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Publication Access status: Open Access , Na-K-Sb photocathodes-synthesis, characterization and application in photoinjector(2026-06-12)State-of-the-art and future accelerators such as energy recovery linacs (ERLs), free electron lasers (FELs) and ultra-fast electron diffraction (UED) facilities have increasing demands on the brightness and robustness of the electron source. To meet these requirements, photocathodes are employed in a variety of material systems. Among them, multi-alkali antimonide photocathodes are promising candidates for high-brightness applications due to their high quantum efficiency (QE) and low intrinsic emittance in the visible range. Na–K–Sb photocathodes exhibit QE values comparable to Cs–K–Sb in the visible range, reaching several percent in the green region, while offering superior thermal robustness—a critical attribute for many applications, especially ERLs. Despite these advantages, the widespread adoption of Na–K–Sb photocathodes has been hindered by poor reproducibility and low success rates, stemming from three fundamental challenges: (1) an exceptionally narrow stoichiometric window for high QE, (2) the extremely low equilibrium vapor pressure of sodium required for the high-QE Na2KSb phase, and (3) insufficient control over key deposition parameters, particularly alkali metal flux rates. In this work, these challenges are systematically addressed through the design and commissioning of a new preparation chamber and the development of an optimized triple evaporation method. Through detailed spectral response and X-ray photoelectron spectroscopy (XPS) characterization, the reproducible growth of Na2KSb photocathodes under highly controlled conditions is demonstrated for the first time on substrates suitable for electron accelerators. A control variable approach establishes clear correlations between growth parameters, stoichiometry, and photoemissive properties. Comprehensive analysis of QE homogeneity, supported by a growth model, enables rapid and precise tuning of photoemissive performance. Furthermore, an extensive XPS database is compiled from over 35 Na–K–Sb photocathodes based on their stoichiometry. Finally, the successful operation of a superconducting radio-frequency (SRF) photoinjector using a Na2KSb photocathode is reported, achieving first beam in the SEALab SRF photoinjector.Source Type:Doctoral ThesisSeries:Schriftenreihe der Arbeitsgruppe des Lehrstuhls für Oberflächen- und Werkstofftechnologie im Institut für WerkstofftechnikVolume number:256 11 - Some of the metrics are blocked by yourconsent settings
Publication Access status: Open Access , Diamond Cloth-Supported Nickel-Manganese Oxide Electrodes for High-Performance Flexible Supercapacitors(2025-11-19)Supercapacitor (SC), one of the most important classes of electrochemical energy storage devices, has attracted considerable attention due to their high power density, fast charging-discharging capabilities, superior cycling stability, and reliable performance. With the rapid advancement of portable and wearable electronics, the demand for high-performance flexible SCs has significantly intensified, thereby driving extensive research into advanced electrode materials, electrolytes, and device architecture. Among them, the development of electrode materials plays a decisive role in determining both electrochemical performance and mechanical flexibility of SCs. Herein, three electrode systems were designed and fabricated in this thesis: flexible diamond cloth (DC), NiMnOx-coated carbon cloth (NiMnOx@CC), and NiMnOx-coated diamond cloth (NiMnOx@DC). Their electrochemical performance was further and comprehensively evaluated by means of various techniques and tools. More details ae demonstrated as follows. The flexible DC was synthesized through the overgrowth of a boron-doped diamond (BDD) film on CC in a microwave plasma enhanced chemical vapor deposition (MWCVD) reactor. A Ti interlayer was first introduced to protect CC from plasma etching while simultaneously enhancing BDD adhesion. The DC electrode demonstrated excellent electrochemical performance, including enhanced capacitance, good rate performance, and exceptional cycling stability. A symmetric pseudocapacitor (PC) assembled from two DC electrodes delivered an energy density of 45.96 µWh cm-2 at a power density of 4.25 mW cm-2, retaining 17.64 µWh cm-2 at 67.93 mW cm-2. Moreover, the symmetric DC electric double-layer capacitor (EDLC) exhibited excellent mechanical flexibility. Pseudocapacitance was then introduced by electrochemical synthesis of Mn-oxide films on CC. This NiMnOx@CC electrode offered better conformality, thickness control, and reproducibility compared with electrodes prepared through hydrothermal methods. The optimized Ni-doped electrode exhibited larger current responses, a wide potential window (1.5 V), and distinct redox peaks, yielding a specific capacitance of 47.1 mF cm-2 at a scan rate of 10 mV s-1 and 42.8 mF cm-2 at a current density of 1 mA cm-2 in 1.0 M Na2SO4, with > 98.5 % coulombic efficiency over 10,000 cycles. Its cycling retention was limited (≈ 16% after 10,000 cycles), constraining practical application. CC was further replaced with DC as the substrate to produce a more stable NiMnOx@DC electrode. The optimal deposition potential of 1.0 V balanced deposition rate and side reactions. The as-fabricated NiMnOx@DC electrode