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We investigate the influence of the geometry and doping level on the performance of n-type silicon nanowire field emitters on silicon pillar structures. Therefore, multiple cathodes with 50 by 50 pillar arrays (diameter: 5 μm, height: 30 μm, spacing: 50 μm) were fabricated and measured in diode configuration. In the first experiment, we compared two geometry types using the same material. Geometry 1 is black silicon, which is a highly dense surface covering a forest of tightly spaced silicon needles resulting from self-masking during a plasma etching process of single crystal silicon. Geometry 2 are silicon nanowires, which are individual spaced-out nanowires in a crownlike shape resulting from a plasma etching process of single crystal silicon. In the second experiment, we compared two different silicon doping levels [n-type (P), 1–10 and <0.005 Ω cm] for the same geometry. The best performance was achieved with lower doped silicon nanowire samples, emitting 2 mA at an extraction voltage of 1 kV. The geometry/material combination with the best performance was used to assemble an integrated electron source. These electron sources were measured in a triode configuration and reached onset voltages of about 125 V and emission currents of 2.5 mA at extraction voltages of 400 V, while achieving electron transmission rates as high as 85.0%.
An inexpensive UV-LED photoacoustic based real-time sensor-system detecting exhaled trace-acetone
(2024)
n this research we present a low-cost system for breath acetone analysis based on UV-LED photoacoustic spectroscopy. We considered the end-tidal phase of exhalation, which represents the systemic concentrations of volatile organic compounds (VOCs) – providing clinically relevant information about the human health. This is achieved via the development of a CO2-triggered breath sampling system, which collected alveolar breath over several minutes in sterile and inert containers. A real-time mass spectrometer is coupled to serve as a reference device for calibration measurements and subsequent breath analysis. The new sensor system provided a 3σ detection limit of 8.3 ppbV and an NNEA of 1.4E-9 Wcm 1Hz 0.5. In terms of the performed breath analysis measurements, 12 out of 13 fell within the error margin of the photoacoustic measurement system, demonstrating the reliability of the measurements in the field.
An Inexpensive Uv-Led Photoacoustic Based Real-Time Sensor-System Detecting Exhaled Trace-Acetone
(2024)
In this research we present a low-cost system for breath acetone analysis based on UV-LED photoacoustic spectroscopy. We considered the end-tidal phase of exhalation, which represents the systemic concentrations of volatile organic compounds (VOCs) – providing clinically relevant information about the human health. This is achieved via the development of a CO2-triggered breath sampling system, which collected alveolar breath over several minutes in sterile and inert containers. A real-time mass spectrometer is coupled to serve as a reference device for calibration measurements and subsequent breath analysis. The new sensor system provided a 3σ detection limit of 6.4 ppbV and an NNEA of 1.1E-9 Wcm-1Hz-0.5. In terms of the performed breath analysis measurements, 12 out of 13 fell within the error margin of the photoacoustic measurement system, demonstrating the reliability of the measurements in the field.
Für den sicheren Betrieb eines Brennstoffzellenfahrzeugs sind Sensoren wichtig, welche die Wasserstoffkonzentration im Fahrzeuginterieur kontinuierlich überwachen, um möglichen Entzündungen bzw. Explosionen vorzubeugen. Dafür wurden von verschiedenen nationalen und internationalen Instanzen bestimmte Kriterien definiert, die Wasserstoffsensoren für einen Einsatz in Brennstoffzellenfahrzeugen erfüllen müssen. Im Jahr 2020gab es noch keinen einzigen kommerziell erhältlichen Sensor, der alle diese Kriterien bedienen konnte.
