Masterstudiengang Mikro- und Nanotechnik
Refine
Document Type
- Master's Thesis (5)
Language
- English (5)
Has Fulltext
- yes (5)
Is part of the Bibliography
- yes (5)
Keywords
- Absorptionswärmepumpe (1)
- Antimaterie (1)
- Astrometrie (1)
- Computersimulation (1)
- Elektronenmikroskopie (1)
- Erdbeschleunigung (1)
- Halbleiterdetektor (1)
- Kryostat (1)
- Nanomechanik (1)
- Nanostruktur (1)
Institute
Correlative Analysis of Surfaces and Nanostructures to Understand their Physics Using AFM and SEM
(2024)
The atomic force microscopy (AFM) and scanning electron microscopy (SEM) are two techniques that now have been used for decades to measure and analyse surfaces. This thesis deals with 1st real in-situ correlative microscopy platform that combines both of these techniques. This unique platform through SEM facilitates to measure the inhomogeneous sample surfaces, on which it is difficult to land the cantilever tip at specific region of interests through a conventional optical visualisation at nanometre resolution. This platform eases the nanostructures’ analysis and directly correlating the AFM and SEM measurement data. In this work different samples with diverse characteristics were measured using a variety of AFM measurement modes and through the assistance of SEM. In doing so, the measurement accuracy, precision and possible applications of the correlative microscopy tool were evaluated. (a.) A polymer blend of two polymers was nanomechanically characterised to benchmark the measurement results of the correlative microscope against other commercial standalone AFMs. The benchmarking is crucial for advancing the development of in-situ AFM measurement modes. (b.) A multi-pulse laser ablated stainless steel surface, which was subjected to ultra-short femtosecond laser pulses, was measured via AFM topography imaging to characterise the evolution of surface morphology. This characterisation not only helps to understand the multi-pulse laser ablation, but it also further provides the experimental knowledge and basis to simulate the models for multi-pulse laser ablation. (c.) 3D structures nanoprinted via Focused Electron Beam Induced Deposition (FEBID) were mechanically characterised to assess their structural stiffness and deformation. This sort of evaluation of FEBID structures is to current knowledge the 1st of its kind. While recording force-distance curves, the structural deformation and tip landing on the FEBID structures was observed live via secondary electron imaging using the SEM. It is also a fantastic example demonstrating the power of correlative AFM and SEM measurements. The results are valuable towards the enhancement of FEBID process.
In many domains of X-ray spectroscopy, such as materials science, bio-imaging and astronomy, large silicon detectors form the heart of the imaging process. Adequate cooling is a prerequisite for an optimum operation. Dark current can thus be minimized, the effects of radiation damage limited, and reverse annealing and thermal runaway avoided [1], [2].
A so-called active interposer (AI), a narrow 76 x 76 mm² silicon cooling plate with an embedded microchannel network designed for large pn-CCDs, which also takes local heat fields into account, is evaluated by equivalent test structures. Unlike the commonly used massive cooling frames, time spent on cool-down and warm-up may being reduced, miss match in coefficient of thermal expansion (CTE) limited and thermal paths are shortened [3]–[5]. It also serves as a silicon circuit board (SiCB) allowing a dense integration into a tiled-like, large-scale focal plane arrangement with little inactive area.
The equivalent test structure was monitored and supplied with coolant by a high-pressure pump in a vacuum chamber. An analytical as well as a CFD model in ANSYS-Fluent were created to verify, compare, and thoroughly investigate the thermal and hydrodynamic parameters. First tests have shown that temperature gradients below 4°C can be achieved at a power dissipation of 22 watts, with an average temperature increase of 7.5 degrees Celsius at Reynolds numbers below 750. Initial conclusions about the microchannel cooling (MCC) capacity could be drawn and the dependence between temperature gradients and flow rates has been verified.
Last but not least, the experiments have provided first indication that an active interposer and the associated supply system are feasible, paving the way for further steps to increase practicality and functionality in cooling of large semiconductor detectors.
The AEgIS collaboration (Antimatter Experiment: Gravity, Interferometry, Spectroscopy) at CERN has assigned itself the goal of directly measuring Earth's gravitational force on antimatter with a precision of 1% for the first time. Neutral antihydrogen atoms enable this measurement and are produced using the highly efficient charge exchange reaction of cold antiproton plasma with laser excited Rydberg-positronium. These atoms are then transported by Stark-acceleration towards a Moiré deflectometer which detects the displacement of the antihydrogen beam caused by gravity.
This Master's thesis is based on AEgIS' goal and focuses on the positron system which is an essential component of the overall experiment. Two main objectives are dealt with. Firstly, the systematical optimization of the positron apparatus consisting of a source, two Surko-type buffer gas Penning-Malmberg traps with rotating walls and a transfer line to the AEgIS antihydrogen apparatus or to a test chamber for sample examination. Secondly, two positron/positronium converter targets are investigated for the first time at AEGIS using single-shot positron annihilation lifetime spectroscopy. Regarding the first objective, a large enhancement of the total positron yield (ten times higher) is achieved which results in a higher positronium output and thus satisfies the needs of a high antihydrogen yield. This is realized by applying horizontal and vertical magnetic correction fields in one of the traps; optimizing the frequency and amplitude of the rotating wall; shielding critical parts with µ-metal sheets from environmental magnetic fields and by adjusting the potentials in the traps and the transfer line. After this, a positron bunch with 20 ns time-spread containing up to 5.7 · 107 positrons within a reasonable accumulation time of ten minutes can be provided for further measurements. The two investigated converter targets for the positronium production are an aerogel target from NASA and a nanochannel converter produced by the University of Trento. The annihilation radiation of the incident positron bunch is detected with a PbF2- Cherenkov detector which shows a slow decreasing signal for tens of nanoseconds for both targets which proves the emission of positronium. The nanochannel converter is slightly more effective and has a positronium lifetime of (90 ± 2) ns, whereas the yield with the aerogel target is smaller and only a lifetime of (59 ± 2) ns is found. These results indicate that the nanochannel converter is best suited for AEgIS but also that additional and more precise measurements will help to improve the positronium yield in the future.
Heat pumps offer a great opportunity of heating buildings on renewable energy sources. A small, cost-efficient and environment-friendly implementation are ammoniawater absorption heat pumps. One of its critical components in terms of efficiency, size and cost is the absorber. In this thesis the physical processes inside an ammoniawater spray absorber chamber were analyzed, modeled and simulated with the objective of optimizing the absorber efficiency by determining the influence of spray and nozzle properties on the absorption process. Effects of drop diameter and velocity were estimated analytically by simple approximations and by calculations in LibreOffice spreadsheets. The joint distribution of drop diameter velocity in a spray produced by a certain nozzle was modeled using both empirical equations and abinitio methods like the maximum entropy formalism (MEF) by the current state of scientific and engineering knowledge. The model of a swirl nozzle thus obtained was implemented in C++ as a part of an OpenFOAM solver. Using an existing, specifically built absorption library, the author was provided with, several simulations were conducted varying the absorption chamber geometries and the nozzle types. The results of these simulation showed that the heat transfer from the drops to the ammonia vapor atmosphere in the absorption chamber and subsequently from the chamber atmosphere to the chamber walls is the crucial parameter determining the absorption rate and the final ammonia concentration. In literature, several approaches can be found to determine the absorption rate of a single, representative drop in a spray, either by analytical or numerical methods. However, so far no study considered the effects of the whole spray consisting of a huge number of drops with varying size and velocity. By the model and setup developed in this work, effects like heating up of the absorber chamber or flow induced by momentum transfer from the drops to the fluid, can be taken into account for the first time.