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In this study, heat-treated and multisurface engineered DIN 1.2367 tool steel was subjected to room and elevated temperature wear tests, and the effect of nitriding on its tribological behavior was investigated. CrN, AlTiN, and CrN/AlTiN coatings with a total thickness of 2 µm were obtained by arc cathodic physical vapor deposition on conventional heat-treated and gas-nitrided steels. The white layer formed during nitriding was removed, and a diffusion layer (100 µm) was achieved in the cross section of the steel having a tempered martensitic matrix. The highest surface hardness was attained with an integral coating (CrN/AlTiN), and surface hardness increased even more after nitriding due to the formation of a multicomponent ceramic layer on top of the diffusion layer. The room temperature wear tests performed against an alumina counterpart revealed that (i) CrN/AlTiN-coated steel had the highest friction coefficient of 0.26, which further increased to 0.33 by nitriding due to the increase in shear strength, and that (ii) with increasing surface hardness, the specific wear rates (W) of the heat-treated and coated steels could be ranked as follows: WCrN/AlTiN < WAlTiN < WCrN. The wear rates decreased when nitriding was carried out prior to coating. In order to simulate the aluminum extrusion conditions, hot wear behavior of the surfaces against AA6080 alloy at 450 °C was investigated. The hot wear tests revealed that (i) high friction coefficients were reached due to the adhesive characteristic of aluminum to the surfaces, (ii) the nitrided and CrN/AlTiN-coated sample exhibited the lowest wear rate among all studied surfaces, and (iii) the film damage on the worn surfaces mostly occurred in the form of droplet delamination.
Gallium nitride (GaN) is a III-V semiconductor, characterized by direct, wide band gap of 3.4 eV at RT. As a material of particular interest for opto- and power electronics applications, it has been thoroughly studied in recent years. Utilization of GaN homoepitaxy in manufacturing of laser diodes (LDs), light-emitting diodes (LEDs), power devices, etc. would be beneficial in terms of reducing defect density, thus improving their lifetime and performance. Yet cost-effective process for providing native GaN substrates has not been established so far.
The focus of this work is put on development of a new method to grow single crystalline GaN layers from Ga vapour. Our approach exploits microwave (MW) plasma as a source of excited nitrogen species, in contrast to classical physical vapour transport (PVT)-based technique, in which ammonia (NH3) serves as a source of reactive nitrogen. Novelty of MW plasma enhanced growth of GaN from vapour lies in MW nitrogen plasma formation in the vicinity of the seed, at moderate pressure (200 – 800 mbar range), and concurrent physical vapour transport of Ga to the growth zone. Simulations of the growth setup (HEpiGaN software) and of the MW plasma source (CST Microwave software) have followed the extensive investigations of material properties. The growth setup and the MW plasma source, with the resonance cavity being its crucial part, have been constructed and implemented into the existing growth reactor.
The stability of MW plasma in function of temperature and pressure has been studied along with its influence on the seed temperature, and thus on the growth conditions. Furthermore, optical emission spectroscopy (OES) has been utilized for in-situ characterization of the growth atmosphere. Studies on the interaction of Ga vapour with the nitrogen discharge were interpreted on the basis of the level structure of lower excited states of Ga.
Deposition experiments have been conducted, using sapphire seeds, GaN, AlN and AlGaN templates, while GaN single crystalline layers have been grown on sapphire and GaN templates. Characterization of GaN layers have been done by various methods, i.e. structure of layers by scanning electron microscopy (SEM), their composition by energy dispersive X-ray spectroscopy (EDX) and secondary ion mass spectrometry (SIMS), and crystal quality by high resolution X-ray diffraction (HRXRD). Results of the characterization together with outcome of OES measurements revealed importance of carbon for the sub-atmospheric MW plasma enhanced growth of GaN from vapour. In addition, this fact was confirmed by experiments in the setup with reduced carbon content. Possible routes for GaN synthesis have been discussed, with the most probable being CN-assisted GaN formation. While CN was detected in the plasma spectra, there was no evidence for the existence of GaN molecules in vapour phase.
