Heavy Ion Elastic Recoil Detection Analysis (HIERDA) is a versatile Ion Beam Analysis technique well suited to multi-elemental depth profiling of thin layered structures and near-surface regions of materials. An existing limitation is the inability to accurately account for the pronounced broadening and tailing effects of multiple scattering typically seen in HIERDA spectra. This thesis investigates the role of multiple large angle scattering in heavy ion applications such as HIERDA, and seeks to quantify its contribution to experimental output. This is achieved primarily by the development of a computer simulation capable of predicting these contributions and using it to classify and quantify the interactions that cause them. Monte Carlo ion transport simulation is used to generate simulated HIERDA spectra and the results are compared to experimental data acquired using the Time of Flight HIERDA facility at the Australian Nuclear Science and Technology Organisat ion. A Monte Carlo simulation code was adapted to the simulation of HIERDA spectra with considerable attention on improving the modelling efficiency to reduce processing time. Efficiency enhancements have achieved simulation time reductions of two to three orders of magnitude. The simulation is shown to satisfactorily reproduce the complex shape of HIERDA spectra. Some limitations are identified in the ability to accurately predict peak widths and the absolute magnitude of low energy tailing in some cases. The code is used to identify the plural scattering contribution to the spectral features under investigation, and the complexity of plurally scattered ion and recoil paths is demonstrated. The program is also shown to be useful in the interpretation of overlapped energy spectra of elements of similar mass whose signals cannot be reliably separated experimentally. The effect of large angle scattering on the transmission of heavy ions through a nano-scale aperture mask, used to collimate an ion beam to a very small beam spot, is modelled using a version of the program adapted to handle the more complex geometry of the aperture mask. The effectiveness of nano-aperture collimation was studied for a variety of ion-energy combinations. Intensity, energy, and angular distributions of transmitted ions were calculated to quantify the degree to which scattering within the mask limits the spatial resolution achievable. The simulation successfully predicted the effect of misaligning the aperture and the beam, and the result has subsequently been observed experimentally. Transmitted ion distributions showed that the higher energy heavier ions studied are more effectively collimated than are lower energy lighter ions. However, there is still a significant probability of transmission of heavy ions with substantial residual energy beyond the perimeter of the aperture. For the intended application, ion beam lithography, these ions are likely to be problematic. The results indicate that medium energy He ions are the more attractive option, as the residual energy of scattered transmitted ions can be more readily managed by customising the etching process. Continuing research by experimentalists working in this area is proceeding in this direction as a result of the conclusions from this work.

The nucleus is a mesoscopic system that retains features from both the quantum and macroscopic worlds. A basic property of a macroscopic body is its shape. Nuclear shapes can be deduced from experimental data as they influence the excitation mode of the nucleus and hence the energies and lifetimes of its excited levels. Various short-lived nuclei were created in fusion-evaporation experiments performed at international heavy-ion accelerator facilities. The emitted γ rays and, in some experiments, also the charged particles and neutrons emitted in the reactions were detected. The studied neutron-deficient isotopes were either selected by the type and number of particles emitted in the reactions, or by using their characteristic decays. The excited states of the different isotopes were extracted from the γ-ray analyses. Spectroscopic properties, such as the lifetimes of the excited states or the angular distribution of the emitted γ rays were measured when possible. The experimentally obtained level schemes together with the other spectroscopic information were used to deduce the excitation modes and the shapes of the studied nuclei. The detector systems are described in the first chapter and in the second chapter some techniques used to extract information from the experimental data are explained. Finally, a brief theoretical overview on the nuclear models which were used to interpret the experimental results is given. / QC 20100621

