-collision events to be very important to prevent island growth to become dominant and increase the surface roughness. Regardless of the deposition method, epitaxial Cu thin films suffer from stacking fault areas (twin boundaries) in agreement with recent experi-mental results. In addition, HiPIMS deposition presents considerable interface mixing while it is negligible in thermal evaporation and dcMS deposition, those present less adhesion accordingly.This work was partially supported by the University of Iceland Research Funds for Doctoral students, the Icelandic Research Fund Grant Nos. 196141, 130029 and 120002023 and the Swedish Government Agency for Innovation Systems (VINNOVA) contract No. 2014-04876

Oblique angle deposition of nickel thin films by high-power impulse magnetron sputtering

Publisher's version (útgefin grein)Background: Oblique angle deposition is known for yielding the growth of columnar grains that are tilted in the direction of the deposition flux. Using this technique combined with high-power impulse magnetron sputtering (HiPIMS) can induce unique properties in ferromagnetic thin films. Earlier we have explored the properties of polycrystalline and epitaxially deposited permalloy thin films deposited under 35° tilt using HiPIMS and compared it with films deposited by dc magnetron sputtering (dcMS). The films prepared by HiPIMS present lower anisotropy and coercivity fields than films deposited with dcMS. For the epitaxial films dcMS deposition gives biaxial anisotropy while HiPIMS deposition gives a well-defined uniaxial anisotropy. Results: We report on the deposition of 50 nm polycrystalline nickel thin films by dcMS and HiPIMS while the tilt angle with respect to the substrate normal is varied from 0° to 70°. The HiPIMS-deposited films are always denser, with a smoother surface and are magnetically softer than the dcMS-deposited films under the same deposition conditions. The obliquely deposited HiPIMS films are significantly more uniform in terms of thickness. Cross-sectional SEM images reveal that the dcMS-deposited film under 70° tilt angle consists of well-defined inclined nanocolumnar grains while grains of HiPIMS-deposited films are smaller and less tilted. Both deposition methods result in in-plane isotropic magnetic behavior at small tilt angles while larger tilt angles result in uniaxial magnetic anisotropy. The transition tilt angle varies with deposition method and is measured around 35° for dcMS and 60° for HiPIMS. Conclusion: Due to the high discharge current and high ionized flux fraction, the HiPIMS process can suppress the inclined columnar growth induced by oblique angle deposition. Thus, the ferromagnetic thin films obliquely deposited by HiPIMS deposition exhibit different magnetic properties than dcMS-deposited films. The results demonstrate the potential of the HiPIMS process to tailor the material properties for some important technological applications in addition to the ability to fill high aspect ratio trenches and coating on cutting tools with complex geometries.The authors would like to thank Dr. Fridrik Magnus for his helpful advice on interpretation of MOKE results. This work was partially supported by the University of Iceland Research Fund for Doctoral students, the Icelandic Research Fund Grant Nos. 130029 and 196141.Peer Reviewe

Photoluminescence study of Si_1-xGe_x nanoparticles in various oxide matrices

We investigate the photoluminescence properties of structures comprising of Si1-xGex nanoparticles (NPs) within SiO2, GeO2, TiO2 and Ta2O5 oxide matrices. Of the investigated structures, it was observed that the structures with GeO2 and TiO2 matrices provide increased spectral response (at ~907 and 844 nm respectively) and increased PL intensity. The improved PL characteristic have been attributed to increased diffusion barrier against oxygen which otherwise would result in formation of unwanted oxide at the film-oxide interface, thereby deteriorating the optical properties.This work is partially funded by the Icelandic Research Fund Grants nos. 218029 and 196141.Pre-print (óritrýnt handrit