Andreas Gollwitzer

Catalyst State Observation via the Perturbation of a Microwave Cavity Resonator

The state of the three-way catalyst (TWC) of a gasoline engine is commonly estimated by measuring the air-to-fuel ratio in the exhaust gas. This indirect method suffers from several disadvantages that one could avoid by directly observing the catalyst state and in particular its oxygen loading. We have investigated an RF approach to this direct measurement problem which is based on the fact that the TWC and its stainless steel canning form a filled cavity resonator. It will be shown that the resonator is perturbed when the TWC changes its conductivity as a function of oxygen loading and that the characteristics of the resonator S-parameters mirror the oxidation/reduction state of the catalyst. Index Terms – Three-way catalyst, TWC, state observation, in situ, cavity resonator, perturbation

On the inverse problem associated with the observation of electrochemical processes by the RF cavity perturbation method

Electrochemical reactions occuring at high temperatures and inside bulk materials are often observed indirectly. For instance, one commonly estimates the state of the three-way catalyst (TWC) in the exhaust pipe of a gasoline engine from the measured air-to-fuel ratios in the exhaust gases entering and leaving the catalyst. Our goal is to directly observe the catalyst state in situ by a non-contact RF approach. It is based on two facts: first, the reacting medium and its stainless steel canning form a filled cavity resonator at microwave frequencies; second, the medium changes its dielectric and conductive properties depending on its electrochemical state. Hence, the resonator is perturbed when the catalyst changes its conductivity as a function of oxygen loading. The estimation of the catalyst state involves the solution to the inverse problem, i. e., the inference of the catalyst material parameters from the measured RF parameters. We will first show that the characteristics of the RF resonator S-parameters indeed mirror the oxidation/reduction state of the catalyst and will then discuss possible approaches to the solution of the inverse problem.

Interferometric Observation of Surface Acoustic Wave Phase Fronts

By combining a homodyne Michelson interferometer with an automatic vector network analyzer (NWA), we have realized a setup capable of observing the amplitude and phase of the particle displacement associated with a propagating surface acoustic wave (SAW). We discuss the hardware and the signal processing details of our setup which is characterized by an upper frequency limit in the gigahertz range, a displacement amplitude resolution on the order of 50 pm and a lateral resolution of about 500 nm. Especially intriguing features of the setup are its insensitivity to the quiescent path length difference of the two interferometer arms and the fact that no calibration is required. The concept is demonstrated by way of 2D pictures (surface scans) of propagating SAWs in interdigital transducer (IDT) regions

A broadly tunable synthesis of linear α-olefins

The catalytic synthesis of linear α-olefins from ethylene is a technologically highly important reaction. A synthesis concept allowing the formation of selective products and various linear α-olefin product distributions with one catalyst system is highly desirable. Here, we describe a trimetallic catalyst system (Y–Al–Ni) consisting of a rare earth metal polymerization catalyst which can mediate coordinative chain transfer to triethylaluminum combined with a simultaneously operating nickel β-hydride elimination/transfer catalyst. This nickel catalyst displaces the grown alkyl chains forming linear α-olefins and recycles the aluminum-based chain transfer agent. With one catalyst system, we can synthesize product spectra ranging from selective 1-butene formation to α-olefin distributions centered at 850 gmol−1 with a low polydispersity. The key to this highly flexible linear α-olefin synthesis is the easy tuning of the rates of the Y and Ni catalysis independently of each other. The reaction is substoichiometric or formally catalytic regarding the chain transfer agent.Linear α-olefins are important bulk chemicals annually produced in megaton scale. Here, the authors report a trimetallic catalyst system for the synthesis of 1-butene and a broad range of α-olefins and achieve control over chain length by tuning the reaction rates of the nickel and yttrium catalysts.

Microcontroller-Based Temperature Compensation Scheme for Two-Port SAW Oscillators

We present a surface acoustic wave (SAW) two-port oscillator extended by a feedforward control circuit, the goal of which is to suppress the effects of the SAW resonator temperature characteristics on the oscillator output frequency. The control circuit consists of a temperature sensor near the SAW resonator the output signal of which is fed to the analog-to-digital converter (ADC) input of a microcontroller. An algorithm implemented on the microcontroller combines the information on the actual temperature and the temperature behavior of all oscillator components (known from previous measurement campaigns) to determine a compensation signal. The latter is fed to a phase shifter inserted in the oscillator feedback loop such that any temperature-induced changes in the round-trip phase shift of the oscillator loop are counteracted. An improvement factor in the frequency stability of 4.4 was demonstrated in the temperature range from 0 to 60 degC and can be further improved to a value of at least 10

Monitoring of Electrochemical Processes in Catalysts by Microwave Methods

Many electrochemical reactions occur at high temperatures and inside bulk materials. Because of the difficulties associated with a direct observation of the reaction, one often resorts to indirect measurement strategies. An example is the three-way catalyst (TWC) in the exhaust pipe of a gasoline engine which stores oxygen when it is abundant in the exhaust gas and releases it later to oxidize noxious gas components such as CO in oxygen-deficient (“lean”) exhaust gases. Currently, the oxygen loading of the TWC is derived indirectly from the output signals of two lambda probes, one upstream and the other downstream of the catalyst, which monitor the air-to-fuel ratio λ in the exhaust gas. We have investigated a microwave cavity perturbation approach towards the direct measurement of the catalyst state. It will be shown that the uptake or release of oxygen in the catalyst is observable in situ via the S-parameters of a cavity resonator.