The Equation of State (EoS) of asymmetric nuclear matter has been explored through the study of mid-rapidity fragment dynamics from the 35 MeV/u $^{70}$Zn $^{70}$Zn, $^{64}$Zn $^{64}$Zn, and $^{64}$Ni $^{64}$Ni systems. The experimental data was collected at the Texas A and M Cyclotron Institute using the 4 NIMROD-ISiS array, which provided both event characterization and excellent isotopic resolution of charged particles. The transverse collective flow was extracted for proton, deuteron, triton, 3He, alpha, and 6He particles. Isotopic and isobaric effects were observed in the transverse flow of the fragments. In both cases, the transverse flow was shown to decrease with an increasing neutron content in the fragments. The (N/Z)sys dependence of the transverse flow and the difference betwen the triton and 3He flow were shown to be sensitive to the density dependence of the symmetry energy using the stochastic mean-field model. A stiff parameterization of Esym(p) was found to provide better agreement with the experimental data. The transverse flow for intermediate mass fragments (IMFs) was investigated, providing a new probe to study the nuclear EoS. A transition from the IMF flow strongly depending on the mass of the system, in the most violent collisions, to a dependence on the charge of the system, for the peripheral reactions, was observed. Theoretical simulations were used to show that the relative differences in the IMF flow are sensitive to the density dependence of the symmetry energy. The best agreement between the experiment and theory was achieved with a stiff Esym(p). A new method was developed in which correlations between the projectile-like and mid-rapidity fragments were examined using a scaled flow. Theoretical simulations were used to show that the scaled flow of the particles was connected to their average order of emission. The experimental results suggest that the mid-rapidity region is preferentially populated with neutron-rich light charged particles and the Z=3-4 IMFs at a relatively early stage in the collision. This work presents additional constraints on the nuclear EoS and insight into the mid-rapidity dynamics observed in Fermi energy heavy-ion collisions.

In relativistischen Schwerionenkollisionen spielen seltsame Teilchen bei der Untersuchung heißer und dichter Materie eine wichtige Rolle. Dazu wird die Produktion von Hadronen mit Strangeness-Inhalt mit Hilfe eines Transportmodells vom Boltzmann-Ühling-Uhlenbeck (BUU)-Typ numerisch behandelt. Aktuelle Messungen der HADES-Kollaboration bezüglich K+- - und phi-Spektren bilden dabei den entsprechenden experimentellen Rahmen. Darüber hinaus wird das doppelt-seltsame Hyperon Xi- unterhalb der freien NN-Produktionsschwelle analysiert. Hadronische Multiplizitäten, Transversalimpuls- und Rapiditätsspektren werden mit neuen experimentellen Ergebnissen verglichen. Weiterhin werden Massenverschiebungen im Medium, die nukleare Zustandsgleichung sowie das mittlere Feld der Nukleonen berücksichtigt. Neben der Untersuchung von Kern-Kern-Stößen bietet sich in Proton-Kern-Kollisionen ein Vergleich mit jüngsten ANKE-Daten im Hinblick auf die phi-Ausbeute an. Mit Hilfe des BUU-Transportcodes wurden Tranzparenzverhältnisse ermittelt und die Absorption von phi-Mesonen untersucht. Dabei sind sekundäre phi-Produktionskanäle, Isospin-Asymmetrie und Detektorakzeptanzen von Bedeutung und werden systematisch für verschiedene Systemgrößen analysiert. Die impulsintegrierten Boltzmann-Gleichungen dienen im Rahmen einer kinetischen Nichtgleichgewichtsdynamik der Beschreibung hochkomprimierter nuklearer Materie auf hadronischem Niveau, wie sie sowohl beim Urknall als auch bei ultra-relativistischen Schwerionenkollisionen auftritt. Diese Theorie wird am Beispiel von Antiprotonen untersucht und unter Berücksichtigung verschiedener Expansionsmodelle numerisch ausgewertet. Dabei wird die Evolution der Proton- und Antiprotondichten bis zum Ausfrieren für SPS- und RHIC-Energien mittels eines hadro-chemischen Resonanzgasmodells als möglicher Lösungsansatz des "Antiproton-Puzzles" analysiert. Diskutiert wird darüber hinaus das Verhalten baryonischer Materie und Antimaterie im frühen Universum und der adiabatische Pfad kosmischer Materie im QCD-Phasendiagramm. / Strange