Stefan F. Wagner

Kinetics of oxygen exchange in strontium titanate

The kinetics of oxygen exchange are of primary importance for the application of titanates as fast resistive oxygen sensors. The sensor’s conductivity is correlated with the oxygen partial pressure pO2 of the surrounding atmosphere: Due to oxygen surface transfer and subsequent diffusion of oxygen vacancies VO··, a pO2 change gives rise to a conductivity change of the sample. While bulk diffusion usually occurs very fast, the surface transfer reaction becomes the rate determining step for thin samples and for low temperatures. We have shown that in the case of acceptor doped SrTiO3 the kinetics of the surface transfer reaction can be strongly influenced through stoichiometric changes brought about by thin coatings of alkaline earth metal oxides (e.g. SrO). In contrast to the commonly used jump method (conductivity response to a sudden pO2 change in the time domain), a model is presented which is based on the frequency-domain analysis of amplitude and phase shift of the response signal obtained from a pO2 modulation in a fast kinetic measurement set-up. This method allows not only for measuring response times in the sub-millisecond range but also for distinguishing between behaviour either controlled by volume diffusion or by surface transfer reaction.

Suspended Liquid Subtractive Lithography: One-step generation of 3D channel geometries in viscous curable polymer matrices

The miniaturization of synthesis, analysis and screening experiments is an important step towards more environmentally friendly chemistry, statistically significant biology and fast and cost-effective medicinal assays. The facile generation of arbitrary 3D channel structures in polymers is pivotal to these techniques. Here we present a method for printing microchannels directly into viscous curable polymer matrices by injecting a surfactant into the uncured material via a steel capillary attached to a 3D printer. We demonstrate this technique using polydimethylsiloxane (PDMS) one of the most widely used polymers for the fabrication of, e. g. microfluidic chips. We show that this technique which we term Suspended Liquid Subtractive Lithography (SLSL) is well suited for printing actuators, T-junctions and complex three dimensional structures. The formation of truly arbitrary channels in 3D could revolutionize the fabrication of miniaturized chips and will find broad application in biology, chemistry and medicine.

Sr(Ti, Fe)O 3-δ Exhaust Gas Sensors

Sr(Ti x , Fe 1-x )O 3-δ solid solutions were found to change their temperature coefficient of resistance (TCR) from negative to positive as iron increasingly substitutes for titanium, with the TCR tending towards zero at × = 0.35. This composition, Sr(Ti 0.65 Fe 0.35 )O 3-δ thus shows temperature independent characteristic. For the development of a planar type sensing element for automotive applications, Sr(Ti, Fe)O 3-δ has to be applied as a thick film. To confirm the sensor characteristic temperature independence (at T = 750…950 °C, p O 2 = 10 −5 …1 bar) and fast response times ( t 90 = 6.5 ms at 900 °C), both key issues of Sr(Ti, Fe)O 3-δ , thick film sensors have to be maintained over the entire lifetime. In this work, the structural and electrical properties of the sensor are investigated with regard to the chemical stability of the sensing element.

Suspended liquid subtractive lithography: printing three dimensional channels directly into uncured PDMS

Polydimethylsiloxane (PDMS) is one of the most widely used polymers for the generation of microfluidic chips. The standard procedures of soft lithography require the formation of a new master structure for every design which is timeconsuming and expensive. All channel generated by soft lithography need to be consecutively sealed by bonding which is a process that can proof to be hard to control. Channel cross-sections are largely restricted to squares or flat-topped designs and the generation of truly three-dimensional designs is not straightforward. Here we present Suspended Liquid Subtractive Lithography (SLSL) a method for generating microfluidic channels of nearly arbitrary three-dimensional structures in PDMS that do not require master formation or bonding and give circular channel cross sections which are especially interesting for mimicking in vivo environments. In SLSL, an immiscible liquid is introduced into the uncured PDMS by a capillary mounted on a 3D printer head. The liquid forms continuous “threads” inside the matrix thus creating void suspended channel structures.

State-of-the-art thermal management demonstrated on cabin air conditioning

P+Z Engineering GmbH has succeeded in creating a virtual version of the highly complicated vehicle air conditioning process for the first time, in hitherto unseen detail, thanks to the development of “Theseus-FE”, a new thermal calculation tool, combined with co-simulation methods. This report highlights the power of this advanced technology, based on an example.

Untersuchungen zur Kinetik des Sauerstoffaustauschs an modifizierten Perowskitgrenzflachen

Der Sauerstoffaustausch zwischen Festkorper und Gasphase ist fur eine Reihe technologischer Hochtemperatur-Anwendungen von groser Bedeutung. In der vorliegenden Arbeit wurde die Kinetik dieser Austauschreaktion am Perowskit-Modellsystem Sr(Ti,Fe)O3 elektrochemisch und mikrostrukturell untersucht. Dabei konnte durch Oberflachenbeschichtungen mit Erdalkaliverbindungen (Ca, Sr oder Ba) eine Erhohung der Sauerstoffaustauschkinetik nachgewiesen werden.