exhibited pronounced redox features and high specific capacitances of 180.5 mF cm-2 at a scan rate of 10 mV s-1 and 228.1 mF cm-2 at a current density of 2 mA cm-2, coupled with exceptional coulombic efficiency approaching 100 % over 10,000 charging-discharging cycles. Furthermore, the NiMnOx@DC electrode exhibited significantly enhanced capacitance retention in comparison to the NiMnOx@CC electrode. The maximum energy density of 72.8 μWh cm-2 and maximum power density of 17.0 mW cm-2 were obtained for a symmetric NiMnOx@DC PC. Simultaneously, the quasi-solid-state device using the NiMnOx@DC electrodes showed excellent mechanical flexibility during the bending tests. In summary, this thesis presents a systematic and stepwise route to design different SC electrodes from DC to NiMnOx@DC, effectively integrating the advantages of CC, BDD, and Ni–Mn oxides. This strategy enables the fabrication of high-performance flexible SCs, which exhibit not only high energy density and rapid power delivery but also excellent long-term stability and mechanical flexibility. These advancements in electrode materials significantly contribute to the realization of practical applications for flexible SCs in portable and wearable electronic devices.Source Type:Doctoral ThesisSeries:Schriftenreihe der Arbeitsgruppe des Lehrstuhls für Oberflächen- und Werkstofftechnologie im Institut für WerkstofftechnikVolume number:241 16 - Some of the metrics are blocked by yourconsent settings
Publication Access status: Open Access , Folded nanocrystalline-stacked WO3 with photo-enhanced functionality for high-performance ammonium-ion storage(2025-11-10)The integration of light harvesting with electrochemical energy storage has been increasingly recognized as a promising pathway to address the global energy crisis. By directly coupling photon absorption with ion storage processes, photo-assisted energy storage systems offer unique advantages over conventional batteries and capacitors. This intrinsic synergy not only enhances device performance but also opens new opportunities for solar-rechargeable energy storage technologies. Among various aqueous energy-storage systems, ammonium-ion-based devices have recently emerged as promising candidates owing to the unique hydrogen-bond-driven insertion mechanism of NH4+, which enables fast ion transport and high reversibility in mild aqueous electrolytes. Integrating such NH4+ storage chemistry with photoactive electrodes could further expand the functionality of aqueous systems toward light-assisted charge storage. Married this context with ammonium-ion hybrid supercapacitors (AIHSs), this concept represents an attractive and novel aqueous energy storage system, thanks to their low cost, intrinsic safety, environmental benignity, and unique hydrogen-bond-driven insertion mechanisms. To achieve these distinctive features, AIHSs particularly need to be coupled with photoactive electrodes featuring both high-performance and light-enhanced charge storage. This dissertation thus focuses on the rational design of WO3-based electrode owning photo-enhanced functionality for efficient ammonium-ion storage. Through the development of folded nanocrystalline–amorphous WO3 structures, WO3/TiO2 heterojunctions, and WO3/BDD composites, this work systematically establishes structure–property–performance relationship and demonstrates new design principles to construct high-performance, solar-responsive aqueous energy storage devices. Three experimental parts are detailed as follows. I) Folded nanocrystalline–amorphous WO3 electrodes: The folded nanocrystalline-stacked amorphous WO3 (a-WO3) electrodes were synthesized via a rapid electrochemical deposition method. They own hierarchical frameworks where ~5 nm crystallites are loosely assembled within an amorphous matrix. Their comprehensive characterization was conducted using microscopy, spectroscopy, electrochemical techniques, and density functional theory simulations. More importantly, the relationship between structural architecture, defect chemistry, and NH4+ storage performance was investigated in detail. The a-WO3 electrodes demonstrated superior electrochemical activity with a capacitance of 2783 mF cm−2 and excellent rate performance compared to crystalline counterparts. The amorphous structure is confirmed to provide abundant grain boundaries, oxygen vacancies, and mixed-valence tungsten states, facilitating rapid NH4+ transport through hydrogen bonding mechanisms. As a demonstration, the a-WO3 electrode was employed to construct a full ammonium-ion hybrid supercapacitor with a polyaniline cathode. This supercapacitor delivers remarkable energy and power densities of 620 mWh cm−2 and 23,980 mW cm−2, respectively, further with 81.5% capacity retention after 3000 cycles. These results highlight the effectiveness of controlled amorphization as a strategy to overcome the intrinsic limitations of crystalline WO3 and achieve both high capacity and durability. All these are shown in Chapter 3. II) WO3/TiO2 heterojunctions for light-assisted energy storage: The WO3/TiO2 heterojunction composites were designed with an aim to harness photo-enhanced energy storage capabilities through synergistic light and electrochemical properties. The band alignment between WO3 and TiO2 was utilized to engineer Type II heterojunctions, which leverage the superior photocatalytic activity of TiO2 and charge separation capability, eventually enhancing