particles play an important role as probes of relativistic heavy-ion collisions where hot and dense matter is studied. The focus of this thesis is on the production of strange particles within a transport model of Boltzmann-Ühling-Uhlenbeck (BUU) type. Current data of the HADES Collaboration concerning K+- and phi spectra provide the appropriate experimental framework. Moreover, the double-strange hyperon Xi- is analyzed below the free NN production threshold. Hadron multiplicities, transverse-momentum and rapidity spectra are compared with recent experimental data. Further important issues are in-medium mass shifts, the nuclear equation of state as well as the mean field of nucleons. Besides the study of AA collisions a comparison with recent ANKE data regarding the phi yield in pA collisions is done. Transparency ratios are determined and primarily investigated for absorption of phi mesons by means of the BUU transport code. Thereby, secondary phi production channels, isospin asymmetry and detector acceptance are important issues. A systematic analysis is presented for different system sizes. The momentum integrated Boltzmann equations describe dense nuclear matter on a hadronic level appearing in the Big Bang as well as in little bangs, in the context of kinetic off-equilibrium dynamics. This theory is applied to antiprotons and numerically calculated under consideration of various expansion models. Here, the evolution of proton- and antiproton densities till freeze-out is analyzed for ultra-relativistic heavy-ion collisions within a hadrochemic resonance gas model acting as a possible ansatz for solving the "antiproton puzzle". Furthermore, baryonic matter and antimatter is investigated in the early universe and the adiabatic path of cosmic matter is sketched in the QCD phase diagram.

No description available.

R??sum?? : L???isolation thermique est essentielle dans de nombreux types de MEMS (micro-syst??mes ??lectro-m??caniques). Elle permet de r??duire la consommation d?????nergie, am??liorer leurs performances, ou encore isoler la zone chaude du reste du dispositif, ce qui est essentiel dans les syst??mes sur puce. Il existe quelques mat??riaux et techniques d???isolation pour les MEMS, mais ils sont limit??s. En effet, soit ils ne proposent pas un niveau d???isolation suffisant, sont trop fragiles, ou imposent des contraintes trop importantes sur la conception du dispositif et sont difficiles ?? int??grer. Une approche int??ressante pour l???isolation, d??montr??e dans la litt??rature, est de fabriquer des pores de taille nanom??trique dans le silicium par gravure ??lectrochimique. En nanostructurant le silicium ainsi, on peut diviser sa conductivit?? thermique par un facteur de 100 ?? 1000, le transformant en isolant thermique. Cette solution est id??ale pour l???int??gration dans les proc??d??s de fabrication existants des MEMS, car on garde le silicium qui est d??j?? utilis?? pour leur fabrication, mais en le nanostructurant localement, on le rend isolant l?? o?? on en a besoin. Par contre sa porosit?? cause des probl??mes : mauvaise r??sistance chimique, structure instable au-del?? de 400??C, et tenue m??canique r??duite. La facilit?? d???int??gration des semiconducteurs poreux est un atout majeur, nous visons donc de r??duire les d??savantages de ces mat??riaux afin de favoriser leur int??gration dans des dispositifs en silicium. Nous avons identifi?? deux approches pour atteindre cet objectif : i) am??liorer le Si poreux ou ii) d??velopper un nouveau mat??riau. La premi??re approche consiste ?? amorphiser le Si poreux en l???irradiant avec des ions ?? haute ??nergie (uranium, 110 MeV). Nous avons montr?? que l???amorphisation, m??me partielle, du Si poreux entra??ne une diminution de sa conductivit?? thermique, sans endommager sa structure poreuse. Cette technique r??duit sa conductivit?? thermique jusqu????? un facteur de trois, et peut ??tre combin??e avec une pr??-oxydation afin d???atteindre une r??duction d???un facteur cinq. Donc cette m??thode permet de r??duire la porosit?? du Si poreux, et d???att??nuer ainsi les probl??mes de fragilit?? m??canique caus??s par la porosit?? ??lev??e, tout en gardant un niveau d???isolation ??gal. La seconde approche est de d??velopper un nouveau mat??riau. Nous avons choisi le SiC poreux : le SiC massif a des propri??t??s physiques sup??rieures ?? celles du Si, et donc ?? priori le SiC poreux devrait conserver cette sup??riorit??. La fabrication du SiC poreux a d??j?? ??t?? d??montr??e dans la litt??rature, mais avec peu d?????tudes d??taill??es du proc??d??. Sa conductivit?? thermique et tenue m??canique n???ont pas ??t?? caract??ris??es, et sa tenue en temp??rature que de fa??on incompl??te. Nous avons men?? une ??tude syst??matique de la porosification du SiC en fonction de la concentration en HF et le courant. Nous avons impl??ment?? un banc de mesure de la conductivit?? thermique par la m??thode ?? 