High-resolution imaging of buried metal interconnect structures in advanced microelectronic products with full-field X-ray microscopy is demonstrated in the hard X-ray regime, i.e., at photon energies > 10 keV. The combination of two multilayer optics—a side-by-side Montel (or nested Kirkpatrick–Baez) condenser optic and a high aspect-ratio multilayer Laue lens—results in an asymmetric optical path in the transmission X-ray microscope. This optics arrangement allows the imaging of 3D nanostructures in opaque objects at a photon energy of 24.2 keV (In-Kα X-ray line). Using a Siemens star test pattern with a minimal feature size of 150 nm, it was proven that features < 150 nm can be resolved. In-Kα radiation is generated from a Ga-In alloy target using a laboratory X-ray source that employs the liquid-metal-jet technology. Since the penetration depth of X-rays into the samples is significantly larger compared to 8 keV photons used in state-of-the-art laboratory X-ray microscopes (Cu-Kα radiation), 3D-nanopattered materials and structures can be imaged nondestructively in mm to cm thick samples. This means that destructive de-processing, thinning or cross-sectioning of the samples are not needed for the visualization of interconnect structures in microelectronic products manufactured using advanced packaging technologies. The application of laboratory transmission X-ray microscopy in the hard X-ray regime is demonstrated for Cu/Cu6Sn5/Cu microbump interconnects fabricated using solid–liquid interdiffusion (SLID) bonding.
In-stream microbial carbon transformation under opposing stresses - drought and sediment transport
(2018)
The mineralization of organic matter (OM) is an important ecosystem service that has come under pressure because of increased frequency of droughts and higher sediment loads in running waters. In particular, lowland streams in temperate regions may experience reinforced sediment transport through migratory ripples and changes of naturally sorted sand and gravel in streambeds towards sand-dominated, homogenized streambed structure. The impact on microbial carbon (C)-transformation from these changes was the main focus of my doctoral thesis, in particular the impact of (i) periodic mechanical disturbance associated with ripple migration (ii) streambed structure homogenization, and (iii) drought in streambeds with sorted or homogenized sediment structure.
In a set of microcosms, the significance of periodic mechanical disturbances for microbial C-transformation was tested. Thereby, the quantity and quality of the OM in the sandy sediments were varied by the addition of leaves and fish feces to the OM-poor sands. The results revealed that periodic mechanical disturbances resulted in significant decrease in microbial respiration to a low and similar level regardless of OM quality contained in sand.
The importance of the streambed structure (sorted vs homogenized) for C-transformation was tested using set of experimental streams. The focus was on the interaction between benthic and hyporheic microbial processes in C-transformation to better understand the consequences of streambed homogenization on microbial function. The results showed that sediment structure determines connectivity between the benthic and hyporheic zones. The lower water exchange in homogenized streambeds and thereby reduced supply of freshly produced bioavailable OM from the benthic to the hyporheic zone, curtailed microbial respiration in the latter affecting the water quality.
The influence of a drought and rewetting was tested on C-transformation in streambeds with a sorted or homogenized sediment structure using experimental streams where one half of the streams were strongly shaded and the other half moderately shaded. The results showed that streambeds affected by droughts, either with sorted or homogenized sediment structure have a similar microbial activity at the first place controlled by shading, whereas microbial composition during drought and its recovery after rewetting was additionally affected by sediment structure.
Overall, this doctoral thesis showed that in sediment transport– and drought-impacted streambeds (i) ripple migration results in decreased C-transformation regardless of the available quality of OM, (ii) homogenization of sorted sediment structure leads to a decrease in microbial C-transformation in the hyporheic zone, and (iii) interaction between sediment structure and shading alters microbial community composition especially critical for resistance and resilience of C-transformation during drought and rewetting.
Violent chronic conflicts, uprooting, and the continuous massive (re)generation of displacement waves constitute the main elements of the Kurds’ history within nation-states’ (changing) geographies. Today, Kurdish-inhabited territories represent palimpsests of involuntary dislocations and relocation stories, constantly (re)written by refuge-seeking/ granting (spatial) practices. Since 2011, contemporary uprisings and wars in Syria and Iraq have inscribed the Kurdistan Region of Iraq (KR-I) territories with a new layer of displacement stories. These territories acted as frontline receptors for consecutive Syrian refugees and Iraqi internally displaced waves seeking protection. Displacees found refuge in the rural, urban and newly set humanitarian camps’ spaces; hence, they (temporarily) anchored and carved some sort of a presence within their receiving sites. The rationale behind such receiving sites' emergence, use and progression in the KR-I have historically oscillated between modernization, discipline, development and humanitarianism.