electrochemical performance of WO3. Through controlled electrochemical deposition parameters, island-like distributed WO3 nanocrystals have been coated on TiO2, ensuring simultaneous exposure of both materials to the electrolyte and meanwhile achieving true synergistic effects rather than simple serial coupling. This island-like growth mode maximizes the heterojunction interface area, promoting efficient charge separation and transport. The TiO2 component provides superior photocatalytic activity and extends the photoactive spectral range, while WO3 offers high-capacity NH4+ storage capability. Comprehensive characterization of these composite electrodes reveals their enhanced light absorption, improved charge carrier dynamics, and accelerated ion transport kinetics under illumination. Under light irradiation, the typical composite material showed 40% capacity enhancement compared to that under dark conditions, with a significantly reduced charge transfer resistance by a factor of 80-85%, a photocurrent density of 0.47 mA cm−2, and substantially improved carrier density and interfacial kinetics. All these validate the effectiveness of heterojunction design in achieving photo-assisted energy storage. These details are summarized in Chapter 4. III) WO3/BDD p-n junction for light-assisted energy storage: This session addresses the persistent conductivity limitations of WO3 electrodes by incorporating boron-doped diamond (BDD), a carbon material known for its wide potential window, high conductivity, and chemical robustness. The strategic combination creates a p–n junction configuration that exploits the ultra-wide bandgap of diamond (5.5 eV) and the superior electronic properties of p-type BDD interfaced with n-type WO3. The intrinsic electric field at the junction enhances charge separation efficiency, while the exceptional conductivity of BDD complements the NH4+ storage capability of WO3. Compared with pristine WO3, the composite delivered a ~1.7-fold higher photocurrent density and the areal capacitance under light was enhanced by nearly 20%. In addition, the electrode maintained over 90% of its capacity after extended cycling, outperforming pristine WO3. Therefore, the WO3/BDD p–n junction delivers significantly enhanced photocurrent response, improved NH4+ storage capacity under illumination, and extended cycling lifetime compared to pristine WO3. The built-in electric field facilitates directional charge transport and suppresses recombination losses, resulting in superior photoelectrochemical performance. These results are demonstrated in Chapter 5. In summary, this thesis provides several successful approaches to design and optimize WO3-based electrode materials for sustainable ammonium-ion energy storage applications in the way of both advanced structural engineering and innovative photo-enhancement strategies. By developing folded nanocrystalline-stacked architectures, engineering Type II and p-n heterojunctions, and integrating photocatalytic functionalities, the great potential of WO3 in achieving high-performance, environmentally friendly energy storage devices have been proved in this thesis. They also feature dual solar energy harvesting and electrochemical storage capabilities. Such systematic investigation of photo-assisted mechanisms clarifies that heterojunction formation enables efficient charge separation, reduces recombination losses, and significantly enhances electrochemical performance under illumination. This work establishes fundamental design principles for photo-responsive electrode materials. Beyond ammonium-ion storage, the strategies presented here open new avenues for multifunctional aqueous devices capable of simultaneously harvesting solar energy and storing ions.Source Type:Doctoral ThesisSeries:Schriftenreihe der Arbeitsgruppe des Lehrstuhls für Oberflächen- und Werkstofftechnologie im Institut für WerkstofftechnikVolume number:237 13 - Some of the metrics are blocked by yourconsent settings
Publication Access status: Open Access , Synthese und Untersuchung nanostrukturierter, katalytischer Beschichtungen auf Keramikschwämmen für das Power-to-Gas-Forschungscluster(2025)This research investigates TiO₂-based catalysts for the CO₂ methanation in the context of Power-to-Gas applications, contributing to long-term energy storage and decarbonization in line with the Paris Agreement. Open-cell ceramic foam carriers coated with nanostructured nickel layers are utilized to enable stable and efficient operation under dynamic conditions. The developed systems exhibit high methane yields close to thermodynamic equilibrium, demonstrate long-term stability and are suitable for economically viable load-change operations.Source Type:Doctoral ThesisSeries:Schriftenreihe der Arbeitsgruppe des Lehrstuhls für Oberflächen- und Werkstofftechnologie im Institut für WerkstofftechnikVolume number:213 4 - Some of the metrics are blocked by yourconsent settings