3 om??ga ?? et l???avons utilis?? pour mesurer la conductivit?? thermique du SiC poreux. Nous avons montr?? qu???elle est environ deux ordres de grandeur plus faible que celle du SiC massif. Nous avons aussi montr?? que le SiC poreux est r??sistant ?? tous les produits chimiques typiquement utilis??s en microfabrication sur silicium. D???apr??s nos r??sultats il est stable jusqu????? au moins 1000??C et nous avons obtenu des r??sultats qualitatifs encourageants quant ?? sa tenue m??canique. Nos r??sultats signifient donc que le SiC poreux est compatible avec la microfabrication, et peut ??tre int??gr?? dans les MEMS comme isolant thermique. // Abstract : Thermal insulation is essential in several types of MEMS (micro electro-mechanical systems). It can help reduce power consumption, improve performance, and can also isolate the hot area from the rest of the device, which is essential in a system-on-chip. A few materials and techniques currently exist for thermal insulation in MEMS, but these are limited. Indeed, either they don???t have provide a sufficient level of insulation, are too fragile, or restrict design of the device and are difficult to integrate. A potentially interesting technique for thermal insulation, which has been demonstrated in the literature, is to make nanometer-scale pores in silicon by electrochemical etching. By nanostructuring silicon in this way, its thermal conductivity is reduced by a factor of 100 to 1000, transforming it into a thermal insulator. This solution is ideal for integration in existing MEMS fabrication processes, as it is based on the silicon substrates which are already used for their fabrication. By locally nanostructuring these substrates, silicon is made insulating wherever necessary. However the porosity also causes problems : poor chemical resistance, an unstable structure above 400???C, and reduced mechanical properties. The ease of integration of porous semiconductors is a major advantage, so we aim to reduce the disadvantages of these materials in order to encourage their integration in silicon-based devices. We have pursued two approaches in order to reach this goal : i) improve porous Si, or ii) develop a new material. The first approach uses irradiation with high energy ions (100 MeV uranium) to amorphise porous Si. We have shown that amorphisation, even partial, of porous Si leads to a reduction of its thermal conductivity, without damaging its porous structure. This technique can reduce the thermal conductivity of porous Si by up to a factor of three, and can be combined with a pre-oxidation to achieve a five-fold reduction of thermal conductivity. Therefore, by using this method we can use porous Si layers with lower porosity, thus reducing the problems caused by the fragility of high-porosity layers, whilst keeping an equal level of thermal insulation. The second approach is to develop a new material. We have chosen porous SiC: bulk SiC has exceptional physical properties and is superior to bulk Si, so porous SiC should be superior to porous Si. Fabrication of porous SiC has been demonstrated in the literature, but detailed studies of the process are lacking. Its thermal conductivity and mechanical properties have never been measured and its high-temperature behaviour has only been partially characterised. We have carried out a systematic study of the effects of HF concentration and current on the porosification process. We have implemented a thermal conductivity measurement setup using the ???3 omega??? method and used it to measure the thermal conductivity of porous SiC. We have shown that it is about two orders of magnitude lower than that of bulk SiC. We have also shown that porous SiC is chemically inert in the most commonly used solutions for microfabrication. Our results show that porous SiC is stable up to at least 1000???C and we have obtained encouraging qualitative results regarding its mechanical properties. This means that porous SiC is compatible with microfabrication processes, and can be integrated in MEMS as a thermal insulation material.