Quadraturinterferometer mit automatischer Fokussierung

Zusammenfassung Oberflächenschwingungen kleiner Amplitude werden häufig interferometrisch gemessen. Um zusätzlich eine hohe örtliche Auflösung zu erreichen, muss der Laserstrahl mit einer stark vergrößernden Linse auf die Oberfläche fokussiert werden. Wegen der kleinen Schärfentiefe einer solchen Linse muss der Abstand zur Oberfläche genau eingehalten werden. Zur Unterdrückung von Störeinflüssen wie der ortsabhängigen Reflektivität der vermessenen Oberfläche und zur Extraktion der absoluten Schwingungsamplitude benutzen wir einen homodynen Aufbau mit zusätzlicher Pfadlängenmodulation. Die Messempfindlichkeit wurde dabei durch die Quadraturinterferometrie, das heißt die zeitsynchrone Erfassung der Intensitäten durch zwei Fotodioden, verbessert. Wir zeigen außerdem einen Signalverarbeitungsalgorithmus, der zur Regelung des Abstands zwischen Linse und Messoberfläche benutzt werden kann. Dies gewährleistet, dass der Laserstrahl optimal auf die Oberfläche fokussiert wird und führt zu einem insgesamt hohen Signal-zu-Rausch-Verhältnis. Abstract Surface vibrations with small amplitudes are often measured by interferometric techniques. For a good lateral resolution, the laser beam has to be focused on the surface through a high- magnification lens. Owing to the small depth of focus of this lens, its distance to the surface has to be controlled both accurately and precisely. In order to eliminate effects of influencing quantities such as the position-dependent surface reflectivity of the device under test, and to extract the absolute vibration amplitude, we use a homodyne setup with additional path length modulation. Its sensitivity is increased by quadrature interferometry, i. e., the synchronous measurement of the intensities detected by two photodiodes. We also present a signal processing technique for controling the distance between lens and surface. This ensures that the laser beam stays focused optimally onto the surface and results in a high overall signal-to-noise ratio.

Kalibrierungsfreie Messung der Amplitude und Phase hochfrequenter Schwingungen

Zusammenfassung Es wird ein neues Verfahren zur interferometrischen Erfassung von akustischen Oberflächenwellen vorgestellt. Die beschriebene Methode erlaubt die Extraktion der absoluten Amplitude und der Phase einer hochfrequenten Schwingung, unabhängig von Oberflächenreflexionsfaktoren, Variationen der Laserleistung oder Sensitivität des Interferometers. Ermöglicht wird dies durch die synchrone Erfassung des Messsignals mittels eines vektoriellen Netzwerkanalysators und eines digitalen Speicheroszilloskops. Die anschließende digitale Signalverarbeitung extrahiert die gewünschte Information aus den Messdaten. Zur Verifikation wurden Simulationen der Anregung akustischer Wellen mit dem Deltafunktionsmodell durchgeführt und mit den Messdaten verglichen. Anhand der Messdaten wird schließlich die anisotrope Slowness-Kurve konstruiert. Diese Information kann zur Bestimmung der Materialparameter von Dünnschichten verwendet werden.

Messsystem zur Erfassung der Phasenfronten von hochfrequenten Oberflächenwellen (Measurement System for Detecting the Phase Fronts of High-Frequency Surface Acoustic Waves)

Es wird ein neues interferometrisches Verfahren zur Erfassung von akustischen Oberflächenwellen (OFW) im Gigahertzbereich mit einer Amplitude unterhalb einem Nanometer vorgestellt. Durch den Einsatz eines vektoriellen Netzwerkanalysators (VNA) ist das zerstörungsfreie und berührungslose Messverfahren in der Lage, sowohl die Amplitude als auch die Phase der Schwingung an jedem Punkt der zu untersuchenden Oberfläche zu bestimmen. Erweiterungen am homodynen Michelson-Interferometer und anschließende digitale Signalverarbeitung machen den Aufbau unempfindlich gegenüber der Längendifferenz der beiden Interferometerarme. Es wird außerdem gezeigt, wie anhand der Messdaten die richtungsabhängige Ausbreitungsgeschwindigkeit der akustischen Wellen ermittelt werden können. We present a noval interferometric approach to measure surface acoustic waves (SAW) at frequencies in the gigahertz range and with amplitudes below one nanometer. By the use of a vector network analyser (VNA), the non-destructive and contact-free measurement method is capable of determining the amplitude and the phase of the vibration at every point of the observed surface. Extensions to the homodyne Michelson interferometer and subsequent digital signal processing make the setup insensitive to the path length difference of the two interferometer arms. The measurement enables to calculate the direction-dependent velocity of the acoustic waves.