Publication Access status: Open Access , Surface Functionalization of Boron/Nitrogen Doped Diamond for Electrocatalytic Nitrate Reduction(2025)Numerous diamond materials have been emerging as a critical material for various applications, due to their unique chemical, physical, and mechanical properties. Among them, conductive diamond films, in most cases doped diamond films (Dia) have been widely employed as electrodes for advanced electrochemical applications. Doping of diamond films refers to the introduction of foreign atoms such as boron, nitrogen, or phosphorus into the diamond lattice, leading to their altered electronic and structural characteristics. Of special interest, a co-doping process involves simultaneous incorporation of two or more dopants (e.g., boron and nitrogen), thus synergistically optimizing the diamond’s conductivity, electrochemical activity, and stability. These co-doped diamond films enhance charge transport, create additional active sites, and improve catalytic efficiency, enabling their broader applications in fields such as energy storage, pollutant degradation, and catalysis. Different from (co-)doped diamond films, diamond composites (DCs) - comprise diamond and other components, thus offering unique opportunities to further leverage these advancements. They effectively combine the outstanding properties of diamond with those of its components, leading to extensive exploration and testing in mechanical, physical, chemical, and biomedical applications. This "1+1 > 2" strategy has proven to be highly efficient for designing and developing advanced diamond-based functional materials. Although DCs offer exceptional chemical and physical properties, the poor electrocatalytic performance of diamond-based electrodes, mainly due to limited active sites, presents a significant challenge. In this context, this thesis addresses above issues by harnessing boron and nitrogen co-doped diamond (BNDD) composites, designing various interlayers and further adding surface modifications – the formation of DCs - to gain enhanced electrocatalytic activity and stability of these diamond materials, and exploring their electrochemical applicaitons in the fields of electrochemical energy storage (here supercapacitors and zinc batteries) and conversion (here electrocatalytic nitrate reduction reaction). This thesis firstly outlines the experimental techniques for synthesizing and characterizing these materials, including magnetron sputtering, electrodeposition, and microwave plasma-enhanced chemical vapor deposition. Its focus then shifts to the preparation and characterization of BDDD films and their composites, including BNDD/Metal/Si, Cu2O@CNF-BNDD (CNF = carbon nano fibers), and partially covered BNDD/Ti/Si. Detailed methods for analyzing the morphology, surface chemistry, and electrochemical performance of these films are discussed, along with their electrochemical applications such as supercapacitor formation, catalytic nitrate reduction and its integration with electrochemical methyl orange (MO) degradation using BNDD films, and Zn-nitrate batteries. The details of these contents are described as follows. In the first experimental system, the BNDD films were grown on a Si substrate with/without Ti and Ta interlayers, resulting in the fabrication of BNDD/Si, BNDD/Ti/Si, and BNDD/Ta/Ti/Si electrodes. After they were characterized using microscopies, spectroscopies, electrochemical techniques, and density functional theory (DFT) simulations, the relationship between their composition, interfacial structure, charge transport, and electrochemical properties was investigated. The BNDD/Ta/Ti/Si electrodes demonstrated faster electron transfer rates and lower resistances for redox probes compared to other diamond electrodes. The formation of TaC facilitated carrier tunneling and increased the concentration of electrically active defects, enhancing its electrochemical performance. As a case study, the BNDD/Ta/Ti/Si electrode was employed as a capacitor electrode to construct a redox-electrolyte-based supercapacitor. This section highlights the importance of interlayer composition and its impact on charge transfer and relevant electrochemical performance of co-doped diamond films, in other worlds how a tailored interlayer design can unlock new capabilities of co-doped diamond (e.g., BNDD) films. In the second experimental system, the BNDD surface was functionalized with carbon nanofibers (CNF) and further with Cu2O. The resultant Cu2O@CNF-BNDD composite was introduced as an efficient electrocatalyst for the electrochemical nitrate reduction reaction (NO3RR). This composite has unique properties of BNDD films, CNFs and Cu2O. For example, the BNDD films have a wider electrochemical potential window that suppresses the competing hydrogen evolution reaction (HER), resulting in a high NO3RR faraday efficiency. The CNFs grown on the BNDD surface regulate the sp2 content, enhancing electrical conductivity and electrochemical activity. Furthermore, the Cu2O catalyst donates electrons to the lowest unoccupied π* orbital of NO3-, making it highly effective for NO3RR. As revealed in DFT calculations, the d-band center of Cu 3d orbitals of reconstructed Cu NPs is shifted to −2.47 eV, balancing the adsorption of nitrogen-contained intermediates, and reducing the overpotential of NO3RR from 1.11 eV to 0.15 eV. Taking the advantage of a eight-electron nitrate-to-ammonia reaction and the exceptional activity of Cu2O@CNF-BNDD, a Zn-nitrate battery system was further proposed, where dual functionality was offered by using NO3RR-generated electrons for both electricity production and ammonia generation. This strategy simultaneously degrades nitrate pollutants and produces valuable chemicals. In the third experimental system, the sluggish oxygen evolution reaction (OER) was proposed to be replaced with a fast photo-electrocatalytic oxidation reaction of methyl orange (MO) with an aim