Self-organization is a hot topic as it has the potential to create surface patterns on the nanoscale avoiding cost-intensive top-down approaches. Although chemists have promising results in this area, ion irradiation can create self-organized surface patterns in a more controlled manner. Different regimes of pattern formation under ion irradiation were described so far by 2D models. Here, two new regimes have been studied experimentally, which require modeling in 3D: subsurface point defect kinetics as well as ion impact-induced melt pool formation. This thesis deals with self-organized pattern formation on Ge and Si surfaces under normal incidence irradiation with heavy monatomic and polyatomic ions of energies up to several tens of keV. Irradiation has been performed using liquid metal ion sources in a focused ion beam facility with mass-separation as well as by conventional broad beam ion implantation. Irradiated samples have been analyzed mainly by scanning electron microscopy. Related to the specific irradiation conditions, investigation and discussion of pattern formation has been divided into two parts: (i) formation of Ge morphologies due to point defect kinetics and (ii) formation of Ge and Si morphologies due to melt pool dynamics. Point defect kinetics dominates pattern formation on Ge under irradiation with monatomic ions at room temperature. Irradiation of Ge with Bi and Ge ions at fluences up to 10^17 cm^(-2) has been performed. Comprehensive studies show for the first time that morphologies change from flat surfaces over hole to nanoporous, sponge-like patterns with increasing ion energy. This study is consistent with former irradiations of Ge with a few ion energies. Based on my studies, a consistent, qualitative 3D model of morphology evolution has been developed, which attributes the ion energy dependency of the surface morphology to the depth dependency of point defect creation and relaxation. This model has been proven by atomistic computer experiments, which reproduce the patterns found in real irradiation experiments. At extremely high energy densities deposited by very heavy ions another mechanism dominates pattern formation. The formation of Ge and Si dot patterns by very heavy, monatomic and polyatomic Bi ion irradiation has been studied in detail for the first time. So far, this formation of pronounced dot pattern cannot be explained by any model. Comprehensive, experimental studies have shown that pattern formation on Ge is related to extremely high energy densities deposited by each polyatomic ion locally. The simultaneous impact of several atoms leads to local energy densities sufficient to cause local melting. Heating of Ge substrates under ion irradiation increases the achievable energy density in the collision cascade substantially. This prediction has been confirmed experimentally: it has been found that the threshold for nanomelting can be lowered by substrate heating, which allows pattern formation also under heavy, monatomic ion irradiation. Extensive studies of monatomic Bi irradiation of heated Ge have shown that morphologies change from sponge-like over highly regular dot patterns to smooth surfaces with increasing substrate temperature. The change from sponge-like to dot pattern is correlated to the melting of the ion collision cascade volume, with energy densities sufficient for melt pool formation at the surface. The model of pattern formation on Ge due to extremely high deposited energy densities is not specific to a single element. Therefore, Si has been studied too. Dot patterns have been found for polyatomic Bi ion irradiation of hot Si, which creates sufficiently high energy densities to allow ion impact-induced melt pool formation. This proves that pattern formation by melt pool formation is a novel, general pattern formation mechanism. Using molecular dynamics simulations of project partners, the correlation between dot patterning and ion impact-induced melt pool formation has been proven. The driving force for dot pattern formation due to high deposited energy densities has been identified and approximated in a first continuum description.