Guided by the urbanism lens, this research aims to narrate the KR-I’s forced displacement territorial biography of involuntary dislocated and (re)located Kurds. This narration chronologically traces these groups' newly (re)shaped receiving sites and how refuge-seeking/granting practices aggregated and (re) articulated the KR-I’s territories. It investigates the ways in which these receiving sites act as seeds of emerging urbanities in such a politically contested region. On the one hand, the research zooms out and repositions these sites' emergence and spatial progression within a broader geopolitical context, socioeconomic conditionalities, uses and meaning subversions due to the ever-changing actors. On the other hand, it zooms closely into the newly set Syrian-Kurdish refugee camps in the KR-I within and beyond predefined (nation-state) belongings in their ever-distorted time-space frames. It securitizes on the ways in which these sites are conceived and inhabited to (re)claim rights “to” and “in” space, territory, history and future. Between the large and small pictures, the research investigates the human and the non-human spatial agency that (re)patches these ruptured Kurds' collective memories and contemporary narratives of where, when, what and with whom (a future) home rests.
This study is an urgent call for the international and local constellation of actors to critically rethink their standardized ready-made solutions and piece-meal spatial interventions in geographies associated with chronic crisis, ever (re)branded under emergency, development and peace ‘help’ banners. It sets the basis for rearticulating and developing further research-based interventions that account for exhausted conflict-ridden (infra)structures, political sensitivities, socio-economic situations and environmental impacts.
The main subject of this work was the investigation of sintering behavior, microstructure, mechanical properties and biocompatibility of metal injection moulded (MIM) Ti-Nb alloys for biomedical applications. Commercially pure titanium (CP-Ti) samples were also fabricated by MIM as a reference. The sintering behavior of MIM Ti-Nb alloys was studied at first, in order to roughly determine the sintering parameters in the following investigations. Dilatometry was applied to investigate the linear shrinkage of MIM Ti-Nb samples from room temperature to 1500 °C at a heating rate of 3 °C/min under argon atmosphere. Various sintering parameters and Nb contents were used to investigate their influences on microstructure and mechanical properties of MIM Ti-Nb alloys by means of density measurements, optical microscopy (OM), X-ray diffraction (XRD), scanning electron microscopy (SEM) and mechanical testing. Transmission electron microscopy (TEM) and high energy X-ray diffraction (HEXRD) measurement were applied to investigate the nature and precipitation of the unexpected titanium carbide precipitates in MIM Ti-Nb alloys. Initial cell adhesion and cell proliferation assays of human umbilical cord perivascular cells (HUCPV) on MIM Ti-Nb alloys were performed for biocompatibility characterization. The results of this work show that MIM Ti-Nb and MIM CP-Ti samples have been successfully fabricated and the as-sintered samples show good shape retention without distortion compared to the green sample. The sintering process of MIM Ti-Nb alloys consists of three main steps – Ti-diffusion step, Ti-Nb-diffusion step and Matrix-diffusion step. With increasing sintering temperatures and time, MIM Ti-Nb alloys exhibit lower porosity and higher Young’s modulus. A higher Nb content in MIM Ti-Nb alloys leads to an increase of carbide area fraction and porosity. The three factors – Nb content, carbide area fraction and porosity – determine the mechanical properties of MIM Ti-Nb alloys. An increase of Nb content and amount of carbides as well as a lowered porosity lead to a higher tensile strength. A decrease of Young’s modulus can be expected with higher Nb content and porosity. A high amount of titanium carbides can result in very poor ductility, but annealing and quenching process can significantly improve the elongation by dissolving the carbides. MIM Ti-Nb alloys exhibit good biocompatibility, indicating their potential for implant applications.
The accelerated urbanization has led to increasing tension on urban land use. In this context, more and more slender high-rise buildings are being built worldwide in pursuit of better economic benefits. However, these structures are susceptible to wind excitation due to their lower first natural frequency. Different passive, semi-active, and active damping systems have been developed to reduce wind-induced structural vibration. Among them, the tuned mass dampers are widely used and proved as a very effective method in practice. However, this system requires a large additional damping mass. This also causes additional reinforcement, which increases the cost and carbon footprint. A huge space near the top story of the building is needed for the installation. In this research, a novel system named distributed-Multiple Tuned Facade Damping (d-MTFD) system is proposed by using specially designed parallel moveable Double-Skin Facade (DSF) outer skin as damping mass. These moveable facade elements can be installed on the upper stories of the high-rise building. Smooth-running guide rail systems are used to achieve the parallel moveability. Multi-objective optimization based on the Genetic Algorithm (GA) is applied to reduce the maximum top floor acceleration (Objective I) and to reduce the maximum facade relative displacement (Objective II) simultaneously. The optimization results for the passive and semi-active systems are presented in the form of the Pareto front. The trade-off between these selected two competing optimization objectives is observed. This approach was first validated in a simulation using a 306 m tall reference building for a wind speed of 13.5 m/s at 10 m above ground level with a return period of 10 years. Acceptable peak accelerations at the top story for hotel use and a maximum facade relative displacement of less than ±0.5 m could be achieved for the benchmark building with the d-MTFD system. For semi-active control, the variable damping coefficient can be achieved by using stepper motors in generator mode. The electrical damping coefficient can be continuously adjusted by the developed power electronics. In addition, electrical energy can be generated and stored in a battery. A full-scale prototype with one parallel moveable facade element was built. Based on the prototype, the functionality of the semi-active control using a stepper motor and its energy harvesting performance was tested by applying Hardware-in-the-Loop (HiL) simulations. Greybox system identification was used to estimate some parameters (spring stiffness, friction, etc.) in the connection. The accurate system identification results ensure further validation using HiL simulations. The HiL simulations successfully demonstrated the feasibility of a self-powered semi-active d-MTFD system.