to address the slow kinetics of the eight-electron NO3RR. In such an integrated system, the anodic MO degradation reaction occurring on a BNDD/Ti/Si electrode provides electrons, while the cathodic reaction performed on a Cu2O/CNF-BNDD cathode reduces nitrate to ammonia. Comprehensive characterization of these composite electrodes demonstrates their impact on electron transfer and electrochemically active surface area for both processes. This "three birds with one stone" strategy not only degrades nitrate pollutants and produces valuable chemicals, but also degrades dye molecules, providing a framework for viable NO3RR and waste degradation processes. In summary, this thesis provides new insights into the design and optimization of BNDD films and their composites for some potential electrochemical energy storage and conversion applications. By enhancing active sites and modifying surface structure of BNDD films, this thesis proves the great appeal of co-doped diamond films and their composite in the aspects of electrochemical energy storage, pollutant degradation, and electrocatalysis, boosting future developments in the synthesis, characterization and electrochemical applications of various diamond materials and related devices.Source Type:Doctoral ThesisSeries:Schriftenreihe der Arbeitsgruppe des Lehrstuhls für Oberflächen- und Werkstofftechnologie im Institut für Werkstofftechnik2 8 - Some of the metrics are blocked by yourconsent settings
Publication Access status: Open Access , Structural dependence of diamond composites on electrocatalytic performance of small molecules(2023)Electrocatalysis of small molecules is a sustainable solution to help the global energy crisis and meanwhile to reduce environmental pollution. The key to reduce energy losses during the electrocatalytic processes is the rational design of efficient and robust electrocatalysts under practical operation conditions. However, commercial metal based catalysts such as Pt/C and RuO2 are suffer from their high costs, scarcity, susceptibility to poisoning, and single reaction sites, limiting their large-scale applications. Moreover, the most common catalytic reactions of small molecules such as oxygen reduction/evolution reaction (ORR/OER), CO2 reduction reaction (CO2RR) and nitrate reduction reaction (NITRR) are multi-electron reactions, where sluggish kinetics exist together with complex reaction pathways. Such traditional single metal active sites are not sufficient to overcome such challenges. Therefore, it is urgent to deeply understand the reaction mechanisms, to design the heterogeneous catalysts, to tune the structure and composition of the multiple components. Diamond-based electrodes have been widely employed in electrochemical sensing and advanced electrocatalytic oxidation of pollutants, due to their broad potential windows, low background currents, high chemical stability, and resistance to toxication. As for the electrocatalysis of small molecules, the broad potential window means the thermodynamic inhibition of hydrogen evolution, which ensures the selectivity of ORR, CO2RR and NITRR. Meantime, the robust structure provides sufficient stability and recycling possibilities in extreme environments. However, the diamond-based electrodes feature poor electrocatalytic performance, due to the lack of active sites. Creation of or the increase of active sites together with the balance of their activity and stability still remains challenging. This thesis combines the density functional theory (DFT) calculations with experiments to construct efficient and robust diamond composites, which are employed as the catalysts for the electrocatalysis of small molecules such as oxygen and nitrate. Deep understanding of the relationship between catalyst structures and electrocatalytic performance is further desired to develop their practical application in energy devices. Concerning the oxygen electrocatalysis, namely, the ORR and OER, the nitrogen doped carbon nanowalls/diamond (N-CNWs/D) composites are synthesized by means of microwave plasma enhanced chemical vapor deposition (MPCVD) technique, consisting of a thin nanodiamond layer at the bottom and a vertically aligned multigraphene on the surface. After annealing post-treatment, the composites feature superior ORR performance (high onset potential of 835 mV vs. Standard hydrogen electrode, RHE) and excellent stability (90% current retention after 20 h). It is attribute to the activated and exposed edge sites and a bonded nanodiamond layer. The Co4N@d-NCNWs/D composites are further synthesized and employed as the ORR/OER bifunctional catalysts. In more detail, the DFT calculations predict that the enhanced metal-supporting interaction between the Co4N and d-NCNWs/D promotes the anchoring and dispersion of the Co4N nanoparticles. It also adjusts the electronic structures of Co and C atoms at the interface, reducing the overpotential of ORR/OER from 0.67 to 0.23 eV. Experimentally, the composites feature superior ORR/OER bifunctional performance (e.g., a small potential gap of 0.75 V). Since these composites are directly grow on carbon cloth collectors, they are utilized as air electrodes. The assembled flexible zinc-air batteries exhibit a high open circuit voltage of 1.41 V and excellent bending stability. With respect to the nitrate electrocatalysis, the copper nanoparticles coated boron-doped diamond mesh (Cu-np@BDD-m) composites are synthesized