R??sum?? : L???isolation thermique est essentielle dans de nombreux types de MEMS (micro-syst??mes ??lectro-m??caniques). Elle permet de r??duire la consommation d?????nergie, am??liorer leurs performances, ou encore isoler la zone chaude du reste du dispositif, ce qui est essentiel dans les syst??mes sur puce. Il existe quelques mat??riaux et techniques d???isolation pour les MEMS, mais ils sont limit??s. En effet, soit ils ne proposent pas un niveau d???isolation suffisant, sont trop fragiles, ou imposent des contraintes trop importantes sur la conception du dispositif et sont difficiles ?? int??grer. Une approche int??ressante pour l???isolation, d??montr??e dans la litt??rature, est de fabriquer des pores de taille nanom??trique dans le silicium par gravure ??lectrochimique. En nanostructurant le silicium ainsi, on peut diviser sa conductivit?? thermique par un facteur de 100 ?? 1000, le transformant en isolant thermique. Cette solution est id??ale pour l???int??gration dans les proc??d??s de fabrication existants des MEMS, car on garde le silicium qui est d??j?? utilis?? pour leur fabrication, mais en le nanostructurant localement, on le rend isolant l?? o?? on en a besoin. Par contre sa porosit?? cause des probl??mes : mauvaise r??sistance chimique, structure instable au-del?? de 400??C, et tenue m??canique r??duite. La facilit?? d???int??gration des semiconducteurs poreux est un atout majeur, nous visons donc de r??duire les d??savantages de ces mat??riaux afin de favoriser leur int??gration dans des dispositifs en silicium. Nous avons identifi?? deux approches pour atteindre cet objectif : i) am??liorer le Si poreux ou ii) d??velopper un nouveau mat??riau. La premi??re approche consiste ?? amorphiser le Si poreux en l???irradiant avec des ions ?? haute ??nergie (uranium, 110 MeV). Nous avons montr?? que l???amorphisation, m??me partielle, du Si poreux entra??ne une diminution de sa conductivit?? thermique, sans endommager sa structure poreuse. Cette technique r??duit sa conductivit?? thermique jusqu????? un facteur de trois, et peut ??tre combin??e avec une pr??-oxydation afin d???atteindre une r??duction d???un facteur cinq. Donc cette m??thode permet de r??duire la porosit?? du Si poreux, et d???att??nuer ainsi les probl??mes de fragilit?? m??canique caus??s par la porosit?? ??lev??e, tout en gardant un niveau d???isolation ??gal. La seconde approche est de d??velopper un nouveau mat??riau. Nous avons choisi le SiC poreux : le SiC massif a des propri??t??s physiques sup??rieures ?? celles du Si, et donc ?? priori le SiC poreux devrait conserver cette sup??riorit??. La fabrication du SiC poreux a d??j?? ??t?? d??montr??e dans la litt??rature, mais avec peu d?????tudes d??taill??es du proc??d??. Sa conductivit?? thermique et tenue m??canique n???ont pas ??t?? caract??ris??es, et sa tenue en temp??rature que de fa??on incompl??te. Nous avons men?? une ??tude syst??matique de la porosification du SiC en fonction de la concentration en HF et le courant. Nous avons impl??ment?? un banc de mesure de la conductivit?? thermique par la m??thode ?? 3 om??ga ?? et l???avons utilis?? pour mesurer la conductivit?? thermique du SiC poreux. Nous avons montr?? qu???elle est environ deux ordres de grandeur plus faible que celle du SiC massif. Nous avons aussi montr?? que le SiC poreux est r??sistant ?? tous les produits chimiques typiquement utilis??s en microfabrication sur silicium. D???apr??s nos r??sultats il est stable jusqu????? au moins 1000??C et nous avons obtenu des r??sultats qualitatifs encourageants quant ?? sa tenue m??canique. Nos r??sultats signifient donc que le SiC poreux est compatible avec la microfabrication, et peut ??tre int??gr?? dans les MEMS comme isolant thermique. // Abstract : Thermal insulation is essential in several types of MEMS (micro electro-mechanical systems). It can help reduce power consumption, improve performance, and can also isolate the hot area from the rest of the device, which is essential in a system-on-chip. A few materials and techniques currently exist for thermal insulation in MEMS, but these are limited. Indeed, either they don???t have provide a sufficient level of insulation, are too fragile, or restrict design of the device and are difficult to integrate. A potentially interesting technique for thermal insulation, which has been demonstrated in the literature, is to make nanometer-scale pores in silicon by electrochemical etching. By nanostructuring silicon in