Titanium and its alloys have been widely used as implant biomaterials due to their suitable combination of mechanical properties and biological compatibilities. At present, about 70-80% of implants are made of metallic biomaterials. Compared with magnesium alloys, stainless steel and cobalt alloys, titanium alloys have a higher specific strength, high corrosion resistance, and excellent biocompatibility. With research, Ti-Nb biomedical titanium alloys have been constantly developed. In the Ti-Nb alloy, the amount of Nb is usually from 16% to 42% (wt.%) which is about 10% to 30% (at.%). As reported, Mn as a trace element to the human body has the potential to be used in bio-materials. Therefore, this study aims at the partial replacement of Nb by Mn to reduce the costs, without deteriorating the mechanical properties. Moreover, it must be ensured good biocompatibility and corrosion resistance. This is the first investigated on Ti-Mn-Nb ternary alloys.
In this work, According to β single-phase field, Ti-xMn-yNb (x=4, 10, 16; y=2, 8, 14, at.%) alloys (arc-melted) have been fabricated. The Ti-Mn-Nb alloys are investigated by optical microscopy (OM), X-ray diffraction (XRD), hardness test, transmission electron microscopy (TEM) and mechanical testing. By screening study on alloy, Ti-10Mn-14Nb (at.%) (Ti-10Mn-23.7Nb (wt.%)) is the optimal alloy with tensile strength (760 MPa) and elongation (10.5%). After that, the Metal Injection Moulding (MIM) is used to prepare Ti-Mn-Nb alloys. The MIM method can greatly reduce the processing cost. MIM Ti-xMn-yNb (x=3, 4, 6; y=1, 2, 4, at.%), Ti-4Mn-14Nb and Ti-10Mn-14Nb alloys are fabricated. Among them, a very good combination of mechanical properties is achieved for MIM processed Ti-4Mn-2Nb (at.%) (Ti-4.5Mn-3.8Nb (wt.%)), namely a YS of 642 MPa, UTS of 725 MPa and high ductility of 16% elongation to fracture. With further investigations, when the yttrium content is 0.1% (at.%), the tensile strength of Ti-4Mn-2Nb-0.1Y (at.%) (Ti-4.5Mn-3.8Nb-0.18Y (wt.%)) is increased to 785 MPa while elongation of 12.9%. These mechanical properties already exceed Ti-6Al-4V (ASTM F2885 Grade 5 undensified).
In the in vitro evaluation, in comparison with MIM pure titanium, human osteoblasts MG63 adhered as well and proliferated on the surface of MIM Ti-Mn-Nb specimens. In the supernatant after cell culture, the Ti-Mn-Nb alloy shows similar osmolality and pH value results as MIM pure titanium. By LDH assay and DNA isolation, the MIM Ti-Mn-Nb alloys are not found to be toxic to MG63 cells.
In the study of corrosion resistance in Hanks’ balanced salt solution (HBSS) at 37 °C, the corrosion current densities as well as the impedance of MIM Ti-Mn-Nb alloys are all better than those of MIM pure titanium and even better than those of MIM Ti-6Al-4V alloy.
The search for textual information, e.g., in the form of webpages, is a typical task in modern business and private life. From a user's point of view, the commonly used systems have matured and established common interaction design patterns such as the textual input box that starts virtually every directed search process.
In comparison, the search for multimedia documents (e.g., images or videos) is still in its early years. In other words, a pre-dominant search strategy has not yet evolved. That is, directed and exploratory search approaches fight for user acceptance.