In der vorliegenden Arbeit wurde für zwei verschiedene Sensoren die Eignung des thermischen Wirkprinzips für die Detektion von Wasserstoff im Kontext dieser Sicherheitskriterien untersucht. Dabei wurden mit einem MEMS-Sensor und einem Mikrodrahtsensor zwei vergleichsweise einfache Geometrien herangezogen, welche den eindimensionalen und zylindersymmetrischen Fall abbilden sollen. Für zwei dynamische Operationsmodi (gepulste Ströme und 3ω-Methode) wurden darauf aufbauend analytische Modelle basierend auf vereinfachten Sensorgeometrien entwickelt, um die Temperaturentwicklung des
Sensorelements in Abhängigkeit der Zeit zu charakterisieren. Diese Studien wurden anschließend anhand von detaillierteren Modellen mithilfe von FEM-Simulationen ergänzt und spezifiziert. Darin konnte gezeigt werden, dass für den gepulsten Betrieb sowohl die Pulshöhe als auch die Zeitkonstante des gemessenen Widerstands- bzw. Temperaturpulses als Messgröße herangezogen werden kann. Beim 3ω-Betrieb konnte zudem eine starke Frequenzabhängigkeit dieser Widerstands- bzw. Temperaturcharakteristik beobachtet werden. Die analytischen und numerischen Studien wurden anschließend durch experimentelle Ergebnisse bestätigt. Im Hinblick auf die zuvor definierten Kriterien stellte sich heraus, dass die Auswertung der Zeitkonstante beim gepulsten Betrieb im Hinblick auf Sensitivität und Auflösungsgrenze der Auswertung der Pulshöhe deutlich unterlegen ist. Beim 3ω-Betrieb können durch die hochintegrierte Messumgebung deutlich höhere Genauigkeiten und Auflösungsgrenzen erzielt werden. Für den MEMS-Sensor wurde bei einer Anregungsfrequenz von 1 Hz mit 55 ppm die bislang geringste in der Literatur berichtete Auflösungsgrenze für einen thermischen Wasserstoffsensor erreicht. Der Leistungsverbrauch ist dabei im Vergleich zum Pulsbetrieb entsprechend des jeweiligen Tastgrads leicht bis mäßig erhöht, die Ansprechzeit wird bei beiden Operationsmodi maßgeblich durch die Sensorgeometrie beeinflusst. Die geometrischen Parameter der jeweiligen Sensoren haben ferner einen starken Einfluss auf die Sensitivität, womit die Divergenz in der
Performance von MEMS-Chip und Drahtsensor erklärt werden kann. Es besteht ferner eine starke Temperatur- und Feuchtigkeitsabhängigkeit des Sensorsignals, welches bei beiden Operationsmodi indirekt proportional zur Wärmeleitfähigkeit des betrachteten Gasgemischs ist. Die Abhängigkeit vom definierten Umgebungsdruck ist lediglich für die
Zeitkonstante relevant, bei der Pulshöhe besteht keine Druckabhängigkeit. Es konnte eine grundsätzliche Eignung des thermischen Messprinzips für den Einsatz eines Wasserstoffsensors im Automobilbereich festgestellt werden. Für eine Anwendung müssen allerdings Referenzsensoren für Temperatur, Feuchtigkeit und Druck implementiert werden und eine Kalibrierung in Abhängigkeit dieser Größen erfolgen. Der 3ωBetrieb ist aufgrund der höheren Genauigkeit zu bevorzugen. Während der MEMSSensor im Hinblick auf Sensitivität, Ansprechzeit und Leistungsaufnahme gut abschneidet, liegen die Vorteile des Mikrodrahtsensors in seiner Robustheit und der vergleichsweise einfachen Realisierbarkeit
The article presents the process of developing a silicon electron source designed for high-vacuum microelectromechanical system (HV MEMS) devices, i.e., MEMS electron microscope and MEMS x-ray source. Technological constraints and issues of such an electron source are explained. The transition from emitters made of carbon nanotubes to emitters made of pure silicon is described. Overall, the final electron source consists of a silicon tip emitter and a silicon gate electrode integrated on the same glass substrate. The source generates an electron beam without any carbon nanotube coverage. It generates a high and stable electron current and works after the final bonding process of an HV MEMS device.
Progress in additive manufacturing is leading to the emergence of new areas of application. Laser Powder Bed Fusion (L-PBF) is increasingly used for the development of metallic medical implants, but for high-risk implants like vascular support structures (stents), surface quality is critical to ensure successful implantation without harming the surrounding tissue and ensure the patients’ health. Therefore, enhancing the surface quality is crucial. Electropolishing is a method for removing surface roughness by smoothing out micro-peaks and valleys. However, L-PBF structures have a high surface roughness due to metal particles adhering on the surface. To achieve a smooth surface for additively manufactured implants like stents using electropolishing, the removal of these particles needs to be studied in more detail.
The objective of this study is to examine the electropolishing mechanism of 316L stainless steel samples additively manufactured through Laser Powder Bed Fusion (L-PBF). The main objective is to investigate the removal properties and surface characteristics during electropolishing. To achieve this, various surfaces were characterized for morphology and roughness during Hull cell experiments. Markings are utilized on the Hull cell sample surfaces to identify points of interest during quasi-in-situ measurements. The surfaces are then analyzed after multiple time steps, applying different currents to investigate particle dissolution. The surface characteristics are analyzed through scanning electron microscopy, and surface roughness is analyzed using laser scanning microscopy.