by means of hot-filament chemical vapor deposition (HFCVD) and magnetron sputtering. After the application of an etching process at a low pressure, Cu nanoparticles are embedded in the BDD-m surface, leading to a strong metal-supporting interaction or a nanoconfined trapping area. The DFT calculations reveal that this strong interaction enhances the adsorption of nitrogen-contained intermediates (e.g., *NO3, *NO2, *N), minimizes the reaction energy through NITRR from 1.06 to 0.91 eV, and raises the energy barrier of competing HER from 0.50 to 0.97 eV at a neutral medium. Consequently, the composite features superior NITRR performance (e.g., nitrate removal of 90.2%and ammonia selectivity of 76.8%). The resultant nano-confined area stabilizes the active sites and enhanced the local adsorption of nitrate, eventually upshifting the diffusion-controlled first-step onset potential (from -0.25 to 0.1 V vs. RHE). In this way, the NITRR mechanism is elaborated in thermodynamic and kinetic processes. Such an electrode promises to assemble liquid flow reactors for sustainable applications. In summary, efficient and robust diamond composites based catalysts are fabricated for the electrocatalysis of small molecules such as oxygen and nitrate. The balance of their activity and stability is achieved via the construction of the rational support structure, the modification of the active sites on the surface, and the enhancement of the metal-supporting interactions. The catalyst design approach proposed in this thesis is promising to be applied for the electrocatalysis of other small molecules and further for the assembly of energy devices of practical applications in future.Source Type:Doctoral ThesisSeries:Schriftenreihe der Arbeitsgruppe des Lehrstuhls für Oberflächen- und Werkstofftechnologie im Institut für WerkstofftechnikVolume number:203 18 - Some of the metrics are blocked by yourconsent settings
Publication Access status: Open Access , Betrachtung der individuellen tribologischen Phänomene im Rahmen der Partikelerosion(2020)The erosion of surfaces caused due to particle impact, for example through sand in desserts, leads to failure of technical equipment components up to the failure of whole technical facilities. This knowledge is an important argument to use scientific methods and experimental research to achieve a deeper insight into the mechanisms which cause these failures. Hence, a substantial aim is to gain an understanding of how such failures can be avoided in future. This work demonstrates a basis by describing and characterizing the main interactions between particles and surfaces. For that purpose, surfaces of different materials are investigated after the impact of a particle collective to identify the individual wear mechanisms. The experimental tests on various surfaces (stainless steel, DLC-coating, copper) are conducted with different conditions of impact, whereby the velocity, the angle and the test duration are varied. Subsequently, methods are introduced which allow examining the interaction of single particles with the surfaces. Various current works analyse the change in adhesion by the surface roughness, whereby the influence of the particle wear is not considered. In the present work, that topic was caught up on and the change of the particle surface due to wear is included in the investigations. The analysis of the results shows that only the consideration of the whole system of the worn surface in combination with the worn particle leads to a change in adhesion. Primarily, this context is based on the fact that every single contact situation differs from position to position and therefore, the particle wear can be neglected. To describe the wear mechanism abrasion, the friction coefficient is determined by a scratch test conducted with the nanoindenter. These tests are implemented on surfaces which are impacted by erosion through particles and on not worn surfaces. Thereby the scratch tests are performed with a commercially available indenter as well as a self-built indenter on which an arbitrary particle can be glued on. The results show that the friction coefficient changes through particle impact and that the self-built indenter is suitable for the measurements. As a further wear mechanism, the fatigue based on surface spalling is examined, which in the past was only possible with great effort. In this work the reference method of the nanoindenter is used which allows the indenter to oscillate with a frequency of 220 Hz and to fatigue the surface. As the obtained results are in good agreement with the “classic” fatigue research, the reference method opens up a new opportunity to investigate fatigue of material’s surfaces. With the current work, a new basis and experimental methods to describe the single mechanisms of erosive wear are developed. On this basis, opportunities are offered to understand cause-effect relationships fundamentally between impacting particles and a surface, which help to make the right decision regarding the material selection for erosive applications.Source Type:Doctoral ThesisSeries:Schriftenreihe der Arbeitsgruppe des Lehrstuhls für Oberflächen- und Werkstofftechnologie im Institut für WerkstofftechnikVolume number:161 11 - Some of the metrics are blocked by yourconsent settings