One further discriminative factor of multimedia information retrieval (MMIR) from traditional text-based information retrieval (IR) is that multimedia documents are not necessarily stored with the help of the same data access paradigm.
From a technical point of view, the use of different data access paradigms complicates the retrieval from such collections because the utilized retrieval model has to support these paradigms.
As a consequence, the main challenges in MMIR - the retrieval engine and the user interaction -- have to be addressed in a holistic way. A holistic theoretic perspective on MMIR/IR research is taken by principle of polyrepresentation (PoP), which forms one half of the theoretic background of this dissertation aiming at the development of a preference-based approach to interactive MMIR. Roughly speaking, the PoP theorizes that representations describing a document are based on various cognitive processes dealing with it, e.g., a title, its color or shape features, its creator, or its date of creation. This multitude of representations can be fused to form a conjunctive cognitive overlap (CO) in which highly relevant documents are likely to be contained. This explicit recommendation discriminates the PoP from typical feature fusion approaches often used in MMIR.
However, the PoP does not answer how a retrieval model has to be implemented in a technical sense which is of interest in the field of computer science. One possibility to implement the PoP are quantum mechanics-inspired IR models such as the commuting quantum query language (CQQL) which is used in this thesis.
CQQL is particularly interesting because it integrates data access paradigms used in the fields of DB and IR. In order to respect the dynamic nature of the search process and information need (IN), CQQL allows the personalization of retrieval results using a preference-based relevance feedback (RF) approach called PrefCQQL, which relies on machine-based learning.
Unique features of the PrefCQQL approach range from the support of negative query-by-example (QBE) documents at query formulation time as well as during the interactive retrieval process to the formulation of weak preferences between result documents to express gradual levels of relevance. In addition, inductive preferences can be used from query formulation time onward to learn new CQQL queries.
In order to evaluate the presented polyrepresentative PrefCQQL approach, two kinds of experiments are conducted: a Cranfield-inspired evaluation of CQQL/PrefCQQL's retrieval effectiveness, which is extended by the utilization of user simulations to better fit the requirements of the evaluation of an adaptive IR system, and a usability study that examines three alternative MMIR system UI prototypes. In order to increase the reproducibility and confirmability of the experiments, the source code to all used programs is made available as a supplement to this dissertation.
The mentioned experiments aim at answering two central questions: first, whether the hypotheses of the PoP can be verified in MMIR, and second, whether a usable interactive MMIR system can be built on the basis of the PoP and PrefCQQL?
To answer the first question, different matching functions that partly follow the recommendations of the PoP are evaluated with six different test collections in both an non-interactive and interactive QBE scenario. The results of this experiment are ambivalent.
In non-interactive MMIR, the experimental data does not provide sufficient justification for the statement that PoP-based matching functions will always surpass single features or other matching functions. For instance, the arithmetic mean, which calculates the average similarity between a query's representations and the documents' representations in the collection, surpasses the conjunction and hence the CO of multiple representations in terms of retrieval effectiveness. Nevertheless, the matching function following the PoP is effectiveness stabler than the best performing single representations per collection. Hence, the CO's retrieval performance is more reliable than the usage of single representations.
In contrast, the predictions of the PoP can be verified in the investigated PrefCQQL-based interactive MMIR scenario. However, it is important to note that also the number of available representations has an impact on the retrieval outcome. That is, if too few representations are present in a matching function, the corresponding IN model in PrefCQQL obviously becomes subject to underfitting eventually lowering its retrieval effectiveness. Unfortunately, when the point of sufficient representations to support PrefCQQL is reached could not be revealed in this dissertation.
The second question is answered with the help of a prototypical MMIR system: the Pythia system, which serves both as proof of concept of the CQQL and the PrefCQQL approach. Furthermore, the system supports different information seeking strategies and a seamless transition between them in order to support users with different kinds of IN.
Bored children begin to draw, do crafts, to fidget - or they do something bad. Others fall silent, withdraw, or become lethargic. Research on school-related boredom has focused primarily on the negative consequences of boredom, such as decreased cognitive performance, motivation or attentativeness, or disruptiveness. These negative aspects of boredom can be contrasted by the notion that boredom can promote creative performance. This paper reflects on boredom's creative and suppressive consequences as an interplay of personality traits and behavioral possibilities in school situations, on the one hand, and as an interplay of situational experiences with constituent developmental processes on the other. It is proposed that boredom is a gauge of the learner's resonance with school content, learning and/or developmental relationships. Boredom indicates a psychological need and its desideratum. Thus, both creative and suppressive potentials are inherent in boredom.