The results show that the electropolishing process preferentially removes the adhering particles present on the surface of the samples. Increasing the current density results in faster particle dissolution and a smoother surface (see Figure 1a and b). The mechanism of material removal of various surface features, as shown in Figure 1 (red circle, yellow arrow and red square), was assessed based on the experimental results of the surface structures seen on the SEM images. It was found that different surface features were removed during the experiment at different polishing times and current densities. The amount of charge flowed was found to correlate with surface morphology.
Based on the obtained results, various surface features (such as large adherent particles, agglomerates of smaller particles, and valleys) and their changes with increasing test duration and current density were observed by quasi-in situ analyses. A reduction in the diameter of round particles adhering to the surface was observed at both low and higher current densities (see Figure 1a red circle a). Increasing the polishing time resulted in leveling of both large particles and valleys (see Figure 1b red square). Also, dissolution of agglomerates of smaller particles occurred at different polishing times as a function of current density and polishing time (see Figure 1a yellow arrow) are observed.
Smoothed surface structures can be observed in regions with equivalent surface charge density (see Figure 2). As a result, comparable surface morphologies may appear at the same area charge density, irrespective of a specific current density. So, it may be adequate to only consider the amount of charge flowed to describe the electropolishing of additive materials.
In conclusion, comprehending the dissolution characteristics of particles on L-PBF surfaces is essential for attaining satisfactory surface finish in electropolishing. The results of this study offer valuable perspectives into the electropolishing mechanism of additively manufactured 316L stainless steel and can guide future investigations on surface finishing and polishing of additive manufactured implants like stents.
Figure 1
In this work, high-current field emission electron source chips were fabricated using laser-micromachining and MEMS technology. The resulting chips were combined with commercially available printed circuit boards (PCBs) to obtain a multichip electron source. By controlling the separate electron sources using an external current control circuit, we were able to divide the desired total current evenly across the individual chips deployed in the PCB-carrier. In consequence, we were able to show a decreased degradation due to the reduced current load per chip. First, a single electron source chip was measured without current regulation. A steady-state emission current of 1 mA with a high stability of ±1.3% at an extraction voltage of 250 V was observed. At this current level, a mean degradation slope of −0.7 μA/min with a nearly perfect transmission ratio of 99% ± 0.4% was determined. The measurements of a fully assembled multichip PCB-carrier electron source, using a current control circuit for regulation, showed that an even distribution of the desired total current led to a decreased degradation. This was determined by the increase in the required extraction voltage over time. For this purpose, two current levels were applied to the electron source chips of the PCB-carrier using an external current control circuit. First, 300 μA total current was evenly distributed among the individual electron source chips followed by the emission of 300 μA per electron source chip. This allows the observation of the influence of a distributed and nondistributed total current, carried by the electron source chips. Thereby, we obtained an increase in the mean degradation slope from +0.011 V/min (300 μA distributed) to +0.239 V/min (300 μA per chip), which is approximately 21 times higher. Moreover, our current control circuit improved the current stability to under 0.1% for both current levels, 300 μA distributed and 300 μA per chip.
Field emitter arrays (FEAs) are a promising component for novel vacuum micro- and nanoelectronic devices, such as microwave power amplifiers or fast-switching X-ray sources. However, the interrelated mechanisms responsible for FEA degradation and failure are not fully understood. Therefore, we present a measurement method for quantitative observation of individual emission sites during integral operation using a low-cost, commercially available CMOS imaging sensor. The emission and degradation behavior of three differently doped FEAs is investigated in current-regulated operation. The measurements reveal that the limited current of the p-doped emitters leads to an activation of up to 55% of the individual tips in the array, while the activation of the n-type FEA stopped at around 30%. This enhanced activation results in a more continuous and uniform current distribution for the p-type FEA. An analysis of the individual emitter characteristics before and after a constant current measurement provides novel perspectives on degradation behavior. A burn-in process that trims the emitting tips to an integral current-specific ideal field enhancement factor is observed. In this process, blunt tips are sharpened while sharp tips are dulled, resulting in homogenization within the FEA. The methodology is described in detail, making it easily adaptable for other groups to apply in the further development of promising FEAs.