this way, its thermal conductivity is reduced by a factor of 100 to 1000, transforming it into a thermal insulator. This solution is ideal for integration in existing MEMS fabrication processes, as it is based on the silicon substrates which are already used for their fabrication. By locally nanostructuring these substrates, silicon is made insulating wherever necessary. However the porosity also causes problems : poor chemical resistance, an unstable structure above 400???C, and reduced mechanical properties. The ease of integration of porous semiconductors is a major advantage, so we aim to reduce the disadvantages of these materials in order to encourage their integration in silicon-based devices. We have pursued two approaches in order to reach this goal : i) improve porous Si, or ii) develop a new material. The first approach uses irradiation with high energy ions (100 MeV uranium) to amorphise porous Si. We have shown that amorphisation, even partial, of porous Si leads to a reduction of its thermal conductivity, without damaging its porous structure. This technique can reduce the thermal conductivity of porous Si by up to a factor of three, and can be combined with a pre-oxidation to achieve a five-fold reduction of thermal conductivity. Therefore, by using this method we can use porous Si layers with lower porosity, thus reducing the problems caused by the fragility of high-porosity layers, whilst keeping an equal level of thermal insulation. The second approach is to develop a new material. We have chosen porous SiC: bulk SiC has exceptional physical properties and is superior to bulk Si, so porous SiC should be superior to porous Si. Fabrication of porous SiC has been demonstrated in the literature, but detailed studies of the process are lacking. Its thermal conductivity and mechanical properties have never been measured and its high-temperature behaviour has only been partially characterised. We have carried out a systematic study of the effects of HF concentration and current on the porosification process. We have implemented a thermal conductivity measurement setup using the ???3 omega??? method and used it to measure the thermal conductivity of porous SiC. We have shown that it is about two orders of magnitude lower than that of bulk SiC. We have also shown that porous SiC is chemically inert in the most commonly used solutions for microfabrication. Our results show that porous SiC is stable up to at least 1000???C and we have obtained encouraging qualitative results regarding its mechanical properties. This means that porous SiC is compatible with microfabrication processes, and can be integrated in MEMS as a thermal insulation material.

In this thesis Langevin approach is applied to analyze the nuclear dissipative dynamics in fission and fusion reactions. In these investigations, the nuclear elongation coordinate and the corresponding momentum are chosen as collective variables. By considering changes in these variables the decay rate of fission and the formation probability of fusion for heavy ion reactions are calculated. These calculations are performed using simulation techniques and the results thus obtained are compared with the corresponding results of analytic solutions.

Multidimensional quantum tunnelling is an important tool that is used in many areas of physics and chemistry. Sub-barrier fusion reactions of heavy-ions are governed by quantum tunnelling. However, the complexity of the structures of heavy-ions does not allow us to use simple one-dimensional tunnelling equations to and the tunnelling probabilities. Instead of this one should consider all the degrees of freedom which affect the phenomenon and accordingly the intrinsic structure or the deformation of the nuclei must be taken into account in the modelling of heavy-ion fusion. These extra degrees of freedom result in a coupling potential term in the Schrodinger equation of the fusing system. In this thesis 16O + 238 U system is considered. Only the rotational deformation of Uranium is assumed and the coupling potential term is calculated for this system by using two diffrent potential types, namely the Woods-Saxon potential and the double folding potential. Using this term in the Schrodinger equation fusion probability and theoretical cross section are calculated. A discussion that addresses then necessity of multidimensional formulation is given. Besides this point the effects of the choice of the potential type are shown.