Publication Access status: Open Access , Microfluidic tools for single cell analysis(2016)Industrial biotechnology plays an important role in the production of fine chemicals, pharmaceuticals and biofuels. Microorganisms are used to convert sustainable resources, e.g. sugars, into high value products. Yet, it has emerged that clonal populations can differ phenotypically. The cause of this heterogeneity is manifold and has been related to microgradients in large scaled bioreactors and the stochasticity in gene expression as well as metabolic pathways. Single cell analysis focuses on unravelling the underlying mechanisms of population based heterogeneity. Here, conventional methods such as flow cytometry and high resolution time-lapse microscopy have been applied intensively for many years. Yet, these methods do not offer the temporal and spatial resolution needed for the investigation the complex processes in cells. Microfluidics has become a promising new tool for studying bacteria populations at the single cell level with reduced reagents consumption and stable environmental conditions through fast exchange of heat and mass. For achieving a better understanding of the underlying processes in microbial population heterogeneity, new tools were integrated into microfluidic devices, such as cell trapping and cultivation as well as the sampling of single cells. In order to study solely single bacteria cells with a high temporal and spatial resolution, a new microfluidic device was established as described in Publication I, allowing the immobilization of single Escherichia coli cells in micron scaled barrier structures. Hence, cell growth can be monitored without the interference of descending cells, which are constantly removed by the media flow holding the cell inside the trap. High-throughput analysis of small bacterial microcolonies of up to 500 cells was realized as shown in Publication II by integrating hundreds of shallow growth chambers into microfluidic channels, restricting the growth of single cells to a monolayer. Mass-transport inside monolayer growth chambers solely relies on diffusion reducing shear forces to a minimum. Hence, growth of single cells in monolayer growth chambers can be followed at a high-resolution. Furthermore, by using automated image analysis cell lineages can be reconstructed and analyzed in high-throughput manner. A novel loading procedure was developed, to guarantee isogenic (clonal) starting conditions in microfluidic monolayer growth chambers - one bacteria cell per chamber (Publication III). Here, an artificial introduced air bubble is applied to temporally distort the flow profile, leading to convective flow inside the monolayer growth chambers. Subsequently, single cells get stuck inside the shallow chambers. After the air bubble is removed mass-transport inside the growth chambers is solely dominated by diffusion again. In this study, the process of air bubble based cell loading as well as loading efficiency was characterized and optimal growth condition after cell loading could been shown. High-throughput cultivation and analysis of small microbial colonies in monolayer growth chambers not allowed for retrieving single cells of interest for further analysis on or outside the microfluidic device. Publication IV introduced a new concept by integrating high-throughput microfluidic single-cell analysis and optical tweezers for sampling single cells from small microcolonies. Optical tweezers applies a highly focused infrared laser beam in order to trap and manipulate micron sized objects and cells with the force of light only. It was shown that single cells of Escherichia coli could be dragged in and out of shallow growth pockets. Characterization of the possible influences due to infrared laser irradiation were carried out and obtained result showed that short exposure timesSource Type:Doctoral ThesisSeries:Schriftenreihe der Arbeitsgruppe des Lehrstuhls für Oberflächen- und Werkstofftechnologie im Institut für WerkstofftechnikVolume number:12542 526 - Some of the metrics are blocked by yourconsent settings
Publication Access status: Open Access , Herstellung von Aluminiumnitrid Dünnschichten mittels Magnetron Sputtern auf Diamant für potentielle Pseudo-Surface-Acoustic-Wave-Anwendungen(2016)In this dissertation high quality, piezoelectrical AlN coatings were synthesized and their suitability as SAW sensor material have been tested. For thin film synthesis reactive radiofrequency magnetron sputtering of high purity aluminum in a N2 atmosphere was utilized. For the optimization of the growth mechanisms and to elucidate the coatings’ structure the process parameters in correlation to comprehensive materials characterization studies were carried out. In detail, the investigated process parameters were rf-power, substrate temperature, composition of process gases and the process pressure. High quality (002)-fibretextured AlN coatings were synthesized at room temperature in pure N2 atmosphere. These films were fully c-axis oriented with a (002)-AlN-rocking-curve exhibiting 0.1° of full width at half maximum. The thickness and surface roughness of the synthesized coatings were 6.6 µm and less than 2 nm, respectively. Measurements by means of nanoindentation displayed hardness values of 21.8 GPa and Young’s modulus of 338 GPa. According to scanning acoustic microscopy the SAW-, SLAW-, and transversal-velocities were 5.7 km / s, 10.9 km / s and 6.7 km / s, respectively. To test the feasibility of PSAW devices based on AlN coatings, nanocrystalline, polished diamond substrates, synthesized by microwave-plasma-enhanced CVD were utilized as substrate material. The AlN coatings’ thickness was simulated and tested successfully to match the needs for a successful PSAW stimulation. To generate surface waves IDT structures were produced in a FIB-SEM by Platinum deposition and electron beam lithography. A symmetrical comb structure with finger thickness of 2 µm each have been chosen corresponding to a wavelength of 8 µm. As per simulation the AlN film thickness should be 720 nm for excitation of the first PSAW mode. The IDTs were arranged to work as a SAW filter showing a resonance frequency at 1 GHz in the case of AlN-Si-systems and 2 GHz in the case of AlN-NCD. Measurements were conducted by employing a frequency generator and an oscilloscope. The corresponding phase velocity of measured PSAWs were 8.2 km / s and 16.1 km / s in the case of AlN-Si and AlN-NCD, respectively. Simulation matched the measurements with a deviation of 1 % only, thus confirming the successful stimulation of the PSAW modes. The results found in this study present the advantage of AlN-NCD compared to other piezoelectric materials. Moreover, the possibility of combining the piezoelectric thin-film material (AlN) with a variety of substrate materials tremendously extends its applications.Source Type:Doctoral ThesisSeries:Schriftenreihe der Arbeitsgruppe des Lehrstuhls für Oberflächen- und Werkstofftechnologie im Institut für WerkstofftechnikVolume number:113 10 - Some of the metrics are blocked by yourconsent settings
Publication Access status: Open Access , Theoretical investigation and experimental confirmation of crystalline and amorphous Si, B, C, N–based hard materials(2013)The newly discovered phase, cubic BC2N, has attracted great interest during the past two decades. This is because the hardness of the novel superhard phase can even reach up to ~ 76 GPa, only behind diamond (~ 100 GPa). It is ranked as the second hardest material ever known, even harder than cubic BN (~ 50 GPa). The fantastic property of superhardness immediately gained wide attention after the findings. However, the crystal structure and atomic configuration of the novel phase have still not been determined so far due to the similar atomic size of B, C, and N. Experimental results of the crystal are also inconsistent. Theoretical calculations are employed to attempt to unveil the crystal structure of the phase by constructing diverse structural models. Due to the difference of the structural models, these calculated results are contradictory as well. Here we propose a solid solution model to search possible structures of the crystalline phase, which are classified by the parameter of the degree of mixture. The powerful feature of our model lies in the aspect that it not only successfully illustrates the discrepancy observed in experiments, but also unifies the inconsistency shown in theoretical calculations. Meanwhile, we extended our model to B–C–N compositions along the C–BN isoelectronic line and broader areas in the ternary B–C–N phase diagram besides BC2N, with the expectation of discovering potential superhard phases that can be comparable to or even harder than cubic BC2N. Indeed, there exist such areas in the ternary B–C–N phase diagram in which they are harder than BC2N. Our prediction of superhard phases can provide general guidance for experimental works to intentionally prepare such superhard phases by regulating correspondingly experimental parameters. In addition, we also extended our model to cubic B–C crystals. The excellent agreement between experimental results and our theoretical ones clearly demonstrates the transferability of our model to other similar covalent crystalline materials. To the best of our knowledge, we performed, for the first time, theoretical calculations of amorphous B–C–N materials at the atomic scale. Based on first–principles calculations, the relation among chemical composition, microstructure, and mechanical properties was established. This is also the first time to build the relation depending on so many chemical compositions of amorphous B–C–N films. The relation can give the general description of the distribution of mechanical properties in the ternary B–C–N phase diagram. Thus it can also guide the experimental works to prepare those compositions with better mechanical properties. We also synthesized amorphous B–C–N films by changing diverse experimental parameters to verify our theoretical results. The obtained compositions in experiments are mostly located in the area that has lower formation energy according to our theoretical calculations, corresponding to those compositions that are easier to obtain. In other words, our experimental works can reproduce the theoretical works well with reasonable accuracy. Besides B–C–N materials, Si–C–N materials were also prepared with the expectation of extracting the relations between mechanical parameters for covalent amorphous materials, such as the relation of hardness and Young§#39;s modulus. If some other mechanical quantities can be found to have good relations with hardness, then the hardness can be indirectly evaluated by theoretical calculations. In addition, the substrate effects were also discussed for such covalent amorphous materials.Source Type:Doctoral ThesisSeries:Schriftenreihe der Arbeitsgruppe des Lehrstuhls für Oberflächen- und Werkstofftechnologie im Institut für WerkstofftechnikVolume number:22 8
